Synthesis, crystal structures, spectral investigations, conformational analysis and DFT studies of N- heterocyclic carbene precursors

Abstract

Three new 2-hydroxyethyl substituted N-heterocyclic carbene (NHC) precursors were synthesized in this study. These NHC precursors were prepared from 1-(alkyl/aryl)benzimidazole and alkyl halides. Their structural characterizations were performed by elemental analysis, 1H NMR, 13C NMR, FT–IR and UV–Vis spectroscopy and single-crystal X-ray diffraction. The spectral features were also characterized by Density Functional Theory (DFT) at B3LYP/Lanl2dz//6-31G++(d,p) basis set. Two most stable conformers belonging to the compounds were found by potential energy surface (PES) scan, and the theoretical ground-state geometries were investigated. Among these conformers, the geometry of the conformer-I for all compounds matches almost well with the experimental results. 1H and 13C NMR chemical shifts were calculated with GIAO approach and compared to the observed ones. Detailed vibrational assignments of the wavenumbers of the conformers were carried out based on the potential energy distribution (PED). Natural bond orbital (NBO) analysis was used to analyze the stability of the molecules arising from hyperconjugative interactions and charge delocalization. The HOMO–LUMO energy gap of the stable conformers was calculated for comparing their chemical reactivity behavior. Molecular electrostatic potential (MEP) diagrams were used to get information about ?the size, shape, charge density distribution and site of chemical reactivity of each stable conformer. The 3D Hirshfeld surfaces and the associated 2D fingerprint plots were also carried out to obtain an insight into the behavior of the interactions in the compounds.