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Bioinspired Iron-Carbene Models of Heme and Non-Heme Enzymes: Electronic Structure and HAA Reactivity

dc.contributor.advisorMeyer, Franc Prof. Dr.
dc.contributor.authorMorganti, Massimiliano
dc.titleBioinspired Iron-Carbene Models of Heme and Non-Heme Enzymes: Electronic Structure and HAA Reactivityde
dc.contributor.refereeSiewert, Inke Prof. Dr.
dc.description.abstractengThe fast depletion of non-renewable energy sources requires modern chemists to develop new industrial processes that are more efficient and environmentally sustainable. In this framework, the functionalization (e.g. hydroxylation) of abundant natural chemical feedstocks to form products of added value is a fundamental goal to achieve and requires increasing research efforts. Hydroxylation of earth-abundant hydrocarbons is a challenging process that natural enzymes can perform at ambient conditions and at impressively high rates. Inspired by natural iron-containing enzymes that activate dioxygen to hydroxylate organic substrates, chemists have developed both heme and non-heme model complexes to understand how natural intermediates can perform such reaction. In this work, two distinct series of non-heme iron-carbene model complexes were synthesized and fully characterized, providing insight on their electronic structure and reactivity. Building on previous results on an organometallic tetracarbene oxoiron(IV) model complex 3, which was investigated as a model for C−H activation (in particular for hydrogen atom abstraction -HAA- reactions), a series of axially substituted analogues bearing trifluoroacetate (complex 4), chloride (5) and tert-butylthiolate (7) trans to the oxo group was synthesized to investigate the effect of axial ligand donation on the reaction rates of HAA. The substituted tetracarbene oxoiron(IV) complexes were fully characterized using UV/Vis, Mößbauer, IRPD spectroscopy, SQUID magnetometry and crystallography (in the case of 4), confirming their electronic structure and S = 1 spin ground state. IRPD investigations supported the presence of progressively stronger axial donors going from 3 to 5, showing a decrease of the Fe=O stretching frequency going from acetonitrile (in 3) to trifluoroacetate and chloride. Complexes 4 and 5 showed similar spectroscopic signatures compared to 3, suggesting that weak axial donors only partially affect the electronic structure of the complexes. However, when the axial position is occupied by a thiolate ligand, as in 7, both the optical spectra and the MB parameters evidence a major change of the electronic structure due to the strong - and - donor properties of the RS− group. The series of complexes was investigated towards activation of the weak C−H bonds of 1,4- cyclohexadiene (CHD), 9,10-dihydroanthracene (DHA) and xanthene. UV/Vis monitoring of the reaction of 4, 5 and 7 with excess CHD in acetonitrile at −40 °C suggested how the presence of axially- bound anions affect the reaction pathway, stabilizing the putative FeIII-OH intermediate and allowing for follow-up reactivity with the nascent substrate organic radical, in contrast to the behavior observed previously for 3. The rates of the HAA with all the substrates were measured in pseudo-first order conditions at temperature between −40 and 25 °C, and activation parameters for the reaction were derived. Interestingly, the presence of strong axial donors decreased the reaction rates compared to 3, in contrast to other known model complexes, possibly due to a different mechanism of the initial interaction with the substrates. Additionally, HAA reactivity studies on the FeIII−O−FeIII dimeric tetracarbene complex 2, deriving from the decomposition of 3 at RT or from the exposition of the corresponding FeII monomer 1 to air, were conducted in order to shed light on a previously reported disproportionation equilibrium that allows for HAA reactivity to take place. For this, scrambling experiments were designed to confirm the presence of the equilibrium in solution. The disproportionation was shown to happened due to the attack of acetonitrile or other potential ligands on 2, triggering the conversion into 1 and 3. A structurally characterized adduct between complex 2 and two molecules of 3 provides support for the attack on the axial free position of 2 being the trigger for the disproportionation. Parallel to the studies on the tetracarbene complexes, the development of a new hybrid organometallic ligand system and its iron complexes was pursued. The novel hybrid macrocycle features both NHC ligation and N-donating groups, together with a redox-non innocent carbazole fragment. The hybrid macrocycle is able to stabilize FeII complexes in two different spin ground states: S = 0 (octahedral complex 12a) and S = 1 (square-pyramidal complex 12b) depending on the nature of axial ligands. Cyclovoltammetry measurements show the possibility to oxidize both complexes twice, with the first oxidation generating the S = 1/2 FeIII complex 13a and the S = 3/2 FeIII complex 13b, and the second removing the electron from the carbazole fragment of the ligand and generating two FeIII -radical cation species. All six complexes were fully characterized using a variety of spectroscopic, structural and magnetochemical techniques, and combination with DFT calculations confirmed the ability of this novel hybrid non-heme system to mimic the typical features of heme-like models. In the last part of the work, a redox series of Fe-NO complexes of the new hybrid ligand was synthesized and initial characterization of the {FeNO}8−6 + {FeNO}6(L●+) series was performed. This new non- heme {FeNO}x series could provide a solid basis to investigate the effect of redox-non innocent ligand scaffold on the electronic structure and reactivity of {FeNO} model
dc.contributor.coRefereeKoszinowski, Konrad Prof. Dr.
dc.contributor.thirdRefereeMata, Ricardo Prof. Dr.
dc.contributor.thirdRefereeStalke, Dietmar Prof. Dr.
dc.contributor.thirdRefereeJohn, Michael Dr.
dc.subject.engBioinorganic Chemistryde
dc.subject.engHeme Enzymesde
dc.subject.engNon-heme Enzymesde
dc.subject.engHydrogen Atom Abstractionde
dc.subject.engElectronic Structurede
dc.subject.engOxoiron Complexesde
dc.affiliation.instituteFakultät für Chemiede
dc.subject.gokfullChemie  (PPN62138352X)de

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