Facile Access to Homo- and Heteroleptic, Triply Bonded Dimolybdenum Hexaalkoxides with Unsaturated Alkoxide Ligands

A series of monodentate, triply bonded Mo-2(OR)(6) complexes [R = MBE (1) (MBE = 2-methylbut-3-ene-2-yl), MMP (2) (MMP = 1-methoxy-2-methylpropane-2-yl), Terp (3) [Terp = 2-(4-methylcyclohex-3-enyl) propane-2-yl], which exhibit C-C double bonds or an ether function in the ligand sphere, were synthesized and characterized by multinuclear (H-1, C-13 and Mo-95) NMR studies. The partial alcoholysis of the latter complexes with neopentyl alcohol (neopentOH) led to the heteroleptic alkoxides Mo-2(OR)(n)(Oneopent)(6-n) (4-7) {n = 2 [for R = tBu (4), MBE (5), MMP (6)], 4 [for R = Terp (7)]}. This concept was further applied to the synthesis ofMo(2)(O2DMH)(2)-(OtBu)(2) (8) (DMH = 2,5 dimethylhexyl) by starting from the Mo-2(OtBu)(6) precursor. The H-1 NMR spectra for the heteroleptic complexes 4-8 show signals that are significantly shifted to a higher field for the RO ligand protons compared to those of their homoleptic analogues. This is the result of a change in the spatial position of the alkoxide ligands (RO) in the homoleptic compared to the heteroleptic complexes that leads to a different magnetic environment for the alkoxide ligands due to the magnetic anisotropy of the Mo-Mo triple bond. Mo-95 NMR studies of the complexes 1-8 show that the resonance strongly depends on the substitution pattern of the alkoxide and that a shift to higher field is observed when going from the tertiary to the primary alkoxides. The molecular structures for 4-8 were determined by single-crystal X-ray diffraction, and all of the complexes show a staggered conformation as well as an asymmetric ligand distribution, which results in unequal Mo-O bond lengths. For the heteroleptic complexes 4-7, the RO ligands (R = tBu, MBE, MMP and Terp, respectively) exhibit the longest bond lengths, which suggests that the position of the ligand strongly depends on the steric congestion of the a-carbon atom of the alkoxide ligand. In 6, the methoxy function enables an intramolecular O -> Mo coordination as is indicated by a Mo-O distance of 2.2464(7) angstrom. This fact is supported by a lengthening of the Mo-Mo triple bond.