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, [2]+, 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 [5]PF6 (M = Pt) and 6 (M = Pd) are obtained if the halide-free imidazolinium salt [2]PF6 is used. In contrast, treatment of the imidazolinium chloride salt [2]Cl with M(PPh3)4 leads to isolation of N-heterocyclic alkyl MII species 3 (M = Pd) and 4 (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 3 and 4, indicating that these species represent an unusual type of arrested C–H activation intermediate. While Pd complex 3 is thermally stable, Pt complex 4 undergoes C–H activation to afford the corresponding NHC-PtII-hydride species [5]Cl upon heating. Additionally, both complexes 3 and 4 undergo rapid C–H activation upon abstraction of the metal-bound halide to form 6 and [5]PF6, respectively. The nature of the bonding in the unusual N-heterocyclic alkyl species is investigated computationally.