A close look at short C-CH₃⋯potassium contacts: Synthetic and theoretical investigations of [M₂Co₂(μ ₃-OtBu)₂(μ₂-OtBu)₄(thf) _n] (M = Na, K, Rb, thf=tetrahydrofuran)

Agostic interactions of the type Si-CH₃⋯M⁺ (M=alkali metal) arc 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(μ₃- OtBu)₂(μ₂-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(μ₃- OtBu)₂(μ₂OtBu)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⋯K distances are in this case a consequence of the packing in the extended solid-state structure.