N-heterocyclic bis-carbene palladium complexes derived from functionalized naphthalimides - Synthesis, Structure elucidation and DFT study

Two new bis-carbene (Pd(NHC)(2)Cl-2 and Pd(NHC)(NHC`)Br-2) complexes derived from substituted 1,8-naphthalimides were synthesized. All complexes were fully characterized by NMR spectra, which indicate trans orientation of the NHC ligands. H-1 and C-13 NMR spectra of complex Pd(NHC)(2)Cl-2 are influenced by existence of trans-syn and trans-anti rotamers. The dynamic behaviour in solution of the complex Pd(NHC)(2)Cl-2 was studied by H-1 NMR spectra in the temperature range of 293-403 K. The rotational barrier is experimentally estimated to 20 kcal/mol at 298 K. The DFT calculations reproduce the experimental data very well. The analysis of DFT calculations suggests electronic origin of the restricted rotation around Pd-C carbene bond. The electron-donating properties of reported system are situated between unsaturated and saturated five-membered ring NHC ligands. (C) 2021 Elsevier B.V. All rights reserved.