Highly oxidized diiron complexes: Generation, spectroscopy, and stabilities

Article abstract of DOI:10.1039/b903500a

Oxidation of the diferric complex [LFe^IIIOFe^IIIL] to the monoradical complex [LFe^IIIOFe^IIIL]⁺ and the diradical complex [LFe^IIIOFe^IIIL]²⁺ is followed by a decay into m

Full text of DOI:10.1039/b903500a

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Highly oxidized diiron complexes: generation, spectroscopy,
and stabilitiesw
Julia Bernhardette Hildegard Strautmann,a Carl-Georg Freiherr von Richthofen,a
Serena DeBeer George,b Eberhard Bothe,c Eckhard Billc and Thorsten Glaser*a
Received (in Cambridge, UK) 18th February 2009, Accepted 23rd March 2009
First published as an Advance Article on the web 7th April 2009
DOI: 10.1039/b903500a
Oxidation of the diferric complex [LFeIIIOFeIIIL] to the
monoradical complex [LFeIIIOFeIIILꢀ]+ and the diradical
complex [LꢀFeIIIOFeIIILꢀ]2+ is followed by a decay into mono-
meric complexes including a highly reactive putative [LFeIVQO]
complex.
recorded during the two-electron oxidation show a continuous
increase in the extinction coefficient at 418 nm. Although the
mono- and the dication are formed one after another, the
spectra exhibit relatively well-defined isosbestic points
implying similar electronic processes for the generation of
1+ and 12+. Re-reduction of 12+ almost restores the spectrum
of 1 (Fig. S1, ESIw), proving the reversibility of this two-
electron oxidation on this timescale. A similar increase in
the absorption at 419 nm was observed during the chrono-
amperometry of monomeric [LFeCl] (2), which reveals the
presence of the ferric diphenoxyl radical species [LꢀꢀFeCl]2+
(22+).10 Thus, 12+ is best described as the diferric diphenoxyl
Methane monooxygenase (MMO) and ribonucleotide reductase
(RNR) are metalloenzymes that activate dioxygen at nonheme
diiron active sites.1 The active intermediates accumulate
oxidation equivalents higher than FeIIIFeIII. While the active
species in MMO, intermediate Q, is proposed to consist of a
high-valent FeIV(m-O)2FeIV core,2 intermediate X of RNR was
radical complex [LꢀFeIIIOFeIIILꢀ]2+
.
first designated a FeIIIFeIII radical species by Mossbauer and
¨
EPR spectra (isotropic EPR signal at g = 2.00).3 But now, the
formulation as a FeIIIFeIV species is commonly accepted.4
There have been numerous efforts to synthesize model
complexes for Q and X.5,6 Our strategy for stabilizing highly
oxidized diiron units involves increasing the electron density at
the iron atoms by using strongly electron-donating ligands7
such as phenolates and tert-amines. Hence, we synthesized the
binuclear ferric complex [LFe(m2-O)FeL] (1, Fig. 1A)8
which exhibits two oxidations at redox potentials that are
uncommonly low for an oxo-bridged binuclear ferric complex
(E1 = 0.27 V, E2 = 0.44 V vs. Fc+/Fc). Herein we present the
generation and spectroscopic characterization of the one- and
the two-electron oxidized species 1+ and 12+, respectively.z
Preliminary electrochemical studies implied that E1 and E2
are quasi-reversible,8 but extensive CV and SW voltammetry
analyses prove E1 and E2 to be reversible (Fig. 1B). The
formation of 1+ was followed by UV/Vis spectroscopy during
a CV of 1 in an OTTLE cell at ꢁ20 1C (Fig. 1C). A new
absorption band at 414 nm arises, consistent with the forma-
tion of the phenoxyl radical complex [LFeIIIOFeIIILꢀ]+.9,10
On the timescale of this experiment (B2 min), the spectra
exhibit isosbestic points. Analogously, the formation of
12+ was followed during chronoamperometry of 1 in DCM
at ꢁ20 1C in an OTTLE cell (Fig. 1D). The UV/Vis spectra
Bulk potentiostatic coulometry of 1 at ꢁ40 1C followed by
UV/Vis spectroscopy demonstrate a reduced stability of 1+ on
the longer timescale of this experiment. The UV/Vis spectra
exhibit isosbestic behavior for 10 min and non-isosbestic
behavior thereafter (Fig. S2, ESIw), indicating a slow decay
of 1+. The low-temperature EPR spectrum of 1+ taken during
a Fakultat fur Chemie, Universitat Bielefeld, Universitatsstrasse 25,
¨
¨
¨
¨
33615 Bielefeld, Germany. E-mail: thorsten.glaser@uni-bielefeld.de;
Fax: +49 (0)521 106 6003; Tel: +49 (0)521 106 6105
b Stanford Synchrotron Radiation Lightsource, SLAC,
Stanford University, Stanford, CA 94309, USA
¨
45470 Mulheim, Germany
¨
w Electronic supplementary information (ESI) available: Synthetic
procedures, details of the physical measurements, UV/Vis spectra
recorded during electrochemical experiments, course of the absorption
at 580 nm during the decay of 1+, Eyring plot, rate constants, XAS-
spectra, and results of the fit of the EXAFS. See DOI: 10.1039/b903500a
Fig. 1 (A) The ligand H2L and the molecular structure of 1;8
(B) electrochemistry of 1 in DCM at ꢁ40 1C, scan rate: 100 mV sꢁ1
,
frequency: 50 Hz. Inset: 10 K X-band EPR spectrum of electro-
chemically generated 1+; (C) UV/Vis spectra recorded during scanning
the oxidative wave of the first oxidation E1 in a CV of 1 at 3 mV sꢁ1 in
DCM in an OTTLE cell at ꢁ20 1C. This experiment is on the timescale
of 2 min. (D) UV/Vis spectra recorded during chronoamperometry of
1 at 0.8 V vs. Fc+/Fc in DCM in an OTTLE cell at ꢁ20 1C. This
experiment is on the timescale of 80 s.
c Max-Planck-Institut fur Bioanorganische Chemie, Stiftstrasse 34-36,
ꢂc
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Chem. Commun., 2009, 2637–2639 | 2637

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the first 10 min exhibits a sharp isotropic signal at g = 2.0050
with 86% spin-concentration (inset Fig. 1B). Despite the
similarity to the EPR spectrum of intermediate X, this iso-
tropic signal corroborates the ligand-centered oxidation of 1+.
In order to study the electronic structures and stabilities of
1+ and 12+ further, 1 was also oxidized chemically by one- and
two-electrons using the aminyl radical [(C6H4Br)3N](SbCl6)
(AR) in DCM. These reactions have been studied by temperature-
dependent UV/Vis spectroscopy under Schlenk condi-
tions. Spectra obtained a few seconds after addition of one
equivalent of the oxidant solution closely resemble those
obtained for electrochemically generated 1+ in Fig. 1C. The
extinction coefficient for the radical band at 414 nm decreases
with time while a new maximum at B580 nm forms (Fig. 2A).
This latter maximum is a characteristic feature of mono-
nuclear ferric complexes of the ligand L2ꢁ, indicating a
cleavage of a Fe–oxo bond.10 The absence of a band around
414 nm indicates that there are no phenoxyl radicals produced
in this decay. Longer measurements, especially at slightly
reaction at ꢁ40 1C reveals first-order dependence with
k = (4.2 ꢃ 0.1) ꢄ 10ꢁ4 sꢁ1 (Fig. S4, ESIw) corresponding to a half-
life at ꢁ40 1C of B27 min. Temperature-dependent measure-
ments (between ꢁ30 and ꢁ60 1C) allowed the determination of
the activation parameters DHz = (51 ꢃ 1) kJ molꢁ1 and
DSz = ꢁ(90 ꢃ 6) J (mol K)ꢁ1 (Fig. S5, ESIw). The negative
value for DSz is indicative of a more complex reaction scheme
than a simple bond cleavage.
Analogously, addition of two equivalents of the oxidant
leads to the direct formation of 12+ (red spectrum in Fig. 2B).
The strong radical band at 418 nm decreases slowly over B12 h
at ꢁ40 1C while a new band at B600 nm with an isosbestic
point around 521 nm forms (blue spectrum in Fig. 2B). This
decay process of 12+ thus produces two mononuclear iron
species of the ligand L2ꢁ. The strong decrease of the radical
band at 418 nm strongly suggests that not both mononuclear
complexes possess phenoxyl radicals as in 12+. However, the
feature at B418 nm is still strong and therefore implies the
presence of a phenoxyl radical in one of the two mononuclear
decay products. With ongoing time, the band at 600 nm
decreases with a concomitant decrease of the radical band at
418 nm. This is indicative of further follow-up reactions. We
refrain from a complete kinetic analysis due to its complexity,
but we have examined a first-order analysis of the first reaction
step. This reveals k = (3.8 ꢃ 0.5) ꢄ 10ꢁ5 sꢁ1 at ꢁ40 1C and the
temperature-dependence of k provides DHz = 68 (ꢃ4) kJ molꢁ1
and DSz = ꢁ(36 ꢃ 10) J (mol K)ꢁ1 as estimates. Interestingly,
the more oxidized species 12+ with a higher reactivity for
electron transfer exhibits a higher stability for Fe–O bond
breaking (t1/2 E 6 h).
higher temperature, reveal
a second follow-up reaction
(Fig. S3, ESIw). The kinetic analysis of the first follow-up
In order to obtain independent spectroscopic signatures for
the electronic structures of 1, 1+, and 12+, we used Mossbauer
¨
(on 57Fe enriched samples) and X-ray absorption spectroscopy
(XAS). Both methods reveal slight differences of 1 as a solid
and in DCM solution indicative of some structural rearrange-
ments in solution as observed for the mononuclear complexes.10
It should be mentioned that even for the solid state data, the
separation of 3 Fe–O components (2 Fe–OPh at 1.92 A and
1 Fe–Ooxo at 1.81 A) is beyond the resolution of the EXAFS
data (Table S2, ESIw). Interestingly, the EXAFS fitting
requires a decrease in the Fe–N distances from 2.25 A in the
solid to 2.14 A in solution. The pre-edge and edge features of
all four spectra are essentially identical (Fig. 2C), indicating
that 1, 1+, and 12+ are ferric complexes. However, the
EXAFS spectra exhibit significant differences (Fig. S6, ESIw).
The Fourier transforms show that the Fe–oxo distances of 1+
and 12+ are decreased as compared to 1 (Fig. 2D, Table S2
(ESIw)), indicating a shortening of the Fe–oxo bonds by
oxidation.
These findings are corroborated by Mossbauer spectra
¨
recorded from specially prepared samples in frozen DCM
(Fig. 2E).z The sample of 12+, taken 15 min after addition
of two equivalents AR at ꢁ30 1C, shows a slightly larger
isomer shift (0.47 mm sꢁ1) than in 1 (0.44 mm sꢁ1), indicating
less covalent bonding. Samples taken after longer reaction
times exhibit an intensity increase of the low velocity line along
with some intensity arising between the two former absorption
lines. The signal-to-noise ratio in the spectraz does not allow
meaningful simulations of individual spectra. However, taking
Fig. 2 (A) UV/Vis spectra of the decay of 1+, recorded at ꢁ40 1C.
(B) UV/Vis spectra of the decay of 12+, recorded at ꢁ40 1C. (C) Fe
K-edge XAS of 1, 1+, and 12+. (D) Fourier transforms of the Fe
K-edge EXAFS of 1, 1+, and 12+. (E) Mossbauer spectra recorded at
¨
80 K after the oxidation of 1 to 12+ at ꢁ30 1C in DCM using two
equivalents of AR recorded on samples quenched at individual times.
ꢂc
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into account the information from the temperature-dependent
UV/Vis spectra, a self-consistent simulation of all spectra
could be performed with two evolving new subspectra with
d = 0.17, |DEQ| = 0.80 mm sꢁ1 (red spectrum) and d = 0.56,
|DEQ| = 1.10 mm sꢁ1 (blue spectrum). The isomer shift of the
red component is consistent with the formulation as a FeIV
species, whereas the blue component is ascribed to a ferric
species of lower covalency.
FeIVQO units12 might indicate the presence of a high-spin
FeIV (S = 2) ion.13
In summary, we have generated and characterized two
highly reactive dinuclear iron complexes, which accumulate
oxidation equivalents as in X and Q. Further studies on the
final products and on the reactivity of the highly oxidized
intermediates with properly chosen substrates are currently
being pursued in our laboratories. In addition, the identifica-
tion of the two main drawbacks of the current ligand, i.e. the
ligand-centered oxidation and dissociation into mononuclear
species, is being used for optimization of the ligand design.
J.B.H.S. is thankful for a doctoral fellowship of the Fonds
der Chemischen Industrie. This publication was made possible
by the Fonds der Chemischen Industrie, the DFG and by
Grant Number 5P41RR001209 from the NCRR, a component
of the NIH. SSRL operations are funded by DOE, BES. The
SMB program is supported by the NIH, NCRR and by
DOE, BER.
All experimental results are consistent with the following
reaction schemes:
Notes and references
z The parent complex 1 is only soluble in DCM. Dissolution of 1 in
other solvents leads to a color change to blue, indicative of the
formation of mononuclear complexes. Thus, all characterizations were
performed in DCM although DCM is a problematic solvent for XAS
One- and two-electron oxidation of 1 results in the fast
generation of the monoradical complex 1+ and the diradical
complex 12+, respectively. Coordinated phenoxyl radicals
donate less charge to the FeIII ions than a coordinating
phenolate ligand. This reduced electron density is compen-
sated by a stronger charge donation from the bridging oxo
ligand.11 This results in an unsymmetrical bonding situation
for the two FeIII–oxo bonds in 1+, while in the diradical
complex 12+, the oxo ligand has to donate more charge to
both FeIII ions in comparison to the precursor 1. The
unsymmetrical Fe–O–Fe bond situation in 1+ already phases
into the unsymmetrical decay of the dimer into two mono-
nuclear complexes. The former bridging oxo ligand remains on
the iron ion with the stronger Fe–oxo bond in 1+, i.e. that with
the coordinated phenoxyl ligand. This would result in the
hypothetical [LꢀFeIIIQO] species, but the absence of a phenoxyl
and Mossbauer spectroscopy due to strong absorptions of X-ray and
¨
g radiation. DCM is usually considered to be opaque for Mossbbauer
¨
radiation (see ESIw for absorber optimization).
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¨
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undergo further decay reactions as evidenced by the time-
dependence of the UV/Vis spectra. The high reactivity of the
[LFeIVQO] despite the stabilization by the strongly electron-
donating ligand set and the weak-field nature of this ligand set
in comparison to ligand environments used for more stable
ꢂc
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Chem. Commun., 2009, 2637–2639 | 2639