Synthesis and characterizations of cyclic octanuclear mixed-valence vanadium(iv,v) clusters with polyoxometalate counterions

Abstract

Hydrothermal reaction of Na₂WO₄, VOSO₄, 2,2′-bpy and H₃PO₄ has afforded in high yield the compound [V^IV₂V^V₆O₁₄(bpy)₈(PO₄)₂][PW₁₁V^VO₄₀](bpy)·12H₂O (1). Compound 1 contains a novel octanuclear mixed valence V(IV,V) cluster, [V^IV₂V^V₆O₁₄(bpy)₈(PO₄)₂]⁴⁺, with [PW₁₁V^VO₄₀]⁴⁻ as counterion. In the vanadium cluster, four V(V) centers are localized and the remaining two V(IV) and two V(V) ions are disordered over four crystallographically equivalent positions. The isostructural compound [V^IV₂V^V₆O₁₄(bpy)₈(PO₄)₂][PMo₁₁V^VO₄₀](bpy)·3H₂O (2) has also been synthesized. Thermodiffractometry experiments indicate that 2 is stable up to 360 °C. Redox activities for both the vanadium and molybdenum centers have been observed by solid-state electrochemical measurements performed on mechanically attached microparticles of 2. Magnetic measurements performed on 1 have shown the occurrence of weak ferromagnetic interactions between the V(IV) centres (J = +0.34 cm⁻¹, H_ex = −JS₁·S₂), and combined with DFT calculations, have allowed to propose a localization of the two V(IV) centers on two of the four equivalent crystallographic sites. Finally high field electron paramagnetic resonance has evidenced the magnetic axial anisotropy of the paramagnetic centers (g_x = g_y = 1.975(3); g_z = 1.939(4)).