Iridium complexes of CCC-pincer bis-N-heterocyclic carbenes, including a newly synthesized trifluoromethyl-substituted complex, were examined as catalysts for the acceptorless dehydrogenation of cyclooctane and n-undecane. Up to 103 turnovers were observed for the dehydrogenation of cyclooctane, and up to 97 turnovers were observed for the dehydrogenation of n-undecane. The catalysts showed high initial turnover frequencies, followed by a gradual loss of activity over 24 h. Experiments indicate that this loss of activity is due to catalyst decomposition rather than product inhibition. Stoichiometric reactivity was investigated for the precatalysts, focusing on the synthesis of dihydride and trihydride complexes as well as the dissociation and addition of neutral ligands.

Three new iridium complexes of meta-phenylene-bridged bis-N-heterocyclic carbene CCC-pincer ligands were synthesized and characterized. For a pincer ligand with 2,6-diisopropylphenyl N-substituents, a six-coordinate iridium(III) complex of the formula Ir(CCC)HCl(MeCN) was formed. In contrast, ligands with t-butyl or adamantyl N-substituents gave five-coordinate iridium(III) complexes of the formula Ir(CCC)HCl. These iridium complexes, along with two previously described iridium complexes, were tested for activity in the catalytic transfer-dehydrogenation of n-octane at 150 °C. The new complexes were inactive for this reaction, while two previously reported catalysts were modestly active: a mesityl-substituted derivative gave 12 turnovers, and a 3,5-di-t-butylphenyl-substituted variant gave 10 turnovers. In contrast, these complexes were shown to be highly active catalysts for the isomerization of terminal alkenes, under conditions much milder than those required for transfer-dehydrogenation.

Iridium complexes containing CCC-pincer m-phenylene-bridged N-heterocyclic carbene ligands were examined as catalysts for alkene isomerization. Complexes containing either mesityl or adamantyl side groups were found to catalyze the isomerization of a number of alkenes to the internal isomers, including 1-octene, vinylcyclohexane, and allylbenzene. Mechanistic studies indicate a surprising dichotomy, apparently caused by ligand steric effects. For the mesityl-substituted catalyst, several lines of evidence provide strong support for isomerization via an iridium allyl hydride intermediate: (1) H–D crossover experiments indicate that 1,3-hydrogen migration is exclusively intramolecular, (2) the catalyst resting state, a π-allyl hydride species, was isolated and serves as a kinetically competent catalyst, (3) NMR experiments indicate that the π-allyl hydride resting state undergoes reversible C–H reductive elimination that is rapid relative to catalytic turnover, and (4) kinetic studies indicate that the isomerization reaction is first order in substrate and catalyst, consistent with turnover-limiting ligand substitution. H–D crossover experiments for alkene isomerization catalyzed by the adamantyl-substituted complex show selectivity for a 1,3-deuterium shift, as well as the intermolecular transfer of hydrogen. These results are consistent with an insertion/elimination mechanism proceeding selectively through a secondary metal–alkyl or with a π-allyl-type mechanism with an unknown pathway for intermolecular hydrogen crossover.

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