Reduction of ruthenium arenecyclopentadienyl complexes - Reactions induced by electron transfer

Ruthenium arenecyclopentadienyl complexes [Ru(eta(5)-C5R5)(eta(6)-arene)](+) (1, R = H, arene = C6H6; 2, R = Me, arene = C6H6; 3, R = H; arene = C6H3Me3; 4, R = Me, arene = C6H3Me3; 5, R = H, arene = C6Me6; 6, R = Me, arene = C6Me6; 7, R = Me, arene = C10H8) and [Ru(eta(5)-C9H7)(eta(6)-C6H6)](+) (8) have been studied by cyclic voltammetry; the complexes are capable of both reduction and oxidation. The reduction peak potential values for 1-6 become more negative by about 31 mV for each Me-group at the arene ring and 61 mV for each Me-group at the cyclopentadienyl ring. Reduction of naphthalene complex 7 proceeds by two one-electron processes; the first one is reversible and the second one is irreversible. Two reversible reduction peaks were observed for indenyl complex 8. The following reactions occur on reduction of benzene complexes 1, 2 and 8 with sodium amalgam in tetrahydrofuran (THF): hydrogen atom addition to and decoordination of benzene ligand as well as dimerization of ligand-to-ligand type. Mesitylene compounds 3 and 4 form dimers [(eta(5)-C5R5)Ru(mu-eta(5):eta(5)-Me3H3C6C6H3Me3)Ru(eta(5)-C5R5)] (14, R = H; 15, R = Me) in both chemical and electrochemical reduction processes. Reaction of [Ru(eta(5)-C5H5)(eta(6)-C6Me6)](+) (5) With sodium amalgam in THF leads to the dimer [(eta(5)-C5H5)Ru(mu-eta(5):eta(5)-Me6C6C6Me6)Ru(eta(5)-C5H5)] (16) as the major. product; products of H-atom addition to both hexamethylbenzene and cyclopentadienyl ligands, [Ru(eta(5)-C5H5)(eta(5)-C6Me6H)] (17) and [Ru(eta(4)-C5H6)(eta(6)-C6Me6)] (18), an also formed in low yields. In the case of permethylated 6 only H-atom addition to hexamethylbenzene was observed and the mixture of endo-H and exo-H isomers [Ru(eta(5)-C5H5)(eta(5)-C6Me6H)] (19a,b) was isolated. Reduction of 7 gives [Ru(eta(5)-C5Me5)(eta(5)-C10H9)] (20). The modes of reaction of 19-electron radicals formed by reduction of 1-8 depend on electronic and steric properties of ligands.