A close look at short C-CH3•••potassium contacts:: Synthetic and theoretical investigations of [M2CO2(μ3-OtBU)2(μ2-OtBU)4(thf)n] (M = Na, K, Rb, thf = tetrahydrofuran)

Agostic interactions of the type Si-CH3 center dot center dot center dot M+ (M=alkali metal) are frequently mentioned in discussions of solid-state structures of trimethylsilyl compounds and the purpose of this work was to elucidate if they also exist in the related tert-butyl species by using density functional theory. The compounds [M2Co2(mu(3)-OtBU)(2)(mu(2)- OtBu)(4)(thf)(n)] (M=Na, n=2; M=K, n=0; M=Rb, n=1) have been synthesised and their crystal structures determined. Close contacts of methyl groups with K atoms are observed in the solid-state structure of [K2Co2(mu(3)-OtBu)(2)(mu(2)-OtBu)(4)], and calculations of the rotational barrier of a tert-butoxy group about the axis through the C-O bond were performed. It was shown that apparent short C-CH3 center dot center dot center dot K distances are in this case a consequence of the packing in the extended solid-state structure.