Postsynthetic Metal Research Articles

Selective Monoborylation of Methane by a Mono Bipyridyl-Nickel(II)Hydride Catalyst.

We report the development of an earth-abundant metal catalyst for methane C‒H borylation. The post-synthetic metalation of bipyridine-functionalized zirconium metal-organic framework (MOF) with NiBr2, followed by treatment of NaEt3BH affords MOF-supported monomeric bipyridyl-nickel(II) dihydride species via active site isolation. The heterogeneous and recyclable nickel catalyst selectively borylates methane at 200 ºC using pinacolborane (HBpin) to afford CH3Bpin in 61% yield with a turnover number (TON) up to 1388. The confinement of the active NiH2-species within the uniformly porous MOF allows selective monoborylation of methane via shape-selective catalysis by preventing the formation of sterically encumbered overborylated products. Unlike MOF-Ni catalyst, its homogeneous control is almost inactive in methane borylation due to its intermolecular decomposition. Our mechanistic investigation, including spectroscopic, kinetic, and control experiments, as well as DFT calculations, revealed that stabilizing mononuclear bipyridyl-nickel dihydride and diboryl species by MOF is crucial for achieving efficient methane borylation via turnover-limiting σ-bond metathesis. This work shows promise in designing MOF-based abundant metal catalysts for the chemoselective functionalization of methane and other inert molecules into valuable chemicals.

Defect strategies in Mn-doped metal–organic frameworks to enhance adsorption-based NOx gas sensing performance

Post-synthetic metalation (PSM) represents a powerful toolkit for modifying the chemical composition of pre-formed metal-organic frameworks (MOFs). However, accessing defective MOFs with exchanged metal nodes via this approach remains challenging. Herein, we report a transmetalation strategy to induce desired metal exchange frustrated flexibility defects in a prototypical MOF platform. Specifically, the pristine Zn-based MOF undergoes selective Zn-to-Mn exchange through a carefully controlled solvothermal transmetalation process. This protocol generates a series of Mn-doped MOFs with increasing Mn integration and the concomitant formation of defect sites within the framework. Detailed structural characterization confirms the successful Mn doping and illuminates the generation of vacancies and distortions around the exchanged Mn nodes. The transmetallated Mn-doped MOF materials exhibit altered chemical environments, as probed by spectroscopic techniques, and display promising sensing activity for NOx through the MOF matrix membrane. The proposed application of the MOF samples as an adsorbent-based chemical sensor includes sensing mechanisms for NO2 gas.

The crystal engineering of 3d-metal with two tetrazole-phenyl-carboxylate linkers and multiple post-synthetic metal exchange

Ligand with mixed-groups contributes to the construction of special structure motif with novel properties, yet it is challenging to understand and regulate the competitive coordination of different functional groups toward metal ions. Herein, the crystal engineering for MOF construction from two linear di-topic linkers of 4-(1H-tetrazol-5-yl)benzoic acid (H2tzba) and the longer 4′-(1H-tetrazol-5-yl)-[1,1′-biphenyl]-4-carboxylic acid (H2tzbba) featuring dual groups of phenyl-carboxylate and tetrazole, with three typical 3d metal ions (Mn2+, Fe2+ and Zn2+) were investigated. A series of five new MOFs, that are [Mn3(tzba)3(dmf)4]·11dmf (1, dmf = N,N-Dimethylformamide), [Mn(tzba)(H2O)] (2), [Zn2(tzba)2(dmf)]·0.5dmf (3), [Fe2(tzba)2(dmf)]·0.5dmf (4) and [Zn2(tzbbz)(gac)(dmf)2]·1.5dmf (5), were synthesized under solvothermal reactions. The modified solvent template and temperature effects lead to 3D framework of 1 (dmf solution under 433 K) with [Mn3(tz)3(COO)3] nodes and 1D hexagonal channels, while the 2 (water solution under 373 K) posssesses compact pillared-layer motif from [Mn2(tz)2] nodes. Under changed metal salt reactents but same solvent of dmf, the generated 3 and 4 have isoreticular and porous pillared-layer structure constituted of [M2(COO)2] nodes. The use of tzbba coupled with a crucial auxiliary ligand of glycollic acid, leads to 5 as porous pillared-layer MOFs with 1D rounded channels. The porous flexible skeleton of 1 constructed from [Mn3] node with four coordinated dmf facilitates post-synthetic metal exchange of the Mn2+ by radius similar metal ions following priority of Cu2+ < Zn2+ < Ni2+< Co2+, with maximum exchange percentage ranging from 17 % to 69 %. Time dependent ICP-AES analysis revealed the quite wide feasible solvents for post-synthetic metal exchange, as well as the fine control on substituting degree based on soaking temperature and time. Particularly, the compatible introduce of four different metal ions support the potential of preparing bi-, tri-, tetra- and penta- metal MOF crystals, that can not be obtained by traditional solvothermal reactions.

Unlocking the potential: strategic synthesis of a previously predicted pyrazolate-based stable MOF with unique clusters and high catalytic activity.

The metal-organic framework (MOF) constructed from [Co4Pz8] clusters (Pz = pyrazolate) and 1,3,5-tris(pyrazolate-4-yl) benzene (BTP3-) ligands was structurally predicted many years ago, and expected to be a promising candidate for various applications owing to its unique clusters and highly open 3D framework structure. However, this MOF has not been experimentally prepared yet, despite extensive efforts were made. In this work, we present the successful construction of this MOF, hereinafter referred to as BUT-124(Co), by adopting a two-step synthesis strategy, involving the initial construction of a template framework (BUT-124(Cd)) followed by a post-synthetic metal metathesis process. The effects of various cobalt sources and solvents were systematically investigated, and an innovative stepwise metathesis strategy was employed to optimize the exchange rates and the porosity of the material. BUT-124(Co) demonstrates high catalytic activity in the oxygen evolution reaction (OER), achieving a competitive performance with an overpotential of 393 mV at a current density of 10 mA cm-2, and also affords remarkable long-term stability during potentiostatic electrolysis in 1 M KOH solution, surpassing the durability of many benchmark catalysts. This work not only introduces a novel MOF material with promising properties but also exemplifies a strategic synthesis approach for pyrazolate-based MOFs, paving the way for advancements in diverse application fields.

(Invited) Chemical Kinetic Method for Active-Site Quantification in Fe-N-C Catalysts and Correlation with Molecular Probe and Spectroscopic Site-Counting Methods

Heterogeneous catalysts consisting of iron cations incorporated into nitrogen-doped carbon (“Fe-N-C”) have received extensive attention as leading alternatives to Pt for the electrochemical oxygen reduction reaction (ORR). Fe-N-C catalysts host mononuclear nitrogen-ligated active centers, FeNx, that frequently coexist with agglomerated Fe species. Although FeNx are the dominant active centers for ORR and other electrocatalytic reactions, the relative accuracies of methods to quantify them are still debated. Here, we develop a kinetic probe-reaction approach to quantify FeNx centers and compare it with spectroscopic and molecular probe methods.Model Fe-N-C catalysts were synthesized to contain mononuclear FeNx species at low loadings (0.1–0.4 wt% bulk Fe) on solvent-accessible surfaces using a postsynthetic metalation approach and their Fe speciation was confirmed by low-temperature 57Fe Mössbauer spectroscopy. The initial rate of oxidation of a water-soluble hydroquinone molecule (per gcatalyst) catalyzed by these model materials correlates linearly with their density of FeNx centers, reflecting their intrinsic turnover frequency (TOF) for this reaction. This TOF, in turn, enables the estimation of the active-site density on any other Fe-N-C catalyst through a simple rate measurement. Kinetically determined FeNx site densities are measured on a suite of fourteen Fe-N-C catalysts with diverse synthetic origins and Fe speciation (0.3–8.4 wt% bulk Fe) and are compared with those estimated by low-temperature 57Fe Mössbauer spectroscopy, CO pulse chemisorption, and electrochemical stripping of NO derived from NO2 −. Kinetic quantifications of FeNx centers correlate well with those obtained from the CO pulse chemisorption method and Mössbauer spectroscopy. The broad survey of Fe-N-C materials also reveals the presence of outliers and challenges associated with each site quantification method. The kinetic method developed here does not require pretreatments that may alter active-site distributions nor specialized equipment beyond reaction vessels and standard analytical instrumentation, offering an attractive complementary approach. Figure 1

Post-synthetic Metalation on the Ionic TiO2 Surface to Enhance Metal-CO2 Interaction During Photochemical CO2 Reduction.

During the photochemical CO2 reduction reaction, CO2 adsorption on the catalyst's surface is a crucial step where the binding mode of the [metal-CO2] adduct directs the product selectivity and efficiency. Herein, an ionic TiO2 nanostructure stabilized by polyoxometalates (POM), ([POM]x@TiO2), is prepared and the sodium counter ions present on the surface to balance the POMs' charge are replaced with copper(II) ions, (Cux[POM]@TiO2). The microscopic and spectroscopic studies affirm the copper exchange without altering the TiO2 core and weak coordination of copper (II) ions to the POMs' surface. Band structure analysis suggests the photo-harvesting efficiency of the TiO2 core with the conduction band edge higher than the reduction potential of CuII/I and multi-electron CO2 reduction potentials. Photochemical CO2 reduction with Cux[POM]@TiO2 results in 30 μmol gcat. -1 CO (79 %) and 8 μmol gcat -1 of CH4 (21 %). Quasi-in-situ Raman study provides evidence in support of CO2 adsorption on the Cux[POM]@TiO2 surface. 13C and D2O labeling studies affirm the {Cu-[CO2]-} adduct formation. Despite the photo-harvesting ability of Nax[POM]@TiO2 itself, the poor CO2 adsorption ability of sodium ions highlights the crucial role of copper ion CO2 photo-reduction. Characterization of the {M-[η2-CO2]-} species via surface tuning validates the CO2 activation and photochemical reduction pathway proposed earlier.

One-pot synthesis of multivariate porphyrinic metal-organic frameworks with the insertion of metallobipyridine and the cooperative catalysis for carbonyl hydrosilylation

A series of multivariate porphyrinic metal-organic frameworks with the insertion of bipyridine were obtained via one-pot synthesis. The single Pt site was introduced into the bipyridine linker of BPy@Co-PCN-224 through post-synthetic metalation. The resultant Pt(BPy)Cl2@M-PCN-224, especially when M was Co(II), could efficiently promote carbonyl hydrosilylation with a turnover number up to 13,140 based on Pt and could be recycled and reused for at least 6 times without a noticeable decrease of activity. Mechanistic studies based on in situ UV–vis, FT-IR and 1H NMR spectra, and kinetic isotope effect (KIE) experiments disclosed that the catalytic reaction was accomplished by the cooperative catalysis of Pt(BPy)Cl2 and Co-PCN-224, in which Pt(BPy)Cl2 was the catalytically active center and meanwhile the Co-PCN-224 could condense and increase the concentrations of ketone molecules in its inner cavities through the interactions between Co-porphyrin and carbonyl group. The plausible reaction pathway was proposed based on density functional theory calculations.

Conversion of Polypropylene into Light Hydrocarbons and Aromatics by Metal Exchanged Zeolite Catalysts.

Polyolefins can be converted into C2-C5 hydrocarbons and benzene-toluene-xylene (BTX) aromatics as high-demand petrochemical feedstocks via catalytic pyrolysis on acidic zeolites. Bro̷nsted and Lewis acid sites are responsible for cracking polyolefins into olefins and subsequent aromatic formation. In this study, we have subjected the parent HZSM-5 zeolite to postsynthetic partial metal exchange with Fe, Co, Ni, Cu, and Ce cations to perturb Bro̷nsted/Lewis acidity. We have investigated these metal-modified HZSM-5 on the catalytic pyrolysis of polypropylene (PP) in a micropyrolyzer connected to a two-dimensional gas chromatograph coupled to a time-of-flight mass spectrometer and flame ionization detector (Tandem Pyrolyzer-GC × GC-TOF-MS/FID setup). Whereas Fe-, Co-, Cu-, and Ce-exchanged zeolites (with 2.5, 2.3, 1.9, and 0.8 wt % metal, respectively) had comparable product yields with the parent zeolite, Ni-exchanged zeolites with Ni content of 0.5 to 2 wt % were associated with enhanced BTX formation (28-38 wt %) compared to that of the parent zeolite (22 wt %). Pyridine-FTIR indicated that the Bro̷nsted/Lewis acid ratio of the parent zeolite decreased upon metal ion exchange. According to Pyridine-TPD, the parent zeolite's medium-strength acid sites were redistributed into weak and strong acid sites in Ni-exchanged zeolites. The higher amount of carbon deposits on Ni-exchanged zeolites compared to the parent and other metal ion exchanged zeolites was attributed to the enhanced aromatization activity by the simultaneous decrease in the Bro̷nsted/Lewis acid ratio and emergence of strong acid sites.

Effect of copper and silver modification of NH2-MIL-125(Ti) on the photoreduction of carbon dioxide to formic acid over this framework under visible-light irradiation

Cu and Ag enhance the photocatalytic activities of metal–organic frameworks (MOFs) toward CO2 conversion because of their CO2 adsorption capacities and effects on the lowest unoccupied molecular orbital (LUMO) overpotentials of MOFs. However, to date, targeted introduction of metals into MOFs to achieve visible (Vis)-light-active photocatalysts for CO2 photoconversion has not been realized. Herein, a series of amine-functionalized Ti MOF (NH2-MIL-125(Ti))-based photocatalysts were successfully synthesized using metalation, incorporation, and photodeposition, allowing Cu and Ag incorporation into NH2-MIL-125(Ti) and attainment of ultraviolet- and Vis-light-active photocatalysts. Notably, the most active photocatalyst obtained by post-synthetic metalation of NH2-MIL-125(Ti) by Cu2+ (MOF_met_0.5%Cu) demonstrated excellent performance in photoreducing CO2 to HCOOH: a conversion rate of 30.1 umolg−1h−1 and quantum yield of 1.18% at 380 nm. Photoconversion of CO2 to HCOOH was further confirmed using 13CO2. The novel approach proposed herein is a significant step toward clean energy production and environmental pollutant elimination.

Catching an Oxo Vanadate Porous Acetylacetonate Covalent Adaptive Catalytic Network that Renders Mustard-Gas Simulant Harmless.

In this work, we illustrated the design and development of a metal-coordinated porous organic polymer (POP) namely VO@TPA-POP via a post-synthetic metalation strategy to incorporate oxo-vanadium sites in a pristine polymer (TPA-POP) having acetylacetonate (acac) as anchoring moiety. The as-synthesized VO@TPA-POP exhibited highly robust and porous framework, which has been utilized for thioanisole (TA) oxidation to its corresponding sulfoxide. The catalyst demonstrated notable stability and recyclability by maintaining its catalytic activity over multiple reaction cycles without any significant loss in activity. The X-ray absorption spectroscopy (XAS) and density functional theory (DFT) analysis establish the existence of V(+4) oxidation state along with the VO(O)4 active sites into the porous network and the most energetically feasible mechanistic pathway involved in the TA oxidation, respectively, indicating the role of electron density associated with vanadium center during the catalytic transformation. Thus, this work aims at the demonstration of versatility and potential of VO@TPA-POP as a porous heterogeneous catalyst for the TA oxidation followed by decontamination of sulfur mustards (HD's) to their corresponding less toxic sulfoxides in a more efficient and greener way.

Three-Dimensional Homochiral Covalent Organic Frameworks with Intrinsic Chiral qzd Topology.

Although a variety of chiral porous framework materials have been reported, there are few examples known to combine molecular chirality, helicity, and three-dimensional (3D) intrinsically chiral topology in one structure, which is beneficial for chirality transfer and amplification. Here, we report the synthesis of the first two 3D covalent organic frameworks (COFs) with an intrinsic chiral qzd topology, which exhibit unusual integration of various homochiral and homohelical features. By imine condensation of 4-connected porphyrin tetraamines and 2-connected enantiopure diene dialdehyde, we prepared two isostructural COFs with a noninterpenetrated qzd topology. The specific geometry and conformation flexibility of the V-shaped diene linker control the alignment of square-planar porphyrin units with rotational linkages and facilitate the creation of homochiral extended porous structures that feature a helical arrangement of porphyrins. Post-synthetic metalation of CCOF 23 with Rh(I) affords a heterogeneous catalyst for the asymmetric Michael addition reaction of aryl boronic acids to 2-cyclohexenone, which shows higher enantioselectivities compared to their homogeneous counterparts, presumably due to the confined effect of helical channels. This finding will provide an impetus to explore multichirality materials, offering new insights into the generation and control of helicity, homochirality, and enantioselectivity in the solid state.

Single Cobalt Ion-Immobilized Covalent Organic Framework for Lithium-Sulfur Batteries with Enhanced Rate Capabilities.

Covalent organic frameworks (COFs) are notable for their remarkable structure, function designability, and tailorability, as well as stability, and the introduction of "open metal sites" ensures the efficient binding of small molecules and activation of substrates for heterogeneous catalysis and energy storage. Herein, we use the postsynthetic metal sites to catalyze polysulfide conversion and to boost the binding affinity to active matter for lithium-sulfur batteries (LSBs). A dual-pore COF, USTB-27, with hxl topology has been successfully assembled from the imine chemical reaction between 2,3,8,9,14,15-hexa(4-formylphenyl)diquinoxalino [2,3-a:2',3'-c]phenazine and [2,2'-bipyridine]-5,5'-diamine. The chelating nitrogen sites of both modules are able to postsynthetically functionalize with single cobalt sites to generate USTB-27-Co. The discharge capacity of the sulfur-loaded S@USTB-27-Co composite in a LSB is 1063, 945, 836, 765, 696, and 644 mA h g-1 at current densities of 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 C, respectively, much superior to that of non-cobalt-functionalized species S@USTB-27. Following the increased current densities, the rate performance of S@USTB-27-Co is much better than that of S@USTB-27. In particular, the capacity retention at 5.0 C has a magnificent increase from 19% for the latter species to 61% for the former one. Moreover, S@USTB-27-Co exhibits a higher specific capacity of 543 mA h g-1 than that of S@USTB-27 (402 mA h g-1) at a current density of 1.0 C after electrochemical cycling for 500 runs. This work illustrates the "open metal sites" strategy to engineer the active chemical component conversion in COF channels as well as their binding strength for specific applications.

Optimizing post-synthetic metal incorporation in mixed-linker MOFs: insights from metalation studies on bipyridine-containing UiO-67 single crystals.

The postsynthetic metalation (PSM) of metal-organic frameworks (MOFs) with intrinsic metal binding sites is an intriguing strategy to introduce catalytic function into MOFs. The spatial distribution of the catalytic sites within the MOF crystal will affect the efficiency of the material, but the factors that govern depth distribution of the introduced metal sites are often not well understood. Herein, we employ Rutherford backscattering spectrometry (RBS) to investigate the metal distribution in a series of post-synthetically metalated mixed linker bpdc/BPY UiO-67 (UiO = Universitet i Oslo, bpdc = biphenyl-dicarboxylate, BPY = 2,2'-bipyridine-5,5'-dicarboxylate) single crystals as a function of linker ratio and metalation time. The RBS spectra reveal large differences in the depth distribution of inserted Ni2+ ions, and core/shell architectures are observed in high BPY materials at shorter incubation times. The incubation times to achieve uniform metal incorporation increases with increasing BPY ratios in the materials, suggesting that the presence of the BPY linkers slow down metal uptake. We propose a combination of ionic interactions and pore clogging, where coordinated ions reduce the available pore space for further ions to diffuse deeper into the framework as reasons for the observed trends. The observations are likely relevant for other mixed-linker MOF systems, and understanding the effect that linker ratios have on PSM and cation distribution will aid in future optimizations of catalytic MOFs.

Pd NP-loaded covalent organic framework for pH-switched Pickering emulsion catalytic dechlorination.

Quinoline carboxylic acid-linked and Pd nanoparticle (NP)-loaded COF nanospheres were constructed via a three-component one-pot Doebner reaction and post-synthetic metalation. The obtained Pd@DhaTAPB-COOH solid stabilizer can greatly promote the pH-switched recyclable Pickering interfacial dechlorination reaction, which sheds light on the bright future of smart Pickering emulsion catalysis.

A Zr-hydroxamate metal-organic framework with intrinsic chelating sites for postsynthetic Pd metalation and Suzuki-Miyaura catalysis.

A highly crystalline and robust Zr-hydroxamate metal-organic framework (MOF) was prepared from a pyrazine-based ligand, featuring abundant N,N' chelating sites. High-degree Pd(II) metalation of the MOF was achieved through straightforward postsynthetic modification, with detailed coordination chemistry elucidated spectroscopically. The Pd-functionalized MOF was then studied as a heterogeneous Suzuki-Miyaura catalyst, through combined experimental/computational methods.

Unravelling co-catalyst integration methods in Ti-based metal-organic gels for photocatalytic H2 production.

The synthesis, characterization and photocatalytic hydrogen evolution reaction (HER) performance of a series of metal-organic gels (MOGs) constructed from titanium(IV)-oxo clusters and dicarboxylato linkers (benzene-1,4-dicarboxylato and 2-aminobenzene-1,4-dicarboxylato) are described. All the MOGs exhibit a microstructure comprised of metal-organic nanoparticles intertwined into a highly meso-/macroporous structure, as demonstrated by cryogenic transmission electron microscopy and gas adsorption isotherms. Comprehensive chemical characterization enabled the estimation of the complex formula for these defective materials, which exhibit low crystallinity and linker vacancies. To gain deeper insights into the local structure, X-ray absorption fine structure (XAFS) spectroscopy experiments were performed and compared to that of the analogous crystalline metal-organic framework. Additionally, the ultraviolet-visible absorption properties and optical band gaps were determined from diffuse reflectance spectroscopy data. The MOGs were studied as light absorbers for the sacrificial photocatalytic HER under simulated solar light irradiation using a platinum co-catalyst by either (1) in situ photodeposition or (2) ex situ doping process, through a post-synthetic metalation of the MOG structure. The chemical analysis of the metalation, along with high-angle annular dark-field scanning transmission electron microscopy, revealed that although the in situ addition of the co-catalyst led to greater HER rates (227 vs. 110 μmolH2 gMOG-1 h-1 for in situ and ex situ, respectively), the ex situ modification provided a finer distribution of platinum nanoparticles along the porous microstructure and, as a result, it led to a more efficient utilization of the co-catalyst (45 vs. 110 mmolH2 gPt-1 h-1).

Imine bond-directed assembly of polyoxometalate-based metal-organic frameworks.

Polyoxometalate-based metal-organic frameworks (POMOFs) are highly effective heterogeneous catalysts that combine the catalytic activity of polyoxometalates (POMs) with the high surface area, tunable porosity, and structural diversity of MOFs. Nevertheless, there is still a lack of a general method to integrate POMs with various transition metal-based building units into POMOFs under mild conditions. In this work, we employed imine bonds to link amino-functionalized Anderson-type POMs with aldehyde-terminated divalent metal clusters, resulting in a series of isostructural POMOFs, M(II)-POMOFs (M = Zn, Co, Mg, or Mn). Furthermore, we used post-synthetic metal exchange and oxidation to transform Zn-POMOF into Fe(III)-POMOF with strong Lewis acidic Fe3+ sites. Notably, both the synthesis and post-synthetic modifications were performed under mild conditions (room temperature, acid-free), preventing the decomposition of the POMs. Compared to M(II)-POMOFs or MOFs without POMs, the combination of Lewis acidic Fe3+ and POMs enhanced its catalytic activity for CO2 cycloaddition with epoxides, enabling efficient synthesis of cyclic carbonates. This versatile synthetic method could broaden the scope of POMOFs, extending their applications in catalysis and beyond.

Topological analysis and control of post-synthetic metalation sites in Zr-based metal–organic frameworks

MOFs formed from 8-connecting nodes and 4-connecting linkers can have the flu, scu and csq topologies. Here we show design criteria for making the rare sqc topology and how topology can be used to generate distinct post-synthetic metalation sites.

Post-Synthetic Modifications of Ti(IV)-Based Metal-Organic Frameworks for Enhanced Photocatalysis and Conductivity

Metal-Organic Frameworks (MOFs) are coordination networks with organic ligands containing potential voids, ideal for guest-host interactions. These unique characteristics make them interesting materials for several different applications, including catalysis. Unfortunately, most MOFs feature metal oxide nodes that are insulting by nature, which limits their potential in electro- and photochemical catalysts. To combat these issues, post-synthetic modifications (PSM) can be used to tune MOF properties. This research reports on different PSMs incorporated into the Ti(IV)-based MOFs MIL-125(Ti) and MIL-125(Ti)-NH2. PSMs that introduce intentional defect states and dopant metal ions, such as Ce(IV), were explored to determine their influence on the photo-degradation of Rhodamine B in aqueous solution. Post-Synthetic Metal Exchange (PSME) was used to incorporate Ce(IV) at the Ti(IV) sites in MIL-125(Ti) and MIL-125(Ti)-NH2, and intentional defect states were introduced using formic acid during MOF synthesis. In addition, conductive organic polymers were utilized to make novel polymer-MOF hybrid composites that showed enhanced charge transport properties compared to pristine MOF. Ultimately, these results show how PSMs can be used to impart conductivity and enhance photocatalysis in the MOFs MIL-125(Ti) and MIL-125(Ti)-NH2.

Symmetry-Mismatched SBU Transformation in MOFs: Postsynthetic Metal Exchange from Zn to Fe and Its Effects on Gas Adsorption and Dye Selectivity.

This research explores the alteration of metal-organic frameworks (MOFs) using a method called postsynthetic metal exchange. We focus on the shift from a Zn-based MOF containing a [Zn4O(COO)6] secondary building unit (SBU) of octahedral site symmetry (ANT-1(Zn)) to a Fe-based one with a [Fe3IIIO(COO)6]+ SBU of trigonal prismatic site symmetry (ANT-1(Fe)). The symmetry-mismatched SBU transformation cleverly maintains the MOF's overall structure by adjusting the conformation of the flexible 1,3,5-benzenetribenzoate linker to alleviate the framework strain. The process triggers a decrease in the framework volume and pore size alongside a change in the framework's charge. These alterations influence the MOF's ability to adsorb gas and dye. During the transformation, core-shell MOFs (ANT-1(Zn@Fe)) are formed as intermediate products, demonstrating unique gas sorption traits and adjusted dye adsorption preferences due to the structural modifications at the core-shell interface. Heteronuclear clusters, located at the framework interfaces, enhance the heat of CO2 adsorption. Furthermore, they also influence the selectivity of the dye size. This research provides valuable insights into fabricating novel MOFs with unique properties by modifying the SBU of a MOF with flexible organic linkers from one site symmetry to another.