Theoretical insights into the relative bonding of normal and abnormal N‐heterocyclic carbenes in [PdCl2(NHCR)2] and [PdCl2(NHCR)(aNHCR)] (R = H, Ph, Mes)

Abstract

Quantum chemical insights into normal Pd‐C2(NHCR) and abnormal Pd‐C5(aNHCR) bonding, dominated by dispersion interactions in N‐hetereocyclic carbene complexes [PdCl2(NHCR)2] (I, R = H; II, R = Ph; III, R = Mes (2,4,6‐trimethyl)phenyl)) and [PdCl2(NHCR)(aNHCR] (IV, R = H; V, R = Ph; VI, R = Mes) have been investigated at DFT and DFT‐D3(BJ) level of theory with particular emphasis on the effects of the noncovalent interactions on the structures and the nature of Pd‐C2(NHCR) and Pd‐C5(aNHCR) bonds. The optimized geometries are good agreement with the experimental values. The Pd‐C bonds are essentially single bond. Hirshfeld charge distributions indicate that the abnormal aNHCR carbene ligand is relatively better electron donor than the normal NHCR carbene ligand. The C2 atom has larger %s contribution along Pd‐C2 bond than the C5 atom along Pd‐C5 bond. As a consequence the Pd‐C2(NHCR) bonds are relative stronger than the Pd‐C5(aNHCR) bonds. Thus, the results of natural hybrid orbital analysis support the key point of the present study. Calculations predict that for bulky substituent (R = Ph, Mes) at carbene, the Pd‐C2(NHCR) bond is stronger than Pd‐C5(aNHCR) bond due to large dispersion energy in [PdCl2(NHCR)2] than in [PdCl2(NHCR)(aNHCR)]. However, in case of non‐bulky substituent with small and almost equal contribution of dispersion energy, the Pd‐C2(NHCR) bond is relative weaker than Pd‐C5(aNHCR) bond. The bond dissociation energies are dependent on the R substituent, the DFT functional and the inclusion of dispersion interactions. Major point of this study is that the abnormal aNHCs are not always strongly bonded with metal center than the normal NHCs. Effects of dispersion interaction of substituent at nitrogen atoms of carbene ligand are found to play a crucial role on estimation of relative bonding strengths of the normal and abnormal aNHCs with metal center. © 2016 Wiley Periodicals, Inc.