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    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.
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    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 van
    Molecular 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.
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    Remarkable enhancement of catalytic activity of Cu‐complexes in the electrochemical hydrogen evolution reaction by using triply fused porphyrin
    (2022) Chandra, Shubhadeep; Singha Hazari, Arijit; Song, Qian; Hunger, David; Neuman, Nicolás. I.; Slageren, Joris van; Klemm, Elias; Sarkar, Biprajit
    A bimetallic triply fused copper(II) porphyrin complex (1) was prepared, comprising two monomeric porphyrin units linked through β-β, meso-meso, β′-β′ triple covalent linkages and exhibiting remarkable catalytic activity for the electrochemical hydrogen evolution reaction in comparison to the analogous monomeric copper(II) porphyrin complex (2). Electrochemical investigations in the presence of a proton source (trifluoroacetic acid) confirmed that the catalytic activity of the fused metalloporphyrin occurred at a significantly lower overpotential (≈320 mV) compared to the non‐fused monomer. Controlled potential electrolysis combined with kinetic analysis of catalysts 1 and 2 confirmed production of hydrogen, with 96 and 71 % faradaic efficiencies and turnover numbers of 102 and 18, respectively, with an observed rate constant of around 107 s-1 for the dicopper complex. The results thus firmly establish triply fused porphyrin ligands as outstanding candidates for generating highly stable and efficient molecular electrocatalysts in combination with earth‐abundant 3d transition metals.
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    ItemOpen 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.