Hydrogenative Carbon Dioxide Reduction Catalyzed by Mononuclear Ruthenium Polypyridyl Complexes: Discerning between Electronic and Steric Effects

The preparation and
isolation of a family of Ru–Cl complexes
containing the deprotonated anionic tridentate meridional ligand (1Z,3Z)-N1,N3-di­(pyridin-2-yl)­isoindoline-1,3-diimine (Hbid) and 1,3-di­(2-pyridyl)­benzene) (Hdpb), namely, [Ru­(bid)­(acac)­Cl], 1d, [Ru­(bid)­(6,6′-Me2-bpy)­Cl], 1e, trans-[Ru­(bid)­(py)2Cl], 2, [Ru­(dpb)­(bpy)­Cl], 3a, and
[Ru­(dpb)­(4,4′-(COOEt)2-bpy)­Cl], 3b,
are reported. All these complexes have been thoroughly characterized
in solution by NMR spectroscopy and for 1d and 1e by single-crystal X-ray diffraction analysis. Furthermore,
the redox properties of all complexes have been investigated using
cyclic voltammetry (CV) and differential pulse voltammetry (DPV).
The capacity of the various complexes to catalyze hydrogenative CO2 reduction was also investigated. Compound 1e is the best catalyst, achieving initial turnover frequencies above
1000 h–1. Kinetic analysis identifies a relationship
between Ru­(III/II) couple redox potentials and initial turnover frequencies.
Finally, DFT calculations further characterize the catalytic cycle
of these complexes and rationalize electronic and steric effects deriving
from the auxiliary ligands.