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)]­(PF6)2 (1(PF6)2), [Os­(dpb)­(Mebip)]­(PF6) (2(PF6), dpb is the 2-deprotonated form of 1,3-di­(pyrid-2-yl)­benzene), [Os­(Mebib)­(ttpy)]­(PF6) (3(PF6), ttpy = 4′-tolyl-2,2′:​6′,2″-ter­pyr­i­dine), and [Os­(dpb)­(ttpy)]­(PF6) (4(PF6)). The other three complexes are [Os­(Mebip)2]­(PF6)2 (5(PF6)2), [Os­(Mebip)­(tpy)]­(PF6)2 (6(PF6)2, tpy = 2,2′:​6′,2″-ter­pyr­i­dine), and [Os­(ttpy)2]­(PF6)2 (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+–72+ 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 3MLCT 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 12+ to 1+ by chemical reduction or electrolysis led to the emergence of the 1MLCT transitions in the visible region.