Catalytic cycle of a divanadium complex with salen ligands in O-2 reduction: Two-electron redox process of the dinuclear center (salen equals N,N'-ethylenebis(salicylideneamine))

In an attempt to provide confirmation for the postulated mechanism of O-2 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)][I-3] (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 x 10(3) cm(-1) (epsilon = 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) Angstrom is consonant with the classification of II as a weakly coupled Type II mixed-valence vandium (alpha = 3.0 x 10(-2)). Oxidation of the cation [(salen)VOV(salen)](+) with O-2 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) sales. Crystal data for III: triclinic space group P (1) over bar (no. 2), a = 14.973(2) Angstrom, b = 19.481(2) Angstrom, c 14.168(2) Angstrom, alpha = 107.00 (1)degrees, beta = 115.56(1)degrees, gamma = 80.35(1)degrees, V = 3561.3(9) Angstrom(3), Z = 4, D-calc = 1.953 g/cm(3), mu (MoK alpha) = 31.74 cm(-1), final R = 0.057 and R(w) = 0.065. Crystal data for IV: triclinic space group P (1) over bar (no. 2), a = 11.923(3) Angstrom, b = 14.25(1) Angstrom, c = 11.368(7) Angstrom, alpha = 112.92(5)degrees, beta = 92.76(4)degrees, gamma = 99.13(4)degrees, V = 1743(1) Angstrom(3), Z = 2, D-calc = 1.537 g/cm(3), mu (CuK alpha) = 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 0.44 V 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.