Direct arylation of heteroarenes by PEPPSI-type palladium–NHC complexes and representative quantum chemical calculations for the compound which the structure was determined by X-ray crystallography

In this study, we synthesized five new PEPPSI-type Pd-NHC complexes with high yields around 78–83%. The structures of all complexes were characterized by FT-IR, ¹H NMR, and ¹³C{¹H} NMR spectroscopies. Further, the structure of 2c was determined by X-ray crystallography. The single-crystal structure of 2c shows that coordination geometry around Pd is distorted square planar. The Pd-NHC complexes were efficient catalysts for the direct C5-arylation of 2-n-butylthiazole, 2-n-butylfuran, and 2-n-butylthiophene with various aryl-bromides and showed high catalytic activity for arylation reaction using only 1 mol% catalyst loading at 130 °C for 1 h. The conversions for substrates containing electron-withdrawing groups were higher than for substituents containing electron-donating groups. In order to evaluate the catalytic activity of the complexes, representative electronic and spectroscopic simulations by DFT method have been performed for 2c. NBO analysis has revealed that the strongest interaction contributing to the lowering of the molecular stabilization energy for 2c is predicted as π(C11-N27) → π*(N26-C28) with E⁽²⁾ = 73.93 kcal mol⁻¹. In addition, FMO analysis has implied that 2c is more chemically reactive because of its lower energy gap value. According to the other quantum chemical parameters, 2c is softer, better electrophile and has more charge-transfer capability than 1c.