Molecular insights into the complex formation between dodecamethylcucurbit[6]uril and phenylenediamine isomers

The complexation behavior of diprotonated phenylenediamine isomers with decamethylene cucurbit[6]uril (Me₁₂Q[6]) in 1:1 and 1:2 stoichiometry was investigated in vacuum and solution using density functional theory (DFT). Among the isomers, o-phenylenediamine (oPDA) forms an exclusion complex in 1:1 stoichiometry, while others form an inclusion complex. In the 1:2 stoichiometry, at least one of the guest molecule lie on the surface of the Me₁₂Q[6] cavity. The structural reorganization was found to depend on the mode of interaction of the guest molecule. In the gas phase, the enthalpy and Gibbs free energy are negative for all the complexes demonstrating the encapsulation process is spontaneous and thermodynamically favorable. In the solution phase, the enthalpy and entropy are negative for complexes with guest molecules meta- and para-isomers of phenylenediamine. For oPDA as a guest, the enthalpy and entropy are positive indicating the complex formation to be nonspontaneous. The MESP isosurface of complexes shows a higher accumulation of charge in the 1:2 stoichiometry, which reduces their stability. AIM analysis shows the interaction between oPDA and Me₁₂Q[6] is due to hydrogen bonding with moderate strength, while for the other isomers, interactions between the benzene group and the Me₁₂Q[6] cavity were noticed. EDA analysis shows the larger contribution is the electrostatic attraction and it decreases with the increase in the guest ratio. The formation of inclusion complexes by mPDA and pPDA and the surface adsorption of oPDA on Me₁₂Q[6] and the higher accumulation of positive charge during solvation in oPDA@Me₁₂Q[6] complexes make them labile in the solution phase.

Graphical abstract