Effect of N-heterocyclic carbene in palladium–tin heterobimetallic catalysis: a DFT supported study on the C3–H functionalization of unprotected indoles†

A new N-heterocyclic carbene (NHC)-based Pd–Sn heterobimetallic complex [(NHC)Pd(μ-Br)SnCl₃]₂ with low steric crowding in the active catalytic region was designed. The reactivity of this complex was compared with a previously reported NHC–Pd–Sn complex (NHC)₂PdBrSnCl₃ towards the C3–H functionalization of indoles with styrenes. The role of NHC in heterobimetallic catalysis was investigated in terms of the density functional theory by comparing the reactivity of these catalysts along with a cyclooctadiene (COD)-based Pd–Sn catalyst (COD)PdClSnCl₃. Thermochemical calculations revealed that the catalytic cycle is thermodynamically favorable in the case of the NHC-based catalysts but not in the case of the COD-based one. An inverse kinetic isotope effect with a k_H/k_D of 0.44 was detected. This alludes to the migration of a hydride from the metal center to the carbon center being the rate-determining step of the reaction. Based on the experimental and computational findings, an appropriate mechanism is proposed.