Physicochemical Factors That Influence the Deoxygenation of Oxyanions in Atomically Precise, Oxygen-Deficient Vanadium Oxide Assemblies

Abstract

Here, we report our findings related to the structural and electronic considerations that influence the rate of oxygen-atom transfer (OAT) to oxygen-deficient polyoxovanadate alkoxide (POV-alkoxide) clusters ([V6O6(OC2H5)12] n ; n = 1–, 0, 1+). A comparison of the reaction times required for the reduction of nitrogen-containing oxyanions (NO x –, x = 2, 3) by the POV-ethoxide cluster in its anionic (1-V 6 O 6 1– ; VIIIVIV 5), neutral (4-V 6 O 6 0 ; VIIIVIV 4VV), or cationic (6-V 6 O 6 1+ ; VIIIVIV 3VV 2) charge state reveals that OAT is significantly influenced by three factors: (1) ion-pairing interactions between the POV-alkoxide and the negatively charged oxyanion; (2) oxidation states of remote vanadyl ions in the Lindqvist assembly; (3) the steric bulk surrounding the coordinatively unsaturated VIII ion. This work provides atomic-level insight related to structure–function relationships that govern the rate of OAT at metal oxide surfaces using polyoxometalate clusters as molecular models.

Physicochemical implications of surface alkylation of high-valent, Lindqvist-type polyoxovanadate-alkoxide clusters

“…22 This attribute has translated to their use as molecular models for the surface reactivity of their nanoscale congeners. [23][24][25] With interest in elucidating the consequences of surface ligation, researchers have developed synthetic methods in order to establish organic-inorganic POM hybrid systems. [26][27][28] In a similar fashion to strategies invoked for ligation of metal oxide nanoparticles, an organic functional group is grafted to the surface of a POM through carboxylate, phosphonate, siloxide or alkoxide linkages.…”

O2 Activation with a Sterically Encumbered, Oxygen-Deficient Polyoxovanadate-Alkoxide Cluster

“…Toward this goal, the electronic properties of calixV 6 O 6 –1 were interrogated via infrared (IR) and electronic absorption spectroscopies (Table , Figures and ). These two techniques, taken in tandem, have been demonstrated to report on the degree of reduction of the cluster core. ,,,− IR analysis of calixV 6 O 6 –1 reveals transitions at 1051 and 960 cm –1 , assigned to ν­(O b –Me) and ν­(VO t ), respectively (Table , Figure ). These values are shifted from those reported for V 6 O 6 –1 (ν­(O b –Me) = 1047 cm –1 ; ν­(VO t ) = 951 cm –1 ) and exhibit significant shouldering, likely due to the incorporation of the calix ligand at the surface of the assembly.…”

“…Isolation of this oxygen-deficient vanadium oxide assembly on preparative scales allows for defect formation and subsequent reactivity to be probed separately. Toward this goal, our laboratory has investigated the reactivity of these oxygen-deficient assemblies; we reported activation of nitrite and nitrate by V 6 O 6 –1 , whereby oxygen atom transfer to the surface of the cluster from the oxyanion results in the formation of NO. , We have also described the reactivity of the oxygen-deficient POV-alkoxide with O 2 ; exposure of V 6 O 6 –1 to O 2 afforded quantitative formation of the fully oxygenated parent cluster, V 6 O 7 –1 . In the latter study, we proposed that O 2 activation by V 6 O 6 –1 proceeds via a multicluster substrate activation pathway; two vanadium­(III) centers bind to opposite ends of O 2 , each providing two reducing equivalents for the complete cleavage of the OO bond.…”

“…Toward this goal, our laboratory has investigated the reactivity of these oxygen-deficient assemblies; we reported activation of nitrite and nitrate by V 6 O 6 −1 , whereby oxygen atom transfer to the surface of the cluster from the oxyanion results in the formation of NO. 53,54 We have also described the reactivity of the oxygen-deficient POV-alkoxide with O 2 ; exposure of V 6 O 6 −1 to O 2 afforded quantitative formation of the fully oxygenated parent cluster, V 6 O 7 −1…”

Polyoxometalate‐Single Atom Catalysts (POM‐SACs) in Energy Research and Catalysis

“…[78] More recently, Matson and colleagues have used vanadium oxo-alkoxide clusters of the general formula [V 6 O 7 (OR) 12 ] n± (R = organic group, e.g., CH 3 ) as molecular models to understand the redox-chemistry and surface oxygen removal of these systems (Figure 7). [79][80][81] The group showed that terminal VO bonds in these clusters can be selectively activated by oxygen atom transfer using oxophilic O-acceptors. In consequence, removal of an oxygen atom from a {V V O} species results in formation of a two-electron reduced {V III } surface site, [79] similar to the Mars-van Krevelen process proposed for solid-state vanadium oxides.…”