Impact of POM's coordination mode and Mo-hybrid constituents on the binding, stability, and catalytic properties of hybrid (pre)catalysts

Abstract

The assembly of Mo^VIO₂²⁺ and methoxy-substituted salicylaldehyde nicotinoyl hydrazone ligands afforded two classes of hybrid polyoxometalates (POMs). In the Class I architectures, [MoO₂(HL¹⁻³)(D)]₂[Mo₆O₁₉]·xCH₃COCH₃ (D = CH₃COCH₃ or H₂O, x = 0 or 2, and L^1–3 = ligands bearing the OMe group at position 3, 4 and 5, respectively), the main driving force for self-assembly is the electrostatic interaction between the components. Class II architectures are composed of a POM anion covalently linked to two Mo-complex units through the terminal O^t or bridging μ₂-O_POM oxygen atoms, as found in Lindqvist-based hybrids [{MoO₂(HL^1–3)}₂Mo₆O₁₉]·xCH₃CN (x = 0 or 2) and the asymmetrical β-octamolybdate-based hybrid [{Mo₂O₄(HL²)(H₂L)}{MoO₂(HL²)}₂Mo₈O₂₆]·CH₃CN·H₂O. Quantum chemical calculations were applied to evaluate the impact of the POM hybrid constituents on the hybrid-type stability, showing that it strongly depends on the ligand substituent position and ancillary ligand nature. Hybrids were tested as catalysts for cyclooctene epoxidation using tert-butyl hydroperoxide (TBHP in water or decane) and with or without the addition of acetonitrile (CH₃CN) as an organic solvent. The catalytic results provided by the use of TBHP in decane are the best ones and classify all the prepared catalysts as very active, with the conversion of cyclooctene >90%, and high selectivity towards epoxide, >80%. We also examined the influence of the ligand structure, POM's hybrid type, and coordination mode on the Mo-hybrid activity and selectivity.