Germanium amidinates

Keywords silicon amidinates, germanium amidinates, hypervalency, low-valency... 

Suitable starting materials for the synthesis of silicon or germanium amidinate complexes are lithium amidinates, which are easily accessible by the reaction of lithium amides with nitriles (A) or by addition of organolithium compounds to carbodiimides (B) (Scheme 1). 

The chemistry of germanium(II) amidinates has been summarized in a review article by Kiihl under the title "N-Heterocyclic germylenes and related... 

A bis(chelate) structure was found for the closely related germylene [MeC(NPr )2]2Ge, which was also made from GeCl2(dioxane) and 2 equivalents of the lithium amidinate (colorless crystals, 81%). The same synthetic approach was used to make bis(amidinato) metal dichlorides of silicon and germanium in high yields (83-95%). Rapid oxidative addition of chalcogen atom sources (styrene sulfide and elemental Se) to the germylene derivatives resulted in a series... 

The monoanionic, bidentate four-electron-donor amidinates are also suitable for stabilizing low oxidation states and/or high coordination numbo-s at germanium centers. The monosubstituted and mononuclear compound 4, as well as compoimd 5, have been isolated and were characterized spectroscopically and structurally. 

This study demonstrates the utility of amidinate ligands for the synthesis of hypervalent silicon and germanium compounds, and also, for the synthesis of low-valent germanium compounds. In addition, surprising results with dianionic amidinate ligands [N-C(R)—NR ] at silicon centers are documented. 

In the case of the monosubstituted amidinate complexes, a crystallographic study of 2c (Fig. I) confirms a monodentate coordination of the amidinate ligand to the silicon center. The C-N bonds are nearly equal. The Si-N bond (1.625(3) A) is remarkably short and indicates some double bond character [6]. The analogous germanium structure (Fig. 2), apart Irom the longer Ge-N bond (1.776(2) A), is quite similar. Though the N-H hydrogen atom has not been located in either case, it seems plausible that an N(2)-H functionality is present. 

Silicon and Germanium Compounds with Amidinate Ligands... 

Summary A series of penta- and hexacoordinate silicon and germanium compounds with amidinate ligands have been synthesized and investigated spectroscopically and structurally. In contrast to silicon, germanium also forms stable compounds with amidinate ligands in the +2 valence state. 

By variation of the substitution pattern of the amidinate ligands and of the silicon or germanium coordination centers, we obtained a number of compounds with a variable degree of Si/Ge-N hypervalent interaction, i.e. compounds with essentially no bones and ones with very strong element-nitrogen bonds ( double bonds ) are formed, providing tetra-, penta- or hexa-coordinated species [4], New or newly structurally characterized examples of silicon amidinate complexes with pentacoordinate silicon are obtained (Eq. 1 Table 1). 

Unsymmetrical amidinate ligands with an N-H functionality have likewise been shown to be suitable ligands for silicon coordination centers [4, 7]. In the case of germanium(ii), a tetrameric compound 4 has been obtained according to Eq. 3 [4]. 

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