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

Abstract

Diorganotin(IV) complexes of the β-diketonato benzoylacetonato ligand were synthesized and characterized with IR and multinuclear (H, C, Sn) 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)°. Each ligand chelates the metal with different donor abilities [Sn−O bond lengths of 2.423(8) A and 2.135(8) A in one ligand, and 2.107(9) A and 2.357(8) A in the other]; results for the 2nd molecule are similar. The coordination arrangement differs from those of related bis(β-diketonato)diorganotin derivatives containing asymmetric ligands, which are characterized by an approximate Cs symmetry, in that both benzoylacetonato ligands point their methyl (and phenyl) substituents across (anti) the metal atom. Chelate planarity and some phenyl-chelate co-planarity 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 180°, 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)2SnPh2, 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. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)