Terpyridine-Based 3D Metal-Organic-Frameworks: A Structure-Property Correlation.

Design, synthesis, and applications of metal-organic frameworks (MOFs) are among the most salient fields of research in modern inorganic and materials chemistry. As the structure and physical properties of MOFs are mostly dependent on the organic linkers or ligands, the choice of ligand system is of utmost importance in the design of MOFs. One such crucial organic linker/ligand is terpyridine (tpy), which can adopt various coordination modes to generate an enormous number of metal-organic frameworks. These frameworks generally carry physicochemical characteristics induced by the π-electron-rich (basically N-electron-rich moiety) terpyridines. In this minireview, the construction of 3D MOFs associated with symmetrical terpyridines is discussed. These ligands can be easily derivatized at the lateral phenyl (4'-phenyl) position and incorporate additional organic functionalities. These functionalities lead to some different binding modes and form higher dimensional (3D) frameworks. Therefore, these 3D MOFs can carry multiple features along with the characteristics of terpyridines. Some properties of these MOFs, like photophysical, chemical selectivity, photocatalytic degradation, proton conductivity, and magnetism, etc. have also been discussed and correlated with their frameworks.

Two- and Three-Dimensional Heterometallic Ln[Ru2-α-Ammonium Diphosphonate] Nets: Structures, Porosity, Magnetism, and Proton Conductivity.

By virtue of its magnetic moment and variable valencies, the D4h paddlewheel ruthenium dimer is a desirable molecular building block for the construction of functional networks. If in addition to the Ru open axial site the organic ligand is designed to allow transversal connections, a wider range of structural possibilities is expected. The organic ligand can also be modified to introduce other functionalities. In the present study we employed a diphosphonate containing a protonated amine, 1-ammoniummethylenediphosphonate [NH3CH(PO3)23-; (Hamdp)], as the ligand to construct paddlewheel Ru2(Hamdp)2 building block. Three networks of different structural dimensionalities were obtained. (H3O)2[RuII2(Hamdp)2] (1) forms one-dimensional chain of S = 1 RuII2 bridged at axial positions through Ru-O bonds. From 1 as a starting material, its reaction with lanthanide ions results in the bimetallic Dy(H2O)3[Ru2(Hamdp)2][Ru2(Hamdp)2(H2O)2][Ru2(Hamdp)(amdp)]0.5·12H2O (2), which has a pillared-bilayer structure and Yb[Ru2(Hamdp)2]2[Ru2(Hamdp)2(H2O)2]·15H2O (3), which is a three-dimensional open framework, both contain mixed-valent RuII/III2 units and show new connection topologies of the Ru2 dimers. After activation 2 does not adsorb N2 and CO2 but takes up 12 H2O. In contrast, 3 take up all three gases. The optimum proton conductivities were moderate reaching 1.44 × 10-6 for 2 and 0.93 × 10-5 S cm-1 for 3 at 95% relative humidity and 55 °C.

Metal-Organic Frameworks and Other Crystalline Materials for Ultrahigh Superprotonic Conductivities of 10-2 †S cm-1 or Higher.

Proton-conducting materials in the solid state have received immense attention for their role as electrolytes in proton-exchange membrane fuel cells. Recently, crystalline materials-metal-organic frameworks (MOFs), hydrogen-bonded organic frameworks (HOFs), covalent organic frameworks (COFs), polyoxometalates (POMs), and porous organic crystals-have become an exciting research topic in the field of proton-conducting materials. For a better electrolyte, a high proton conductivity on the order of 10-2  S cm-1 or higher is preferred as efficient proton transport between the electrodes is ultimately necessary. With an emphasis on design principles, this Concept will focus on MOFs and other crystalline solid-based proton-conducting platforms that exhibit "ultrahigh superprotonic" conductivities with values in excess of 10-2  S cm-1 . While only a handful of MOFs exhibit such an ultrahigh conductivity, this quality in other systems is even rarer. In addition to interpreting the structural-functional correlation by taking advantage of their crystalline nature, we address the challenges and promising directions for future research.

Polycarboxylate-Templated Coordination Polymers: Role of Templates for Superprotonic Conductivities of up to 10-1 †S cm-1.

Three coordination polymers (CPs) have been synthesized based on a [Co(bpy)(H2 O)4 ]2+ chain (bpy=4,4'-bipyridine) by a template approach. The frameworks are neutralized by different templated polycarboxylate anions (furan di-carboxylate (fdc) in Co-fdc, benzene tri-carboxylate (btc) in Co-tri and benzene tetra-carboxylate (btec) in Co-tetra). These templates with different degrees of protonation and ionic carrier concentration played significant role on crystal packing as well as formation of well-directed H-bonded networks which made these CPs perform well in proton conduction (PC). The PC value reaches to 1.49×10-1  S cm-1 under 80 °C and 98 % relative humidity (R.H.) for Co-tri, which is the highest among CPs/MOFs/COFs and is an example of conductivity in the order of 10-1  S cm-1 . Co-tri and Co-tetra are excellent proton conductors at mild temperature (40 °C) and 98 % R.H. (conductivities up to 2.92×10-2 and 1.38×10-2  S cm-1 , respectively).

A microporous MOF with a polar pore surface exhibiting excellent selective adsorption of CO2 from CO2-N2 and CO2-CH4 gas mixtures with high CO2 loading.

A microporous MOF {[Zn(SDB)(L)0.5]·S}n (IITKGP-5) with a polar pore surface has been constructed by the combination of a V-shaped -SO2 functionalized organic linker (H2SDB = 4,4'-sulfonyldibenzoic acid) with an N-rich spacer (L = 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene), forming a network with sql(2,6L1) topology. IITKGP-5 is characterized by TGA, PXRD and single crystal X-ray diffraction. The framework exhibits lozenge-shaped channels of an approximate size of 4.2 × 5.6 Å2 along the crystallographic b axis with a potential solvent accessible volume of 26%. The activated IITKGP-5a revealed a CO2 uptake capacity of 56.4 and 49 cm3 g-1 at 273 K/1 atm and 295 K/1 atm, respectively. On the contrary, it takes up a much smaller amount of CH4 (17 cm3 g-1 at 273 K and 13.6 cm3 g-1 at 295 K) and N2 (5.5 cm3 g-1 at 273 K; 4 cm3 g-1 at 295 K) under 1 atm pressure exhibiting its potential for a highly selective adsorption of CO2 from flue gas as well as a landfill gas mixture. Based on the ideal adsorbed solution theory (IAST), a CO2/N2 selectivity of 435.5 and a CO2/CH4 selectivity of 151.6 have been realized at 273 K/100 kPa. The values at 295 K are 147.8 for CO2/N2 and 23.8 for CO2/CH4 gas mixtures under 100 kPa. In addition, this MOF nearly approaches the target values proposed for PSA and TSA processes for practical utility exhibiting its prospect for flue gas separation with a CO2 loading capacity of 2.04 mmol g-1.

A new set of Cd(ii)-coordination polymers with mixed ligands of dicarboxylate and pyridyl substituted diaminotriazine: selective sorption towards CO2 and cationic dyes.

On the basis of a mixed ligand system of L(NH2)2 (6-(pyridin-4-yl)-1,3,5-triazine-2,4-diamine) and dicarboxylic acids, three new Cd(ii) coordination polymers viz. {[Cd0.5(tdc)0.5(L(NH2)2)0.5(H2O)]·DMF·H2O}n (1), {[Cd0.5(bdc)0.5(L(NH2)2)(H2O)]·DMF·H2O}n (2), and {[Cd(ipa)(L(NH2)2)(DMF)]·H2O}n (3) (tdcH2 = thiophene-2,5-dicarboxylic acid, bdcH2 = benzene-1,4-dicarboxylic acid, ipaH2 = benzene-1,3-dicarboxylic acid) were synthesized under diverse reaction conditions and characterized by single crystal X-ray diffraction, PXRD, elemental analysis, IR spectroscopy and TGA. While 1 and 2 revealed 1D chain structures, 3 acquired a 2D square net structural arrangement. Gas adsorption measurements of the desolvated framework 3 showed a moderate uptake of CO2 under ambient conditions with good selectivity over N2 and CH4. The solid state luminescence properties were studied for all three coordination polymers. Moreover, a dye adsorption study on 3 exhibited selective adsorption towards a cationic dye.