Three-center-four-electron halogen bond enables sub-ppm-level non-metallic complex catalysis

The three-center-four-electron halogen bond (3c4e X-bond) presents a new design concept for catalysis. By integrating halogen(I) (X+: I+ or Br+), bis-pyridyl ligand NN, and non-nucleophilic counter anion Y. [N···X···N]SbF6, we developed non-metallic complex catalysts, [N···X···N]Ys, which exhibited outstanding activity and facilitated the Mukaiyama–Mannich-type reaction of N-heteroaromatics with sub-ppm-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 attribute the robust catalytic activity of [N···X···N]Y to the unique feature of the 3c4e X-bond for binding chloride: i) the partial covalent nature transforms [N···X···N]+ complexation to sp2CH 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 a new application of 3c4e X-bonds to catalysis via halogen(I) complexes.