Reporting a new class of divanadium(V) compounds connected by an unsupported hydroxo bridge

Dinuclear oxovanadium(V) compounds [LV(V)O(mu-OH)OV (V)L](PF6) [H2L = N,N'-tert-ethylene bis(salicylideneimine) (H 2Salen) and its derivatives] ( 1- 3) have been obtained by aerial oxidation of V (IV)OL precursors in THF in the presence of added NH 4PF 6. The oxidized vanadium(V) probably extracts an OH (-) ligand from the residual moisture in the solvent and is retained as an unsupported hydroxo-bridge between the metal centers of these compounds as confirmed by single-crystal X-ray diffraction analyses. The molecules of 1- 3 have centrosymmetric structures with each vanadium center having a distorted octahedral geometry. The bridging OH (-) group is located trans to the terminal VO t bond. The latter exerts strong trans labilizing influence to set the participating vanadium centers apart by about 4.1 A. These separations are by far the largest (e.g., V...V#, 4.131 A in 1) among all binuclear compounds containing an unsupported hydroxo bridge reported to date. The complexes retain their identity also in solution as established by (1)H NMR spectroscopy. Electrochemically, the behaviors of 1-3 are quite interesting as studied by cyclic voltammetry in acetonitrile, each undergoing three (except 3) nearly reversible metal-based reductions, all in the positive potential range (e.g., at E (1/2) = 0.57, 0.39, and 0.04 V versus Ag/AgCl reference for 1) as indicated by steady state voltammetry. The electrode process at 0.39 V appears to involve a single-step two-electron transfer as revealed from the normal and differential pulse voltammetric data and probably includes a combination of V(V)-V(IV) <--> V(III)-V(IV) mixed oxidation states. Compounds 1-3 thus provide a unique example of divanadium compounds in which the metal centers are linked by an unsupported hydroxo-bridge.