The halogen(I) complex of astatine: a theoretical perspective on structural and bonding properties

Halonium ions (X⁺) can interact with two Lewis bases to form linear 3c4e halogen-bonded halogen(I) complexes ([D⋯X⋯D]⁺), which have been found to be useful in organic synthesis and supramolecular chemistry. However, current research is limited to lighter halogens (F, Cl, Br, and I) and does not include the At element owing to the lack of stable isotopes for experimental studies. Herein, we explore the structural and bonding properties of an At-mediated 3c4e halogen(I) complex ([D⋯At⋯D]⁺) and the effects of various Lewis bases, substituents, and halogens using relativistic density functional theory (DFT) and the coupled-cluster approach with single, double and perturbative triple excitation (CCSD(T)) calculations. Theoretical calculation results show that At, similar to other halogens, can form a linear [D⋯At⋯D]⁺ structure with equal bond lengths from the halonium ion to two donor atoms. The physical nature of the interaction and electronic structure of the At-mediated 3c4e halogen(I) complex are the same as those of the halogen(I) complexes of light halogens. Interestingly, the positive correlation between the polarizability of the halogen and the interaction between D and [D⋯X]⁺ fragments (X = F to At) observed at the scalar relativistic level does not hold when considering spin-orbit coupling effects on the At atom. This work deepens the understanding of the halogen bonds of At, and the stable [D⋯At⋯D]⁺ structure offers new insights into At coordination chemistry and the relevant experimental study of radiolabeling of At.