Porous Metal–Organic Polyhedra: Morphology, Porosity, and Guest Binding

Abstract

Designing porous materials which can selectively adsorb CO₂ or CH₄ is an important environmental and industrial goal which requires an understanding of the host-guest interactions involved at the atomic scale. Metal-organic polyhedra (MOPs) showing permanent porosity upon desolvation are rarely observed. We report a family of MOPs <b>(Cu-1a</b>, <b>Cu-1b</b>, <b>Cu-2</b>), which derive their permanent porosity from cavities between packed cages rather than from within the polyhedra. Thus, for <b>Cu-1a</b>, the void fraction outside the cages totals 56% with only 2% within. The relative stabilities of these MOP structures are rationalized by considering their weak nondirectional packing interactions using Hirshfeld surface analyses. The exceptional stability of <b>Cu-1a</b> enables a detailed structural investigation into the adsorption of CO₂ and CH₄ using <i>in situ</i> X-ray and neutron diffraction, coupled with DFT calculations. The primary binding sites for adsorbed CO₂ and CH₄ in <b>Cu-1a</b> are found to be the open metal sites and pockets defined by the faces of phenyl rings. More importantly, the structural analysis of a hydrated sample of <b>Cu-1a</b> reveals a strong hydrogen bond between the adsorbed CO₂ molecule and the Cu(II)-bound water molecule, shedding light on previous empirical and theoretical observations that partial hydration of metal-organic framework (MOF) materials containing open metal sites increases their uptake of CO₂. The results of the crystallographic study on MOP-gas binding have been rationalized using DFT calculations, yielding individual binding energies for the various pore environments of <b>Cu-1a</b>.