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

Abstract

The concerted transfer of protons and electrons enables the activation of small-molecule substrates by bypassing energetically costly intermediates. Here, we present the synthesis and characterization of several hydrogenated forms of an organofunctionalized vanadium oxide assembly, [V 6 O 13 (TRIOL NO 2 ) 2 ] 2− , and their ability to facilitate the concerted transfer of protons and electrons to O 2 . Electrochemical analysis reveals that the fully reduced cluster is capable of mediating 2e − /2H + transfer reactions from surface hydroxide ligands, with an average bond dissociation free energy (BDFE) of 61.6 kcal/mol. Complementary stoichiometric experiments with hydrogen-atom-accepting reagents of established bond strengths confirm that the electrochemically established BDFE predicts the 2H + /2e − transfer reactivity of the assembly. Finally, the reactivity of the reduced polyoxovanadate toward O 2 reduction is summarized; our results indicate a stepwise reduction of the substrate, proceeding through H 2 O 2 en route to the formation of H 2 O. Kinetic isotope effect experiments confirm the participation of hydrogen transfer in the rate-determining step of both the reduction of O 2 and H 2 O 2 . This work constitutes the first example of hydrogen atom transfer for small-molecule activation with reduced polyoxometalates, where both electron and proton originate from the cluster.

Molecular Capacitors: Accessible 6- and 8-electron Redox Chemistry from Dimeric “Ti(I)” and “Ti(0)” Synthons Support-ed by Imidazolin-2-Iminato Ligands.

“…For instance, while side-on N2 coordinated titanocene complexes [(η 5 -C5H2-1,2,4,-Me3)2Ti]2(µ,η 2 ,η 2 -N2) (N-N = 1.216(4) Å), 74 and [(η 5 -C5H3-1-i Pr-3-R)2Ti]2(µ,η 2 ,η 2 -N2) (R = Me, N-N = 1.217(6) Å; R = i Pr, N-N = 1.226(6) Å) 75 were found to be paramagnetic at room temperature with moderately activated (N2) 2-bridging units; the titanium dimer 76 which was suggested to be neutral despite this complex exhibiting a low effective magnetic moment of 1.37 μB and formally having 5 metal-based electrons available for donation to the N2. This is significantly differentiated from the dimeric zirconium and hafnium dinitrogen species {(η 5 78 and {[η 5 79 all of which are described as possessing (N2) 2moieties.…”

“…Discrete molecular compounds capable of achieving multielectron transfers, outside of molecular electrochemical catalysts, are important for modeling the electron transfer chemistry of enzymatic systems, [1][2] enabling new charge transport layers in solar devices 3 and anodic materials for batteries, 4 while also effecting such biologically relevant transformations as O2 and CO2 reduction. [5][6][7] Extended aromatic π-systems such as fullerenes are competent charge carriers C60 n-(n = 0 -6) [8][9] but are limited in their ability to accomplish the chemical transformation of small molecules. With regards to this latter aspect, multi-metallic systems have demonstrated most promise.…”

“…12 With respect to the earlier 3d metals, Matson et al have demonstrated the successful implementation of a vanadium polyoxometalate cluster for the reduction of O2 to H2O. 5 Reduction of [V6O13(TRIOL NO2 )2] 2-(TRIOL NO2 = [(OCH2)3C(NO2)] 3-) by 6electron and proton equivalents yields [V6O7(OH)6(TRIOL NO2 )2] 2-, where the latter product reacts with excess O2 to produce 3 equiv. of H2O.…”

“…Highly reduced multi-metallic assemblies containing earlymetals that can promote redox transformations involving 4 or more electrons are few in number. Cloke et al have shown that the Ti=Ti pentalene sandwich complex (μ-η 5 :η 5 -Pn)2Ti2 (Pn = [1,4-( i Pr3Si)2C8H4] 2-) reacts with a host of small molecules to perform 2 and 4-electron reductions. 19,[23][24][25][26][27] As an example, (μ-η 5 :η 5 -Pn)2Ti2 reacts with 1 equiv.…”

“…Cloke et al have shown that the Ti=Ti pentalene sandwich complex (μ-η 5 :η 5 -Pn)2Ti2 (Pn = [1,4-( i Pr3Si)2C8H4] 2-) reacts with a host of small molecules to perform 2 and 4-electron reductions. 19,[23][24][25][26][27] As an example, (μ-η 5 :η 5 -Pn)2Ti2 reacts with 1 equiv. of CO2 to produce [(η 5 :η 5 -Pn)Ti]2(μ-O), 24 releasing an equiv.…”