Oxovanadium(IV) Silsesquioxane Complexes

Abstract

In the context of a potential modeling of reduced oxovanadium species occurring on the surfaces of silica-supported vanadia catalysts in the course of its turnover, the incompletely condensed silsesquioxane H₃<b>^c-pentylT₇</b> was reacted with Cl₄V(THF)₂ (where THF = tetrahydrofuran) in the presence of triethylamine. Precipitation of 3 equiv of HNEt₃Cl seemed to point to the clean formation of [(<b>^c-pentylT₇</b>)(V^IVCl)] (<b>1</b>), which was supported by electron paramagnetic resonance studies performed for the resulting solutions, but further analytical and spectroscopic investigations showed that the processes occurring at that stage are more complex than that and even include the formation of [(<b>^c-pentylT₇</b>)(V^VO)]₂ as a side product. Storage of a red-brown hexane solution of this product mixture reproducibly led to the precipitation of blue crystals belonging to the chloride-free compound [(<b>^c-pentylT₇</b>)₂(V^IVO)₃(THF)₂] (<b>2</b>), as revealed by single-crystal X-ray diffraction. Performing the same reaction in the presence of 2 equiv of pyridine leads to an analogous product, where the THF ligands are replaced by pyridine. Subsequent investigations showed that the terminal oxo ligands at the vanadium centers are, on the one hand, due to the presence of adventitious water; on the other hand, the [<b>^c-pentylT₇</b>]³⁻ ligand also acted as a source of O²⁻. The results of SQUID measurements performed for <b>2</b> can be interpreted in terms of a ferromagnetic coupling between the vanadyl units. Exposing <b>2</b> to a dioxygen atmosphere resulted in its immediate oxidation to yield the V^V complex [(<b>^c-pentylT₇</b>)(V^VO)]₂, which may model a fast reoxidation reaction of oxovanadium(IV) trimers on silica surfaces.