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1. Modelling local structural and electronic consequences of proton and hydrogen-atom uptake in VO 2 with polyoxovanadate clusters

   https://doi.org/10.1039/D1SC02809J  

   Chakraborty, Sourav ; Schreiber, Eric ; Sanchez-Lievanos, Karla R. ; Tariq, Mehrin ; Brennessel, William W. ; Knowles, Kathryn E. ; Matson, Ellen M. ( October 2021 , Chemical Science)

   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 the local structural and electronic consequences of the installation of hydrogen-atom-like dopants in VO 2 , and challenge current perspectives of the operative mechanism of electron–proton co-doping in these materials. 

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2. Site-selective halogenation of mixed-valent vanadium oxide clusters

   https://doi.org/10.1039/D0DT01077D  

   Maiola, Michela L. ; Petel, Brittney E. ; Brennessel, William W. ; Matson, Ellen M. ( January 2020 , Dalton Transactions)

   Here, we expand on the synthesis and characterization of chloride-functionalized polyoxovanadate-alkoxide (POV-alkoxide) clusters, to include the halogenation of mixed-valent vanadium oxide assemblies. These findings build on our previously disclosed results describing the preparation of a mono-anionic chloride-functionalized cluster, [V 6 O 6 Cl(OC 2 H 5 ) 12 ] 1− , by chlorination of [V 6 O 7 (OC 2 H 5 ) 12 ] 2− with AlCl 3 , aimed at understanding the electronic consequences of the introduction of halide-defects in bulk metal oxides ( e.g. VO 2 ). While chlorination of the mixed-valent POV-ethoxide clusters was not possible using AlCl 3 , we have found that the chloride-substituted oxidized derivatives of the Lindqvist vanadium-oxide clusters can be formed using TiCl 3 (thf) 3 with [V 6 O 7 (OC 2 H 5 ) 12 ] n ( n = 1−, 0) or WCl 6 with [V 6 O 7 (OC 2 H 5 ) 12 ] 0 . Characterization of the chloride-containing products, [V 6 O 6 Cl(OC 2 H 5 ) 12 ] n ( n = 0, 1+), was accomplished via 1 H NMR spectroscopy, X-ray crystallography, and elemental analysis. Electronic analysis of the redox series of Cl-doped POV-alkoxide clusters via infrared and electronic absorption spectroscopies revealed all redox events are localized to the vanadyl portion of the cluster, with the site differentiated V III –Cl moiety retaining its reduced oxidation state across a 1.9 V window. These results present new synthetic routes for accessing chloride-doped POV-alkoxide clusters from mixed-valent vanadium oxide precursors. 

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3. Oxygen-atom vacancy formation and reactivity in polyoxovanadate clusters

   https://doi.org/10.1039/D0CC05920J  

   Petel, Brittney E. ; Matson, Ellen M. ( November 2020 , Chemical Communications)

   null (Ed.)

   Reducible metal oxides (RMOs) are widely used materials in heterogeneous catalysis due to their ability to facilitate the conversion of energy-poor substrates to energy-rich chemical fuels and feedstocks. Theoretical investigations have modeled the role of RMOs in catalysis and found they traditionally follow a mechanism in which the generation of oxygen-atom vacancies is crucial for the high activity of these solid supports. However, limited spectroscopic techniques for in situ analysis renders the identification of the reactivity of individual oxygen-atom vacancies on RMOs challenging. These obstacles can be circumvented through the use of homogeneous complexes as molecular models for metal oxides, such as polyoxometalates. Summarized herein, a sub-class of polyoxometalates, polyoxovanadate–alkoxide clusters, ([V 6 O 7 (OR) 12 ] n ; R = CH 3 , C 2 H 5 ; n = 2−, 1−, 0), are explored as homogeneous molecular models for bulk vanadium oxide. A series of synthetic strategies have been employed to access oxygen-deficient vanadium oxide assemblies, including addition of V(Mes) 3 (thf), tertiary phosphanes, and organic acids to plenary Lindqvist motifs. We further detail investigations surrounding the ability of these oxygen-deficient sites to mediate reductive transformations such as O 2 and NO x 1− ( x = 2, 3) activation. 

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4. Accelerated rates of proton coupled electron transfer to oxygen deficient polyoxovanadate–alkoxide clusters

   https://doi.org/10.1039/D3QI00129F  

   Cooney, Shannon E. ; Schreiber, Eric ; Brennessel, William W. ; Matson, Ellen M. ( May 2023 , Inorganic Chemistry Frontiers)

   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 derived from the O-atom vacancy is shown to play an important role in influencing H-atom uptake at the cluster surface, lowering activation barriers for H-atom transfer. 

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5. Acid-Induced, Oxygen-Atom Defect Formation in Reduced Polyoxovanadate-Alkoxide Clusters

   https://doi.org/10.1021/jacs.0c03864  

   Schreiber, Eric ; Petel, Brittney E. ; Matson, Ellen M. ( May 2020 , Journal of the American Chemical Society)

   Here, we present the first example of acid-induced, oxygen-atom abstraction from the surface of a polyoxometalate cluster. Generation of the oxygen-deficient vanadium oxide, [V6O6(OC2H5)12]1−, was confirmed via independent synthesis. Spectroscopic analysis using infrared and electronic absorption spectroscopies affords resolution of the electronic structure of the oxygen-deficient cluster (oxidation state distribution = [VIIIVIV 5]). This work has direct implications toward the elucidation of possible mechanisms of acid-assisted vacancy formation in bulk transition metal oxides, in particular electron−proton codoping that has recently been described for vanadium oxide (VO2). Ultimately, these molecular models deepen our understanding of protondependent redox chemistry of transition metal oxide surfaces. 

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