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.