Susan Sen

High Proton Conductivity by a Metal–Organic Framework Incorporating Zn8O Clusters with Aligned Imidazolium Groups Decorating the Channels

A novel metal-organic framework, [{(Zn(0.25))(8)(O)}Zn(6)(L)(12)(H(2)O)(29)(DMF)(69)(NO(3))(2)](n) (1) {H(2)L = 1,3-bis(4-carboxyphenyl)imidazolium}, has been synthesized under solvothermal conditions in good yield. It shows a Zn(8)O cluster that is coordinated to six ligands and forms an overall three-dimensional structure with channels along the crystallographic a and b axes. The imidazolium groups of the ligand moiety are aligned in the channels. The channels are not empty but are occupied by a large number of DMF and water molecules. Upon heating, these solvent molecules can be removed without breakdown of the overall structure of the framework as shown by variable-temperature powder X-ray diffraction patterns. Of great interest is the fact that the compound exhibits high proton conductivity with a low activation energy that is comparable to those of Nafion presently used in fuel cells.

Substitution at the metal center of coordination polymers in single-crystal-to-single-crystal (SC-SC) transformation

With the explosive growth in the synthesis of coordination polymers, several systems are now available which exhibit transformation without losing single crystallinity. Among different types of single-crystal-to-single-crystal (SC-SC) transformations, those that involve the first coordination sphere of the metal centers are only a few in number. Nevertheless, they constitute an important case of SC-SC transformation because they are likely to activate small molecules inside the pores, can lead to separation of geometrical isomers, and so on. In this highlight, we have focused our attention on several examples of SC-SC transformations at the metal center. As the crystallinity is maintained in these cases, great insights into the entire process can be obtained that are potentially important in the development of new and technologically useful nano-scale devices and sensors. Besides, they can help in designing new catalytic systems.

Binding of various anions in laterally non-symmetric aza-oxa cryptands through H-bonds: characterization of water clusters of different nuclearity

The X-ray crystallographic study of various anionic complexes have been performed on two laterally non-symmetric aza-oxa cryptands Lo and Lm having different cavity dimension. The anions are bound through various N/C/O–H⋯X (X = anion) bonding interactions with the cryptand receptors. For the chloride complex of Lo (complex 1), the encapsulated chloride resides perfectly on C3 axis and prefers to stay at the ‘oxa’ end despite H- bonding interactions with protonated ‘aza’ N atoms. In case of the perchlorate (complex 2), the anion remains outside the cavity. The anions occupy the cavity created by four arms of two cryptand molecules and also the space provided by the two arms of a single cryptand molecule. Although, Lm has a larger cavity, perchlorate still remains outside (complex 3). To probe preferences for encapsulation, cryptand Lm is allowed to react with three different binary mixtures of acids. For the case of HCl–H2SO4 and HCl-HNO3 binary systems (complex 4 and 5, respectively), Cl− is incorporated in the cavity leaving SO4− or NO3− outside. In case of HBr-HNO3 mixture, NO3− is preferred over Br− in the cavity (complex 6). In each case for Lm, the encapsulated anion is displaced away from ‘oxa’ end to the ‘aza’ end of the cavity. Interestingly, in 2–6 the secondary amino N in the bridges and the bridgehead N at the ‘aza’ end gets protonated. In these compounds, water clusters of various nuclearity have been identified.

Role of spacer in single- or two-step FRET: studies in the presence of two connected cryptands with properly chosen fluorophores

Two cryptand molecules are connected via p-xyloyl, benzene-1,4-dicarbolyol and 9,10-dimethylene anthracene. Each cryptand is further derivatized with fluorophores such that electronic absorption of one fluorophore overlaps emission of the other. This way, three different systems L(1), L(2) and L(3) have been synthesized to get a better view of the effect on the distance in single- and two-step fluorescence resonance energy transfer (FRET) process. These molecules are probed for FRET in the presence of a metal ion as input. L(1) and L(2) exhibit poor performance in single step FRET while in the case of L(3), a large two-step FRET process is operational with Cu(II) or Hg(II) as input.

Reversible Switching between Highly Porous and Nonporous Phases of an Interpenetrated Diamondoid Coordination Network that Exhibits Gate‐Opening at Methane Storage Pressures

Herein, we report that a new flexible coordination network, NiL2 (L=4-(4-pyridyl)-biphenyl-4-carboxylic acid), with diamondoid topology switches between non-porous (closed) and several porous (open) phases at specific CO2 and CH4 pressures. These phases are manifested by multi-step low-pressure isotherms for CO2 or a single-step high-pressure isotherm for CH4 . The potential methane working capacity of NiL2 approaches that of compressed natural gas but at much lower pressures. The guest-induced phase transitions of NiL2 were studied by single-crystal XRD, in situ variable pressure powder XRD, synchrotron powder XRD, pressure-gradient differential scanning calorimetry (P-DSC), and molecular modeling. The detailed structural information provides insight into the extreme flexibility of NiL2 . Specifically, the extended linker ligand, L, undergoes ligand contortion and interactions between interpenetrated networks or sorbate-sorbent interactions enable the observed switching.

Readily accessible shape-memory effect in a porous interpenetrated coordination network

An interpenetrated flexible metal-organic material exhibits only the second example of a shape-memory effect in a porous material. Shape-memory effects are quite well-studied in general, but there is only one reported example in the context of porous materials. We report the second example of a porous coordination network that exhibits a sorbate-induced shape-memory effect and the first in which multiple sorbates, N2, CO2 and CO promote this effect. The material, a new threefold interpenetrated pcu network, [Zn2(4,4′-biphenyldicarboxylate)2(1,4-bis(4-pyridyl)benzene)]n (X-pcu-3-Zn-3i), exhibits three distinct phases: the as-synthesized α phase; a denser-activated β phase; and a shape-memory γ phase, which is intermediate in density between the α and β phases. The γ phase is kinetically stable over multiple adsorption/desorption cycles and only reverts to the β phase when heated at >400 K under vacuum. The α phase can be regenerated by soaking the γ phase in N,N′-dimethylformamide. Single-crystal x-ray crystallography studies of all three phases provide insight into the shape-memory phenomenon by revealing the nature of interactions between interpenetrated networks. The β and γ phases were further investigated by in situ coincidence powder x-ray diffraction, and their sorption isotherms were replicated by density functional theory calculations. Analysis of the structural information concerning the three phases of X-pcu-3-Zn-3i enabled us to understand structure-function relationships and propose crystal engineering principles for the design of more examples of shape-memory porous materials.

Efficient CO2 Removal for Ultra-Pure CO Production by Two Hybrid Ultramicroporous Materials

Removal of CO2 from CO gas mixtures is a necessary but challenging step during production of ultra-pure CO as processed from either steam reforming of hydrocarbons or CO2 reduction. Herein, two hybrid ultramicroporous materials (HUMs), SIFSIX-3-Ni and TIFSIX-2-Cu-i, which are known to exhibit strong affinity for CO2 , were examined with respect to their performance for this separation. The single-gas CO sorption isotherms of these HUMs were measured for the first time and are indicative of weak affinity for CO and benchmark CO2 /CO selectivity (>4000 for SIFSIX-3-Ni). This prompted us to conduct dynamic breakthrough experiments and compare performance with other porous materials. Ultra-pure CO (99.99 %) was thereby obtained from CO gas mixtures containing both trace (1 %) and bulk (50 %) levels of CO2 in a one-step physisorption-based separation process.