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Entropy-Driven Ligand Exchange in a Rotationally Flexible Dinuclear Fe(II)–Fe(II) Complex
Authors
P. Porta, B. Kintzel, B. Weber, M. Busch, D. Sorsche
Year of publication
Published in:
Inorganic chemistry
In metalloenzymes, precise control of metal–metal distance and coordination environment enables challenging catalytic transformations. This often involves hydrogen bonding in the second coordination sphere. Although many homogeneous systems aim to mimic these features, the capture and characterization of transient coordination events remains a challenge. Exploring the first di-iron complex of the rotationally flexible dinucleating ligand 1,1′,5,5′,6,6′-hexamethyl-4,4′-bis(picolinimino)-2,2′-bibenzimidazole, report an unexpected, entropy-driven ligand-exchange equilibrium which we describe by means of variable temperature studies in solution, key species fully structurally characterized in crystallo, and a mechanistic study in silico. This comprehensive experimental and computational characterization of the ligand and complex revealed how the central C–C bond enables adaptation to different metal–metal distances. Cooling a methanol solution of the complex induces a color change from green to blue which is attributed to the reversible substitution of chlorido ligands by methanol. Density functional theory calculations suggest that this ligand exchange is driven by the change in entropy inherent to the reduction of temperature. Under controlled conditions, electrochemical analysis reveals two accessible redox events: a fully reversible redox process at −0.05 V vs Fc/Fc ⁺ and a series of irreversible reduction reactions at more negative potentials. The resemblance of these redox features with those in established iron catalysts highlights the potential of this system to support catalytic transformations under certain conditions.
Determining the binding strength of phenolic anchoring groups on hydrated WO 3 surfaces
Authors
M. Knodt, L. Mayer, E. Gareis, F. von der Haar, G. Hörner, L. Leppert, B. Weber, R. Marschall, S. Kümmel
Year of publication
Published in:
Physical chemistry, chemical physics : PCCP
WO ₃ has many properties that make it a promising anode material for photoelectrochemical water splitting. However, its practical use is limited by its moderate efficiency. Anchoring a co-catalyst to the WO ₃ surface can be a way of substantially improving in particular the kinetics of the oxygen evolution reaction, and thus the overall efficiency. This requires identifying anchoring groups that reliably bind to WO ₃ in the presence of water. Based on first-principles calculations we here determine whether catechol, resorcinol or hydroquinone bind to a hydrated R45° (001) WO ₃ surface. Furthermore, we calculate the binding to clusters of WO ₃ to study the effects of non-ideal surface structures, and we gain insight into the electronic interaction between a ligand and the cluster. We discuss different levels of approximation to calculate the anchoring of a molecule to a surface. For all adsorbates studied here, we find that dispersive and especially covalent binding to a hydrated surface is in energetic competition with surrounding water molecules. However, structural defects in the WO ₃ surface have the potential to enable covalent binding, for which strong electronic interaction is to be expected. The theoretical results for catechol are in agreement with an experimental Raman and contact angle analysis with a porous WO ₃ photoanode.
Synthesis and characterisation of homo-and heterobinuclear transition-metal complexes
Author
F. Walter
Year of publication
Ziel der Arbeit war die Synthese von Dinuklearen Übergangsmetallkomplexen, die hinsichtlich des Einsatzes als Photosensibilisatoren oder Photokatalysatoren untersucht werden sollten. Ein literaturbekanntes molybdänbasiertes System sollte synthetisch variiert und als dinukleares System mit Kupfer(II) hinsichtlich Wasseroxidation untersucht werde. Der Fokus liegt auf Kupfer(I)-basierten Systemen, die vor allem vielversprechend für reduktive Katalyse sind. Neue dinukleare Kupfer(I)-Komplexe wurden mit vier literaturbekannten Brückenliganden synthetisiert und hinsichtlich ihrer strukturellen, optischen und elektrochemischen Eigenschaften untersucht. Außerdem wurde die Synthese Heterodinuklearer Komplexe mit anderen Übergangsmetallen untersucht. Basierend auf Erkenntnissen aus diesen Kapiteln wurde ein neues Ligandsystem mit variabler Substitution entwickelt und strukturell, sowie optisch charakterisiert. Außerdem wurde auch von dem neuen Ligandsystem, das mit PDAOA/PDAPA abgekürzt wird, dinukleare Kupfer(I)-Komplexe synthetisiert. Diese zeigen vielversprechende Eigenschaften für den Einsatz als Photosensibilisatoren, da sie breite Absorption im sichtbaren Spektrum und gute Redoxeigenschaften aufweisen. Neben diesen Komplexen stechen auch die Eigenschaften der Mononuklearen Kupfer(I)-Phenanthrolindion Komplexe heraus, die ebenfalls sehr gute optische Eigenschaften aufweisen. Diese sind strukturell sehr vielversprechend für den Einsatz als Photosensibilisatoren in Multikomponentensystemen.
Heterobimetallische Koordinationsverbindungen mit Aminosäure-Dithiocarbamat-Liganden
Author
P. Köhler
Year of publication
Die gezielte Synthese heterobimetallischer Koordinationspolymere (CPs) und -netzwerke (CNs) ist aufgrund der schwierig zu steuernden Metall-Ligand-Konnektivität oft nicht trivial. Deshalb besteht großes Forschungsinteresse an der Entwicklung neuer funktioneller Liganden, die selektiv an die gewünschten Metallzentren koordinieren. Die auf zehn Originalarbeiten und zwei Übersichtsartikeln basierende kumulative Habilitationsschrift beschäftigt sich mit der Etablierung von Aminosäure-Dithiocarbamaten (AA-DTCs) für die Anwendung als Bausteine für heterobimetallische Koordinationsverbindungen. Das Hauptaugenmerk lag auf möglichst porösen CNs, die sich als funktionelle Metal-Organic Frameworks (MOFs) eignen könnten. Dabei haben sich AA-DTC-Liganden als nützlich erwiesen, die sich von verschiedenen kommerziell erhältlichen α-Aminosäuren ableiten. Durch die stringente physikalisch-chemisch Charakterisierung der AA-DTCs in Form ihrer Alkalimetallsalze sowie verwandten O-Methylxanthaten wurde ein detailliertes Bild zu diesen beiden Stoffgruppen erhalten. Die genannten AA-DTC-Liganden eignen sich gut für die effiziente Synthese verschiedener d-/s-, d-/p- und d-/d-heterobimetallischer Produkte nach verschiedenen Strategien. Von den thiophilen Übergangsmetallen erwiesen sich unter anderem Ni(II), Pd(II), Pt(II) aufgrund ihrer definierten quadratisch-planaren Koordination als nützliche Bausteine für CNs. Zusätzlich wurden in diesem Kontext Cu(II)-, Au(III)- und Ag(I)-Komplexe mit AA-DTC-Liganden untersucht. Die Festkörperstrukturen der Zielprodukte hängen vordergründig von der Wahl des oxophilen Metalls ab. Mit Closed-Shell-Ionen des s-Blocks sowie Cd2+ bilden sich salzartige Produkte, die je nach Lewis-Acidität als solvensseparierte Ionen oder als Netzwerke mit irregulär koordinierten M2-Ionen vorliegen. 3d-Metallionen und Organozinn(IV)-Fragmente bilden aufgrund ihrer regulären Koordinationspolyeder definierte kleine Moleküle, CPs oder CNs unterschiedlicher Porosität.
Magnetic Structure of the High-TN Antiferromagnet La0.5Sr3.5Fe3O7.5F2.6: a Temperature-Dependent Mößbauer Spectroscopy and Neutron Diffraction Study
Authors
A. Bivour, J. Jacobs, R. Köferstein, F. Daumann, G. Hörner, B. Weber, C. Ritter, W. Schnelle, C. Felser, S. Ebbinghaus
Year of publication
Published in:
Zeitschrift für anorganische und allgemeine Chemie
La ₀.₅ Sr ₃.₅ Fe ₃ O ₇.₅ F ₂.₆ is one of the few n = 3 members of Ruddlesden–Popper oxyfluorides. This compound was recently found to be an antiferromagnet with T N being well above room temperature. In this contribution, temperature-dependent neutron diffraction and Mößbauer spectroscopy were applied to get detailed information on the nuclear and magnetic structures of La ₀.₅ Sr ₃.₅ Fe ₃ O ₇.₅ F ₂.₆ . The nuclear structure was refined in the orthorhombic space group Cmce (No. 64), while for the magnetic structure space group P2 ₁ /c (No. 14.75) was derived. The antiferromagnetic ordering of the iron moments was found to persist in the very wide temperature range of 1.7–470 K without any structural transitions. While neutron diffraction revealed the loss of long-range antiferromagnetic ordering to occur above 470 K, Mößbauer spectroscopy showed that short-range ordering remains up to 525 K.
Spin Crossover around Room Temperature: Effects of Remote Ligand Symmetry Breaking within Jäger-Type Iron(II) Complexes
Authors
F. Daumann, L. Gottwalt, P. Köhler, G. Hörner, B. Weber
Year of publication
Published in:
Zeitschrift für anorganische und allgemeine Chemie
In a bottom-up approach, we studied the feedback of slight molecular modifications on the spin crossover (SCO) properties of otherwise structurally conserved iron(II) complexes. There, the ligand backbone of a C ₂v symmetric, planar-directing N ₂ O ₂ ²⁻ Jäger platform was modified through remote substituents (R = CN, F, CF ₃ ), reducing symmetry to C s . Both electronic influences, through mesomeric activation/deactivation and inductive electronic effects, were studied. From these ligands, a series of three Jäger-type iron(II) complexes [Fe1-3(py) ₂ ] has been prepared and characterized by IR spectroscopy, SQUID magnetometry, and ⁵⁷ Fe Mössbauer spectroscopy. Bulk solid measurements are supplemented by ¹ H, ¹⁹ F, and ¹³ C NMR spectroscopy in fluid solution and density functional theory (DFT) modeling of the molecular properties. While full SCO occurs close to room temperature in all three compounds, the SCO profiles lack magnetic bistability previously observed with the symmetric parent complex for R = H. Single crystal XRD analysis of the fluorido substituted complex [Fe2(py) ₂ ] associates the very gradual SCO profile with the simultaneous effects of syn and anti rotamers of noncoordinate carbonyls and H/F crystallographic site disorder.
Substitution matters: Comparison of four N,1,4-tri(methoxy-2-hydroxybenzyl)-DAZA ligands as potential PET/CT liver imaging agents
Authors
A. Zenner, T. Winkens, M. Pomraenke, S. Wiegand, M. Freesmeyer, F. Hüttner, W. Weigand, B. Weber, J. Greiser
Year of publication
Published in:
Nuclear medicine and biology: official journal of the Society of Radiopharmaceutical Sciences
Four regioisomers of [68Ga]Ga-TMoS-DAZA, a PET/CT radio tracer for liver function imaging, were studied as candidates of potentially improved hepatobiliary biokinetics. Liver uptake and biliary clearance behavior were compared using an in ovo model based on ostrich eggs for PET/CT imaging and ex vivo biodistribution analysis. The tracer lipophilicity was evaluated via logD determination. The experiments showed remarkable differences between the four tracers concerning their maximum liver uptake, percentage of tracer cleared via the biliary tract and their respective logD values.
Thermal- and light-induced valence tautomerism with a concerted spin transition in an iron tris(diimine) complex
Authors
J. Janetzki, D. Brown, F. Daumann, I. Ismail, R. Gable, M. Hay, R. Mulder, A. Starikova, B. Weber, M. Giansiracusa, C. Boskovic
Year of publication
Published in:
Chemical Science
The switching phenomena of spin crossover (SCO) and valence tautomerism (VT) are respectively dominated by iron(ii) and cobalt-dioxolene systems. To explore new possibilities for SCO or VT, the redox-active α-diimine ligand bis((phenyl)imino)acenaphthene (Ph-BIAN), which can adopt neutral (L⁰), monoanionic (L˙−), and dianionic (L²−) states, was paired with zinc, cobalt, manganese and iron to give [M(Ph-BIAN)₃](BPh₄)₂ (M = Zn (1), Co (2), Mn (3), Fe (4)). Compounds 1, 2 and 3 adopt a temperature invariant MII-(L⁰)₃ state, (2 and 3 are high spin (HS)) in the solid- and solution-states. Electrochemical measurements show the metal controls the degree of electronic communication between the Ph-BIAN ligands. In stark contrast to 1, 2 and 3, compound 4 adopts the LS-FeIII-(L˙−)(L⁰)₂ (LS = low spin) tautomeric form as the ground state in both the solid-state and in solution. Combined variable temperature solid- and solution-state structural, Mössbauer and electronic spectroscopy, and magnetic measurements, show that 4 undergoes the thermally-induced VT process LS-FeIII-(L˙−)(L⁰)₂ ⇌ HS-FeII-(L⁰)₃, the only example of VT accompanied by a concerted spin transition in an iron complex with a redox-active ligand. Solid-state photomagnetic measurements suggest that the VT interconversion is also induced by light. Light-induced VT has not been previously observed for complexes other than cobalt-dioxolene, and is potentially afforded here by the unique spin-state change that results in large differences in the Fe-N bond lengths for the two valence tautomers. This study introduces a new example of VT, and suggests that optically-induced VT can be displayed by iron systems, opening alternate pathways toward molecular switches that can be controlled with multiple stimuli.
Magneto-structural correlation in pseudotetrahedral Cobalt(II) complexes
Author
M. Pohle
Year of publication
This dissertation explores magneto-structural correlations in pseudotetrahedral cobalt(II) complexes with bidentate [N2O2] donor sets, aiming to establish design principles for single-molecule magnets (SMMs). For spin S = 3/2 systems, the anisotropy barrier U, derived from the zero-field splitting parameters D and E, is used as a reliable magnetic descriptor. Structural variation is quantified via two parameters: the elongation ϵT and the dihedral angle δ. Ten cobalt(II) complexes with eleven coordination geometries were synthesised and characterised using magnetic susceptibility, magnetisation, continuous-wave EPR, and frequency-domain Fourier-transform terahertz EPR (FD-FT THz-EPR) spectroscopy. This powerful, rarely applied technique enables direct measurement of spin transitions associated with U, providing significantly higher accuracy and reliability than conventional, model-dependent methods. A linear correlation was observed between ϵT and U, while δ influences the rhombicity |E/D|, which affects the quantum tunnelling of magnetisation. Two outlier complexes showed significant kink distortion, captured by a newly defined angle κ, which also correlates with U. These findings yield three design rules: (i) axial elongation increases magnetic anisotropy; (ii) twisting the coordination sphere towards a quadratic planar structure raises rhombicity and tunnelling rates; (iii) kink distortion must be controlled due to its non-linear impact. The approach offers transferable insights for designing SMMs with tailored properties.
Structure-reactivity relationship in Titanium(IV) coordination chemistry
Author
B. Rezaei Kheirkhah
Year of publication
Many researchers have dedicated their efforts to developing metal-organic complexes capable of catalyzing hydrofunctionalization reactions such as hydroamination and hydrophosphorylation in recent decades. Their objective is to ensure that these complexes exhibit a high degree of selectivity and reactivity. Researchers have concentrated their efforts on Group IV metal complexes, especially the bis-(amido) complexes obtained from carboxamide proligands, in hydroamination reactions. Furthermore, they utilize the bis-(chloro) Group IV complexes obtained from Schiff bases for the alkene polymerization. In our research, we synthesized a tetradentate N2O2-Schiff base ligand and a carboxamide. The reaction of the synthesized ligands with tetrahydrofuran adducts of TiCl4 and ZrCl4, as well as with homoleptic tetrakis(dimethylamido) Group IV reagents such as Ti(NMe2)4, Zr(NMe2)4, and Hf(NMe2)4, resulted in the formation of the corresponding Group IV metal complexes. All reactions were monitored using various NMR spectroscopic techniques. Furthermore, all compounds produced in this experiment were characterized by both NMR measurements and X-ray single crystal diffractometry. Overall, two ligands and six Group IV coordination complexes were synthesized and characterized: H2L1 (1), [TiCl2L1]•Hex (2), [ZrCl2L1] (3), [Ti(NMe2)2L1´] (4), HL2 (5), [Ti(NMe2)3L2] (6), [Zr(NMe2)2(L2)2] (7), and [Hf(NMe2)2(L2)2]∙5Tol (8). The influence of metal cation size on ligand binding modes was investigated. Additionally, the potential application of complex (6) in hydroamination and hydrofunctionalization reactions was examined.
Daumann, F., Hörner, G., Köhler, P., Weber, B. Half the Step, Twice the Mystery─Kinetically Controlled Spin Crossover in an Anticooperative Pyridine-Coordinate Jäger-Type Iron(II) Complex Cryst. Growth. Des., 2025, 25, 4565-4576. https://doi.org/10.1021/acs.cgd.5c00477External link
Janetzki, J. T., Brown D. S., Daumann, F.,Haseena Ismail, I., Gable, R.W., Hay M.A., Mulder, R.J.,Starikova, A. A., Weber, B., Giansiracusa, M.J., Boskovic, C. Thermal- and light-induced valence tautomerism with a concerted spin transition in an iron tris(diimine) complex Chem. Sci., 2025, Advance Article https://doi.org/10.1039/D4SC07798AExternal link
Zander, J., Daumann, F., Loukrakpam, R., Roth, C., Weber, B. Marschall, R.* Correlations of Calcination Temperature with the Catalytic Properties of CuFe2O4 for the Synthesis of Green Fuels, Adv. Energy Sustainability Res. 2025, 2400281. LinkExternal link
2024
Sila, N., Dürrmann, A., Weber, B., Heinemann, F. W., Irrgang, T., Kempe, R.* A Selective Iron(I) Hydrogenation Catalyst, J. Am. Chem. Soc. 2024, 146, 26877-26883. LinkExternal link
Bivour, A.; Jacobs, J.; Daumann, F.; Hörner, G.; Weber, B.; Ritter, C.; Ebbinghaus, S.* Structure and Magnetic Properties of the n = 3 Ruddlesden-Popper-Oxyfluoride La0.5Sr3.5Fe3O7.5F2.6, Inorg. Chem. 2024, 63, 20427–20437. LinkExternal link
Hörner, G.; Weber, B.* Switching Spin-States: Spin Crossover vs. Coordination-Induced Spin-State Switching, Eur. J. Inorg. Chem. 2024, e202400471. LinkExternal link
Dürrmann, A.; Hörner, G.; Baabe, D.; Heinemann, F. W.; de Melo, M.; Weber, B.* Cooperative spin crossover leading to bistable and multi-inert system states in an iron(III) complex. Nat Commun. 2024, 15, 7321. LinkExternal link
Egner, S.; Akintola, O.; Plass, W.; Liebing, P.; Schlörer, N.; Hörner, G.*; Weber, B.* Axial Perturbation of Intermediate Spin (S=3/2) Iron(III) Complexes with Square Pyramidal N4X Coordination: Solid State Structures and Electronic Properties. Z. Anorg. Allg. Chem. 2024, 650, e202400061. LinkExternal link
Schreck, C.; Schönfeld, S.; Liebing, P.; Hörner, G.; Weber, B.* Binucleating Jäger-type {(N2O2)2}4− ligands: magnetic and electronic interactions of Fe(II), Ni(II) and Cu(II) across an in-plane TTF-bridge† Dalton Trans. 2024, 53, 9092-9105. LinkExternal link
Yadav, J.; Gupta, A.; Mondal, A.; Daumann, F.; Hörner, G.; Weber, B.*; Konar, S.* Exploring the Magnetic Interactions in Two Novel Cyanide-Bridged Homo- and Heterometallic Hexadecanuclear Complexes. Chem. Eur. J., 2024, e202400321. LinkExternal link