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

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.