A comparison between (a/n-NHC)PdX2(pyridine) and (a/n-NHC)2PdX2 (X = I, Cl) type complexes of abnormal fused-bicyclic imidazo[1,2-a]pyridine based N-heterocyclic carbene (a-NHC) and of normal imidazole based N-heterocyclic carbene (n-NHC) ligands in the Suzuki-Miyaura coupling reactions

Abstract

A comparison between (a/n-NHC)PdX2(pyridine) (1b, 2b and 3) and (a/n-NHC)2PdX2 (X = halide) (1c, 2c and 4) type complexes of abnormal fused-bicyclic imidazo[1,2-a]pyridine framework derived N-heterocyclic carbenes (a-NHC) and of the ubiquitous normal imidazole based N-heterocyclic carbenes (n-NHC) in Suzuki-Miyaura coupling reactions, revealed near comparable yields between the two (a/n-NHC)PdX2(pyridine) and (a/n-NHC)2PdX2 type complexes of the (a-NHC) and the (n-NHC) ligands. Indeed, the Density Functional Theory (DFT) studies performed on all of the (a/n-NHC)PdX2(pyridine) (1b, 2b and 3) and (a/n-NHC)2PdX2 type complexes (1c, 2c and 4), indicated that as the latter (1c, 2c and 4), with two (a/n-NHC) ligands bound to the metal center were only marginally electron rich than the former (1b, 2b and 3), containing only one (a/n-NHC) ligand bound to metal center, no correlation of the electron richness of the metal centers with the catalysis yields was observed for these complexes. In this regard, the (a-NHC)PdI2(pyridine) (1b and 2b) and the (a-NHC)2PdI2 (1c and 2c) type complexes of two new abnormal fused-bicyclic imidazo[1,2-a]pyridine framework derived N-heterocyclic carbenes (a-NHC) have been synthesized and structurally characterized.