Experimental and theoretical studies of the molecular and crystal structures of trialkoxy- and chlorodialkoxy-stibanes †

Abstract

The molecular structures of triisopropoxystibane, Sb(OiPr)3, and chlorodiisopropoxystibane, SbCl(OiPr)2, were determined in the solid state by single crystal X-ray diffraction. Sb(OiPr)3 forms discrete centrosymmetric dimers in the solid state via Sb⋯O–Sb interactions, leading to pseudo trigonal bipyramidal configurations of the four co-ordinate Sb atoms, while SbCl(OiPr)2 forms chains via Sb⋯O–Sb and Sb⋯Cl–Sb bridges, resulting in five-co-ordinate Sb atoms with pseudo octahedral configurations. Comparison of the solid state structures and the density functional optimized molecular structures of Sb(OMe)3, SbCl(OMe)2 and their dimers revealed a steady increase of the average Sb–O bond lengths with the co-ordination number of Sb, and mutual trans effects of the ligands. Standard enthalpies of dimer formation from density functional calculations are −23.8 and −69.7 kJ mol−1 for [Sb2(μ-OMe)2(OMe)4] and [Sb2Cl2(μ-OMe)2(OMe)2], respectively, and −42.7 kJ mol−1 for [Sb2(μ-Cl)2(OMe)4]. A natural bond orbital analysis reveals that n(O)–σ*(Sb–O) and n(O)–σ*(Sb–Cl) interactions are the main contributions to the inter-monomer bonding in the O-bridged dimers, of Sb(OMe)3 and SbCl(OMe)2, respectively, while n(Cl)–σ*(Sb–O) plays no significant role in the Cl-bridged dimer of SbCl(OMe)2. IR and Raman spectra of Sb(OiPr)3 indicated molecular association in the solid and liquid phase, but dissociation into monomers in non-polar solvents.