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

Here, we present an investigation of the thermochemistry of proton uptake in acetonitrile across three charge states of a polyoxovanadate-alkoxide (POV-alkoxide) cluster, [V6O7(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 [V6O7(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 [V6O6(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, [V6O7(OMe)12]n+1 and [V6O6(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 pKa 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.


■ INTRODUCTION
The interactions of cations with the surface of reducible metal oxides (RMOs) is an important area of research, with implications in the advancement of energy storage technologies 1,2 and the development of catalysts. 3,4 In reports on cation uptake into bulk and nanocrystalline materials, analytical techniques such as X-ray diffraction, electrochemistry, and a host of spectroscopies have been used to describe how the bulk structure of a metal oxide is altered upon uptake of cationic species, as well as ascertain the mechanism of cation insertion. 5−9 Although current analytical techniques provide critical information on global material properties, they do not provide atomically precise detail, for which researchers turn to computational modeling. 5,7,9−11 To empirically obtain atomiclevel insight into these types of interactions, alternatively researchers can turn to the study of molecular model complexes.
Polyoxometalates (POMs) are molecular assemblies composed of multiple redox-active transition-metal oxyanions linked together by bridging oxide units to form threedimensional structures. The peripheral morphology of POMs resembles the surface of RMOs, with alternating terminal and bridging oxygen atoms composing the surface of the assembly. 12−16 However, unlike bulk solids, POMs exhibit solubility in organic and aqueous solution, rendering these clusters amenable to analysis using analytical techniques reserved for homogeneous systems. Given the relevance of POMs as homogeneous models for bulk systems, there has been great interest in understanding the role of cations in mediating the electrochemical properties of these assemblies. 17−25 Charge-compensating interactions between the surface of POMs and protons have been reported to result in the coalescence of individual one-electron-oxidized (1e − ) redox events into multielectron−multiproton processes located at potentials anodically shifted from those reported for the parent complex. [18][19][20]22,24,25 Of note, the affinity of the POM surface for protons has been described as dependent on the degree of reduction of the cluster, with more reduced variants of the metal oxide assembly possessing acid association equilibrium constants with an order of magnitude higher than that of their oxidized counterparts. [18][19][20][21][22]25 Theoretical and experimental studies have demonstrated that the interactions of protons with the surface of the POM occur at nucleophilic, bridging oxido (μ 2 -O 2− ) sites following reduction of the parent ion. 24,26−28 The reactivity of terminal oxo moieties in POMs toward protons, on the other hand, has not been as prominently observed. Besides our recent report on an ethoxide-bridged polyoxovanadate cluster (vide infra), only one example of the formal protonation of terminal oxido groups of a POM has been reported to date. 29−32 In a series of papers, the sixelectron reduction of a Keggin-type polyoxotungstate, [XW 12 O 60 ] y− (X = Si, B, H 2 ), in an acidic medium was shown to produce a charge-separated complex featuring three "W IV O 5 (OH 2 )" sites on a single cluster face. The surfacebound aquo ligands were shown to be labile; refluxing the reduced species in organic solvent results in dissociation of the water moiety from the W IV center, exposing three oxygen-atom vacancies. The cluster is subsequently able to perform deoxygenation of dimethyl and diphenyl sulfoxide, triphenylarsine oxide, and nitrosobenzene. 32 Whereas this work has intriguing potential implications in catalysis and materials chemistry, very little progress has been made in the formation of oxygen-atom defects via the protonation of other reduced POMs.
Our research group has been investigating the synthesis and reactivity of a family of organofunctionalized polyoxovanadate clusters first reported by Hartl and co-workers. 33 This subset of POMs is unique because they possess reduced metal centers and low overall charge. The hexavanadate assembly features 12 bridging alkoxide ligands that flank the surface of the Lindqvist core that stabilize reduced variants of the cluster and saturate nucleophilic μ 2 -O 2− sites at the surface of the assembly. This, in turn, inhibits these moieties from participating in strong interactions with cations, providing opportunities to isolate and explore the reactivity of terminal oxido ligands.
To this end, we recently reported the reactivity of the reduced polyoxovanadate-alkoxide (POV-alkoxide) cluster [V IV 6 O 7 (OEt) 12 ] 2− with an organic acid. 34 The addition of 1 equiv of triethylammonium tetrafluoroborate ([HNEt 3 ][BF 4 ]) to [V IV 6 O 7 (OEt) 12 ] 2− results in the formation of a 1:1 mixture o f a c l u s t e r w i t h a n o x y g e n -a t o m v a c a n c y , [ V I I I V I V 5 O 6 ( O E t ) 1 2 ] 1 − , and the 1e − s p e c i e s [V IV 5 V V O 7 (OEt) 12 ] 1− . In our original report, we include a proposed mechanism for the aforementioned reaction: protonation of a terminal V IV O moiety at the surface of the cluster is believed to result in the formation of a transient, hydroxide-substituted POV-alkoxide (e.g., [V 6 O 6 (OH)-(OEt) 12 ] 1− ), which subsequently disproportionates via proton-coupled electron transfer (PCET) to form [V III V IV 5 O 6 (OEt) 12 ] 1− and [V IV 5 V V O 7 (OEt) 12 ] 1− (Scheme 1). Further evidence in support of this mechanism was disclosed in our recent publication describing the reactivity of a silylium ion with the reduced vanadium oxide assembly; 35 here, it is important to note that silylium ions have been described ubiquitously as bulky surrogates for protons. 36 On the basis of the proposed mechanism for proton-induced oxygen-atom-defect formation at the surface of POV-alkoxide, the pK a of the transient hydroxide-substituted assembly [V 6 O 6 (OH)(OMe) 12 ] 1− is envisaged as an important thermochemical parameter in defining the reactivity of the cluster with organic acids. As such, this work targets elucidation of the basicity of terminal oxido moieties in POM architectures to establish trends for activation of these distinct surface sites in metal oxide assemblies. Here, we report the pK a dependence of the acid reactivity with a series of POValkoxide clusters, [V 6 O 7 (OMe) 12 ] n (n = 2−, 1−, 0). The three clusters are identical in structure, differing from one another only in the oxidation state. Our findings demonstrate that despite possessing lower overall charges for the metal oxide cluster, terminal oxido moieties in POV-alkoxides exhibit sufficient basicity in an organic solvent across multiple charge states to bind and react with protons. Taken as a whole, this work asserts the role that the electronic structure (i.e., oxidation-state distribution) plays in dictating the cluster basicity, providing design criteria for the development of metal oxide nanomaterials with regioselective and tunable proton uptake characteristics.
■ RESULTS AND DISCUSSION pK a Dependence of the Reactivity of 1-V IV 6 O 7 2− with Protons. To deepen our understanding of the thermochemistry of proton uptake at the surface of POV-alkoxide clusters, we set out to elucidate the pK a dependence of acid-induced oxygen-atom-vacancy formation. We opted to shift our focus from the POV-ethoxide cluster to its methoxide-substituted congener, 1-V IV 6 O 7 2− , to minimize activation barriers associated with proton transfer as a result of steric clashes between organic acids and the surface of the cluster (note that previous work has established that the electronic structure of the hexavanadate core is conserved regardless of the identity of the bridging alkoxide ligand 41−44 Figures S1 and S2). This outcome mirrors the reactivity reported previously for the POV-ethoxide cluster and thus confirms analogous behavior of the two distinct organofunctionalized assemblies in the presence of a weak organic acid.
As described in the introduction of this paper, the proposed mechanism for acid-induced defect formation in POV-alkoxide clusters invokes the formation of a transient hydroxidesubstituted assembly, [V 6 O 6 (OH)(OMe) 12 (Figure 1). Notably, the dimethylammonium cation is engaged in two types of hydrogen-bonding interactions, bridging two clusters to form a dimer. One proton of the organic acid forms a single hydrogen bond with a terminal oxido moiety, while the second proton is bifurcated between two adjacent bridging alkoxide ligands at the surface of another cluster molecule. The hydrogen-bonding motif is mirrored between the two clusters, with two organic acid molecules bridging two discrete hexavanadate anions.
The interactions with the bridging alkoxide ligands are notable but not totally unprecedented. While the proposed Scheme 2. Acid-Driven Oxygen-Atom-Vacancy Formation via the Reaction of 1-V mechanism of cluster protonation is framed under the assumption that the alkoxide ligands will prevent cation pairing interactions at bridging oxygen atoms within the assembly, previous work from our laboratory investigating the interactions of POV-alkoxide clusters with alkali ions has shown that these oxygen atoms possess some degree of nucleophilic character. 46 12 ] in MeCN, partial conversion to a 1: Figure S3). We hypothesize that the introduction of heat disrupts the hydrogen-bonding interactions with the face of the cluster, driving protonation of the cluster surface. As such, we infer that the hydrogen-bonding interactions between the ammonium salt and the oxygen atoms of bridging alkoxide ligands are not influential in the general reactivity of 1-V IV 6 O 7

2-
with the other organic acids reported in this work. The existence of a hydrogen bond between the dimethylammonium cation and a vanadyl moiety is a significant finding, as this result offers additional support for the proposed mechanism of acid-induced defect formation involving protonation of a terminal oxido ligand. The O1··· N1 interaction of 2.818(3) Å can be classified as a hydrogen bond of moderate strength, characteristic of a primarily electrostatic interaction between the organic acid and cluster surface (Table 1). 47 Notably, the V1O1 bond distance is slightly elongated [1.6227(17) Å vs VnO t (avg; 1-V IV 6 O 7 2− ) = 1.606(1) Å] in the presence of an organic acid, albeit not to the extent consistent with formal reduction of the VO t bond. 35 Given the scarcity of proton interactions with terminal oxido ligands in POM research, we became interested in elucidating the pK a dependence of proton interactions with terminal vanadyl moieties within POV-alkoxide clusters. Unfortunately, the instability of [V 6 O 6 (OH)(OMe) 12 ] 1− prohibits direct measurement of the acidity of the OH bond. 48−51 Instead, we invoke the quantification of disproportionation products as a means of assessing the extent of reaction for a given acid/ cluster charge combination, under the assumption that the hydroxide-substituted assembly converts quantitatively to a 1: In so doing, we can determine the strength of the organic acid required to react with half of the dianionic cluster by calculating the pK a of an organic acid whose addition to 1-V IV 6 O 7 2− would yield a 2:1: For the purpose of this work, we describe this value as the pK a of the conjugate acid of 1-V IV 6 O 7 2− , the transient hydroxide-functionalized assembly, [V 6 O 6 (OH)-(OMe) 12 ] 1− , and use this value to describe the basicity of the cluster surface.
To quantify the strength of the acid required for surface activation of 1-V IV 6 O 7 2− , we explored the reactivity of the reduced assembly with a series of organic acids, all of which possess acid dissociation constants that have been reported in MeCN (Table S1). 45 . As such, monitoring the change in the absorptivity at either of these wavelengths following the addition of 1 equiv of acid can serve as a spectroscopic handle to determine the extent of reaction.
To probe the acid dependence on defect formation, we evaluated the absorption spectrum of the crude reaction mixture upon the addition of 1 equiv of a series of organic acids (pK a = 12.53−22.60) to complex 1-V IV 6 O 7 2− (Figure 2 and Table S2). The molar absorptivities at 386 and 1000 nm are unchanged following the addition of the weakest acids evaluated (pK a values ranging from 19.62 to 22.60). The addition of marginally stronger acids (pK a = 18.82−19.35) yields a large increase in the absorbance at both wavelengths, with stronger acids (pK a = 12.53−17.96) displaying a minimal effect on further reactivity. The plateau in the molar absorptivity indicates complete consumption of acidic protons, with pK a values of less than 17.96. The generated plot of the where 253.95 and 1868.15 M −1 cm −1 are the lower and upper bounds of the function, respectively, pK a is the strength of the applied acid, and 19.30 and 639.41 are constants derived from the fit of the curve (eq 1 and Figure 2b). Using Beer's law, the molar absorptivity at 386 nm of the 2:1:1 speciation produced when a proton source activates half of the available 1-V IV 6 O 7 2− , resulting in a reaction mixture containing a 2:1:1 ratio of 1- (Table S2; see the Supporting Information for additional details pertaining to this calculation). Solving the pK a value of the organic acid, following whose addition results in conversion of half of the dianionic assembly, is achieved using this molar absorptivity (eq 1 and Figure 2b). On the basis of the assumptions described above, we correlate this value to the pK a value of the transient hydroxide-functionalized assembly [V 6 O 6 (OH)(OMe) 12 ] 1− in MeCN (pK a = 19.28). A plot was generated using the molar absorptivities for the aforementioned reactions at 1000 nm to confirm the product speciation; using this wavelength, a pK a value of [V 6 O 6 (OH)(OMe) 12 ] 1− of 19.31 was found, within error of that resolved using the absorption data at 386 nm ( Figure S7 and Table S2).  18,24,25,56 have very low affinity for protons. This observation is counterintuitive, as one would expect that the increase in electrostatic forces would play a substantial role in proton uptake. We believe the high proton affinity of our modestly charged cluster can be explained by two features of the assembly. First, the POValkoxide is comprised of group V metal ions, which are known to be more basic than their group VI counterparts. 27,57−59 This has been reflected in several reports on the protonation of vanadium-and niobium-functionalized polyoxotungstates and molybdates. 19−21,27,58−62 In all cases, protonation of the heterometallic site(s) is preferred, as revealed by structural, electrochemical, and theoretical analyses. This is due to the lower oxidation state of the d 0 group V ion, which is unable to compensate for the charge of the bound O 2− ligands, as demonstrated on isostructural, homometallic POMs comprised of different metal ions. 57 The increased partial negative charge resides on the oxide ligands surrounding the group V ion(s) in the heterometallic structure, resulting in increased basicity at these sites.
In addition, the high proportion of vanadium(IV) centers within the POV-alkoxide promotes enhanced proton uptake in this assembly in comparison to other POMs. POMs are typically isolated with fully oxidized metal ions. The d 0 metal centers withdraw electron density from coordinated oxygen atoms, reducing the nucleophilicity of the surface oxido moieties despite the large negative charge of the assembly. In contrast, the dianionic form of the cluster is composed entirely of tetravalent (d 1 ) vanadium ions. Because all of the vanadium ions comprising the cluster core contain one d electron, the metal centers are more electropositive than their oxidized congener. 63,64 This further increases the electron density at the terminal oxido moieties, rendering these sites more reactive with protons from the surrounding medium.
Charge-State Dependence of the Surface Basicity of POV-alkoxide Clusters. The methoxide-substituted POValkoxide cluster has been structurally and spectroscopically characterized in four distinct charge states, namely, complexes ). 33,41,43 Interested in interrogating the chargestate dependence on proton uptake, we next investigated the reactivity of organic acids with a series of POV-alkoxides in higher oxidation states. These studies are predicated on the assumption that higher oxidation states of the POV-alkoxide cluster (e.g., [V 6 O 7 (OMe) 12 ] n ; n = 1−, 0) proceed via a similar mechanism of surface protonation of a vanadyl moiety to generate [V 6 O 6 (OH)(OMe) 12 ] n+1 , followed by disproportionation to generate [V 6 O 7 (OMe) 12 ] n+1 and [V 6 O 6 (OMe) 12 ] n+1 . We opted to narrow the scope of our investigations to the reactivity of complexes 3-V IV 5 V V O 7 1− and 4-V IV 3 V V 2 O 7 0 with organic acids because the proposed products of proton uptake have been shown to be stable in MeCN. 33,41,43,65,66 Upon exposure of the monoanionic POV-alkoxide complex   4 ], were noted, suggesting no interaction between the cluster and this proton source ( Figure S8).
Hypothesizing that the change in cluster charge state may be associated with a decreased basicity of the surface vanadyl moieties, we opted to investigate the reactivity of 3-V IV 5 V V O 7 1− with a stronger acid. When 1 equiv of pyrazolium tetrafluoroborate ([HPz][BF 4 ]; pK a = 9.1) 45 was added to the monoanionic POV-alkoxide cluster, the resultant 1 H NMR spectrum contains chemical shifts corresponding to the 1e − assembly 4-V IV 4 V V 2 O 7 0 (δ = 21.7 ppm) and the neutral, oxygen-deficient cluster 6-V III V IV 4 V V O 6 0 (δ = 25.3, 18.2, and −12.6 ppm; Figure 3).
To quantify the basicity of the cluster surface, we explored the reactivity of 3-V IV 5 V V O 7 1− with a series of organic acids of varying strength (pK a = 6.79−17.96). In the case of the more oxidized assembly, an alternative analytical approach was needed, as all three clusters contained within the reaction mixture To our surprise, in monitoring of the 1 H NMR spectra of a series of reactions exposing complex 3-V IV 5 V V O 7 1− to 1 equiv of various organic acids, only four paramagnetically broadened and shifted signals were observed as opposed to the expected five signals (Figure 3a). Three of these signals correspond to the expected oxygen-deficient assembly, complex 6-V III V IV 4 V V O 6 0 , and appear to increase in intensity proportionately with the strength of the added organic acid. Interestingly, the fourth resonance is observed between 21.7 and 23.4 ppm, corresponding to the reported chemical shift of neither 3- As the pK a value of the added organic acid decreases, the chemical shift of the broad resonance moves upfield, toward the established value of 4-V IV 4 V V 2 O 7 0 (δ = 21.7 ppm), settling when the pK a value of the organic acid added is less than or equal to 9.44 (Figure 3a). In addition, upon assessment of the relative integration of the paramagnetically shifted resonance at 18.2 ppm compared with those at 25.3 and 23.4−21.7 ppm (taken together because of overlap in reactions with weak acids), it is observed that the speciation in acid addition reactions differs very little in reactions between 3-V IV 5 V V O 7 1− and acids with pK a values below 9.44 (Table S3). The relative integrations of these NMR signals for reactions with strong acids (pK a < 9.44) presents an approximate 1:4 ratio between the peak at 18.2 and the two more-downfield shifts, respectively, matching what is expected for a 1:1 mixture of 4-V IV 4 V V 2 O 7 0 and 6-V III V IV 4 V V O 6 0 . The lack of significant differences in the relative integrations for spectra of reactions with strong acids indicates that the addition of an organic acid with a pK a value lower than 9.44 would be expected to result in complete conversion of complex 3-V IV 5 V V O 7 1− and that the region where the pK a value of the acid dictates the extent of conversion should fall between 9.44 and 14. 24. Whilea comparison of the relative integrations of paramagnetic protons provides general information into this pK a -dependent reactivity window, we do note that because of the likely differences in the relaxation time for each resonance comparing signal integrations is not an accurate measure of the extent of reactivity of the parent POV-alkoxide.
The observed fluxionality of the chemical shift of the single broad resonance, however, presents an opportunity to quantify the contents of the reaction mixture. We hypothesized that electron transfer exceeding the time scale of the 1 H NMR experiment between the unreacted monoanionic POV-alkoxide cluster 3-V IV 5 V V O 7 1− and the 1e − assembly formed following cluster protonation, 4-V IV 4 V V 2 O 7 0 , may explain the observed behavior. 42 12 ] n (n = 1−, 0) versus pK a of the added organic acid was generated ( Figure  3b). This plot reveals a sigmoidal relationship, which can be fit by the following equation: ( ) 23 ; δ = 16.7 ppm) and three additional signals at 21.4, 13.7, and −11.8 ppm ( Figure 5). Given the stoichiometry of the reaction, we hypothesized that this unfamiliar set of resonances in the 1 H NMR spectrum of the crude reaction mixture might correspond to the cationic, oxygen-atom-deficient POValkoxide cluster [V 6 O 6 (OMe) 12 ). Indeed, an independent synthesis of this complex was achieved by the oxidation of 6-V III V IV 4 V V O 6 0 with 1 equiv of AgBF 4 in dichloromethane (DCM). The resultant 1 H NMR spectrum reveals the same three-peak pattern as that observed in the acid addition reaction, albeit with a significant impurity peak at 21.7 ppm corresponding to , likely formed as a consequence of the reactive nature of oxygen-atom-deficient clusters with adventitious water ( Figure S10).
With product speciation in hand, we were able to determine the acid strength required to react with 50% of the neutral cluster in solution ( Figure 5). It is important to note that, again, rapid electron transfer between the neutral and cationic states of the fully oxygenated Lindqvist POV-alkoxide results in a shift of the 1 H NMR signal for this complex ( Figure S11 and see the Supporting Information for addition details). Analysis  of the resultant 1 H NMR spectra of a series of cluster/acid combinations produces a relationship similar to those in the previous two examples, where certain acids (pK a = 7.02−9.1) do not produce any oxygen-atom defects and stronger proton donors (pK a = 1.28−5.98) gradually convert 4-V IV 4 V V 2 O 7 0 to a mixture of 5-V IV 3 V V 3 O 7 1+ and the oxygen-deficient assembly 7-V III V IV 3 V V 2 O 6 1+ (Figure 5a). Plotting the chemical shift of the signal corresponding to a mixture of [V 6 O 7 (OMe) 12 ] n (n = 0, 1+) versus pK a of the added organic acid reveals the anticipated sigmoidal relationship, which is fit by the function ( ) 21 where 21.93 and 16.79 ppm are the upper and lower bounds, respectively, pK a is the applied acid strength, and 4.97 and 6.94 are constants (Figure 5b). The chemical shift of the expected 2:1 ratio of 4-V IV 4 V V 2 O 7 0 and 5-V IV 3 V V 3 O 7 1+ present after conversion of half the initial neutral cluster with an acid is 20.24 ppm; this value corresponds to a pK a value for the purported protonated POV-alkoxide cluster of 5. 51.
As described at the outset of this work, the experimentally determined pK a values for the purported hydroxide functionalized POV-alkoxide clusters, [V 6 O 6 (OH)(OMe) 12 ] n (n = 1−, 0, 1+), can be used to describe the basicity of the parent, fully oxygenated assembly. Analysis of the data reported reveals a trend, with reduced derivatives of the organofunctionalized vanadium oxide assembly possessing a significantly increased affinity for protons in MeCN compared to their oxidized congeners. This result is consistent with previous studies describing the relative basicity of POMs across multiple charge states, in which cluster reduction has been shown to enhance proton association constants. For instance, Wedd and coworkers showed that reduction of the Wells−Dawson complex [S 2 W 18 O 62 ] 4− in the presence of acidic protons yields anodically shifted redox waves, indicating PCET to the cluster surface. The reduced species [S 2 W 18 O 61 (OH)] n (n = 5−, 6−, 7−) feature proton association constants of 2 × 10 −1 , 2 × 10 8 , and 3 × 10 13 M −1 , respectively, in MeCN. 18 The large equilibrium constants for the most reduced variants of the cluster suggest that as electrons are added to the assembly, the surface of the cluster dramatically increases in its affinity for protons. Similar results have been noted for the αand βisomers of the Keggin-type polyoxotungstate [PW 12 O 40 ] 3− , where the addition of electron density to the assembly corresponds to an increased basicity of the cluster surface. 24,25,56 As the POV-alkoxide cluster is oxidized, the number of vanadium(V) centers contained within the hexavanadate core increases, decreasing the average electropositivity of the VO sites within the cluster. Therefore, because 3-V IV 5 V V O 7 1− and 4-V IV 4 V V 2 O 7 0 contain one and two d 0 metal centers, respectively, their average electropositivities will be lower than that of the fully reduced 1-V IV 6 O 7 2− , resulting in observed decreases in the basicity for each electron removed. However, because both oxidized clusters are composed of a majority of reduced vanadium centers, the average electropositivity of the core metal ions facilitates measurable proton uptake to the modestly charge assemblies. This further supports the notion that the electronic structure [i.e., the ratio of vanadium(V)/ vanadium(IV) ions contained within the vanadium oxide assembly] has a more substantial impact on the basicity of the polyoxovanadate surface (as opposed to molecular charge).
It is worth mentioning that in our previous work investigating charge-compensation reactions between POValkoxide clusters and alkali ions, we observed that these cations do not interact with the surface of the cluster in its monoanionic and neutral charge states. 46 This observation renders the reactivity of the monoanionic and neutral POValkoxide clusters with protons particularly intriguing. We propose two possible, nonexclusive, explanations for the observed reactivity. The first pertains to the irreversibility of the protonation reaction. Generally, POM protonation has been shown to be a reversible equilibrium process able to be quantified by association constants extracted from electrochemical analyses. 18,24,25,56 In our system, the acid−base equilibrium is disrupted by rapid disproportionation of the protonated cluster. Considering Le Chatelier's principle, this drives the equilibrium to favor its protonated form. The irreversible proton transfer enhances the basicity of the more oxidized clusters, resulting in proton uptake for clusters that do not feature surface coordination of alkali ions. Separately, the reducibility of protons may facilitate proton uptake to modestly charged clusters. Alkali ions require high potentials to be reduced by an electron donor; for example, the potential of the Li 0/+ couple is −3.05 V versus standard hydrogen electrode. 75 Because the reduction of protons is accessible at much lower potentials and dependent on the strength of the proton donor, 76 the POV-alkoxide cluster may more readily transfer electron density to the proton, resulting in the reactivity being better described as that of the vanadium oxide assembly with hydrogen atoms, diminishing the importance of electrostatic forces.

■ CONCLUSIONS
In this work, we have quantified the basicity of a POV-alkoxide cluster in a variety of charge states in MeCN. This property is presented as the pK a value of the transient hydroxideterminated species [V 6 O 6 (OH)(OCH 3 ) 12 ] n (n = 1−, 0, 1+), providing a thermochemical descriptor for acid-induced oxygen-atom-vacancy formation on the Lindqvist core. The molecular vanadium oxide studied in this work exhibits regioselectivity toward protonation at terminal oxido moieties, imparted by the saturation of bridging oxides with organic groups, giving rise to the formation of an atypical terminal vanadium hydroxide that rapidly converts to an oxygen-atom vacancy poised for downstream reactivity. Our findings indicate that the basicity of terminal oxido moieties at the surface of the assembly is primarily a consequence of vanadium-ion electropositivity and can be tailored by tuning the ratio of oxidized and reduced metal ions (∼7 pK a units per electron lost). We also found that cluster interactions with protons vastly differ from those with alkali ions because of the chemical irreversibility of the protonation reaction as well as the redox activity of the added cationic species. Overall, the cluster's structural and electronic characteristics serve as atomically precise descriptors for nanocrystalline metal oxide design criteria (i.e., capping ligand density and oxidation state distribution), suggesting that control over these features should allow for careful tuning of the material activity with protons.

■ EXPERIMENTAL SECTION
General Considerations. All manipulations were carried out in the absence of water and oxygen using standard Schlenk techniques or in a UniLab MBraun inert-atmosphere drybox under a N 2 atmosphere. All glassware was oven-dried for a minimum of 4 h and cooled in an evacuated antechamber prior to use in the drybox. Solvents were dried and deoxygenated on a Glass Contour System (Pure Process Technology, LLC) and stored over activated 3 Å molecular sieves purchased from Fisher Scientific prior to use. [ n Bu 4 N]BH 4 , AgOTf, AgBF 4 , a tetrafluoroboric acid ether complex, thiophenol, triphenylamine, 4-chloro-2-nitroaniline, 2,6-dichloroaniline, 3-nitroaniline, diphenylamine, pyrazole, 4-bromoaniline, 2,2′bipyridine, trimethylammonium chloride, and dimethylammonium chloride were purchased from Sigma-Aldrich and used as received. 2-Chloropyridine, 2-bromopyridine, 4-N,N-dimethylaniline, pyridine, 2methylpyridine, 4-methoxypyridine, 4-N,N-dimethylpyridine, triethylamine, N,N,N′,N′-tetramethyl-1,3-propylenediamine, piperidine, and pyrrolidine were purchased from Sigma-Aldrich, dried over KOH under an atmosphere of Ar, distilled, and stored over activated 3 Å molecular sieves prior to use. 4-Bromoaniline was purchased from Alpha Aesar and used as received. 65 and 6-V III V IV V V O 6 066 were synthesized according to literature precedent. 1 H NMR spectra were recorded at 400 and 500 MHz on Bruker DPX-400 MHz and DPX-500 spectrometers, respectively, locked on the signal of deuterated solvents. All chemical shifts were reported relative to the peak of the residual hydrogen signal in deuterated solvents. MeCN-d 3 was purchased from Cambridge Isotope Laboratories, degassed by three freeze−pump−thaw cycles, and stored over fully activated 3 Å molecular sieves. Electronic absorption measurements were recorded at room temperature in anhydrous MeCN in a sealed 1 cm quartz cuvette with an Agilent Cary 60 UV− vis spectrophotometer. Mass spectrometry analyses were performed on an Advion ExpressionL Compact mass spectrometer equipped with an electrospray probe and an ion-trap mass analyzer (instrument error: ±0.1 amu). Direct injection analysis was employed in all cases with a sample solution in MeCN 12 ] were mounted on the tip of a thin glass optical fiber (goniometer head) and mounted on a XtaLab Synergy-S Dualflex diffractometer equipped with a HyPix-6000HE HPC area detector for data collection at 223 K. The structure was solved using SHELXT-2018/2 77 and refined using SHELXL-2018/3. 78 Modified ). The synthesis of the oxidized cluster was adapted from a previous report. 43 A 20 mL scintillation vial was charged with 4-V IV 4 V V 2 O 7 0 (0.150 g, 0.190 mmol) and 5 mL of DCM. While stirring, a suspension of AgOTf (0.054 g, 0.21 mmol, 1.1 equiv) in DCM was added to the cluster-containing solution. The solution was stirred for 20 h, after which the formation of a gray precipitate was observed. The solution was filtered over a bed of Celite (2 cm) on a medium-porosity glass frit. The precipitate was washed with DCM (3 × 2 mL), and the resultant green solution was dried under reduced pressure to yield 5- Acidification of Organic Bases. Acidification was performed using a preparation derived from literature precedent. 76 A 10 mL solution of base in Et 2 O was charged in a 20 mL scintillation vial with a magnetic stirbar. The solution was vigorously stirred, and 0.95 equiv of HBF 4 · Et 2 O was added dropwise. Caution! This reaction is exothermic. Take care to add HBF 4 ·Et 2 O very slowly with stirring to avoid the solution boiling and splashing out of the vial. Upon the addition of HBF 4 ·Et 2 O to the stirring solution, a white solid immediately precipitated. The resultant powder was washed with Et 2 O (3 × 10 mL) and recrystallized from a mixture of 10:1 Et 2 O/MeCN. The recrystallized material was filtered over a medium-porosity frit, dried under reduced pressure overnight, and stored over P 2 O 5 prior to use. The material purity was confirmed via 1 H and 13 C NMR spectroscopy. For previously reported acids, the referenced NMR data are noted in Table S1. 51 The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c02937. 1 H NMR, electrospray ionization mass spectrometry, and electronic absorption spectra of POV clusters and acid addition reactions, charge-state calibration curves, and pK a calculations for cluster charge states (PDF) Accession Codes CCDC 2110688 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.