Reductive dimerization of triruthenium clusters containing cationic aromatic N-heterocyclic ligands.

The cationic cluster complexes Ru(3)(mu-H)(mu-kappa(2)N,C-L(1) Me)(CO)(10) (1(+); HL(1) Me=N-methylpyrazinium), Ru(3)(mu-H)(mu-kappa(2)N,C-L(2) Me)(CO)(10) (2(+); HL(2) Me=N-methylquinoxalinium), and Ru(3)(mu-H)(mu-kappa(2)-N,C-L(3) Me)(CO)(10) (3(+); HL(3) Me=N-methyl-1,5-naphthyridinium), which contain cationic N-heterocyclic ligands, undergo one-electron reduction processes to become short lived, ligand-centered, trinuclear, radical species (1-3) that end in the formation of an intermolecular C--C bond between the ligands of two such radicals, thus leading to neutral hexanuclear derivatives. These dimerization processes are selective, in the sense that they only occur through the exo face of the bridging ligands of trinuclear enantiomers of the same configuration, as they only afford hexanuclear dimers with rac structures (C(2) symmetry). The following are the dimeric products that have been isolated by using cobaltocene as reducing agent: [Ru(6)(mu-H)(2){mu(6)-kappa(4)N(2),C(2)-(L(1) Me)(2)}(CO)(18)] (5; from 1(+)), [Ru(6)(mu-H)(2){mu(6)-kappa(4)N(2),C(2)-(L(2) Me)(2)}(CO)(18)] (6; from 2(+)), and [Ru(6)(mu-H)(2){mu(4)-kappa(8)N(2),C(6)-(L(3) Me)(2)}(CO)(18)] (7; from 3(+)). The structures of the final hexanuclear products depend on the N-heterocyclic ligand attached to the starting materials. Thus, although both trinuclear subunits of 5 and 6 are face-capped by their bridging ligands, the coordination mode of the ligand of 5 is different from that of the ligand of 6. The trinuclear subunits of 7 are edge-bridged by its bridging ligand. In the presence of moisture, the reduction of 3(+) with cobaltocene also affords a trinuclear derivative, [Ru(3)(mu-H)(mu-kappa(2)N,C-L(3') Me)(CO)(10)] (8), whose bridging ligand (L(3') Me) results from the formal substitution of an oxygen atom for the hydrogen atom (as a proton) that in 3(+) is attached to the C(6) carbon atom of its heterocyclic ligand. The results have been rationalized with the help of electrochemical measurements and DFT calculations, which have also shed light on the nature of the odd-electron species, 1-3, and on the regioselectivity of their dimerization processes. It seems that the sort of coupling reactions described herein requires cationic complexes with ligand-based LUMOs.