Electrochemical reduction of Ru(η6-arene)(κ3-tris(pyrazolyl)methane) dicationic complexes

One-electron reduction of [Ru(η6-arene)(κ3-HCPz3)]2+ complexes (arene = p-xylene, p-cymene, or hexamethylbenzene, Pz = pyrazolyl) was observed at approximately −1.4 V vs. ferrocene in nonaqueous media. Cyclic voltammetry data showed no direct evidence of a hapticity change for either the arene or the tris(pyrazolyl)methane (TPM) ligand in the electron-transfer step. The resulting 19-electron radicals underwent reactions at the arene, giving persistent cyclohexadienyl ruthenium complexes which could be oxidized back to the original complex in an overall chemically reversible process. The dominant reaction for the hexamethylbenzene-containing radical was shown to be an arene-based dimerization (kdim = 4 (±3) × 103 M−1 s−1) which occurs in competition with an H-atom capture, most likely from solvent. The behavior of the 19-electron radicals is similar to that of their all-carbocyclic mixed-sandwich analogues Ru(arene)(C5R5) [O.V. Gusev, M.A. Ievlev, T.A. Peganova, M.G. Peterleitner, P.V. Petrovskii, Y.F. Oprunenko, N.A. Ustynyuk, J. Organomet. Chem. 551 (1998) 93].

The ruthenium hexamethylbenzene tris(pyrazolyl)methane (TPM) complex [Ru(η6-C6Me6)(κ3-HCPz3)]2+ undergoes a one-electron reduction (E1/2 = −1.46 V vs. ferrocene in acetone/0.1 M [NBu4][PF6]) to a 19-electron complex which undergoes radical reactions at the arene ligand. The primary product is a dimer dication, coupled at the aryl groups, which can be anodically oxidized back to the original 18-electron complex.Figure optionsDownload as PowerPoint slide