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.