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.