The search for new hydrogenation catalyst motifs based on N-heterocyclic carbene ligands

A series of new iridium complexes containing N-heterocyclic carbene ligands (NHCs) has been prepared and tested for hydrogenation of primary (1-octene), secondary (cyclohexene), tertiary (1-methylcyclohexene) and quartenary (2,3-dimethyl-2-butene) alkenes. Taking inspiration from Crabtree's catalyst, [Ir(COD)(PCy3)(py)]PF6, the labile pyridine ligand was exchanged with an NHC ligand, and was found to produce catalysts with similar activities and rates. Further refinement of the ligands produced the optimal version of the iridium phosphine/NHC combination: Using a smaller phosphine, tri-n-butylphosphine, a saturated NHC and the non-coordinating anion, tetrakis(3,5-bis(trifluoromethylphenyl))borate (BARF), a highly active catalyst that has substantially better longevity and stability, but similar activity, to the parent Crabtree's catalyst was prepared in three steps. The effect of the BARF counteranion was explored, and was shown to improve even Crabtree's catalyst, and is thus a general phenomenon for iridium catalysts in non-polar solvents. Mechanistic studies suggest that the catalyst resting state is a dimeric complex, presumably hydride-bridged, that dissociates into catalytically active species under catalytic conditions. Thus, the catalyst dead-end for Crabtree's catalyst, hydride-bridged dimers and trimers, may in fact be a reversible resting state for the iridium complexes described here. (c) 2005 Elsevier B.V. All rights reserved.