Guest Adsorption in the Nanoporous Metal-Organic Framework Cu3(1,3,5-Benzenetricarboxylate)2: Combined In Situ X-ray Diffraction and Vapor Sorption

The structure of the nanoporous metal-organic framework Cu-3(BTC)(2) (BTC = 1,3,5-benzenetricarboxylate) with a variety of molecular guests was studied in situ using single crystal X-ray diffraction. By collecting crystal structure data for a series of guests within the same host crystal, insights into the molecular interactions underpinning guest adsorption processes have been gained. Adsorption behaviors are influenced strongly by both enthalpic and entropic thermodynamic, as well as interpore steric (size-exclusion) effects, and we note correlations between guest attributes and these effects. Vapor adsorption measurements revealed a guest uptake capacity inversely proportional to guest size. Correspondingly, structural results show that guests reside in the smallest pores accessible to them. Interpore steric effects for larger guests cause these to be excluded from the smallest pores, and this corresponds to lower total uptake. Both hydrophilic and lipophilic small guests adsorb favorably into the 5 angstrom diameter smallest pore of the material, with the number of guests in these pores dependent on guest size and their location, in turn dependent upon both guest guest interactions and competition between hydrogen-bonding interactions at the apertures of the smallest pore and lipophilic interactions at the center of the smallest pore. Hydrophilic guests with lone electron pairs interact preferentially with the coordinatively unsaturated Cu sites of the desolvated framework, with the number of these depending on steric interactions between neighboring bound guests and guest flexibility. Guest coordination at the Cu sites has a significant effect on the framework structure, increasing the Cu center dot center dot center dot Cu distance in the dinuclear unit, with the Cu-3(BTC)(2) unit cell being smaller when guests that do not coordinate with the Cu are present, and in the case of cyclohexane, smaller than for the desolvated framework. Overall, our comprehensive structural study reconciles Cu-3(BTC)(2) adsorption properties with the underlying guest host and guest guest interactions that gives rise to these.