Hydrogen-atom (H-atom) transfer at the surface of heterogeneous metal oxides has received significant attention owing to its relevance in energy conversion and storage processes. Here, we present the synthesis and characterization of an organofunctionalized polyoxovanadate cluster, (calix)V6O5(OH2)(OMe) 8 (calix = 4- tert -butylcalix[4]arene). Through a series of equilibrium studies, we establish the BDFE(O–H) avg of the aquo ligand as 62.4 ± 0.2 kcal mol −1 , indicating substantial bond weaking of water upon coordination to the cluster surface. Subsequent kinetic isotope effect studies and Eyring analysis indicate the mechanism by which the hydrogenation of organic substrates occurs proceeds through a concerted proton–electron transfer from the aquo ligand. Atomistic resolution of surface reactivity presents a novel route of hydrogenation reactivity from metal oxide surfaces through H-atom transfer from surface-bound water molecules.
This content will become publicly available on May 2, 2024
Accelerated rates of proton coupled electron transfer to oxygen deficient polyoxovanadate–alkoxide clusters
Anionic dopants, such as O-atom vacancies, alter the thermochemical and kinetic parameters of proton coupled electron transfer (PCET) at metal oxide surfaces; understanding their impact(s) is essential for informed material design for efficient energy conversion processes. To circumvent challenges associated with studying extended solids, we employ polyoxovanadate–alkoxide clusters as atomically precise models of reducible metal oxide surfaces. In this work, we examine net hydrogen atom (H-atom) uptake to an oxygen deficient vanadium oxide assembly, [V 6 O 6 (MeCN)(OCH 3 ) 12 ] 0 . Addition of two H-atom equivalents to [V 6 O 6 (MeCN)(OCH 3 ) 12 ] 0 results in formation of [V 6 O 5 (MeCN)(OH 2 )(OCH 3 ) 12 ] 0 . Assessment of the bond dissociation free energy of the O–H bonds of the resultant aquo moiety reveals that the presence of an O-atom defect weakens the O–H bond strength. Despite a decreased thermodynamic driving force for the reduction of [V 6 O 6 (MeCN)(OCH 3 ) 12 ] 0 , kinetic investigations show the rate of H-atom uptake at the cluster surface is ∼100× faster than its oxidized congener, [V 6 O 7 (OCH 3 ) 12 ] 0 . Electron density more »
- Award ID(s):
- 2154727
- Publication Date:
- NSF-PAR ID:
- 10417576
- Journal Name:
- Inorganic Chemistry Frontiers
- Volume:
- 10
- Issue:
- 9
- Page Range or eLocation-ID:
- 2754 to 2765
- ISSN:
- 2052-1553
- Sponsoring Org:
- National Science Foundation
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We report the synthesis and characterisation of a series of siloxide-functionalised polyoxovanadate–alkoxide (POV–alkoxide) clusters, [V 6 O 6 (OSiMe 3 )(OMe) 12 ] n ( n = 1−, 2−), that serve as molecular models for proton and hydrogen-atom uptake in vanadium dioxide, respectively. Installation of a siloxide moiety on the surface of the Lindqvist core was accomplished via addition of trimethylsilyl trifluoromethylsulfonate to the fully-oxygenated cluster [V 6 O 7 (OMe) 12 ] 2− . Characterisation of [V 6 O 6 (OSiMe 3 )(OMe) 12 ] 1− by X-ray photoelectron spectroscopy reveals that the incorporation of the siloxide group does not result in charge separation within the hexavanadate assembly, an observation that contrasts directly with the behavior of clusters bearing substitutional dopants. The reduced assembly, [V 6 O 6 (OSiMe 3 )(OMe) 12 ] 2− , provides an isoelectronic model for H-doped VO 2 , with a vanadium( iii ) ion embedded within the cluster core. Notably, structural analysis of [V 6 O 6 (OSiMe 3 )(OMe) 12 ] 2− reveals bond perturbations at the siloxide-functionalised vanadium centre that resemble those invoked upon H-atom uptake in VO 2 through ab initio calculations. Our results offer atomically precise insight into themore »
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Here, we present the first example of reductive silylation for oxygen defect formation at the surface of a polyoxometalate. Upon addition of 1,4-bis(trimethylsilyl)dihydropyrazine (Pyz(SiMe 3 ) 2 ) to [V 6 O 7 (OMe) 12 ] 1− , quantitative formation of the oxygen-deficient vanadium oxide assembly, [V 6 O 6 (OMe) 12 ] 1− was observed. Substoichiometric reactions of Pyz(SiMe 3 ) 2 with the parent cluster revealed the mechanism of defect formation; addition of 0.5 equiv. of Pyz(SiMe 3 ) 2 to [V 6 O 7 (OMe) 12 ] 1− results in isolation of [V 6 O 6 (OSiMe 3 )(OMe) 12 ] 1− . This reactivity was extended to reduced and oxidized forms of the cluster, [V 6 O 7 (OMe) 12 ] n ( n = 2-, 0), revealing the consequences of modifying the oxidation states of remote transition metal ions on the stability of the siloxide functional group, and thus the extent of reactivity of the cluster surface with Pyz(SiMe 3 ) 2 . The work offers a new understanding of the mechanisms of surface activation of reducible metal oxides via reductive silylation, and reveals new chemical routes for the formation of oxygen atom vacancies inmore »
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