Coordination Compounds of (IV) Formed with Soft and Hard Donor Atoms : A Look into the Past and Present Work

R.V. Singh'*, Pratibha Gupta1, Pratibha Chaudhary' and C.N. Deshmukh2

'Department of Chemistry, University of Rajasthan, Jaipur - 302 004, India E-mail: kudiwal(a>dataii\fosys.net; Fax : +91-141-2708621 Department of Chemistry, Vidyabharti Mahavidyalaya, Amravati - 444 602.

ABSTRACT

An account of approximately hundred germanium and organogermanium derivatives have been included in this review. The unimolar and bimolar substitution products have been characterized by elemental analyses, conductance measurements, molecular weight determinations and spectral studies, viz., IR, 'H NMR, 13C NMR, U.V. and mass spectra. Thermal stability has been explained on the basis of TGA data. From the analyses of these studies the donor sites of the ligands are located and geometries of the donor environment around the Ge(IV) acceptor centre have been proposed. Based on these studies, trigonal bipyramidal and octahedral geometries have been proposed for the resulting derivatives. The resulting coloured solids are soluble in most of the common organic solvents. These were found to be monomers, as evidenced by their molecular weight determinations. The low values of molar conductance of the resulting metal complexes in anhydrous DMF show them to be non-electrolytes in nature. The spectral data suggested that the ligands act in monofunctional bidentate, bifunctional tridentate and Afunctional tetradentate manners, coordinating through the hard nitrogen and soft sulfur atoms. All such important aspects, including the biological properties about the germanium complexes synthesized in our laboratory as well as other laboratories, are discussed in brief.

CONTENTS 1. Introduction 2. Organogermanium Compounds with Schiff Bases: Experimental 3. Other Germanium(IV) Complexes: Experimental 4. Results and Discussion for Organogermanium-Schiff Base Compounds 5. Results and Discussion for Other Germanium(IV) Complexes 6. Biological Aspects 7. References

93 Vol. 28, No. 2, 2005 Coordination Comopunds of Germanium(lV) Formed with Soft and Hard Donor Atoms: A Look into the Past and Present Work

1. INTRODUCTION

Germanium with the electronic configuration, Is2, 2s2p6, 3s2p6d"', 4s2p2 is an analogue of carbon, silicon, tin and lead. In the tetravalent state, all the members of this group form covalent compounds and, except carbon, coordination number six is commonly exhibited by these elements with sp3d2 hybridization. The existence of coordination number five has also been reported in some of the germanium l\l, organotin 111 and silicon /3/ compounds. Germanium in the divalent state has been reported to exhibit coordination numbers four and three. For example, reacts with 1,4-dioxane to yield Ge(C4H802)Cl2, which is stable in air and hydrolyzed to Ge(OH)2 by water /4/. It decomposes at 140-210°C. A critical review of the literature revealed that, in the past several years, there has been a growing interest in the study of the various coordination compounds of germanium, and especially with nitrogen donor ligands. Germanium(IV) halides readily act as Lewis acids to give molecular complexes, e.g., GeX4(X=halogen) reacts with pyridine and isoquinoline to give 1:2 complexes, GeX4.2L (where, L is the Lewis base molecule). and germanium tetrabromide react with 2,4-pentanedione with the evolution of hydrogen /bromide gas to yield germanium bis-(2,4-pentanedionato)dihalide derivatives 151. Further, catechol and germanium tetrachloride in the presence of pyridine yield an octahedral complex 161 (I):

(I)

On heating the dipyridine complex in DMF a new complex (II) gets precipitated 111.

DMF

DMF

(II)

94 R. V. Singh et al. Main Group Metal Chemistry

With p-aminobenzoic acid, ethyl-p-aminobenzoate and o- and m-H2NC6H4COOH, germanium tetrachloride form 1:4 and 1:6 complexes. The infrared spectra suggest that in the 1:4 complexes, the two atoms in the inner coordination sphere are in the trans position. Mutterties et al. /8,9/ treated the tetrahalides of germanium with Ν,Ν'-dimethylaminotroponine (A), or + preferably its lithium salt, in to give an octahedral A3Ge chelate. These cationic chelates were found to be thermodynamically unstable towards hydrolysis and could not be prepared in aqueous media. On + the other hand, tropolone(T) and germanium tetrachloride gave T3Ge as the sole chelates product in aqueous media, while in non-aqueous media T2GeCl2 was obtained and this could then be reacted with water to give T)Ge+. Kenney et al. /10-14/ have reported the synthesis of germanium phthalocyanines. These compounds are important because few metal phthalo cyanines are known in which the central element has as large an electronegativity as germanium. Also, they provide an opportunity for the study of hexa-coordinated germanium when it is bonded to six atoms, of which four are the nitrogen atoms, and the germanium atom can be assumed to be a planar arrangement. Due to the great stability of the phthalocyanine ring system, this unusual, partly predetermined, hexa-coordination is preserved under a wide variety of conditions. Comparatively, simpler systems of germanium having hexa-coordination are described by Aggarwal et al. /15,16/ using 4d Orbitals: germanium in increases its covalency from four to six with

GeF4 acting as a Lewis acid. In this manner, GeF4 forms 1:2 complexes with ethers, acetone and methylalcohol /17/. Similarly, it also forms 1:1 adducts with ethylenediamine and 1:2 complexes with acetonitrile, ammonia, hydrazine, pyrrolidine and piperidine. The nitrogen-containing complexes are white in colour, non-volatile and thermally stable solids and insoluble in hydrocarbons. Hexa-coordinated compounds of germanium with phenyl- and p-nitrophenylhydrazine having an octahedral trans configuration have also been prepared. Similar complexes of substituted 2,5-dihydroxy-p- benzoquinones, substituted o-diphenols and dimethylsulfoxide and hexachlorofluoro-hydroxo-oxyfluoro- and aluminogermanates have been attempted. Though the literature records a large number of -containing germanium compounds /18/, Kraus and coworkers were the first to describe germanium-nitrogen derivatives which appeared in Johnson's article /19/. This area, however, remained unexplored for a long' time, and until recently only a score of organogermanyl amines were described along with some organogermanium isocyanides, isocyanates and isothiocyanates. The growing interest in the Si-N and Sn-N bonds recently gave rise to a rapid development of the study of Ge-N bonded derivatives /20-23/.

Nitrogen-containing compounds of the types R3GeNH2, (R3Ge)3N, (R3Ge)2NH, R2GeNH and RGeN have been described in Johnson's article /19/. Anderson /21/ reported the preparation of dialkylamine derivatives from dimethyl or diethylamine and alkyl (aryl) halogermanes. Trimethylnitratogermane was prepared by Schmidt and Ruidsch /24/ in 1961 and its use has been suggested as an additive in rocket or engine fuels.

They have also reported the synthesis of (Me3Ge)2NH and (Me2GeCl)3N from their corresponding halogermanes /25/ and ammonia. Satge et al. /26/ studied the synthesis and reactivity of a number of organogermanium derivatives having Ge-N bonds. Rijkens and Vander Kerk27 also prepared a number of organogermanium compounds containing

95 Vol. 28, No. 2, 2005 Coordination Comopunds of Germanium(IV) Formed with Soft and Hard Donor Atoms: A Look into the Past and Present Work

Ge-N bonds. Abel /28/ reported the synthesis of (diethylamino)trimethylgermane and Yoder and Zuckerman /29/ studied the amination and transamination reactions of group XIV elements. However, a survey of the literature indicated that, except for a few publications on the Schiff base complexes of germanium, systematic studies with these ligands are lacking. There are some reports on reactions of germanium tetrachloride and germanium tetraisothiocyanate with benzylidene-o-aminophenol, o-hydroxyanils of aromatic aldehydes and aromatic azomethines /30-32/. Germanium is the middle element of the periodic group XIV. The other elements in the group have extensive and important biochemistries. The organometallic chemistry of germanium differs from that of silicon only slightly. However, germanium has been the least studied of these elements; nevertheless, its role in the biological processes is quite extensive. The biological effects of inorganic germanium compounds resemble those of the silicon analogues, whereas the effects of organogermanium compounds resemble more those of tin and lead. Dealing with biological aspects of inorganic germanium compounds, Simpson et al. /33/ reviewed the effect of Ge02 on the development of freshwater sponges. Plants absorb and interact with and a noticeable change on the growth of tomatoes /34-36/ has been observed. The compound isopropoxy germatrane, (CH3)2CH-0-Ge[C0CH2CH2)3N] stimulates the healing of wounds in rats, possibly by abetting the reparative function of connective tissues /37/. Several reports have included organogermanium compounds as members of the biologically active organometal series. The following cyclic compound(III) shows bactericidal and fungicidal activities /38Λ

F F / F F F

H3C CH3

(III)

Okawa and Kubo39 studied the antimicrobial activity against different microorganisms of compound(IV):

C02H

Cl3GeCH2CH

CH3

(IV)

96 R. V. Singh et al. Main Group Metal Chemistry

Triorganogermanium compounds have been fairly well studied and show appreciable activity /40/ against bacteria and fungi, while unsymmetrical tetraorganogermanes are found to be more active. A fair amount of work has been done on the germacyclic compounds like (V) /41/.

(V)

In an extension to the related germanium complexes, the bis(benzenediolato)chlorogermanates and bis(benzenediolato)fluorogermanates(Vl) have been structurally characterized by Holmes et al. /42,43/.

X

X = CI and F

(VI)

The sulphur compounds of germanium have also been studied extensively /44,.45/. The crystal and

molecular structures of the complex, formed by the reaction of GeCl4 with the disodium salt of ethylenediamineteraacetic acid in hot water, have been determined by single crystal X-ray diffraction techniques /46/. Some synthetic aspects of germanium compounds have also been reported by Zuckerman et al. /47/, Singh et al. /48-57/ and others /58-61/. A review by Melnik et al. /58/ covers some four hundred and fifty organogermanium compounds. The Ge atom is mostly in oxidation states +4 or +2, with a few examples of mixed valencies and also an example of + 1. G)ordination states and geometries range from seven (pentagonal bipyramidal), six, five (trigonal bipyramidal), to three (mostly trigonal) and two (bent), with the most common being four (tetrahedral). The lower coordination number compounds (two, three and some four) have Ge atoms in a+2 , and most of the four and all the higher have the Ge atom in a +4 oxidation state. The CsF/18-crown-6 mediated reactions of disubstituted acetylenes Me^SiOCGeMej with carbonyl compounds in were studied by E. Lukevics et al. 1591. Almost two hundred and fifty heterometallic

97 Vol. 28, No. 2, 2005 Coordination Comopunds of Germanium(IV) Formed with Soft and Hard Donor Atoms: A Look into the Past and Present Work germanium coordination and organometallic derivatives have been included in this review. There are some forty metals, up to fifteen metal atoms per cluster unit, found with germanium in complexes and clusters. The nuclearities are mostly from bi- to tridecameric, with examples of pentadecamers and polymers. The most common donor atom is carbon (methyl and phenyl), so the majority of the derivatives are organometallics. The non-transition heterometallic compounds are mostly colourless or yellow. Those with transition metal elements range from yellow to red with colours deepening through brown to black. Summaries of metal- metal distances are given in each section, and correlations are drawn between structural parameters /60/. In the following pages some work reported from our laboratories has been described in brief with suitable references."

2. ORGANOGERMANIUM COMPOUNDS WITH SCHIFF BASES: EXPERIMENTAL

2.1. Reactions of ethylorthogermanate with monofunctional bidentate Schiff bases, N-[alkyl or aryl] salicylaldimines:

Ethylorthogermanate was reacted in a 100 ml round bottomed flask containing 40 ml of dry benzene and the calculated amount of the ligand was added. The contents were refluxed using a fractionating column and the ethanol liberated in the reaction was removed azeotropically with benzene. The progress of the reaction was ascertained by the estimation of ethanol in the azeotrope. The solvent was removed and the product dried under vacuum. The ligands used are:

1: N-[ethyl]salicylaldimine (G>H λ ,ΝΟ),

2: N-[n-propyl]salicylaldimine (C10HnNO),

3: N-[isopropyl]salicylaldimine (C10H13NO)*

4: N-[n-butyl]salicylaldimine (C„H15NO),

5: N-[isobutyl]salicylaldimine(CnH15NO)*,

6: N-[sec-butyl]salicylaldimine(CnHi5NO)**,

7: N-[t-butyl]salicylaldimine(Ci1H15NO)\

8: N-[phenyl]salicylaldimine(CuHnNO),

9: N-[o-tolyl]salicylaldimine(Cl4H13NO),

10: N-[m-tolyl]salicylaldimine(C14H,.,NO)*,

11: N-[p-tolyl]salicylaldimine(C14H,.iNO)**,

12: N-[o-anisyl]salicylaldimine(C14HuN02),

13: N-[m-anisyl]salicylaldimine(Ci4HuN02)*,

14: and N-[p-anisyl]salicylaldimine(C|4HuN02)**, which have been reported earlier /52/.

" Note: In all the products, the superscripts *, " and " indicate isomers (same molecular formula but different names/structures) of the imines. The letters i, c and e refer, respectively to the imines, their compounds and their derived exchange products, with the same serial numbers.

98 R. V. Singh et al. Main Group Metal Chemistry

The above reactions were also carried out in 1:1, 1:3 and 1:4 molar ratios, and even on refluxing a product similar to the one obtained in 1:2 molar ratio reaction could be isolated,

lc Ge(OC2H5)2(C9HU)NO)2 (Yellow solid), 2c Ge(OC2H5)2(C10H12NO)2 (Yellow solid),

3c Ge(OC2H5)2(C|0H12NO)2* (Yellow solid), 4c Ge(OC2H5)2(C11H14NO)2 (Brownish-yellow semi-solid),

5c Ge(OC2H5)2(C11H14NO)2' (Yellow semi-solid), 6c Ge(OC2H5)2(CuH14NO)2" (Yellow solid),

7c Ge(OC2H5)2(C11H14NO)2 (Yellow solid), 8c Ge(OC2H5)2(CuH,0NO)2 (Yellow solid),

9c Ge(OC2H5)2(C14H12NO)2 (Yellow solid), 10c Ge(OC2H5)2(C14H12NO)2* (Light yellow solid),

11c Ge(OC2H5)2(C14Hl2NO)2" (Reddish-yellow solid),

12c Ge(0C2H5)2(C|4H12N02)2 (Brownish-yellow solid), 13c Ge(0C2H5)2(C14H12N02)2* (Yellow solid) and

14c Ge(0C2H5)2(C14H12N02)2" (Greenish-yellow solid)

2.2. Exchange reactions of diethoxygermanium complexes of mono-functional bidentate Schiff bases with 2-methylpentane-2,4-diol:

To a benzene solution of diethoxygermanium complex of monofunctional bidentate Schiff bases, an equimolar amount of 2-methylpentane-2,4-diol was added. The binary azeotrope of ethanol-benzene was removed and the excess of benzene distilled at a high reflux ratio. The reaction product was then dried at 40- 50°C/0.05 mm for two hours,

le Ge(C9H10NO)2(C6H12O2) (Yellow semi-solid), 2e Ge(C,nH12N0)2(C6H1202) (Yellow viscous liquid),

3e Ge(C,nHl2N0)2*(C6H1202) (Yellow semi-solid), 4e Ge(C,,H14N0)2(C6Hl202) (Yellow semi-solid),

5e Ge(C,|H,4N0)2'(C6H1202) (Yellow semi-solid), 6e Ge(C:iH,4NO)2" (C6Hl202) (Yellow semi-solid),

7e Ge(CnH14N0)2*(C6H,202) (Yellow semi-solid), 8e Ge(C,.1HloNO)2(C6H1202) (Yellow solid),

9e Ge(C14Hl2N0)2(C6H,202) (Yellow solid), lOe Ge(Cl4Hl2N0)2*(C6H1202) (Yellow solid),

lie Ge(C14H12N0)2"(C6H1202) (Yellow solid), 12e Ge(C14H12N0)2(C6H1202) (Yellow solid),

13e Ge(C14Hl2N02)2*(Q,H1202) (Yellow solid, sparingly) and

14e Ge(C14Hl2N0)2(C6H1202) (Greenish-yellow solid).

2.3. Reactions of ethyl-orthogermanate with bifunctional tridentate Schiff bases,

CH3COCHCCH3NHXOH, QH5COCHCCH3NHXOH, O-HOQRTC (CHJ):NX"OH, HOC6H4CH:NX"OH

or 2-HCX:,0H6CH:NX""OH, [where,

X = -CH2CH2-, -CH2, -CH-, -JCH2-CH2-CH2-, -|CH-CH2-, O-C6H4 and 4-N02C6H3

CH, CH2-CH3

X'=X"' = -CH2CH2-, -CH2-CH-, -

CH3 CH2-CH3

99 Vol. 28, No. 2, 2005 Coordination Comopunds of Germaninm(lV) Formed with Soft and Hard Donor Atoms: A Look into the Past and Present Work

CH3 I

X" = X"" = -CH2CH2-, -CHjjCH-, -CH2-CH2CH2-, -C-CH2- and -CH-CHr] ,

CH3 CH3 CH,-CH,

The weighed amount of ethylorthogermanate was taken in a 100 ml R.B. flask containing 40ml of dry benzene and the requisite amount of the Schiff base was added to this solution. The reaction mixture resulted in a yellow coloured solution accompanied by a slightly exothermic change. The contents were then refluxed and the course of all these reactions was ascertained by estimating the liberated ethanol. On its completion, the resulting products were rendered free of the solvent under reduced pressure (40-50°C/0.05mm) for two to three hours.

The 1:2 molar products of ethylorthogermanate with:

15i 4-(2-hydroxyethyl)-amino-3-pentene-2-one(C7H1-,N02),

16i 4-(2-hydroxy-1 -propyl)-amino-3-pentene-2-one(C8H15N02),

17i 4-(3-hydroxy-l-propyl)-amino-3-pentene-2-one(C8H15N02)*,

18i 4-(l-hydroxy-2-butyl)-amino-3-pentene-2-one(C9H17N02),

19i 4-(2-hydroxyphenyl) amino-3-pentene-2-one(CnHnN02),

20i 3-(2-hydroxyethyl) amino-1 -phenyl-2-butene-1 -one(Ci2Hi5N02),

21i 3-(2-hydroxy-1 -propyl)-arnino-3-pentene-2-one(C13H17N02),

22i 3-(3-hydroxy-1 -propyl)-amino-1 -pehnyl-2-butene-1 -one(C]3H17N02)*,

23i 3-(l-hydroxy-2-butyl)amino-l-phenyl-2-butene-l-one(C14H19N02),

24i 3-(2-hydroxy-phenyl)amino-1 -phenyl-2-butene-1 -one (Ci6Hi5N02),

25i o-hydroxy-acetophenone-2-hydroxyethylimine(CioH11N02),

26i o-hydroxy-acetophenone-2-hydroxy-1 -propyl-imine(Ci ι Η15N02),

27i o-hydroxy-acetophenone-3-hydroxy-l-propylimine(CnH15N02)*,

28· o-hydroxy-acetophenone-2-methyl-2-propylimine(C12H17N02),

29i o-hydroxyacetophenone-l-hydroxy-2-butyl-imine(C12H17N02)*,

30i N-[2-hyderoxyethyl]salicyladimine (C9HnN02),

31i N-[2-hydroxy-1 -propyl]salicylaldimine(Ci0Hi3NO2),

32i N-[3-hydroxy-l-propyl]salicylaldimine(Ci0H13NO2)*,

33i N-[l-hydroxy-2-butyl] salicylaldimine(CnH15N02),

34i N-[2-hydroxyphenyl]salicylaldimine(Ci3H1 iN02),

35i N-[2-hydroxyethyl]-2-hydroxy-l-naphthaldimine(Ci3H13N02),

36i N-[2-hydroxy-l -propyl]-2-hydroxy-l -naphthaldimine(C14H15N02),

37i N-[3-hydroxy-1 -propyl]-2-hydroxy-1 -naphthaldimine(Ci4Hi5N02)*,

38i N-[l-hydroxy-2-methyl-2-propyl]-2-hydroxy-l-naphthaldimine(Ci5H17N02) and

39i N-[l-hydroxy-2-butyl]-2-hydroxy-l-naphthal-dimine(Ci5H17N02)*, were isolated. These are :

15c Ge(C7HnN02)2 (Yellow solid), 16c Ge(C8H13N02)2 (Yellow solid),

17c Ge(C8H13N02)2(Brown solid), 18c 0ε(0,Η,,Ν02)2 (Highly-yellow viscous liquid),

100 R. V. Singh et al. Main Group Metal Chemistry

19c Ge(CnHnN02)2 (Faint-yellow solid), 20c Ge(C12H13N02)2 (Yellow solid),

21c Ge(C,3H15N02)2 (Yellow solid), 22c Ge(C13H15N02)2* (White solid),

23c Ge(C14H17N02)2 (Yellow solid), 24c Ge(C16H13N02)2 (Dark-yellow solid),

25c Ge(C10H„NO2)2 (Yellow solid), 26c (Ge(CnH13N02)2 (Yellow solid),

27c GeiCnHuNO^' (Light-yellow solid), 28c Ge(C12H15N02)2 (Yellow solid),

29c Ge(C12H15N02)2* (Yellow solid), 30c Ge(C9H9N02)2 (Yellow solid),

31c Ge(C,0HnNO2)2 (Yellow solid), 32c Ge(C10HnNO2)2* (Light-yellow solid),

33c Ge(C,, Η 13N02)2 (Light-yellow solid), 34c Ge(C,3H9N02)2* (Reddish-yellow solid),

35c Ge(C,3H„N02)2 (Yellow solid), 36c Ge(C14H13N02)2 (Dim-yellow solid),

37c Ge(C14H13N02)2* (Dark-yellow solid), 38c Ge(C15H15N02)2 (Yellow solid),

39c Ge(C15H15N02)2* (Yellow solid)

2.4. Reactions of ethylorthogermanate with sulphur containing bifunctional tridentate Schiff bases,

CH3COCHCCH3NHYSH and HOC6H4C(CH3):NY'SH or 2-HOC6H4CH:NY"SH : Where:

Y = -CH2CH2- and o-C6H4, Y' = Y"= -CH2CH2-

Ethylorthogermanate was taken in benzene and the required amount of the Schiff base was added to it. The mixture was refluxed for a suitable period and the benzene ethanol azeotrope was fractionated off. After the distillate attained a temperature of 80°C, the excess of benzene was pumped out and the compound dried at 40-50°C/0.05 mm. The reactions of ethylorthogermanate with

40i 4-(2-thiophenyl)amino-3-pentene-2-one (CIIHI3NSO)3,

41 i o-hydroxyacetophenone-2-thioethylimine(C! 0H13NSO),

42i N-salicylideneanthranilic acid (C14HuN03),

43i N-2-hydroxy-l-naphthylideneanthranilic acid (C]8H13N03),

44i salicylaldehydethiosemicarbazone(C8H9N3SO) and

45! 2-hydroxy-l-naphthaldehydethiosemicarbazone (Ci2HuN3SO) were carried out in 1:2 molar ratio. All the resulting compounds were found to be sparingly soluble in benzene.

40c Ge(C„HuNSO)2 (Dark-brown solid), 41c Ge(C10HnNSO)2 (Brownish-yellow solid),

42c Ge(CI4H9N03)2 (Yellow solid), 43c Ge (C,8H, ,N03)2 (Light-yellow solid),

44c Ge (C8H7N3SO)2 (Yellow solid) and 45c Ge(C12H9N3SO)2 (Yellow solid).

101 Vol. 28, No. 2, 2005 Coordination Comopunds of Germanium(IV) Formed with Soft and Hard Donor Atoms: A Look into the Past and Present Work

2.5. Reactions of ethylorthogermanate with bifunctional tetradentate Schiff bases,

CH3COCHCCH3NHZHNCH3CCHCOCH3,

QHsCOCHCCHJNHZ'HNCHJCCHCOQHS, HOC6H4CH:NZ"N:CHC6H4OH

or HOC6H4CCH3:NZ"N:CCH3C6H4OH: where:

Ζ = CH2-CH2-, Z' = CHrCH-, Z" = -CH2-CH2-CH2-, including

CH3

46i Ν ,N '-ethylenebis[2,4-pentanedioneimine](Ci2H2oN202),

471 N,N'-l,2-propylene bis[2,4-pentanedioneimine](Ci3H22N202),

48i N,N'-l,3-propylenebis[2,4-pentanedioneimine](C13H22N202)*.

49i N,N'-ethylenebis[l-phenyl-l,3-butane-dioneimine](C22H24N202),

50i N,NM,2-propylenebis[l-phenyl-l,3-butane-dioneimine] (C23H26N202),

51i N,N'-l,3-propylenebis-[l-phenyl-l,3-butane-dioneimine] (C23H2fiN202)*,

52i N,N'-ethylenebis[o-hydroxyacetophenoneimine] (ClxH20N2O2),

53i N,N'-l,3-propylenebis[o-hydroxyacetophenoneimine] (Ci9H22N202),

54i Ν,Ν'-ethylenebis [salicylaldimine] (C16H16N202) and

55i N,N'-l,3-propylenebis[salicylaldimine] (C17H1KN202), the compounds:

46c Ge(0C2H5)2(C,2H18N202) (Brown solid), 47c Ge(OC2H5)2(Cl3H2oN202) (Brown solid),

48c Ge(OC2H5)2(C,3H20N2O2)' (Reddish-yellow solid), 49c Ge(0C2H5)2(C22H22N202) (White solid),

50c Ge(0C2H5)2(C23H24N202) (Brown solid), 51c Ge(0C2H5)2(C23H24N202)* (Yellow solid),

52c Ge(0C2H5)2(C,8H18N202) (Brownish-yellow solid), 53c Ge(OC2H5)2(C19H20N2O2) (Yellow solid),

54c Ge(0C2H5)2(C,6H14N2O2) (Dark-yellow solid) and 55c Ge(0C2H5)2(C17H16N202) (Yellow solid) were synthesized.

2.6.Exchange reactions of diethoxygermanium derivatives of bifunctional tetradentate Schiff bases with 2-methylpentane-2,4-diol yielded,

46e Ge(Cr,Hi202)(Ci2Hli(N202) (White solid), 47e GeiQHuOzXCuH^NzOz) (Dim-brown solid),

48e Ge(C6H12O2)(C13H20N2O2)* (Brown solid), 49e Ge(C6H1202) (C22H22N202) (White solid),

50e Ge(C6H1202)(C23H24N202) (Brown solid), 51e Ge(QH,202)(C23H24N202)* (White solid),

52e Ge(C6H1202XC,xHIKN202) (Light-yellow solid),

53e Ge(C6H1202)(CiyH2nN202) (Yellow solid, sparingly), 54e Ge(C6H1202XClflHl4N202) (Yellow solid) and

55e Ge(C6Hi202)(C17H,6N202) (Yellow solid) complexes.

102 R. V. Singh et al. Main Group Metal Chemistry

3. OTHER GERMANIUM(IV) COMPLEXES: EXPERIMENTAL

3.1. Triorganogermanium(IV) complexes of monofunctional bidentate thiosemicarbazones and benzothiazolines derived from heterocyclic ketones

A weighed amount of triphenylchlorogermane was added to the calculated amount of the ligand in 1:1 molar ratio using dry THF as the solvent in presence of Et3N as hydrogenchloride acceptor. After complete precipitation of the Et3N.HCl formed during the course of the reaction, the excess of the solvent was removed and the complexes were dried in vacuo. The complexes were purified by repeated washing with cyclohexane and methanol. The complexes are Ph3Ge(C8H9N4S) (Brown), Ph3Ge(C7H8N3SO) (Reddish),

Ph3Ge(C7H8N3S2) (Brown), Ph3Ge(C13H„N2S) (Yellowish), Ph3Ge(C,2H10NSO) (Dark brown) and

Ph3Ge(C12H10NS) (Brown).

3.2. Germanium(IV) complexes of monofunctional bidentate semicarbazones and thiosemicarbazones derived from heterocyclic ketones

Germanium tetrachloride was dissolved in methanol and it was then added to the methanolic solution of the ligand in 1:2 molar ratio under perfectly anhydrous conditions. The mixture was refluxed for 2-3 hours. The addition reaction takes place with a change of colour in solution. The resulting product was then isolated by removing the excess of the solvent under reduced pressure and dried for 3-4 hours.

3.3. Germanium(IV) complexes of monofunctional bidentate benzothiazolines derived from heterocyclic ketones

A weighted amount of germanium tetrachloride was taken in dry methanol in 100 ml R.B. flask and the requisite amount of the ligand was added to it in 1:2 molar ratio. The reaction mixture was refluxed over a fractionated column for 3-4 hours. The excess of the solvent was removed under reduced pressure and the compound so obtained was dried in vacuo. The complexes were then repeatedly washed with dry cyclohexane and methanol so as to ensure their purity. The new compounds isolated are GeCl4(CgH10N4O)2

(Greenish), GeCl4(C7H9N302)2 (Off-white), GeCl4(C7H9N3SO) (Dark-yellow), GeCl4(C13H,3N30)2

(Creamish), GeCl4(C8H10N4S)2 (Yellow), GeCl4(C7H9N3SO)2 (Dark yellow), GeCl4(C7H9N3S2) (Grey),

GeCl4(C13H I3N3S)2 (Dark-brown), GeCl4(C,3H12N2S)2 (Yellow), GeCl4(C12H„NSO)2 (Brown),

GeCl4(C,2HNNS2)2 (Orange) and GeCl4(C18H15NS)2 (Brown).

103 Vol. 28, No. 2, 2005 Coordination Comopunds of Germanium(IV) Formed with Soft and Hard Donor Atoms: A Look into the Past and Present Work

4. RESULTS AND DISCUSSION FOR ORGANOGERMANIUM - SCHIFF BASE COMPOUNDS

4.1. Properties of the Obtained Complexes

4.1.1. Germanium(IV) complexes of monofunctional bidentate Schiff bases (SBH): The 1:2 molar reactions of ethylorthogermanate with monofunctional bidentate Schiff bases, having the η donor system Ν OH, yielded Ge(OC2H2)2(SB)2 type of complexes. The resulting diethoxy bis-Schiff base germanium complexes have been obtained either as coloured solids or semisolids. The ebullioscopic determinations of the molecular weights in boiling benzene show them to be monomers, thereby indicating a stable hexacoordinated state for the central germanium atom(VIII).

OC2H5

OC2H5

(VIII)

As evidenced by the low molar conductance values (8-15 ohm"1 cm2 mol"1), the conductivity measurements in DMF have shown the non-electrolytic nature of the germanium Schiff base derivatives. The reactions in 1:2:1 stoichiometry of ethylorthogermanate with monofunctional bidentate Schiff bases and 2-methylpentane-2,4-diol have yielded Ge(SB)2(C6H1206) type of unsymmetrical derivatives. The reaction was carried out in two steps. In the first step the ethylorthogermanate was reacted with monofunctional bidentate Schiff bases in 1:2 molar ratio and, after the liberation of two moles of ethanol,

Ge(OC2H5)2(SB)2 type of derivative was obtained. It was then reacted with 2-methylpentane-2,4-diol in equimolar ratio and the remaining two moles of ethanol were also liberated, resulting in the formation of

Ge(SB)2(C6H1202) type of products. It may, however, be mentioned that, although Ge(OC2H5)4 was reacted with SBH in varying molar ratios, only 1:2 type of products could be isolated in each case. Further, on refluxing Ge(OC2H5)4, SBH and C6Hi402 in the medium of anhydrous benzene, only derivatives of the type

Ge(SB)2(C6HI202) were obtained. The resulting new compounds are either yellow solids or semisolids and these were purified by repeated washing with solvent ether. The compounds, which are initially highly viscous liquids (termed as semisolids), tend to solidify on keeping for a few days in a vacuum desiccator. Their purity was further ascertained by T.L.C. As compared to the 1:2 derivatives of the same Schiff base, these asymmetrical derivatives have higher melting points or decomposition temperatures.

4.1.2. Germanium(IV) Complexes of Afunctional tridentate Schiff bases (HONOH) The reactions of ethylorthogermanate in 1:2 molar ratios with bifunctional tridentate Schiff bases

104 R. V. Singh et al. Main Group Metal Chemistry

HO Ν OH yielded the complexes Ge(0 Ν 0)2. Similar reactions took place with HO Ν SH type of ligands. All the newly synthesized products have been found to be coloured solids or semisolids and these were purified either by crystallization from a solution mixture of benzene and chloroform or by repeated washing with solvent ether/hexane. The molecular weight determinations in boiling chloroform show them to be monomers and the following structures having hexa-coordinated germanium (IX) may be suggested for such compounds:

(IX)

The molar conductance values as determined in DMF at the concentration ΙΟ"3 Μ show these derivatives to be non-electrolytes. In the case of reactions of ethylorthogermanate with bifunctional tridentate Schiff bases derived from 2,4-pentanedione, it was observed that these reactions are slower than those with the Schiff bases of 1 -phenyl-1,3-butanedione. This may be due to the -I effect of the phenyl ring, whereby the polarization of the enolic proton of the ligand is enhanced. The N-[hydroxyalkyl or aryl] salicylaldimines are, however, the most reactive of all the above species and this may be due to the presence of the phenolic group in the ligand moiety.

4.1.3. Germanium(IV) complexes of bifunctional tetradentate Schiff bases (BTHJ: The bifunctional tetradentate Schiff bases have been found to react with ethylorthogermanate in 1:1 molar ratio, giving rise to the formation of Ge(OC2H5)2(BT) type of complexes. The reaction, in general, may be represented as follows:

Ge(OC2H5)4 + BTH2 --> Ge(OC2H5)2(BT) + 2C2H5OH

With the view to prepare compounds of the type Ge(BT)2,1:2 molar reactions were also carried out in the presence of excess of the Schiff bases, but these were found to be unsuccessful, probably due to steric factors. The resulting new compounds are coloured solids, mostly soluble in chloroform and non-electrolytes in DMF. The ebulliosopic determinations of molecular weights in boiling benzene of chloroform show them to be monomers, thereby indicating a stable hexacoordinated state of germanium in these compounds(X).

105 Vol. 28, No. 2, 2005 Coordination Comopunds of Germanium(lV) Formed with Soft and Hard Donor Atoms: A Look into the Past and Present Work

.OC2H5

OC2H5

(X)

Further, exchange reactions of Ge(OC2H5)2(BT) type of derivatives with 2-methylpentane-2,4-diol were also carried out to examine the labile nature of the ethoxy groups and these may be depicted as follows:

Ge(OC2H5)2(BT) + C6H1402 -> Ge(C6H1202)(BT) + 2C2H5OH

Similar to the parent bis-ethoxy-Schiff base derivatives of germanium, the resulting new compounds are also non-electrolytes in nature in the medium of DMF. These are also soluble in chloroform and quite stable in the open atmosphere.

4.2. Spectral Studies

4.2.1. Infrared spectra The infrared absorption frequencies of the germanium Schiff base derivatives along with their tentative assignments was an attempt to establish bonding in these complexes. The free OH group absorbs at 3730- 3520 cm"1 and the absorption of the free NH group occurs at 3400-3200 cm'1. However, in the Schiff bases used during the present investigations, broad and intense weak bands have been observed in the region 3400- 3100 cm"', and these are comparatively on the lower side. This lowering may be due to both types of hydrogen bonding. In the case of a few monofunctional bidentate and bifunctional tetradentate Schiff bases, the stretching frequency of the hydrogen bonded OH is shifted considerably to the lower side and overlaps with the vCH vibrations, exhibiting a broad band in the region, 3050-2900 cm"1. Freedman /61/ has observed similar absorption bands in the region, 2800-2600 cm'1 in the spectra of salicylideneaniline and Ν, N' ethylene bis(salicylaldimine). An examination of the Fisher-Hirshfelder models of 2,4-pentanedione indicates that the hydrogen bonding can not occur in the 'anti' form and is only possible in the case of the 'syn' form /62/ (XI).

106 R. V. Singh et al. Main Group Metal Chemistry

OH R Η HO

C C

C CH3 H3C 'CH3 H3C RN

Η 'Syn' form 'Anti' form

XI

The disappearance of vOH or vNH band in the corresponding Schiff base derivatives is an indication of the bonding of germanium to phenolic or alcoholic oxygen as well as nitrogen of the azomethine group. In the case of the Schiff bases derived from thiosemicarbazide, a strong band at 3440 cm"1 may be assigned to

VNH2 and this remains almost unchanged in the corresponding germanium complexes. It therefore indicates the non-participation of this functional group in the complex formation. A strong band in the sulphur- containing Schiff bases is also observed in the 2580-2560 cm"1 region and this may probably be assigned to vSH. The disappearance of this band in the corresponding germanium complexes shows the coordination of germanium to sulphur. The infrared spectra of the ligands derived from aminoacids show a band of medium intensity at 1700 cm"1 and this may be assigned to the antisymmetric carbonyl stretch of the -COOH group.63 This, however, totally disappears in the infrared spectra of the complexes. A strong band is observed at 1600 cm'1 in the complexes and this may be due to the antisymmetric stretch of the coordinated carboxylate ion. A strong band in the 1635-1605 cm'1 region in the Schiff bases is characteristic of the azomethine group and this remains almost unaltered or is shifted towards the higher frequency region in the germanium complexes. The shifting of this band to the higher side probably indicates that coordination due to the π- electrons could have led to an increase in the frequency of the azomethine stretching vibrations. A band of medium intensity in the region, 1070-1040 cm"' in the case of the Schiff bases of thiosemicarbazide, may be assigned to C=S stretch /64/ and this is found to be absent in the resulting Ge(IV) complexes. The disappearance of this band along with the 3320 cm'1 band (due to vNH) in the complexes indicates that coordination takes place through both sulphur and nitrogen atoms of the ligands. The maximum intensity bands in the regions 1040 - 1010 and 900-875 cm"1 may be assigned to Ge-O-C absorptions /65/ and vGe-0 band /66/, respectively. The assignment of metal-nitrogen stretching vibrations in the compounds, which contain nitrogen in addition to the carbon, hydrogen and oxygen, is rather difficult on account of strong coupling between the various modes. However, the appearance of a strong band at 690 cm"1 in the present cases may be ascribed to the Ge-N coordinate bond /68/. Schiff bases derived from ß-diketones and aliphatic or aromatic amines or diamines have been shown to exist in the keto-amine form rather than the enolimine or the ketoimine form. On the other hand, Schiff bases derived from aromatic amines and salicylaldehyde or 2-hydroxy-1 -naphthaldehyde have been shown to exist as an equilibrium mixture of the ketoimine and enolimine forms and these results are in good agreement with

107 Vol. 28, No. 2, 2005 Coordination Comopunds of Germanium(IV) Formed with Soft and Hard Donor Atoms: A Look into the Past and Present Work the present observations.

4.2.2. NMR spectra The proton signals observed at δ 12.95 and 4.80 ppm in the ligands are due to the phenolic and alcoholic OH, respectively. The disappearance of these signals in the spectra of the corresponding germanium complexes provides a strong evidence for the desired complexation through these groups. The methine and methyl proton signals are shifted downfield (δ 8.24 and 1.33 ppm respectively), leaving the methylene proton signal almost unaltered. The resonance signals for these protons are observed initially at δ8.20, 1.20 and 3.50 ppm, respectively in the spectra of the ligands. The deshielding of these nuclei rather indicates that germanium acquires the stable hexa-coordinated environment as a result of donation of a lone pair of electrons by the nitrogen of the ligand moiety. The coordination of germanium to nitrogen is further supported by the spectrum of a representative derivative. In that case, the two signals observed at δ3.98 and 1.55 ppm in the spectrum are due to the methylene and methyl protons, respectively of the two ethoxy groups. The signal at δ 11.03 ppm in the spectrum of the ligand is due to the presence of quadrupole on the nitrogen atom. This resonance signal, however, disappears in the spectrum of its 1:2 derivative, thereby showing the deprotonation of the NH group resulting in the coordination of nitrogen to the germanium atom. Further, a considerable downfield shift in the proton signals of the amine residue of the ligand moiety in the spectra of the derivatives indicates the formation of a coordinate bond between nitrogen and germanium, thereby causing a change in the electronic environment around the same protons. Similar observations have been made in the spectra of all the remaining compounds synthesized during the course of the present investigations, and these are in full agreement with the structures proposed for these derivatives.

4.2.3. Mass spectra of germanium Schiff base complexes The mass spectral fragmentations of the complexes: germanium bis[o-hydroxyacetophenone-l-hydroxy- 2-butylimine], germaniumbis[N-(2-hydro-xyethyl)salicylaldimine] and germaniumbis[N-(2-hydroxy-1 - propyl)salicyl-aldimine] have been studied and these support the structures indicated for the above complexes. Germanium occurs in several isotopic forms and out of these three having mass numbers 70, 72 and 74 are important. The abundance of 74Ge is 37.1%. In the mass spectra, a particular ion consisting of germanium shows the expected combination of the peaks corresponding to the various isotopes of germanium. The molecular ion peak of germanium bis(o-hydroxyacetophenone-l-hydroxy-2-butylimine) is obtained at m/z 484 (calcd. on the basis of 74Ge), and peaks at 482 and 480 are also observed having the expected abundances. The molecular ion undergoes fragmentation by mainly two schemes - one leading to the ion consisting of germanium, while the other scheme leads to the ions characteristic of the Schiff base. The ion ++ ++ [Ci2H1502NGe] observed at m/z 279 eliminates ethane molecule to give the ion [C10H9O2NGe] at m/z f+ 249. The ion [C^H^NGe]" also the forms ion [C8H7ONGe] at m/z 207, and this loses a methyl radical + + to form the ion [C7H4ONGe] at m/z 192. The latter splits off germanium to form ion [C7H4ON] at m/z 118 and this finally gives ion QHjC^ at m/z 91.

108 R. V. Singh et al. Main Group Metal Chemistry

The molecular fragments according to the second scheme form the ion at m/z 175 and this loses a + hydrogen atom to form the ion [CnH12ON] at m/z 174. The latter splits off an ethylene molecule, + accompanied by a migration [CnH^ON]" to form the ion [CuH12ON] at m/z 146. This.ion forms the most + + abundant peak in the spectrum. The ion [C9H8ON] eliminates CH3CN to form the ion C7H50 at m/e 105, ++ + which subsequently loses CO to give ion [C10H9O2NGe] at m/z 77. The latter finally changes into ion C4H3 at m/z 51. Similarly, the mass spectra of germanium bis-[N-(2-hydroxyethyl)salicylaldimine] and germanium bis[N-(2-hydroxy-l-propyl)salicylaldimine] show molecular ion peaks at m/z 400 and 428, respectively, with base peaks at m/z 207. With the view to ascertain the thermal stability, the TGA of 1:2 germanium complexes of N-[2-hydroxy- 1 -propyljsalicylaldimine, N-[o-hydroxyphenyl] salicylaldimine, 3-(2-hydroxyphenyl)amino-1 -phenyl-2- butene-1 -one,N-[2-hydroxyethyl]-2-hydroxy-1 -naphthaldimine and o-hydroxyacetophenone-2-hydroxy- ethylimine along with the first two ligands were carried out. The TGA curve of the ligand N-[2-hydroxy-1 - propyljsalicylaldimine does not show any change up to a temperature of 200°C, and thereafter a heavy loss in its weight indicates its decomposition. The corresponding germanium complex has, however, been found to be stable up to 300°C. It finally gets converted to Ge02 at 600°C. Similarly, the TGA curve of the Schiff base, N-[o-hydroxyphenyl]salicylaldimine has shown it to be quite stable up to a temperature of 200°C, and afterwards a heavy loss in weight due to its decomposition is noted. The corresponding germanium complex has been found to be stable up to 180°C. Thereafter, a slow decomposition takes place up to 380°C and this is followed by a rapid decomposition up to 450°C. It finally gets converted into Ge02 at 750°C. On the other hand, the TGA curve of germanium bis[3-(2-hydroxy-phenyl)amino-l-phenyl-2-butene-l- one] does not show any change up to a temperature of 200°C, and after that a slow decomposition up to 350°C, followed by a rapid change up to 400°C, has been observed. The resulting product at 740°C has been identified to be Ge02. The thermograms drawn by plotting percentage residual weight against temperature for germanium bis-[N-(2-hydroxyethyl)-2-hydroxy-1-naphthaldimine] and germanium bis-[o-hydroxy- acetophenone-2-hydroethylimine] indicated that the initial decomposition temperatures of these complexes are 260 and 280°C, respectively. A comprehensive index of thermal stability, the integral procedural decomposition temperature (IPDT), was proposed by Doyle69 and Bajaj et al. /70/. This index places all materials on a common basis and provides the most valid comparison of different complexes. Accordingly, from IPDT values it is clear that the overall thermal stability of the second complex is higher then that of the first complex A.

5. RESULTS AND DISCUSSION FOR OTHER GERMANIUM(IV) COMPLEXES

5.1. Triorganogermanium(IV) Complexes of Monofunctional Bidentate Thiosemicarbazones and Benzothiazolines Derived from Heterocyclic Ketones

Triphenylchlorogermane has been found to react with the above mentioned ligands. The reactions involved in the preparation of these derivatives may be depicted as shown below:

109 Vol. 28, No. 2, 2005 Coordination Comopunds of Germaniwn(IV) Formed with Soft and Hard Donor Atoms: A Look into the Past and Present Work

Ph,GeCl + NnSH ™F > Ph,Ge(NnS) + Et,N.HCl Et3N

(where NnSH represents the donor system of the ligand)

All the resulting products are coloured solids, soluble in methanol, DMF and DMSO. These are insoluble in CC14. The molecular weight determinations show that these complexes are monomeric in nature. The conductance measurements (15-20 ohm"1 cm2 mol"1) of 10"3M solutions reveal their non-electrolytic behaviour. The electronic spectrum of 2-acetylpyridine thiosemicarbazone exhibits a band at ca. 380 nm, assignable to η-π* transitions of the >C=N chromophore. However, in the spectrum of germanium complex, this shifts slightly to the lower wavelength side, possibly due to the coordination of azomethine group to the germanium atom. Other bands observed at ca. 285 and 305 nm in the case of thiosemicarbazone remain approximately at the same position in the spectrum of the germanium complex. In the IR spectra of 2-acetylpyridinethiosemicarbazone and benzothiazoline, a broad and strong band observed in the region 3300-3100 cm'1 can be attributed to v(NH) modes. In the case of thiosemicarbazones there appears a band at ca. 1050 cm"1, due to v(C=S) vibrations, which shifts towards lower wave number side in the spectrum of germanium complex. The absence of bands due to v(SH) and v(>C=N) in the case of second ligand are indicative of the benzothiazoline structure of the ligand rather than the Schiff base form /71/72/. The bands due to v(NH) vibrations disappear in the spectra of germanium complexes, indicating the chelation of sulphur with the metal atom. A new band at ca. 1585 cm'1 is also observed in the spectra of germanium complexes and can be assigned to v(>C=N) vibrations. The presence of (>C=N) band suggests that the resulting germanium complexes are the metal Schiff base derivatives, as the benzothiazoline ring rearranges to give the Schiff-base derivatives in the presence of metal ion. In the spectra of germanium complexes, the appearance of v(Ge<—N) /32/ and v(Ge-S) /74-76/ vibrations 1 at ca. 650 and 418 cm" are the indication of complex formation. Further, new bands due to Ph3Ge /77,78/ group appear at ca. 1100, 700, 455 and 325 cm"1. The signal due to NH proton of the ligand thiosemicarbazone appears at 510.64 ppm and is found to be absent in the spectrum of the corresponding complex, again supporting the deprotonation of the NH group during the course of coordination. In the case of benzothiazoline the signal due to the NH proton appears at 54.55 ppm and this, however, disappears in the spectrum of the corresponding germanium complex, indicating the deprotonation of the NH group on complexation with the germanium atom. Further, the ligand thiosemicarbazone shows a downfield shifting for aromatic protons in the spectrum of its complex, thereby indicating the coordination of azomethine nitrogen to the germanium atom. The downfield shift in the position of the azomethine proton (-C=N) signal is also a strong evidence for the formation of a coordinate bond between nitrogen and germanium atoms. The appearance of the signal due to NH2 protons at the same positions in the ligand and its complex shows the non-involvement of this group in coordination. 13C NMR spectra of 2-acetylpyridinethiosemicarbazone and benzothiazoline and their corresponding germanium complexes were also recorded in methanol. The shifts of the carbons attached to sulphur and

110 R. V. Singh et al. Main Group Metal Chemistry

nitrogen indicate their coordination with, the germanium atom. Thus, on the basis of the foregoing spectral studies, a trigonal bipyramidal structure (XII) may be proposed for the resulting complexes.

(XII)

5.2. Germanium(IV) Complexes of Monofunctional Bidentate Semicarbazones Derived from Heterocyclic Ketones

The reactions of germanium tetrachloride with 2-acetylfuran-, 2-acetylthiophene-, 2-acetylpyridine- and 2-acetylnaphthalene semicarbazone, in 1:2 molar ratios, in completely dry methanol yielded complexes of the n n type GeCl4.(N OH)2 (where, N OH represents the donor set of the semicarbazone molecule). These reactions are quite facile and occurred instantaneously, with the formation of a solid mass which is removed by filtration. All the resulting complexes are coloured solids, soluble in common organic solvents, monomeric and show non-electrolytic behaviour in dry DMF. The mode of bonding in these addition products has been deduced on the basis of electronic, infrared and NMR ("Η and ,3C) spectral evidences. In the electronic spectrum of 2-acetylthiophene semicarbazone, a band at ca. 380 nm assignable to η-π* transitions of the azomethine linkage gets shifted to the lower wavelength in the germanium complex, thereby indicating the coordination of nitrogen of the azomethine group. Further, two bands at ca. 280 nm and 300 nm, due to π-π* transitions in the ligand, remain approximately at the same positions in the spectrum of the germanium complex. In the IR spectrum of 2-acetylthiophene semicarbazone, a sharp band observed at ca. 1620 cm"1 due to v(>C=N) group shifts to the lower frequency (ca. 20 cm"') region in the germanium complex, indicating the participation of the azomethine nitrogen in bonding. A broad and strong band at ca.3300 - 3100 cm"1, due to v(NH) / (OH) modes of vibrations in the ligand, appears as such in the corresponding germanium complex, indicating the non-involvement of this group during complexation. Further, some additional bands at ca.680 and 340 cm"1 in the complex may be assigned to v(Ge<—N)79 and v(Ge-Cl)80 vibrations, respectively. The appearance of (>C=0) stretching vibrations at ca. 1720 cm"1 is the additional confirmation for non- involvement of the functional group in coordination.

Ill Vol. 28, No. 2, 2005 Coordination Comopunds of Germaninm(IV) Formed with Soft and Hard Donor Atoms: A Look into the Past and Present Work

The bonding pattern discussed above gets further support by the 'H NMR spectral studies of the same ligand and its corresponding germanium complex.''7. The strong points which confirm the suggested structure for the germanium complex have been observed. The broad signal exhibited by the ligand due to (NH) proton also appears in the corresponding complexes at the same position, indicating the non-involvement of this functional group in coordination. In the spectrum of the complex, the considerable downfield shifting in the position of the methyl protons is strong evidence for participation of the azomethine nitrogen in bonding with germanium atoms. The appearance of signals due to NH2 protons at the same positions in the ligand and its complex, confirms the non-involvement of this group in coordination. \ The 1JC NMR spectra of 2-acetylthophene semicarbazone and its corresponding germanium complex were also recorded. The signals due to the carbons attached to the ketonic and azomethine groups in the ligand appear at δ 165.20 ppm and 151.00 ppm, respectively. However, in the spectra of corresponding germanium complex, the same carbons appear at 165.10 ppm and 145.80 ppm, respectively, indicating the coordination of azomethine nitrogen to germanium atom. All these studies clearly indicated an octahedral structure for these derivatives.

5.3. Germanium(IV) Complexes of Monofunctional Bidentate Thiosemicarbazones Derived from Heterocyclic Ketones

The reactions of germanium tetrachloride with the monofunctional bidentate thiosemicarbazones in 1:2 n molar ratios in dry methanol proceed instantaneously with the precipitation of a solid mass, GeCl4.(N SH)2. The resulting derivatives are coloured solids, soluble in DMF and DMSO and insoluble in chloroform and

CC14. The molecular weight determinations show them to be monomeric in nature. The conductance measurements in dry DMF at the room temperature show that the complexes are almost non-electrolytes. In the electronic spectrum of 2-acetylpyridine thiosemicarbazone, a band due to (>C=N) chromophore at ca. 380 nm shifts to a shorter wave length in the complex and appears at ca. 340 nm, and thus supports the coordination of azomethine nitrogen to the germanium atom. Further, two bands at ca. 280 nm and 305 nm remain approximately at the same positions in the spectrum of germanium complex. In the IR spectra of the ligands, a broad band in the region 3300-3100 cm'1 is due to v(NH) vibrations.

The strong bands observed at ca. 3430 and 3350 cm"' are due to vlsy and vsym modes of NH2 group, which remain at the same position in the spectra of complexes. The appearance of v(C=S) vibrations at ca. 1050 cm" ' in the ligands remain unaltered in the corresponding complexes. Further, a sharp and strong band at 1600 + cm"' in the spectra of ligands, due to v(>C=N) group, shifts towards the lower wave number (ca. 10 cm"1) side, indicating the coordination of azomethine nitrogen to the germanium atom. This is further supported by the appearance of v(Ge<—N)K1 vibrations at ca. 685 cm"'. The presence of v(NH) and v(Ge-Cl) bands in the solid state spectra of the complexes is additional confirmation for the non-involvement of functional groups in bond formation. In the 'Η NMR spectra, a sharp singlet observed for NH proton at 510.64 ppm in the spectrum of the ligand also remains as such in the spectrum of the corresponding germanium complex, indicating the non- participation of this functional group in bond formation. The methyl protons attached directly to the

112 R. V. Singh et al. Main Group Metal Chemistry

azomethine moiety, appearing at δ 1.80 ppm, show a downfield shift of ca 50.40 ppm in the spectrum of the complex. This further supports the formation of (Ge<—N) bond in these type of complexes. The signal observed for NH2 protons at 52.84 ppm in the complex again supports the non-participation of this group during complexation. The marked shifting in the position of the 13C NMR signal for the carbon constituting the azomethine group clearly indicates the participation of azomethine nitrogen in coordination. Thus, on the basis of above spectral evidence, the hexa-coordinated structure has been proposed for the resulting germanium complexes.

5.4. Germanium(IV) Complexes of Monofunctional Bidentate Benzothiazolines Derived from Heterocyclic Ketones

The addition reactions of germanium tetrachloride with mono-functional bidentate benzothiazolines

(B2tH2) in 1:2 molar ratios in dry methanol proceeded with the formation of GeCl4(BztH)2 type of complexes. All these newly synthesized complexes are coloured solids having sharp melting points and are soluble in DMSO, DMF and CHCI3. The molecular weights determined by the Rast camphor method show them to be monomers. The molar conductances of these complexes in DMF at room temperature lie in the range 15 - 20 ohm"' cm2 mol"1, suggesting that these behave as non-electrolytes. In the electronic spectra of the complexes, a band at ca. 410 nm, assignable to η-π* electronic transitions of the azomethine group, does not appear in the spectra of the ligands. This indicates that the benzothiazoline ring rearranges to give the Schiff-base derivatives in presence of the germanium atom. Further, two bands due to σ - σ* and π - π* benzenoid transitions are observed at ca. 270 nm and 310 nm, in the spectra of the complexes as well as in the spectra of the ligands. In the infrared spectra of the ligands, the absence of v(SH) band in the region 2700-2500cm"', and the appearance of a strong and sharp v(NH) band in the region 3400-3200 cm"1, are strong evidence for the existence of the benzothiazoline structure. However, in the spectra of germanium complexes, the disappearance of the v(NH) band and the presence of a v(SH) band indicate that the benzothiazoline ring rearranges to give azomethine structure in the presence of germanium, which latter finally acts as a monobasic bidentate ligand. Further, in the case of germanium complexes, a strong band ca. 1580 cm"1 may be ascribed to the coordinated >C=N group. Some new bands, due to v(Ge<-N)73 and v(Ge-Cl) /78/ at ca. 680 cm'1 and 340 cm"1, respectively, are additional confirmation for the formation of these complexes. 'H NMR spectra of 2-acetylpyridine benzothiazoline and its germanium complex have been recorded in DMSO-c^. The signal at 5 4.55 ppm in the case of ligand is due to NH proton. The disappearance of this signal in the spectra of the complex and appearance of a new signal at δ 5.50, due to SH proton, indicate that the benzothiazoline ring rearranges in the presence of germanium atom to give the Schiff base derivatives. The azomethine proton signal due to (-C=N-) group at 52.15 ppm in the ligand undergoes deshielding and was observed at 5 2.35 ppm in the germanium adducts. This again confirms the coordination of the azomethine nitrogen to the germanium atom. The ' C NMR spectral data of the germanium compounds with benzothiazolines clearly indicated the involvement of the nitrogen atom in the complex formation.

113 Vol. 28, No. 2, 2005 Coordination Comopunds of Germanium(IV) Formed with Soft and Hard Donor Atoms: A Look into the Past and Present Work

Thus on the basis of the above spectral studies, the hexa-coordinated environment around the germanium atom may be proposed in the resulting derivatives. However, it has been reported that if these reactions are allowed under microwave irradiation then the products are formed quickly in much less time and with higher yields. Thus, the microwave method is more useful than the ordinary synthesis under reflux. Similarly Seifullina et al. /82/ have established the structures of diphenylcarbazone derivatives of germanium(IV). Stable heterocyclic Schiff base derivatives of this element along with tin and lead have also been synthesized and characterized by Agustin and coworkers /83/.

6. BIOLOGICAL ASPECTS

The field of organo-germanium chemistry is becoming increasingly important. Certain germanium compounds have a low mammalian toxicity, but a marked activity against certain bacteria, which makes them useful as chemotherapeutic agents. Twenty years have passed since the original synthesis of an organic germanium compound. During that period, organic germanium has been used clinically in many parts of the world to treat a wide spectrum of illnesses, and has been the subject of extensive research in many disciplines : pathology, biochemistry, pharmacology, immunology, oncology and neurochemistry. Organic germanium has been used in a broad spectrum of regimes - on its own, with diet and stress counselling, and as a drug in clinical trials of cancer therapy, in conjunction with chemotherapy, radiation therapy and surgery. Drawing these sources of information together gives quite a solid overview of the fundamental aspects of organic germanium. The safety of organic germanium has been well-documented, as have its health-promoting effects in many diseases, including cancer and arthritis. Case histories, and in some instances, clinical trials, have documented organic germanium's therapeutic effects in treating the following conditions : • Rheumatoid Arthritis and Rheumatism • Cancer - Colon, Prostate, Breast, Lung, Ovarian, Cervical • Leukemia • Asthma • Diabetes • Malaria • Senile Osteoporosis • Mental Disorders - Depressive Psychoses, Schizophrenia, • Pain • Digestive Disorders - Gastritis, Ulcers • Influenza • Cardiac Disorders - Angina, Hypertension, Arteriosclerosis, Apoplexy, Cardiac Infarction • Circulatory Disturbances - Raynaud's Disease • Parkinson's Disease

114 R. V. Singh et al. Main Group Metal Chemistry

• Cerebral Sclerosis • Skin Eruptions - Warts, Corns, Eczemas, Burns, Herpes • Epilepsy • Old-Age Infirmities • Amyloidosis • Myelo-Optico-Neuropathy • Eye Diseases - Glaucoma, Black Cataracts, Detached Retinas, Inflammation of the Retina and Optic Nerves, Behcet's Disease.

Measurement of urinary Ge can detect occupational exposure to inorganic Ge and its compounds. It is prudent to recommend the monitoring of renal variables in workers exposed to Ge /84/. A histological comparison of the liver, spleen, bone marrow, circulating young erythrocytes, and differential count in mature male and female albino rats receiving germanium dioxide with their litter controls not receiving this compound was made /85/. The Journal of Microbiology and Immunology has recently reported that organic germanium compound promotes increased interferon production. The International Archives of Allergy and Applied Immunology reported that it has helped the restoration of impaired immune responses. This interferon-stimulating effect puts organic germanium in a select group of true immunostimulants. (Additional agents are Vitamin C, Ε and Coenzyme QIO.) There are additional reports relating to life-threatening illnesses. The research has moved organic germanium from anecdotal curiosity to scientific fascination. The apparent versatility of organic germanium in normalizing health and alleviating major human diseases and dysfunctions suggest that it acts at a fundamental level of life function. It is clearly apparent that organic germanium will become the most important supplement in the American diet in years to come. Documented studies about organic germanium have been reported in the following medical journals: • Anticancer Research • British Journal of Cancer • Journal of Interferon Research • Microbiological Immunology • International Archives of Allergy and Applied Immunology • International Journal of Radiation Biology • Journal of Biological Response Modifiers.

ACKNOWLEDGEMENT

The authors are thankful to U.G.C., C.S.I.R. and Central D.S.T., New Delhi as well as state D.S.T., Rajasthan Government, Jaipur for financial assistance. Some part of this review has been prepared with the help of recently sanctioned CSIR Grant No. 01 (1956)/04/EMR-II. One of the authors (Dr. C.N. Deshmukh) is highly grateful to honourable Dr. Devi Singh Sekhawat, who always blessed and encouraged her. Mrs.

115 Vol. 28, No. 2, 2005 Coordination Comopimds of Germaniam(IV) Formed with Soft and Hard Donor Atoms: A Look into the Past and Present Work

Deshmukh is also thankful to Dr. K.N. Patil and Dr. K.G. Khamare for their encouragement and moral support throughout this work.

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