Theoretical study on the interactions of halogen‐bonds and pnicogen‐bonds in phosphine derivatives with Br2, BrCl, and BrF

The MP2 ab initio quantum chemistry methods were utilized to study the halogen-bond and pnicogen-bond system formed between PH2X (X = Br, CH3, OH, CN, NO2, CF3) and BrY (Y = Br, Cl, F). Calculated results show that all substituent can form halogen-bond complexes while part substituent can form pnicogen-bond complexes. Traditional, chlorine-shared and ion-pair halogen-bonds complexes have been found with the different substituent X and Y. The halogen-bonds are stronger than the related pnicogen-bonds. For halogen-bonds, strongly electronegative substituents which are connected to the Lewis acid can strengthen the bonds and significantly influenced the structures and properties of the compounds. In contrast, the substituents which connected to the Lewis bases can produce opposite effects. The interaction energies of halogen-bonds are 2.56 to 32.06 kcal·mol−1; The strongest halogen-bond was found in the complex of PH2OH•••BrF. The interaction energies of pnicogen-bonds are in the range 1.20 to 2.28 kcal·mol−1; the strongest pnicogen-bond was found in PH2Br•••Br2 complex. The charge transfer of lp(P) σ*(BrY), lp(F) σ*(BrP), and lp(Br) σ*(XP) play important roles in the formation of the halogen-bonds and pnicogen-bonds, which lead to polarization of the monomers. The polarization caused by the halogen-bond is more obvious than that by the pnicogen-bond, resulting in that some halogen-bonds having little covalent character. The symmetry adapted perturbation theory (SAPT) energy decomposition analysis showes that the halogen-bond and pnicogen-bond interactions are predominantly electrostatic and dispersion, respectively.