Benjamin Kintzel

ZUSAMMENFASSUNG: Frequenzbereichs‐Fourier‐Transformations‐THz‐ESR‐Spektroskopie ermöglicht einen direkten Zugang zu kinetisch eingefrorenen High‐Spin ‐( HS )‐Zuständen in Eisen(II)‐ Spin‐Crossover ‐(SCO)‐Komplexen. Mithilfe des temperaturinduzierten Einfangens angeregter Spinzustände (temperature‐induced excited spin‐state trapping, TIESST) wird die elektronische Struktur des HS ‐Zustands bei tiefer Temperatur trotz eines diamagnetischen Low‐Spin ‐( LS )‐Grundzustands zugänglich. Unterstützt durch Ab‐initio ‐Berechnungen erlauben feldabhängige Spektren die Aufspaltung des HS ‐Grundzustandsmultipletts zu entschlüsseln und liefern Hinweise auf eine Spin‐Phononen‐Kopplung, die durch einen „avoided‐crossing“‐Hamilton‐Operator beschrieben wird. Die THz‐ESR‐Spektroskopie quantifiziert darüber hinaus den verbleibenden HS ‐Anteil (γ _HS ) und detektiert Verschiebungen magnetischer Übergänge, die von den Spinzuständen der umgebenden Matrix abhängen. Diese Ergebnisse zeigen, dass HS ‐Zentren in gemischten HS / LS ‐Zuständen unterschiedliche lokale Gitterumgebungen erfahren, und eröffnen einen experimentellen Zugang zum Einfluss der Spinzustandsverteilung auf SCO‐Prozesse. Kooperative Schaltszenarien, einschließlich Domänenbildung und alternierender HS / LS ‐Anordnungen, lassen sich auf diese Weise unterscheiden, so dass feldabhängige THz‐ESR‐Spektroskopie einen direkten Zugang zu gitterabhängigen Effekten in SCO‐Materialien bietet.

Frequency‐domain Fourier‐transform (FD‐FT) THz‐EPR spectroscopy provides direct access to kinetically trapped high‐spin ( HS ) states in iron(II) spin crossover (SCO) complexes. Using temperature‐induced excited spin‐state trapping (TIESST), the low‐temperature electronic structure of the HS state is accessed despite a diamagnetic low‐spin ( LS ) ground state. Field‐dependent spectra resolve the splitting of the HS ground‐state manifold and provide evidence for spin–phonon coupling, described by an avoided‐crossings Hamiltonian supported by ab initio calculations. THz‐EPR further quantifies the remaining HS fraction () and reveals shifts of magnetic transitions that depend on the surrounding spin‐state matrix. These results show that HS centers experience distinct local lattice environments in mixed HS / LS states and provide experimental access to the influence of spin‐state distribution on SCO processes. Cooperative switching scenarios, including domain formation and alternating HS / LS arrangements, can thus be distinguished. Field‐dependent THz‐EPR spectroscopy therefore provides direct access to lattice‐dependent effects in SCO materials.

The thermal decomposition of the compounds M ₂ [Pt(L) ₂ ] (M = Ca, Sr, Ba, Zn, Cd; L = N ‐dithioato‐iminodiacetate) was investigated by thermogravimetry and powder X‐ray diffraction. The pristine hydrated compounds can be fully dehydrated by heating without decomposition. Structural elucidation of Ca ₂ [Pt(L) ₂ ]·4H ₂ O and Zn ₂ [Pt(L) ₂ ]·8H ₂ O·3 acetone was performed by single‐crystal X‐ray diffraction, whereas the structures of Ba ₂ [Pt(L) ₂ ]·2H ₂ O, anhydrous Ba ₂ [Pt(L) ₂ ], Zn ₂ [Pt(L) ₂ ]·2H ₂ O, and Cd ₂ [Pt(L) ₂ ]·2H ₂ O were determined from powder X‐ray diffraction data. The Ca, Sr, and Ba compounds form ionic 3D networks with irregular coordination environment of the alkaline‐earth metal center, whereas the Zn and Cd compounds display a more regular five‐ or six‐coordination by oxygen donors. The pristine hydrates form crystallites with diameters in the lower µm range. Combustion at 600°C results in the formation of crystalline MSO ₄ (for M = Ca, Sr, Ba), Zn ₃ O(SO ₄ ) ₂ , or CdSO ₄ /CdO mixture. For Zn, further heating to 1000°C yields ZnO, whereas CdO sublimes under these conditions. The combustion products retain their original particle morphology for the alkaline‐earth derivatives, whereas they are not mechanically stable in the Zn and Cd cases. For the particles formed at 600°C, a homogenic distribution of nanocrystalline platinum is assumed, sintering to larger spherical particles in the 100 nm diameter range upon heating to 1000°C.

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.

Recent studies have highlighted the biological significance of 8-hydroxyquinoline-2-carboxylic acid (8-HQA) as a metal chelator. In this work, several techniques were applied for the study of the interaction of 8-HQA with vanadium(IV/V) oxidometal ions at a temperature of T = 298.2 K and an ionic strength of I = 0.20 mol dm^-3 in KCl_(aq). The redox behavior of the chemical system was defined, and the stability constants of the formed complexes were determined by H⁺-ion selective electrode potentiometry and UV-vis spectrophotometric titrations, while nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopies and mass spectrometry provided stoichiometric and structural information. The formation of both oxidovanadium(IV) and dioxidovanadium(V) ML complexes with 8-HQA was observed in aqueous solution, with both complexes being particularly stable in acidic conditions. The dioxidovanadium(V) compound is stable even at neutral pH, but dimeric or tetrameric hydrolytic species are predominant under alkaline conditions. In contrast, oxidovanadium(IV) complexes undergo oxidation as the pH increases. Nevertheless, under strictly anaerobic conditions, the complexation of oxidovanadium(IV) by 8-HQA can also occur at pH > 6.0. The nature of the oxidovanadium(IV) and dioxidovanadium(V) ML complexes, representing the major species formed in solution, was further investigated by DFT calculations.

The reduction of the iron(II) half-sandwich precursors [Cp'FeN(R)(SiMe₃)] (R = SiMe₃, 2,6-di-iso-propylphenyl (dipp)) with KC₈ yields the one-dimensional polymeric iron(I) complexes {[Cp'Fe(N(SiMe₃)₃)]K}_n (4) and {[Cp'Fe(N(dipp)(SiMe₃))]K}_n (5). Addition of 18-crown-6 breaks up these chains and affords the monomeric salts [Cp'Fe(N(SiMe₃)₂)][K(18-crown-6)(thf)₂] (6) and [Cp'Fe(N(dipp)(SiMe₃))][K(18-crown-6)(thf)₂] (7). All four compounds were structurally characterized by single-crystal X-ray diffraction analysis and adopt an S = 3/2 spin ground state. Zero-field ⁵⁷Fe Mössbauer spectra uncover slow magnetic relaxation at low temperatures in each complex. Complementary CASSCF/NEVPT2 calculations reveal an S = 3/2 ground-state Kramers doublet that is ca. 100 cm^-1 below the first excited state. These findings are confirmed by results from DC susceptibility and magnetization experiments. The slow relaxation of magnetization is further characterized via AC susceptibility measurements, unveiling detectable slow relaxation of magnetization at zero external field for 4-6. Complex 4 exhibits an unusual field dependence of the quantum tunneling of magnetization, akin to exchange-bias effects, which are commonly observed in lanthanide dimers.

Spin-electric effects are crucial for quantum technologies, offering several advantages over standard magnetic field-based spin control. Seeking a mechanism independent of spin-orbit interaction, here we report the detection of a spin-electric effect in the [Cu₃(saltag)(py)₆]ClO₄ spin triangle. The effect is investigated by electron paramagnetic resonance under electric field modulation on single crystals. The anisotropy of the magnetic response to the electric field is addressed, and comprehensive ab initio calculations are performed to elucidate its origin. We demonstrate that when the electric field is applied in the plane of the triangle, the dominant contribution to the observed spin-electric signal arises from a variation of the isotropic exchange interaction. Our combined theoretical and experimental approach demonstrates that, in our system, there is no evidence of antisymmetric exchange (Dzyaloshinskii-Moriya) interaction, confirming that electric-field control of magnetic exchange is achievable in the absence of significant spin–orbit coupling. Moreover, we underscore the crucial role of the bridging ligand, which opens new avenues for chemically optimizing spin–electric coupling.