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

The highly unusual Mo(VI) thioether complexes [MoO2X2(L-L)] (X = Cl or Br; L-L = MeS(CH2)(2)SMe or EtS(CH2)(2)SEt) were obtained by reaction of MoO2X2 with L-L in rigorously anhydrous CH2Cl2 solution. Similar reaction of MoO2Cl2 with the diselenoether MeSe(CH2)(2)SeMe gives the very reactive [MoO2Cl2{MeSe(CH2)(2)SeMe}] as a yellow solid. These compounds are very moisture sensitive and were characterised by IR, diffuse reflectance UV-vis and multinuclear (H-1, C-13{H-1}, Se-77 and Mo-95) NMR spectroscopy. The data are consistent with distorted 6-coordination at Mo(VI) via trans X ligands, mutually cis oxo groups and a chelating dithio- or diseleno-ether ligand. Variable temperature H-1 and C-13{H-1} 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 Mo-95 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 Angstrom. 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.