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

This report describes the synthesis of a new NNSe pincer ligand and its palladium pincer complexes. Surprisingly, the reactivity of the ligand toward the palladium precursor is base-dependent. In the presence of the Et₃N base, a mononuclear pincer complex was formed whereas in the absence of a base, a dinuclear pincer complex was formed. The complexes were used as catalysts for decarboxylative direct C−H heteroarylation of (hetero)arenes. Among the complexes, the dinuclear complex was found to be more reactive. Only 2.5 mol % catalyst loading was needed to activate a broad substrate scope.

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 palladium pincer complex (C1) was isolated, while in the presence of 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 the 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 (43–87 % 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 (hetero)arenes.

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