Ruthenium Complexes of Triazole-Based Scorpionate Ligands Transfer Hydrogen to Substrates under Base-Free Conditions

The first ruthenium complexes of bulky tris­(triazolyl)­borate (Ttz) ligands were synthesized, fully characterized, and studied as transfer hydrogenation catalysts. The structures of the complexes were (η6-arene)­RuCl­(N,N), where in each case N,N is a κ2-Ttz or bis­(triazolyl)­borate (Btz) ligand (arene = p-cymene (1, 3, 5, 6), benzene (2), C6Me6 (4); N,N = TtzPh,Me* (1, 2), TtzMe,Me (3, 4), Ttz (5), Btz (6)). All but 5 were crystallographically characterized, and notably for 1 and 2 a rearranged ligand structure is observed (as indicated by an asterisk). These complexes were all effective catalysts for transfer hydrogenation of aryl ketones in isopropyl alcohol with base co-catalyst, with rates that were accelerated by moisture-free conditions. Complexes 1 and 2 are also effective catalysts for base-free transfer hydrogenation, and with 1 hydrogenation of several base-sensitive substrates was demonstrated. The ability of 1 to serve as a hydrogenation catalyst without base is attributed primarily to steric bulk, and a preliminary mechanism for formation of that active catalyst is proposed.