Lehrstühle für Anorganische Chemie

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Structural control of highly efficient thermally activated delayed fluorescence in carbene zinc(II) dithiolates
(2023-12-08) Mitra, Mousree; Mrózek, Ondřej; Putscher, Markus; Guhl, Jasper; Hupp, Benjamin; Belyaev, Andrey; Marian, Christel M.; Steffen, Andreas
Luminescent metal complexes based on earth abundant elements are a valuable target to substitute 4d/5d transition metal complexes as triplet emitters in advanced photonic applications. Whereas CuI complexes have been thoroughly investigated in the last two decades for this purpose, no structure-property-relationships for efficient luminescence involving triplet excited states from ZnII complexes are established. Herein, we report on the design of monomeric carbene zinc(II) dithiolates (CZT) featuring a donor-acceptor-motif that leads to highly efficient thermally activated delayed fluorescence (TADF) with for ZnII compounds unprecedented radiative rate constants kTADF=1.2×106 s−1 at 297 K. Our high-level DFT/MRCI calculations revealed that the relative orientation of the ligands involved in the ligand-to-ligand charge transfer (1/3LLCT) states is paramount to control the TADF process. Specifically, a dihedral angle of 36–40° leads to very efficient reverse intersystem-crossing (rISC) on the order of 109 s−1 due to spin-orbit coupling (SOC) mediated by the sulfur atoms in combination with a small ΔES1-T1 of ca. 56 meV.
Towards squaramide-based coordination cages with catalytic activity
(2025) Yalçin, Ertuğrul; Clever, Guido H.; Schäfer, Lars
This thesis explores two main topics: the computational study of strain in heteroleptic coordination cages and the incorporation of squaramide functionalities into coordination cages to investigate their host-guest properties. The computational study aimed at expanding the understanding of the thermodynamic driving forces behind the non-statistical self-assembly of cis-[Pd2L2L´2] and trans-[Pd2L2L´2] nanocages. Previous studies on Pd(II)-based homo- and heteroleptic cages composed of a carbazole ligand with either an elongated or non-elongated phenanthrene ligand were analyzed using NMR spectroscopy, ESI-MS and single-crystal X-ray analysis. The findings of these studies raised the question of whether differences in ring strain could be reproduced through gas-phase electronic energy comparisons via DFT geometry optimization. To address this, cage-to-cage transformations were examined, and ligands excised from heteroleptic cages were compared with their fully relaxed counterparts to assess strain in self-assembled coordination cages. This work extends these studies to acridone- and inverted carbazole-based ligands paired with the respective phenanthrene ligands. The calculations revealed that acridone- and inverted carbazole-based ligands preferentially form heteroleptic coordination cages with elongated phenanthrene ligands rather than with the shorter phenanthrene ligands. Additionally, the incorporation of diketopyrrolopyrrole-based ligands in trans-[Pd2L2L´2] cages featuring self-penetrated motifs was examined. Computational studies demonstrated that the release of strain upon transformation of a self-penetrated homoleptic diketopyrrolopyrrole cage to a heteroleptic cage plays a significant role in driving the preferential formation of the heteroleptic structures. Hydrogen bonding is a key noncovalent interaction that influences molecular recognition and catalytic processes. Squaramides are well-known hydrogen bond receptors and catalysts, possessing both hydrogen bond donor and acceptor groups. The second focus of this thesis is the heteroleptic cage formation of squaramidebased ligands with square-planar Pd(II) cations. Poorly soluble squaramide-based ligands assembled into a distinct assembly in DMSO-d6. To enhance the likelihood of observing the effects of hydrogen bonding on host-guest interactions, derivatives of squaramide-based ligands were synthesized and formed analogous heteroleptic cis-[Pd2L2L´2] cages in CD2Cl2. The resulting assemblies were analyzed using NMR spectroscopy, ESI-MS and single-crystal X-ray crystallography. When two squaramide ligands were combined, the formation of a heteroleptic cage was observed. X-ray analysis revealed that the hydrogen bond donor functionalities of one squaramide is oriented outward, with intramolecular hydrogen bonding occurring within the cage. This raised the question of whether such intramolecular hydrogen bonding contributes to a cooperative effect in guest binding. To investigate this, a reference cage was designed by pairing a squaramide-based ligand with a benzothiadiazole-based ligand, forming a distinct heteroleptic cage with the same topology. Computational studies suggested that the cooperative effect of intramolecular hydrogen bonding is more pronounced when two squaramide units are involved in intramolecular hydrogen bonding. To validate this, binding constants for neutral guest molecules were determined via 1H NMR titration experiments and compared across the two different cage systems. The results confirmed a cooperative effect in guest binding, particularly for caffeine and β-nitrostyrene. Additionally, the host-guest properties of the assembled coordination cages were examined with chloride, disulfonate and phosphate anions. However, attempts to determine binding constants for anions to the solely squaramide-based heteroleptic cage in CD2Cl2 revealed broadening in the 1H NMR spectra, suggesting aggregation, likely due to ion pairing effects. Nevertheless, binding constants for two disulfonates with the benzothiadiazole-based cage were successfully obtained, demonstrating a binding constant that is an order of magnitude stronger than for other palladium-based cages. In addition, the incorporation of 1,3-squaramides with a piperazine linker into coordination cages with Pd(II) was investigated. The self-assembled structures and their host-guest complexes were analyzed using NMR spectroscopy, ESI-MS and single-crystal X-ray crystallography. A ringlike topology with the molecular formula [Pd3L6]6+ was characterized. Binding constants for Allura Red and ferrocene-1,1’-disulfonate were determined via 1H NMR titration experiments. Cyclic voltammetry experiments showed that the redox event of Pd3L6 is electrochemically reversible, whereas the free ligand exhibits no reversibility.
An air‐ and moisture‐stable zinc(II) carbene dithiolate dimer showing fast thermally activated delayed fluorescence and Dexter energy transfer catalysis
(2023-01-13) Mrózek, Ondřej; Mitra, Mousree; Hupp, Benjamin; Belyaev, Andrey; Lüdtke, Nora; Wagner, Dorothee; Wang, Cui; Wenger, Oliver S.; Marian, Christel M.; Steffen, Andreas
A dimeric ZnII carbene complex featuring bridging and chelating benzene-1,2-dithiolate ligands is highly stable towards air and water. The donor-Zn-acceptor structure leads to visible light emission in the solid state, solution and polymer matrices with λmax between 577–657 nm and, for zinc(II) complexes, unusually high radiative rate constants for triplet exciton decay of up to kr=1.5×105 s−1 at room temperature. Variable temperature and DFT/MRCI studies show that a small energy gap between the 1/3LL/LMCT states of only 79 meV is responsible for efficient thermally activated delayed fluorescence (TADF). Time-resolved luminescence and transient absorption studies confirm the occurrence of long-lived, dominantly ligand-to-ligand charge transfer excited states in solution, allowing for application in Dexter energy transfer photocatalysis.
Comprehensive study of the enhanced reactivity of turbo-Grignard reagents
(2023-03-27) Hermann, Andreas; Seymen, Rana; Brieger, Lukas; Kleinheider, Johannes; Grabe, Bastian; Hiller, Wolf; Strohmann, Carsten
Since its introduction in 2004, Knochel's so called Turbo-Grignard reagents revolutionized the usage of Grignard reagents. Through the simple addition of LiCl to a magnesium alkyl an outstanding increase in reactivity can be achieved. Though the exact composition of the reactive species remained mysterious, the reactive mixture itself is readily used not only in synthesis but also found its way into more distant fields like material science. To unravel this mystery, we combined single-crystal X-ray diffraction with in-solution NMR-spectroscopy and closed our investigations with quantum chemical calculations. Using such a variety of methods, we have gained insight into and an explanation for the extraordinary reactivity of this extremely convenient reagent by determining the structure of the first bimetallic reactive species [t-Bu2Mg ⋅ LiCl ⋅ 4 thf] with two tert-butyl anions at the magnesium center and incorporated lithium chloride.
Microporous, crystalline, and water‐processable framework materials of organic amphiphile salts
(2023-05-18) Frentzel-Beyme, Louis; Kolodzeiski, Pascal; Terlinden, Kai; Henke, Sebastian
Porous framework materials are of major importance for a wide range of technologies. Nevertheless, many of these materials lack processibility as they are typically synthesized under rather harsh conditions and obtained as microcrystalline powders that cannot easily be coated or deposited from solution. Herein, a new approach to water-processable metal–organic framework materials is presented. The materials are based on amphiphilic organic building blocks consisting of polar carboxylate groups and non-polar alkyl chains connected to a rigid aromatic core. The amphiphilic building blocks assemble to porous framework structures via bonding to kinetically labile sodium ions from concentrated aqueous solution. The obtained crystalline materials, termed amphiphile salt frameworks , are thermally and mechanically stable (some derivatives up to 365 °C and up to at least 4000 bar hydrostatic pressure), exhibit persistent microporous channels accessible to several gases (N2, CO2, propane, propylene, n-butane), and can be reversibly assembled/disassembled by crystallization from or dissolution in water. Systematic variation of the hydrophobic side chains of the amphiphile building blocks allows extracting structure-property relationships and first design rules for this new class of water-processable microporous framework materials.
Coinage metal complexes for near infrared emission and energy transfer photocatalysis
(2024) Maity, Sabyasachi; Steffen, Andreas; Henke, Sebastian
Herein I present comprehensive studies on synthesis, structure, and photophysical behavior of a series of luminescent CuI phosphide (-PPh2) complexes supported by various mono- and bidentate phosphines displaying TADF behaviour. Depending upon the stoichiometry used two heteroleptic dimers and four heteroleptic polynuclear CuI clusters could be isolated from the self-assembly reaction of CuI-phosphide and phosphine ligand. These complexes are green-to-red (1-2 & 4-6) and NIR (3) photo-emitters with emission maxima varying over the range λmax = 515 to 890 nm. Moderate to high photoluminescence quantum yield (Ф = 20-44% ), with a relatively high radiative rate constant of up to 3 × 10^6 s^-1, and short lifetimes τ = 0.1-34 μs were observed. Compounds 2 and 5 exhibit pronounced thermochromism and evidence of dual TADF behaviour originating from two different types of excited states. I also report a new sterically demanding, electrophilic [2.2]isoindolinophanyl-based carbene (iPC) that bears a [2.2]paracyclophane moiety. The iPC ligand is a very potent π-chromophore, which participates in low energy IL-“through-space”-CT transitions in [Au(iPC)2]OTf (8) in the visible. The steric demand of the iPC leads to a high stability of 8 against air, moisture, or solvent attack, and ultralong-lived green phosphorescence with a lifetime of 185 μs is observed in solution. The beneficial photophysical and electronic properties of the iPC ligand, including a large accessible π surface area, were exploited by employing highly efficient energy transfer (EnT) photocatalysis in a [2+2] styrene cycloaddition reaction using 8, which outperformed other established photocatalysts in comparison. In addition I also describe a series of linear copper(I) carbene carbazolate complexes (9-12) bearing the [2.2]isoindolino-phanyl-based carbene (iPC) ligand as a potent excited state π-acceptor. The compounds show efficient TADF from ligand-to-ligand charge transfer (1/3LLCT) states with quantum yields of up to 0.8 and exceptional kr of 0.8-1.9×10^6 s^-1 that are among the fastest for CuI emitters, outcompeting traditional triplet emitters based on Ir^III and Pt^II.
Towards fast circularly polarized luminescence in 2‐coordinate chiral mechanochromic copper(I) carbene complexes
(2023-06-05) Muthig, André M. T.; Wieland, Justin; Lenczyk, Carsten; Koop, Stefan; Tessarolo, Jacopo; Clever, Guido H.; Hupp, Benjamin; Steffen, Andreas
A series of chiral mechanochromic copper(I) cAAC (cAAC=cyclic (alkyl)(amino)carbene) complexes with a variety of amide ligands have been studied with regard to their photophysical and chiroptical properties to elucidate structure-property relationships for the design of efficient triplet exciton emitters exhibiting circularly polarized luminescence. Depending on the environment, which determines the excited state energies, either thermally activated delayed fluorescence (TADF) from 1/3LLCT states or phosphorescence from 3LLCT/LC states occurs. However, neither chiral moieties at the carbene nor at the carbazolate ligands provide detectable luminescence dissymmetries glum. An exception is [Cu(phenoxazinyl)(cAAC)], showing orange to deep red TADF with λmax=601–715 nm in solution, powders and in PMMA. In this case, the amide ligand can undergo distortions in the excited state. This design motif leads to the first linear, non-aggregated CPL-active copper(I) complex with glum of −3.4 ⋅ 10−3 combined with a high radiative rate constant of 6.7 ⋅ 105 s−1.
Multi-cage systems and dynamic transformations
(2024) Benchimol, Elie; Clever, Guido H.; Ribas, Xavis
Biological systems use multi-component low-symmetry receptors with dynamic structures operating in networks far from equilibrium to perform complex functions. While coordination cages have taken great inspiration from the structure of these receptors by harnessing their cavities, the use of their dynamic nature and function is still in its infancy. The introduction of heteroleptic cages has brought the possibility of forming low-symmetry assemblies but also of going towards multi-functional species. However, several of the previously described features are missing to this day in order to get closer to biological complexity and function. Hence, this thesis aims to introduce a new concept in the field: multi-cage systems. The complexity does not only arise anymore from the number of components in a single assembly but from the number of discernable assemblies and their structure within a population of cages. A new type of self-sorting namely heteromeric completive self-sorting is coined and defines cage populations where multiple heteroleptic structures coexist orthogonally. This is further extended to the creation of cage systems in which heteroleptic and homoleptic hosts coexist. This concept allows a further step towards mimicking biological systems as it enables the possibility of both intramolecular and intermolecular transmission of information. Multiple coexisting assemblies can in theory perform orthogonal, cooperative or antagonist functions opening new avenues for the applications of cages in systems chemistry. The proof of concept of multi-cage systems is introduced by taking advantage of a singular diketopyrrolopyrrole (DPP) ligand, which can adopt distinct conformations and thus offers the possibility of forming coexisting cages with different topologies. One of these topologies is the trans-figure-of-eight Pd2A2B2 heteroleptic structure and is central to most of the results of the thesis. The two DPP ligands form the eight by adopting an S shape conformation and interlacing in the middle of the assemblies, occupying the cavity. It allows a precise arrangement of ligands, giving also robust heteroleptic cages, and owns an inherent chirality as the “8” can adopt P or M chirality. Not only this ligand is fostered for its structure and variability of conformation but it is also a well-known dye with strong luminescence. Therefore, we demonstrate its use for the emergence of a new multi-cage self-sorting as well as its utilization for the formation of single multi-chromophore assemblies with outstanding photophysical properties thanks to intramolecular energy transfer. A second system presenting the premises of functionality is then introduced. Taking advantage of guest affinity, we show that an increase in the number of components of a system results in the simplification of its self-sorting outcome. This can be seen as a case of “simplexity”. A heteroleptic cage can eventually coexist with a homoleptic host-guest complex and subsequently uptake another anionic guest orthogonally to the first binding event. This is defined as guest segregation in a multi-cage system and differs from the classical narcissistic self-sorting. Further harnessing the slight guest affinity for one of the homoleptic species finally allows the creation of a system where three host-guest complexes coexist orthogonally without shuffling their components or guests. Eventually, the concept of cage population is extended to dynamic mixtures in which competition for resources between components allows the emergence of chiral memory and out-of-equilibrium phenomenon. We use a chiral ligand derived from the Tröger base structure to both induce chirality on pristine fullerene C60 and C70 and drive the formation of a unique chiral diastereoisomer of the figure-of-eight species. Because of the high affinity of this ligand to form a host-guest complex with fullerene, it is then possible to create a transformable multi-cage system in which the chiral figure-eight ‘releases’ its outer ligands and exchanges them for non-chiral ones. However, we still observe a chiral answer in circular dichroism witnessing the (partial) retention of chirality of the central “8” created by the DPP ligands. We then take advantage of the differences in both thermodynamic and kinetic behaviors to drive the transformative system out of equilibrium and demonstrate the transient formation of the chiral figure-eight before its decay to reform the thermodynamically preferred complex with fullerene. This thesis introduces the concept of populations, networks, segregated functions, and transformative out-of-equilibrium behavior in the field of cages.
Engineering soluble diketopyrrolopyrrole chromophore stacks from a series of Pd(II)‐based ravels
(2023-07-17) Regeni, Irene; Chowdhury, Rituparno; Terlinden, Kai; Horiuchi, Shinnosuke; Holstein, Julian J.; Feldmann, Sascha; Clever, Guido H.
A strategy to engineer the stacking of diketopyrrolopyrrole (DPP) dyes based on non-statistical metallosupramolecular self-assembly is introduced. For this, the DPP backbone is equipped with nitrogen-based donors that allow for different discrete assemblies to be formed upon the addition of Pd(II), distinguished by the number of π-stacked chromophores. A Pd3L6 three-ring, a heteroleptic Pd2L2L′2 ravel composed of two crossing DPPs (flanked by two carbazoles), and two unprecedented self-penetrated motifs (a Pd2L3 triple and a Pd2L4 quadruple stack), were obtained and systematically investigated. With increasing counts of stacked chromophores, UV/Vis absorptions red-shift and emission intensities decrease, except for compound Pd2L2L′2, which stands out with an exceptional photoluminescence quantum yield of 51 %. This is extraordinary for open-shell metal containing assemblies and explainable by an intra-assembly FRET process. The modular design and synthesis of soluble multi-chromophore building blocks offers the potential for the preparation of nanodevices and materials with applications in sensing, photo-redox catalysis and optics.
Chiroptical recognition of carboxylates with charge‐neutral double‐stranded zinc(II) helicates
(2023-07-30) Kalarikkal, Malavika G.; Drechsler, Christoph; Tusha, Gers; Schäfer, Lars V.; Van Craen, David
Chirality analysis of small molecules for the determination of their enantiopurity is nowadays ruled by streamlined chromatographic methods which utilize chiral stationary phases. Chiroptical probes which rely on host–guest interactions are so far overshadowed by the latter but have the benefit of depending only on common spectroscopic techniques such as CD spectroscopy to distinguish enantiomers and to quantify their ratio. Interest into this receptor-based approach is constantly rising because non-invasive high-throughput screenings with a minimal waste production can be performed. In this study we investigate the possibility to utilize metal-based containers in form of charge-neutral helicates able to recognize anions for this purpose. Key building block of the helicates are triazole units which show rotational freedom and give rise to either a meso-structure or a racemic mixture of the right- and left-handed complex. A chiroptical response of the probe is observed upon recognition of chiral mono- or dicarboxylates and chirality analysis of tartrate is conducted by CD spectroscopy.
Complex systems based on coordination cages
(2024) Drechsler, Christoph; Clever, Guido H.; Kast, Stefan M.
Koordinationskäfige entstehen durch die Kombination von bis-monodentaten organischen Liganden und Metallkationen. In unserer Gruppe konzentrieren wir uns auf laternenförmige PdII2L4-Spezies, wobei L ein gebogener, bis-monodentater Ligand ist. Diese Käfige haben vier Öffnungen, in die kleine, meist anionische, gastähnliche Moleküle passen. Die Systemchemie befasst sich mit komplexen chemischen Mischungen, die biologische Systeme nachahmen sollen. Koordinationskäfige, die Enzymen ähneln, können dabei helfen, da sie durch dynamische Koordinationsbindungen und flexibles Gastverhalten anpassungsfähig sind. Die zunehmende Komplexität solcher Systeme stellt die Analytik vor Herausforderungen. Die Massenspektrometrie, insbesondere in Kombination mit der Ionenmobilitätsspektrometrie, ist ein wertvolles Werkzeug zur Untersuchung komplexer Mischungen. Im ersten Teil dieser Arbeit wird die Trapped Ion Mobility Spectrometry verwendet, um die Struktur von Palladium(II)-Koordinationskäfigen zu untersuchen. Die experimentell ermittelte Ionenmobilität K wird in den Kollisionsquerschnitt umgewandelt. Für die theoretische Untersuchung wird ein in silico Modell des Koordinationskäfigs entweder geometrisch optimiert oder in einer kurzen Molekulardynamik-Simulation verwendet. Der theoretische Kollisionsquerschnitt wird dann mit dem experimentellen Wert verglichen, um die Übereinstimmung zu überprüfen. Ein besonderes Augenmerk liegt auf den flexiblen Eigenschaften von Alkylseitenketten, die Fehler in der Querschnittsberechnung verursachen können. Daher wurde eine Reihe von Käfigen mit unterschiedlich langen Seitenketten synthetisiert, gemessen und verglichen. Diese Ergebnisse geben Aufschluss über das Verhalten dieser Seitenketten in der Gasphase, wobei die London-Dispersionskräfte eine wichtige Rolle spielen. Solche Dispersionskräfte können Kompaktierungen der Käfige in der Gasphase verursachen. Im zweiten Teil der Arbeit werden komplexe Mischungen von Käfigen mit photoschaltbaren Einheiten untersucht. Heteroleptische Käfige mit photoschaltbaren Dithienylethen (DTE)-Einheiten wurden synthetisiert und charakterisiert. Durch Bestrahlungsexperimente konnten verschiedene Käfigmischungen mit unterschiedlicher Komplexität erzeugt werden. Im dritten Teil wird die langsame Bildung von doppelt verschränkten Käfigen untersucht. Dabei dient der monomere Käfig mit großem Hohlraum als vorübergehende Kapsel für ein großes Gastmolekül. Dieses Konzept der transienten Gastbindung wird hinsichtlich seiner Eigenschaften und Grenzen erforscht.
Chromophoric coordination cages
(2024) Walther, Alexandre; Clever, Guido H.; Lützen, Arne
One of the principal reasons for the extensive study of coordination cages in the last few decades is the promise they hold for their use as artificial analogues to biological systems, and more specifically artificial enzymes. Indeed, like this class of catalytically active proteins, coordination cages and capsules possess a central cavity able to bind guest molecules. In turn, it has been shown by the supramolecular community that catalytic processes can happen in coordination cages, by the specific arrangement of guest molecules in the cavity. However, unlike enzymes, coordination cages have been seen as rather conformationally or topologically static, in contrast to the vast changes ternary and quaternary structure that may take place upon the binding of a substrate to an enzyme. Moreover, the cavity of non-chiral homoleptic coordination cages do not allow for the asymmetric catalysis enabled by the chirotopic and low-symmetry catalytic pocket of enzymes. Therefore, to better mimic biological systems with coordination cages, the study of low-symmetry systems that are responsive to external stimuli is of interest. In the first project of this thesis, I present an azulene-based Pd2L4 lantern-shaped cage that transforms into a Pd4L8 tetrahedral complex upon addition of benzene disulfonate guests. This geometrical change is clean, efficient, and rapid. The final structure was determined by single crystal X-ray crystallography. The use of azulene, a simple, but coloured two-ring aromatic hydrocarbon allows for an easy monitoring of the progress of the transformation by eye only. Using a second coordination cage based on methylene blue, the transformation was shown to be reversible, thanks to the transfer of the guest from the tetrahedron to the newly added cage, which has a higher affinity for the disulfonates. This experiment demonstrates several key principles of advanced coordination cage chemistry, such as multi-component non-statistical systems of narcissistically self-sorting cages, or guest transfer. Several other azulene-based ligands were synthesised as well, but none of their corresponding homoleptic assemblies could undergo the same cage-to-cage transformation, highlighting the sensitivity of the first system to shape and size. A second azulene-based family of ligands was prepared next, but this time with a chiral amino-biazulene backbone, isomeric to the widely adopted BINOL moiety. Two ligands were thus synthesised, the first of them bearing two pyridine donor groups. Using the racemate of the first ligand for the formation of the Pd(II)-cage ultimately resulted in the formation of a single meso-trans isomer of the Pd2L4 cage in acetonitrile and DMSO, instead of a statistical mixture. X-ray structure analysis of the cage unexpectedly revealed that discrete solvent molecules were responsible for the non-statistical arrangement, by acting as tethers between the amino groups of the ligands, through hydrogen bonding. This is the first time that single solvent molecules were shown to directly influence the outcome of the chiral self-sorting of a coordination cage. This observation was supplemented with experimental and computational models. A larger ligand with 7-isoquinoline groups was also synthesised. Due to the larger donor groups, the amino groups are pulled further away from each-other in the final coordination assembly. This greater distance means that the solvent molecules were not able to bridge the ligands anymore, and therefore a statistical mixture was observed in this case. Finally, thiophene- and and thieno[3,4b]pyrazine-based ligands were used to potentially recreate the cage-to-tetrahedron transformation described in the first part. The transformation was successful due to the similar geometry imparted by the five-membered rings. In addition, the thieno[3,4b]pyrazine-based ligands were observed to be highly luminescent, and the resulting Pd2L4 cages were as well. This is an interesting result, as the fluorescence of palladium(II) assemblies are generally quenched, and therefore their use as fluorescent probes can be limited. Moreover, the emission colour of those ligands could be modulated by electron withdrawing or donating substituents.
Modulation of porosity, melting and glass formation in zeolitic imidazolate frameworks
(2024) Xue, Wenlong; Henke, Sebastian; Clever, Guido
The phase transition of metal–organic frameworks (MOFs) from a crystalline to a liquid and a glassy state represents a promising avenue for the development of a novel class of amorphous materials. In contrast to their crystalline counterparts, liquid and glassy MOFs exhibit excellent processability, a highly advantageous property for industrial applications where the formation of specific shapes, such as pellets or thin films, is often a prerequisite. Furthermore, unlike conventional glasses, the utilization of coordination chemistry allows for the implementation of rational design concepts, thereby facilitating the fine-tuning of the desired properties. In this thesis, Solvent-assisted linker exchange (SALE) as a versatile method for the preparation of melt-quenched MOF glasses from proto typical MOFs, such as ZIF-8. Two additional organic linkers are incorporated into the non-meltable ZIF-8, yielding high-entropy linker-exchanged ZIF-8 derivatives that undergo a phase transition from crystalline to liquid and glassy states upon thermal treatment. This strategy opens new pathways for creating meltable and glass-forming MOFs from the extensive pool of non-meltable MOFs documented in the literature. Moreover, Post-synthetic ball-milling (PSBM) is explored as a suitable alternative to melt-quenching in MOF glass formation. MOFs can be transformed into glasses in just 5 minutes by mechanical milling at room temperature. This PSBM method can also transform crystalline MOFs in the glassy state that do not melt due to low thermal stability. This approach broadens the spectrum of potential applications, paving the way for the development of multifunctional MOF glass composites and expanding their use into more diverse fields. Additionally, Single-component glass-ceramics (SCGC) explored from a Cd-based MOF exhibit unique structural and thermal properties. Upon reheating the Cd-based MOF glass, it undergoes partial recrystallization into a single component glass-ceramic state with potentially enhanced mechanical properties — a feature not observed in previous melt-quenched MOF glasses. Overall, this thesis offers new insights into the porosity of MOF glasses, their formation mechanism, the preparation of MOF glass-ceramics and a generic method for the mechanochemical preparation of MOF glasses.
Highly porous metal-organic framework liquids and glasses via a solvent-assisted linker exchange strategy of ZIF-8
(2024-05-24) Xue, Wen-Long; Kolodzeiski, Pascal; Aucharova, Hanna; Vasa, Suresh; Koutsianos, Athanasios; Pallach, Roman; Song, Jianbo; Frentzel-Beyme, Louis; Linser, Rasmus; Henke, Sebastian
By combining the porosity of crystalline metal-organic frameworks (MOFs) with the unique processability of the liquid state, melt-quenched MOF glasses offer exciting opportunities for molecular separation. However, progress in this field is limited by two factors. Firstly, only very few MOFsmelt at elevated temperatures and transform into stable glasses upon cooling the correspondingMOF liquid. Secondly, theMOF glasses obtained thus far feature only very small porosities and very small pore sizes. Here, we demonstrate solventassisted linker exchange (SALE) as a versatile method to prepare highly porous melt-quenched MOF glasses from the canonical ZIF-8. Two additional organic linkers are incorporated into the non-meltable ZIF-8, yielding high-entropy, linker-exchanged ZIF-8 derivatives undergoing crystal-to-liquid-to-glass phase transitions by thermal treatment. The ZIF-8 glasses demonstrate specific pore volumes of about 0.2 cm3g–1, adsorb large amounts of technologically relevant C3 and C4 hydrocarbons, and feature high kinetic sorption selectivities for the separation of propylene from propane.
The halogen bond to ethers - prototypic molecules and experimental electron density
(2024-08-05) Schmidt, Annika; Krupp, Anna; Kleinheider, Johannes; Binnenbrinkmann, Tamara M. L.; Wang, Ruimin; Englert, Ulli; Strohmann, Carsten
Halogen bonds to dialkyl ether molecules have remained largely unexplored. We here address the synthesis and the structural chemistry of the first halogen-bonded noncyclic alkyl ethers, combining 1,4-diiodotetrafluorobenzene and the prototypic or commonly used ethers dimethyl ether, tetrahydrofuran, and methyl-tert-butyl ether as halogen acceptors. Two different structural motifs based on moderately strong halogen bonds were obtained: Discrete trimolecular aggregates are formed, and unexpected halogen-bonded supramolecular chain adducts feature oxygen-bifurcated halogen bonds with 1:1 donor:acceptor ratio. Both structure types may be selectively obtained even for the same ether by adjusting the stoichiometry in the crystallization experiments. The geometric features of the etheric oxygen center were found to be flexible, in contrast to the almost linear geometry about the halogen donor atom. A high-resolution X-ray diffraction experiment on the extended adduct of dimethyl ether allowed us to study the electronic details of the acceptor-bifurcated I···O···I halogen bonds. The electron density in the bond critical points and derived properties such as the Laplacian indicate essentially electrostatic interactions and explain the geometrical flexibility of ethers in halogen bonds. Our studies demonstrate the great versatility of ethers as halogen bond acceptors, that can occur in many geometrical arrangements and whose contribution to nature’s structural designs should not be underestimated.
CycloSiFA: the next generation of silicon-based fluoride acceptors for positron emission tomography (PET)
(2023-10-18) Mawick, Matthias; Jaworski, Carolin; Bittermann, Jens; Iovkova, Ljuba; Pu, Yinglan; Wängler, Carmen; Wängler, Björn; Jurkschat, Klaus; Krause, Norbert; Schirrmacher, Ralf
The ring-opening Si-fluorination of a variety of azasilole derivatives cyclo-1-(iPr2Si)−4-X−C6H3−2-CH2NR (4: R=2,6-iPr2C6H3, X=H; 4 a: R=2,4,6-Me3C6H2, X=H; 9: R=2,6-iPr2C6H3, X=tBuMe2SiO; 10: R=2,6-iPr2C6H3, X=OH; 13: R=2,6-iPr2C6H3, X=HCCCH2O; 22: R=2,6-iPr2C6H3, X=tBuMe2SiCH2O) with different 19F-fluoride sources was studied, optimized and the experience gained was used in a translational approach to create a straightforward 18F-labelling protocol for the azasilole derivatives [18F]6 and [18F]14. The latter constitutes a potential clickable CycloSiFA prosthetic group which might be used in PET tracer development using Cu-catalysed triazole formation. Based on our findings, CycloSiFA has the potential to become a new entry into non-canonical labelling methodologies for radioactive PET tracer development.
Nonstatistical assembly of multicomponent [Pd2ABCD] cages
(2024-01-19) Wu, Kai; Benchimol, Elie; Baksi, Ananya; Clever, Guido H.
Self-assembled hosts, inspired by biological receptors and catalysts, show potential for sustainable synthesis, energy conversion and medicine. Implementing multiple functionalities in the form of distinguishable building blocks, however, is difficult without risking narcissistic self-sorting or statistical mess. Here, we report a systematic series of integratively self-assembled heteroleptic cages in which two square-planar PdII cations are bridged by four different bis-pyridyl ligands A, B, C and D via synergistic effects to exclusively form a single isomer — the lantern-shaped cage [Pd2ABCD]. This self-sorting goal — forming just one out of 55 possible structures — is reached under full thermodynamic control and can be realized progressively (by combining progenitors such as [Pd2A2C2] with [Pd2B2D2]), directly from ligands and PdII cations, or by mixing all four corresponding homoleptic cages. The rational design of complex multicomponent assemblies that enables the incorporation of chemical moieties in a modular approach will advance their potential applications in functional nanosystems.
Modular enhancement of circularly polarized luminescence in Pd2A2B2 heteroleptic cages
(2023-02-21) Tessarolo, Jacopo; Benchimol, Elie; Jouaiti, Abdelaziz; Hosseini, Mir Wais; Clever, Guido H.
Metal-mediated assembly allows us to combine an achiral emissive ligand A with different chiral ligands (such as B) in a non-statistical fashion, obtaining Pd2A2B2 heteroleptic cages showing circularly polarized luminescence (CPL). By using the ‘shape complementary assembly’ (SCA) strategy, the cages are exclusively obtained as cis-Pd2A2B2 stereoisomers, as confirmed by NMR, MS and DFT analyses. Their unique chiroptical properties derive from the synergy of all the building blocks. Ligand B imparts the chiral information of its aliphatic backbone, comprising two stereogenic sp3 carbon centres, to the overall structure, causing CD and CPL signal induction for the chromophore on ligand A. The heteroleptic cage shows CPL with a |glum| value of 2.5 × 10−3, which is 3-times higher than that for a progenitor based on aromatic helical building block H, thus opening a rational route towards optimizing the CPL properties of self-assembled nanostructures in a modular way.
Systematic studies on heteroleptic coordination cages
(2024) Ebbert, Kristina E.; Clever, Guido H.; Merten, Christian
Supramolecular coordination cages based on square-planar Pd(II) cations in combination with bis-monodentate ligands equipped with pyridine, respectively isoquinoline donor groups, form a subclass of coordination cages and assemblies that has been extensively studied. In recent years, efforts towards the development of functional coordination cages have been made, with the usual approach to achieve this goal being the incorporation of a functional backbone followed by careful choice of linker and donor-groups. Tuning the donor-groups and linkers allowed the synthesis of heteroleptic coordination cages by designing shape-complementary ligands (SCA). Herbey, this approach has been utilized to form a wide array of heteroleptic coordination assemblies and analyze them in regard to their structure, solvent susceptibility and functional group tolerance. Furthermore, ion-mobility mass spectrometry has been established as a powerful tool in the analysis of complex mixtures of up to ten structurally related supramolecules.
Functional heteroleptic coordination cages based on a modular approach
(2024) Platzek, André; Clever, Guido H.; Hiller, Wolf
This work describes the rational design of a family of endohedrally functionalized heteroleptic coordination cages of the type Pd2A2B2. These were assembled out of a modular toolbox of banana shaped ligands A based on carbazole and shape complementary ligands B with different aromatic backbones and lengths. The obtained structures were characterized by NMR spectroscopy and ESI mass spectrometry as well as for some examples with X-ray diffraction in the solid state. With installing different functional groups to the free secondary amine at the carbazole moiety, namely amides and methyl substituents a variety of coordination cages with unique properties inside the cavities were formed. The coordination cages were investigated in their ability to bind hydrogen bond acceptors. Another goal of this work was the NMR spectroscopic investigation of coordination assemblies. With the help of tailor-made NMR experiments new cage compounds were characterized. The main focus was laid on DOSY NMR experiments.

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