Solvent-Dependent Switch of Ligand Donor Ability and Catalytic Activity of Ruthenium(II) Complexes Containing Pyridinylidene Amide (PYA) N‑Heterocyclic Carbene Hybrid Ligands

Chelating ligands
incorporating both N-[1-alkylpyridin-4­(1H)-ylidene]­amide
(PYA) and N-heterocyclic carbene (NHC) donor sites
were prepared and used for the synthesis of ruthenium­(II) complexes.
Cyclic voltammetry, NMR, and UV–vis spectroscopy of the complexes
indicate a solvent-dependent contribution of the limiting resonance
structures associated with the ligand in solution. The neutral pyridylidene
imine structure is more pronounced in apolar solvents (CH2Cl2), while the mesoionic pyridinium amide form is predominant
in polar solvents (MeOH, DMSO). The distinct electronic properties
of these hybrid PYA-NHC ligands in different solvents have a direct
influence on the catalytic activity of the ruthenium center, e.g.,
in the dehydrogenation of benzyl alcohol to benzaldehyde. The activity
in different solvents qualitatively correlates with the solvent permittivity.