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



A series of monodentate, triply bonded Mo2(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 (1H, 13C and 95Mo) NMR studies. The partial alcoholysis of the latter complexes with neopentyl alcohol (neopentOH) led to the heteroleptic alkoxides Mo2(OR)n(Oneopent)6–n (47) {n = 2 [for R = tBu (4), MBE (5), MMP (6)], 4 [for R = Terp (7)]}. This concept was further applied to the synthesis of Mo2(O2DMH)2(OtBu)2 (8) (DMH = 2,5 dimethylhexyl) by starting from the Mo2(OtBu)6 precursor. The 1H NMR spectra for the heteroleptic complexes 48 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. 95Mo NMR studies of the complexes 18 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 48 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 47, 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 α-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) Å. This fact is supported by a lengthening of the Mo–Mo triple bond.