Boosting CO2 and benzene adsorption through π-hole substitution in β-diketonate Cu(ii) complex within non-porous adaptive crystals

Abstract

The effect of quadrupole moments in non-porous adaptive crystals of fully, partially, and non-fluorinated β-diketonate Cu(II) complexes on CO₂ and hydrocarbon adsorption was systematically investigated using structurally similar models with distinct electronic properties. The fully fluorinated complex significantly enhanced CO₂ adsorption, particularly at low pressures (<0.1 P/P₀), achieving a 1 : 1 stoichiometric ratio through quadrupole interactions, where the positively polarized regions of electrostatic potentials (ESPs) on the Cu(II) center and pentafluorophenyl rings facilitated CO₂ binding via its quadrupole nature. The perfluorinated complex also exhibited a stepwise vapor adsorption of benzene (C₆H₆), exhibiting distinct hysteresis and a 1 : 3 stoichiometric ratio, driven by M⋯π and π-hole⋯π interactions. In contrast, the partially fluorinated complex and non-fluorinated [Cu(dbm)₂] (dbm = dibenzoylmethanido⁻) showed significantly reduced adsorption capabilities, reflecting the critical role of quadrupole moments and charge distribution in molecular recognition. The poor guest insertion of hexafluorobenzene (C₆F₆) into the perfluorinated complex highlighted the impact of electrostatic repulsion between similarly positive quadrupole moments. The gas adsorption studies further demonstrated differences in the kinetics and adsorption behavior of CO₂, C₂H_n (n = 2, 4, 6), and aromatic vapors, underscoring the importance of quadrupole design. These findings provide a rational framework for the development of advanced host–guest materials tailored for selective adsorption and separation applications.