A Homoleptic Trisbidentate Ru(II) Complex of a Novel Bidentate Biheteroaromatic Ligand Based on Quinoline and Pyrazole Groups: Structural, Electrochemical, Photophysical, and Computational Characterization

We
synthesized a new homoleptic, tris-bidentate complex [Ru­(QPzH)3]2+ based on the novel biheteroaromatic, 8-(3-pyrazolyl)-quinoline
ligand QPzH. The QPzH ligand was designed to reduce the distortions
typically observed in complexes incorporating the 8-quinolinyl group
into the ligand framework. This was indeed observed, and was also,
as anticipated, found to facilitate the formation of tris-homoleptic
Ru­(II) complexes; [Ru­(QPzH)3]2+ is the first
reported tris-homoleptic complex with ligands based on the 8-quinolinyl
group. The synthesis can either result in a statistical 3:1 mer/fac ratio of the complex, or, through
controlled exposure to light, be tweaked to allow isolation of the
pure mer isomer only. X-ray crystallography reveals
three nonequivalent ligands, with significantly less strain than other
quinoline-based bidentate ligands. The complex exhibits a nearly octahedral
coordination geometry but shows large differences in bond lengths
between the Ru core and the quinoline and pyrazoles, respectively.
The Ru–N­(pyrazole) bond distances are ∼2.04 Å,
while the corresponding distances for Ru–N­(quinoline) are ∼2.12
Å. Structural, photophysical, electrochemical, and theoretical
characterization revealed a mer-Ru­(II) complex with
a low oxidation potential (0.57 V vs ferrocene0/+) attributed
to the incorporation of the pyrazolyl group, a ground state absorption
that is sensitive to the local environment of the complex, and a short-lived 3MLCT excited state.