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 H ₃ L _a and H ₃ L _b are reported. Using [MoO ₂ (acac) ₂ ], H ₃ L _a 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 H ₃ L _b produces the dinuclear complex Mo2b regardless of the solvent used. Heavy‐oxygen labelling of Mo2a with H ₂ ¹⁸ O 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 ¹⁸ O) and ν(Mo ¹⁸ OMo) stretching vibrations. Mo1a has a distorted octahedral Mo centre with an O ₃ N donor set from H ₂ L _a ⁻ , 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) ¹ H 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.