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

Article abstract of DOI:10.1039/c3dt50523e

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.