Krahe, Oliver Moritz: Experimental and theoretical studies of mononuclear high-valent nitrido-iron and oxo-iron complexes. - Bonn, 2016. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Oliver Moritz Krahe}},
title = {Experimental and theoretical studies of mononuclear high-valent nitrido-iron and oxo-iron complexes},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2016,
month = may,

note = {Nitrido-iron and oxo-iron complexes are important species in manifold biological and industrial processes. While nitrido-iron complexes are intermediates in biological and industrial nitrogen fixation, oxo-iron complexes are known to be active species in biological oxidation reactions. Therefore, in the work presented herein, mononuclear nitrido-iron(V) complexes are studied as well as a novel oxo-iron species that shows distinct differences when compared to other complexes reported so far. The aim of studying the (spectroscopic) properties of those well-defined model systems is to benefit from their understanding when more complex systems are investigated, whose exact structures are not known in detail. Spectroscopic and theoretical methods are applied that individually provide valuable information, but the combination of both facilitates the interpretation of data and permits an even more detailed understanding of the systems. Proposed species can be further confirmed or ruled out by comparison of their calculated spectroscopic properties with experimental results.
Azido-iron(III) complexes that serve as precursors for the photolytic formation of nitrido-iron(V) complexes are studied in Chapter 4. Absorption bands are assigned to various LMCT-transitions by TD-DFT calculations, and bands observed in the visible region are confirmed to originate from azide ligand to metal transitions by resonance Raman spectroscopy. It has been assumed that excitation into these bands results in the elimination of N2, thereby yielding high-valent nitrido-iron(V) complexes, while photolysis with UV-light results in the reductive elimination of an azide radical giving ferrous complexes. However, photolysis experiments with visible (470 nm) and UV-light (304 nm) on frozen solution Mössbauer samples of [FeIII(N3)cyc-ac]PF6 have shown that both wavelengths result in the corresponding nitrido-iron(V) complex, whereas time-resolved IR measurements on fluid solution samples performed after photolysis with UV-light (266 nm) have revealed the formation of a ferrous species and an azide radical. Upon allowing the frozen solution sample of [FeV(N)cyc-ac]PF6 to thaw it immediately decays. Interestingly, the analog complex [FeV(N)TMC-ac]PF6 with methylated cyclam ligand obtained by photolysis of frozen solution samples of the parent azido-iron(III) complex [FeIII(N3)TMC-ac]PF6 was shown to be stable in liquid solution. However, photolysis of the [FeIII(N3)TMC-ac]PF6 complex in liquid solution did not yield the iron(V) complex. The chemoselectivity of photolysis is, therefore, found to be more complex than a simple wavelength dependence, with the collated data indicating that the state of aggregation determines which photoproducts are observed.
Previously reported iron(V) complexes and one new tetragonal nitrido-iron(V) species are studied in detail in Chapter 5. The first nitrido-iron(V) species reported were five coordinate porphyrin complexes, for which an S=3/2 ground state was assumed. The same ground state spin was deduced for the first non-heme iron(V) complex [FeV(N)(N3)cyc]+. However, combining the previously published spectroscopic data with state of the art calculations led to revised S=1/2 ground state assignment.
High-level multiconfigurational ab initio calculations have demonstrated, that even though the used ligand systems are not genuinely symmetric, the nitrido-iron(V) species exhibit a nearly degenerate 2B2[(dxy)2(py*/dyz)1] ground state separated by only very few hundred wavenumbers from the 2B1[(dxy)2(px*/dxz)1] excited state.
In certain transition metal complexes the strong nitrido p-ligand raises the energy of the dxz/dyz-orbitals above the dx2-y2-orbital. However, for the studied tetragonal nitrido-iron(V) complexes this is not the case, as demonstrated by the calculations that result in a 2B2[(dxy)2(py*/dyz)1] ground state and not the alternative 2A1[(dxy)2(dx2-y2)1] state. This is in line with the fully consistent interpretation of the spectroscopic data assuming a 2B2[(dxy)2(py*/dyz)1] ground state.
EPR spectra for a series of tetragonal nitrido-iron(V) complexes (g3g2≈g1 The fleeting nature of tetragonal nitrido-iron(V) complexes rendered their reactivity very challenging to study. However, it was shown that the tetragonal nitrido-iron(V) complexes [FeV(N)cyc-ac]+ and [FeV(N)(N3)cyc]+ decay via a reductive nitrogen coupling mechanism yielding ferrous complexes an dinitrogen, which is the microscopic reverse of the Haber-Bosch process.
A novel oxo-iron(IV) complex [FeIV(O)(NHC)4(EtCN)]2+ was computationally studied and compared to previously published oxo-iron(IV) complexes in Chapter 6. While all previously studied tetragonal oxo-iron(IV) complexes and most other tetragonal oxo-transition metal complexes show the “classical” dxydxz,dyz< dx2-y2< dz2 d-orbital splitting, a dxy< dxz,dyz< dz2< dx2-y2 arrangement is found for the aforementioned species. The new complex [FeIV(O)(NHC)4(EtCN)]2+ and the complex [FeIV(O)(TMC)(MeCN)]2+, which is the most widely studied “classical” system, show a 3A2[(dxy)2(dxz,dyz)2] doublet ground state, however, the excited quintet state was shown to be involved in CH-activation reactions by oxo-iron(IV) complexes. Notably, the distinct d-orbital splitting results in an 5B1[(dxy)1(dxz,dyz)2(dz2)1] excited quintet state for the new complex and 5A1[(dxy)1(dxz,dyz)2(dx2-y2)1] excited quintet state for the complex [FeIV(O)(TMC)(MeCN)]2+.
A computational study of hydrogen abstraction reactivity for the complexes [FeIV(O)(NHC)4(EtCN)]2+ and [FeIV(O)(TMC)(MeCN)]2+ demonstrated significant differences between the two. While for the complex [FeIV(O)(TMC)(MeCN)]2+ and all other existing oxo-iron(IV) complexes the well-established concept of two-state reactivity successfully describes the electronic structure in the underlying mechanism, this is not the case for [FeIV(O)(NHC)4(EtCN)]2+. More specifically, due to its distinct electronic structure of the latter complex in the quintet state, the usual lowest energy 5s-pathway for hydrogen abstraction reaction is not accessible. Instead, for the [FeIV(O)(NHC)4(EtCN)]2+ complex it was shown that the reaction barrier is lowest in the triplet ground state.},

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