CATALYTIC CYCLE OF A DIVANADIUM COMPLEX WITH SALEN LIGANDS IN O-2 REDUCTION - 2-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 dipheny l disulfide, a series of divanadium complexes containing salen ligand (salen = N,N'ethylenebis(salicylideneamine)) were prepared, characteri zed, 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 acetoni trile with excess tetrabutylammonium iodide and by electroreduction of I followed by anion exchange with tetrabutylammonium triiodide. The c omplex 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 classificati on 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 i n dichloromethane yielded spontaneously the deep blue, mixed valent, d ivanadium(IV, V) species [(salen)VOVO-(salen)](+) which was structural ly characterized both as its triiodide (III) and perchlorate (IV) sale s. 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)d egrees, 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 d ata 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 = 17 43(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 [(sal en)VOV(salen)](3+) as a high-valent complex whose reversible two-elect ron 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 mode l of a multielectron redox cycle in oxidative polymerization.