Synthesis and characterization of Os(II)(dpop′) (dpop′ = dipyrido(2,3-a;3′,2′-j)phenazine) complexes with 2,2′-bipyridine(bpy); 2,2′-bipyrimidine(bpm) and 2,3-bis(2-pyridyl)pyrazine(dpp)

New Os(II) complexes including [Os(dpop′)2](PF6)2 (dpop′= dipyrido(2,3-a;3′,2′-j)phenazine) and a series of mixed ligand [Os(dpop′)(N–N)Cl]PF6 (N–N = 2,2′-bipyridine(bpy); 2,2′-bipyrimidine(bpm) and 2,3-bis(2-pyridyl)pyrazine(dpp)) were synthesized. The Os dπ → dpop′ π∗ MLCT transitions for [Os(dpop′)2]2+ are observed at lower energy than for Os dπ → tpy π∗ (tpy = 2,2′:6′,2″-terpyridine) and Os dπ → tppz π∗ (tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine) (The ligand abbreviations tpd, tpp and tpypz have also appeared in the literature for 2,3,5,6- tetrakis(2-pyridyl)pyrazine in addition to tppz.) MLCT transitions in the comparative [Os(tpy)2]2+ and [Os(tppz)2]2+ complexes. The Os dπ → dpop′ π∗ MLCT transitions are observed at lower energy in mixed bidentate ligand N–N systems compared with [Os(dpop′)2]2+. Cyclic voltammetry shows more positive osmium oxidation, and less negative ligand reduction potentials for [Os(dpop′)2]2+ as compared to [Os(tpy)2]2+ and [Os(tppz)2]2+ complexes. The osmium oxidation potentials in mixed ligand [Os(dpop′)(N–N)Cl]+ complexes are at less positive potential than for the [Os(dpop′)2]2+ ion. NMR results show different chemical shifts for ring protons either trans or cis to dpop′ in mixed ligand systems, and also show two geometrical isomers for the [Os(dpop′)(dpp)Cl]+ complex. The [Os(dpop′)(dpp)Cl]+ geometric isomer with the pyrazine ring of dpp trans to dpop′ is found more predominate by 1.0/0.7 over the isomer with the pyrazine ring of dpp cis to dpop′ and that inter-conversion of geometric isomers does not occur in room temperature solution on the NMR timescale.