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)­(vegi^R)]­PF₆ (<b>2</b>) (cod =1,5-cyclooctadiene, vegi^R = bidentate N-heterocyclic carbene) as well as their isomerized complexes [Ir­(1-κ-4,5,6-η-C₈H₁₂)­(NCCH₃)­(vegi^R)]­PF₆ (<b>3</b>) 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)­(vegi^<i>n</i>Pr)]­PF₆ (<b>7a</b>) 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 (<b>8</b>) (2,2-bipy = 2,2′-bipyridine)