Neutral and Cationic V^IV/V^V Mixed-Valence Alkoxo-polyoxovanadium Clusters [V₆O₇(OR)₁₂]ⁿ⁺ (R = −CH₃, −C₂H₅):  Structural, Cyclovoltammetric and IR-Spectroscopic Investigations on Mixed Valency in a Hexanuclear Core

The alkoxo-polyoxovanadium clusters [V₆O₇(OR)₁₂] (R = −CH₃, −C₂H₅) are fully alkylated polyoxometalate derivatives comprising a hexavanadate core with the vanadium ions organized in an octahedral fashion, a classic isopolyoxometalate structure (Lindqvist) which as an entity is not known for vanadium. The clusters are highly redox-active compounds, displaying a large number of thermodynamically stable redox isomers of which the chemical syntheses and structural characterization of the neutral and cationic V^IV/V^V mixed-valence species [V^IV_(4-n)V^V_(2+n)O₇(OR)₁₂]ⁿ⁺[SbCl₆]_n (R = −CH₃, n = 0, 1; R = −C₂H₅, n = 0, 1, 2) are presented here. Neutral and positively charged clusters remain exceptional in the field of polyoxometalate chemistry. Results obtained from cyclic voltammetry, infrared spectroscopy, and from valence sum calculations conducted on X-ray structural data classify these clusters as class II mixed-valence compounds. Their highly symmetrical molecular structures make them particularly interesting as model compounds for the investigation of intervalence charge transfer and electron delocalization in the hexanuclear core. Furthermore, the large number of isostructural redox isomers affords a high variability in d-electron content. Accordingly, a dependency could clearly be established between the extent of electron delocalization and the V^IV/V^V ratio in a cluster species. A further interesting observation concerns the neutral ethoxo compound [V^IV₄V^V₂O₇(OC₂H₅)₁₂] (3) which exhibits a crystallographic phase transition accompanied by the conversion from a structure at 173 K with fully localized valencies to a room-temperature modification displaying complete d-electron delocalization.