Heteroleptic 5,5′-disubstituted-2,2′-bipyridine complexes of ruthenium(II): spectral, electrochemical, and structural investigations

The optical absorption and luminescence spectra, the electrochemical behavior, and the X-ray crystal structure of a series of three heteroleptic Ru(II) complexes in comparison to [Ru(bipy)3]2+ are reported. The complexes examined are of the type [Ru(bipy)2(bipy′)](PF6)2 with bipy=2,2′-bipyridine and bipy′=5,5′-diamino-2,2′-bipyridine (4), diethyl-2,2′-bipyridine-5,5′-dicarboxylate (5) or 5,5′-bis(ethoxycarbonylamino)-2,2′-bipyridine (6). Absorption bands and redox waves are fully assigned. The position of bands or half-wave potentials can be correlated with the electron-withdrawing/donating properties of the bipy′ ligand. The relative emission intensities of complexes with 4 and 6 can be modulated through the hydrogen-bonding capabilities of the solvent (MeOH and H2O). The cyclic voltammograms between +1.5 and −2.2 V show a reversible metal-oxidation wave and two to four reduction waves, attributed to successive reductions of the different ligands. Ligand 4 can only be oxidized. Correlations between spectroscopical and electrochemical data give a linear relation for hνmaxabs, hνmaxem versus ΔE1/2. A comparison with complexes with the analogous 4,4′-disubstituted-2,2′-bipyridine ligands reveals pronounced differences in the position of the MLCT-absorption and emission bands due to the 5- or 4-position of the substituent.