Sc3+-triggered oxoiron(IV) formation from O2 and its non-heme iron(II) precursor via a Sc3+-peroxo-Fe3+ intermediate

Article abstract of DOI:10.1021/ja402645y

We report that redox-inactive Sc³⁺ can trigger O₂ activation by the Fe^II(TMC) center (TMC = tetramethylcyclam) to generate the corresponding oxoiron(IV) complex in the presence of BPh ₄^- as an electron donor. To model a possible intermediate in the above reaction, we generated an unprecedented Sc³⁺ adduct of [Fe^III(η²-O₂)(TMC)]⁺ by an alternative route, which was found to have an Fe³⁺-(μ- η²:η²-peroxo)-Sc³⁺ core and to convert to the oxoiron(IV) complex. These results have important implications for the role a Lewis acid can play in facilitating O-O bond cleavage during the course of O₂ activation at non-heme iron centers.

Full text of DOI:10.1021/ja402645y

Communication
pubs.acs.org/JACS
Sc3+-Triggered Oxoiron(IV) Formation from O2 and its Non-Heme
Iron(II) Precursor via a Sc3+−Peroxo−Fe3+ Intermediate
Feifei Li,†,§ Katherine M. Van Heuvelen,†,∥ Katlyn K. Meier,‡ Eckard Munck,*,‡ and Lawrence Que, Jr.*,†
̈
†Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota 55455, United
States
‡Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
S
* Supporting Information
Scheme 1. Proposed Mechanism for the Formation of 4 from
1 and O2
ABSTRACT: We report that redox-inactive Sc3+ can
trigger O2 activation by the FeII(TMC) center (TMC =
tetramethylcyclam) to generate the corresponding
−
oxoiron(IV) complex in the presence of BPh4 as an
electron donor. To model a possible intermediate in the
above reaction, we generated an unprecedented Sc3+
adduct of [FeIII(η2-O2)(TMC)]+ by an alternative route,
which was found to have an Fe3+−(μ-η2:η2-peroxo)−Sc3+
core and to convert to the oxoiron(IV) complex. These
results have important implications for the role a Lewis
acid can play in facilitating O−O bond cleavage during the
course of O2 activation at non-heme iron centers.
here is much current interest in investigating the ability of
Tredox-inactive metal ions to modulate redox reactions by
virtue of their Lewis acidity, particularly with respect to their
possible roles in O2 evolution1 and activation.2,3 For example,
the oxygen-evolving complex of Photosystem II requires a
redox-inactive Ca2+ ion to produce O2.1 In addition, redox-
Figure 1. Reaction of 0.96 mM 1 with NaBPh4 and Sc(OTf)3 in
aerobic CH3CN at 0 °C. (A) UV−vis spectral changes observed with 1
equiv of NaBPh4 and 1 equiv of Sc(OTf)3. Inset: structure of the TMC
inactive ions have been found to affect the stabilities and
reactivities of high-valent metal−oxo complexes in biomimetic
systems2 and to accelerate O2 activation by FeII and MnII
complexes.3 In the latter case, heterobimetallic O2 adducts and
high-valent metal−oxo species are presumably involved but
have not been observed. We previously demonstrated that
[FeII(TMC)(NCCH3)]2+ (1) (TMC = 1,4,8,11-tetramethylcy-
clam) reacts with O2 in CH3CN in the presence of
−
ligand. (B) Plot of the yield of 4 vs equivalents of BPh4 in the
presence of 1 equiv of Sc3+. Inset: plot of the yield of 4 vs equivalents
−
of Sc3+ with 1 equiv of BPh4 .
the reaction was carried out with 18O2, the m/z 477 peak
showed an upshift of 2 units (Figure S2), confirming that the
oxo moiety of 4 was derived from O2 and that O−O bond
cleavage must occur for the formation of 4 from 1 and O2.
−
stoichiometric H+ and BPh4 to form [FeIVO(TMC)-
(NCCH3)]2+ (4).4 Herein we report that a redox-inactive
Sc3+ ion can replace the strong acid in this reaction to trigger the
formation of 4. An unprecedented Sc3+ adduct (3) of [FeIII(η2-
O2)(TMC)]+ (2) was trapped by an alternative route,
spectroscopically characterized, and found to convert to 4
(Scheme 1).
−
Further investigation demonstrated that both Sc3+ and BPh4
are required for the formation of 4 from 1, as addition of either
BPh4 or Sc3+ alone to 1 in air-saturated CH3CN solution did
−
not elicit any detectable change in the UV−vis spectrum. In
addition, the yield of 4 was linearly correlated with the amount
Complex 1 is air-stable in acetonitrile solution for days.
However, the addition of 1 equiv of Sc(OTf)3 together with 1
equiv of NaBPh4 to an aerobic solution of 1 resulted in the
formation of 4 in >70% yield over the course of ∼1 h at 0 °C,
as indicated by its signature near-IR band at 820 nm (Figure
1A).5 Electrospray ionization mass spectrometry (ESI-MS)
analysis of the solution revealed the evolution of a prominent
peak at m/z 477.0 that was assigned to the {[FeIV(O)(TMC)]-
(OTf)}+ ion on the basis of its position and isotope distribution
pattern [Figure S1 in the Supporting Information (SI)]. When
−
−
of BPh4 added, plateauing at 1.0 equiv of BPh4 (Figure 1B).
−
1H NMR studies of the final solution showed that BPh4 had
decomposed to give 1,1′-biphenyl (Figure S3) with a
stoichiometry of 0.95 0.15 equiv relative to 1, demonstrating
that BPh4− provides the two electrons needed to convert 1 and
O2 into 4. On the other hand, a substoichiometric amount of
Received: March 14, 2013
© XXXX American Chemical Society
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dx.doi.org/10.1021/ja402645y | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Sc3+ was sufficient for the maximal formation of 4 (Figure 1B
inset), suggesting that Sc3+ can act somewhat “catalytically”.
As shown in Figure 1A, no intermediates were evident in the
UV−vis spectra during the conversion of 1 to 4.6 To account
for the role of Sc3+ in this transformation, we propose the
formation of a Sc3+−peroxo−Fe3+ adduct that is reminiscent of
−
the FeIII−OOH species proposed in the H+ and BPh4 -
promoted generation of 4 from O2 and 1.4,7 To test this
hypothesis, Sc(OTf)3 was added to a solution of the blue
FeIII(η2-O2) complex 2 (purified via precipitation as its BPh4
salt; see the SI for details), which resulted in the immediate
generation of a magenta intermediate, 3, and its subsequent
conversion to 4 in ∼70% yield over the course of ∼1 h at −10
°C (Figure 2A).
Figure 3. (left) EPR spectra of 2 (blue)7a and 3 (red) at 2 K and a
microwave power of 0.2 mW. (right) Mossbauer spectra of 3 at 4.2 K
̈
in MeCN recorded in parallel applied fields of (A) 0.5 and (B) 8.0 T.
The red lines in (A) and (B) are theoretical curves based on eq 1 in
the SI using the following parameters: D = +1.3 cm−1, E/D = 0.18, g0 =
2.00, Ax/gnβn = −20.0 T, Ay/gnβn = −20.6 T, Az/gnβn = −19.9 T, ΔEQ
= 0.50 mm/s, η = −0.5, δ = 0.47 mm/s. The Mossbauer sample
̈
contained 90% 38 and 10% FeIVO species (blue line).
Table 1. Spectroscopic Comparison of FeIII(TMC)−Peroxo
5
Complexes (S = /2) in CH3CN
Figure 2. (A) UV−vis spectral changes upon addition of 3 equiv of
Sc3+ to 1.5 mM purified 2 (ε835 = 650 M−1 cm−1) in CH3CN at −10
°C, instantly generating 3 (ε520 = 780 M−1 cm−1), which in turn
decayed to 4. (B) UV−vis changes upon titration of 1.5 mM 2 with
Sc3+ (0, 0.5, 1.0, 1.5, 2.0, and 9.0 equiv) in CH3CN at −40 °C.
λmax
(nm)
ΔEQ
δ
D
pre-edge
area
(mm/s) (mm/s) (cm−1
)
E/D
ref
7a
2
3
5
835
520
500
−0.92
0.50
0.58
0.47
0.51
−0.91
0.28
0.18
0.097
17.9
14.4
22.4
a
1.3
−
7a
0.20
2.5
a
This work.
What is the identity of complex 3? It exhibits a λmax of 520
nm (ε520 = 780 M−1 cm−1), as established from its UV−vis
closer to that of 2 with an η2-peroxo ligand than that of 5 with
an η1-OOH ligand.
spectrum (Figure 2A) and Mossbauer analysis. The large blue
̈
shift observed for the peroxo → Fe(III) charge-transfer band of
2 (λmax = 835 nm) is reminiscent of that seen upon protonation
of 2 to form [FeIII(TMC)(η1-OOH)]2+ (5) in CH3CN,7a
indicating partial neutralization of the negative charge of the
peroxo ligand. Titration of 2 with Sc(OTf)3 showed that 1
equiv of Sc(OTf)3 was nearly sufficient to cause the 835 nm
band of 2 to disappear, suggesting a 1:1 stoichiometry for the
Sc3+ adduct of 2 (Figure 2B). The EPR spectrum of 3 shows
Analysis of the extended X-ray absorption fine structure
(EXAFS) data for 3 provided additional structural insight. Best
fits revealed four N scatterers at 2.18 Å and four C scatterers
each at 3.00 and 3.15 Å (Figure 4 and Table S2); all of these
features arise from the TMC ligand and have distances close to
those found for 2 (Table 2). In addition, there is an O subshell
at 1.98(1) Å arising from the peroxo ligand. Notably, the Fe−O
distance (rFe−O) in 3 is significantly longer than the distance of
1.91 Å found for 2,7a implying that the addition of Sc3+
5
features at g = 9.1, 5.1, 3.6, and ∼2, consistent with an S = /2
Fe(III) center with an E/D ratio of 0.18 (Figure 3 left),
compared with E/D = 0.28 and 0.097 for 2 and 5,7a
respectively. The Mossbauer spectra of 3 (Figure 3 right) are
̈
typical of high-spin Fe(III); their analysis is described in the SI,
and the Mossbauer parameters are listed in Table 1 and the
̈
Figure 3 caption. A comparison of the spectroscopic properties
in Table 1 shows that 3 is quite different from 2 and 5,
indicating that Sc3+ significantly affects the properties of the
peroxoiron(III) unit.
We also carried out Fe K-edge X-ray absorption spectroscopy
(XAS) studies to investigate the structural features of 3.
Complex 3 exhibited an Fe K-edge at 7125.3 eV and a pre-edge
feature at 7113.3 eV, which are comparable to those of 2 and 5
obtained in CH3CN (Figure S4 and Table S1).7a The pre-edge
feature of 3 has an area of 14.4(6) units, compared with 17.9
for 2 and 22.4 for 5 (Table S1). As the pre-edge area reflects
the extent to which the iron center deviates from
centrosymmetry, the coordination environment of 3 must be
Figure 4. Fourier transform of the Fe K-edge EXAFS data for 3 over a
k range of 2−14 Å−1. The inset shows k3χ(k) vs k data. The solid black
lines represent the experimental data, while the red dashed lines
correspond to the best fit with two O at 1.98 Å and four N at 2.18 Å
(fit 22 in Table S3).
B
dx.doi.org/10.1021/ja402645y | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Table 2. Comparison of Structural and Raman Data for S = /2 FeIII−Peroxo Complexes
Communication
5
complex
rFe−N (Å)
rFe−O (Å)
1.98, 1.98
νO−O (cm−1
)
refs
b
a
3
2.18
807
−
non-heme FeIII−η2-peroxo
816−827
826 (825)
830−891
870 (868)
7, 15
a
2 (2′)
2.20 (2.21)
1.91, 1.91 (1.91, 1.91)
7a (7b)
c
non-heme FeIII−η1-peroxo
7, 16
a
5 (5′)
6
2.15 (2.16)
2.17
1.92 (1.85)
1.89
7a (7b)
17
(heme)FeIII−(μ-η2:η1-O2)−CuII
2.09
1.92, 2.09
788−808
747−767
9a, 9b
9a, 9b
(heme)FeIII−(μ-η2:η2-O2)−CuII
2.09
1.94, 2.09
a
b
c
2, 3, and 5 in CH3CN; 2′ and 5′ in 3:1 (v/v) acetone/CF3CH2OH. This work. Also see Table S4 in the SI of ref 7a.
significantly weakens the iron−peroxo interaction. This 0.07 Å
lengthening is inconsistent with conversion of the η2-peroxo
ligand to an η1 isomer, as the related η1-peroxo complexes 5
and [FeIII(TMCS)(η1-O2)] (6) [TMCS = 1-(2-mercaptoeth-
yl)-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane] have
shorter Fe−O distances (Table 2). CuII adducts to (η2-
peroxo)iron(III) porphyrin complexes also have one short Fe−
O bond (∼1.93 Å) in a highly unsymmetric η2-peroxo ligand
that binds to the iron.9 Thus, the 0.07 Å lengthening of rFe−O in
3 relative to that in 2 favors a symmetric η2-peroxo binding
mode for 3. This conclusion is also supported by a comparison
of fits 7 and 8 in Table S2, where the two-O subshell in fit 7 has
a σ2 value of ∼4, while the one-O subshell in fit 8 has a σ2 value
of −0.4. The negative σ2 value for the latter indicates that either
a bond is more rigid than would be expected for its distance or
to date (Table 2). Relative to its precursor 2,7a 3 has a νO−O
that is downshifted by 19 cm−1 and a νFe−O that is upshifted by
50 cm−1,12 consistent with retention of the η2 binding mode of
the peroxo ligand. Taken together, the spectrosopic data lead us
to propose an Fe3+−(μ-η2:η2-O2)−Sc3+ core for 3, analogous to
the Ni2+−(μ-η2:η2-O2)−K+ core found in a complex charac-
terized crystallographically by Limberg, Driess, and co-work-
ers.13,14
With the nature of 3 characterized, an important question
that remains is whether 3 is involved in the conversion of 1 to 4
by O2 activation. The requirement for both Sc3+ and two
electrons to trigger O2 activation of 1 suggests the likely
formation of a Sc3+−peroxo−Fe3+ species such as 3 as an
intermediate (Scheme 1). However, the fact that this species
did not accumulate during O2 activation (Figure 1A) suggests
that 3 may correspond to a more stable isomer of the actual
intermediate involved in the O2 activation reaction. Never-
theless, 3 represents a rare example of a heterobimetallic
complex bridged by a peroxo ligand9,13 and the only one to date
that involves a non-heme iron center.
that there are too few scatterers associated with that shell.10
A
negative σ2 value was also found when only one O scatterer
(instead of two) was used in fitting the EXAFS data for 2. Our
EXAFS results thus demonstrate that the binding of Sc3+ retains
the symmetric side-on binding mode of the peroxo ligand in 3
but increases rFe−O by 0.07 Å.11
The spectroscopic characterization of 3 as a complex with an
Fe3+−(μ-η2:η2-O2)−Sc3+ core provides a plausible mechanism
for a Lewis acid to promote O−O bond cleavage. This insight
points to another role the second iron center can play in diiron
enzymes besides serving as an electron source: functioning as a
Lewis acid to facilitate the formation of high-valent iron−oxo
intermediates such as Q and X in the respective oxygen
activating cycles of methane monooxygenase and class 1A
ribonucleotide reductases.18 This report of the Sc3+−peroxo−
Fe3+ intermediate 3 also augments the recent literature focused
on the effects of redox-inactive Lewis acidic metal ions on redox
transformations.1−3 Prominent among these are their accel-
erative properties in oxidations by high-valent metal−oxo
complexes discovered by Fukuzumi and Nam2a−f as well as the
role of Ca2+ in forming an O−O bond from water during
photosynthesis.1 Relevant to the latter, Borovik recently
showed that group-II metal ions (MII) can enhance the rates
of O2 activation by FeII and MnII complexes to afford well-
characterized MII−(μ-OH)−(MnIII/FeIII) products, presumably
via heterobimetallic O2 adducts.3 The present results
demonstrate that Sc3+ can “turn on” the activation of O2 at a
non-heme iron center and that a transient Sc3+−peroxo−Fe3+
species related to 3 could be a viable intermediate leading to
O−O bond cleavage.
The final key piece of evidence for the identity of 3 was
provided by resonance Raman spectroscopy. Laser excitation
into the intense 520 nm band of 3 revealed two prominent
peaks at 807 and 543 cm−1 (Figure 5) that correspond to νO−O
and νFe−O modes, respectively. These assignments were
corroborated by 18O labeling, which resulted in respective
downshifts of 45 and 23 cm−1 that correlate well with Hooke’s
Law predictions for these modes and support the presence of
an iron-bound peroxo ligand in 3. The νO−O of 3 is the lowest
of any non-heme high-spin peroxoiron(III) complex observed
ASSOCIATED CONTENT
■
S
* Supporting Information
Syntheses; physical methods; ESI-MS, H NMR, and XANES
Figure 5. Resonance Raman spectra of 3 prepared in CH3CN with
H216O2 (red) and H218O2 (black) obtained with 514.5 nm excitation at
100 mW. The 16O − 18O difference spectrum is shown in blue. S =
solvent-derived peaks.
1
figures; and details of XAS analyses. This material is available
free of charge via the Internet at http://pubs.acs.org.
C
dx.doi.org/10.1021/ja402645y | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
3629. (b) Chufan
́
, E. E.; Puiu, S. C.; Karlin, K. D. Acc. Chem. Res. 2007,
AUTHOR INFORMATION
Corresponding Author
emunck@cmu.edu; larryque@umn.edu
■
40, 563. (c) Chishiro, T.; Shimazaki, Y.; Tani, F.; Tachi, Y.; Naruta, Y.;
Karasawa, S.; Hayami, S.; Maeda, Y. Angew. Chem., Int. Ed. 2003, 42,
2788.
Present Addresses
(10) Scott, R. A. In Physical Methods in Bioinorganic Chemistry:
Spectroscopy and Magnetism; Que, L., Jr., Ed.; University Science
Books: Sausalito, CA, 2000; pp 465−503.
§F.L.: Brookhaven National Laboratory, Upton, NY 11973.
∥K.M.V.H.: Harvey Mudd College, Claremont, CA 91711.
(11) We attempted to include in fits of 3 a Sc scatterer at ∼3.7 Å. Fits
13 and 14 in Table S2 show that a Sc scatterer at 3.8 Å with a
reasonable Debye−Waller factor (σ2 ≈ 4) could be added but gave
only a slight improvement in the goodness of fit. Similar results were
obtained in the EXAFS analysis of a Sc−O−Co complex (see: Pfaff, F.
F.; Kundu, S.; Risch, M.; Pandian, S.; Heims, F.; Pryjomska-Ray, I.;
Haack, P.; Metzinger, R.; Bill, E.; Dau, H.; Comba, P.; Ray, K. Angew.
Chem., Int. Ed. 2011, 50, 1711 ).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(Grant CHE1058248 to L.Q.) and the National Institutes of
Health (Grant EB001475 to E.M. and postdoctoral fellowship
GM093479 to K.M.V.H.). F.L. acknowledges a doctoral
dissertation fellowship from the University of Minnesota.
XAS data were collected at beamline X3B of the National
Synchrotron Light Source at the Brookhaven National
Laboratory and beamline 7-3 of the Stanford Synchrotron
Radiation Lightsource, both supported by NIH and DOE. We
thank Dr. Jason England and Ms. Jennifer Bigelow for
assistance and discussions.
(12) At first glance, the 50 cm−1 upshift in the Fe−O vibration in
going from 2 to 3 may appear to contradict the observed lengthening
of the Fe−O bond distance deduced from the EXAFS analysis, but the
18O shifts found for the respective Fe−O vibrations were quite
different (−15 vs −23 cm−1). The downshift for 3 is as calculated for a
diatomic Fe−O oscillator, but the smaller shift for 2 indicates mixing
of the diatomic Fe−O vibration with other vibrational modes. Thus,
the use of a direct comparison of the frequencies to deduce the Fe−O
bond distance is not valid in this case.
(13) Yao, S.; Xiong, Y.; Vogt, M.; Grutzmacher, H.; Herwig, C.;
̈
Limberg, C.; Driess, M. Angew. Chem., Int. Ed. 2009, 48, 8107.
(14) A related Fe3+−(μ-η2:η2-O2)−H+ core was postulated by Nam
for a short-lived (<2 ms) species (λmax = 527 nm) observed at −40 °C
upon treatment of 2 with strong acid in its conversion to 5.7b
(15) (a) Girerd, J.-J.; Banse, F.; Simaan, A. J. Struct. Bonding 2000, 97,
145. (b) Roelfes, G.; Vrajmasu, V.; Chen, K.; Ho, R. Y. N.; Rohde, J.-
U.; Zondervan, C.; la Crois, R. M.; Schudde, E. P.; Lutz, M.; Spek, A.
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