Modeling the Oxygen Vacancy at a Molecular Vanadium(III) Silica

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Modeling the Oxygen Vacancy at a Molecular Vanadium(III) Silica–Supported Catalyst Teng Zhang, Albert Solé-Daura, Sarah Hostachy, Sébastien Blanchard, Céline Paris, Yanling Li, Jorge J. Carbó, Josep M. Poblet, Anna Proust, and Geoffroy Guillemot J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09048 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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Journal of the American Chemical Society

Modeling the Oxygen Vacancy at a Molecular Vanadium(III) Silica–Supported Catalyst Teng Zhang,† Albert Solé-Daura,‡ Sarah Hostachy,†,# Sébastien Blanchard,† Céline Paris,§ Yanling Li,† Jorge J. Carbó,‡ Josep M. Poblet,‡* Anna Proust†* and Geoffroy Guillemot†* †

Sorbonne Université, CNRS, Institut Parisien de Chimie Moléculaire, IPCM, F-75005 Paris, France

‡

Universitat Rovira i Virgili, Department de Química Física i Inorgànica, Marcel·lí Domingo 1, 43007 Tarragona, Spain

§

Sorbonne Université, CNRS, De la Molécule aux Nano-objets: Réactivité, Interactions et Spectroscopies, MONARIS, F-75005 Paris, France KEYWORDS: Vanadium, redox chemistry, polyoxotungstate, silanol, site-isolated model, density functional theory (DFT) ABSTRACT: Here we report on the use of a silanol-decorated polyoxotungstate, [SbW9O33(tBuSiOH)3]3− (1), as a molecular support to describe the coordination of a vanadium atom at a single-site on silica surfaces. By reacting [V(Mes)3·thf] (Mes= 2,4,6-trimethylphenyl) with 1 in tetrahydrofuran, the vanadium(III) derivative [SbW9O33(tBuSiO)3V(thf)]3− (2) was obtained. Compound 2 displays the paramagnetic behavior expected for a d2-VIII high spin complex (SQUID measurements) with a triplet electronic ground state (ca. 30 kcal·mol-1 more stable than the singlet, from DFT calculations). Compound 2 proves to be a reliable model for reduced isolated–vanadium atom dispersed on silica surfaces [(≡Si–O)3VIII(OH2)], an intermediate that is often proposed in a Mars-van Krevelen type mechanism for partial oxidation of light alcohols. Oxidation of 2 under air produced the oxo-derivative [SbW9O33(tBuSiO)3VO]3− (3). In compound 2, the d2–electrons are localized in degenerated d(V) orbitals whereas in the electronically analogous bireduced-[SbW9O33(tBuSiO)3VO]5−, 3·(2e), one electron is localized on d(V) orbital and the second one is delocalized on the polyoxotungstic framework, leading to a unique case of a bi-reduced heteropolyanion derivative with completely decoupled d1-V(IV) and d1-W(V). Our body of experimental results (EPR, magnetic measurements, spectroelectrochemical studies, Raman spectroscopy) and theoretical studies highlights (i) the role of the apical ligand coordination, i.e. thf (σ-donor) vs. oxo (π-donor), in destabilizing or stabilizing the d(V) orbitals relative to the d(W) orbitals, and (ii) a geometrical distortion of the O3VO entity that causes a splitting of the degenerated orbitals and the stabilization of one d(V) orbital in the bireduced compound 3·(2e).

to formaldehyde.12,13,14,15 In the latter case, a plausible and often proposed mechanistic pathway corresponds to an oxidationreduction process referred to as a Mars–van Krevelen type of mechanism. The redox cycle involves (i) the dissociative alcohol adsorption on a V-O-M bond (M= metal support) to form an alcoholate at the vanadium and M-OH followed by (ii) an irreversible H-transfer from the α-carbon atom leading to the formation of aldehyde and water and to the reduction of the catalyst. Finally, (iii) re-oxidation of VOx is promoted by lattice oxygen and/or by oxygen from air. This mechanism therefore requires the formation of a vanadium(III) complex. Modeling a low-valent vanadium in such a tris–silanol environment is rather difficult and no valuable molecular example has been reported.16,17 We therefore took advantage of a molecular model that some of us recently introduced, built on multi-vacant polyoxometalates (POMs), to stabilize a vanadium(III) metal ion in a rigid tris– siloxido environment and produce a relevant model of an oxygen vacancy in vanadia lattice.

1 – INTRODUCTION Vanadium oxides deposited onto a large-surface area oxide represent a class of heterogeneous catalysts that has been wellstudied.1 The catalytic efficiency of these materials has been shown to be dependent on both the nature of the support (montmorillonite, silicates, zirconia, anatase) and the vanadium density at the surface.2,3,4 The formation of isolated VO4 entities, meaning a {V=O} functionality grafted to the surface in a pseudotetrahedral geometry, prevails at low surface coverage whereas polyvanadate and bulk V2O5 form at higher surface density. Probably one of the most ubiquitous metal oxo species, vanadium oxo has been reported as key intermediate in a large variety of catalytic oxidative transformations, such as epoxydation of alkenes, oxidation of alcohols and SO2, C–C bond cleavage of αhydroxyketones, α-hydroxyethers and also oxidative degradation of lignin models.5,6,7 Isolated oxovanadium groups, supported on SiO2, ZrO2 and TiO2, play also an important role in two reactions of great interest in chemical industry, i.e. the oxidative dehydrogenation of light alkanes8,9,10,11 and the partial oxidation of alcohol, notably methanol

Multi-vacant POMs possess nucleophilic oxygen atoms and have been extensively exploited as multi-dentate ligands to coordinate

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in a larger variety of organic solvents, and particularly in tetrahydrofuran.

extra transition metal cations. A wide variety of transition metal substituted POMs (TMSPs) has thus been reported these last decades.18,19 Multi-vacant POMs also provide thermally and oxidatively robust backbones to build organic-inorganic hybrid platforms.20,21,22 Functionalization of vacant [α-XW9O34-x]9- and [γPW10O36]7- with t-butyl trichlorosilane thus yields the silanoldecorated polyoxotungstates [α-XW9O34-x(tBuSiOH)3]3− (X= P, x= 0; X= Sb, x= 1) and [PW10O36(tBuSiOH)2]3−.23 These in turn display a rigid and geometrically preorganized set of silanol functionalities that subsequently leads to transition metal complexes with constrained geometry. At variance with the TMSPs mentioned above, the transition metal site is isolated from the POM backbone by the organic shell. We recently reported the vanadium and titanium complexes, [XW9O34−x(tBuSiO)3VO]3−, and [XW9O34[PW10O36(tBuSiO)2VO(iPrO)]3− t i 3respectively, in an original approach to x( BuSiO)3Ti(O Pr)] modeling active sites of titanium or vanadium-containing single site heterogeneous catalysts for epoxidation.24,25 As a result, a reliable structure-activity relationship was unambiguously assessed by a combined use of spectroscopic studies and kinetic investigations. It is worth mentioning that in the complexes [SbW9O33(tBuSiO)3M(L)]3−, the hybrid platform not only reproduces the chemical environment (three silanol functions) but also the exact geometry found in β-crystoballite, which is considered the best model for silica.26 This is particularly evidenced by very similar Si-Si and M-Si interatomic distances (see Figure S1).

2-1 VANADIUM(III) DERIVATIVES Preparation and characterization. The most popular synthetic routes for the generation of electron-rich metals bonded to an oxo environment are (i) the complexation of high valent metals, most often used as chloride derivatives, followed by their chemical reduction, and (ii) the use of organometallic precursors, such as homoleptic alkyl or aryl precursors. The latter procedure avoids the formation of halide salts, derived from the metathesis reaction, and is therefore more appropriate for the generation of coordinatively unsaturated d2–vanadium(III) metal. This was achieved here by reacting [V(Mes)3·thf] with (THA)3[SbW9O33(tBuSiOH)3], THA1, in tetrahydrofuran. The reaction led to a bluish solution from which compound (THA)3[SbW9O34(tBuSiO)3VIII(thf)], THA-2, could be isolated (Scheme 1). Scheme 1. Preparation of complexes 2 and 3; counter ions = tetrahexylammonium.

Beyond the structural model aspect, POMs are also electron acceptors that can reversibly store and release multiple electrons with only minor structural reorganization, a feature that has been largely exploited in electrocatalysis.27,28,29,30,31 As such, the coordination of a low-valent metal might thus imply an innersphere electron transfer to the polyoxotungstic framework as observed in photo-activated photosensitized-POM dyads for applications in artificial photosynthesis.32,33,34 To assess the electronic interplay between the remote low-valent metal site and the POM backbone, we proceeded to the metallation of [SbW9O33(tBuSiOH)3]3− (1) by using the reactive [VIII(Mes)3·thf] (Mes= 2,4,6-trimethylphenyl) and studied the electronic structure of the resulting complex (2). Oxidation of compound 2 in solution led to the known oxovanadium(V) derivatives [SbW9O33(tBuSiO)3VO]3- (3), from which we report here a complete spectroelectrochemical study. In addition, special efforts were devoted to developing a reliable synthetic methodology for controlled chemical reduction of this parent compound 3, by means of sodium-naphthalenide solution35 to properly isolate the one-electron and the two-electron reduced derivatives. The location of the “extra” electrons in [SbW9O33(tBuSiO)3V(thf)]3−, [SbW9O33(tBuSiO)3VO]4− and [SbW9O33(tBuSiO)3VO]5− was clearly assigned on the basis of EPR analyses and magnetic measurements and supported by a theoretical study, which pointed out the critical role played by the electronic feature of the apical ligand, thf vs oxo.

The assignment of the chemical structure of THA-2 was first based on in situ NMR monitoring of the reaction, which showed both the formation of mesitylene and the disappearance of the signals related to the starting ligand THA-1. The NMR pattern is strongly affected by the paramagnetism of the compound and 51V NMR only indicates a very broad signal centered at -500 ppm (Figure S2). Secondly, complex 2 displays the paramagnetic behavior expected for a d2-vanadium high-spin complex. Its magnetic susceptibility was measured in the temperature range 1.9 – 250 K and data are shown in Figure 1. The χMT product is nearly constant throughout the temperature range 50 – 250 K with a value that matches the one expected for two unpaired electron (S= 1). The decrease observed at low temperature can be attributed to the presence of zero field splitting and/or small intermolecular interactions. This electronic structure was also confirmed by the UV-Vis spectrum of a solution in tetrahydrofuran which profile is similar to that of [VCl4]– species (Figure S3).

2 – RESULTS In this study, we used the silanol-decorated polyoxotungstate [α–B–SbW9O33(tBuSiOH)3]3(1), isolated as a tetrahexylammonium (THA) salts in order to increase its solubility

As expected, THA-2 is very sensitive to air and is therefore readily reoxidized at room temperature to afford complex 3, the vanadium-oxo derivative, which has been formerly described by us

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and which electronic features will be discussed hereafter (section 2.2). Reoxidation could be also achieved by using styrene oxide as an oxygen atom donor. In all cases, the formation of anion 3 was evidenced by 51V and 1H NMR (see experimental section and Figure S4). Alternatively, compound 2 could also be obtained by reacting [SbW9O33(tBuSiO)3VO]3- (3) with [V(Mes)3·thf]. Electron and oxygen-atom transfers probably proceed through the formation of a μ-oxo bridged {V-O-V} dimer, which evolves towards the formation of tris-mesityl oxovanadium(V) and [SbW9O33(tBuSiO)3VIII(thf)]3- (2) as revealed by NMR spectroscopy (see experimental section and Figure S5).36 This strategy was very recently applied by Matson et al. to generate an oxygen-atom vacancy at a polyoxovanadate-alkoxides as molecular model of heterogeneous reducible metal-oxides.37

Figure 2. Frontier molecular [SbW9O33(tBuSiO)3V(thf)]3- (2).

orbital

scheme

for

Such a tripodal arrangement of three (≡SiOH) can be likened to the tris(alkoxide) platform based on sterically hindered alkoxides39 reported by the group of Nocera for the preparation of low coordinate metal complexes.40 It is worth highlighting that in 2, the siloxide groups are linked to a threefold symmetric scaffolds [SbW9O33]9- as in the case of the atrane systems and particularly the tris(2-aminoethyl)amine ligands functionalized by bulky silyl groups well developed by Schrock and coworkers.41 However, we should mentioned at this stage that the distance between the antimony and the mean plane of the three oxygen atoms (3.22 Å) precludes any interaction with the embedded metal center.42 Overall, the collected data are consistent with the successful formation of (THA)3[SbW9O33(tBuSiO)3VIII(thf)], THA-2. Considering the thf molecule as a weakly bonded ligand, THA-2 can be described as a reliable molecular model for a surface oxygen vacancy associated with the formation of reduced vanadium atoms in a vanadia lattice.43 This also demonstrates the ability of (THA)3[SbW9O33(tBuSiOH)3] to accommodate low-valent vanadium(III) without any inner-sphere electron transfer to the tungsten atoms. In this scenario the role of the apical ligand coordination might be of particular interest. We could expect that by replacing the pure σ-donor by a π-donor ligand, the d(V) orbitals could be sufficiently destabilized to promote an electron delocalization onto the d(W) orbitals of the polyoxotungstate scaffold. We therefore decided to draw a comparison with the redox features of the vanadium-oxo derivatives.

Figure 1. χMT product per vanadium as a fonction of the temperature for [SbW9O33(tBuSiO)3V(thf)]3- (2).

Electronic structure. Computational studies were performed to understand the electronic structure of anion 2. Figure 2 contains a schematic representation of its structure and its frontier molecular orbitals. The absence of electron transfer to the empty d(W) is supported by the calculations that showed a clear preference for populating V rather than W orbitals. Since the two single occupied molecular orbitals are basically degenerate, the ground state of the anion is a triplet in excellent agreement with its magnetic properties. The singlet electronic state was found to be more than 30 kcal·mol-1 less stable. This finding has some similarities with the recent observation reported by Streb and coworkers on the stabilization of low-valent Fe(I) in a high valent V(V) oxide cluster.38 We have also computed the UV-vis spectrum, observing that the main absorption band is centered at 280 nm, which corresponds to p(O)→d(W) transitions (Figure S3). In the visible region, the absorbance is weak and is associated to transitions involving vanadium orbitals, which is also in good agreement with the experimental observation. Of particular interest, calculations also described a planar trigonal arrangement of the siloxido groups around the vanadium center with a calculated (OSi–V–OSi) angle of 117.6° and a thf ligand only weakly bonded to the metal [cald. d(V– OTHF)= 2.05 Å and (OSi-V-OTHF) = 95.7°] (computed interatomic distances are reported in Figure S1). Hence the bonding energy was computed to be only 11.1 kcal·mol-1, suggesting that a ligand substitution is plausible.

2-2 VANADIUM(V)–OXO DERIVATIVE Considerations about the molecular structure. Following a synthetic methodology akin to the route (i) mentioned above and previously published procedures,24,25 we have prepared [α–B– SbW9O33(tBuSiO)3VO]3- (3) as a tetrahexylammonium salt (Scheme 1). As described previously, compound 3 represents a reliable structural model for an oxidovanadium group of single-site heterogeneous catalysts (Figure S1).44 Whereas the C3 symmetry is commonly found in a large variety of metal complexes assuming trigonal bipyramidal ligand field, it is worth mentioning at this stage that only seldom examples of pseudotetrahedral early transition metal-oxo complexes [(L)3MO] incorporating either the sterically crowded amido (N[R]Ar)– (R= tBu, Adamentyl)45 or alkoxido

3



(OR)– (R= C(Me)tBu2) ligands resemble the structures of complex 3.46,47,48

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By comparison, the cyclic voltammogram of the vanadium-oxo derivative THA-3 is characterized by a first wave appearing at a more positive, E½ = –0.45 V vs SCE, that we could reasonably assign to a reduction primarily centered on the vanadium center element. It is worth mentioning however that this redox potential is more negative than those of V-TMSPs59,60,61,62 and closer to the values generally encountered for reduction events located at the polyoxotungstic core. Figure 4b displays the evolution of the UVvis spectrum of a solution of THA-3 in acetonitrile when potentials of -0.8 V (red), -1.3 V (light blue) and -1.8 V (dark blue) vs SCE were successively applied. The different profiles that were obtained clearly confirm that the first reduction event occurs at the vanadium center, resulting in an almost colorless solution as revealed by the appearance of a band of very low intensity centered at ca. 650 nm and attributed to a V→V d-d transition. The second and third reduction events are tungsten-centered as shown by the intense bands due to IVCT, which are almost identical to those of the mixed–valence compounds obtained when reducing THA-1 by one and two electrons (Figure 4a). Besides, the gap between the second and third reduction events in 3, ΔE½= 0.481 V, is similar to the gap between the first and second reduction events in 1, ΔE½= 0.487 V. In order to get a deeper understanding of the redox properties observed for these compounds, we have analyzed the electronic structures of these anions from DFT calculations.

Electrochemical Studies. To get insights into the electrochemical properties of 3, we first carried out cyclic voltammetry and spectroelectrochemical studies. Figure 3 displays the cyclic voltammograms of [α–B–SbW9O33(tBuSiO)3VO]3- (3) in acetonitrile along with the one of [α–B–SbW9O33(tBuSiOH)3]3(1). Table 1 gathers the half-wave potentials and peak-to-peak separations. In the reduction part, the starting ligand 1 undergoes three reversible electrochemical events, each one corresponding to a monoelectronic process located at the tungstic framework. This behavior is typical for POMs that are able to store (and release) several electrons under minor structural reorganization. In such heteropolyanions the electrons are delocalized over all or part of the tungsten oxide framework by rapid intramolecular electron transfer.28,49,50,51,52,53 The redox potentials mainly depend on i) the nature of the constitutive metals (W, Mo, V) and ii) the global charge, and to a lesser extent on the nature of the solvent and the cations.49 Reduction at the tungsten oxide core is accompanied by a deep blue coloration that produces a spectrum with intense bands in the Vis-Near IR region, typical of Inter Valence Charge Transfers (IVCT) from W(V) to W(VI), as represented in Figure 4a for compound 1.54,55,56,57,58

Figure 3. Cyclic voltammograms of (THA)3[SbW9O33(tBuSiO)3VO], THA-3 (top), and (THA)3[SbW9O33(tBuSiOH)3], THA-1 (bottom), as 5×10-4 M solutions in acetonitrile carried out with TBAPF6 as supporting electrolyte (10-1 M) in a standard three-electrode cell, composed of a glassy carbon working electrode, a platinum counter electrode, and a saturated calomel reference electrode (SCE) at a scan rate of 100 mV·s-1.

Table 1. Half -Wave Potentials (V vs. SCE) and Peak to Peak Separation (mV).a 0/I

I/II

ΔE½

II/III

ΔE½

THA-1

-0.702 (72) -1.189 (74) 0.487 -1.931 (78) 0.742

THA-3

-0.455 (94) -0.998 (76) 0.543 -1.479 (78) 0.481

Figure 4. (a) Evolution of the UV-Vis spectrum of a 5×10-4 M solution of THA3[SbW9O33(tBuSiOH)3], THA-1, in acetonitrile during electrolysis at -0.9 V (light blue) and -1.5 V (dark blue) vs SCE; (b) Evolution of the UV-Vis spectrum of a 5×10-4 M solution of THA3[SbW9O33(tBuSiO)3VO], THA-3, in acetonitrile during electrolysis at -0.8 V (red) -1.3 V (light blue) and -1.8 V (dark blue) vs SCE.

a) The notation in upper case roman numerals refers to the number of added electrons: 0 corresponds to the fully oxidized species, I, II and III to the 1e- 2e- and 3e- reduced species, respectively.

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Electronic structures of vanadium-oxo derivatives. Figure 5 contains a frontier orbital scheme for the fully oxidized [SbW9O33(tBuSiO)3VVO]3-. As expected, the HOMO of the molecule corresponds to the lone pair of the trivalent ion SbIII. In this case, the replacement of the thf ligand in 2 by the weak-field oxo ligand destabilizes the d(V) orbitals, which become higher in energy than the three unoccupied orbitals with main contribution of d(W) atomic orbitals. This would indicate that the reduction of the compound should involve the POM framework instead of the vanadium center.

Figure 6. Schematic representation and DFT structures computed for the two structures of anion [SbW9O33(tBuSiO)3VIVO]4-, s-3·(1e) and d-3·(1e), and the lowest in energy structure of [SbW9O33(tBuSiO)3VIVO]5-, d-3·(2e). Spin density distribution for each anion is represented on the optimized DFT geometry. Relative Gibbs free energies for s-3·(1e) and d-3·(1e) in parenthesis are given in kcal·mol-1.

The subsequent reduction of species 3·(1e) leads to the 2ereduced anion 3·(2e). In concordance with the results obtained for 3·(1e), the species with two electrons delocalized on the tungstate framework is clearly destabilized (by more than 10 kcal·mol-1) with respect to the optimized structure with one electron located at the V center and a second electron delocalized among the d(W) orbitals (Figure 6 and Figure S6). The computed absorption spectrum for d-3·(2e) (Figure S7) shows an intense band in the visible region that corresponds to W → W transitions in concordance with the observed UV visible spectrum of [SbW9O33(tBuSiO)3VO]3– during electrolysis. 2-3 CHEMICAL REDUCTION OF VANADIUM(V)OXO COMPOUNDS AND MAGNETIC CHARACTERIZATION

Figure 5. Frontier molecular orbital scheme for the fully oxidized anion [SbW9O33(tBuSiO)3VO]3- (3).

Chemical reduction of (THA)3[SbW9O33(tBuSiO)3VO], THA-3. To get further evidences on the electronic structures of the reduced forms of anion 3 we decided to proceed to its chemical reduction to recover solids freed from electrolytes. Applying a rational synthetic methodology for the non-aqueous reduction of POMs is not trivial.64 Most of the reduced POMs have been prepared (i) from hydrothermal reactions, which main drawbacks are poor control on both the stoichiometry and the degree of reduction, and (ii) from oxometallates precursors in the presence of aqueous hydrazine solutions or phosphanes.65,66,67,68 Use of stoichiometric amount of reductive agent in drastic non-aqueous conditions is extremely scarce,69,70 the most relevant examples being reported by Errington et al and Matson et al.71,72 This required first rigorous drying of the starting compounds,24 then addition of one equivalent of sodium-naphthalenide, as a 0.5 M thf solution, to a solution of dehydrated THA-3 in thf at room temperature led to a bluish solution from which the one-electron reduced species depicted as THA-3·(1e), was isolated as a grey solid. Addition of two equivalents of sodium-naphthalenide to a solution of THA3·(1e) in thf led to a deep blue solution. The dark blue twoelectron reduced compound THA-3·(2e), could be also isolated (Scheme 2).

The apparent discrepancy originates in the coordination of the vanadium center. In a typical vanadium substituted polyoxotungstate with a generic formula {XVVWVIOm}n-, the orbital accommodating the electron is of dxy type since the metal is in a distorted octahedral environment.53,63 Conversely, in the present hybrid polyoxotungstate, the VV site has a pseudotetrahedral coordination. Directly related to this fact, we were able to locate two structures with different symmetry at the O3V=O moiety for the one-electron reduced species of anion 3, s-3·(1e) and d-3·(1e) (s- stems for symmetric, and d- for distorted). If the local C3v symmetry of the O3V=O moiety is preserved, s-3·(1e), the electron is (de)localized among the tungsten centers as it can be anticipated from the two degenerated LUMOs of the fully oxidized anion. However, if the distortion of the O3V=O moiety is permitted a second minimum lower in energy can be found, d-3·(1e), which localizes the extra electron on the vanadium center as shown in Figure 6. This distortion provokes a splitting of the d(V) orbitals degenerated in the symmetric structure, destabilizing one of them and stabilizing the other one below the d(W) orbitals (see discussion part), and thus, allowing the reduction to be V centered. Spin density representations for the two structures computed for 3·(1e) are given in Figure 6.

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Scheme 2. General scheme for the preparation of the one-electron and the two-electron reduced species THA-3·(1e) and THA3·(2e), respectively.

spectroelectrochemical study. The magnetic susceptibility of complex 3·(1e) was measured in the temperature range 1.9 – 300 K and the χMT product from 1.9 to 120 K is shown in Figure S12. Above 120 K the reliability on the measured magnetization as a function of the time was not satisfying enough because of a large diamagnetic contribution. The temperature dependence of the χMT product for 3·(1e) matches with a d1-V(IV) center being nearly constant throughout the whole temperature range 2 – 120 K. The decrease observed below 10 K can probably be attributed to the presence of small intermolecular interactions.

Raman Spectroscopy. Isolated vanadium-oxo groups deposited onto large-surface area oxides is an example of single-site heterogeneous catalysts that has been studied for long time by Raman spectroscopy.73,74,43,44,3 In such systems, three bands have been generally assigned to the terminal {V=O} stretch at 1064(sh), 1033 and 923 cm-1 and three others have been observed only by resonance enhancement. Recent investigations by Stiegman et al.75 indicated that the two former bands are mainly the result of interfacial Si-O-V stretches (ν3 mode of a O3V=O moiety),76 whereas only the weak band at 923 cm-1 is dominated by the terminal V=O stretch. The Raman spectra of the silanol-decorated POMs, collected off-resonance at 458 nm, are dominated by two W-O specific modes at 970 and 991 cm-1. Most apparent in the Raman spectrum of the starting compound 1 is an intense band centered at 953 cm-1, which could tentatively be assigned to the Si– OH stretches (observed at ∼977 cm-1 in pure silica). Upon coordination of the {V=O} moiety, the latter band disappeared and a new and typical band associated to the presence of vanadium oxo group is observable at 1018 cm-1 (Figure S8). This band, of low intensity, is rather broad and the assignment is most probably consistent with an overlapping of two contributions, which should correspond to the previously observed modes at 1033 and 1064 cm1 73 . The band at 1018 cm-1 is also similar to the one observed in analogous homogeneous system [{(tBuO)3SiO}3VO] (1038 cm1 77 ). In the one-electron reduced compound 3·(1e), the mode at 1018 cm-1 is only slightly shifted to the low wavenumbers and is enlarged. This is in line with the distortion of the O3V=O moiety and the filling by one electron of a d(V) orbital with partial antibonding d(V)-p(O) character (see Figure 8 and discussion part) which consequently affects the Si–O–V stretching modes. Concerning the W-O specific modes, the reduction by one electron of compound 3 has only a light effect (enlargement of the band at 990 cm-1), whereas the spectrum of the polyoxometallic framework is more affected by the second electron reduction (shift of about 10 cm-1). EPR and magnetic analysis. X-Band CW-EPR experiments and magnetic susceptibility analyses were perform to probe the electronic structure of the various reduced species. The 10 K EPR spectrum of 3·(1e) (Figure 7, top) showed two sets of octet hyperfine structure typical of the coupling of an axial S= ½ spin system with the vanadium I= 7/2 nuclear spin. Simulation of the spectrum (Figure S9) led to classical g-values (gperp=1.975; gpar=1.900) and hyperfine coupling constants (Aperp= 180 MHz, Apar=525 MHz) for a d1-Vanadium(IV) ion,78,79 in agreement with the proposed vanadium centered first reduction wave in the

Figure 7. X-band EPR spectrum at 10 K of complexes a) THA-3·(1e), b) (THA)3Na[SbW9O33(tBuSiO)3VO], THA-1·(1e) and c) (THA)3Na[SbW9O33(tBuSiOH)3], (THA)3Na2[SbW9O33(tBuSiO)3VO], THA-3·(2e).

For comparison, we also prepared the monoreduced (THA)3Na[SbW9O33(tBuSiOH)3] THA-1·(1e) by addition of one

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Journal of the American Chemical Society vanadium(III) entity, as shown by the formation of compounds [SbW9O33(tBuSiO)3VO]3- (3) and [SbW9O33(tBuSiO)3V(thf)]3(2), respectively. The synthesis of both compounds is particularly relevant to the context of partial oxidation of alcohol. Indeed, the reaction of dioxygen at the V(III) metal center in 2, affording the V(V)-oxo derivative 3, models the adsorption of dioxygen at a vacant reduced vanadium atom in vanadia lattice, a mechanism step that is often reported and well accepted.15 In our specific case, the re-oxidation step in solution most probably proceeds through the formation of a bimetallic peroxide-species, whereas in the solidstate migration of a O atom through the surface is proposed to occur. As POM acts as an electron acceptor lattice, as TiO2 could do in VOx/TiO2 metal-oxides, the question of an inner-sphere electron transfer to the POM framework in compound 2 may naturally arise. The question was answered by analyzing the role of the apical ligand coordination in tuning the overall ligand field. As thf is a σ-donor ligand it does not destabilize V orbitals as much as a π-donor oxo ligand does. As a consequence, compound 2 possesses two degenerated low-lying singly occupied molecular orbitals (triplet ground state) centered on the vanadium atom whereas in compound 3 the lowest unoccupied molecular orbitals are delocalized over tungsten atoms. This notwithstanding, EPR and magnetic measurements evidenced that the reduction occurred at the vanadium center, thus leading to a vanadium(IV)-oxo species.

equivalent of sodium-naphthalenide, as a 0.5 M thf solution, to a solution of dehydrated THA-1 in thf at room temperature. The 10 K EPR spectrum of the resulting powder displays an axial EPR spectrum with gpar=1.855 and gperp= 1.795 (Figure 7, middle, and Figure S10). The latter spectrum is reminiscent of those of monoreduced polyoxotungstates described in the literature.80,81 Moreover, it is known for these mixed-valent blue POMs that while the electron is trapped on one W-site at low temperature, a thermally activated electron hopping can occur at higher temperatures, leading to a broadening of the EPR spectra. Similarly, upon raising the temperature, the EPR spectrum of THA-1·(1e) is modified toward a more rhombic system (Figure S10), strongly suggesting that THA-1·(1e) also belongs to class II of Robin & Day's classification. Interestingly, the 10 K EPR spectrum of the doubly reduced 3·(2e) (Figure 7, bottom) appears to be mostly similar to that of 3·(1e), except for the 360-380 mT region where new features appear. This spectrum could be simulated as the sum of the spectrum of 3·(1e) with that of a second axial S= ½ spin system (g2par=1.87, g2perp=1.80), clearly originating from a W-based reduction process. Upon raising the temperature, the latter showed the same type of broadening as THA-1·(1e), while the Vanadium (IV) features remain identical (Figure S11). Thus, the doubly reduced species can best be described as a d1-V(IV) / d1-W(V) system. Finally, as the EPR spectrum appears as the sum of the individual components, it strongly suggests no or little magnetic exchange between the V(IV) and W(V) centers, that is in line with the calculated spin density distribution in 3·(2e) (Figure 6). This latter point was also evidenced by the temperature dependence of the χMT product of 3·(2e), which is again nearly constant throughout the temperature range 2 – 200 K with a plateau at χMT= 0.46 cm3.K.mol-1 (Figure S12, bottom). The fact that the measured value is lower than the expected one (χMT= 0.67 cm3.K.mol-1) could be attributed to the presence of a fraction of the one-electron reduced species 3·(1e) among the sample of 3·(2e) probably due to partial reoxidation during the sample preparation.

Distortion at the VO4 entity. As revealed by the calculations, the peculiarity of complex 3 is its ability to undergo a distortion that provokes a splitting of the degenerated d(V) orbitals, destabilizing one of them and stabilizing the other one below the d(W) orbitals (Figure 8). This allows for the reduction of the V center. This splitting was ascribed to releasing or increasing the antibonding πinteraction between d(W) and p(O) orbitals upon distortion, which stabilizes or destabilizes the respective molecular orbitals. Consequently, the spin density distributions in O3V=O moiety computed for anion d-3·(1e) and d-3·(2e) (Figure 6) look like the shape of the occupied V orbital. In addition, this distortion explains the decrease in the first reduction potential of 1 by incorporating the vanadium fragment to yield 3. Otherwise, (as reported in Table S1) the energy level associated to the LUMO of the non-distorted C3v species is too high to be filled at such low potentials as those presented in Table 1. Here, it is important to mention that in both species 1 and the symmetric isomer of 3, the 1e-reduced species accommodate the extra electron in the same region of the polyoxotungstate framework (Figure S13). Nevertheless, species 1 permits a higher contribution of the equatorial tungsten centers to the orbital in which the electron is placed. This fact was ascribed to a lower geometric strain due to the lack of the VO fragment.

3 – DISCUSSION This report deals with the particular redox features of a vanadium atom anchored to the non-innocent ligand [SbW9O33(tBuSiOH)3]3-. This platform exhibits a number of significant peculiarities that make it quite unique among the wellestablished ligand systems that are capable of mimicking a heterogeneous siloxide environment, e.g., polyhedral oligomeric silsesquioxanes,16,17,82 calix[n]arenes,83,84 and heteropolyanions.85,86 First, the set of silanol moieties is geometrically pre-organized by the threefold symmetric scaffold [SbW9O33]9- constraining the metal ion in a trigonal environment. Second, the bulky tert-butyl groups at the silicon atoms form a steric protection that prevents the formation of oligomers and allows creating a site-isolated model. Such preorganized environment is quite uncommon in coordination chemistry and very difficult to design by organic chemistry.87 Third, the tungstic framework in [SbW9O33(tBuSiO)3]6- is a multi-electron acceptor that can reversibly store and release a large number of electrons under minor structural reorganization. Role of the apical ligand. The macrocyclic environment provided by the platform [SbW9O33(tBuSiOH)3]3- is able to accommodate a vanadium(V)-oxo but also a low-valent d2-

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In this report, we have described the ability of the silanol decorated polyoxotungstate [SbW9O33(tBuSiOH)3]3− to stabilize a low-valent d2-vanadium(III) ion in a trigonal coordination environment. Indeed, [SbW9O33(tBuSiO)3V(thf)]3− (2) proves to be a reliable model for a reduced vanadium atom dispersed on silica or anatase surfaces, [(≡M–O)3VIII(OH2)] (M= Si or Ti). Such V(III) entity represents a putative intermediate in a Mars-van Krevelen type of mechanism for partial oxidation of light alcohols. When exposed to air, compound THA-2 is readily oxidized to the V(V)-oxo derivative [SbW9O33(tBuSiO)3VO]3− (3). Beyond the structural model aspect, one fascinating facet of the polyoxotungstate-based ligand [SbW9O33(tBuSiOH)3]3− is its ability to act as a multi-electron acceptor. In compound [SbW9O33(tBuSiO)3V(thf)]3− the d2–electrons are localized in degenerated d(V) orbitals whereas in the electronically analogous bi-reduced [SbW9O33(tBuSiO)3VO]5− the two electrons are completely decoupled, one being localized on d(V) orbital and the second delocalized on the polyoxotungstic framework. Our body of experimental and theoretical results evidences two critical points in this scenario: i) the role of the apical ligand coordination, i.e. thf (σ-donor) vs. oxo (π-donor), in tuning the destabilization of the low-lying d(V) orbitals, and ii) a geometrical distortion of the O3VO moiety that provokes a splitting of the degenerated orbitals and the stabilization of one d(V) orbital in compound 3. These results emphasize the potency of our molecular model for understanding the active surface sites, specific intermediates or chemical pathways in related heterogeneous systems. In this context, it is worth noting that [SbW9O33(tBuSiOH)3]3− (1) not only reproduces the chemical environment but also the exact geometry found in β-crystoballite, considered as the best model for silica.

Figure 8. Schematic representation of the molecular orbital diagram showing the effect of the distortion of the O3V=O moiety in vanadiumoxo derivatives. Red dashed lines emphasize antibonding interactions.

The subsequent reduction of species 3·(1e) leads to the 2ereduced anion 3·(2e). In concordance with the UV-vis, EPR and magnetic measurements, calculations describe the electronic structure of 3·(2e) as a triplet ground state: one electron is localized in the V center and the other one is delocalized over W centers with higher contribution of the distal ones, as the spin density distribution shows (Figure 6). Calculations also confirmed that a species with two electrons delocalized on the tungstate framework is clearly destabilized (by more than 10 kcal·mol-1) with respect to the former one. The magnetic properties of bi-reduced POMs have attracted the attention of chemists and physicists since the eighties.80,88 Baker and co-workers analyzed in detail the magnetic properties of mixedvalence heteropoly blues showing that the diamagnetism of 2ereduced POMs can be attributed to a ring current of the paired delocalized “blue” electron(s) circulating.89,90 It is worth mentioning that reduced fullerenes with an even number of electrons also exhibit an important antiferromagnetic coupling.91,92,93 Detailed theoretical analysis94,95 has demonstrated that electron hopping integrals between neighboring centers and electron/electron repulsion are the main factors that determine the energy differences between singlet and triplet in 2e-reduced POMs, in which the singlet can be stabilized with respect to the triplet up to more than 200 meV.96 On the contrary, in the 2-electronreduced K2Na6[GeV14O40], the two electrons are “trapped” on each side of the POM framework.68,97 Thus, the electron distribution observed for the bi-reduced anion 3·(2e) follows a similar pattern than that of GeV14 or anions with localized paramagnetic centers.

EXPERIMENTAL SECTION General procedures. All reactions were carried out in an inert argon atmosphere using standard Schlenk techniques. Tetrahydrofuran and acetonitrile were dried using solvent purification systems or by distillation under nitrogen from appropriate drying agents. All solvents, including deuterated acetonitrile, were then degassed by several freeze-pump-thaw cycles and stored over activated molecular sieves. (iPrO)3VO was purchased from Aldrich and used as received. [V(Mes)3·thf]100 and the silanol derivative (n-Hex4N)3[(α-B-SbW9O33)(tBuSiOH)3] (THA–1) were prepared according to previously reported procedures.25 1H NMR, 31 P{1H} NMR and 51V NMR spectra were obtained at room temperature in 5 mm o.d. tubes on a Bruker AvanceII 300 spectrometer equipped with a QNP probehead or on a Bruker AvanceIII 600 spectrometer equipped with a BBFO probehead. The Raman spectra were recorded using a Horiba Jobin Yvon LabRam HR 800 spectrometer equipped with edge filters, a 1800 lines/mm grating and a Peltier cooled CCD detector. The excitation wavelength was the 458 nm line of an Ar+ Laser (Innova 90C, Coherent Inc.). The laser power at the sample was around 500 µW. Raman scattering was collected via an Olympus microscope equipped with a long working distance 50× objective, allowing a laser spot size of about 5 µm. The time of exposition was about three times 300 s to improve the signal to noise ratio. Elemental analyses were performed by the « Service de microanalyses » from the ICSN–CNRS, Gif-sur-Yvette, France. X-band EPR spectra were recorded under non-saturating conditions on a Bruker Elexsys 500 spectrometer equipped with an Oxford Instrument continuous-flow liquid helium cryostat and a temperature control system. Simulations of EPR spectra were performed using the Easy Spin Pepper program.101 Magnetic susceptibility was measured on a Quantum Design MPMS5SQUID susceptometer. Corrections were applied for diamagnetism calculated from Pascal constants or measured on diamagnetic THA-3 compound.

This notwithstanding, compound 3·(2e) represents a unique case of a bi-reduced heteropolyanion derivative with decoupled d1V(IV) and d1-W(V). In our system, the siloxide shell plays a primary role, forcing the V(IV)-oxo moiety and the one electron reduced polyoxotungstate backbone to be completely decoupled. This scenario differs totally from conventional transition metalsubstituted polyoxometalates and the bicapped [PMo12O40(VO)2]n98,99 in which magnetic coupling between the extra metallic moieties is mediated by the highly reduced polyoxomolybdate scaffold. 4 – CONCLUSION

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Journal of the American Chemical Society CH2CH3 THA). 51V NMR (CD3CN, 157.9 MHz, 300 K) δ − 730 (ω½ = 200 Hz).

Spectroelectrochemical studies. Cyclic voltammetry studies were carried out on 10-3 M deoxygenated solution of the desired compound in CH3CN with TBAPF6 as supporting electrolyte (0.1 M) in a standard three- electrode cell, composed of a glassy carbon working electrode, a platinum counter electrode, and a saturated calomel reference electrode (SCE). Spectroelectrochemical studies were conducted in a cell using a bed of mercury as working electrode and equipped with a UV-Vis immersion probe. UV-visible spectra were recorded on a Jasco V-670 equipped with an ETC-717 Peltier module. DFT calculations. The DFT analysis of the electronic structures was carried out with Gaussian 09 rev. A02 package.102 All the calculations, including geometry optimizations and frequency calculations were performed at the B3LYP level of theory,103,104,105 including the effects of the solvent (THF; ε = 7.43) by means of the IEF-PCM continuum solvent model,106 as implemented in Gaussian 09. LANL2DZ pseudopotential107 was used to describe V, W and Sb centers, while double ζ Pople’s type 631G(d,p) basis set was used for the remaining atoms.108,109,110 Such methodology has been successfully used by some of us to provide an accurate description of electronic structures and redox properties of both fully oxidized and reduced polyoxometalates.63 A data set collection of computational results is available in the ioChem-BD repository111 and can be accessed via https://doi.org/10.19061/iochem-bd-2-29. Dehydration of THA–1. Samples of THA3[SbW9O33(tBuSiOH)3] were dehydrated by heating at 210°C for 3 h under dynamic vacuum (10-3 mbar) and stored under inert atmosphere before use. Complete dehydration may be controlled by thermo gravimetric profiles that show no mass loss below 270°C. NMR analyses after heating treatment indicated that the structures were not modified. Synthesis of THA3[SbW9O33(tBuSiO)3V(thf)] (THA-2). Dehydrated THA3[SbW9O33(tBuSiOH)3] (477 mg, 0.130 mmol) was placed in a Schlenk tube and, under argon atmosphere, dried THF (10 mL) was added to give a colorless solution. At room temperature, [V(Mes)3·thf] (75 mg, 0.16 mmol) was then added and the solution turned to dark blue grey. The solution was stirred overnight, then concentrated to 5 mL and then layered with diethylether (ca. 10 mL). The resulting deep colored solid was finally collected and dried under vacuo, yielded ca. 60 % of THA-2. In all attempts, a very small amount of mesitylene was detected in the final compound by NMR analysis. χMT = 1.02 cm3.K.mol-1. UV-vis (CH3CN): λ= 630 nm (ε= 3.2 × 103 M-1 cm-1). THA3[SbW9O33(tBuSiO)3V(thf)] (THA-2) could also be generated from THA-3: To a solution of dehydrated THA3[SbW9O33(tBuSiO)3VO] (49 mg, 0.013 mmol) in CD3CN (2 mL) and under argon was added [V(Mes)3·thf] (1.2 eq., 0.4 mL of a 0.04 M THF solution). NMR analysis of the reaction mixture after 2h at room temperature reveals the complete disappearance of THA-3 and the formation of [(Mes)3VO]: 1H NMR (CD3CN, 400 MHz, 300 K) δ, ppm: 6.67 (s, 6H, ArH Mes), 2.55 (s, 18H, o-Me from Mes), 2.19 (s, 9H, p-Me from Mes). 51V NMR (CD3CN, 157.9 MHz, 300 K) δ, ppm: + 877 (ω½ = 85 Hz). Oxidation of THA-2 to THA-3: A solution of THA-2 freshly prepared under argon in dried CD3CN was then allowed to stand under air to rapidly give a yellowish solution. NMR analyses (51V and 1H) revealed the quantitative formation of THA-3. Alternatively, oxidation of THA-2 occurs in the presence of styrene oxide under argon at 343 K: monitoring the reaction by NMR allowed observing the formation of THA-3 and also the release of styrene. Synthesis of THA3[SbW9O33(tBuSiO)3VO] (THA-3). Dehydrated THA3[SbW9O33(tBuSiOH)3] (2.60 g, 0.71 mmol) was placed in a Schlenk tube and, under argon atmosphere, dried THF (30 mL) was added. At room temperature VO(OiPr)3 (251 µL, 1.07 mmol) was then added and the resulting solution was stirred overnight. The volatiles were removed under vacuum and the solid was recrystallized from a solution in THF layered with diethyl ether. Crystals grew after two days standing at room temperature. The crystals were collected, washed with diethyl ether and dried under vacuum (1.60 g, 60 %). Elemental analysis calcd (%) for C84H183N3O37SbSi3VW9 (3738.9): C 26.98, H 4.93, N 1.12; found: C 26.50, H 4.78, N 1.06. 1H NMR (CD3CN, 300.13 MHz, 300 K) δ, ppm: 3.11 (m, 24 H, NCH2 THA), 1.63 (m, 24 H, NCH2CH2 THA), 1.35 (m; 72 H, (CH2)3CH3 THA), 1.25 (s, 27 H, CH3 tBuSi), 0.91 (t, 2JH,H = 6.9 Hz; 36 H,

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXX Interatomic distances in anions 2, 3 and β-crystoballite, UV-vis spectrum and computed UV-vis spectrum of THA-2, 51V NMR spectra of compounds THA-2, THA-2 under air and THA-3, 1H NMR spectra of THA-1 and THA-3, 51V NMR spectrum of [(mes)3VO], representations of the electronic structures computed for anion 3·(2e), computed UV-vis spectra for compounds THA-3, THA-3·(1e) and THA-3·(2e), Raman spectra of anions compounds THA-1, THA-3, THA-3·(1e) and THA-3·(2e), experimental and simulated EPR spectra of THA-1·(1e), THA-3·(1e) and THA-3·(2e) recorded at different temperatures, magnetic measurements for isolated compounds THA-3·(1e) and THA-3·(2e), comparison of the computed spin densities and computed half-wave potentials for anions 1·(1e) and s-3·(1e) (PDF).

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected]

ORCID Anna Proust: 0000-0002-0903-6507 Geoffroy Guillemot: 0000-0002-2711-8514 Josep M. Poblet: 0000-0002-4533-0623 Albert Solé-Daura: 0000-0002-3781-3107 Jorge J. Carbó: 0000-0002-3945-6721 # Present address: Leibniz-Institut für Molekular Pharmakologie, Robert-Rössle Strasse 10, 13125 Berlin, Germany.

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS T.Z. acknowledges the Sorbonne Universités – China Scholarship Council program for a Ph.D. grant. S.B. and A.P. are thankful to the RENARD network (IR-RPE CNRS 3443) and more specifically to Dr. J.-L. Cantin from INSP (UMR 7588, CNRS - Sorbonne Université) for access to the EPR spectrometer. G.G. also acknowledges Pr. Pierre Gouzerh and Richard Villanneau for fruitful discussions. J.M.P. thanks the Spanish Ministry of Science (CTQ2017-87269-P and UNRV15EE-3935), the Generalitat de Catalunya (2017SGR629) and the URV for generous support. J.M.P. also thanks the ICREA foundation for an ICREA ACADEMIA award.

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Modeling the Oxygen Vacancy at a Molecular Vanadium(III) Silica

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