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

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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 [Mo 2 (μ-O)(O) 4 (pin F ) 2 ] (pin F = perfluoropinacolate), 1, which possess a singly bridged, ″open″ {Mo 2 O 5 } 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 = NMe 4 + ( 1a ), {K(18c6)} + ( 1b ), and HNEt 3 + ( 1c ), were prepared from MoO 3 and H 2 pin F . Reduction of 1 yields Mo(5+) dimers, (A) 2 [Mo 2 (μ-O) 2 (O) 2 (pin F ) 2 ] ( 2a, 2b, and 2c ), which adopt a ″closed″ {Mo 2 O 4 } 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 19 F NMR spectroscopy reveal 2 to be an off-cycle species, inert to oxidation by O 2 but reoxidized by in-situ- generated H 2 O 2 . The necessity of PCET is confirmed by a +0.9 V anodic shift in the Mo 6+ reduction potential of 1a upon protonation and by the catalyst’s inertness toward OAT with PPh 3 . 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 [V 2 (μ-O) 2 (O) 2 (pin F ) 2 ] 2–, {V(5+)} 2, proposes differences in the mechanisms based on the {M x O y } cores.

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