Synthesis, spectroscopic and structural properties of hexavalent molybdenum complexes with thio- and seleno-ether ligands

Abstract

The highly unusual Mo(VI) thioether complexes [MoO₂X₂(L–L)] (X = Cl or Br; L–L = MeS(CH₂)₂SMe or EtS(CH₂)₂SEt) were obtained by reaction of MoO₂X₂ with L–L in rigorously anhydrous CH₂Cl₂ solution. Similar reaction of MoO₂Cl₂ with the diselenoether MeSe(CH₂)₂SeMe gives the very reactive [MoO₂Cl₂{MeSe(CH₂)₂SeMe}] as a yellow solid. These compounds are very moisture sensitive and were characterised by IR, diffuse reflectance UV-vis and multinuclear (¹H, ¹³C{¹H}, ⁷⁷Se and ⁹⁵Mo) NMR spectroscopy. The data are consistent with distorted 6-coordination at Mo(VI) viatrans X ligands, mutually cis oxo groups and a chelating dithio- or diseleno-ether ligand. Variable temperature ¹H and ¹³C{¹H} NMR data indicate fast pyramidal inversion at the coordinated chalcogen atoms occurs at room temperature, but cooling slows this process to reveal resonances consistent with the meso and DL forms. The ⁹⁵Mo NMR spectra are single resonances in the region 200–300 ppm, as expected for Mo(VI) complexes, and show inverse dependence of the chemical shifts upon both halide and chalcogen type. Crystal structures of three of the dithioether complexes are described and provide unequivocal evidence for Mo(VI) thioether coordination, confirming chelation of the dithioether through long Mo–S interactions of ca. 2.7 Å. Attempts to extend the range of compounds by using other chalcogenoether ligands failed, indicating that to obtain complexes involving these extremely mis-matched metal ligand combinations requires both the favourable 5-membered chelate ring and small terminal alkyl substituents on the chalcogen.