Transition Metal Chelates with Schiff Bases Derived from Salicylaldehyde

TRANSITION METAT CHETATES \ryITH SCHIFF BASES DERIVED FROM SATICYTATDEHYDE AND DIAMINO ETHANE

ALVIN PAUL SUMMERTON, a.sc.

UNIVERSITY OF ADELAIDE, DEPARTMENT OF PHYSICAL AND INORGANIC CHEMISTRY

November 1978 1tu¡artol q''-'t tÇ14 Pl.l CONTENTS Page

SUMII{ARY

STATEMENT

ACKNOhTLEDGEMENTS

ABBREVIATIONS

CHAPTER 1: Introduction t

PART Ï

CHAPTER 2: Preparation and characterization of salen complexes of iron (III) 6

CHAPTER 3: Solut.ion studies 30

CHAPTER 4: Moessbauer .spectra 57

PART II

CHAPTER 5: Preparation, characterization and solid state properties 84

CHAPTER 6: Solution studies 115

CHAPTER 7: Crystal structure of Fe (saen) 2C1-.H2O 143

APPENDTX I: Computer Programs APPENDÏX II: Structure Factor Tables APPENDTX III: Publication

BIBLTOGRAPHY SUM¡{ARY The following study of transition metal complexes with Schiff base ligands comprises two related studies.

PART I Complexes of the quadridentate ligand, sa1en.

A number of complexes of general formula Fe(III)salenX, where X = Cl, Br, I, NO3, N3, NCS and C6H5COO, have been prepared. Of these species, two (X = NO3 and N3) have not been previously reported and only one (X = Cl) has been intensively studied. In addition, two novel octahedral complex salts

KlFesalen(CN)21 and Fe(saen) 2C1-.H2O, which involve a low spin central metal ion, have been obtained. The salt Fe(saen) 2CI. H2O has several unusual properties which have been examined in Part II of this thesis.

This j-nvestigation of the salen complexes has involved: (1) confirmation of structure by microanalysis, i.r. spectra and magnetism; (2\ a sÈudy of the u.v.-visible solution spectra; (3) deterinination of the Moessbauer spect.ral parameters.

The solution spectra indicated that the species were involved in dissociative equilibria which were utilized to estimate the relative stability of the covalent bond between the metal and the anionic ligand occupying the fifth coordination position. fn contrast to previous studies, where a correlation between magnetic properties and the Moessbauer data was indicated, the results of this work suggested that the magnitude of the quadrupole splitting may be related to the nature of the Fe-X bond. PART TI Comple xes of the trldentatê liqand , saen.

cationic salts of general formura M(rrr) (saen) 2x.nT)o, where X = CI, Br, I, NO3, NCSrC1O4, BF4 and. pF6r n = 0 or l, have been prepared and confirmed. by microanarysis for M = cr, Fe and Co. The properties of these salts have been studied. in the sorid state utitiri\g a variety of techniques incruding i.r spectra, x-ray powder diffraction, magnetic susceptibility and dehydration in vacuo. The characteristic i.r. spectra obtained allowed classification of the sarts into one of three general spectral types. This classification was found to be dependent on the nature of both the metal and anion in addition to the presence or absence of water of hydration

The chloride salts of aIr three comprex metar cations v¡ere found to be isostructural monohydrates of particurar interest. The water morecule appeared to be unusual in that it: (1) produced two sharp absorptions in the O-H stretching region of the i.r. spectrum; (2) stabilized the low spin state of Fe(III) in Fe(saen) zCI. Hzo.

The solution properties of the salts \^rere studied via n.m-r. and u.v.-visible spectra, conductance and magnetic susceptibility measurenrents. In contrast to the resul-ts obtained in water and alcohol, association of the chloride sal-ts was observed in the aprotic solvent DMSO, wíth the evidence supporting the conclusion that the water morecule also remained bound.

rn order to rationalize the behaviour of the water morecule, the molecular structure of Fe(saen) 2Cl-.H2O was determined by the single crystal x-ray diffraction technique. The water molecule was found to be strongly bound into the crystal lattice via unusual, specific hydrogen bonding interactions. These specific ínteractions involve one cation ammine hydrogen atom with the water oxygen and one water hydrogen atom with the anion. The mode of bonding of the water molecule has enabled the above- mentj-oned unusual features to be explained. STATEMENT

This thesis contains no materiat which has been accepted for the award of any other degree or diploma in any university, and to the best of my knowled.ge and belief, contains no material previously published or written by another personr gxc€pt where due r,eference is made in the text of the thesis.

A. P. SUMMERTON ACKNOWLEDGEMENTS

My sincerest thanks are extended to all who have been involved with my research project friends, colleagues and students at both the S.A. Institute of Technotogy and the University of Ade1aide. I am grateful to prof. R.V. Cu1ver of S.A.I.T. and Prof. D.O. Jordan of the University for making available to me the facilities to undertake this worki to Dr. Mike Snow and Mr. John Vrlestphalen for assistance with the crystat structure, and to Mrs. Trish McMurtrie for the typing of this thesís. The expertise, interest and friendship of Mr. Keith Johnson of S.A.I.T. has been appreciated immensely. The guidance, patience, consideration and advice f have received from my supervisor, Dr. Alex Diamantis, over a number of years, has proved of inestimable value in completing this research.

Finally, I am indebted to my parentsr my wife Kathryn and daughters, Lynda and Kerry, to whom I wish to dedicate this work. ABBREVIATIONS

The following abbreviations have been used in the text of this thesis: acac Acetylacetonate

C6H5COO Benzoato

DMF Dimethyl formamide

DMSO Dimethylsulphoxide

Et'OH Ethanol en 1,2 diaminoethane (ethylenediamine )

MeOH Methanol ophen 2 aminoaniline (o-phenylenediamine ) pn L12 diaminopropane (propylenedi amine)

sal salicylaldehydato

Structures of the Schiff base ligands involved in this study:

salen

N N \ I CH z-H zc o- saen

\ /N"z CH -H c 2 2 salpn o- -o

T H-I{ c 2 H 3 saloph-en -o

=\J I CHAPTER I

1.1 TNTRODUCTION

From an historical viewpoint, chemistry received recog- nition as a scientific discipti-ne in its own ríght as recently as L789, with the establishment of the first Chair in chemistry(1). Prior to this, it rnras merery considered to be a branch of Medical science. The profound infl-uence of the 'biologicar biasr in scientific thought is readily reflected in the controversy surrounding the 'vitar force' theory, which many scientists regard as having significantly retarded the advancement of organic chemistry during the early nineteenth c"rrtrrry(2). An indication of the animosity generated by this controversy is provided by Berzerius, whor orr being informed of wohrerrs urea synthesis, is reported Èo have stated that 'whosoever started on the path of, immortal fame with urine was (3). bound to end in i¡'

chemistry has rapidly developed d.uring the ensuing period, with the inevitabre accumuration of a vast mass of knowledge. rt is hardry surprising that sub-division into major areas, such as Analytical, rnorganic, Physicat, organic and Biochem- istry, has occurred in an effort to quantify this knowledge. Such areas have become effectively isolated tiisciplines today, and increasing speciatization in narrower and narrower fields appears the norm for postgraduate research.

It is interesting to noÈe an apparent reversal in this trend where emphasis is placed on the interaction between supposedly isolated disciprines. As a particurar example, the 2 rapidry developing field of Bio-inorganic chemistry overlaps significantly into areas traditionalry associated with Bio- chemistry and Biology(4). rn this field, the especiat expertise of the coordination chemist is applied to specific problems, of major importance to the 'rife sciences' , such as the chemistry of the important naturalry occurring metarloprotein= (4). A particurarly useful approach utirizes ,model compoundsr, whereby, through the invention, synthesis and.detailed. study of suitable simpre systems, some insight into the workings of the natural system may be achieved(4).

A large number of the model systems involve the complexes rfamily' of a of ligands collectively referred to as 'schiff bäses'. such ligands contain the azomethine group (-Rc=N-), and one class of such compounds which has been intensively studied involves the NrN'-ethyrene bis (saricylideneimine) dianion, traditionally abbreviated as sa1en. cobart comprexes of Èhis liganit provide a typicar iIl-ustration of the apprication of such compounds as biorogicar models. co(rr)saten (5) (a) is able to absorb oxygen reversibty , and 'be (6) (b) may reduced to (co (r) saren) - , which reacts with arkyl halides to form complexes formulated as Rco(rrr)salen. The alkyl group, R, is directry bonded to the cobalt through the carbon .to*(7). similar reactions have been observed with a naturarly occurring system involving a coenzyme of vitamin (t). "r,

Arthough schiff base complexes have proved of significance in the understanding of biological systems, Èhe chemistry of such compounds is of considerable interest in its own right 3 and has occupied a central role in the development of modern coordination chemistry(9). of all the complexes studied, those with salicylaldimine ligands, and in particular salen, have received the most attentiorr(9). rn fact a recent review, devoted solely to the c.ompounds of sa1en, cites 155 major references covering a six year period(10). No doubt this is due, in part, to thè fact that salen complexes have been obtaified htith many metals, including the majority of the transition elemenÈs. However, a significant factor must be the diversity in the chemical behaviour of these compounds, as illustrated. in the review article by Hobday and Smith(10).

1.2 RESEARCH PROGRAM - Aim and Rationale

The primary aim of this research study is the determination of the lvloessbauer parameters for a series of iron(III) complexes and the elucidation of the correlation, if ârry, between these and other information which may be related to the bonding of ligands

A convenient. system, for such a study, would be a series of iron (III) complexes in which the ligand occupying one coordination site may be varied whilst the remaining positions have the same coordination environment. The series selected is that of general formula Fe(III)salenX, where X is the uni- dentate ligand varied.

Salen is a quadridentate ligand, and the iron (III) salenX complexes would be expected to be square pyramidal. The structure determinat,ion of the chloro-complex showed it to be a dimer wittr assymetric bridging between the monomers via 4 inÈermolecular interactions involving the central ion and an adjacent phenolic oxygen. This results in essentially octahedral coordination as ishown in the following diagram(10'11),

x o N

N At the time this study was commenced (1970), only a few members of the series had been reported (x = c1(12), Br(13) and c6H5coo(14) )1. Furthermore, there was substant,iar disagreement in the Moessbauer data available. The published values for the chloro complex covered a range of 0.57 to 0.7I mm/sec. for the centre shift and 0.71 to 1.45 mm/sec.

for the quadrupore spritting(15 'L6'17) .

In broad outline, then, the proposed research program involves the preparation and characterization of the series Fe(III)sa1enX, determinat.ion of the Moessbauer parameters and any other data which may be relevant to the bonding of X. This study is presented in Part I (Chapters 2, 3 and 4) of this thesis.

In Part II (Chapters 5, 6 and 7) , the properties of complexes of the tridentate ligand, saen, are investigated. The interest in this ligand arose as a consequence of the

(r73) tDuring the course of this study the complexes with X = f and NCS (L741 have been reported. 5 observation of severar unusual aspects in the properties of the low spin hydrate , of formula Fe (III) (saen) 2CL.HZO. In parLicular, t,he interest centred on the water molecure which appeared to be responsible for unusual features in the i.r. spectra as well as the stabilization of the row spin state.

tl PART I

Complexes of the quadridentate ligand, salen. 6

CTIAPTER 2

Preparation and characÈerization of sa.l_en complexes of iron (III) .

2.L PREPARATTON

(i) Appl-ication of published meLhods The complex FesalenCl may be synthesized by the direct reaction of anhydrous ferric chloride, Fec13, with the schiff base, salenH2, in the stoichiometric ratio 1:1, utilizing an organic solvent such as acetone or ethanol (13) . This reaction may be described by the equat,ion: FeCl3 + sa1enH2 + I'esaIenCI + 2HC\. Tþis procedure gives high yields. of quite pure product provid,ed small quantities are involved (<0.01 mole) .

In this study, larger quantities (c.a. 0.1 mole, 36 grams) were required as a starting material. Attempts to apply this direct reaction on such a scale resulted in low yields (<308) of impure solid contaminated with free salicylaldehyde. . Purification of the impure product by solvent extraction with acetone gave, in addition to crystalline FesalenCl, white, acetone insoluble crystals. The latter compound was identified as the hydrochloride salt of I,2 diaminoethane, êûr and is denoted hereafter as enH2C72. This information suggested that hyd.rolysis of salenH2 had occurred.

A spectral investigation of the hydrolysis of salenH2 (discussed in detail in Chapter 3, section 3.3 (b)) shows that irreversible hydrolysis of salenH2 occurs rapidly in acidic solutions. Clearly, then, the problem as outlined 7 above, is a resurt of the difficulty in eriminating HCr when large scare preparations are involved. A synthetic procedure of general application to salicylaldimine complexes recommenrfs reaction of the metar ion and schiff base in the presence of added b.="(9). Attempted syntheses utilizing sodium hydroxide, ethoxide, carbonate .and acet.ate, all resurted in the formation, in near quantiÈative yield, of the well known oxo species, (Fesa1en)ZO. The spectral dàta in 3.3(b) confirm that in the presence of a strong base, such as sodium hydroxide, the di- anj-on, salen2-, is probably present in solution. It would appear that rapid hydrolysis of any FesalenX, formed. in solution, Èo (Fesalen)20 is caused by Èhe excess of added alkali.

However, the spectrum of salenH2 is not affected. by the presence of a hundredfold molar excess of en. In fact the only noticeable effect of en is an increase in the sorubility of salenH2. As the hydrolysis of salenH2 in acid solution may be represented as: H2O) , salenH2 + 2HCL 2 salicylaldehyde + enH2C12 , add.ition of a moderate excess of en to the reaction mixture could be expected to react with the free acid in preference to the hydrolysis.

Syntheses attempted utilizing the Fe (III) salt, salenH2 and en in the mole ratio 1:l:1.2 proved successful.

(ii) Reaction schernes for the synthesis of FesalenX Scheme A - modified direct reaction. FeX3 + salenH2 + en + Fesalenx + enH2X2

where X = CI, Br, NO3 or C6H5COO. B With Èhis modified procedure, the reaction proceeded smoothly giving high yields of product utitizíng either anhydrous or hydrated Fe (III) salts (e.9. FeC13.6H2O, FeBr3.6H2O and Fe (NO3) 3.9H2o) .

Scheme B - metathesis EtoH) FesalenC1 + Mx Fesalenx + MCI

where X = N3 and I (M - K or Na), NCS and Br (M = K, Na or NH4).

Any salt MX can be used provided KspMX > KspMCl in ethanol, which was the solvent utilized. Alternatively, FesaIenNO3 can replace FesalenCl, and acetone may be used as solvent,. Attempts to prepare FesalenX with X = CH3COO, NO2, OH or OEt (M = Na) resulted in the formation of (Fesalen) 20.

Purification of the products, FesalenX, by solvent extraction is discussed in the experimenÈa1 section of this Chapter.

(iii) Novel Compounds

(a) Dj-cyâno cornpl-ex Metathetical reactions using cyanide salts resulted in the formation of the green anionic species lFesalen(CN)2J-. EtoH) Fesalenx + McN M lFesalen(cN) 2] + Mx where [ = any of the ligands Cl, Br, T., NCS, NO]r N3 or C5H5COO and M = NH4 or K.

Alternatively, the oxo compound may be used: EtoH) (Fesalen) ZO + McN M [Fesalen (cN) 2l + ttou 9 (b) Product of reaction with I ,2 diaminoethane

Reaction of FesalenCL with en yielded a deep blue crystalline solid with a formula corresponding to tFe(rrr)salen.enlcl .Hzo. rt was suspected that Èhis species was a mixed bidentate-quadrid.entate cherate similar to the well studied complex, Cosalenacac, but in fact proved to be the bistridentate complex Fe (III) (saen) 2CL.H2O. A more detailed. discussion of the properties of complexes with the ligand saen is presented in Part III, however, some information relevant to the Fe (rrr) chloro comprex is discussed in this part of the Èhesis.

2..2 CHARå,CTERTZAT.I ON OF C'OMPT,EXES

(i ) Analyses

The data obtained for the elements carbon, hydrogen, nitrogen and iron are presented in Table 2.1.

The analytical data confirm the compounds as formulated.

(ii) Solid state studies

Further characterization of these compounds was effected by measurement of the following solid state parameters: (a) Magnetic moments; (b) U.V. -visible spectra; (c) fnfra-red spectra; (d) Moessbauer spectra. The Moessbauer spectra and data are discussed separately in Chapter 4. TABLE 2.I Analytical Data

Compound Formula Calculated Found

(a) FesalenX C16H14N2o2FeX c H N Fe c H N Fe

X=CI C16H14N202FeCI s3.7 4.0 7.8 15. 6 53.s 4.0 7.9 15.5 Br C15H14N202FeBr 47 .8 3.5 7.0 1.3.9 47.9 3.6 7.r 13. 6 I C15H14N2O2FeI 42.8 3.1 6.2 L2.4 42.9 3.2 6.2 L2.4 ' No3 C16H14N305Fe 50.0 3.7 10 .9 14 .5 50.0 3.7 r0.9 L4 .4 NCS C17H14N302FeS 53.7 3.7 11.1 ]-4.7 53.7 3.7 11. I L4 .6 N3 C15H14N5O2Fe 52.8 3.9 L9.2 L5.2 52.7 4.0 19 .0 L5.2 C6H5COO C23H19N204Fe 62 .3 4.3 6.3 L2.6 61. 9 4.3 6.3 12.7

(b) (Fesalen) 2O CZZHZgN405Fe 58.2 4-3 8.5 16.9 58.2 4.3 8.4 17.0

(c) M IFesalen (CN) 2l 14 = NH4 CtgHZ2N602Fe 55.r 4.6 17.9 L4.2 54.4 4. 5 17 .6 14. 0 M=K C1gH14N4O2FeK 52.3 3.4 13.6 13.5 52.L 3. 6 L3.4 13. 6

(d) Fe (saen) 2CL.H2O C18H24N4O2FeC1 49.6 s.6 12.9 L2.8 49.3 5.6 )-2.8 L2.8 IO

(a) Magnet.ic moments

The Fe (III) ion has five 3d electrons and the electronic ground state may have five, three or one unpaired electron(s). As Llne i-rn4 .91 configuration only occurs in a tetragonally distorted fierd, the possible erectronic configurations in an essentially octahedral ligand. field are Lzs3 en2 with a total (18). spin, s, of Z, or tzgs, s = , Measurement of magnetic susceptibilities, prêferably over a range of temperatures, (18) readily allows the determination of S in the ground state .

As no contribution from the orbital angular momentum is expected for a high spin (s = |l system the experimental varues of the magnet,ic moment, !e.ff , should be close to the spin only value of 5.92 B.M. Sígnificant deviations of Lreff from this value have been observed. and are usually ascribed to dimer formation, as established for (Fesalen3L) Z by an X-ray structure determination(19). rn such cases the observed range (13) for uef f is 5.0 to 5.4 B.M. .

1 The.low spin (S = ,) case is expected to result in values of Ueff of the order of 2 8.M., which show considerable temperature dependence.

In this study, magnetic susceptibilities have been determined at room tpmperature only and as such must be considered of limited value as the sole means .of evaluating the ground state configurations. Ho\oever, as add.itional information, in the form of Moessbauer data, is available, the variable temperature magnetic studies \^rere not considered essential in this study. 11 The magnetic suscepti'bilities and moments are tabulated in Tab1e 2.2.

On t.he basis of the observed magnetic moments, three categories of iron(III) Schiff base complexes have been - (10.11) proposed' . (1) Spin free, paramagnetic systems with treff near 5.92 8.M., e.g. the monomer, FesalenCl.

(2) lileakly coupled. antiferromagnetic systems with Ueff =

5.2 B ¡I{. , ê .9. the dimer, (FesalenCl) 2 . (3) Medium coupled antiferromagnetic systems with Ueff g 2 B.M., ê.9. (Fesalen) 20.

Thus the majority of complexes studied for this work appear to be dimeric (X = CI, Br, I, NCS and N3) and two monomeric (X = C5H5COO and NO3).

The two compounds with Ueff near 2 B.M. require further consideration. The above proposal suggests that these compounds are similar to (Fesalen)20, i.e,. dimers with antif.rro- magnetic exchange occurring between two high spin (s = |l iron (III) centres resulting in a pseudo low spin value for l.leff . In fact, these compounds are bot.h believed to be low I spin (S = ;), although the single temperature magnetic measurements in no way provides conclusive evídence.

A more pertinent argument is provid.ed by the coordination environment of iron(IIT) in these complexes. In Fe(saen)2CI. H2O, the iron(III) is involved in an octahedrally coordinated (20) cation, Fê (saen) 2+ . The interatomic distances in this TABI,E 2.2

Colour of finely l,Iotrar Susceptibitity Compound T"K Ueff (a) Ueff divided solid Xt,t (corrected) (b) (published) (13) FesalenCl violet-bror¡rn t52 458 293.2 5.33 5.34 ( 13) Br black L54 s94 295.2 s.39 s. 40 I black 14s 5L7 293.L 5.2L

No3 violet r83 134 293.6 5. 85

NCS chocolate brown 150 343 295.5 5.31

N3 red brown 151 034 29 4.L 5.33 (14) C6H5CO deep red 188 009 293.9 s.93 5.88

K IFesalen (CN)Z ] bright green 23 739 293.L 2.t0

Fe (saen) 2CL.H2O blue 23 606 296 .6 2.rL

(a) estimated error t .03 B.M. (b) S.I. units, i.e. x 16-12*3*o1-1 I2 cation are consistent with a low spin Fe (IfI) complex, as discussed later in this thesis. The formulation of^ the anionic species IFesalen(CN)21 is indicative of octahedral coordination also, and it would appear that the possibility of antiferromagnetic coupling is precluded in such complexes. Additional evidence provided by Moessbauer spectral data, âs discussed in Chapter 4, strongly support the proposition that both complexes are indeed low spin.

(b) U.v.-visible spectra - reflectance and muIl

For a d5 ion in a cubic field, such as a high spin, octahedrally coordinated iron(III) species, the electronic ground state is designaÈed, as 64. As no other spin-sextuplet energy levels occur in this system, aII 'd-d' transitions are spin forhidden and would be expected to result in very low intensity transitions in the visible region(18). In addition such transitions may well be obscrlred by intense metal-ligand charge transfer absorptions, which are known to occur at (rB 2t) rerativery row energi-es ' .

AII of the compounds studied in this work are intensely coloured, suggesting that such charge transfer absorptions probably occur. In particular, the crystalline samples of the FesalenX series are extremely dark, almost b1ack, needles. When finely divided, the solids have a characteristic hue as described in Table 2.2.

The solid state spectra (reflectance and mull) of the complexes show a broad absorption in the visible and near ultraviolet. A representative selection of the muII spectra Figure 2.I MuII Spectra

(a)

(¡) o

(E -o L o (b) o -o

(c)

(d )

ó50 T5O 85O 350 450 550 wavelen gth (nm.) (a) FesalenCl (b) FesalenC6H5COO (c) Fe (saen) 2CI -H2o

(d) K IFesalen (CN) Z I 13 are reproduced in Figure 2.L. The major contribution to the spectra of the FesalenX type of compound (spectra (a) and (b) ) must be the metal ligand charge transfer absorpt,ion(s). However, in two cases (X = NO3 and C6H5COO) the presence of 'weak humps' in the broad envelope may indicate the presence of 'd-d' type transit.ions. It is felt that, in generalr flo meaningful information may be obtained from these spectra.

The two remaining spectra do have features which may prove of interest. The complexes involved are the low spin octahedral species discussed above. In S = | systems spin- allowed d-d transiÈions from the 'r, ground state are well known(18). The prominent bands centred at 640 nm (re(saen)2cI.

HZO - spectrum (c)) an-d 750 nm (KlFesalen(CN)Zl spectrum (d) ) may be assigned to such transitions, with the relatively high intensity being rationalized in terms of the effect of 'mixing' (rB) of charge transfer with Èhe 'd-d' absorptions .

The .infra-red spectra of all compounds have been obtained over the range 41000 4OO .*-I. Many similar features have been observed, including: (1) alt have a multiptet near 750 cm-l, (2\ all salen complexes have two prominent bands of similar intensity between 400 and 5OO cfr-I, (3) all salen complexes have no prominent bands of significance above 2 1200 cm-l other than a similar series of very weak C-H stretch vibrations between 2,800 and 3rl0O cm-I.

rn the region from 11800 to 400 each spectrum is "*-1 Figure 2.2 Infra-red Spectra

(a) SalenH2

(b) Fe (salen) 20 (c) FêsalenCI (d) FesaIenI (e) FesaIenNO3 (f) FesalenNCS (s) FesaIenN3

(h) K lFesalen (CN) Z ]

(i) Fe (saen) 2CL.H,2O Figure 2.2 (a) (c)

(a)

(b)

(c)

r8 1ô 14 12 10 I 6 4 wavenumber (x lO2) cm-l Figure 2.2 (d) (f)

(d)

(e)

(f)

r8 r6 1¡l 12 ro I 6 4

wavenumber (x 102) cm-l Figure 2.2 (g) (i)

(e)

(h)

(.)

10 IG 1a t2 I 6 4 wavenumber (x 102) cm-I I4 unique and may thus be considered as characteristic of the individual complex. A representative selection of these spectra have been reproduced in Figure 2.2.

Comparison of the spectrum of the free ligand sa1enH2, (a), with those of the salen complexes, (b) - (h), clearly show that many of the salen ligand vibraÈional bands are affected markedly on chelation. There appears to be little value ín attempting to assign all spectral bands to specific vibrational transitions as the similarities in the spectra are vastly outnumbered by the differences. These spectra have been used as a means of identifying qualitatively species isolated from various reactions.

However, several spectra do contain prominent bands, frequently outside the range I,800 400 which may be - "*-1, used to confirm that coordination has occurred between the Iigand X and the metal. Relevant data are reproduced in Tab1e 2.3, and all band positions are consistent with those expected for coordinated X.

As a further example, in a more detailed examination of the spectrum of the complex FesalenNCS below 900 .*-1, several additional bands, not present in the similar spectrum of FesalenCl, can be observed. These are: (a) at 848 c*-1, âs a shoulder on the band. at 852 cm-I; (b) a weak doublet, ât 485 and 478 c*-1.

For the system M-I{CS, bands are found between 870-820 .*-1 (v"-r) and 485-475 (Qvcs), whereas for M-SCN the corres- "*-1 TABLE .3

-1 -'t re lative và (cm ) x ucmt intensity (a) (b) Reference

N3 2055 0.91 ,45 (22) p. 176

No3 15 35 I.34 =30 (22) p. 161

C6H5COO 688 0.76 7 (d) 719 0.98 11 r420 1.06 a 50 1500 0.87 =10 15 15 0.91 =30

CN 2Lts 0.62 10 (22¡ p. 16 6 (Z off) 2L35 0.27 10

Notes: observed intensity of peak (a) Relative IntensitY = intensity of major peak in 750 cm multiplet (b) v\ = peêk width at half-height. Where approximate values of \lz are given, the value has been estimated from overlapping bands.

(d) Assignment of bands based on comparison with the i.r. spect.ra of C5H5COO-Na+ and Pe (C5H5COO) 3. 15 ponding transitions are 760-200 .*-r (u"_.),and 470-430 (23). "*-1 (ô*.r) This evidence confirms the proposition that the thiocyanate group is coordinaÈed to the metal via the nitrogen atom.

2.3 STRUCTURE OF COMPLEXES TN TTIE SOLTD STATE

The characterization outlined above, confirms that the compounds are as formulated in Table 2.L. Two aspects of the structure of these complexes requiring further clarification are: (i) What is the arrangement of the cyanide ligands in the - anion lFesalen (Ctrt¡ ,1 cis or trans?

(i.i ) Are the species of the series Fesalenx penta-coordinate monomers or pseudo-octahedral dimers?

(i ) The Anion F'esa

The assignment of the configuration, cis or trans, of dicyano complexes may be decid.ed on the basis of the number of bands observed in c=N stretch region of the i.r. spectrum. on theoretical grounds, the trans configuration spectrum is expected to have a single bandr ês is the case in known trans- dicyano comprexe s(24) . on the other hand, two closely spaced bands are indicative of the cis configuration as has also been (25i oþservecl'. -

The Chromium and cobalt analogues of the iron (III) salt (26 have been prepared '27 ) and the trans configuration has been assigned, apparently arbitrarily, to the complex KlCosalen(CN)Z] (28). A sample of this compound has been prepared, and the l6

i.r. spectrum examined. The spectra of both the Fe (rrr) and Co (III) salen dicyano salts are very similar in the 'characteristic' 1,800 400 .*-r region and the c=N stretch region.

The assignment of a trans configuration t.o these dicyano species could perhaps be justified on the basis that salen acts as a pranar, quadridentate ligand. However this justification is clearry in error as at least two species, namery co(rrr) salen.."."(29) and dioxo Mo(vr)sarerr(30), have been shown to have e non planar salen moiety. rn such non pranar systems, the n.m.r. signar assigned to protons in the azomet,hine group (31) consists of two resonances . No splitting could be observed in the n.m.r. spectrum of KlCosalen(CU)21, which indicates *rãt tfre salen moiety is not coordinated in the same r^ray as in Cosalen. acac.

In addition, a partially characterized complex formutated as KlFesalophen(cN)zl has been prepared, and the i.r. spectrum of this compound arso has two sharp bands in the clN stretch region aL 2L20 and 2140 The ligand salophen "*-1. is compretely conjugated and is known to exhibit planar coordination only(31). This dicyano complex must have a trans configuration.

Laser Raman spectra of the three dicyano species have been attempted, but, due to decomposition of the Fe (IIf) complexes, meaningful data have only been obtained for KlCosalen(CN)21 with sharp absorptions at 2LZ6 and 2I5O cil-l, which is the same as in the i.r. spectrum. The theoreticar considerations leading to the pred5-ction of tw-o bands in the i.r. spectrum of a cis dicyano system, require that the bands L7 should be found at energies some 100 cxn-I higher than in the i.r. spectra. CIearIy then these two bands do not occur simply as a result of a cis configuration.

The Fe (III) anions readily undergo decomposition in aqueous solut.ions, whereas the Co(III) species may be recrystallized from water. Consequently, a series of salts of the Co(III)salen(CN)2 aníon have been prepared.with a number of different cations. The band positions and relative intensities appear anion dependent. The data are tabulated in Tab1e 2.4.

No pattern can be elucidated from this data in terms of cation charge or size, although it would appear reasonable to suggest that packing in the crystal lattice would be influenced by such variables. Therefore it is proposed that the two bands observed in the C=N stretch region arise because of 'solid statet or tlatticer effects.

Such behaviour is not uncommon in polynuclear cyano complexes. The spectra of hexacyano species would be expected to have one band only in the c=N stretch region Q2) . However, in the complexes K3 tCo (CN) 01 , K3 [Fe (CN) 5 ] and K4 [Fe (CN) e ] , three, one and seven bands are observed respectivery Qz¡ . This is attributed to a 'lattice' effectQz¡.

The dicyano compounds would appear to have a trans arrangement of the cyanide ligands and a square planar Schiff base moiety. The presence of two bands from the C=N stretching vibrations is best rationalized as a solid state (or lattice) TABT,E 2.4

Data for salts with the Co (III) salen (Cu¡, anion

Cation C=N stretchin frequencies fntensity * (cm- ? ) ratio

K+ 2130 (vk=19¡ , 2155 (v%=10 ) Iz2 + Ag 2L25 (sh) , 2I'75 (v\=20) 1:0.06

Co (saen) 2+ 2140 (tsÐz=20) , 2200 (v%=30 ) 1:0.13

Cosalen* 2L45 (v\=20') , 2200 (vL=30) I :0 .14

J- Et4N' 2140 (tt\=20) , 2200 (v%=30) l:0.23 ca2+ 2r4o (vÐr=29¡ , 2200(vL=30) 1:0.13 cu2+ 2L40 (v\=20) , 2195(v%=30) l:0.11 zn2+ 2L45 (v1=20) , 2180 (Éh) , 2200 (và=40) 1:0.08

Fe3+ 2140 (v\=20) , 2L95 (vL=30) 1: 0 .13

* intensity ratio Intensity of higher energy absorption = Intensity of low energy absorption 18 effect, indicating the two cyano ligands are not in equivarent envìronments in the crystal lattice.

(ii) FesalenX series

Previous studies of the chloro complex have established that either dimeric, (Fesalencl)2, or monomeric, Fesalencr, species may be obtained ón recrystalrization, depending on the (32¡ sorvent serected . The bromo comprex apparentry behaves in the same wây, with the monomeric species having one or more (16) solvent molecules incorporated in the crystar ratti". . The analytical data presented in Table 2.I confirm that in the series of FesalenX prepared in this studyr rro solvent molecules a{e present. rt would appear that solvent incorporation is affected markedly by the rate at which the crystalline species are formed, with the sorvent adducts occurring only when the procedure is very srow. Thus the rapid recrystalrization, by solvent extraction, used in preparing this series of complexes, would suggest that dimeric species only wourd be expected.

The assignment of the structure, based on the magnetic moment of the complexr €rs discussed in section 2.2(ii), indi- êated that the dimeric structure appeared probabre for x = c1, Br, T.t NCS and N3, and the monomeric for X = NO3 and C6H5COO. An additionar criterion has been proposed on the basis of i.r. spectra with supporting evidence from Moessbauer spect.r(16). fn several cases the magnetic and spectral criteria resulted in differing concrusions, suggesting 'that at l-east for complexes having magnetic susceptibilities near the spin-onry va1ue, the agreement between the magnetic susceptibitities and the suggested dimeric model, is probably fortuitous,(16). I9

The fundamental premise involves the assignment of an intense band in the i.r. spectra, near 850 to an Fe-o-Fe "*-I, vibration. The complex (Fesalen) 20 has an Fe-O-Fe linkage, and an intense band, near 825.*-1, which has been assigned

to the assymetric Fe-o-Fe stretching vibrat,ion(33'34) The presence of such a band in the spectra of FesatenX complexes, (16) would then be indicative of a dimeric structure .

In order to test this proposition, the i.r. spectra of a number of compounds have been examined in some detail in the region of 850 The positions and retative intensities "*-1. of the spectral bands are tabulated in Table 2.5.

The M(II)sa1en complexes, prepared by previously described procedures (where M = co(5), Cu and Ni(35)) known (10) ^r" to have dimeric structur"" . These complexes do have relatively intense bands near 850 however so do "*-1, al-I of the other species includ.ing the free ligand, salenH2.

In the case of the known monomer, Fê(saen) 2cl.u2o(201 , it is not possible for there to be an Fe-O-Fe interaction of the type proposed for (FesalencL)2(1I). Furthermore, the Ni(II) complex, denoted in Table 2.5 as Nisalen, is known from the X-ray struct.ural determination to have 'a dimeric structure formed by direct metal-metal interactions' (10).

Consequently, it is suggested that the assignment of a dimeric or monomeric structure solely on the basis of i.r. assignments, is, to say the least, somewhat dubious. Due to the similarity of the spectra, it would appear more reasonable TABLE 2.5

I.R. SPECÎRAL BAI{DS NEAR 850 CM-I

Structure f2,l

1 ) Salen Hz 9o2(.25) 878 ( . 20) 8621.67¡( 778(.54) 760(.75) 7s2 ( r .0) 74s ( .96)

r ( Cu Salen 9l .70) 860(.54) 8s2(.s6) 793(.29) 763(.66) 758(.84) 7s2(.9s) 745 ( .83) 73s ( 1.0) D

( Co Salen 908 .44 ) 85s(.3s) 849(.23) 797 (. r0) 762(.48) 756(.7L) 75r(.87) 746(.65) 734(1.0) D

Nl Salen 9os (.70) 85s(.40) 848(.44) 832 ( . 17) 800 ( . 25) 7so ( . 8s) 740 ( . 96) 733(1.0) 727 (.92',) D

(Fe Selen) z O 906(.80) 860(.42) 826(.8D(3) 7e2(.98) 759(.93) 751(r.o) 742(.83) 7 32(.77)

Fe Salen CI 906 ( .58) 8el(.18) 866(.20) 8s5(.11) 80r (.43) 79s(.4r) 767 (.e6) 758 ( r .0) 74s(.38) 738(.3r)

Fe Salen Br 9os ( .60) 889(.27) 862(.22) 8s1(.22) 798(.s7) 77 6(.84) 75s (r.0) 739(.46) AI)

Fe Salen I e05(. se) 888(.22) 862(.19) 8s0(.17) 827 (. 15) 797(.47) 772(.77) 7s3( r.o) 739(.49) U

Fe Salen lþ¡ eo5(.41) 87 2( .06) 862(.t6) 827 ( .09) 802(.60) 778(.97) 760(r.0) 745(.46) U

Fe Salen C5II5Cfi) eo4(.62) 863(.70) 850(.38) 820(.05) 801 (. s0) 796( . sl) 7s6( l .0) 740(.39) 734(.40) 7ts(.eut1) U

( Fe Salen N3 905 .49) 888(.21) 870(.19) 8s0(.19) 798(.41) 789 ( .27 ) 776(.46) 7ss( 1.0) 7 40 (.44) U

Fe Salen NCS 906(.54) 890(.24) 869(.30) 852(.t7) 8o0(.37) 766(.83) 758(r.0) 739 ( . 20) U

K[Fe Salen(CN)z ] 902(.49) 8s8 ( .23) 826 ( . 06) 789 ( .4 s) 770( l .0) 760(.87) 7 47 ( .28) M

Fe(Saen) zCl,H2O 897 ( .56) 8s7 ( .38) 7e6 ( .56) 782(.23) 769 (.88) 7se ( l .0) 747 (.47) 7 40(.7 2) M

Ì'¡OTES: (1) reraÈlve fntenslties shor¡n fn parentheses

relaÈive LntenslÈy æasured intensLty of proolnenÈ bard fn -7 cm nulÈ lplet (2) Abbrevlarlons for scructure: D = dinerfc, AD - aeerûed dlmerlc, U - unkno¡¡n, Ìl - rcnonerlc. (s) Fe-O-Fe band l¡r (Fe Salen)zO

(4) C5H5COO llgard bard. 20 to assign bands in the 820-890 .*-1 region to salen ligand vibrations, together with the bands at 904!7 | 795t9 cm'-1 and the multiplet near 750 .*-1. Vrlhilst the hypothesis that an Fe-O-Fe linkage may be associated with an i.r. band is va1id, the corollary may not necessarily be so.

It appears improbable that a definite assignment of either a dimeric or monomeric structural arrangement to each of the members of the series FesalenX can be decided on the basis of relatively simple data. The only positive method would be to d.etermine the structure of, each complex by X-ray diffraction techniques. such studies wourd require considerable expenditure of time and effort and may, in fact, be unnecessary, at least for the purposes of this study. There is evidence to suggest that the Moessbauer spectra of monomeric and dimeric species may not be significantly different. This suggestion is explored in detail in Chapter 4.

2.4 EXPERTMENTAL

(i) StarÈing materials

The puríty of the chemicals used did not appear to be critical to the success of the preparative methods. In general, A.R. grade chemicals were used as starting materials and solvents.

(ii¡ Preparations

(a) SalenH2

The condensation reaction between salicylaldehyde and 2L Lr2 diaminoethaner êrr, is known to occur readily in near quantitative yiera(5). A typicar example of the rigand preparation forlows: A sorution of L22.1g of salicyraldehyde

(r mole) in 3oo c*3 of ethanol in a l1itre beaker, r^ras stirred vigorously and warmed to 6ooc. 30g en (0.5 more) in 300 cm3 of ethanol was slowry added, and a yerlow crystalrine solid formed rapidry. The reaction mixture was then gently boiled for L5-20 minutes, allowed to cool overnight and the sorid, collected by vacuum filtration, dried for 24 hours at

7 ooc.

Yield 1269 (94?) M.pr. L26oc.

Analysis for CI6H1ONZOZ, in quadruplicate: Caic. C7I.62, H6.01, NIO.44s" Found C7L.6L, H6.02, NL0.44Z

samples of salenH2 have been used successfulry as a secondary standard in C, H and N determinations.

(b) Fesal-enX complexes

Scheme A - direct reaction General method FeX3, salenH2 and en in mole ratio 1:l:L.2. Finery divided sarenH2 r¡ras suspended in a vigorously stirred ethanol-en solution. A sorution of the appropriate Fe (III) salt in ethanol was then slowly added to the suspension. stirring was continued for 30 minutes, and the deepry coroured 'reacÈion mixture allowed to stand for a further 30 minutes

before vacuum filtration. Any sorid product collected was retained, the filtrate evaporated to dryness by rotary evaporation and the solid product collected. 22 For [ = Cl, Br and C6H5COO' the solid product obtained from the first filtration contained almost aII the crude FesalenX, whereas the nitrate was found in the rotary evaporation residue only.

In alt cases, the crude product was separated from the enH2X2, also formed, by extraction in a Soxhlet extraction system with either acetone or chloroform as so.lvent. The crystalline products, obt,ained on cooling the resultant solutions, $tere air dried for 24 hours and analyzed.. The samples were found to be essentially pure, and solvent free, after one such extraetion' as confirmed by the analyses in Table 2.I.

Example: FesalenNo3 (S- mofar scale)

FeNO3 .9H2O, 409¡ salenH2 , 26.89¡ en 7g¡ ethanol 3OO + 300 cm3. Yield of crystalline product 28.39 (74.22) .

AIl samples \^¡ere contaminated to some extent with the hydrolytic product, (Eesalen)ZO. Provided the preparation was completed to the solid crude product stage wíthin 2 Eo 3 hours, hydrolysis was minimized. As (Fesalen)ZO is only very slightly soluble in acetone and chloroform, the traces of this complex remained in the extraction thimble with the enH2X2, and contamination of the purified FesalenX could not be detected.

Scheme B - metathesis General method - FesalenCl or FesalenNO3 * excess MX. A suspension of the Fe (III) salen complex in acetone or 23 ethanol, containing at least a fivefold molar excess of the appropriate MX, \^ras stirred for 24 hours. The solid product was collected, washed with cold water (to remove MC\/MNO3), dried, and then purified by solvent extraction, as above.

Example: Fesalenl. 2.Og FesalenNO3 + I0g NaI in 50 cm3 of acetone were stirred for 24 hours at room temperature. The black solid formed was collected, washed with 2 x IO cm3 portions of cold demineralized water, then 2 x I0 cm3 portions of diethyl ether and air dried for 3 hours. On recrystaltization by solvent extraction with chloroform, 1.19 (472) of Fesalenl \^rere obtained.

On mixing cold aqueous solutions of NH4C6H5COO and FeCI3.6H2Or âr orange-brown precipitate of Fe (C6H5COO) 3 formed. The solid was collectea Uy filtration, d.ried overnight at 70oC, and used without purification. This benzoate salt is insoluble in cold water, but appreciably soluble in hot alcohol(36).

(d) As indicated earlier, agueous solutions of FesalenX species slowly undergo hydrolysis to (Fesa1en) 20. The hydrolysis occurs rapidly in the presence of alkali and appears to be quantitative when PH >. I0.

Residual solutions from the preparation of Fesalenx species were diluted with Lz2 ethanol-water, treated with NH4OH solution (S cm3 of solution) and allowed conc. "*37t00 to stand for 2-3 hours. The red, crude solid (Fesa1en)20 was 24 corrected, air dried and recrystallized blr prolonged sorvent extraction with acetone. Approximately 24 hours of extraction were required per gram of crystalrine (Fesalen) 20.

üIith very st.rong arkari, €.9. NaoH, pH > 13, breakdown of the complex to solid Fe2O3 $ras observed.

(e) re (saen) 2CI.H2o

Preparative methods for this comprex are discussed in detail in chapter 5. The sample studied in this section was prepared as follows: 8.9g FesaIenCI (f, mole) + 29 en ($ mofe) in tgg .*3 of ethanol, rnrere refluxed, with stirring for 2 hours. The vioret solution was filtered hot and the firtrate alrowed to coor overnight. The deep blue crystallíne solid was colrected, washed with .2 x 10 cm3 portions of diethyt ether and air dried for 24 hours.

Yield: 8.39 (768) .

(f) Dicyano complexes

(1) MlFesalen (CN) Z l

The method employed was essentially that of the metathetical reaction scheme outlined above, with a major modification in the treatment of the reaction producÈ. MlFesalen(cN)zl salts are insoluble in the majority of sorvents (with the exception of DMSO) and al1 recrystarlization attempts have proved unsuccessfur. rn fact the only solid product isolated from such attempts was (Fesalen) 20. 25 The crude product obtained. from the metathetical reaction r¡ras thoroughly washed with cold water, removing all traces of MCN and MX, then ethanol ancl the product dried overnight at. 70oc. The dried green sorid was analyzed directly (yield r00B).

The NH4CN reagent btas prepared as an alcoholic solution, by stirring an equimolar suspension of KCN and NH4CI in ethanol for 24 hours. After allowing the suspension to settte, the supernatant liquid was" used d.irectly as the medium for the metathesis.

Alcohol solubility data(36) NH4cl, 0 .69/100 cm3; NH4CN, v. soluble; KCN, 0.88g/LO0 cm3; KCl, slightly soiubte

(2) K lFesalophen (CN¡ , ¡

The procedure as for K [Fesalen (CU¡ , ] r¡ras used, with FesalophenC1 as starting material. FesalophenCl was prepared as prevj-ousIy described(13) .

- (3 ) K lCosalen (CN) 21

The deep orange crystals were prepared by a previously described method(28).

(4 ) Salts of ICosalen (Cu¡ 21-

Hot, aqueous solutions containing a molar excess of the cations Ag*, co(saen)2+, cosalen*, Et4N+, c^2*, cu2t, zn2t and Fe3+ were added to separate hot solutions of Kleosalen(CN)Zl. On cooling, precipitates of varying hue formed, were collected, 26 washed r¡rith arcohor and air dried. No further purification \^ras attempted.

(iii) Maqnetic susceptibilities

The magnetic susceptibility of each complex was determined by the Guoy method, using a sauter model 4L4 barance, and an Alpha Scientific model 7600 electromagnet with current regurated power supply producing a field strength of c.a. 0.gT (It = tO4 gauss).

From the data obtained, the molar susceptibilityr XMr and the Bohr magneton number, ueffr have been estimated using the method of calcutation outlined by Marr and Rockett(37). Diamagnetic corrections (S.f. units) were obtained from this

re ference .

(iv) Infra-red spectra

The i.r. spectrar âs nujol and hexachlorobutadiene mulls, were obtained on a Pye-unicam moder splloO spectrophoÈometer.

The spectra reproduced in this thesis $rere obtained by photographing directty recorded spectra, covering the appropriate region.

(v) UItra-víolet visible spectra

Refrectance spectra were obtained on a pye-unicam model sP500 manuar model, fitted with a reflectance attachment.

The nujol mull spectra \^rere obtained on a pye-Unicam 27 sPr700 recording spectrophotometer, fitted with a variable path length ceII (RIIC BM-3). The mull was introduced into the cell, and the windows adjusted to produce a uniform translucent layer c.a. 0.01 to 0.05 mm thick.

(vi ) Analyses

(a) c HandN

Elementar carbon, hydrogen and nitrogen determinations were performed on a Perkin-Elmer model 240, elemental analyser, at S.A.I.T. by the author. The instrumenÈ was calibrated with the e' H, N, microanalyticar standard, acetaniride, and a se"condary standard, salenH2. The minimum number of anaryses per sample was three.

(b) Metal analyses

Ttso techniques were used for iron analyses:

(1) Gravimetric

Approximatery 100 mg samples, weighed accuratery, \^rere ignited in an electric furnace (50ooc) for c.a. 2 hours until the residues (Fe2O3) were at constant weight.

Alternativery, the weight of the residue from mícro- analysis may be used. This method is not reriable¡ âs sampres frequently rsputter' on combustion. About 60% of the metal anaryses by this method agreed with the controrred ashing procedure.

(2) Atomic AbsorÞtion Spectroscopy (A.A.s. ) 28

An acidified, aqueous solution of ah: complex (c.a. 40 mg/ 100 .*3) can be analysed for Fe directly by A.A.s. without interference from the ligand. The instrument utilized in this study was a varian-Techtron model AA4, set for the anarytical Fe line aÈ 37L.99 nm.

(c ) Chloride anall¡ses

The complexes Fesalencl and Fe(saen) 2cL.H2o were both analyzed for CI- by potentiometric titrations against standard AgNo3 sorution. Approximately 200 mg (accurately weighè.d) of complex $tere wetted with alcohol (c.a. 5 cm3) and dissolved in loo cm3 of dilute HNo3 (5 .*3 tM HNo3/r00 cm3). rn both cases the solutions remained highly coloured.

' CI- in Fe (saen) 2CL.H2O was then determined by direct titration against 0.02M A9No3. The end.point was obtained potentiometrically, via a sirver wire placed in the test solution with a caromel reference electrode electricalty connected via an ammonium nitrate salt bridge. Calc.. 8.14 Found 8.15

Attempts to determine Cl- in FesalenC1 by direct titration were unsuccessful no clear endpoint could be obtained. How- ever, successful analyses were obtained using the back titration technique. An approx. 2 molar excess of AgNo3 was added and the excess Ag+ determined by titration against standard (0. 02M) NaCI.

These observations are consistent r¡¡ith the suggestion 29 that the chloride anion remains bonded, ât least to some extent, to the iron in FesaIenCI, even in dilute aqueous solution. Calc . 9.92 Found 9.83 30

CHAPTER 3 Solution Studies

3.1 INTRODUCTION

The solid state data, presented and discussed in Chapter 2, has provided sufficient information to formulate the complexes but may not be correlated directly with parameters associated with the nature of the Fe-X bond. Frequently, investigations of the properties of a compound in solution will provide relevant d,at,a for example, dissociation constants.

Such studies have proved pertinent in the case of Fesalen Cl, ft has been shown that. this complex, which is of limited solubility ín methanol and water, is monomeric in chloroform (19) solution, and non-conducting in nitromethane . These observations confirm the presence of an Fe-CI covalent bond in the complex(19). Further, solvated monomeric species may be obtained on recrystallization from rcoordinatingr solvents such as chloroform, nitromethane, methanol, pyridine and methyl (321 cyanrcte

The analysis for chloride (experimental section, Chapter 2\ suggests that at least partial dissociation of the Fe-Cl bond does occur in aqueous solution. Consequently it was anticipated that determination of solution properties, in particular conductances and u.v.-visible spectrar mây well prove useful in estimating the rstrengthr of the respective Fe-X bonds in Èhe series of complexes. 3I

3.2 CONDUCTA}TCE

The interpretation of conductance data, in terms of the possibre structures of coordination compounds, has been used extensively for over 80 years. Data obtained in aqueous solution allowed Werner and co-workers to elucidate the (38) structures of numerous ammine complexes . ïn recent years, conductance measurements in organic solvent have found widespread applicaÈion as a means of ascertaining the magnitude (38) of the ionic charge on complex species .

Conduct.ance measurements in methanol, in the concentration range 3 to 7o x t0-5M, have been obtained for arl FesalenX and other compounds of interest.. Due to the low solubility of severaL of the FesalenX species, the molar conductances, ÂM, have been interpolated to 5 x tO-4¡l rather than the convent- ional concentration of tO-3u. The data obtained are tabulated in Table 3.1, together with some single measurements in DMF.

For I:I electrolytes, the suggested range of values of il¡a (at 1O:3U) are 80-115 for methanol, 65-90 for DMF(38). At face value, the results obtained suggest. that all species investigated are essentiatly 1:I electrolytes in these solvents, however, closer examination reveals the situation to be more complex. Specifically: (I) The value of Â¡1 for the known 1:1 electrolyte nt4n+C1- occurs near the lower limit of the suggested range whereas thoie for the FesalenX species, with the exception of X = C6H5COO, are found to be closer to the upper limit. TABLE 3.I Molar conductivity, AM (Scm2mol-I), at 25oC

Methanol DMF

Species M M Ä¡a FesalenX 5xI0-4 X=Cl 9L.2 9.5x10-4 74.0 Br 99.5 I IL2.I No¡ 92.6 1 .26x10-3 77 .7 NCS 88.7 9.0x10-4 6s.5 .N3 80.4 C6H5COo 59.6

Fe (saen) 2Cl.H2O 78.1 Et4N+C1- 77 .5 1. 37x10-3 zg. g 32 (2) The timiting equivalent conductances, Ao, for the anions CI-, Br-, I- and NO3- (H2O at 25oC) are 75.5,77.4,75.g, 70.6 scm2mor-r respectively(39) . The values in nrethanor are expected to be approximatery 0.7 times these aqueous values, and show a similar trend(40). The observed spread in the values obtained is much greater than thaÈ ant.icipated fo,r completely ionized species.

(3) Fe(saen) TCI.H2O has a similar value of À¡4 to that of Et4N+cl-, and is known to be ionic. (The crystar struct- (20) ure and chemical studiês of Fe (saen) 2cL.H2o are discussed in Part rr of this thesis.) The difference between the mode of bonding the anion, x, in the Fe(saen)2 and Fesaren species, ,is il'!ustrated by the procedures used for halide analysís (experimental, Chapter 21. In the case of Fe(saen) 2cr.Hzo, the cI- may be determined by direct. argentometric titration in arcoholic solution, whereas for FesalenX (X = Cl, Br, I and NCS) ttre back titration technique must be employed, even in aqueous solution. Hydrolysis of FesalenX species, and the consequent cleavage of the Fe-X bond, occurs slowly and does not go t,o completion. Clearly, then, the bonding in FesalenX involves a covalent Fe-x bond and complete dissociation in solution does not occur.

These resurts and observations suggest that the species Fesalenx, in arl probability, are invorved in raòþer complex solution equilibria in which solvolysis may be of significance. such equilibria would be difficult to resolve by conductance studies alone. 33 3. 3 U.V.-VTSIBLE .SPECTRA

In t.he visible and ultra-violet region, the absorption spectrum of a complex originates, in general, from three types of electronic t.ransition, vl-z. excitation of the metal ion, excitation within the ligand and charge transfer excitation. As outlined in Section 2.2(j-i-) (f¡, metal excitation is not expected to be of significance for Fe (ITI) high spin complexes.

The conjugated system of ligands such as salen, would be expected to involve transitions such as n -) o*r rì + T* and fi + Tr* in the spectral region above 200 nm. Complexation would be expected to result in changes in the energy and intensity of the ligand absorptions, although these changes would be expected to be small. The spectrum resulting from ligand transitions would be similar to that of the protonated rigand, salenu2(4L) .

Charge transfer transitions may well be of significance as the intense colour of many complexes, particularly those of (4I) Fe (rrr) , is freguently associated with such transitions . During the preparation of the series of complexes, FesalenX, intensely coloured solutions have been observed, varying somewhat in hue depending both on X and the solvent utilized. Solvent and anion (X) dependent spectra have indeed been obtained.

In order to rationalize such spectra, it is necessary as a first step to resolve which absorption bands may be considered as arising from the ligand transitions, as distinct from those also involving the coordinated metal. 34 (a) SÞectra of the Iiqand

A number of workers have undertaken spectral studies of salenH2 utilizLng a variety of solvents(42,43,44,45). As substantial improvements in spectrometer technology have occurred during the rast 20 years, a check of this earrier data was considered worthwhile. The results obtained in this work are compared in Tabre 3..2 r^rith the earrier data, using methanol as the solvent in aII cases.

Four absorption bands have been observed between 2OO and 4I0 nm and in this study have been labelled as l,r to À,*, in increasing order of energy, i.e. decreasing,waverength. As a measure of the intensity.of each band, the logarithm of the respective molar extinction coefficients, Iog e, is incruded (where availabte) after each absorption maximum.

The assignment of bands \2, trs and À,* to T -) T* type transitions is consisÈent with the partial assignments given in references 37 and 38. rn particular, Àz is consistent with the ?T + Tr* transition of the imine (-ttC=U-) chromophore (43'45'46'47). The bands Às and À+ may be assigned to n + r* transitions associated with the substituted aromatic ring(46,47) .

The band near 405 nm has been the subject of some controversy. It has been suggested that this band is an n + r* transition, infruenced by the 'hydrogen bonding between the nitrogen atom and the phenoric oxygen in the ortho position' (42¡ . rn the sotid state spectrum (as a nujol mull) intense bands occur at 2I0, 256 and 325 nm on1y, i.e. Àr is absent. The intensity and position of band Àr is very solvent dependent TABI,E 3.2 Methanol solution spectra of sa1enH2

Source Àr ).2 ¡.3 Àa Concentration

This study 40s (3.23) 317 (3.93) 255 (4.36) 2L5 (4 . 61) :3-8 0x10-5tvt

Ref. 45 406 (3.20) 320(3.8s) 262 (4.33) =220 (4 . 65 ) 8-75xl0-51,1

Ref.44 408 (3.2r) 320 =265 not stated Ref . 42 403(3"r0) 318(3.8s) not stated 35 and is only observed in sorution spectra where the sorvent (43) morecure is a weak Lewis base . rt would appear that the ),r band is consistent with an n + T* transition and it has been assigned as such in this study, as has the corresponding (+Z¡ transition (405 nm) in the closely related ligand salpnH2 .

(b) Related. studies

The spectrar investigations were extended with the view to: (i) investigating the conditions for acid hydrolysis of salenH2 as mentioned in Section 2.L;

(ii ) determining whether or not the free tridentate ligand saenH exists in solution.

Representative spectrar data for sarenH2 in the presence of a 100-ford morar excess of acid (Hcr), arkari (NaoH) and en are tabulated in Table 3.3. The data for a similar study with saricylaldehyde are arso incruded in Tabre 3.3. However, as salicylaldehyde contains one chromophore per molecule whereas sarenH2 has two, the values of e for the salicylaldehyde study have been multiplied by 2 Lo allow direct comparison of the log e values.

The positions and intensity of the absorption bands suggest: (1) the absorbing species present in solutions (a) , (b) and (c) is the same - probably sa1enH2; (2) salenH2 is probably hydrolyzed in acidic solution to salicylatdehyde (and enH22*), (3) salenH2 is distinct from salicylaldehyde in alkalíne solutions. TABLE 3.3 Spectral data for salenH2 and salicylaldehyde study

* Solution Àl À.2 ).e l,¡

(a) salenH2 (3-80x10-5u) 40s ( 3.23) 317(3.e3) 25s (4.36) 2rs (4.6L) (b) salenH2 (6-40x10-5M) with excess en 40 4 (3 .20) 315 (3.91) 254(4.30) 2ls (4.6 8) (c) salicylaldehyde (3-40x10-5u) with excess en 40s ( 3.23) 314 ( 3.94) 254 (4.37) 2L3 (4.s7) (d) salicylaldehyde (3-10xI0-5M) 325(3.94) 2s4(4.37) 2L4 (4.s7) (e) salicylaldehyde (5-50xtO-5¡¿) with excess H+ 325(3.93) 2s6 (4. 38 ) 2L3 (4.57) (f) salenH2 (5-60x10-5u) with excess H* 325(3.e0) 2s7 (4.36) 2r3 (4.56) (g) salenH2 (2-3OxlO-SM) with excess OH- 380 (4 .0 3) 265 (4.r0) 2L5 (4 .7 4) (h) salicylaldehyde with excess OH- 37e (4.22) 264 (4.26) 225 (4.66)

* The values of 1og e for 1,,* are uncertain due to the solvent absorption band observed below 220 nm. 36 On increasing gradually the amount of OH-, the band near 380 nm can be shown to originate from the band near 315 nm for salenH2 and that near 325 nm for salicylaldehyde. In both cases, Èhe band near 265 nm arises from that near 255 nm in the neutral solutions. These spectral changes may be reversed by neutralization of-the excess alkali.

The changes noted in the spectrum of salenH2 as the amount of acid is gradually increased, suggest that the hydrolysis to salicylaldehyde and enH22+ is the only observable reaction taking p1ace. The reaction cannot be reversed by neutralization. It would appear that rapid, irreversible hydrolysis of salenH2 occurs in alcoholic solutions when the pH is below 5 and small amounts of water are present.

There is no evidence to suggest that the free ligand saenH exists in solution. AIl attempts to obtain the free ligand have been unsuccessful, with salenH2 being the only product isolated. It would appear that the ligand exists only when coordinated to a metal. As explored further in Part II, the conversion of salen complexes to saen complexes, and vice versa, occurs readily with the most stable reaction product being formed.

Although formally the dianion, salen2- (spectrum (g) ) is the species coordinated to the metal, the band positions and intensities would be expectedr âs stated earlier, to be (41) similar to those of salenH2 . Thus bands could be anticipated near 2I5, 260, 320 and 400 nm with extinction coefficients 37 of the order of 104. The Àr band, which has been assigned to an I + T* transition modified by hydrogen bon,iling, would not be observed for two reasons: (1) t,he phenolic hydrogen atom responsibre for the interaction is no ronger present in the anionic species; and (2) the non-bonded electrons on the imine nitrogen are coordinated to the central metal ion.

thus, while n + T* transitions may be observed, they must certainly occur at a much higher energy (Iower r) than 400 nm.

The spectrum of Zn(II)salen, where the only transitions oôcurring would be essentially those of the ligand, has been studied and bands were found near 350 nm (Àz), 260 nm (fr¡ (45) The spectrum of the similar complex, Zn(II)salpn, b¡as found to be almost identical with the ).2 band at 348 nm and no evidence (48) of an n + 'rT* transitior, .

spectrar anarysis of the methanor solution circular dichroism'spectra of Ni(II)sa1pn, a ¿8 square planar complex, has been recently performed(48,49) . Three categories of transit,ion have been suggested:

(1) Ligand transitions, with e > LOA, below 350 nm. (2) sorvent dependent charge transfer transitions, of the d + n* type, with e = 7x103 ,r".t 400 nm. (3) Magnetic dipole allowed 'd-d' transitions (e = 200) above 500 nm.

As previously outlined in Section 2.2 (¡i) (b) , for a high spin ¿5 syst.em the 'd-d' transitions, which are both 38 LaporLe and spin forbidden, would be e>rpected. to be of very Iow intensity (e < 1) if observed at att(18). However, a large number of Fe (III) complexes are known to be intensely coloured, as are the complexes in this study, with the colour being ascribed to charge transfer transitions. It has been'shown that Fe (rlr) complexes with dipyridyl, dimethyl glyoximate and other conjugated ligands show Ligand + Metal charge transfer (50) (n * d type) bands . rt appears reasonable. to suggest that such transitions may well be responsible for the intense colour of the FesalenX complexes.

Recently the methanol solution spectra of several iron(III) chelates, including FesalenCl, have been studied(5f¡. rntense bánds near 500 and 400 nm (e > IO3) have been assigned to'd-d' absorptions, with the intensity rationalized in terms of rborrowing from low-Iying charge transfer bands in the imine (51). ligand' Further, it was suggested that the u.v. spectra may provide structural information for Schiff base complexes of iron (IiI) in that: (1) pentacoordinate complexes show two bands in the visible regiön;

(2) hexacoordinate complexes show four bands, two between 400 nm and 500 nm with e > 103 and two weaker bands at longer

\âtavetengttsFl) .

This proposition is not in agreement with observations made in this study.

(i) Methanol spectra of Fe (saen)2+ (r'ig. 3.1(a) )

The methanol spectra of this cation have been obtaíned, 39 utilizing the complex Fe (saen) 2cL.H29(20) . The cation has octahedral- coordination and, as discussed in Part II, has the high spin d5 configuration in solution. The chromophoric group, ví2. an imine conjugated to an aromatic ring with a 4 phenolic oxygen in the ortho position, is considered sufficiently similar to salen to allow comparisons to be made. The spectra obtained, which obeyed Beer's Law over the concentration range 4 to 80 x 10-5tt, are characÈerized by the following absorption maxima: 500 nm (1og e = 3.44), 332 (3.87), 262 (4.32) and 230 (4.64) - a shoulder $¡as observed at 2g2 nm (Iog e = 3.83).

The positíon and intensity of the bands at 332, 262 and

23'O nm compare favourably with those obtained for the \.2 , tr s and À+ bands with sa1enH2, reinforcing the suggestion that these bands are essentially ligand transitions.

A single band only can be observed between 400 and 850 Dil, suggesting that the d-d assignment as outlined above may be in error.

(ii ) Methanol spectra o r FesaIenX

On examination, the spectra obtained appear to fall into two broad types:

Type I - [ = Cl, Br, Lt NO¡ and ]trCS (Fig. 3.1(b) ) The spectra are characterized by distinct bands at 505 nm and

near 325, 260 and 230 rrmr with prominent shoulders at 390 nm and near 300 nm. Substantial deviations from Beer's Law were observed on dilution (over the concentration range 2 - 40 x

1o-5M) . Figure 3.1 l:,lethanol solution spectra

o o c G .cr L o .o lt (a)

(b)

350 550 27o 750 850 wavelength (nm.)

(a) Fe (saen) 2Cr.Íl2o (b) Fesarenllo3 (c) r'esarenc6H5coo 40

Type II X = N3, C6H5COO (nig. 3.1 (c) ) These spectra are characterized by distinct bands near 320, 260 and 230 nm with prominent shoulders at 390 nm and near

500 and 300 nm. The deviations from Beer's La\nr \nrere not as great as for Type I

As deviations,from Beerrs Law have been observed, it must be considered that all absorptions cannot be attributed to a single species. CIearIy, this is not consistent with the assignment of the bands at 505 and 390 nm to d-d type transitions nor, for that matter, to charge transfer transitions in the solvated FesalenX species.

" However, as indicated earlier, partial dissociation of the Fesa1enX species occurs in solution, and the spectra of FesalenX and Fesalen* would be expected to be different. In particular, a Ligand + Metal charge transfer band would occur at a lower energy (longer wavelength) in the cationic species (50) than in the neutral molecule . Consequently, it is proposed that the band near 500 nm is characteristic of a charge transfer transition in the Fesalen* species whilst a band. near 4OO nm corresponds to the equivalent transition in the associated species, FesalenX. The position and int.ensity of these bands (50) would be expected to show considerable solvent dependen"" .

The above observations and discussion suggest that the equilibrium reaction FesalenX Fesalent + X- may be readily confirmed by spectral studies, in which the solvent or anion are varied. 4L (1) Solvent Such spectral studies were performed with chloroform, rnethanot and aqueous solutions. The results indicate that: (a) the position of the bands attributed to charge transfer absorptions are sensitive to the electron donating ability of the solvent. As expected, the charge transfer absorptions occur at longer r^/avelengths (lower energy) with

the weakly coordinating solvent, chloroform(50) .

(b) the'assignment of the bands near 500 and 400 nm is consistent with the cationic and associated species

respectively. In the presence of a substantial (Z 10 fold) molar excess of the anion X- (X = CI and NCS), the band at 505 nm (MeOH solution) decreased in intensity, whitst that at 390 nm increased, with an increase in the concentration of X-.

(c) the extent of the equilibrium reaction is solvent dependent, with the dissociatj-on being more complete in agueous solution. All spectra in aqueous solution are of Type I, irrespective of X, whereas in chloroform solution the majority are of Type II. Such solution spectra may be used to assess the extent of the dissociation.

The equilibrium reactionr âs proposed above, is described by the equation

K lFesalen*l tx- ¡ lFesalenXl

where K is the equilibrium or dissociation constant.

Assuming that the intensity of the band at 505 nm is related to lFesalen+] and that at 390 nm is related to lFesalenX] 42 then, as a first approximation, (esos)2 Kcr Ê¡go for methanol solutions.

2 Thus the ratio (ero r) /esso would be expected to reflect any anion dependent trend. in the event of dissociation for the Fesa1enX species in'solution.

TABLE 3.4 Dissociation of Fesa1enX in methanol solution (c.a. 4 x 1o-4M)

x c1 Br I NO3 NCS N3 C6H5COO

euor:esgo 0.985 0.998 r.0t2 0.996 0.983 0.464 0.385 2, e, 0.970 0.996 L.024 0.992 0.966 (e s o s ) , o 0.215 0.148 (xto-3 )

The ratios in Tabre 3.4 which are rel-ated onry approximately to the absolute value of K, suggest Èhat in methanol the comprexes of Type r dissociate to a much greater extent than those of Type rr. clearry FesalenN3 dissociates more readiry than Fesarenc5H5coo, but the rerative values for Type r (i.e. [ = CI, Br, ft NO3 and. NCS) do not positively establish whether the apparent order is real or not. rn order to differentiate between those species with close ratios, the chl-oroform spectra have been examined.

(iii) Studies in chl-oroform

The spectra in chloroform for Type I (X = NO3r CI, NCS), with a representative of Type II for comparison (X = N3), are reproduced between 390 and 700 nm. All spectra were found to Figure 3.2

Nos

o o c (E -o L o q lt N 3

NCS

cr 400 450 500 550 óoo wavelength ( nm.)

Chloroform sol-ution spectra of FesalenX

a 43 be similar in the region 250 to 400 hftr with absorption maxima at 270 and 330 nm.

Above 400 nm significant features may be readily observed. Att spectra have a shourder near 420 îrtrr and it appears reasonable to suggest that this band corresponds to that at 390 nm in the methanol spectra.

üIith respect to the prominent band found near 500 nm in methanor solutionr ro single equivalent band can be found in the chloroform spectra. Anion dependent maxima \^/ere observed, however, ât 448 nm ()( = NCS), 480 nm (X = CI) and 532 nm (X = tttO3). No equivalent peak was found for X = N3.

By assuming that a band near 430 nm correl-ates with the FesalenX transition, and that at 532 nm with Fesaren*r ân interpretation, equivarent to that for the methanol spectra, may be proposed.

(1) The observation of bands at 448 and 480 for X = NCS and Cl respectively, may be rationalized in terms of the additivity of overlapping absorption bands, i.e. the absorbance at any particular energy (or wavelength) is the sum of the contributions from a number of transitions which have a finite absor- (52) bance at that energy . By assuming that these bands have a Gaussian distribution of the absorbance with energy, it has proved possibre to carcurate spectra similar to those in Fig. 3.2 utilizing four absorbance bands at 532, 435t 330 and zio nm respectively. The procedure employed is discussed in the experimental section of this Chapter (3.5 (iv) ) . However, the assumptions made in these calculations are equivalent to 44 proposing that the degree of dissociation for the Fesalenx species are respectively 7oz (X = No3) r 542 (x = c1), 5og (x = NCS) and 202 (X = N3).

(21 The (r. ratios of ,, )2 /¿ + t o , as estimated from the spectra' are respectiveJ-y L.28 (x = No3) r 0.494 (x = c1),. 0.407 ([ = NCS) ¡ 0.216, (x = N3). These ratios crearly itlus- trate the trend suggested above.

(3) The conductance of each of the chloroform solutions has been measured. The results obtained are tabulated below:

x N3 NCS c1 No3

Â¡a (Scm2mo1-I¡ non-conducting =0.1 =0.I =0.2

These values of .A¡4 are extremely'row, but are comparabre with the rimited data avairable, ê.g. for the l:r erectrolyte nt4N+c1- in chroroform at a simirar concentration, ÂM =0.4 (40). scm2mol-1 These results indícate that the assumptions made above, in order to rationarize the spectra, are probably vaIid.

(d) Devlations froin Beerrs Law on dilution

The above discussion supports the proposition that solution equilibria may be utilized to rationalize the u.v.-visible solution spectra of the FesalenX series.

It would be expected that, for a simple equilibrium reaction, such as that proposed, ví2.

FesalenX i- tr'esalen* + X rdegree the of dissociation' would increase on dilution. The 45 concentration of Fesalêrt, relative to that of FesalenX, woul-d be expected to be increasingly rarger the more dilute the solution. For the methanol solutíon spectra, the band at 505 nm shourd become increasingry more intense, relative to that at 390 Dilr and should be reflected in an increasing ratio euou/ergo on dilution.

In ['ig. 3.3, the spectra of FesalenCl in me'thanol, between 370 and 625 Drtrr have been reproduced. Data relevant to this study follow in Tabte 3.5.

TABLE 3.5 Variation, on dilution, of FesalenCl methanol solution spectra

^ t^ SpecÈrum Concentration esou e 390 ¿sos/Lg9o

(1) 3.5 x10-4¡¡ 3.21 xIO 3 3. 05 x10 3 1.05 (21 2.8 2.98 3.00 0.99

(3) 2.L 3. 00 3.10 0.97

(4) L.4 2.56 3. 14 0.82

(s) 0.7 2.26 3.57 0.6 3

(6) 0. 3s 1.57 4 .00 0. 39

Clear1y the ratio ruou/r*o follows a trend in direct opposition to that predicted. This apparent anomaly could be rationalízed in a number of ways. (1) The criteria proposed above for the identification of the species are incorrect. (21 The intensity of the 390 nm band is accentuated by overlap of lower \¡ravelength bands. Figure 3.3

1.5

t.3

l.t

(¡) o 7 o lt o U' .cr 5 0) (2)

.3 (3) G) I (s (ó)

400 450 500 550 óoo wavelength ( nm.) Variation of FesalenC1 methanol solution spectra on dilution: Spectrum123456

Concentration 3.5 2.8 2.L I. ¿, 0 .7 0 . 35 (x 1o-4M) 46 (3) The equilibrium is more comprex than that proposed.

Suggestion (3) is considered the major reason for the apparent anomaly, and an indication of the complexity of the sorution equiribria may be provided in the following observations. rn Table 3.'4, the ratio euou!e ago obtained for a c.a. 4 x 1O-4¡rt methanol qolution of FesalenCl is stated as 0.9g5. From Tabre 3.5, the ratio for a 3.5 x ro-4¡,t sorution is 3.2L/ 3-05, i.e. 1.05. on examination of the rerevant spectral records, the only apparent experimentar variation was in the supprier of the methanol. That the spectra appear sensitive to small changes in the concentration or nature of sorvent impurities is in itserf anomalous. The major impurity present in spectral grade methanol wourd'be water, probably absorbed from the atmosphere, due to the known hygroscopic behaviour (53) of metharrol .

Earlier in t.his work, it has been stated (section 3.3 (c) (ii) ) that spectral studies indicate the dissociation is more complete in aqueous solution. For such studies, the solvent utilized waslr,r *"thanor-water, whereas in spectral grade rethanor, thq maximum water content is of the order of o-o5t(54). However, even with a water content of 0.05?, the morar concentration of water in the sorvent is 0.02gM, i.e. approximately 100 Èimes the concentration of any of the species involved in the dissociation reaction. Traces of water coul-d be significant, if a reaction occurs in solution in which water may be involved. ft is known that in aqueous solution, FesalenX may be hydroryzed to the binuclear oxo bridged species, accor- (33) ding to the equation - 47

2 FesalenX + H2O + (Fesalen)2o+ + 2H+ + 2X- Presumably this occurs via an int.ermediate species, FesalenOH.

seared methanol sorutions, prepared for Lhis study over two years ãgo, do not appear to have undergone any spectrally observable deterioration and no precipitate of (Fesaren) 20 can be observed. However, with a r:r methanor-water mixture as sorvent, (Fesalen)20 can be observed after 24 hours. clearly then, the water present in the sorvent is not the only species responsible for the spectral anomalies observed, but serves to 'magnifyr the solution equilibria occurring.

consequently it is proposed that a further equiribrium o"".,t" in solution, in which the solvent methanol is involved. The sorvorytic equilibrium proceeds according to the equation Fesalen*+MeoH FesalenoMe+H+ = -

Solvolysis has been confirmed experimentally as follows:

(I) In the presence of increasing amounts of acid, HC1O4, maintaining lFesalenX] constant throughout, the intensity of the peak at 505 nm increases to a maximum, whereas that of the 390 nm peak decreases. This behaviour is irlustrated in Fiq. 3.4, for X = C6H5COO. Similar results were obtained for X = Cl, NCS and N3.

(2) In t.he presence of a 3-fo1d molar excess of H*, the spectra followed the Èrend on dilution as expected from the simple equilibrium proposal.

This proposed two-step solution equilibrium accounts for the anomal-ous features of the spectral study. clearly the Figure 3.4

1.4

1.2

t.o

o o c (g lt o Ø 6 .ct ål 4 ( 2 ) (r)

2 (s) (ó)

o 400 450 500 550 óoo wavelength (nm.)

Spectra of FesalenC6H5COO in aqueous methanol, in the presence of added HC1O4

Spectrum 1 2 3 4 5 6

lFe complexl L.72 L.72 L.72 L.72 0.86 0.43 x to -4¡r

tHClo4 l 0 0 . 45 0 .90 3.62 5 .43 7 .24 x I0- 4 M 48 deviations from Beer's Law observed, are entirety consistent with the proposed assignment of the spectrar bands. The band at 505 nm, in methanol, may be attributed to the ion, Fesalen+, whilst that at 390 nm corresponds to the equivalent transition in the associated species FesalenX and FesalenOMe

Additional confirmation of these equilibria has been obtained by measuring the response of a glass electrode imm- ersed in the methanol solutions. rt has been assumed that such measurements indicate the 'pH' of these solutions.

TABLE 3.6

Solute Concentration pH

pure MeOH 8.30 FesalenCl 7. 00x10-4u 6.20

FesalenBr 4 . 36xI0-4¡r 6. 10

Fesalenl 4.72xIO-Avt 6 .0s

FesalenNO3 3. 91x10-4u 6. 15

FesalenNCS 6 .45x10-4¡l 6.30

FesalenN3 3.54xl0_4t,t 7 .20

FesalenC6H5COO 3 .4 3x10-4¡l 7.25

(Fesaen) 2Cl-..HZO 4.64xl.}-4vt B. 30

Although the measured pH must be consid.ered of limited theoretical vaIue, it does confirm that hydrogen ions have been released through a solvolytic reaction. Further, as the solvolysis occurs in methanol solution, the equilibrium constant of the solvolysis, KsoI., is given by the relation 49

a lFesalenoMe I tH+ l tH*l' Ksol = @ [!'esaren' I

i.e. p Fesalent PKsoI 2pH -i-

Now, as K=o1 must be the same for all FesalenX, the solution pH is dependent on lFesalen*] as the only variable. C1earty, then, the lower the pH of the solution, the greater the concentration of Fesalen*. Thus the pH values may be used to indicate the extent of dissociation of the FesalenX species in solution.

Similar effects have been observed in EIOH and H2O, and it is probable that such equilibria would occur in any amphi- pi'otic solvent. The equilibria inay be summarized in a general form, as

FesaIenX Fesalen* + X

Investigations in both basic and acidic amphiprotic solvents may \^/e11 allow a more positive differentiation of the extent of the various equilibria. Such an investigation has not been undertaken in Èhis study because the information required, viz. the apparent ease of dissociation of the Fe-X bond, may be obtained from the data available.

3.4 DTSCUSSTON

The data presented above in Sections 3.3(c) and (d), in particular that in Tables 3.4 and 3.6, when considered in toto, support the conclusion that the extent to which dissociation occurs varies according to the nature of X, i.e. may weII 50 reflect the strength of the Fe-X bond,

The apparent ease of dissociat.ion of the species FesalenX to Fesalen* and X- follows the order:

C6H5COO - N3 << I Presumably this is the reverse order of the strength of the Fe-X bond.

In an attempt to rationalize the apparent sequence of bond strengths, correlations have been sought with a number of known effects, in particular those obtained from spectroscopic investigations. Trnro well established examples, which are ligand d.ependent and of possible relevance, are the spectrochemical and nephelauxetic series. In both cases, the position of each ligand in the series has been determined from spectral measurements of the observed 'd-d' transitions(55'56).

(a) Spectrochernical series

The energy of separation, A, between the ground and first excited stater ilây be obtained from the spectra of a series of transition metal compounds. ft has been observed that for many transition metal cations, ên approximatety constant (neutral molecule and anion) ligand series occurs. This ligand series cannot be rationalízed simply in terms of obvious properties such as the anion acid dissociation constants, Kâ, or the electrostatic field strength, 10D9, operating on the central metal d electrons. The generally accepted rationalization, based on the molecular orbital model, is that the values of A, as observed, folrow the order(56) ' 51

very sÈrong o donors saturated neutraf very rÀ¡eak O donorS and/or fi acceptors molecules and,/or fi donor anions e.g. CN ¡ CO¡ NO2 e.g. NH3, H2O e.g. Cl- ,Bt-,I- rSO42-

Although there is disagreement in some of the pubrished dat,a, the generally occupied positions of the ligands rerevant to this study (arranged in order of increasing are(55,56,57, 59,59,60). ^)

(b) Nephelauxetic series

The interel-ectron repulsion, frequentry expressed as the Racah parameter, g (cm-l) , may also be obtained from spectroscopic measurements. The value of B, which is extremely sensitive to changes in the electronic environment of the metal ratom', increases with both z, the atomic number, and the oxidation state(56). From the d.ecrease in the varue of B for

complex species, when compared to the gaseous metar ion, a rigand series may be built up(5e ). This series, t.ermed the nephelauxetic series, rêflects the polarizability of the ligand. The covalent character of the metal-ligand bond may \^re1l be related to this polarizabirity(56'60). The generalry accepted positions of the relevant ligands in this case rt.(56,58,59,60)

I- > N3 > Br- > SCN- > CI- > NCS- > RCOO > H2O The observed sequence of apparent strengths for the Fe-x bond, in the FesalenX series, cannot be rationarized in terms of either of these effects.

In the case of the Fe (III) -X bond, a more accurate re- 52 flection of the coordinating 'ability' of the anionic ligand, X-, may hrell be provided by this experimentally determinable property. The covalent Fe-X bond would be expected to be predominantly of 'o character' and consequently the stabitity of this bond may well correlate with the stability of the H-X bond.

The Hard-Soft Acid Base (HSAB) theory proposed by Pearson, rhas proved. useful in accounting for and predict.ing Èhe stability (61¡. of coordination compound=' According to this theory, the (62) ions H+ and Fe3* are both classified as hard u"i¿" and would thus be expected to show similar trends. Whilst it is not clear what influence, if any, the presence of the coordinated anion ligand salen2- may have on the hardness of the central metal ion, it appears unlikely that the Fesalen* ion would be considered soft. In general, those species classified as 'soft' acids appear to be complexes in which significant n bonding occurs between the metal and ligand(61). Such bonding is un- Iikely to be of significance in the Fe(III) case.

Howeverr âs data is available for both 'hard' and 'soft' acid equilibria with many of the ligands of interest to this study, both cases are tabulated in Table 3.5. The cations selected are H* as the hard acid and methyl mercury, MeHg+, âs the soft acid.

In both cases, it can be seen that the stability of the M-X bonds (where I{ = Hg or H) follow the order:

oH- >> The position in this series, of the anions C5H5COO-and NO3- is not definite. Hor¡rever r it appears reasonable to suggest that TABT,E 3.5 Basicity of ligand X towards hard (H +) and soft (trleug+) acidic cations

(61,63) Anion, X- (62) PK tH+l PK lueug+l

c1- 5.26 12.25

Br 7 .26 l-5.62 r 7.76 r8.10

No¡- 0. 37

NCS- 2.70 6.7

N3- 6 .46 r.3

C5H5COO s.94 oH- L5.74 6.3

Notes: I Equilibria

f (i) ivteng+.HZO +HX i- MeHgX + H3O' J- (ii) H2o + HX x + HgO'

K1 tMeHgXl tH¡O+l R2 tX-l tH30+1 @ tH2ol tHxl and pK -1og K.

(62) 2 Thiocyanate ligand coordination, through S in MeHgX

and through N in HX Q3) . 53 C6H5COO- would be near the N3- ion, simply on the basis of their relative pK¡¡¡+l values.

The case of NO3- is not as clear cut, as much contradict- ory evidence as to the coordinating ability of this ion has been presented..

(1) From the acid dissociation data, the order suggested is

NOrJ > Cl- > Br- > I-.

(2) Substitutíon reactions involving the replacement of H2O in tco(rrr) (en¡ 2(No2)H2ol'* o, *t, suggest that the co-X (6a¡ bond strength follows the sequence . SCN- > Cl- e Br- - NO¡-.

(3) I'or tCo(III) (NH¡)S xl species, the Co-X bond strength order is(64).

> NCS c1- I Br NO 3

Thus it would appear reasonable to suggest that the ofder of bond strengths suggested for the Fe-X bonds in the FesalenX series, ví2.

C6H5COO = N3 >> NO¡>I is consistent with that expected from the anion acid dissociation constant data.

3.5 EXPERTMENTAL

(i) All solvents utilized for the conductance and spectral measurements were of spectral grade.

(ii) Conductance Inêasurements

Conductance measurements hrere made on a phillips Model 54 PR9500 Cond.uct,ivity meter, utilizing a conductance ce11 fitted

wíth a 824 cone. Approximatery 20 cm3 of test solution was placed in a loo .*3 test tube, with 824 socket, and the conductance cell inserted. The tube and contents \^rere then thermo- statted at 25oC for 15 minutes before measurement.

sorutions vrere prepared using A grade vorumetric flasks. The conducÈance cell was calibrated against aqueous solutions L/e, of A.R. Kcr cell constant, 0.g43 cm-r. The conductances have been corrected for the solvent conductance and Â¡4 calcu1- ated using the formula rr!Í _ L 103 ^ t. Ã. T-- wtrere C = conductance or resistance (ohm¡ -1 M = molar concentration.

(iii ) ul-traviolet visible spectra

Solution spectra \Arere obtained on a Perkin-Elmer l4odel 402 recording u.v.-visible specÈrophotometer, caribrated as per the manufacturer's recoilrmendations, with a reproducibility in absorbance readings of t .5? and an accuracy of t rå. Absorbance varues \^rere estimated to three significant figures and obtained directly from the recorded spectra. varues of the molar extinction coefficient, ¿, hrere calculated from the formula 10A e d where A = Absorbancei C molar concentration; and L = path length in mm 55 The values of e have been expressed to three significant figures throughout this work. clearry the uncertainty ries in the third figure.

For alI studies for which results appear in the same Table, the same sample of solvent has been utilized. AII sorutions were prepared and stored in tightly stoppered volumetric flasks.

(iv) calculated Ctra

Each spectral band may be described by the Gaussian (65) distribution function - (v 2 . A = e 2- v¡,/ôn) whereA = Absorbance; e = extinctioncoefficient; v = frequency (cm-I); vn = frequency at maximum absorbance of nth band; 6n = halfwidth (v¿) of nth band (cm-I).

The spectrumr âs obtained, may be considered as the sum of a number of overlapping bands with the absorbance, at any particular frequency, being described by the relation(65): n Atotal xAi i=1

For this study n = 4, and the values of the respective parameters v¡ and ôn, as obtained from the original spectra, are: v1 I8,8OO cm-I (532 nm), 61 27OO = = "*-1 v2 = 23,000 cm-l (435 nm) r ô2 = 25oo cm-l v3 = 3O,3OO c*-I (330 nm) r ô3 = 34oo .rn-I v4 = 37,700 cm-1 (270 nm) , ô4 = 3800 cm-I 56 Ttre values of the ext.inction coefficient parameters, which are of the order of 103 to I04, setected for the calculation need only be in the same ratio as the experimentat values. rn this case the values of e3 and e4 chosen urere 60 and roo respectivery and were herd constant throughout. The values of e1 and e2 were varied between 0 and 40, such that eI+eZ=40.

Close approximations of the observed spectra were obtained over the range of interest, víz.18,OOO.*-1 (555 nm) to 25,000 c*-1 (4OO nm).

The values of el and e2 ut.ilized, together with the positions of the calculated and experimental maxima, are tabulated below:

-l -1 x CaIc. VI (cm ) Exp. v1(crn )et e.>

No3 L9,2OO l_8,800 28 12 c1 20,800 20,800 2I.6 18.4 NCS 22,3OO 22,3OO 20 20 N3 shoulder from shoulder from 8 32 22,4OO - 24,3OO 22,2OO - 24,4OO 57

CTIAPTER 4 Moessbauer Sþectra

(15 69) 4.L THE PRINCÏPLES OF }4OESSBAUER SPECTROSCOPY '66 '67 '68'

(i) Source of radiatiôn, ene rgy and modulat.ion

Until comparatively recently, the physical behaviour of the atomic nucleus r,'las considered to be independent of chemical bonding. The widespread application of nuclear magnetic resonance spectroscopy to chemical problems highlights the fallacy of this belief. Another resonance phenomenon involving atomic nuclei, the Moessbauer effect, arso yields significant

information regarding chemical bonding(68) .

Moessbauer spectroscopy is similar to other spectroscopíc techniques in that a source of erectromagnetic radiation, an absorber and a detector measuring the incident radiation intensity are required. Transitions occurring between nuclear energy leve"ls are studied and the photon energies involved are typically in the range 10 to 2OO L.v(70). The radiation detector is essentially a garnma ray spectrom"t.r(66) and the radiation source a rádioactive nuclide.

The nucleus involved in this study is that of the isotope of iron of atomic mass 57 , denoted ."5Ë.ì ' which has a natural (7r¡ abundance of 2.Lg* . The required excited nuclear states of 57c" ''7r" ' result from the decay of the radionuclide P7"o has (7I) a half life (t\l of 270 aays and decays according to the (72) following scheme z 5B

7 z

electron capture

2nd 5 excited state 2 T = 9x10-o -sec

(91?)

3 Ist excited state 2 T = lO-7sec

1 Ground sta z where Y1 = 136.47 keV, Y2 = L22.06 keV, (71). YM- I' 4.4L keV (Moessbauer transition) The half integral values listed to the right of each leve1 are the values of the nuclear spin guantum number, I.

Thre absolute value of the 14.4 keV Moessbauer transition is known accurately to only three significant figures r âs there is considerable disagreement about the fourth in published data(73). However, the transition is extremely weII defined as it produces a very sharp line with a natural line width, fnat , of 4.6 x 10-12 ev(74) . Thus the relative energy of the transition is defined to better than one part ín LOL2, with an absolute value near 14.4 keV. The radiation may be considered as effectively monochromatic.

For resonant absorptíon of this monochromatic radiation 59 to be observed, it is necessary to modulaLe the radiation source in such a r¡/ay that a range of energies may be scanned. This may be achieved by utirizing the Dopprer effect which aI1ows the source energy to be varied srightly depending on the relative velocity of the source with respect to the stationary absorber. such a variation is d.escribed by the re lation: (60¡ -Ec "E tE Yr(å) Y Y where v = velocity of source, c = velocity of light and tE^. "E^.YY and are the transition energies for the source and absorber respectively.

" Experimentally, the, source is oscillated through a range of velocities within the limits +I0 mm/sec to -10 mm/sec, covering the region required for resonant absorption witrr5Tpe: nuclei in the majority of cases. This velocity range corresponds to an energy scan of + 4.8 to - 4. g x l0-7 eV.

Resonant absorption of quanta of the appropriate energy produces absorption minima on a plot of incident radiation intensity agai'nst energy, i.e. the Moessbauer spectrum. rn common with other types of spectroscopy, there are three main characteristics of interest in the spectra. These are the width, position and murtíplet structure for the absorptions.

(ii-¡ Line width

rdeally the line width of the observed absorption is simpry twice the natural line width, i.e. 2frr"t(75) where fnat is calculated from the uncertainty principle by 60 Inat = h/2n'r and t is the lifetirne of the excited state.

In piactice the observed line width, fobs, is always larger than 2fnat, sometimes by an order of magnitude'(75). There are a number of causes, three of which are of importance in the practical requirements of the Moessbauer experiment.

(a) Recoil of ernitter/absorber atom

Emission/absorption of the high ener:gy gamma quanta wourd be e>çected to be accompanied by significant movement or recoil of the atom concerned. The fraction of the avail- aþIe energy lost to a free recoi.ling atom is quite smarl and

rcf7t" is of the order of 0.00 2 ev06) -

This is significant when compared. with Inat (4.6 x Io-I2 ev) and such a variation in 8.,. wourd lead to broadening of the sharp t.ransition line to the extent where resonant absorption could not be observed. This probrem is prevented by ensuring that both e¡nitter and absorber nucrei are herd tightly into a crystal lattice so a large proportion of the transit.ions are (771 effectively recoil-free .

(b) Therrnal motion

As discussed above, motion of atomic nuclei leads to line broadening. consequentry it would be expected that vibration of nucrei in the crystal lattice should also contribute somewhat. This is observed and the fraction of trans- (78) itions which are recoil free shows temperature dependence . 6I For Èhe majority of Moessbauer nuclides, this effect necess- itates that both the source and absorber be maintained at low temperatures (78oK or lower). Fortunately, witn56e l: the recoil-free fraction is significant, and consequently arlows spectra to be obtained, over a wide range of temperat.rr"" (78) Data can be obtained readiry ror5ã"' at room temperature.

The absorber thickness contributes to the line width and Èhe difference between fobs and 2fnat shows a direct relation- (68) ship to this thickness . Corrections can be made , if required, but are not of great significance if the absorber is less than I mm thick.

(d) other effects

There are a number of other causes of line broadening which are more difficult to evaluate. Exampres include non- homogeneity of the chemical environment of the resonant atoms in the source and/or absorber, and. localized magnetic effects which may average out over the sample.

The value of the ratio fsys/2fnat provides a measure which aIlows assessment of the 'quality' of the data obtained from a Moessbauer spectrometer (7s¡

(iii) Position ahd mul-tiplet structure

As different compounds have been observed to give characteristic spectra, it may be reasonably concluded that the nuclear energy leveIs are sensitive to the extra-nuclear en- 62

víronment. The interactions responsible are those between the nuclear charge distribution and the electrostatic and magnetic (80) fields produced by the erecÈron crouds . The effects of such interactions may be summarized diagrammatically as follows (81).

3 f=t z

3 1 I z f=t z

EF Eobs uo, "e, Energy

1 Isomer I Quadrupole z Free Atom shifr Splitting

(a) Isomer (or Chernical ) shift

The electrostatic interaction between the nucleus and electrons ûhich have a finite probability of being in the region of the nucreus resurts iq a slight shift of both the ground and excited energy leveIs relative to those of a free atom. Thus the isomer shift may be defined as the difference in energy between that of the observed transition, Eobs, and that of the free atom, EF. The free atom situation is difficult to achieve experimentally and the isomer shift is usually expressed relative to the position of a standard absorber. Two materials are in common use natural iron and sodium nitroprussid.e (Na2[Fe(cN)5Nol .2H2o), with the latter being the preferred referenc" (82¡. 63

(b) dr ole s l_ifti (0S or AEQ)

A further interaction of an electrostatic nature occurs between the nuclear quadrupole moment, Q, and the electrostatic field gradient, E.F.G., due to t,he geometry of the erectronic charge around the nucleus. The quadrupole splitting is the separation between the two peaks observed in the spectrum, i.e. Q.S. = Eg2 The isomer shift, in this case, is obtained from the centroid of the two peaks. An alternati-ve, and possibly preferable, (83). term is the centre shift, C.s. The abbreviations to be used in this work are and C.S. ^Ee

The degeneracy of the nuclear energy states may be lifted in a magnetic field giving rise to 2I + 1 levels in each case. The allowed transitions between the ground and excited states produce six line spectra. In general, this requires the application of a substantial external magnetic field and such measurements have not been attempted in this study.

(iv) Rel-ationshi of the Moessbauer arameters to the chemical environrnent

The influence of the chemical bonding of the iron atom is reflected mainly in the magnitude of the isomer/centre shift and the quadrupole splitting parameters.

(a) Centre shift (c.S. )

This electrostatic interaction may be calculated using a classical, quantum mechanical treatment. Electrons in s orbitals 64 have a significant 'near-nuclear' population and the magnitude of the shift follows a linear rerationship Lo the s electron density, the shift increasing with a decrease in the density. ,ì

P and d electrons, arthough they have zero density at the nucleus, also influence the shift. Hartree-Fock calculations, for different fl con'figurations, show a marked increase in the nucrear erectron density with a d.ecrease in the value of n(84). This arises indirectly because the orbital of the 3s electrons, which spend a fraction of their time further from the nucl-eus than the 3d erectrons, expands significantry with increasing n. on this basis the range of shifts for various dn configurations should show the trend ¿6 ¡ ¿5 ¡ ¿4, and the varues obtained for th; following Fe ions confirm this(85): d6 (re2+) range 1.5 I.6 mm/sec ¿5 (Fe 3+) range 0.7 0. B mm/sec d4 (reo42-) 0.6 mm/sec

rn the case of compounds with significant covarent character, the paÈtern is more compticated, due to modification of s, p and d electron densities by covalent bonding. shifts vary between 0 and 0.65 mm/sec with rittre or no dependence on the formal oxidation state (metallic iron falrs in this ranqe (86). also) This may be rationarized using the pauri erectro- neutrality principle which suggests that the charge may be 'adjusted' between the central metal atom and ligands such that the effective charge on the metar atom is between -r and *i(80¡.

This arises from the important bonding interactions ror5ã., r namery o-bonding and n-back bonding, which both contribute to a change in the 4s and 3d popurations from those in 'free' iron compounds. 65

calculations of the shift varues for covalently bonded comprex morecures are of limited vatidity but the use of relative shifts to estimate the relative changes in the valence orbital populations finds wide apprication. An increase in the isomer shift corresponds to an increase in the 3d or a decrease in the 4s electron density(87).

(b) euadrupole spli*ing (Aeg¡

The magnitude of AEQ is proportional to the electric field grad.ient (E.F.G. ) which interacts with the quadrupore moment, Q, of the nucreus. The nine term E.F.G. tensor, with proper selection of the coordinate axes, reduces to three non-zero terms - one component along each of the three axes. As these terms are interdependent, further reduction to two independent parameters can be effected. The two terms are g, the field gradient component in the Z direction, and n, the assymetry parameter (a combination of the three terms). Thus

AEQ 4"2qQ¡ * Ér' (88) where e is the protonic charge and varues of n Iie between o and t.

rdeally the requíred information from e.s. data is the magnitude of q and n and the sign of q. Although AEQ may be readily measured, the reguired data cannot be obtained directly from the powder spectra. speciar techniques may be used to (89) determine the sign of q and to estimate n . To a first approximation, the n term may be neglected as values lie between I and 1.15 66 The E.F.G. term can be conveniently d.ivided into two components with

9rat. 9val. where glat. is the contribution from external ligand charges and qy¿1. the contribution from valence electrons. (R and y- are the Sternheimer anti-shielding factors, which allow for the effecÈ of the inner, non-valence electrons on glat. and gval. These magnify the contributions to q by a factor of about 7.)

These quantities can be evaluated if the crystal structure and valence orbital populations are known. As the latter is not usuatly the case, such calculations are difficult. Ho\,vever, (89) generarizations may be made ' (1) If the electronic and ligand charges have cr¡bic synunetry, QIat.=9val.=0. (2) If the 3p orbitals are equally populated, gval. = 0. (3) ff the ligand charges are of equal magnitude in an octahedral arrangement, glat. = 0.

The valence electron contribution term can be further subdivided Qval. = 9c.f. tgm.o. gc. f. = valence contribution considering a crystal field model with NO overlap of ligand and metal orbitals a temperature dependent term. gm.o. = valence contribution considering metal-ligand covalent bonding only a temperature independent term. ,67 (89) For octahedral complexes, the dominant term will ¡" , Qc.f. for Fe (II) high spin {trn4en2) and Fe (fII) Iow spin

{trns ) . em.o. for Fe (Ir) low spin (tZg6) and Fe (Irr) high spin

Itrn3enz) .

rn summary, the anticipated results for octahedrar iron ('9) comprexes .r. :

. assymetric Around state (Fe (II) high spin, Fe (III) low spin) large AEe, in the range I.7 to 3.6 mm/sec. . synìmetric ground state (Fe (II) low spin, Fê (f f f ) high spin - smal1 AEQ, in the range 0 to 0.7 mm/sec.

These expectations are esséntially confirmed by the experimentar results, with the major anomalies occurring in the second case. Many complexes, with a symmetric Around state, have substantial Q.S. values but this is generally attributable to significant distortion from octahedral (89) symmetry .

(v) The additivl principle and correlations with bondins

As indicated above, prediction of expected Moessbauer parameters by calculation finds somewhat limited apprication. In the main, these parameters have been rationalized using semi-empirical methods. One of these, the additivity modeI, has been of particular use in interpreting the spectra of (89) Sn (ïV) , Fe (Iï) tow spin and Fe (lL,Ir) compounds .

The basic premise of the model is that the parameters may be regarded, ât least to a first approximationr âs the sum of 6B independent contributions - one from each of the bonded ligands. Tables of partial quadrupole splittings, p.e.s., have been developed for Fe (II) low spin and Sn(fV) compounds and a table of partial centre shifts¡ p.c.s.r for the Fe(II) low spin case (er¡.

The possible bonding factors which are expected to influence the value of AEQ are inequalities in o or n bonding (ø*.o.) and ligand charges (qf"t.). For Fe(II) low spin, eval. is expected to be the dominant contributor to the E.F.G. with o bonding inequalities of more i-mportance than n bonding, al- (92¡ though n acceptor ability may be significant . The centre shift for Fe (rr) low spin, reflects both the o änd n bonding abilities of rigands, with a d.ecrease in c.s. correlat.ing with an increase in o donating ability and,/or r (92) acceptor ability . using the partial field strengths of a series of ligands (calculated from the optical spectra of co(rrr) complexes) a correration between the p.c,s. value and the ranking of the ligand on the spectrochemical series has (92¡ been observed .

The additivity principre, which has proved of varue in the rationarization of some Moessbauer data, is e>çected to (89) find application in the Fe (rrr) high spin case . Atthough a rarge amount of data for this system appears in the literature, there appears to have been no systematic aÈtempt to evaluate such correlations.

rn broad outline then, the object of this section of the research program is to determine the Moessbauer parameters for 69

the series of high spin complexes of generar formula Fe (rrr) salenx. This data will then be compared. with that obtained in the preceding chapters with the prime objective being the erucidation of any correlation between the Moessbauer parameters

and information related to the bonding of the ligand x. As indicated above, the Fe(rrr) high spin system would be expected to show similarities to the Fe (rr) 1ow spin system and thus correrations courd well involve spectrochemical or nephelauxetic effects.

4.2 REVIEVü OF PUBLTSTTED DATA . Correlati on with Structure

For a systematic study as proposed, the preferred environment for the Fe (IIf) atom would involve octahedral coordination where the ligand atoms in five of the positions remain constant and the coordinating group in the remaining position may be varied systematicalty. The 'famiry' of compounds of formula FesalenX would be suitabl-e if atl complexes hrere dimeric and thus effectivery six coordinate. A simirar theoretical consideration to that discussed for octahedral coordination has been appried Èo pentacoordinate monomeric complexes (16). Thus it is considered that the exact structure of the complexes in the sotid state is not vital for a comparative study, if either the structure can be assumed to be the same for all x or the Moessbauer data obtained for known monomeric/dimeric species do not differ significantly.

The data available for FesalenX (X = CI or Br) and solvated adducts are tabulated in Table 4.L. TABLE 4.7 Room temperature Ivloessbauer parameters of Fe (f f f ) high spin complexes with salen

Compound Structure c.s . AEQ Re ference (Fesalen3l-) 2 dimer .65 r .0r r.45 I .01 (16) .62 r .02 1. 38 ! .02 ( 17) .66 1.40 ( r3) FesalenCl. 2MeNO2 monomer .58 r .01 1. 30 r .01 (16 ) XMeNO2 .61 1. 34 2CHC13 .63 r .0r 1. 40 + .01 (16) 2MeOH .64 r .01 I.45 t .01 (16) (Fesalengr) Z dimer (assumed) .67 t .0r r.63 + .01 (r6) Fesa1enBr. 2CHC13 .65 r .01 1.65 + .01 (r6) 2MeOH dimer. (assumed) .67 r .02 1.6 3 + .01 (16) monomer (assumed) .67 1 .01 0.87 + .0r (16) 2MeNO2 .62 r .01 1. 61 + .01 (16) MeNO2 .67 r .01 1.13 + .01 (16) 7o The variation in the parameters (FesalenCI) obtained for 2 is consistent with the limitations expected for the Moessbauer technique and these results agree wer-r with each other(69). All samples were prepared in the same wayr, ví2. recrystarriz- ation from acetone, and it would be reasonabre to expect that the average of these values represents an accurate estimate of

the parameters. Normal- statistical pract.ice would require a minimum of three standard deviations for a 90Íà.confidence (93) intervar and as such, the expected Moessbauer parameters for (FesaIenCI)2 are C.S. :0.64 t .03 mm/sec, AEe = 1.41 t .06 mm/sec. Applying similar statisticar criteria, it wourd appear that there is not a significant difference between (a) (for X = Cl), the dimer and CHC13 and MeOH adducts; and (b) (for X = Br), the dimer, CHCI3 and one of each of the MeOH and MeNO2 adducts.

Furthermore, there is considerable uncertainty as to the structure (i.e. monomer or dimer) of several of these complexes. The assignment of structure, based on i.r. spectral criteria, has been criticized in chapter 2, and the Moessbauer data obtained(16) would appear to contradict several of the structural assignments based on magnetic data(11). only two structures may be regarded as definite, viz. (Fesalencl)z and the nitromethane adduct, because the crystal and morecurar structures (L9,94) have been determined by X-ray diffraction techniques . consequently, it is suggested that structural assignrnents based on criteria other than X-ray studies should be considered as suspect in the case of rFesalenX' type complexes. 7L the deÈermination of the crystal structure of each rFesalenxr comprex was considered to be outside the scope of this study. In order to rationalize the Moessbauer parameters obtained., it has been assumed that the environment of the metat atom in all FesalenX species is simirar, wiÈh the major difference arising from thê Fe-X bond. This assumption wourd be considered valid if .either (a) all species have a similar solid state structure,

viz. that of the dimer (FesalenCL) Z; or (b) association in the solid state does not influence the magnitude of the Moessbauer parameters markedly.

It appears that (b) may we]l be a reasonable proposition. rhe uoessbauer data for the known monomeric species, FesaIenCI. 2MeNo2, do not differ greatry from the data obtained for the dimer. Furthermore, the majority of the solvated species, particularly those wíth weakly coordinating solvents, have similar Moessbauer parameters to those of the non-solvated (16) dimers . rt would appear that structural assi gnments based

on data, other than a crystal structure, must be regarded as doubtful. All species obtained in this work have been shown to be non-solvated (Chapter 2) and there is no evidence to suggest any pronounced structural dissimilarities.

4.3 MOESSBAUER PARAMETERS

A summary of the relevant data obtained from the Moess- bauer spectra is presented in Table 4.2.

The data obtained in this study are in reasonable ag'ree- ment with the previously published values (X = CI and Br) in TABLE 4.2 (a) Room Temperature Moessbauer Parameters (b)

Compound c.s. AEg yþ(c) I Absoryrio¡ (d)

(i) Reference sta¡¡dards

Sodium nitroprusside 0.001.01 L.73 0.45 8.1 Stainless steel(enríched) -0. L6!.O2 0.43 2I (ii) FesalenX

X = Cl 0.651.03 1 . 351.06 0.53 I 7

Br 0.'78t.03 1. 64t . 05 0.48 I 6

r 0.77!.03 I. 781.05 o -49 I 1

NO: 0.78t.03 I.62!.O5 0.5I 2 I

NCS 0.671.03 1. 26t.0s o -52 2 6

- N¡ O.77!.O4 I .081. 07 0.5 7 I 0

C5,H5COO 0.68t. 04 0 . 8It.08 0.60 1 I

(iii) Novel Compòr:nds

K lFesalen (CN) 2 ] O. 31t .02 2.r5!.04 0.36 2.O lFe (saen) 2)cL.H2O O-42!.O2 2.75!-04 0.43 4.O lFe(saen)ZlI 0.591.04 0.351.08 o.62 0.8

Notes:

(a) Room temperature 294t1. soK.

(b) units mn/sec, with centroid of sodium nitroprusside spectrum as reference (zero velocity) point. (c) yfu = peak v¡idth at half height - average value of two peaks (where apþficalte) .

(d) BAbsorption values average of two peaks (where applica.ble) . 72 Table 4,.I, with the exception of the C.S. ,r"In" for the bromo complex. However the values for both the chloro and bromo complexes agree more favourably with those summarized in a (69) recent review article .

4.4 DISCUSSION

(i) spin state

The relevance of the Moessbauer parameters has been discussed earlier in this Chapter. One application, of significance to this study, is the determination of the spin state of Fe (III) in the complexes.

A summary of relevant obsetvations follows: Maqnitude of auo(15,33,69) (1) Fe (III) low spin, S = , (assymetric ground state) large, > 2rm/sec. ^Eg (2) Fe (III) high spin, S = + (symmetric ground state) (a) octahedral symmetry AEQ small, < 0.7 mm/sec (b) distorted octahedral symmetry 0.7 < AEQ < 2 mm/sec (15,33,69) Centre Shift (c. s. ) The range of C.S. values is not clearcut for the Fe(III) low spin state, with values being found in the range 0 to 0.6 mm/sec. However in the majority of cases, the C.S. range is 0 to 0.45 mmr/sec. 73 For the Fe(III) high spin state, the C.S. values are significantly higher than in the low spin case and are found to Iie between 0.55 and 0.85 mm/sec

(a) Octahedral complexes

In the case of those complexes which can be considered as having essentiatly octahedral symmetry, the Moessbauer data unambiguously confirm the spin state of Fe (III) ¡ âs illustrated by: Compound peff (b.u.y c.S. Spin state ^Ee Fe (saen)2l 6.06 0.59 0.35 S = 5/2 KlFesaIen(CN)2J 2.L0 0.31 2.I5 S = I/2 Fe (saen) 2CL.H2O 2.LL 0 .42 2.75 S = I/2 (Fesalen)2o(33) I.B7 o.5B o.g2 S = 5/2

Clearly the Moessbauer data discount the possibility that the low value of the magnetic moment observed with

K tFe (salen) (cN) Z I and Fe (saen) 2cL.H2o arises f rom antiferromagnetic coupling of two Fe(III) S = | states, similar to that established for (Fesa1en) 20. The two compounds must then be considered as having the Fe(III) in the 'true' Iow spin (S =ll state.

(b) Distorted octahedral cotnplexes

The data for the seri es of compounds of formula FesalenX, I'aIl clearly indicate tha involve Fe(IïI) in the S = | state.

(ii) Distortion from octahedral s

The magnitude of the AEQ values for the Fesa1enX series, suggest significant distortion from octahedral symmetry. 74 Calligaris and co-workers have related this distortion from octahedral sl"mmetry to deviations from planarity in the coordinated tetradentate ligand, salen(95). Such deviations have been attributed to interactions between this ligand and the apical ligand, X. This argument has been supported by the observation that the 'more bulky' the ligand X, 'the larger (95). the distortions in the tetradentate ligand' rt would appear that this argument is not correct because the benzoate Iigand, C5H5COO, must be considered as the 'bulkiestr of those studied yet it prod.uces by far the lowest AEg value. Further, it is difficult to rationalize the polyatomic ligands NCS and N3 which woul-d appear larger than Cl(96). ïn both cases the AEg value is lower than that of the chloro complex. Clearly the cause of the distortion from octahedral - symmetry is not related simply to the size of the tigand X. Whether the magnitude of AEQ correlates precisely hrith the distortion of the planar salen moiety cannot be positively decided without structural evidence for all of the complexes. It would appear, however, that this must be considered a possibility.

(iii) Correlation of AEQ values

The ord.er of magnitude of AEg observed in this study is: I > Br - NO3 > Cl > NCS > N3 ,> C6H5COO. This order correlates with that observed for the apparent extent of dissociation, in solution , of the Fe-X bond in Fesa1enX, as discussed in Chapter 3. This would suggest that the magnitude of AEg may well be related to the nature of the Fe-X bond. The relationship would be such that the stronger the f'e-X bond (i.e. the greater the covalent character) the smaller the value of observed. ^Ee

As discussed above, the lifting of the degeneracy of the 75 nucrear energy levels, which resurts in quadrupole splitting, occurs as a result of the interaction between the nuclear quadrupole moment and the electric field gradient, E.F.G. The E.F.G. arises from two fundamental sources(89). (1) the charges on the coordination sphere ions or ligands; (2) the erectrons in the incompletely firred shelrs of the atom itself. The coordination sphere charges contribute providing the symmetry is lower than cr:bic.

In Èhe case of Èhe Fesa1enX series, where the electronic arrangement has been established as the same, the variations in ÂEQ must arise from variations in the effective charge on the distant atoms of the rigands'. rt would appear reasonabl_e to propose that the bonding between the saren ligand and the meÈal is substantially covalent whereas the Fe-x bond is much more polar in nature. This is consistent with the observations of Chapter 3. As a consequence the E.F.G. must be substantially assymetric, with a major component directed. along the Fe-X bond. The larger the magnitude of the effective charge on x, the greater the E.F.G. and thus AEg.

The order of magnitude of AEe for the various X is consistent with the order observed for the porarity of the Fe-X bonds. rn this respect the observations for the Fe(rrr) high spin system parallel those outlined above (4.1(iv) and (v)) for the Fe (II) Iow spin system,

(iv) Correlation of C.S. values

The values of C.S. for the FesalenX series fall within 76 the range 0.65 to 0.78 mm/sec" whire this range supports the assi'gnment of the s = | spi-n state for Fe(rrr), it would appear too narrow to alrow the erucidation of any anion dependent trend in these centre shifts. At first grance it would appear that the c.s. varues faIl into two d.istinct groups, vLz. 0.67 t .02 mrt/sec (X. = CI, NCS and C6H5COO) and 0.77 t .01 mm/sec (x = Br, T-, No¡ and N3). No correlation can be found with any parameter which could be associated with the Fe-x bond, incruding all those discussed in the preceding chapters.

Furthermore' the C.S. values should be considered in the light of r"""rrra'tions as to their respective absolute values. The error estimates in Table 4.2 indicate the reproducibility (or precision) of this parameter and do not truly reflect its accuracy. The uncertainty arises as a consequence of the experimental procedure whereby individual spectra were obtained for each unknown compound. and reference sample. rt is worth

noting that the uncertainty only applies to parameters, such as the centre shift, which have been obtained from different spectra. The procedure utilized is discussed in more detair in the experimental section of this Chapter.

. rt is felt therefore that the data obtained do not con- clusivery confirm whether or not there is a significant difference between any of the C.S. values. Indeed it is possible that the c.s. value may $rell be independent of the nature of the Fe-x bond in such a series of Fe (rrr) high spin complexes. This is contrary to the expectation, based on analogy with the Fe (rr) low spin case , of a correration with the spect.rochemical series. clearly data of a higher accuracy are required to resolve this situation. The application of an 77

internar standard technique simirar to that employed in N.M.R. spectroscopy, with the increased availabirity of computing facilities, may well now be a feasible method of achieving this aim- rn addit.ion, studies at rower temperatures (e.g. c-a- 100oK), where the recoir fraction is known to be, higher than at room temperaturer ffiây welr facilitate such studies.

4.5 ; EXPERIMENTAL

(1) Sumtnary

The Moessbauer spectrometer, constructed at s.A.r.T., was operated in the constant accereration mode to produce spectra covering a velocity range of c.a. +5 to -5 mm/sec. Alr data were obtained in the form of typewritten digitar output which was handplotted to give the ind.ividual spectra. The position of absorption minima was estimated. with the aid of computer curve fitting programs. Detairs of the construction, calibration and experimental procedure are discussed in the following sections.

(2) Moêssbauer sþectrometêr

The following block diagram (fig. 4.1) summarizes the components of the spectrometer.

The spectrometer was based on the modurar Nucl_ear Data series 1100 Anaryzer system, consisting of the followingr modul-es:

(a) 512 channer memory, data handling and analog to digital converter which comprise the basic murtichannel analyzer (McA) . 78 (b) ND520 PAD, preamplífier, amplifier and discriminator. (c) ND522 SCA, single channel analyzer (energy selector). (d) Fairchild Model 701 Oscilloscope (visuat display). (e) ND316 Autofínger drive for IBM model 721 Selectric

Typewriter (digital output) . (f) Stabilized power supply incorporated in mounting rack.

V.loc¡ty Dr¡v. u f cô Pow¡ r Tr¡n¡duc¡r Octacto. lr¡n¡duca r Supply

Co¡l¡ r"ro.l

Summing Pow¡¡ Pul¡o Anpl¡t¡.r Amplilirr Anpl¡f icr

Wlveform Mult¡chånncl Enorcy Gan!r!toi Analyecr Sel actor

Figure 4.,| Vi.u.l O¡g¡tål NDrr00 D¡.pl6y Output Sy¡tom

The components constructed at s.A.r.T. are discussed in detair beLow:

(i) 'Detêctor

A sealed, gas-filled, proportional counter was used to detect the 14.4 keV Moessbauer transition. This type of detector has been shown t,o provid.e optimum resolution for row energy (< 30 keV) y radiation(97), and the principtesofcounter (98) design and construction have been described by Culhane .

The counter gas chamber, constructed from 10 mm Al plate, w'as rectangular in shape (150 x I50 x 230 mm, volume c.a. 5.1 litres) and also acted as the electrical cathode. A tungsten 79 wire (0.05 mm diameter), acting as the anode, r¡ras operated at a potential of c.a. 1500V provided by a philtips pW4O22 stabilized power supply. The radiation entered .the detector chamber via an Al foil window 0.05 mm thick and 60 rnm in diameter. The counter gas utilized was a 908 Argon -'lOE Methane (P10) mixture at approximatety atmospheric pressure. Replacement of the counter gas may be effectedr âS required, via needre valves and the seared system was shown to have a useable lifetime of c.a. 28 days.

The counter was purged and refilled before each unknown sample was studied.

(ü¡ El-ectromechanical syStem

The drive unit consisted of two mechanicalty coupled transducers, viz.

Drive transducer - magnet and coil assembly of a Vüharf- dale 250 run high compliance low frequency loud speaker; Velocity transducer - Sanborn Irfodel 6LV2. The configuration of the drive unit was essentially as previously descrj-bed by cohen et aI(99) and zane(100) ' The unit was enclosed in an AI housing incorporating the Pb collimator, radiation source and appropriate Pb shielding.

(iii) Wave form seneratoT and sumrni ng atnplifier

The ultrastable triangular hrave generator developed by (I0I) Cohen was modified to make use of current electronic technology, with the recommended operational amplifiers being replaced by,Fairchild UA739C integrated circuits. These I.C.'s 80 required few additional stabilizing components and the circuit was essentially as published(10l). The synchron .zing signal was obt.ained from the triangular generator as a positive purse to suit the Nuclear Data M.C.A.

The linearity of the final velocity r^rave form was optimized by the rshape compensation'technique of zane(100). This invorves 'mixing' a parabolic'wave form, obtained by integrating the reference triangular \^rave form, with the original signar. The optimization procedure $ras forrowed visuarly on an oscilloscope connected to the independent auxilliar:y ouÈput of the 6LVZ transducer.

The summing amplifier was of a simpte variable gain design based on the pA739C I.C., with the 'mixed' signal introduced into the non-inverting input and negative feedback from the velocj-ty transducer coupled to the inverting input.

(iv) Power ampl-ifier

The drive transd.ucer $ras driven by a power amplifíer based on the General Electric PA246 I.C. Dírect coupling was employed throughout to minimize phase changes in this final stage. The amplifier power supply was derived from the positive and negative rails of the stabilized mounting rack supply utilizing the symmetric voltage technique such that D.C. component in the output voltage wave form was less than 1 mV.

(v) Re fe nce Ínâteri 1s calibration

Iron foil, 0.L25 mm thick, of natural isotopic composition. 8I Stainless steel foil, Q.25 mm thick. The above calibration foils r,rrere provid.ed wittr the Moessbauer source. Sodium Nitroprusside - Fluka, purum grade.

(vi) Moessbauer source

Ê-a -'Co in Pd matrix, obtained from the Amersham Radio- chemical Centre, England. The activity of the source \¡ras initially 10.8 mCi, with a natural line width, f, of 0.097 mm/sec.

(vii) Absorbers

" The metal foils were'fasteried directly in position with adhesíve tape. Polycrystalline samples were placed in a 12 mm diameter hole in a 25 run square, 0.6 mm thick AI plate and held in place witTr adhesive tape. The sample was then attached to the lead collimator between the source and proportional counter window.

(3) rinêntal Procedure

(i) rnitiaL itnization

The constant acceleration mode of operation requires that the source velocity changes linearly with time, i.e. the output from the velocity transducer produces a triangular wave form. For this reason the 6LV2 transducer output wave form was observed via the independent auxilliary output, i.e. the output not involved in the feedback loop. 82 The drive transducer was optimized at a frequency of 9.5 hz to produce a slnnmetric triangular vrave approximately 140 mV peak to peak on the C.R.O. screen. This iorresponds to a velocity range of c.a. +6 to -6 mm/sec as the 6LV2 transducer calibration was quoted as 12 mV/nun.=""-1. ,

An appropriate,energy window (14.4 t 2 kev) was then selected utilizing the M.C.A.'in Èhe pulse height analysis mode. The spectrum of the natural iron foil was then obtained and the positions of the six absorption minima determined and checked against the published values to ensure linear operation. Minor adjustments were made as required.

' The velocity range $ra's then decreased slightly to 120 mV peak to peak (or +5 to -5 mm/sec) such that the inner four lines of the natural íron spectrum only r^rere resolved. These absorptions ï¡ere found between channels 80 and. 430 and the positions utilized as a final check of the linearity of the system.

(ii) Calibrat,lon

. Spectra of the three calibrants were obtained with c.a. 2 x I05 countsr/channel being accumulated in each case. The positions of thê absorption minima r,irere then determined and the calibraÈion constant (velocity increment/channel) calculated from the literature values. The data have been listed in Table 4.3.

(iii) sþe ctra of unkno\{n compounds

The spectra of the samples described in Chapter 2 were obtained in a similar manner, with c.a. 5 x 105 counts/channel TABI,E 4.3

(a) Iine Velocity rran,/sec(b) Channel Number

Iron (2') 3. 340 105t1

Iron (3) 1 .096 2L6 + I Nitropn:ssíde (1) , 0.865 226.5 t 0.5

Iron (centroid) 0.258 257.5 t 1 (c) Stainless steel 0.161 262 t 1. s

Nitroprusside (centroi¿) 0.000 269.25 + 0.5

Iron (4) -0.580 298 t I

Nitroprusside (2) -0. 865 3L2 t 0.5

Iron (5) -2 -820 409 + T

Calibration constant I channel = O.O2O3 t .O0OI mm/sec.

Notes:

(a) Spectral lines have been nrmibered in accordance with the accepted

criteria for the natural iron spectrum, viz. the first line (iron (l) ) is that for¡rd at the rnost positive velocity.

þ) All verocities have been erçressed relative to the centroid of the nitroprusside spectn¡n which has been assigned an arbitrary value of zero. The r¡erocity data has been taken from references (r02),

(97) and (69) .

calibration checks talcen throughout this study. The estimated error of t 1.5 channels covers the total spread of values. 83 being accumulated for each.

(iv) Treatment of data and errors

The data accumulated in the 5t.2 channel memory consisted of six digit numbers which may be addressed in sequence and displayed on the C.R.O. or typed onto a sheet of paper.

From this raw data, the position of the ábsorption minima is required with some accuracy. This is usually achieved by computerized curve fitting techniques, for which purpose programs specific to the nature of the individual spectrometer are generally developed. Due to the lack of computing experience on- the part of the author the procedure adopted involved a compromise to allow the use of an existÍng 'Iibrary' program.

The raw data was first hand-plotted in graphical form (count against channel number) and the approximate positions of the minima visually estimated. The position of each minimum was then accurately deter:míned by fitting the experimental points 20 channels each side of the estimated position to a Lorentzian line shape function. The minimum value of this function was expressed to the nearest 0.1 of a channel. The standard deviation of the experimental data in the linear portion of the spectrum was then estimated and on the basis of this information, the position of each minimum has been rounded to the nearest 0.5 channel. The errors have been calculated as three standard deviations and accordingly expressed as for the minima, covering the range 0.001 to 0.003 mm/sec. A summary of the data obtained appear in Tables 4.3 and 4.4. TABT,E 4 .4

Positions of minima Sample c. s. Line (I) Line (2)

(a) Fesal-enX X=Cl 237.3 204!L 270.5lL Br 230.5 l90rr 27Lll I 23L.3 187.5tr 275!L Nog 230.5 190.5 r1 270.stl-

NCS 236 205r1 269!L

N3 23L 205!2 257 .5!2

C6H5COO 236 216!2 256!2

(b) K IFesa1en (CN) Z I 254 201r.5 3071.5

Fe (saen) 2CL.H2O 248.8 18It.5 316.51.5 Fe (saen) 2I 240.3 233.5!2 249!2

C.S. = Ltr,ine (1) + Line (2)l PART II

Transition metal complexes with the tridentate ligand, saen, are discussed in the following Chapters. 84

CHAPTER 5 Preparation, characterization and sorid state properties.

5.1 INTRODUCTTON

(a) Rationale

This study was undertaken in order to rationalize several unusual aspects of the chemical behaviour of the complex Fe (saen) 2cr.Hzo. some of the properties of interest, observed when studying this compound in the solid state, appeared to be directly attributable to the presence of a water molecure.

The following comparison of properties of Fe (saen) 2cL.H2o and Fe(saen)2I, provides an illustration of the features of the behaviour of the rhydrated' species which hrere considered anomalous. (1) The i.r. spectra of each complex differ substantial-ly both in the O-H stretching and the fingerprint regions. (2) The chloride salt has a low spin magnetic monentt, with Ueff = 2.IL B.M. at room temperatrrr"('g) , whereas the (17). iodide is high spin, with Ueff = 6.06 B.M. t The room temperature value of Ueff, which will be discussed later in this section, does not necessarily indicate the S - \ spin sÈate for the Fe (III) . The additional supporting evidence is discussed in the following Chapters. Recently, a variable temperature study of this complex has been undertaken by Dr. K. Murray (Monash University, Victoria). The data obtained confirm the low spin state, with Ueff values in the range 1.99 1.80 B.M. over the temperature range 313 90.5oK. In addition, the Ueff values are field independent and exhibit Curie -Weiss behaviour (0 = 18oK). Dr. K. Murray - private communication, May 1978. B5 (3) In solution, the magnetic moment of the chloride salt, Ueff = 5.96 8.M., is close to that expected for Fe(III) (18) high spin compounds . (4) The i.r. and u.v.-visible spectra, in methanol solution, of the iodide and chloride salts are indistinguishable. (s) Interconversion of one species to the other by metathetical reactions. occurs readily. (6) The chloride salt may be reversibly dehyd.rated under vigorous conditions (e.9. heating the solid in a vacuum for several hours). The dehydrated complex is high spin, with Ueff > 5.8 8.M., and has an i.r. spectrum identical to that of the iodo compound.

These observations support the conclusion, alluded to in Part I of this thesis, that in solution both salts generate the Fe(saen)2+ cation. Iron(III) complex ions are known to be (103) labile , and this may be readily seen in Fe (rïI) Schiff base systems. For example, the solution reaction between FesalenX and en to give Fe(saen)2+ o""rrrs rapidly, as also does * the hydrolysis of Fe (saen) to (Fesalen) ZO. Consesuently the solution properties outlined above could be rationalized in terms of such lability. However the solid state properties suggest that the low spin complex Fe (saen) 2CI.H2O is not a simple hydrated species. Indeed the evidence strongly supports the proposition that the two complexes are not related at aII. In particular the four major observations, felt to be anomalous, with respect to the chloro complex, are:

(I) the presence of two sharp absorptions in the O-H stretching region of the i.r. spectrum; (2) the substantial differences in the fingerprint region; B6 ,(3) the extreme conditions required for dehydration; (4) the apparent stabilization of the low spin state by the water molecule.

These four areas of concern, which courd not be resolved by chemical methods, have in fact been rationalized by the sÈructural determination discussed later in this th""i=(20)

In an attempt t.o solve the problem by chemical means, a series of complexes of general formula t4(rrr) (saen) 2x.n[2o have been prepared and studied utilizing a variety of techniques. The preparation, characterization and chemical properties of these complexes are discussed in the following sections.

(b) Previously published work

OrConnor and West have described the preparation of several complexes of formula Cr (III) (saen) 2X, where X = Brr I and CIO4, from tris(salicylaldehydato) chromium (III), (104) cr (sa1) 3 . The crystal structure determination of the iodo complex has confirmed the proposed octahed.ral coordination by the two tridentate ligands in the meridional configuration (105). The Fe(rrr) analogue, Fe(saen)2r, has also been described(17).

A Cobalt (III) complex, proposed as a mixed bidentate- quadridentate complex tCo (III) salen.enJClO4, has been reported (106) by Dey and De . rn this study the complex has been shown to be Co(saen) 2CIO4. The meridional configuration of Co(saen)2+ has been confirmed recentry by Benson et al(107'108). B7 It has been suggested that heterochelates of Mn(III) formulated as tMn(III)salen.enlX, exist. Dey and Ray suggested that these compounds, in common with those now established as involving Èhe cation Co(saen)2+, had structures similar to the (I09) mixed bidentate-quadridentate chelate, cosalen. acac . The spectra and chemical properties of these Mn(III) complexes do not support the existence of either a mixed bidentate-quadridentate or a bistridentate configuration.

5.2 (i) Preparation of cotnplexes of qeneral formula,

M(III) (saen) 2X..nH2O

, The preparative methods employed involved refluxing an ethanolic solution of a suitable transition metal complex in the presence of a slight molar excess of en. The aromatic moiety was present as either free or coordinated salen or as coordinated salicylaldehyde. The source of the anion was either the transition metal complex or an appropriate salt which \^ras subsequently added to the reaction mixture. For convenience the methods have been divided into five categories:

(a) M(III)sa1enX as reactant

EtOH M(III)sa1enX + en re7iu;) M(III) (saen) 2X.nH2o

M - Cr and Co, [ = CI, Br, I; M = Fe, X = Cl, Bf, I-, NO3, NCS.

(b) M(IIr) (sa1) 3 as reactant

EtOH (sat)3 (saen) salenH2 M(III) + en #re t_Lux M(rrr) 2+ + 88 On addition of an aqueous solution containing the appropriate anion, the crystalline product M(III) (saen) 2X.nH20 was obtained;

M = Cr, X = Cl, Br , I-, NCS, C1O4. |l[ = Fe, X = Cl, OH, Br, I, NCS, NO3 and C1O4.

The Co (II) complex was oxidized to Co (III) by atmospheric oxidation or aqueous hydrogen peroxide.

co(rr)salen + en + H2o2.:+þ co(rrï) (saen)2+

The crystalline salts Co(IIf ) (saen) 2X.nH2O hrere obtained on addition of an aqueous solution containing the appropriate anion.

X = CI, Br, I, NO3, NCS, C1O4, BF4 and pF6.

(d) Co (III) ammine complexes as reactants

Aqueous solutions of the Co(III) complexes \^rere slowly added Èo a refluxing ethanol solution of salenH2 and en.

The reflux was continued untit the evolution of NH3 ceased. and the crystalline product obtained on cooling the reaction mixture.

Co(III) (NH3)eX¡ + salenH2 + en -> Co(III) (saen) 2X.nH2O + + 2NH4X + 4NH31

X = Cl, B.r, T, NO3.

Similar results \^¡ere obtained with other Co(III) ammine complexes, e.9. tCo(III) (uH3)5H20lCI¡. 89 (e) (Fesalen) 20 as reactant EtOH (Fesalen) + 2Fe (saen) 20 en ;ffià 2+ Addition of the appropriate anion resulted in crystallization of the required salt as before.

X = Cl, OH, Br, T, NCS, NO3 and CIO4.

Purification of the reaction products \^ras effected by solvent extraction with chloroform or a l:l (v/v) ethanol- diisopropyl ether mixture. In many cases such purifi-:cation \^ras found to be unnecessary as the majority of reaction products, particularly those from methods (a), (c) and (e), analyzed correctly without further purification. In one case, when M = Co, X = PF6, analyses on recrystallized products showed a poor correlation with calculated values, probably due to thermal instability.

(ii) Attempted preparations

(a) Manganese (III)

The preparation, outlined in Ref. (109), was essentially similar to method (c) above, üsing Mn(II)salen as starting material. Following the procedure as reported, solid reaction products rttere obtained, horn/ever, the i.r. spectra and analyses did not correlate with those reported(109). purification of the reaction product by recrystallization from chloroform g'ave weII formed, golden brown crystals which had a different i.r. spectrum from that of the reaction product. Analysis of these recrystallized compounds suggested they were Mn (III) sa1enX (X = Cl, Br and C1O4). The i.r. spectrum of the chloro complex 90 correlated precisery with the spectrum published by Boucher(110)

After a prolonged reflux (48 hours) of Mn(IIT)salenCl with en in EIOH, Mn(III)salenCl was the only observable product. clearJ-y Mn(rrr) sarenx cannot react to give ¡f:lt(rrr) (saen) 2x. rn additionr orr drying the complex formulated as Mnsalen.en.cr at 100oC for 12 hours, the dried solid had an id.entical i.r. spectrum to that of Mn(III)salenC1.

It is suggested the.fefore that the Mnsalen.en.X species are not bidentate-quadridentate chelates but are simply Mn(III)salenX complexes with en of crystallization.

(b) M ÏT saen cl-e s

Attempts to prepare these neutral bivarent transition metar complexes (M = co, cu and Ni) from M(rr)salen, resuÌted in no apparent reaction. The starting material_s \^rere recovered unchanged after prolonged reflux. For Cu(II) and Ni (II) , the refrux was carried out with no special precautions whereas for Co(II) the procedure required an inert (uZ) atmosphere.

5.3 CHARACTERTZATION OF COMPLEXES

(i ) AnaI ses of unds of general formula f

M (IrI) (saen) 2X. nH O viz. M.C gHZZN¿O2.X.nH2O

(a) Fe saen) cI.H o

For this complex, analyses were used as a 'quality controlr check for the 'purity' of numerous preparations. To date, 18 consistent analyses have been obtained and 9t statistical carculations confirm that the mean varues can be expressed to a higher degree sf precision than usually accepted. For this reason the analysis for thj-s compound is presented to 2 decimal places throughout.

Analysis for C1gH24ñ4O3FeCI Calculated C 49.62 H 5.55 N 12.86 Fe L2.82 cr 8.14 Found C 49.58 H 5.52 N 12.87 Fe 12.79 cI 8.15

All other analyses are expressed to one decimar place and have been performed at least in triplicate.

(b) M(TII) (saen) 2CL.H2o

M=Cr Calculated c 50.r H 5.6 N 13.0 Cr 12.0 cl 8.2 Found c 49.9 H 5.6 N 13.0 Cr I2.2 CI 8.I

M=Co Calculated c 49.3 H 5.5 N L2.8 Co 13.4 c1 8.I

Found c 49.5 H 5.5 N 12.6 Co 13. 4 CI 8.2

In all other cases, analyses for carbon, hydrogen and nitrogen only have been obtained. Estimates for metal content from the combustion residues have been included in the following table, where applicable. In some cases, particularly when X = ClO4, explosive combustion occurred and no metal estimates have been attempted.

The data are tabulated in Table 5.I. TABLE 5. I (a) Analyses for M(III) (saen) 2X.nH2O

Calculated Found

MX n c H N M c HN M

Fe OH 1 51.8 6.0 13.4 L3.4 51. 1 5.8 l-3.2 13-3

Fe Br 0 46-8 4.8 T2.L L2.T 47.O 4.9 12.L L2.2

Fe NO¡ 0 48.7 5.O 15.8 L2-6 48.9 5.r 16.0 t2.4

Fe NCS o 51.8 5.0 15.9 L2.7 5r.5 5.1 15.6 L2.9

Fe CIO4 0 44.9 4.6 11.6 11.6 44.7 4.6 II.8

Cr NCS 0 52.3 5.1 16.1 1r.9 5r.9 5.2 16 .0 rt. I

Co Br t 44.7 5.0 1r.6 12.2 44 -7 4 -9 11.5 L2 .L COI I 40.8 4.6 r0.6 rl.1 40.8 4.4 10.6 r0.9

Co NO¡ I 46.5 5.2 15.1 L2.7 46.5 5.2 15 .0 13.0

Co NCS 0 5r.5 5.0 . 15. B 13.3 51.1 5.0 15.8 r3.0

Co Cl04 I 43.0 4.8 11.1 LI.7 42.8 4.7 LL.2 (b) co C1O4 0 44.6 4-6 11. 6 12.2 44.6 4 6 LT.]

(b) co BF4 0 45.8 4.7 1r.9 L2.5 45.6 4 7 L2.O (c) ao PPo 0 40.8 4-2 10.6 11.1 4L.3 4.3 r0.7 10.7

Notes: (a) Analyses for compounds previously described, ví2. M = Cr, X = Br, Tt CIO4 and ltl = Fe, X = I have not been included. (b) Compounds recrystallized from CHCl3. (c) Analyses for Co(saen) ZPFA, in particular recrystallized

samples, \^rere erratic, with carbon analyses being = 2% high. The data presented here was obtained with a sample obtained from a CHCI3 solution which had not been heated. 92 (ii¡ Infra red spectra

(a) Classification of speCtra

The i.r. spectra of all complexes have been obtained. In contrast to the characteristic spectra of the. salen complexes,

discussed in Chapter 2, the spectra of the M(III) (saen) 2X.nH2O complexes appeared to faII into three distinctive groups. Each spectrum has been classified into one of these groups, utilizing as criteria for classification the number'and position of

absorptions between 4000 and 3050 c.-1. I

For each spectral type, Lhe characteristic absorptions are: TyÞeA(n=1) Four bands near 3620 (v\ = 20) , 3400 (và = 40), 3200 (v% = 40) and 3too cm-I (và 40 = "m-1). TypeB(n=0) Two bands near 3230 (v4 50) and 3130 (v1 = "*-1 = 30 .*-1). TypeC(n=0or1) Two bands near 3330 (v\ = 30) and 3280 cm-I (v'< .' 30 c*-1). When n = L, a weak, broad absorption (v\ = 2OO cm-I) is observed near 3500 "*-I.

In Fig. 5.1 examples of each spectral type in the region 4O0O 3050 have been reproduced. "*-1

The classification of the complexes of formula M(III) (saen) 2X.nH2O according to spectral type is presented below in Table 5.2. Figure 5.,| l.r. spectra, O-H and N-H stretching regron.

Type

40 38 3ó 34 32 30 waveno. ( x to2 ) cm.-l 93

TABLE 5.2

Spectral Classification of M(III) (saen) 2X.nHrO

Spectral ÍYpe M=Cr M:Fe M=Co

A X = CIr Br: X=ClrOH X = Cl, Bx, T¡ NO3

B X = I, NO3, NCS X = Br, ï¡ NO3, NCS X=NCS

c X = CIO4 X = CIO4 X = CIO4, BF4, PF6

The rspectral type' is dependent on the nature of both the metar and anion. However, for the three metars studied,

when X = CI, the spectrum is Type A, when X = NCS it is Type B and when x = C1O4, Type C.

Discounting absorptions due to the anion (vi-z. X = NO3, NCS, C1O4, PF6 and BF4) within each type the spectra are essentially similar in the region 1800 400 cm-l, but different from the other types. The spectra of the chloride sartsr âs

reproduced in Fig. 5.2, illustrate the simirarity of the Type A spectra.

As typical examples of the three spectral types, spectra of the Co(III) complexes have been reproduced in Fig. 5.3. For Type A, X = Br and for Type B, X = NCS. The Type C spectrum, as shown, has been obtained by combining the individual spectra for x = CIo4, BF4 and PF6, omitting the broad anion absorptions found in the range 1150 1O5O .*-I when X = CIO4, 1050 - 950 cm-I when X = BF4 and 900 - 800 cm-I when X = pF6. Figure 5.2 I.r. apectra of the isostructural chloride salts

r6 l4 12 t0 6 4 wavenumber (x lo2) cm-I Figure 5.3 Representative examples of the three spectral types

t8 r6 t¿t t2 r0 I 6 4 wavenumber (x lO2) cm-l 94

, (b) Bands between 30OO 40OO and "*-I

The classification of the M(III) (saen) 2X.nH2O spectra into three distinctive types, strong,ly suggests that the solid state environment of the cationic species, within each class, is similar.

As a focal point for comparison of these spectra, the Tlpe B case has been selected because the structure of the complex Cr(saen)2r has been determined(105). Hydrogen bonding occurs between the anion and cation such that all four hydrogen atoms on the -NH2 groups interact with adjacent iodide anions. The positions of the N-H stretching absorptions between 3050 anq 3350 c*-l for the Cr(III)saen complexes with X = Br, I and C1O4 have been discussed in terms of a simple anion dependent hydrogen bonding model by o'Connor and west(I04).

It would appear from the current study that this rationalization is inadequate in that for the Type A and B spectra, where hydrogen bonding would be expected to result in significant 'shiftsr, the observed positions of the two N-H stretching absorptions are essentially independent of the nature of either the metal or the anion.

(I) Type B and C spectra

For convenience in the following discussion, the spectra have been subdivided into two groups. In the spectral region above 3OOO cm-l, the spectra of both Type B and. C complexes exhibit two wetl-defined absorptions which have been assigned as N-H stretching transitions. The relevant data have been listed in Table 5.3. 95 As indicated in Part I of this thesis, there is no evi-

dence supporting the existence of the free ligand, saenH. A'11 attempts to prepare it have proved unsuccessful. As a consequence, L12 diaminoethane(en) has been selected as a reference to allow rationalization of changes ir u*_" on coordination. In common with saen complexes, two absorptions occur in the N-H stretching region in the en spectra. In CCI4 solution, the vN-n posiÈions are at 3390 and 3310.*-1 and are shifted to 3370 and 3300 .*-1 respectively in the liquid film spectrum. Presumably the shift is caused by inter-molecular hydrogen bonding in the liquid phase.

A further decrease in the frequency of these absorptions would be observed on coordínation(ll1).

TABLE 5.3 M(III) (saen)ZX complexes: N-H stretching frequencies (cm-l)

M X Band positions

Tlpe B Cr r 3205 3110 Cr No3 3220 3l_30

Cr NCS 3205 3I15 Fe Br 32L5 3115 Fe I 32l-5 31t5

Fe NO: 3225 31 35

Fe NCS 32L5 31 t5 Co NCS 3205 3r15

TYpe C Cr ClO¡ 3295 3250 Fe CIO4 3295 3255 Co cIO4 3305 3255

Co BF4 3 315 3265

Co PFo 3 340 3290

In all cases, the values of vu_u obtained indicate that

coordination has occurred between the metal ion and the -NHZ 96 group. However, the magnitude of the 'shift' for the Type C complexes is substantially less than for Type B. The anions in the Type C complexes are considered unlikely to be involved in strong hydrogen bonding interactions and the higher values for u¡q_¡r, relative to those for Type B, indicate such interactions are minimal..

The anions of the Type B complexes are known to frequently be involved in substantial hydrogen bonding interactions which may be reflected in a significant shift of the ur_" absorptions to lower frequencies. Indeed for the series of complexes of formula tCo(III) (UH3)6JX3, the position(s) of vu_H show extreme sensitivity to the nature of X, as shown in the follow- (r12) ing dat,a : X C1O4 NO3 I Br cl u*_" 3320, 3240 3290, 3200 3150 3120 3070

These values probably reflect the relative rstrengthr of the H-X 'bond'.

In contrast to the above, the ur_* values obtained for the Type B complexes appear remarkably insensitive to the nature of X. In view of the known structure, this appears somewhat unusual in that it suggests that all of the H-X 'bonds' are of similar 'strength'.

Further, it woul-d appear that the vu_H values which do seem to follow a trend of the type expected, are observed when X = C1O4, BF4, PF6 and NO3. Thus the apparent'hydrogen bonding ability' of the anions indicated follows the order: Br-I=NCS>NO3>> 97 The reason for this observation is not cIear, but it is possible that the apparent 'levelling' of the hydrogen bonding interactions may weII result from the packing of the aations and anions in the crystal lattice. The structural information available is considered in Chapter 7.

(2) Type A spectr4

In addition to two N-H stretching bands between 3100 and 3250 .*-1, tr^Io additional bands are found at higher frequencies. The data for the four band positions are tabulated in Table 5.4.

TABLE 5.4 Type A, M(III) (saen) 2X.H2O complexes: Absorptions in N-H and O-H stretching regions

M x Additional bands vn-u (cm-t)

Cr c1 3 615 3355 3175 310 5

Cr Br 3 610 3390 31 80 3110

Fe c1 3620 3 365 316 5 310 5

Fe OH 3605 3370 316 0 3115

Co c1 36 0s 337 0 316 s 3r05

Co Br 3600 3405 316 5 310 5

Co ï 3600 3420 316 5 3I0 5

Co No¡ 3605 3425 3245 314 5

The positions of vu_H follow a similar pattern to that discussed above for the Type B complexes, i.e. the uu_' values appear insensitive to the nature of either l[ or X, implying that the H-X bonds are also of similar strength in this case. 98 Of interest, hohrever, is the further observation that vlu_u values for X - NO3 are apparently independent of the spectral type with band positions of 3235 t 10 and 3135 r 10 .*-1. On the basis of this observation it may be expected that all nitrate sa1Ès have similar solid. state struct.ures. The i.r. spectra between 1900 .and 4OO suggest thaÈ this is not so, "*-1 as the Type A spectrum differs in many respects from those of Type B.

With respect to the Type A spectra, the major problem of interest is the rationalization of the two additional sharp bands above 3350 cm-l. All Type A complexes are monohydrates and. it appears reasonable to assume that the two bands are asðociated with the presence of a twater molecule'.

Water may be incorporated into the crystalline salt in the form of molecules trapped in the lattice (lattice water) or coordinated to the metal ion(s) (coordinated water) (113). In general, a broad absorption, observed in the i.r. spectrum between 3550 and 32oo is consid.ered typical of either case(113) Lattice water in particular may usually be removed under relatively mild conditions, e.g. heating for a few hours at

110oc (atmospheric pressure) .

The Type C complex, Co (saen) 2CIO4, has been obtained as a monohydrate from aqueous ethanol and as the anhydrous salt by drying at llOoC for 3 hours or by recrystallization from chloroform. The presence of the water molecule in the hydrate has been demonstrated by a weak, broad i.r. band (3500 to 3300 cm-l¡ with the remainder of the i.r. spectra of both species being indistinguishable . 99 As discussed later in this Chapter, the water molecule in the hydrated chlorides with M = Fe and Cr, may be removed under vigorous conditions whereas for M = Co dehydration could not be confirmed. The water molecule is tightly held and it appears unlikely that it is simply lattice water. It'is highly improbable that the waÈer molecule could be coordinated to the metal ion as this would require a seven coordinate metal ion. Furthermore, on recrystallization from'coordinatingr solvents such as DMSO and a1cohol, the monohydrated species only are obtained, thus negating the possibility of a labile coordination site such as would be expected with a M-OH2 bond.

However, the observation that monohydrate species T^rere invariably recovered suggested that the water molecule was involved in a very strong, specific interaction in these

M (III) (saen) 2X.H2O complexes.

Recehtly, a similar effect was observed in a series of copper complexes with the ligand N,N'-bis (2-pyridylmethylene) - ethane-I,2diamine (abbreviated as bpe). The Cu(fI)bpe complexes $¡ere formulated as monohydrates or alcoholates and gave rise to i.r. spectra with sharp bands near 3400 and 3250 cm-I. In these complexes, addition of ROH (R = H, Me, Et) across one of (rr4) the azomethine groups has occurred , l-.e \ROHI C=N ----+ H-C-NH /\l\ HOR

A similar reaction h/as considered to be feasible in the Type A complexes. An i.r" band near 3600 would be consis- "m-1 tent with a 'free' (i.e. nonhydrogen bonded) O-H stretching 100 vibration and a band near 3400.*-1 would correlate with a

' free! N-H stretching vibration(115)

Attempts to verify this proposition profoundly influenced the direction of the ensuing research program.. Much of the chemical evidence obtained may be rationalized in terms of this model, however, the problem could not be conclusively resolved by chemical studies äIone. Consequently, the crystal structure determination of the complex Fe(saen)2CI.H2O, as presented in Chapter 7, was undertaken.

This structure determination has unambiguously established that this Type A cômplex is, in fact, a true hydrate. Briefly, thé water molecule is bound into the crystal lattice such that only one hydrogen atom is involved in a hydrogen bonding interaction with the anion(20). The i.r. bands may be rationalized in terms a hydrogen bonded O-H stretch near 3400 and a of "*-1 'free' o-H stretch near 3600 cm-I(116).

The position of the hydrogen bonded O-H stretch reflects an anion depend.ent trend with the order of uo_H (bonded) being NOg > I > Br > CI. This correlates with the expected order predicted from studies such as that with the Co(III) hexammine (rr2) complexes_-_- r '

5.4 FURTHER SOLID STATE STUDTES

In addition to the analytical and i.r. spectral data discussed above, investigations of further properties of these saen complexes, in the solid state, \^Iere undertaken and information of particular interest obtained from: 101 (i) magnetic moments

(ii ) X-ray powder diffraction photographs

(iii ) dehydration of the Type A monohydrates.

(i) Magnetic moments

(a) General comments

The data obtained in the majority of cases, was quite (117) predictable, as summarized below . (1) when M = Crr Ue¡¡ values were in the range 3.7 3.9 B.M. (2) aII Co(III) complexes \^rere diamagnetic (3) the Type B and C complexes of Fe (III) were found to be high spin, with Ueff near the spin only value of 5.92 B.M.

The only unusual behaviour \^ras observed with the low spin complex, Fe (saen) 2CL.HZO.

(b) Magnetic susceptibility of Fe (IIT) (saen) z C1 . H2o

As p4eviously reported in Chapter 2, Ueff for this complexr ês determined initially, hras z.LL B.M. As a consequence of the more detailed study undertaken for Part II of this thesis, the magnetic susceptibility of a number of samples of this complex have been measured. Consid.erable variation in the value o¡ Ie¡¡ was observed, and it has been established that this variation can be related to the rhistory of the sample,. By studying the susceptibility of 'ana1ytically pure' samples, the following generalizations may be made:

(1) samples obtained from the initiat reaction mixture have 102 Ieff = 1.93 t .01 B.M. (2) on exposure to the atmosphere, Þeff gradually increases

with time to c.a. 2.3 B.M. (after six months) . (3) samples purified by recrystalrization from chroroform

or an ethanol-diisopropyl ether mixture. have Ueff = 2.IL t .01 B.M.

These observations may be explained by considering the effect on the apparent value of Þeff of 'contamination' with smarl quantities of a high spin comprex. As can be seen from the data in Tab1e 5.5, 53 contamination will increase Ueff from I.93 to 2.3L B.M.

TABLE 5.5

Effect of a high spin contaminant on Ueff of a low spin compJ_ex

x0I234567891020

Ueff L.93 2.OL 2.09 2.16 2.24 2.31 2.37 2.44 2.5O 2.57 2.63 3.IB where x = å of high spin contaminant

In aqueous solution, Ueff of Fe (saen) 2CI.H2O is 5.96 B M consequentl.y it appears reasonabre to propose that the 'high spin contaminant' is, in fact, the high spin form of the complex whichr âs shown later (Section 5.4 (iii)), is simply the anhydrous complex.

It is worth noting that with as much as 10% 'contamination' with this high spin form no significant difference in the i.r. spectrum or the analysis could be detected. For example, consider the following analytical data: Calc. (O% high spin) C 49.62 H 5.55 N 12.86 Fe 12.92 r03 Calc. (I0? high spin) C 49.84 H 5.53 N I2.9I Fe I2.BB Found C 49.58 H 5.52 N L2.97 Fe L2.79

Clearly, the value of the magnetic susceptibility is a far more sensitive means of assessing the 'purity' of sampres of Fe (saen) 2cL.H2o than the other readily avaitatte techniques.

(ii) X-ray pot^¡der d.iffraction

The powder photographs of a number of the species of formula M(III) (saen) 2X.nH2O have been obtained.

The experiment.al procedure allowed. simultaneous determination of four separate samples, thus facilitating direct visual comparison of the diffraction patterns. From these patterns, reproduced diagrammatically in Fig. 5.4, the forrow- ing conclusions have been made:

(1) When X = CI, the solids are isostructural, (M = Cr, Fê and Co). (2) !ùhen X = Br, M = Co and Cr, the solids are probably isost.ructural and similar to the chloride salts. (3) When X = I, !l = Cr and Fe, the solids are not isostructural, nor are they similar to the chloride salts.

On this basis it would appear that the crystal structures of all three chloride sarts are identical, and are similar to the Type A bromides, as previously suggested by the i.r. spectra. In the case of the iodide sa1ts, the solid state arrangement is not the same even though the Type B i.r. spectra are similar. Figure 5.4 Diagrammatic representation of powder dif fraction photographs (a) (b) (c) (d)

1 0

7 I

---+ (a) M=FerCrorCo;[=CI (b) |l[=CrorCo; X=Br (c) M = Feì X = I (d) M=Cr; X=T 104 (iii) Dehydration of Type A coiqìpIêxes

Attempts have been made to dehydrate the isostructural chlorides, estimating the extent of the dehydration (if any) by following changes in the weight and i.r. spectra of samples. In the case of the Fe (III) complex, changes in the magnetic susceptibiliÈy have also been utilized.

(a) Fe (saen) cI.H o

Quantitative dehydration was achieved by heating the sample in a vacuum pistol for four hours at 1I0oC.

(r) The observed weight change was 4.2gso, compared with the . value as calcurated for a monohydrate of 4.132. water

was rapidry absorbed from the atmosphere (509 within 15 mins., 752 within 30 mins. with quantitative absorption after 3 hours). rn addition, quantitative absorption of water occurred with a dehydrated sampre stored overnight over phosphorus pentoxide ín a dessicator.

(2) The i.r. spectrum of the dehydrated species was characterized'by the absence of the vo_H bands and a general similarity to the Type B spectrum. After 30 mins. exposure to the atmosphere, the spectrum of the partially re- hydrated species was essentially that of the original Type A complex.

(3) A sample of the monohydrate was dissolved in D2O, filtered and 'freeze dried' overnight. The i.r. spectrum of the resurtant violet sorid was indistinguishable from that of the Type B complex Fe (saen) 2I. After a few minutes, it was noted that the mu1l, a deep violet colour 105 initially, hras slowly changing to a bright blue colour. This colour change was accompanied by changes in the i.r. spectrum, and after a few hours the spectrum obtained was of Type A. Presumably, this was a result of water adsorption from the mulling agent (nujql).

In addition, a.weak dor:blet was found in these spectra between 2300 and 24OO cfn-I. Bands in this region would (118) be assigned to o-D and N-D stretching vibrations .

(4) A sample of the dehydrated complex r,,ras exposed to DzO saturated NZ gas. The i.r. spectrum was essentially similar to the Type A spectrum with four prominent new bands at 2305, 2385, 24gO and 2650 cm=l respectively. The general 'shape' of this series of bands was essent- ia1ly similar to the O-Il and N-H stretching absorptions between 3IOO and 3620 .*-I. on exposure to the atmosphere, the bands at 24gO and 2650 .*-I rapidly 'disappeared' (over a period of 10 minutes) with a correspondir,rg increase in intensity for the bands at 3365 and 3620 .*-1.

Clearly, then, deuterium exchange had occurred with both the ammine and water hydrogen atoms and the bands may be assigned as: 2650.*-l rfree' o-D stretch 24gO cm-I 'hydrogen bonded' O-D stretch 2385, 2325 N-D stretch (doublet) "^-I The 'water' molecule appears to be 1abile with respect to isotopic exchange, whereas the ammine groups appear relatively inert. 106 (s) A sample of the monohydrate was placed in a Guoy tube and dehydrated in the vacuum pistol at 110oC. At approximately 24 hour intervals, the tube and contents were sealed under an atmosphere of dry N2 and the magnetic susceptibility determined. Over a period of 7 days, the value of Ìreff increased from 1.93 B.M. to c.a.5.8 B.M. with a corresponding weight loss of 42.

(b) cr ( sirêh) r 9l-H2O and Co (saen) 2CI.H2O

Dehydration of the Cr(III) complex, under the same conditions as for Fe (III) , required 8 hours for quantitative removal of H2O. The weight and r.r. spectral changes \^rere as described for the Fe (III) case.

, In contrast, after 24 hours at 110oC in vacuo, no weight loss could be detected in the Co(III) case. Ho\^rever, evidence for the removal of H2O was obtained indirectly. After heating the complex for I hours at 110oC in the vacuum pistol, D2O saturated NZ Ì¡tas introduced and the complex allowed to stand in this atmosphere overnight. The changes in the i.r. spectra were similar to those noted above, with the exception that the N-D bands were much weaker than in the case of Fe (III) . Ex- change with atmospheric moisture also appeared to occur more slowIy than with the Fe(III) complex as the O-D bands r¡/ere still prominent after one hour exposure to the atmosphere. In addition, it would appear that heating in vacuo is a necessary prerequisite to deuterium exchange as exposure of the Co (ITI) monohydrate to the D2O saturated atmosphere overnight did not result in the presence of detectable O-D peaks in the i.r. spectrum. 107 However, with all three complexes, deuterium exchange occurs readily in solution, as discussed in the next Chapter.

5.5 CONCLUSIONS

The complexes of formula M(III) (saen) 2CL.H2O, and presumably other Type A hydrates, all have a water molecule strongly bound, via hydrogen bonding interactions in the crystat structure. In contrast, to the 'normalt behaviour ôf water in hydrat.ed complexes, the water molecule in these complexes appears to be unusual in that (a) it gives rise to two sharp i.r. absorptions, assigned as uo_" 'free' and uo_u 'bondedr. (b) the 'strength' of the bonding interactions is high and

appears to be comparable with covalently bonded -OH groups. (c) in the case of the Fe (III) complex, it apparently stabilizes the low spin Fe (IIf) state and is responsible for the difference in colour observed, in the solid state,

with the hydrated and dehydrated samples of the Type A Fe (III) salt.

ft would be expected that the behaviour of the complex salts of Types A, B and C should be essentially similar in solution and thus present fewer problems in the interpretation of the data obtained. The solution properties of the series of complexes are d.iscussed in the fol-Iowing Chapter.

5.6 EXPERTMENTAL

(i) Starting materials r08 Tn general, AR grade chemicals hrere used as starting materials and solvent.s.

(a) M (Ir) saLen species

(1) The preparation of Co(fI)salen \^ras essentially as described by Bailes and Calvirr(5).

Salicylaldehyde , L2.2g (0.1 mo1) \iras dissolved in 150 c*3 of EtOH and heated under reflux. 3.09 of en (0.05 mol) in 50 cm3 of EIOH r¡rere added slowly. A solution of

L2.5g of Cobalt acetate tetrahydrate (0.05 mo1) in 60 cm3 of H2O Ì^ras warmed to 60oC, filtered, and then slowly added to the vigorously stirred refluxing solution of salenH2. The reaction mixture was refluxed for a further hour then allowed to cool slowly. The red crystalline solid product was filtered, washed and dried. Yield L3.29; 808.

(2) The equivalent Mn(II) complex was prepared by a similar procedure utilizi.ng Manganese acetate tetrahydrate. The yellow solid product was obtained in approximately 508 yield, and was observed to be air sensitive in solution (ro) as reported previou=Iy .

(b) M (TIT) salenx species

(r) I[ = Fe (III) . The complexes of formula FesalenX have been described together with (Fesa1en) 20 in Chapter 2 (section 2.4\ of this work. 109 (2) M = Co(III), Co(III)sa1enX.2H2O where [ = Cl, Br, I.

The filtrate from the preparation of Co(II)salen as described above was rendered slightly acidic to pH test

paper (pH c.a. 4-6) with the appropriate dilute acid, HX. On standing overnight, dark green-brown crystals of CosalenX.2H2O formed, \^¡ere filtered off and dried.

(3) Ir[ = Cr(III), X = CI.

CrsalenCl.2H2O was prepared as described by Coggin et (12o) ar .

An aqueous solution of CrCI3.6H2O was reduced with zi,nc amalgam under an atmospheré of N2. The aqueous solution of Cr(II) was then added to a stirred, hot alcohol solution containing the stoichiometric quantity of salenH2. The reaction mixture was stirred vigorously for a further hour, then allowed to cool whereupon the brown solid product formed. The crude product was purified by recrystallization from water.

(4) M = Cr(III), X = Br and I.

These salts r¡rere prepared by metathesis by adding a 10-

fold molar excess of the appropriate sodium salt to a hot agueous solution of the chloride salt. On cooling crystals of the appropriate salt of [Crsa1en (HZO) Z]+ were obtained. 110 (c) Co(III) hexammine salts, Co(NH3)6X3 where X - Cl, Bx, I and NO3

The complexes were prepared as described in Inorganic (121) syntheses .

(d) M (III) (sal) 3

The Cr(III) complex was. prepared as described by O'Connor and Vtest(104), and the Fe(III) complex as described by

van den Bergen et aL(L22) .

(ii) M(TrT) (saen) 2X.nHo O complexes

The preparative methods as outlined in Section 5.2 were found to be essentially similar with respect to procedure and yields. Consequently typicat examples are considered in the following more detailed discussion.

(a) M IÏ sa1enX as reactant

The preparation of Fe(saen) 2Cl-.H2O as described in Chapter 2 (section 2.4(íi-) (e)) may be regarded as typical.

The yield of product obtained was improved significantly by concentrating the filtered reaction mixture (utilizíng a rotary evaporator) to about half the initial volume.

Yields of products were within the range of 60-80å for M = f'e and Co, and 40-503 for M = Cr. 111 (b) M(TTT) (sal) 3 as reactant

Preparation of Cr(saen) 2NCS. A solution of Cr (sal) 3, 4.2g ( . 0I mol), and en, 1.5g (.025 mol), in I00.*3 of EtOH, \^ras refluxed for one hour. A

solution of NH4CNS, I.5g (.02 mol), in c.a. 20.*3 of H2O was

then added and the refrux continued for a further hour. The hot reaction mixture was then fittered, allowed to cool and concentrated by srow evaporation for c.a. 2 days. A yield of 2.499 (59?) of red crystaltine product was obtained.

The following preparations of co(saen) 2x.H2o ilrustrate the advantage of oxidizi-ng the co(rr)saren with Hzoz rather than air.

(I) Co(II)salen, 10.89 (.033 mot), and en, 4.0g (.067 mot),in 2oo cm3 of EtoH \Àrere heated under reflux until no more red solid Co(II)salen could be observed (c.a. 20 hours) and a yellow-brown solution only was present. 2.7q of solid NH4cr (.05 mol) was then added and reflux continued until the evolution of NH3r âs detected by damp indicator paper, had ceased (c.a. 24 hours). On cooling the reaction mixture, 9.9g (672,1 of cryst,alline Co(saen) 2CI.H2O was obtained.

(2) Co(Ir)salen, 5.4g, and en, 2.0g, in 100 cm3 ntog \¡rere heated under reflux with stirring. A solution of 2 cm3 of r00 vol. H2oz in c.a. 10 cm3 of H2o was added dropwise over a period of 10 minutes. At this stage a yerlow brown sorution only could be observed and a solution of 2 cm3 of conc. HBr in

5 cm3 of H2O was added. The reflux was continued for 30 L12 minutes and the hot reaction mixture filtered. On cooling 5.8g (722) of olive brown crystalline Co(saen) 2Br.H2O was obtained.

(d) Co (III) ammine complexes as reactants

To a refluxing solution of salenH2, 6.79 (.025 mo1), and €nr 3.0g (.05 moI)rin 2OO cm3 of EIOH, âî aqueous solution

( (50 cm3) containing Co (NH3) e CI¡ , 6 .7g .025 mol) r wês slowly added over a period of c.a. 30 minutes. The reaction mixture was refluxed until evolution of NH3 had. ceased (c.a. 2 hours), filtered hot and the filtrate allowed to stand overnight. 'WeIl formed crystals of Co(saen) 2CI.H2O were obtained. Yield 9.2g; B4?.

(e) (Fesalen) rO as reactant

(Fesalen)ZO, 6.6g (.01 mol), and en, 2.49 (.04 mol), in IO0 cm3 of EIOH r^tere heated under reflux for one hour. An aqueous solution of NH4CIO4 was prepared by rendering a solution of 2 cm3 of BO? HCIO4 in 10 cm3 of H2O slightly alkaline with conc. NH4oH (added. dropwise). The solution of NH4c1o4 was then added slowIy to the refluxing reaction mixture and the reflux continued for 2 hours. The resulting violet solution was filtered hot and allowed to stand overnight.

Well formed violet-black crystals of Fe(saen) 2CIO4 r¡tere obtained. Yield 4.89; 508.

(iii) The analyticat data, i.r. spectra and magnetic susceptibilities were obtained as previously described in section 2.4. 1r3

The solution susceptibility of Fe (saen) 2CI.H2O was estimated using the Evanrs n.m.r. technique(119), and is described in more detail in Chapter 6.

(iv) x-Tay povrder diffrac tion photoqraphs

The 'powder photographs' were obtained on Kodak X-ray film strips mounted in a Nonius Guinier-De Wolff camera and. required c.a. 30 minutes exposure to filtered Cu Ko radiation. Finely powdered samples of the complexes were mounted on the sample holder with adhesive tape.

(v) Dehydrat,ion experiments

(a) Vacuum pistol

The system was based on a Gallenkamp heated vacuum dessicator fitted with ground glass adaptors to allow introduction of dry or D2O saturated N2 as required. A 'Quickfit' test tube was used as a reaction vessel and all operations subseguent to the experiment were performed in an atmosphere of dry N2. For example, the test tube was sealed with a tight fitting ground glass stopper inside the pistol under a positive flow of the gas.

(b) f.T. spêctra

All mulls hrere prepared in a portable glove box under dry N2.

The D2O solution of Fe (saen) 2CL.H2O was placed in a small 114 (20 cm3) 'Quickfit' test tube and aÈtached to a vacuum Iine. The solution was frozen with liquid nitrogen then exposed to a vacuum for c.a. 12 hours. The mull fox the i.r. spectral work was then prepared as in (b) above. r15

CHAPTER 6

Solution studies of the series of complexes of general formula M(III) (saen) 2X.nH2O.

6. I INTRODUCTION

rt is apparent t,hat much of the anomarous behaviour of the sartsr âs discussed in chapter 5, may be rationalized in terms of the strong hydrogen bonding interactions observed in the solid state. The occurrence of hydrogen bonds is not unexpected in view of Èhe presence of a primary amine function- ar group in the ligand. However, it does seem reasonable to expect that the solution properties of the salts would be simpler to interpret because of dissociation into the cation, M(rrt) (saen)2+, and the anion, x-. Ready dissociation in solution has already been encountered with the anion analyses of the salts (chapters 3 and 5). The solution properties, then, would be'expected to be characteristic of the cationic species in particurarrwith the anion behaving as an armost independent entity.

6.2 GENERAL SOLUTION PROPERTIES

In general, the solution properties studied have, r¡rith one important exception, confirmed the above expectation. rn the solvents dimethyl sulphoxide, DMSO, and its deuterated anarogue, DMSo-d6, the results obtained require considerable

discussion. This is done later in this chapter, folrowing a generar discussion of the properties in solvents such as water, methanol, ethanol and chloroform. 1r6 (i) I.r. spectra

The chloroform solution spectra of a number of complexes of Types A, B and C were found to be virtually indistinguishable. No dor:bÈ this may be attributed, in part, to the rather limited solubility of the compounds in chloràform and solvent absorptions in the fingerprint region.

. However, in the region of the spectrum above 3000 cû-1, where solvent absorptions are negligible, all spectra obtained showed distinctive vu-s bands. In Chapter 5 the considerable sensitivity of these absorptions to hydrogen bonding effects has been discussed. The band positions in the solution spectra r¡rere remarkably similar to those of the Type C solid state spectra, ví2. near 3250 and 3300 No metal or anion "*-1. dependent effects \^rere observed. These observations support the proposition that the spectra obtained relate to an isolated solvated cation.

In addition, the monohydrate complexes had a relatively sharp band near 3600 .*-l (v'¿ = 40 .*-l), presumably due to a (118) free water molecule .

(ii ) .Conduc!ances

Utilizing methanol as solvent, the values of À¡4 obtained for aII complexes, at concentrations of I0-3M, hlere found to be within the range 75 115 Scm2mol-l, as expected for 1:1 (38) electrolytes . rn contrast to the Fe (rrï) salenx series discussed in Chapter 3, no evidence of partial association was observed, e.9. IL7 (a) the 'pH' of all solutions vras 8.32, i.e. identical_ to the tpHt value for pure methanol;

(b) plots of Â¡4 against C4, over the concentration range 5x10-3 to 1o-5M, \^rere linear in agreement with idear r:I electrolyte behaviour.

(iii ) U. V. -visible spectra

The spectra of all complexes in methanor arso proved relatively simple to interpret. rn arr cases, prominent rigand absorptions were observed below 400 nm, and for the cr (rrr) species weak bands, previousry ascribed to I d-d' type transitions, were found near 500 nm(104'108). The corresponding 'd-d' transitions urere found near'500 and 600 nm for the (IoB) Co (rrr) salts .

The Fe (rrr) complexes gave rise to spectra indistinguishable from that of Fe (saen) 2cL.H2o, as described in chapter 3.

(iv) Magnetic suscept.ibility of Fe (III ) complexes

The magnetic susceptibility of solutions of Fe (saen) 2CI. H2o have been measured, employing the n.m.r. technique described by Evans(119)- rn aqueous sorutionr ueff has been determined as 5.96 B.M. and similar values have been estimated for sorutions in chloroform and methanol. The Type B complexes (x = Br and r) arso have sorution magnetic moments near the spin only value of 5.92 B.M.

(v) Proton n.m.r. spectra of Co ( IJ_J_ ) complexes

Attempts were made to obtain proton n.m.r. spectra of all llB the different salts of Co(III) (saen)2+. These attempts were unsuccessful in deutero-chloroform, cDc13, due to the low solubility of the compounds. rn addition, suitable spectra could not be obtained in deutero-nethanol, cD3oD, and deuterium oxide, D2o, because of rapid proton exchange between the relativery labile ammine protons and the solvents.

However, the spectra of.alt three types of salt in the sorvents deutero-ethanor, c2D5oD, and trifluoracetic acid, CF3COOH' v/ere similar and thus consistent with the proposition of the existence of the simple cation, co(saen)2+, in sorution. The positions and assignments of the spectral bands are discussed in the following section.

The spectra of the Co (III) salts in DMSO-d6 initially presented. consid.erable difficulty in their interpretation but have been rationarízeð, by considering the probabirity of appreciable association in solution. A more detailed discussion of the results obtained from the n.m.r., conductance, u.v.- visible and magnetic susceptibility studies is presented in the following section, with particutar emphasis on the case of DMSO as solvent.

.N.M. 6. 3 PR.OTON R. SPECTRA

(i) Ligand spectra

Due to the non-availability of the free ligand, saenH, the spectra of sarenH2 and free and coordinated en have been utilized as a basis for the assignment of the observed band positions in the spectÈa of salts of the cation Co(saen)2+. 119 (a) SaIenH2

Disregarding absorptions obviousry attributable to the solvent utilized, alr solution spectra of salenH2 consisted of four distinct. groups of bands. The band positions and integrated intensities allowed ready interpretatiòn because the salenH2 molecule has only four major environments for the six-

Èeen hydrogen atoms ,'ví2. phenolic O-H(2) , Aromatic C-H(g) , imíne CH=N(2) and methylene =N-CHZ (4) .

The positions and. assignments of the resonances for the salenH2 spectra are tabulated below in Table 6.I.

(b) Resonances for the -CIIZ-ug^ moiet from free and coordinated en

The resonances and assignments for en in a number of solvents and for co (en) 3r3H2o in DMSo-d6 are taburated below in Tab1e 6.2.

The resonance at $ = 2.65 t .02 ppm may be considered to be characte.ristic of the methylene protons in the -CHZ-NHZ group and could reasonably be anticipated for the saen spectra. However) the position of resonances for -NHz protons would be expected to bb uncertain. rn part, this may be attributed to the effects of metal-nitrogen coordination which would, in (123) general, result in a downfield shift of the resonance . A more important variable effect would result from solvent interactions with the -NHZ protons which have been well documented in the literature(L24) . TABLE 6.1 Assignment of salenH2 spectra Solvent -CH=N Ql Aromatic (8) =N-Ctir (4) cDcl3 8.26 (r.e8) 7 .4-6.6 ( 8. 00) 3 .82 (3.e8) DMSO-d6 8.49 (2. 11) 8.1-7.1 (8.00) 3.92 (4.10)

Notes: ir

(1) Integrated areas have been included in parentheses follow- f l, ing the resonance position (0 in ppm from TMS). i- I

I (2) The broad multiplet near 7 ppm, typical in 'shape' and

position to that frequently attributed to aromatic I (I23) protons , has been assigned as an aromatic multiplet of integrated area 8.00. (3) The phenolic O-H resonances were found downfield near $ = 13 (CDC13) and $ = 14 (DMSO-d6).

TABLE 6.2 Assignment of free and coordinated en spectra

Solvent -cH -NH2_ CC14 2.65 I. 00 CDCt3 2 .66 I.27 DMSO-d6 2 .65 1. 70 neat'liquid 2.63 1.55 Co (en) 3I3. H2O in DMSO-d6 2.66 (12.00) 4.74 (rr.78)

Notes: (1) -NH2 resonances in free en assigned. on the basis of the observed solvent depenclence of the resonance position(124)

(2) The spectrum of the Co (III) salt included an additional band at g = 3.37 (2.32) attributed to H2o(125). (3) The coordinated -NH2 resonance at $ = 4.74 was confirmed

by deuterium exchange with ozo(l-26) . L20 (c) Free water

The position of the resonance associated with the water molecule in the Type A hydrated salts would also be expected to show substantial solvent dependence. The variation in the position of the H2O resonance has been examinäd for all solvents of interest over a range of concentration. The data are tabulated. below in Table 6.3.

(ii) Spectra of salts of Co (saen¡ 2+ ín Ps OD and CFa cooH

The positions and intensities of the proton resonances of two of Èhe members of the series Co(saen)2X.nH2O ví2. X = Cl and C1O4 have been tabulated below in Table 6.4. In patt (a) of this table defÍnite band assignments, based on the resonances in the ligands discussed above, have been included. The positions and intensities of the other resonances only have been presented.

,. The intense band at 5.15 in the C2D5OD spectra is mainly due 'to H2O in the solvent (integrated intensity 9.2 in solvent spectrum).' The increased intensity in the spectra of the complexes may be attributed to absorption of atmospheric moistuie(I27) plus the water of hydration in the case of the chloride salù.

The overlapping bands, resulting in the broad absorption, must be attributable to the remaining unassigned ligand protons, and it appears reasonable to suggest that the signals for the four =N-CE2 protons and the four -NHZ protons are involved. TABIE 6.3 Position of H2O n.m.r. resonances in the concentration range 0-I0? H2O (v/v)

Solvent C2D5OD Dzo DMSO-d6 CF 3COOH

Range (ô) 5.2-4.7 4 .67 3.2r-3.43 11.5-9 . 3

Notes: (r) The range shown is in order of increasing concentration. (2) only one resonance signal was observed in the case of cF3cooH, due to rapi,d exchange between the -cooH and H2o protons. spectra of the co(rrr) salts in cF3cooH wirl not show H2O resonances below ô - 10 ppm.

TABLE 6.4 Proton n.m.r. spectra of the sarts of the cation co(saen)2+

(a) Complex Solvent -CII=N Aromatic -ClI2-NH2 Co (saen) 2CI.H2O CF3COOH 8.54 (2.O2) 6.8-7.7(8.00) 2.8s(4.01) C2D5oD 8. 38 (2 . 10) 6 .2-7.4 ( 8.00) 2 .60 (4. rO)

Co (saen) 2CLO4 CF3COOH 8.48 (2.0s) 6.9-7.7(8.00) 2 . 8s (4.0s) C2D5oD 8.40 (2.L2) 6 .3-7. 4 ( 8. 0o) 2 .62 (4.l-5)

(b) Additional bands for both complexes: (1) cF3cooH - broad absorption 3.2-4.8(B.r) consisting of overrapping bands, with the major component centred at $ = 4.45 having c.a. 2/3 total integrated area. (2) C2D5OD (a) broad absorption 3.9-4.G(7.5) consisting of overrapping bands, with the major component centred aÈ $ = 4.8 having c.a. 2/3 total integrated area (b) intense sharp band at g = 5.15 integrated intensity anion dependent with area 11.6 when [ = C1O4 and 13.4 when X = Cl. L2I Ammine protons often give rise to broad absorptiorr=(r28, L29) , which may be rationalized in two T¡rays:

(1) moderately slow chemical exchange of protons. The exchange rate, R, is defined as +, where 2.c is the lifetime of the proton in a given environment. rf .r is comparable wiÈh the uncertainty in the time of rneasurement At, it can be shown, via the Heissenberg uncertainty principle, that the uncertainty in the energy of the resonance is reflected in an increased tínewidth with (128) v4=T.,L-t . This chemical exchange may involve replacement of an ammine proton with one from another comprex morecule or a sorvent molecule or impurity (such as H2o).

(2) broadening due to guadrupole effects. If a pair of coupled nucrei undergo moderately rapid relaxatíons of nucrear spin state due to quadrupore effects, the multiplet of one of the nuclei may be broadened by a large amount(128). The nucleus Nl4 relaxes moderately rapidly and produces inherently broad proton resonance signals via coupled nuclei. For example, the n.m.r. spectrum of

formamide consists of 3 broad proton resonances and may be sharpened considerably by ¡14 decoupling such that the anticipated multiplet structure can be observed(130).

Thus the broad absorptions in the range 3.2-4.9 ppm (CF3COOH) and 3.9-4.6 ppm (c2D5oD) may be attributed, in part, to the -NH2 protons. The prominent sharper band. near 4.45 (cF3cooH) or 4.18 (c2D5oD) is considered consistent with the -C=N-CH2 resonance. The observed integrated areas may be rational|zed in terms of the overlapping of the resonances. 122 Ttre resonance signals observed for the methylene protons of both types =N-cH2 and -cH2-NHz are somewhat broader than in the 'free' rigand cases. This was also observed with the

spectra of the Co(en) g3* salt investigated earlier (6 (i) (c) ) and such broadening may be rationalízed. by qyadrupore'coupling effects invorving the co59 nucleus (r00? natural abundance). such coupling wourd also increase the rwidth' of the -NHz resonances (129).

(iii) Spêctra of Co (IIl) saen comptexes in DMSo-d6

In contrast to the spectra in CF3COOH and C2D5OD, the proton n.m.r. spectra of the series of complexes of formula co(saen)2X.nH2o utilizing DMSo-d6 as solvent are not anion independenÈ. Representative spectra have been reproduced in Fig. 6.1.

Spectrum (a) n = 1, X - tr, NO3, ClO4 Spectrum (b) n = L, [ = Cl. Similar when X = Br. Spectrum (c) n = 0, X = NCS, CIO4, BF4, PF6.

The positions of the resonance signals are tabulated below in Table 6.5 , with the r-rnambiguous assignments in part (a) . The positions and intensities of the other resonances, listed in part (b), ^are discussed. in the subsequent text.

The weak resonance near 3.2 ppm in the spectra of the anhydrous species may be reasonably assigned to traces of H2o in the sorventr âs the spectral grade solvent has a weak peak at $ = 3.2L ppm. Consequently the peak near 4.06 ppm must be assigned to the 4 NH2 protons plus the ¿ =N-CII2 protons which Figure 6.I

H1 n.m.r. spectra of Co(saen) 2X.nH2O

(a)

(b)

(c)

\í ro I 6 a 2 o

Chemical shift (õ) ppm. TABLE 6.5

Proton n.m.r. specÈra of Co (III) (saen) 2X.nH2O in DMSO-d6

(a) Unambiguous assignme nts for all spectra

-CFN Aromatic multiplet . -cþ-NH2 8.46!.02(2.0r.r) 6.2-7.st.-s(8.00) 2.46!.02(4.0r.I)

(b) x Resonance I 2 3

f, NOlr CIO4 4 .09!.02 ( 8.0r . 1) 3.36!.04(2.0r.1)

c1 s.4s(r.98) 4 .10 3.55 Br 4.3s(1.e) 4.08(6.1) 3.4s (2.0s)

NCS, PF6r BF4 4.06!.02(8.1r.1) 3.20!.02(0.2)

Notes:

(t) When X = C1O4, n= 1, i.e. Type C hydrate investÍgated.

(2) When X = Cl, the total area for resonances 2 and 3 - 8.03. 123 appear to exhibit raccidental coincidence' i.e. both resonances (126) occur at almost the same frequency and cannot be resolved .

It is worth noting that the position of resonance 2 in the D¡.[SO solution spectra is similar to that.of the major component of the broad absorption observed in the CF3COOH and

C2D5OD solution spectra. The absorptions due to the =N-CH2 protons in these solvents are shifted slightly downfield and the -NH2 signals are somewhat broader in comparison to the DMSO-d6 spectra. Both observations may be rationalized in terms of the hydroxylic nature of'the solvents CF3COOH and C2D5OD, with the proton deshielding resulting from hydrogen bonding and the broadening of the -NH2 signal being attributed (L28 tci proton exchange with the solvènt 'r29) .

The DMSO-d6 spectra of the monohydrated species, with [ = T, No3 and C1o4, may be interpreted in a similar manner. Resonance 2 corresponds to the -NH2 and =N-CH2 protons and resonance 3 to Èhe water molecule.

However the interpretation of the spectra for X = CI and Br is complicated somewhat by the presence of the additional band, resonance I. Clear1y, resonance 2 involves the four

=N-CH2 protons, but the integrated area (c.a. 6) suggests that only two of the -NHZ protons resonate at this energy. The broad absorption downfield, with an integrated area, c.a. 2, is consistent with the two remaining -NHZ protons. Resonance 3 is assigned to the H2O molecule. L24 From the above considerations, the spectral data for most of the complexes are consistent with the existence of the cation, Co(saen)2*, in DMSO-d6 solution. Hov,rever, the spectra obtained with X = C1 and Br suggest that in these two cases at Ieast, the situation is more complicated. Bearing in mind the strong hydrogen bond.ing interactions, observed in the solid state, it would seem to be vatid to propose that the cation and anion may remain associated in solution. Such association would involve the maintenance of hydrogen bonds between the anion and ammine hydrogens on solution in DMSO. In contrast to the solid state case, the solvent molecules would be competing with the anion for the available hydrogen bonding sites. Consequently it is not surprising that the only cases where the cation- ariion interaction appears to be Significant is with those anions known to be able to form strong hydrogen bonds, ví2. { = CI and Br. Furthermore, the magnitude of the separation between the two sets of -NHZ resonances, viz. Cl > Br, is consistent with this suggestion.

(iv) Confirmation of ion pairing in solution

A simple test to confirm the above proposition would be to investigate the effect of excess anion on the position of the -NH2 respnance. In a molecule containing exchangeable protons, the observed n.m.r. signal may be the average of the (128) signals for different environments . For a solution equilibrium such as Co(saen)2x ¡-:== Co(saen) 2+ + X-, the -NHZ protons in the ion pair and cation would be expected to have different environments. The position of the signal resulting from averaging the two signals, would be dependent on the population of each site (128) . consequently the position of l-25 the -NHz resonance should be influenced by the concentration of the anion present, and the peak should move in the direction of the resonance position for the associated species, if such an effect occurs in this case.

A saturated sol-ution of dried LiBr in DMSo-d6 was added to a solution of Co(saen)2Br.H2O, and the spectra obtained. The position of the broad -NHz resonance at 4.35 ppm moved

downfield as the Br- concentration was increased, to a maximum observed value of 5.1 ppm in the presence of a threefold molar excess of Br-. This observation strongly supports the proposed ion pair association in DMSO.

' simirar spritting of -NHz iesonances has recently been observed with salts of Co(medierr)23* in DMSo-d6. The anion dependence of the magnitude of the splitting has been rationalized in terms of hydrogen-bonded ion pair associatiorr(l'29) .

(v) Resonances attributed to tìtater

As can be seen from Table 6.S (U) and Fig. 6.1, the position and linewidth of the H2o resonance for the chloride salt is substantially different from that of the other monohydrates. ro fact the v\ varues for the H2o resonances are respectively 4 (X = Cl) and 2.5 (X = Br) times that of the resonance for the perchlorate salt. rn addition the band position is somewhat downfield, suggesting that the electron density experienced by both H2o protons is ress than that of the free water morecule in DMSO. rt would appear that the v/ater molecule may al-so be involved in the ion pair association via hydrogen bonding. 126 rn order to ascertain the validity of this suggestion, the proton n.m.r. spect.ra of Co(saen) ZC1.H2O solutions have been obtained under a variety of conditions.

(a) Retnoval or addition of H2O

A solution of Co(saen)2CL.HZO in DMSO-d6 was progress_ ively dried over molecular seives and spectra obtained at regular intervars over a period of 48 hours. The totar area between $ = 3.0 and 4.7 ppm relative to the aromatic multiplet (8.00 assumed) \^ras utilized to estimate the rerative number of H2o molecures/cation. Additional spectra of the chloride salt were obtained in the presence of a twofold and threefold molar excess of added water. For purposes of comparison, a similar study of the perchlorate salt was performed. A representative selection of relevant data have been tabulated in Tabre 6.6 and selected spectra, âs indicated, have been reproduced in Fig. 6.2

TABLE 6.6 H2O proton resonances in Co (saen) 2X spectra spectrum x ôHzo shape Relative area(r)l::"t mors Hzo (a) c1 3.55 broad 8.03 r.0 (b) c1 3.65 broad 7.6L 0.8 (c) CI 3.77 broad 7 .28 0.6 ( (d) ir =3.85 shoulder 2 ) 6.96 0.5 c1 3. 3B sharp 9.08 2.0 (e) c1 3. 36 sharp 10 .06 3.0 Clo4 3. 36 sharp 10 .12 1.0 (f) C1O4 3. 36 sharp 9.05 0.5 Notes: (t) Total area = integrated area between g = 3.0 and 4.7 for X = CI, area -. 4 =N-CE2 + , *9, + 2H2O protons for Figure 6.2 H2O proton resonances

(a,

(b)

(c)

(d)

ro 86. 2 0 Chemical shift (õ) ppm. Figiure 6.2 (cont.)

(e)

(f)

(s) r0 I 6 4 2 0

Chemical shift (ô) ppm. 127 x = CIO4, area: 4 =N-CE2 + *EZ + 2VO protons. (2) ôHZO assumed - shoulder on the broad band centred at 4 .l ppm.

(b) Deterrnination ô solvenL of 2 O in the only

Before attempt.ing to interpret the above behaviour it was considered to be essential that the spectra of H2O in DMSO-d6 be investigated over a wide range of concentrations. A series of spectra r^rere obtained by adding increasing volumes of a 3:1 molar mixture of H2O and DMSO-h6 to 1.OO .*3 of the DMSO-d6. The DMSO-h6 was used to provide a secondary reference signal (viz. ôCH3) in case the TMS reference signal became too weak due to the limited solubilitlr of T¡4S in aqueous solutions. Over the concentration range studied, this pre- caution proved unnecessary and ôCH3 was found to vary stightly from $ = 2.44 ppm for the pure solvent to 2.62 ppm for the neat 3:1 mixture.

The position of 6HZO and the approximate concentration of H2O have been tabulated in Tab1e 6.7. Spectrum (g) of Fig. 6.2 illustrates the typically sharp resonance signals observed throughout the study.

The position of the H2O resonance, ôg2O, exhibits an essentially linear dependence on the concentration of water, tHZOl, up to about 4M. The concentration of all Co(III) complexes investigated was of the order of 1-0% w/v (i.e. c.a. 0.2M). Thus the concentration of H2O in all of the spectral studies must be less than lM. Consequently 6UZO should lie between 3.2I and 3.30 ppm if the H2O molecule in sol-ution is TABLE 6.7

Water resonances in DMSO solution

Volume added ôHzo t H2ol

o cm3 3.2L 0 0.005 3.22 0.L2 0.0I0 3.23 0.24 0.015 3.24 0.35

0.050 3. 30 1.1 0.07s 3.35 L.7

0.100 3. 3B 2.4 t. 000 4. 01 I2

3 ¡ 1 mixture 4 .39 24 l-28 essentially 'free' , and for most of the solutions studied (those without added water), 6"ZO would be expected to be near 3.23 ppm.

rn all cases, ôu2o is observed downfield from this value and clearly the presence of the complex must influence the position of the H2o ,resonance. rt wourd appear reasonable to propose that this deshielding may result from a hydrogen bonding interaction between the cation, anion and water morecule in DMSO solution.

However, in the chloride salt spectra, the shift in ô"ZO is much greater than that found with other anions, and it be- cómes more pronounced when the solution is progressivery dehydrated. These observations, coupled with the broadening of the H2o resonance suggest strongly that a three-way association between the cation, chloride ion and water molecure occurs in DMSO solution.

A similar proposition appeaTs to apply for the bromide sart also, but it is unlikery that appreciable association occurs with ot.her anions.

6 .4 CARBON 13 n .m. r. SPECTRA

The Cl3 ,r.m.r. spectra of the DMSO solutions have been obtained for x = c1, No3 and c1o4. Making arrowances for the solvent resonance bands, only 9 separate ligand cr3 resonances of almost equal intensity could be observed. This strongly suggests that the carbon skeletons of the two bistridentate ligands are in equivalent environments. The chemical shift l.' P. Þp, TABTE 6.8 rt F: P, c13 chemical shift data, and assignments, for saen complexes 0Jr o' H9' oFi F1 X Solvent I 2 3 4 5 6 7 I 9 .)ol 9J @tQ No3 DMSO/IO? C6D5 '166.5 165.s L35.2 134.5 12 3.0 1r9.9 ILA.4 61.6 43.0 o tt P. (D c1 DMSo/IO% C6D6 166.I 165.5 135 .1 134. r L22.8 r19.8 114.0 61. I 39.8 oÞHlJ. rQ¡{ c1 DMSO-d6 L66.2 l-65.4 135.r r34.1 L22.7 119 .9 TL4.2 o 6I.7 39.6 rtO (QOoFl CIo4 DMSO-d6 166.s L65 .4 135. 3 134.5 I23.0 119 .8', 114.6 6L.7 42 .3 o9r cftú¿ i'P. (DÞ Assignment - based cl, c7 c2,3r415,6 cll ce,17 Cg, Ctg FJTQl.. 10 ,16 t ,L2rL3,L4r:-5 {o\o t\) on references (L29, o \ Aromatic carbons H. I-J N =\J c I C-NH2 úo 131, 132 ) c- 5(D õ fl Ê, l-l F1 (D ctP Note: Atomic labelling scheme described in Fig. 7.2 (or Fig. 1, Ref. (20)). p, prrfts. HP. pro (t (n ts.ocf (qÞÊ o(n=à rtþ\ .tr|/Jp, riJ o pr FJ 130 On the basis of this data, it would appear that the environments of the carbon atoms, in a1l complexes, are essentially the same, with the possibre exception of c-NH2. H-owever, any conclusions regard'ing the respective values of the C-NH2 chemical shifts must be considered as d-ubious because the DMSO solvent produces an intense resonance centred- at 4;-.4 ppm. Attempts to oþtain c13 spectra in other sorvent systems have proved unsuccessful due to the limited sorubility of the complexes.

6.5 CONDUCTANCE STUDÎES OF DMSO SOLUTIONS

The molar conductivity, AM, for a tO-3M solution of a r.:l electroryte in'DMSO, would be expected to rie within the range 23-42 scm2mol-r(38) . Furthermore, a prot of 11,¡4 against cL wourd be expected to closely apnroximate a straight rine over a considerable concentration range, rimited only by the solubility limit and the approach of the experímental values to that of the pure solvent(¡g). sorutions of saen comprexes in methanol exhibited such behaviour (Section 6.2(ii)).

In the previous section, the n.m.r. spectra of the Co (IrI) (saen) 2 chloride salt strongly supported an ion-pair association.in DMSO. An association of this type should be evident in a conductance study over a wide range of sorute concentrations, and wourd be refrected in significant deviations from the behaviour expected for a 1:1 electrolyte.

For this study four complex salts v¡ere investigated, vrz. Co (saen) 2CIO4.H2O, Co (saen) ZCLO+, Co (saen) 2CI.H2O and f'e(saen) 2CI.H2O. In all cases, the values of Âg at tO-3U 131 were in the range expected. for a 1:1 electrolyte, however, ât higher concentrations significant deviations $rere observed for the chloride sa1ts. The data obtained have been tabulatecl below in Tab1e 6.9 , and a plot of L¡4 against CÞz reproduceC in Fig. 6.3.

The conductivity data and graphical plot support an un- equivocal associative equilibrium for both chloride salts and essentially complete dissociation for the perchlorates.

It would further appear reasonable to propose that both of the chloride salts are less than 30% dissociated in c.a. 0.2M solution. In the case of the Co(III) salt, the interpretation suggested for the proton n.m.r. spectra is supported by the conductance data because almost 80tà of the 'salt' is involved in an ion-pair association at the concentration of the n.m.r. test solutions.

It is worth noting the importance of determining the molar conductance over a range of concentrations. A single concentration conductance measurement (at tO-3M) could reasonably be interpreted as implying the chloride salts are I: I electrolytes in DMSO solution.

6.6 U.V...VTSTBLE SPECTRA OF DMSO SOLUT'IONS OF !ÉE

FE (ITT) SAEN COMPLEXES

(i) Fe (saen) 2CL.H2O

The ion-paired association established in the preceding section for DMSO solutions of the complex Fe (saen) 2C1-.H2O, is known to occur in the solid. state. In conjunction with the TABLE 6.9 Conductivity data at 25oC for saen complexes

(saen) (a) Co (saen) 2CL.H2O (b) Fe (saen) 2C:-..H2o (c) Co(saen) 2CIo4 (d) Co 2CIo4.HZo

3.73 0. 230 4.77 0.2L8 17.0 0.203 t6 .8 0.247

6.54 0.0836 8.86 0 .07 32 24.6 0.0561 30.6 0.0168

15. 0 0.0134 16 .4 0.0156 28.0 0.019 3 33. I 0 .00208 26.4 0. 00231 27.8 0.00237 30.0 0.00665

34 .6 0.000384 34.4 0.000476 3r.2 0.00243 33.0 0.00062

Au 31.2 at 10-3¡'t Iryl 32.2 at t0-3Pt Â¡a 3L.7 at tO-3Pl Â¡a 34. 4 at 10-3M

Notes: (a) Units t\¡r1 Scm2mol-l , C mol. dm- 3 (b) Ä¡4 values at C - tO-3U obtained by extrapolation from Fig. 6.3 Figure 6.3 Plot of conductivitY data

¡â I (a) Co ( saen) cI. H2O 2 (b) Fe ( saen) c1. H2O 2 (c) Co ( saen) cIo4 2 (d) Co ( saen) clo4."2o 2

(ü

(' a

s o

E o -o (tr a

(' rô aI Io o F

|n 0 úl o lô o rô o (r| c' (\l $l ? F L32 hydrogen honded water molecule, these interactions have been shown to stabilize the low spin state for Fe (rrr) . Arthough no n.m.r. studies were possible with the paramagnetic l'e(saen)2+ cation, the conductance studies above suggest that the isostructural Fe (III) and Co (III) chloride salts have similar associative interactions. Thus it would be reasonable to expect that the involvement of the water molecule in the ion- pair interaction may also apply to the Fe (III) chloride salt in DMSO solution. Consequently it was anticipated. that evidence for the existence of the low spin Fe (fII) species may be obtained from the u.v.-visible spectra of the DMSO solutions. Indeed the concentrated DMSO solution utilized for the conductance study was observed to have a distinct blue hue, in cöntrast to the deep violet colour of methanol solutions.

The spect.ra of DMSO solutions of Fe(saen) 2CL.H2O have been obtained and were observed to be concentration dependent over the range 0.0009 to 0.213M. Representative spectra have been reproduced in Fig. 6.4, the absorbance scale being normalized as outlined in Section 6.8(vi). The spectra may be simply rationalízed in terms of two overlapping bands centred near 510 and 600 nm respectively and the concentration dependence is reflected in the variation of the ratio Esro!Eeoor âs tabulated below in Table 6.10.

TABLE 6.10 Concentration effect in the u.v.-visible spectra of Fe(saen) 2CI. Hzo in DMSO solution Concentration 0.2L3 0.0213 0.0042 0.0009 esroieooo 0.99I 1.0 8 I. 18 r.25

The data are consistent with ion-pair association, where Figure 6.4 U.v.-visible spectra of DI,ISO solutions

(¡) (r) _o o -o o Ø -o (2)

(g)

(4)

(s)

4so 500 5so óoo ó50 wavelength ( nm.)

Fe (saen) 2CL.H2O¡ (1) 0.2r3, (2) 0 .0213, ( 3) 0.0043, (4) 0 .0009 !r. Fe ( saen) , NCS ; (5) 0.161 M" 133 the associated species is characterized by the absorption at 600 nm and the cation by one at 510 nm.

However, the energy difference between the two electronic (c.a. 3000 is larger than would be antici- transitions "*-1) pated for equilibria not involving the central metal ion directly. In fact no variation in the electronic spectra of the Cr (III) and Co (III) saen complexes could be observed under simil-ar conditions to those of the Fe(III) case. The origin of the two bands must then be attributable to some 'change' involving the central metal ion, Fe (Irr) , which does not occur with either Cr (III) or Co (III) .

In contrast to these other metal complexes, Fe (saen) 2+ salts exist in two distinguishable spin states. Indeed the mulI spectrum of Fe(saen) 2CI.H2O is characterized by a band near 640 nm (Chapter 2) whereas the methanol solution spectrum has a prominent band near 5I0 nm (Chapter 3). Thus the observed results can be rationalLzed in terms of the associative equilibrium in which the associated species contains I Fe (III) in the S = z (Iow spin state) and the cation contains 5 Fe (III) in the S = z (high spin state) .

As a consequence, the average magnetic moment of Fe(III) in the DMSO solutions would be expected to exhibit concentration dependence consistent with the evidence from the conductance and spectral measurements, viz. Ueff should increase on dilution. The variation of Ì.reff with concentration has been determined utilizing both the Gouy and Evanrs n.m.r. techniques. The data obtained have been listed in Tab1e 6.11. 134 TABLE 6.11 Variation of ileff with concentration of Fe (saen) 2CI.H2O in D¡ISO

Concentration (M) 0.213 0. rt9 0.0424 Heff (4.u.¡ 2.58!.02 3.24r.05 3.8r0.1

The observed variation of Ueff with concentration appears to validate the proposed equilibrium. The values of Ueff obtained. suggest the proportion of the high spin (dissociated) species present in solution increases from c.a. t0? (0.2M) to c.a. 4OZ (O.O4M) which corlelates, to a first approximation, with the conductance data.

(ii) Other Fe (TII) saen complexes

The DMSO solution spectra of the Type B complexes Fe(saen)2X, with X = Br, I, NCS and NO3, have been obtained. These spectra also show two bands but no variation on dil-ution was could be detected. In all cases, the ratio Esr.o !e s o o observed to be L.25t0.01 and the spectra \^/ere indistinguishable from spectrum (d) of Fig. 6.4. In addition, the values of Ueff for c.a. 0.214 solutions were found to be near 3.9 8.M., suggesting the presence of both low and high spin species in

DMSO solutions of these Type B complexes also.

(iii) Discussion

From the magnetic and spectral data considered above it would appear that attributing the behaviour of the monohydrate Type A complexes to association in solution is inadequate. Vühilst it is tempting to rationalize the observations in terms of the presence or absence of association, it is clear that 135 the low spin value for the Type B complexes cannot be accounted for in this way because:-

(1) the spect.ral data showed no concentration dependence; (2) the n.m.r. spectra of the Co(fff ) analogues showed no

evidence of ion-pair association -, (3) the Type B complexes have solid state values of Ueff near 5.g2 8.M., i.e. an S = ] spin state.

Information of particular relevance to this problem has been provided by the solution behaviour of salts of the cationic +. (133) complex, Fê (rrr) (sa12trien) Trueedle and lrlilson have examined a number of salts of this cation and reported the observation of !variable temperature magnetic, electronic spectral and tt1 n.m.r. properties commensurate with a 2T (low spin);=64 (high spin) equilibrium for Fe(ïrr) in a tetragonally distorted oct.ahedral field environment'. Relevant (133) observations incruded .

(1) The equilibrium was solvent dependent and attributed to specific solvent interactions with N-H in the complex. DMSO was one of the ten solvents used and showed the strongest interaction. (2) Ion-pair association, although not reported, rmust be considered as potentially importan¡' (133). (3) A number of salts of general formula Fe (sal2trien) X.nH2O were reported. The effect of the anion and degree of hydration of the value of Ueff \^Iere noted t ê.g. when X=PF6 (n= 0), Ueff =5.81; X=CI (n:2), Ueff = L.94¡ [ = trl (n = I), Ueff = 2.33 B.M. In the case of the iodide salt, other workers obtained a value Ìleff = 1.81 when 136 (r34 n = 1.5 ) It was concluded that both 'anion and. hydration effects' are likely to be 'operative and non-mutually exclusive' (r33) influences on the spin state in such complexes .

Such solvent-cation interactions can pr.ovicle an explanation for the magnetism and electronic spectra of the Fe(saen)2+ salts in DMSO. Thus the Type B salts, which would be expected to be essentially completely ionized in solution, produce the solvated. cation, Fê(saen)2+-ol,tsO, on1y. The solvated cations then consist of an equilibrium mixture of 1ow spin and high spin isomers, estimated from the resultant magnetic moment of 3.9 B.M. to be in the ratio 60:40 respectively.

". The situation with the chloride salt is cornplicated further by the additional associative equilibrium in that the solution contains an associated Iow spin species plus the solvated cation. From the conductance data for an approximately 0.2M solution of Fe (saen) 2CL.H2O in DMSOr c.â. 742 association is indicated. Eor such a solution containing 742 ion-pairs (uef f = 1.93) and, 262 Fe (saen) 2'-otnlso (uef f = 3.9) , the resultant average value of the magnetic moment is calculated as 2.59 B.M. The agreement with the value measured for the 0.2IBM solution, viz. 2.58 B.M. (Tab1e 6.11) may merely be fortuitous, but obviousty lends weight to the above proposition.

6.7 CONCLUSTON

The spin state behaviour of salts of the ¡[(III) (saen)2 + cations in solution is clearly dependent on ion-pair association and/or cation-solvent interactions. These interactions are of particular importance when DMSO is the solvent utilized and r37 this observation could be attributed to the following:

(I) DMSO is well known as a 'strongly coordinating solvent' due primarily to its basicity with respect to hydrogen and metal ions and the ability to form hydrogen bonds with acidic hydrogen atoms. Further, i.t would appear

that DMSO is a Lewis base of comparable strength to H2O, and may in fact be stronger(135). (2) As a solvent, DMSO is particularly effe'ct.ive in its ability to solvate cations but relatively poor with respect to small anions. For example, the anion Cl-

is less solvated in DMSO than in methanol, whereas C1O4- is more solvated in DMSo(f35). This is generally attributed to the aprotic nature of DMSO and the consequent Iack of stabilizing hydrogen bonds with smaI1 anions. (3) The extent of the ion-pair association is favoured by high concentration of the salt. Consequently, the observation of ion-pairing in DMSO may simply be due to the much greater solubility of the complexes in the solvent. The solubility of Fe (saen) 2CI.H2O in DMSO is approximately 10 times that in H2O and 100 times that in methanol. (4) Attempts to obtain the chloride salts containing molecules of solvent of crystallization other than H2O were unsuccessful. This indicates that the Lewis base strength of the solvent cannot be the major contributing factor to the occurrence of the solvated cationic salts because, on this basis, DMSO solvates could be anticipated.

+ Thus the behaviour of the M(III) (saen)2 salts in DMSO may be rationalized in terms of any one or more of the above r38 observations (1) to (3). In particular, the strong hydrogen bonding interaction between the cation ammine hydrogens and the oxygen of the DMSO may be sufficient to increase the crystal fietd splitting sufficiently to increase the low spin Fe (IIf) isomer population significantly. The anion s.olvation effects and the increased solubility probably both contribute to the ion-pair association, observed.

However, the existence of the ion-pair associated low spin species in the case of the Fe(saen) 2CL.H2O requires further consideration. The n.m.r. spectra of the Co(III) analogue suggested that the H2O molecule remains associated under conditions where the DMSO solvent could reasonably be expected to- displace HZO. Thus the 1ow spin Fe (III) associated species may in fact be rmolecular aggregates' with the H2O remaining bound onto the cation. This may result from either the relat.ive size of H2O compared with DMSO or the aprotic nature of the solvent. Clearly, the role of the water molecule requires further investigation.

6.8 EXPERTMENTAL

In general, the solvents utilized were A.R. grade with

the exçeption of the following n.m.r. solvents: C2D5OD and CDC13 Merck-Uvasol (992 min) , D2O Merck-Uvasol (99.752 min) , CF3COOH Koch-Light Puriss and DIIISO-d6

C.E.A.-France (99.8% min) .

(ii ) Solution studies in solvents other than DMSO

Conductance, i. r. and u. v. -vis . spectral data \^rere 139 ohtained as described in Chapter 3 (Section 3.5).

(iii) N.M.R. Spectra

(a) Hl' spectra

The p.m.r. spectra were obtained on an Hitachi perkin Elmer Model R20B operated under the forlowing conditions: He L.4087T, v 60.0 MHz, TMS internal reference,

probe temperature 34oC, sample tube size 3mm.

The spectra reproduced in this work were recorded on blank paper and photographically reduced. In aIl cases, saturated solutions of solid ligands ancl complexes were utir- i zed.

(b) c13 sþectra

The broad-band. proton decoupleð, 22.625 ltEtz carbon-I3 n.m.r. spectra in DMSO hrere obtained on a Bruker HX-908 spectrometer, operating in Fourier transform mode, connected to a Nicolet BNC-12 computer with 8192 data table. The spectrometer vtas locked to deuterium and a reference signal obtained from c.a. I3 CeDe (shift of central peak L28.5 ppm from TMS) introduced into tþe c.a. r:1 DMso-d6 and DMSo-h6 sorvent utilized. sorution concentrations used \^rere c.a. 10% w/v in r0 mm tubes at 28oC.

(iv) conductance mêasurernênts in DMSo

Cond,uctances were measured on a precision conductivity bridge with a cathode ray tube nu11 point indicator. The 140 instrument was built at the University of Adelaide and allowed measurement of resistances to a precision better than 0. tU.

The DMSO solvent was dried over molecular sieves and all operations performed in an atmosphere of dry .N2. The 'conductance cell and A grade volumetric glassware used for solution preparations were dried at IIOoC for at least 2 hours and cooled in a dessicator for 30 minutes prior to use.

(v) Ma tic susce ibirir of Fe( 1r) lexes in solution

(a) Gouy rnethod

The magnetic susceptibitity of each solution was determined by the Gouy method utilizirig the equipment described in Chapter 2. The sample tube was calibrated with a NiCI2 solution as described by Marr and Rock"t(37).

The solutions of the complexes were prepared as described above for the conductance study. The sample tube, and contents, was sealed with a tight-fitting plastic stopper to prevent absorption of moisture by the hygroscopic solvent.

AIt data were corrected for solvent diamagnetism and corrections ior the ligand atoms applied as previously described (Section 2.4(iii)). The error estimates in Table 6.II were obtained by summing all uncertainties in the measured weights.

(b) Evahs n.m.r. technique (1re) The technique, as originally described by Evans involved measurements in aqueous solution, utilizi,ng c.a. 10? 141 w/v tertiary butanol-v¡ater as solvent and reference. The difference between the respective methyl resonances may be related to the solution paramagnetism by the relation with the mass susceptibility of the solute, Xgì

3Av (do ds) xg + xo + xo m m where Âv separation between CH3 resonances (uz¡

v f requency of spectrometer ( 6 0. 0 ¡nHz)

m mass solute/cm3 solution xo= mass susceptibility of solvent do and ds are the respective densities of the solvent and solution respectively.

The test solut.ion was placed in a standard n.m.r. tube

and a capillary containing the solvent only introduced. Vühen the tube was placed in the sample spinner, the capillary assumed a position such that both the sample tube and the capillary vrere concentric. The frequency difference, Av, between the two sharp resonance lines was estimated with a Taked.a-Riden TR3824-X frequency cor:nter and accurately measured from the graphical plots. The values of Av (c.a. 20-50 Hz) were determined in this way with an accuracy of 4 significant figures

In addition, the sharp resonances observed with methanol (CHg) and chloroform were found to be suitable for such studies. The methyl proton resonance for the DMSO solutions was observed to be significantly broader than the appropriate resonance for the other solvent systems. Presumably this may be attributed to the cation-solvent interaction as previously r42 discussed. As a consequence the measured values of Av could only be considered to, êt best, 3 significant figures, although due to the higher concentration of the complexes in D¡4SO, the values of 1reff were of the same order of precision as the data obtained via the Gouy method.

(vi) U.v.-visibl-e spectra of DMSO solutions

The solutions prepared for the conductance and magnetic susceptibility studies r,irere utilized.. Spectra vrere obtained over the range 400 850 nm with a Pye-Unicam SP1700 recording spectrometer fitLed \^/ith a variable path length cell. The ce11 paÈh length was adjusted to produce an absorbance at 5I0 nm of c.a. 1.00 in each case.

As the accuracy of the actual path length was not checked, accurate estimates of molar extinction coefficients have not been presented in this study, although the general order of €sro and. e.oo appears to be I-2 x 103. rt was considered preferable to ind.icate the extent of the equilibrium via the ratio ÊrrotE.oo as this quantity is not affected by uncertainty in the ceI1 path lenqth. 143

CTIAPTER 7

The crystal structure of Fe (III) (saen) 2CI.H2O

7.L TNTRODUCTTON

The unusual features of the physical and chemical behaviour of the Type A monohydrates of general formula M(rrr) (saen) 2x.H2o (M = Fê, cr and. co) have been documented in the preceding chapters. The chroride sarts have been shown to be isost.ructural, and as such the structure determination of any of these complexes should provide the required information. The Fe (III) complex has been selected because of the additional interest in the s = b spin state. Thus the principle ai_ms of this structure determination are threefold, being: (1) to verify the suspected bistridentate configuration; (21 to obtain structural parameters for the low spin state; and (3) to ascertain the mode of bonding of the water molecule.

The structure of the Fe (ffÏ) salt was initially presented at the COMO 7 conference (Melbourne L977). Subsequently, (108) Benson et ar published the structure of the Type A comprex Co(saen) 21.H2O, which also showed the meridional bístridentate configuratiop of essentially pranar ligands and the presence of the waÈer molecule as a simple hydrate. No attempt was made to investigate the role of the water in this structure.

A report on the structural determination of Fe (IlI) (saen) Z- Cl.H2O has recently been published(20), and. consequently the presentation in this thesis will differ somewhat in format from that ln the publication and the preceding chapters. The 144 experimental procedure will be discussed concurrently with the results in an endeavour to explain the significance of each. The final structural data will be then considered in relation to the rationalization of the solution equilibria, magnetic and

electronic specÈra of Fe (saen) 2C]-.HZO.

7.2 PRELTMTNARY TN\IESTTGATTON

(i) Crystal selecti on and mounting

The complex, Fe(saen) 2CI.H2O, was prepared as described earlier and specimens recrystallized from I:1 ethanol- diisopropyl ether mixt.ure were examined. The crystals were reasonably large, well formed plates which could not be cleaved ea'sity - sampres tended to 'shatter' or creave micaceously. A large fragment of one plate was found, attached to a thin glass fibre and mounted on a goniometer head following the procedure outrined. by stout and. Jensen(136). carefur examination of the mounted crystar suggested that three mutually perpendicular axes could be observed. A sketch of the shape and dimensions of the fragment, with the directions of the apparentry orthogonal axes, is presented in Fig. 7.L.

(ii¡ Oscillation o hs

(a) Outline of technique

In this procedure, the crystal is mounted in the Weissen- berg camera, with the rotation axj-s along an apparent crystal axis. rf correctry atigned, when the crystal is rotated while being irradiated, the diffracted X-ray beam wilt intersect a surrounding cyrinder of film such that the resulting photograph Figure 7.I Crystal fragrment dímensions

v t0.f 5mm. fibre c rysta

s$ \ 6 145 r^rill consist of a series of 'spotst (or 'refrections') arranged (t:z¡. in parallel rows termed 'layer lines' The rotation technique suffers from the disadvantage that the complete reciprocal lattice is condensed into two dimensions and frequentry too many reflections occur(137). rn addition, exposure times required are of the order of six hours or more. The oscillation technique involves p,artial rotation over a much smalrer angre (t5o to 2Oo) and thus produces a more readily interpreted photograph with a shorter exposure time.

Rotation and oscillation photographs may be used to(138). (1) align the crystal; (2) measure the cell 'edge' along the axis of rotation;

( 3-) obtain preliminary information about the crystal symmetry.

(b) Allgnrnent

The goniometer head and crystal were mounted in the weissenberg camera and opticarry arigned such that the axis of rotation was the apparent crystal axis along the z direction, with the crystar centred in the x-ray beam. The specimen was then irradiated with unfiltered cu radiation for I hour, usingr an oscillation range of 17.50. The crystal was then rotated

I80o and írr¿diated for a further 30 minutes with the same oscillation range. On developing the film it was found that the zero level lines did not correspond. corrections for the arc settings of -0.50 and *2.80 (arcs A and B respectively) (136) were calculated utilizíng the method described by Stout and (138) Jensen . A subsequent double oscillation photograph after application of these corrections showed that the crystal was now aligned. L46 tc) Axial lensth (13e)

From a single oscillation photograph (range t7.50) with filtered Cu K6¡ radiation, the perpend.icular distances between the layer lines r^ras measured. The distance between the zero and nth layer, Çn, may be related to 6t by tÈe relation eL = + . An average value for 61 of 0.1286 reciprocal lattice units (r.I.u.) was obtained. The interplanar spaci.g, d, is equal to X/et, where L is the wavelength of the radiation (À = 1.5418 Â for Cu Ks). In this case a value of L2.O Â, was obtained for the unit celI dimension along the axis of rotation (i.e. crystal z axis). Thus the cell dimension, e, is approximately 12 . O .û, f or Fe ( III ) (saen) 2CI .H2o.

(r40) (d) Symmelry

The number of possible symmetries for the intensity- weighted reciprocal lattice is smaller than those for the crystal structure. As a result, oscillation photographs exhibit one of five possible syrunetries, vi-z. I, T, ffix, *y and mm.

In this study, the oscillation photographs were found to be of mm symmetry. Thus the reciprocal lattice must have at least one milror plane perpendicular to the axis of rotation and therefore must belong to the monoclinic class or higher(140) Thus a mirror plane must be directed along the z axis in the crystal.

(r41) (iii ) lrleissenberg photographs - unit celI parameters

The major disadvantage with rotation and oscillation l-47 photographs is that the information contained in an entire reciprocar lattice plEne is condensed j-nto a one dimensionar layer line . Ut,ili zíng the Ìrleissenberg technique, a single reciprocal lattice plane may be mapped onto an entire sheet of film, altowing simplified indexing of the.reflection"(tCZ)

From the oscillat.ion data, the values of the required parameters the screen shift, s, and the inclination angle, il, - \¡rere calculated as described by Stout and Jensen(143). The calculated settings were checked with a rot.ation photograph and corrected when necessary.

Weissenberg photographs were obtained. for the levels '1 "- 0 to 4, utilizíng Cu Kç¡ filtered radiation, an oscillation of t95o and c.a. 48 hour exposure time.

From the photographs obtainedr ân initial examination yietded the following resurts and conclusions(14r'L42) - (1) The axes appeared to be mirror lines dividing the festoons into distorted mirror images. This suggests, when considered in conjunction with the oscillation

photographs, that the crystal has mmm point group symmetry.

(2) The axes hrere found to be 90o apart. Fo1lowing on from (1) above, the crystal symmetry must be orthorhombic or higher.

The zero level Ïrleissenberg photograph may be utilized to provide measurements of the reciprocal lattice parameters att and bx, from which the unit ceII dimensions, a and b, may be 148 calculated. The reciprocal lattice parameters \^lere obtained utili zi-:ng the published chart t Pc32(L44) .

The blr axis had a spacing of 0.160 r.I.u., giving b = 9.63 Â,. The zero level spacing for the ax axis was 0.097 r.I.u. but the level 2 photograph indicated that. the true a* spacing r¡ras in fact half this varue, i.e. on the zeyo revel photograph, only alternate reflections were observed along øt. The value of a then is 31.8 Â.

Thus the crystal symmetry is orthorhombic and approximate unit ceIl dimensions are a = 3L.8, b = 9.63, c = L2.O Â. These ceII dimensions probably involve errors of the order of 2-32 due to indeterminate film shrinkage followinq development(139).

(iv) Space group

Examination of the Weissenberg photographs, âs outlined above, indicated that the crystal had mmm point group symmetry. The photographs \^rere then indexed as described by Stout and Jensen(I42), and examined for systematic absence of reflections. From these observations, suflrmarized below, the conditions reguired for reflections of general index (hkL) \^rere determined.

(1) Zero level photograph (L = 0) a* axis, alternate ref lections absent when h oð.d. There- fore for the hOO reflections, the condition required for reflection is that the value of the h index is even, i.e. h = 2n. b* axis similar Lo a* axis in that reflections absent when k odd. Thus the condition for reflection of OkO requires k = 2n. t49 (2) Level 1 photograph (L = 1) a* axis - all reflections absent. bx axis reflections with k odd absent. Condition for OkL reflection then requires k = 2n.

(3) Level 2 photograph (L = 2) a* axis reflections present at half spacing of a* axis in zero level photograph. This confirms the condition for hOO ash=2n.

General hk2 reflections for k = 2rt, reflections observed at half spacing of the corresponding festoons in the zero level photograph. Thus for hk} reflections the conditions forreflectionrequireh=2n.,'i

(4) Leve1 3 photograph (L = 3) a* axis a1l reflections absent as for level 1. This suggests that for hOL reflections, the conditions for reflection require | - 2n.

From this d.ata, the following conrditions for reflection apply: hkL no general conditions hoo l¿ - 2n oko O=2n n hoL /, - 2n OkL k = 2n hko h=2n From these üIeissenberg photographs r rro conditions may be deduced for OOL reflections as these cannot be observed because of the X-ray beam stop which excludes all reflections withbothhandk-O. 150 However, the conditions for reflection for lattice planes with indices hoL, okL and, hko indicate the presence of three (145) orthogonal glide planes as major symmetry elements . This determines the Laue group as mflrm which, considered with the above reflection conditions and mmm point gr.oup symmetry, uniquery determines rhe space group as pbea, at] ,146). [o]l "".

The next stage of the Structural determination required collection of intensity data for each reflection, and for this purpose, a Stoe-lrleissenberg automatic dif fractometer fitted with a graphite monochromator \^/as utilized.

7.3 STRUCTU.RE DETERMTNATTON

(i) Collection of intensity data

The automatic diffractometer is of the integrating Weissenberg type (i.e. two circle) which allows each reflected rspot' visible on the Vüeissenberg photographs to be examined in turn. For each layer, the inclindtion angIe, It is set and. optimized, and each reflection is sequentially examined by rotation about the axes.

These rotations and the required counter position are calculated and set by a dedicated minicomputer. Providing the unit ce11 parameters introduced into the computer programming are correct, the intensity of each reflection may then be automatically collected and recorded.

(a) Check of a1I pararneters

Ihe goniometer head and previously aligned crystal \^/ere 151 mounted in the automatic diffractometer and. centred in the X-ray beam. After confirming the alignment of the specimen, the cell parameters obtained earlier r,.rere introduced into the Stoe supplied control program. These parameters were checked utilizing ur scans of the hOO, OkO and OOZ re.flections. The unit cell data from the photographs were found to be slightly in error. The corrected data with estimated errors in parentheses (g standard deviations, i.e. 3o) as calculated from the observed variations, have been tabulated below: a. = 32.694 (22), b = IO.O83 (7), c = L2.274 (8) Â 0=ß=y=90O.

The estimated errors represent an accuracy of 0.07å which agrees well with the previously demonstrated capabilities (14 8)'. of the equipment , ví2. O. 05?

Utilizíng the above data, the unit ceII volume, U, is

4046.2 (82¡ å3. The formula weight of Fe (saen) 2CL.H2O is 435.72 and. the densityr ês measured by the zero flotation technique(149), is L.429 (r) gm.c*-3. The number of rmolecules' in the unit ce1-1-, Z, may be cal-culated as 7.99 (1), i.e. I = 8.

(b) Reflectíon data

Typically, the operating routine for an automatic diffractometer involves the following sequence of operations: (I) read. setting information; (2) move to background; (3) count background intensity, hl, for time L¡ (4) scan through reflection by rotation of a crystal o scan in equal steps such that total counting time is 2t and 152 the total count is n; (s) count background, n2, beyond the reflection for time t.

The intensity of the reflection, I, will be given by the relation

(n +n ) I=n 1 2

The Stoe control program operates in a similar manner with the additional facility of allowing a preliminary observation of the reflection peak count rate. On the basis of this prescan either an attenuator may be inserted (>4'000 counts per sec.) or the measuring time may be increased in multiples of two up to a fact,or of 8. fhis ensures that all reflection data has the same statistical significance.

The basic step counting time for each 0.0Io of the scan range is 0.5 sec., with L2.5 sec. for each background count. Reflections with rates less than 8 counts per second are automatically rejected as weak.

For upper level reflections' the scan range, Ao, is varied according to the formula: At¡=A+Bsinp/Lan(V/2) where u = equi-inclination angle and rJ.r = detector angle(I48)

The values of A and B utilized for this study were 1.0 and 0.6 respectivety. The reflection data for each of the levels | - 0 to | - 7 were collected and record,ed on paper tape. For each level¡ ân intense reflection was selected as a reference and the intensity data automatically neasured

after every 30 reflections - 153

(ii ) Treatment of data

(a) Structure factor (rso)

In order to solve the crystal structure, i.e. locate the positions of atoms in the unit ceII, the fundamenÈal information required for each set of planes with lltiller index (hkLl is the structure factor modulus , lPtlrt l. No\^r if the atomic positions are known, these structure factors may be calculated theoretic- aIIy. Thus the procedure employed. to solve the structure involves calculation of the structure factors for a series of trial structures and comparison with the values of the corresponding observed factors. The trial structures are 'improved' until a close correlation between the calculated and observed factors occurs.

The values of the structure factors may be deduced from the intensity data, utilizing the relationship

Altt<, I hkL = K'Lhkr. Ptkt ltroTl2 where K is a constant for the experiment.

+ Pfrkt, polarization equal ao I ços2O the factor, is 2 i.e. depends on the angle of diffraction only.

Lntt, the Lorentz factor, is specific for the experiment and depends on the motion of the crystal d.uring the recording.

Ant

(b) Data reduction

The paper tape output from the diffractometer consisted of diffractometer count rates for the two backgrounds and the reflection Èogether with the information on the counting times, attenuators and detector and crystal angles. This reflection d.ata v/as processed on a CDC6400 computer, utilizing the program AUPTP, to give intensities with standard deviations (o2) , corrected for Lorentz and polarization effects, normalized on the basis of the standard reflection for each leve1t. In ad.dition the values of the structure factor and standard deviation (o) for each reflection $/ere determined during this processing.

The intensity data, and structure factors, I^Iere then corrected for crystal absorption effects utilizíng the program ABSCOR. The value of the linear absorption coefficient, 0, r^ras calculated from the relation: O - gxt x Oi fi = 8.36 c*-1 where pxt density of cryst.al = L.429 gcm-3 (151) 0i ^absorption coefficient for element i fi fraction of element i in compound (from analysis).

fFootnote A programming error in relation to the first level was recently diècovered iñ that the standard reflection was incorrectly assigned. The value of 02 for the correct level one standard is approximately one half that of the reflection actually used. The data has been checked and found to have no significant effect on the magnitude of the residual factor, R, for the structural determination. 155

Finally, the data for each level were scaled together by (L52), the non-iterative reast-squares method of Rae utili zing the program AULACT. The processed data was stored on a magnetic tape file for use in the operations following.

A total of 2L70 reflections were obtained of which 15g7 had intensities greater than three standard deviations in the intensity measured, i.e. T > 3o(I). These non-zero reflections were used to solve the structure.

(iii) Determination of trial struct ,rr.(153,154)

(a) Location of Fe atom " rn order to determine the crystal structure, it is necessary to propose an initial triar structure so that the calculated structure factors may be compared with the observed factors. As the complex under investigation involves a central metal atom/ion, a large proportion of the scattering of the x-ray beam wilr be caused by the electrons of this heavy atom. (153,154) Such heavy atoms may be located from patterson maps .

The Patterson function is a Fourier series directry related to the refrection intensities and wi]l'prod.uce maxima at the ends df vectors between the atom locations, i.e. for every distinct pair of maxima in the electron density Râp, there is a maximum in the 'patterson density'. In greneral, the most prominent peaks will be those representing the vectors between the atoms with the rargest atomic numbers. The intensity of the vector peak may be related to the atomic number, z, for the atoms consideredrby finding the sum of the z2 values (lsa) for the atoms concerneu . L56 . 2 Nor^¡ for Fe-Fe , EZ = L352, Fe-Cl 965, CI-CI 578, Fe-O 740, Fe-N 725, Fe-C 712.

Thus, the most prominent peaks in the Patterson map will be those representing the Fe-Fe vectors, with. the vectors between other atomic species and Fe being substantially weaker.

The Patterson map \^ras produced utitizing the program FORDAP (Patterson option). From the computer tisting of the 100 most intense peak positions, those representing the vectors between the I symmetry related Fe atoms in the unit cell \^rere located by applying the principles outlined in Stout and Jensen(154) to the particular case of the Pbca space group. Thè coordinates of the Fe atom, i'n unit cell dimensions, were determined as */u = 0.15t Y/a = 0.035 '/" = o.ot5.

Attempts r/üere made to locate other atoms from the Patter- son map. In retrospect, such attempts were overambitious in view of the complications implied by the relative values of Fe-Cl t,z2 above. However, several of the suspected vectors for I were assigned, suggesting the coordinates for Cl \^rere near vVz ^/u = 0.22, ..- /b = 0.10, '/. = 0.49.

(b) location of CI atom

A difference,!'ourier ftâp, (I), based on the (Fobs Fcalc) method of synthedis(r55'15'6) \^ras prod.uced with the program FORDAP (Fourier option). The resulting electron density map r^ras expected to include aII significant regions of electron density in the unit cell region selected, other than that 157 associat.ed with the Fe atom. One of the most intense peaks */u in the listing (=geÂ-3) had coordinates = 0.22I, ,/o = 0.090 '/^,c = 0.500 and was assigned to t,he Cl aÈom/ion.

(157,158) (c) Refinement of atomic sitions for Fe d cl

The iterative least-squares method of refinement is based on the prediction, from error theory, that if errors in the measured values of F¡7rL are random and follow the Gaussian Iaw, the best atomic and thermal parameters will be those which result in a minimum va1ue, for the case of a linear system, in the relation x w (lrou=l - lF.rr"l)2 where w is the 'weight' for a pafticular observation, and is proportional to the reciprocal of the square of the probable error for that observation.

This relation can be expanded to cover the 'real' or non- linear situation, providing a reasonable set of trial parameters is available, whereupon Èhe function minimized becornes r w (lrou"l - nllr""r"l)2

The parameter å t= the scale factor required to ensure the values of F""1. and Fs5s are on the same relative scale.

The fulI-matrix, least squares program, FUORFLS, requires additional information regarding atomic scattering factors, fs, and estimates of the atomic therma'I parameters. Scattering factor tables were obtained from previously published data(14B' l5r'159'160). The atomic positions determined above for Fe and CI were utilized. The eight scale factors required were ls8 estimated as 0.L32. lrtre isotropic temperature factors were assigned varues of 2.0 and 3.0 for Fe and cI respectively and w was taken as unity.

The probabirity of the moder selected being correct or reasonable may be assessed from changes in the value of the residual index, R, during the refinement. (lrepsl R= r - lFcarcl) x AF Fo

The varue of R shourd decrease, idearly to zero, during the refinement, however, progress of the refinement may be estimated qualitatively , for a centrosymmetric structure, from the foltowing(161) . (lj Q = 0.83 for a random arrangement of atoms (2') R " 0.45, moder not hoperess but substantiar changes required (3) R - 0.35, model may be reasonable and wil_l refine (4) R < 0.25, mod.el probably correct

A 'well behaved structure' shourd refine to R < 0.2 and_ in most cases to R < 0.r(16T).

The initiar varue of R for this study, based on the Fe and cl parameters, was 0.592. After two cycres of refinement of the scale factors and atomic positions, the R varue was 0.423. The atomic positions were altered slightry but the scale factors increased significantly to between o.2o and,0.22. Due to the reratively smooth refinement it was fert that the Fe and CI atoms \^rere placed very near to the correct positions, and the value of R may probably be attributed to the inadequate model used. ls9

(d) Location of O N and C at.oms

A further difference Fourier map, (II), was obtained at this stage over one-eighth of the unit cerr. A1l atoms except for Ee and cr were expected to be present in this electron density map. The remaining 25 non-hydrogen àto*" \^rere located from the electron density printout as follows: (1) The non-symmetry related peaks of the first 40 listed were located. (2) A three-dimensional scale 'ba1l and stick' model was constructed and the potential atomic positions examined visually. (3) obviously spurious peaks r^rere rejected, bearing in mind - the possibre molecular structures consistent with the chemical data obtained earlier. (4) Where necessary, potential 'atoms' hrere shifted into the model utilizing the symmetry transformations for the Pbca space group.

The three oxygen, four nitrogen and eleven of the carbon atoms \^rere found to correspond to the first eighteen peaks in the printout. The remaining seven carbon atoms were found to be interspersed among symmetry rerated peaks between peaks zo and 35. Two. potentíar atomic positions only were rejected as spurious, and in all cases the electron density of the peaks selected was > 3.5 Â,-3. "

(e) Refinement. of atomic pos'itions

These additional atomic positions, with arbitrary isotropic temperature factors of 2.30, were introduced into 160 further cycles of refinement. After six cycres invorving the simultaneous refinement of the scale and temperature factors with the atomic positions, arl variations in the parameters were less than the respective est.imated standard deviations, i.e. the refinement had effectivery converged to an overall value of R of 0.153.

At this stage, a further difference Fourier synthesis ilâp, (rrr), showed that there were no significant peaks in the electron density above r.1 e Å-3, thus confirming that arr of the major (non-hydrogen) atoms had been located.

(L62) (f) Anisotroþic thertnal moti on

For the refinement thus far, isotropic temperature factors only have been considered with the scattering factors for each atom being mod.ified by multiplication by the express- ion: ( e^-B sin2o) /x2 where B is the isotropic temperature factor which typically has values between 2 and 5 in organic molecuÌes(162). These values correrate with those obtained in this study. For this type of correction to fo, it, has been assumed that al-r atomic vibrations a.re spherically symmetric, which, atthough it facilitates the speed of the computations, is obviously an inadequate approximation to the 'real' thermar motion of the atoms in molecules. The more realistic anisotropic model abandons the assumption of spherical symmetry and the single thermal parameter, B, is replaced by six parameters which describe the size and orientation of the vibration eltipsoid.. 161 The control data for the FUORFLS program was modifíed to allow anisotropic refinement for all atoms. The processing time requirements necessitated that the structural refinement proceeded with arternate cycles of scale factor - atomic position and thermal parameter optimization. Duqing these operations, difficutty was experienced with the anisotropic refinement in that three of the caçbon atoms could not be refined. The factors for C10, C15 and C17 continually refined to negative values. Consequently the refinement was modified to altow isotropic refinement of the temperature factors for these atoms, whereupon the refinement proceeded smoothly.

In addition, correction terms, and Af"rwere included ^f' to'a1low for the effects of anomalous dispersion in the case of the Fe and Cl atoms. The data were obtained from reference (l_60).

Convergence to an R value of 0.125 was achieved with four cycles of refinement.

(f ) Illldrogen atotn positions

The positions of the twenty-two ligand hydrogen atoms \^rere calculated wi^th the program PLANEH. These calculated positions Ì¡/ere inc}uded in the FUORFLS control data, together with arbitrary isotropic temperature factorsrB, selected such that B¡¡ = 1 t BN or C. Refinement of the O, N and C atom parameters on1y, þ¡as continued and convergence to ft = 0.114 occurred rapidly (Z cycles). Slight movement of the ligand N and C atoms was observed, which was consistent with contraction of the ligand bonds, due to the inclusion of the hydrogen atoms. L62 The positions of the ligand hydrogens were then recalcu- lated on the basis of these final atomic positions utilizing the following bond-length criteria: (1) aliphatic C-H bond = 1.02 å'. (21 N-H bond = 0.91 å.. (3) aromatic C-H = 0.92 Å.

A final difference Four'ier map (IV) showed no peaks with electron density > 0.83 e Å-3.

The Fourier maps (III) and (fV¡ were then examined for signifícant peaks in the electron density in the region of the waÈer oxygen atom 03, i.e. within 1.5 Å. In map III three prbminent peaks Ì^Iith d,ensity > 0.65 t"t. found to be ".å,-3 near atom 03. T"v¡o of the intense peaks (density > 0.65 e Å-31 in map IV corresponded to those in map III. These two peaks have been assigned to the hydrogen atoms of the water molecu1e.

The final positional and thermal parameters for the major atoms have been tabulated in Table 7.L, with those for the hydrogen atoms in Table 7.2.

The final observed and calculated structure factor tables have been included4 in the appendices.

(g) Interat,ornic distances and angles

,; The interatomic distance, AB, between two atoms A and B' with orthogonal coordinates (x1, Yl, zL) and (x2, Y2, zZ) respectively may be carcurated from the formura(163) ' AB = t(x, *")'+ (y, 'Yr)" + (zt - ,r)')\ TABIJ I. Positional antl Thcrmal Parameters (x f 04). The form of the anisotropic thermal etlipsoid is Exp [-(p11h2 + poû + I's,12 + 2pphk + 2pphl + 2&zskJ)l

Atom xlz vlb zlc 9t Pz 9n 9tt 9ø

Fe ts22 (r) 3s7 (3) 2s 1 (3) 4.6 (0.3) ss.9 (2.7) 27.9 (3.9) -0.6 (0.8) 0.s (0.8) 3.4 (2.6) cl 2204 (2) 871 (6) 4890 (7) 7.7 (0.6) 865 (6.1) 80.6 (9.8) 3.8 (1.s) -6.2 (1.7) -9.6 (6.1) (4) (14) J o(1) 1026 (4) -428 (14) -r73 (r3) 6 (l) 90 (16) 26 (19) 2 (4) -2 -11 g, o(2) 1264 (4) r72r (L3) 10s6 (r4) 5 (1) 86 (r7) 29 (19) 7 (4) 2 (3' -11 (13) cr o(3) 1473 (6) 1328 (16) t62 (r9) 15 Q' 78 (18) 137 (28) 10 (6) 1 (7) 7 (r8) o N(1) lss3 (s) -636 (16) 1s92 (16) 4 (2) 68 (19) 38 (24) 2 (s) 0 (s) 0 (1s) N(2) ls18 (s) r34',t (r4) -1119 (16) 7 (2) 4s (17) 29 (23) -10 (s) s (s) -30 (14) N(3) 20',t2 (s) 1064 (18) 7r3 (2O) 5 (2) 67 (19) ss (26) _l (s) _4 (5) ls (16) : N(4) 1796(s) -10s8 (17) -681 (20) 7 (2) 3s (16) 68 (2',t) -1 (s) -4 (s) 16 (16) (6) (6) (2r) c(1) 757 (6) -972 (20) srB (22) 7 (3) 67 (24) 36 (3s) l1 6 16 (27) (37) (6) (7) (24) c(2) 367 (7\ -1223 (23) r00 (26) 7 (2) 98 s7 1 4 -4 c(3) 87 (7) -18s 2 (30) 822 (30) 7 (3) 167 (40) s 1 (43) 4 (8) s (8) -2t (30) (29) c(4) 17e (8) -226s (28) 1872 (32) 7 (3) t42 (37) 80 (47) -12 (8) 13 (8) -t2 (42) (7) (e) (26) c(s) ss9 (9) -2044 (26) 2266 (29) t3 (3) 80 (26) 92 -6 2 -1 (18) c(6) 866 (6) -1402 (r7) Ls82 (22\ 8 (2) 29 (19) 43 (33) 4 (s) I (6) -7 (7) (20) c(7) t26s (7) -1220 (2t) 2149 (2s) 9 (2) 63 (2r) 49 (3S¡ e (6) -3 13 c(8) 19s9 (8) -s68 (23) 2163 (31\ 9 (2) 67 (2s) 77 (38) 4 (6) -1 (7) 2 (23) c(e) 2134 (7) 793 (2s) 1923 (26) 8 (2) 9s (26) 43 (3s) -8 (7) -e (7) -2 (22) c(10) 1044 (6) 2690 (19) 633 (23) a 2.3s (0.43) c(l1) 768 (8) 343s (26) t3t3 (24) l1 (3) 99 (26) 24 (33) I (7) 3 t7) 3 (23) c(r2) s48 (7) 44s9 (2r) 9rr (26) 11 (3) s7 (23) 40 (34) L3 (7) e (7) -20 (2ð) (28) (37) (7) (7) (2s) c(l3) s80 (?) 4827 (24) -202 (29) 7 (2) 92 7L e -13 -19 c(14) 816 (7) 4L03 (21) -888 (26) 9 (2) 37 (2O) 88 (37) 2 (6',) | (7) 9 (20) a c(1s) 1060 (6) 30s6 (20) -484 (23) 2.40 (0.42) c(16) 1316 (6) 2446 (23) (26) 7 (2) 72 (22) ss (36) (6) _1 (7) 5 (20) -1332 z -3 c(17) 1 768 (8) 7s7 (24) -r988 (26) 3.s2 (0.sr) (7) (23) c(18) t7r7 (6) -734 (23) -1882 (27) 4 (2\ 88 (26) l0l (38) 4 (6) 2 28 ¿Isotropic thermal parameter refinement only for C(I0), C(15), and C(l7). TABLE III" Interatomic Di¡tances a¡d Angles with Thri¡ Staridatd Deviations.

Distances Fe{l 4.37 (.01) Â ,c(3){(4) 1.39 (0.4) Ä c(l1){(12) 1.3s (.04) Â TABLE II. Hydrogen Atoms. Po¡itb¡¡l (X 104) a¡d lsot¡opic Fe-O (3) 4.06 (.02) c(4){(s) 1.35 c(l2rc(13) 1.42 Thermal Pa¡ameters.s Fe-O (1) 1.88 c(sx(6) 1.46 c(l3x(14) 1.36 Fe-o (2) 1.89 c(6x(1) 1.42 c(14){(1s) L.4L Fe-N(1) 1.93 c(6rc(7) r.49 c(1s){(10) 1.42 xlz vlb zlc B (Â2) q) Fe-N(2) 1.96 c(7)-N(1) 1.31(.03) c(1src(ú) t.47 g Fe-N(3) 2.02 N(1X(8) 1.5 0 c(16)-N(2) 1.32 (.03) c(2) H 29s -994 -600 3.s0 o Fe-N(4) 2.04 c(8H(e) l.s2 (.04) N(2){(1?) r.47 c(3) H t7s -r997 574 3.50 to o(1){(1) 1.34 (.03) c(eF¡(3) l.s2 (.03) c(17){(18) l.s2 (.04) c(4) H 15 -2683 2293 3.s0 c(1){(2) 1.40 (.04) o(2rc(10) t.32 c(l8FN(3) 1.s3 (.03) c(s) H 624 -229',t 2966 3.50 c(2){(3) t.42 c(I0){(11) r.44 (.04\ c(7) H I 306 -1504 2853 3.s0 c(8) H(1) 2194 1876 4.s0 -1288 N Angfes c(8) H(2) t920 -688 2982 4.50 c(9)H(1) 2438 810 2to6 4.s0 9o o(1)-Fe-o(2) 93.7 (2.0)" c(s){(6){(l) 119.5 (2.5)" c(e) H(2) 1986 L493 23',t3 4.s0 O(1)-Fe-N(l) 93.6 c(1){(6){(7) 127.6 c(l1) H 741 3208 2036 3.50 : O(l)-Fe-N(2) 88.3 c(6){(7)-N(1) 116.3 (2.0) c(l2) H 375 4923 r364 3.s0 (¡) o(1)-Fe-N(3) 175 .8 c(7FN(1){(8) 114.s c(l3) H 441 5556 -457 3.50 o(1)-Fe-N(4) 86.0 N(1){(8){(e) 106.5 c(I4) H 817 4295 -1621 3.s0 O(2)-Fe-N(1) 87.4 c(8){(e)-N(3) 107.6 c(16) H 13 3s 2832 -2009 3.s0 o(2)-Fe-N(2) 94.3 C(9)-N(3)-Fe t09.2 c(17) H(1) t669 1070 -2732 4.50 o(2)-Fe-N(3) 89.7 Fe-O(2rc(10) t25.2 c(I7) H(2) 2068 1013 -1891 4.s0 o(2)-Fe-N(4) t77.2 o(2){(10){(ls) t23.s c(l8) H(1) t922 -t215 -2366 4.50 N(lFFe-N(2) t77.3 c(lsrc(10){(11) rl6.s (2.s) c(I8) H(2) t427 -1006 -2095 450 N(1)-Fe-N(3) 84.1 c(l0){(1lX(12) r21.3 N(3) H(1) 2273 654 324 3.50 N(1)-Fe-N(4) 9s.3 c(11){(r2rc(13) 120.7 N(3) H(2) 2083 1953 s90 3.50 N(2)-Fe-N(3) 93.9 c(12){(13}{(14) 120.0 N(4) H(l) 2070 -L062 -550 3.s0 3.s0 N(2)-Fe-N(4) 8 3.0 c(13){(r4){(1s) r20.3 N(4) H(2) 1690 -1868 -5 l8 N(3)-Fe-N(4) 90.8 c(l4){(ls){(10) 120.9 o(3) H(1) 3468 145 216 3.s0 Fe-O(l){(1) r24.3 c(10){(ls){(16) 126.5 o(3) H(2) 33 18 332 ?8 3.s0 o(l){(l){(6) 123.0 c(1src(16)-N(2) l r9.8 (2.0) tAil c(6){(r){(2) r20.? (2.5) c(16)-N(2){(17) 118.2 ,- parameters calculated except the positional parameters c(1){(2){(3) 1 16.0 N(2){(17){(18) 106.0 fo¡ O(3) H(1) and O(3) H(2). c(2){(3){(4) t24.9 c(17){(18FN(4) 106.0 c(3){(4){(s) I 18.9 C(18)-N(4)-Fe 108.4 c(4){(s){(6) 119.9 N3{l-Na 52.0 163

The bond angle, O, between three atoms A, B and C may be calculated. from the interatomic distances AB, BC and AC according to the relation(163).

-1 (as 2+ 2 BC 2 0=cos AB AC The errors invotrved in these calculations may be estimated in terms of Èhe standard deviations of the atomic positions if these are known

In Table 7.3 the interatomic distances, angles and estimated standard deviations for the non-hydrogen atoms are tabulated. These values were calculated with the computer program BLANDA.

Table 7.4 contains the interatomic distances and angles for the potential hydrogen bonding interactions. These values hrere calculated from the above equations. No attempt has been made to estimate errors in this case due to the way in whi-ch these atomic positions have been determined.

Finally, the Figures 7.2 and 7.3 present a perspective víew of the complex showing the atom labelIing scheme and a stereoscopic view of the molecular structure. Both figures were obtained from CALCOMP plots, utilizing the thermal ellipsoid control program ORTEP, followed. by photographic reduction.

(h) tude of residual- index R

The final value of R obtained on oonvergence of the refinement, ví2. 0.114, is substantially higher than expected in view of previous structural determinations in the same Figure 7.2

Perspective view showing the atom labelling scheme

,,.C(4) Cttl (ó) (7')

(2) Ctsl c (2,

( Ott 4) (e) t (rs) t Httl Q tzl (r 7) (3)

H tzl Qtrol Ntzl \l Qtttl ìl (rc) ìì lì ll (r¡) Q(tz) Qlrc) 7 (l ) 1 Qtrsl ( 2 )

Otsl Figure 7.3 Stereoscopic view of the molecular structure with thermal ellipsoidE of non-hydrogen atoms drawn at 50* probabitity

ffi4

¡l

ry L64 laboratory. For example, the data for the complex tris(biquani- dine) cobalt (III) trichloride monohydraLe, which has a similar orthorhombic space group, was obtained on the same equipment following a similar procedure. The structure was refined utilizing the same computer programs to a final R value of o.o2L(148) .

It is believed that the comparatively hiqh value of R for this study may be att.ributed to a number of cumulative sources of error

(a) The morphology of the crystal fragment examined must be considered unfavourable in terms of size and shape. A major reþuiremenÈ, for satisfactory collection of data, is that the crystal be of approximatêly uniform dimensions and of such a shape that the whole crystal may be uniformly bathed in radiation of uniform intensity. An ideal crystal would have a spherical shape c.a. 0.3 mm in diameter(164). The crystals utilized in this study could not be shaped readily and tended to cleave micaceously. The fragment selected was felt to represent an optimum compromise realising that considerable errors might be anticipated in the observed reflection intensities.

(b) Because of the f1at, plate-Iike morphology of the fragment, it was mounted on a relatively thick, c.a. 0.5 Ítm diameter, glass fibre which would be expected to give rise to high background scatter a further source of error.

Fe (saen) 2C1.H2O was 2.IL B.M., which suggested slight contam- 165

ination with the high spin anhydrous species. The homogeneous appearance of crystal fragments under the microscope indicated that the high spin form of the complex was rel-ativery evenly distributed throughout Èhe crystarline mass. As some of the atomic parameters for the high spin moiety would be expected to differ from those of the low spin hydrate (Section 7.4), further errors in the reflection data may result.

An R value of 0.114 then, may be considered to be indicative of a structural d.etermination in which the atomic parameters are essentially correct and represent a very close approximation to the 'truer molecurar structure. rt is worth noting that the ligand. bond rengths rie within one standard deviation of the corresponding bonds in the cr(rrr) and co(rrr)

complexes of the M(III) (saen) 2+ cation. In addition the R value compares favourably with those obtained for these similar structures viz. Q = 0.114 (14 = Cr) and R = 0.098 (105'1oB). (M = co)

7.4 DTSCUSSION

(i ) Conf i ration of the cation

Figure 7.2 clearly shows that the iron atom, in the cation Fe (saen) 2*, is involved in octahedral coordination with the two tridentate saen ligands in the meridional configuratíon. In view of the fact that this configuration has also been estab- tished for cr(saen)21(Ios) and Co(saen) 2r.H2o(IoB), it would appear reasonable to propose that all of the salts of general formula M(III) (saen) 2X.nHlO involved in this study, contain the cation mer-M(III) (saen) 2+. 166

(ii¡ s in state of Fe(TII)

The low spin state for Fe (f f r¡ in Fe (saen) 2CL.H2O, which was initially proposed on the basis of the room temperature magnetic suscepÈibility and Moessbauer data, is further supported by this structural determination.

In a recent review article, the published structures of complexes with the related. Iigand salen have been discussed and compared(95). of particular interest to this study is the apparent influence of the nature of the metal on the metal-donor atom distances. When complexes of similar structure are com- (95) pared, the following observations may be made '

(a) The metal-phenolic oxygen distances appear to be relatively insensitive to the nature of the metal. The variation observed appears to be attributable to the type of structure in the complex, âs would be expected, because bonding in the tdimers' occurs via the phenolic oxygen. Thus no useful generalization may be deduced for these interatomic distances.

(b) on the other hand, the metal-imine nitrogen distances do show a clear trend. A clear correlation between these interatomic -distances and the number of unpaired electrons on the central metal ion, as tabulated below, would seem to appIy.

TABLE 7 .5 Metal-imine nitrogen bond lengths

Met.al Co (III¡ çq(II) Cr (III) re (rrr)

Range of bond lengths tÂl t.88-t.91 1.e3-r.9s 2 .00-2 .01 2 .06-2.rO

No. unpaired elecÈrons 0 I 3 5 ]-67 An alternative correlation may be observed with the effective ionic radii of the metal ions. Recently published data (e.g. Tab1e 3.6 in Ref. 60 (1978), Second Edition) suggest that the order of the radii is Co(Itt) = Fe(III) low spin < Cu(II) < Cr(III) < Fe(III) high spin. Either, or indeed both, ef these correlations with the M-N(imine) distances may be applied in the following consideration.

The M-N(imine) interatomic distances obtained in this study, I.93 and 1.96 Å, are substantiatly less than the range for Fe(III) high spin, ví2. 2.06-2.I0 Å. This apparent contraction cannot be simply attribuLed to the difference in the ligand because the M-N(imine) distances for the Cr(III) and Co('IfI) salts correlate with those in the corresponding salen comprexes(105'r08). clearly then, the above evidence must be considered as confirming the low spin state for Fe (fff¡ in the salt Fe (saen) 2CJ-.H2O.

Similar Fe-N(imine) interatomic distances have been observed for the low spin (Ueff = 2.00 B.M.) sulphur containing analogue, bis (2-aminoethyl thiosalicylideneiminato) iron(III) chloride, Fê(mben) zcL(165'166) . Presumably the low spin state of Fe (mben) 2CL, which appears to resemble the complexes desig- nated as Type-B in this study, may be rationalized in terms of the presence of the sulphur atom in the tridentate ligand.

(iii) t^later mol-ecuÌe - mod.e of bonding

The water is present in the crystal as a lattice water molecule which lies in a well d-efined positionr âs indicated by the relatively small thermal ellipsoid of 03 in Figs. 7.2 168 and 7.3. The thermal parameters for 03 appear to be approximately half those of a structure known to be a disordered (148). hydrate Further, the water molecule appears to be bound into the lattice in a somewhat unusual manner in that one hydrogen atom is apparently not involved in hydrogen bonding. The proposed hydrogen bonding interactions have been illustrated in Fig. 7.4, with thq rhydrogen bonds' represented as dotted lines.

The reasoning behind this proposal has been based on criteria established, through wide experience, as indicative of hydrogen bondirg(L67'168). Alr of the interatomic distances, (A-B), tabulated in Table 7.4 are within the expected range fõr suóh interactions but the 'potential' bond between O¡Ht and the chloride anion does not meet the additional criterion that (l0z¡. the A-H-B angle be 'within about 30o of a straight angle'

In addition, the simple rationalization of the i.r. spectra of the Type A monohydrates discussed in Chapter 5, Iends weight to the above proposal regarding the mode of bonding of the water molecule iri such complexes.

Furthermore, the specific nature of these interactions involving the water molecule, clarify the observations discussed in Chapter 6. Clearly the aprotic nature of the solvent DMSO precludes the involvement of this molecule in a similar interaction.

1iv) RoIe of the water molecule

The crystal structure does not inmediately clarify the Hvdroqen bondincl Figure 7.4, Table 7.4 Interactions illustrated by dotted lines. )

// // //

ll tl lr \\ lt ,// lr \\\ lt II \ lr ¡l t I rl rl rl tl r\ \\ ¡\ \\ ù

A H B A-B A-H H..B Angle

o (3) H (1) cl 3.1s Å 1.1e Å 2.4s Å 113'

o (3) H (2) cL 3.ls L.T2 2.L0 153

N (3) H (1) c1 3.23 0.91 2 .36 160

N(3) H(2) c1 3.28 0.9r 2.39 166

N (4) H (1) cl 3. 35 0.91 2 .44 175

N(4) H(2) o (3) 2.97 0.91 2.07 L67 169 role of the water molecule in the stabilization of the low spin state for Fe (fff) , however it is known that relative minor changes in the coordination sphere may result in spin (1681 crossover'--"'. Consequently, it is proposed that variations in the environment of the ammine hydrogen atoms may be responsible for the necessary modification to the crystal field which leads to stabilization of the low spin state. The ammine protons are believed to be of significance because of the anion dependent hydrogen bonding effects evident from the proton n.m. r. spectra (Chapter 6 ) .

The ideal situation for the following discussion would involve a comparison of the structural data for the hydrated and anhydrous chloride salts of the cation fe(saen)2+. As data for the anhydrous species have not been, and probably cannot be, obtained, it is necessary to consider the data available for complexes of Types A and B. Thus the iodide salts of the

Cr (III) and Co (III) cations have been selected as a basis for comparison.

In both cases, the anion is involved in four close con- tact interactions which may be considered as hydrogen bondst, thus for the Type B complex (Cr(III) ) al1 four are N-H....I interactions, whereas for the Type A (Co(III)) complex only

tFootnote In reference (20), the statement that the rstructure of the nonhydrated species Cr (saen) 2T shows that only two of the ammine hydrogens on the cation are able to form hydrogen bonds' is incorrect.. A cursory glance at the ORTEP diagrams for this Structure (10S) certainly èuggested that this \^Iasathe case. Howeverr ârI atom search, based on the published atomic positions, revealed that symmetry transformed iodide ions have close con- tacts with the ammine nitrogen atoms indicative of aI1 four ammine hydrogen aÈoms being ínvolved in hydrogen bonds. 170 three are N-H....I, with the additional interactions O-H... -I and N-H....O (water) . The relevant interatomic distances have been tabulated below in Table 7.6. In addition, the corresponding interatomic distances for Fe (saen) 2CL.H2O and- Fe (mben) 2CL (166) have been included, together with the Co(III) and Cr(III) ammine halides as lis.ted below.

(170) (a) (t) Co (III) (en):CIg.2'8H2o and (t) Cr (III) (en) ¡CI¡.3H2o racemates with significant N-H....cl interactions but no significant N-H. . . .O or O-H. . . .CI interactions. (r71) (b) (+) o - Co (III) en3C13 .H2o significant N-H. ...CI, N-H... .O, O-H- - - -Ct interactions.

TABLE 7.6 Hydrogen bonding interactions for metal-ammine halide salts

N-H....X o-H....x N-H....o N-X distances O-X distances N-O distances o o o OO range A mean A range A ÍEan A me¿tn A

Cr (saen) 2I 3.68 - 3.89 3.74 3.54 2.93 Co(saen) 2f .H2o 3 .61 - 3.'74 3.65 >4 2.98 tCo (en) 2AP. I .H2O 3.62 - 3.66 3.64 lCo (en) ¡Cl¡. 2'tlu2o 3.26 - 3.46 3. 35 >4 >4 3.25 2.98 tCo (en) 3C13.H2O 3.r2 - 3.43 3. 3t 3.21-3.30 >4 tCr (en) 3CI3. 3H2O 3.21 - 3-43 3. 35 >4 3. 15 2.97 Fe (saen) 2C1--H2O 3.23 3. 35 3.27 Fe (mben) 2Cl 3.29 3.62 3. 38

From the data in Table 7 -6'. (I) For the hydrated ammine salts, the N-H....o (water) hydrogen bonds, âs estimated from N-o interatomic I7I distances, appear to be of similar strength. (2) The O-H....CI bond in Fe(saen) 2CI.H2O does not appear to be unusually strong. (3) For the Co(III) and Cr(III) tris-en salts, â11 N-H....CI

bonds are of similar strength to those in Fe(saen) 2CL.H2O. (4) The N-H....I interactions for the Cr(saen)2+ and Co(saen)2+ iodide salts do appear to be significantly different. This is also the case wittr the N-H....Cl interactions

for the chloride salts of Fe (saen) 2+ and Fe lmben) 2+.

In each case, the values for the N-X interatomic distances have been calculated from the published dtomic positions to complete and to check the published. data. The iodide salts .rå considered first. Eor Cr(saen)2I, the N-I d.istances are 3.68, 3.77, 3.80 and 3.91 Å whereas the three distances in

Co (saen) 2T.H2O are 3.61, 3.6 2 and. 3.7 4 Å. According to the criteria relating to the rstrengthr of hydrogen bondirg, N-I distances between 3.6 and 3.8 å, may be considered as indicating a strong hydrogen bond(168). Thus in cr(saen)2r only three of the four N-H....I interactions are strong hydrogen bonds while one is substantially weaker. In Co(saen) 2I.H2O aL1- four of the ammine hydrogen atoms are involved in strong hydrogen bonds with three involved in N-H....I interactions and the other strongly bound to the r¡rater molecule via the N-H....O interaction.

In the case of Fe(saen)2CL.H2O, applying the criterion for a strong bond that N-cI distances be less than 3.4 Å(168), it must be concluded that aII four ammine hydrogens are involved in strong interactions, three N-H....CI (3.23, 3.28 and r72 3.35 Ål and one N-H....O (Z.gl i) . V.¡ith the anhydrous complex Fe (mben)zc]-t three of the N-H....cl ínteractions are strong (166 (3. 30, 3. 30 and 3.2g Å) , and one (3.62 Ål weak ) . on dehyd.ration of the Fe(saen) 2CL.HZO, ,a high spin species with a Type B i.r. spectrum was obtained.. It would .appear that the interactions for this species resemble those in Fe (mben) zcL.

From the above discussion it is clear that the major differences between the Type B and Type A complexes may be re- Iated to the number of ammine protons involved in strong hyd.rogen bonds, viz. 4 in Type A, 3 in Type B. Clearly the water molecule in the Type A monohydrates plays an important role in that its incorporation into Lhe molecular structure re-surts in an additional strong hydrogen bonding interaction with an ammine proton when compared with the Type B case.

During recent years there has been much interest shown in spin-crossover systems as shown by the large number of publications and review articles in the titerature for b-oth Fe(II) and Fe(IfI) complexes. In both cases the data presented suggest that the spin state is extremely sensitive to apparently minor variations in the ligand(169). The presence of an additional strong hydrogen bonding interaction would be expected to result in"a slight increase in the electron density on the ammine nitrogen,and thus enhance the crystal field experienced by the central metal ion to a slight extent.

That such a subtle interaction could 'tip the balance' in. favour of the low spin state is somewhat surprising, but clearly this is the case. 173

Thus it is concluded that the water molecule must be considered responsible for the observation of the low spin

state in Fe (III) (saen) 2CL.H2O and in addition stabilizes that spin state. To the best of the author's knoroledge, this represents the first case where not only spin:crossover but also spin state stabi,lization has been established as resulting from the incorporation of an apparently símple lattice water molecule into a molecular structure. APPENDIX I Computer programs utilized in the structural cleterminatÍon.

AUPTP - Program to read and process thê paper tape output from a Stoe automatic diffractometer. R.J. Hitl, Ad,elaide University, 1973.

AULAC - Program for interlayer scaling, sorting and editing data. M.R. Snow, Adelaide University, 1973.

ABSCOR - A crystallographic absorption correction program employing the Analytical method of H. Tompa, Acta Cryst. t9 (196s) 1014. Modification by M.R. Taylor, Flinders University, 1971.

FUORFLS - Local modification of ORFLS. Vl.R. Busing, K.O. Martin and H.A. Levy, Report ORNL-TM-35 Oak Ridge National Laboratory, (L962), Oak Ridge Tennessee.

FORDAPB Loca1 modification of FORDAP.

A. Zalkin, Lawrence Radiation Laboratory (1962) , Livermore California.

ORTEPA - Local modification of ORTEP. C.K. Johnson, Report ORNL-3794, Oak Ridge National Laboratory (1965).

PLANEH - Local modification of PLANET. J.F. Blount, University of Sydney, (1966).

BLANDA J. F. Blount, University of Sydney (1966). APPENDIX II Structure Fact,or TabIes

H K FOBS FEAL H K FoÊS FC/rt r'l FORC ËcAt_ *+L F grt{l{r# ?2 ? 356 46i I 641 frIS U 3 0 0 ?4 2 372 4 jt+ ¡0 5q8 64t 4 3óle 3538 2lt 2 ô3q óF5 t7 264 25t U 5 0 0 7-{1 ? I7t l0c l4 t5t 84 6 I 753 I Ér96 30 ? 72r) 7tn I É-ì 15? 444 U 7 0 0 2 3 4?c lFl ln ?64 ló0 I 761 738 4 3 75c Ft7 7.n 543 621 U I 100 t) E6 3 2150 71 ?? 114 26e ^O I 0 902 R50 I 3 350 300 ?4 t5l 145 UI I 0 0 IO i 692 7l+4 26 ?67 352 I ? ?.c0 ?-72 E t2 3 e0ta zo7 t 2B lq9 307 3 0 t4 3 823 75q t.f 30 q4 96 I t82 q69 I 4 2180 2?-t7 t6 3 645 674 ? l0n2 UI 5 0 0 tB 3 75q 716 LJ 4 0 102 6 2 t25 ? 0q5 20 3 dgn sz3 á 667 7l¿ I q UI 7 0 0 ??. 3 Ì81 80 1 455 ì 528 I I 7o+ 7?8 24 3 330 747 l0 5it 53ì UI I 4? 0 ?6 3 21,h ?-4\ l? 63C 713 20 777 Bt I ?t1 3 75r 76'7 il 1/+ 64 ?t4 U¿I 0 n 30 3 345 746 U lrì ti2 l7t 2? 459 553 0 4 148"r ì4lfì il lrì 0 137 u23 135 (t 7 4 t23s ì zr c U 7n ô 6n 24 4?5 45q 4 4,4:lq 442 ?.7 3Bì 35'| q U?5 0 fl 6 4 3s7 2q? 74 ftl 57q 26', 504 490 I 4 75¡ 7qf. U ?6 g8 85 u27 0 0 ì0 4 fr8q 6CR ?A jÊt 4(lo U?8 0 13? l2 4 2?? lR? 3tr 2 rìc lcc ?9 150 n t4 4 347 7?7 tì 2r I 2Bn 30 909 q42 r.l t6 4 l.tt I '15 il 7 q4 187 E 2 r 132 t2-rÉ' l8 4 '¿0t\ 7-L\ T¡ ?74 B4 4 964 q9:t 2o 4 I I t4 illr ll ¿i 0 5l 6 I 078 I 054 '¿2 4 47n 447 IJ I l3q c5 I t980 tq37 24 4 837 9¡/r l0 157 108 10 It76 It5ì U 2.6 0,, ?n IJ T2 _0 24 le 9tB q4 I pB 4 ?00 7i" ll ì. /a ll2 lz. l4 305 30rr 30 4 14ì 7q ;t É' ?06 100 U I6 67 4g 2 5 Ilól ì 111 tÊ 44q ì7Ír t8 57t s53 4 5 49ì 451 U '2n lt6 t86 313 25 tì U h 5o 144 t, ?7 n l7 20 "17? ?,? 360 ?84 ß 5 824 U '2/+ n t?ì tl 't t, 24 0 l, 10 5 I2-{ Á'ì ll ?6 0 3) ?(, 352 742 Hlz 5 t2el I 2ar rJ 28 0 3l U ?8 0 7cl l4 5 +31 53î l0 210 l3? '3 U 30 0 4a 16 5 ylt+ tÁs 7 t+f'¡ i36 0 25i4 ?a8 ¡ 1fl 5 7 4I 7C6 4 71A ¿36 z 2341 ?125 70 5 ?0Á ì 0l å t+6C 49'l 4 ftot 551 27 5 ¿5? 25q ír ttqq 1205 6 I 08e InZ9 t, 24 5 I0(r Bt 1n 4cn 4s6 E B 832 I 78? 26 5 1(: t+ ?46 l2 533 539 I q6c 't4 E l0 t5?7 153 6 2Í) 5 486 ìqÁ I2q l2 676 677 u 30 50 l6 ?70 i95 t4 7c)9 904 0 6 I t4'ì ! "06107 tB ?eo ).?q qq6 E l6 t7 t?_ 176? 2 6 105¿+ ?n 744 I l4 l8 467 48'ì 4 6 66q 7t t ?? 641 66(1 20 l0t8 Q8u+ l, 6 6n 8q ?4 454 4r51 H K FOFS FCAL h K FOF]5 FC l\1"" H K FOÉ]S FaÂt U ?6 I 0 7? ?7 363 39? l?- 33 rì 14q 0 l0 159 ??T 28 338 255 l-3 I 186 t 201 2 l0 164 6l+ U?9 0 cE t4 49 r) rilg 4 l0 664 622 30 ?- l() 301 I5 9sq qq2 6 t0 441 398 3I ?85 247 ló l9c J 1+6 I IO 307 2-65 U3? 0 4 l/ 9?7 qa7 l0 TO 45ó 464 33 598 601 1B 507 /+86 L?. l0 ??l 2o( u34 12t 215 19 84f, a?q l4 l0 ?,60 t0? 35 425 422 t-l 20 l?Ê 2g l6 t0 ?45 ?56 0 I 0q3 I 0,14 pl 3?4 lza I8 l0 143 237 I 6os 64? 7? ?41 214 ?0 l0 509 5?t 2 1035 1 034 73 I06n ì 04q ?.? l0 ?r2 I7ó 3 e76 c97 24 512 h7a U ? II 0 85 4 458 q00 t., ?-5 Il7 5fì 4 tl ?74 ?61 5 2åcq 2e96 Z6 25r ?06 6 ?85 It t8l U6 0 tl ?7 frá 1 7lq I II 230 ?3? 7 556 484 28 l8o 217 l0 lt 254 142 ¡t ?.90 77F. 79 307 ? âC' T? ll 184 l3n I r72e lTtr t.l 30 57 t6 440 j64 [, l4 ll 245 3:l I t0 3l 11 ì rìq u 16 IT 0 lìf. tl 8t.I 7 4?. il 3? n 1) l8 rl 366 4q3 l2 45? 466 t.l _t3 0 R1 0 l2 ?0? 2ôl t3 785 78 t 0 5 0,4 46n U? l2 t?T 77 l4 254 I '¿tl^ ??q 4 l2 33 t 375 t5 33ó 370"32 2 407 46(t 6 l? 403 376 lÉ, 202 140 1 l3c) l2íì UB t? 063 T7 7-61 t 69 4 3?,t+ 1Á tì l0 l2 33c 425 18 175 22t',, 5 I l2q i't rÁ ett 84o 6 *+t = 1 ++** lq +?^ ?'ze I I t628 16t2 20 23? 183 7 4j2 44q 2 I 937 q5? ?T 1t72 IO84 U I 0 4A 3 I 27Zr ?7 4tl uez 0 1Ér 9 oRa zoa t, 4 I 0 Â0 23 340 40ì. t, t0 lta Áq E I 378 343 24 157 100 tl 5:t7 616 U6 I 0fl 25 26j Zrì I l2 417 j l0 E7 I e68I ?79L ?.() L4¿ ?47 l3 5or +7n B I r00t 900 ?7 208 3t -1 l4 4lì 146 q I l07s tltT ?8 _ì6 3 ?4^ 15 526 É?j Ir) l 9?o 962 ?e 20t 2 311 l6 J3ì a'7 ¿+ Ìt I 594 620 u30 0 8q t.t t7 I3r A¡ T?. I 4ßtì 4ri3 31 3t8 3l I I f'l 2 0,i 16" I3 I 48e 559 3? t7n 4r4 lc) bq7 A71 l4 I 236 t lb 33 ?21 I c)q (l ?0 l/¡ c4 /rH l5 I 38fi 4t.l u34 f) ?l I00r) r 06 ì l6 I 958 e69 UI 0 t? ?2 It+t+ ?h l7 I l3I3 130 I ? 7q0 26Ë; t, 7i fì lìa l8 I 47 4 49e 3 ì 798 t824 l.l 74 rl ì5 le I 600 65 4 Ëìlt) 7C5 ?5 53r E/+l I -¿-''l U?0 I sB l?7 5 765 7 26 l8e 171 2t t 464 4É,9 U6 0 1j Z'l 35¿* zqq 7 IJ ?? T 58 8t 1000 103r 7A i 7q 177 ?j I I 123 I 0e3 B 332 343 ?e l')5 ?4? 24 I 335 35ö I 1¿7 3,6? U 30 tlÉ1 16t+ ?s I 7?2 70c l0 673 61'7 U 3t 4 ír qÁ 0 ?8 t, ?6 1 304 io¿ u ll i2 4 n l2q H K FOBS FC t_ H K F0¡lS F EAL H K ITOHS F(:,rl- 't rJ t 0 5l ?4 t3c t3b tJ 23 tì 064 7 ??-9 144 ?5 52t 461 I I Iô5 ìçrtì q 3 353 363 u ?,6 0 L54 2 226 5q U4 83 9O ?7 392 3q6 3 I 357 3cq 5 1?7 305 U?8 0 qb 4 q 371 ?cq q U6 0 I4 ?e 224 2e3 s 26? ?2\ 7 7 ?_n ó40 Ul 0 85 U6 I 0 t?? I Bt3 R3l Ua 0 ?? '7 I 4ll4 401 q 476 45Á U3 0 ZB u8 I 43 R t t5ß 234 4 168 T0 U9 9 t30 125 l0 q u 750 796 5 I54 6l 10 ??s 3'ì rJ t?. 3t 145 î1 144 25b L' tt a fì0 l nq q l3 I07C I 0ó8 1 425 408 UI? 0 62 l4 250 64 B lStr 55 l3 9 3oq 315 1q Ì57 88 I 553 5t+7 l¿l I 152 64 l6 253 406 tJ l0 0 78 I5 s I 8 0 ?'.1/+ t7 30e 337 tl ?-9c ?_65 ló I ?12 ?1 1q ?ql ?56 T2 ?45 790 u 17 c 0 I42 t9 24fj ?3F l3 I37 11) tB q 159 t 7R 2a ÌR8 I24 tJ l4 94 l? ('' le q j7 4 277 g ll ?l 0 I5 2t(, 2?9 UO t0 t 32 166 ?_? 2g? e5?_ l6 300 35e l IO 541 5ql LI 23 q4 84 tJ t7 0 5q IJ? IO 0 ?l ?4 I5t 204 lfì 29? 265 rJ3 l0 0 12'ì TJ 2ã 0 4(t lrì ?3? 120 l¡ l0 t6? lon U ?6 n 4l ?n 2qz 174 5 l0 48F 46(-, ?"'l 32? 26tl (J ?r 0 ó5 U6 10 0 ì 52 '210 7 ?11 t43 25 ?t 200 l0 ?tì5 23 t ?<7 6?_l 67n 2i 3f:a 2tJ I t0 193 I f.2 IJ 30 n 7- ?4 152 ?25 9 10 ?tt4 2rl ¡ 3l ?7n I94 U?5 t0r .Jz u l0 l0 ç rì4 14 53t 4q3 0 t8t 47 ?(, 165'ln lt l0 I 12e 197 U0 7? t? l0 160 I o3 U? 't,?61 6t llt3 t 107 u tl t0 tì4 lqÉl I $+L t, 3 47 112 IJ? 0 l1ì1 Z*#+rrr 4 65q 596 LJI t) aI 3 0 lt62 1l2l q 7oL 650 Ut+ 0 I n4' 4 0 1 341 l?c7 5 0 96C c21 fJ r?t l0tl 5 7q5 ttY U 6 7 5tq 536 r''t 2fc l9e 0 0 1-rtt rì 404 415 'f 23q ?.07 7 0 lTfr a0 q 64c (tlz R t90 2ì0 I 0 lì02 731 rì U 10 126 156 59e 58v I 0 ?t7 ?\7 85C 86t Io 3q7 27n 10 0 I 133 l0/+2 tt qr4 ja r.J l7 ln? t36 lt 607 U Ir 0 ?/t e tl 13 rf) r47 t85 9I t2 0 A8a ?60 t2 ßì64 L' t4 0 t2t l3 195 I',¿7 l3 0 85 z l5 73C T ?_l tJ t4 t I4 ttl g l4 0 ì017 lt44 1å ?-i4 283 l5 86t 8aY € tS 0 I 054 cgi It r7 CB TIì 1., I l'' 60 I t5 l6 0 ? 44c) ?36t+ IJ lF 67 r2'l I7 42C 4?5 U t7 0 0 I tn l9 287 )?t u l¡l 0 t'l t8 0 9t7 BlrS U 20 125 r21 lc tclÌ tl2 Iq 0 t 87 2¿',1 7t 35q 346 l"J 2n 4I É)5 20 0 ll43 itÉ'r U 22 0 6n rJ Zl 3t r?t 2r 0 7tO qt5 '¿0¡, 2a ?tt 3 u?2 0 4A ?2 0 ?q0 4ñt+ H FORS K FC r\l H K FOqS tcAt- H K FOHS Fr.Iìl fl 23 40? 4?'4 ? 2n3 r30 29 3 1/5 a tct 24 94? 940 2 66çt 634 3O 3 56q 559 1., 25 o 86 lo 7 t617 r62t "tr .l tSft I A'f ?6 897 qnQ 1l ? r¡q 6+'l 3? -t 25ó 2lo 27 ö53 r,l4 tl l2 ?CA 7,56 ll 33 3 I .16 ilI ' ¡+82 qcq 28 l3 ?. 852 855 0 + 8iì 93 lö3 u29 2(t l4 ? 1?16 1207 LJ ì 4 123 ñY 30 66¿ 630 l6 2 ¿on :19') IJ 7 4 0 9j 3I 294 777 lh (t4q 4 qct 2 | tI0'l 3 3?3 32+ 3? soB 1'l 2 3al j95 lt 4 t450 I4''lr 33 217 164 1n 2 lch ?35 5 4 45e 49t" rJ 34 sì tìt la ? 5ì2 576 t, f1 4 4?. rJ 'f1¡ì 2oc 35 84 tc2 ?.() ¿ 624 '7 4 752 7¡1 t 86n n4g 2ì ? 47q 431 n 4 42+ 453 ? Ite3 \'¿as 2? 2 ?46 339 Cì 4 3q3 j t'ì 3 ótr 64q lt ?,1 2 I na 128 l0 4 353 +nv 55 '2 U4 56 ?4 \Az rr: ¡ 1t 4 4lì A 499 5 265 77 2s ' 3zl 32tt t2 4 446 lt5 U6 0 77 7t\ 2 4nq 393 l3 4 572 '5/l 7 9¿7 c00 Ll rlQ 7_7 '?0 179 u 1,4 4 H4 I 815tì R?e ll 7q 7o 125 l5 4 t5n I t¿ tJ9 l,f 4? 4ñ pl 2o 2n l.J 16 4 0 12 l0 83r Âzq 3rt '¿ 67s 672 '767 I7 4 417 4h?_ tl r77 I n3 3t n 25a ll l8 4 0 )7 T2 jqÉ.r "lgq ?'77 32 7 44q 474 !, la 4 0 th l3 7n7 3ì ;4 t Bo 6l 2n 4 rl3 I Brt rJ Ì4 8¡i 77 tJ 3¿. q6U 7n l.4c 2t 4 4¿l 3ll(r l5 l Tfr I -ì I ln l8s ?2 4 259 -lal '2 ló z?4 1q ì 4ft3 57q 2j 4 44t 3¿r3 36¿t l7 2ql tl ì ì0 88 24 4 3r)6 7_7 V l8 32'l ?411 (ç ì I7,r6 I 79c ?\ 4 4 JS 451 u 5q q t9 2ll T 976 954 76 4 t¡12 137 ?t) 5o¡l 4c0 t, 6 tn ?Q4 tJ 27 4 0 I9 ¿l I 8') ?41 "l '1 z. aø 7_94 ?B 4 1 il7 7te ?? 321 qt tl fì 'ì ì +n 106 2q 4 ?n1 l9¿+ qs (, c) t, ?3 0 I I an t33 tJ ;to 4 0 94 24 'l t, I05 102 l0 6¡e 627 3t l+ ?qt l{ì t, 25 ô lll lì I i00 32? tJ iz 4 0 ).23 rt ?_6 q4 tl I LP ì tzcz t 787 I 5 ??c) l')l u27 Ht+ q4 q lj I 519 4ql ? 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5 Inuganlø Cldmlco Acþ,27 (1918) 123-128 123 @Ebevie¡ Sequoia S.4., Leuwrne - Printed in Switzerland

The Crystal Structure of Bis[N-(2-aminoethyl)salicylaldiminatol iron(III) Chloride Monohydrate, a Low Spin Iron(III) Complex Stabilized by Lattice Water [ 1l

A. P, SUMMERTON School of Chemícøl Technology, S, A. Institute of Technology, Levels Campus, Pooralø, South Austr¿lia, 5098 A. A. DIAMANTIS and M. R. SNOW Depaftment of Physicøl ønd Inorganic Chemistry, Universìty of Adelaide, Adelaide, South Austratia, 5001 Received luly 15, 1977

The crystal structure ønd molecular conftguration compounds can be dehydrated by heating at 100 "C of bís[N-( 2-ominoethyl)salicyløldiminatoliron(III ) in a vacuum for æveral hours. It is not possible to chbride mornhydrate, Fe(III)(Saen)2A.H2O, has dehydrate the cobalt complex. been determined by X-rry dífftaction technþues. The (3) Evidence from n.m.r. and conductance studies crystals øre orthorhombic with spoce goup Pbca and suggests the water remains bound in solution. cell dimensíons a= 32.694(22), b = 10.083(7) and (a) The iron complex is low sph (tr¡err. = 2.ll c = 12.274(8)Ã¡. The structurc was solved by the B.M.) but high spin when dehydrated and in solution heøIy atom technique and retìned by the full-møtrix úterr.) = 5.96 B.M.). Iron complexes with n = 0 are least-squares method with 1587 reflections to wt R high spin (e.g. Fe(Saen)el, ¡.¿"rr. = 6.06 B.M. [2] ). factorof0.l14 (onF). This structure determination was undertaken in The iron øtom is octahedrally coordirat'ed by the order to ascertain the mode of bonding of the water two tridentate ligands ín the meridional conftgura- molecule, which stabilizes the low spin state of iron- tion. Strong hydrogen bonding occut! between the (IID. ømmíne hydrogens, wøter molecale and chloride bn, Details of the preparation, spectra and chemical properties accounting for the stabilization of the low spin state of the series of compounds will be the subject ofa later publication. ín the hydrøted solid over the high spin stste Íound when dehydrøted. Sa len

Introduction

A series of compounds of general formula M(III)- =N (Saen)2X.nH2O has been prepared for M = Cr, Fe c Hr-H, and Co. The value of n can be I or 0 depending on the anion X. The chlorides of all three metalJ are monohydrates, and powder diffraction data indicated Saen that all three compounds are isostructural. These o- complexes, as well as other hydrates in the series, show unusual feature¡ arising from the presence of the water molecule. A brief summary follows: Hr- (l) The water molecule gives riæ to two intense bands in the O-fl stretch region of the i.r. spectrum. The fi¡st band occurs at 3620 cm-r and is extremely narrow with a peak width at half-height (v¡ ¡r') of 20 Experimental crnll. The second is slightly broader (vtn = 5b cm-t) and occurs between 3,350 and 3,450 cm-t. The posi- Crystals suitable for structure determination !,vere tion of this latter band is different for each metal and prepared by reaction of Iron(III)Salen Chloride and also varies with the nature of X. 1,2 diaminoethane in ethanol. Subsequent recrystal- (2) E:rperiments conducted on the isostructural lization from ethanol-düsopropyl ether gave deep chloride salts showed that the iron and chromium blue plates. t\) TABLE I. Positional and Thermal Paraneters (X 104). The form of the anisot¡opic thermal ellþsoid is Exp [-@¡1h2 + gptkt + p:rrlt + 2p12hk + 2pl3hl + 2Ézrkl)l ò

Atom xla vh zlc Érr 92 Fs 9tz Ptt 9ø (0.8) 3.4 (2.6) Fe ls22 (1) 3s7 (3) 2s r (3) 4.6 (0.3) ss.9 (2.7) 27.9 (3.9\ -0.6 (0.8) 0.s (1.s) (1.7' (6.1) cl 220ø. (2) 871 (6) 4890 (7) 7.7 (0.6\ E65 (6.r) 80.6 (9.8) 3.8 -6.2 -9.6 (4) (4) (14) o(l) to26 (4) 428 (r4\ -173 (13) 6 (l) 90 (16) 26 (19) 2 -2 -11 (3) (l o(2) L264 (4) r7¡1 (13) 1056 (14) 5 (1) 86 (12¡ 29 (19) 7 (4',) 2 -11 3) (l o(3) 3473 (6) 1328 (16) 162 (19) l5 Q) 7E (18) r3't (28) 10 (6) I (7) 7 8) (24) (s) (s) 0 (15) N(l) lss3 (s) -636 (16) 1592 (16) 4 (2) 68 (19) 38 2 0 (s) (s) (14) N(2) 1sl8 (s) 1347 (r4) -1119 (16) 7 (2\ 4s (17) 29 (23\ -10 s -30 (s) (16) N(3) 2072 (s) 1064 (r8) 7r3 (20) 5 (2) 67 (19) 5s (26) -1 (s) -4 l5 'l (27) (s) (s) l6 (l 6) N(4) 1796(s) -1058 (17) -681 (20) (2) 3s (16) 68 -1 4 (24) (3s) (6) (6) l6 (21) c(1) 7s7 (6) -912 (20) stg (22) 7 (3) 67 36 11 6 (37) (6) (7) (24) c(2) 367 ('t) -1223 (23\ 100 (26) 7 (2) 98 (2t¡ s7 7 4 -4 (8) (8) (30) c(3) 87 (7) -18s2 (30) 822 (30) 7 (3) 167 (40) s l (43) 4 s -21 (8) (29) c(4) 179 (8) (28) t8'12 (32) 1 (3) t42 (37) 80 (41) -r2 (8) 13 -12 -226s (e) (26) c(s) ss9 (9) (26) 2266 (29) 13 (3) 80 (26\ 92 (42) -6 (7) 2 -1 -2044 (6) (l c(6) 866 (6) (r7) rs82 (22\ 8 (2) 29 (19) 43 (33) 4 (s) I -7 8) -1402 (7) (20) c(7) t26s (7) (21) 2149 (2s\ 9 (2\ 63 (21) 49 (35) e (6) -3 l3 -1220 (7) ) (23) c(8) 19s9 (8) (23) 2163 (31) 9 (2) 67 (2s) 77 (38) 4 (6) -1 -s68 (7) _1 (22) c(e) 2134 ('r) 793 (2s) 1923 (26) -8 (2) 9s (26) 43 (3s) -8 (7) -e a c(10) 1044 (6) 2690 (19) 633 (23\ 2.3s (0.43) (7) (7) 3 (23) c(I1) 768 (8) 343s (26) I 31 3 (24) l1 (3) 99 (26) 24 (33) I 3 (7) (7) (2Ò) c(12) s48 (7) 44s9 (2r) 9tr (26) 1l (3) s7 (23\ 40 (3+¡ 13 9 -20 (7) (2s) c(13) 580 (7) 4827 (24) (29) 7 (2)i 92 (28) 7r (37) 9 (7) -13 -19 -202 (7) (20) c(r4) 816 (7) 4103 (21) (26) 9 (2) 37 (20) 88 (37) 2 (6) 1 9 -888 a c(ls) 1060 (6) 30s6 (20) -484 (23) 2.40 (0.42) (22) (6) (7\ (20) c(16) 1316 (6) 2446 (23\ -1332 (26) 7 (2) 72 ss (36) -3 -2 s a ¿ c(17) l 768 (8) 7s7 (24) -1988 (26) 3.s2 (0.s1) r7r't (6) (23) (27) 4 (2\ 88 (26) r01 (38) 4 (6) 2 (7) 28 (23) c(18) -734 -1882 \ tlsotropic thermal parameter refinement only for C(10), C(15), and C(17).

È\ * ¿

€ Struc tw e of Fe (I il ) (Sa en) 2Cl. H 2O l2s

TABLE IL Hydrogen Atoms. Positional (X l0a) and Isotropic The¡mal Parameters.a

xlÃ ylb zlc B (Â2)

c(2) H ¡9s -99{ -600 3.5 0 c(3) H 175 -1997 574 3.s 0 c(4) H l5 -2681 2293 3.50 c(s) H 624 2966 3.s0 -2297 '285 c(7) H I 306 -1504 3 3.s 0 c(8) H(l) 2t94 -1288 1876 4.s0 c(8) H(2) 1920 -688 ,2982 4.s0 c(9)H(l) 2438 810 2t06 4.50 Sú, c(9) H(2) 1986 t493 2373 4.5 0 c(ll) H 74t 3208 2036 3.50 c(I2) H 375 4923 I 364 3.5 0 c(l3) H 44t 555 6 _457 3.5 0 c(14) H 8t7 4295 -1621 3.s 0 c(I6) r{ I 33s 2832 -2009 3.s 0 c(17) H(1) 1669 I 070 -2732 4.50 c(r7) H(2) 2068 l0l 3 -1891 4.50 c08) H(l) t922 -l2ls -2366 4.s0 c(I8) H(2) 1427 -1006 -209s 4.s0 -r'S N(3) H(l) 2273 6s4 324 3.s0 N(3) H(2) 2083 l9s 3 590 3.s0 (þ N(4) H(r) 2070 -1062 -550 3.5 0 N(4) H(2) 1690 -1868 -5 l8 3.5 0 o(3) H(l) 3468 t4s 2t6 3.5 0 o(3) H(2) 33 l8 332 78 3.50 rAll i"¡ paramete¡s calculated exccpt the positional paratncters fo¡ O(3) H(l) and O(3) H(2).

Qor

ltt Figurç 2. Slercoscopic vicw of thc molccular structurc with tftcrntal clli¡tsr¡ids drarvn at 50% protrability. (exccpt for H r) atonrs, whcre thc scalc is arbitrary).

The crystal data are: Space Group, Pbca; M = 435.7; a = 32.694(22); o {lt å = 10.083(7): = 12.274(8) Ã; i = 4046.2 A3 : D" = 1.430; D^ "- 1.43(1) gcnr-3 (by florarion); Z = 8;p(MoKo) = 8.4 cm-r ; À(MoKa) = 0.7107 Å. l (ta, ll Intensities were rneasured on the diffractonleter with MoKa radiation, utilizing the <¿ scan technique c(l controlled by variable scan and scan tilne routines lt.) .:' as prevjously described [3] . A single crystal, rnounted Qr along c, (0.15 X 1.0 X 1.5 nrrn3) was used and data for hkO-7 collected, yielding a total ol2718 rcflcc- Qrrt tions, includfurg 1587 non-zero rcflcctions Il > 3o(l)] used to solve Figure l. Perspective view showing the atom labclling thc structure. Absorption, Lorcntz and scherne. polarization co¡rections were applicd.

Prelimin a ry ÌVeissen berg photographs (CuKa radia- Structure Determination tion Ni filter) established the space group, and the parameters cell were estimated from the c.r scans of The three dimensional Patterson function enabled hOO, OkO and OOI reflections on a Stoe Weissenberg location of the iron position. The chlo¡ide ion, automatic diffractometer. carbon, nitrogen and oxygen atom positions were A. P. Summerton, A. A. Diamontis and lvl, R' Snow 126

TABLE III. Interatomic Distances and Àngles with Their standa¡d Deviations'

Distances (0.4) c(1 l)-c(12) 1.3s (.04) A Fe{l 4.37 (.ol) Á, c(3x(4) 1.39 Â c(12)<(r3) 1.42 Fe-O (3) 4,06 (.02) Ç(48(s) 1.35 c(13){(14) 1.36 Fe-O (1) 1.88 c(sx(6) t.46 r.41 Fe-O (2) 1.89 c(6x(1) t.42 c(l4){(1s) 1.42 Fe-N(1) 1.93 c(6){(7) t.49 c(ls){(10) |.47 Fe-N(2) 1.96 c(7)-N(l) 1.3 I (.03) c(ls){(16) c(16)-N(2) 1.32 (.03) Fe-N(3) 2.02 N(r)-{(8) 1.50 N(2){(17) r.4'l Fe-N(4) 2.04 c(8){(9) l .s 2 (.04) c(l 7)-c(l 8) l.s2 (.04) o(l){(l) 1.34 (.03) c(9)-N(3i r.s 2 (.03) c(r8)-N(3) 1.s 3 (.03) c(1){(2) 1.40 (.04) o(2){(10) r.32 c(2)-c(3) 1,42 c(l0){(11) 1.44 (.04)

Angles (2.5)" 93.7 (2.0f c(5){(6){(l) I19.5 o(1)-Fe-o(2) 127.6 93.6 c(l){(6){(7) o(l)-Fe-N(l) t 16.3 (2.0) o(l)-Fe-N(2) 88.3 c(6)-c(7)-N(l) 114.5 o(l)-Fc-N(3) 175.8 c(7)-N(1){(8) 86,0 N(r){(8){(e) 106.5 o(l)-Fe-N(4) 107.6 o(2)-Fe-N(l) 87.4 c(8)-c(9)-N(3) I 09.2 O(2)-Fe-N(2) 94.3 C(9)-N(3)-Fe 12.s.2 o(2)-Fe-N(3) 89.7 trc-O(2){(10) t't7.2 o(2){(lo){(15) 123.-s o(2)-Fe-N(4) l l6.s (2.s) N(l)-Fe-N(2) 177.1 c(ls){(to){(ll) 121.3 N(l)-Fe-N(3) 84.l c(10){(11){(12) 120-7 N(l)-Fe-N(4) 95 .3 c(r l){(12){(13) 120.0 N(2)-Fc-N(3) 93.9 c(r2) {(13) {(14) t 20.3 N(2)-Fe-N(4) 8 3.0 c(13)-c(14){(ls) 120 9 N(3)-Fe-N(4) 90.8 cl04) c(ls)-c(lo) 26.5 124.3 c(lo)-c(ls)4(16) I Fe-o(l){(l) l r9.8 (2.0) o(l){(1)-c(6) r23.0 c(rs){(16)-N(2) c(6){(l){(2) 120.7 (2.s) c(16)-N(2){(17) r 18.2 c(l)-c(2)-c(3) 116.0 N(2){(17){(18) 106.0 c(2){(3){(4) 124.9 c(17){(18)-N(4) 106.0 c(3){(4){(s) 118.9 C(18)-N(4)-Fe 108.4 c(4){(s){(6) 119.9 N3-Cl-Na s 2.0

TABLE, IV. lntcratontic Distanccs and Ângles for Potcntial positions oI all ligatrd lrydrogort iltorì¡s \\'crt'clllculat- Hydrogen Bonds, A-Ì1 "'' "8. ed and includcd in furthcr cyclcs of rclì¡ìcrììclìt' After convergence to a lural rR value of 0'l 14 [4] , a AHB A-B A-H H' 'B Ansles furtl.rer difference syntlìesis revealed thc trvo peaks ncar atonl O(3) rvere still prescrlt' Tltese ¡rcltks rvr're o(3) H(l) ct 3,15 Â l.19 A 2,49 l 13" assigned to hydrogctt atorrrs O(3) Il(l) antl O(3) o(3) H(2) ct 3.15 t.r2 2.t0 ^ r53 Ì{(2). Scattering factors were takerl froru rcf. 5. those N(3) H(r) cl 3.23 0.91 2.36 160 for iron and chlorine bcing corrected lor thc elfccts N(3) H(2) Cl " 3.28 0.91 2.39 166 of anonralous scattering (Lf', Af ") [6] ' All data 2.44 175 N(4) H(1) ct 3.3s 0.91 processing was carried out on a CDC 6400 coluputer 2.07 t67 N(4) H(2) o(3) 2.e'l 0.91 at Adelaide University with prograrrls described previously [3] . obtained from three dimensional difference Tables I to IV show the fìnal pararrleters and refinement molecular geolnetty, with estirlratcd standlrd devia' ors for all tion in parentheses. The atom labelling scherue is shown in Figure l. Final observed and calculated ;ïl:::å:: structure factor tables are available.* Fourier Synthesis showed no peaks of height-greater than 1.03 e ,F3. Two of the more intense peaks were in the region of atom O(3)' At this stage, the *Copies arc availablc on application to the lìditor-in{hief t2't Structurc of Fe ( Iil ) (Saen ) 2Cl' H 20

Discursion Although the crystal structure does not clarify the role played by the water molecule in stabilization The cation clearly hæ a bis'tridentate ligand of the low spin state, it is known that small changes arrangement in common with the recently determin' in the crystal field ca¡r bring about crossover [15] . As èd structures of Cr(IIt{Saen)zI [7] and Co(III)' a posible explanation, it is suggested that the environment of the ammine hydrogens may be res- (Saen)2l.H2O [8] , in,the meridional confìguration. ' The possible hydrogen bonding interactions ponsible for such a variâtion in crystal field. between the complex cation, water-molecule and The crystal structure of the non-hydrated chloride ion are listed in Table IV. According to species C(Saen)2I shows that only two of the aocepted criteria, regarding interatomic distances and ammine hydrogens on the cation are able to form angles for such interactions [9], all ntay be consider- hydrogen bonds Í71. The high spin complex ed as hydrogen bonds, with the exception of the. Fe(Saen)2I has been assumed to have a similar O(3FH(I)'....C1 interaction where the angle of molecular geometry on the basis of the similarity of 1l3o is far too low. Fþure 2 shows the hydrogen the respective i.r. spectra [2] . Figure 2 clearly shows are bonding interactions as broken lines. The water mole- that in this study all four ammine hydrogens cule has specific interactions with both the complex involved in interactions, three with chloride ions, one and thç anion and lies in a well defined position in the with the water molecule. This would be expected to lattice. One hydrogen atom of the water molecule result in a slight increase in electron density on the lteld interacts with the anion whereas the other may be ammine nitrogen, and enhance the crystal considered as effectively 'free' of significant inter- experienced by the central metal ion. actions. Much interest has been shown of late in spin-cross- In hydrates where both hydrogen atoms are over systems and consequently a number of review known to be involved in hydrogen bonds, the G-H articles have appeared in the literature for both stretching band is observed as a single broad envelope iron(Il) and (II) complexes. In both cases, the data with-a band width typically of the order of ,200 presented suggest that the spin state is extremely cm-r [10]. sênsitive to apparently minor variations in the ligand Holever, it is well known that O-H stretching [15] . In the complex described above then, it is bands can.be extremely sharp. In discussions of the perhaps not surprising that such a subtle interaction O-H stretching region of the infra'red, many authors as that between a ligand and a lattice water molecule assign a very sharp band between 3650 and 3590 may be suffìcient to 'tip the balance'in favour of cm-r to 'free' O-H [10, I l] . Examination of the stabilization of the low spin state. published i.r. spectra of alcohols reveals the band width is often as low as v¡¡2 = 20 cm-r [12] . The Acknowledgements observed in this complex must sharpness of the bands The authors are grateful to Mr. J. Westphalen, the mode of bonding of the water mole- be related to School of Applied Physics, S.A.I.T., for æsistance in particular the presence of only one hydrogen- cule, in obtaining the initial Weissenberg photographs. bonded arm. A simple qualitative description of the observed spectrum ii to ãsign the band at 3620 cm-l References as a free O-H stretch and the other as a hydrogen bonded O{I stretch. I Presented, in part, at the R.A.C.I, Division of Coo¡dina- The low spin btate of this iron(III) complex, tion and Metalorganic Chemistry, Seventh Conference (COMO 7), La Trobe University, Victoria, February supported by the large quadrupole which is further o977). splitting of 2.75 mm seCt obtained frorn the Möss' 2 A. Yan den Bergen, K. S. Mu¡ray, B. O. West and A. N. bauer spectrum [13], is of interest because of its Buckley, I Chem. Soc. A, 2051 (1969). effect on the Fe to donor-atom distances. As recently 3 M. R. Snow,z{cfa C>ystallogr.,34 1850 (1974). The unusually high value is part due to the discussed review article on the related Salen 4 R in in a uirfavourably thin crystal plate chosen and hþh back- series, the metal to iniine-nitrogen bond distance is ground scatter f¡om the thick glass f,rbre to which it was sensitive to the nature of the metal [l4l.It appears attached. However the value compares favourably with that such distances are dependent on the number of those for similar structures, e.8', R = 0.114 [ref, (7)] and (8)]. unpaired electrons on the metal. In the case of high- R = 0.098 [ref, Clystallogr., Sect. A, metal imine- 5 D. T. Cromer and J. B, Marn, Acta spin iron(III) Salen complexes, the to 24, 32r (1968). nìtrogen distances are in the nnge 2.06-2.10 A, 6 D. T. Ctomet,Acta Cvystallogr., /8, 17 (1965). wherèas in this study the range is 1.93-1.96 Â. This ? A. P. Gardner, B, M. Gatehouse and J. C.B,While,Acta app4rent contraction is not due to the difference in C'rystøllogr., Sect. 8, 27, 1505 (19? 1). T. H. Benson, M. S. Bilton and N. S. Grdl,Aust. J. Chem. ligand as the corresponding distances for Co(lII) and 8 30,26r (1977\. the Salen C(III) Saen complexes agree with those in 9 G. H. Stout and L. H. Jensen, "X-ray Structure Deter- æries [7, 8] . It must then be considered as consistent mination", Mc Millan, London (1968), p. 303 and refe- with the presence of a low spin central metal ion. rences cited the¡ein. t28 A, P, Summerton, A, A, Diattøntís and M, R. Snow

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