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

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/P 0), 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.