Lehrstühle für Anorganische Chemie

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    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.
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    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.
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    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.
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    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.
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    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.
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    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.
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    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.
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    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.
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    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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    Synthese, Charakterisierung und photophysikalische Untersuchungen von Kupfer(I)-Emittern im Zusammenhang mit Exzitonenkopplung und die Synthese heteronuklearer Systeme auf Basis von Kupfer(I)- und Gruppe(IV)-Metallen
    (2023) Janiak, Lars Immanuel; Steffen, Andreas; Henke, Sebastian
    Das Design molekularer Emitter auf Basis von häufig vorkommenden Metallen, wie z.B. Kupfer(I), ist eine Herausforderung der gegenwärtigen Forschung und wird in dieser Arbeit thematisiert. Lumineszente Materialien in z.B. OLEDs enthalten häufig teure Edelmetalle mit örtlich begrenztem Vorkommen, wodurch aktuell eine geopolitische Abhängigkeit entsteht und in Zukunft der Bedarf für die Massenprodukte nicht abgedeckt werden kann. Ein weiteres Problem von OLEDs ist die Stabilität und Lichtsättigung, die aufgrund niedriger strahlender Ratenkonstanten die Lebenszeit des Bauteils begrenzen. Organometallkomplexe bieten den Vorteil einer hohen Spin-Bahn-Kopplung (SOC), wodurch Singulett- mit Triplettzuständen koppeln und eine effiziente Phosphoreszenz erreichen. Nachteilig ist jedoch, dass Phosphoreszenz sich durch verhältnismäßig lange Lebenszeiten auszeichnet. Diese Problematik kann mit dem alternativen und vielversprechenden Emissionsmechanismus der thermisch aktivierten verzögerten Fluoreszenz (TADF) umgangen werden, der Zugang zu deutlich kürzeren Lebenszeiten und hohen strahlenden Ratenkonstanten 𝑘𝑟 ermöglicht. Einen weiteren Lösungs-ansatz bietet die Verwendung des physikalischen Phänomens der intramolekularen Exzitonenkopplung, welches in dieser Arbeit anhand ausgewählter Kupfer(I)-Komplexe untersucht wurde. So wurde der Einfluss eines weiteren Metallzentrums in Organometallverbindungen analysiert, um das Übergangsdipolmoment μ zu erhöhen, das ein direktes Maß für die Erlaubtheit eines Übergangs darstellt. Hierzu wurden Pyrimidin- und Benzimidazol-basierte Brückenliganden verwendet, die durch zwei bidentate Koordinationsstellen eine hohe räumliche Nähe beider Metallzentren ermöglichen. Im Rahmen dieser Arbeit wurde zudem mit S-BINAP, POP und Xantphos der Einfluss verschiedener bidentater Phosphanliganden, aber auch des N-heterocyclischen 2,4-Di-isopropylphenyl-substituierten ARDUENGO-Carbens, auf die photophysikalischen Eigenschaften im Festkörper untersucht. Insbesondere bei bpm-basierten Kupfer(I)-Komplexen wurden kürzere Lebenszeiten mit gleichbleibend niedrigen Quantenaus-beuten detektiert, während für die strahlende Ratenkonstante 𝑘𝑟 bei pmbim- und bzim-basierten Systemen keine Zunahme zu beobachten war. Des Weiteren wurden einige phosphoreszierende Blaulichtemitter auf bzim-Basis mit Quantenausbeuten von bis zu 𝛷 = 0.61 charakterisiert. Zudem wurden bpm-basierte bimetallische Komplexe mit ungewöhnlichen reversiblen Reduktionspotentialen isoliert, welche sich als potenzielle Ein-Elektronen-Transferreagenzien ausweisen. Zusätzlich wurden photokatalytische Umwandlungen mittels DEXTER-Energietransfer von bzim-basierten Kupfer(I)-Komplexen erfolgreich durchgeführt. Eine weitere Herausforderung stellt die Bildung des anti-MARKOVNIKOV-Produkts in der Hydroaminierung da. Aufgrund verschiedener thermodynamischer und kinetischer Barrieren ist das anti-MARKOVNIKOV-Produkt in der Hydroaminierung auf direktem Weg schlecht zugänglich. Gruppe(IV)-Metalle sind in der Lage diese Reaktion zu katalysieren. Aufgrund ihrer geringen Toleranz gegenüber funktionaler Gruppen und geringer Ausbeuten des anti-MAR-KOVNIKOV-Produkts jedoch limitiert. Im zweiten Teil der Arbeit wurden daher heteronukleare Systeme auf Basis von Kupfer(I)- und Gruppe(IV)-Metallen untersucht, welche in Zukunft als bimetallische Photokatalysatoren für die Hydroaminierung eingesetzt werden sollen. Priorität lag hierbei auf der Synthese und Isolation von derartigen Metallkomplexen. Während sich Gruppe(IV)-Amidokomplexe nicht für die Umsetzung mit Kupfer(I)-Komplexen eigneten, konnte durch den Wechsel auf Gruppe(IV)-Halogenidkomplexe eine erste heteronukleare Spezies auf Basis von Cu(I) und Zr(IV) nachgewiesen werden. Zudem konnten weitere neuartige Gruppe(IV)-pmbim-Verbindungen und ein trimetallischer Komplex auf Basis von einem Ti(IV)-Atom und zwei Cu(I)-Atomen isoliert werden. Letzterer zeichnet sich durch eine intensive Absorption über das gesamte sichtbare Spektrum aus.
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    Tuning the high-pressure phase behaviour of highly compressible zeolitic imidazolate frameworks: from discontinuous to continuous pore closure by linker substitution
    (2022-02-04) Song, Jianbo; Pallach, Roman; Frentzel-Beyme, Louis; Kolodzeiski, Pascal; Kieslich, Gregor; Vervoorts, Pia; Hobday, Claire L.; Henke, Sebastian
    The high-pressure behaviour of flexible zeolitic imidazolate frameworks (ZIFs) of the ZIF-62 family with the chemical composition M(im)2−x(bim)x is presented (M2+=Zn2+, Co2+; im−=imidazolate; bim−=benzimidazolate, 0.02≤x≤0.37). High-pressure powder X-ray diffraction shows that the materials contract reversibly from an open pore (op) to a closed pore (cp) phase under a hydrostatic pressure of up to 4000 bar. Sequentially increasing the bim− fraction (x) reinforces the framework, leading to an increased threshold pressure for the op-to-cp phase transition, while the total volume contraction across the transition decreases. Most importantly, the typical discontinuous op-to-cp transition (first order) changes to an unusual continuous transition (second order) for x≥0.35. This allows finetuning of the void volume and the pore size of the material continuously by adjusting the pressure, thus opening new possibilities for MOFs in pressure-switchable devices, membranes, and actuators.
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    Synthese und Studien photoaktiver Übergangsmetallkomplexe
    (2023) Koop, Stefan; Steffen, Andreas; Henke, Sebastian
    Der Fokus dieser Dissertation lag in der Synthese und Untersuchung photoaktiver Übergangsmetallkomplexe. Im ersten Projekt wurde die Darstellung neuartiger NHC-Übergangsmetallkomplexe auf Basis von Mangan und Vanadium und ihre Eignung als photoinduzierte, schaltbare Radikalspezies untersucht. Neben dieser wurde im zweiten Projekt die Synthese bimetallischer Photokatalysatoren auf Basis von Kupfer und Rhodium/Iridium und deren Eignung für Hydroaminierungsreaktionen untersucht. Es gelang der Einblick in die Synthese und Darstellung vier neuartiger bimetallischer Komplexe. Im letzten Projekt dieser Promotionsarbeit wurde die Synthese und Untersuchung von photolumineszenten Zn(II)-Komplexen in niedrigen Koordinationsmodi, welche Carbenliganden als π-Chromophore tragen. Neben der erfolgreichen Synthese trigonaler Zn(II)-Komplexe, konnte ein Einblick in deren photophysikalischen Verhaltensweisen gewonnen werden. Diese Ergebnisse liefern eine neue Klasse von seltenen phosphoreszierenden Zn(II)-Komplexen, die für photokatalytische Anwendungen interessant sein könnten.
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    Breathing porous liquids based on responsive metal-organic framework particles
    (2023-07-14) Koutsianos, Athanasios; Pallach, Roman; Frentzel-Beyme, Louis; Das, Chinmoy; Paulus, Michael; Sternemann, Christian; Henke, Sebastian
    Responsive metal-organic frameworks (MOFs) that display sigmoidal gas sorption isotherms triggered by discrete gas pressure-induced structural transformations are highly promising materials for energy related applications. However, their lack of transportability via continuous flow hinders their application in systems and designs that rely on liquid agents. We herein present examples of responsive liquid systems which exhibit a breathing behaviour and show step-shaped gas sorption isotherms, akin to the distinct oxygen saturation curve of haemoglobin in blood. Dispersions of flexible MOF nanocrystals in a size-excluded silicone oil form stable porous liquids exhibiting gated uptake for CO2, propane and propylene, as characterized by sigmoidal gas sorption isotherms with distinct transition steps. In situ X-ray diffraction studies show that the sigmoidal gas sorption curve is caused by a narrow pore to large pore phase transformation of the flexible MOF nanocrystals, which respond to gas pressure despite being dispersed in silicone oil. Given the established flexible nature and tunability of a range of MOFs, these results herald the advent of breathing porous liquids whose sorption properties can be tuned rationally for a variety of technological applications.
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    Ultra‐long lived luminescent triplet excited states in cyclic (alkyl)(amino)carbene complexes of Zn(II) halides
    (2022-05-18) Mrózek, Ondřej; Gernert, Markus; Belyaev, Andrey; Mitra, Mousree; Janiak, Lars; Marian, Christel M.; Steffen, Andreas
    The high element abundance and d10 electron configuration make ZnII-based compounds attractive candidates for the development of novel photoactive molecules. Although a large library of purely fluorescent compounds exists, emission involving triplet excited states is a rare phenomenon for zinc complexes. We have investigated the photophysical and -chemical properties of a series of dimeric and monomeric ZnII halide complexes bearing a cyclic (alkyl)(amino)carbene (cAAC) as chromophore unit. Specifically, [(cAAC)XZn(μ-X)2ZnX(cAAC)] (X=Cl (1), Br (2), I (3)) and [ZnX2(cAAC)(NCMe)] (X=Br (4), I (5)) were isolated and fully characterized, showing intense visible light photoluminescence under UV irradiation at 297 K and fast photo-induced transformation. At 77 K, the compounds exhibit improved stability allowing to record ultra-long lifetimes in the millisecond regime. DFT/MRCI calculations confirm that the emission stems from 3XCT/LEcAAC states and indicate the phototransformation to be related to asymmetric distortion of the complexes by cAAC ligand rotation. This study enhances our understanding of the excited state properties for future development and application of new classes of ZnII phosphorescent complexes.
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    Chelating phosphorus–an O, C, O-coordinating pincer-type ligand coordinating PIII and PV centres
    (2022-07-12) Gock, Michael; Lutter, Michael; Pintus, Anna; Schollmeier, Dieter; Arca, Massimiliano; Lippolis, Vito; Jurkschat, Klaus
    The sequence of reactions of the phosphorus-containing aryllithium compound 5-t-Bu-1,3-[(P(O)(O-i-Pr)2]2C6H2Li (ArLi) with Ph2PCl, KMnO4, elemental sulfur and elemental selenium, respectively, gave the aryldiphenylphosphane chalcogenides 5-t-Bu-1,3-[(P(O)(O-i-Pr)2]2C6H2P(E)Ph2 (1, E=O; 2, E=S; 3, E=Se). Compound 1 partially hydrolysed giving [5-t-Bu-1-{(P(O)(O-i-Pr)2}-3-{(P(O)(OH)2}C6H2]P(O)Ph2 (4). The reaction of ArLi with PhPCl2 provided the benzoxaphosphaphosphole [1(P), 3(P)-P(O)(O-i-Pr)OPPh-6-t-Bu-4-P(O)(O-i-Pr)2]C6H2P (5i) as a mixture of the two diastereomers. The oxidation of 5i with elemental sulfur gave the benzoxaphosphaphosphole sulfide [1(P), 3(P)-P(O)(O-i-Pr)OP(S)Ph-6-t-Bu-4-P(O)(O-i-Pr)2]C6H2 (5) as pair of enantiomers P1(R), P3(S)/P1(S), P3(R) of the diastereomer (RS/SR)-5 (5b). The aryldiphenylphosphane 5-t-Bu-1,3-[(P(O)(O-i-Pr)2]2C6H2PPh2 (6) was obtained from the reaction of the corresponding aryldiphenylphosphane sulfide 2 with either sodium hydride, NaH, or disodium iron tetracarbonyl, Na2Fe(CO)4. The oxidation of the aryldiphenylphosphane 6 with elemental iodine and subsequent hydrolysis yielded the aryldiphenyldioxaphosphorane 9-t-Bu-2,6-(OH)-4,4-Ph2-3,5-O2-2,6-P2-4λ5-P-[5.3.1.0]-undeca-1(10),7(11),8-triene (7). Both of its diastereomers, (RR/SS)-7 (7a) and (RS/SR)-7 (7b), were separated as their chloroform and i-propanol solvates, 7a⋅2CHCl3 and 7b⋅i-PrOH, respectively. DFT calculations accompanied the experimental work.
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    Untersuchung von Zn(II)-Komplexen als preisgünstige Triplett-Emitter
    (2023) Kuhnt, Julia; Steffen, Andreas; Henke, Sebastian
    Die Entwicklung nachhaltiger Alternativen zu etablierten Emittermaterialien basierend auf Iridium(III) und Platin(II) stellt eine aktuelle Herausforderung dar. Zink(II) bietet aufgrund der hohen Verfügbarkeit eine vielversprechende Möglichkeit, den Schutz seltener Metalle und damit die Verfügbarkeit aller chemischen Elemente für zukünftige Generationen zu garantieren. Allerdings erschwert die hohe Redox-Stabilität von Zink(II) das Design effizienter Emitter, sodass nur ein geringes Forschungsinteresse auf diesem Gebiet besteht. Im Fokus dieser Arbeit stehen literaturbekannte Zn(II)-Emitter, die nach heutiger Betrachtung Fragen bezüglich ihrer Emissionsmechanismen aufwerfen. Durch umfangreiche Studien werden diese erfolgreich aufgeklärt und das versteckte Potential der Verbindungen herausgearbeitet. Für diese Untersuchungen ist es unabdingbar, auf die Komplexität multiexponentieller Zerfälle einzugehen und es wird eine Vorgehensweise für die Analysen basierend auf gemittelten Lebenszeiten herausgearbeitet. Hierfür werden drei Annahmen formuliert und validiert, die die Beteiligung der einzelnen Lebenszeitkomponenten in demselben Emissionsmechanismus, die Unabhängigkeit von der Anregungswellenlänge und die Temperaturunabhängigkeit des Extinktionskoeffizienten beinhalten. Mithilfe der photophysikalischen Daten werden zudem kinetische Methoden angewendet, um strahlende und strahlungslose Ratenkonstanten für einen ineffizienten Emitter abzuschätzen und so Emissionsmechanismen vorherzusagen.
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    Revisiting ligand-to-ligand charge transfer phosphorescence emission from zinc(II) diimine bis-thiolate complexes: it is actually thermally activated delayed fluorescence
    (2022-09-04) Lüdtke, Nora; Kuhnt, Julia; Heil, Tabea; Steffen, Andreas; Marian, Christel M.
    In this work we revisit and re-evaluate the photophysical behavior of the prototypical complex [Zn(SC6H4-4-R)2(phen)] as the most in-depth studied type of Zn(II)-based triplet state emitters. Previous reports suggest population of ligand-to-ligand charge transfer (LLCT) states via phenanthroline localized ππ* states, with an energy barrier between the 3ππ* and the bk;1/3LLCT states requiring thermal activation. Besides very weak prompt fluorescence, the dominant radiative mechanism was attributed to phosphorescence. Our photophysical studies, including temperature-dependent quantum yield determination and time-resolved luminescence measurements, reveal a high radiative rate constant kr=3.5×105 s−1 at room temperature and suggest thermally activated luminescence as the major emission path. High-level DFT/MRCI calculations confirm this assignment and provide deeper insight into the excited-state kinetics, including rate constants for the (reverse) intersystem crossing processes. Thus, our study demonstrates that further optimization of the photophysical properties of this type of Zn(II) triplet exciton emitter bears great potential for future application in devices.
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    Towards substrate-reagent interaction of Lochmann-Schlosser bases in THF: bridging THF hides potential reaction site of a chiral superbase
    (2022-09-13) Brieger, Lukas; Schrimpf, Tobias; Scheel, Rebecca; Unkelbach, Christian; Strohmann, Carsten
    The metalation of N,N-dimethylaminomethylferrocene in THF by the superbasic mixture of nBuLi/KOtBu proceeds readily at low temperatures to afford a bimetallic Li2K2 aggregate containing ferrocenyl anions and tert-butoxide. Starting from an enantiomerically enriched ortho-lithiated aminomethylferrocene, an enantiomerically pure superbase can be prepared. The molecular compound exhibits superbasic behavior deprotonating N,N-dimethylbenzylamine in the α-position and is also capable of deprotonating toluene. Quantum chemical calculations provide insight into the role of the bridging THF molecule to the possible substrate–reagent interaction. In addition, a benzylpotassium alkoxide adduct gives a closer look into the corresponding reaction site of the Lochmann–Schlosser base that is reported herein.
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    A silicon-stereogenic silanol - 18O-isotope labeling and stereogenic probe reveals hidden stereospecific water exchange reaction
    (2022-09-27) Langenohl, Felix; Rösler, Jonas; Zühlke, Sebastian; Kirchhoff, Jan-Lukas; Strohmann, Carsten
    A silicon-stereogenic aminosilanol was isolated in excellent diastereomeric ratio and the absolute configuration was determined. The silanol is configurative and condensation stable in solution and shows stereoselective transformations with a clean stereospecific pathway in follow-up reactions, which leads to the isolation of a silicon-stereogenic zinc complex and siloxane compounds. Investigations with 18O-labelled water and mass spectrometry analysis revealed an otherwise hidden exchange of oxygen atoms of silanol and water in solution that proceeds with retention of the configuration at the silicon center. This novel combination of a stereochemical probe and isotopic labeling in a silicon-stereogenic compound opens new analytic possibilities to study stereochemical courses of reactions with the aid of chiral silanols mechanistically.
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    Über die Reaktivität gezielt ausgebildeter Lithiumalkylaggregate
    (2023) Kleinheider, Johannes; Strohmann, Carsten; Däschlein-Gessner, Viktoria
    Im Rahmen dieser Arbeit konnte das Verständnis von Aggregation und Reaktivität in ihrem direkten Zusammenhang ausgeweitet werden. Dabei wurde das Konzept der „frustrierten“ Aggregation entwickelt. Dieses baut auf der Annahme auf, dass sich durch die gezielte Anwendung von stöchiometrischen Mengen an Liganden in unpolarer, aprotischer Lösung eine gezielte Aggregation erreichen lässt, die die Öffnung einer Koordinationsstelle über das Spalten einer schwachen Koordination ermöglicht. Auf diese Weise lässt sich eine ungewöhnlich hohe Zugänglichkeit einer freien Koordinationsstelle am Lithiumalkyl erreichen, die eine ungewöhnlich hohe Reaktivität des Aggregates zur Folge hat. Die Überlegungen beruhen auf dem CIP-Effekt, welcher die Reaktivität und Selektivität von Lithiumalkylreaktionen mit der entsprechenden Vorkoordination verbindet. Folglich konnten auch statistische Einflüsse bei der Bildung von Vorkoordinationen beobachtet werden, welche bei energetisch geringen Unterschieden zwischen den jeweiligen Komplexen auftraten und in Abhängigkeit zum Aggregationsgrad der reaktiven Spezies standen. Anhand von Hexamethylaceton konnte zudem das Verständnis der gezielten Aggregatbildung erweitert werden, indem durch die sterische Überfrachtung in der Vorkoordination der Ablauf einer Additionsreaktion an das Keton unterbunden werden konnte. Stattdessen war es möglich eine selektive Lithiumhydrid-Transferreaktion zu beobachten, die auch mittels in situ FTIR-Spektroskopie verfolgt werden konnte. Durch die Zuhilfenahme quantenchemischer Berechnungen konnte der Reaktionsmechanismus dieses meist übersehenen Reaktionstypus aufgeklärt werden.