The role of water molecular structuring in the formation of the inclusion compounds based on cucurbit[8]uril and trans-[Co(en)2Cl2]+, trans-[Ru(en)2Cl2]+ complexes: a DFT examination

This paper presents the results of studying the structural features of inclusion compounds based on the cucurbit[8]uril macrocycle and the cobalt(III) and ruthenium(III) complexes (trans-[Co(en)₂Cl₂]⁺ and trans-[Ru(en)₂Cl₂]⁺) within the framework of the density functional theory taking into account the course of the reaction in the aqueous solution. Formation thermodynamics of the complexes in the cavitands was evaluated by taking into account the most probable number of water molecules inside cucurbit[8]uril. In this methodology, the complexation was considered as a substitution reaction in which the guest complex displaces partially or completely the water molecules that are located inside the cavity. The water molecules present in the cavitand were shown to play an important role in the fixation of the guest complex inside the cavity due to the hydrogen bonds with the oxygen portals. The role of water molecules in the fixation of the complex in the cavity of the macrocycle with the formation of a particular structure is shown. The symmetrical arrangement of the fixing water molecules in the portals leads to a more symmetric arrangement of the metal complex in the cucurbit[8]uril cavity, while their asymmetric localization leads to a significant turn of the complex. Comparison of the main geometric characteristics according to the calculation and X-ray structural analysis showed good agreement. The thermodynamic parameters of the formation of the inclusion compound are estimated. It is shown that the energy contribution of macrocycle deformation to the energetics of formation of the inclusion compound is insignificant.