| Abstract
| - The reaction of [Os(H)(Br)(CO)(PPh3)3], 5, with 2-(phenylazo)pyridine (pap) in boiling dry heptane has affordedthe 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 abovesynthesis, the radical complex [Os(abp•-)(Br)(CO)(PPh3)2], 6b, becomes the sole product. It is proposed that 6 isformed via homolytic cleavage of the Os−H bond in 5; in the formation of 7, the Os−Br bond of 5 is heterolyticallycleaved. 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 extendedHückel HOMO in a model of 6 is found to be ∼70% azo-π* 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 → LUMOtransition, the LUMO being 95% pyridine-π* in character. One-electron paramagnetic 6 displays well-definedanisotropic 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 inthe 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 voltammetricreduction potentials of the 6+/6 couple follow the order 6a+/6a< 6b+/6b, and the carbon monoxide stretchingfrequencies 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.
- The title radical complexes of type A (Ar = phenyl, pyridyl) have been isolated by the reaction of [Os(H)(Br)(CO)(PPh3)3] with azopyridines. In the Ar = pyridyl complex the N−N length is 1.35(2) Å. The spin-bearing orbital is primarily azo-π* in character but with a small metal contribution (B) which is expressed in EPR anisotropy near g = 2 (C). Aerial oxidation of A in polar media has afforded the nonradical congener A+ isolated as PF6- salts (E1/2, A+/A, ∼−0.35 V).
|
