Two-Coordinate 13-Electron PdI Complexes Stabilized by the Bulky DIMeIHeptCl NHC Ligand.

Abstract

Monomeric, two-coordinate 13-electron Pd^I complexes are exceptionally rare due to their intrinsic instability and propensity for dimerization. In this study, we report that the bulky N-heterocyclic carbene (NHC) ligand DiMeIHept^Cl (1,3-bis[2,6-bis(1-propylbutyl)phenyl]-4,5-dichloro-1,3-dihydro-2H-imidazol-2-ylidene) enables the generation and stabilization of a series of such species. Reduction of the dichloro Pd^II precursor affords PdCl(DiMeIHept^Cl) (3), which serves as a platform for accessing Pd(OtBu)(DiMeIHept^Cl) (4), Pd[N(SiMe₃)₂](DiMeIHept^Cl) (5), and Pd(NH-2,4,6-tri-tertbutyl-C₆H₂)(DiMeIHept^Cl) (6). These complexes adopt pseudolinear or bent geometries, as established by EPR spectroscopy, x-ray crystallography, and DFT calculations. Complexes 3 and 4 react with nucleophiles by an associative mechanism, which becomes impossible for sterically over-crowded 5. Kinetic and spectroscopic studies reveal divergent decay pathways, with 3 undergoing disproportionation and 4-6 decomposing via Pd─X bond homolysis to generate Pd⁰ species and transient organic radicals. Notably, these Pd^I complexes do not undergo oxidative addition with aryl halides, excluding Pd^I/III reactivity under these conditions. Electronic structure analysis shows the unpaired electron resides primarily on Pd and the X ligand, with minimal NHC involvement. These findings provide insight into low-coordinate Pd^I intermediates that may prove relevant to Pd(NHC) precatalyst activation.