Synthetically persistent, self assembled [VIV2VV4] polyoxovanadates: Facile synthesis, structure and magnetic analysis

Abstract

Slow diffusion in a H-tube at room temperature of a methanolic solution of [VO(acac)₂] (Hacac = acetylacetone) and 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy) into an aqueous solution of sodium pyrophosphate (Na₄P₂O₇) resulted in the serendipitous formation of X-ray quality crystals of mixed-valent, hexameric oxovanadates of general formula [V₆O₁₂(OCH₃)₄(L)₄]·solv [L = 1,10-phenanthroline (phen) for 1 · 2CH₃OH · 4H₂O (1a), and 2,2′-bipyridine (bipy) for 2 · 4H₂O (2a)]. These were characterized by single-crystal X-ray diffraction, IR, elemental and thermogravimetric analysis (TGA). A facile, rationalized synthetic route for the isolation of 1a and 2a could be established following structural determination, involving NaOH in place of Na₄P₂O₇ as pH modulator. The use of distilled water (pH 7) as methanolic co-solvent also resulted in crystallization of the two complexes, proving the presence of a base in the reaction scheme is not vital, with slightly pH-depended yields noted for 2a only. A survey of the literature revealed the occurrence of several other procedures, from classical methods to hydrothermal routes, leading to different solvates of 1, the crystal structure of 2 being unreported in any form to date. The precise nature of the molecular assembly in these type of hybrid organic-inorganic poly-vanadates is contradictory in published reports. On the basis of newly acquired high resolution crystal data and supported by magnetic investigation of the samples, we propose herein a formulation as [(V^IVO)₂(V^VO₂)₄(μ₃–O)₂(μ–OCH₃)₄(L)₄], with two oxovanadyl(IV) and four dioxovanadyl(V) units per molecule. A net ferromagnetic coupling between the two isolated V^IV metal centers was measured with literature-consistent J values of +16.1(1) and +19.7(1) cm⁻¹ for 1a and 2a, respectively [H = −JS_A·S_B + S_A·D·S_B + βH (g_AS_A + g_BS_B)], suggesting that crystal packing forces do not significantly influence the magnetic properties of this class of materials. A facile route toward the synthesis of the fully-oxidized [V^V₄O₈(CH₃O)₄(bipy)₂] and [V^V₄O₆(CH₃O)₆(acac)₂] tetraoxovanadates is also reported.