Ruthenium(II) and osmium(II) complexes with new terdentate polyquinoline and cyclometalating ligands. Synthesis, NMR characterization, luminescence properties, and electrochemical behavior

Ru(II) and Os(II) complexes with new polyquinoline and cyclometalating terdentate ligands were synthesized and fully characterized by 1H and 13C NMR. The new ligands are 2,6-bis(7'-methyl-4'-phenyl-2'-quinolyl)pyridine (bmpqpy) and its N-C--N cyclometalating homolog, 1,3-bis(7'-methyl-4'-phenyl-2'-quinolyl)benzene (bmpqb-H); the complexes studied are [Ru(bmpqpy)2](PF6)2 (1), Ru(bmpqpy)Cl3 (2), [Ru(bmpqpy)(bmpqb)](PF6) (3, bmpqb- is the anion of bmpqb-H), [Ru(bmpqb)(terpy)](PF6) (4, terpy = 2,2':6',2''-terpyridine), [Ru(bmpqpy)(tppz)](PF6)2 (5, tppz = 2,3,5,6-tetra(2'-pyridyl)pyrazine), [Os(bmpqpy)2](PF6)2 (6), Os(bmpqpy)Cl3 (7), and [Os(bmpqpy)(bmpqb)](PF6) (8). The absorption spectra, redox behavior, and luminescence properties (both at 77 K in a rigid butyronitrile matrix and at room temp. in a fluid MeCN soln.) of the bis-terdentate complexes were studied. The complexes have noticeable absorption properties throughout the visible region, due to spin-allowed metal-to-ligand charge-transfer (MLCT) transitions. In the Os-contg. complexes, spin-forbidden MLCT transitions contribute significantly to the visible light absorption. The complexes undergo metal-centered oxidn. processes and ligand-centered redn. processes in the potential window studied (-2.0 to +1.6 V vs. SCE). All the bis-terdentate complexes exhibit luminescence at 77 K and 1, 4, 5, 6, and 8 also exhibit luminescence at room temp. in the near-IR. The luminescence originates in all cases from triplet MLCT excited states. The lack of room temp. luminescence of 3 (as well as the very weak room temp. luminescence of 1) is attributed to the presence in 3 (and in 1) of two terdentate polyquinoline ligands, which leads to increased steric hindrance around the metal with respect to the terdentate polypyridine ligands in octahedral complexes. Because of their properties, these complexes are interesting species in terms of light-harvesting compds. and can be regarded as useful energy traps when inserted into supramol. arrays.