Isolation of N‑Heterocyclic Alkyl Intermediates en Route to Transition Metal N‑Heterocyclic Carbene Complexes: Insight into a C–H Activation Mechanism

Abstract

An imidazolinium cation has been incorporated into an arene-linked diphosphine pincer ligand, <b>[2]</b>^<b>+</b>, and the metalation of this ligand has been investigated via direct imidazolinium C–H activation to Pd⁰ and Pt⁰. The expected NHC-ligated metal-hydride species <b>[5]­PF</b>_<b>6</b> (M = Pt) and <b>6</b> (M = Pd) are obtained if the halide-free imidazolinium salt <b>[2]­PF</b>_<b>6</b> is used. In contrast, treatment of the imidazolinium chloride salt <b>[2]­Cl</b> with M­(PPh₃)₄ leads to isolation of N-heterocyclic alkyl M^II species <b>3</b> (M = Pd) and <b>4</b> (M = Pt), in which the imidazolinium C–H bond remains intact. Interestingly, there are no apparent agostic interactions between the imidazolinium protons and the metal centers in <b>3</b> and <b>4</b>, indicating that these species represent an unusual type of arrested C–H activation intermediate. While Pd complex <b>3</b> is thermally stable, Pt complex <b>4</b> undergoes C–H activation to afford the corresponding NHC-Pt^II-hydride species <b>[5]­Cl</b> upon heating. Additionally, both complexes <b>3</b> and <b>4</b> undergo rapid C–H activation upon abstraction of the metal-bound halide to form <b>6</b> and <b>[5]­PF</b>_<b>6</b>, respectively. The nature of the bonding in the unusual N-heterocyclic alkyl species is investigated computationally