Carbon Dioxide Capture by a Metal–Organic Framework with Nitrogen-Rich Channels Based on Rationally Designed Triazole-Functionalized Tetraacid Organic Linker

A semirigid tetraacid linker <b>H</b>_<b>4</b><b>L</b> functionalized with 1,2,3-triazole was rationally designed and synthesized to access nitrogen-rich MOFs for selective adsorption of CO₂. The cadmium MOF, that is, <b>Cd-L</b>, obtained by the reaction of <b>H</b>_<b>4</b><b>L</b> with Cd­(NO₃)₂, is found to be a 3D porous framework structure that is robust to desolvation. Crystal structure analysis reveals channels that are decorated by the triazole moieties of <b>L</b>. Gas adsorption studies show that <b>Cd-L</b> MOF permits remarkable CO₂ uptake to the extent of 99 and 1000 cc/g at 1 and 30 bar, respectively, at 0 °C. While literature survey reveals that <b>MIL-112</b>, constructed from a 1,2,3-triazole functionalized linker, exhibits no porosity to gas adsorption due to structural flexibility, the results with <b>Cd-L</b> MOF described herein emphasize how rigidification of the organic linker improves gas uptake properties of the resultant MOF

Metal-Mediated Self-Assembly of a <i>Twisted</i> Biphenyl-Tetraacid Linker with Semi-rigid Core and Peripheral Flexibility: Concomitant Formation of Compositionally Distinct MOFs

A semi-rigid organic linker, namely, 3,3′,5,5′-tetra­kis­(4-(α-carboxy)­meth­oxy­phenyl)-2,2′,6,6′-tetra­methoxy-1,1′-biphenyl (<b>H</b>_<b>4</b><b>L</b>), was designed and synthesized to access metal–organic frameworks (MOFs). While the <i>ortho</i>-methoxy substituents in the biphenyl core of <b>H</b>_<b>4</b><b>L</b> were surmised to twist the aromatic planes and impart porosity to the resultant MOFs, the (α-car­boxy)­meth­oxy­phenyl moieties at the periphery were envisaged to enable requisite flexibility for metal–ligand coordination polymerization. The reactions of <b>H</b>_<b>4</b><b>L</b> with Cd, Mn, and Zn salts indeed yielded MOFs, i.e., <b>Cd-L</b>, <b>Mn-L</b>, <b>Zn-Lsqc</b>, and <b>Zn-Ldia</b>, with interesting structural features and unusual inorganic SBUs. In particular, the reaction of <b>H</b>_<b>4</b><b>L</b> with ZnI₂ in DMF at 90 °C over 2 days led to concomitant formation of a pair of <i>compositionally distinct</i> Zn-MOFs, i.e., <b>Zn-Ldia</b> and <b>Zn-Lsqc</b>, each of which could be accessed exclusively by controlling the reaction conditions. The diversity observed in the structures of MOFs formed with the linker <b>H</b>_<b>4</b><b>L</b> with a limited number of metal ions sufficiently emphasizes the importance of the attributes of the linker in the formation of 3D MOFs; the latter are desirable from the stability and exfoliation points of view when the MOFs are explored for applications such as gas storage, catalysis, etc. In corroboration of our previous results, it emerges that the flexibility that is built into the structure of the organic linker, both at the central core and at the periphery, leads to concomitant formation of compositionally distinct MOFs in addition to diverse SBUs and disparate framework topologies

Diverse Metal–Organic Materials (MOMs) Based on 9,9′-Bianthryl-Dicarboxylic Acid Linker: Luminescence Properties and CO₂ Capture

A fluorescent organic linker, namely, 10,10′-bis­(4-carboxyphenyl)-9,9′-bianthryl (<b>H</b>_<b>2</b><b>L</b>), was rationally designed and synthesized to access luminescent metal–organic materials (MOMs). A series of structurally diverse MOMs was synthesized with the diacid linker <b>H</b>_<b>2</b><b>L</b> by reacting it with main group, transition, and lanthanide metal ions under different conditions. Among them, <b>Zn-L</b> MOM is a 1D polymeric chain, while <b>Cd-L</b> is a 2D structure in which 4,4′-bipyridyls mediate the formation of 2D networks by linking up the 1D metal–carboxylate chains. The <b>Pb-L</b> MOM is found to be a 2D polymeric net, while <b>Sr-L</b>, obtained under similar reaction conditions, is a noninterpenetrated 3D polymeric structure; the extension from 2D to 3D framework occurs by mediation of Cl^– ions. Notably, the reaction of <b>H</b>_<b>2</b><b>L</b> with the lanthanide ions yielded isostructural 3D MOFs, i.e., <b>Tb-L</b>, <b>Eu-L</b>, <b>Sm-L</b>, <b>Nd-L</b>, <b>La-L</b>, <b>Pr-L</b>, <b>Gd-L</b>, and <b>Yb-L</b>, which are noninterpenetrated and porous. The Ln-MOFs are highly robust and stable to solvent exclusion. The representative Ln-MOFS, viz., <b>Tb-L</b>, <b>Eu-L</b>, and <b>Sm-L</b>, are shown to exhibit gas adsorption at ambient temperatures; the CO₂ uptake capacities are found to be in the range of the highest values observed for Ln-MOFs to date. All the MOMs, including lanthanide MOFs, exhibit linker-based luminescence in the solid state

Tetraarylbiphenyl as a New Lattice Inclusion Host by Structure Reductionism: Shape and Size Complementarity Based on Torsional Flexibility

3,3′,5,5′-Tetrakis­(2,6-dimethyl-4-methoxyphenyl)­biphenyl (<b>TB</b>) was designed as a lattice inclusion host based on a structure reductionistic approach. <b>TB</b> binds guests in two different domains based on the premise that the two phenyl rings of the biphenyl core remain coplanar in the solid state. The versatility of <b>TB</b> as a host has been demonstrated by its ability to include different guest molecules in the crystal lattice, as revealed by X-ray crystal structure analyses. One observes preference for guest location in the concave domain with the trough domain compromised. The host <b>TB</b> is found to exploit torsional freedom about the central σ-bond between the two phenyl rings of the biphenyl core to adopt discrete structures that are complementary in terms of shape and size to include a given guest. In other words, <b>TB</b> behaves like a “molecular chameleon” that undergoes structural adaptation in response to the guest via torsional twist about the central σ-bond. Quite remarkably, the inclusion compounds with different guests having similar torsional angles between the phenyl rings of the biphenyl core are found to be isostructural. The torsion-induced structural morphosis in response to the guest is found to completely offset binding in the trough region