Bidentate mesoionic triazolylidene NHC–amine ligands for ruthenium-catalysed transfer hydrogenation: impact of ligand architecture and coordination mode

Ruthenium(ii) complexes containing donor-functionalised NHCs in combination with pendant non-aromatic amine moieties (NHA) have been explored as catalysts for transfer hydrogenation (TH). They typically show low to moderate activity, while mechanistic insight remains limited. In this work, a new NH₂-functionalised 1,2,3-triazolium ligand precursor L·2HBr is presented to access mesoionic NHC–NHA systems. Reaction of L·2HBr with [Ru(OAc)₂(PPh₃)₂] affords the complexes [Ru(OAc)(PPh₃)₂L]Br (1), [RuBr(PPh₃)L2]Br (2) and the isomeric complex 2′, which have been characterised by NMR spectroscopy, mass spectrometry, and, for 1 and 2, by single-crystal X-ray diffraction. In the TH of acetophenone to 1-phenylethanol using NaOiPr in iPrOH, complex 1 shows activity at the upper end of reported Ru–NHC–NHA systems, reaching a TOF_max of 4200 h⁻¹ under reflux conditions. The bis-ligated complex 2 is substantially more active, reaching near-quantitative conversion within minutes at S/C = 500 and 80–100 °C, and its highest observed TOF_max is 24 000 h⁻¹ at S/C = 5000 under reflux conditions. These results demonstrate that NH₂-functionalised mesoionic NHC ligands provide a viable platform for Ru-catalysed TH and highlight the strong influence of ligand architecture on catalytic performance.