Amine-Based MOF for Precious Metal Remediation.

Due to the continuous growth rate of the electronic industry, hi-tech companies depend on mining and extracting precious metals to meet the public demand. The high turnover of modern devices generates an alarming amount of electronic waste (e-waste), which contains more precious metals than mined ores and therefore needs efficient recovery procedures. A highly stable homopiperazine-derived Cd-MOF, poly-[Cd(H2L)]·9H2O, with a protonated amine ligand core, exists as a twofold interpenetrated 3D framework with 1D channels into which the N+-H bond is directed. The geometry of these channels appears to be suitable to host square planar metalate complexes. Under acidic conditions, [MCl4]x- anions containing Au, Cu, Ni, and Pt, representing common components of e-waste under extraction conditions, were tested for capture and recovery. Cd-MOF exhibits remarkable selectivity and uptake performance toward Au with an adsorbent capacity of 25 mg g-1ads and shows a marked selectivity for Au over Cu in competitive experiments. The adsorption mechanism of Au appears to be predominantly physical adsorption at the surface of the material.

Mixed donor, phenanthroline photoactive MOFs with favourable CO2 selectivity.

Mixed donor phenanthroline-carboxylate linkers were combined with MnII or ZnII to form photoactive MOFs with large pore apertures. The MOFs display high CO2 adsorption capacities, which consequently causes structural framework flexibility, and align with favorable metrics for selective CO2 capture. The photophysical properties of the MOFs were investigated, with the MnII MOF giving rise to short triplet LMCT lifetimes.

Coordination polymers of a bis-isophthalate bridging ligand with single molecule magnet behaviour of the CoII analogue.

A linear diamine-bisisophthalate bridging linker N,N'-bis(1,3-dicarboxyphenyl-5-methylene)-1,3-dimethylpropanediamine, designed to incorporate amine/ammonium functionalities in the core of the ligand, has been isolated as the pentahydrate of its dihydrochloride salt (H6L)Cl2·5H2O. Using this compound, four new coordination polymers have been formed, namely poly-[M(H2L)]·4.5H2O (1M, where M = Co, Zn, Cd) and poly-[Cd(H2L)(OH2)]·DMF·7H2O (2). Compounds 1M are isostructural 2D coordination polymers that contain 1D channels occupied by water molecules. In the case of 1Co these form a well ordered hydrogen-bonding network as determined by single crystal X-ray studies. Compound 2, synthesised under similar conditions, is a 1D coordination polymer in which the metal is partially solvated. DC and AC magnetic studies of 1Co, which posseses a mononuclear cobalt(ii) node, revealed single molecule magnet behaviour (SMM) with an effective barrier height Ueff of 37.7 K and τ0 = 1.02 × 10-9 s, among the highest reported for CoII coordination polymers.

Enhancing Multicomponent Metal-Organic Frameworks for Low Pressure Liquid Organic Hydrogen Carrier Separations.

The resurgence of interest in the hydrogen economy could hinge on the distribution of hydrogen in a safe and efficient manner. Whilst great progress has been made with cryogenic hydrogen storage or liquefied ammonia, liquid organic hydrogen carriers (LOHCs) remain attractive due to their lack of need for cryogenic temperatures or high pressures, most commonly a cycle between methylcyclohexane and toluene. Oxidation of methylcyclohexane to release hydrogen will be more efficient if the equilibrium limitations can be removed by separating the mixture. This report describes a family of six ternary and quaternary multicomponent metal-organic frameworks (MOFs) that contain the three-dimensional cubane-1,4-dicarboxylate (cdc) ligand. Of these MOFs, the most promising is a quaternary MOF (CUB-30), comprising cdc, 4,4'-biphenyldicarboxylate (bpdc) and tritopic truxene linkers. Contrary to conventional wisdom that adsorptive interactions with larger, hydrocarbon guests are dominated by π-π interactions, here we report that contoured aliphatic pore environments can exhibit high selectivity and capacity for LOHC separations at low pressures. This is the first time, to the best of our knowledge, where selective adsorption for cyclohexane over benzene is witnessed, underlining the unique adsorptive behavior afforded by the unconventional cubane moiety.

CUB-5: A Contoured Aliphatic Pore Environment in a Cubic Framework with Potential for Benzene Separation Applications.

One prominent aspect of metal organic frameworks (MOFs) is the ability to tune the size, shape, and chemical characteristics of their pores. MOF-5, with its open cubic connectivity of Zn4O clusters joined by two-dimensional, terephthalate linkers, is the archetypal example: both functionalized and elongated linkers produce isoreticular frameworks that define pores with new shapes and chemical environments. The recent scalable synthesis of cubane-1,4-dicarboxylic acid (1,4-H2cdc) allows the first opportunity to explore its application in leading reticular architectures. Herein we describe the use of 1,4-H2cdc to construct [Zn4O(1,4-cdc)3], referred to as CUB-5. Isoreticular with MOF-5, CUB-5 adopts a cubic architecture but features aliphatic, rather than aromatic, pore surfaces. Methine units point directly into the pores, delivering new and unconventional adsorption locations. Our results show that CUB-5 is capable of selectively adsorbing high amounts of benzene at low partial pressures, promising for future investigations into the industrial separation of benzene from gasoline using aliphatic MOF materials. These results present an effective design strategy for the generation of new MOF materials with aliphatic pore environments and properties previously unattainable in conventional frameworks.

Anisotropic Thermal and Guest-Induced Responses of an Ultramicroporous Framework with Rigid Linkers.

The interdependent effects of temperature and guest uptake on the structure of the ultramicroporous metal-organic framework [Cu3 (cdm)4 ] (cdm=C(CN)2 (CONH2 )- ) were explored in detail by using in situ neutron scattering and density functional theory calculations. The tetragonal lattice displays an anisotropic thermal response related to a hinged "lattice-fence" mechanism, unusual for this topology, which is facilitated by pivoting of the rigid cdm anion about the Cu nodes. Calculated pore-size metrics clearly illustrate the potential for temperature-mediated adsorption in ultramicroporous frameworks due to thermal fluctuations of the pore diameter near the value of the target guest kinetic diameter, though in [Cu3 (cdm)4 ] this is counteracted by a competing contraction of the pore with increasing temperature as a result of the anisotropic lattice response.

Insights into Selective Gas Sorbent Functionality Gained by Using Time-Resolved Neutron Diffraction.

An understanding of the atomic-scale interactions between gas sorbent materials and their molecular guests is essential for the identification of the origins of desirable function and the rational optimization of performance. However, characterizations performed on equilibrated sorbent-guest systems may not accurately represent their behavior under dynamic operating conditions. The emergence of fast (minute-scale) neutron powder diffraction coupled with direct, real-time quantification of gas uptake opens up new possibilities for obtaining knowledge about concentration-dependent effects of guest loading upon function-critical features of sorbent materials, including atomic structure, diffusion pathways, and thermal expansion of the sorbent framework. This article presents a detailed investigation of the ultramicroporous metal-organic framework [Cu3 (cdm)4 ] as a case study to demonstrate the variety of insights into sorbent performance that can be obtained from real-time characterizations using neutron diffraction.

Selectivity differences of coordination compound stationary phases for polyaromatic hydrocarbons and polar analytes in gas and liquid phases.

A precursor solution to the metal-organic framework (MOF), Cd(1,4,7,10-tetrakis(4-carboxybenzyl)-1,4,7,10-tetraazacyclododecane), was aged in a surface-functionalised 250µm-diameter capillary, to generate a film containing the constituents of the bulk phase crystalline material. EDX analysis revealed a higher ratio of ligand elements (C, N and O) to Cd in the capillary coated phase compared to that achieved from bulk synthesis in a vial. Separation performance of the coated capillary for gas chromatography (GC) was compared to the performance of the original MOF crystals packed in tips for liquid elution chromatography (LEC) towards the separation of polycyclic aromatic hydrocarbons (PAHs) and small probe analytes with different properties. This study showed that different retention mechanisms were observed for the same set of analytes in gas and liquid phases. Independently of the trend of vapour pressure difference, the GC result showed less specific size selective retention, but with stronger retention towards larger PAHs. A linear solvation energy relationship correlation further revealed enhanced retention contributions (positive stationary phase descriptor values) by cavity formation/dispersion interactions and H-bonding with acid functionality of the probe analytes in both GC and LEC (though stronger in LEC), whilst H-bonding with basic functional groups slightly increased retention in GC; retention was strengthened by dipole-dipole/dipole-induced dipole interactions only in LEC.

Porous Polyrotaxane Coordination Networks Containing Two Distinct Conformers of a Discontinuously Flexible Ligand.

A new divergent homopiperazine-derived ligand N,N'-bis(4-carboxyphenyl)-1,4-diazacycloheptane H2L has been prepared, containing a semirigid saturated heterocyclic core which is capable of providing multiple distinct bridging geometries. Reaction of H2L with zinc acetate in DMSO gives a two-dimensional parallel interpenetrated polyrotaxane structure 1 in which the loops and rods are formed by the bent cis-(eq,ax) twist boat and trans-(ax,ax) twist chair conformers, respectively. By matching the distances between the solvated metal sites in the structure of 1, a related material 2 can be prepared incorporating the pillaring ligand trans-1,2-bis(4-pyridyl)ethylene bpe. Compound 2 displays a similar polyrotaxane interpenetration mode, permitted by the presence of both cis and trans ligand conformers, but displays a three-dimensional 2.69 topology related to the dia diamondoid network. The guest exchange and gas adsorption properties of both materials were investigated; while compound 1 displays poor stability to guest exchange and negligible gas uptake, the higher connectivity microporous compound 2 shows facile guest exchange and a surprisingly high CO2 capacity of 12 wt % at 1 bar and 273 K, and a zero-loading enthalpy of adsorption of -32 kJ mol-1. High-pressure adsorption isotherms also show moderate physisorption of H2 and CH4 within the material.

Coordination Chemistry and Structural Dynamics of a Long and Flexible Piperazine-Derived Ligand.

A long and highly flexible internally functionalized dipyridyl ligand α,α'-p-xylylenebis(1-(4-pyridylmethylene)-piper-4-azine), L, has been employed in the synthesis of a series of coordination polymer materials with Co(II), Cd(II), and Ag(I) ions. In poly-[Cd(L)(TPA)] 1 and poly-[Co(L)(IPA)], 2, (TPA = terephthalate, IPA = isophthalate) the ligand adopts a similar linear conformation to that seen in the structure of the unbound molecule and provides a long (2.6 nm) metal-metal bridging distance. Due to the mismatch of edge lengths with that provided by the carboxylate coligands, geometric distortions from the regular dia and (4,4) network geometries for 1 and 2, respectively, are observed. In poly-[Ag2(CF3SO3)2(L)], 3, the ligand coordinates through both pyridine groups and two of the four piperazine nitrogen donors, forming a high-connectivity 2-dimensional network. The compound poly-[Ag2(L)](BF4)2·2MeCN, 4, a porous 3-dimensional cds network, undergoes a fascinating and rapid single-crystal-to-single-crystal rearrangement on exchange of the acetonitrile guests for water in ambient air, forming a nonporous hydrated network poly-[Ag2(L)](BF4)2·2H2O, 5, in which the well-ordered guest water molecules mediate the rearrangement of the tetrafluoroborate anions and the framework itself through hydrogen bonding. The dynamics of the system are examined in greater detail through the preparation of a kinetic product, the dioxane-solvated species poly-[Ag2(L)](BF4)2·2C4H8O2, 6, which undergoes a slow conversion to 5 over the course of approximately 16 h, a transition which can be monitored in real time. The reverse transformation can also be observed on immersing the hydrate 5 in dioxane. The structural features and physical properties of each of the materials can be rationalized based on the flexible and multifunctional nature of the ligand molecule, as well as the coordination behavior of the chosen metal ions.

Two Unusual Nanocage-Based Ln-MOFs with Triazole Sites: Highly Fluorescent Sensing for Fe3+ and Cr2 O7 2- , and Selective CO2 Capture.

Luminescent metal-organic frameworks (LMOFs) containing fluorescent probes for the detection of pollutants such as organic solvents and heavy metals are becoming increasingly important, with lanthanide-MOF (Ln-MOF) materials receiving greater attention owing to the possibility of achieving fine-tuned luminescent properties. Herein, two unusual isostructural nanocage-based three-dimensional Ln-MOFs, 1-Ln (Ln=Tb, Eu), are constructed, using a new diisophthalate ligand with active Lewis basic triazole sites. Selective gas adsorption, especially the removal of CO2 from CH4 , a primary component of natural gas and biogas, is desirable in terms of both economic and environmental considerations. 1-Eu is found to exhibit highly efficient luminescent sensing for Fe3+ cations and Cr2 O7 2- anions, as well as selective CO2 capture over CH4 .

Coordination Polymers/MOFs: Structures, Properties and Applications.

Spotlight on coordination polymers: ChemPlusChem is pleased to present its special issue on coordination polymers/metal-organic frameworks (MOFs), guest-edited by Stuart Batten, Banglin Chen and Jagadese J. Vittal. This issue features MOFs through various applications as sensors and in drug delivery, gas storage, catalysis etc.

A Luminescent Zinc(II) Metal-Organic Framework for Selective Detection of Nitroaromatics, Fe3+ and CrO4 2- : A Versatile Threefold Fluorescent Sensor.

A new ZnII metal-organic framework, [Zn6 (L)3 (DMA)4 ]⋅5 DMA (H4 L=[1,1':3',1''-terphenyl]-3,3'',5,5''-tetracarboxylic acid, DMA=dimethylacetamide), has been synthesized and characterized. The structure contains a three-dimensional 3,4,4,6-connected net with (4.62 )2 (66 )(66 )(42 .610 .83 ) topology and displays selective detection of nitrobenzene, CrO4 2- and Fe3+ ions. The present work thus indicates that this metal-organic framework could be a prospective candidate for developing novel luminescence sensors for the selective sensing of nitrobenzene, which can be used as a precursor for explosives. Furthermore, the photoluminescent properties of this material in different solvents and with various analytes were investigated and corroborated by theoretical calculations. The results were in good agreement with the experimental solvent-dependent luminescence behavior.

A combined experimental and computational study of novel nanocage-based metal-organic frameworks for drug delivery.

Three new metal organic frameworks (MOFs) with chemical formulae [(CH3)2NH2] [Sm3(L1)2(HCOO)2(DMF)2(H2O)]·2DMF·18H2O (1), [Cu2(L2)(H2O)2]·2.22DMA (2) and [Zn2(L1)(DMA)]·1.75DMA were synthesized and structurally characterized. 1 and 2 show a classical NbO-like topology and have two types of interconnected cages. 3 exhibits an uncommon zzz topology and has two types of interconnected cages. These MOFs can adsorb large amounts of the drug 5-fluorouracil (5-FU) and release it in a progressive way. 5-FU was incorporated into desolvated 1, 2 and 3 with loadings of 0.40, 0.42, and 0.45 g g(-1), respectively. The drug release rates were 72%, 96% and 79% of the drug after 96 hours in 1, 120 hours in 2 and 96 hours in 3, respectively. Grand Canonical Monte Carlo (GCMC) simulations were performed to investigate the molecular interactions during 5-FU adsorption to the three novel materials. The GCMC simulations reproduced the experimental trend with respect to the drug loading capacity of each material. They also provided a structural description of drug packing within the frameworks, helping to explain the load capacity and controlled release characteristics of the materials. 5-FU binding preferences to 1, 2 and 3 reflect the diversity in pore types, chemistry and sizes. The calculated drug load is more related to the molecular properties of accessible volume Vacc than to the pore size.

Coordination polymers from a highly flexible alkyldiamine-derived ligand: structure, magnetism and gas adsorption studies.

The synthesis and structural, magnetic and gas adsorption properties of a series of coordination polymer materials prepared from a new, highly flexible and internally functional tetrakis-carboxybenzyl ligand H4L derived from 1,2-diaminoethane have been examined. The compound poly-[Ni3(HL)2(OH2)4]·2DMF·2H2O 1, a two-dimensional coordination polymer, contains aqua- and carboxylato-bridged trinuclear Ni(II) clusters, the magnetic behaviour of which can be well described through experimental fitting and ab initio modelling to a ferromagnetically coupled trimer with a positive axial zero-field splitting parameter D. Compound poly-[Zn2L]·2DMF·3H2O 2, a three-dimensional coordination polymer displaying frl topology, contains large and well-defined solvent channels, which are shown to collapse on solvent exchange or drying. Compound poly-[Zn2(L)(DMSO)4]·3DMSO·3H2O 3, a highly solvated two-dimensional coordination polymer, displayed poor stability characteristics, however a structurally related material poly-[Zn2(L)(bpe)(DMSO)2]·DMSO·3H2O 4 was prepared under similar synthetic conditions by including the 1,2-bis(4-pyridyl)ethylene bpe co-ligand. Compound 4, containing small one-dimensional solvent channels, shows excellent structural resilience to solvent exchange and evacuation, and the evacuated material displays selective adsorption of CO2 over N2 at 273 K in the pressure range 0-1 atm. Each of the coordination polymers displays subtle differences in the conformation and binding mode of the ligand species, with switching between two distinct conformers (X-shaped and H-shaped), as well as a variable protonation state of the central core, with significant effects on the resulting network structures and physical properties of the materials.

A cyclic dodecanuclear cobalt cluster based on a derivative of the rhodamine 6G dye with unusual magnetization.

A novel cyclic dodecanuclear cobalt cluster which is bridged via a derivative of the rhodamine 6G dye is presented. The work provides a new and effective strategy for the design and development of novel magnetic materials based on high-nuclearity metal clusters decorated by the rhodamine dye.

Synthesis and structure of new lanthanoid carbonate "lanthaballs".

New insights into the synthesis of high-nuclearity polycarbonatolanthanoid complexes have been obtained from a detailed investigation of the preparative methods that initially yielded the so-called "lanthaballs" [Ln(13)(ccnm)(6)(CO(3))(14)(H(2)O)(6)(phen)(18)] Cl(3)(CO(3))·25H(2)O [α-1Ln; Ln = La, Ce, Pr; phen = 1,10-phenanthroline; ccnm = carbamoylcyanonitrosomethanide]. From this investigation, we have isolated a new pseudopolymorph of the cerium analogue of the lanthaball, [Ce(13)(ccnm)(6)(CO(3))(14)(H(2)O)(6)(phen)(18)]·C(l3)·CO(3) (ß-1Ce). This new pseudopolymorph arose from a preparation in which fixation of atmospheric carbon dioxide generated the carbonate, and the ccnm ligand was formed in situ by the nucleophilic addition of water to dicyanonitrosomethanide. From a reaction of cerium(III) nitrate, instead of the previously used chloride salt, with (Et4N)(ccnm), phen, and NaHCO(3) in aqueous methanol, the new complex Na[Ce(13)(ccnm)(6)(CO(3))(14)(H(2)O)(6)(phen)(18)](NO(3))(6)·20H(2)O (2Ce) crystallized. A variant of this reaction in which sodium carbonate was initially added to Ce(NO(3))(3), followed by phen and (Et(4)N)(ccnm), also gave 2Ce. However, an analogous preparation with (Me4N)(ccnm) gave a mixture of crystals of 2Ce and the coordination polymer [CeNa(ccnm)4(phen)3]·MeOH (3), which were manually separated. The use of cerium(III) acetate in place of cerium nitrate in the initial preparation did not give a high-nuclearity complex but a new coordination polymer, [Ce(ccnm)(OAc)(2)(phen)] (4). The first lanthaball to incorporate neodymium, namely, [Nd(13)(ccnm)(4)(CO(3))14(NO(3))(4)(H(2)O)(7)(phen)(15)](NO(3))(3)·10H(2)O (5Nd), was isolated from a preparation similar to that of the second method used for 2Ce, and its magnetic properties showed an antiferromagnetic interaction. The identity of all products was established by X-ray crystallography.

Ferrocene-appended ligands for use in spin crossover-redox "hybrid" complexes of iron(II) and cobalt(II).

In a study of multifunctional ('hybrid') molecular materials, with one function being spin-crossover and the second being reversible redox behaviour, we describe ferrocene-appended ligands and their d(6) and d(7) complexes trans-[Fe(II)(FTP)2(NCS)2] (1), [Co(II)(FTTP)2](ClO4)2·2(MeCN) (2) and [Fe(II)(FTTP)2](ClO4)2·Et2O (3) (where FTP = 4-(2-pyridyl)-1H-1,2,3-triazol-1-ylferrocene and FTTP = 4'-ferrocenyl-2,2':6',2''-terpyridine). The structures, magnetism and solution electrochemistry are described. Complex 1 remains high-spin, 2 displays gradual, incomplete spin crossover and 3 remains low-spin between 350-2 K. The electrochemical results show that one-electron oxidations at the ferrocene group, located external to the coordination site, occur at more positive potentials than the 'inner' M(II/III) couple in 1 and 2, but not in 3, and this has implications for retaining and influencing spin transitions at the M(II) centres, in future.

Metal-organic frameworks as stationary phases for mixed-mode separation applications.

Polymorphic metal-organic framework (MOF) materials offer a platform for small-scale separation of complex mixtures of polycyclic aromatic hydrocarbons (PAHs) and polar compounds. Retention factors show dependence on both analyte dimensions and polarity, suggesting mixed-mode separation, allowing complete resolution of some analytes from multi-component mixtures.