Adducts of the supraicosahedral stannacarborane 1,6-Me₂-4,1,6- closo-SnC₂B₁₀H₁₀; Synthetic, structural and computational studies

The 13-vertex stannacarborane 1,6-Me₂-4,1,6-closo-SnC ₂B₁₀H₁₀ reacts with the Lewis bases 2,2'-bipyridine, 1,10-phenanthroline, 4,4'-dimethybipyridine and 4,4'-diphenylbipyridine, in toluene, to precipitate bright-yellow adducts 1,6-Me₂-4-(L₂)-4,1,6-closo-SnC₂B ₁₀H₁₀ (1-4, respectively) in good yield. Compounds 1-3 have been characterised by NMR spectroscopy and single-crystal X-ray diffraction. Compound 4 is somewhat less stable in solution but has also been studied crystallographically. Adduct formation retains the docosahedral structure of 1,6-Me₂-4,1,6-closo-SnC₂B₁₀H ₁₀ but is accompanied by polyhedral distortion arising from significant increases in Sn-C and Sn-B2 distances. This distortion is described in terms of either a slipping or hinging of the {SnL₂} fragment. In the molecular structures of 1-4 it is evident that the Sn lone pair of electrons is stereochemically active since the L₂ ligand is clearly inclined with respect to the polyhedron. Both the inclination and orientation of L ₂ are rationalised by the results of DFT calculations on 4,1,6-closo-SnC₂B₁₀H₁₂. Calculations on models of the adducts 1, 3 and 4 reproduce the structural distortion that accompanies adduct formation, but appear to underestimate somewhat the strength of Sn-L ₂ bonding. © The Royal Society of Chemistry 2009.