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

Abstract

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)-pinF (pinF = perfluoropinacolate) complexes are reported herein. Monomeric V­(IV) complex (Me4N)2[V­(O)­(pinF)2] (1) and dimeric (μ-O)2-bridged V­(V) complex (Me4N)2[V2(O)2(μ-O)2(pinF)2] (3a) are prepared in water under ambient conditions. Monomeric V­(V) complex (Me4N)­[V­(O)­(pinF)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 VIV/V couple at 20 mV (vs Ag/AgNO3), whereas a quasi-reversible VIV/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(pinF)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(pinF)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.

Concerted Multiproton–Multielectron Transfer for the Reduction of O2 to H2O with a Polyoxovanadate Cluster

“…16,52 By comparison, examples of multielectron/multiproton transfer using molecular MO x assemblies are rare. 53 From Figure 3A, an OCP of −0.9893 V vs Fc +/0 at a 1:1 ratio of V 6 O 7 (OH) 6 to V 6 O 9 (OH) 4 is found. We can convert this potential to be referenced against H + /H 2 using eq 3: 47 °= °+…”

“…The ability to transfer multiple H-atom equivalents through a concerted proton electron transfer (CPET) mechanism is beneficial to small-molecule activation reactions in which PCET is operative, due to the fact that many energy-poor substrates require the addition of multiple electrons and protons to produce their energy-rich, reduced forms. , In principle, the use of complexes capable of storing multiple e – /H + pairs allows for an increase in the efficiency of small-molecule activation by avoiding high energy intermediate states involving the transfer of a single hydrogen atom, as well as improved selectivity in product formation. , Recent work has established the efficacy of reduced MO x surfaces for the transfer of multiple equivalents of H-atom equivalents in PCET reactions. , By comparison, examples of multielectron/multiproton transfer using molecular MO x assemblies are rare …”

“…While no other examples have examined the bond strength of surface O−H bonds in POMs, the thermochemistry of other classes of "MO x " clusters have been reported. For example, bis(μ-oxo)dicopper(III) 56 and bis(μ-oxo)divanadium(V) 53 complexes have been reported to abstract hydrogen atoms from weak C−H bonds. In both examples, the BDFE(O−H) of the bridging hydroxide ligands of the resultant 2e − /2H + reduced assembly are greater than 71 kcal/mol, on the basis of the reactivity of these complexes with sacrificial hydrogen atom 57 In all cases, the experimentally established BDFE(O−H) values of the polynuclear metal oxide clusters are significantly larger (>10 kcal/mol) than that of complex V 6 O 7 (OH) 6 .…”

Oxygen-Atom Defect Formation in Polyoxovanadate Clusters via Proton-Coupled Electron Transfer

“…16 Additionally, iron oxide dimers have been shown to direct H-atom reactivity toward the oxido moiety spanning the two metal centers. 17 More recently, reports from our group 18 (Figure 1) and others 19 describe the ability of multimetallic vanadium oxide complexes to facilitate the activation of E−H (E = O, N, C) bonds via PCET. Notably, all of these examples have focused on PCET to bridging oxide moieties, resulting in the formation of surface μ 2 -OH ligands.…”

HAA by the first {Mn(iii)OH} complex with all O-donor ligands

“…Ligand uorination also increases water tolerance via the lower alcohol pK a values of up to 14 orders of magnitude. 32 In some cases, transition metal complexes with uorinated ligands have been able to oxidize C-H 31,33 and O-H bonds. 31 In this work we present the synthesis and characterization of several Mn complexes supported by the peruoropinacolate (pin F ) ligand.…”