Surface-Inspired Molecular Vanadium Oxide Catalysts for the Oxidative Dehydrogenation of Alcohols: Evidence for Metal Cooperation and Peroxide Intermediates

Reversible PCET and Ambient Catalytic Oxidative Alcohol Dehydrogenation by {V=O} Perfluoropinacolate Complexes

A new air-stable catalyst for the oxidative dehydrogenation of benzylic alcohols under ambient conditions has been developed. The synthesis and characterization of this compound and the related monomeric and dimeric V(IV)- and V(V)-pin^F (pin^F = perfluoropinacolate) complexes are reported herein. Monomeric V(IV) complex (Me₄N)₂[V(O)(pin^F)₂] (1) and dimeric (μ-O)₂-bridged V(V) complex (Me₄N)₂[V₂(O)₂(μ-O)₂(pin^F)₂] (3a) are prepared in water under ambient conditions. Monomeric V(V) complex (Me₄N)[V(O)(pin^F)₂] (2) may be generated via chemical oxidation of 1 under an inert atmosphere, but dimerizes to 3a upon exposure to air. Complexes 1 and 2 display a perfectly reversible V^IV/V couple at 20 mV (vs Ag/AgNO₃), whereas a quasi-reversible V^IV/V couple at -865 mV is found for 3a. Stoichiometric reactions of 3a with both fluorenol and TEMPOH result in the formation of (Me₄N)₂[V₂(O)₂(μ-OH)₂(pin^F)₂] (4a), which contains two V(IV) centers that display antiferromagnetic coupling. In order to structurally characterize the dinuclear anion of 4a, {K(18C6)}⁺ countercations were employed, which formed stabilizing K···O interactions between the counterion and each terminal oxo moiety and H-bonding between the oxygen atoms of the crown ether and μ-OH bridges of the dimer, resulting in {K(18C6)}₂[V₂(O)₂(μ-OH)₂(pin^F)₂] (4b). The formal storage of H₂ in 4a is reversible and proton-coupled electron transfer (PCET) from crystals of 4a regenerates 3a upon exposure to air over the course of several days. Furthermore, the reaction of 3a (2%) under ambient conditions with excess fluorenol, cinnamyl alcohol, or benzyl alcohol resulted in the selective formation of fluorenone (82% conversion), cinnamaldehyde (40%), or benzaldehyde (7%), respectively, reproducing oxidative alcohol dehydrogenation (OAD) chemistry known for VO_x surfaces and demonstrating, in air, the thermodynamically challenging selective oxidation of alcohols to aldehydes/ketones.

Catalytic Applications of Vanadium: A Mechanistic Perspective

The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C-C and C-O bond cleavage, carbon-carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.

A study of oxidizing centers in carbon nanotubes by solid-state NMR

The adsorbed trimethylphosphine is oxidized to trimethylphosphine oxide by the oxygen-containing groups on the surface of functionalized CNTs. The concentration of oxidizing center is selectively measured.