Dioxomolybdenum(VI) Complexes Supported by Fluxional Bis(amide)‐Functionalized Aminophenolate Ligands

Benchtop-stable dioxomolybdenum(vi) complexes supported by the new bis(amide) functionalized aminophenolate proligands H3La and H3Lb are reported. Using [MoO2(acac)2], H3La forms the mononuclear complex Mo1a in MeOH and the dinuclear, μ-oxo–bridged complex Mo2a in aprotic solvents. In contrast, the more sterically hindered ligand H3Lb produces the dinuclear complex Mo2b regardless of the solvent used. Heavy-oxygen labelling of Mo2a with H218O confirms that water promotes oxo ligand exchange and μ-oxo bridge formation, as evidenced by IR shifts of approximately 50 cm–1 to a lower wavenumber for the ν(Mo18O) and ν(Mo18OMo) stretching vibrations. Mo1a has a distorted octahedral Mo centre with an O3N donor set from H2La−, two oxides and a methanolate. One of the amide arms remains non-coordinating. Mo2a and Mo2b have similar coordination spheres, with methanolate replaced by a μ-oxo bridge. Mo1a shows hydrolytic instability and poor solubility. Contrarily, the more stable Mo2a is fluxional in solution at room temperature (RT) and decomposes above 65 °C. However, low-temperature (–45 °C to –85 °C) 1H NMR reveals distinct signals for the coordinated and non-coordinating amide arms. Tungsten analogues W1a (mononuclear) and W2b (dinuclear) are isostructural with their molybdenum counterparts. In all complexes, the ligand platform exhibits hemilabile behaviour and solvent-dependent nuclearity.