Comparing {Mo₂O₅} and {V₂O₄} Cores in Aerobic Catalysts for Oxidative Alcohol Dehydrogenation

Multisite molybdenum oxides are key heterogeneous catalysts for oxidative alcohol dehydrogenation (OAD), motivating the development of molecularly defined homogeneous models. Herein, we report a series of dimeric molybdenum(6+) complexes, (A)₂[Mo₂(μ-O)(O)₄(pin^F)₂] (pin^F = perfluoropinacolate), 1, which possess a singly bridged, ″open″ {Mo₂O₅} core and represent, to the best of our knowledge, the first examples of dimeric complexes for aerobic, homogeneous, Mo-catalyzed OAD. Three salts, where A = NMe₄⁺ (1a), {K(18c6)}⁺ (1b), and HNEt₃⁺ (1c), were prepared from MoO₃ and H₂pin^F. Reduction of 1 yields Mo(5+) dimers, (A)₂[Mo₂(μ-O)₂(O)₂(pin^F)₂] (2a, 2b, and 2c), which adopt a ″closed″ {Mo₂O₄} core featuring a Mo-Mo bond. In catalytic OAD, 1a oxidizes activated alcohols whose yield depends on the substrate's C-H BDE. Mechanistic studies using ¹⁹F NMR spectroscopy reveal 2 to be an off-cycle species, inert to oxidation by O₂ but reoxidized by in-situ-generated H₂O₂. The necessity of PCET is confirmed by a +0.9 V anodic shift in the Mo⁶⁺ reduction potential of 1a upon protonation and by the catalyst's inertness toward OAT with PPh₃. SCXRD characterization of 1a, 1b, and 1c and of reduced dimers 2a, 2b, and 2c is reported herein. Comparison with the previously reported vanadium(5+) OAD catalyst [V₂(μ-O)₂(O)₂(pin^F)₂]^2-, {V(5+)}₂, proposes differences in the mechanisms based on the {M_xO_y} cores.