03 Fakultät Chemie
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/4
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Item Open Access Mesoionic imines (MIIs) : strong donors and versatile ligands for transition metals and main group substrates(2022) Rudolf, Richard; Neuman, Nicolás I.; Walter, Robert R. M.; Ringenberg, Mark. R.; Sarkar, BiprajitWe report the synthesis and the reactivity of 1,2,3‐triazolin‐5‐imine type mesoionic imines (MIIs). The MIIs are accessible by a base‐mediated cycloaddition between a substituted acetonitrile and an aromatic azide, methylation by established routes and subsequent deprotonation. C=O‐stretching frequencies in MII-CO2 and -Rh(CO)2Cl complexes were used to determine the overall donor strength. The MIIs are stronger donors than the N‐heterocyclic imines (NHIs). MIIs are excellent ligands for main group elements and transition metals in which they display substituent‐induced fluorine‐specific interactions and undergo C-H activation. DFT calculations gave insights into the frontier orbitals of the MIIs. The calculations predict a relatively small HOMO-LUMO gap compared to other related ligands. MIIs are potentially able to act as both π‐donor and π‐acceptor ligands. This report highlights the potential of MIIs to display exciting properties with a huge potential for future development.Item Open Access Activation of aromatic C-F bonds by a N‐heterocyclic olefin (NHO)(2020) Mandal, Debdeep; Chandra, Shubhadeep; Neuman, Nicolás I.; Mahata, Alok; Sarkar, Arighna; Kundu, Abhinanda; Anga, Srinivas; Rawat, Hemant; Schulzke, Carola; Mote, Kaustubh R.; Sarkar, Biprajit; Chandrasekhar, Vadapalli; Jana, AnukulA N‐heterocyclic olefin (NHO), a terminal alkene selectively activates aromatic C-F bonds without the need of any additional catalyst. As a result, a straightforward methodology was developed for the formation of different fluoroaryl‐substituted alkenes in which the central carbon-carbon double bond is in a twisted geometry.Item Open Access N,N′‐ethylene‐bridged bis‐2‐aryl‐pyrrolinium cations to E‐diaminoalkenes : non‐identical stepwise reversible double‐redox coupled bond activation reactions(2020) Nayak, Mithilesh Kumar; Stubbe, Jessica; Neuman, Nicolás I.; Narayanan, Ramakirushnan Suriya; Maji, Sandipan; Schulzke, Carola; Chandrasekhar, Vadapalli; Sarkar, Biprajit; Jana, AnukulThis work presents a stepwise reversible two‐electron transfer induced hydrogen shift leading to the conversion of a bis‐pyrrolinium cation to an E‐diaminoalkene and vice versa. Remarkably, the forward and the reverse reaction, which are both reversible, follow two completely different reaction pathways. Establishing such unprecedented property in this type of processes was possible by developing a novel synthetic route towards the starting dication. All intermediates involved in both the forward and the backward reactions were comprehensively characterized by a combination of spectroscopic, crystallographic, electrochemical, spectroelectrochemical, and theoretical methods. The presented synthetic route opens up new possibilities for the generation of multi‐pyrrolinium cation scaffold‐based organic redox systems, which constitute decidedly sought‐after molecules in contemporary chemistry.Item Open Access Electrochemistry and spin‐crossover behavior of fluorinated terpyridine‐based Co(II) and Fe(II) complexes(2023) Nößler, Maite; Jäger, René; Hunger, David; Reimann, Marc; Bens, Tobias; Neuman, Nicolás I.; Singha Hazari, Arijit; Kaupp, Martin; Slageren, Joris van; Sarkar, BiprajitDue to their ability to form stable molecular complexes that have tailor-made properties, terpyridine ligands are of great interest in chemistry and material science. In this regard, we prepared two terpyridine ligands with two different fluorinated phenyl rings on the backbone. The corresponding CoII and FeII complexes were synthesized and characterized by single-crystal X-ray structural analysis, electrochemistry and temperature-dependent SQUID magnetometry. Single crystal X-ray diffraction analyses at 100 K of these complexes revealed Co-N and Fe-N bond lengths that are typical of low spin CoII and FeII centers. The metal centers are coordinated in an octahedral fashion and the fluorinated phenyl rings on the backbone are twisted out of the plane of the terpyridine unit. The complexes were investigated with cyclic voltammetry and UV/Vis-NIR spectroelectrochemistry. All complexes show a reversible oxidation and several reduction processes. Temperature dependent SQUID magnetometry revealed a gradual thermal SCO behavior in two of the complexes, while EPR spectroscopy provided further insights on the electronic structure of the metal complexes, as well as site of reduction.Item Open Access Directed design of a AuI complex with a reduced mesoionic carbene radical ligand : insights from 1,2,3-triazolylidene selenium adducts and extensive electrochemical investigations(2021) Beerhues, Julia; Neubrand, Maren; Sobottka, Sebastian; Neuman, Nicolás I.; Aberhan, Hannes; Chandra, Shubhadeep; Sarkar, BiprajitCarbene‐based radicals are important for both fundamental and applied chemical research. Herein, extensive electrochemical investigations of nine different 1,2,3‐triazolylidene selenium adducts are reported. It is found that the half‐wave potentials of the first reduction of the selones correlate with their calculated LUMO levels and the LUMO levels of the corresponding triazolylidene‐based mesoionic carbenes (MICs). Furthermore, unexpected quasi‐reversibility of the reduction of two triazoline selones, exhibiting comparable reduction potentials, was discovered. Through UV/Vis/NIR and EPR spectroelectrochemical investigations supported by DFT calculations, the radical anion was unambiguously assigned to be triazoline centered. This electrochemical behavior was transferred to a triazolylidene‐type MIC‐gold phenyl complex resulting in a MIC‐radical coordinated AuI species. Apart from UV‐Vis‐NIR and EPR spectroelectrochemical investigations of the reduction, the reduced gold‐coordinated MIC radical complex was also formed in situ in the bulk through chemical reduction. This is the first report of a monodentate triazolylidene‐based MIC ligand that can be reduced to its anion radical in a metal complex. The results presented here provide design principles for stabilizing radicals based on MICs.Item Open Access Precursor molecules for 1,2-diamidobenzene containing cobalt(ii), nickel(ii) and zinc(ii) complexes : synthesis and magnetic properties(2024) Hunger, David; Suhr, Simon; Bayer, Valentin; Albold, Uta; Frey, Wolfgang; Sarkar, Biprajit; Slageren, Joris vanMolecular magnetic materials based on 1,2-diamidobenzenes are well known and have been intensively studied both experimentally and computationally. They possess interesting magnetic properties as well as redox activity. In this work, we present the synthesis and investigation of potent synthons for constructing discrete metal-organic architectures featuring 1,2-diamidobenzene-coordinated metal centres. The synthons feature weakly bound dimethoxyethane (dme) ligands in addition to the 1,2-diamidobenzene. We characterize these complexes and investigate their magnetic properties by means of static and dynamic magnetometry and high-field electron paramagnetic resonance (HFEPR). Interestingly, the magnetic and magnetic resonance data strongly suggest a dimeric formulation of these complexes, viz. [MII(bmsab)(dme)]2 (bmsab = 1,2-bis(methanesulfonamido)benzene; dme = dimethoxyethane) with M = Co, Ni, Zn. A large negative D-value of -60 cm-1 was found for the Co(ii) synthon and an equally large negative D of -50 cm-1 for the Ni(ii) synthon. For Co(ii), the sign of the D-value is the same as that found for the known bis-diamidobenzene complexes of this ion. In contrast, the negative D-value for the Ni(ii) complex is unexpected, which we explain in terms of a change in coordination number. The heteroleptic Co(ii) complex presented here does not feature slow relaxation of the magnetization, in contrast to the homoleptic Co(ii) 1,2-diamidobenzene complex.Item Open Access Cooperative effects in multimetallic complexes applied in catalysis(2021) Maity, Ramananda; Birenheide, Bernhard S.; Breher, Frank; Sarkar, BiprajitThe field of multimetallic catalysis is rapidly developing and some multimetallic complexes catalyze organic transformations to yield the desired products in more efficient ways owing to the combined action of different metals in a cooperative fashion. This Concept article describes the recent advances of cooperative catalysis playing in multimetallic systems such as homo‐multimetallic complexes with short metal‐metal distances, homo‐multimetallic complexes with long metal‐metal distances, hetero‐multimetallic complexes and metallocene‐based multimetallic complexes with special attention towards redox‐switchable catalysis. Examples are illustrated in which the use of multimetallic complexes show clear enhancement of catalytic outcomes when compared with the sum of their corresponding mononuclear counterparts. Furthermore, several examples are discussed showing the effects of electronic communication in cooperative systems.Item Open Access Identification of the irreversible redox behavior of highly fluorescent benzothiadiazoles(2020) Rietsch, Philipp; Sobottka, Sebastian; Hoffmann, Katrin; Hildebrandt, Pascal; Sarkar, Biprajit; Resch‐Genger, Ute; Eigler, SiegfriedRedox switches are applied in various fields of research, including molecular lifts, electronic devices and sensors. Switching the absorbance between UV and Vis/NIR by redox processes is of interest for applications in light harvesting or biomedicine. Here, we present a series of push‐pull benzothiadiazole derivatives with high fluorescence quantum yields in solution and in the crystalline solid state. Spectroelectrochemical analysis reveals the switching of UV‐absorption in the neutral state to Vis/NIR absorption in the reduced state. We identify the partial irreversibility of the switching process, which appears to be reversible on the cyclic voltammetry timescale.Item Open Access Isomerization reactions in anionic mesoionic carbene-borates and control of properties and reactivities in the resulting CoII complexes through agostic interactions(2020) Stubbe, Jessica; Neuman, Nicolás I.; McLellan, Ross; Sommer, Michael G.; Nößler, Maite; Beerhues, Julia; Mulvey, Robert E.; Sarkar, BiprajitWe present herein anionic borate‐based bi‐mesoionic carbene compounds of the 1,2,3‐triazol‐4‐ylidene type that undergo C-N isomerization reactions. The isomerized compounds are excellent ligands for CoII centers. Strong agostic interactions with the “C-H”‐groups of the cyclohexyl substituents result in an unusual low‐spin square planar CoII complex, which is unreactive towards external substrates. Such agostic interactions are absent in the complex with phenyl substituents on the borate backbone. This complex displays a high‐spin tetrahedral CoII center, which is reactive towards external substrates including dioxygen. To the best of our knowledge, this is also the first investigation of agostic interactions through single‐crystal EPR spectroscopy. We conclusively show here that the structure and properties of these CoII complexes can be strongly influenced through interactions in the secondary coordination sphere. Additionally, we unravel a unique ligand rearrangement for these classes of anionic mesoionic carbene‐based ligands.Item Open Access Mechanistic and kinetic investigations of on/off (photo)switchable binding of carbon monoxide by chromium(0), molybdenum(0) and tungsten(0) carbonyl complexes with a pyridyl‐mesoionic carbene ligand(2022) Boden, Pit J.; Di Martino‐Fumo, Patrick; Bens, Tobias; Steiger, Sophie T.; Marhöfer, Daniel; Niedner‐Schatteburg, Gereon; Sarkar, BiprajitThis work tackles the photochemistry of a series of mononuclear Cr0, Mo0 and W0 carbonyl complexes containing a bidentate mesoionic carbene ligand of the 1,2,3‐triazol‐5‐ylidene type. FTIR spectroscopy, combined with density functional theory calculations, revealed a clean photo‐induced reaction in organic solvents (acetonitrile, pyridine, valeronitrile) to give mainly one photoproduct with monosubstitution of a carbonyl ligand for a solvent molecule. The highest photodissociation quantum yields were reached for the Cr0 complex under UV irradiation (266 nm). Based on previous investigations, the kinetics of the dark reverse reactions have now been determined, with reaction times of up to several hours in pyridine. Photochemical studies in the solid state (KBr matrix, frozen solution) also showed light‐induced reactivity with stabilization of the metastable intermediate with a free coordination site at very low temperature. The identified reactive species emphasizes a mechanism without ligand–sphere reorganization.
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