Osmium Bisterpyridine Complexes with Redox-Active Amine Substituents: A Comparison Study with Ruthenium Analogues

Five osmium complexes with redox-active amine substituents, [Os(ttpy)(Ntpy)](PF6)2 (1(PF6)2), [Os(Ntpy)2](PF6)2 (2(PF6)2), [Os(ttpy)(NPhtpy)](PF6)2 (3(PF6)2), [Os(Ntpy)(NPhtpy)](PF6)2 (4(PF6)2), and [Os(NPhtpy)2](PF6)2 (5(PF6)2), have been prepared, where ttpy is 4'-tolyl-2,2':6',2″-terpyridine, Ntpy is 4'-(di-p-anisylamino)-2,2':6',2″-terpyridine, and NPhtpy is 4'-(di-p-anisylaminophen-4-yl)-2,2':6',2″-terpyridine. X-ray crystallographic data of 2(PF6)2 and 4(PF6)2 are presented. These complexes show rich visible absorptions attributed to the singlet metal-to-ligand charge-transfer ((1)MLCT), triplet MLCT, and intraligand charge-transfer transitions. Complexes 3(PF6)2 and 5(PF6)2 show weak emissions around 720 nm at room temperature. All complexes show stepwise oxidations of the osmium ion and the amine segment. However, the redox potentials and the order of the Os(III/II) and N(•+/0) processes vary significantly, depending on the electronic nature of the amine substituents. In the singly oxidized state, either Os(II) → N(•+) MLCT or N → Os(III) ligand-to-metal charge-transfer transitions in the near-infrared region have been observed. For complexes 2(PF6)2, 4(PF6)2, and 5(PF6)2 with two amine substituents, no evidence has been observed for the presence of osmium-mediated amine-amine electronic coupling. Density functional theory (DFT) and time-dependent DFT calculations have been performed to complement these experimental results. The one-electron-oxidized forms 3(3+) and 5(3+) show distinct electron paramagnetic resonance (EPR) signals in CH3CN at room temperature. However, complexes 1(3+), 2(3+), and 4(3+) are EPR silent under similar conditions. In addition, a comparison study has been made between these osmium complexes and the previously reported ruthenium analogues.