Smooth C(alkyl)–H bond activation in rhodium complexes comprising abnormal carbene ligands

Abstract

Rhodation of trimethylene-bridged diimidazolium salts induces the intramolecular activation of an alkane-type C-H bond and yields mono-and dimetallic complexes containing a formally monoanionic C,C,C-tridentate dicarbene ligand bound to each rhodium centre. Mechanistic 10 investigation of the C alkyl -H bond activation revealed a significant rate enhancement when the carbene ligands are bound to the rhodium centre via C4 (instantaneous activation) as compared to C2-bound carbene homologues (activation incomplete after 2 days). The slow C-H activation in normal C2-bound carbene complexes allowed intermediates to be isolated and suggests a critical role of acetate in mediating the bond activation process. Computational modelling supported by 15 spectroscopic analyses indicate that halide dissociation as well as formation of the agostic intermediate is substantially favoured with C4-bound carbenes. It is these processes that discriminate the C4-and C2-bound systems rather than the subsequent C-H bond activation, where the computed barriers are very similar in each case. The tridentate dicarbene ligand undergoes selective H/D exchange at the C5 position of the C4-bound carbene exclusively. A mechanism has 20 been proposed for this process, which is based on the electronic separation of the abnormal carbene ligand into a cationic N-C-N amidinium unit and a metalla-allyl type M-C-C fragment.

Normal and Abnormal N-Heterocyclic Carbene Ligands

“…54 Further support for a metal-mediated reaction trajectory in the acidolysis of complex 15 has been obtained when the Lewis acid is exchanged from H + to Ag + . 54 Further support for a metal-mediated reaction trajectory in the acidolysis of complex 15 has been obtained when the Lewis acid is exchanged from H + to Ag + .…”

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

“…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. 226…”

Computational Studies on Heteroatom-Assisted C–H Activation and Functionalisation at Group 8 and 9 Metal Centres

“…BP86(MeCN) calculations suggest that the greater reactivity seen with the abnormal NHC is due to an easier dissociation of iodide that is required, rather than the subsequent AMLA-6 C-H activation step [22]. A subsequent study considered the different behaviour seen in the H/D exchange reactions of benzene at cis-Ir(acac) 2 (κ 2 -O 2 CR) complexes, 7, depending on whether R ¼ CH 3 (multiple exchange) or CF 3 (single exchange) [24].…”