Universität Stuttgart
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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 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.Item Open Access Spin crossover and fluorine‐specific interactions in metal complexes of terpyridines with polyfluorocarbon tails(2023) Nößler, Maite; Neuman, Nicolás I.; Böser, Lisa; Jäger, René; Singha Hazari, Arijit; Hunger, David; Pan, Yixian; Lücke, Clemens; Bens, Tobias; Slageren, Joris van; Sarkar, BiprajitIn coordination chemistry and materials science, terpyridine ligands are of great interest, due to their ability to form stable complexes with a broad range of transition metal ions. We report three terpyridine ligands containing different perfluorocarbon (PFC) tails on the backbone and the corresponding FeII and CoII complexes. The CoII complexes display spin crossover close to ambient temperature, and the nature of this spin transition is influenced by the length of the PFC tail on the ligand backbone. The electrochemical properties of the metal complexes were investigated with cyclic voltammetry revealing one oxidation and several reduction processes. The fluorine-specific interactions were investigated by EPR measurements. Analysis of the EPR spectra of the complexes as microcrystalline powders and in solution reveals exchange-narrowed spectra without resolved hyperfine splittings arising from the 59Co nucleus; this suggests complex aggregation in solution mediated by interactions of the PFC tails. Interestingly, addition of perfluoro-octanol in different ratios to the acetonitrile solution of the sample resulted in the disruption of the Furn:x-wiley:09476539:media:chem202301246:chem202301246-math-0001 F interactions of the tails. To the best of our knowledge, this is the first investigation of fluorine-specific interactions in metal complexes through EPR spectroscopy, as exemplified by exchange narrowing.Item Open Access Isolation, characterization and reactivity of key intermediates relevant to reductive (electro)catalysis with Cp*Rh complexes containing pyridyl‐MIC (MIC=mesoionic carbene) ligands(2023) Bens, Tobias; Walter, Robert R. M.; Beerhues, Julia; Lücke, Clemens; Gabler, Julia; Sarkar, BiprajitIn recent years, metal complexes of pyridyl‐mesoionic carbene (MIC) ligands have been reported as excellent homogeneous and molecular electrocatalysts. In combination with group 9 metals, such ligands form highly active catalysts for hydrogenation/transfer hydrogenation/hydrosilylation catalysis and electrocatalysts for dihydrogen production. Despite such progress, very little is known about the structural/electrochemical/spectroscopic properties of crucial intermediates for such catalytic reactions with these ligands: solvato complexes, reduced complexes and hydridic species. We present here a comprehensive study involving the isolation, crystallographic characterization, electrochemical/spectroelectrochemical/theoretical investigations, and in‐situ reactivity studies of all the aforementioned crucial intermediates involving Cp*Rh and pyridyl‐MIC ligands. A detailed mechanistic study of the precatalytic activation of [RhCp*] complexes with pyridyl‐MIC ligands is presented. Intriguingly, amphiphilicity of the [RhCp*]‐hydride complexes was observed, displaying the substrate dependent transfer of H+, H or H-. To the best of our knowledge, this study is the first of its kind targeting intermediates and reactive species involving metal complexes of pyridyl‐MIC ligands and investigating the interconversion amongst them.Item Open Access The best of both worlds : combining the power of MICs and WCAs to generate stable and crystalline CrI‐tetracarbonyl complexes with π‐accepting ligands(2023) Bens, Tobias; Walter, Robert R. M.; Beerhues, Julia; Schmitt, Manuel; Krossing, Ingo; Sarkar, BiprajitHere we present stable and crystalline chromium(I) tetracarbonyl complexes with pyridyl-MIC (MIC=mesoionic carbene) ligands and weakly coordinating anions (WCA=[Al(ORF)4]-, RF=C(CF3)3 and BArF=[B(ArF)4]-, ArF=3,5-(CF3)2C6H3). The complexes were fully characterized via crystallographic, spectroscopic and theoretical methods. The influence of counter anions on the IR and EPR spectroscopic properties of the CrI complexes was investigated, and the electronic innocence versus non-innocence of WCAs was probed. These are the first examples of stable and crystalline [Cr(CO)4]+ complexes with a chelating urn:x-wiley:09476539:media:chem202301205:chem202301205-math-0001 accepting ligand, and the data presented here are of relevance for both the photochemical and the electrochemical properties of these classes of compounds.