Combined solution calorimetric and quantum mechanics studies of reactions involving saturated and unsaturated N-heterocyclic carbene (NHC) ligands show that the difference in their relative bond dissociation energies is very small (1 kcal·mol-1). Structural and computational studies reveal small metric parameter differences. These observations in conjunction with relative reactivity profiles of NHC-modified ruthenium-based olefin metathesis catalysts suggest that very small changes in the donor properties of the NHC ligands can translate into significant differences in catalytic properties.

The synthesis and structural characterization of a series of palladium complexes bearing N-heterocyclic carbenes (NHC) as supporting ligands are described. The reaction of commercially available [Pd(allyl)Cl]2 and isolated or in situ generated NHC leads to monomeric palladium complexes where one NHC is bound to the metal center, as indicated by spectroscopic and single-crystal X-ray diffraction studies. The relative reactivity trend for these complexes as catalysts in aryl amination is discussed in terms of ligand steric properties, which vary as a function of imidazole-nitrogen substituents and perturbation resulting in modulation of ring planarity. The concept of buried volume is used to quantify the steric demand of each NHC in the corresponding complexes.

A variety of palladated PCP pincer complexes are covalently tethered onto polymeric and silica supports via either amide or ether linkages and are evaluated in the Heck reaction of iodobenzene and n-butyl acrylate. The decomposition under reaction conditions of all complexes studied is established through poisoning and kinetic studies. Furthermore, the initial steps of the decomposition pathway of PCP as well as SCS pincer Pd(II) complexes are proposed and validated using in situ NMR, mass spectroscopy, and XAS as well as computational methods. These findings together with our previous reports strongly suggest that all Pd(II) pincer complexes are simply precatalysts during the Heck reaction that decompose to form catalytically active Pd(0) species.