Si-, Ge-, and Sn-Centered Anions

The most fundamental issues of the structures of heavier group 14 element-centered anionic derivatives R3EM (R = alkyl, aryl, silyl E = Si, Ge, Sn, Pb M = alkali or alkaline earth metals) turned out to be the questions of their aggregation states (monomeric, dimeric, or oligomeric), nature of the E-M bond (covalent or ionic), and configuration of the anionic centers E (tetrahedral, pyramidal, or planar). The most important experimental techniques that are widely used to clarify these questions are NMR spectroscopy and X-ray diffraction analysis. 

Decisive properties of the phosphinomethanides are their stability towards reduction, the separation of the negative charge of the anionic ligand (centered at the formal carbanion) from the electron rich E(II) (E = Si, Ge, Sn) center by the phosphane donor atoms, the thus generated additional electrostatic stabilization and the four-membered chelate ring formation [1]. 

Bochkarev et al. 16 prepared hyperbranched dendrimers via polymerization of tris(pentafluorophenyl)germanium anions resulting in branched macromolecules with Ge-branching centers. Building block propagation was attributed to metal anion, aromatic substitution of a phenyl, para-fluoro-moiety. Thus, treatment of Ge-monomer 8 with excess Ge-anion afforded hyperbranched macromolecules with an internal framework resembling tetrakisGedendrimer 9 (Scheme 8.4). Subsequently, Sn and Si were employed for hyperbranched dendrimer construction. 

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