Charge-State Dependence of Proton Uptake in Polyoxovanadate-alkoxide Clusters

Abstract

Here, we present an investigation of the thermochemistry of proton uptake in acetonitrile across three charge states of a polyoxovanadate-alkoxide (POV-alkoxide) cluster, [V 6 O 7 (OMe) 12 ] n ( n = 2–, 1–, and 0). The vanadium oxide assembly studied features bridging sites saturated by methoxide ligands, isolating protonation to terminal vanadyl moieties. Exposure of [V 6 O 7 (OMe) 12 ] n to organic acids of appropriate strength results in the protonation of a terminal V=O bond, generating the transient hydroxide-substituted POV-alkoxide cluster [V 6 O 6 (OH)(OMe) 12 ] n +1 . Evidence for this intermediate proved elusive in our initial report, but here we present the isolation of a divalent anionic cluster that features hydrogen bonding to dimethylammonium at the terminal oxo site. Degradation of the protonated species results in the formation of equimolar quantities of one-electron-oxidized and oxygen-atom-efficient complexes, [V 6 O 7 (OMe) 12 ] n +1 and [V 6 O 6 (OMe) 12 ] n +1 . While analogous reactivity was observed across the three charge states of the cluster, a dependence on the acid strength was observed, suggesting that the oxidation state of the vanadium oxide assembly influences the basicity of the cluster surface. Spectroscopic investigations reveal sigmoidal relationships between the acid strength and cluster conversion across the redox series, allowing for determination of the proton affinity of the surface of the cluster in all three charge states. The fully reduced cluster is found to be the most basic, with higher oxidation states of the assembly possessing substantially reduced proton affinities (∼7 p K a units per electron). These results further our understanding of the site-specific reactivity of terminal M=O bonds with protons in an organic solvent, revealing design criteria for engineering functional surfaces of metal oxide materials of relevance to energy storage and conversion.

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

“…These signals are consistent with those reported for the acetonitrile-bound, oxygen-deficient POV-alkoxide cluster, [V 6 O 6 (OCH 3 ) 12 (MeCN)](OTf) ( V 6 O 6 1+ ). 24 Similar results were observed with Me 2 HQ. We note improved product conversion at early time points using t Bu 2 HQ as the H-atom source; however, in all reactions, the major product observed is complex V 6 O 7 0 ( vide infra ).…”

“…Previous work has reported that the surface basicity of POV-alkoxides is dependent on the oxidation state of the assembly, two factors that influence the BDFE­(E–H), as outlined by the Bordwell equation . This observation led us to hypothesize that terminal oxido moieties in vanadium oxide clusters might also be reactive with H-atoms.…”

“…Previous work has reported that the surface basicity of POV-alkoxides is dependent on the oxidation state of the assembly, two factors that influence the BDFE(E–H), as outlined by the Bordwell equation. 24 This observation led us to hypothesize that terminal oxido moieties in vanadium oxide clusters might also be reactive with H-atoms. To directly probe the reactivity of POV-alkoxides with H-atom donors, we first investigated the reactivity of [V 6 O 7 (OCH 3 ) 12 ] 1– ( V 6 O 7 1– ) with 5,10-dihydrophenazine (H 2 Phen; BDFE(N–H) avg.…”

“…To work around this constraint, we turn to previous work from our group that has shown the direct relationship between the oxidation state of the cluster and its affinity for protons in acetonitrile. 24 In this work, the reactivity of the fully oxygenated cluster, [V 6 O 7 (OCH 3 ) 12 ] 2– , with organic acids of varying strengths (p K a = 22.6–12.5) was evaluated; the addition of acids of p K a values higher than 18 revealed incomplete conversion of [V 6 O 7 (OCH 3 ) 12 ] 2– to the disproportionation products, V 6 O 7 1– and V 6 O 6 1– . Under the assumption that the conversion of half of [V 6 O 7 (OCH 3 ) 12 ] 2– corresponds to a “half-way” point for cluster acidification, we can approximate the p K a of the hydroxide-substituted species, [V 6 O 6 (OH)(OCH 3 ) 12 ] 1– .…”

“…Upon introduction of protons to the dianionic assembly, [V 6 IV O 7 (OR) 12 ] 2– , the formation of an O-atom vacancy at the surface of the polyoxovanadate is observed. 23 , 24 These studies provide a distinct perspective on mechanisms of acid-induced surface activation of nanoscopic metal oxide materials, as these clusters lack the nucleophilic surface bridging oxido moieties commonly invoked in charge compensation following the reduction of POMs (i.e., protonation). This attribute of POV-alkoxides allows for control over site-specificity in PCET by directing reactivity to terminal oxido moieties.…”