Atomic Transfer in Halogen-Bonded Complexes Mediated by Polarizing Environments: Mimicking Intra- and Intermolecular Effects in a Series of Cocrystals of N-Bromosaccharin with Pyridines

The good halogen bond donor N-bromosaccharin (NBrSac) has been cocrystallized with four para -substituted pyridines (Py X ) as good halogen bond acceptors of different strengths (PyCN, PyCF 3, PyCO 2 Me, and PyMe). Their crystal structures have been determined and permitted to observe the formation of adducts with a significantly weakened N Sac ···Br bonding interaction with respect to the NBrSac donor and the formation of a strong Br···N Py interaction. The topological properties at the N Sac ···Br···N Py bond critical points (BCPs) of the adducts frozen at experimental geometries indicate the increasing strength of the studied acceptors along the series PyCN < PyCF 3 < PyCO 2 Me < PyMe, correlating with the structural distances and the deeper negative molecular electrostatic potential values in the individual molecules. The polarizability calculated in individual molecules clearly points out the atomic dipole moments of the N Sac, Br, and N Py atoms, increasing in magnitude within the adducts and demonstrating their sensitivity to external electric fields mimicking molecular environments. In order to explore the effect of polarizing environments on the N Sac ···Br···N Py halogen bonding motif, an external electric field has been applied (−40 × 10 –4 < ε < 100 × 10 –4 a.u.) to adducts extracted from experimental geometries with all atoms frozen except Br, which has been optimized as a function of ε. It has been shown that an effective electric field ranging from 1.28 to 2.96 GV m –1 is necessary to recover the experimental position of the Br-atom in the adducts (from that calculated in gas-phase optimization), providing an evaluation of the effect of the crystalline environment. At any investigated geometry (i.e., within the full range of applied fields), the adducts exhibit a significant covalence degree at both donor and acceptor sides (measured by 1 < | V |/ G < 2), in addition to a nonnegligible delocalization index (DI(N Sac |N Py )) in the range of 0.085–0.099. These features indicate that the adducts should be considered as a unique molecular entity rather than two while pointing to an assembly with a small but nonnegligible contribution of a three-center four-electrons interaction. The formal border of the halogen atom transfer between donor and acceptor moieties is established by the topological magnitudes of ρ and | V |/ G at both BCPs, as well as that of DI, which all balance almost simultaneously in donor and acceptor regions. The position of the halogen atom within the adduct is straightforwardly driven by the external polarization induced by ε, permitting control of the significant variation of the dipole moment of the adducts.

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