On the structural diversity of [K(18-crown-6)EPh3] complexes (E = C, Si, Ge, Sn, Pb): synthesis, crystal structures and NOESY NMR study

Abstract

A series of homologous potassium triphenylelement complexes [K(18-crown-6)EPh₃] 6a–e of group 14 elements (E = C, Si, Ge, Sn, Pb) was synthesised by alkoxide induced heterolytic cleavage of boron–element compounds. The complexes 6a–e are isolated as storable solids possibly useful as sources of nucleophilic [EPh₃]⁻ moieties. The solid state structures of 6a–e were established by X-ray crystal structure determination. Whilst all structures can be described as polymeric chains consisting of alternating [K(18-crown-6)]⁺ and [EPh₃]⁻ units, the interaction within each chain varies systematically with the coordination properties of E. For Si and Ge, classical E–K coordination along with secondary phenyl–K interactions are characteristic, whilst for Sn and Pb, potassium coordination via the phenyl π-system is observed due to inefficient coordination by the free electron pair localised in an ‘inert’ s-orbital. The carbon derivative is exceptional as the central sp²-hybridised carbon atom gives rise to extensive charge delocalisation and coordination via these partially charged π-systems. A ¹H-¹H NOESY NMR spectroscopic study in THF-d₈ suggests appreciable anion/cation interactions for Si to Pb and hence the presence of contact ion pairs.