Monometallic osmium(II) complexes with bis(N-methylbenzimidazolyl)benzene or -pyridine: a comparison study with ruthenium(II) analogues.

Seven bis-tridentate osmium complexes with Mebib or Mebip (Mebib is the 2-deprotonated form of 1,3-bis(N-methylbenzimidazolyl)benzene and Mebip is bis(N-methylbenzimidazolyl)pyridine) have been prepared, and their electrochemical and spectroscopic properties are compared with ruthenium structural analogues. Among them, four complexes have the [Os(NCN)(NNN)]-type coordination, including Os(Mebib)(Mebip)2 (1(PF6)2), Os(dpb)(Mebip) (2(PF6), dpb is the 2-deprotonated form of 1,3-di(pyrid-2-yl)benzene), Os(Mebib)(ttpy) (3(PF6), ttpy = 4'-tolyl-2,2':6',2"-terpyridine), and Os(dpb)(ttpy) (4(PF6)). The other three complexes are Os(Mebip)22 (5(PF6)2), Os(Mebip)(tpy)2 (6(PF6)2, tpy = 2,2':6',2"-terpyridine), and Os(ttpy)22 (7(PF6)2) with the [Os(NNN)(NNN)]-type coordination. Single crystals of 2(PF6) and 6(PF6)2 have been obtained, and their structures are studied by X-ray crystallographic analysis. The Os(II/III) redox potentials of 1(PF6)2 to 7(PF6)2 progressively increase from +0.04, +0.23, +0.24, +0.36, +0.56, +0.79 to +0.94 V vs Ag/AgCl, which are 200-300 mV less positive relative to the Ru(II/III) potentials of their ruthenium counterparts. The highest occupied molecular orbital energy levels of 1(+)-7(2+) are calculated to vary in a descending order. The ruthenium and osmium complexes have singlet metal-to-ligand charge-transfer (MLCT) transitions of similar energies and band shapes, while the osmium complexes display additional (3)MLCT transitions in the lower-energy region. Complexes 6(PF6)2 and 7(PF6)2 emit weakly at 780 and 740 nm, respectively. Complex 1(PF6)2 was synthesized as the oxidized Os(III) salt because of the low Os(II/III) potential. The transformation of 1(2+) to 1(+) by chemical reduction or electrolysis led to the emergence of the (1)MLCT transitions in the visible region.