Hypercoordinated organogermanium compounds

Hypercoordination of silicon in organosihcon compounds has often been associated with a considerable low frequency shift of Si signals. It is expected that the same effect will be observed for the Ge NMR spectra of hypercoordinated organogermanium compounds. The Ge NMR spectra of tris(o-substituted phenyl)germanes were examined and it was found that the Ge chemical shifts (summarized in Table 11) are not necessarily correlated with hypercoordination. 

R 661 Y. Takeuchi, M. Nishikawa and Y. Sugiyama, Synthesis and Structure of Some Hypercoordinated Organogermanium Compounds , Yuki Gosei Kagaku Kyokaishi, 2004,62,727 R 662 T. Tanaka, Fundamental of Density Functional Theory and Its Applications to Molecular Spectroscopy , Bunseki, 2004,12,717... 

The summary of all these characteristic points to a pentacoordinated organogermanium compound where no steric enforcement is involved to enhance the hypercoordination. The rotation about the S—CH2 bond, which should be feasible, would move the CO2H group far apart from germanium and should be noted. This in turn appears to suggest that the Ge—O interaction is strong enough to lead to hypercoordination. 

Recent advances in Ge NMR spectroscopy are reviewed. Following the introduction (Section 1), Section 2 describes recently determined chemical shifts and linewidths for a variety of organogermanium compounds which range from normal (tetravalent) to hypercoordinated (up to heptavalent) species. The role of linewidth as evidence for hypercoordination is discussed. In Section 3, coupling constants, relaxation times and relaxation mechanisms are discussed in detail, showing that in some cases a mechanism other than quadrupolar is involved. In the last section, high-resolution solid-state Ge NMR spectroscopy is covered which has not been treated in previous reviews. 

In the course of their investigations on organogermanium compounds with anion transport capability, these authors noticed that aryl substituent(s) on germanium are effective in enhancing its Lewis acidity. Since the observed anion transport capability is most likely due to the hypercoordination in which the germanium atom can be involved with anions, chloride ion in particular, a suitable method of evaluation of germanium hypercoordination is needed. 

Although compounds 89-93 exhibit a substantial low frequency shift as compared with 66, compound 95 shows much the same low frequency shift. Hence, this shift should be due to a -7-011601 rather than to hypercoordination. It is interesting to note, however, that there is a rough linear relation l oe = 0.025 5si — 0.049 R = 0.972)) between the Ge chemical shifts of 89, 92, 93, 66 and Si chemical shifts of related silanes. Such linear relations have long been known to exist between related organogermanium and organosiUcon compounds... 

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See also in sourсe #XX -- [ ]

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