Catalytic Cycle of a Divanadium Complex with Salen Ligands in O₂ Reduction:  Two-Electron Redox Process of the Dinuclear Center (salen = N,N‘-Ethylenebis(salicylideneamine))

In an attempt to provide confirmation for the postulated mechanism of O₂ reduction in vanadium-mediated oxidative polymerization of diphenyl disulfide, a series of divanadium complexes containing salen ligand (salen = N,N‘-ethylenebis(salicylideneamine)) were prepared, characterized, and subjected to reactivity studies toward dioxygen. A divanadium(III, IV) complex, [(salen)VOV(salen)][I₃] (II), was yielded both by treatments of solutions of [(salen)VOV(salen)][BF₄]₂ (I) in acetonitrile with excess tetrabutylammonium iodide and by electroreduction of I followed by anion exchange with tetrabutylammonium triiodide. The complex II was characterized by a near-infrared absorption at 7.2 × 10³ cm^-1 (ε = 60.1 M^-1 cm^-1 in acetonitrile) assigned to an intervalence transfer band. A crystallographically determined V(III)−V(IV) distance of 3.569(4) Å is consonant with the classification of II as a weakly coupled Type II mixed-valence vanadium (α = 3.0 × 10^-2). Oxidation of the cation [(salen)VOV(salen)]⁺ with O₂ in dichloromethane yielded spontaneously the deep blue, mixed valent, divanadium(IV, V) species [(salen)VOVO(salen)]⁺ which was structurally characterized both as its triiodide (III) and perchlorate (IV) salts. Crystal data for III:  triclinic space group P1̄ (no. 2), a = 14.973(2) Å, b = 19.481(2) Å, c = 14.168(2) Å, α = 107.00 (1)°, β = 115.56(1)°, γ = 80.35(1)°, V = 3561.3(9) ų, Z = 4, D_calc = 1.953 g/cm³, μ (MoKα) = 31.74 cm^-1, final R = 0.057 and R_w = 0.065. Crystal data for IV:  triclinic space group P1̄ (no. 2), a = 11.923(3) Å, b = 14.25(1) Å, c = 11.368(7) Å, α = 112.92(5)°, β = 92.76(4)°, γ = 99.13(4)°, V = 1743(1) ų, Z = 2, D_calc = 1.537 g/cm³, μ (CuKα) = 57.69 cm^-1, final R = 0.042 and R_w = 0.061. The complexes III and IV were deoxygenated in strongly acidic nonaqueous media to produce [(salen)VOV(salen)]³⁺ as a high-valent complex whose reversible two-electron redox couple (VOV³⁺/VOV⁺) at 0.44V vs Ag/AgCl has been confirmed. Its ability to serve as a two-electron oxidant provided a unique model of a multielectron redox cycle in oxidative polymerization.