Protic N‐Heterocyclic Carbene Ligands in Ruthenium‐Catalyzed Transfer Hydrogenation

ABSTRACT

While N‐heterocyclic carbenes (NHCs) have emerged as privileged ligands in organometallic chemistry and homogeneous catalysis, protic variants have remained remarkably underdeveloped despite the potential of proximal N–H sites for cooperative reactivity. Here we synthesized a series of ruthenium(II) complexes [RuCl ₂ (cym)(NHC)] containing NHC ligands with varied N─H content (NH, NH 5a , NH, NMe 5b , NMe, NMe 5c ) and ring size (five‐membered 5a–c , six‐membered 6a ; cym = para‐ cymene). NMR spectroscopy and x‐ray diffraction reveal distinct effects of N‐substitution and ring expansion on ligand geometry, N─H⋯Cl hydrogen bonding, and MeCN insertion. In transfer hydrogenation catalysis, the activity follows the trend NMe, NMe > NMe, NH > NH, NH, which mirrors catalyst stability under basic conditions, indicating that catalyst robustness rather than NH‐mediated cooperativity governs transfer hydrogenation activity. Under base‐free conditions, the diprotic NH, NH system leads to competitive H ₂ formation, thus diverting hydride transfer away from substrates and emphasizing the relevance of controlling competing hydride transfer pathways.