Iridium complexes of bulky CCC-pincer N-heterocyclic carbene ligands: Steric control of coordination number and catalytic alkene isomerization

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.