The nature of the central halide encapsulation in bambusuril hosts (BU[6]). Structural and interaction energy insights in BU[6]·X− (X = Cl, Br, I) from relativistic DFT calculations

Abstract

Insights into host–guest complexation is a relevant issue towards the formation of efficient supramolecular arrays. Here, we explore the favorable encapsulation of halide anions in the representative bambus[6]uril host, with properties contrasting to the well explored in the cucurbituril family. Bambus[6]uril enables a positively charged cavity supporting the encapsulate halide ions at the center of the structure via 12C-H···X− hydrogen bonds interactions (X = F, Cl, Br and I). Our results unravel the nature of such interaction where both electrostatic and Pauli repulsion terms are relevant in the efficient encapsulation, with an increasing contribution of the orbital term for the iodine counterpart. The dispersion term contributes to a lesser extent, in contrast to the cucurbituril case. From fluorine to iodine, the contribution of different dipole–dipole interactions becomes more relevant, in agreement to the softer character of the host. Moreover, a sizable encapsulation barrier is found for F− and Cl− cases, which requires to be overwhelmed to ensure central coordination, leading to an overall bond dissociation energy (BDE) in the order Br− ∼ I− > Cl− > F−, in agreement to the experimental trend. In addition, only BU[6]·I− pair appears with a packing coefficient (PC) within the suggested favorable range (PC = 55–68%), which favors its larger aggregation constant found experimentally. A host structure breathing is suggested between D3d ↔ C1 structures, providing a flexible cavity in the bambusuril family able to maximize host–guest interactions, as observed in the case of BU[6]·I−.