Tetradentate C-scorpionates for iron(IV)oxo model systems and spin switch applications

Keisers, Kristina; Herres-Pawlis, Sonja (Thesis advisor); Oppel, Iris Marga (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

Scorpionates are a ligand class that has been successfully applied for the synthesis of model systems for copper-containing enzymes. If they can also be used for iron-based model systems is the fundamental question of this thesis. In the center of scientific discussion are iron(IV)oxo systems. Starting from bis(pyrazolyl)bipyridinylmethane ligands, functionalization at three locations is investigated and subsequently a new heteroscorpionate with a bis(pyridinyl) function is developed. The ligands and their resulting complexes are characterized in solid state and in solution and successively tested with oxygen transfer reagents. Only iron(II) salts with weak and non-coordinating anions are used. The spin state of the dissolved iron(II) complexes of the known ligands HC(Pz)2BiPy and HC(MePz)2BiPy strongly depend on the used solvent. The complex [Fe(HC(Pz)2BiPy)(MeCN)(OTf)](OTf) demonstrates temperature-induced switching behavior via spin crossover (SCO) in MeCN and a change of the spin state induced by coordination (CISSS) in methanol. A new bismeridional coordination motif for the ligand HC(Pz)2BiPy is found. The cation [{HC(Pz)2BiPy}2]2+ is obtained with three different anions. The sterically demanding methyl groups in HC(MePz)2BiPy prevent this bismeridional coordination. Both systems do not react with oxygen transfer reagents. Furthermore, the structure of the known complex [Fe(HC(Pz)2BiPy)Cl2] is found to be also dependent on the solvent. It decomposes in alcohols under oxidation of the ligand to [(Fe(BiPyCOO)Cl)2(ยต-OAlc)2]. New fluorinated bis-(pyrazolyl)bipyridinylmethanes are developed with a new CCl4-free synthesis, but so far, no iron complexes are obtained. For tert-butyl substituted ligands, fivefold coordinated complexes are obtained, which do not react with oxygen transfer reagents, yet. Two new hybrid ligands of guanidinoquinolines and bis(pyrazolyl)methanes are developed. Their complexes react with oxygen transfer reagents. Instead of a Fe(IV)oxo complex, only self-hydroxylation products are identified, indicating extremely short-lived Fe(IV)oxo intermediates. Finally, backbone functionalized ligands are examined. While pyrazole-based ligands (MeC(Pz)2BiPy) are not suitable to stabilize a Fe(IV)oxo unit, a pyridine-based ligand (MeC(Py)2Phen) is synthesized, which successfully stabilizes a Fe(IV)oxo species. Extensive comprehensive studies using geometric analysis, spectroscopy, and density functional theory (DFT) indicate octahedral compression/deformation for the bis(pyrazolyl)methane complex, caused by shorter distances between the pyrazolyl donors and the central carbon atom. These ligands wrap the iron center to a lesser extent. This results in an altered orbital splitting of the iron(II) complexes, which might influence the reactivity with oxygen transfer reagents.

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