title: Multivalent Iron Coordination Complexes with Custom-Tailored N-Heterocyclic Carbene Ligands for the Stabilization of Unusual Oxidation States
Creators: name: Keilwerth, Martin
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024

description: Based on previous studies of the K. Meyer group, the intention of this thesis was the utilization and modification of tripodal tris-[(3-mesitylimidazol-2-ylidene)¬alkyl]amine N-heterocyclic carbene ligands for the generation and isolation of multi¬valent iron coordination complexes, followed by an in-depth spectroscopic and analytic investigation. The resulting insights into the molecular and electronic structures of these novel complexes and their reactivity will be supportive in the process of understanding the role of iron nitrido, imido or nitrosyl intermediates in biological reactions. This will potentially lead to an enhanced mechanistic comprehension applicable in synthetic processes. As exemplified in Chapter 1, tripodal N-heterocyclic carbene (NHC) ligands are promising candidates for these challenging tasks in many respects. They are capable of stabilizing low- and high-valent metal sites. Likewise, this class of ligands is easily adaptable by introducing variable bridging anchoring units or sterically demanding substituents perpendicular to the tris-(NHC) carbene plane, leading to a cavity that can accommodate for a variety of axial ligands, e.g., small nitrogen containing molecules. Thus, even reactive ligands — such as terminal iron nitrides — can be stabilized and protected from decomposition. This thesis has three main research projects: i) the investigation of the molecular and electronic structure of a series of mid- to low-valent iron nitrosyl complexes derived from nitrous oxide activation chemistry [P-1], ii) the examination of the nature of the Fe–N bonding interaction and its reactivity within a series of high-valent iron nitrido complexes [P-2, P-4], and iii) the investigation and reactivity of different series of mid- to high-valent iron imido complexes [P-3]. Hereafter, these projects are explained in more detail. Chapter 3.1 details about the synthesis and characterization of a mid- to low-valent {Fe−NO}6−9 iron nitrosyl complex series, including a reactive {Fe−NO}10 intermediate [P-1]. For all complexes in this chapter, the 1st-generation tripodal N-heterocyclic carbene ligand tris-[2-(3-mesitylimidazol-2-ylidene)ethyl]amine (TIMENMes) is utilized, which is known to support iron in high and low oxidation states. Based on previous studies, a reaction of the well-known [(TIMENMes)Fe]2+ precursor complex with an NO source results in the formation of the {Fe−NO}6 compound [(TIMENMes)¬Fe(NO)(solv)]3+. The stepwise chemical reduction leads to a series of iron nitrosyl complexes [(TIMENMes)Fe(NO)]n (n = +3 to −1). All complexes are characterized by sc-XRD analyses, temperature and field-dependent SQUID magnetization methods, as well as 57Fe Mössbauer, IR vibrational, UV/vis electronic absorption, multinuclear NMR, and dual-mode EPR spectroscopy. Complementary DFT analyses, based on the experimental findings, provide insight into the electronic structures of all compounds and allow for the assignment of the physical oxidation states to the iron center and the NO ligand. In addition, an alternative preparation of the {Fe−NO}8 complex is attempted via oxygenation of the [(TIMENMes)Fe(N)]1+ nitride complex, thereby extending the synthetic routes to {Fe−NO}n complexes. Chapter 3.2 details the examination of a series of high-valent iron nitrido complexes and the reactive nature within the iron nitrogen bonding [P-2, P-4]. For this purpose, the modified 2nd-generation tripodal N-heterocyclic carbene ligand tris-[(3-mesityl-imidazol-2-ylidene)methyl]amine (TIMMNMes) is developed. In this chapter, it is utilized to transcend the limitation of the [(TIMENMes)Fe(N)]1+ complex, which is only isolable in the +IV oxidation state and inserts the nitrido moiety upon oxidation. In analogy to the TIMENMes system, the [(TIMMNMes)Fe(N)]1+ complex can be synthesized. On account of the proposed enhanced steric and electronic flexibility of the TIMMNMes ligand, the iron(IV) nitride complex is stepwise oxidizable giving further insight into the nature and reactivity of high to even very-high oxidation states. Herein, a series of iron nitrido complexes is accessible, ranging from Fe(IV) to Fe(VII). This approach is further underpinned by single-crystal X-ray diffraction analyses as well as 57Fe Mössbauer, EPR, and UV/vis electronic absorption spectroscopy combined with quantum chemical calculations. Chapter 3.3 details the investigation and reactivity of different series of mid- to high-valent iron alkyl imido complexes [P-3]. To prevent the insertion chemistry observed for the TIMENMes iron imido complex upon synthesis, the herein reported iron alkyl imido complexes are supported by the newly developed TIMMNMes ligand. One common method for the preparation of such imido complexes is the reaction of the Fe(I) precursor with an organic azide to yield the corresponding trivalent iron imide. An alternative synthetic route to the respective imido complex, particularly for imides with primary alkyl substituents, can be achieved by alkylating the respective Fe(IV) nitrido complex. Both pathways result in stable iron imido complexes, which can be stepwise oxidized and/or reduced leading to isostructural series of [(TIMMNMes)¬Fe(NR)]n (n = +3 to 0). Accordingly, these complexes are characterized by single-crystal X-ray diffraction analyses, SQUID magnetization, and electrochemical methods as well as 57Fe Mössbauer, IR vibrational, UV/vis electronic absorption, multinuclear NMR, EPR, and X-ray absorption spectroscopy. As iron imido complexes exist in a manifold of different electronic structures, ultimately controlling the diverse reactivities, the studies are complemented with quantum chemical calculations, providing additional insight into the electronic structures of all these complexes.