Adducts of the supraicosahedral stannacarborane 1,6-Me-2-4,1,6-closo-SnC2B10H10; synthetic, structural and computational studies

The 13-vertex stannacarborane 1,6-Me-2-4,1,6-closo-SnC2B10H10 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-Me-2-4-(L-2)-4,1,6-closo-SnC2B10H10(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-Me-2-4,1,6-closo-SnC2B10H10 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 L-2 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.