Syntheses, X-ray Structures, and Solution Properties of [V₄O₄{(OCH₂)₃CCH₃}₃(OC₂H₅)₃] and [V₄O₄{(OCH₂)₃CCH₃}₂(OCH₃)₆]:  Examples of New Ligand Coordination Modes

Tetranuclear vanadium complexes with alkoxy ligands, [V₄O₄{μ,μ,μ₃-(OCH₂)₃CCH₃}₂(OCH₃)₆] (1) and [V₄O₄{μ-(OCH₂)₃CCH₃}{μ,μ₃-(OCH₂)₃CCH₃}{μ,μ,μ₃-(OCH₂)₃CCH₃}(OR)₃] (R = C₂H₅ (2), R = CH(CH₃)₂ (3), R = CH₃ (4)), were synthesized by reacting VO(OR)₃ and H₃thme (H₃thme = 1,1,1-tris(hydroxymethyl)ethane) in alcohol. Complex 1 crystallized in the monoclinic space group P2₁/n with a = 9.646(4) Å, b = 11.502(3) Å, c = 11.960(3) Å, β = 90.20(3)°, V = 1326.9 (7) ų, Z = 2 and R (wR₂) = 0.045 (0.143). Complex 2 also crystallized in the monoclinic space group P2₁/n with a = 8.290(8) Å, b = 12.237(2) Å, c = 29.118(4) Å, β = 89.455(9)°, V = 2954(3) ų, Z = 4, and R (wR₂) = 0.049 (0.126). Both 1 and 2 are neutral, discrete complexes possessing a common [V₄O₁₆]^12- core, which consists of four vanadium(V) atoms chelated by two (1) or three (2) tridentate thme^3- ligands and by six (1) or three (2) RO^- groups. Compound 1 exhibits a crystallographically required inversion center; in contrast, complex 2 exhibits no crystallographically imposed symmetry, and its three trialkoxy ligands each coordinate differently (one thme^3- is coordinated in a new coordination mode with the oxygens in a terminal, doubly-bridging and triply-bridging mode). Both compounds 1 and 2 maintain their structures in solution, although compound 1 also forms a second minor species upon dissolution. Sequential exchanges of the RO^- groups in complexes 2 and 3 were investigated by ⁵¹V and ¹H NMR spectroscopy. For example, [V₄O₄(thme)₃(OC₂H₅)₃] will react with CH₃OH to generate [V₄O₄(thme)₃(OCH₃)₃] (4). These reactions were found to be reversible. The time scale of the alcohol exchange reactions were found to vary depending on the vanadium center that is undergoing the exchange.