Azo anion radical complexes of osmium and related nonradical species

Abstract

The reaction of [Os(H)(Br)(CO)(PPh₃)₃], 5, with 2-(phenylazo)pyridine (pap) in boiling dry heptane has afforded the azo anion radical complex [Os(pap^•-)(Br)(CO)(PPh₃)₂], 6a, as the major product and [Os(pap)(H)(CO)(PPh₃)₂]Br, 7, as a minor byproduct. Upon replacing pap by the better π-acceptor azo-2,2'-bipyridine (abp) in the above synthesis, the radical complex [Os(abp^•-)(Br)(CO)(PPh₃)₂], 6b, becomes the sole product. It is proposed that 6 is formed via homolytic cleavage of the Os-H bond in 5; in the formation of 7, the Os-Br bond of 5 is heterolytically cleaved. The X-ray structures of 6b and 7·CH₂Cl₂ have been determined. In 6b, the N-N length is 1.35(2) Å, consistent with the anion radical description; in 7·CH₂Cl₂ the length is 1.27(1) Å. The spin-bearing extended Hückel HOMO in a model of 6 is found to be 70% azo-p in character associated with a small metal contribution. An electronic band observed in the range 600-700 nm in solutions of 6 is assigned to the HOMO → LUMO transition, the LUMO being 95% pyridine-π^∗ in character. One-electron paramagnetic 6 displays well-defined anisotropic EPR features near g = 2.00. The anisotropy arises from the metal character of HOMO and is magnified by the large spin-orbit coupling in osmium. In a moisture-free environment 6 is indefinitely stable in the solid state, but in CH₂Cl₂-MeCN solution 6a is rapidly oxidized by air, affording [Os(pap)(Br)(CO)(PPh₃)₂]⁺, 6a⁺, which has been isolated as the diamagnetic PF₆- salt; 6b+PF₆^- has been similarly prepared. The voltammetric reduction potentials of the 6⁺/6 couple follow the order 6a⁺/6a < 6b⁺/6b, and the carbon monoxide stretching frequencies follow the order 6a < 6b and 6a⁺ < 6b⁺. These trends are consistent with the π-acidity order pap < abp. Crystal data are as follows: (6b, C₄₇H₃₈BrN₄OOsP₂) monoclinic, space group P2₁/c (no. 14), a = 10.215(4) Å, b = 17.634(7) Å, c = 22.473(8) Å, β = 97.67(3)°, Z = 4; (7·CH₂Cl₂, C₄₉H₄₂BrCl₂N₃OOsP₂) monoclinic, space group P21/n (no. 14), a = 15.323(7) Å, b = 15.201(6) Å, c = 19.542(7) Å, β = 92.51(3)°, Z = 4.