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 (1MLCT), 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 OsIII/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 33+ and 53+ show
distinct electron paramagnetic resonance (EPR) signals in CH3CN at room temperature. However, complexes 13+, 23+, and 43+ are
EPR silent under similar conditions. In addition, a comparison study
has been made between these osmium complexes and the previously reported
ruthenium analogues.