Single-Atom Ligand Changes Affect Breathing in an Extended Metal–Organic Framework

Two-Photon 3D Printing in Metal-Organic Framework Single Crystals

Two-photon polymerization (TPP) is a micro/nano-fabrication technology for additive manufacturing, enabling 3D printing of polymeric materials using ultrafast laser pulses. In this work, two-photon polymerization is realized inside a metal-organic framework (MOF) crystal. Intricate structures are built in the porous crystal to create a microstructure-in-crystal hybrid. Furthermore, the MOF can be removed by acid treatment to release the printed structure. The two-photon polymerization inside the crystal has the potential for MOF sensing device fabrication and data storage applications. In the future development, printing different materials in the same MOF crystal for creating functional 3D devices is hoped.

Unfolding the Role of Building Units of MOFs with Mechanistic Insight Towards Selective Metal Ions Detection in Water

The potential emergence of fluorescence-based techniques has propelled research towards developing probes that can sense trace metal ions specifically. Although luminescent metal-organic frameworks (MOFs) are well suited for this application, the role of building blocks towards detection is not fully understood. In this work, a systematic screening by varying number of Lewis basic (pyridyl-N atoms) sites is carried out in a series of isostructural, robust UiO-67 MOFs, and targeting a model metal ion-Fe³⁺ . All the three fluorescent MOFs are seen to present quenching response towards Fe³⁺ ions in water. However, UiO-67@N exhibits highly selective and sensitive response, whereas emission of both UiO-67 and UiO-67@NN is quenched by several metal ions. Detailed experimental and theoretical mechanistic investigation is carried out in addition to demonstration of UiO-67@N being able to sense trace amount of Fe³⁺ ions in synthetic biological water sample. Further, UiO-67@N based mixed-matrix membrane (MMM) has been prepared and employed to mimic the real time Fe³⁺ ions detection in water.

A series of entangled MOFs constructed from flexible dipyridyl piperazine and rigid dicarboxylate: interpenetration, self-penetration, and polycatenation

By employing a flexible dipyridyl piperazine and a rigid linear dicarboxylate, four MOFs with different entangled structures involving a 3D inclined polycatenane and a 3D self-penetrated framework incorporating cyclic [3]catenane were isolated.

The structures of 1:1 and 1:2 adducts of phosphane-tricarbo-nitrile with 1,4-di-aza-bicyclo[2.2.2]octa-ne

In the structures of 1:1 and 1:2 adducts of phosphanetricarbo-nitrile (C3N3P) with 1,4-di-aza-bicyclo-[2.2.2]octane (C6H12N2), the 1:1 adduct crystallizes in the ortho-rhom-bic space group, Pbcm, with four formula units in the unit cell (Z' = 0.5). The P(CN)3 unit lies on a crystallographic mirror plane while the C6H12N2 unit lies on a crystallographic twofold axis passing through one of the C-C bonds. The P(CN)3 moiety has close to C 3v symmetry and is stabilized by forming adducts with two symmetry-related C6H12N2 units. The phospho-rus atom is in a five-coordinate environment. As a result of the symmetry, the two trans angles are equal so τ5 = 0.00 and thus the geometrical description could be considered to be square pyramidal. However, the electronic geometry is distorted octa-hedral with the lone pair on the phospho-rous occupying the sixth position. As would be expected from VSEPR considerations, the repulsion of the lone-pair electrons with the equatorial bonding electrons means that the trans angles for the latter are considerably reduced from 180° to 162.01 (4)°, so the best description of the overall geometry for phospho-rus is distorted square pyramidal. The 1:2 adduct crystallizes in the monoclinic space group, P21/m with two formula units in the asymmetric unit (i.e. Z' = 1/2). The P(CN)3 moiety lies on a mirror plane and one of the two C6H12N2 (dabco) mol-ecules also lies on a mirror plane. The symmetry of the P(CN)3 unit is close to C 3v. There are three P⋯N inter-actions and consequently the mol-ecular geometry of the phospho-rus atom is distorted octa-hedral. This must mean that the lone pair of electrons on the phospho-rus atom is not sterically active. For the 1:1 adduct, there are weak associations between the phospho-rus atom and one of the terminal nitro-gen atoms from the C≡ N moiety, forming chains in the a-axis direction. In addition there are weak C-H⋯N inter-actions between a terminal nitro-gen atoms from the C≡N moiety and the C6H12N2 mol-ecules, which form sheets perpendicular to the a axis.

Linker Expansion and Its Impact on Switchability in Pillared-Layer MOFs

Linker elongation is an important method to systematically adjust porosity and pore size in isoreticular MOFs. In flexible structures, this approach opens the possibility for the systematic analysis of the building blocks and their contribution to the overall flexible behavior enabling tuning of the framework responsivity toward molecular stimuli. In this work, we report two new compounds isoreticular to the highly flexible pillared layer structure DUT-8(Ni) ([Ni₂(2,6-ndc)₂(dabco)]_n, 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicylo[2.2.2]octane). Aromatic linker 2,6-ndc was substituted by longer carboxylic linkers, namely, 4,4'-biphenyldicarboxylate (4,4'-bpdc) and 4,4'-stilbenedicarboxylate (4,4'-sdc), while the dabco pillar was retained. The structural response of the new compounds toward the desolvation and adsorption of various fluids was studied using advanced in situ PXRD techniques, demonstrating distinct differences in the flexible behavior of three compounds and disclosing the impact of linker structure on the framework response. Theoretical calculations provide mechanistic insights and an energetic rationale for the pronounced differences in switchability observed. The energetics of linker bending and linker-linker dispersion interactions govern the phase transitions in investigated MOFs.

Octafluorobiphenyl-4,4′-dicarboxylate as a ligand for metal-organic frameworks: progress and perspectives

While metal-organic frameworks based on aromatic carboxylates are very numerous and well investigated, the chemistry of their fully fluorinated analogues is at the very beginning. This minireview aims at summarizing all metal complexes with octafluorobiphenyl-4,4′-dicarboxylate (oFBPDC2−) anion and in particular, porous coordination polymers, their syntheses, crystal structures and functional properties highlighting the importance of further investigation of such systems.

Hydrogen-Bonding Linkers Yield a Large-Pore, Non-Catenated, Metal-Organic Framework with pcu Topology

Pillared paddle-wheel-based metal-organic framework (MOF) materials are an attractive target as they offer a reliable method for constructing well-defined, multifunctional materials. A drawback of these materials, which has limited their application, is their tendency to form catenated frameworks with little accessible volume. To eliminate this disadvantage, it is necessary to investigate strategies for constructing non-catenated pillared paddle-wheel MOFs. Hydrogen-bonding substituents on linkers have been postulated to prevent catenation in certain frameworks and, in this work, we present a new MOF to further bolster this theory. Using 2,2'-diamino-[1,1'-biphenyl]-4,4'-dicarboxylic acid, BPDC-(NH₂)₂, linkers and dipyridyl glycol, DPG, pillars, we assembled a MOF with pcu topology. The new material is non-catenated, exhibiting large accessible pores and low density. To the best of our knowledge, this material constitutes the pcu framework with the largest pore volume and lowest density. We attribute the lack of catenation to the presence of H-bonding substituents on both linkers.

Diversity-Oriented Metal Decoration on UiO-Type Metal-Organic Frameworks: an Efficient Approach to Increase CO2 Uptake and Catalytic Conversion to Cyclic Carbonates

A library of metallo-bipyridine UiO-type metal-organic frameworks (MOFs) has been successfully synthesized by postmetallation of a wide range of metal complexes into bidentate bipyridine moieties. Then, a systematic investigation is devoted to a catalytic evaluation of the resultant MOFs containing a binary Lewis acid function for the synthesis of cyclic carbonates from epoxides and carbon dioxide (CO2). The result indicated that the metal-grafted MOFs exhibit improvement in terms of CO2 uptake capacity and catalytic activity in comparison with their nonmetallated counterparts. The comprehensive investigation provides a valuable insight into the synergetic effects of MOF functionalities including metal node, grafted metal, and its counterion in the cycloaddition reaction. Furthermore, the metal coordination modulation due to its benefits such as being a solvent-free process, nearly full conversion to cyclic carbonates, high selectivity and high CO2 uptake, applying atmospheric CO2 pressure, and excellent stability and easy recyclability of the catalyst demonstrates them as promising candidates for practical utilization of CO2 conversion into value-added chemicals.

Understanding Hysteresis in Carbon Dioxide Sorption in Porous Metal-Organic Frameworks

Two new isostructural microporous coordination frameworks [Mn₃(Hpdc)₂(pdc)₂] (1) and [Mg₃(Hpdc)₂(pdc)₂] (2) (pdc^2- = pyridine-2,4-dicarboxylate) showing primitive cubic (pcu) topology have been prepared and characterized. The pore aperture of the channels is too narrow for the efficient adsorption of N₂; however, both compounds demonstrate substantially higher uptake of CO₂ (119.9 mL·g^-1 for 1 and 102.5 mL·g^-1 for 2 at 195 K, 1 bar). Despite of their structural similarities, 2 shows a typical reversible type I isotherm for adsorption/desorption of CO₂, while 1 features a two-step adsorption process with a very broad hysteresis between the adsorption and desorption curves. This behavior can be explained by a combination of density functional theory calculations, sorption, and X-ray diffraction analysis and gives insights into the further development of new sorbents showing adsorption/desorption hysteresis.

Metal–organic frameworks based on octafluorobiphenyl-4,4′-dicarboxylate: synthesis, crystal structure, and surface functionality

Six Zn coordination polymers have been synthesized and investigated in terms of their surface properties.

Metal-Organic Frameworks with Internal Urea-Functionalized Dicarboxylate Linkers for SO2 and NH3 Adsorption

Introduction of a urea R-NH-CO-NH-R group as a seven-membered diazepine ring at the center of 4,4'-biphenyl-dicarboxylic acid leads to a urea-functionalized dicarboxylate linker (L1^2-) from which four zinc metal-organic frameworks (MOFs) could be obtained, having a {Zn₄(μ₄-O)(O₂C-)₆} SBU and IRMOF-9 topology (compound [Zn₄(μ₄-O)(L1)₃], 1, from dimethylformamide, DMF) or a {Zn₂(O₂C-)₄} paddle-wheel SBU in a 2D-network (compound [Zn₂(L1)₂(DEF)₂·2.5DEF], 2, from diethylformamide, DEF). Pillaring of the 2D-network of 2 with 4,4'-bipyridine (bipy) or 1,2-bis(4-pyridyl)ethane (bpe) gives 3D frameworks with rhombohedrally distorted pcu-a topologies ([Zn₂(L1)₂(bipy)], 3 and [Zn₂(L1)₂(bpe)], 4, respectively). The 3D-frameworks 1, 3, and 4 are 2-fold interpenetrated with ∼50% solvent-accessible volume, albeit of apparently dynamic porous character, such that N₂ adsorption at 77 K does not occur, while H₂ at 77 K (up to ∼1 wt %) and CO₂ at 293 K (∼5 wt %) are adsorbed with large hystereses in these flexible MOFs. The urea-functionalized MOF 3 exhibits an uptake of 10.9 mmol g^-1 (41 wt %) of SO₂ at 293 K, 1 bar, which appears to be the highest value observed so far. Compounds 3 and 4 adsorb 14.3 mmol g^-1 (20 wt %) and 17.8 mmol g^-1 (23 wt %) NH₃, respectively, which is at the top of the reported values. These high uptake values are traced to the urea functionality and its hydrogen-bonding interactions to the adsorbents. The gas uptake capacities follow the specific porosity of the frameworks, in combination with pore aperture size and affinity constants from fits of the adsorption isotherms.

Five 1D to 3D Zn(ii)/Mn(ii)-CPs based on dicarboxyphenyl-terpyridine ligand: stepwise adsorptivity and magnetic properties

Five coordination polymers with different dimensional structures have been solvothermally synthesized by utilizing H₂dtp ligand. Complexes1and2reveal strong solid-state luminescence, and complexes3–5display antiferromagnetic exchange.

Isolation of a structural intermediate during switching of degree of interpenetration in a metal-organic framework

A known pillared layered metal-organic framework [Co2(ndc)2(bpy)] is shown to undergo a change in degree of interpenetration from a highly porous doubly-interpenetrated framework (2fa) to a less porous triply-interpenetrated framework (3fa). The transformation involves an intermediate empty doubly-interpenetrated phase (2fa') which has been isolated for the first time for this kind of phenomenon by altering the conditions of activation of the as-synthesized material. Interestingly, all the transformations occur in single-crystal to single-crystal fashion. Changes in degree of interpenetration have not been explored much to date and their implications with regard to the porosity of MOFs still remain largely unknown. The present study not only provides a better understanding of such dramatic structural changes in MOF materials, but also describes an original way of controlling interpenetration by carefully optimizing the temperature of activation. In addition to studying the structural mechanism of conversion from 2fa to 3fa, sorption analysis has been carried out on both the intermediate (2fa') and the triply-interpenetrated (3fa) forms to further explain the effect that switching of interpenetration mode has on the porosity of the MOF material.

Structure-directing factors when introducing hydrogen bond functionality to metal–organic frameworks

Introduction of functional groups into an isoreticular series of MOFs may be complicated by noncovalent interactions between interpenetrated nets inducing differing topologies.

A non-regular layer arrangement of a pillared-layer coordination polymer: avoiding interpenetration via symmetry breaking at nodes

A coordination terpolymerization strategy is exploited to generate a non-interpenetrated pillared-layer coordination polymer with a non-regular layer arrangement. The role of breaking nodal symmetry is discussed.

Solvothermal synthesis, structure, and fluorescence properties of three d10 polymers assembled from semi-rigid V-shaped aza-bridged multicarboxylate

Hydrothermal reactions of azanediyldibenzoic acid and M(OH)₂ with different ancillary ligands afford three polymers. These polymers show great different structures and properties originated from the variation of second ligands and coordination modes.

Ce(III) doped Zr-based MOFs as excellent NO2 adsorbents at ambient conditions

New hybrid cerium modified zirconium based metal-organic frameworks (MOFs) were synthesized. The as-received materials were evaluated as adsorbents of NO2 in either moist or dry conditions. The surface of the initial and exhausted samples was characterized using XRD, SEM-EDX, nitrogen adsorption, thermal analysis, and FTIR. It was found that the addition of Ce(+3) slightly affects the growth of the framework and introduces new features to Zr-MOF. The shapes of the octahedral crystals are changed, and they are interwoven with rod-flake-like sheets. The extent of the interconnection, and thus the extent of the hybrid MOF formation, depends on the Zr to Ce ratio. The alterations in the surface chemistry and texture are reflected in the amount of NO2 adsorbed. The narrow pore channels present in these new materials enhance adsorption in either moist or dry conditions. The amount of NO2 adsorbed on the Ce-doped MOF increases over 25% in dry conditions in comparison with the unmodified MOF. Exposure of Ce-UiO-66 to NO2 results in a development of porosity. Regardless the conditions, the XRD patterns indicate the stability of this new hybrid MOF upon NO2 adsorption. Interactions of NO2 with MOF result in the formation of nitrate and nitrite species associated either with metals or with organic ligands.