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HUNG, Yann, 1948- STUDIES ON TRANSITION METAL ION COMPLEXES OF MULTIDENTATE LIGANDS. I . KINETICS OF AQUATION OF DICHLORO-TETRAAMINE COMPLEXES OF COBALT(III) INVOLVING MACROCYCLIC COMPLEXES. I I . STEREORESTRICTIVE CHELATION—METAL COMPLEXES OF SPARTEINE AND BISPIDINE.
The Ohio State U niversity, Ph.D., 1976 Chemistry, inorganic
Xerox University Microfilms,Ann Arbor, Michigan 48106
0 1976
YANN HUNG
ALL RIGHTS RESERVED STUDIES ON TRANSITION METAL ION COMPLEXES OF MULTIDENTATE LIGANDS.
I. Kinetics of Aquation of Dichloro-tetraamine Complexes of Cobalt(IU) Involving Macrocyclic Complexes.
II. Stereorestrictive Chelation—Metal Complexes of Sparteine and Bispidine.
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University
By Yann Hung, B. S.
•M-l-l-H-H-H-
The Ohio State University 1976
Reading Committee: Approved By Dr. D. K. Busch Dr. Eugene P. Schram Dr. Gary G. Christoph J § . f (■ i A dviser ACKNOWLEDGEMENT
I would like to express my sincere appreciation to those individuals who helped me to learn, especially my parents and Professor Daryle H. Busch. CURRICULUM VITAE
November 18 1948 B orn—Taipei, Taiwan 1970 B. S ., National Taiwan University, Taipei* Taiwan 1970-1973 Teaching Assistant, Department of Chemistry, The Ohio State University, Columbus, Ohio 1973-1974 Stauffer Chemical Company Fellowship 1974-1976 Research Assistant, The Ohio State Univer sity, Columbus, Ohio 1976 Ph.D ., The Ohio State University, Columbus Ohio
FIELD OF STUDY
Major Field: Inorganic Chemistry
Studies in Coordination Chemistry, Professor Daryle H. Busch TABLE OF CONTENTS Page No.. Acknowledgement ...... ii Curriculum Vitae...... iii List of Tables ...... viii List of Figures ...... x
Kinetics of Aquation of Dichloro-tetraamine Complexes of Cobalt(IIl) Involving Macrocyclic Ligands.
I. Introduction ...... 1
Aquation of Cobalt Complexes ...... 6 Aquation of trans-Complexes ...... 7 Aquation of cis-C omplexe s ...... 4 5 Hg2+ Induced Aquation ...... 17
Base Hydrolysis of Cobalt(H3) Complexes ...... 18
Anation of Cobalt(H3)...... 22
Statement of the Problem ...... 24
n. Experimental...... 25
Physical Measurements ...... 25
Syntheses ...... 26
K inetics ...... 31
HI. Results and Discussion ...... 34
Syntheses and Characterization of Complexes ...... 35 trans-[Co(MAC)Cl;]+ Complexes ...... 35 Isomers of trans-[Co( [lSjaneN^C^]"*"...... 51 Isomers of trans-[Co( [lGjaneNJ Cl?]* ...... 58 cis-[Co(MAC)X^]+ Complexes ...... 60 TABLE OF CONTENTS
Page No. Kinetic Studies...... 67 Aquation of trans-[Co(MAC)Cl?]+ Complexes ...... 68 Case I. Aquation of trans-[Co([i3]aneN4)Cl2]+...... 68 Case II. Isomerization of trans-[Co([l3]aneNa)(HgO)2]3t ...... 76 Case HI. Aquation of trans(I)-[Co([l5]aneNJC1?]+ ...... 76 Case IV. Aquation of trans(ID-[Co([l5]aneNa)Cl2]+...... 79 Case V. Conversion of isomer II to isomer I of trans-[Co([l5]- aneN4)Cl(H20)]2+ in aqueous solution ...... 82 Case VI. Aquation of trans-[Co([l6]aneN4)Cl2]C104...... 85 Aquation of cis-[Co(MAC)Cl?]+ Complexes...... 85 Case I. Aquation of cis-[Co([12]aneNa)C 12]+ ...... 85 Case n. Aquation of cis-[Co( [!3]aneNa) Cl2]+ ...... 90 Case m . Isomerization of cis-[Co([l3]aneN/i)Cl2]Cl to trans- [Co( [13]aneN4)Cl2]Cl in M ethanol...... 92 Discussion of the Kinetic Behavior of trans-[Co(MAC)Cl23+...... 92 The Kinetic Behavior of cis-[Co(MAC)Cl2]+ Complexes...... 1 0 3
IV. Sum m ary...... 1 0 7
V. Bibliography^ ...... 109
Stereorestrictive Chelation-Metal Complexes of Sparteine and N,Nt-Dimethyl- bispidine.
I. Introduction ...... 114
H. Experimental ...... 118
Materials ...... 118
Physical Measurements ...... 118
Syntheses ...... 119
K inetics ...... 426
v TABLE OF CONTENTS
Page No. HI. Results and Discussion...... 127
Strain Energy Calculations ...... 128
Syntheses and Characterizations...... 131 Case I. Co(L)X2 C om plexes...... 131 Case H. Ni(L)X2 Complexes ...... 142 Case HI. Cu(L)X2 Complexes ...... 155 Case IV. Pd(L)X2 Complexes ...... 164 Case V. [Co(Me4N[i4]aneN4)Cl]C104...... 171 Comparison of Metal-Chloride and Metal-Nitrogen Stretching M odes...... 171
Kinetic Studies ...... 173
IV. Summary...... J.77
V. Bibliography ...... 178 LIST OF TABLES P a rt I
Table Page No.
1. Ligand Field Parameters for trans-Diacidotetraamine Complexes of [Co([l3-l6]aneN4)Cl2]+ and [Ni([14-16^6^Cl2]. 3
2 . Ideal Metal-Nitrogen Bond Lengths and Planarity of the Macrocyclic Ligands. 3
3. Minimized Strain Energies for the Free Macrocycles. 4
4. Activation Parameters and Steric Courses of the Aquation of Co(L)ACln+. 8
5. Rate Constants and Steric Courses of Aquation of Dichlorotetraaminecobalt(III) Complexes. 9
6. Rates for Base Hydrolysis of [Co(AA)2LCl]n . 20
7. Wavelengths Where the Aquation Was Followed. 32
8. Selected Infrared Spectral Bands for trans-[Co(MAC)C 12]+X. 37
9. Infrared Spectral Bands for_trans- [Co(MAC)C12].Com plexes in CH2-Rocking and Metal-Nitrogen Stretching Regions. 43
10. Far-Infrared Data for the Complexes trans -[Co(MAC)Cl>]+. 44
11. Electronic Spectral Data for trans-[Co(MAC)XY]n+. 46
12. Possible Stereoisomers for trans-tCojUsjaneNJCl?]*. 54
13. Conformational Energies of Some trans-[Co([l5laneN/t)Cl>]+ C onform ers. 54
14. Conformational Energies of Some trans-[Co([l6]aneN^)Cl9]+ Conformers. 60
15. Molar Conductivities for the Complexes _cis-[Co^(MAC)X2]Y 61
16. Infrared Data for cisy[Co(MAC)X^]Y, N-H and CC^2~ Modes. 62 v ii LIST OF TABLES P a r t I
Table Page No. 17. Infrared Absorption Bands for the cis-Complexes in the CH2-Rocking and M-N Stretching Region. 63
18. Electronic Spectral Data for cis_-[Co(L)X2] Complexes. 64
19. Half-Wave Potentials of cis_-[Co(MAC)Cl2]+ in Acetonitrile. 66
20. Half-Wave Potentials for trans-[Co(MAC)Cl2]+ in Acetonitrile. 48
21. Aquation Data for trans-[Co([l3]aneN/))Cl9]+ in 0.1 N HNC^. 68
22. Aquation Data for jtrans(l)-[Co([i5]aneN4)Cl2]+ in 0.1 N HNC^. 79
23. Aquation Data for_trans(H)-[Co([l5janeN4)Cl2]+ in 0.1 N HN03. 82
24. Aquation Data for cis-[Co([l2, l3]aneN4)Cl2]+ in 0,1 N HNC^. 86
25. Rate Constant and Activation Parameters for the Aquation of Some Complexes of the Form trans-[Co^(L)Cl2j+. 94
26. Minimized Strain Energies of trans-[Co(MAC)Cl?]+. 97
27. Ideal Metal-Nitrogen Bond Lengths and Planarity of the Macrocyclic Ligands. 100
28. Kinetic Data for the Aquation of cis-[Co^L)CI9] . 103
29. M olar Conductivities of the Complexes tr a n s - [Co(MAC) C1P]X. 36 P a r t II
1. Selected Infrared Absorption Bands for the M(L)X2 Complexes. 130
2. Magnetic Moments and Molar Conductivities for Co(l)X2. 132
3.... E lectronic Spectral Data fo r Cb(L)X^. 133
4. Magnetic M oments and Molar Conductivities of Ni(L)X^ Com plexes. 144 v iii LIST OF TABLES P a rt II
Table Page No. 5. Electronic Spectra of Ni(L)X2 Complexes in Acetonitrile. 147
6. Magnetic Moments and Molar Conductivities for Cu(L)X2. 154
7. Electronic Absorptions of Copper(II) Complexes. 158
8. Electronic Spectra of Pd(L)X2 Complexes in Acetonitrile. 165
9. Infrared Spectra Data for the M'etal-Donor Atom Modes for the Co(IQ, Ni(I3), and Cu(H) Complexes 170
10. Rate of Dissociation of [Ni(DMBsp)]2 and [Cu(DMBsp)r in Perchloric Acid. 174
11. Kinetic Data for the Rate of Dissociation of Complexes of Some Tertiary Amines in Acid. 176
i x LIST OF FIGURES P a rt I
Figure Page No. 1. Homologous, Fully Saturated, Tetraaza-Macrocyclic Ligands. 2
2. Some Tetradentate Ligands. 14
3. Infrared Spectra of trans-[Co([l53aneN4)Cl?3NQt Isomers. 49
4. Infrared Spectra of trEms_-[Co([l6]aneN4)Cl2]C104 Isomers. 56
5. The Energy Diagrams for Low Spin d6 Oh and D4i4 Symmetry. 45
6. Electronic Spectra of trans-[Co([15 janeN^Cl?]NOs Isomers in M ethanol. 50 + 7. Possible Isomers for [l5]ane N4 in trans-[Co([l5]aneNj)Cl9.] „ 53
8. Structure of trans(I3)-[Co([l5]aneN4)Cl9!]+. 55
9. E lectronic Spectra of tran s - [C o ([ 16 3 aneN^) C1? ] (Cl O4) Iso m ers. 57
10. Possible Isomers for [i6]aneN4 in trans-[Co([l6]aneN4)Cl23+. 59
11. Changes in Absorption Spectra for the Aquation of trans- [Co([i3]aneN4)Cl2]+. 69
12. Sample First Order Plot of Kinetic Data. 74
13. Changes in Absorption Spectra for the Second Aquation of tran s-[Co([l3]aneN4)Cl2]+. 71
14. Changes in Absorption Spectra for the Isomerization of j£ans-[Co([l3]aneN4)(H20)2]3+ to cis-diaquo Complexes. 75
15. Changes in Absorption Spectra for the Aquation of trans- (D-[Co([i5]aneN4)Cl2]+. 77
x LIST OF FIGURES P a r t I Figure Page No. 16. Changes in Absorption Spectra for the Aquation of trans(Il)-[Co([l53aneN^)Cl9]+. 80
17. Changes in Absorption Spectra for the Isomerization of trans(II)-[Co([l5]aneNd)Cl(H^O)32+ to transGMCodTSjaneN^Cl- (H20)]2+. 84
18. Infrared Spectra of trans(I])-[Co([l5]aiieN4)Cl2]C104 in N-H and N-D Stretching Region. 83
19. Changes ill Absorptoin Spectra for the Aquation of cis-[Co([l2]~ aneN4)Cl2]+. 87
20. Changes in Absorption Spectra for the Aquation of cis-[Co([l3]- aneN4)Cl2]+. 91
21. The Strain E nergies of the tr a n s - [Co(MAC)CI?3+ Complexes vs Rate Constant Plot. 95
2 2 . The Isokinetic Plot of Some Related cis-Complexes. 104
23. Some Infrared Vibration Modes. 42
24. The Van't Hoff Plot of the Aquation of trans_-[Co([l3]aneN4)Cl2]+. 72
25. The van’t Hoff Plot of the Aquation of trans(h-[Co([l5]aneNA)Cli)]~<~. 78
26. The van’t Hoff Plot of the Aquation of trans(H)-[Co([l5laneN^)- Cl2]+. 81
27. The van't Hoff Plot of the Aquation of cis-[Co([12JaneN^Cl2]+. 88
28. The van’t Hoff Plot of the Aquation of cisr [Co([l3]aneN4)Cl2]+. 89
29. Far-Infrared Spectra of trans-[Co(MAC)X)]~1' Complexes. 39
x i LIST OF FIGURES P a r t II Figure Page No.
1. Some Bidentate Ligands. 115
2. Electronic Spectra of Co(sp)X^ in Acetonitrile. 135
3. Term States for d7 Ion in Td and C2v Symmetry. 138
4. Electronic Spectra of CoL(OAc)2 in Acetonitrile. 141
5. Electronic Spectra of Ni(sp)X2 in Acetonitrile. 149
6. Electronic Spectra of Ni(DMBsp)X2 in Acetonitrile. 150
7. E lectronic Spectra of NiLXg in A cetonitrile. 152 X = OAc or NOj.
8. Electronic Spectra of Ni(DMBsp)(NCS)2. 153
9. Electronic Spectra of Cu(sp)X^ in Acetonitrile. 156
10. Electronic Spectra of Cu(DMBsp)X2 in Acetonitrile. 157
11. Esr spectrum of Cu(sp)Cl2 in CH2C12-CHC13 (1:1) at 77° K. 159
12. Electronic Spectra of CuL(OAc)2 in Acetonitrile. 160
13. E lectronic Spectra of [Cu(DMBsp) (OH)]2(C104)2 . 163
14. Electronic Spectra of PdLX2 in Acetonitrile. 166
15. Far Infrared Spectra of MLX2 Complexes. 167
- 16. Electronic Spectra of [Co(Me4^[l4]aneN4)Cl]C104. 172
17. Esr Spectrum of Cu(sp)(OAc)2 in CH2C12-CHC13 (1:1) at 77° K. 161
x ii LIST OF FIGURES P a rt II
Figure Page No. 18. Term States for d8 Ion in Td and C2v Symmetry. 145
19. Ideal Structure of Co(DMBsp)Cl2. 129
20. Electronic Spectra of Co(DMBsp)X2 in Acetonitrile. 136 PART I Kinetics of Aquation of Dichloro-tetraamine Complexes of Cobalt(in) Involving Macrocyclic Ligands.
INTRODUCTION
Transition metal complexes of synthetic macrocyclic ligands have attracted much interest because of their fascinating stereochemistry and properties and because they are potential model systems for those biologically important natural complexes that contain macrocyclic ligands. For any given metal ion, the properties of the complexes of synthetic macrocyclic ligands, such as the rates of hydrolysis, oxidation-reduction potentials, coordination number, etc., depend, to a large extent, on the size of the macrocyclic ring, on the positions and degree of ligand unsaturation, and on the presence of ring substituents. Fourteen-membered macrocyclic ligands have attracted the most attention, principally because of their relative ease of preparation. Although the unsubstituted fully-saturated ligands, 1,4,7, iO-tetraazacyclododecane, [12]aneN4 (structure 1), and 1,4,8, il-tetraaza- cyclotetradecane, [l4]aneN4 (structure III), have been known for some time, only recently have these and the ligands 1,4,7,10-tetraazacyclotridecane, [l3]aneN4 (structure ID, 1,4,8,12-tetraazacyclopentadecane, [l5]aneN4 (structure IV), and 1,5,9,13-tetraazacyclohexadecane, [l6]aneN4 (structure V), been synthesized by procedures that provide substantial yields in return for reasonable effort. Thenickel(ID and cobalt(ni) complexes of this series of ligands have been prepared and characterized by Martin. 1 It was found that the Dq*^ values of trans-[Co([l3-l6]atmN4)Cl?]+ and trans-[Ni([l4-l6]aneN4)Cl2]
1 [12] J [13] cN N < N N-
[l2]aneN4 [l3]aneN4 I II
[14]
[l4 ]an e N 4 HI
3 [16]
[l5]aneN4 [l6]aneN4 IV V
F ig u re 1.
Homologous, Fully Saturated Tetraaza-M acrocyclic Ligands 3
XV series (Table 1) increase as the ring gets smaller with Dq of [Co([i3JaneN4)Cl2] XV and Dq of [Nid^JaneN^C^] being the largest of their series, respectively. [Ni([’i3]aneN4)Cl2] exists in cis-configuration and cannot be compared.
Table i Ligand Field Parameter for trans-Diacidotetraamine Complexes of [Co([l3-l6]aneN4)Cl2]+ and [Ni([i4-i6]aneN4)Cl2]. ______D q ^ cm ' 1______Ligand Co3+ Ni2 [13]aneN4 2750 [14]aneN4 2480 1480 [!5]aneN4 2360 1250 [!6]aneN4 2250 1110
Therefore, the mechanical constraint on the metal-nitrogen distance exhibits a profound effect on the extent of metal-donor interaction. Louis DeHayes has calculated3 the ideal metal-nitrogen bond lengths for the macrocycles (Table 2). As the number of atoms decreases the hole in the
Table 2 Ideal Metal-Nitrogen Bond Lengths and Planarity of the Macrocyclic Ligands. Ring Size Average ideal bond lengths (A) Average deviation from ideal NA plane (A) ______[12] 1.83 0.41 [13] 1.92 0.12 [14] 2.07 0.00 [15] 2.22 0.14 [16] 2.38 0.00 4
center decreases. Overly large ions, therefore, cannot fit into the ring without causing extreme distortion from the square planar geometry; this is exemplified by [Ni([l3]aneN4)Cl2] which is in a folded cis-configuration. DeHayes has also calculated3 strain energies for the macrocycles [i2]aneN4-[i6]aneN4 (Table 3). The macrocycles were assigned a planar geometry, with the nitrogens occupying the same plane as the square planar complex but without a metal ion in the center. The strain energy, U, is defined as
U = U(y) + U(0) + U(jZf) + U(d) (1)
where U(y) is the energy associated with bond stretching, U(0) is the energy resulting from bond angle deformations, U(ff) is the energy involving bond torsions, and U(d) is the energy arising from the nonbonded or Van der Waal's interaction in the molecule. The specific conformations used were [l2]aneN4, 6^6^; [i3]aneN4, 6^6y; [l4]aneN4, 6y^y; [i5]aneN4, 6.y^y; [ i6]aneN4, by^y, where b and ^ refer to chiralities and y refers to the chair form of the six- membered ring.
Table 3 Minimized Strain Energies for the Free Macrocycles. Ring size RNB 0 0 H [12] 0.52 -1.70 6.75 3.04 8.61 [13] 0.67 -1.94 4 .4 7 3.52 6.72 [14] 0.81 -0 .7 9 3.68 1.36 5.06 [15] 0.98 0.10 4 .85 3.20 9.12 [16] 1.17 -1.05 5.71 5.71 13.63
According to the calculations, [i4]aneN4 can assume the planar conformation with the least strain of all the macrocycles considered. The redox potentials of the [Co(ti3-l6]aneN4)Cl2]C104 series have also been studied.4 It was found that for this series, the half-wave potentials of the Co(m)/Co(II) couple fall in the o rd e r [13] ~ [14] > [15] > [16], with the 16-m em - bered ring complex being the easiest to reduce. Again, this is the effect of 5
ring size. The larger cobalt(II) ion fits better in the larger i5- and 16-mem- bered rings. Therefore, complexes of [i5]aneN4 and [i6]aneN4 are easier to reduce. The rates of acid displacement of the macrocyclic ligands have been studied.5 It was found that for the series [Ni([l3-l6]aneN4)(H20)2]2 , the rates of dissociation follow the order [16] > [15] > [13] » [14], under the same conditions. The cobalt(IH) complexes of these ligands present a unique opportunity to study axial ligand substitutions and cis-trans isomerizations. These complexes, being constrained to some degree by the cyclic ligand system, should provide meaningful insight into stereochemical effects on the course of substitution reactio n s. A large number of kinetic studies have been performed on the kinetically inert complexes of cobalt(III) and the mechanisms of substitution reactions involving replaceable ligands have been extensively studied.6 »7 >8 >9 »10 In aqueous solution, three types of substitution reactions are known for octahedral cobalt(HI) complexes: 1. Aquation: the replacement of a ligand by water. CoA4LXn+ + H20 CoA4L(H20 )(n + 1)+ + X" - When X is chloride or azide, the aquation of the complex is promoted + by Hg2 or HN02, respectively. 2. Base hydrolysis: the replacement of a ligand by a hydroxide group. CoA4LXn+ + OH" -* CoA4L(OH)n+ + X" 3. A nation: the replacement of coordinated water by another ligating agent, often an anion. CoA4L(H20)(n+1)+ + X" -» CoA4LXn+ + H20 In the formula, CoA4LXn+, A are inert ligands, L is the so-called orienting group and X is the leaving group. Aquation
The aquation of cobalt(IU) complexes has been studied extensively. The mechanism for aquation is generally described as dissociative in nature;, that is, bond breaking is more important than bond making in the transition state, and the process goes by an SN i mechanism (eq. 2, 3).
M - X Sl-?W M + X (2) M + Y f-st MY (3)
The generation of a five coordinated species, M, occurs in the rate determining step. However, in some cases, especially when the orienting ligand is capable of ttbonding, such as N02”, CN7 etc., two kinds of reaction mechanism have been proposed; one, dissociative (Sjsfi) and the other, associative (Sn 2). In general, the S^j2 mechanism is less favored by most workers and indeed is relatively poorly documented. Some characteristics of Sjq i mechanisms are as follows: 1. The rate of reaction is independent of the incoming group. 2. Increasing the size of the inert ligand or the leaving group will increase the ra te . 3. The nature of the leaving group will affect the rate profoundly. The five coordinated intermediate, Co(CN)52- involved in the aquation of [Co(CN)5X]3- has been detected by Haim, Grassie, and Wilmarth, where X is Br", I~, NCS", and N3- . 14>12»13 Pearson et al14 have studied the rates of aquation of several dichloro-tetraamine cobalt(IO) complexes in order to evaluate general steric effects. They found that the rate order increases as the ligands become more bulky. That the aquation of complexes containing chloride and azide groups are promoted by Hg2+ and nitrous acid indicates the facilitated formation of the five coordinated intermediate by its conversion into a better leaving group.
All of these observations support an Sjj i mechanism for the aquation of cobaltdD) com plexes. 7
There are two possible geometries for the five coordinate intermediate. Namely, trigonal bipyramidal and square pyramidal. It is believed48 that the energy difference between the two is not large. Hence, the intermediate can adopt either structure. Usually, the geometry of the products provides some information about the geometry of the intermediate, since no geometric rearrangement is expected for the square-pyramidal intermediate while isomerization is expected for trigonal-bipyramidal intermediate. Tobe48 noted that in most cases involving aquation where the isomer distribution changes, a large £S^ is observed (Table 4). He, therefore, proposed that the higher entropies of activation are diagnostic of a trigonal-bipyramidal intermediate and that the lower values are indicative of tetragonal pyramidal intermediates which do not produce isomerization for complexes with four amine groups cis to the leaving group. Attempts49 to predict which compounds should experience geometric rearrangements .during the aquation reaction by calculating the energies of the intermediates, with different structures such as trigonal bipyramide, tetragonal pyramide and pentagonal bipyramide, have not been too successful.
Aquation of trans-complexes. — The dichlorotetraamine cobalt(IO) complexes of a series of linear tetradentate ligands have been synthesized28’29 and their aquation reactions studied (Table 5).24>25>26 The cation trans-[Co- (trien)Cl2]+ aquates stereospecifically to the ff-cis- [Co(trien)(H^O)Cl]2+ ion and an Sn 2 mechanism was proposed by Sargeson and Searle24 to explain this phenomenon. They proposed some degree of bond formation with water at the face of the octahedron and that the accompanying edge displacement of the terminal NH2 is responsible for this particular aquation. 8
Table 4 Activation Parameters and Steric Courses of the Reactions n+ (n+l)+ trans-CoLACl + H2Q - CoLAH2Ov ' + Cl .
L A AH^ A ^ $ steric kcal mol cal deg mol change (en)2 OH 25.9 +20 75 (en)2 Cl 26.2 +14 35 (en)2 Br 24.9 + 3 50 (en)2 n 3 22.5 0 20 (en)2 NCS 30.2 + 9 60+10 trien Cl 25.5 +16 100 d,£-2,3,2-tet Cl 25.1 + 9 >95 (en)2 n h 3 23.2 -11 0 (en)2 CN 2 2 .5 - 2 0 (en)2 n o 2 20.9 - 2 0 [i4]aneN4 OH 18.1 - 7 0 [l4]aneN4 Cl 24.6 - 3 0 meso-2, 3,2-tet Cl 2 3 .7 - 1 0 cis-CoLACln+ + H20 - CoLAH20(n+1)+ + Cl" (en)2 OH 23.0 +10 0 (en)2 Cl 21.5 - 5 0 (en)2 B r 23.5 + 5 0 (en)2 NCS 20.1 -14 0 (en)2 n3 21.3 - 4 0 (en)2 n h 3 24.5 - 6 0 (en)2 n o 2 21.8 - 3 0 (en)2 c h 3n h 2 25.9 - 2 0 (en)2 c h 3c h 2nh 2 25.5 - 4 0 (en)2 (CH3)2CHNH2 24.3 - 4 0 (en)2 ch3ch=chnh2 25.6 - 4 0 (en)2 cich2ch2nh2 25 .7 - 5 0 (en)2 CH3CH(C1)CH2NH2 25.6 - 5 0
a-trie n Cl 21 .0 - 6 0 /3-trien Cl 20.3 - 3 0 trien n h 3 23 .7 -10 0 cyclam Cl 18.3 - 6 0 /3-2,3,2-tet Cl 22.3 2 0
Data are taken from ref. 48. Table 5 Rate Constants and Steric Course of Aquation of D ichlorotetraam inecobalt(ffl) Com plexes at 25° C, pH = 1.
Complex k( (sec *) steric course (# cis) trans- [Co(trien) Cl*] 3.5 x 10-3 100 tran s -m e so- [C o(2,3 ,2 -tet) Cl* ] 1.5 x 10"5 0 tran s-d , 4-[Co(2,3 ,2 -tet) Cl*]+ 2 .9 x 10"4 50 + 20$ trans-[Co(3,2, 3-tet)Cl^] 5.3 x 1(T5 0 cis-/3- [Co(trien) Cl2]+ 1.5 x 10"3 0 cis-a- [Co(trien) Cl2]+ 1.6 x iO -3 0 cis-[Co(2,3,2-tet)Cl*] + 1.1 x 10-3 0
trien = triethylenetetraamine 2.3.2-tet = 1,4,8,11-tetraazaundecane 3.2.3-tet = i, 5,8,12-tetraazadodecane + trans-[Co( trien) Cl2] c ls-ff- fC o( tr.ien) (H20) Cl]+
However, most of the remainder of the complexes with linear tetradentate ligands, e.g., ti?ans-meso-[Co(2,3,2-tefc)Cl?] and trans_-[Co(3,2,3-tet)Cl2] aquate with complete retention of the trans-configuration. An exception occurs with trans-d, ;,-[Co(2,3,2-tet)Cl9]+, which is similar to trans-[Co- "f" (trien) Cl2] in configuration and aquates to a 50:50 mixture of the trans- and ff-c-is-[Co(2,3,2 -tet) (H2 O) Cl]2 species. Niththyananthan and Tobe28 proposed another model, which is the original Sjvy i mechanism, involving a trigonal- bipyramidal intermediate state to explain the aquation of both trans-[Co(trien) - + + C12J and trans-d, ^-[Co(2,3,2-tet)Cl2] by considering whether the site where the incoming water enters is "hydrophilic M or "hydrophobic". As shown in Scheme i, there are three regions of attack for the incoming water. They are not, however, equivalent. From an examination of molecular models, path two is the most favorable since it does not offset the strain that has been released, also the incoming water enters in an area near the hydrophilic amine protons, rather than at the hydrophobic area of the methylene protons as occurs in path 3. Therefore, in the case of trans-[Co(trien)Cl2] , the product arises from path two which gives the ff-cis-complex. Since the additional carbon atom in the ligand 2,3,2-tet serves to reduce the strain, paths one and two are both likely, hence both trans and ff-cis -products are formed. HaO | SCHEME 1 HaN
Poon and Tobe31 studied the aquation of the trans-dichloro(l, 4 ,8,11- tetr aaz acyc lote trade cane) c obalt( HI) cation (structure HQ and found that this compound aquates to an equilibrium mixture of trans-dichloro- and trans- aquochloro complexes with Kgq 0.008 M at 60.5° C. The rate constant was found to be 1.1 x i0-6 sec-1 at 25° C which is some thirty times slower than the rate for the corresponding strain free bis-ethylenediamine complex. Furthermore, the aquation proceeds with complete retention of configuration while that of the ethylenediamine complex takes place with 35$ conversion to the cis-compound.32 The macrocyclic complex of [l4]aneN4 does not differ significantly in ligand field strength from the ethylenediamine complex, DqU4]aneN ^480 and D q^ 2530 cm-1, but is much more rigid in structure. The aquation of other trails-{Co([ 14]aneNJ AX] complexes has also been studied. 19>20>21 it was found that these complexes' also aquate with complete retention of configuration to an equilibrium mixture which is different from their corresponding ethylenediamine analogues. Also, the rate of aquation of the macrocyclic complexes is invariably slower than their bis-ethylenediamine analogues. 12
Chau and Poon34 investigated the hydrolysis of trans-m eso-[Co(Men[l4]aneNJ- ■f* Cl2] and trans-(d,j£)-[Co(Me6tl4]£meN4)Cl2] (structure VI) where Me6[l4]aneN4 is 5,7,7,12,14,14-hexamethyl-l,4,8,1 1-tetraazacyclotetradecane. They found that the aquation rates for these two isomers differ by a factor of 4.5 and both are faster than the aquation of trans-[Co([l4]aneN,1)Cl?] . k4 is 9.3 x iO-4sec 1 for the meso complex and 4.2 x lO-3sec-i for the d,j I complex. An examination of molecular models shows that the two chlorides of the racemic isomers are not equivalent while they are equivalent in the meso isomer (structures VII and VIII). The radioactive chloride exchange reaction shows that the two chlorides in
V II V III meso-isomer racemic-isomer
d,j&~MeG[l4]aneN4 are not equivalent, k4 is 4.2 x 10"3 and 3.4 x iO-4sec_1. The rates differ by a factor of twelve. The same land of study showed that the two chlorides in meso- Me6[l4]aneN4 are equivalent, k4 = 9.3 x lO^sec-1. Compa rison of the results for the meso and d, SL-Mee[ 14]aneN4 complexes with that of the [l4]aneN4 complex indicates that the rates of aquation for the methylated species are at least one or two orders of magnitude faster than that for the unmethylated complex. The repulsions between chloride and methyl groups 13 malce the "expel" of chloride easier. This strongly indicates that the aquation of the macrocyclic complexes follows an Sj^l mechanism. Kernohan and Endicott?5 have studied the aquation kinetics of a complex with an unsaturated macrocyclic ligand, trans-[Co(meso-MeR[l4j-4, ll-dieneN^- Cl2] (structure DO, and found that the rate is about one hundred times faster than the rate for the corresponding saturated complex, trans-meso-[Co(Mefi- [l4]aneN,1)Cl2]+. This increase in reactivity has been attributed to the presence of cobalt(IH) -N(imine) bonds which stabilize the five-coordinate intermediate by increasing electron delocalization between cobalt and the ligand. Wan and Poon36 + found thatjtrans-[Co(dt-CH3)Cl2] (structure X) aquates so fast that it cannot be followed by conventional techniques. However, they claimed7 to be able to iso late and characterize the compound trans-[Co(CR-CH3)(OIi)](Cl, CIO4). Another macrocyclic derivative whose aquation kinetics has been studied7 is trans-[Co(Me2[i4]-4, ll-dieneN4)Cl(N02)J+(structure XI). At 25° C this compound aquates with a rate constant of 4.4 x iO-4sec-1, which is comparable to that of the trans-[Co(Mee[l4]-4, ll-diene)Cl(N02)]+ complex (k4 = 5.5 x iO-4sec-1). The additional methyl groups on the unsaturated macrocyclic ligand seem to have little effect on the aquation rate in the case of unsaturated macrocycles. An examination of molecular models shows that the axial methyl groups (structure XH) are out of the way of the axial ligands. Hence, the rate of aquation is not influenced by the extra methyl groups with the unsaturated ligand. This contrasts
Me
M e
R R = H or Me XH to the difference in rates summarized above for related saturated ligand deriva tiv es. 14
H n
Me6[l4]aneN4 Me6[l4]-4, 11 -d ie n e N 4
VI IX
•CH
C R -C H 3
X
ETE Me2[l4]-4, 1 l-dieneN 4 XV XI
Figure 2. Some Tetradentate Ligands 15
Lee and Poon38 tried to apply a Hammet relationship39 to the aquation data n-f- for the trans-[CoLACl] complexes where L =(NH3)4, (en)2, [l4]aneN4, or Me6[l4]aneN4 and A = CF, N02”, NCS”, CN~, N3 , or N1I3. They were able to obtain straight lines for these data by defining = log(k^/lt£]) [l4]aneN4* The Hammet equation is then log( k^/k^) ^ = J’l^A- Treating the same set of data by another method, they were able to study the eis effect, i.e ., the inplane ligand effect, by defining ajg = l°g(kjg/kj-^^^ and then plotting log(kL/k[i4]aneN4)A vs Again a straight line with f±\ close to unity was observed. These results demonstrate that the aquation of these complexes proceeds with the same mechanism. Since the aquation of cobalt(II]) amine complexes goes by an S^i mechanism, , the aquation process for macrocyclic complexes is therefore also dissociative in nature. Eade et al40 have suggested a correlation between the energy of a skeletal vibrational mode and the rate of aquation. They assigned inplane N-Co-N bonding frequency in the infrared spectra of [Co^(L)X2] where X = Cl“ or B r”, L = {tmd|, ,($-bn)2, (i-bn^, (NH3)4, (NH3)(diene), (e.n)(tmd), (pn)(tmd), (pn)2, (en)(pn), (3,2,3-tet) (en)2, and (2,3,2-tet), and compared them with the aquation rate constants by plotting log kj vs 6ncoN* T*ie rate constant increases as 6NqoN decreases. The data shows a reasonably good correlation between the rates and the ligand flexibility. Aquation of cis-complexes. — Results indicate that all the cis-compounds so far studied aquate with retention of configuration; that is, the product is the cis-chloroaquo complex. In general, a five-coordinate square-pyramidal intermediate is proposed for this reaction. cis-Dichloro complexes generally aquate faster than trans-compounds. Conceivably the electronic repulsion between these chlorides is responsible for the acceleration of rate. However, the trans isomer of [Co(en)2(NO^)Cl]+ aquates faster than the cis isom er, 17 since the large trans labilizing effect of the N02" group overshadows the cis effect. 16
-f* cis-[Co([i4]aneN4)Cl2] has been prepared and investigated by Poon and Tobe.41 The aquation of this cation proceeds via a three stage reaction path; the aquation to cis-chloroaquo complex ion, then further to cis-diaquo complex ion, and, last, the isomerization and anation to a mixture of trans-chloroaquo and trans-dichloro complexes. Further investigation42 of the last stage indicated + that the presence of cis-[Co([l4]aneN4)(OH)(H20)]2 in equilibrium with the related diaquo species is responsible for the isomerization. The aquation of the cis-dichloro cation proceeds some 15,000 times faster than that of the trans-isomer. This is due to the lower aH^ in the case of the cis isomer. Lower AH^ is usually observed for cis compounds. Kernohan and Endicott43 investigated the acid hydrolysis of the cis-carbo- nato complexes [CoLCOs] , where L = Meg[l4]aneN4 (structure VI) or Mee[l4]- 4, ii-dieneN4 (structure IX). The immediate product appears to be cis-[CoL- (H20)2]3 , which then rapidly isomerizes to the trans-isomer. In the case of the MeG[14]aneN4 complex, the isomerization is Cl~-catalyzed. Both of these compounds hydrolyze more slowly than the corresponding bis(ethylenediamine) complex, with the order being [Co(Me6[l4]aneN4)CO^]+ (2.82 x 10-3sec-1) « [Co(Me6[l4]-4, ll-dieneN4)C03]+ (2.35 x lO ^sec"1) « [Co(en)2CQ3]+ (2 x i02 sec-1) at 73° C. The difference in rate between these complexes has been attributed43 to the repulsion between the methyl groups and the fifth and sixth coordination sites of the complex. Some cis-dichloro complexes of linear tetradentate ligands (trien and 2,3,2-tet) have been synthesized and their kinetics investigated.24’26 fi-cis-[Co(2,3,2-tet)Cl2]+ reacts in two stages, aquation and isomerization. The final product of the isomerization is an equilibrium mixture of trans- m eso- [Co(2, 3, 2-tet)Cl?]+ and trans - me so - [C o( 2,3,2 - tet) Cl( H? Q) ]2 . While the B-cis complex of trien (structure XIV) aquates to an equilibrium mixture of 85$ of the B-c is diaquo and i£$ of the jS^cis-cldoroaquo complex, the reaction of a-cis-isom er (structure XIQ) produces 80$ of the diaquo and 20$ of the chloroaquo complexes with the diaquo ion isomerizing to some extent to the 17
more stable g^cis-isomer.
XIH XIV
*+* Hg.-2 induced aquation. — Bifano and Linck45 have studied the Hg(H)- n+ promoted aquation of cis- and trans-CoA^XCl where A A = en, A = NH3, X = NH3, py, H20, N3", or NC^“. If kHg2+ is the rate constant for the promoted reaction and k^p that for aquation, the graph of log kHg2+ vs log Icqj- is & reasonably good straight line with a slope of about 0.6. This indicates that the same structural factors are important in determining the activation free ener gies for the spontaneous and promoted aquation reactions. Steric studies48’47 on the induced aquation of some cis- and trans-bis- (ethylenediamine) complexes indicate a different stereochemical course from that of the spontaneous aquation. However, two lands of induced aquation, Hg2+ induced aquation on chloro-complexes and IINO^ induced aquation on azido- complexes, have been found to proceed via the same stereochemical pathway. Thus, the induced aquation mechanism involves a common five-coordinate intermediate; however, the spontaneous reactions do not always involve the same intermediate. 18
Base Hydrolysis
Base hydrolysis of cobalt(IIQ complexes, has been extensively studied. 10’56 Several mechanisms57’58, 59have been proposed, but the Sjvjl CB mechanism (substitution, nucleophilic, unimolecular, conjugate base) is best supported by the available experimental evidence and is therefore widely accepted. The Sj\jl CB mechanism involves the initial attack of base on an acidic hydrogen of an inert ligand, commonly an amine, to form the conjugate base of that ligand (an amide) which then promotes the aquation reaction (equation 4-7). + k i + [Co(NH3)5C1]2 + OH" £ [Co(NH3)4(NH2)C1] + H20 (4) fa st k
j . kcb [Co(NH3)4(NH2)C1] - [Co(NH3)4(NH2)]2 + Cl“ (5) slow
■prt [Co(NH3)4(NH2)]2 + H20 - [Co(NH3)4(NH2)(H20)]2 (6)
+ f a s t + [Co(NH3)4(NH2) (H20) ]2 [Co(NH3)5(OH)]2 (7)
The process involves a preequilibrium followed by the rate determining dissociation step, equation 5. The rate expression derived by applying the steady state approxim ation to [Co(NH3)4(NH2)C1] is
kcbkl f0H”^ [complex] Hate. , , i r/'-'vtt —i (®) k_i + kiLOH J k-j Under the preequilibrium condition K = so that
kc^K [OH- ] [complex] Rate = 1. + KlOH-j------(9> According to this mechanism, the complex must have an amine, or similar proton to form the conjugate base, in order to react by this mechanism and such complexes should be subject to general base catalysis. When kcjj (eq. 5) is comparable to the rate of formation of the amido conjugate base (kj) the 19 reactant should show less isotope exchange with D20 than the product. + + Studies50 on trans-[Co(bipy)2Cl2] , trans - [Co(/3-pic) 4C12] and trans- [Co- (tt-pic)4Cl2]+ indicate that amines without acidic protons are insensitive to base hydrolysis. Pearson et al51 investigated the effect of different bases on the sub stitution reaction of tr a n s - [Co(en)9(NQ>)Cl]+ with NC^~ in dry dimethylsulfoxide. Results show that the bases, OH-, or piperidine, catalyze the reaction to produce the same product, trans-[Co(en)2(N02)2] . Poon and Tobe52 demonstrated the enhanced isotope exchange for the amine proton of the product of the base hydrolysis of trans-[Co(D4-[l4]aneN4)Cl2]+. A comparison of the amount of exchange in the recovered unreacted complex and the product indicated some 20} more exchange had occurred in the product. A common phenomenon of the base hydrolysis of cobalt(IH)-amine complexes is the stereochemical change in the product (Table 6). Since an Sj^lCB mechanism is widely accepted for the reaction, a five-coordinate intermediate with trigonal- bipyramidal structure is generally ascribed to be the shape of the intermediate.56 The base hydrolysis of some tetradentate complexes has been thoroughly studied.53’54 Base hydrolysis for these complexes is considerably faster than that of the bis-bidentate complexes (Table 6). Ahmed and Tobe55 in a detailed study on the base hydrolysis of m eso-trans-[Co(2,3,2-tet)Cl2]+ found that the activating deprotonation occurs at the secondary nitrogen atom. Base hydro- *4* lysis of trans-d, j?,-[Co(2,3,2-tet) Cl2] produces the me so-trans form only, suggesting that inversion of the secondary nitrogen has occurred. The results of iso topic labeling experiments suggest that the rate determining step is the de- protonation of the secondary amine. For the d,£-2,3,2-tet complex is greater than 20 k_4 (equations 4 and 5). Hence, the base hydrolysis of the tetraamine complexes differs from that of the bidentate amine complexes in that the hydrolysis of the latter complexes proceeds with deprotonation being a preequilibrium step instead of being rate-determining as for the tetradentate species. Studies54 done in substituted pyridine buffers indicate that the base hydrolysis of the meso- and d,f-2, 3,2-tet complexes is susceptible to general Table 6 Rates for Base Hydrolysis of [Co(AA)2LCl]n+0 (AA)2 L k2(0° C) JvH sec"1 steric course
(en)2 trans-OH .017 6 (en)2 tra n s-C l 85 >95 (en)2 tra n s-B r 110 >95 (en)2 trans-NCS .35 24 (en)2 trans-N H 3 1.25 36 (en)2 trans-NC^ 0.080 94 (en)2 cis-O H .37 <3 (en)2 cis-Cl 15.1 63 (en)2 cis-Br 23 60 (en)2 cis-NCS 1.40 20 (en)2 cis-NH3 0.50 22 (en)2 cis-NH2CH3 0.17 (en)2 cis-NO^ 0.32 33 (en)2 cis-Cl i,oooa (en)2 trans-Cl 3,000a trien ^ -c is-C l 200, 000a cyclam tra n s-C l 67,000 100 meso-2,3,2-tet trans-Cl 61,000 100 aat2 5 ° C. Data taken from ref. 56. 2 'i base catalysis. Furthermore, the authors suggested that the enthalpy of acti vation can provide preliminary information on the rate determining step. A mixed N,S ligand complex trans-[Co(ETE)Cl?]+ (structure XV)30 gives k(base hydrolysis) = 1600 sec-1 at 13.5° C which is considerably slower than the tetraamine 2,3 ,2-tet ligands. This is not too surprising.since the ligand contains no secondary amine groups as does 2,3 ,2-tet. Another interesting complex, [Co(tren)(NH3)Cl]2+, has been synthesized and the base hydrolysis of this compound studied.60 Two isomers have been separated p- and t-
m
NH
£- [C o( tren) (NH3) Cl]2+ t_- [Co(tren) (NH3) Cl]2+
[Co(tren)(NH3)Ci]2+. The rates of base hydrolysis of these two isomers differ by a factor of i04 which cannot be explained by the small change in electronic structure or by the strain energy difference of 0.8 kcal mole-1. However, the geometry of the t-isomer is such that the tertiary amine is trans to the leaving chloride group. The only source for the formation of the conjugate base is from the hydrogens on the nitrogens cis to the chloride. On the other hand, for the p-isomer, amine hydrogens cis- and trans- to the chloride are available. The faster rate occurs when the intermediate is generated by deprotonation at the N atom trans to the leaving group; i.e ., for the p-isomer. 2 2
A nation
Anation reactions have not been as extensively studied as aquation and base hydrolysis reactions. The rates of anation of the aquo-amine cobalt(ID) complexes with different anions, Yn , in aqueous solution were found to be proportional to the concentrations of Y11 but to level off at higher concentra tions of Y° . The rate law can be expressed as
a[^n~][RIIoOm+l Rate = 1 + b[/"j (10)
The anation of [Co(NH3)5(H20)]3+with a s e ris of anions, S042 , Cl , SCN~, H2P04~, N3~ has been investigated.61’22*62 The data indicates that the reaction is dissociative in nature since the rate-range varies by only a factor of five for the series. This suggests that the rate is independent of the incoming group, indicating that anation does not involve an Sn 2 mechanism. In the cases of + — — anation of [Co(NH3)5(H20)]3 byN3~, H2P04", Cl- , and S042 anions, evidence for ion-pair formation has been obtained.61*22 The reaction mechanism is then
RH2Om+ + Y11- £ {RH2Om+,Y11-} (11) fa st
{RH2Om+, Yn_} ^ RY*m ~"*+ + HzO (12) slow
kYK [^1“][RH2Om+] Rate ------(13) 1 + K t^ 1-]
However, data for the anation of trans- and cis_-[Co([l4]aneN4)Cl(H20)]2 with Cl" and NCS" as the anating species are better explained by a disso- eiation-competition mechanism, equations 14 to 16. . ‘ 23
m+ ^ m+ RH20 - R + H20 (14) slow
1Y1 ~f* k *** -i m + R + H20 - RH20 (15) fa st Rm+ + V1' \ Rli(m-")+ (16) fast
1X1+ where R is a five-coordinate intermediate. Although substitution reactions of octahedral cobalt(IH) complexes in aqueous solution are classified into three categories according to their products, they can be considered as one general reaction from a mechanistic point of view.6 The generalized reaction can be written as: K eq Complex + Reagent £ reactive species (17) fa st I?(Sn 1) reactive species -* product (18) slow , . , „ . kK[reagent][complex] lor which Rate - T+ <19)
In the limiting cases when K[reagent]» 1 Rate = ktcomplex] (20) and when 1 » K[reagent] Rate = kK [reagent] [complex] (21) 24
Statement of the Problem
Since some of synthetic macrocyclic complexes of cobalt(IIl) have exhibited unusual behavior, such as slow aquation rates, fast base hydrolysis rates, etc., a more extensive investigation of complexes of this class appeared to be needed. Since investigations other than kinetic studies had shown that ring-size can affect the properties of complexes greatly, the cobalt(IH) complexes of the fully saturated, substituent free macrocycles [i3]aneN4, [i5]aneN4, and [i6]aneN4 have been investigated in the studies reported here. These studies provide specific information with respect to the effect of ring size on kinetic behavior. The cobalt(III) complexes [Co(MAC)X2]C104 have been prepared and their spectral and electrochemical properties have been examined. Martin et al2 xy have found that the in-plane ligand field strength, Dq , varies with the ring XV size, the order being Dq [i3]aneN4> [i4]aneN4> [i5]aneN4> [i6]aneN4. Conversely, Dq , the axial ligand field strength for the same coordinating anion, decreases in the reverse order. 1 The reduction potentials of the trans- [Co- (MAC)C12]C104 complexes are in the o rd er I3 ~ i4 < i5 < 16 with 16-m em bered macrocyclic complexes being the easiest to reduce. It has been the goal of the present work to investigate the aquation reaction of these complexes, "h [Co (MAC)C12] and to correlate the results with other properties of the complexes. The most general question raised in these studies is Mhow does the ring size of the inert macrocyclic ligand affect the stereochemistry, kinetics, and mechanism of aquation reactions of cobalt(ffl) ?M EXPERIMENTAL
M aterials.
All the chemicals used for synthetic purposes were reagent grade. No additional purification was performed. Triethylenetetraamine, N, N'-bis(3- aminopropyl)-i,3-diaminopropane and 1,3-dibromopropane were purchased from Eastman Organics. N, N'-bis(3-aminopropyl)-i, 2-diaminoethane was prepared according to the procedure of Barefield. 16 p-Toluenesulfonylchloride was purchased from Eastman Organics. Dowex 1X8 200-400 mesh anion exchange resin in the chloride form and AG 50W-X2 100-200 mesh cation exchange resin in hydrogen form were purchased from Bio Rad Labs. [i2]aneN4 was a gift from Dr. T. Atkins.
Physical Measurements.
Infrared Spectra. — Perkin-Elmer Models 337 or 457 spectrophotometers were employed to record infrared spectra. Samples were run as Nujol mulls or as KBr or Csl pellets. Polystyrene standards were used to calibrate spectra. Conductivities. — Conductivity measurements were made with an Industrial Instruments, Inc. Model RC 16B conductivity bridge on~ i0“3 M solution at room temperature. A cell with cell constant of 0.1 was used. Magnetic Moments. — Magnetic measurements were performed on solid samples by the Faraday Method with a Cahn electrobalance and a Varian electromagnet operating at a current of 3.5 amps. Measurements were made at room temperature under helium at a pressure of 35 mm. The system was calibrated with Hg[Co(NCS)4], Diamagnetic corrections for the ligands were made with Pascal's constants.23
25 26
Electronic Spectra. — A Cary Model 14R spectrophotometer was used to record the electronic absorption spectra. Solution spectra were obtained in either one or five cm cells at room temperature. Elemental Analyses. — Most elemental analyses were performed by Galbraith Labs, Inc., Knoxville, Tennessee. Some nitrogen analyses were obtained by Mr. W. Sehammel at The Ohio State University using a Coleman Model 29 Nitrogen Analyzer'.
Syntheses
[13, !5andi6]aneN^. — These ligands are prepared by a modification of the procedures of Martin.1 Four steps are involved to synthesize the ligands 1,4,7, iO-tetraazacyclotridecane, [l3]aneN4; 1,4,8,12-tetraazacyclopenta- decane, [l5]aneN4; and 1,5,9,13-tetraazacyclohexadecane, [l6]aneN4. 1. Tosylated linear tetraamines. — N,N',N",N,n-tetra-pi-toluenesul- fonyl derivatives of triethylenetetraamine, (2,2,2),N,N,-bis(3-aminopropyl)- 1,2 -diaminoethane(3,2,3), and N, N'-bis(3-aminopropyl) -1,3-diaminopropane (3,3,3) were starting materials for the [13,15, and l6]aneN4 macrocyclic rings, respectively. To a solution of the appropriate linear tetraamine (0.1 mol) and sodium hydroxide (0.4 mol) in water (100 ml), there is added dropwise, a solution of p-toluenesulfonylchloride (0.4 mol) in ether (400 ml). The mixture is then stirred for an hour at room temperature. The tosylates separate as white solids and are recrystallized from large volumes of metha nol (1 1.). Yields are 80-90$. 2. N,NLN'L N'^-tetrafo-toluenesuIfonyl) Derivatives of [13,15, and 16]- aneN,t. — Two equivalents of sodium ethoxide are added to ~50 g of the tosylated linear tetraamine in 200 ml of boiling ethanol. After boiling for 20 minutes, the ethanol is removed by rotary evaporation to yield the disodium salt of the tosylated linear tetraamine. A 0.1 M solution of the sodium salt 27
in DMF is heated to ~110° C, and one equivalent of 1,3-dibromopropane (0.2 M in DMF) is added dropwise over a period of one hour while vigorously stirring the solution. The volume of DMF is then reduced to one-fourth the initial volume. Then a volume of water equal to 1-2/3 that of the initial DMF volume is slowly added to yield a tacky off-white precipitate. The product is recrystallized from hot benzene. However, in the case of 16-membered ring derivatives, not all of the tosylated [l6]aneN4 goes into benzene. A white product is precipitated by reducing the volume of the benzene and adding ethanol followed by letting it stand at room temperature for one hour. Yields vary from 40-60/ with the yield of the tosylated [i3]aneN4 being the lowest (40/). 3. Hydrolysis of tosyl groups. — The tosylated [13,15, and l6]aneN4 is dissolved by heating it in 30/ hydrobromic-acetic acid (lg/50 ml), which is prepared by adding 9 volumes of glacial acetic acid to 16 volumes of 47/ hydrobromic acid. The mixture is refluxed for two days except for the tosy lated [l6]aneN4 which has low solubility and consequently is refluxed for four days or until all of the material appears iridescent. The volume of the solution is reduced to one-tenth its initial volume. After cooling, ether-ethanol (1:1) is added to separate a solid which is washed repeatedly with ether and ethanol. The product is air-dried on the frit. Yield, ~90/. 4. Free ligand extraction. — The tetrahydrobromide salt of [13,15, and l6]aneN4 is dissolved in a minimum amount of water and is neutralized with a slight excess of sodium hydroxide. Several extractions with chloroform are then carried out. The chloroform is rotary evaporated, ether is added and then rotary evaporated leaving behind the solid ligand. Sometimes, especially in the case of [i3]aneN4, refrigeration is required to bring about solidification of the ligand. White needles are obtained. Yield, 70-90/. cis;- and trans_-[Co([i3]aneN4)Cl2]Cl. — Cobalt(H) chloride • 6-hydrate (1.0 g) and a slight excess of [l3]aneN4 (0.8 g) are warmed in methanol for ten minutes, conc. IlCl is then added dropwise to the solution until the red-brown 28 solution turns green (about 1 ml). Air is bubbled through the solution for two hours and the volume of solution is reduced. At this point green crystals separate. The green trans-compound is recrystallized from methanol. Further reduction of the volume of the original filtrate results in isolation of the red cis-compound. Combined yield: 50$ . Anal. Calcd. for [ColCgH^N^cyci: C, 30.75; H, 6.3i; N, 15.94; Cl, 30.25; Co, 16.76. Found: trans-compound: C, 30.52; H, 6.24; N, 15.66; Cl, 30.99; Co, 16.53; cis-eompound: C, 30.70; H, 6.85; N, 15.70; Cl, 29.78; Co, 16.96. The nitrate salt of the trans isomer is obtained by adding ammonium nit rate to the methanolic solution of the chloride salt. Green needles are obtained. Anal. Calcd. for [Co(C9H22N4)Cl2](N q): C, 28.59; H, 5.86; N, 18.52; Cl, 18.75; Co, 15.58. Found: C, 28.72; H, 5.88; N, 18.44; Cl, 18.69; Co, 15.66. "J" trans-[Co( [!5]aneN^) Cl? j complexes (isomers I and 10. — Because of the cobalt(H) contamination, the perchlorate salt is synthesized and then changed to other salts. Cobalt(H) chloride 6-hydrate (1.0 g) and free ligand [l5]aneN4 (0.9 g) are warmed in methanol for ten minutes. Cone. HC1 is added dropwise and air is then bubbled for at least two hours. Several drops of perchloric acid is added to precipitate the product as the perchlorate salt. The perchlo rate salt is recrystallized first from acetonitrile and then from water. At this point, the product is about 95$ pure isomer I (brown in color). The brown perchlorate salt is dissolved in acetonitrile and passed through a Dowex IX 8 200-400 mesh, anion exchange column (Cl- form) at a rate of 10 sec/drop. The eluent is concentrated by rotary evaporation. Some green solid (isomer I® precipitates. This is filtered and the volume of the filtrate is further reduced in order to precipitate the tan isomer (D as the chloride salt. The tan chloride salt is dissolved in methanol and ammonium nitrate is added to produce crystals of the nitrate salt. 29
The perchlorate salt of trans_-[Co([l5]aneN4)Cl2] is dissolved in acetonit rile containing several ml of water, and warmed with lithium carbonate with stirring for two hours. The solution turns violet. The excess lithium carbo nate is removed by filtration. Cone. HC1 is added dropwise (2 ml) and the solution is warmed for thirty minutes. At this time the color has changed to green. The solid product, which is some 90$ isomerically pure green isomer, is dissolved in acetonitrile and passed through a Dowex 1X8 (200-400 mesh) anion exchange column (Cl- form). The chloride salt of green isomer (10 is obtained by reducing the volume of the eluent. The nitrate salt can be obtained in the same way as described for isomer I. Anal. Calcd. for [CofCuHaeN^cyNQ: C, 32.53; H, 6.45; N, 17.24. Found: C, 31.98; H, 6.55; N, 17.37 (isomer 0. Anal. Calcd. for [CotCuH^N^C12]C1: C, 34,80; H, 6.90; N, 14.76. Found: C, 34.38; H, 7.24; N, 14.41 (isomer D). trans_-[Co([l6]aneN4)Cl2]C104 (isomers I and H). — Five-tenths gram of [l6]aneN4 is dissolved in methanol to which is added 0.52 g of cobalt(II) chlo ride 6-hydrate. The resulting magenta solution is warmed for 20 min, a slight excess of lithium perchlorate is then added and the solution is warmed for another ten minutes. The methanol is then removed by rotary evaporation and the solid is redissolved in chloroform and filtered. Evaporation followed by dissolution in chloroform is repeated once more. Bromine is then added drop- wise to the chloroform solution until the magenta color is completely gone (several drops) and brown solids separate from the solution. The product is filtered and washed with chloroform. The brown product is dissolved in hot methanol to which a slight excess of lithium chloride is added. Green isomer II is obtained from the cooled solution. Brown isomer I is obtained by reducing the volume of the filtrate. Both are recrystallized from acetonitrile. Combined yield: 50$. Anal. Calcd. for [CofC^I^gN^cyClQp C, 31.49; H, 6.17; N, 12.24. Found: C, 31.28; H, 5.92; N, 12.04 (isomer 0- C, 31.40; H, 6.20; N, 12.21 (isomer 10. trans-[Co([l33aneN4)Cl(H20)](PF6)2. — trans_-[Co([l3]aneN4)Cl2]Cl is dissolved in 0.1 M trifluoromethanesulfonic acid. Ag(CF3SC>3) is added in 2:1 ratio (Ag(CF3S03) :trans-compound). The solution is then stirred for four hours at room temperature. After the addition of ammonium hexafluorophosphate, the solution is filtered over filter aid and is freeze-dried to 2 ml. The product is recrystallized from acetone. The yield is low. Anal. Calcd. for [Co(C3H22- N4)C1(H20)](PF6)2: N, 9.52. Found: N, 9.65. trans-[Co([l3]aneN4)Cl(NQ1)]C104. — This compound is obtained in the same way as described for toans-[Co([l3]aneN4)Cl(H20)](PFG)2 except 0.1 N nitric acid and silver nitrate are used. NH4PF6 is also replaced by LiC104. Green crystals are obtained from recrystallization in acetonitrile. Anal. Calcd. for [Co(C9H22N4)C1(N03)JC104: C, 24.45; H, 5.02; N, 15.84. Found: C, 24.58; H, 5.07; N, 15.72. cis-[Co([l3,14,15, l6]aneN4)CQj]C104. — The perchlorate salt of the appropriate compound trans_-[Co(macroeycle)Cl2] is dissolved in water and warmed with lithium carbonate for several hours. The excess Li2C03 is filtered and the filtrate is concentrated to obtain the product. The product is recrystal lized from methanol. Anal. Calcd. for [C©(CgH^N,^COjlClC^ (orange): C, 29.68; H, 5.48; N, 13.84. Found: C, 29.61; H, 5.11; N, 13.66. Calcd. for [Co(C10H24Nli)CQ]ClO4 • 2H20 (pink violet): C, 29.06; H, 6.2i; N, 12.32. Found: C, 28.83; H, 5.90; N, 11.66. Calcd. for [Co(C11H26N4)C03]C104 • 3/2H20 (violet): C, 31.35; H, 6.36; N, 12.19. Found: C, 31.45; II, 6.18; N, 12.15. Calcd. for [Co(C12H28N4)CC^]C104 (red-violet): C, 35.11; H, 5.89; N, 12.60. Found: C, 34.74; H, 6.43; N, 12.50. cis-tCod^laneN^Cl^Cl. — Five-tenths gram of [l2]aneN4 and 0.7 g of CoCl2 • 6H20 are warmed in methanol for ten minutes. The solution is acidified with conc. HC1 and air is bubbled through it for one hr. Purple crystals separate and are recrystallized from methanol. Yield, 70$. The nitrate salt is obtained by adding NH4N03 to the methanolic solution. Anal. Calcd. for 3 i
[Co(C8H20N4)Cl2]Nq: C, 26.39; H, 5.54; N, 19.23. Found: C, 26.24; H, 5.21; N, 19.36. -J" Attempts to synthesize cisj[Co([i5, i6]aneN4)Cl2] . — cis-[Co([l5, l6]ane- -j* N4)COs] was acidified in water with conc. HC1 and warmed for thirty minutes. At that time, the solution turned green. The product was trans-isomer.
K inetics.
The aquation of trans-[Co( [13,15, l 6]aneN4)Cl2]+ and cis-[Co([l2,13]- aneN4)Cl2] was investigated at five different temperatures over a range of 20° C in 0.1 N IINOj, ionic strength, 0.1. The acid solution was prepared by diluting the concentrated acid with boiled demineralized double distilled water and was standardized against Na2C03 withbromcresol green as indicator. Speetrophotometric method. — The Cary Model 14R spectrophotometer was used to follow the change in absorption during the aquation of cis- and trans_-[Co(macrocycle)Cl2]+ complexes, except for the [l6]aneN4 derivatives. Cells with path lengths of 10 cm were used for the reactions of the trans -comp- lexes while 5 cm cells were used for the aquation of the cis-complexes. The reaction was followed by setting the wavelength at an optimum value and the absorption at this wavelength was then recorded as a function of time by running the chart at a known speed. The optimum wavelength either corresponds to the isosbestic point of the second successive reaction, where there is no complication due to the second reaction, or to the wavelength where the change of absorption is the greatest. The first aquation reaction was followed at two different wavelengths (Table 7). 32
Table 7 Wavelengths Where the Aquation Was Followed. Complex wavelength, nm trans-[Co([i3]aneN4)Cla] 560, 620 trans.-( 1) -[Co([i5]aneN4)Cl2]+ 400, 688 transr(n)-[Co([l5]aneN4)Cl2]+ 560, 670 cis~[Co([i2]aneN4)Cl2]+ 340, 590 cis-[Co([i3]aneN4)Cl2]+ 340, 490
The solid sample of complex was weighed in the spectrophotometer cell. 0.1 N acid solution was brought to reaction temperature and reaction was initiated by injection of the acid solution into the reaction cell. Temperature was controlled within + 0.1° C by circulating the water from a constant temp erature bath through the compartment surrounding the cell. Kinetic runs were carried out at five different temperatures over a range of 20° C. Graphs of ln(Aoo - Af) vs time were straight lines for at least two half-lives for all the reaction investigated. Enthalpies and entropies of activation were calculated from the rate constants using Arrhenius equation. The aquation of trans_-[Co([i6]aiieN4)Cl2]C104 was followed by stopped-flow method. A Durrum-Gibson Model 110 stopped-flow apparatus interfaced with a Nova mini-computer for data acquisition, storage, and evaluation was used. The instrument was fitted with Kel-F components and a 2 cm light path. Acetonitrile used in the stopped-flow measurements was dried over 4A Linde molecular sieves and then distilled. Two solutions, one containing the complex dissolved in acetonitrile and the other containing 0.2 N HNOj, were rapidly mixed in the mixing chamber of the instrument and the reaction "I" was followed at 680 nm for isomer I of toansj[Co([l6]aneN4)Cl2] and 670 nm for isomer n of the same complex. Rate constants were calculated by a curve fitting method. 33
Titrimetric studies of chloride release. — Weighed samples were mixed with the acid solution which had previously been brought to the reaction tempera ture, and placed in a water bath at the reaction temperature. Samples were withdrawn at appropriate times and quenched with ice. Each sample was then passed through cation exchange resin in the acidic form and eluted with ice-cold water. The eluent was titrated with AgN03 solution using a Corning Model 12 pH meter and a silver wire indicator electrode and Calomel reference electrode. Alternatively, the reaction mixture was titrated directly after quenching without passing through the cation exchange resin. The results were identical. The chloride concentrations were also determined by Volhard’s method. Reactions with mercuric ion. — A weighed amount of mercuric acetate was + dissolved in 0.1 N nitric acid to make a 0.01 M Hg2 solution. This solution was preequilibrated in the water bath and was injected into the reaction cell which had previously been loaded with a solution of. the complex. The spectrum was scanned after ca. 10 sec. to obtain the spectrum of the aquoehloro complex. The isosbestic points for the second sequential reaction were located by repeatedly scanning the spectrum during the course of long reaction times. PART I RESULTS AND DISCUSSION
Cobalt(IQ) complexes with fourteen-membered macrocyclic ligands have displayed some unusual kinetic properties, such as slow aquation rates and fast base hydrolysis rates. Therefore, it is important to extend the investigation to compounds with different sizes of the rings and to study how the rates of aquation vary with ring size. The spectrochemical studies of trans-[Co(MAC)C12]C104 XV and trans-[Ni(MAC)C12] have shown that the Dq' values increase as the ring becomes smaller, and strain energy calculations3 show that the 14-membered ring fits cobalt(III) best and that increased strain occurs as the ring size increases or decreases. Electrochemical studies4 on toans-[Co(MAC)Cl2]C104 also show a trend with half-wave potential for the Co(in)/Co(Il) couple in the order [i3]aneN4 ~ [l4]aneN4 > [i5]aneN4 > [l6]aneN4, with trans-[Co([ 16]aneN4)Cla]- C104 being easiest to reduce. Furthermore, studies5 on the dissociation of the ligand from [Ni(MAC)KCIO^ in acidic media show that the rate varies tremendously with ring size, [i6]aneN4 » [l5]aneN4 > [l3]aneN4 » [l4]aneN4. Therefore, it is to be expected that the rate of aquation of the complexes, *{* [Co (IvIAC)C12J would display a meaningful trend. The chloride or nitrate salts ■j* of trans-[Co([l3, l5]aneN4)Cl2] , and cis-[Co([l2, lSlaneN^Cl,] and the salt trans-[Co([l6]aneN4)Cl2]C104 have been synthesized in order to study the approp riate aquation reactions. During the study, two conformational isomers of trans-tCo^iSjaneN^Cl^ and trans-[Co([l6]aneN4)Cl2] have been separated “f" and the isomerization of isomer II to isomer I of trans-[Co([l5]aneN4)Cl2] has been studied. A series of cis-carbonato complexes, cis_-[Co([l3- l6]aneN4)CC^]- C104 has also been prepared in order to study the ring size effect on the cis-
34 35
complexes. In this chapter, the results of these studies are reported and discussed. The section is divided into two major parts. The first covers the synthesis and characterization of the complexes and the second, the kinetic studies on the complexes. In section I, the infrared and electronic spectral properties and electrochemical properties of these complexes are reported and discussed. Also, the configurations of the isomers of trans- 4 4* [Co([i5]aneN4)aneN4)Cl2] and trans - [Co( [16] aneNJCl?] are proposed. In section II, the aquation reactions of trans-[Co([i3,15, l6]aneN(1)Cl2]+ and cis_-[Co([l2, l3]aneN4)Cl2]+, the isomerization of trans-lCodlSjaneNJ- (I-I20)2]3+ to cis-diaquo complex, and the isomerization of trans-(H)- 4* [Co([l5]aneN4)Cl(H20)]2 to isomer I are reported and discussed.
Synthesis and Characterization of Complexes
A series of complexes of the type trans-[Co( MAC) Cl? ]X where X = Cl- , NO3-, or CIO4- , and where MAC = [l3]aneN4, [l5]aneN4, or [i6]aneN4, have been synthesized. The trans-complexes with [l5]aneN4 and [l6]aneN4 have been shown to form conformational isomers and these were separated as discussed in the experimental section. £is-[Co(MAC)C03]C104 where MAC = 4" [ l3-l6]aneN4 and cis-[Co([l2, l3]aneN4)Cl2] have also been synthesized. 4* 4 trans-[Co(MAC)Cl2] complexes. — The complexes, trans-[Co(MAC)Cl?3 , were generally prepared by the reaction of cobalt(Il) chloride 6-hydrate with the free ligand in methanol or water, followed by aerial oxidation. Because of cobalt(H) contamination, it was not possible to prepare pure trans-[Co([l5]aneN4- C12]C1 directly, even in the presence of excess ligand, nor could the pure compound be obtained from the reaction of Na3[Co{CG3)3] • 3H20^63^ with the ligand hydrochloride salt. Therefore, the chloride salt was prepared from the analogous perchlorate salt by anion exchange column chromatography. tram-[Co([l3]aneN4)Cl(H20)](PF6)2 and trans-[Co([l3]aneN4)Cl(N03)]C104 were also prepared in order to augment the studies on the steric course of the 36
Table 29 Molar Conductivities cl of the Complexes trans-[C o( M A C) C12 ]X.
Complex solvent Am ohm-1cm2mole-1 trans-[Co([l3]aneN4)Cl2]NO} m ethanol 77 tra n s (3) -[Co( [l5]aneN4) C12]NOj m ethanol 84 tra n s(ID -[Co( [!5]aneN4) C12]C1 nitromethane 81 trans(l) -[Co( [l6]aneN4) C12]C104 acetonitrile 132 transJU)-[Co( [l6]aneN4) C12]C104 acetonitrile 145 tra n s-[Co( [l3]aneN4) C1(N03) ]C104 acetonitrile 152 w ater 282
Concentrations are ~i.O x 10"3M, 25° Table 8 Selected Infrared Spectral Bandsa for trans-[Co(MAC) Cl2 ] X.
Complex VN-H cm " ‘ v NOT c m ~ 1 trans.-[C o( [ 13 ] aneN^ C12 ]C 1 3155 S , s p tr a n s - [Co([l3]ajieN/i)Cla]NClh 3268 s ,s p 1350 s, 1050 s,825 mw tra n s(l) -[Co( [i5]aneN4) C12]NOj 3200 s, sp 1360 s , 1040 s,826 m tra n s(II) -[Co( [ l5]aneN4) C12]NC^ 3 160 s ,s p 1370 s , 1055 s,823 m trans(X>-ICo( t l 6]aneN4) C12]C104 3236, 3175 s,sp tran s(II) -[Co( [lGjaneN^ C12JC104 3247(sh), 3174 s, tr a n s - [Co( [l3]aneN4) CKNO,) ]C104 3257 s, sp 1506 s, s p ,1263 s, sp
S i KBr pellets, abbreviations; s = strong, vs = very strong, ms = medium strong, m = medium, sh - shoulder, sp = sharp, w = weak. 38
aquation reaction of trans-[Co([l3]aneN4)Cl2]+„ Elemental analyses of the complexes confirm their formulas. They are all diamagnetic with magnetic moments of 0.4 ~ 0.6 B. M. Molar conductivities (Table29) indicate that the complexes behave as 1:1 electrolytes in solution. The expected range for a 1:1 electrolyte in methanol is 75-110 ohm-1cm2mole-1; 145-160 ohm-1cm2mole-1, in acetonitrile, and 75-95 ohm“1cm2mole-1, in nitrome thane.44 The infrared spectra of the nitrate salts show a strong sharp N-H stretching peak around 3200 cm-1 with a broad tail at lower energies. In some cases, the N-H peak appear as a doublet, indicating some hydrogen bonding interaction between the N-H groups and the uncoordinated nitrate anion. The free anion, N03", absorbs at~l380 cm-1 (V3), ~1050 cm-1 (vi), and~826 cm-1 (v2)64 in the spectra of the [l3]aneN4 and [i5]aneN4 derivatives. That the N03~ group acts as a monodentate anion in trams-[Co([l3]aneN4)Cl(N03)]C104 is confirmed by the infrared spectrum . Strong absorptions a t 1506 cm -1 and 1263 cm -1 are a ttri buted to the asymmetric stretching mode of the coordinated nitrate ion (C2 v. sym m etry) . 27 [Co(TAAB)(N0 3)2]N03 exhibits peaks at 1510 cm -1 and 1256 cm -1 for the unidentate anion.65 Although the splitting of the asymmetric stretching mode of the NO^~ group in [Co([l3]aneN4)Cl(NO^)]+ is large, it is evident from the electronic spectrum and from other regions of the infrared spectrum that this compound exists in the trans configuration with the nitrate group serving as unidentate ligands. The position of the N-H absorption for the trans- complexes usually varies with changes in the counter anion. The perchlorate and nitrate salts display vn-H at higher energies than does the chloride salt (Table 8). Poon66 has shown that cis- and trans-[Co([i4]aneN4)X2]Ycomplexes can be distinguished from each other by a comparison of their infrared spectra in the CII2 rocking region at 800-910 cm-1. The cis-[l4]aneNA complexes display at least five bands spread fairly evenly between 800 and 910 cm-1, while the trans-[!4]aneN4 complexes show two bands near 900 cm-1 and one near 810 J ______I------1------1— I i______t______i______i i 600 500 400 250 600 500 400 250 cm1
Figure 29. Far-Infrared Spectra of_tran£-[Co(MAC)X2]+Complexes
A. trans-[Co(f 13]aneN4)Br2l C._trans-[Co([l4]aneN4)Br2] + + B. _trans^-[Co([l3]aneN4)Cl2] D. trans-[Co([l4]aneN4)Cl2] 40
E
600 500 400 250 cm-i
600 500 400 cm Figure 29. Cont'd + E.trans-[Co([l5]aneN4)Br2] F.trans(l)-[G o([i5]aneN 4)Cl2] _|_ G. trans(ll)-[C o([l5]aneN4)Cl2] 41
J i 600 500 400 250 cm1
Figure 29. Cont'd
H. _trans(n)-[Co([l6]aneN4)Cl2]C104 I. _trans(l)-[Co([l6]aneN4)Cl2]C104 42.
cm-1. It has also been suggested 67’ 08 that the Co-N stretching modes which occur in the 500-600 cm -1 region can be used to distinguish between cis- and trans isomers. cis-Complexes have at least four bands in this region while trans complexes have no more than three bands. Therefore, the absorption peaks for the trans-complexes in these regions are listed in Table 9 and in other parts of this chapter, these features will be compared with the spectra of the cis-complexes, mainly the carbonato complexes. The metal-chloride antisymmetric V 0 O_ d vibration, which usually occurs between 300 and 400 cm-1, ^ . has also been assigned (Table 10) by comparing the spectra of the corresponding dibromo- and dichloro-complexes in this region (Fig. 29). In general, this band is medium strong and sharp compared to other bands which occur in this region. An attempt has also been made to assign the in-plane ( S]\jmn) ou^ plane (r^MISf) deformation modes.(Table 10). These tentative assignments follow those reported for trans-[Co(NIi 3)4Cl2] (69) and considerations of band intensities and the flexibilities of the macrocyclic ligands. Because of the nonflexibility of macrocyclic ligands, the in-plane bending mode should be restricted, hence it should occur at higher energies than the out of plane mode.
ttnmn
Figure 23 Selective Infrared Active Skeletal Vibrations of trans-[Co(MAC)Cl2]+. vCo~Cl’ ttmnM> ^ d %MN 3X6 presented in Table 10. While the structural dependences of these vibrational modes are not easily rationalized in detail, one general observation may be of substantial significance in imderstanding the 43
Table 9 *J* Infrared Spectral Bands for trans-[Co(MAC)Cl2] Complexes in CH2-Rocking and Metal-Nitrogen Stretching Regions.
Complex CH9 rocking modes v M—n aiodes J ra n s - [C o( [ 13 ]aneN4) C1 2 ]C 1 825m, 862s, 880m 5l2m, 539m, 550sh, 565n trans.-[Co( [l3]aneN4) C1 2]NC^ 820m, 876s, 893m trans-[Co( [l 4 ]aneN 4) C12]C1 818 s , 888 s,906s 505m, 52 lw , 558w a trm is-fCo([l4]aiieN, 1)Cl?3NO/ 8 l 8 s, 888 s, 905s trans(!Q-fCo([l5]aneN 2)Cl23NQ? 8 l3w, 837w, 892m, 882w 537w, 550w trans(I3) -[Co( [lSjaneN^ C1 2]NC^ 8l5w, 83 iw, 837w, 875m 539w, 560w trans(I) -[Co( f.l 6]aneN4) C 12]C104 893m trans(I3)-[Co([l6]aneNa)Cl 2]C104 806m\v, 873sh, 893ms 575w, 590w tra n s-[Co( [l3]aneN4) Cl(NO,) ]C10 4 793w, 820m, 873ms, 893m 511m, 540w, 550w, 560m uRef. 66. Table 10 •p Far-Infrared Data for the Complexes trans-[C o (MAC)C19] .i
MAC vco-ci asy c 111" 1 TTNMN cm_1 6NMN cm ~ [l3]aneN 4 338 290 301 (sh) [l4]aneN 4 340 306 280 [i5]aneN4, isomer I 332 282 301 (sh) [l5]aneN4, isomer II 338 280 [ l6]aneN4, isomer I 360 282 [ i 6]aneN4, isom er II 367 284 325
Spectra were obtained using Csl pellets. 45
bahavior of macrocyclic complexes. The data of Table 10 show that ttnmn is a maximum for [i4]aneN 4 while 6NMN is a minimum for the same species. This correlates with the demonstrated fact that only the 14-membered ring + encloses the Co 3 ion in a strain-free fashion. The vibrational data appear to indicate that it is most difficult for the metal to vibrate out of the plane when the ring fits best and that it is easiest for the metal to vibrate in the plane when the ring fits best. Thus, while the vibrational frequencies do not show a quan titative correlation with the strain energies discussed in the introduction, a qualitative relationship can be inferred. The two isomers of Jrans-[Co([l5]aneN 4)Cl2]NOj display in frared spectra distinctively different from each other (Fig. 3), while the two isomers of trans-[Co([l 6]aneNA)Cl 9 ]ClQ/1 do not (Fig. 4). The properties of the isomers will be discussed subsequently. The trans-eobalt(III) complexes discussed here are examples of strong ligand field, low-spin complexes of symmetry no higher than D ^. The energy diagrams for Oh and D4h symmetry are shown in Figure 5.
29 6Ds-5/4Dt
16B
*T, 2g |35/4D t
]ODq-C 10DqXy-C >9 ’g ° h D4h r F ieure 5 The Energy Diagrams for Low Spin d 6 Oh and D4h Symm etry. Table 11 VJ+ r\ Electronic Spectral Data for trans-[Co(MAC)XY] .
Complex - *Alg(kK) % s - *Alg (kK) (*: !g +IB2g)« - Dq^cra-1 DqZcm -1 • (kK) ------trans_-[C o( [13 ] aneN^ C1 2]C1 16.67 (33)b 23.70 (114) m asked 2760 1332 16.39 (33)° 23.42 (130) m asked 2722 1316
tra n s - [Co( [l 4 ]aneN 4) C12]C1 16.00 (40) 21.00 (30) ~25.00(sh) 2480 1479 15.87 (30.2)C ~22.72(sh) m asked 2652 1282
tran s(I) - [Co( [l5]aneN4) Cl 2]NO^ 15.27 (40)b 19.23 (69) 23.70 (106) 2303 1511 15.13 (36)C 19.42 (67) 23.26 (87) 2322 1464
trans(H) -[Co( [l 5]aneN 4) C12]C1 15.63 (27)b 20.41 (38) 24.10 (62) 2421 1465 15.29 (36)C 20.41 (54) 23.81 (64) 2421 1397
tra n s ( I)—[Co( [16] aneN4) Cl 2 ] C104 14.73 (57)d 18.69 (34) 23.26 (99) 2249 1457
trans(Il)-[Co( [lelaneN^ C1 2]C104 15.11 (4i)d 19.61 (35) 23.87 (76) 2341 1441 J trans-[C o( [lSlaneN^ CKNQ,) ]C10 4 17.09 (32) 23.98 (128) m asked 2778 1400 I7.24e 24.10 m asked 2790 1418
trans-[Co([l 3 ]aneN/i)Cl(H 9 Q) ](PF6)2 17.24 (30)6 24.10 (122) m asked 2790 1418
sl b e d © Absorptivity in parentheses. In methanol. In conc. HC1. In acetonitrile. In 0.1 N HNC^. 47
Two d-d transitions are expected for true octahedral complexes (*T4g ♦- *A4g and *T2g ♦- *A4„); however, when the symmetry is lowered to D4h the *T4g and lT2g states both split into two states. Hence, more than two bands are expected.
Usually, three bands are observed; i.e ., ^ g 3- ♦- XAig> i^2g «- ^ig and (*Egk + *B 2g) ♦- ^jg . The ligand field strength parameter lODq can be calculated70 according to the following equations.
lODq*7 = v. + C (22) 2g
Dt = - a ^ E g 3 - V (23)
DqZ = Dq*7 - 7/4Dt (24)
C is roughly constant and usually assumed to have the value 3800 cm-1. n+ The electronic spectral absorption maxima for trans-[Co(MAC)XY] are presented in Table 11. The in-plane ligand field strength decreases as the macrocyclic ring size increases. The Dq value for the [l 6]aneN 4 complex XV is smaller than that for the corresponding [i5]aneN 4 complex. The Dq values of isomer II of trans_-[Co([i5]aneN 4)Cl2]+ (2421 cm-1), and of isomer II of trans-[Co- ([l6]aneN 4)Cl2]+ (2341 cm-1) are larger than those of their corresponding isomer I, (2303 cm -1 for complexes of [l5]aneN4, 2249 cm -1 for complexes of [l 6]aneN4). This could imply that the "effective ring size" is different in the two isomers of each of trans_-[Co([l5]aneN 4)Cl2]+ and trans^[Co([i 6]aneN 4)Cl2]+ with isomer II of each of these complexes acting as a smaller ring and therefore, causing a stronger X V X V metal-nitrogen interaction; i.e ., larger Dq J values. It is also noted that Dq J of tra n s -(ID- [Co([ 1 6]aneNJC1?]+ is larger than that of trans-((D-[Co([l5]aneNj)Cl2] .
The vi band for the trans complexes shifts to lower energy in conc. HC1 for every compound investigated; however, the v 2 and v 3 bands do not shift in Cl this manner. The calculated Dq values are invariably lower under these circumstances. Hence, the axial ligand field strength is weakened, presu mably through interactions with the polar solvent. 48
Table 20 ■f ^ Half-wave Potentials for trans-[Co(MAC)Cl?] in Acetonitrile.
_ n r , , i i „ MAC Co /C o V
[l3janeN 4 - 0.66 rev ersib le [!4]aneN 4 -0.69 rev ersib le [15]aneN4, (1) -0.38 reversible (ID -0 .4 7 quasi-rever sible [16]aneN4, (D -0.15 irreversible (ID - 0 .1 1 irreversible a vs Ag/AgN0 3 .1 M solution; n-Bu 4NBF4 as supporting electrolyte. 4000 3000 2000 1400 1000 500 250 W AVENUM BERS (cm* ) Figure 3. Infrared Spectra of_trans-[Co(.[l5]aneN 4)C l2](N03) A. Isomer I B. Isomer II
co ABSORBANCE gur re u ig F 6 . ectoni r _ta -Co(l aneN n ]a ([l5 o s-[C tran f_ o tra c e p S ic n tro c le E -. 400 I I . somer II r e m o Is B. I r e m o Is . A i. - 4 - 500 LNGT n ) (nm TH G ELEN V A W 4 )'C lj(N 0 3) in M ethanol ethanol M in 3) 0 lj(N )'C -Ji— 600 4 - 700 5i
The half-wave potentials of the Co(in)/Co(n) couple are reported in Table 20 . 4 In general, the Co(IIl) -> Co(H) reduction are reversible when examined by cyclic voltammetry. The data indicates that the [i4]aneN 4 complex is the hardest to reduce while the [l 6]aneN 4 complex is the easiest to reduce. These data parallel qualitatively the fit of the Co 3 ion to the macrocyclic ligand as indicated by strain energy calculations. Thus, when + + the ring is much too large for Co 3 , i.e., fits Co2 better, the ease of reduction increases. (E ly/2 becomes less negative.) It is noted that Ely£ + for trans-(l) ~[Co([l5]aneNfl)Cl?] is less negative than that for isomer II (-0.38 vs -0.47V), indicating that isomer I is easier to reduce than isomer II. This implies that the "effective ring size" of the ligand is different in the two isomers with isomer II acting as a smaller ring and therefore, causing Co3+ to be harder to reduce. This is consistent with the electronic spectral data discussed above. However, there is not much difference in the half wave potentials of the isomers of [l 6]aneN 4 complexes. The complex trans.-[Co([l 3]aneN 4)Cl(NO^)]C104 exhibits different molar conductivities and d-d bands in acetonitrile and in water. In acetonitrile, A m ~ *52 ohm“ 1cm2m ole “1 indicating that, in this solvent, the nitrate anion is coordinated. In water, Am = 282 ohm- ^ m 2 mole-1, indicative of a 2:1 electro lyte . The spectrum of this compound in water is the same as that of trans-[Co([l3]aneN,i)Cl(HgO)j2+, i.e ., band maxima at 580 and 415 nm. The labile N03- group is replaced by water in aqueous solution. Isomers of trans-[Co([i5]aneN 4)Cl2] . — The two isomers exhibit different colors, ligand field strengths, infrared spectra (Fig. 3), electronic spectra (Fig. 6), reduction potentials, and solubilities. The chloride salt of isomer I is significantly more soluble in acetonitrile than is that of isomer II, while the nitrate salt of isomer I is slightly less soluble in methanol than the nitrate of isomer n. It is the great difference in solubility in acetonitrile that made possible the separation of these two isomers in pure form. 52
Isomer I (the tan compound) shows four medium to weak CH 2 rocking bands in its infrared spectrum while isomer II (green compound) has only three peaks. Isomer I also has more bands in the region 250-400 cm-1. However, isomer H displays a medium strong band at 775 cm -1 where isomer I does not. In methanol, isomer I displays bands in the visible spectral region at 15.27 kK (s =40), 19.23kK (e = 69.3) and 23.70 kK (e = 106). The Dq values calculated from equations 22 to 24 are Dq2^ = 2303 cm -1 and DqZ = 1511 cm-1; isomer II shows bands in the visible region at 15.63 kK (e = 27), 20.41 kK (e = 38) and 24.10 kK (e = 62). All are shifted to higher energies and are less intense than the corresponding bands of isomer I. The in-plane ligand field parameter DqXy for the macrocyclic ligand is 2421 cm -1 and DqZ for the axial ligand Cl“ is 1465 cm -1 for isomer H. Hence, the macrocyclic ligand exerts a stronger effect in isomer n. While the axial chloride has a stronger ligand field in isomer I. The electronic and infrared spectra prove that these two compounds both have the trans-configuration, they are configurational isomers not geometrical ones. Unfortunately, the poor resolution of the NMR spectra did not provide further information regarding their isomeric structures. It is significant that a change in conformation of the ring can exert as large an effect on DqXy as a change in ring size by one member. . The two isomers also display different electrochemical behavior and this has been described earlier. In methanol the original green solution of isomer H gradually changes to a brown color, indicating an isomerization reaction to a mixture of isomers I and II. The percentage of each isomer at equilibrium in methanol has been calculated from their extinction coefficients at 520 and 420 nm. This gives 67 + 3! of isom er I and 33 + .3# of isom er II at 25° C. The equilibrium constant for equation 25 is then calculated to be 2 . Keq isomer II isomer I (25) methanol 53
Isomer I can be converted to isomer II by reacting with lithium carbonate and then acidifying with conc. HC1 acid (eq. 26). Li2CO^ conc. HC1 isomer I (brown) -* violet -» isomer II (green) (26) A> H20
We note with interest that the isomer that is slightly favored thermodynami cally (isomer 3) exerts the weaker in-plane ligand field strength (2303 cm -1 vs 242 i cm -1 for isomer II). Since I and n are configurational isomers, they most probably differ in the orientation of the hydrogens on their secondary amine groups with respect to tlie metal-nitrogen plane. In theory, six basic forms are possible for the conformers (Figure 7) on the basis of these N-H orientations.
a b c . d e f F igure 7 ■f Possible Isomers for [i5]aneN 4 in trans-[Co([l5]aneNi)Cl;>] .
The ”+M and ,,-n signs represent hydrogens above or below the metal-nitrogen plane, respectively. The numbers are the numbers of carbons in the chelate rings. Table 12 lists these possible stereoisomers of trans-[Co([l 5]aneN 4)Cl2]+ along with their chelate ring conformations. The conformations of six-membered chelate rings are indicated by either T (for twist) or C (for chair),while g (for gauche) and p (eclipsed) refer to the five-membered chelate ring. It is well known that the chair form is the most stable form for the six-membered ring , 33 whiie the gauche form is the one preferred for five- membered rings. However, an examination of the molecular models indicates that severe bond angle distortions occur when three six-membered chelate 54
Table 12 Possible Stereoisomers for trans-[Co([l5]aneN 4)Cl2J .
Isomer Chelate ring conformations a C C C p b C C T g c C T T p d C T C g e T C T p f T T T g
rings having chair conformations are fused together. For this reason, form a is unlikely. Forms b and d both have one twist, one gauche and two chair conformations for the chelate rings. It is highly likely that one of these two spe cies suffers the least strain and is therefore most stable. Dr. Susan Jackels lias calculated the conformational energies of these two forms. The results are shown in Table 13. The strain energy of form b is slightly less than that
Table 13 Conformational Energies of Some trans-[Co([l5]aneN 4)Cl2]+ C onform ers.
Conform er R_ NB 9 0 H configuration
b 21.26 5.32 8.02 3.73 21.33 YY^
d 4.52 6.26 8.69 3.36 22.83 §y ^Y 55 of form d. Therefore, form b which has three hydrogens pointing up and one down is the most stable one and is assigned to isomer I. This is different from the complex of [i4]aneN 4 which exists in a conformation of type d. The strain energy calculations are of limited accuracy and it cannot be considered certain that the predicted structure is correct when the strain energy differences are quite small. The precursor of isomer II is the cis-carbonato complex. For this reason, the configuration of isomer n should be related to the favored confi gurations for the cis-structure. In this case, the basic forms a to f in Fig. 7 can also be applied. It is possible, in concept, to turn each of these trans- forms into one or two cis analogs by the displacement of a nitrogen along an edge of the octahedron . 76 Nevertheless, some of the cis-forms are sterically impossible or very highly strained. By using Dreiding stereomodels the amount of strain can be estimated qualitatively. Of the possible isomers only form f is clearly strain-free. All other possible forms are folded with severe interaction between the atoms or other extreme distortions. There fore, the cis-configuration arises from the configuration that has two diago nally opposed hydrogens on the same side of the Co-N 4 plane and with the other two hydrogens on the opposite side. Since acidification of the carbonato compound will not promote inversion of the secondary amine groups, isomer II is expected to adopt the same N-H configuration as its precursor cis-compound. The structure in Fig. 8 is therefore assigned.
F igure 8 + Structure of Isomer II of trans-[Co([l5]aneN^)Cl?] . 56 WAVENUMBERS (cmc m 1) 4000 3Q00 2000 1500 1300 1200
Figure 4. Infrared Spectra of trans-[Co([l6]aneN 4)C l2]C104 A. Isomer I B. Isomerll —I------1------1------1------1______I______- 4 - 400 500 . 600 700 WAVELENGTH (nm)
F ig u re 9. Electronic Spectra of trans-[Co(f l61aneNA)Cl2l(ClQt) in Acetonitrile A. Isomer I B. Isomer II 58
Since isomer II is expected to involve considerably more strain energy than XV isomer I, it is concluded that the greater Dq for isomer n arises from the strain energy in a manner similar to that previously reported by Martin et al 2 for [Co([l 3]aneN 4)Cl2]+. The Isomers of trans-[Co([l 6]aneN 4)Cl2]C104. — Metathesis of the bromochloro complex (experimental section) of [i 6]aneN 4 with lithium chloride in hot methanol gives two crops of crystals. A green material precipitates from the cooled solution and a brown compound is obtained by reducing the volume of the filtrate. The infrared spectra of these two compounds are compared in Fig. 4. Because of the strong absorption due to the perchlorate ion at ~ 1100 cm-1, some of the ligand bands are obscured. When this is taken into account, there is almost no difference in the two infrared spectra. The visible absorption maxima for these compounds are, however, significantly different both with regard to band positions and intensities. Therefore, the two isomers differ in their color and in the separations between their electronic states. The electronic spectra are presented in Fig. 9. In acetonitrile, isomer I (brown compound) displays bands at 14.73 kK (e = 57), 18.69 kK (e = 34) and 23.26 kK (s = 99). The calculated Dq values are 2249 cm -1 for the macrocyclic ligand and 1457 cm -1 for the axial ligand (chloride). Isomer II (green) is characterized by band maxima at 15.11 kK (e = 41), 19.61 kK (e = 35) and 23.87 KK (e = 76), D q ^ = 2341 cm -1 and Dq^* = 1441 cm -1. The stronger in-plane ligand field is accompanied by a weaker axial ligand field. Isomer II like isomer II of trans-[Co([l5]aneN^)Cl?] , could be obtained by acidifying the cis-carbonato compound. In methanol, isomer ii slowly isomerizes to a mixture of isomer I and II. However, due to the sparing solubility of isomer II in methanol, the absorptivity of this compound has not been obtained. Hence, it is not possible to calculate the percentage of each isomer at equilibrium, nor to calculate the equilibrium constant. Never theless, the isomerization is clearly shown by the color change of the 59
solution and by the shifting of electronic absorption bands to longer wave lengths. Attempts to investigate the isomerization to isomer I in water failed because of the easy reduction of the cobalt(M) in the diaquo-complex to cobalt(D). This is not surprising since the half-wave potential for Co(H0-Co(D) couple of_trans-[Co([l6]aneN 4)Cl2]C104 is -0.15 V in acetonitrile. Of the macrocyclic cobalt(IU) complexes investigated, the 16-membered ring complexes are the easiest to reduce. The configurations of the isomers have been assigned, tentatively, on the basis of models and conformational energy considerations, as described earlier for the 15-membered ring derivatives. The possible conformers for the [ l 6]aneN 4 derivatives reduce to four forms because of the equal numbers of carbon atoms in each chelate ring (Fig. 10).
4- 3 + 3 3 3 3 3 +■ 4- 3 + 3 n IV Figure 10
Possible Isomers for [l 6]aneN 4 in trans-[Co([l 6]aneNa)Cl;l .
Of the four conformers, forms H and m both have two twist and two chair conformations for the chelate rings. Again, it is likely that one of these two is the least strained. The results of conformational energy calculations” are shown in Table 14. The strain energy of conformer III is less than that of conformer II but the two values are rather close. Consequently, isomer I is tentatively assigned this configuration wh ich is also the basic form of the stable [i4]aneN4. complex .71 All four forms are possible in the cis-configura- tion, however, IV is the least strained in the cis -configuration. Therefore, isomer II is assigned configuration IV. It is again true that the more strained structure is associated with the higher ligand field strength for the macrocyclic ligand. 60
Table 14 Conformational Energies of Some trans-[Co([l6]aneN4)Cl2] Conformers. conformer R_ NB 9 H_ configuration n 6.26 10.36 13.87 6.09 36.57 yyM HI 7.81 10.18 10.98 6.60 35.56 6 Y ^ Y
cis- fCo(MAC)X?]Y-complexes. — A series of cis-carbonato complexes has been synthesized as perchlorate salts with the macrocyclic ligands, [i3-i6]aneN4. cis-[Co(MAC)Cl2]X complexesX = Cl” or NC^-, have also been prepared where MAC is [l2]aneN4 or [l3]aneN4. The carbonato complexes were prepared by reacting the appropriate trans-dichloro complex with lithium carbonate in water. cis-[Co([l2, l3]aneN4)Cl2]Cl were synthesized by mixing the free ligand with cobalt(U) chloride 6-hydrate in metlianol, followed by aeration of the solution for one hour. Alternatively, cis-[Co([l2]aneNj)Cl2]Cl could be prepared by the metathesis of the corresponding dinitro complex.72 Elemental analyses of the complexes confirm their compositions. All the complexes are diamagnetic. For cis-[Co([l4]aneN4)C03]C104 • H20 and cis-tCoUlSjaneN^CQjClQi • 3/2H20 the presence of water is confirmed by a broad band around 3450 cm-1 in the infrared spectrum. Molar conductivities are collected in Table 15. The expecied molar conductivity range is 100 to 150 ohm~1cm2mole_1 in water and 75 to 110 ohm-1cm2mole~1 in methanol for 1:1 electrolytes. Each cis-complex displays more than one N-H stretching band, due to the low symmetry of the molecule. Table 16 gives v n -H values for the cis-complexes. Since the C032- serves as a bidentate ligand, the symmetry is no higher than C2V. The absorption bands of the C032- ligand are assigned following the notation of Nalcamoto.64 v2 is obscured by the strong C104" peak in all of the perchlorate salts. The COj2~ ligand bands are also listed in Table 16. 61
Table 15 Molar Conductivities for the Complexes cis-[Co (MAC)X2]Y.
Complex Solvent A m olun_1i c is - [C o( [ 13 ] aneN4) C ]C 104 w ater 81 m ethanol 81 cis.-[Co([i4]aneN4)CQj]C104 w ater 83 m ethanol 83 DMFa 65 c is- [C o( [ 15] aneN^ C O^CIC^ w ater 90 cig.-[Co( [l6]aneN4)CC^]C104 w ater 95 cis-[Co([l3]aneN4)Cl2]Cl m ethanol 77 cis-[C o( [ 12 ] aneN4) C12]N03 m ethanol 99
EL N, N-dimethylformamide . In this solvent, the expected molar conductivity range is 75-95 ohm~Icm2mole~1 for 1:1 electrolytes.
/ Table 16 Infrared Data for cisr [Co(MAC)X2]Y, N-H and CO 32" Modes.
Complex VN-H cm_1 v C -0 cm 1 cis-[Co([i3]aneN4)C03]C104 3257 s,sp, 3205 s,b 1653 s, sp , 1605 s, b 831 m, 752m, 676 m cis-[C o( H4]aneN4) COa ]C104 3250 s,sp, 3180 s,b 1655 s, sp, 1605 s, b 825 m, 750m, 676w cis-[Co( [l5]aneN4)C033C104 3255 s, sp, 3200 b 1662 s, sp, 1621 s, b 828m, 750m, 676m cis-[Co( [l6]aneN4) C0^]ClO4 3257 sh, 3236 s, sp, 3165 s 1656 s, sp, 1613 s, b 828m, 752m, 672m cis-[Co( [i3]aneN4) C12]C1 3215 s, sp, 3195 s, 3l06 s, b cis-[Co([i2]aneN4)Cl2]Cl 3145 s, sp, 3164 s, 3238 s, sp
.05tN5 Table 17 o Infrared Absorption Bands for the eis-Complexes in the CH2-Rocking and M-N Stretching Region.
Complex CHp-rocking cm"1 M-N stretching cm-1 cis-[Co([l33aneN^)Cl9]Cl 807m, 8i5w, 833m, 850sh, 871m, 885w, 525w, 546w,b, 566w, 578w 890m, 9l3w cisj[C o ([l 3]aneN 4)CC^]C104 8l3w, 820w, 828^s, 847m, 866w, 873sh, 575w, 562m, 545sh, 5l8\v, 899w 498w
cis-[Co([l4]aneN;i)CC^]C10/t 8l0m, 825^m, 845w, 860w, 876w, 890w 5l8w, 545w, 560w, 590w
cis>-[Co([l 5]aneN 4)C0 3]C104 808w, 82lw, 827 s, 845w, 872m, 890\v, 520m,w, 545w, 560w, 585w 905w
cis_-[Co([l 6]aneN 4)C0 3]C104 789w, 826w, 889w, 903m 558w, 520m, 505w
cis-[Co([l23aneN4)Cl23Cl 802sh, 810s, 830ms, 860sh, 867m 5i0 w,b, 545sh, 565w, 595sh
aAs nujol mull. Overlaps with the Co32- absorption. 64
Table 18 *}• £ Electronic Spectral Data for cis-[Co(L)X9] Complexes. Complex band max. (kK) Dq cm-1
lT2 g - lAlg j, p [Co([i2]aneN4)CGj] 18.87 (280) 27.17 (210) 2267 [Co( [13 JaneN^ C C^] 19.96 (178) 28.09 (133) 2376 [Co([l4]aiieN4)CO^]+ 19.23 (154) 27.40 (140) 2303 [Co([l5]aneN4)CQ}]+ 18.94 (138) 27.32 (182) 2274 [Co([l6]aneN4)CQj]+ 18.52 (146) 26.67 (197) 2232 /3-[Co(trien)CO^] 19.72 (178) 27.93 (140) 2352 a-[Co(trien)CC^] 19.88 (120) 28.01 (103) 2368 j8-[Co(3,2,3-tet)CO^]+ 19.23 (127) 27.78 (125) 2303 [Co( [12 laneN^ Cl2] b l8 .3 5 (170) 27.03 (sh) 2215 ° 1 8 .18 (222) 25.91 (208.8) [Co( [lSjaneN^ Cl2] b l8 .5 2 (124) m asked 2232 C18.59 (150.5) 25.32 (160) [Co([l4]aneN4)Cl23+ d 17.92 (105) m asked 2172
ci Id c d 0 In water. In methanol. In cone. HC1. Ref. 41. Ref. 72. 65
The infrared bands in the CH2 rocking region (800-910 cm-1) and the metal nitrogen stretching region (500-600 cm-1) are collected in Table 17. Comparison of the numbers and positions of bands for the cis- and trans-compounds (Table 9 and 17) in the region 800-910 cm-1 and 500-600 cm-1 shows that infrared spectral properties can be used to distinguish these cis- and trans-compounds as suggested by Poon66 and by Hughes et al.G7,G8 This is especially useful when the electronic sprctrum is ambiguous. In general, cis-compounds exhibit more bands than do the trans-compounds in these infrared regions, with the intensities being somewhat weaker in the case of the cis-complexes. The cis-complexes generally have five or more fairly evenly distributed bands between 800 and 9i0 cm-1 and at least four bands between 500 and 600 cm-1. On the other hand, the trans-compounds usually exhibit two bands close to 900 cm-1 and one close to 800 cm-1. However, in the region 500-600 cm-1 four bands were observed for the trans-[l3]aneISh complex (Table 9). Although other trans-complexes exhibit the expected number (i. e ., three) of bands in this region (500-600 cm-1), they are all very weak. It is concluded that the CH2rocking region is reasonably reliable as a criterion for distinguishing between these cis- and trans-complexes. For the cis-complexes, two d-d transitions are observed in the visible spectra. This is the result of the smaller magnitude of the splittings of the energy states that occurs for cis-isomers. The splitting is approximately half that seen for the trans-isomers (D4h symmetry). Therefore, the lower energy band vi corresponds to the transition JTjg «- •‘Ajg and the higher energy band V 2 involves the transition *T2g .- *Aig. The ligand field parameter Dq (average ligand field strength) can be calculated from eq. 27
lODq = vTlg + C (27) where C is 3800 cm-1. The band maxima, extinction coefficients and the cal culated Dq values for the cis-complexes are listed in Table 18 along with those of some cis-carbonato complexes of linear tetraamine ligands. The positions 66 of the bands vary only slightly with changes in ring size, the total range being 4* only 144 cm-1 as compared to 511 cm"1 for trans-[Co(MAC)Cl2] . TheDq values lie in the order [i3]aneN4 > [i4]aneN4 > [l5]aneN4 ^ [l2]aneN4 > [l6]aneN4, and some of the differences are indeed small. Thus, the energies of the electronic transitions of the carbonato complexes show little sensitivity toward ring size. That ring size for the cis-complexes has little effect on the energies of the electronic transitions has also been observed for the oxalato complexes of the linear tetradentate ligands. The electrochemical behavior of the cis-[Co(MAC)Cl2] complexes, where MAC = [l2]aneN4, [i3]aneN4, and [i4]aneN4, has been examined by Miss Kathy Holter. The half-wave potentials for the Co(III)-Co(II) couples are listed in Table 19. Unlike the analogous trans-compounds (Table 20), the complex containing the smallest ring system is not the hardest to reduce. E y 2 is -0.68 V for cis-tCoCClSlaneN^Clj,]4^ while the 12-membered ring compound, which has E i/2 - 0.49 v, is the easiest to reduce. The 14-membered ring complex exhibits an (-0. 54 V) very similar to that of the 12-membered ring.
Table 19 Half-Wave Potentials of cis-[Co(MAC)Cl2] in Acetonitrile.a MAC > Co(rn)-Co(i]} ;;/2 v Co(ID-Co(D E^2 V [12]aneN4 -.49 irreversible -2.08 irreversible [l3]aneN4 -.68 quasi-re versible -2.06 ir re ver sible [!4]aneN4 -.54 quasi-reversible -2.15 irreversible cl vs Ag/AgN03 (0.1 M) reference electrode. 0 .1M t-Bu4NBF4 as supporting electrolyte. 67
Kinetic Studies
In this section, the kinetic behaviors of the aquation and isomerization reactions of the macrocyclic complexes are reported and discussed. The section is divided into two subsections treating the trans- and cis-compounds, respectively. Within each subsection, the kinetics of each complex is reported separately. Finally, the overall results are discussed in terms of structural changes. ■f Where possible, the aquation kinetics of trans- [Co( [X]aneN;))C1,] and + cis-[Co([X]aneN 4)C12 j were examined over a temperature range of 20° C. The first sequential reaction is aquation for all of the compounds and involves the replacement of one of the chlorides by water. The rate of the first “t* aquation step for trans-[Co(MAC)Cl,] follows the trend [i 6]aneN 4 > [i5]aneN 4 > [i3]aneN 4 with the rate constants differing from 3 sec -1 to i0~ 4sec_1 at 25° C. The rate constants for the first aquation step for the cis-complexes do not vary much (a factor of 2). The isomerization of isomer II to isomer I for trans-[Co([l5janeN 4)Cl2]+ and the isomerization of trans-[Co( [13]aneN4)- (H20)2]3+ to the cis-diaquo complex have also been investigated in substantial detail. All aquation reactions were studied under pseudo-first order conditions since one of the reactants, water, was used as the solvent (eq. 28)i
[Co (MAC)C12]+ + h 2o [Co(MAC)C1(H 20)]2 + C1‘ (28)
The rate of this reaction can be expressed as: -d[Co(MAC)Cl,]+ Rate dt ki[Co(MAC)Cl2] (29)
Eq. 29 after integration becomes eq. 30.
In [Co(MAC)Cl2+]t = k (30) [Co(MAC)Cl 2+]0
+*4* . "I* w here [Co(MAC)C1 2 ]0 is the initial concentration and [Co(MAC)C1 2 is that 68 at time t. Hence any physical properties which are proportional to the concent ration can be used to follow the reaction. Spectrophotometry is most popular and has been used in most of these studies. Iq in eq. 30 is determined as the slope of the graph of ln(At - AJ vs tim e.
Aquation of trans-[C o (MAC)C12]+. — The first-order rate constants for the aquation of trans - [C o(MA C) C1? ]1+ complexes in 0.1 N nitric acid at different temperatures are reported in Tables 21, 22, and 23. The activation parameters are reported in Table 25. Aquation of trans-[Co([l3]aiieN 4)Cl23+o — The aquation of trans-[Co([l3]~ aneN 4)Cl2J+ in 0. i N nitric acid proceeds in two stages as indicated by two spectrophotome trie ally distinctive changes. The first stage is the solvolytic displacement of one coordinated chloride (eq. 31). This has been verified by the titration of liberated chloride. The amount of chloride liberated, which
+ k l + trans_-[Co([l 3]aneN 4)Cl2] + H20 <1 tra n s - [C o( [ 13] aneN^) C1(H 9 O) ]2 k -i + Cl" (31)
Table 21 -}• o Aquation Data for trans-[Co( [13laneN^ Cl2] in 0.1 N HNO 3. temperature °C k 4 sec " 1 2 5 .0 (6 .8 + .2) x 10 ‘ 4 25.0b 5.4 “ x 10 "4 25.0 3.4 x lO"3 ° 25.0 2.2 x 10",-3 d 30.0 (1.29 + 0.05) x lO-3 35.0 2.52 + 0.06 x i0 “3 4 5 .0 5 .7 + .2 x 10 "3
EL b At least three rims unless otherwise specified.+ standard deviation, in 0.1 NCF 3SO3H. Cin 0.07 N HC1. din 0.04 N HCl,~jj. = 0.09. —4 ------.------1------.------1__ 400 500 600 700 WAVELENGTH (nm)
Figure 11. Changes in Absorption Spectra for The Aquation of trans-[Co([l3]aneN4)Cl2] was titrated at three different time-intervals, shows successive increases with tim e.
time (min) [Cl~]/[complex]n 20 0 .5 37 0.65 60 0.8
Second reaction stage is the solvolytic displacement of the second coordinated chloride by water and the concomitant isomerization of the product, trans-[Co([l3]aneN4)(H20)2]3+ to a mixture of the cis-diaquo and cis-chloroaquo complexes, (eq. 32).
trans-[Co([l3]aneNa)Cl(H90)]2+ + H20 2 trans-[Co([i3]aneN4)(H20)2]3+ (32) it cis-[Co([i3]aneN4)Cl(H20)]2+ «* cis-[Co([l3]aneN,)(H9Q)9]3+
Titration of the free chloride in the reaction mixture at the end of the reaction (several days) gives 2.6 mole of free chloride. The electronic spectra taken at different times during the first, aquation reaction are characterized by a well defined isosbestic point at 588 nm (Fig. 11). At the end of the first reaction, the visible spectrum showed band maxima at 415 nm (e = 122) and 580 nm (e = 30). The spectrum is identical + to that of the trans-[Co([l3]aneNA)Cl(H9Q)]2 complex synthesized by direct means. There was no evidence to indicate the formation of the cis-chloroaquo complex. Thus, the first aquation proceeds without steric change. Titration of the free chloride after ten half-lives indicated that aquation was 75$ complete for the chloride salt, and 91$ for the nitrate salt. The equilibrium constant Keq is calculated to be 0.014 + 0.003 M (average of two calculations) tr a n s - [Co(Cl3]aneN^)Cl9]+ tr a n s - CCo([l3janeN,)Cl(H9Q)]2+ + C l" (33) + at 25° C. Both the chloride and nitrate salts of trans-[Co([13]aneN^ Cl2] were used for the kinetic study of the displacement of chloride and the results for kj are comparable. Since in the acidic medium, the second reaction 400 500 600 WAVELENGTH (nm) Figure 13. Changes in Absorption Spectra for The Second Aquation of_tran£-[Co([13]aneN4)Cl2]+ In k x
-7
- 6
-5
-4
3,0 31 32 33 3.4
xlCf3 1/T ( K_1)
Figure 24. The van’t Hoff Plot of The -Aquation of _trans_-[Co([l3]aneN4)Cl2]+ 73
is very slow (see following section), there is no interference from the second reaction stage. The first reaction was monitored at 560 and 620 nm, where the absorption change is greatest, and the results were the same. This indi cates that no steric change occurs during the aquation.74 The first-order graph of ln(At - A J vs time is linear for at least two half-lives (Fig. 12), kj at 25° C is 6.8 x l0“4sec-1. Rate constants are reported in Table 21. The activation parameters were calculated from the Arrhenius equation. The slope of the graph of -lnkj vs 1/T(°K-1) (Fig. 24) is - which can be d converted to All by the equation d AH = E a - RT (34)
The intercept b of the graph of -lnlq vs 1/T allows for the calculation of the d entropy of activation, a S via eq. 35.
b = InA = 1 + ln ~ ~ + (35) U XX
where k_ is Boltzmann constant and h is Planck's constant. The calculated values for the aquation of trans-[Co([l3]aneN4)Cl2] are ah = 25.6 kcalmole-1 and AS^ = 12.6 cal deg"1 mole-1. In order to more completely evaluate the system, the first aquation process was also examined in the presence of excess chloride ion (0.07 N and 0.04 N at 25° C). k0ks was calculated as before from the graph of ln(At. - A^ vs t. The slope of the graph of k0bs vs [Cl-] gives k_* ( in eq. 31) which is found to be 3.5 x lO ^sec'1. The intercept is kj which is 8 x I0“4sec-1 (compared to 6.8 x 10“4sec -1 obtained from the slope of the ln(A{;- A») vs t graph, in the absence of excess chloride). The equilibrium constant Keq (eq. 31) is then calculated to be 0.019 M compared to Keq determined from the chlo ride titration of 0.014 + 0.003 M (two runs) at 25° C. The second sequential reaction is characterized by isosbestic points at 577,453 and 393 nm (Fig. 13). However, toward the end of this reaction, the isosbestic points disappear. The spectrum at very long times corresponds 74f
3.5
3.0
2.5
2 .0
0 400 1200 1600 2000 2400 time (secs.)
Figure 12. Sample First Order Plot of Kinetic Data F ig u re 14. C hanges in A b so rp tio n S p e q tra fo r th e Is o m e riz a tio n of of n tio a riz e m o Is e th r fo tra q e p S n tio rp so b A in hanges C 14. re u ig F ABSORBANCE rn_s[ [3]ne )H0f o_ci-Co(l a )H0)J 4)(H20 N e ]an ([l3 o is-[C c _ to 4)(H20^f eN ]an ([l3 o s-[C _ tran 400 LNGT n ) (nm TH G ELEN V A W 500 600 cn -4 mainly to that of the cis-diaquo complex max at 495 and 365 nm) which has been obtained independently by acidifying cis-[Co([l3]aneNa)CQJ in aqueous solution with perchloric acid. Chloride titration demonstrated that chloride ion is released during this second reaction stage. Further confirmation of the nature of the reaction is provided by the fact that it is accelerated by the presence of mercuric ion. is i.9 x iO-5sec-1 at 35.0°C, and 3.2 x lO“5sec-1 at 40.0 0 C for the second spontaneous aquation process. The examination of the steric course of the second aquation step is complicated because of the fast isomerization (see next paragraph) of the trans-diaquo species. However, the aquation is considered to be stereoretentive in view of the rigid nature of the maerocyclic ligand and the results of studies with trana_-[Co([i3]aiieN,1)(H20)2]3 . + Isomerization of trajis-[Co([i3]aneN4)(H20)2]3 . — Because the product of
“h the second stage of aquation of trans-[Co([l3]aneN4)Cl23 is mainly a mixture of cis-complexes, it was important to find out how rapidly the trans-diaquo complex isomerizes to the dys-compound under the same condition as were used for the kinetic study discussed previously, trans -[Co( [ 13]aneN,t) (H9O)9]3 + was produced in solution by passing the trans-[Co([l3]aneNd)Cl;».]1 complex through an anion exchange column (OH form). The pH of the eluent was adjusted to about i with nitric acid. The isomerization of the pure diaquo complex was then followed by repeatedly scanning the spectrum between 650 and 330 nm. Two well-defined isosbestic points were observed at 385 and 448 nm (Fig. 14). It was found that kjs0 = 5.4 x iO^sec-1 at 25° C and 3 x l0-3sec-1 at 45° C. These values are about sixty times faster than the observed rate constants for the second stage of the aquation of the trans-dichloro complex. It is therefore to be expected that the second stage of the aquation should yield the cis-diaquo com plex. •J* Aquation of trans(b-[Co([l5]aneNA)Cl^] . — The tan compound aquates to the chloroaquo-complex and the spectral scans show isosbestic points at 630 , 562, and 468 nm (Fig. 15). The second aquation stage is characterized by isosbestic points at 400 and 460 nm. Hence, the first aquation reaction was 400 500 600 700
WAVELENGTH (nm)
Figure 15. Changes in Absorption Spectra for The Aquation of_trans^-(l)-[Co([l5]aneN4)Cl2]^" .-3 78
In kj
-6
-5
-4
-2
3.13.0 3.2 3.3 3.4 • 0 " 1 xlO3 1/T ( K)
Figure 25. The van't Hoff Plot of The Aquation of trans (l)-[Co([l5]aneN4)Cl2] 79
followed at 400 nm where the second aquation process does not interfere. Runs followed at 688 nm were also carried out. The results are reported in Table 22. Chloride titration after ten half-lives indicates that the aquation is complete
Table 22 Aquation Data for tran.s(l)-[Co([l5]aneN,1)Cl?]+ in 0.1 N HNO^.3- temp. C° k4 x i03sec-1 25.0 1.16 + 0.06 30.0 2.03 + 0.06 35.0 3 .1 + 0 .2 40.0 4 .9 + 0 .6 44.9 7 .9 + 0 .8
S i At least three runs were carried out. + standard deviation.
(>99^). The product, the trans-chloroaquo complex, exhibits band maxima at 625, 500, and 408 nm. Iq for the first aquation is 1.16 x iO^sec-1 at 25° C, AH 19.3 kcal m ole-1 and aS -7 .4 cal deg- 1mole-1 a t 25° C . The van't Hoff plot is shown in Fig. 25. There is no complication in the aquation reactions of this compound. The second aquation process was not studied. Aquation of trans(ID - CCo([l5]aneN,i)Cl?] . — The sp ectral change accompanying the aquation of the green compound is characterized by the ob servation of isosbestic points at 425 and 621 nm (Fig. 16) for the first reaction stage. That one of the coordinated chloride ions is completely released is confirmed by chloride titration at the end of the reaction. The mole ratio is [AgNO3]used/[complex]0 = 5‘ = 2,0 where trans(D)-[Co([15]- aneN4)Cl2]Cl was the reactant. At the end of this first process, the spectrum shows bands at 615, 481, and 400 (sh) nm. As in the other cases, the presence of these three bands indicates that the product has the trans configuration. The first order rate constant k4 has the value 9.9 x 10“3sec-1 at 25° C. The rate constants are reported in Table 23. The values were obtained by 1______I______L______!_____ J ----- _J ------1 400 500 600 700 WAVELENGTH (nm) + Figure 16. Changes in Absorption Spectra for The Aquation of trans(lI)-FCo(r 15laneN4Cl?1 81
In kj 6
5
4
-3
2
3.13.2 3.3 3.4 3.5 xlO3 1/T(0K"X)
Figure 26. The van't Hoff Plot of The Aquation of trans(ll)-[Co([l5]aneN4)Cl2] 82
following tiie reaction at 560 and 670 nm. AH is 15.8 kcalmole-1 and
Table 23 Aquation Data for trans(H)-[Co([i5janeN/t)Cl;>]+ in 0.1 N HNO?.a temperature °C kj sec-1 15.2 4 .9 + 0 .5 b x i 0 - 3 20.0 6 .3 + 0 .3 x 10~3 25.0 9.9 + 0.5 x 10~3 30.0 1.88 + 0.06 x lO"2 35.0 2 .5 + 0.2 x 10“2
9 . ]} At least three runs, standard deviation.
AS^ is -14.5 caldeg-1niole-1 at 25° C. The van't Hoff plot is shown in Fig. 26. Conversion of isomer II to isomer I in aqueous solution. — After the completion of the displacement of one of the coordinated chloride ions from isomer n, the spectral bands shift to lower energies with isosbestic points at 559 and 602 nm (Fig. 17). The final spectrum associated with this process is similar to that of the product from the first aquation step of isomer I with Amax at 408, 500, and 629 nm. Therefore the reaction corresponds to the isomerization of the chloroaquo complex of isomer II to that of isomer I. It was found that the rate constant for isomerization is 2.5 x lO'^sec"1 at 30.0° C. No effort was made to determine the activation parameters. However, deuterium isotope exchange reactions were carried out in order to further demonstrate that this isomerization occurred with inversion of one or more secondary nitrogen atoms. A small amount of sample was dissolved in 0.1 N HNG3-D20 solution previously brought to 30.0° C, to make a 0.002 M solution. This solution was then placed in a water bath at 30.0° C for 45.8 minutes (one half-life for the isomerization at this temperature) and then quenched in ice. The complex was isolated as the perchlorate 83
WAVELENGTH (micron)
X 4 x i 3000 2500 WAVENUMBERS (cm-1)
Figure 18. Infrared Spectra of trans-(n)-[Co([l5]aneN4)Cl2]C104 o A. in 0. IN HN03-D20 at 3C‘0C 45.8 m ins. B. in 0.1 N HNO3-D2O at 30 C 300 m ins. ABSORBANCE gur 7 Chne i Abs pton Specta f Te somerzaton of n tio a riz e m o is The r fo tra c e p S n tio rp so b A in hanges C 17. re u ig F 400 _trans (II) - - [ (II) C o ([ 15 N4) ]_trans C ane 1( (I) - [ H2C o O) o t ([ ] 15 _trans N4)] ^ C ane 1( H2 O) ]2+ 500 LNGT n ) (nm TH G ELEN V A W 600 700 00 salt by adding lithium perchlorate to the solution and freeze-dried. The infrared spectrum exhibited a small N-D stretching peak at 2400 cm-1 (Fig. 18), indicating that the breaking of nitrogen-hydrogen bonds had occurred to some extent in acidic media. Another portion of the sample was isolated after 300 min.in the reaction bath. The infrared spectrum (Fig. 18) shows more deuterium exchange has occurred as the relative intensity of vn-d/- VN-H *ms increased. Hence nitrogen inversion is not prohibited at low pH hereby allowing for the isomerization to the more stable isomer. Aquation of trans-[Co([l6]aneN4)Cl2]C104 complexes. — Both of these isomers aquate so fast that it was not possible to follow their reactions by the method that was used for the investigation of the other compounds. Conse quently, the reaction was studied in a slightly different environment, a 1:1 mixture of water/acetonitrile, by the stopped-flow method. Due to solubility problems, the aquation of isomer II (green complex) was not investigated thoroughly. At 25° C from measurements at 670 nm, k4 is estimated to be ~3 sec-1. The first aquation of isomer I (brown compound) was followed at 680 nm w'here the absorption change is greatest. At 25.0° C, k4 is 2.57 + 0.09 sec-1. The only other temperature where the rate was measured was 19.8° C, where k4 was found to be 2.27 + 0.07 sec-1, suggesting that AH7^ is sm all. Aquation of cis-[Co(MAC)Cl2] complexes. — Since attempts to prepare cis-dichloro complexes containing [l5]aneN4 and [i6]aneN4 failed, only the cis-dichloro complexes containing [i2]aneN4 and [l3]aneN4 were investigated. The rate constants for aquation of these two compounds at different temperatures are collected in Table 24. The aquation reactions of these compounds proceed without complication, as reported in the following sections. "f" Aquation of cis-[Co([l2]aneN4)Cl2] . — Both chloride and nitrate salts were used for the kinetic studies. The aquation reaction again proceeds in two stages. The first is characterized by one isosbestic point at 535 nm (Fig. 19) and the second stage, by an isosbestic point at 427 nm. Table 24 ■f* Q Aquation Data for cis-[Co([l2, i3]aneN4)Cl2] in 0.1 N HNOj.
Complex temp. °C k4 sec-1 c is - [Co( [i2]aneN4) C12J 15.0 1.7 + 0.1 X 10-3 25.0 4.2 + 0.2 X i0 “3 25.0 4.3 X 10"3 35.0 8.9 + 0.8 X 10"3 40.0 1.53 0.02 X 10“2
cis_-[Co( [l3]aneN4) Cl2] 15.8 3.1 + 0.1 X i0 “3 20.0 5.6 + 0.3 X io"3 n n a & 25.0 9.5 + 0.2 X io -3 25.0 10.2 + 0.5 X 10-3 b, e 25.0 9.5 + 1.0 X io -3 CM t 1 O 30.0 1.49 + 0.05 X H 35.0 2.49 + 0.09 X io -2
3 b Average of at least three runs. + Standard deviation. Average of two runs. CIn 0. i N HC1. dIn 0.1 N C.F3SQ5H. _J ______I______I______1______I— 400 500 600
WAVE LENGTH (nm)
Figure 19. Changes in Absorption Spectra for The Aquation of cis-fCo(f 12laneNd)Cl21 88
in kj 7
6
5
-4
3
2
1
3.0 3.3 3.4 3.5 xio3 i/t(°k "1)
Figure 27. The van't Hoff Plot of The Aquation of c i s -[ C o ( r i2laneN 4)C l?.l In k
-6
3.2 3.3 3.4 3.5 l/T xlO3 (°K”1) Figure 28. The Van't Hoff Plot of Th^ Aquation 90
After 10 ty2at 25° C, the free chloride released in the first step was determined by potentiometric titration with silver nitrate. The chloride salt gives 1.85 M free chloride while the nitrate salt gives 0.93 M [Cl-]. Therefore Keq = 0.012 M (average of two determinations). The aquation reaction was followed at 340 and 590 nm and both give the same rate indicating that no other reaction occurs. The rate constants are reported in Table 24. The presence of excess chloride (0.1 N) did not affect the reaction rate as lq is the same as those in 0.1 N HNOa at 25° C. The cis-chloroaquo complex absorbs at 530 and 370 nm. lq at 25° C is 4.2 -1 JL x lO^sec-1; AH being 13.5 kcal mole-1 and AS , -24.4 cal deg-1mole-1. The van’t Hoff plot is shown in Fig. 27. Aquation of cis-[Co([l3]aneN;i)Cl?]Cl. — Isosbestic points at 528 and 455 nm w ere observed both in 0 .1 N HNOj (Fig. 20) and 0.1 N HC1. The first order rate constant for the aquation in 0.1 N HC1 is about the same value as that in 0.1 N HN03, indicating that the equilibrium (eq. 28) lies far to the right. In 0.1 N nitric acid, the second stage is characterized by three isosbestic points at 513, 436, and 378 nm. Chloride titration indicates that the second aquation is not complete, 2.61 mol of [Cl- ] was found in the solu tion at equilibrium. The cis-chloroaquo complex has peak maxima at 384 and ' 520 nm . The first reaction was followed at 340 and 490 nm. At 25° C lq is t 1.02 x 10-2sec-1, AH 15.2 kcal mole-1, and aS -17.4 cal deg-1mole-1. The van't Hoff plot is shown in Fig. 28. The second aquation step has kQbs ~ 2.4 x i0-5sec-1 at 30.0° C in 0.1 N CF3SO3H and 1.7 x l0“4sec-1 at 40.0° C in 0.1 N HNOj. The second reaction is about a thousand times slower than the first aquation process (lq 1. 5 x l0-2sec-1 at 30.0° C) at this acid strength. ______I______i______I______i______!------1 400 500 600
WAVELENGTH (nm)
Figure 20. Changes in Absorption Spectra for ^The Aquation of cis_-[Co([13]aneN4)Cl2] 92
Isomerization of cis-[Co([i3]aneN4)CI2]Cl to_trans-fCo([l3]aneN4)Cl2]Cl in methanol. — The pink solution of cis-[Co([l3]aneN,i) Cla]Cl in methanol gradually changes color to green. At room temperature the half-life for isomerization is about one hour. At equilibrium, the mixture contains mainly the trans-[Co- ([i3]aneN4)Cl2]Cl complex. + § e q + cis-[Co([l3]aneN4)Cl2] mgthanol trans~^c°^:*-3JaneN^c^1 (36)
The equilibrium constant for eq. 36 is calculated from the absorptivities of the compounds involved at 600 and 510 nm and the value is 8.0 +0.1. The trans- complex was isolated as the nitrate or the perchlorate salt. The infrared spec trum of this product was indistinguishable from that of the trans-compound synthesized directly. Discussion of the kinetic behavior of trans-[Co(MAC)Cl?3+. — The *)■ aquation of complexes of the formula trans-[Co(MAC)Cl2] have been studied in 0.1 N HNOj as described in earlier sections. The first stage of the aquation of all the trans-compounds is the release of one of the coordinated chloride ions with retention of the trans-geometry. The rate constants for this stage fall in the order [l6]aneN4 > [l5]aneN4 > [l3]aneN4. The second stage of the reaction varies from system to system. The complex of [i3]aneN4 aquates (equation 37) to the trans-diaquo species which isomerizes and anates to cis-diaquo and cis- chloroaquo complexes. The aquation product of isomer I of trans-[Co([l5]aneN/t- Cl2]+ aquates to the diaquo-product with no side reactions (eq. 38). Isomer II of 15-membered ring system isomerizes to the first aquation product of isomer I (eq. 39). The complex of [l6]aneN4 aquates to release the second chloride (eq. 40).
^rans-[Co([l3]aneN4Cl2l+ 2 trans-[Co([l3]anel>h)C1(H9Q)]2+«* cis-[Co([lSjaneN^ + + f (H20)2F+ cis-[Co([lSlaneN^CKIl^O)]2 _cis[Co([l3]aneN4)(H20)2]3 (37)
trans(D -[Co([l5]aneISh)Cl9l+ -» trans(I)-[Co([l5]aneN4)Cl(H,0)]2+ XT trans(I)-[C o([l5]aneN 4)(H20)2]3+ (38) 93
trans(H) -[Co( [l5]aneN4) Cl2]+ - trans(ID-[Co([i5]aneN4)Cl(H20)]2+ it irans(])-[Co([i5]aneN4)Cl(H20)]2+ (39)
-J- f n *j» trans-tCoCLlGjatieN^Cl,] trans-[Co([l6]ancN,)Cl(H9Q)]2 -
tra n sr [Co([i6]aneN4)(H20)2]3+ (40)
The rates of aquation of the first chloride of the trans-dichloro complexes are collected in Table 25 along with the activation parameters AH and AS . Data for the corresponding complex of [i4]aneN4 and that for related linear tetra- amines, bis(ethylenediamine). and bis(trimethylenediamine) complexes are also included for comparison. The rate of aquation of the unsubstituted, fully “f* saturated macrocyclic complexes trans~[Co(MAC)Cl?] can be arranged in the o rd er:
[i4]aneN4 < [i3]aneN4 < [l5]aneN4 < [i6]aneN4 2,3,2,3 2,2,3,2 3,2,3,3 3,3,3,3
While that of the open chain tetraamine complexes follows the order:
2,2,2-tet > 2,3,2-tet > 3,2,3-tet
Since the macrocyclic ligands can be considered to bo derivatives of the linear tetraamines, some similarities in their rates of aquation are to be expected. It is anticipated from other properties of the complexes of the homologous cyclic ligands that strain energies will contribute to the differences in their behavior. This is also expected for the linear tetradentate deri vatives. Unfortunately, the complex trans-[Co(3,3,3-tet)Cl2]+ has not been successfully synthesized. This limits quantitative comparisons. Nevertheless, the trans-complexes of 2,3,2-tet and 3,2,3-tet are relatively strain free3 while that of 2,2,2 is strained due to the small number of carbon atoms in its chelate rings and it is this species which aquates most rapidly. The trans-complex of 3,3,3-tet would be expected to be strained because of the presence of three carbon chains between each pair of nitrogens. Therefore, the rate for the 3,3,3-tet complex would 94
Table 25 Rate Constants and Activation Parameters for the Aquation of Some TTT + Complexes of the form trans-[Co (L)C12] . , , Steric change L ki(at 25°C)sec~1 AHkcai/mole AScal/degmole (% cis) [l3]aneN4 6.76 x 10“4 25.6 12.6 0 [l4]aneN4a 1.1 x 10~6 24.6 -3 [l5]aneN4 1.16 x i0~3 19.6 -7 .4 0 (isom er 1) [l5]aneN4 9.92 x iO"3 15.8 -1 4 .5 0 (isom er D) [!6]aneN4 2.5 7 0 (isom er 1) trien 3.5 x IO-3 26.0 +15.5 100 b meso-2, 3,2-tet 1.5 x IO"5 24.6 +1 0 1-. d,*-2,3,2-tet 2 .9 x 10“4 2 4 .7 +9 50 3,2,3-tet0 5.3 x 10“5 24.5 +4 0 (en)2d 3 .2 x IO"5 26.2 +14 35 (tn)2° 5.33 x IO-2
^ef. 31. bRef. 26. CRef. 25. ‘W . 32.eRef. 15. 95
-15
-10
-5
15 25 35 H s tr a in 4- Figure 21. The Strain Energies of The trans-^Co(MAC)Cl2] complexes vs Rate Constants a;t 25 C 96
be expected to be much faster than those of the strain-free complexes of 3,2,3-tet and 2,3,2-tet. Also lq for trans-[Co(tn)2Cl2]+ is a thousand times -f* larger than that for trans.-[Co(en)2Cl2] . With this in mind, it is interesting to note that lq for the 15-membered ring system is a thousand times larger than that for the strain-free 14-membered ring system and IO3 times smaller than that for the 16-membered ring system and that each pair of complexes compared differs by substitution of one six-membered chelate ring for a five- membered chelate ring. •f The minimized strain energies for the complexes _trans-[Co(MAC)Cl2] have been calculated by Dr. Susan Jackels and are listed in Table 26.77 It is obvious that when several five-membered rings are fused together,([13JaneN^, the resulting complex is strained because the complete ligand is too short for facile, planar tetradentate chelation. The largest sources of strain arise from bond angle stretching and torsional deformation as indicated by the quantities 9 and 0 listed in Table 26. In contrast, the fusion together of several six-membered chelate rings produces crowding in the structure. Huge non bonding interactions and distortions of the bond angles result. Hence trans- [Co([l6janeN4)Cl2j is the least stable complex and it is the substance which gives the most rapid rate of reaction, as reported above. The graph of lnki vs Hgtrain energy is a fairly good straight line with slope of 0.6 (Fig. 21) indi cating that the strain energy is a dominant factor in the aquation of these macrocyclic complexes. That the strain energy is a dominant factor in the kinetic behavior of the fully saturated macrocyclic complexes is also demonstrated by the kinetic behavior of [Ni(MAC)(H20)2]2 complexes. The rate of dissociation of the macrocyclic ligand from the complex in 0.3 N HC104, at an ionic strength 0.5 a t 25° C, follows the ord er:
[l4]aneN4 < [l3]aneN4 < [l5]aneN4 < [l6]aneN4 2.0 x i0-5sec-1 6.4 x l0-5sec-1 9.3 sec-1 and 0.18 sec-1 Table 26 Minimized Strain Energies of_toans-[Co(MAC)Cl2] . MAC R NB _0_ JL H_ Conform er
[i3]aneN4 2.01 4.97 7.37 5.39 19.74 8^ 6y [i4]aneN4 1.95 2.77 5.60 1.20 11.53 gyA-y [i5]aneN4(l) 4.26 5.32 8.02 3.73 21.33 yy6^ [i5]aneN4(II) 3.49 7.50 7.69 6.79 25.47 [i6]aneN4 7.81 10.18 10.98 6.60 35.56 6yXy 98
The rate of dissociation of the 14-membered ring complex is too slow to be Com_cl Ni.fi measured. The graph lnk1 vs lnk^ is a fairly good straight line with'slope — 1. This strongly suggests that the two reactions, aquation of tra n s -[Co'^(MAC)Cl?]+ and the dissociation of [Ni**(MAC)(H20)2]2+, are influenced by the same set of strain energy consideration. Since a ring could not conceivably be removed from the metal ion without folding, this suggests that for the reactions studied here the more highly strained stiuctures could involve some folding of the ring in the transition state. All of the macrocyclic complexes that have been investigated at this tim e,31*34’35*37 whether the ligand is saturated, unsaturated, substituted, or not substituted exhibit a characteristic property. That is the retention of configuration for the macrocyclic ligand in the product of aquation. This is consistent with the assmnption that the intermediate in the aquation reaction of the macrocyclic complex has the tetragonal pyramidal geometry. For cases where the strain energy is modest, this is easily understood in view of the stereorestrictive nature of the ligand, a properly which could oppose the trigonal bipyramidal structure for the five-coordinate intermediate state.31 If the ring fits in a strain free fashion, the enthalpy of the transformation from octahedral to trigonal bipyramidal in the macrocyclic complex is considerably less favorable than that to the tetragonal pyramide because the ligand molecule is not required to rearrange in the latter case. Tobe proposed48 that the structure of the intermediate formed during substitution of Co(IH) could be distinguished by using the value of the entropy of activation. Positive values of AS were taken as evidence for a trigonal bipyramidal intermediate while a negative entropy of activation was assumed to imply a tetragonal pyramidal intermediate. According to Tobe's concept, trans-[Co(fl3laneNd)Cl2l+ which has AS^ = 13 cal deg-1mole-1 would be assigned a trigonal bipyramidal intermediate. Simplistically viewed, this would lead to the prediction that the complex would I
99
undergo geometric isomerization during aquation. To the contrary, this compound aquates with retention of the tr ans -configuration. Interestingly, the activation parameters for the [i3]aneN4 and trien complexes are very similar
(AH 25.6 kcalmole-1, a S 13 caldeg-1mole-1 for the former; 26 kcalmole-1 and 16 cal deg-1m ole-1 for the latter), and the values for both differ substantially from those of their homologs. The activation parameters are i also similar to those of bis(ethylenediamine) complex (a H 26.2 kcalmole , J: i . AS 14 caldeg mole ). This could be considered as another sign that an additional factor influences the kinetic behavior of the small ring compounds in addition to those affecting the larger ring compounds. It is possible that the [l3]aneN4 complex actually forms an approximately trigonal bipyramidal intermediate but that the conformation of the ligand is such that it is constrained to return to the trans structure rather than fold further to form a detectable amount of the cis structure in the final product. The conformational energy calculations77 on complexes of [l3]aneN4 show that the conformer which has two amine hydrogens on the six-membered ring pointing toward one side of the coordination plane, and the other two pointing toward the other side is the least strained. This conformation is represented in Table 26. Earlier studies76 have shown that complete folding of tetraaza-macrocycles to form cis[M(MAC)X>T can occur only when a pair of hydrogens on trans nitrogens are located on the same side of the coordination plane. We therefore suggest that strains relieved in forming a transition state that resembles a trigonal bipyramid but that steric constraints still lead only to retention of the trans configuration during the overall substitution process. The enthalpy of activation of the i3-membered ring derivative is slightly higher than that of the [i4]aneN4 complex, but this is compensated by the favorable entropy of activation. Because the enthalpies of activation of the 15-membered ring derivatives and (apparently) the 16-membered ring derivatives are much lower than that of the [l4]aneN4 derivative , the dissociation rate is tremendously increased. Since the sources of strain 100 energy for the complex of [i3]aneN4 are different from those for the complexes of the [i5]aneN4 and [l6]aneN4, (E q and for [i3]aneN4 and E N B and E q for [l5]aneN4 and [i6]aneN4) tlie observed distinction in parameters is easily rationalized. The half-wave potentials for the Co(III)/Co(IQ couples for the series trans-[Co(MAC)Cl?]+ have the values: [i4janeN4 (-0.69 V) < [±3]aneN4 (-0.66 V) < [i5]aneN4 (-0.38 V) < [i6]aneN4 (-0.15 V) with the 14-membered ring complex being the hardest to reduce. This data provides another example of the ring size effect wherein the variations resemble those observed here. The calculated ideal metal-nitrogen bond lengths and deviations from planarity of .the various macrocyclic ligands based on strain energy calculations, are listed in Table 26. The Co(ffl) ion is too small to fit into a 16-membered ring without extensive contraction of the hole. Similar but less extensive contraction is necessary for the 15-membered ring complex. Therefore, it is easier to reduce Co(HI) to the larger Co(ll) ion in these complexes, thereby releasing the strain resulting from contraction. The 13-membered ring complex is
Table 27 Ideal Metal-Nitrogen Bond Lengths and Planarity of the Macrocyclic Ligands. Ring Size Average Ideal Bond Length A Average deviation from the ideal ISb plane, A
13 1.92 0.12 14 2.0 7 0.00 15 2.22 0.14 16 2.38 0.00 again a special case. Because of the shorter chain, the ring cannot accomo date the metal ion without having the metal ion deviate from the N4 plane. Therefore, the complex exhibits a behavior that is not expected directly io i
from the rationale stated previously for the 16- and 15-membered ring systems. A simple and useful point of view follows from recognizing that the small 13-membered ring may force a change in the stereochemistry of the reduced member of the couple. Thus Co(E0 may be displaced from the plane in its complex with [l3]aneN4 or it may have a structure in which the ligand is more heavily folded. Limited data on high spin Ni(II) and Fe(II)79 suggest that the larger rings allow 5-coordination more generally than does the 13-mem-* bered ring which seems to favor 6-coordinated structures in which the ligand is folded. In a preceding section the V£0_ci asymmetric stretching band has been identified in the infrared spectra of the trans-complex (Table 10). The values are scattered. No relationship has been found between vco-Cl anc* Cl ring size, rate of aquation or Dq values. However, it is noted that
reaches a maximum and that 6 n m N reaches a minimum for the complex of [l4janeN4, a feature that is consistent with the strain energy relationships. Eade et al40 were able to correlate the in-plane bonding mode SjsriVIlSr -h the rates of aquation of several linear amine complexes, _trans-[CoA4Cl2J but the values for 6NMN f°r the trans-[Co(MAC)Xa]+ complexes are scattered.
A trend in hjvjmn (out-of-plane deformation) was observed for the complexes understudy here, [i4]aneN4 > [l3]aneN4 > [l5]aneN4 ~ [l6]aneN4. While the sequence is approximately monotonic with the rate of aquation, a strong corre lation cannot be claimed. Since strain energy does correlate with the aquation rate, a simple model causes one to suspect that tinmn s^ould correlate. This follows from the simplest model for relief of strain energy when [Co (MAC)C12] aquates by an Sjji mechanism. The suggested tetragonal intermediate (and related transition state) could most easily distort by movement of the metal atom out of the N4 plane, thereby permitting adjustment in ligand parameters so as to lower the strain energy. 102
+ The isomerization of trans-[Co([l3]aneN^)(H?Q)?]3 can be justified by this method. The five-coordinate intermediate is distorted toward trigonal-bipyra midal structure in order to release the strain. The incoming water can enter by three pathways; paths one and two lead to
cis-diaquo complex, while path three leads to the trans-complex. Path three is less likely because of the "hydrophobic" property of the carbon chains. Therefore, the isomerization is observed. Since cis- and trans-diaquo complexes have different conformations (Table 26 and earlier discussions), it appears that inversion of the nitrogen has to occur sometime during the reaction. The reason that the isomerization did not occur during the second aquation stage could be explained by arguing that the chloride ion is still in the vicinity of the coordination site, hence it prevents the ring from distort ing too far from tetragonal pyramidal geometry. 103
From the strain energy calculations, the trans complex of [l4]aneN4 is the le ast strained of the series tra n s - [Co( [XIaneN^) Y0]n+. [i4]aneN4 fits Co(III) almostperfectly;2 therefore, the complex resists distortion. This can be correlated with the high energy o f the out-of-plane deformation mode in the infrared spectrum, by the slow rate of aquation, by the absence of trans to cis isomerization and by the higher reduction potentials (harder to reduce). "f* The kinetic behavior of cis-[Co(MAC)Cl9] complexes. — The aquation of cis-[Co([12 or l3]aneN4)Cl2J+ has been studied in 0.1 N acid medium at several different temperatures. As is generally true of such isomers, the cis^-complexes aquate with retention of the cis-eonfiguration. The aquation data for cis-[Co- (MAC)C12]+ and several other related complexes are collected in Table 28. The rates of aquation of the cis-macrocyclic complexes do not differ as dramatically
Table 28 TTT + Kinetic Data for the Aquation of cis-[Co (L)C12] .
L k itsec"1) at 25° C aH^ ^Ca/^mole AS^ca^degmole AG^Ca/mole
[l2]aneN4 4.2 x 10~3 13.5 -24 20.8 [l3]aneN4 1.0 x IO"2 15.2 -17 20.4 [l4]aneN4 1.6 x 10-2 18.3 - 6 20.1 jS-trienb 1.5 x IO-3 20 .5 - 3 21.9 &2,3,2-tet0 1.1 x 10-3 22.3 2 21.7 (en)2d 2 .4 x 10"4 21 .5 - 5 23.0
aFrom Ref. 41. bRef. 24. °Ref. 26. ‘W . 32.
as do the rates for the trans-complexes. The rate constants only differ by a factor of four from [l4]aneN4 to [l2]aneN4. The activation enthalpies of the cis-complexes follow the order [l2]aneN4 < [l3]aneN4 < [l4]aneN4; however, the order for the activation entropies follows the reverse trend. This results -2 0 -10 0 AS
Figure 22. The Iso-kinetic Plot of Some Related cis-Complexes 105
in a fairly constant activation free energy, hence a small range for the rates. The cis-dichloro complexes of macrocyclic ligands and the similar linear tetra- amine complexes display an isokinetic relationship. That is, the enthalpies and entropies of activation vary linearly (Fig. 22), with the slope of 366. The slope has units of absolute temperature and is equal to the isokinetic temperature (At that temperature the compounds would react at the same rate). This beha vior suggests that variation in a single structural parameter (and its effect) dominates the variation in rate.18 It has previously been suggested48 that all cis-complexes aquate via tetragonal pyramidal intermediate states. Since no steric change occurs during aquation, this is the structure expected for the intermediate. The kinetic behavior reported here and the electronic spectral behavior described earlier have demonstrated that the cis-complexes are insensitive to the size of the macrocyclic ring. From this it may be concluded that when chelated in folded conformation, macrocycles do not exhibit the distinctive effects that derive from excessive strain energies. To reflect back on the behavior of
the trans complexes of the composition [Co (MAC)C12] , these conclusions support the view that the relief of strain energy may accompany the dissociation of a chloride ligand. Some folding of the macrocycle in the 5-coordinate transition state may produce species in which the macrocyclic ligand can chelate while experiencing relatively little strain energy. There remains to be considered the very tidy isokinetic behavior found in the rates of aquation of cis-[Co(MAC)C12] . Since the effect involves both strong enthalpy and entropy changes, the varying phenomenon most probably is external in character.18 The entropy is expected to be relatively insensitive to such factors as stereorestrictivity which vary internally with respect to the structure of the complex. It is therefore highly rational to suggest that the interaction generating the isokinetic behavior among cis-[Co(MAC) C12] complexes derives from solvent interactions. One might anticipate more extensive solvation in the polar transition state, in which case /\S would become 106 more negative as the hydrophobic structural component decreased in amount; 4 i . e . , a S should become more negative as ring size decreases. This is a rather compelling argument that the solvational effects of ring size are most simply and clearly seen in cis complexes. It is interesting in this regard to recall that Margerum and associates75 have tended to emphasize solvational effects in discussing the dissociation of macrocyclic ligands from metal complexes. The relatively greater importance of stereorestrietivity in those reactions has been pointed out by Jones et al.78 and also demonstrated in the unpublished results of Martin and Callahan.5 SUMMARY
E arlier studies1 >3 >4 >5 »6 on Co(IID and Ni(D) complexes of macrocyclic ligands which showed that chemical and physical properties, such as ligand field strengths, strain energies, redox potentials and rates of displacement of the macrocyclic ligand vary as the size of the ring, provided the motivation for further studies on macrocyclic ring size effects. The effect of this 4" structural parameter on the rates of aquation of the complexes [Co (MAC)C12] has been evaluated in this study. -}• A series of trans-[Co(MAC)Cl2] complexes where MAC = [l3]aneN4, [i5]aneN4, and [l6]aneN4, have been prepared and the conformational isomers of [l5]aneN4 and [i6]aneN4 have been separated and characterized. The isomers display different colors, solubilities, infrared and electronic spectra, Dq values, and electrochemical properties. Complexes of cis_-[Co([l3-i6]aneN4)C03] C104 and cis-[Co([l2, iSjaneN ^cyci have also been synthesized and characterized. 4* XV trans-[Co([l3-16]aneN4)Cl2] display Dq values in the order [l3]aneN4 > [l4]aneN4 > [i5]aneN4 > [l6]aneN4, with the total difference being 511 cm-1, while Dq values for the related cis-carbonato complexes only differ by 144 cm-1. The aquation reactions were studied in 0.1 N nitric acid at several different temperatures (range of 20° C). Rate constants fall in the order [i4]aneN4 < [l3]aneN4 < [l5]aneN4 < [l6]aneN4 for trans-[Co([13-16 janeNJCl?] and they range over a factor of i06. The trend parallels the calculated strain energies of the complexes in the appropriate conformation^7 and the rates of dissociation of [Ni(MAC)(H20)2]2+. However, the rate of aquation of the cis-complexes 4" [Co([i2-l4]aneN4)Cl2] does not differ much, only a factor of four. That the chemistry of the folded compounds is insensitive to the size of the macrocyclic ring is also demonstrated in the Dq values of the cis-carbonato complexes. The 107 108 ring size effect on the rate of aquation of the cis complexes is strictly a solvational phenomenon while the effect in the case of the trans complexes deri ves mainly from the stereorestrictivity imposed by strain energy changes. The isomerization of trans-[Co([l3]aneN4)(H20)2]3 to the cis-diaquo compound has also been studied. It was found that this reaction is much faster than the release of the chloride from the complex, trans_-[Co([l3]aneN4)Cl(H20)]2 , therefore, the observed product for the aquation of trans-chloroaquo complex of [l3]aneN4 is a mixture of cis-diaquo and cis-chloroaquo complexes. Isomer II of trans- [Co([l5]aneN^)Cl(H^O)]2 also isomerizes to the more stable isomer I. This reaction requires ionization of a proton from an amine group of the macro- cyclic ligand, followed by inversion in chirality of that nitrogen atom. P a rt I BIBLIOGRAPHY
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Do W. Watkins and D. H. Busch, submitted for publication. PART n Stereorestrictive Chelation—Metal Complexes of Sparteine and N, N'-dimethylbispidine.
INTRODUCTION
The macrocyclic tetradentate ligands have demonstrated how the rigid structure of the ligand will influence the chemical and physical properties of the resulting complexes. It is important to extend the studies to complexes of other stereorestrictive ligands with different numbers and dispositions of coordination sites, especially to those which serve as bidentate ligands. The ligands of lower dentate character provide the ultimate test for concepts relating the stereorestrictivity and bidentate ligands are the subject of the present study. Some studies1’2,3 have been done on complexes of small ring ligands as piperazine (structure J), 1,4-diazacycloheptane (dach) (structure I® and 1,5-diazacyclooctane (daco) (structure IU). It was found that with Ni(D) and Cu(H) daco produces planar complexes regardless of the anion or solvent present while the geometry of the Cu(3D complexes of dach is dependent on the solvent and the anions present. The nickel complex of daco is very stable. It is rather resistant to attack by chloride, bromide, or cyanate in aqueous solution, and decomposition is not observed on solution in 12 N HC1;1 however, the addition of cyanide will replace the ligand. This behavior is similar to that of the nickel complex of [l4]aneN4. It has been suspected that the unusual stability of daco complex comes partly from the blocking of the axial coordina tion positions by the methylene hydrogens (structure VIU) and this geometric factor has been confirmed by X-ray crystal structure determination on the
114 115
-N' .N
'N p ip e ra z in e 1, 4-diazacycloheptane 1, 5-diazacyclooctane I n in
tE O f— i ^ 1 e M ey = = C 0 2Et
3, 31, 5, 5'-tetram ethyldipyrromethene- 4, 41 -dicarboxylate
IV
CH.
s p a r te in e N, N'-dimethylbispidine
V VI Figure 1. Some Bidentate Ligands 116
nickel complex of 1,5-diazacyclooctane N, N’-diacetate (daco DA).4 The ML 2 complexes of dipyrromethene(IV) have also been studied.5 Because the bulky 5,5'-methyl groups prevent the two ligands from bonding in a square-planar geometry, the cobalt and nickel complexes are tetrahedral and the copper complex of this ligand exhibits a distorted tetrahedral geometry which has been confirmed by X-ray crystal structure determination.5 The stereochemistry and properties of MLX2 complexes of N,N,N', N '-tetra- methylated ethylenediamine (Me4en), 1,2-propylenediamine (Me4pn), and trim ethylenediam ine (Me4tn) have also been synthesized w here M is Co(II) or Ni(H).7 It was found that the tendency to form pseudotetrahedral struc tures is in the order JVIe4en< Me4pn< Me4tn. Apparently, the substitution on the N-atoms has a large influence on the structure. The rate of dissociation of en or Me4 en from their aquonickel(H) or aquocopper(II) complexes has been studied44.45*46 in acidic media. It was found that k(j for [Ni(en)]2 and [Ni(Me4en)j2 are nearly the same (k^ ~ 0.15 sec-1 at 25° C), while k^ for [Cu(Me4dn)]2+ is smaller than that for [Cu(en)]2+ (38.5 sec -1 vs ~ 115 sec-1 at 25° C). Sparteine (structure V) and 3,7-dialkyl-3,7-diazabieyclo[3,3,1]- nonane(N, N’-dialkylbispidine) (structure VI) are similar to daco but have a bridge connecting the methylene groups on different sides of the ring. This obviates the blocking of the axial coordination positions by protons; however, the presence of substituent groups on the nitrogen will force the geometry of the complex away from square-planar toward a tetrahedral structure. Also, 117 it is noted that these are steric hindered tertiary amines; therefore, the Ni(II) and Cu(H) complexes are expected to be less reactive than those complexes of Me4en. It is our goal to synthesize and study the reactivities of the metal complexes of these ligands. However, during the study, the publication by Mason and Peacock41 describing a number of metal complexes with sparteine appeared; their compounds include complexes of the type M(sp)X2 + + + *hi where M - Co 2, Ni 2, Cu 2, and Zn 2; sp = sparteine; X = CF, Br“, or F . Boschmann et al37 at the same time studied the rate of hydrolysis of the copper(D) complexes of three sparteine diastereoisomers in the form Cu (L)C12, where L = (+)/3-isosparteine, (-)-sparteine, and (-)ft-isosparteine. EXPERIMENTAL
M aterials
All the solvents and chemicals were reagent grade and were used without further purification. Methanol and acetonitrile were dried over appropriate Linde molecular sieves. Sparteine sulfate was purchased from K and K Labs., Inc., Plainview, N.Y. Methylamine hydrochloride was purchased from MCB. i-methyl-4-piperidone obtained from Aldrich Chemical Company, Inc., was used without purification. Paraformaldehyde was obtained from Fisher Scientific Company.
Physical Measurements
Electronic Spectra. — Visible and near-infrared absorption spectra were obtained on solution in matched silica cells or as mulls on filter paper using a Cary Model 14R recording spectrophotometer. Infrared Spectra. — The infrared spectra were obtained using Perkin- Elmer Models 337 and 457 recording spectrophotometers on nujol mulls or Csi pellets. Polystyrene standards were used to calibrate the spectra. Conductivities. — An Industrial Instruments Model RC 16B conductivity bridge was used to obtain the conductivites of the complexes. Measurements were taken at room temperature at 1000 cps on~0.00i M solutions. Magnetic Moments. — Magnetic moments were measured at room temperature by the Faraday method.42 The system was equipped with a Calm electrobalance and was calibrated with Hg[Co(NCS)4]. Diamagnetic corrections for the ligands were made using Pascal's constants.8
118 119
Electron Spin Resonance Spectra. — The esr spectra were obtained with a Varian V-4500 Model EPR spectrophotometer operating in the X band. The instrument is equipped with a dual cavity; a sample of DPPH was used in the reference cavity and its g value was taken to be 2.0037. The g values were calculated by the approximate method of Kneubuhl9 and are accurate to + 0.005. Mass Spectra. — Mass spectral measurements were carried out by Mr. Richard Weisenberger of The Ohio State University on an MS-9 spectrometer, at an ionizing potential of 70 eV, using a direct insertion probe. Elemental Analyses. — Elemental analyses were performed by Chema- lytics, Inc., Tempe, Arizona or by Galbraith Labs, Inc., Knoxville, Tenn. Molecular Weights. — Molecular weights of the complexes were determined using a Mechrolab Inc. Vapor pressure osmometer Model 301A. Concentrations were between 0.01 and 0.06 M in chloroform. Electrochemical Measurements. — The measurements were perfor med by Dr. M. Rakowski and Miss K. Holter. Computer Calculations. — The Ohio State University Instructional and Research Computer Center IBM 370 Model 165 computing systems were used to perform all calculations. All programs used were written by L. J. D eH ayes.48
Syntheses
N, N'-dimethylbispidine and N-ethyl-N'-methylbispidine. — These compounds were synthesized according to the procedure of Douglass and Ratliff10 except that CH3NH2 • HCl is used to synthesize N,N'-dimethylbis pidine. [J. Org. Chem., 33, 355 (1968)]. 120
Extraction of Sparteine. — Due to the ease of sparteine to form oxide, sparteine was isolated just before the reaction with metal salts. Five grams of sparteine sulfate is dissolved in a minimum amount of water and is neutralized with 25 ml of 10$ NaOH. The aqueous layer is extracted with five 20 ml portions of ether. The ether extract is combined and dried over MgS04„ The residue after rotary evaporation is used without further puri fication. Dichloro-N, N'-dimethylbispidinecobaltdD, [Co(DMBsp)Cl?] and dichlorosparteinecobalt(II), [Co(sp)Cl?]. — The syntheses of the cobalt(Il) complexes are all carried out under a N2 atmosphere. Anhydrous cobalt(D) chloride (. 2 g) is dissolved in acetonitrile and 10 ml of triethylorthoformate is added. The blue solution is then refluxed for thirty minutes. Ligand, in a 1:1 equivalent amount, is added and the solution is stirred for twenty minutes. After reducing the volume, ethanol is added to induce the formation of blue crystals. Yield ~ 70$. The products are recrystallized from
acetonitrile. Anal. Calcd. for Co(DMBsp)Cl2, Co (C9HI3N2)C12: C, 38,05; H, 6.39; N, 9.86; Cl, 24.96; Co, 20.74. Found: C, 38.06; H, 6.56; N, 9.86; Cl, 24.89; Co, 20.35. Anal. Calcd. for Co(sp)CI2, Co(C10H26N2)- Cl2: C, 49.47; H, 7.20; N, 7.69; Cl, 19.47; Co, 16.18; Found: C, 49.36; H, 7.06; N, 7.66; Cl, 19.58; Co, 16,06. Mass Spectrum: p= 363, p + 2 = 365. Co(DMBsp)Bp> and Co(sp)Br 9 . — The same procedure is followed as for the chloride derivatives, except that anhydrous cobalt(If) bromide is used. The blue products are recrystallized from ethanol. Yield: ^ 65$. Anal. Calcd. for Co(C9H18N2)Br2: C, 28.98; H, 4.S6; N, 7.51; Br, 42.85; Co, 15.80. Found: C, 28.97; H, 5.11; N, 7.51; Br, 42.70; Co, 15.66. Anal. Calcd. for Co(C15H26N2)Br2: C, 39.76; II, 5.74; N, 6.19; Br, 35.30; Co, 13.01. Found: C, 40,03; H, 5.81; N, 6.31; Br, 35.05; Co, 12.87. CoIDMBspXCH-jCO?)? and CotspHCILCO?)?. — The sam e procedure is applied for these complexes, except that cobalt(D) acetate 4-hydrate is used and the reaction mixture in methanol is refluxed for two hours before 121
the addition of the ligand. Violet crystals are obtained. Co(DMBsp)(CH3C02)2 is re crystallized from xylene. Yield, dimethylbispidine complex: 75$. The sparteine complex is re crystallized from toluene. Yield: 90$. Anal. Calcd. for CotCgHigN^CHgCO^: C, 47.13; H, 7.30; N, 8.46. Found: C, 47.09; H, 7.17; N, 8.57. Molecular weight (by osmometer) 330.8 (calcd. 331.275). _Anal. Calcd. fo r Co(C15H26N2)(CH3CC^)2: C, 55.49; H, 7.79; N, 6,8i. Found: C, 55.40; H, 7.66; N, 6.71; m.p. 158° C. Co(DMBsp)I) and Co(sp)I?. — One gram of the acetato complex is dissolved in methanol and 2 ml of hydroiodic acid is added dropwise to the solution. The blue product precipitates from the solution and is recrystallized from ethanol. Yield: ~8i$. Anal. Calcd. for Co(C9H18N2)]^: C, 23.15; H, 3.89; N, 6.00; I, 54.35; Co, 12.62. Found: C, 21.44; H, 3.87; N, 5.84; I, 55.11; Co, 12.40. Anal. Calcd. for Co(Ci5H26N2)] 2: C, 32.92; H, 4.76; N, 5.12; I, 46.42; Co, 10.77. Foimd: C, 32.82; H, 4.75; N, 5.01; I, 46.25; Co, 10.06. Co(DMBsp)(NCS)9 and Co(sp)(NCS)9. — Metathesis of the diiodo derivative with sodium thiocyanate in ethanol in the presence of air results in the royal blue dithiocyanato compounds, which is recrystallized from methanol. Anal.
Calcd. for Co (C9H18N2)(NCS)2: C, 40.12; H, 5.51; N, 17.01; S, 19.47; Co, 17.89. Found: C, 41.09; H, 5.74; N, 16.35; S, 18.44; Co, 17.15. Anal.
Calcd. for Co (C15H26N2)(NCS)2: C, 49.89; H, 6.36; N, 13.70; S, 15.65. Found: C, 49.80; H, 6.50; N, 13.94; S, 15.89. Dichloro N, N1 -dimethylbispidinenickel(II), [Ni(DMBsp)Cl9], dichloro sparteine- nickel(Il), [Ni(sp)Cl9j, and dichloro N-ethyl-N'-methylbispidinenickeldD, [Ni- (EMBsp)Cl9]. — Four tenths of one gram of NiCl2 • 6H20 is refluxed in a mixture of 30 ml of triethylorthoformate and 60 ml of acetonitrile for two hours, then ligand in 1:1 ratio is added to the solution. The solution turns to pinkish color. The volume is reduced and ethanol is added leading to the deposit of purple crystals. The product is re crystallized from ethanol (for both compounds). Yield: — 30$. Anal. Calcd. for Ni(C9H18N2)Cl2: C, 38.08; H, 6.39; N, 9.87; Cl, 24.98; Ni, 20.68.
Found: C, 37.96; H, 6.43; N, 9.79; Cl, 25.17; Ni, 20.51. Anal. Calcd. for 122
Ni(C15H2eN2)Cl2: C, 49.50; H, 7.20; N, 7.70; Cl, 19.48; Ni, 16.13. Found: C, 49.24; H, 7.43; N, 7.59; Cl, 19.28; Ni, 16.26. Mol. wt. 344.4 (calc. 363. 7). Mass spec, p = 362,p + 2 = 364. Anal. Calcd. for [Ni(EMBsp)Cl2], Ni(C10H20N2)Cl2: C, 40.32; H, 6.77; N, 9.40; Cl, 23.80; Ni, 19.71. Found: C, 40.12; H, 6.77; N, 9.33; Cl, 23.64; Ni, 19.87. Ni(DMBsp)Br» and Ni(sp)Br9. — The same procedure is applied for the syntheses of these compounds, except NiBr • 3H20 is used. Purple crystals are obtained after recrystallizing from ethanol. Anal. Calcd. for
N i(C 9 H igN 2) B r 2 : C, 29.00; H,4.87; N , 7.51; Br, 42.87. Found: C, 29.40; H, 4.92; N, 7.67; Br, 42.44. Anal. Calcd. for Ni(C15H26N2)Br2: C, 39.78; H, 5.79; N, 6.09; Br, 35.29. Found: C, 39.87;H, 5.83; N, 6.19; Br, 35.13. Mass Spec., p = 450. Ni(DMBsp)(CI-I?,COo)o and Ni(sp)(CH^CO?)2. — These compounds are made by the same method as the dichloro derivatives, except nickel(U) acetate 4-hydrate is used. Green crystals are obtained. Anal. Calcd. for Ni
Ni, 10.74. Found: C, 32.76; H, 4.91; N, 5.09; I, 46.29; Ni, 11.07. Mass Spec, p = 546. Ni(DMBsp) (NCS)9. — Metathesis of the dichloro derivative with sodium thiocyanate in a mixture of acetonitrile and ethanol results in the green thiocyanato compound. Anal. Calcd. for Ni(C9H18N2)(NCS)2: C, 40.14; H, 5.51; N, 17.02; S, 19.48; Ni, 17.84. Found: C, 40.04; H, 5.5l; N, 16.93; S, 19.46; Ni, 17.86. Cu(DMBsp)Cl? and Cu(sp)Cl?. — Three hundred milligrams of copper(Il) chloride 2-hydrate is dissolved in methanol and excess triethylorthoformate is added. This solution is refluxed for one hour and the ligand is added in a 1:1 ratio. The green crystals are recrystallized from ethanol: Yield:
~ 70$. Anal. Calcd. for Cu (C9H13N2)C12: C, 37.44; H, 6.29; N, 9.70; Cl, 24.56; Cu, 22.01. Found: C, 37.68; H, 6.34; N, 9.59; Cl, 24.11; Cu, 21.87. m.p. 178-180° C. Mol. Wt. 306.8 (calc. 288. 697). (m/e 287). Anal. Calcd. for Cu(Cl5H26N2)Cl2: C, 48.85; H, 7.11; N, 7.60; Cl, 19.22; Cu, 17.23. Found: C, 48.56; H, 7.27; N, 7.70; Cl, 19.22; Cu, 17.39. m .p . 167° C. Cu(DMBsp)(CH^CO?)? and Cu(sp)(CH^CQ9)9. — The same procedure is followed as for the chloride derivative, except acetonitrile is used as solvent and the acetate salt was used Shiny blue crystals are obtained. Anal. Calcd. fo r Cu (C9H18N2)(CH3C02)2: C, 46.49; H, 7.20; N, 8.34: Found: C, 45.09; H, 6.93; N, 8.12. Mol. wt. 354 (calc. 335.885). Calc, for Cu(C15H26N2)- (CH3C02)2: C, 54.87; H, 7.70; N, 6.74; Cu, 15.28. Found: C, 54.94; H, 7.61; N, 6.71; Cu, 15.24. Cu(DMBsp)Br-> and Cu(sp)Br?. — Hydrobromic acid is added dropwise (several drops) to the ethanolic solution of the diacetato derivative. Yellow crystals are obtained by recrystallizing from ethanol. Anal. Calcd. for Cu(C9IIi 3N2)Br2: C, 28.63; H, 4.80; N, 7.42; Cu, 16.83. Found: C, 28.96; H, 4.74; N, 7.28; Cu, 16.70. Anal. Calcd. for Cu(Cl5H26N2)Br2: C, 39.34; H, 5.68; N, 6.12; Br, 34.97; Cu, 13.88. Found: C, 39.17; H, 5.86; N, 124
6.09; Br, 34.85; Cu, 13.86. m.p. 157° C. Cu(DMBsp)!?. — The same procedure is followed as for the dibromo compound except that hydroiodic acid is used. Reddish brown crystals are obtained. Anal. Calcd.; C, 22.92; H, 3.85; N, 5.94; I, 53.82; Cu, 13.47. Found: C, 23.19; H, 3.85; N, 5.50; I, 52.96; Cu, 12.84. The attempt to make Cu(CI5H26N 2)]2 failed. The hydroiodic salt of sparteine was obtained. [Cu(CflHiaN9) (OH) ]9X9, X = C107 o r BF7. — Copper(Il) p erch lo rate 6-hydrate in 15 ml of absolute ethanol is added to the ethanolic solution of excess ligand. A precipitate forms immediately. Deep lavendar crystals are obtained by recrystaRizing from acetonitrile. Anal. Calcd. for Cu(C9H18N 2)- (OHMClQj): C, 32.34; H, 5.73; N, 8.38; Cl, 10.61; Cu, 19.01. Found: C, 32.42; H, 5.75; N, 8.26; Cl, 10.52; Cu, 19.25. Mol. wt. 700 (calc, for [Cu(C9HlaN2)(0H)(C104)]2,678.5) at .0091 g/iO ml concentration.
Zii(Ci;;H9rN9)Cl9. — This compound is synthesized by the same method as for the corresponding copper complex. The white product is recrystallized from chloroform. Anal. Calcd.: C, 48.60; H, 7.02; N, 7.56; Cl, 19.17; Zn, 17.65. Found: C, 48.68; H, 7.03; N, 7.44; Cl, 18.91; Zn, 17.71. m.p. 276° C. Pd(DMBsp)Cl9, Pd(EMBsp)Cl9, and Pd(sp)Cl9. — Palladium chloride 1.8 g (.001 mole) and LiCl 1.0 g (.02 mole) are heated in 50 ml of 1-butanol with magnetic stirring until a nearly clear solution is formed and the solution is then filtered.43 After the addition of an appropriate amount of the ligand (1:1 ratio), the solution is stirred for several hours. The dark precipitates are filtered and the volume is reduced. Ethanol is added to deposit orange crystals which are recrystallized from acetonitrile. Anal. Calcd. for PdtCgHjghyCl;,: C, 32.60; H, 5.47; N, 8.45; Cl, 2l;38; Pd, 32.09. Found: C, 32.38; H, 5.53; N, 8.22; Cl, 21.39; Pd, 31.40. Anal. Calcd. for
Pd(C i0H2oN2)Cl2: C, 34.76; H, 5.83; N, 8.11; Cl, 20.52; Pd, 30.79. Found: C, 35.15; H, 5.60; N, 8,03; Cl, 20.39; Pd, 29.10. Anal. Calcd. for 125
Pd(C15H26N2)Cl2: C, 43.76; H, 6.37; N, 6.80; Cl, 17.22; Pd, 25.85. Found: C, 43.51; H, 6.21; N, 6.86; Cl, 17.24; Pd, 24.4. Pd(DMBsp) (NCS)9. — Metathesis of the dichloro derivative with sodium thiocyanate in a mixture of acetonitrile and ethanol results in yellow crystals. Anal. Calcd: C, 35.06; H, 4.82; N, 14.87; S, 17.02; Pd, 28.24. Found: C, 35.39; H, 4.89; N, 14.82; S, 16.21; Pd, 28.60. Co(Me/t^[l4]aneN^)ClC10/1. — The ligand Me4N[l4]aneN4 is synthesized according to Barefield's method11 [Inorg. Chem., 12_, 2435 (1973)]. Five hundred milligrams of anhydrous cobalt(D) chloride is dissolved in a mixture of methanol and triethylorthoformate and the solution is refluxed for one hour. A methanolic solution of Me4^[i4]aneN4 is added. Lithium perchlorate is added to deposit violet crystals. Anal. Calcd.: C, 37.35; H, 7.16; N, 12.44. Found: C, 37.49; H, 7.53; N, 12.53. A ttem pts to synthesize [M(DMBsp)?32+ com plexes. — D ouglass and Ratliff reported10 the synthesis of Cu(DMBsp)2(C104)2 by adding ethanolic solution of Cu(C104)2 * 6H20 (10 mmoles in 30 ml of absolute ethanol) to a solution of DMBsp (20.0 mmoles in 20 ml of absolute ethanol). Therefore, the same procedure was followed. However, only the complex [Cu(DMBsp)- -f (0 H)]2(C104)2 was isolated and identified. Attempts to prepare [Cul^r by reacting Cu(BF4)2 • 4 acetonitrile12 with excess ligand in ethanol results in the same hydroxyl complex. Reactions of Cu(C9H18N2)X2, where X = Cl", or Br“, with excess ligand were also carried out in methanol. A green compound which has a strong infrared peak around 1550 cm-1 was obtained. No further effort was made to characterize this compound. Metal ions of cobalt(H), and palladium(Il) were also tried. Co(BF 4)2 • 6 acetonitrile12 was refluxed in a mixture of methanol and triethylorthofor mate for 20 min.and was mixed with excess ligand. A brown compound was obtained. No effort was made to characterize this compound. Another trial was performed by refluxing Co(BF4)2 • 6 acetonitrile in a mixture of 2,2- dimethoxypropane and acetonitrile for 2 hrs. After mixing with excess 126
ligand, magenta crystals were obtained which gives a strong infrared absorption peak at 3600 cm-1, a hydroxyl compound. A trial with palladium(ll) complex was then performed. Pd(DMBsp)Cl2 was dissolved in chloroform and AgNOa was added. The mixture was brought to reflux for 30 min. This was filtered. Orange crystals were obtained after reducing the volume. However, the elemental analyses fit the starting material PdtCgHjgNgJC^. A hydroxyl compound was obtained when mixing Pd(DMBsp)Cl2 with excess ligand in the presence of NaC104. This compound is suspected to be [Pd(C9H18N2)(OH)]2(C104)2, however, no effort was made to confirm this.
K inetics
The reactions were followed with a Cary Model 14R recording spectro photometer. Temperature control within + 0.1° C was achieved by circulating water from a thermostat through the reaction chamber. Samples in acetonitrile were brought to tine reaction temperature and injected into aqueous solution previously placed in the reaction vessel, a i cm silica cell. The solutions generally contained about 6$in acetonitrile. A solution of 0.08 N perchloric acid was made by diluting conc. HC104 with double distilled water and standardized against sodium carbonate using bromcresol green as indicator. Reaction of the nickel complexes were monitored at 370 nm while the wavelength was set at 790 nm for the copper compounds. In all cases, the graph lnfA^ - A J vs t is linear for at least two half-lives. Kinetic runs were carried out at four temperatures over a range of 25°. Enthalpies and entropies of activation were calculated from the rate cons tants using the Arrhenius equation. RESULTS AND DISCUSSION
Macrocyclic complexes have been found to exhibit some special chemical and physical properties such as unusual geometries, unusual oxidation states of the metal ion, and increased kinetic stability, etc. It is important to extend these studies to complexes containing other stereorestrictive ligands, such as bidentate ligands. Sparteine and N, N'-dialkylbispidine are tertiary amines which can act as bidentate ligands and an examination of molecular models shows that metal complexes of these compounds (MLX2) are less strained if a tetrahedral geometry is adopted. This will be discussed subsequently. Furthermore, it was thought that the rigid geometry of the ligands would be reflected in the kinetic behavior of the complexes. Therefore, a series of metal complexes of the type MLX2 where L is sparteine (sp), N, N'-dimethylbispidine (DMBsp), and N-ethyl-N’-methylbispidine (EMBsp) have been synthesized and investigated. The metalions are cobalt(II), nickel(U), copper (U), zinc(H), and palladium(Il), where X is chloride, bromide, iodide, thiocyanate, acetate, or nitrate. The rate of dissociation of Ni(DMBsp)Cl2 and Cu(DMBsp)Cl2 have also been studied. Barefield and Wagner have synthesized nickel(H), copper(H), and zinc(I0 complexes of 1,4,8, ll-tetram ethyl-1,4,8, li-tetraazacyclotetradecane (structure YD). 11 They reported that these complexes are five-coordinate in
1,4,8,11-tetramethyl-i, 4,8, ii-tetraazacyclotetradecane VII the presence of a suitable anion or solvent. The nickel(Il) complex is kinetieally 127 128
labile compared to complexes of other ligands of the 14-membered ring class. Buxtorf et al47 reported the spectral property of the cobalt(Il) complex with the same ligand and the equilibrium studies in aqueous solution. They concluded that the cobalt(D) complex is five-coordinate in water. However, no solid compound was isolated. Therfore, another subject of this study is to synthesize cobalt(D) complex of Me4N[i4]aneN4 and study the chemistry of this compound. In this chapter the syntheses and characterizations of the complexes are reported and the stereochemistry is discussed. Properties of the complexes are discussed in several subsections according to metal ions. The kinetic studies on the rate of dissociation of M(DMBsp)Cl2, where M = Ni(H) and Cu(II), in acidic media are also reported and discussed in the later section of this chapter. Strain energy calculations. — The calculations were performed using the classical model of the strained molecule. Program written by L. DeHayes was used.48 The strain energy is defined as
U = U r + U6 + U0. + UnB (9) w here Ur is the energy associated with bond stretching, Uq is the energy resulting from bond angle deformation, is the energy involving bond torsions, and UjvTg is the nonbonded or van der Waals interactions in the molecule. The calculation starts with idealized square planar geometry of Co(DMBsp)Cl2 where the ca. strain energy is 263.1 keal/mole. The geometry after minimi zation of the strain energy is a slightly distorted tetrahedral which is shown in Fig. 19. The minimized strain energy is i. 11 kcalmole”1. Results are listed below:
Compound R Q 0 NB H_ TSa G Co(DMBsp)Cl2 0.73 4.49 0.54 -4.64 1.11 33.1 -32 a2 9 8 .15° K 129
(0.52,1.82,1.31)
(1.94,0.18,-1,23)
(-1. 99, 0. 0, 0. 02) (0 . 0 , 0 . 0 , 0 . 0 )
.07,-1.85,0.77)
F igure 19
Ideal Structure of Co(DMBsp)Cl2 130
Table 1 Selected Infrared Absorption Bands for the Complexes, MLX2. SL
Compl ex. v C- n cnT* yc _s cm"1 vc-Oasvni v C - O s y m vN-Oasy Co(sp)(NCS)2 2083, 2066 789,850 Co(DMBsp)(NCS)2 2064 798 Ni(DMBsp) (NCS)2 2126,2066 722,822 Pd(DMBsp) (NCS)2 2100,2083 830 Co(sp)(CH3C02)2 1575 1370 Co(DMBsp) (CII3C 02)2 1563 1414 Ni(sp)(CH3CO^)2 1550 1410 . Ni(E)MBsp) (CHgCCy 1557 1420 Cu(sp)(CH3Cq>)2 1600 1400 Cu(DMBsp) (CH3C02)2 1575 1404 Ni(sp)(NOj)2 1562 1275 806b
aNujol mull. bOut-of-plane deformation. 131
Syntheses and Characterizations.
The compounds are synthesized by mixing metal salts with ligands in a + 1:1 ratio under anhydrous condition.- When the metal ion is Co2 , reaction is carried out under N2 atmosphere. The elemental analyses agree with the proposed formulas. Complexes of the bidentate ligands MLX2 are all non-electrolytes with very low molar conductivities in the acetonitrile solution (Table 2, 4, 6, and 8). No N-H stretching bands are observed in the infrared spectra since the bidentate ligands are tertiary amines. CoLX) complexes. — The anions Cl", Br", I“, NCS- , and CH3C02“ have been incorporated into the cobalt(I0 complexes of sparteine (sp) and N, N'-dimethylbispidine (DMBsp). Some infrared bands of the anionic ligands are reported in Table 1. The thiocyanate groups exhibit a strong broad, slightly split band at 2083 and 2066 cm"1 for the sparteine complex and at 2064 cm"1 for DMBsp complex. The thiocyanate group is coordinated with N bonding to the cobalt ion as indicated by its infrared spectrum at the C-S stretching frequency.13»14>15 A band around 820 cm"1 (880-760 cm"1) can be attributed to M-NCS while band around 700 cm"1 (694-723 cm"1) indicates M-SCN. No band around 700 cm”1 was found for either of the dithiocyanato cobalt(D) complexes of the stated bidentate ligands. Furthermore, bands at 789 cm"1 arid 850 cm"1 for the sparteine complex and at 798 cm-1 for the DMBsp complex are attributed to C-S stretching frequencies of the N-bonded NCS. The low 1S another indication15 that these are isotMocyanato comp lexes with nitrogen bonded to the metal. The acetato-complexes show vc_o at 1575, 1370 cm"1 (Co(sp)(OAc)2), and 1563, 1414 cm"1 (Co(DMBsp)(OAc)2). The compounds are all monomers as confirmed by the molecular weight determination on Co(DMBsp)(OAc)2 and by the mass spectral results. The parent peak was found at m /e 363 for Co(sp)Cl2. All the cobalt(II) complexes other than diacetato compounds exhibit tetrahedral geometries. This is evidenced by their magnetic moments (Table 2) and their electronic Table 2 ct b c Magnetic Moments and Molar Conductivities * for Co(L)2%. complex B. M. A m ohm Co(sp)Cl2 4.54 0.1 Co(sp)Br2 4.33 2.2 Co(sp)I2 4.46 22.8 Co(sp)(NCS)2 4.31 7.8 Co(DMBsp)Cl2 4.52 2 .5 Co(DMBsp)Br2 4.43 1 Co(DMBsp)^ 4.76 9.3 Co(DMBsp) (NCS)2 3 .6 100 Co(sp)(CH3C02)2 4.70 3 .7 Co(DMBsp) (CH3C 02)2 4.86 3.2 ct b Solid state at room temperature. Measurements were taken on ~ 0 „ 001 M acetonitrile solution at room temperature. cThe expected range for a 1:1 electrolyte is 145-160 ohm”‘cm2mole-1 in acetonitrile. Table 3 a Electronic Spectral Data for Co(L)5^. -1 Compound Color absorption kK(e) B cm -1 Dq cm
Co(sp)Cl2 blue 6.8 (100), 7.75 (127), 10.64 (83), 722 493 16.39 (352,), 17.54 (263), 18.18 (272)
Co(sp)Br2 blue 7.46 (92), 10.31 (34), 16.26 (389) 724 461 16.95 (318), 17.79 (331)
Co(sp)l2 blue 7.14 (89), 10.20 (52), 15.15 (370) 708 449 15.63 (370), 16.13 (347), 16.95 (444)
Co(sp)(NCS)2 blue 8.77 (233), 11.36 (145), 18.87 (414)sh, 681 518 16.95 (742)
Co(DMBsp)Cl2 blue 7.87 (78), 11.11 (24), 16.26 (402), 715 499 17.70 (255), 18.18 (209)
Co(DMBsp)Br2 blue 7.58 (86), 10.90 (30), 16.20 (445), 719 474 16.90 (361), 17.70 (300)
Co(DMBsp)]^ blue 7.20 (116), 10.40 (56), 15.30 (431), 702 456 15.70 (454), 16.20 (519), 16.90 (561)
Co(DMBsp) (NCS)2 blue 8.93 (100), 11.90 (36), 16.67 (643), 663 515 18.52 (l65)sh
Co(sp)(OAc)2 violet 6.85 (19.6), 9.01 (21.6), 17.86 (117.6) CO 20.00 (92.7) CO (cont.) Table 3 (Continued)
Compound Color absorption kK(e) B cm "1 Dq cm " 1 Co(DMBsp)(OAc)2 violet 8.13(11), 18.18 (58), 20.83 (44 ), 31.25 (sh)
SLIn acetonitrile. ABSORBANCE 20 gur . ectoni r f (p) i onirl itrile n to e c A in 2 o(sp')X C of tra c e p S ic n tro c le E . 2 re u ig F XNS . C C. D. =I X . D r B = X . C =C1 B.X X=NCS . A VNMBR (kK) BERS AVENUM W 10 gur El r c Specta o C( .pX2 n Acet tie itrile n to e c A in 2 B.sp)X M Co(D of tra c e p S ic n tro c le E . 0 2 re u ig F ABSORBANCE XNS XC C. D. =I X . D r B = X . C X=C1 . B X=NCS . A 5 10 15 VNMBR (kK) BERS AVENUM W 136 137 transition absorptions (Table 3). The expected magnetic moments for tetrahedral Co(II) complexes fall between 4.2 and 4.7 B. M. 16 The magnetic moments for the-halogeno compounds range from 4.3 to 4.7 B.M. and are within the expected range. The orbital contribution is small, as is generally found for tetrahedral complexes. 17 Therefore, the moments alone indicate a tetrahedral configuration. The acetato-complexes exhibit higher moments, pgff = 4.70 B.M. for Co(sp)(OAc)2 and 4.86 B.M. for Co(DMBsp)(OAc)2, which fall into the expected range for octahedral complexes, (5.i ~ 4 .7 B.M.). Electronic spectra of the halogeno-complexes. — The halogeno- and pseudo halogeno-compounds are royal blue and are soluble in organic solvents. Their electronic spectra are shown in Fig. 2 and 20 and data are collected in Table 3. The absorption maxima are the same in several organic solvents (acetonitrile, chloroform and nitromethane) and in solid mulls; therefore, the compounds retain the same structure in the solid state and in solution. The intensities of the bands are high, as would be expected for tetrahedral compounds, e being of the order of i02. As indicated earlier the moments are consistent with the tetrahedral structure, they are also appreciably larger than expected for square planar (2.1-2.9 B. Ml) geometry, since the latter are usually low spin complexes. The electronic spectra are very similar to that of the tetrahedral C0CI42- ion. 18 However, due to the presence of two different pairs of ligands, the effective symmetry of the compound is no more than C2v. In C2V symmetry, the degeneracy of the Tj and T2 energy states is completely lifted (Fig. 3) .19 Each T state is split into one A and two B states. In general, the 4T2 ♦- 4A2 transition in Td symmetry is not observed because it occurs at energies below the limits of available instruments. Two higher energy multip le bands, 4Ti(P) *- 4A2V3 and 4Ti(F) *- 4A2V2 are observed. The spectral proper ties of CoA2B2 complexes have been investigated extensively. 20>21»22 Three components of V2 in the near infrared region are generally observed for CoL^X). However, only two components of v2(4T1(F)«- 4A2) are observed in solution for these compounds, except that three bands are observed for -TV A
4 b 2 _^-r _ _ _ ._____ / ' 4- *2 x: a, *B;
-4A2------— • % -
Td c2V
F ig u re 3 .
7 Term States for d Ion in Td and C-zr S y m m etry 139
Co(sp)Cl2. Presumably, this is due to the superimposition of the tvvo components in lower energy. The splitting is about 3 kK for the V2 band and about 2 kK for the \$ band. With the same anionic ligand, the lower energy band of V2 does not depend on L, while the higher energy component is lower in energy in the case of the sterically more hindered sparteine complexes. The same phenomena have previously been observed for other C0L2X2 complexes where L is a series of 2-substituted pyridine derivatives.22 The Dq values have been calculated20 assuming tetrahedral symmetry instead of the true C2v symmetry. The V2 and V3 transition energies are assumed to correspond to the centers of gravity of the multiple bands and the relatively low intensity shoulders, or peaks, on the high energy side of the sb band are ignored. By doing this, the calculation is simplified and the absolute values obtained for Dq and B are not highly accurate. It follows that only the trends and not absolute magnitudes will be discussed. The V2 band occurs around 8 kK and vs around 17 kK. Dq and B values are calculated according to equations 1, 2, and 3 and are reported in Table 3.
V2 = 15 Dq + 7.5 B - Q (1)
v3 = 15 Dq + 7.5 B + Q (2) Q = 1 /2 [(6 Dq - 15 B)2 + 64 Dq2]1/2 (3)
The observed mean Dq values for each series containing a specific amine fall in the sequence I” < Br" < Cl" < NCS“. This is the usual spectrochemical series for cobalt(D). The B values, which are associated with the nephelauxetic effect, fall in the sequence Cl- ~ Br" > I" > NCS". This corresponds approximately to the reverse of the variation in the intensities of the \§ bands, an observation reported for many other complexes. The higher intensity is a result of increasing covalency, which also leads to lower B values. For the same coordinating anion, the Dq of sparteine complex is invariably smaller than that of the DMBsp complex as would be expected for the more sterically hindered amine. 140
Lever and Nelson21 have investigated a series of tetrahedral bis-amine cobalt halides, (Amine)2CoX2, where the amine is quinoline, isoquinoline, 2,6-dimethylpyrazine, and substituted pyridine, etc. The range of Dq values for the chloride complexes examined is 460 ~ 472 cm-1. The values for sparteine and DMBsp complexes are 493 cm"1 and 512 cm-1 respectively, higher than those of most tertiary amines. This could be due to the shortened metal- nitrogen bond distance as is displayed by certain cobalt(IU) complexes with macro- cyclic ligands. The 13-membered ring, being the smallest, exerts the strongest, ligand field effect on the metal ion (see page 2). That the geometry of the complex is governed by the ligand N, N' -dimethylbispidine is supported by the results of strain energy calculations. The geometry provided for dichloroN, N'-dimethyl- bispidine-cobalt(II) by minimization of the strain energy is tetrahedral (see earlier section). Electronic spectra of diacetato complexes. — The violet diacetato comp lexes exhibit electronic spectra (Fig. 4) that differ from those of the halogen complexes. The band intensities are lower with extinction coefficients below 100 A cm-1mole"1. Also, a broad visible band is observed which can be resolved into two components with Av~ 2.5 kK. The band in the near-infrared region is assigned as the vi (4T2g 4Tig) transition required for octahedral ligand field symmetry. The band around 19 kK is assigned as the V3(4Tig(P) ♦- 4Tjg) transition. The 4A2g *- 4Tig transition, being a two-electron transition, is not observed. That the complex is in lower symfnetry than octahedral is revealed by the higher extinction coefficients (~ 102) for V3 as compared to those found for regular octahedral geometry (5 ~40 a cm"1mole"1). The splitting of the vj band in the spectrum of Co(sp)(OAc)2 also constitutes evidence for lower symmetry. Some nitrate complexes24 of cobalt(H) exhibit similar electronic spectra and also lower magnetic moments when compared to octahedral cobalt complexes. 1------1------■------1------1______1______I______I______I ______I______I______I______I______!______I______|______I______I______|______1 25 20 15 10 WAVE NUMBERS (kK) **• **• 5 Figure 4. Electronic Spectra of CoL(OAc)2in Acetonitrile A. Co(DMBsp)(OAc)2 B. Co(sp)(OAc) 2 M- 142
The electronic spectra of the aeetato-complexes can also be interpreted in terms of a tetrahedral structure, especially Co(sp)(OAc)2 which displays two near infrared bands. These bands could be regarded as components of the v2 band (4T4(F) 4A2). The magnetic moment which is lower than those usually observed for regular octahedral complexes is attributed to the distortion toward tetrahedral. It has been postulated25 that the six-coordinated nitrate complex can be regarded as having pseudotetrahedral structure with the bidentate nitrate ion being considered to function in the same way as such large uninegative anions as Cl- and Br- . Some haloacetate complexes of cobalt(II), Co(pyridine)2(CXxH3_xC02)2 where x= 1,2, or 3, X = Cl or F, have been investigated by Lever and Ogden5.0 The similarity of the sparteine and DMBsp complexes to those six-coordinated complexes provides further evidence for the assumption that the acetate group acts as bidentate ligand.
3
Co(DMBsp)(OAc)2 displays a half-wave oxidation potential at 0.74 V vs Ag/AgCl in acetonitrile in the presence of 0.1 M n-Bu4NBF4 as supporting electrolyte. The oxidation is irreversible with 360 mV. NiLX9 complexes. — The complexes NiLX2 have been synthesized by adding the ligand to a refluxing solution of nickel salt in acetonitrile and triethylorthoformate as described in the experimental section. The experi ments have been performed for L = sparteine, N,N’-dimethylbispidine,. or N-ethyl-N'-methylbispidihe, and X = Cl", Br , I , NCS , N03-> or CH3C02- . 143
Molecular weight determinations on Ni(sp)Cl2 and Ni(DMBsp)(OAc)2 indicate that these complexes are monomeric, while the molar conductances show them to be nonelectrolytes (Table 4). The infrared bands of some of the anionic ligands are reported in Table 1. The DMBsp thiocyanato-complex displays two well separated C-N stretching bands at 2126 and 2066 cm-1 and two C-S stretching bands at 722 and 822 cm '1. The values are higher than those found for monomeric nickel complexes and presumably indicate that the thiocyanate groups act as bridges.26 The structure of the thiocyanate compound will be discussed in a later section. The vc_o stretching m odes in the diacetato complex occur a t 1550 and 1410 cm -1 for Ni(sp)(OAc)2 and 1557 and 1420 cm-1 for Ni(DMBsp)(OAc)2. The nitrate groups in Ni(sp)(NOa)2 exhibit a large splitting of the asymmetric N-0 stretching band. \&Sy is observed at 1562 and 1275 cm-1. The out-of-plane deformation at 806 cm '1 is also observed. The large splitting of vasy the lowering in energy of the deformation band indicate that the nitrate groups serve as bidentate lig an d s.27 The magnetic moments of the nickel complexes are reported in Table 4. Data on their molar conductivities are also listed in Table 4. The values of the magnetic moments are perhaps more ambiguous in suggesting the structures of the nickel complexes than in the case of the cobalt complexes. The theoretical value of jjyff for tetrahedral nickel complexes is 4.1 ~ 3.5 B. M. For octa hedral complexes it is ~3.2 B.M. The acetato- and nitrato-complexes show lower magnetic moments (3.0 to 3.2 B.M.) indicating they are octahedral in geometry. The values for the halogen complexes are in the range from 3.2 to 3.6 B.M. which is slightly lower than expected for tetrahedral structures. However, the electronic spectra show that the complexes are tetrahedral. This will be discussed later. The moments of halogenosparteine complexes are closer to 3.5 B.M. while those of the corresponding DMBsp complexes occur near 3.3 B.M. This may indicate that the structure of the sparteine complexes is more nearly tetrahedral. The bulky groups on sparteine may weaken the 144
Table 4 cl Magnetic Moments and Molar Conductivities of Ni(L)X2 Complexes.
com plex Ueff B .M . Am ohm-1cm2 mole' Ni(sp)Cl2 3.46 .4 Ni(DMBsp)Cl2 3.38 1.8 N i(sp)Br2 3.46 1.2 Ni(sp)I2 3.56 12.6 Ni(DMBsp)Br2 3.32 30 Ni(DMBsp) (NC^)2 3.20 38 Ni(DMBsp) (Ac)2 3.21 0 Ni(sp)(Ac)2 2.98 3 .4 Ni(sp)(NG,)2 3.05 3.8 Ni(EMBsp) 012 3.40 3 .0 Ni(DMBsp)I2 3.46 48 a b Solid sample at room temperature. In acetonitrile at room temperature. 145
3 b ; 3-r 3 , T, B.2 33A
3 A 3A: 2 4
3 ^ -
- 3 T 2 3 B ,‘ 3 A-
3A - 3 t. 3b;
b2
Td C2V
F ig u re 18.
Term States for d8 Ion in Td and CzV S y m m etry 146
ligand field due to the nitrogen atoms, thereby making them more similar to the anionic ligands. The low magnetic moment presumably arises from the low symmetry in these M(I^)X2 compounds. Electronic spectra of the halogen complexes. — The similarities of the visible and near-infrared spectra (Fig. 5.6 and Table 5) of the halogen comp lexes to those of the known pseudo-tetrahedral complexes of the type Nil^X^8’29 indicate that these new substances are tetrahedral in structure. The spectra in acetonitrile solutions are the same as those of the solid mulls. Hence, the compounds have the tetrahedral configuration both in the solid state and in solu tion. Although the actual symmetry of the molecule is no higher than C2v, the electronic spectra essentially display the same pattern as do true tetrahedral complexes (Fig. 5,6). Hie energy diagram is shown in Fig. 18.30 The T* and T2 states are split into three bands in the lower symmetry (C2V) ligand field. The absorptions around 8 kK are assigned as components of the vi transition (3T2 (F) <- 3T1). The doublet at 10 kK to 12 kK is ascribed to transitions to the mixed state 3A2 and *A2 (C2v) arising from 3A2 and % states in tetrahedral symmetry, V2(3A2 «- 3T1). The sharp band on the higher energy side is assigned as n^. The bands at 16 ~ 20 kK are components of the v3(3Ti(P) <- ST1(F)) transi tion. The Dq and'B values are calculated according to equations 4, 5, and 6.
Vi = 5 Dq - 7.5 B + 1/2[225 B2 + 100 Dq2 + 180 Dq B]1/2 (4) v;> = 15 Dq - 7.5 B + 1/2[225 B2 + 100 Dq2 + 180 D q B ] 1/ 2 (5 ) vs = [225 B2 + 100 Dq2 + 180 Dq B ]1/2 (6)
Because of complications in locating the center of gravity of vi at very low
energy, it is better to use V2 and V3 in calculating Dq and B v a l u e s . The results are reported in Table 5. Again, the usual speetrochemical . s e r i e s is observed and Dq values can be arranged in the sequence Cl" > Br” > I". B values occur in the reverse order. Invariably, the sparteine complexes experience w eaker mean ligand fields than do those of DMBsp. N-ethyl-NT-methylbispidine is a we aim r ligand than DMBsp owing to bulkiness. Table 5 Electronic Spectra of NiLX2 Complexes in Acetonitrile.
Complex color absorption max (kK) Dq cm -1 B cm -1 (e molar absorptivity)
Ni(sp)Cl2 purple 85i (24), 10.93 (58), ii.83 (58) 640 928 16.39 (25.4), 20.50 (162)
Ni(sp)Br2 purple 8.51 (27), 10.64 (62), 11.50 (70) 617 914 16.13 (46), 20.00 (209)
Ni(sp)I2 brown 8.47 (35), 10.31 (89.7), 10.99 (97), 597 828 16.26 (83), 18.87 (220)
Ni(DMBsp)Cl2 violet 8.33 (41.6), 10.87 (89), 11.98 (102), 646 925 16.67 (37), 20.41 (230)
Ni(DMBsp)Br2 violet 8.47 (30), 10.93 (80), 11.69 (91.3) 630 907 17.54 (41), 20.00 (243.5)
Ni(DMBsp)I2 brown 8.62 (29), 10.42 (88), 11.11 (109.4) 598 855 16.39 (72), 18.69 (313) a8.62, 11.11, 16.13, 18.70
Ni(DMBsp)(NCg)2 green 7.87 (22), 11.63 (sh), 15.20 (36), 20.41 (19), 7.41, 11.49, 15.38, 20.00 (sh), 22.73 (sh), b7.l4, 12.34, 14.71, 21.28
w N1CI4"2 C 7.55 (21), 11.63 (6), 14.25 (160), 15.24 (160), 490 791 ^ 15.95 (cont.) Table 5 (Continued)
Complex color absorption max (kK) Dq cm B cm' (e molar absorptivity) N i(DMB sp) (A c) 2 green b9.1 (12), 15.2 (18), 25.61 (33) 910 899
Ni(sp)(Ac)2 green 8.62 (16), 14.71 (24.5), 25.38 (33) 862 939
Ni(sp)(NOb)2 green 9.52 (8.6), 15.15 (23), 25.64 (34.5) 952 816
Ni(EMBsp)Cl2 violet 8.3 (40), 10.64 (82), 11.9 (95), 16.39 (38), 20.41 (238)
aMull. bIn chloroform. °Ref. 23, H U £ < CQ § w ffl / V \A <;
20 1.5 10 5 WAVE NUMBERS (kK)
Figure 5. Electronic Spectra of Ni(sp)X2 in Acetonitrile A. X=Br B. X=I C. X=C1 WAVENUMBERS (kK) F ig u re 6 . Electronic Spectra of NiLX2 in Acetonitrile A. Ni(DMBsp)l2 B. Ni(DMB sp)Br2 , C. Ni(EMB sp)Cl2 D. Ni(DMBsp)Cl2 151
Electronic spectra of diacetato- and dinitrato-complexes, Ni(L))X?, — Some bidentate nitrato amine nickel complexes have been studied and reported to be six-coordinate with bidentate nitrate groups.24 The d-d spectra (Fig. 7 and Table 5) of the green acetato- and nitrato-complexes are similar to those previously reported compounds consequently, they are easily assigned octahedral structures. The bands around 9, 15 and 25 kK are assigned to the transitions 3T2g(F) <- 3A2g(F), 3Tjg(F)<- 3A2g(F) and3Tig(P)«- 3A2g(F), respectively. The Dq and B values are calculated accordi ng to equations 7 and 8 .
vj = 10 Dq (7) V2 + V3 = 15 B + 30 Dq (8)
The bands for the DMBsp complex (Table 5) are all shifted toward higher energies than those of the corresponding sparteine complex. Indeed, the Dq value is 910 cm-1 for Ni(DMBsp)(OAc)2 while it is only 860 cm-1 for Ni(sp)(OAc)2. Dq for Ni(sp)(NOa)2 (952 cm-1) exceeds that for Ni(sp)(OAc)2 indicating nitrate is a stronger ligand than acetate, assuming the ligand field strength of sparteine to be constant. Since molecular weight determination shows that Ni(DMBsp)(OAc)2 is monomeric, the nitrate and acetate groups must be bidentate in order to achieve the six-coordinated octahedral structure. Ni(DMBsp) (NCS)9. — The infrared spectrum of the green compound shows two \£j-N modes at 2126 and 2066 cm"1 and two C-S stretching bands at 822 and 722 cm-1. The magnetic moment in the solid state at room temperature is 3.20 B.M. The observed molar conductivity in acetonitrile is 38 ohnT^cn^mole"1, somewhat higher than the usual values for nonelectrolyte but still far too low to indicate the possibility of a 1:1 electrolyte (145 ~ 160 olrm”1cm2mole~1 in acetonitrile). Visible and near-infrared spectra in solid mulls and in acetonitrile and chloroform solutions show bands at about the same positions, but the relative band intensities vary. This is shown in Fig. 8. The bands at 21.2 kK and WAVENUMBERS (kK) Figure 7. Electronic Spectra of NiEX2 in Acetonitrile A. Ni(&p')(N03)2 B.Ni(DMBsp)(OAc)2 C. Ni(DMB-sp)(OAc)2 ABSORBANCE gur El r c Specta o N(MBs)NCS2 . l B i HjN C i CHCI3 in C. jCN CH in B. ull M A. S)2 C sp)(N B Ni(DM of tra c e p S ic n tro c le E . 8 re u ig F 25 20 VNMBR (kK) BERS AVENUM W 15 10 T ab le 6 a Magnetic Moments and Molar Conductivities for Cu(L)X2. complex |jeff B. M. Am ohm“1cm2mole“1 Cu(sp)Cl2 1.91 1 Cu(sp)Br2 1.90 6 Cu(DMBsp)Cl2 1.83 7.9 Cu(DMBsp)Br2 1.88 0 Cu(DMBsp)I2 2 .05 50 Cu(sp)(OAc)2 1.94 1.2
Cu(DMBsp) (OAc)2 1.86 1 [Cu(DMBsp)(OH)]2(C104)2 1.79 126 cl In acetonitrile at room temperature. The expected range is 145-160 ohm-1 i cm2mole-1 for a 1:1 electrolyte in acetonitrile. 155
11.5 kK can be attributed to the complex in a tetrahedral structure and those at 24 and 15.5 kK are consistent with octahedral symmetry. Such a structure would arise by polymerization through thiocyanate bridges. In acetonitrile, there'appears to be less tetrahedral monomer present than in chloroform, as evidenced by the relative band intensities at 11.5 and 15.5 kK. The thio- cyanato amine nickel complexes7 of some other tertiary amines Me4en, Me4pn and Me4tn are all green and are polymeric, displaying typical octahedral Ni(D) reflectance spectra. Due to the similarity of Ni(DMBsp)(NCS)2 to those complexes (vq_jj infrared spectrum, magnetic moments and the electronic spectrum), Ni(DMBsp)(NCS)2 is considered to be polymeric in the solid state and a mixture of tetrahedral and octahedral forms in solution. Nickel(H) has less tendency to form the tetrahedral structure than does cobalt(H). This is demonstrated by the thiocyanato and acetato complexes of these metal ions. The acetato-complex of Co(H) is much distorted from octahedral toward tetrahedral and CoL(NC§)2 are monomeric tetrahedral compounds, while nickel acetato complexes are much like the usual octahedral complex at least in their electronic spectral and magnetic properties, and Ni(DMBsp)(NCS)2 is polymeric and octahedral in the solid state. CuLX) complexes. — The magnetic moments of halogeno and acetato- complexes of copper(D) with L = sparteine and N,N'-dimethylbispidine fall within the range (1.8 - 2.0 B.M .)31 (Table 6) expectedfor mononuclear copper(Il) complexes. Molecular weight measurements on Cu(DMBsp)Cl2 and Cu(DMBsp)(OAc)2 confirm this argument. Again, the properties of these complexes will be discussed in two subsections for the halogen and acetato com plexes. Halogeno-complexes. — The d-d spectra (Fig. 9,10 and Table 7) of these complexes consist of one band around 7 kK and another occurring at about 11 kK. While hardly diagnostic, this is consistent with a quasi-tetrahedral structure.32 CuCl42" and CuBr42- (D2d symmetry) exhibit bands at ~ 5 kK F ig u re 9. E le c tro n ic S p e c tra of Cu( sp)X 2 in A c e to n itrile A . X=C1 B . X X r = B . B X=C1 . A itrile n to e c A 2in sp)X Cu( of tra c e p S ic n tro c le E 9. re u ig F ABSORBANCE VNMBR (kK) BERS AVENUM W 10 N ✓ C71 O) ABSORBANCE gur 0 El r c Specta o CuDMBs) i onirl X= B. C-.X=I r B = X . B 1 =C .X A itrile n to e c A in 2 sp)X B M u(D C of tra c e p S ic n tro c le E 10. re u ig F VNMBR (K) (kK BERS AVENUM W 10 cn i s • 158
Table 7 Electronic Absorptions of Copper(II) Complexes. complex solvent absorption max. (kK) color (e molar absorptivity)
Cu(sp)Cl2 acetonitrile 7.81(191), 12.66(183) green
Cu(sp)Br2 acetonitrile 7. 51(246), 12.34(283) orange
Cu(DMBsp)Cl2 acetonitrile 7.75(114), 11.63(161) green
Cu(DMBsp)Br2 acetonitrile 6.76(140), 11.40(257) orange
Cu(DMBsp)I2 m ull 10.50,15.0(sh), 17. 5,19.3, 27,5 brown acetonitrile 10.20(214) brown
Cu(DMBsp) (OAc)2 acetonitrile 12.80(208) blue
Cu(sp)(OAc)2 acetonitrile 13.89(230) blue
[Cu(DMBsp) (OH) 12(010^2 acetonitrile 16.13(137),26.32(133) lavender chloroform 17.24(~ 150), 2 6 .67(~ 100) m ull 17.90, 27.00 50 G
0 Figure 11. Esr Spectrum of Cu( sp)Cl2 in CH2Cl2- CHCl3( 1:1) at 77 K J. 15 10 WAVENUMBERS (kK)
F ig u re 12 . Electronic Spectra of C u L(OA c )2 in Acetonitrile A. L=sp B. L=DMBsp cs o 50 G
0 Figure 17. Esr Spectrum of Cu(sp)(OAc)2 in CHaClz-CHC^l :1) at 77 K 162
(2E ♦- 2B2) and ~ 9 kK (2A4 + 2B4 *- 2B2). In C2V symmetry the degeneracy of the E state is lifted, therefore, four bands are expected. The observed broad bands presumably indicate the existence of more than one band under the enve lope. The electron spin resonance spectrum of Cu(sp)Cl2 at liquid nitrogen temperature in a frozen 1:1 mixture of dichloromethane and chloroform shows nuclear hyperfine splittings due to G3Cu, 65Cu (3/2 = D in the parallel region (Fig. 11). The g„ and gx values are 2.292 and 2.066, Azz is 105 G. No splittings were observed in the solid sample at room temperature
(g|( 2.297, gj_2.070). Acetato complexes. — The blue acetato complexes show vc_oasy 1575 and 1580 cm -1 and \C-Ogyjn *404 cm -1 for Cu(DMBsp)(OAc)2 and 1600 and 1580 cm -1 and vc-O sym a* *400 cm -1 for Cu(sp)(OAc)2. The separations of these bands are 171 and 200 cm-1, respectively. The magnetic moments are 1.86 B.M. for Cu(DMBsp)(OAc)2 and 1.94 B.M. for Cu(sp)(OAc)2. One broad d-d transition is observed at~ 12.8 kK with e about 200 jjcm-Imole_1 (Fig. 12). However, this does not provide information about the geometry of the complexes. The compounds are monomeric like their cobalt(D) and nickel(II) analogues. Therefore, it is assumed that the acetate groups coordi nate in a bidentate fashion the copper complexes as well. The electron spin resonance spectrum of Cu(sp)(Ae)2 in frozen chloroform and dichloromethane mixture (1:1) shows a hyperfine splitting due to 63Cu, 65Cu (I = 3/2) in the z direction. g(J is 2.274 and g^ 2.052. Azz = 150 G (Fig. 17). C u(DMB sp) (OH) C 10,4. — Cu(DMBsp)(OH)C104 has a magnetic moment of 1.79 B.M. and its infrared spectrum shows a sharp vo-H at 3633 cm-1. The acetonitrile solution of this lavendar compound is dark green and its molar conductivity in this solvent suggests that it is a 1:1 electrolyte
(Am = 126 ohm"Icm2mol“1). However, dilution studies in acetonitrile indicate that it is a 2:1 electrolyte. The slope A from the graph of (Ao - Ae) vs ^Tis ABSORBANCE gur 3 El r c Specta o [uDyBs ( ]( 042 i H1 B Mul i C3 -§ ' N .CH3C in . C ull M B. CHC13 in . A 4)2 l0 )]2(C H )(0 sp [Cu(DlylB of tra c e p S ic n tro c le E 13. re u ig F 25 W AVENUM BERS (kK) (kK) BERS AVENUM W 0 2 15 10 h - 164
1000. The calculated A from the equivalent conductivity at infinite dilution (Ao) and other constants appropriate for acetonitrile33 is 351 for a 1:1 electro lyte and ~ 955 for a 2:1 electrolyte. Furthermore, the molecular weight of this compound in CHC13 is 700, indicating a dimeric compound. Hence, the appropriate formula is [Cu(DMBsp)(0H)]2(C104)2 with bridging hydroxyl groups. The electronic spectra in solid mulls and in acetonitrile and chloroform solution are shown in Fig. 13 and the numbers are reported in Table 7. The similarity of the spectrum in chloroform and in the solid mull, (bands at 17.05 and 26.7 kK in CHC13 and 17.9 and 27.0 kK for mull spectrum) indicates that in this solvent the compound retains the solid state structure. This is suggested to be a square planar structure since the band at ~ 17 kK is much higher in energy than the corresponding bands for the tetrahedral compounds. In the presence of a potential coordinating solvent, acetonitrile, the band maximum shifts to lower energy presumably due to the coordinating of acetonitrile to a square pyramidal compound.34 Attempts to obtain an esr spectrum of this compound have failed. PdLX? complexes. — The chlorocomplexes of sparteine, N.N'-dimethyl- bispidine, and N-e thyl-N' - me thylbi spidine and the thiocyanato complex of DMBsp have been prepared. They are all orange yellow in color, diamagnetic and nonelectrolytes in acetonitrile solutions. However, in water the molar conductivity of Pd(DMBsp)Cl2 is 214 ohm-1cm2mole-1 indicating that the chlo
rides are replaced by water. Am for a 2:1 electrolyte is expected to be 210-260 in water. The d-d electronic spectrum of Pd(DMBsp)Cl2 in water also shifts to higher energy as would be expected since water is a stronger
ligand than chloride. The thiocyanate group shows vc _n bands at 2100 and
2083 cm-1 and ^c-S bands at 830 cm-1, indicating that the nitrogen atom is bonded to the palladium. The compounds display typical square-planar electronic spectra (Table 8, Fig. 14) with bands around 25 kK. Three spin allowed d-d transitions are expected. *A2g *- ^ g , *Big «- ^jg, and 165
Table 8 Electronic Spectra of Pd(L)X2 Complexes in Acetonitrile. complex color absorption max. (kK) AM°hni-1cm2mole_1 (e molar absorptivity) ' Pd(sp)Cl2 orange 23.80 (585) 15
Pd(DMBsp)Cl2 orange 25.30 (338 13 27.40 (499)a 2 l4 ‘
Pd(EMBsp)Cl2 orange 25.00 (418) 9
Pd(DMBsp)(NCS)2 yellow 30.30, sh (2389) 18 aIn w ater. ABSORBANCE F ig u re 14. E le c tro n ic S p e c tra of BdLX 2 in A c e to n itrile A. Pd(D M B sp )C12 sp B M Pd(D A. itrile n to e c A in 2 BdLX of tra c e p S ic n tro c le E 14. re u ig F . e ) 2 BdDMBs)NCS2 Pd( p) l2 )C sp B M (E d P . D S)2 C sp)(N B M d(D B . C l2 ')C (ep d P B. 35 30 VNMBR (kK) BERS AVENUM W 25 20 Gi a 1/ — 1 I------1____ 1 . 500 400 250 c m 1 500 400 250 cm~‘
B
F ig u r e 15. Far Infrared Spectra of: A. Co('sp)Br2 C. Co(DMBsp)Br2 B. Co(sp)Cl2 D. Co(DMBsp)Cl2 168
J------1------1------1 I ! I . 500 400 250 cm 500 400 250 cm
Figure 15. Cont'd E. Ni(sp)Br2 G. Ni(DMBsp)Br2 F.. Ni(sp)Cl2 H. Ni(DMBsp-)Cl2 169
600 500 400 250 crn
t F ig u re 15. C o n t'd I. Cu(DMBsp)Br2 J. Cu(DMB sp)Cl2 170
T able 9 Q Infrared Spectral Data for the Metal-Donor Atom Modes for theC o(D ), Ni(U), and Cu(D) Com plexes. compound M-Cl cm -1 M-N cm -1 Co(DMBsp)Cl2 329, 309 (471, 481), 500 Co(sp)Cl2 335, 305 435, 463 Ni(DMBsp)Cl2 322, 292 (480, 490), 500(sh) Ni(sp)Cl2 318, 296 429, 465 Cu(DMBsp)Cl2a 284 (broad) 479, 503 Cu(sp)Cl2^ 272, 288 468, 438 a b The Csl pellet changed color slightly. From S. Choi, R. D. Bereman, and J. R. Wasson, J. Inorg. Nucl. Chem., 37, 2087 (1975). cIn Csl pellet. Spectrum is calibrated against polystyrene. 171
*Eg - ^jg. However, it is not possible to assign these bands due to the interference of charge transfer absorptions. It has been suggested?5 that the lowest energy electronic transition can be used to compare the ligand field strengths toward palladium(D). Therefore, the amines can be arranged in the spectrochemical sequence sparteine < N-ethyl-N'-methylbispidine < N, N'-di- methylbispidine. Comparison of metal chloride and metal nitrogen stretching m odes.— Metal chloride stretching bands have been assigned by the comparison of the infrared spectra of the bromo and chloro complexes between 250-600 cm"1 (Fig. 15, Table 9). The metal-nitrogen bands have also been assigned (Table 9). The band positions indicate that the Co-Cl bonding is the strongest, Ni-Cl being the next followed by Cu-Cl bond. The M-N bonding can be arranged as Cu-N > Ni-N > Co-N for sparteine complex, but the bands are all at the same positions for DMBsp complex. Comparison in the same metal complex shows that M-Cl bonds are stronger for DMBsp complex while M-N bonds are invariably weaker for sparteine complex. This is consistent with the spectral data. [Co(Me,iN[l4]aneN/t)CllC10/t. — The molar conductivity of the purple crys tals of this compound in nitromethane is 85 ohm_1cm2mole-1 indicating that it is a 1:1 electrolyte. The molar conductivities of 1:1 electrolytes in nitro methane fall between 75 and 95 ohm- 1cm2mole_1. The magnetic moment is 4.58 B .M ., a value in the range observed for five-coordinate Co(2I) complex (4. 5-5.5 B.M .).23 15 WAVENUMBERS (kK) Figure 16. Electronic Spectra of [Co(Me^[M]aneN 4)C l]C 104 A. Mull B. in CH 3NO2 C. in CH3CN 173
No splittings of the perchlorate bands were observed in the infrared spectrum. Further, the electronic spectrum is the same in solution and in solid mull. All this points to a five-coordinate cobalt(II) complex. Due to the restrictive struc ture of the macrocyclic ligand, it is unlikely to form a trigonal bipyramidal compound. The electronic spectral bands are observed at 19.60 (65), 16.80 (34) and 11.70 (34) kK in nitromethane. The spectra in solution and solid mull are shown in Fig. 16. This compound is reversibly oxidized in acetonitrile, with the half-wave potential being 0.72 V vs Ag/AgN03 in the presence of 0.1 M Bu4NBF4 as the supporting electrolyte. Attempted syntheses of bis-diamine complexes. — The attempts to synthesize Cu(DMBsp)2(C104)2 failed. Douglass and Ratliff reported the synthesis of Cu(DMBsp)2(ClC>4)210 by mixing Cu(C104)2 • 6H20 and DMBsp in absolute ethanol. However, the only compound isolated following their method is [Cu(DMBsp)(0H)]2(C104)2, a hydroxyl bridged dimer. Other metal ions such as cobalt and palladium have also been tried without success. It seems that in the absence of coordinating anions, the metal ion has a great tendency to pick up hydroxyl groups and form OH- bridged dimers. However, Pd(D) probably is capable of forming [PdL2]X2 due to the larger size of the metal ion provided the right conditions could be found. [Pd(Me4en)2](NC^)2 has been synthesized.36 In earlier studies it was also not possible to isolate the bis complexes of Ni(IJ) and Cu(H) with tetram ethylated ethylenediam ine.36
Kinetic Studies
In order to study how the rigid structure of the ligand affects the reactivities of the complex, the following study was performed. The rate of dissociation of the amine, DMBsp, from the Ni(D) and Cu(D) complexes was studied in 0.09 N aqueous perchloric acid at several different temperatures.
M(DMBsp)(H20)42+ + H20 5+ DMBsp * .2H+ + M(H20)62+ (10) 174
Table 10 Rate of Dissociation of Ni(DMBsp)Cl2 and Cu(DMBsp)Cl2 in Perchloric Acid. Compound Tem p. kj sec"*
Cu(DMBsp)Cl2 20.0°Cb 3.5 + 0.2 x 10"3 25.0 5.3+0.4 x 10-3 2 5 .0 J 5.8 +0.21 x l0~3 30.0 J 8.2 +0.5 x 10“3 4 4.0 b 2 .3 + 0 .2 x 10-2
Ni(DMBsp)Cl2 25.0° C 6.7 + 0.2 x 10"5 25 .0 6.6 + 0 . i x i 0 - 5 30.0 ° 1.4 + O.lxlO-4 40.0° C 4.7 + 0.2 x 10-4 47.6 ° 1.28 + .09 x i0“3
a[H+] = .048, p =.09. b[H+] = .08 N, ^ = .08. C[H+] = .09 N, ^ = 0.09 d[H+] = .04, (j, = . 09 with NaC104. 175
Chloro-complexes were used and the reaction was followed at 370 nm for the Ni(B) complex and at 790 nm for the Cu(D) complex. The results are reported in Table 10. Enthalpies and entropies of activation were calculated: a H = z/L 11 7^ 24.8 kcal/mole; fiS = 5.8 cal deg mole for the Ni(II) complex and a H =
13.8 kcalmole-1, a s^ - “23 cal deg-1mole-1 for Cu(DMBsp)Cl2. The rate constants for Ni(DMBsp)Cl2 at [H+] = 0.05 N and 0.08 N are the same and the + same is true of the rate constants for the Cu(H) complex at [H ] 0.09 N and 0.04 N. The dissociation of Co(E) complexes is so fast under the same condition that it is not able to study the reaction by the same method. The hydrolysis of Cu(sp)Cl2 has been investigated by Boschmann et al37 in neutral water, kj of Cu(sp)Cl2 at 25.0° C is 2.6 x lO^sec-1. At the same temperature, Cu(DMBsp)Cl2 shows kt, 5.3 x 10-3sec-1 in 0.08 N acid. In view of the very different conditions for measurement and since it is well known that the rates of dissociation of amine complexes are acid-dependent,46 it is not proper to com pare the values for these two complexes. The rates of dissociation of [Ni(Me4en)(H20)4]2+ and [Cu(Me4en)(H20)4]2 have been studied.44 >45>46 The results of the present study and those of the study on complexes of Me4en are summarized in Table 11. It is noted that the rates of dissociation of the compounds of DMBsp are very much slower than those of the simple tertiary amine complexes. This can be justified by consi dering the rigid structure of the ljg and, DMBsp. Because N atoms are part of the fused rings, it is hard to break a single metal-nitrogen bond by a simple vibration or rotation. Therefore, the rate of dissociation is retarded by effects which are related to ligand conformation and rigidity. It is also noted that the activation energies of the complexes of DMBsp are higher than their corresponding Me4en complexes. In conclusion, complexes of M(L)X2 where M = Co(H), Ni(H), Cu(ID, Zn(H), L = sparteine, N,N'-dimethylbispidine, or N-ethyl-N’-methylbispidine and X = monodentate anionic ligands, i.e ., Cl~, Br , I- , have been prepared and char acterized. They are tetrahedral or distorted tetrahedrals in structure, as 176
Table 11 Kinetic Data for the Rate of Dissociation of Complexes of Some Tertiary Amines in Acid at 25° C. r i Complex sec-1 -E kcalmole log A sec-1
Ni(DMBsp) Cl2a 6 .7 x 10-5 25.4 14.4 [Ni(Me4en)]2+ ^ 1.4 x 10-1 20.9 14.5 Cu(DMBsp)Cl2C 5.3 x 10"3 14.4 8 .3 [Cu(Me4en)]2 ^ 38.5 . ' 12.0 10.4 Cu(sp)Cl2b 2 .6 x lO -3
a3h 0.09 N HC104. bfci H2Q. CIn 0.08 NHC104. d [n 0 .5 N HNC^.
predicted by the strain energy calculations. When X = potential bidentate anionic ligands such as CH3C02“ or N03 , the complexes MLX2 exhibit distorted octahedral structures. When palladium(D) is the metal ion, the complexes are square planar; either due to the larger metal ion size, or be cause of the extreme electronic stabilization that favors the planar structure for palladium(II). The effect of the rigid character of the ligand on the reactivities of the complexes has been demonstrated in the kinetic studies of the dissociation of the ligand in the acidic media. The great enhancement of stability toward dissociation is a result of the stereochemistry of the ligand. SUMMARY
Earlier studies on macrocyclic compounds (Part I) has shown that the stereorestrictivity of the ligand dominates the reactivities of the complexes. It is expected that the stereorestrictivity of the ligand would not only be displayed by the macrocyclic tetradentate ligands but also by other lower dentate ligands. In order to study how the stereochemistry of ligands other than macrocyclic compounds affects the chemistry of their complexes, metal complexes of the form MLX2 where L is a bicyclo tertiary amine such as sparteine, N,N'-di- methylbispidine and N-ethyl-N'-methylbispidine have been synthesized. From the physical measurements, i.e., infrared spectra, visible spectra, magnetic moments, conductivities and molecular weight determinations, it has been shown that the halogeno-complexes of Co(IQ, Ni(H), and Cu(33) are all tetrahedral or distorted tetrahedral in structure, while that of palladium is square-planar. The complexes of the potential bidentate anions NO^ and CH3C02~ of the first row transition metal ions are six-coordinated with biden tate anionic ligands. The ligand field strengths of the halogens lie in the usual spectrochemical order NCS~ > Cl“> Br~ > I” and bidentate nitrate is a stronger ligand than acetate ion. The less bulky amine DMBsp is the strongest ligand of the three and the more bulky sparteine is the weakest among them. + + The rates of dissociation of Cu(DMBsp)]2. and [Ni(DMBsp)]2 in acidic media have been investigated. Rate constants at 25° C are found to be 5.3 x 10-3 sec-1 and 6.6 x iO-5sec-1, respectively. a H and a S values are 13.8 lccal- mole-1, -23 cal deg-1mole-1 [Cu(DMBsp)]2 and 24.8 kcalmole-1 and 6caldeg-1mole-1 [Ni(DMBsp)]2+. This is a lot slower than the rate of disso ciation of other Ni(H), Cu(II) complexes. This study has demonstrated that stereorestrictive ligands other than macrocyclic ligands can affect both the geometries of the complexes and also their reactivities.
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