Cyclometalation of (2R,5R)-2,5-Diphenylpyrrolidine and 2-Phenyl-2-imidazoline Ligands with Half-Sandwich Iridium(III) and Rhodium(III) Complexes

Abstract

Cyclometalation of (2R,5R)-2,5-diphenylpyrrolidine (1) with [(η5-Cp*)MCl2]2 using NaOAc in CH2Cl2 at room temperature, followed by cationization with KPF6 in CH3CN, cleanly yielded the cationic cyclometalated amines 2a,b (a, M = Ir; b, M = Rh). This constituted an improvement with regard to another cyclometalation pathway, which had led to unwanted oxidation of the ligand to an imine. Compounds 2a,b were characterized by elemental analysis and X-ray diffraction of single crystals; the latter gave evidence of an R configuration of the metal center, and a λ envelope conformation of the five-membered metallacycle. Successful application of the NaOAc/CH2Cl2 methodology to the cycloiridation and cyclorhodation of the imidazolines 3–5 led to the neutral half-sandwich chloro complexes 6–8, which were completely characterized, with yields ranging from 44% to 82%. In these complexes, the labile NH proton was localized in a γ position with regard to the metal. From the enantiopure chiral ligands 4 and 5, both diastereoisomers of the four compounds 7a,b and 8a,b were identified in solution by NMR; they were in equilibrium in solution. Complexes 6b, (SM)-7a,b, and (RM)-8a,b were characterized by radiocrystallography, which showed that the metallacycles were planar. Complexes 6–8 were tested as catalysts for the reduction of acetophenone and imines by hydrogen transfer.