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

Abstract

The isolation of the oxygen-deficient, polyoxovanadate-alkoxide (POV-alkoxide) cluster, [ n Bu4N]­[V6O6(OMe)12(MeCN)], and its subsequent reactivity with oxygen (O2), has demonstrated the utility of these assemblies as molecular models for heterogeneous metal oxide catalysts. However, the mechanism through which this cluster activates and reduces O2 to generate the oxygenated species is poorly understood. Currently it is speculated that this POV-alkoxide mediates the four-electron O–O bond cleavage through an O2 bridged dimeric intermediate, a mechanism which is not viable for O2 reduction at solid-state metal oxide surfaces. Here, we report the successful activation and reduction of O2 by the calix-functionalized POV-alkoxide cluster, [ n Bu4N]­[(calix)­V6O6(OMe)8]­(MeCN)] (calix = 4-tert-butylcalix­[4]­arene). The steric hindrance imparted to the open vanadium site by the calix motif eliminates the possibility of cooperative, bimolecular O2 activation, allowing for a comparison of the reactivity of this system with that of the nonfunctionalized POV-alkoxide described previously. Rigorous characterization of the calix-substituted assembly, enabled by its newfound solubility in organic solvent, reveals that the incorporation of the tetradentate aryloxide ligand into the POV-alkoxide scaffold perturbs the electronic communication between the site-differentiated vanadium­(III) ion and the cluster core. Collectively, our results provide insight into the physiochemical factors that are important during the O2 reduction reaction at oxygen-deficient sites in reduced POV-alkoxide clusters.

Isolation of a Side-On V(III)-(η2-O2) through the Intermediacy of a Low-Valent V(II) in a Metal–Organic Framework

“…The reduced framework, 2 , is a rare example of a vanadium­(II)-bearing MOF, and the methodology for its synthesis may be transferrable to other systems. The reactivity of 2 with O 2 to form superoxo-bearing 3 is quite surprising because dioxygen-bound vanadium complexes are typically difficult to isolate in the absence of bulky steric protection to prevent bimolecular reactivity . This difference demonstrates the benefits of site isolation within the heterogeneous MOF support.…”

“…The reactivity of 2 with O 2 to form superoxo-bearing 3 is quite surprising because dioxygen-bound vanadium complexes are typically difficult to isolate in the absence of bulky steric protection to prevent bimolecular reactivity. 39 This difference demonstrates the benefits of site isolation within the heterogeneous MOF support. We anticipate that this methodology of postsynthetic reduction will enable the access of other low-valent, coordinatively unsaturated metal species within MOFs.…”

Synthesis and oxidation catalysis of a difluoride-incorporated polyoxovanadate and isolation of active vanadium alkylperoxo species

“…Recently, Matson's group developed the oxygen-atom vacancy formation and oxygenation system with polyoxovanadate-alkoxide clusters. 27,28 By the reaction with reductant, one of the terminal oxo ligands was removed, and the oxygen-deficient site of V 3+ in the cluster reacted with O 2 to form the oxygenated cluster. During the transformation, two electron reduction and oxidation of the cluster were observed.…”

“…These molecular level findings can act as a molecular model for the bulk reducible metal oxides. 27,28 Polyoxovanadates exhibit structural diversity because of the various coordination environments of tetrahedral VO 4 , square pyramidal VO 5 , and octahedral VO 6 . 29,30 Polyoxovanadates accommodate different types of template anions at the centres of the clusters.…”

Mechanistic insight into rapid oxygen-atom transfer from a calix-functionalized polyoxovanadate

“…15 In particular, we became interested in studying the reactivity of a POV-alkoxide functionalized with a calix ligand (calix = 4-tert-butylcalix [4]arene), as these clusters have been demonstrated to possess electronic structures influenced by the polydentate ligand. 16 To access the calixfunctionalized analogue to the [V 6 O 7 (OR) 12 ] cluster, oxidation of the previously reported assembly, [(calix)V 6 O 6 (OMe) 8 L] (1, L = MeCN; structural characterization obtained by single crystal X-ray diffraction, see Fig. S1 and Tables S1, S2, ESI †), with iodosyl benzene was performed (Scheme 1).…”