Unveiling the non-standard chemical bonds in PdII and PdIV- CH2- bridged Bis-N-heterocyclic carbene complexes type catalyst: DFT investigation Jean Moto Ongagna 1 , Abel Idrice Adjieufack 2 , Désiré Bikele Mamaa 1 1 Computational and Theoretical Chemistry Unit, Faculty of Science, University of Douala, Cameroon, 2 Physical and Theoretical Chemistry Laboratory, University of Yaoundé I, Cameroon The most important fact in the past years is the synthesis of natural products compounds promoted by transition metal-catalyzed C-H bond activation/functionalization/cyclization. Whereas activation is facilitated by a higher acidity and more favorable reactant–catalyst interactions, the activation of bonds for the formation of C-C or C-X ( X=N, O…) bonds is still a significant and stimulating challenge in various areas of organic synthesis 1 . For that, the palladium-catalyzed activation of unactivated methyl or methylene C-H bonds has been an effective strategy for the construction of carbo- and heterocycles 2 . Usually, the operating mode of these systems is referred to as homogeneous which, although very efficient, are usually very difficult to recover and recycle when the ligand is weakly coordinated. To solve this issue, it is well-recognized that bis-N-heterocyclic carbene ligands have provided a new paradigm to the design of homogeneous catalysts. In this study, two class of CH2-bridged bis- N-heterocyclic carbene [bis(nNHC)=Ln) and (Bis(aNHC)=La)] ligands were installed on to both Pd II and Pd IV oxidation state with halide ( Cl and Br) counterions generating different bis-chelating-NHC complexes of the general formula and respectively to normal and abnormal types. In this consideration, eight complexes have been investigated using DFT study at the B3LYP/GEN level of theory to understanding Ln − Pd or La − Pd and Pd– X bond properties. The geometrical structure data shows that, the bonds lengths categorized close to experimental data 3 and all complexes was indicated a flexible boat-type conformation in a slightly distorted square-planar and tetrahedral environment around Pd II and Pd IV respectively. The calculated electronic results reveal that the nature of the Pd oxidation state (+2 and +4) as well as the coordinating halide and type of ligand determine the strength of the Ln − Pd or La − Pd and Pd– X bonds. These bonds can be fine-tuned to achieve better catalytic activity through type of ligand. In effect, this can be done by making the Pd–ligand bond much stronger through σ-donation and π-back donation and also by making the Pd–X bond weaker by choosing a suitable halide (X). References
1. N. Y. S. Lam, K. Wu, J.-Q. Yu, Angew. Chem. Int. Ed. 2021, 60, 15676–15790. 2. F. Ferlin, L. Anastasiou, N. Salameh et al. ChemSusChem. 2022, 15, 1 – 7. 3. A. S. McCall, H. Wang, J. M. Desper, and S. Kraft, J. Am. Chem. Soc. 2011, 133, 1832–1848.
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