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(pinF)2] (pinF = 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 H2pinF. Reduction of 1 yields Mo(5+) dimers, (A)2[Mo2(μ-O)2(O)2(pinF)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(pinF)2]2–, {V(5+)}2, proposes differences in the mechanisms based on the {MxOy} cores.