Halogen bonds with N-heterocyclic carbenes as halogen acceptors: a partially covalent character

N-heterocyclic carbenes (NHCs) are important ligands in organometallic catalytic reactions. This study focuses on the halogen bonds with NHCs as the electron donors. The results show that NHCs are better electron donors in halogen bonds. Our interest is how to make a halogen bond having a partially covalent property, which depends on the strength of halogen bonding. The covalent property of halogen bonding is related to the nature of the halogen donor. Iodine is favourable to form a halogen bond with covalent property than chlorine and bromine. The covalent property of halogen bond is greatly affected by substituents. Strong electron-donating groups in NHCs could reinforce the covalent attribute of bromine bonding, whereas strong electron-withdrawing groups in NHCs make iodine bonding lose the covalent nature. The covalent property of halogen bond is further heightened through the cooperative effect between the carbene-halogen bond and another interaction. This covalent property results in a much short binding distance and a prominent bond elongation. The covalent property of halogen bond has been analysed with the energy density, electron local function, and electron density shifts.