Mono and Dinuclear Palladium Pincer Complexes of NNSe Ligand as a Catalyst for Decarboxylative Direct C-H Heteroarylation of (Hetero)arenes.

Abstract

This report describes the synthesis of a new NNSe pincer ligand and its mono and dinuclear palladium(II) pincer complexes. In the absence of a base, a dinuclear pincer palladium complex (C1) was isolated, while in the presence of an Et3N base a mononuclear palladium pincer complex (C2) was obtained. The new ligand and complexes were characterized using techniques like 1H, 13C{1H} Nuclear Magnetic Resonance (NMR), Fourier Transform Infrared (FTIR), High-Resolution Mass Spectrometry (HRMS), Ultraviolet-visible (UV-Visible), and cyclic voltammetry. Both complexes showed pincer coordination mode with a distorted square planar geometry. The complex C1 has two pincer ligands attached through a Pd-Pd bond in a dinuclear pincer fashion. The air and moisture-insensitive, thermally robust palladium pincer complexes were used as the catalyst for decarboxylative direct C-H heteroarylation of (hetero)arenes. Among the complexes, dinuclear pincer complex C1 showed better catalytic activity. A variety of (hetero)arenes were successfully activated (48-87% yield) using only 2.5 mol% of catalyst loading under mild reaction conditions. The PPh3 and Hg poisoning experiments suggested a homogeneous nature of catalysis. A plausible reaction pathway was proposed for the dinuclear palladium pincer complex catalyzed decarboxylative C-H bond activation reaction of heteroarenes.