Three-center-four-electron halogen bond enables non-metallic complex catalysis for Mukaiyama-Mannich-type reaction

The three-center-four-electron halogen bond (3c4e X-bond) presents a fundamental design concept for catalysis. By integrating halogen(I) (X⁺: I⁺ or Br⁺), the bis-pyridyl ligand NN, and a non-nucleophilic counteranion Y, we developed non-metallic complex catalysts, [N···X···N]Ys, that exhibited outstanding activity and facilitated the Mukaiyama-Mannich-type reaction of N-heteroaromatics with parts-per-million-level catalyst loading. The high activity of [N···X···N]SbF₆ was clearly demonstrated. NMR titration experiments, CSI-MS, computations, and UV-vis spectroscopic studies suggest that the robust catalytic activity of [N···X···N]Y can be attributed to the unique ability of the 3c4e X-bond for binding chloride: i) the covalent nature transforms the [N···X···N]⁺ complexation to sp² CH as a hydrogen-bonding donor site, and ii) the noncovalent property allows for the dissociation of [N···X···N]⁺ for the formation of [Cl···X···Cl]^-. This study introduces the application of 3c4e X-bonds in catalysis via halogen(I) complexes.