Combining Structural with Functional Model Properties in Iron Synthetic Analogue Complexes for the Active Site in Rabbit Lipoxygenase

Iron complexes that model the structural and functional properties of the active iron site in rabbit lipoxygenase are described. The ligand sphere of the mononuclear pseudo-octahedral cis-(carboxylato)­(hydroxo)­iron­(III) complex, which is completed by a tetraazamacrocyclic ligand, reproduces the first coordination shell of the active site in the enzyme. In addition, two corresponding iron­(II) complexes are presented that differ in the coordination of a water molecule. In their structural and electronic properties, both the (hydroxo)­iron­(III) and the (aqua)­iron­(II) complex reflect well the only two essential states found in the enzymatic mechanism of peroxidation of polyunsaturated fatty acids. Furthermore, the ferric complex is shown to undergo hydrogen atom abstraction reactions with O–H and C–H bonds of suitable substrates, and the bond dissociation free energy of the coordinated water ligand of the ferrous complex is determined to be 72.4 kcal·mol–1. Theoretical investigations of the reactivity support a concerted proton-coupled electron transfer mechanism in close analogy to the initial step in the enzymatic mechanism. The propensity of the (hydroxo)­iron­(III) complex to undergo H atom abstraction reactions is the basis for its catalytic function in the aerobic peroxidation of 2,4,6-tri­(tert-butyl)­phenol and its role as a radical initiator in the reaction of dihydroanthracene with oxygen

Stress‐induced Domain Wall Motion in a Ferroelastic Mn3+ Spin Crossover Complex

Domain wall motion is detected for the first time during the transition to a ferroelastic and spin‐state ordered phase of a spin crossover complex. Single crystal X‐ray diffraction and resonant ultrasonic spectroscopy (RUS) revealed two distinct symmetry‐breaking phase transitions in the mononuclear Mn 3+ compound [Mn(3,5‐diBr‐sal 2 (323))]BPh 4 , 1. The first at 250 K, involves the space group change Cc → Pc and is thermodynamically continuous, while the second, Pc → P1 at 85 K, is discontinuous and related to spin crossover and spin‐state ordering. Stress‐induced domain wall mobility was detected as softening of the phonon modes at the Pc → P1 transition

Mononukleare Fe(III)-Hydroxid- und Fe(II)-Aqua-Komplexe als Reaktive, Strukturelle Analoge für das Aktive Zentrum der Kaninchen-Lipoxygenase

This thesis describes the synthesis and extensive characterization of mononuclear cis-(carboxylato)(hydroxo)iron(III) and cis-(carboxylato)(aqua)iron(II) complexes among others and illuminates their capability to engage in hydrogen atom transfer reactions via reactivity studies with suitable substrates. The employed carboxylates include benzoate, p-nitrobenzoate, and p-methoxybenzoate. Additionally, the first example for a solution-stable mononuclear cis-di(hydroxo)iron(III) complex is presented, the extensive characterization of which aims to contribute to the identification of spectroscopic markers and a better understanding of the role of the carboxylate ligand in the above-mentioned complexes. The cis-(carboxylato)(hydroxo/aqua)iron(III/II) complexes match the coordination environment and the electronic properties of the active iron site in the resting state of rabbit lipoxygenase as well as of the reaction intermediates postulated for the enzymatic mechanism. In addition to being excellent structural and electronic models, the cis-(carboxylato)(hydroxo)iron(III) complexes display reactivity in abstracting hydrogen atoms from (weak) O–H and C–H bonds of suitable substrates, thus proving themselves to be worthy functional model complexes for lipoxygenases. The findings are supported with extensive structural, spectroscopic, spectrometric, magnetic, and electrochemical investigations as well as with quantified thermodynamic and kinetic parameters to allow for an adequate comparison between the derivatives with varying carboxylate ligands and to other works. Moreover, the reactivity investigation for the cis-(benzoato)(hydroxo)iron(III) (the first example found) was exemplary accompanied by a thorough theoretical study (done by external cooperation partners), which validates the experimental results and identifies an underlying concerted protoncoupled-electron-transfer (cPCET) mechanism for the cis-(carboxylato)(hydroxo)iron(III) complexes – analogous to the one suggested for the enzyme. The synthesis and study of a functional structural model complex is extremely challenging and rarely successful. Thus, this result alone represents a significant scientific advancement for the field, as no such model for lipoxygenases had been precedented prior to this project. The in-depth studies with derivatives of the initial cis-(benzoato)(hydroxo/aqua)iron(III/II) complexes further contribute to this advancement by illuminating structure-function relations.Diese Arbeit beschreibt u.a. die Synthese und umfangreiche Charakterisierung von mononuklearen cis-(carboxylato)(hydroxo)Eisen(III)- und cis- (carboxylato)(aqua)Eisen(II)-Komplexen und beleuchtet deren Fähigkeit, H-atom Transferreaktionen mit geeigneten Substraten durchzuführen. Zu den verwendeten Carboxylatliganden im Rahmen dieser Untersuchungen gehören Benzoat, p-Nitrobenzoat und p-Methoxybenzoat. Weiterhin wird in dieser Arbeit das erste Beispiel für einen mononuklearen cis-Di(hydroxo)eisen(III)-Komplex vorgestellt, welcher in Lösung nachweislich stabil ist. Dessen umfangreiche Charakterisierung trägt nicht nur zum besseren Verständnis der Rolle des Carboxylatliganden in den o.g. Komplexen bei, sondern beschreibt außerdem seine spektroskopischen Eigenschaften, welche der Identifikation solcher Spezies in zukünftigen Studien dienlich sein könnten. Die cis-(carboxylato)(hydroxo/aqua)eisen(III/II)-Komplexe entsprechen der Koordinationsumgebung und den elektronischen Eigenschaften des aktiven Zentrums der Kaninchen-Lipoxygenase. Dies gilt sowohl für den Ruhezustand als auch für die, für den enzymatischen Mechanismus postulierten, reaktiven Zwischenprodukte. Die genannten Komplexe sind nicht nur hervorragende strukturelle und elektronische Modelle, sondern zeigen auch eine Reaktivität mit (schwachen) O–H und C–H Bindungen geeigneter Substrate und erweisen sich damit als würdige funktionale Modellkomplexe für Lipoxygenasen. Die Ergebnisse dieser Arbeit werden durch umfangreiche strukturelle, spektroskopische, spektrometrische, magnetische und elektrochemische Untersuchungen sowie durch quantifizierte thermodynamische und kinetische Daten unterstützt, um einen adäquaten Vergleich zwischen den Derivaten mit verschiedenen Carboxylat-Liganden und anderen Arbeiten zu ermöglichen. Für die Reaktivitätsstudien des cis(-benzoato)(hydroxo)Eisen(III) Komplexes wurde weiterhin in externer Kooperationsarbeit eine gründliche theoretische Studie durchgeführt, die die experimentellen Ergebnisse validiert und einen zugrunde liegenden konzertierten protonengekoppelten Elektronentransfermechanismus (cPCET) für die Reaktivität dieser Komplexe identifiziert – analog zu dem Mechanismus, der für das Enzym vorgeschlagen wird. Die Synthese und Untersuchung eines funktionalen strukturellen Modellkomplexes ist äußerst herausfordernd und gelingt nur selten. Daher stellt dieses Ergebnis allein schon einen bedeutenden wissenschaftlichen Fortschritt für das Gebiet dar, besonders da es vor dem Abschluss dieses Projekts noch kein solches Modell gab. Die weiterführenden Synthesen von Derivaten der initialen cis(-benzoato)(hydroxo/aqua)eisen(III/II)-Komplexe und deren Untersuchungen tragen weiter zu diesem Fortschritt bei, indem sie die Struktur-Funktions-Beziehungen vertieft herausstellen

Interview with Prof. Dr. Benjamin List: Nobel Laureate in Chemistry 2021

In an interview with Benjamin List, winner of the 2021 Nobel Prize in Chemistry, members of the Young Chemists' Network (JCF) of the German Chemical Society (GDCh) asked him about his science, his career, and the academic system. Benjamin List, Director at the Max-Planck-Institut fur Kohlenforschung in Germany, was awarded the Nobel Prize together with David W. C. MacMillan (Princeton University, USA) for the development of asymmetric organocatalysis. After studying chemistry at the Free University of Berlin, he received his doctorate from Goethe University in Frankfurt. He discovered the amino acid proline to be an efficient catalyst and thus co-founded the field of organocatalysis. In 2016, he received the Gottfried Wilhelm Leibniz Prize, which is considered the most important research award in Germany

Thermal and Magnetic Field Switching in a Two‐Step Hysteretic Mn(III) Spin Crossover Compound Coupled to Symmetry Breakings

International audienceA Mn spin crossover complex with atypical two-step hysteretic thermal switching at 74 K and 84 K shows rich structural-magnetic interplay and magnetic-field-induced spin state switching below 14 T with an onset below 5 T. The spin states, structures, and the nature of the phase transitions are elucidated via X-ray and magnetization measurements. An unusual intermediate phase containing four individual sites, where are in a pure low spin state, is observed. The splitting of equivalent sites in the high temperature phase into four inequivalent sites is due to a structural reorganization involving a primary and a secondary symmetry-breaking order parameter that induces a crystal system change from orthorhombic→monoclinic and a cell doubling. Further cooling leads to a reconstructive phase transition and a monoclinic low-temperature phase with two inequivalent low-spin sites. The coupling between the order parameters is identified in the framework of Landau theory

Giant Magnetoelectric Coupling and Magnetic-Field-Induced Permanent Switching in a Spin Crossover Mn(III) Complex

International audienceWe investigate giant magnetoelectric coupling at a Mn spin crossover in [MnL]BPh (L = (3,5-diBr-sal)323) with a field-induced permanent switching of the structural, electric, and magnetic properties. An applied magnetic field induces a first-order phase transition from a high spin/low spin (HS-LS) ordered phase to a HS-only phase at 87.5 K that remains after the field is removed. We observe this unusual effect for DC magnetic fields as low as 8.7 T. The spin-state switching driven by the magnetic field in the bistable molecular material is accompanied by a change in electric polarization amplitude and direction due to a symmetry-breaking phase transition between polar space groups. The magnetoelectric coupling occurs due to a γη coupling between the order parameter γ related to the spin-state bistability and the symmetry-breaking order parameter η responsible for the change of symmetry between polar structural phases. We also observe conductivity occurring during the spin crossover and evaluate the possibility that it results from conducting phase boundaries. We perform ab initio calculations to understand the origin of the electric polarization change as well as the conductivity during the spin crossover. Thus, we demonstrate a giant magnetoelectric effect with a field-induced electric polarization change that is 1/10 of the record for any material

CCDC 2077026: Experimental Crystal Structure Determination

Related Article: Emiel Dobbelaar, Christian Rauber, Thorsten Bonck, Harald Kelm, Markus Schmitz, Matina Eloïse de Waal Malefijt, Johannes E. M. N. Klein, Hans-Jörg Krüger|2021|J.Am.Chem.Soc.|143|13145|doi:10.1021/jacs.1c0442

CCDC 2077030: Experimental Crystal Structure Determination

Related Article: Emiel Dobbelaar, Christian Rauber, Thorsten Bonck, Harald Kelm, Markus Schmitz, Matina Eloïse de Waal Malefijt, Johannes E. M. N. Klein, Hans-Jörg Krüger|2021|J.Am.Chem.Soc.|143|13145|doi:10.1021/jacs.1c0442

Domain Wall Dynamics in a Ferroelastic Spin Crossover Complex with Giant Magnetoelectric Coupling

International audiencePinned and mobile ferroelastic domain walls are detected in response to mechanical stress in a Mn3+ complex with two-step thermal switching between the spin triplet and spin quintet forms. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy on [Mn-III(3,5-diCl-sal(2)(323))]BPh4 reveal three distinct symmetry-breaking phase transitions in the polar space group series Cc -> Pc -> P1 -> P1((1/2)). The transition mechanisms involve coupling between structural and spin state order parameters, and the three transitions are Landau tricritical, first order, and first order, respectively. The two first-order phase transitions also show changes in magnetic properties and spin state ordering in the Jahn-Teller-active Mn3+ complex. On the basis of the change in symmetry from that of the parent structure, Cc, the triclinic phases are also ferroelastic, which has been confirmed by resonant ultrasound spectroscopy. Measurements of magnetoelectric coupling revealed significant changes in electric polarization at both the Pc -> P1 and P1 -> P1((1/2)) transitions, with opposite signs. All these phases are polar, while P1 is also chiral. Remanent electric polarization was detected when applying a pulsed magnetic field of 60 T in the P1 -> P1((1/2)) region of bistability at 90 K. Thus, we showcase here a rare example of multifunctionality in a spin crossover material where the strain and polarization tensors and structural and spin state order parameters are strongly coupled

CCDC 2077036: Experimental Crystal Structure Determination

Related Article: Emiel Dobbelaar, Christian Rauber, Thorsten Bonck, Harald Kelm, Markus Schmitz, Matina Eloïse de Waal Malefijt, Johannes E. M. N. Klein, Hans-Jörg Krüger|2021|J.Am.Chem.Soc.|143|13145|doi:10.1021/jacs.1c04422 cis-aqua-(benzoato)-(3,7-di-t-butyl-3,7-diaza-1,5(2,6)-dipyridinacyclooctaphane)-iron(ii) perchlorat