Tuning Carbon Dioxide Adsorption Affinity of Zinc(II) MOFs by Mixing Bis(pyrazolate) Ligands with N-Containing Tags

The four zinc(II) mixed-ligand metal-organic frameworks (MIXMOFs) Zn(BPZ)x(BPZNO2)1-x, Zn(BPZ)x(BPZNH2)1-x, Zn(BPZNO2)x(BPZNH2)1-x, and Zn(BPZ)x(BPZNO2)y(BPZNH2)1-x-y (H2BPZ = 4,4′-bipyrazole; H2BPZNO2 = 3-nitro-4,4′-bipyrazole; H2BPZNH2 = 3-amino-4,4′-bipyrazole) were prepared through solvothermal routes and fully investigated in the solid state. Isoreticular to the end members Zn(BPZ) and Zn(BPZX) (X = NO2, NH2), they are the first examples ever reported of (pyr)azolate MIXMOFs. Their crystal structure is characterized by a three-dimensional open framework with one-dimensional square or rhombic channels decorated by the functional groups. Accurate information about ligand stoichiometric ratio was determined (for the first time on MIXMOFs) through integration of selected ligands skeleton resonances from 13C cross polarized magic angle spinning solid-state NMR spectra collected on the as-synthesized materials. Like other poly(pyrazolate) MOFs, the four MIXMOFs are thermally stable, with decomposition temperatures between 708 and 726 K. As disclosed by N2 adsorption at 77 K, they are micro-mesoporous materials with Brunauer-Emmett-Teller specific surface areas in the range 400-600 m2/g. A comparative study (involving also the single-ligand analogues) of CO2 adsorption capacity, CO2 isosteric heat of adsorption (Qst), and CO2/N2 selectivity in equimolar mixtures at p = 1 bar and T = 298 K cast light on interesting trends, depending on ligand tag nature or ligand stoichiometric ratio. In particular, the amino-decorated compounds show higher Qst values and CO2/N2 selectivity vs the nitro-functionalized analogues; in addition, tag "dilution" [upon passing from Zn(BPZX) to Zn(BPZ)x(BPZX)1-x] increases CO2 adsorption selectivity over N2. The simultaneous presence of amino and nitro groups is not beneficial for CO2 uptake. Among the compounds studied, the best compromise among uptake capacity, Qst, and CO2/N2 selectivity is represented by Zn(BPZ)x(BPZNH2)1-x

Synthesis and structural characterization of metal azolate/carboxylate frameworks incorporating the 1-H-pyrazol-3,4,5-tricarboxylate ligand

The tetratopic ligand 1-H-pyrazol-3,4,5-tricarboxylic acid (H4PZTC) has been used for the first time to prepare the new metal azolate/carboxylate (MAC) frameworks [Co3(HPZTC)2(H2O)6]·2H2O (MAC-1), [Co(H2PZTC)(DMF)(H2O)]2 (MAC-2) and [Cd2(PZTC)(H2O)2] (MAC-3), along either conventional or solvothermal routes. As assessed by thermogravimetric analysis, before decomposition the three MAC frameworks undergo partial decarboxylation over 200 °C. Powder X-ray diffraction unveiled 1-D chains alternating monomeric and dimeric units in MAC-1, 1-D hydrogen bonded strands of dimeric units in MAC-2 and a 3-D non-porous network in MAC-3

Crystal structure of cis-bis(μ-β-alanine-κ2O:O′)bis-[trichloridorhenium(III)](Re-Re) sesquihydrate

The structure of the title compound, [Re2Cl6(C3H7NO2)2]1.5H2O, comprises a dinuclear complex cation [Re - Re = 2.2494 (3) Å] involving cis-oriented double carboxylate bridges, four equatorial chloride ions and two weakly bonded chloride ligands in the axial positions at the two rhenium(III) atoms. In the crystal, two complex molecules and two water molecules constitute hydrogen-bonded dimers, while an extensive hydrogen-bonding network involving the groups of the zwitterionic ligand is important for generation of the framework. An additional partially occupied water molecule is disordered over two sets of sites about a symmetry centre with a site-occupancy ratio of 0.3:0.2

1,3-Bis(1,2,4-triazolyl)adamantine-based Coordination Polymers

The hybrid organic-inorganic materials known as coordination polymers are continuously gaining ground of scientific research, especially due to the interesting and promising functionalities they possess, e.g. magnetic, optical, electrical, redox, and luminescence properties, as well as, when permanent porosity is present, gas adsorption or separation, catalytic activity, and drug delivery. The reaction of the flexible ligand 1,3-bis(1,2,4-triazolyl)adamante (tr2ad, Scheme 1) with chlorides of different late transition metals, either following conventional routes or under solvothermal conditions, afforded the coordination polymers having the general stoechiometric formula of the type M(tr2ad)Cl2 (M = Zn,1; Cu, 2; Cd, 3; Ni, 4; Co, 5). Preliminary ab initio X-ray powder diffraction analyses revealed that 1 developes into a 1-D polymeric chain, while 2 features a 2-D polymeric structure. Thermal-gravimetric analyses (TGA) showed relevant thermal robustness of all these materials, peaking up to the onset of decomposition set at 350 °C

Carbon Dioxide Capture and Utilization with Isomeric Forms of Bis(amino)-Tagged Zinc Bipyrazolate Metal\u2013Organic Frameworks

Aiming at extending the tagged zinc bipyrazolate metal\u2013organic frameworks (MOFs) family, the ligand 3,3\u2019-diamino-4,4\u2019-bipyrazole (3,3\u2019-H2L) has been synthesized in good yield. The reaction with zinc(II) acetate hydrate led to the related MOF Zn(3,3\u2019-L). The compound is isostructural with its mono(amino) analogue Zn(BPZNH2) and with Zn(3,5-L), its isomeric parent built with 3,5-diamino-4,4\u2019-bipyrazole. The textural analysis has unveiled its micro-/mesoporous nature, with a BET area of 463 m2 g 121. Its CO2 adsorption capacity (17.4 wt. % CO2 at pCO2 = 1 bar and T = 298 K) and isosteric heat of adsorption (Qst = 24.8 kJ mol 121) are comparable to that of Zn(3,5-L). Both Zn(3,3\u2019-L) and Zn(3,5-L) have been tested as heterogeneous catalysts in the reaction of CO2 with the epoxides epichlorohydrin and epibromohydrin to give the corresponding cyclic carbonates at T = 393 K and pCO2 = 5 bar under solvent- and co-catalyst-free conditions. In general, the conversions recorded are higher than those found for Zn(BPZNH2), proving that the insertion of an extra amino tag in the pores is beneficial for the epoxidation catalysis. The best catalytic match has been observed for the Zn(3,5-L)/epichlorohydrin couple, with 64 % conversion and a TOF of 5.3 mmol(carbonate) (mmolZn) 121 h 121. To gain better insights on the MOF-epoxide interaction, the crystal structure of the [epibromohydrin@Zn(3,3\u2019-L)] adduct has been solved, confirming the existence of Br c5 c5 c5(H) 12N non-bonding interactions. To our knowledge, this study represents the first structural determination of a [epibromohydrin@MOF] adduct