Isolation of N-heterocyclic alkyl intermediates en route to transition metal N-heterocyclic carbene complexes: Insight into a C-H activation mechanism

摘要

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 Pd~0 and Pt~0. The expected NHC-ligated metal-hydride species [5]PF _6 (M = Pt) and 6 (M = Pd) are obtained if the halide-free imidazolinium salt [2]PF_6 is used. In contrast, treatment of the imidazolinium chloride salt [2]Cl with M(PPh_3)_4 leads to isolation of N-heterocyclic alkyl M~(II) 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-Pt~(II)-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]PF_6, respectively. The nature of the bonding in the unusual N-heterocyclic alkyl species is investigated computationally.