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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. 

The selective uptake of lithium ions is of great interest for chemists and engineers because of the numerous uses of this element for energy storage and other applications. However, increasing demand requires improved strategies for the extraction of this element from mixtures containing high concentrations of alkaline impurities. Here, we study solution phase interactions of lithium, sodium, and potassium cations with polyoxovanadate-alkoxide clusters, [V 6 O 7 (OR) 12 ] (R = CH 3 , C 3 H 7 , C 5 H 11 ), using square wave voltammetry and cyclic voltammetry. In all cases, the most reducing event of the cluster shifts anodically as the ionic radius of the cation decreases, indicating increased stability of the reduced cluster and further suggesting that these assemblies might be useful for the selective uptake of Li + . Exploring the consequence of ligand length, we found that the short-chain cluster, [V 6 O 7 (OCH 3 ) 12 ], irreversibly binds Li + in the presence of excess potassium (K + ) and exhibits an electrochemical response in titration experiments similar to that observed upon the addition of Li + to the POV–alkoxide in the presence of non-coordinating tetrabutylammonium ions. However, in the presence of excess sodium (Na + ), the cluster showed only a modest preference for lithium, with exchange between sodium and lithium ions governed by equilibrium. Extending these studies to [V 6 O 7 (OC 5 H 11 ) 12 ], we found that the presence of the pentyl ligands allows the assembly to irreversibly bind Li + in the presence of Na + or K + . The change in mechanism caused by surface functionalization of the clusters increases the differential binding affinity for more compact cations, translating to improved selectivity for Li + uptake in these molecular assemblies. 

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. 

Lindqvist polyoxovanadate‐alkoxide (POV‐alkoxide) clusters are excellent candidates for applications in energy storage and conversion due to their rich electrochemical profiles. One approach to tune the redox properties of these cluster complexes is through substitutional cationic doping within the hexavanadate core. Here, we report the synthesis of a series of tungsten‐substituted POV‐alkoxide clusters with one and two tungsten atoms. Soft landing of mass‐selected ions was used to purify heterometal POV‐alkoxides that cannot be readily separated using conventional approaches. The soft landed POV‐alkoxides are characterized using infrared reflection‐absorption spectroscopy and electrospray ionization mass spectrometry. The redox properties of the isolated ions are examined using an in situ electrochemical cell which enables traditional in vacuo electrochemical measurements inside of an ion soft landing instrument. Although the overall cluster core retains redox activity after tungsten doping, vanadium‐based redox couples (V^V/V^IV) are shifted substantially, indicating a pronounced effect of a heteroatom on the electronic structure of the core.

 

Polyoxovanadate (POV) clusters are an important subclass of polyoxometalates with a broad range of molecular compositions and physicochemical properties. One relatively underdeveloped application of these polynuclear assemblies involves their use as atomically precise, homogenous molecular models for bulk metal oxides. Given the structural and electronic similarities of POVs and extended vanadium oxide materials, as well as the relative ease of modifying the homogenous congeners, investigation of the chemical and physical properties of pristine and modified cluster complexes presents a method toward understanding the influence of structural modifications ( e.g. crystal structure/phase, chemical makeup of surface ligands, elemental dopants) on the properties of extended solids. This review summarises recent advances in the use of POV clusters as atomically precise models for bulk metal oxides, with particular focus on the assembly of vanadium oxide clusters and the consequences of altering the molecular composition of the assembly via organofunctionalization and the incorporation of elemental “dopants”. 

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.