Electrochemical and alkali metal reduction studies of silicon,germanium, and tin derivatives of cyclooctatetraene : Properties conferred by cyclooctatetraenide dianions by group IV substituents

The electrochemical behavior of trimethylsilyl-, trimethylgermyl-, trimethylstannyl- and t-butylcyclooctatetraenes has been examined by the techniques of polarography and cyclic voltammetry in THF and hexamethylphosphoramide (vastly preferred) solutions. Chemical reduction by means of potassium in ND₃ has also been studied. Evidence is presented that all four compounds undergo reduction to their respective dianions. However, the energetics of (Me₃)MCOT^? and (Me₃)MCOT^2? formation vary widely, as do the rates and mechanism of dianion disappearance under the conditions of electrochemical measurement. The available kinetic data suggest, for example, that destruction of the t-Bu and trimethylgermyl substituted dianions proceeds via protonation and/or disproportionation pathways. In contrast, the silyl and stannyl examples experience metal-ring carbon cleavage reactions at yet faster rates. Interestingly, the E¹_{$\text{1}\text{2}$} values give evidence of being quite well correlated by σ_p. Furthermore, when the magnitude of the E¹_{$\text{1}\text{2}$}-E²_{$\text{1}\text{2}$} gap are plotted vs the covalent radii of the M atoms, a straight line is obtained for the Si, Ge and Sn examples; C deviates widely. This effect is believed to have its origin in resonance of the M←C_π type involving the d orbitals of M which, of course, is not possible in the carbon case. The significance of these results as well as the other observations made in the course of this work is discussed.