Oxygen activation and catalytic aerobic oxidation by Mo(iv)/(vi) complexes with functionalized iminophenolate ligands

Abstract

Synthesis of molybdenum(VI) dioxido complexes 1–3, coordinated by one or two functionalized iminophenolate ligands HL1 or HL2, bearing a donor atom side chain or a phenyl substituent, respectively, allowed for systematic investigation of the oxygen atom transfer (OAT) reactivity of such complexes towards phosphanes. Depending on stoichiometry and employed phosphane (PMe₃ or PPh₃), different molybdenum(IV) and molybdenum(V) complexes 4–7 were obtained. Whereas molybdenum(IV) complexes 4 and 5, bearing a terminal PMe₃ ligand, readily reacted with molecular O₂ to form oxido peroxido complexes 8 and 9, phosphane free μ-oxido bridged dinuclear molybdenum(V) complexes 6 and 7 proved to be stable towards oxidation with molecular O₂ under ambient conditions. Single-crystal X-ray diffraction analyses revealed different isomeric structures in the solid state for dioxido complexes 1 and 2 in comparison with oxido phosphane complex 5, dinuclear oxido μ-oxido complex 6 and oxido peroxido complexes 8 and 9, pointing towards an isomeric rearrangement during OAT. Compounds 1 and 2 were furthermore tested for their ability to catalyze the aerobic oxidation of PMe₃ and PPh₃. A significant difference in catalytic activity has been observed in the oxidation of PMe₃, where complex 1 bearing donor atom functionalized ligands led to higher conversion and selectivity than complex 2 coordinated by phenyl iminophenolate ligands. In the oxidation of PPh₃, complex 2 leads to higher conversion compared to 1. In a control experiment, phenyl-based dinuclear μ-oxido complex 7, derived from complex 2, was found to be catalytically active, which suggests a lower energy barrier for disproportionation into [MoO(L)₂] and [MoO₂(L)₂] in comparison with methoxypropylene based compound 6, a prerequisite for subsequent reactivity toward molecular O₂.