Christian J. Doonan

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.

Crystals as molecules: postsynthesis covalent functionalization of zeolitic imidazolate frameworks.

A new crystalline zeolitic imidazolate framework, ZIF-90, was prepared from zinc(II) nitrate and imidazolate-2-carboxyaldehyde (ICA) and found to have the sodalite-type topology. Its 3D porous framework has an aperture of 3.5 A and a pore size of 11.2 A. The pores are decorated by the aldehyde functionality of ICA which has allowed its transformation to the alcohol functionality by reduction with NaBH4 and its conversion to imine functionality by reaction with ethanolamine to give ZIF-91 and ZIF-92, respectively. The N2 adsorption isotherm of ZIF-90 shows a highly porous material with calculated Langmuir and BET surface areas of 1320 and 1270 m2 g(-1). Both functionalized ZIFs retained high crystallinity and in addition ZIF-91 maintained permanent porosity (surface areas: 1070 and 1010 m2 g(-1)).

Exceptional ammonia uptake by a covalent organic framework

Covalent organic frameworks (COFs) are porous crystalline materials composed of light elements linked by strong covalent bonds. A number of these materials contain a high density of Lewis acid boron sites that can strongly interact with Lewis basic guests, which makes them ideal for the storage of corrosive chemicals such as ammonia. We found that a member of the covalent organic framework family, COF-10, shows the highest uptake capacity (15 mol kg⁻¹, 298 K, 1 bar) of any porous material, including microporous 13X zeolite (9 mol kg⁻¹), Amberlyst 15 (11 mol kg⁻¹) and mesoporous silica, MCM-41 (7.9 mol kg⁻¹). Notably, ammonia can be removed from the pores of COF-10 by heating samples at 200°C under vacuum. In addition, repeated adsorption of ammonia into COF-10 causes a shift in the interlayer packing, which reduces its apparent surface area to nitrogen. However, owing to the strong Lewis acid-base interactions, the total uptake capacity of ammonia and the structural integrity of the COF are maintained after several cycles of adsorption/desorption.

Crystalline Covalent Organic Frameworks with Hydrazone Linkages

Condensation of 2,5-diethoxyterephthalohydrazide with 1,3,5-triformylbenzene or 1,3,5-tris(4-formylphenyl)benzene yields two new covalent organic frameworks, COF-42 and COF-43, in which the organic building units are linked through hydrazone bonds to form extended two-dimensional porous frameworks. Both materials are highly crystalline, display excellent chemical and thermal stability, and are permanently porous. These new COFs expand the scope of possibilities for this emerging class of porous materials.

Reticular Synthesis of Covalent Organic Borosilicate Frameworks

This paper reports the synthesis and characterization of a new crystalline 3D covalent organic framework, COF-202: [C(C6H4)4]3[B3O6 (tBuSi)2]4, formed from condensation of a divergent boronic acid, tetra(4-dihydroxyborylphenyl)methane, and tert-butylsilane triol, tBuSi(OH)3. This framework is constructed through strong covalent bonds (Si-O, B-O) that link triangular and tetrahedral building units to form a structure based on the carbon nitride topology. COF-202 demonstrates high thermal stability, low density, and high porosity with a surface area of 2690 m2 g-1. The design and synthesis of COF-202 expand the type of linkage that could be used to crystallize new materials with extended covalent organic frameworks.

Isoreticular Metalation of Metal―Organic Frameworks

Sequential covalent transformation and metalation were performed on (Zn(4)O)(3)(BDC-NH(2))(3)(BTB)(4) with maintenance of crystallinity and porosity. Reaction of (Zn(4)O)(3)(BDC-NH(2))(3)(BTB)(4) with 2-pyridinecarboxaldehyde in toluene at room temperature for 5 days resulted in the formation of the extended crystalline structure (Zn(4)O)(3)(BDC-C(6)H(5)N(2))(3)(BTB)(4), which possesses iminopyridine moieties covalently bound to the organic links of the framework. Subsequent reaction with PdCl(2)(CH(3)CN)(2) in CH(2)Cl(2) at room temperature for 12 h yielded the metalated metal-organic framework (Zn(4)O)(3)(BDC-C(6)H(5)N(2)PdCl(2))(3)(BTB)(4). Both functionalized materials retained high crystallinity and were permanently porous with high surface areas [3200 and 1700 m(2) g(-1) for (Zn(4)O)(3)(BDC-C(6)H(5)N(2))(3)(BTB)(4) and (Zn(4)O)(3)(BDC-C(6)H(5)N(2)PdCl(2))(3)(BTB)(4), respectively.].

Biomimetic mineralization of metal-organic frameworks as protective coatings for biomacromolecules.

Enhancing the robustness of functional biomacromolecules is a critical challenge in biotechnology, which if addressed would enhance their use in pharmaceuticals, chemical processing and biostorage. Here we report a novel method, inspired by natural biomineralization processes, which provides unprecedented protection of biomacromolecules by encapsulating them within a class of porous materials termed metal-organic frameworks. We show that proteins, enzymes and DNA rapidly induce the formation of protective metal-organic framework coatings under physiological conditions by concentrating the framework building blocks and facilitating crystallization around the biomacromolecules. The resulting biocomposite is stable under conditions that would normally decompose many biological macromolecules. For example, urease and horseradish peroxidase protected within a metal-organic framework shell are found to retain bioactivity after being treated at 80 °C and boiled in dimethylformamide (153 °C), respectively. This rapid, low-cost biomimetic mineralization process gives rise to new possibilities for the exploitation of biomacromolecules.

Mechanisms of gold biomineralization in the bacterium Cupriavidus metallidurans

While the role of microorganisms as main drivers of metal mobility and mineral formation under Earth surface conditions is now widely accepted, the formation of secondary gold (Au) is commonly attributed to abiotic processes. Here we report that the biomineralization of Au nanoparticles in the metallophillic bacterium Cupriavidus metallidurans CH34 is the result of Au-regulated gene expression leading to the energy-dependent reductive precipitation of toxic Au(III)-complexes. C. metallidurans, which forms biofilms on Au grains, rapidly accumulates Au(III)-complexes from solution. Bulk and microbeam synchrotron X-ray analyses revealed that cellular Au accumulation is coupled to the formation of Au(I)-S complexes. This process promotes Au toxicity and C. metallidurans reacts by inducing oxidative stress and metal resistances gene clusters (including a Au-specific operon) to promote cellular defense. As a result, Au detoxification is mediated by a combination of efflux, reduction, and possibly methylation of Au-complexes, leading to the formation of Au(I)-C-compounds and nanoparticulate Au0. Similar particles were observed in bacterial biofilms on Au grains, suggesting that bacteria actively contribute to the formation of Au grains in surface environments. The recognition of specific genetic responses to Au opens the way for the development of bioexploration and bioprocessing tools.

Postsynthetic Modification of a Metal–Organic Framework for Stabilization of a Hemiaminal and Ammonia Uptake

In our study, we show by solid-state (15)N NMR measurements that an important zirconium metal-organic framework (UiO-66) with amino-functionalized links is composed of a mixture of amino and -NH(3)(+)Cl(-) salt functionalities rather than all amino functionality to give a composition of Zr(6)O(4)(OH)(4)(BDC-NH(2))(4)(BDC-NH(3)(+)Cl(-))(2) (UiO-66-A). UiO-66-A was postsynthetically modified to form a mixture of three functionalities, where the hemiaminal functionality is the majority species in UiO-66-B and aziridine is the majority functionality in UiO-66-C. UiO-66-A-C are all porous with surface areas ranging from 780 to 820 m(2)/g and have chemical stability, as evidenced by reversible ammonia uptake and release showing capacities ranging from 134 to 193 cm(3)/g.

Post-synthetic structural processing in a metal-organic framework material as a mechanism for exceptional CO2/N2 selectivity

Here we report the synthesis and ceramic-like processing of a new metal-organic framework (MOF) material, [Cu(bcppm)H2O], that shows exceptionally selective separation for CO2 over N2 (ideal adsorbed solution theory, S(ads) = 590). [Cu(bcppm)H2O]·xS was synthesized in 82% yield by reaction of Cu(NO3)2·2.5H2O with the link bis(4-(4-carboxyphenyl)-1H-pyrazolyl)methane (H2bcppm) and shown to have a two-dimensional 4(4)-connected structure with an eclipsed arrangement of the layers. Activation of [Cu(bcppm)H2O] generates a pore-constricted version of the material through concomitant trellis-type pore narrowing (b-axis expansion and c-axis contraction) and a 2D-to-3D transformation (a-axis contraction) to give the adsorbing form, [Cu(bcppm)H2O]-ac. The pore contraction process and 2D-to-3D transformation were probed by single-crystal and powder X-ray diffraction experiments. The 3D network and shorter hydrogen-bonding contacts do not allow [Cu(bcppm)H2O]-ac to expand under gas loading across the pressure ranges examined or following re-solvation. This exceptional separation performance is associated with a moderate adsorption enthalpy and therefore an expected low energy cost for regeneration.