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

Abstract

The independent synthesis of the biscarbene complexes [Ir(cod)(vegi R )]PF 6 (2) (cod =1,5-cyclooctadiene, vegi R = bidentate N-heterocyclic carbene) as well as their isomerized complexes [Ir(1-κ-4,5,6-η-C 8 H 12 )(NCCH 3 )(vegi R )]-PF 6 (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)(F 3 CCO 2 )(H)(vegi nPr )]PF 6 (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)]PF 6 (8) (2,2-bipy = 2,2′-bipyridine).

Generation of Annelated Dicarbenes and Their Alkali-Metal Chelate Complexes in Solution: Equilibrium between Hetero- and Homoleptic NHC Lithium Complexes

“…The carbene–Li distances of 2.22–2.23 Å match those of complex E but are longer than those typically reported for NHC-Li complexes (2.09–2.20 Å) . The bite angle C1–Li–C8 is 80.1°, which is a value also found for vegi R complexes of transition-metal complexes. ,, The C1–C8 carbene distance of 2.89 Å is only slightly shorter than the value for the imidazolium salt 1b (3.04 Å) and the calculated value (DFT) for the free carbene 5b (2.98 Å) (see the Supporting Information). It lies right within the broad range of 2.67–3.26 Å observed for this ligand system so far .…”

Computational Studies of Carboxylate-Assisted C–H Activation and Functionalization at Group 8–10 Transition Metal Centers

“…223 Further mechanistic studies on this carboxylate-induced COD isomerisation were performed by Kunz on the related [Ir(C^C)(COD)] + system, 175, where C^C is a chelating bis-NHC ligand (see Figure 75). 225 Experimental studies showed that formation of the isomerised product [Rh(C^C)I2(OAc)] complexes (where C^C = bidentate 'normal' and 'abnormal' NHCs linked by a -(CH2)3-unit) exhibit C(sp 3 )-H activation of the linker's central CH2 group. This process occurs more readily with the abnormal NHCs, and BP86(MeCN) calculations suggest these induce an easier predissociation of iodide, rather than having any direct effect on the AMLA/CMD C-H activation step.…”

Chiral Discrimination in Rhodium(I) Catalysis by 2,5-Disubstituted 1,3a,4,6a-Tetrahydropenatalene Ligands—More Than Just a Twist of the Olefins?

“…This surprisingly low selectivity combined with the facile isomerization/racemization of 3b upon treatment of 10 with HCl (vide infra) was suggestive of a rhodium-mediated isomerization of the ligand also during catalysis. 23…”