Design and Synthesis of a Palladium(II) Complex of a CNHCNN Pincer-Type N-Heterocyclic Carbene Ligand: Application towards the Oxidative Amidation of Aldehydes with 2-Aminopyridines

Abstract

This report describes the synthesis of a [CNHCNN]-pincer-type palladium(II) complex bearing a N-heterocyclic carbene (NHC)-derived pincer CNN ligand. The ligand and palladium pincer complex were characterized with the help of 1H, 13C{1H} NMR, Fourier transform infrared (FTIR) spectroscopy, high-resolution mass spectrometry (HRMS), ultraviolet–visible (UV–vis) spectroscopy, and X-ray photoelectron spectroscopy (XPS) techniques. The coordination mode of the ligand with the palladium was confirmed using single-crystal X-ray diffraction studies. The complex possesses a distorted square planar geometry around the palladium center. The Pd(II) pincer complex was used as a catalyst for the oxidative amidation of aldehydes with 2-aminopyridines. Notably, only 1.0 mol % catalyst loading is required to activate a wide range of substrates under mild reaction conditions. The protocol showed excellent tolerance toward a diverse range of functional groups with good to very good yields (up to 88%) of amidation products. A plausible mechanism of the amidation reaction is proposed on the basis of control experiments.