O 2 Activation with a Sterically Encumbered, Oxygen-Deficient Polyoxovanadate-Alkoxide Cluster.

The isolation of the oxygen-deficient, polyoxovanadate-alkoxide (POV-alkoxide) cluster, [ ⁿ Bu ₄ N][V ₆ O ₆ (OMe) ₁₂ (MeCN)], and its subsequent reactivity with oxygen (O ₂ ), 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 O ₂ 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 O ₂ bridged dimeric intermediate, a mechanism which is not viable for O ₂ reduction at solid-state metal oxide surfaces. Here, we report the successful activation and reduction of O ₂ by the calix-functionalized POV-alkoxide cluster, [ ⁿ Bu ₄ N][(calix)V ₆ O ₆ (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 O ₂ 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 O ₂ reduction reaction at oxygen-deficient sites in reduced POV-alkoxide clusters.