Helge Reinsch

Metal–organic frameworks (MOFs) can provide a variety of nanocompartments for the confinement of guest molecules. Furthermore, the emissions of fluorescent molecules can be tuned by confinement. In this study, a solvent‐free “bottle‐around‐ship” method is used to encapsulate perylene and 9,10‐dimethylanthracene, two polycyclic aromatic hydrocarbons, in the MOF ZIF‐8. Luminescence color tuning is achieved, including white‐light emission, when controlling the loading of only a single type of guest. Photophysical analysis suggests that the variations in luminescence result from various guest arrangements in the nanocompartments, as well as host–guest interactions. Because of the tight confinement of the guests, this host–guest system displays excellent luminescence thermal stability.

Two different metal‐organic frameworks containing 2,5‐pyrazinedicarboxylate (PzDC²⁻) as linker molecule and hexanuclear {Hf₆(μ₃‐O)₄(μ₃‐OH)₄} clusters were obtained under solvothermal reaction conditions at 120 °C. Formic acid as the solvent leads to the formation of Hf‐UiO‐66‐PzDC ([Hf₆(μ₃‐O)₄(μ₃‐OH)₄(OH)₄(H₂O)₄(PzDC)₄]), while the reaction in a water/formic acid mixture resulted in the crystallisation of Hf‐CAU‐22‐PzDC ([Hf₆(μ₃‐O)₄(μ₃‐OH)₄(μ‐OH)₂(OH)₄(H₂O)₄(PzDC)₃]). The UiO‐66 compound is a new member of the large class of UiO‐66‐type MOFs. Hf‐CAU‐22 contains a chain of condensed {Hf₆O₄(OH)₄(μ‐OH)₄} clusters as the IBU, which is new to Hf‐MOF chemistry. For both MOFs, a detailed characterization, i. e. structure refinement from powder X‐ray diffraction data, ¹H‐NMR‐spectroscopy, elemental and thermogravimetric analyses and N₂ sorption measurements, was carried out. Hf‐UiO‐66‐PzDC undergoes a dehydration around 215 °C and framework decomposition starts at 315 °C. From N₂ sorption measurements, a maximum BET surface area of 460 m²/g was determined. The dehydration of Hf‐CAU‐22‐PzDC starts at 200 °C and the framework decomposition at 325 °C. The BET surface area was determined to 170 m²/g.

Following the concept of isoreticular chemistry, we carried out a systematic study on Ga-containing metal–organic frameworks (MOFs) using six V-shaped linker molecules of differing sizes, geometries, and additional functional groups. The linkers included three isophthalic acid derivatives (m-H₂BDC-R, R = CH₃, OCH₃, NHCOCH₃), thiophene-2,5-dicarboxylic acid (H₂TDC), and two 4,4′-sulfonyldibenzoic acid derivatives (H₂SDBA, DPSTA). The crystal structures of seven compounds were elucidated by a combination of model building, single-crystal X-ray diffraction (SCXRD), three-dimensional electron diffraction (3D ED), and Rietveld refinements against powder X-ray diffraction (PXRD) data. Four new Ga-MOFs that are isoreticular with their aluminum counterparts, i.e. Ga-CAU-10-R (Ga(OH)(m-BDC-R); R = OCH₃, NHCOCH₃), Ga-CAU-11 (Ga(OH)(SDBA)), and Ga-CAU-11-COOH (Ga(OH)(H₂DPSTC)), were obtained. For the first time large single crystals of a MOF crystallizing in the CAU-10 structure type could be isolated, i.e. Ga-CAU-10-OCH₃, which permitted a detailed structural characterization. In addition, the use of 5-methylisophthalic acid and thiophene-2,5-dicarboxylic acid resulted in two new Ga-MOFs denoted Ga-CAU-49 and Ga-CAU-51, respectively, which are not isostructural with any known Al-MOF. The crystal structure of Ga-CAU-49 ([Ga₄(m-HBDC-CH₃)₂(m-BDC-CH₃)₃(OH)₄(H₂O)]) contains an unprecedented rod-shaped inorganic building unit (IBU) of the formula _∞¹{Ga₁₆(OH)₁₈O₆₀}, composed of corner-sharing GaO₅ and GaO₆ polyhedra. In Ga-CAU-51 ([Ga(OH)(C₅H₂O₂S)]) chains of alternating cis and trans corner-sharing GaO₆ polyhedra form the IBU. A detailed characterization of the title compounds was carried out, including nitrogen gas and water vapor sorption measurements. Ga-CAU-11 was the only compound exhibiting porosity toward nitrogen with a type I isotherm, a specific surface area of a_S,BET = 210 m²/g, and a micropore volume of V_mic = 0.09 cm³/g. The new MOF Ga-CAU-51 exhibits exceptional water sorption properties with a reversible S-shaped isotherm and a high uptake around p/p₀ = 0.38 of m_ads = 370 mg/g.

A new gallium based metal‐organic framework, denoted as Ga‐MIL‐53‐FcDC, with the chemical formula [Ga(OH)(FeC₁₂H₈O₄)] was synthesized using the ferrocene containing linker molecule 1,1’‐ferrocenedicarboxylic acid (H₂FcDC, FeC₁₂H₁₀O₄). The porous nature of the compound could be confirmed by nitrogen sorption and a specific surface area of 270 m²/g was determined. The persistence of the ferrocene complex inside the structure was confirmed by Mössbauer‐, EPR and UV/VIS‐spectroscopy. Ga‐MIL‐53‐FcDC shows structural flexibility depending on which guest molecule is located in the pores of the compound. The mechanism of structural flexibility was analyzed by means of powder X‐ray diffraction adsorbing pyrazine or iodine. The flexibility of the crystal structure can be attributed to the torsion of the GaO₆ octahedra in the IBU respective to each other and the torsion of the carboxylate groups of FcDC²⁻ relative to the aromatic ring.

Here, we have described the design, preparation and detailed characterization of a new acetoxy functionalized aluminium based metal–organic framework (MOF) called CAU-10-OCOCH₃ (1) (CAU stands for Christian-Albrechts-University). The desolvated compound was employed for the detection of perborate in a pure aqueous environment. The presented MOF based perborate sensing probe (1) was synthesized by employing 5-acetoxyisophthalic acid and AlCl₃·6H₂O as the linker molecule and metal salt source, respectively, in DMF/H₂O medium at 120 °C for 12 h. The material (1′) showed a very selective fluorescent turn-on response towards perborate in aqueous medium with the coexistence of several competitive analytes. A dramatic increment (65 fold) in emission intensity of the probe was observed within 5 min of the addition of perborate. A chemo-selective reaction between perborate and the acetoxy functionality and subsequent hydrolysis of the acetoxy group to the hydroxy group is the main cause of the turn-on nature of detection. The material showed a detection limit of 1.19 μM. The probe was also applied for the recognition of perborate in several environmental water samples. The material is the first ever MOF based probe for selective detection of perborate.

Acetic acid, an alternative green solvent, was utilized for the solvothermal synthesis of four 2D materials of composition [Zr₂O₂(OAc)₂(BDC-F)], [Zr₂O₂(OAc)₂(BDC-F₄)], [Zr₂O₂(OAc)₂(BDC)], and [Zr₂O₂(OAc)₂(NDC)] (BDC, terephthalate; BDC-F, 2-fluoroterephthalate; BDC-F₄, tetrafluoroterephthalate; NDC, 2,6-naphthalenedicarboxylate). The first three compounds were subsequently reacted with terephthalic acid in solid-state reactions to form porous MIL-140A-type metal–organic frameworks and mixed-linker derivatives ([ZrO(BDC)_1–x(BDC-Y)_x], x = 0–0.18, Y = F, F₄). The reaction kinetics of the formation of MIL-140A were investigated with the aid of time-resolved synchrotron and temperature-resolved in-house X-ray powder diffraction experiments. Thorough compositional analyses and solid-state NMR spectroscopic experiments were used to assess the crystallographic ordering of the different linker molecules. Additionally, acetic acid-based routes for the direct synthesis of MIL-140A-NO₂ and a novel MIL-140A-(CH₃)₂ derivative were discovered.

Following the strategy of installing porosity in coordination polymers predefined by linker geometry, we employed the new tetratopic linker molecule 1,1,2,2-tetrakis[4-phosphonophenyl]ethylene (H₈TPPE) for the synthesis of new porous metal phosphonates. A high-throughput study was carried out using Ni²⁺ and Co²⁺ as metal ions, and a very strong influence of the reactor size on the product formation is observed while maintaining the same reaction parameters. Using small autoclaves (V = 250 μL), single crystals of isostructural mononuclear complexes of the composition [Ni(H₃DPBP)₂(H₂O)₄] (1) and [Co(H₃DPBP)₂(H₂O)₄] (2) are formed. They contain the linker molecule H₄DPBP (4,4′-diphosphonobenzophenone), which is formed in situ by oxidation of H₈TPPE. Using autoclaves with a volume of V = 2 mL, two new 3D metal–organic frameworks (MOFs) of composition [Ni₂(H₄TPPE)(H₂O)₆]·4H₂O (CAU-46) and [Co₂(H₄TPPE)(H₂O)₄]·3H₂O (CAU-47) were isolated in bulk quantities, and their crystal structures were determined from three-dimensional electron diffraction (3D ED) and powder X-ray diffraction data. Using even larger autoclaves (V = 30 mL), another 3D MOF of the composition [Co₂(H₄TPPE)]·6H₂O (Co-CAU-48) was obtained, and a structure model was established via 3D ED measurements. Remarkably, the isostructural compound [Ni₂(H₄TPPE)]·9H₂O (Ni-CAU-48) is only obtained indirectly, i.e., via thermal activation of CAU-46. As the chosen linker geometry leads to the formation of MOFs, topological analyses were carried out, highlighting the different connectivities observed in the three frameworks. Porosity of the compounds was proven via water sorption experiments, resulting in uptakes of 126 mg/g (CAU-46), 105 mg/g (CAU-47), 210 mg/g (Ni-CAU-48), and 109 mg/g (Co-CAU-48).

A new layered mesoporous Zr-MOF of composi-tion [Zr₃₀O₂₀(OH)₂₆(OAc)₁₈L₁₈] was synthesized employing 5-acetamidoisophthalic acid (H₂L) in acetic acid as the solvent. The new MOF, denoted as CAU-45, exhibits a honeycomb structure of stacked layers which comprise both hexa- and dodecanucelar zirconium clusters. Its structure was solved from submicron-sized crystals by continuous rotation electron diffraction (cRED). Liquid phase exfoliation and size selection were successfully performed on the material.

Electron and powder diffraction were combined to elucidate the structure of the new coordination polymer [Sc₂(FcDC)₃] based on 1,1′-ferrocenedicarboxylate (FcDC²⁻), denoted as CAU-50. Remarkably, three different conformers of the very same linker molecule are observed, two of which serve as connectors for the scandium cations while one conformer acts as capping agent.

A microwave-assisted synthesis method for Ce(IV)-based MOFs crystallizing in the MIL-140 structure has been developed. Three different linker molecules, i.e. terephthalic acid (H₂BDC), 2-chloroterephthalic acid (H₂BDC-Cl) and 2,6-naphtalenedicarboxylic acid (H₂NDC) that have previously been used for the synthesis of Ce-UiO-66 which contains hexanuclear Ce-O clusters as the inorganic building unit (IBU), were employed. Under solvothermal reaction conditions (140 °C) with acetonitrile as the solvent the compounds Ce-MIL-140-BDC, -BDC-Cl and -NDC, with the general composition [CeO(linker)] were obtained as microcrystalline products. For all three MOFs an extended purification process had to be carried out. The MOFs were fully characterized and the structure of Ce-MIL-140-BDC was refined against PXRD data using the Rietveld method. In contrast to Zr-MIL-140-BDC a symmetry reduction to the space group P is observed. The MIL-140 structure type is built up by infinite CeO₇ polyhedra that are interconnected by dicarboxylate ions to generate 1D pores. For Ce-MIL-140-BDC the highest specific surface area of a_sBET = 222 m² g⁻¹ is observed and the MOF is thermally stable up to 370 °C. This new synthetic route to Ce(IV)-MOFs avoids the formation of the previously extremely dominant hexanuclear IBU, and paves the way for higher IBU diversity in Ce(IV)-MOFs.