Browsing by Author "Nößler, Maite"

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Open Access
Azide‐substituted 1,2,3‐triazolium salts as useful synthetic synthons : access to triazenyl radicals and Staudinger type reactivity
(2023) Stein, Felix; Suhr, Simon; Singha Hazari, Arijit; Walter, Robert; Nößler, Maite; Sarkar, Biprajit
Mesoionic carbenes (MIC) are a popular class of compound that are heavily investigated at the moment. The access to cationic MICs, and the ability of MICs to stabilize radicals are two highly attractive fields that have hardly been explored until now. Here the synthesis and characterisation of three different cationic azide-substituted 1,2,3-triazolium salts, used as building blocks for studying their reactivity towards triphenylphosphine are reported, where the reactivity is dependent on the nature of the starting triazolium salt. Furthermore, the cationic triazolium salts were used to develop a series of unsymmetrical MIC-triazene-NHC/MIC’ compounds, which can be readily converted to the radical form either by electrochemical or chemical methods. These radicals, which display NIR electrochromism, were investigated using a battery of techniques such as electrochemistry, UV/Vis/NIR and EPR spectroelectrochemistry, and theoretical calculations. Interestingly, the MIC plays an important role in the stabilization of the triazenyl radical, particularly in a competitive role vis-à-vis their NHC counterparts. These results shed new light on the ability of MICs to stabilize radicals, and possibly also on their π-accepting ability.
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, Biprajit
Due 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.
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, Biprajit
We 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.
Open Access
Ruthenium complexes of polyfluorocarbon substituted terpyridine and mesoionic carbene ligands : an interplay in CO2 reduction
(2023) Stein, Felix; Nößler, Maite; Singha Hazari, Arijit; Böser, Lisa; Walter, Robert; Liu, Hang; Klemm, Elias; Sarkar, Biprajit
In recent years terpyridines (tpy) and mesoionic carbenes (MIC) have been widely used in metal complexes. With the right combination with a metal center, both of these ligands are individually known to generate excellent catalysts for CO2 reduction. In this study, we combine the potentials of PFC (PFC=polyfluorocarbon) substituted tpy and MIC ligands within the same platform to obtain a new class of complexes, which we investigated with respect to their structural, electrochemical and UV/Vis/NIR spectroelectrochemical properties. We further show that the resulting metal complexes are potent electrocatalysts for CO2 reduction in which CO is exclusively formed with a faradaic efficiency of 92 %. A preliminary mechanistic study, including the isolation and characterization of a key intermediate is also reported.
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, Biprajit
In 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.