Computational simulation of anion binding association mechanisms contributing toward rotation of pyrrole rings in dipyrrolyldiketone BF2 complexes

Abstract

Dipyrrolyldiketone BF₂ complexes have a characteristic association mechanism with anions; an anion is tightly captured by the NH bonding of the two rotated pyrrole rings and the CH bonding of the backbone in dipyrrolyldiketone BF₂ complexes. In order to elucidate this anion association mechanism in dipyrrolyldiketone BF₂ complexes, the relative stability of several conformers of the dipyrrolyldiketone BF₂ complex and anion complexes are initially calculated by quantum mechanical density functional theory (DFT). Second, molecular dynamics (MD) simulations were performed for systems comprising a dipyrrolyldiketone BF₂ complex and Cl⁻ with several types of countercations in CH₂Cl₂. From our DFT calculations, it was observed that isomerization could be classified by whether Cl⁻ changes its position relative to the dipyrrolyldiketone BF₂ complex or not. From the MD simulations, we found the size effect of countercations: a large cation is released from dipyrrolyldiketone BF₂ complexes more frequently than smaller cations. Meanwhile, when Na⁺ is employed as the cation, the formation of a seven-membered ring-like cluster suppresses both the isomerizations. Furthermore, MD simulations suggest that the presence of an anion is critical to stabilize the two rotated pyrrole rings. Hence, from our MD simulations, we have clearly found that the size of the cation affects the isomerization processes.