A Mechanistic Study on the Reaction of Non-Heme Diiron(III)-Peroxido Complexes with Benzoyl Chloride

Article abstract of DOI:10.1002/ejic.202100711

Dinuclear iron peroxido complexes are important intermediates for selective oxidation reactions. A detailed kinetic study of the reaction of benzoyl chloride (BzCl) with a dinuclear iron non-heme cis end-on peroxido complex with the ligand EtHPTB (N,N,N′,

Full text of DOI:10.1002/ejic.202100711

European Journal of Inorganic Chemistry
Accepted Article
Title: A Mechanistic Study on the Reaction of Non-Heme Diiron(III)-
Peroxido Complexes with Benzoyl Chloride
Authors: Markus Lerch, Andreas Achazi, Doreen Mollenhauer,
Jonathan Becker, and Siegfried Schindler
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100711
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01/2020

10.1002/ejic.202100711
European Journal of Inorganic Chemistry
FULL PAPER
A Mechanistic Study on the Reaction of Non-Heme Diiron(III)-
Peroxido Complexes with Benzoyl Chloride
Markus Lerch,[a] Andreas J. Achazi,[b] Doreen Mollenhauer,[b] Jonathan Becker,[a] Siegfried Schindler*[a]
[a]
Prof. Dr. S. Schindler, Dr. J. Becker, M. Lerch
Institute of Inorganic and Analytical Chemistry, Justus-Liebig-Universität Gießen
Heinrich-Buff-Ring 17, 35392 Gießen, Germany
E-mail: siegfried.schindler@anorg.chemie.uni-giessen.de
https://www.uni-giessen.de/fbz/fb08/Inst/iaac/schindler
Prof. Dr. D. Mollenhauer, Dr. A. Achazi
[b]
Institute of Physical Chemistry, Justus-Liebig Universität Gießen
Heinrich-Buff-Ring 17, 35392 Gießen, Germany
Supporting information for this article is given via a link at the end of the document.
Abstract: Dinuclear iron peroxido complexes are important
intermediates for selective oxidation reactions. A detailed kinetic study
of the reaction of benzoyl chloride (BzCl) with a dinuclear iron
non-heme cis end-on peroxido complex with the ligand EtHPTB
(N,N,N′,N′-tetrakis[(N-ethyl-2-benzimidazolyl)methyl]-2-hydroxy-1,3-
diamino-propane) had been performed. The starting complex, the iron
peroxido complex, can be obtained either by reaction of the iron(II)
complex with O2 or instead, applying the corresponding iron(III)
complex together with hydrogen peroxide. Using low temperature
stopped-flow measurements allowed to obtain activation parameters
and in combination with a Hammet plot it was possible to postulate a
mechanism for the formation of a perbenzoate complex prior to its
decomposition. Furthermore, the direct reaction of the dinuclear
iron(III) EtHPTB complex with peracetic acid was analyzed.
Additionally, in comparison a mononuclear non-heme iron complex
with the ligand bztpen (N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethane-
1,2-diamine) was investigated as well.
co-ligand).[5c] While under these conditions the formed peroxido
complex is only persistent for a short time, it is possible to
increase its stability by preparing it from the corresponding iron(III)
complexes, [Fe2(RHPTB)(X)m]n+ in combination with hydrogen
peroxide,[5b] This reaction was investigated in acetonitrile as well
as in methanol leading to the same results.[6] Furthermore, the
reaction
of
the
mononuclear
iron(III)
complex
[Fe(bztpen)(OMe)](ClO4)2, (3(ClO4)2; bztpen = N-benzyl-N,N',N'-
tris(2-pyridylmethyl)ethane-1,2-diamine), with hydrogen peroxide
had been analyzed.[7] An end-on hydroperoxido complex was
formed that could be transformed into a side-on peroxido species
after the addition of a base.[7b] Interestingly, the corresponding
iron(II) complex did not react with dioxygen (neither with carbon
monoxide), only with nitrogen oxide a reaction occurred.[7c]
Introduction
Scheme 1. a) MeBzim-Py, b) RHPTB (R = Et: EtHPTB), c) bztpen.
Selective oxidation/oxygenation reactions of organic substrates
are important in the lab and in industry.[1] Efforts to substitute
oxidants that are either expensive and/or quite toxic, e.g.
chromium(VI) compounds, led to an intensive research on the
development of functional model complexes for the active sites of
metalloenzymes, especially oxygenases.[2] For example
cyanobacterial aldehyde deformylating oxygenase (ADO)
catalyzes the conversion of fatty aldehydes into alkanes and
formate.[3] Recently Kaizer and co-workers reported the complex
[FeIII2(µ-O2)(MeBzim-Py)4(MeCN)2]4+ as a functional model for this
enzyme by applying an iron complex with the ligand MeBzim-Py
(2-(2'-pyridyl)-N-methylbenzimidazole; Scheme 1). In that context
they furthermore pointed out the mechanistic versatility of
peroxido-diiron(III) intermediates.[4]
With regard to the formation of percarboxylate species as
important intermediates in oxygenation reactions we report herein
our results on the reaction of the iron(III) peroxido complex with
EtHPTB as ligand and benzoyl chloride (BzCl) together with some
related reactions.
Results and Discussion
The iron(III) complex [Fe2(EtHPTB)(OH)2(MeOH)2](ClO4)3
(1b(ClO4)3) was prepared in good yields by reacting the ligand
EtHPTB with iron(III) perchlorate and Et3N in methanol. The
molecular structure of 1b is shown in Figure 1 (crystallographic
data of 1b(ClO4)3 are reported in the Supporting Information;
Figure S2). The molecular structure of this complex is very similar
to the complex [Fe2(EtHPTB)(H2O)2(OMe)(MeOH)]-(ClO4)4 that
had been reported previously by Avenier et al.[8] Here, water
molecules are coordinated instead of hydroxide anions and one
We and others have been investigating in great detail the
reactivity of iron complexes with the ligand RHPTB (Scheme 1)
towards dioxygen and hydrogen peroxide.[5] The dinuclear iron(II)
complex [Fe2(EtHPTB)(OBz)]2+ (1a, R = Et; EtHPTB = N,N,N′,N′-
tetrakis[(N-ethyl-2-benzimidazolyl)methyl]-2-hydroxy-1,3-
diamino-propane) reacts with O2 to a cis end-on peroxido complex
[Fe2(EtHPTB)(µ-1,2-O2)(OBz)(X)]n+
(2a,
X = solvent
or
1
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10.1002/ejic.202100711
European Journal of Inorganic Chemistry
FULL PAPER
of the methanol ligands is deprotonated. A direct comparison of
the two structures reveals that the angle spanned between the
iron centers and the alcoholate oxygen is somewhat larger (angle
Fe1-O1-Fe2: Avenier 131.1°; this work 128.5°) and this also
results in a greater distance between the iron centers (distance
Fe1-Fe2: Avenier 3.65 Å; this work 3.61 Å). In contrast to our
synthetic protocol described herein, the authors did not apply
triethylamine to prepare the iron(III) complex and did not
recrystallize the product. Furthermore, 1b can be compared with
[Fe2(HPTB)(µ-OH)(NO3)2](NO3)2 that had been investigated
previously.[9]
Figure 3. Time-resolved UV-vis spectra (Δt = 0.03 s) of the reaction between
complex 1b (c = 0.4 mmol L-1) and hydrogen peroxide (c = 20.0 mmol L-1) at
25.0 °C in methanol (concentrations after mixing). The inset shows the
time-trace of the reaction.
Absorbance vs. time traces under pseudo-first-order conditions
([H2O2] >> [1b]) could be fitted using single exponential functions.
As observed previously a plot of the obtained rate constants kobs
vs. the hydrogen peroxide concentration at different temperatures
exhibited a linear dependence without an intercept (Figure 4) and
thus leading to an overall second-order rate law:
Figure 1. Complex 1b [Fe2(EtHPTB)(OH)2(MeOH)2]3+ (left). Molecular structure
(ORTEP drawing; 50% probability) of the cation of 1b(ClO4)3 x 3 MeOH.
Uncoordinated 3 perchlorate anions and 3 methanol molecules are omitted for
clarity (right).
Reaction of 1b with H2O2
When a solution of 1b(ClO4)3 was reacted with hydrogen peroxide
in methanol, a color change from a pale yellow color to a turquoise
color was observed, which indicates the formation of an iron
peroxido complex (2b, Scheme 2) as described previously
(Figure 2).[5b]
(eq. 1)
An Eyring plot of the derived second-order rate constants over a
much larger temperature range from -50.0 to +25.0 °C (Figure 5)
allowed the calculation of the activation parameters to
ΔH≠ = 46 ± 1 kJ mol-1
and
ΔS≠ = -40 ± 2 J mol-1 K-1.
Data
compare reasonable well with previous results of the
[Fe2(HPTB)(µ-OH)(NO3)2](NO3)2 with ∆H≠ = 53 ± 3 kJ mol-1 and
∆S≠ = -17 ± 1 J mol-1 K-1 (see Table 1).[5b]
Figure 2. Reaction of 1b with hydrogen peroxide, followed by the addition of
benzoyl chloride.
ESI-MS measurements support the assumption that the formation
of
the
peroxido
complex
{[Fe2(EtHPTB)(µ-1,2-
O2)(OH)(H2O)]+MeOH}2+ is the main species under these
conditions (compare Figure S5 and S6). One equivalent of H2O2
is enough for complete formation of 2b (Figure S9). The reaction
in methanol is quite fast (completed within 1 s at RT) and the
turquoise-colored solution is stable for a few minutes under these
conditions. As previously described, this reaction can be followed
by applying stopped-flow techniques.[5b] Time-resolved spectra
are shown in Figure 3.
Figure 4. Plot of the rate constants kobs versus H2O2 concentration at different
temperatures. Complex concentration: c = 0.2 mmol L-1. Second-order rate
constant k2(25°C) = 568 ± 2 L mol-1 s-1.
The peroxido complex is characterized by the main absorbance
at 605 nm with
a
molar attenuation coefficient
ε
of
2200 L mol-1 cm-1 under our conditions which compares well with
data of the literature with other solvents.[5b,5c]
2
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10.1002/ejic.202100711
European Journal of Inorganic Chemistry
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linear correlation, it was necessary to plot kobs vs. c(BzCl)2
(Figure 7) leading to a third-order rate law with kobs = k3 c(BzCl)2
(Equation 2).
(eq. 2)
Third-order rate constants obtained here (k3(25°C) = 9.7
± 0.3 × 105 L2 s-1 mol-2) compare well with those measured for
other reactions that showed a third-order rate law e. g. NO + O2
with
a
rate constant of 2.1 × 106 L2 s-1 mol-2 at 25 °C[10]
(depending on the conditions slightly different values)[11] in
aqueous solutions and non-aqueous media were obtained.[12]
Figure 5. Eyring plot for the reaction of complex 1b (c = 0.4 mmol L-1) with H2O2
(c = 20 mmol L-1) in the temperature range from -50.0 to 25.0 °C.
Reaction of 2b with benzoyl chloride
The reaction of the peroxido complex 2b with BzCl was
investigated in methanol because no reaction was observed in
acetonitrile (only decomposition of 2b was detected).
Furthermore, we did not assert an influence of different R-groups
in RHPTB (Scheme 1) on the reactivity. Iron(III) complexes with
the unsubstituted ligand HPTB (R = H) and BnHPTB (R = Bn)
showed the same reactivity towards H2O2 and the subsequent
reaction with benzoyl chloride. As shown in Figure 2 a rapid color
change to a gold-brown-colored solution was observed when 2b
was reacted with benzoyl chloride in excess. Time-resolved
UV-vis-spectra were recorded in stopped-flow measurements at
low temperatures (Figure 6).
Figure 7. Plot of kobs vs. c(BzCl)2 at different temperatures. Concentration
c(2b) = 0.2 mmol L-1.
Since the sole kinetic interpretation does not yet provide any
evidence that exactly two benzoyl chloride molecules react with
the peroxido complex, a substoichiometric addition was also
carried out here. A clear linearity can be seen up to the second
equivalent benzoyl chloride added (Figure S10). This observation
supports the assumption that exactly two molecules of benzoyl
chloride react with 2b during the reaction with benzoyl chloride.
Furthermore, to exclude that benzoyl peroxide is an active
reagent herein (it is synthesized in industry from benzoyl chloride
with hydrogen peroxide in alkaline solutions), several test
reactions with ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-
sulfonic acid) diammonium salt) were performed. ABTS is a
reagent that is used to detect radicals and selectively
peroxycarboxylic acids in the presence of H2O2.[13] Benzoyl
peroxide reacts rapidly with ABTS, however, much slower in
comparison with the reaction of complex 2b with BzCl and
hydrogen peroxide. BzCl together with hydrogen peroxide in
methanol with a small amount of NaOH only reacts quite slowly
with ABTS. Benzoyl peroxide itself does not react with complex
2b. These experiments clearly indicate that the reaction occurs
with BzCl.
Figure 6. Time-resolved UV-vis spectra (Δt = 7.4 s) of the reaction between 2b
(c = 0.2 mmol L-1, obtained by premixing 1b with an excess of H2O2) and BzCl
(c = 20.0 mmol L-1)
at
-80.0 °C.
Molar
attenuation
coefficient
ε445 = 4300 L mol-1 cm-1. The inset shows the time-traces of the reaction.
The decay of the absorbance maximum at 605 nm of 2b
(premixed immediately prior to the measurement applying an
excess of H2O2) can be observed while at the same time a new
compound is formed, characterized with an absorbance
maximum at 445 nm. Two isosbestic points are observed at 415
and 515 nm. Absorbance vs. time traces again could be fitted
perfectly well to a single exponential function, however a plot of
kobs vs. c(BzCl) turned out to be nonlinear. Instead, to obtain a
From these results we propose that an intermediate, I1, is formed
from 2b and BzCl. The Intermediate I1 stands in a rapid
equilibrium with the peroxido complex 2a which then reacts in a
much slower reaction (rate-determining step, RDS) to the
metastable product I2 according to Scheme 2. Other third-order
reactions in solution have been described previously (e.g.)[14] with
a similar mechanistic scenario. After the very fast formation of I2,
3
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10.1002/ejic.202100711
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plot leads to a V-shape from the two linearly fitted points that
intersect near by a σ-value of 0 (unsubstituted BzCl).
much slower decomposition reactions took place including the
formation of products P such as benzoic acid, methyl benzoate,
benzoic acid anhydride as well as iron(III) chloride (detected as
[FeCl4]-). These products could be identified by ESI-MS
measurements. An ESI-MS measurement of a dilute mixture of
2b with BzCl in a ratio of 1:2 (measurement performed instantly
after
mixing)
showed
the
possible
formation
of
direct
[Fe2(EtHPTB)(OOBz)(OH)2]2+ with m/z = 502.1 as
a
derivative of I1 (Figure S7). The formation of methyl benzoate and
benzoic acid anhydride could be a hint of a free radical
mechanism (iron(IV)oxido/benzoato radical-pathway), as these
products are due to recombination reactions.
Figure 8. Hammett plot for the reactions of 2b with para-substituted
BzCl-derivatives (substituent = -N(Me)2, -OMe, -Me, -Cl, -NO2) at -60.0 °C in
MeOH.
Overall it is obvious, that electron-withdrawing (pCl, pNO2) and
electron-donating (pN(Me)2, pOMe, pMe) substituents slowed
down the reaction. The V-shape indicates a change in the reaction
mechanism, in which the electron-rich pN(Me)2-BzCl is more
nucleophilic (ρ = -1.996) and the electron-poor pNO2-BzCl is
more electrophilic (ρ = 0.477). This provides valuable information
about the location of the RDS and transition state (TS). The strong
negative ρ-value clearly indicates a build-up of a positive charge
in the TS of the RDS, whereas the slightly positive ρ-value
indicates a build-up of a positive charge in the TS of the RDS. In
combination with the slightly positive activation entropies of the
reaction between 2b and pNO2-BzCl resp. pN(Me)2-BzCl
(indicating a dissociative interchange mechanism) two different
reaction pathways can be proposed. A possible mechanism for
both reaction pathways is presented in Scheme 3.
Scheme 2. Proposed reaction mechanism for the formation of I2.
For a more detailed analysis we applied para-substituted BzCl
derivatives with the electron-withdrawing groups -Cl, -NO2 and
electron-donating groups -N(Me)2, -OMe, -Me for the reaction with
2b. Besides BzCl (temperature range between -80.0 and 0.0 °C)
we performed measurements with pN(Me)2-BzCl and pNO2-BzCl
in a temperature range between 15.0 and 50.0 °C. From the
temperature dependence the corresponding Eyring plots
(Figure S11) allowed calculations of the activation parameters
with BzCl: ΔH≠ = 40 ± 1 kJ mol-1 and ΔS≠ = 10 ± 3 J mol-1 K-1;
pN(Me)2-BzCl: ΔH≠ = 69 ± 1 kJ mol-1, ΔS≠ = 39 ± 2 J mol-1 K-1;
pNO2-BzCl: ΔH≠ = 67 ± 3 kJ mol-1, ΔS≠ = 21 ± 8 J mol-1 K-1 (see
Table 1). The slightly positive activation entropies indicate a
dissociative interchange mechanism.
Hammett Plot
The Hammett correlation (a linear free energy relationship) is a
widely used method to evaluate the impact of electrosterical
effects on reactions. This correlation allows conclusions to be
drawn about the transition state of the rate-determining step
(RDS) of the observed reaction. Figure 8 shows the Hammett plot
for the reaction of 2b with para-substituted benzoyl chloride. In
this study only para-substituted benzoyl chlorides were used, due
to possible steric hindrance of meta-substituents. The Hammett
Scheme 3. Postulated mechanisms in dependence on different benzoyl
chloride derivatives (electron-withdrawing or electron-donating groups in para
position).
In the reaction pathway with pN(Me)2-BzCl, the C-Cl bond is
severely weakened or even broken in the TS of the RDS, which
is due to the high positive charge on the carbonyl carbon. On the
4
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10.1002/ejic.202100711
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other hand, the slightly positive ρ-value in the pNO2-BzCl reaction
this point it could be possible that to some part acetonitrile is
coordinated that needs to dissociate prior to the bonding of BzCl,
leading to a reaction step independent of c(BzCl) and thus
causing an intercept. However, it also could be caused by the
equilibrium between 2a and I1.
pathway can be interpreted as
a partially accomplished
nucleophilic attack on the carbonyl carbon of the benzoyl chloride.
In any case, it can be confirmed that benzoyl chloride is actively
involved in the RDS of the whole reaction sequence.
In contrast to our system, no effect on para-substitution was
observed with the system of Kripli and co-workers, where a
ρ-value of zero was obtained.[4] Therefore, for their reaction
mechanism a nucleophilic attack of the peroxido-diiron(III)
complex as rate-determining step had been excluded.
Table 1. Summary of molar attenuation coefficient, rate constants and activation
parameters of the kinetically investigated reactions in this study.
Figure 9. Time-resolved UV-vis spectra (Δt = 0.14 s) acquired from
a
double-mixing stopped-flow experiment in MeOH (5 % MeCN) as cosolvent.
The intermediate 2a was generated by mixing 75 µL of solution 1a
(c = 0.4 mmol L-1) with 75 µL oxygen-saturated methanol solution
(c = 10.4 mmol L-1). The first mixed solution was aged for 3 s. In the second
mixing the generated peroxido complex 2a reacted with 75 µL BzCl solution
(c = 40 mmol L-1) at -70 °C to trace the chromophore at λmax = 445 nm
(intermediate I2) and the decay of the peroxido complex 2a at λmax = 605 nm.
The inset shows the time-traces over the course of 10 s.
ND = not determined. [a] k3 third-order constant. [b] at -60 °C. [c] at -65 °C.
Reaction of iron(II) complex 1a with O2 and benzoyl chloride
As described in the introduction, the iron(II) complex
[Fe2(EtHPTB)(OBz)]2+ (crystallographic data of 1a(BPh4)2 are
reported in the Supporting Information; see Figure S1) reacts with
dioxygen to form the peroxido complex 2a (Scheme 2). Therefore,
to gain further support for the proposed mechanism of the reaction
of 2b with BzCl we also investigated the reaction of 2a with BzCl
(the benchtop experiment is reported in the Supporting
Information, Figure S12 and S13). To accomplish this we used a
double-mixing stopped-flow unit with four syringes (syringe 1:
solution of 1a; syringe 2: solvent saturated with dioxygen; syringe
3: solvent saturated with argon; syringe 4: solution of BzCl).
Because of the poor solubility of 1a(BPh4)2 in MeOH, the reaction
was performed with 5 % MeCN as co-solvent. Time-resolved
UV-vis spectra of this reaction are presented in Figure 9.
DFT calculations
To gain further insights into the reaction from 2b with benzoyl
chloride via the (assumed) I1, 2a and (assumed) I2 to give a
possible product P, a computational analysis was performed. The
equilibrium geometries and Gibbs energies of the involved
complexes in solution were calculated with DFT for the
concentrations used in the experiment (see Computational
details). The results of the calculations are shown in Scheme 4.
A plot of the kobs-values vs. BzCl-concentration allowed a linear fit
with an intercept
C
(Figure S13) leading to an overall
second-order rate law:
(eq. 3)
The reaction is a bit faster than the reaction of 2b with BzCl.
However, this is easy to understand because here a benzoate is
already coordinated to the complex and therefore only has to
react further with BzCl. Overall, this supports our postulated
mechanism (Scheme 2). The reaction was measured in a
temperature range between -70.0 and -40.0 °C and from an
Eyring plot (Figure S14) of the second-order rate constants the
activation
parameters
could
be
calculated
to
ΔH≠ = 36 ± 1 kJ mol-1 and ΔS≠ = -33 ± 6 J mol-1 K-1 (see Table 1).
The slightly negative activation entropy indicates an interchange
associative mechanism, again in accordance with our postulated
mechanism. The intercept we observed for the plot of the
kobs-values vs. c(BzCl) (Figure S14) is a bit more difficult to
interpret. While we think we can exclude a reversible reaction at
Scheme 4. Gibbs energies relative to 2b in conjugation with BzCl. c(BzCl) =
0.04 mol L-1. The electronic energies are calculated with B3LYP*-D3(BJ)/def2-
TZVP COSMO(methanol). The ro-vibrational contributions are calculated with
5
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PBE0-D3(BJ)/def2-SVP, def2-TZVP. All iron complexes are in an undecet spin
state (S = 5).
acid was prepared according to the synthesis reported by Sitko
et al., however, so far no crystal structure of this compound had
been reported.[15] We obtained crystals of perdecanic acid and the
molecular structure together with crystallographic data are
presented in the Supporting Information (Figure S4 and
Table S4).
The low-spin state with 0 unpaired electrons (singlet state with
total spin quantum number state S = 0), the triplet state (S = 1),
quintet state (S = 2), the septet state (S = 3), nonet state (S = 4)
and the high-spin state with 12 unpaired electrons (tridecet state
with total spin quantum number S = 6) were energetically
unfavorable for 2b, I1-a, 2a and I2-a. Instead, the undecet spin
state (S = 5) is most favorable for these iron complexes. The other
iron complexes (Scheme 4) are very similar to 2b, I1-a, 2a and
I2-a; therefore, we calculated them in an undecet spin state
(S = 5) as well. For the first step we assume that 2b reacts to I1-a
(see Scheme 4). The reactions is exergonic and a Gibbs energy
of -48.1 kJ mol-1 is gained. Three additional isomers (I1-b to I1-d)
have been calculated for this step but all are energetically less
favorable. I1-a and I1-c are in agreement with the structures
proposed in Scheme 2. The benzoate radical in complexes I1-c
and I1-d is only loosely attached to the rest of the complex via
p-p interaction and C-H···O hydrogen bonds. Thus, the iron atoms
remain FeIII. Furthermore, the spin state of the benzoate radical
should have nearly no effect on the rest of the complex due to the
large distance or loose attachment. Indeed the nonet spin state
(S = 4) for I1-c and I1-d is energetically only +0.1 kJ mol-1 higher
than the undecet spin state (S = 5) for the same molecular
structure.
When 1b was reacted with an excess PAA in methanol, an
orange-brown-colored solution formed (Figure S15) and peracetic
acid adduct compounds could be detected in an ESI-MS spectrum
(Figure S8). A m/z ratio at 1123.1 shows the presence of the
peracetato complex [Fe2(EtHPTB)(OOAc)(OH)](+2ClO4)+. In
Figure 10 the characteristic UV-vis bands of the intermediates
(1a + O2 + BzCl, 1b + H2O2 + BzCl and 1b + PAA) are compared.
Comparison with data from the literature for UV-vis data for
iron(III) peroxyacetate complexes (in acetonitrile) are in
accordance with our data and therefore also support our
assignment of a peroxybenzoate complex discussed above.[16]
Efforts to crystallize and structurally characterize the
intermediates I1/I2 have been unsuccessful. So far (to the best of
our knowledge) only one example of a structurally characterized
mononuclear non-heme iron(III) complex with a coordinating
percarboxylic acid (PAA) had been reported.[17] However its
UV-vis spectrum is completely different to the other known
peroxyacetate iron complexes.
Next an intramolecular formation takes place in I1-a, water is
released, methanol binds to one iron atom resulting in 2a. The
reaction is exergonic and a Gibbs energy of -78.0 kJ mol-1 is
gained. From there, we assume that 2a reacts to I2-a and a Gibbs
energy of -33.8 kJ mol-1 is gained. The shortest bond distance
between an oxygen atom of the benzoate radical and the FeIV=O
group of the undecet I2-a is 2.99 Å. This is a very long, and
therefore, a weak bond. For comparison the bond distance
between the benzoate anion and the Fe-Cl group of the undecet
I2-a is only 1.96 Å. Like for I1-c and I1-d, the spin state of the
loosely attached benzoate radical has nearly no effect on the rest
of the I2-a complex. Hence, the nonet spin state is energetically
very close to the undecet spin state (only +0.5 kJ mol-1 higher for
the same molecular structure). As shown in Scheme 4, the
benzoate radical in I2-a can react to a peroxybenzoate (I2-b). This
intramolecular reaction is exergonic by -21.5 kJ mol-1 and both the
third oxygen of the peroxybenzoate and the chloride form weak
bonds to the iron, which is connected to the peroxybenzoate.
These results are in agreement with the proposed reaction in
Scheme 2.
Figure 10. Comparison of the UV-vis spectra. The spectra of the monitored
reactions were used, which showed the maximum absorption at 445 nm and
605 nm.
Time-resolved UV-vis spectra of the reaction of 1b with an excess
of PAA (formation of I2’) at low temperatures are reported in the
Supporting Information (Figure S15). A plot of kobs values vs.
c(PAA) could be fitted linear through the origin (Figure S16)
leading to a second-order rate law (Equation 4).
A possible next step is the intramolecular formation of a
hypochlorite (P-a). The hypochlorite can reorient itself forming
P-b, which is the most stable complex found in this study. The
reaction from 2a over I2-a, I2-b, and P-a up to P-b is
thermodynamically favorable. The formations of I2-a, I2-b, and
P-a involve either the breaking or formation of an oxygen-oxygen
bond as a key step. Thus, possible reaction barriers can be
expected to be similar in all these cases.
(eq. 4)
An Eyring plot at temperatures between -70.0 and -40.0 °C
(Figure S17)
provides
the
activation
parameters:
ΔH≠ = 32 ± 1 kJ mol-1 and ΔS≠ = -48 ± 3 J mol-1 K-1 (see Table 1)
indicating an interchange associative mechanism.
Reactions of 1b with peracids
To gain a better understanding of the overall mechanistic scenario
1b was furthermore reacted with percarboxylic acids. We
excluded perbenzoic acid due to the danger involved with this
compound and chose peracetic acid (PAA), m-chloroperbenzoic
acid (mCPBA) and perdecanoic acid (PDA) instead. Perdecanoic
Reaction of 1b with mCPBA was sluggish and was not
investigated further.
A similar observation was described
previously by Furutachi and co-workers.[17] PDA was applied in
order to be able to react a highly pure peracid with complex 1b
6
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10.1002/ejic.202100711
European Journal of Inorganic Chemistry
FULL PAPER
and thus to exclude other interfering components such as water.
When analyzing time-resolved UV-vis spectra, only the formation
of a peroxido adduct could be detected at low concentrations of
percarboxylic acids. This is probably due to the direct oxidation of
the coordinated hydroxides. However, applying high
concentrations of the peracids PAA, mCPBA or PDA the
formation of gold-brown-colored product solutions was observed.
While the PAA adduct is stable for weeks at low temperatures the
solutions of the mCPBA and PDA adducts turned to a blue green
colored solution at -80 °C in less than an hour. Furthermore, PAA
as well as mCPBA were tested for their reaction towards ABTS.
The reaction turned out to be quite slow: about 3 min for peracetic
acid and approximately 10 min for mCPBA.
under aerobic conditions in methanol by combining
bztpen,Fe(ClO4)3 and Et3N in a ratio of 1:1:1. Crystals were
obtained and the molecular structure together with
crystallographic data are reported in the Supporting Information
(Figure S3 and Table S3). The molecular structure compares well
with [FeIII(bztpen)(OMe)](PF6)2 reported previously.[18] It is well
known from previous work that this complex reacts with hydrogen
peroxide to a purple-colored end-on hydroperoxido complex,
[Fe(bztpen)(OOH)]2+.[7b] Time-resolved UV-vis spectra of this
reaction are reported in the Supporting Information (Figure S21)
and kinetic data fit well with our previous results. However, in
contrast to the reaction of 2b with BzCl the reaction of
[Fe(bztpen)(OOH)]2+ with BzCl is complex (time resolved UV-vis
spectra of this reaction are reported in the Supporting Information,
Figure S22), indicating additional reactions take place that so far
could not be analyzed.
Reactions with NEt3
If the percarboxylate complex formed by the reaction of 2b + BzCl
was reacted with NEt3, a peroxido complex formed (the benchtop
reaction is shown in Fig S18). Time-resolved spectra of the
reaction of NEt3 with premixed 2b + BzCl were obtained again
with our double-mixing stopped-flow unit with four syringes and
are presented in Figure 11. A plot of kobs vs. [NEt3] showed a linear
dependence with intercepts (Figure S19).
In a subsequent reaction with a base such as Et3N the
hydroperoxido complex reacts to the corresponding side-on
peroxido complex (turquoise). Interestingly, while the
hydroperoxido complex reacts quite fast with benzoyl chloride the
side-on peroxido complex did not react at all with BzCl.
Summary and Conclusion
In summary, we have studied the kinetics of the formation of
peroxido complex 2b (FeIII complex 1b + H2O2) and the
subsequent reaction with benzoyl chloride in methanol in detail.
We assume that the chromophore formed with λmax = 445 nm is
an iron(IV)oxido/carboxylato radical species I1 that alternatively
had been described as an iron(V)oxido-/carboxylato species (for
PAA systems) according to the following equilibria presented in
Scheme 5.[16a,16b,16c,16e,16f,16g,16h,16i,19]
Figure 11. Time-resolved UV-vis spectra (Δt = 0.42 s) acquired from
a
double-mixing stopped-flow experiment in MeOH. The intermediate I2 was
generated by mixing 75 µL of a premixed solution of 2b (c = 0.4 mmol L-1) with
75 µL H2O2 solution (c = 20 mmol L-1). The first mixed solution was aged for 5
s. In the second mixing the generated intermediate I2 reacted with 75 µL of an
Et3N solution (c = 80 mmol L-1) at 0 °C to trace the decay at λ = 445 nm (I2) and
the formation of a peroxido complex at λmax = 550 nm. The diagram in the right
corner shows the time-trace over the course of 20 s.
Scheme
iron(III)/percarboxylato-,
iron(V)oxido/carboxylato-species.
5.
Illustration
of
the
possible
equilibria
radical-
between
and
iron(IV)oxido/carboxylato
This indicates a parallel or back reaction, most likely based on an
acid/base equilibrium under the conditions applied.
It turned out that the peroxido complex 2a, formed in the reaction
between FeII complex 1a and O2, was also able to react in the
same way to form the chromophore at λmax = 445 nm.
An Eyring plot obtained from the k2 values of the main reaction
(Figure S20) did lead to the activation parameter
ΔH≠ = 49 ± 1 kJ mol-1, ΔS≠ = 9 ± 4 J mol-1 K-1. With an activation
entropy close to 0 this reaction follows an interchange pathway.
The same reaction behavior was observed when premixed
1a + PAA (instead of 2b + BzCl) was reacted with NEt3.
In contrast to previous publications that reported formation of
iron(III)percarboxylates (mainly peracetate) in acetonitrile, our
intermediates I1/I2 herein were formed in the protic solvent
methanol. Detailed kinetic studies were performed for the reaction
of the peroxido complexes with benzoyl chloride and furthermore,
the reaction of the iron(III) complex 1b with peracetic acid.
Reaction of [Fe(bztpen)OOH]2+ with BzCl
From our kinetic investigations including a Hammet analysis we
could propose a mechanism for the formation of the reactive
To compare our results of a dinuclear iron complex system with a
mononuclear iron complex we also investigated the reaction of
the iron(III) complex with bztpen as ligand (Scheme 1). The
complex [Fe(bztpen)(OMe)](ClO4)2 (3(ClO4)2) has been prepared
intermediates, which is supported by
a
thermodynamic
consideration of various intermediates using DFT calculations.
They showed that the iron(III) species are more stable than the
7
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10.1002/ejic.202100711
European Journal of Inorganic Chemistry
FULL PAPER
resolution-of-identity (RI) approximation for
J (MARI-J, with default
species with higher oxidized iron. The intermediates finally
decompose to an almost colorless mixture of products that only in
part could be identified.
parameters), Fermi-smearing, the “multiple grid” m4 for the quadrature of
exchange-correlation terms,[24f] and the dielectric continuum solvent model
COSMO[25] (simulating methanol, relative permittivity εr = 32.6, refractive
index nD = 1.3288) using the Turbomole 7.5 software package.[26] Several
studies have shown that for iron complexes, B3LYP with 15 % HF
exchange (called B3LYP*)[24c] is the density functional that comes closest
to the experimental data.[24c,27] Furthermore, Kepp[27b] showed that the
results for B3LYP* (with 15 % HF exchange) can be improved by adding
the D3(BJ) dispersion correction of the original B3LYP density functional.
The ro-vibrational contributions are calculated for all molecules at the
PBE0-D3(BJ)/def2-SVP, def2-TZVP[24,28] level seminumerical with the
NumForce script including the fast contribution of the solvent in Turbomole
7.3 software package.[29] The def2-SVP[24f,28d] basis set was used for all
atoms except iron. For iron the def2-TZVP[24e] was employed. For MARI-J
the default parameter from older Turbomole versions were used to speed
Experimental Section
General: All solvents were distilled before use. Extra dry and oxygen-free
solvents were distilled using desiccants under an argon atmosphere.
Commercially available chemicals were used without further purification.
Abbreviations of the supplier: Alfa Aesar (AA), CARL ROTH (CR), J&K
Scientific (J&K), PanReac AppliChem ITW Reagents (PRAC), ACROS
ORGANIC (AO), Sigma Aldrich (SA), FluoroChem (FC). Methanol extra
dry
(AO;
99.9 %),
acetonitrile
extra
dry
(AO;
99.9 %),
4-(dimethylamino)benzoyl chloride (J&K; 97 %), 4-methoxybenzoyl
chloride (J&K; 99 %), 4-methylbenzoyl chloride (J&K; 98 %), benzoyl
chloride (SA; 99 %), 4-chlorobenzoyl chloride (J&K; 99 %), 4-nitrobenzoyl
chloride (SA; 99 %), 1,3-diamino-2-propanol-N,N,N',N'-tetraacetic acid
(TCI; 98 %), 1,2-phenylenediamine (AA; 98 %), 2-chloromethyl-pyridine
hydrochloride (FC; 97 %), N-benzylethylenediamine (AA; 98 %),
trifluoromethanesulphonic acid (abcr; 99 %), iron powder (AA; 99 %),
iron(III) perchlorate nonahydrate (SA; 98 %), iron(III)triflate (AA; 90 %),
benzoic acid (AA; 99 %); decanoic acid (TCI; 98.0 %), hydrogen peroxide
(CR; 50 % solution in H2O), peracetic acid (PRAC; 15 % solution in H2O),
urea hydrogen peroxide (SA; 97 %), 2,2'-azino-bis(3-ethylbenzothiazoline-
up calculations: precision: 10-6, lmaxmom: 10, thrmom: 10-18
. The
frequency calculations also verified the found molecular structures as
minima (at the PBE0-D3(BJ)/def2-SVP, def2-TZVP level of theory). At the
B3LYP*-D3(BJ)/def2-TZVP level of theory various spin states (S = 0, S =1,
S = 2, S = 3, S = 4, S = 5 and S = 6) were tested and the undecet spin
state was found to be the most stable.
The same concentration was used as in the experiment starting from the
peroxide complex 2b: Methanol 24.66 mol L-1, benzoyl chloride
0.04 mol L-1, and all the iron complexes 0.4 mmol L-1. The experiments
starting from 2a were performed with an benzoyl chloride concentration of
20 mmol L-1, the results are given in Scheme 4. The concentration is
included via the translational entropy. This results in the following
correction term Gcorr for the Gibbs-Energy:
6-sulfonic
acid)
diammonium
salt
(AA;
98 %).
Pure
iron(II)triflate•diacetonitrile salt was synthesized according to the
literature.[20]
Physical Measurements: 1H- and 13C-NMR spectra were recorded on a
Bruker Avance II 200 spectrometer (1H at 200 MHz; 13C at 50 MHz), Bruker
Avance II 400 spectrometer (1H at 400 MHz; 13C at 100 MHz) and Bruker
Avance III HD 400 spectrometer (1H at 400 MHz; 13C at 100 MHz) in
deuterated solvents using TMS as internal standard. The 1H- and 13C-NMR
spectra were calibrated against the proton and carbon signals of
tetramethylsilane.
(eq. 5)
with the molar gas constant R, the temperature T and the new (Vmnew) and
old (Vmold = 0.024789598 m3) volume of 1 mol of substance.
Synthesis of N,N,N′,N′-Tetrakis(2-benzimidazolylmethyl)-2-hydroxo-
1,3-di-aminopropane (HPTB): The synthesis was performed according to
the procedure by McKee et al.[22] 5.00 g diamino-2-propanol-N,N,N′,N′-
tetraacetic acid (15.5 mmol) and 10.07 g o-phenylenediamine
(93.09 mmol) were grounded together. The powder was taken up in a
250 mL Schlenk-flask and heated to 180 °C over 1.5 h. After cooling to
RT the reddish glass was grounded and dissolved in 250 mL 4 mol L-1 HCl.
After a few minutes a colorless solid precipitated. The solids were filtered
and washed with ice-cold water. The filter residue was dissolved in 250 mL
water. The solution was basified by adding 5 mol L-1 NaOH solution. After
10 min the colorless solid was filtered and washed a few times with water
until the filtrate shows no alkaline reaction. The filter residue was air dried
and then dissolved in 200 mL of hot acetone. At room temperature the
mixture was mixed with 100 mL H2O. The precipitation was completed at
4 °C over night. The solids were filtered and washed with water. The filter
residue was first air-dried and then dried in a desiccator over P2O5 under
vacuum. The product was obtained as voluminous colorless powder
(8.615 g, 14.10 mmol, yield 91 %). 1H-NMR (400 MHz, DMSO-d6, TMS):
δ = 7.60-7.53 (m, 8H, Ar-CH), 7.25-7.18 (m, 8H, Ar-CH), 6.37 (s[br],
Ar-NH+H2O), 4.19 (q, 8H, CH2), 2.81 (q, 2H, CH2), 2.57 (q, 2H, CH2).
13C-NMR (400 MHz, DMSO-d6, TMS): δ = 152.5 (s, Ar-Cq), 136.5 (s,
Ar-Cq), 122.5 (s, Ar-CH), 114.5 (s, Ar-CH), 66.6 (s, CH), 58.6 (s, CH2),
ESI-MS
Measurements:
Electrospray-ionization
MS
(ESI-MS)
measurements in MeOH were performed on a Bruker micro-TOF mass
spectrometer. When measuring any percarboxylato species, the
corresponding solutions were pre-cooled to a temperature just above the
melting point of methanol using liquid nitrogen. After mixing of the complex
and the reagent, the cooled solution was measured immediately.
Stopped-Flow-Technique: Kinetic studies of the reactions of hydrogen
peroxide with iron(III)complexes were recorded on a modified Hi Tech
SF-3L low-temperature stopped-flow unit (modification for possible
double-mixing measurements; Salisbury, U.K.) equipped with a J&M
TIDAS 16-500 diode array spectrophotometer (J&M, Aalen, Germany).
The kinetic data were treated by a global analysis fitting routine using the
program Kinetic Studio (v.4.0.113798, TgK Scientific) and/or by extracting
single absorbance vs. time traces at different wavelengths. These traces
were fitted to single-exponential functions using the integrated J&M
software Kinspec. Hydrogen peroxide solutions were prepared by adding
hydrogen peroxide (50 %, cerimetric titration) with micropipets to the
solution. In order to investigate the Intermediates I1/I2 using the
stopped-flow technique, complex 1b was treated with an excess of H2O2.
This premixed solution is taken up in the stopped-flow syringes and used
as usually. For measurements where moisture and or air/oxygen should
be avoided, the stopped-flow syringes were prepared in a glovebox by
MBraun under an argon atmosphere. The reaction between 1a and O2 and
the subsequent reaction with benzoyl chloride was performed with the
double-mixing stopped-flow option. Dioxygen concentration in a saturated
methanol solution has been reported to be 10.4 mmol L-1 at 25 °C.[21]
51.9
(s,
CH2),
MS
(ESI;
in
MeOH):
m/z = [M + 2Na]2+ = 393.23, [M + H]+ = 611.30, [M + Na]+ = 633.28.
Synthesis of N,N,N′,N′-Tetrakis(2-(1-ethylbenzimidazolyl-methyl))-2-
hydroxo-1,3-di-aminopropane (EtHPTB): In a 100 mL round-bottom
flask 1.344 g fine-powdered NaOH (33.60 mmol) was suspended in 50 mL
tetrahydrofuran. 4.885 g HPTB (8.000 mmol) was added and stirred for
30 h at room temperature to completely dissolve the ligand. Afterwards
10 mL bromoethane (14.6 g, 134 mmol) was added. The mixture was
stirred for 48 h at room temperature. The milky suspension was
evaporated to dryness and the faint yellow residue was extracted 3 times
Computational Details: All structure optimizations were performed with
B3LYP*-D3(BJ)/def2-TZVP,[24]
the
multipole
accelerated
8
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10.1002/ejic.202100711
European Journal of Inorganic Chemistry
FULL PAPER
with 25 mL chloroform. The combined extracts were dried with sodium
sulfate, filtrated and the filtrate was reduced to a minimal volume. The
concentrated solution was added dropwise to 250 mL n-hexane under
vigorous stirring to precipitate the product. The formed suspension was
filtrated, the filter residue was under vacuum. The product was obtained
(m, 3H, 3xAr-CH), 7.32-7.17 (m, 5H, 5xAr-CH), 7.14-7.07 (m, 3H,
3xAr-CH), 3.78 (s, 4H, 2xCH2), 3.72 (s, 2H, -CH2), 3.59 (s, 2H, -CH2),
2.82-2.62 (m, 4H, 2xCH2). 13C-NMR (400 MHz, CDCl3, TMS): δ = 150.0 (s,
Ar-CH), 148.9 (s, Ar-Cq), 136.3 (s, Ar-CH), 128.8 (s, Ar-Cq), 128.2 (s,
Ar-CH), 126.9 (s, Ar-Cq), 122.8 (s, Ar-CH), 121.8 (s, Ar-CH), 121.6 (s,
Ar-CH), 60.8 (s, 2xCH2), 60.6 (s, -CH2), 59.0 (s, -CH2), 52.2 (s, -CH2), 51.4
(s, -CH2). MS (ESI): m/z = [M + H]+ = 424.25.
as
a
colorless powder (5.343 g, 7.391 mmol, yield 92 %). 1H-NMR
(400 MHz, CDCl3, TMS): δ = 7.71-7.64 (m, 4H, 4xAr-CH), 7.25-7.16 (m,
12H, 12xAr-CH), 5.06 (s[br], 1H, OH), 4.20 (m, 16H, 4xCH2+4xCH2), 3.77
(p, 1H, CH), 2.73 (dd, 4H, 2xCH2), 1.11 (t, 12H, 4xCH3). 13C-NMR
(400 MHz, CDCl3, TMS): δ = 151.4 (s, Ar-Cq), 142.3 (s, Ar-Cq), 134.7 (s,
Ar-Cq), 122.6 (s, Ar-CH), 121.9 (s, Ar-CH), 119.5 (s, Ar-CH), 109.5 (s,
Ar-CH), 68.9 (s, CH), 60.3 (s, CH2), 52.0 (s, CH2), 38.2 (s, CH3). MS (ESI):
Synthesis of [Fe(bztpen)(OMe)](ClO4)2 (3): CAUTION! All perchlorate
salts in this study should be handled with care because of their potential
explosiveness. In a 25 mL round-bottom flask 847 mg bztpen (2.00 mmol)
were dissolved in 15 mL MeOH. A solution of 1.03 g Fe(ClO4)3 × 9H2O
(2.00 mmol) in 5 mL MeOH was added dropwise to this mixture. With
vigorous stirring, the solution was triturated with 291 µL Et3N (2.10 mmol).
After 15 min the product was precipitated by adding Et2O to the yellow
solution. The crude product was recrystallized in hot MeOH. The yellow
crystals were filtered off and dried under vacuum. The obtained crystals
were suitable for single crystal X-ray diffraction. The product was obtained
as yellow powder (1.10 g, 1.36 mmol, yield 68 %). MS (ESI):
m/z = [M + MeO]+ = 541.22. Elemental analysis calcd. (%) for
C28H32Cl2FeN5O9(+H2O): C(46.24), H(4.71), N(9.63). Found: C(46.22),
H(4.40), N(9.54).
m/z = [M + 2H]2+ = 362.22,
[M + Na]+ = 745.4060.
[M + 2Na]2+ = 393.17;
[M+H]+ = 723.43;
Synthesis of [Fe2(EtHPTB)(OBz)](BPh4)2 (1a): The synthesis was
performed under inert conditions. 361 mg EtHPTB (0.50 mmol) and
50.5 mg Et3N (0.50 mmol) were dissolved in 10 mL MeOH. A solution of
436 mg Fe(OTf)2 × 2MeCN (1.00 mmol) in 3 mL MeOH was added to the
ligand solution. After 5 min a solution of 61 mg and 50.5 mg Et3N
(0.50 mmol) was added dropwise to the complex solution. A precipitate
was formed immediately. After 20 min the suspension was filtered and the
filter residue was washed with MeOH and Et2O. The crude product was
dissolved in a minimal amount of acetone and stirred with 342 mg NaBPh4
(1.00 mmol) overnight. After the solution was filtered, Et2O was added until
the solution became slightly cloudy. After two days at -30 °C light green
crystals grew, which were suitable for single crystal X-ray diffraction
analysis. The product was obtained as a slightly green powder (414 mg,
Synthesis of perdecanoic acid (PDA): Synthesized according to
literature.[15] In
a 250 mL round-bottom flask 10.0 g decanoic acid
(58.1 mmol) were dissolved in 20 mL conc. sulfuric acid (ω = 98 %). The
mixture was cooled in an ice bath to keep the temperature around 10 °C.
Under stirring and proceeded cooling 5 mL of a H2O2 solution (ω = 50%)
was added dropwise over the course of 20 min. The temperature should
not rise above 20 °C during the addition. After the addition, the mixture
was allowed to stir for another 50 min at 10 °C. The reaction mixture was
then slowly diluted with 75 mL ice-cold water, in which the temperature
should not rise about 30 °C. The product was extracted five times with
15 mL Et2O using a separatory funnel. The combined organic phase was
extracted twice with 15 mL water. The organic phase was dried over
Na2SO4 and the filtrate was removed on a rotary evaporator at 25 °C
(CAUTION!). The colorless residue was dissolved in 100 mL n-hexane and
crystallized at -30 °C. overnight. The crystals were filtered off and rinsed
with cold n-hexane. The product was carefully dried under vacuum. The
product was obtained as colorless crystals (6.77 g, 36.0 mmol, yield 62 %).
1H-NMR (400 MHz, CDCl3, TMS): δ = 11.49 (s[br], 1H, -CO3H), 2.42 (t,
2H, -CH2), 1.70 (quint, 2H, -CH2), 1.40-1.19 (m, 12H, 6xCH2), 0.88 (t,
3H, -CH3). 13C-NMR (400 MHz, CDCl3, TMS): δ = 174.7 (s, Cq), 31.8
(s, -CH2), 30.4 (s, -CH2), 29.3 (s, -CH2), 29.2 (s, -CH2), 29.1 (s, -CH2), 28.9
(s, -CH2), 24.6 (s, -CH2), 22.7 (s, -CH2), 14.1 (s, -CH3). MS (ESI):
m/z = [M + H]+ = 189.15, [M + Na]+ = 211.13.
0.26 mmol, yield 51 %).
MS (ESI, 0.1 % formic acid):
m/z = [Fe2(EtHPTB)(OBz)(CHO2H)]+ = 1000.37,
[Fe2(EtHPTB)
(CHO2H)]2+ = 439.67.
Synthesis of [Fe2(EtHPTB)(OH)2(MeOH)2](ClO4)3 (1b): CAUTION! All
perchlorate salts in this study should be handled with care because of their
potential explosiveness. In a 250 mL round-bottom flask 2.184 g EtHPTB
(3.000 mmol) were dissolved in 100 mL MeOH. A solution of 3.098 g
Fe(ClO4)3 x 9 H2O (6.000 mmol) in 50 mL MeOH was added to this
mixture. After 5 min 304 mg Et3N (3.00 mmol) was added and stirred for
10 min. To the dark red solution another portion of 607 mg Et3N
(6.00 mmol) in 50 mL MeOH was added dropwise. The orange suspension
was stirred for 30 min and then heated under reflux conditions for 30 min.
After hot filtration of the solution the product was crystallized at room
temperature. The crystals were filtered off and recrystallized twice in hot
MeOH. The yellow crystals were filtered off and dried under vacuum. The
obtained crystals were analyzed by single crystal X-ray diffraction. The
product composition was determined by elemental analysis. The product
was obtained as amber-colored powder (1.439 g, 1.170 mmol, yield 39 %).
MS
(ESI;
in
MeOH):
m/z = [Fe2(EtHPTB)(OMe)3]2+ = 463.17,
The crystallographic data have been deposited with the Cambridge
Crystallographic Data Centre as (CCDC) No. 2088347 (for 3), 2088348
(for 1b(ClO4)3), 2088349 (for 1a(BPh4)2) and 2101789 (for PDA).
[Fe2(EtHPTB)(OMe)2(ClO4)]2+ = 497.14;[Fe2(EtHPTB)(OMe)3(ClO4)]+ =
1025.29. Elemental analysis calcd. (%) for C45H59Cl3Fe2N10O17(+2H2O):
C(42.69), H(5.02), N(11.06). Found: C(42.50), H(4.85), N(11.19).
Synthesis of N-benzyl-N,N',N'-tris(2-methylpyridyl)-ethylenediamine
(bztpen): The synthesis was performed according to the procedure of
Acknowledgements
Duelund et al.[23]
N-benzylethylenediamine (5.00 mmol) and 2.63 g 2-Chloromethylpyridine
hydrochloride (16.0 mmol) were dissolved in mixture of 7.5 mL
dichloromethane and 7.5 mL water. A solution of 1.28 g NaOH (32 mmol)
in 5 mL water was prepared. A third of this solution was added for the first
3 days. After the addition was complete, the solution was stirred for an
additional 3 days at room temperature. The product was extracted 3 times
with 10 mL dichloromethane. The combined organic phase was extracted
with brine. The dichloromethane extract was dried over Na2SO4. The
solvent was evaporated and the crude product was purified by column
In
a
50 mL
Schlenk-flask
751 mg
We
gratefully
acknowledge
support
by
the
a
Justus-Liebig-Universität Gießen. A. J. A. and D. M. wish to
express thanks for the support by the administrators of the
JustHPC-cluster of the Justus-Liebig-Universität, Gießen.
Keywords: Iron(IV)oxido Complexes • Kinetics • Reaction
Mechanism • Substituent Effects • Time-Resolved Spectroscopy
chromatography
with
silica
as
stationary
phase
(eluent:
[1] a) J. Piera, J.-E. Bäckvall, Angew. Chem. Int. Ed. 2008, 47, 3506–3523; b)
J. Dong, E. Fernández-Fueyo, F. Hollmann, C. E. Paul, M. Pesic, S.
Schmidt, Y. Wang, S. Younes, W. Zhang, Angew. Chem. Int. Ed. 2018, 57,
9238–9261.
dichloromethane/MeOH). The product was obtained as a pale yellow solid
(1.36 g, 3.21 mmol, yield 64 %). 1H-NMR (400 MHz, CDCl3, TMS):
δ = 8.52-8.44 (m, 3H, 3xAr-CH), 7.60-7.53 (m, 3H, 3xAr-CH), 7.49-7.39
9
This article is protected by copyright. All rights reserved.

10.1002/ejic.202100711
European Journal of Inorganic Chemistry
FULL PAPER
[2] A. L. Feig, S. J. Lippard, J. Am. Chem. Soc. 1994, 116, 8410–8411.
[3] U. K. Bagha, J. K. Satpathy, G. Mukherjee, C. V. Sastri, S. P. de Visser,
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10.1002/ejic.202100711
European Journal of Inorganic Chemistry
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A rapid reaction between a dinuclear iron(III)peroxido complex and benzoyl chloride was observed in methanol. Time-resolved UV-vis
spectra at low temperatures showed a clean decay of the peroxido complex with simultaneous formation of an intermediate that we
assign as an iron(IV)oxido/benzoato radical species.
11
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