Carolyn B. Knobler

High-Throughput Synthesis of Zeolitic Imidazolate Frameworks and Application to CO2 Capture

A high-throughput protocol was developed for the synthesis of zeolitic imidazolate frameworks (ZIFs). Twenty-five different ZIF crystals were synthesized from only 9600 microreactions of either zinc(II)/cobalt(II) and imidazolate/imidazolate-type linkers. All of the ZIF structures have tetrahedral frameworks: 10 of which have two different links (heterolinks), 16 of which are previously unobserved compositions and structures, and 5 of which have topologies as yet unobserved in zeolites. Members of a selection of these ZIFs (termed ZIF-68, ZIF-69, and ZIF-70) have high thermal stability (up to 390°C) and chemical stability in refluxing organic and aqueous media. Their frameworks have high porosity (with surface areas up to 1970 square meters per gram), and they exhibit unusual selectivity for CO2 capture from CO2/CO mixtures and extraordinary capacity for storing CO2: 1 liter of ZIF-69 can hold ∼83 liters of CO2 at 273 kelvin under ambient pressure.

Multiple Functional Groups of Varying Ratios in Metal-Organic Frameworks

Many Mixed Linkers in MOFs Crystallization can separate different molecules because different molecules cannot generally be accommodated equally well in the same crystal lattice. However, in metal-organic framework (MOF) compounds, the organic linkers do not pack closely to other parts of the lattice, so it may be possible to mix several linkers that are derivatives of a parent compound with the same end groups. Deng et al. (p. 846) show that zinc-based MOFs can be made that mix 1,4-benzenedicarboxylate and up to eight of its derivatives in a random fashion. The effects of such mixing on porosity and absorption characteristics is nonlinear; in one case, a mixed-linker compound was four times better for selecting CO2 versus CO compared with the best MOF bearing only one of the component linkers. The adsorption characteristics for mixed linkers can exceed that expected from just combining the single-linker compounds. We show that metal-organic frameworks (MOFs) can incorporate a large number of different functionalities on linking groups in a way that mixes the linker, rather than forming separate domains. We made complex MOFs from 1,4-benzenedicarboxylate (denoted by “A” in this work) and its derivatives -NH2, -Br, -(Cl)2, -NO2, -(CH3)2, -C4H4, -(OC3H5)2, and -(OC7H7)2 (denoted by “B” to “I,” respectively) to synthesize 18 multivariate (MTV) MOF-5 type structures that contain up to eight distinct functionalities in one phase. The backbone (zinc oxide and phenylene units) of these structures is ordered, but the distribution of functional groups is disordered. The complex arrangements of several functional groups within the pores can lead to properties that are not simply linear sums of those of the pure components. For example, a member of this series, MTV-MOF-5-EHI, exhibits up to 400% better selectivity for carbon dioxide over carbon monoxide compared with its best same-link counterparts.

Control of Pore Size and Functionality in Isoreticular Zeolitic Imidazolate Frameworks and their Carbon Dioxide Selective Capture Properties

Five new crystalline zeolitic imidazolate frameworks (ZIFs), ZIF-78 to -82, were prepared from zinc(II) nitrate and mixtures of 2-nitroimidazole and five different functionalized imidazoles and were found to have the GME topology. These structures, along with three previously reported GME ZIFs, constitute a series of highly porous materials with Brunauer-Emmet-Teller surface areas ranging from 620 to 1730 m(2)/g. The pore diameters and apertures vary incrementally from 7.1 to 15.9 A and 3.8 to 13.1 A, respectively, and the functionalities decorating the pores vary from polar cyano- and nitro- groups to nonpolar alkyl groups. The variability expressed in these materials makes them highly attractive for study as gas-separation media. Selectivity values calculated for separation of CO(2) and CH(4) predict that the ZIFs with polar functionality, ZIF-78 (10.6:1) and -82 (9.6:1), retain CO(2) gas to a greater degree than the other members of the GME series and BPL-activated carbon. These predictions are borne out in dynamic breakthrough studies, which confirm the increased capacity of ZIF-78 and -82 and demonstrate the promise of this class of materials.

Docking in metal-organic frameworks

MOFs with Guest Rooms Metal organic framework (MOF) solids typically boast large pores in their crystal lattices that can accommodate molecular guests and are useful as compact media for gas storage. However, guest binding has generally relied upon nonspecific interactions with the framework components. Li et al. (p. 855) have now prepared MOFs that incorporate macrocyclic ethers into the structural ligands comprising the framework walls within which certain cationic guests can bind quantitatively and site-specifically, akin to the molecular docking of drug molecules within protein receptors. Guest molecules bind site-specifically in macrocycles incorporated into the ligands of a metal organic framework solid. The use of metal-organic frameworks (MOFs) so far has largely relied on nonspecific binding interactions to host small molecular guests. We used long organic struts (~2 nanometers) incorporating 34- and 36-membered macrocyclic polyethers as recognition modules in the construction of several crystalline primitive cubic frameworks that engage in specific binding in a way not observed in passive, open reticulated geometries. MOF-1001 is capable of docking paraquat dication (PQT2+) guests within the macrocycles in a stereoelectronically controlled fashion. This act of specific complexation yields quantitatively the corresponding MOF-1001 pseudorotaxanes, as confirmed by x-ray diffraction and by solid- and solution-state nuclear magnetic resonance spectroscopic studies performed on MOF-1001, its pseudorotaxanes, and their molecular strut precursors. A control experiment involving the attempted inclusion of PQT2+ inside a framework (MOF-177) devoid of polyether struts showed negligible uptake of PQT2+, indicating the importance of the macrocyclic polyether in PQT2+ docking.

Synthesis and Structure of Chemically Stable Metal−Organic Polyhedra

Two imidazolate-metal based rhombic dodecahedra (termed MOP-100 and MOP-101) were designed and prepared from [(NH(3))(4)Pd(NO(3))(2)] and hydrogen tetrakis(1-imidazolyl)borate or hydrogen tetrakis(4-methyl-1-imidazolyl)borate in a concentrated ammonium hydroxide solution at 85 degrees C. Both rhombic dodecahedra show unusual chemical stability in acidic and basic solutions as well as common organic solvents. Permanent porosity was examined by gas adsorption studies. From the N(2) isotherm for MOP-101, the Langmuir and BET surface areas of MOP-101 were calculated to be 350 and 280 m(2) g(-1), respectively. Anion exchange experiments confirmed the internal cavities of such polyhedra are accessible.

Host-guest complexation : 43. Synthesis and binding properties of a macrocycle composed of two phenanthrolines and two sulfonamide units

Abstract Treatment of 2,9-bis(chloromethyl)-1,10-phenanthroline with toluenesulfonamide and K 2 CO 3 in HCON(CH 3 ) 2 gave macro-cycle 1 (21%) composed of two phenantroline units bonded to each other through two CH 2 N(Ts)CH 2 bridges. The 18-membered inner ring system contains six nitrogens, each of which is separated by two carbons. A crystal structure of 1.2H 2 O indicated the two faces of the phenanthrolines approach each other to place their four nitrogens nearly in a plane bridged by a water dimer held in position by bifurcated hydrogen bonds. The two ArSO 2 N groups are turned outward, and the hydrogens of their attached CH 2 groups are oriented inward toward the water dimer, which serves as a guest. Complexation was indicated by addition of a variety of salts to a solution of 1 in (CD 3 ) 2 SO, which casued changes in the 1 H NMR by as much as 1.5 ppm. Complexing ions include Li + . Na + , K + , Rb + , Cs + , NH 4 + , Ag + , Cu + , Tl + , Zn 2+ , Cd 2+ , Hg 2+ and Pb 2+ noncomplexing ions are Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , UO 2 2+ , and Ce 3+ . A correlation between the chemical shifts of the CH 3 and tolyl aryl protons of the host and the ionic radii of the guests is interpreted in terms of three different host conformations in the various complexes. Equilibrations of the guest cations between 1 and cryptaplexes of Na + , K + , Rb + , Cs + , and NH 4 + picrates in (CD 3 ) 2 SO provided the following estimates of the -ΔG° values of binding of these salts by 1: 8.4, 8.5, 6.8, -1 , respectively.

Metal–Organic Frameworks of Vanadium as Catalysts for Conversion of Methane to Acetic Acid

A catalytic system combining the high activity of homogeneous catalysts and the ease of use of heterogeneous catalysts for methane activation is reported. The vanadium-containing metal-organic frameworks (MOFs) MIL-47 and MOF-48 are found to have high catalytic activity and chemical stability. They convert methane selectively to acetic acid with 70% yield (490 TON) based on K(2)S(2)O(8) as an oxidant. Isotopic labeling experiments showed that two methane molecules are converted to the produced acetic acid. The MOF catalysts are reusable and remain catalytically active for several recycling steps without losing their crystalline structures.

All-Carbon Molecules: Evidence for the Generation of Cyclo[18]carbon from a Stable Organic Precursor

The unambiguous structural characterization of a single-sized all-carbon molecule requires its chemical synthesis. For cyclo[18]carbon, ab initio calculations predict a relatively stable, cyclic D9h ground state geometry with alternating C-C (1.36 angstroms) and C≡C (1.20 angstroms) bonds. The synthesis and x-ray crystal structure of a direct precursor to C18 are described. The analysis of laser flash heating experiments on this precursor by time-of-flight mass spectroscopy shows a sequence of retro-Diels-Alder reactions leading to C18 as the predominant fragmentation pattern. Structural evidence is provided for the generation of an all-carbon molecule from a well-characterized organic precursor.

Aromatic Polyhedral Hydroxyborates: Bridging Boron Oxides and Boron Hydrides

No explosion, but per-B-hydroxylation occurs if the icosahedral boron hydrides [closo-B12 H12 ](2-) (see picture), [closo-CB11 H12 ](-) , or closo-1,12-(CH2 OH)2 -1,12-C2 B10 H10 are refluxed in 30 % hydrogen peroxide. Thus, the three isoelectronic species [closo-B12 (OH)12 ](2-) , [closo-1-H-1-CB11 (OH)11 ](-) , and closo-1,12-H2 -1,12-C2 B10 (OH)10 were obtained. ○=BH, ○=BOH.

A CAMOUFLAGED NIDO-CARBORANE ANION : FACILE SYNTHESIS OF OCTA-B-METHYL-1,2-DICARBA-CLOSO-DODECABORANE(12 AND ITS DEBORATION REACTION

Facile, efficient, and economical is the exhaustive methylation of ortho-carborane, its isomers and its derivatives under Friedel–Crafts conditions utilizing MeI and AlCl3. Regioselectively, ortho-carborane yields the 4,5,7,8,9,10,11,12-octamethyl derivative 1 which has been degraded by using KOEt under autogenous conditions. The resulting camouflaged nido anion can be protonated (2⋅H) and finally converted to the per-B-methylated C2B9closo cluster 3.