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Synthetic Supercontainers Exhibit Distinct Solution versus Solid State Guest-Binding Behavior

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posted on 21.05.2014 by Feng-Rong Dai, Uma Sambasivam, Alex J. Hammerstrom, Zhenqiang Wang
The phase-dependent host–guest binding behavior of a new family of synthetic supercontainers has been probed in homogeneous solution and at liquid–liquid, solid–liquid, and solid–gas interfaces. The synthetic hosts, namely, type II metal–organic supercontainers (MOSCs), are constructed from the assembly of divalent metal ions, 1,4-benzenedicarboxylate (BDC) linker, and sulfonylcalix[4]­arene-based container precursors. One member of the MOSCs, MOSC-II-tBu-Ni, which is derived from Ni­(II), BDC, and p-tert-butylsulfonylcalix­[4]­arene (TBSC), crystallizes in the space group R3̅ and adopts pseudo face-centered cubic (fcc) packing, whereas other MOSCs, including TBSC analogue MOSC-II-tBu-Co, p-tert-pentylsulfonylcalix­[4]­arene (TPSC) analogues MOSC-II-tPen-Ni/Co, and p-tert-octylsulfonylcalix­[4]­arene (TOSC) analogues MOSC-II-tOc-Ni/Mg/Co, all crystallize in the space group I4/m and assume a pseudo body-centered cubic (bcc) packing mode. This solid-state structural diversity is nevertheless not reflected in their solution host–guest chemistry, as evidenced by the similar binding properties of MOSC-II-tBu-Ni and MOSC-II-tBu-Co in solution. Both MOSCs show comparable binding constants and adsorb ca. 7 equiv of methylene blue (MB) and ca. 30 equiv of aspirin in chloroform. In contrast, the guest-binding behavior of the MOSCs in solid state reveals much more variations. At the solid–liquid interface, MOSC-II-tBu-Co adsorb ca. 5 equiv of MB from an aqueous solution at a substantially faster rate than MOSC-II-tBu-Ni does. However, at the solid–gas interface, MOSC-II-tBu-Ni has higher gas uptake than MOSC-II-tBu-Co, contradicting their overall porosity inferred from the crystal structures. This discrepancy is attributed to the partial collapse of the solid-state packing of the MOSCs upon solvent evacuation. It is postulated that the degree of porosity collapse correlates with the molecular size of the MOSCs, i.e., the larger the MOSCs, the more severe they suffer from the loss of porosity. The same principle can rationalize the negligible N2 and O2 adsorption seen in the larger MOSC-II-tPen-Co and MOSC-II-tOC-Ni/Mg/Co molecules. MOSC-II-tPen-Ni features an intermediate molecular size and endures a partial structural collapse in such a way that the resulting pore dimension permits the inclusion of kinetically smaller O2 (3.46 Å) but excludes larger N2 (3.64 Å), explaining the observed remarkable O2/N2 adsorption selectivity.

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