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

A series of Ru(ll) and Os(ll) complexes with new polyquinoline and cyclometalating terdentate ligands has been synthesized and fully characterized by H-1 and C-13 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)Cl-3 (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)Cl-3 (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 temperature in a fluid acetonitrile solution) of the bis-terdentate complexes have been investigated. The complexes have noticeable absorption properties throughout the visible region, due to spin-allowed metal-to-ligand charge-transfer (MLCT) transitions. In the Os-containing complexes, spin-forbidden MLCT transitions contribute significantly to the visible light absorption. The complexes undergo metal-centered oxidation processes and ligand-centered reduction processes in the potential window investigated (-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 temperature in the near-IR. The luminescence originates in all cases from triplet MLCT excited states. The lack of room temperature luminescence of 3 (as well as the very weak room temperature 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 compounds and can be regarded as useful energy traps when inserted into supramolecular arrays.