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.     

Understanding adsorption and interactions of alkane isomer mixtures in isoreticular metal-organic frameworks

Novel metal-organic frameworks (MOFs) may lead to advances in adsorption and catalysis owing to their superior properties compared to traditional nanoporous materials. A combination of the grand canonical Monte Carlo method and configurational-bias Monte Carlo simulation was used to evaluate the adsorption isotherms of C4-C6 alkane isomer mixtures in IRMOF-1 and IRMOF-6. The amounts of adsorbed linear and branched alkanes increase with increasing pressure, and the amount of branched alkanes is larger than that of the linear ones. The locations of the alkane isomer reveal that the Zn4O clusters of the IRMOFs are the preferential adsorption sites for the adsorbate molecules. The interaction energy between the Zn4O cluster and the adsorbate is larger than that between the organic linker and the adsorbate. It was further confirmed that the Zn4O cluster plays a much more important role in adsorption by pushing a probe molecule into the pore at positions closer to the Zn4O cluster. It is difficult for branched alkane molecules to approach the Zn4O cluster of IRMOF-6 closely owing to strong spatial hindrance. In addition, the adsorption selectivity is discussed from the viewpoints of thermodynamics and kinetics, and the diffusion behavior of n-butane and 2-methylpropane were investigated to illustrate the relationship between diffusion and adsorption.     

Pyrazolate-based cobalt(II)-containing metal-organic frameworks in heterogeneous catalytic oxidation reactions: elucidating the role of entatic states for biomimetic oxidation processes

Crystal structures of two metal-organic frameworks (MFU-1 and MFU-2) are presented, both of which contain redox-active Co(II) centres coordinated by linear 1,4-bis[(3,5-dimethyl)pyrazol-4-yl] ligands. In contrast to many MOFs reported previously, these compounds show excellent stability against hydrolytic decomposition. Catalytic turnover is achieved in oxidation reactions by employing tert-butyl hydroperoxide and the solid catalysts are easily recovered from the reaction mixture. Whereas heterogeneous catalysis is unambiguously demonstrated for MFU-1, MFU-2 shows catalytic activity due to slow metal leaching, emphasising the need for a deeper understanding of structure-reactivity relationships in the future design of redox-active metal-organic frameworks. Mechanistic details for oxidation reactions employing tert-butyl hydroperoxide are studied by UV/Vis and IR spectroscopy and XRPD measurements. The catalytic process accompanying changes of redox states and structural changes were investigated by means of cobalt K-edge X-ray absorption spectroscopy. To probe the putative binding modes of molecular oxygen, the isosteric heats of adsorption of O(2) were determined and compared with models from DFT calculations. The stabilities of the frameworks in an oxygen atmosphere as a reactive gas were examined by temperature-programmed oxidation (TPO). Solution impregnation of MFU-1 with a co-catalyst (N-hydroxyphthalimide) led to NHPI@MFU-1, which oxidised a range of organic substrates under ambient conditions by employing molecular oxygen from air. The catalytic reaction involved a biomimetic reaction cascade based on free radicals. The concept of an entatic state of the cobalt centres is proposed and its relevance for sustained catalytic activity is briefly discussed.     

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.     

Probing the Lewis acidity and catalytic activity of the metal-organic framework [Cu3(btc)2] (BTC=benzene-1,3,5-tricarboxylate)

An optimized procedure was designed for the preparation of the microporous metal-organic framework (MOF) [Cu3(btc)2] (BTC=benzene-1,3,5-tricarboxylate). The crystalline material was characterized by X-ray diffraction, optical microscopy, SEM, X-ray photoelectron spectroscopy, N2 sorption, thermogravimetry, and IR spectroscopy of adsorbed CO. CO adsorbs on a small number of Cu2O impurities, and particularly on the free CuII coordination sites in the framework. [Cu3(btc)2] is a highly selective Lewis acid catalyst for the isomerization of terpene derivatives, such as the rearrangement of alpha-pinene oxide to campholenic aldehyde and the cyclization of citronellal to isopulegol. By using the ethylene ketal of 2-bromopropiophenone as a test substrate, it was demonstrated that the active sites in [Cu3(btc)2] are hard Lewis acids. Catalyst stability, re-usability, and heterogeneity are critically assessed.     

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).     

Chemical and thermal stability of isotypic metal-organic frameworks: effect of metal ions

Chemical and thermal stabilities of isotypic metal-organic frameworks (MOFs) like Al-BDC (Al-benzenedicarboxylate called MIL-53-Al), Cr-BDC (MIL-53-Cr) and V-BDC (MIL-47-V), after purification to remove uncoordinated organic linkers, have been compared to understand the effect of the central metal ions on the stabilities of the porous MOF-type materials. Chemical stability to acids, bases, and water decreases in the order of Cr-BDC>Al-BDC>V-BDC, suggesting stability increases with increasing inertness of the central metal ions. However, thermal stability decreases in the order of Al-BDC>Cr-BDC> V-BDC, and this tendency may be explained by the strength of the metal-oxygen bond in common oxides like Al(2)O(3), Cr(2)O(3), and V(2)O(5). In order to evaluate precisely the stability of a MOF, it is necessary to remove uncoordinated organic linkers that are located in the pores of the MOF, because a filled MOF may be more stable than the same MOF after purification.     

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-.     

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.