Coordination Compounds of Germanium(IV) Formed with Soft and Hard Donor Atoms : a Look Into the Past and Present Work
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Coordination Compounds of Germanium(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, germanium dichloride 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). Germanium tetrachloride and germanium tetrabromide react with 2,4-pentanedione with the evolution of hydrogen chloride/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 chlorine 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 chloroform 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 germanium tetrafluoride 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 oxygen-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 germanium dioxide 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