Examining the effects of different ring configurations and equatorial fluorine atom positions on CO₂ Sorption in [Cu(bpy) ₂SiF₆]

Simulations of CO₂ sorption were performed in a metal-organic material (MOM) that is part of a "SIFSIX" family of compounds that has remarkable carbon dioxide capture and separation properties. The MOM considered here has the formula [Cu(bpy)₂SiF₆] (bpy = 4,4′-bipyridine). This hydrophobic MOM is both water-stable and CO ₂-specific with significant sorption capacity under ambient conditions. The crystal structure reveals bpy rings and equatorial fluorine atoms in multiple possible orientations; the static disorder has been modeled based on single-crystal X-ray diffraction data revealing several possible relatives of atoms in the crystal structure. With regards to the bpy rings, the structure can be interpreted as two pyridyl rings with coplanar configurations within a unit cell (configuration 1), a twisted bpy ring conformation in which orthogonal pyridyl rings have C₄ symmetry about the Cu²⁺ ion (configuration 2), and a twisted bpy ring conformation in which the two orthogonal pyridyl rings are facing one another within a unit cell (configuration 3). Further, the equatorial fluorine atoms can be positioned such that all atoms are eclipsed with the square grid (position A), oriented at a 21.3 angle with respect to the square grid (position B), and oriented at a 45 angle with respect to the square grid (position C). It was observed that experimental data for CO₂ sorption were only consistent with sorption into configurations 1 and 3 with any of the possible equatorial fluorine atom positions at ambient temperatures, although simulations using position A produced slightly higher uptakes in these bpy ring configurations. It is demonstrated that the orientation of the bpy rings in configurations 1 and 3 allows more space for the sorbate molecules and thus promotes favorable MOM-sorbate interactions, resulting in isotherms in line with the experimental results. The results from this study suggests that [Cu(bpy)₂SiF ₆] in either configuration 1 or 3 with CO₂ present in the pores at ambient temperatures is consistent with experimental sorption measurements and crystal structure data.