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.