The acid-mediated isomerization of iridium(III) complexes with cyclometalated NHC ligands: kinetic vs. thermodynamic control.

The isomerization of iridium(III) complexes with metalated N-heterocyclic carbene (NHC) ligands was studied. The fac isomers of complexes with 1-phenyl-3-methylbenzimidazolin-2-ylidene or 1-phenyl-3-benzylbenzimidazolin-2-ylidene ligands are transformed cleanly into the mer isomers when solutions of the complexes are treated with first HNTf 2 and then NEt 3 . The transformation can be accomplished within a few minutes and the side product (NEt 3 H)(NTf 2 ) is easy to separate. Spectroscopic and structural analyses indicate that the isomerization proceeds by protonation of the carbene ligand at the metalated phenyl group, accompanied by a fac → mer rearrangement of the carbene donors. An iridium complex with a 1-phenyl-1,2,4-triazolo[4,3- f ]phenanthridine-based carbene ligand could not be isomerized under similar conditions, most likely because of its reduced conformational flexibility.