dc.description.abstracteng | The 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 complexes. | de |