Benzene-templated hydrothermal synthesis of metal-organic frameworks with selective sorption properties

In this paper, we report two metal-organic frameworks [Co3(ndc)3(bipyen)(1.5)]H2O (1) and [Co2(ndc)2bipyen)]C6H6.H2O (2) (bipyen=trans-1,2-bis(4-pyridyl)ethylene, H2ndc=2,6-naphthalenedicarboxylic acid). These compounds were both synthesized from identical hydrothermal reaction conditions except that benzene was added to the reaction for 2. Crystal structures show that the two compounds have triply interpenetrated three-dimensional frameworks and these frameworks have the same primary structure of a two-dimensional network of interconnected [Co2(O2CR)(4/2)] (R=naphthalene group) paddle-wheels and bridging bipyen ligands. Both compounds have guest water molecules and, in addition, 2 has guest benzene molecules. Structural transformations of the host accompanied guest removal, which can be monitored by powder X-ray diffraction. N2 adsorption data of 2 show that there are two different types of pores corresponding to the benzene and water pores. Upon exposure to vapors of several organic molecules, the heat-treated sample of 2 adsorbs benzene and cyclohexene, but does not adsorb toluene, (o-, m-, and p-)xylenes, cycloheptatriene, or cyclohexane.     

Flexibility and sorption selectivity in rigid metal-organic frameworks: the impact of ether-functionalised linkers

The functionalisation of well-known rigid metal-organic frameworks (namely, [Zn(4)O(bdc)(3)](n), MOF-5, IRMOF-1 and [Zn(2)(bdc)(2)(dabco)](n); bdc = 1,4-benzene dicarboxylate, dabco = diazabicyclo[2.2.2]octane) with additional alkyl ether groups of the type -O-(CH(2))(n)-O-CH(3) (n = 2-4) initiates unexpected structural flexibility, as well as high sorption selectivity towards CO(2) over N(2) and CH(4) in the porous materials. These novel materials respond to the presence/absence of guest molecules with structural transformations. We found that the chain length of the alkyl ether groups and the substitution pattern of the bdc-type linker have a major impact on structural flexibility and sorption selectivity. Remarkably, our results show that a high crystalline order of the activated material is not a prerequisite to achieve significant porosity and high sorption selectivity.     

Mixed-ligand metal-organic frameworks with large pores: gas sorption properties and single-crystal-to-single-crystal transformation on guest exchange

Two isomorphous 3D metal-organic frameworks, {[Cu2(BPnDC)2(bpy)].8 DMF.6 H2O}n (1) and {[Zn2(BPnDC)2(dabco)].13 DMF.3 H2O}n (2), have been prepared by the solvothermal reactions of benzophenone 4,4'-dicarboxylic acid (H2BPnDC) with Cu(NO3)(2).2.5 H2O and 4,4'-bipyridine (bpy), and with Zn(NO3)(2).6 H2O and 4-diazabicyclo[2.2.2]octane (dabco), respectively. Compounds 1 and 2 are composed of paddle-wheel {M2(O2CR)4} cluster units, and they generate 2D channels with two different large pores (effective size of larger pore: 18.2 A for 1, 11.4 A for 2). The framework structure of desolvated solid, [Cu2(BPnDC)2(bpy)]n (SNU-6; SNU=Seoul National University), is the same as that of 1, as evidenced by powder X-ray diffraction patterns. SNU-6 exhibits high permanent porosity (1.05 cm3 g(-1)) with high Langmuir surface area (2910 m2 g(-1)). It shows high H2 gas storage capacity (1.68 wt % at 77 K and 1 atm; 4.87 wt % (excess) and 10.0 wt % (total) at 77 K and 70 bar) with high isosteric heat (7.74 kJ mol(-1)) of H2 adsorption as well as high CO2 adsorption capability (113.8 wt % at 195 K and 1 atm). Compound 2 undergoes a single-crystal-to-single-crystal transformation on guest exchange with n-hexane to provide {[Zn2(BPnDC)2(dabco)].6 (n-hexane).3 H2O}n (2hexane). The transformation involves dynamic motion of the molecular components in the crystal, mainly a bending motion of the square planes of the paddle-wheel units resulting from rotational rearrangement of phenyl rings and carboxylate planes of BPnDC2-.     

Elucidating gating effects for hydrogen sorption in MFU-4-type triazolate-based metal-organic frameworks featuring different pore sizes

A highly porous member of isoreticular MFU‐4‐type frameworks, [Zn₅Cl₄(BTDD)₃] (MFU‐4l(arge)) (H₂‐BTDD=bis(1H‐1,2,3‐triazolo[4,5‐b],[4′,5′‐i])dibenzo[1,4]dioxin), has been synthesized using ZnCl₂ and H₂‐BTDD in N,N‐dimethylformamide as a solvent. MFU‐4l represents the first example of MFU‐4‐type frameworks featuring large pore apertures of 9.1 Å. Here, MFU‐4l serves as a reference compound to evaluate the origin of unique and specific gas‐sorption properties of MFU‐4, reported previously. The latter framework features narrow‐sized pores of 2.5 Å that allow passage of sufficiently small molecules only (such as hydrogen or water), whereas molecules with larger kinetic diameters (e.g., argon or nitrogen) are excluded from uptake. The crystal structure of MFU‐4l has been solved ab initio by direct methods from 3D electron‐diffraction data acquired from a single nanosized crystal through automated electron diffraction tomography (ADT) in combination with electron‐beam precession. Independently, it has been solved using powder X‐ray diffraction. Thermogravimetric analysis (TGA) and variable‐temperature X‐ray powder diffraction (XRPD) experiments carried out on MFU‐4l indicate that it is stable up to 500 °C (N₂ atmosphere) and up to 350 °C in air. The framework adsorbs 4 wt % hydrogen at 20 bar and 77 K, which is twice the amount compared to MFU‐4. The isosteric heat of adsorption starts for low surface coverage at 5 kJ mol^?1 and decreases to 3.5 kJ mol^?1 at higher H₂ uptake. In contrast, MFU‐4 possesses a nearly constant isosteric heat of adsorption of ca. 7 kJ mol^?1 over a wide range of surface coverage. Moreover, MFU‐4 exhibits a H₂ desorption maximum at 71 K, which is the highest temperature ever measured for hydrogen physisorbed on metal–organic frameworks (MOFs).     

High gas sorption and metal-ion exchange of microporous metal-organic frameworks with incorporated imide groups

Metal-organic frameworks (MOFs), {[Cu(2)(bdcppi)(dmf)(2)]·10DMF·2H(2)O}(n) (SNU-50) and {[Zn(2)(bdcppi)(dmf)(3)]·6DMF·4H(2)O}(n) (SNU-51), have been prepared by the solvothermal reactions of N,N'-bis(3,5-dicarboxyphenyl)pyromellitic diimide (H(4)BDCPPI) with Cu(NO(3))(2) and Zn(NO(3))(2), respectively. Framework SNU-50 has an NbO-type net structure, whereas SNU-51 has a PtS-type net structure. Desolvated solid [Cu(2)(bdcppi)](n) (SNU-50'), which was prepared by guest exchange of SNU-50 with acetone followed by evacuation at 170 °C, adsorbs high amounts of N(2), H(2), O(2), CO(2), and CH(4) gases due to the presence of a vacant coordination site at every metal ion, and to the presence of imide groups in the ligand. The Langmuir surface area is 2450 m(2) g(-1). It adsorbs H(2) gas up to 2.10 wt% at 1 atm and 77 K, with zero coverage isosteric heat of 7.1 kJ mol(-1), up to a total of 7.85 wt% at 77 K and 60 bar. Its CO(2) and CH(4) adsorption capacities at 298 K are 77 wt% at 55 bar and 17 wt% at 60 bar, respectively. Of particular note is the O(2) adsorption capacity of SNU-50' (118 wt% at 77 K and 0.2 atm), which is the highest reported so far for any MOF. By metal-ion exchange of SNU-51 with Cu(II), {[Cu(2)(bdcppi)(dmf)(3)]·7DMF·5H(2)O}(n) (SNU-51-Cu(DMF)) with a PtS-type net was prepared, which could not be synthesized by a direct solvothermal reaction.     

Pillaring of CdCl2-like layers in lanthanide metal-organic frameworks: synthesis, structure, and photophysical properties

A hydrothermal reaction of lanthanide salts, pyridine-2,3-dicarboxylic acid, benzene-1,4-dicarboxylic acid, and water gave rise to a new series of three-dimensional mixed carboxylates (homocyclic and heterocyclic) of lanthanides with the general formula [M2(H2O)4][{C5H3N(COO)2}2{C6H4(COO)2}], M=La (I), Pr (II), and Nd (III). The structure consists of M2O14N2 dimeric units connected by pyridine-2,3-dicarboxylate moieties to form two-dimensional layers that are pillared by terephthalate units. The structures also possess two co-ordinated water molecules, which are arranged to form one-dimensional helical chains and can be reversibly adsorbed. The connectivity within the layers closely resembles that of the CdCl2 layered structure with 3(6) topology. To the best of our knowledge, this is the first observation of CdCl2 topology in lanthanide metal-organic framework compounds. Partial substitution of La3+ in I by Eu3+ and Tb3+ (2 and 4 %) gives rise to characteristic red/pink or green emission, which suggests a ligand-sensitized metal-centered emission. The Nd compound III shows interesting UV and blue emission through an up-conversion process.     

Functional mixed metal-organic frameworks with metalloligands

Immobilization of functional sites within metal-organic frameworks (MOFs) is very important for their ability to recognize small molecules and thus for their functional properties. The metalloligand approach has enabled us to rationally immobilize a variety of different functional sites such as open metal sites, catalytic active metal sites, photoactive metal sites, chiral pore environments, and pores of tunable sizes and curvatures into mixed metal-organic frameworks (M'MOFs). In this Minireview, we highlight some important functional M'MOFs with metalloligands for gas storage and separation, enantioselective separation, heterogeneous asymmetric catalysis, sensing, and as photoactive and nanoscale drug delivery and biomedical imaging materials.     

Fluorescent composite hydrogels of metal-organic frameworks and functionalized graphene oxide

GO MOFs! Azobenzoic acid functionalized graphene (A-GO) can act as a structure-directing template that influences hydrogel formation together with metal-organic frameworks (MOFs). Zn(2+) MOFs of pyridine derivatives work as framework linkers between the A-GO sheets (MOF-A-GO, see figure). MOF-A-GO exhibits a strong fluorescence enhancement upon gel formation. In addition, MOF-A-GO selectively recognizes trinitrotoluene.     

Hydrogen storage in microporous metal-organic frameworks with exposed metal sites

Owing to their high uptake capacity at low temperature and excellent reversibility kinetics, metal-organic frameworks have attracted considerable attention as potential solid-state hydrogen storage materials. In the last few years, researchers have also identified several strategies for increasing the affinity of these materials towards hydrogen, among which the binding of H(2) to unsaturated metal centers is one of the most promising. Herein, we review the synthetic approaches employed thus far for producing frameworks with exposed metal sites, and summarize the hydrogen uptake capacities and binding energies in these materials. In addition, results from experiments that were used to probe independently the metal-hydrogen interaction in selected materials will be discussed.     

Porous interpenetrated zirconium-organic frameworks (PIZOFs): a chemically versatile family of metal-organic frameworks

We present the synthesis and characterization of porous interpenetrated zirconium-organic frameworks (PIZOFs), a new family of metal-organic frameworks obtained from ZrCl(4) and the rodlike dicarboxylic acids HO(2)C[PE-P(R(1),R(2))-EP]CO(2) H that consist of alternating phenylene (P) and ethynylene (E) units. The substituents R(1),R(2) were broadly varied (alkyl, O-alkyl, oligo(ethylene glycol)), including postsynthetically addressable substituents (amino, alkyne, furan). The PIZOF structure is highly tolerant towards the variation of R(1) and R(2). This together with the modular synthesis of the diacids offers a facile tuning of the chemical environment within the pores. The PIZOF structure was solved from single-crystal X-ray diffraction analysis. The PIZOFs are stable under ambient conditions. PIZOF-2, the PIZOF prepared from HO(2)C[PE-P(OMe,OMe)-EP]CO(2)H, served as a prototype to determine thermal stability and porosity. It is stable up to 325 °C in air as determined by using thermogravimetry and powder X-ray diffraction. Argon sorption isotherms on PIZOF-2 revealed a Brunauer-Emmett-Teller (BET) surface area of 1250 m(2) g(-1) and a total pore volume of 0.68 cm(3) g(-1).     

A family of chiral metal-organic frameworks

Chiral metal–organic frameworks with a three‐dimensional network structure and wide‐open pores (>30 Å) were obtained by using chiral trifunctional linkers and multinuclear zinc clusters. The linkers, H₃ChirBTB‐n, consist of a 4,4′,4′′‐benzene‐1,3,5‐triyltribenzoate (BTB) backbone decorated with chiral oxazolidinone substituents. The size and polarity of these substituents determines the network topology formed under solvothermal synthesis conditions. The resulting chiral MOFs adsorb even large molecules from solution. Moreover, they are highly active Lewis acid catalysts in the Mukaiyama aldol reaction. Due to their chiral functionalization, they show significant levels of enantioselectivity, thereby proving the validity of the modular design concept employed.     

A new approach to construct a doubly interpenetrated microporous metal-organic framework of primitive cubic net for highly selective sorption of small hydrocarbon molecules

A new approach has been realized to construct a three‐dimensional doubly interpenetrated cubic metal–organic framework Zn₂(PBA)₂(BDC) ? (DMF)₃(H₂O)₄ ( UTSA‐36 , HPBA=4‐(4‐pyridyl) benzoic acid, H₂BDC=1,4‐benzenedicarboxylic acid) through the self‐assembly of the pyridylcarboxylate linker 4‐(4‐pyridyl) benzoate and bicarboxylate linker 1,4‐benzenedicarxylate with paddle‐wheel [Zn₂(COO)₄]. The activated UTSA‐36 a exhibits highly selective gas sorption of C₂H₆, C₂H₄ and C₂H₂ over CH₄ with the Henry law’s selectivities of 11 to 25 in the temperature range of 273 to 296 K attributed to the unique 3D intersected pore structure of about 3.1 to 4.8 Å within the framework, indicating that UTSA‐36 a is a potentially very useful and promising microporous material for such industrially important separation of C₂ hydrocarbons over methane.     

Route to a family of robust, non-interpenetrated metal-organic frameworks with pto-like topology

A combination of topological rules and quantum chemical calculations has facilitated the development of a rational metal–organic framework (MOF) synthetic strategy using the tritopic benzene‐1,3,5‐tribenzoate (btb) linker and a neutral cross‐linker 4,4′‐bipyridine (bipy). A series of new compounds, namely [M₂(bipy)]₃(btb)₄ (DUT‐23(M), M=Zn, Co, Cu, Ni), [Cu₂(bisqui)_0.5]₃(btb)₄ (DUT‐24, bisqui=diethyl (R,S)‐4,4′‐biquinoline‐3,3′‐dicarboxylate), [Cu₂(py)_1.5(H₂O)_0.5]₃(btb)₄ (DUT‐33, py=pyridine), and [Cu₂(H₂O)₂]₃(btb)₄ (DUT‐34), with high specific surface areas and pore volumes (up to 2.03 m³ g^?1 for DUT‐23(Co)) were synthesized. For DUT‐23(Co), excess storage capacities were determined for methane (268 mg g^?1 at 100 bar and 298 K), hydrogen (74 mg g^?1 at 40 bar and 77 K), and n‐butane (99 mg g^?1at 293 K). DUT‐34 is a non‐cross‐linked version of DUT‐23 (non‐interpenetrated pendant to MOF‐14) that possesses open metal sites and can therefore be used as a catalyst. The accessibility of the pores in DUT‐34 to potential substrate molecules was proven by liquid phase adsorption. By exchanging the N,N donor 4,4′‐bipyridine with a substituted racemic biquinoline, DUT‐24 was obtained. This opens a route to the synthesis of a chiral compound, which could be interesting for enantioselective separation.