Communication Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Oxygen-Atom Vacancy Formation at Polyoxovanadate Clusters: Homogeneous Models for Reducible Metal Oxides Brittney E. Petel, William W. Brennessel, and Ellen M. Matson* Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
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ABSTRACT: We report the first example of oxygenatom vacancy formation at the surface of a polyoxometalate, highlighting the ability of a polyoxovanadatealkoxide cluster, [V6O7(OCH3)12]1−, to function as a homogeneous model for reducible metal oxides. The removal of an oxide ion from [V6O7(OCH3)12]1− results in the formation of a reactive vanadium(III) cation within t he m ultimetallic fram ework. Generatio n of [V6O6(OCH3)12]1− is confirmed by 1H NMR, infrared and electronic absorption spectroscopies, as well as electrospray ionization mass spectrometry. The consequences of oxygen atom removal on the electrochemical profile of the assembly are assessed, revealing that stabilization of the reduced cluster is achieved through delocalized electron density. The oxygen-atom vacancy permits activation of O2, demonstrating the ability of polyoxovanadate-alkoxide clusters to serve as both structural and functional models of reducible metal oxides.
Figure 1. Polyoxovanadate-alkoxide clusters as models for oxygenatom vacancy formation and reactivity at the surface of reducible metal oxides.
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educible metal oxides (RMOs) catalyze the multielectron conversion of energy-poor, gaseous substrates to chemical fuels.1 Illustrative examples include the reduction of CO2 to CO and CH3OH, which are chemical processes of fundamental importance for emission abatement.2 The remarkable reactivity of RMOs in heterogeneous catalysis has been attributed to their ability to generate coordinatively unsaturated cationic sites, through the removal of surface oxygen atoms (Figure 1).3−9 These surface-exposed sites of reduction have been predicted, by theory, to play an integral role in the activation of small, gaseous molecules. Substrates are proposed to bind to oxygen-deficient lattice sites, enabling the formation of deoxygenated products through chemical reactions with reduced metal ions. Despite progress in the development of RMOs as catalysts for multielectron transformations, robust experimental support for structural and electronic factors that contribute to the formation and reactivity of anionic vacancies is lacking. This is principally due to variations in surface properties of bulk materials via oxygen-atom exchange and surface reconstruction, rendering in situ spectroscopic resolution of single-atom vacancies challenging.4,10,11 These obstacles can be circumvented through the use of homogeneous molecular model complexes.12−16 Discrete metal-oxide clusters have fewer sites available for reduction, allowing for unambiguous determination of oxygen-atom removal. Additionally, the solubility of molecular model complexes provides access to alternate © XXXX American Chemical Society
spectroscopic handles for probing the structural and electronic consequences of defect formation on the chemical reactivity of RMOs. The structural composition and tunable magnetic, photochemical, and electronic properties of polyoxovanadate clusters make them ideal for modeling bulk metal-oxide materials.17−23 In particular, polyoxovanadate-alkoxide (POV-alkoxide) clusters are distinctly qualified to serve as homogeneous surrogates for RMOs (Figure 1). With six terminal vanadyl moieties, bridged by methoxide ligands, the structure of the hexavanadate cluster, [V6O7(OCH3)12]1− (1-V6O7−), resembles the surface of bulk metal-oxide materials.24 Additionally, POV-alkoxides have rich electrochemical properties, owing to accessible VIV/VV redox couples, and a large degree of electronic delocalization across the cluster core.25−27 In analogy, RMOs feature metal ions that are capable of fluctuating between multiple oxidation states. Their energetically accessible d-orbitals accommodate added electron density, facilitating the removal of surface oxide ions. Additionally, the semiconducting properties of RMOs afford a delocalized electronic structure that is effectively modeled by the mixed-valent hexavanadate cluster.28 Despite the analogous properties of RMOs and POV-alkoxides, generation of oxygenReceived: May 21, 2018 Published: June 26, 2018 A
DOI: 10.1021/jacs.8b05298 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
reduction (Figure 3). The loss of intervalence charge transfer (IVCT) bands associated with the mixed-valent vanadium ions
atom vacancies on the surface of these molecular, metal-oxide clusters has not been reported. The similarities between the POV-alkoxide cluster and RMOs prompted investigation into the synthetic accessibility of oxygen-atom vacancies with 1-V6O7−. Exposure of the hexavanadate cluster to an equivalent of VIII(Mes)3 (Mes = 2,4,6-trimethylbenzene) results in an immediate color change from green to brown-red (Scheme 1). Characterization of the Scheme 1. Synthesis of 2-V6O6−
Figure 3. Electronic absorption spectra of 1-V6O7−, 2-V6O6 and 3V6O6OTf.
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crude reaction mixture by H NMR spectroscopy reveals formation of the expected diamagnetic byproduct, [OVV(Mes)3], consistent with removal of a single oxygenatom from the POV-alkoxide cluster (Figure S1).29,30 Analysis of the paramagnetic region of the 1H NMR spectrum shows consumption of 1-V6O7− (δ = 23.2 ppm), and formation of a new species, with signals located at +25.3, +23.9 and −15.5 ppm (Figure 2, Figure S2). The observation of three
(VV/VIV) of complex 1-V6O7−, coupled with the observation of a weak transition at 526 nm (ε = 473 M−1 cm−1), are consistent with formation of a reduced POV-alkoxide scaffold.27,31 To assess the electrochemical effect of oxygen atom removal on the hexavanadium cluster, the redox profile of 2-V6O6− was probed using cyclic voltammetry (CV) (Figure S5). The CV of 2-V6O6− in dichloromethane reveals three evenly spaced, quasireversible redox events (E1/2 = −0.502, + 0.031 and +0.716 V vs Ag/Ag+), assigned to sequential oxidation of vanadyl ions. A fourth, irreversible redox event is observed at Ep = +1.365 V, suggesting vacancy formation results in oxidative instability of the POV core at high potentials. The evenly spaced redox events of 2-V6O6− (ΔE1/2 = 0.533 to 0.685 V) suggest retention of the delocalized electron structure upon vacancy formation. Though polyoxometalates have been touted as homogeneous models for bulk metal oxides, investigations into the parallel reactivity of the molecules and materials have focused primarily on the photophysical and electronic consequences of heterometal incorporation (“dopants”).32−34 Vacancy formation in these metal-oxide clusters has traditionally been relegated to the removal of an entire [MOx]n− subunit, resulting in the generation of a lacunary structure.35,36 Accessing an oxygen atom vacancy in the homometallic POV-alkoxide cluster represents the first example of the cleavage of a single MO bond at the surface of a polyoxometalate. Discovery of this type of chemical reactivity for metal-oxide clusters opens the possibility of using polyoxometalates as molecular models for in situ surface defect formation during catalysis with RMOs. Attempts to crystallize complex 2-V6O6− were thwarted by the long-term, solution stability of the reduced cluster under conditions evaluated for crystallization. We postulated that saturating the coordination sphere of the apical vanadium ion in 2-V6O6− would afford a stable analog of the complex. Additionally, a large anion would prevent isopositional disorder within the crystal structure. Following previously reported strategies for purification of a site-differentiated, heterometallic POV-alkoxide cluster,37 oxidation of 2-V6O6− was performed with AgOTf (OTf = trifluoromethylsulfonate).
Figure 2. 1H NMR spectra of 1-V6O7− and 2-V6O6− (CD3CN).
paramagnetically shifted resonances is consistent with vacancy formation, as the removal of an oxygen atom results in reduction of the symmetry of the cluster (Oh → C4v). A prominent signal in the electrospray ionization mass spectrum (ESI-MS) corresponds to the reduced cluster [V6O6(OCH3)12]1− (2-V6O6−) plus methanol (Figure S3, m/ z = 806 amu), further supporting defect generation. The infrared (IR) spectrum of 2-V6O6− contains features indicative of the structural resilience of the hexavandadium core under reducing conditions (Figure S4). Bands corresponding to ν(ObCH3) (1047 cm−1; Ob = bridging oxo) and ν(VOt) (951 cm−1; Ot = terminal oxo) confirm that no degradation of the multinuclear assembly occurs upon removal of a terminal oxygen atom. Notably, small shifts in energies of both bands from that of 1-V6O7− (ν(ObCH3) = 1047 cm−1, ν(VOt) = 953 cm−1) are observed, denoting a net, twoelectron reduction of the cluster core.25 The electronic absorption spectrum of 2-V6O6− provides further evidence of B
DOI: 10.1021/jacs.8b05298 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Journal of the American Chemical Society Gratifyingly, addition of 3 equiv AgOTf to 2-V6O6− results in the generation of the desired product, [V6O6(OCH3)12]OTf (3-V6O6OTf), in good yield (81%, Scheme 2), as confirmed by 1 H NMR and ESI-MS (Figures S6−S7).
Evaluation of the structural perturbations of 3-V6O6OTf shows a resemblance to predicted VO bond distances following vacancy formation in the bulk material, V2O5 (Figure 4b).38,39 Theoretical models of two-layered V2O5 substructures predict that upon cleavage of a VVOt bond, transient formation of a VIII ion is observed.6 The reduced vanadium cation puckers inward, toward a neighboring, electrophilic VVOt moiety, resulting in the formation of a new VIVO VIV linkage. This behavior is mimicked by the movement of V1 in the structure of 3-V6O6OTf, indicating that Oc plays an important role in charge redistribution upon vacancy formation. Bond valence sum calculations indicate that the reduced vanadium ion retains its trivalent oxidation state upon addition of AgOTf (Table S2).24,40 As a result, two VIV ions, located in the equatorial plane of the cluster, are oxidized to VV. These calculations are consistent with the molar absorptivity of the IVCT bands of complex 3-V6O6OTf in its electronic absorption spectrum (Figure 2).31 Oxidation of the hexavanadium core is further confirmed through IR via shifts in v(V Ot) and v(ObCH3), which denote the presence of two V(V) ions (Figure S9).25,31 Notably, the BVS values calculated for individual vanadium ions within the Lindqvist core deviate, slightly, from assigned valence states, providing further evidence for a delocalized electronic structure.41−43The retention of the electron-rich VIII ion across oxidation states of complex 2-V6O6− in the presence of two VV moieties is surprising, given the large comproportionation constants for the POV-alkoxide cluster (Kc = 5.5 × 109 − 9.4 × 1011, RTln Kc = nF[ΔEp]).31,44−46 The site-differentiated, low-valent vanadium ion embedded within the delocalized, metal-oxide cluster primes the system for multielectron substrate activation. Given the physical and electronic similarities between the POV-alkoxide and RMO surfaces, we hypothesized that the oxygen-atom vacancy in 2-V6O6− would mediate stoichiometric chemical transformations invoked in heterogeneous small molecule activation. The reduction of O2 by heterogeneous catalysts is a key step in chemical transformations of relevance to oxidation catalysis, electrocatalysis, and corrosion.47,48 Exposure of 2-V6O6− to 1 atm of O2 at 50 °C results in the restoration of an oxygen anion to the vacant site in quantitative yield, as confirmed by 1H NMR and ESI-MS (Figures S10 and S11). Formation of 1-V6O7− is consistent with the transfer of two-electrons from the cluster to the substrate. However, to afford OO bond cleavage, four electrons are required, suggesting that O2 reduction proceeds through cooperative substrate activation by two molecules of 2-V6O6− (Scheme 3). To ensure the restored oxygen atom was not a result of cluster degradation, 1 atm of 18O2 was added to 2-V6O6−. ESI-MS reveals a shift in the parent mass of 1-V6O7− by 2 atomic mass units, corresponding to 1-V6O618O− (m/z =
Scheme 2. Synthesis of Complex 3-V6O6OTf
X-ray analysis of 3-V6O6OTf reveals, as expected, a hexavanadium core, with a triflate anion coordinated to a single, site-differentiated vanadium center (Figure 4a, for full
Figure 4. Comparison of (a) molecular and (b) bulk structures of oxidized and reduced models of V2O5.The molecular structures of the hexavandate clusters have hydrogen atoms, the tetrabutylammonium counterion of 1-V6O7−, and the triflate anion of 3-V6O6OTf removed for clarity.
structure and crystal parameters, see Figure S8 and Table S1). Each vanadium cation of 3-V6O6OTf occupies a unique position within the unit cell, providing an opportunity to understand the structural consequences of vacancy formation. Analysis of the bond metrics of complex 3-V6O6OTf reveals striking changes in the VO distances within the cluster core. Upon removal of a surface oxygen atom, the reduced vanadium ion is pulled toward the center of the cluster, indicated by a shortened V1Oc (Oc = central μ6-oxygen atom) distance of 2.08(4) Å (Table S3), as compared to that of 1-V6O7− (VOc (avg) = 2.25 A).24 Further evidence for the movement of the apical vanadium ion is apparent in contractions of V1Ob Vn (n = 2, 3, 4, 5) bond angles, averaging ∼105°.
Scheme 3. Reactivity of 2-V6O6− with O2
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DOI: 10.1021/jacs.8b05298 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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792, Figure S12). A separate control experiment confirms that degradation of 2-V6O6− does not occur under the relevant reaction conditions, indicating substrate is required for the formation of 1-V6O7− (Figure S13). In this paper, we have demonstrated the formation of an oxygen-atom vacancy at the surface of a metal-oxide cluster. This work has direct implications for modeling the chemical reactivity of reducible metal oxides in heterogeneous catalysis with homogeneous, molecular model complexes. Reactivity of the oxygen-deficient cluster toward molecular oxygen is evaluated, revealing the propensity of the surface-defect to facilitate activation of oxygenated gaseous substrates. Ongoing investigations are focused on extending the reactivity of 2V6O6− to other gaseous molecules and chemical contaminants, ultimately determining the mechanism of substrate activation through kinetic analysis.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b05298. Materials, experimental procedures, characterization data for all compounds (1H NMR, ESI-MS, IR, CV, UV−vis) and bond valence sum calculations(PDF) Crystallographic data for 3-V6O6OTf (CIF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
William W. Brennessel: 0000-0001-5461-1825 Ellen M. Matson: 0000-0003-3753-8288 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge Dr. Feng Li and Lauren VanGelder for conducting the preliminary experiments that led to the evolution of this project. The authors also appreciate guidance from Dr. Olaf Nachtigall in performing the bond valence sum calculations. The authors acknowledge generous financial support from the University of Rochester.
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DOI: 10.1021/jacs.8b05298 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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DOI: 10.1021/jacs.8b05298 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
