A novel configuration of a benzoylacetonato-diorganotin species is modified by an electron withdrawing substituent on tin. Synthesis, IR and NMR spectroscopy, structure and ab-initio studies

Diorganotin(IV) complexes of the beta-diketonato benzoylacetonato ligand were synthesized and characterized with IR and multinuclear (H, C, Sri) NMR spectroscopy. The X-ray diffraction study of bis(benzoylacetonato)di-tert-butyltin(IV) shows two independent molecules in the crystallographic unit cell. The metal polyhedron is a distorted octahedral (Skewed Trapezoidal Bipyramidal) with the trans angle C-Sn-C of 151.5(5)degrees. Each ligand chelates the metal with different donor abilities [Sn-O bond lengths of 2.423(8) Angstrom and 2.135(8) Angstrom in one ligand, and 2.107(9) Angstrom and 2.357(8) Angstrom in the other]; results for the 2nd molecule are similar. The coordination arrangement differs from those of related bis(beta-diketonato)diorganotin derivatives containing asymmetric ligands, which are characterized by an approximate C-s symmetry, in that both benzoylacetonato ligands point their methyl (and phenyl) substituents across (anti) the metal atom. Chelate planarity and some phenyl-chelate coplanarity was observed. Hartree-Fock (HF) and Density Functional Theory (DFT) calculated structures are in good agreement with those obtained using X ray analysis. DFT and HF calculated structures of bis(benzoylacetonato)diphenyltin(IV), having a similar connectivity as bis(benzoylacetonato)di-tert-butyltin(IV), differ markedly: the phenyls subtend a C-Sn-C bond angle of 180degrees, all Sn-O bond lengths are equal and the tin coordination sphere is a regular octahedron with the ligand bzac being isobidentate. This arrangement is caused by the phenyl electronic withdrawal effect. However, IR data for this compound show several Sn-O bands inconsistent with this configuration and suggest a different connectivity. Additional DFT structural calculations for (bzac)(2)SnPh2, performed on the connectivity resembling the "normal" syn geometry, show a conformational energy slightly lower than that of the centrosymmetric arrangement. Low temperature NMR spectra illustrate the possible rearrangements in solution. (C) Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.