Osmium Hydrazido and Dinitrogen Complexes.

Abstract

[Os(tpy)(bpy)(NH(3))](PF(6))(2) (1) was oxidized electrochemically in the presence of a series of amines in aqueous solutions buffered to pH 7. With secondary aliphatic amines, electrolysis gave [Os(tpy)(bpy)(NNR(2))](PF(6))(3) (3); number of electrons n = 4.6-5.0. 3 was reduced to [Os(tpy)(bpy)(NNR(2))](PF(6))(2) (2) in aqueous and nonaqueous solutions with n = 1.0. The structures of 2 were determined by X-ray crystallography for NR(2) = diethylamide (2a) and morpholide (2c) and were found to exhibit bent hydrazido(2-) coordination (Os-N-N = 137 degrees ). The salts crystallized in the triclinic system, space group P&onemacr;. For 2a, a = 9.004(1) Å, b = 9.796(1) Å, c = 20.710(2) Å, alpha = 88.78(2) degrees, beta = 85.43(2) degrees, gamma = 86.22(2) degrees, and Z = 2. For 2c, a = 9.632(8) Å, b = 21.229(9) Å, c = 9.039(5) Å, alpha = 97.41(4) degrees, beta = 94.28(5) degrees, gamma = 85.07(5) degrees, and Z = 2. Solutions of 2 were protonated in strongly acidic media to give hydrazido(1-) complexes. The pK(a) of the protonated form of 2a is 0.90 +/- 0.01. Reduction of 2 in aqueous solutions of pH <1 gave 1 and NH(2)R(2)(+) with n = 4.0. At higher pH, there is evidence for an Os(II) hydrazine intermediate. Oxidation of 3 by one electron afforded transiently stable species which decomposed to give [Os(tpy)(bpy)(NCCH(3))](3+) in acetonitrile solution. Pseudo-first-order rate constants of 8.1 +/- 0.9 s(-)(1) and 0.200 +/-.005 s(-)(1) were estimated by cyclic voltammetry on solutions of 2a and 2b(NR(2) = piperidide), respectively. Oxidation of 1 at pH 7, in the presence of primary aliphatic amines or ammonia, occurred with n = 5.9-6.2, and generated [Os(II)(tpy)(bpy)(N(2))](PF(6))(2) (4).