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Azo Anion Radical Complexes of Osmium and Related Nonradical Species

1999; American Chemical Society; Volume: 39; Issue: 2 Linguagem: Inglês

10.1021/ic9908322

1520-510X

Kausikisankar Pramanik, M. Shivakumar, Prasanta Ghosh, Animesh Chakravorty,

Radioactive element chemistry and processing

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 pi-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) A, consistent with the anion radical description; in 7.CH2Cl2 the length is 1.27(1) A. The spin-bearing extended Huckel HOMO in a model of 6 is found to be approximately 70% azo-pi* 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-pi* 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 pi-acidity order pap < abp. Crystal data are as follows: (6b, C47H38BrN4OOsP2) monoclinic, space group P21/c (no. 14), a = 10.215(4) A, b = 17.634(7) A, c = 22.473(8) A, beta = 97.67(3) degrees , Z = 4; (7.CH2Cl2, C49H42BrCl2N3OOsP2) monoclinic, space group P2(1/n) (no. 14), a = 15.323(7) A, b = 15.201(6) A, c = 19.542(7) A, beta = 92.51(3) degrees, Z = 4.