Redox chemistry and electronic properties of 2,3,5,6-tetrakis(2-pyridyl)pyrazine-bridged diruthenium complexes controlled by N,C,N'-biscyclometalated ligands.

To investigate the consequences of cyclometalation for electronic communication in dinuclear ruthenium complexes, a series of 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz) bridged diruthenium complexes was prepared and studied. These complexes have a central tppz ligand bridging via nitrogen-to-ruthenium coordination bonds, while each ruthenium atom also binds either a monoanionic, N,C,N'-terdentate 2,6-bis(2'-pyridyl)phenyl (R-N(wedge)C(wedge)N) ligand or a 2,2':6',2''-terpyridine (tpy) ligand. The N,C,N'-, that is, biscyclometalation, instead of the latter N,N',N''-bonding motif significantly changes the electronic properties of the resulting complexes. Starting from well-known {Ru(tpy)}(2)(mu-tppz) (tpy = 2,2':2'',6-terpyridine) (3) as a model compound, the complexes {Ru(R-N(wedge)C(wedge)N)}(mu-tppz){Ru(tpy)} (R-N(wedge)C(H)(wedge)N = 4-R-1,3-dipyridylbenzene, R = H (4a), CO(2)Me (4b)), and {Ru(R-N(wedge)C(wedge)N)}(2)(mu-tppz), (R = H (5a), CO(2)Me (5b)) were prepared with one or two N,C,N'-cyclometalated terminal ligands. The oxidation and reduction potentials of cyclometalated 4 and 5 are shifted negatively compared to non-cyclometalated 3, the oxidation processes being affected more significantly. Compared to 3, the electronic spectra of 5 display large bathochromic shifts of the main MLCT transitions in the visible spectral region with low-energy absorptions tailing down to the NIR region. One-electron oxidation of 3 and 5 gives rise to low-energy absorption bands. The comproportionation constants and NIR band shape correspond to delocalized Robin-Day class III compounds. Complexes 4a (R = H) and 4b (R = CO(2)Me) also exhibit strong electronic communication, and notwithstanding the large redox-asymmetry the visible metal-to-ligand charge-transfer absorption is assigned to originate from both metal centers. The potential of the first, ruthenium-based, reversible oxidation process is strongly negatively shifted. On the contrary, the second oxidation is irreversible and cyclometalated ligand-based. Upon one-electron oxidation, a weak and low-energy absorption arises.