Triazolylidene-stabilized aNHC–P Ru(II) complexes: electronic effects and high activity in transfer hydrogenation

Homogeneous transfer hydrogenation (TH) has emerged as an efficient and atom-economical strategy for the catalytic reduction of carbonyl compounds. Among the most active systems, the abnormal NHC–phosphine (aNHC–P) ruthenium complex [Ru(OAc)(aNHC–P)₂]Br has shown exceptional activity in TH of acetophenone. However, a base-induced NHC isomerization pathway has been observed as a potential deactivation route under strongly basic conditions. To address this limitation, we developed two different abnormal NHC–phosphine ruthenium complexes. Both are designed to suppress the deactivation route while retaining high catalytic performance: Blocking the normal carbene site (C2-methylation) or replacing the imidazolylidene moiety with a 1,2,3-triazolylidene modulates the electronic properties of the bifunctional ligand framework and suppresses the isomerization pathway under catalytic conditions. Structural and computational data support a distinct electronic profile for the triazolylidene complex, involving reduced σ-donor character and enhanced Ru → carbene/ring backbonding. In the TH of acetophenone, this complex reaches TOFs of 1.1 × 10⁶h⁻¹, surpassing the benchmark Ru(II) NHC catalyst while retaining remarkable stability under strongly basic conditions (NaOiPr). These results suggest that electronically optimized aNHC–P ligands, especially triazolylidenes, provide a robust and highly active platform for Ru(II) TH catalysis.