Deciphering the influence of PdII and PdIV oxidation states on non-standard chemical bonds within bis(jN-heterocyclic carbene) complexes: insights from DFT

Abstract

Bis-N-heterocyclic carbene ligands (bis(NHC)) have introduced a new approach to designing homogeneous and heterogeneous catalysts, demonstrating the versatility of ligand concepts in catalysis. This study presents a computational analysis of palladium (+II and +IV) complexes containing either a normally (bis(nNHC)) or an abnormally (bis(aNHC)) bound CH₂-bridged bis-N-heterocyclic carbene ligand; in addition, ancillary ligands are permuted from chlorides (X = Cl) to bromides (X = Br). Density functional theory at the B3PW91/6-31G(d)/Lanl2DZ level in the gas phase was used to investigate the electronic structure and bonding properties of bis(NHC)Pd^IIX₂ and bis(NHC)Pd^IVX₄ for bis(NHC) palladium(II) dihalide and palladium(IV) tetrachloride complexes, respectively. Results indicate that all of the palladium complex structures prefer a flexible boat-type conformation with an average C_2v symmetry, according to bond property (C_carbene–Pd and Pd–Cl[Br]) analysis. The strength of these bonds depends on coordinating halide ions (Cl⁻ and Br⁻), the type of ligand (bis(nNHC) and bis(aNHC)), and the palladium oxidation state (+II and +IV). Analysis of thermodynamic parameters (ΔH⁰, ΔG⁰, and ΔE_bind) shows an increase in values from an abnormal to normal chelating mode in tetrahalides, whereas the opposite is observed for dihalide complexes. The lower π-backbonding ability of the metal, which is influenced by the quantity and size of halide ions involved, could be one possible explanation for this deficiency.