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Synthesis, Stability, and (De)hydrogenation Catalysis by Normal and Abnormal Alkene- and Picolyl-Tethered NHC Ruthenium Complexes

A series of p-cymene and cyclopentadienyl Ru(II)-aNHC complexes were synthesized from 2-methylimidazolium salts with either an N-bound alkenyl (1, 3) or picolyl tether (6, 7). The C(5)-Me substituted alkenyl-tethered analogues (2, 4) were also synthesized. Ag-mediated C(2)-dealkylation was a prominent side reaction that led to the formation of normally bound NHC Ru(II) complexes, which in selected cases were isolated (5, 8). A C(4)- over C(2)-selectivity for ruthenium binding was established by protecting the C(2)-position with an iPr group on the imidazolium precursor, for which unique p-cymene (9) and cyclopentadienyl (10) Ru(II)-aNHC derivatives were synthesized. All complexes were applied in the transfer hydrogenation of ketones and in secondary alcohol oxidation, with higher catalytic activity for the p-cymene over the cyclopentadienyl systems, as well as the alkenyl- over the picolyl-containing aNHC complexes.