Three-center-four-electron halogen bond enables non-metallic complex catalysis for Mukaiyama–Mannich-type reaction at ppm-level catalyst loading

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, 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 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 suggest that the robust catalytic activity of [N···X···N]Y can be attributed 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 for catalysis via halogen(I) complexes.