Identifying and Rationalizing the Conditions for the Isomerization of 1,5-Cyclooctadiene in Iridium Complexes by Experimental and Theoretical Mechanistic Studies

The independent synthesis of the biscarbene complexes [Ir­(cod)­(vegiR)]­PF6 (2) (cod =1,5-cyclooctadiene, vegiR = bidentate N-heterocyclic carbene) as well as their isomerized complexes [Ir­(1-κ-4,5,6-η-C8H12)­(NCCH3)­(vegiR)]­PF6 (3) is described. We elucidated acetic acid as the catalyst and coordinated acetonitrile as the thermodynamic driving force for this cod-isomerization. By using the stronger trifluoroacetic acid, we isolated complex [Ir­(cod)­(F3CCO2)­(H)­(veginPr)]­PF6 (7a) as an intermediate of the isomerization. From H/D exchange experiments as well as DFT calculations, we conclude that after formation of the Ir–H complex, an olefin insertion, followed by a concerted metalation-deprotonation step and a coordination of acetonitrile, is the mechanistic pathway. On the basis of our findings, we were able to carry out the cod-isomerization for the first time also for the less-electron-rich complex [Ir­(2,2′-bipy)­(cod)]­PF6 (8) (2,2-bipy = 2,2′-bipyridine).