Pd(ii) NCSe–pincer complexes for regioselective cross-dehydrogenative coupling of arylthiophenes with hetero(arenes)†

Abstract

Herein, we have presented the synthesis of two new NCSe pincer ligands by the reaction of 3-((phenylselanyl)methoxy)benzaldehyde (P) with alkyl amines (^tbutylamine (L1) and 1-adamantylamine (L2)). The reaction of ligands L1 and L2 with PdCl₂(CH₃CN)₂ precursor resulted in the formation of palladium pincer complexes C1 and C2, respectively. The newly designed compounds were characterized using various spectroscopic and analytical techniques like ¹H and ¹³C{¹H} nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) spectroscopy, high-resolution mass spectrometry (HRMS), and ultraviolet-visible (UV-visible) spectroscopy. The coordination mode of the ligand with palladium metal was authenticated with the help of single-crystal X-ray diffraction studies. Both palladium complexes showed distorted square planar geometry around the palladium center having NCSe pincer coordination mode. The NCSe palladium pincer complexes were utilized as a catalyst for regioselective cross-dehydrogenative coupling (CDC) of 2-arylthiophenes with hetero(arenes). Intriguingly, only 2.5 mol% catalyst loading is adequate for achieving higher yields (65–89%) of heterocoupled products. A broad substrate scope of hetero(arenes) showed excellent tolerance towards the developed protocol under mild reaction conditions and short reaction time. The poisoning experiments showed the homogeneous nature of the catalysis process. The plausible reaction mechanism was studied using control experiments and time-dependent HRMS studies. Among the complexes, the sterically bulky adamantyl side arm containing palladium pincer complex (C2) showed a higher yield of CDC reaction.