Disproportionation Equilibrium of a μ-Oxodiiron(III) Complex Giving Rise to C?H Activation Reactivity: Structural Snapshot of a Unique Oxoiron(IV) Adduct

Article abstract of DOI:10.1002/anie.201900683

μ-Oxodiiron(III) species are air-stable and unreactive products of autoxidation processes of monomeric heme and non-heme iron(II) complexes. Now, the organometallic [(L^NHC)Fe^III-(μ-O)-Fe^III(L^NHC)]⁴⁺ complex 1 (L^NHC is a macrocyclic tetracarbene) is shown to be reactive in C?H activation without addition of further oxidants. Studying the oxidation of dihydroanthracene, it was found that 1 thermally disproportionates in MeCN solution into its oxoiron(IV) (2) and iron(II) components; the former is the active species in the observed oxidation processes. Possible cleavage scenarios for 1 are shown by scrambling experiments and structural characterization of an unprecedented adduct of 1 and oxoiron(IV) complex 2. Kinetic analysis gave an equilibrium constant for the disproportionation of 1, which is very small (K_eq=7.5±2.5×10^?8 m). Increasing K_eq might by a useful strategy for circumventing the formation of dead-end μ-oxodiiron(III) products during Fe-based homogeneous oxidation catalysis.

Full text of DOI:10.1002/anie.201900683

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Accepted Article
Title: Disproportionation Equilibrium of a µ-Oxodiiron(III) Complex
Giving Rise to C–H Activation Reactivity: Structural Snapshot of a
Unique Oxoiron(IV) Adduct
Authors: Franc Meyer, Claudia Cordes (nee Kupper), Massimiliano
Morganti, Iris Klawitter, Claudia Schremmer, and Sebastian
Dechert
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10.1002/anie.201900683
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Disproportionation Equilibrium of a µ-Oxodiiron(III) Complex
Giving Rise to C–H Activation Reactivity: Structural Snapshot of a
Unique Oxoiron(IV) Adduct
Claudia Cordes (née Kupper),# Massimiliano Morganti,# Iris Klawitter, Claudia Schremmer, Sebastian
Dechert, Franc Meyer*[a]
Abstract: µ-Oxodiiron(III) species typically represent the air stable
and unreactive products of autoxidation processes of monomeric
heme and non-heme iron(II) complexes. Herein, we report that the
organometallic [(LNHC)FeIII–(-O)–FeIII(LNHC)]4+ complex 1 (LNHC is a
macrocyclic tetracarbene) shows reactivity in C–H activation without
addition of further oxidants. We studied the oxidation of
dihydroanthracene and found that 1 thermally disproportionates in
MeCN solution into its oxoiron(IV) (2) and iron(II) components; the
former represents the active species in the observed oxidation
processes. Possible cleavage scenarios for 1 are evidenced by
scrambling experiments and by structural characterization of an
unprecedented adduct of 1 and oxoiron(IV) complex 2. Kinetic
analysis gave an equilibrium constant for the disproportionation of 1,
which expectedly is very small (Keq = 7.5 ± 2.5 10−8 M). Increasing
Keq might by a useful strategy for circumventing the formation of
dead-end µ-oxodiiron(III) products during Fe-based homogeneous
oxidation catalysis.
hydrogen atom abstraction (HAA) by 2. Now, further studies
have shown that these HAA processes do not terminate at the
FeIII–O–FeIII species 1, but slowly proceed in subsequent
substrate conversion leading to a final iron(II) product. Our
working hypothesis was that this unexpected reactivity of 1,
which occurs even in the absence of additional oxidant and in
the dark, involves a pre-equilibrium in which 1 disproportionates
into
2
and ferrous
3
(Scheme 1). While
a
thermal
disproportionation equilibrium has recently been reported for a
µ-oxodiiron(IV) complex,[14] it is unusual for µ-oxodiiron(III)
compounds (with few exceptions[5]) and has mostly been
reported to happen upon light irradiation.[2,7,8] Therefore we
decided to compare the reactivity of
1
and
2
using
dihydroanthracene (DHA) as model C-H substrate.
Upon exposure of iron(II) complexes to O2, in many cases
reactive intermediates can be observed, such as monoiron- or
diiron-peroxo as well as high-valent oxoiron(IV) species; these
intermediates have attracted much interest during the last
decades, owing to their relevance in biological and technical
oxidation and oxygenation processes.[1] At rt, however, in case
of molecular systems, these species usually evolve to form
stable µ-oxodiiron(III) products.[2,3] While the resulting FeIII–O–
FeIII compounds have been used as catalysts for conversions
such as transesterification or acylation,[4] they are commonly
rather inert in solution and under aerobic conditions, and their
reactivity and oxidizing potential is typically too low for oxidation
reactions such as C–H activation or oxygen atom transfer
(OAT).[2,5,6] Only upon addition of external oxidants such as H2O2
or peroxy acids, or via photoexcitation,[7,8] can they be re-
activated for catalysis.[9,10]
Scheme 1. Proposed reaction scheme of the C–H activation of DHA mediated
by µ-oxodiiron(III) complex 1.
For oxoiron(IV) species 2 kinetic data are available,[13] and a
k2 value of 0.76 M-1 s-1 at −40 °C was determined for the
selective oxidation of DHA to anthracene; this translates into k2
= 35 M−1 s−1 at rt using activation parameters from Eyring
analysis and DFT calculations.[13] With these data in hand, we
now studied the reaction of 1 with an excess of DHA. UV/vis
spectroscopy was not a useful tool for monitoring the C-H
activation of DHA by 1, because of significant band overlap for 1
(367 nm), anthracene (338, 356, 376 nm), and product 3
(340 nm). The slow reaction was thus followed by NMR
spectroscopy using 10, 15 and 20 equiv. of DHA. In this case,
the rate of anthracene formation is basically not affected by the
substrate concentration (see Figure S14), in agreement with the
rate depending on the pre-equilibrium rather than the excess of
substrate. Chemical equilibria can be manipulated by the
addition of one of the involved species. In the present case,
addition of the parental iron(II) complex 3 was most promising
since it is not an active species for OAT or C–H activation. A
similar strategy was also pursued in a recent study of the
Recently we presented a unique organometallic oxoiron(IV)
compound with macrocyclic tetracarbene (LNHC
)
ligation,
[(MeCN)(LNHC)FeIV=O]2+ (2),[11] whose electronic structure differs
from the one of typical N-coordinated oxoiron(IV) species.[12,13]
This oxoiron(IV) intermediate decomposes at rt yielding the
organometallic µ-oxodiiron(III) complex [(LNHC)FeIIIOFeIII(LNHC)]4+
(1), which is also the product that eventually results from
[a]
Dr. C. Cordes (née Kupper), M. Morganti, Dr. I. Klawitter, C.
Schremmer, Dr. S. Dechert, Prof. Dr. F. Meyer
Universität Göttingen, Institut für Anorganische Chemie
Tammannstr. 4, D-37077 Göttingen (Germany)
E-mail: franc.meyer@chemie.uni-goettingen.de
Homepage: http://www.meyer.chemie.uni-goettingen.de
#
These authors contributed equally to this work
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201900683
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disproportionation of a FeIV-O-FeIV dimer.[14] Addition of iron(II)
complex 3 to the mixture (1.6, 3.2, and 6.4 equiv.) led to a
significant retardation of the reaction, in line with a shift of the
disproportionation equilibrium towards the µ-oxo complex 1
(Figures 1 and S16).
slow
formation
of
the
scrambled
µ-oxo
species
[(Me8LNHC)FeIIIOFeIII(LNHC)]4+ (1Me8) that has different macrocyclic
ligands at the two metal ions (Figures 2 and S8). It should be
noted that, depending on the ionization conditions, splitting of 1
into 2 and 3 is generally observed during mass spectrometry
experiments, similar to what was reported by Collins et al. for
their FeIV-O-FeIV system.[17] Still, a fresh mixture of the two µ-
oxodiiron(III) species 1 and 1Me16 did not show significant signals
corresponding to the scrambled product, indicating that
scrambling occurs gradually within the reaction mixture and not
just during the MS measurement. Because an influence of the
donating properties/nucleophilicity of the solvent on the cleavage
of the Fe−O−Fe bridge might be expected, the scrambling
experiment was also performed using butyronitrile (PrCN) as
solvent; however, only minor differences were observed for the
time dependence of relative MS signal intensities (Figure 2,
right). Unfortunately, the solubility of highly charged 1, isolated
and used as its triflate salt, in less polar solvents is negligible,
which prevents comparative studies in non-donor media.
Figure 1. Plot of initial rates versus concentration of 3 for the reaction of 1 (5
mM) with DHA (35 mM) and different amounts of added 3 at rt. The curve
represents a fit to the data points using equation (i) to determine k1 and k−1 for
the disproportionation of 1.
Since oxoiron(IV) complex 2 abstracts one hydrogen atom
from DHA, two molecules of 2 are needed to form one
anthracene molecule, hence the maximum yield for these
conversions is 0.5 equiv. anthracene with respect to 1 equiv. of
µ-oxo compound 1. NMR monitoring of the reaction in presence
of 1.6 equiv. DHA until completion evidenced a selective
reaction without any side products and with full conversion
(essentially 100% of the expected yield). A plot of the initial
reaction rates, obtained from linear fits to the data points for the
formation of anthracene, versus [3]0 clearly shows the expected
rate decrease with increasing amounts of added 3 (Figures 1
and S16). The dependence of initial rate on [3]0 was fitted using
eq. (i) which allowed for the determination of k1 [s−1] and k−1 [M−1
s−1] as well as the equilibrium constant Keq [M] for the
disproportionation of 1 (eq. (ii)).[14,15]
Figure 2. ESI(+)-MS studies of MeCN and PrCN solutions containing 1 and
1Me16 (4.3∙10−2 mM) with spectra recorded during 180 min after mixing. Left:
selected spectra for PrCN after 6 min, 60 min and 120 min showing peaks for
[(LNHC)FeIIIOFeIII(LNHC)(OTf)3]+
[(Me8LNHC)FeIIIOFeIII(LNHC)(OTf)3]+
(at
(at
m/z
m/z
=
1271),
and
=
1383),
[(Me8LNHC)FeIIIOFeIII(Me8LNHC)(OTf)3]+ (at m/z = 1495). Right: time traces for the
intensity of the peak at m/z = 1383 normalized by its value after 180 minutes
from mixing, for PrCN (black) and MeCN (red); see SI for details.
[
]
푘1푘2 ퟏ [DHA]
In a second scrambling experiment, 1Me16 and oxoiron(IV)
complex 2 (with the non-methylated ligand LNHC) were mixed at
−30°C.[18] ESI mass spectra recorded over 1200 min showed the
gradual formation of 1Me8 (Figures S11). We assume this
scrambling reaction to occur via attack of the FeIV=O unit at a Fe
atom of the FeIII-O-FeIII core, trans to the -oxo ligand (Scheme
2). DFT calculations had shown that the square-pyramidal
coordinated ferric ions in 1 adopt an intermediate spin state
(S = 3/2) with electron configuration (Fe-dxy)2π*(Fe-dxz,yz)2σ*(Fe-
dz²)1σ*(Fe-dx²-y²)0.[16,19] Thus, the antibonding dz² orbital is singly
occupied and five-fold coordination without a MeCN ligand in the
axial position trans to the -O is favored over six-fold
coordination. For comparison, the metal ions in trans-1,2 end-on
disulfido-bridged [(MeCN)(LNHC)FeIIISSFeIII(LNHC)(MeCN)]4+ are
coordinated octahedrally with additional MeCN ligands, in
agreement with their S = 1/2 spin states (electron configuration
(Fe-dxy)2π*(Fe-dxz,yz)3σ*(Fe-dz²)0σ*(Fe-dx²-y²)0), but this disulfido-
bridged iron(III) dimer readily decomposes in solution with loss
of Sx.[19] Consequently, we propose that axial coordination (trans
to the -O) at 1 (or 1Me16) by either MeCN, PrCN or by the
FeIV=O unit of 2 induces cleavage of the Fe–O–Fe bridge.
푟푎푡푒 =
(i)
[
]
푘−1 ퟑ + 푘2[DHA]
1
1.3 ∙ 10−5
푘1
푠
−8
(
)
퐾푒푞
=
=
= 7.5 ± 2.5 ∙ 10
M
(ii)
푙
푘−1
174 푚표푙 푠
As expected, Keq is very small, which explains the huge
difference in reaction time when starting from 1 or 2, and why 2
cannot be detected spectroscopically in solutions of 1 or as an
intermediate during the reactions of 1 with DHA. The value for µ-
oxodiiron(III) complex 1 is similar to Keq = 9.7·10-7 M determined
by Gupta et al. for their µ-oxodiiron(IV) compound.[14] Using the
Gibbs equation (ΔG = –RTlnKeq), this value can be translated
into a Gibbs free energy of about 40 kJ mol-1 (9.6 kcal mol-1) for
the disproportionation of 1 at rt.
To corroborate the presence of
equilibrium a scrambling experiment was performed starting
a disproportionation
from
1
and
a
similar
µ-oxo
with
complex
LNHC ligand
[(Me8LNHC)FeIIIOFeIII(Me8LNHC)]4+ (1Me16
)
a
derivative that has all imidazol-2-yliden groups methylated at the
backside 4- and 5-positions.[16] Recording a series of ESI mass
spectra of a MecN solution containing both 1 and 1Me16 revealed
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We can conclude that in MeCN solution the organometallic
µ-oxodiiron(III) complex 1 is in equilibrium with the oxoiron(IV)
species 2 and the ferrous complex 3. This is a rare example of a
thermal disproportionation equilibrium of a FeIII-O-FeIII system
leading to a reactive high-valent iron-oxo intermediate without
addition of further oxidants.[2,9,10] An unprecedented adduct
between 1 and 2 featuring a FeIV=O∙∙∙FeIII–O–FeIII∙∙∙O=FeIV core
could be isolated and structurally authenticated, which we
propose to visualize the substitution step leading to FeIV=O/FeIII–
O–FeIII scrambling. As a result of the disproportionation, we
observed reactivity (albeit very slow) of the µ-oxodiiron(III)
complex 1 in C–H activation. Kinetic analysis of DHA oxidation
allowed for the determination of the equilibrium constant Keq for
the disproportionation, which expectedly is very small. The
particular ability of NHC ligands to stabilize both high and low
oxidation states of metal ions[22] likely is a key factor favoring the
disproportionation. Molecular design to increase Keq might be a
rewarding strategy for the development of Fe-based
homogeneous oxidation catalysts that circumvent the formation
of off-cycle dead-end µ-oxodiiron(III) products. Indeed, iron
complexes ligated by macrocyclic tetracarbene ligands are now
emerging as rugged catalysts for, e.g., epoxidation and
aziridination chemistry.[23]
Scheme 2. Proposed scenario for the scrambling reactions between 1Me16 and
2, or between 1Me16 and 1 (after partial disproportionation of 1 according to
Scheme 1).
A snapshot visualizing this scenario could be obtained by X-
ray structure analysis of a crystalline adduct [2∙1∙2](OTf)8 with a
FeIV=O∙∙∙Fe–O–Fe∙∙∙O=FeIV core, obtained via
a
co-
crystallization experiment from a mixture of 1 and 2 in MeCN
(Figure 3).[20] 2∙1∙2 contains a central µ-oxodiiron(III) entity (1,
Fe2–O2–Fe2’) flanked by two oxoiron(IV) units (2, Fe1−O1) in
an overall linear array (<(Fe1–O1∙∙∙Fe2) 178.2°); the
components are closely interlocked in the adduct (Figure 3,
bottom).[21] The Fe1−O1 bond (1.673(4) Å) and the Fe2−O2
bond (1.769(2) Å) in 2∙1∙2 are only slightly longer than the Fe-O
bonds in parent 2 (1.661(3) Å) and 1 (1.752(2) Å),[11] respectively,
and the O1∙∙∙Fe2 distance between the two components (3.19 Å)
is much longer. However, it is well conceivable that reshuffling of
the Fe−O bonds (and the MeCN) in an adduct 2∙1 according to
Scheme 2 to give 1∙2 is facile, leading to the observed
scrambling in solution. The structure of the adduct 2∙1∙2
suggests a highly nucleophilic character of the O-atom in the
oxoiron(IV) complex 2, in line with its unusually poor OAT
reactivity towards substrates such as phosphines or
thioethers.[13]
Acknowledgements
This work was supported by the Fonds der Chemischen
Industrie (Kekulꢀ scholarship to C.C.), the DFG (IRTG 1422
“Metal Sites in Biomolecules: Structures, Regulation and
Mechanisms”), and the University of Göttingen.
Keywords: Iron-oxo complexes • C-H activation • N-heterocyclic
carbene ligands • Iron • Kinetics • Organometallic Chemistry
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Figure 3. Structure of the adduct 2∙1∙2 in solid state (top: ball and stick
representation; bottom: space filling model). Anions and hydrogen atoms have
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probability thermal ellipsoids.
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[8]
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material contains besides the adduct the individual compounds 1 and 2
in variable amounts (note that the reactive oxoiron(IV) complex 2
gradually degrades into 1 in solution).
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slightly varied values for kobs, k2 and the concentrations.
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Entry for the Table of Contents
COMMUNICATION
Claudia Cordes (née Kupper),
Massimiliano Morganti, Iris Klawitter,
Claudia Schremmer, Sebastian Dechert,
Franc Meyer*
Page No. – Page No.
Disproportionation Equilibrium of a µ-
Oxodiiron(III) Complex Giving Rise to
C–H Activation Reactivity: Structural
Snapshot of a Unique Oxoiron(IV)
Adduct
Scrambling experiments and kinetic analyses evidence thermal disproportionation
of a tetracarbene ligated a µ-oxodiiron(III) complex in solution, enabling C-H
activation via the resulting oxoiron(IV) intermediate. Structural characterization of a
unique adduct between the oxoiron(IV) and µ-oxodiiron(III) species provides a
snapshot of the scrambling process.
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