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

Stephen P. Argent, Ivan da Silva, Alex Greenaway, Mathew Savage, Jack Humby, Andrew J. Davies, Harriott Nowell, William Lewis, Pascal Manuel, Chiu C. Tang, Alexander J. Blake, Michael W. George, Alexander Markevich, Elena Besley, Sihai Yang, Neil R. Champness, Martin Schröder

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 (Cu-1a, Cu-1b, Cu-2), which derive their permanent porosity from cavities between packed cages rather than from within the polyhedra. Thus, for Cu-1a, 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 Cu-1a enables a detailed structural investigation into the adsorption of CO2 and CH4 using in situ X-ray and neutron diffraction, coupled with DFT calculations. The primary binding sites for adsorbed CO2 and CH4 in Cu-1a 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 Cu-1a 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 Cu-1a.

Physics of Nanostructured Materials
External organisation(s)
University of Nottingham, Diamond Light Source Ltd, University of Manchester, Rutherford Appleton Laboratory
Inorganic Chemistry
No. of pages
Publication date
Peer reviewed
Austrian Fields of Science 2012
104003 Inorganic chemistry, 103018 Materials physics
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