Electron-transfer properties of an efficient nonheme iron oxidation catalyst with a tetradentate bispidine ligand

Article abstract of DOI:10.1002/anie.200904427

(Graph Presentation) The oxidation catalyst [ (L) Fe^IV=O] (see picture; L is a tetradentate bispidine ligand) has the highest known one-electron reduction potential for a Fe^IV=O species (0.73 V vs. SCE). Investigations into the electron-transfer kinetics show a linear correlation with the bond dissociation energy of the organic substrates.

Full text of DOI:10.1002/anie.200904427

Communications
DOI: 10.1002/anie.200904427
Bioinorganic Enzyme Models
Electron-Transfer Properties of an Efficient Nonheme Iron Oxidation
Catalyst with a Tetradentate Bispidine Ligand**
Peter Comba,* Shunichi Fukuzumi,* Hiroaki Kotani, and Steffen Wunderlich
The interest in nonheme iron systems has lead to an
increasingly detailed knowledge of the coordination geo-
metries, electronic structures, and reaction mechanisms of
oxygenases and halogenases (for example TauD and SyrB2),
and this is due to numerous studies involving the biological
processes, small molecule model systems, and quantum-
chemical model studies.[1–7] The thorough analysis of a variety
of model systems has yielded a detailed understanding of the
nature of the catalytically active high-valent iron oxo
intermediates.[2,4–8] Current focal points in experimental and
Scheme 1. Structure and proposed geometry of the ligand L (left) and
theoretical modeling of the enzyme reactions are ambiguities
in the degree of protonation, the oxidation and spin states of
the catalytically active high-valent iron complex, and their
relation to the reactivities (FeIV vs. FeV; S = 1 versus S = 2; O
versus OH versus OH2).
of complex 1 (right).
been thoroughly studied by computational (molecular
mechanics, DFT) and experimental methods, and it was
shown to be enforced by the rigid adamantane-type ligand
backbone. A variety of X-ray crystal structures have been
used to show that tetra- and pentadentate derivatives of L
with different donor sets have an elastic coordination sphere
as a consequence of the flat potential energy surface with
various close-to-degenerate minima. The rigid and relatively
large cavity of the bispidines is one reason for their high metal
ion selectivity, and these ligands exhibit uncommon depend-
encies of the stability constants.[17,18] Their iron chemistry has
been developed in the fields of alkane and alkene oxidation
and biomimetic nonheme–halogenase reactivity.[19–23] Com-
Important parameters for the characterization of the high-
valent iron oxo complex and for the interpretation of its
reactivity are 1) the reduction potential Ered of the ferryl
complex,[9–11] 2) the kinetic barrier for the electron-transfer
process, 3) the basicity of the oxidized and reduced iron oxo
species,[12] and 4) the energy gap between the various spin
states, because computational studies indicate that the
potential energy for oxidation reactions on the high-spin
surface has lower barriers, whereas most low-molecular-
weight biomimetic FeIV O systems, in contrast to the
=
enzymes, have an intermediate spin (S = 1) ground
state.[6,13–15]
putational studies related to the bispidine–FeIV O species
=
Herein, we report the electron-transfer properties of an
=
(Scheme 1).[16] The coordination chemistry of ligand L has
reveal a very small energy gap between the intermediate-spin
(S = 1) and high-spin (S = 2) electronic configurations.[21–23]
Furthermore, the reorganization energy of the electron
transfer between the oxo FeIV and the oxo FeIII species is
expected to be small owing to the rigid ligand backbone,
which is a possible reason for the exceptionally high reactivity.
FeIV O complex with the tetradentate bispidine ligand L
[*] Prof. Dr. P. Comba, S. Wunderlich
Universitꢀt Heidelberg, Anorganisch-Chemisches Institut, INF 270
69120 Heidelberg (Germany)
Therefore, we report herein the fundamental electron-trans-
Fax: (+49) 6226-546-617
E-mail: peter.comba@aci.uni-heidelberg.de
2+
fer properties of [FeIV O(L)(NCMe)] (1).
=
The high-valent complex 1 is generated quantitatively
with 1.2 equivalents of iodosylbenzene diacetate (PhI(OAc)2)
and stabilized at 238 K in concentrations of up to 5 ꢀ 10ꢀ4 m.
The absorption maximum of 1 (760 nm; e = 130 Lmolꢀ1 cmꢀ1)
Prof. Dr. S. Fukuzumi, Dr. H. Kotani
Department of Material and Life Science
Graduate School of Engineering, Osaka University
SORST (Japan) Science and Technology Agency (JST)
Suita, Osaka 565-0871 (Japan)
is typical for an S = 1 FeIV O system and consistent with the
=
and
solution magnetic moment of 3.01 B.M. that was determined
by the Evans method (this value is close to the spin-only value
of 2.83 for two unpaired electrons).[24–26] At higher temper-
Department of Bioinspired Science, Ewha Womans University
Seoul 120-750 (Korea)
Fax: (+81) 6-6879-7370
atures 1 is unstable, and at higher concentrations an inactive
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
III
FeIII O Fe dimer forms.[27] A series of ferrocene derivates
ꢀ ꢀ
[**] Financial support by the German Science Foundation (DFG) and a
Grant-in-Aid (No. 19205019) from the Ministry of Education,
Culture, Sports, Science and Technology (Japan), are gratefully
acknowledged.
(ferrocene (Eox = 0.38 V vs. SCE), bromoferrocene (0.54 V),
acetylferrocene (0.62 V), and dibromoferrocene (Eox
=
0.69 V))[10] were used as one-electron reductants to reduce
the ferryl complex 1 in dry acetonitrile to the corresponding
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200904427.
FeIII O compound [Eq. (1)].[28] A series of organic substrates
ꢀ
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Chemie
Ket
Br2Fc þ ½FeIVðOÞðLÞðNCMeÞꢁ
Br Fcþ þ ½FeIIIðOÞðLÞðNCMeÞꢁþ
AcFc+ lead to identical second-order rate constants (see the
Supporting Information). This result supports our interpreta-
2þ
ƒ!
ƒ
2
ð1Þ
tion and analysis that 1, with an absorption at 760 nm, is
indeed the reactive species and is reduced to FeIII O. A plot
ꢀ
(10-methyl-9,10-dihydroacridine, xanthene, dihydroanthra-
cene, and fluorene) was used to promote the proton-coupled
of the driving force versus the rate constants of the electron-
electron transfer (PCET) to FeIII/II OH.
transfer processes involving FeIV O and the ferrocene
ꢀ
=
The electron transfer from dibromoferrocene (Br2Fc) to 1
is confirmed by the UV/Vis spectral changes, with an increase
of the absorption observed at 690 nm for the Br2Fc+ cation
and the concomitant decrease of the absorption of 1 at
760 nm. The electron-transfer equilibrium of the reaction was
studied at various Br2Fc concentrations (0.1–2.0 mm, [1] =
0.2 mm). The plot in Figure 1 indicates that Br2Fc+ is
derivatives in acetonitrile at 238 K is shown in Figure 2,
where the log ket values are plotted against the ꢀDGet values
(ꢀDGet = exp(EredꢀEox) in eV). The data is well-fitted by the
Figure 1. a) Observed spectral changes in the electron-transfer reaction
of Br2Fc with 1 (2ꢁ10ꢀ4 m) in oxygen-free MeCN at 238 K. b) Concen-
tration of Br2Fc+ observed in the electron-transfer reaction of Br2Fc
with 1 (2ꢁ10ꢀ4 m) as a function of the initial concentration of Br2Fc.
Figure 2. Driving force dependence of the rate constants (logket) for
the electron transfer from different ferrocene derivates to 1 in oxygen-
free MeCN at 238 K.
generated quantitatively under the conditions used, and was
used to determine the equilibrium constant Ket of the
electron-transfer reaction to be 5.9 (see the Supporting
Information for details). This value and the known potential
of Br2Fc were used to determine the one-electron reduction
potential of 1, namely Ered = 0.73 V vs. SCE [Eq. (2)].
solid line in light of the Marcus theory of adiabatic outer-
sphere electron transfer [Eq. (3)], where Z is the collision
2
ð3Þ
ket ¼ Z exp½ꢀðl=4Þð1 þ DGet=lÞ =kB Tꢁ
frequency taken as 1 ꢀ 1011 Lmolꢀ1 sꢀ1, l is the reorganization
energy of the electron-transfer process, kB is the Boltzmann
constant, and T is the absolute temperature (see referen-
ces [30,31] for the Marcus analysis of electron-transfer
reactions with large l values).[32]
Ered ¼ Eox þ ðRT=FÞln Ket
ð2Þ
This one-electron reduction potential is the highest
reported to date for FeIV O compounds. A previous similar
=
From the plot of DGet versus ket in Figure 2, the
reorganization energy l was calculated to be 2.05 eV
(197.8 kJmolꢀ1); this value is significantly lower than values
=
study on FeIV O compounds with other tetra- and pentaden-
=
tate ligands reported Ered values of 0.39 V vs. SCE for the
complex of tmc (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclo-
tetradecane), 0.49 V for bn-tpen (N-benzyl-N,N’,N’-tris(2-
pyridylmethyl)ethane-1,2-diamine) and 0.51 V for N4py
(N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine),
and these values are significantly lower than Ered = 0.73 V vs.
SCE obtained herein for 1.[10]
for other FeIV
O complexes studied previously (2.37–
2.74 eV).[10] This result is not unexpected because of the
rigid bispidine backbone. The one-electron reduction of 1
(S = 1) to FeIII O (S = 3/2) is a spin-allowed process, and the
ꢀ
spin-state change is not believed to make a large contribution
to l.
The kinetics of the one-electron transfer between the
intermediate-spin (S = 1) ferryl complex 1 and a variety of
ferrocene derivates were analyzed by pseudo first-order
kinetics using an excess of the corresponding ferrocene
derivative. The observed rate constants kobs increase linearly
with the ferrocene concentrations, and the second order rate
constants ket were determined from the slopes of the curves.
All spectrophotometric experiments show an isosbestic point
between the absorption of the corresponding ferrocenium
cation derivative (615–690 nm) and 1 (760 nm). For acetyl-
ferrocene (AcFc), where the two absorptions are well-
separated (640 and 760 nm, respectively), the decrease of
the absorption for 1 and the increase of the absorption of
The product of the one-electron transfer, [FeIII(O)(L)]+,
has a high basicity and will immediately be protonated in
oxidation reactions of organic substrates.[11,12] This protona-
tion will lead to the thermodynamically more stable hydroxo
complex [FeIII(OH)(L)]2+ and will, as a consequence of a
larger equilibrium constant, lead to a higher value of Ered
(proton-coupled electron transfer, PCET). Such a PCET
reaction is observed between 4-tert-butylphenol (Eox = 1.66 V
vs. SCE) and 1, but no reaction occurs with 4-nitrophenol
(Eox = 2.24 V vs. SCE).[33] In the course of the reaction, the
formation of an absorption at 685 nm occurs, which is
proposed to be an FeIII phenoxyl radical complex (see
Supplementary Information).[34] The second-order rate con-
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Communications
change in the presence of various concentrations of dibromoferro-
cene at 238 K using a Hewlett Packard 8453 spectrophotometer with a
quartz cuvette (path length = 10 mm). Typically, a deaerated MeCN
solution of Br2Fc was added by means of a microsyringe to a
deaerated MeCN solution containing 1. The concentration of Br2Fc+
was determined from the absorption band at lmax = 690 nm (e = 3.9 ꢀ
102 Lmolꢀ1 cmꢀ1). The e value of Br2Fc+ was confirmed by the
electron-transfer oxidation of Br2Fc with an excess of 2,3-dichloro-
5,6-dicyano-p-benzoquinone.
Kinetic measurements were performed on a UNISOKU RSP-601
stopped-flow spectrometer equipped with a MOS-type highly sensi-
tive photodiode array or a Hewlett Packard 8453 spectrophotometer.
Rates of electron transfer from ferrocene derivatives and different
organic substrates to 1 at 238 K were monitored by the rise of the
absorption band owing to product formation (e.g. for AcrH2 to AcrH+
at 357 nm or for DHA to anthracene at 377 nm) or the decay of 1,
respectively. All kinetic measurements were carried out under pseudo
first-order conditions in which concentrations of substrates were
maintained to be more than ten-fold in excess of 1.
stants k’H with the organic substrates 10-methyl-9,10-dihy-
droacridine (AcrH2), xanthene, dihydroanthracene (DHA),
and fluorene with different bond-dissociation energies were
determined.[35,36] The reaction between the NADH analogue
AcrH2 and 1 leads to formation of the acridinium ion (AcrH+)
with an absorption maximum at 357 nm. This reaction is a
consequence of hydride transfer, and the corresponding
mechanism has been thoroughly studied.[37] It is interesting
to note that Eox of AcrH2 is higher than that of Br2Fc (0.81 V
and 0.69 V vs. SCE, respectively),[38] but ket increases from
10.5 to 475.9 Lmolꢀ1 sꢀ1 in the case of the PCET process. A
kinetic isotope effect (KIE) of 2.3 was observed when AcrH2
was replaced by the doubly deuterated substrate (AcrD2), and
this effect is much smaller than that observed in the N4py-
based system.[37] The hydrogen abstraction is thus the rate-
determining step, but in the present case, the barrier for the
ET process is very similar in energy. The dependence of the
logk’H values and the BDE of the organic substrates is shown
in Figure 3.[35,39]
Received: August 7, 2009
Revised: December 11, 2009
Published online: March 12, 2010
ꢀ
Keywords: bioinorganic chemistry · C H activation ·
electron transfer · enzyme models · iron oxo complexes
.
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Experimental Section
Commercially available reagents, such as ferrocene (Tokyo Chemical
Industry Co., Ltd.) were of the highest available grade and used
without further purification unless otherwise noted. Acetonitrile was
dried according to the literature procedures and distilled under argon
prior to use.[40] Preparation and handling of air-sensitive materials was
done under an inert atmosphere. NMR Spectra were recorded on a
400 MHz (1H), BRUKER AVANCE II 400 instrument with TMS as
internal reference; d in ppm, J in Hz. The oxo FeIV complex 1 was
prepared by the reaction of [FeII(L)(OTf)2] (0.1–0.5 mm) with
1.2 equiv of PhI(OAc)2 in deaerated MeCN at 238 K. [FeII(L)(OTf)2]
was synthesized as reported.[41]
Spectrophotometric redox titration: Electron transfer from
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dibromoferrocene to 1 (2 ꢀ 10ꢀ4 m) was examined from the spectral
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ꢀ
product of the PCET (FeIII OH) were analyzed by X-Band EPR
ꢀ
(see the Supporting Information). The observation of clean one-
electron reduction processes (quantitative transformation of the
ferryl complex and clean isosbestic points for the pure ET
reaction; see Figure 1 for the reaction with Br2Fc and the
Supporting Information for the other reductants) supports the
ꢀ
[39] The kH values are divided by the number of equivalent target C
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