Spectroscopic and DFT Characterization of a Highly Reactive Nonheme FeV-Oxo Intermediate

Article abstract of DOI:10.1021/jacs.7b11400

The reaction of [(PyNMe₃)Fe^II(CF₃SO₃)₂], 1, with excess peracetic acid at -40 °C generates a highly reactive intermediate, 2b(PAA), that has the fastest rate to date for oxidizing cyclohexane by a nonheme iron species. It exhibits an intense 490 nm chromophore associated with an S = 1/2 EPR signal having g-values at 2.07, 2.01, and 1.94. This species was shown to be in a fast equilibrium with a second S = 1/2 species, 2a(PAA), assigned to a low-spin acylperoxoiron(III) center. Unfortunately, contaminants accompanying the 2(PAA) samples prevented determination of the iron oxidation state by M?ssbauer spectroscopy. Use of MeO-PyNMe₃ (an electron-enriched version of PyNMe₃) and cyclohexyl peroxycarboxylic acid as oxidant affords intermediate 3b(CPCA) with a M?ssbauer isomer shift δ = -0.08 mm/s that indicates an iron(V) oxidation state. Analysis of the M?ssbauer and EPR spectra, combined with DFT studies, demonstrates that the electronic ground state of 3b(CPCA) is best described as a quantum mechanical mixture of [(MeO-PyNMe₃)Fe^V(O)(OC(O)R)]²⁺ (～75%) with some Fe^IV(O)(^?OC(O)R) and Fe^III(OOC(O)R) character. DFT studies of 3b(CPCA) reveal that the unbound oxygen of the carboxylate ligand, O2, is only 2.04 ? away from the oxo group, O1, corresponding to a Wiberg bond order for the O1-O2 bond of 0.35. This unusual geometry facilitates reversible O1-O2 bond formation and cleavage and accounts for the high reactivity of the intermediate when compared to the rates of hydrogen atom transfer and oxygen atom transfer reactions of Fe^III(OC(O)R) ferric acyl peroxides and Fe^IV(O) complexes. The interaction of O2 with O1 leads to a significant downshift of the Fe-O1 Raman frequency (815 cm^-1) relative to the 903 cm^-1 value predicted for the hypothetical [(MeO-PyNMe₃)Fe^V(O)(NCMe)]³⁺ complex.

Full text of DOI:10.1021/jacs.7b11400

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Article
Spectroscopic and DFT Characterization of a
V
Highly Reactive Nonheme Fe-oxo Intermediate
Ruixi Fan, Joan Serrano-Plana, Williamson N. Oloo, Apparao Draksharapu, Estefania Delgado-
Pinar, Anna Company, Vlad Martin-Diaconescu, Margarida Borrell, Julio Lloret-Fillol, Enrique
García-España, Yisong Guo, Emile L. Bominaar, Lawrence Que, Miquel Costas, and Eckard Münck
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11400 • Publication Date (Web): 20 Feb 2018
Downloaded from http://pubs.acs.org on February 21, 2018
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Spectroscopic and DFT Characterization of a Highly Reactive
V
Nonheme Fe ꢀoxo Intermediate
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†
‡
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Ruixi Fan, Joan SerranoꢀPlana, Williamson N. Oloo, Apparao Draksharapu, Estefanía
¥
‡
‡
‡
DelgadoꢀPinar, Anna Company, Vlad MartinꢀDiaconescu, Margarida Borrell, Julio
&
,
¥
,†
,†
LloretꢀFillol, Enrique GarcíaꢀEspaña,* Yisong Guo,* Emile L. Bominaar,* Lawrence
,
§
,‡
,†
Que Jr.,* Miquel Costas,* Eckard Münck,*
†
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh,
Pennsylvania 15213, United States
‡
Grup de Química Bioinspirada, Supramolecular i Catàlisi (QBISꢀCAT), Institut de Química
Computacional i Catàlisi (IQCC), Departament de Química, Universitat de Girona, C/ M.
Aurèlia Capmany 69, 17003 Girona, Catalonia, Spain
§
Department of Chemistry and Center for Metals in Biocatalysis, 207 Pleasant Street SE,
University of Minnesota, Minneapolis, Minnesota 55455, United States
&
Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and
Technology, Avinguda Països Catalans 16, 43007, Tarragona, Spain.
¥
Grup de Química Supramolecular, Institut de Ciència Molecular, Departament de Química
Inorgànica, Universitat de València, 46980, Paterna, (Valencia), Spain
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ABSTRACT
The reaction of [(PyNMe )Fe (CF SO ) ], 1, with excess peracetic acid at ꢀ40 °C generates
II
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3
3 2
a highly reactive intermediate, 2b(PAA), that has the fastest rate to date for oxidizing
cyclohexane by a nonheme iron species. It exhibits an intense 490ꢀnm chromophore
associated with an S = ½ EPR signal having gꢀvalues at 2.07, 2.01, and 1.94. This species
was shown to be in a fast equilibrium with a second S = ½ species, 2a(PAA), assigned to a
lowꢀspin acylperoxoiron(III) center. Unfortunately, contaminants accompanying the 2(PAA)
samples prevented determination of the iron oxidation state by Mössbauer spectroscopy.
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Use of MeOꢀPyNMe3 (an electronꢀenriched version of PyNMe ) and cyclohexyl
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peroxycarboxylic acid as oxidant affords intermediate 3b(CPCA) with a Mössbauer isomer
shift δ = ꢀ0.08 mm/s that indicates an iron(V) oxidation state. Analysis of the Mössbauer and
EPR spectra, combined with DFT studies, demonstrates that the electronic ground state of
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b(CPCA) is best described as a quantum mechanical mixture of [(MeOꢀ
V
2+
IV
III
PyNMe )Fe (O)(OC(O)R)] (≈ 75%) with some Fe (O)(•OC(O)R) and Fe (OOC(O)R)
3
character. DFT studies of 3b(CPCA) reveal that the unbound oxygen of the carboxylate
ligand, O2, is only 2.04 Å away from the oxo group, O1, corresponding to a Wiberg bond
order for the O1–O2 bond of 0.35. This unusual geometry facilitates reversible O1–O2 bond
formation and cleavage and accounts for the high reactivity of the intermediate when
compared to the rates of hydrogen atom transfer and oxygen atom transfer reactions of
III
IV
Fe (OC(O)R) ferric acyl peroxides and Fe (O) complexes. The interaction of O2 with O1
ꢀ1
leads to a significant downshift of the FeꢀO1 Raman frequency (815 cm ) relative to the
ꢀ1
V
3+
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03ꢀcm value predicted for the hypothetical [(MeOꢀPyNMe )Fe (O)(NCMe)] complex.
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INTRODUCTION
High valent oxoiron species are nature’s tool for functionalizing inert molecules such as
1–6
aliphatic CꢀH bonds. Oxoiron(IV) species have been identified as the CꢀH cleaving agents
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–9
in several mono and dinuclear nonheme iron enzymes. Parallel efforts with oxoiron(IV)
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synthetic models of these enzymes have shown that they can break strong CꢀH bonds,
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although at reaction rates that still fall short when compared with enzymatic systems.
Higher oxidation states are accessed in many cytochrome P450s and peroxidases via a
species known as Compound I that is best described as an oxoiron(IV)ꢀporphyrin cation
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radical. Oxoiron(V) species have been proposed as the reactive intermediate in the
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5–17
catalytic cycle of Rieske oxygenase enzymes,
although direct detection of this species
has not been reported. Synthetic nonheme oxoiron(V) compounds are rare (Scheme 1).
Tetraamido macrocyclic ligands (TAML) have for the first time allowed the stabilization,
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8–23
spectroscopic characterization and reactivity analysis of an oxoiron(V) species,
and only
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V
recently has an oxoiron(V) imido complex been described. [(L)Fe (O)(R)] (L = neutral
polyamine ligand, R = OH or O CR) species have been proposed to be reactive
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2
5–28
intermediates in the oxidation of hydrocarbons.
Their implication in these reactions has
been deduced based on indirect evidence such as product analyses with substrate probes and
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isotope labelling, and by computational methods.
Besides, such oxoiron(V) species have
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2–34
34–36
been detected in trace quantities by cryospray massꢀspectrometry
or by EPR,
but no
V
definitive characterization has been reported. In some instances, claimed Fe species have
3
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37,38
been shown or reinterpreted to be ferric species.
Scheme 1. Spectroscopically characterized oxoiron(V) complexes.
V
2+
Putative [(L)Fe (O)(O CR)] intermediates (where L stands for a neutral tetradentate
2
aminopyridine ligand) are of interest because there is mounting evidence that they perform
the stereoꢀretentive hydroxylation of aliphatic CꢀH bonds, a distinctively difficult but
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powerful reaction in organic synthesis and biology.
Stereospecific hydroxylation entails
two difficult steps. The first is the breakage of a strong CꢀH bond, usually considered inert
towards common organic reagents, via a fast hydrogen atom transfer reaction, creating a
carbon centered radical. In a second step, this extremely shortꢀlived radical reacts with the
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hydroxyl ligand, avoiding escape into the solvent.
We have recently described the generation of a highly reactive S = ½ species (called 2b by
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SerranoꢀPlana et al ) from the reaction of [(PyNMe )Fe (CF SO ) ], 1, with peracetic acid
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(PAA) at cryogenic temperatures (Scheme 2).
Species 2b has gꢀvalues at 2.07, 2.01 and
1
.94, which are difficult to reconcile with a lowꢀspin ferric assignment; spectral simulation
of the signal showed that it accumulated to approximately 40% of the iron in the sample.
EPR also indicated that this species is in fast equilibrium with S = ½ species 2a (≈ 5% of the
Fe). On the basis of its gꢀvalues at 2.20, 2.19 and 1.99, species 2a was assigned as
III
2
2+
[
(PyNMe )Fe (κ ꢀOOAc)] . Product analysis, EPR and massꢀspectrometry experiments
3
V
2+
initially led us to formulate 2b as [(PyNMe )Fe (O)(OAc)] . The Fe(V) species described
3
in this work differs from those with tetraanionic tetraamido ligands such as TAML in having
a neutral supporting ligand, which leads to the observation of extraordinary oxidation
reactivity. 2b oxidizes hydrocarbons at a record fast rate among synthetic nonheme iron
systems and reproduces the reactivity of P450 Compound I in terms of reaction rates and
stereospecificity. Because of the presence of at least six other paramagnetic species in the
reaction mixture, the Mössbauer spectra of 2b, crucial for the assignment of its oxidation
state, could not be identified.
Herein, we describe critical improvements that have enabled a full spectroscopic and
computational characterization of the complex of interest. By replacing the PyNMe ligand
3
with the electronꢀenriched version MeOꢀPyNMe and using cyclohexyl peroxycarboxylic
3
acid (CPCA) as the oxidant, we were able to generate the corresponding species in nearly
5
0% yield. Our combined spectroscopic and DFT analysis suggests an unprecedented
electronic structure for a highꢀvalent iron complex, which in turn provides insight into its
unique reactivity.
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Scheme 2. Proposed intermediates formed in the reactions of 1 and 1’ with various peracids (PAA =
peracetic acid, CPCA = cyclohexyl peroxyacid, PNA = pernonanoic acid.
RESULTS
The spectral properties of the intermediate of interest depend on the combination of
ligand/oxidant (Table 1). Below we use the following designations: 2 for PyNMe and 3 for
3
MeOꢀPyNMe complexes. The oxidant used is indicated in parenthesis, e. g. 2(PAA),
3
3
(CPCA), 3(PNA) etc. The UVꢀvis absorption spectrum of 3(CPCA) shown in Figure 1
shows two principal absorption features in the visible region, a more intense feature with
λmax = 520 nm and a weaker band near 700 nm. These features are blueꢀshifted in 2(CPCA)
upon changing the supporting ligand from MeOꢀPyNMe to PyNMe (Table 1). Comparison
3
3
among complexes in the 3 series shows that the nature of the R group on the peracid also
affects the observed absorption maxima slightly.
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Figure 1. UVꢀvis spectra of [(MeOꢀPyNMe )Fe (O)(OC(O)cy)] , 3(CPCA), in 3:1 acetone:MeCN
3
in a 1ꢀcm cuvette. Solid lines show progressive formation of 3(CPCA) upon addition of 10 equiv.
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ꢀ1
ꢀ1
CPCA to 0.5 mM [(MeOꢀPyNMe )Fe (CF SO ) ] at – 60 °C. ε(λ = 520 nm) ≈ 7,500 M cm . The
3
3
3 2
max
bands at 520 nm and 700 nm increase monotonically over 200 s upon addition of CPCA.
Table 1. Comparison of some spectroscopic properties of complexes 2 and 3.
ꢀ1 a
Raman data (cm )
(815/829 intensity ratio)
g = 2.07/g = 2.7
signal intensity
ratio
UVꢀVis data
λmax (nm)
Samples
18
[
OꢀPNA data]
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3
2
(PAA)
490, 680
504, 685
815/829 (8:1)) (exc 561 nm)
815/829 (8:1) (exc 561 nm)
8:1(EPR)
11:1(EPR)
:1(Mössbauer)
2
(CPCA)
7
c
b
8
15 [783] /829 [?] (11:1)
exc 515 nm)
9:1 (EPR)
2
(PNA)
496, 685
(
10:1 (Mössbauer)
8
12/820 Fermi doublet
29 not obsvd (exc 515 nm)
3(PAA)
508, 683
520, 700
N/A
8
8
12/820 Fermi doublet
829 not obsvd (exc 515 nm)
3
(CPCA)
50:1 (EPR)
40:1 (EPR)
b
813/821 Fermi doublet [784]
829 not obsvd
3(PNA)
512, 685
(
exc 515 nm)
a
18
18
Entries in square brackets correspond to data obtained with OꢀPNA.
b
ꢀ1
18
ꢀ1
While the 771ꢀcm peak is observed in the spectra of 2( OꢀPNA) and 3( OꢀPNA), the 829ꢀcm
peak is not observed in 3( OꢀPNA). Therefore it cannot be connected to the 829ꢀcm peak.
16
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EPR Results. Intermediates 2 and 3 exhibit S = 1/2 EPR signals that represent two
subspecies a and b, which are present in different ratios depending on the supporting ligand
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and are involved in a fast equilibrium (Table 1). Subspecies a has a lowꢀspin Fe center,
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whereas b, as shown below, is predominantly Fe . The main text focuses on 3b(CPCA),
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while corresponding results for 2b(CPCA) are presented in SI. The subspecies of 2(PAA),
2(CPCA) and 2(PNA) occur in the ratio [b]:[a] ≈ 8ꢀ10:1, while subspecies a is almost
absent (<2% of total Fe) in 3(CPCA). Figure 2 shows Xꢀband spectra of 3(CPCA)
57
containing Fe in natural abundance (2.2%, panel A) and enriched to 95% (panel B).
Unenriched 3(CPCA) exhibits an S = ½ EPR signal with gꢀvalues at 2.07, 2.01 and 1.94, the
4
3
same gꢀvalues as those reported for 2b(PAA). This signal is thus associated with
III
3
b(CPCA). The lowꢀspin Fe species 3a(CPCA) was essentially absent in the sample, i.e.
by spectral simulations we found [3b(CPCA)]:[3a(CPCA)] ≈ 50:1, which greatly simplifies
the Mössbauer analysis as the features of 3a(CPCA) represent only 1% of the iron in the
Mössbauer spectrum (the Mössbauer spectrum of 2a(CPCA) is hidden under the features of
2
b(CPCA), see Section III of SI). Importantly, samples of 3b(CPCA) were free of other S =
III
½
species but contained various highꢀspin Fe species (EPR spectra are shown in Figures
S19ꢀ22).
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Figure 2. Xꢀband EPR spectra of 3b(CPCA) recorded at T = 15 K. Samples were prepared by
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adding 10 equivalents of CPCA to a 1 mM solution of [(MeOꢀPyNMe )Fe (CF SO ) ] at ꢀ70 °C.
3
3
3 2
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(
A) Fe in natural abundance (2.2%); full scan and low field region are shown in Figures S19 and
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S20. (B) Spectrum of 3b(CPCA) 95 % enriched with Fe. The red lines are SpinCount simulations
for g = 2.010, g = 2.067, g = 1.941. For the line shapes it was assumed that the gꢀvalues have a
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z
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spectrum of the enriched sample was simulated with the same gꢀvalues and line width parameters,
57
adding a Fe magnetic hyperfine interaction term. The results are listed in Table 2. Instrumental
conditions: (A) 0.2 mW, (B) 0.002 mW, microwave power (Change of microwave power is
accidental, not dictated by saturation considerations); 0.3 mT, modulation amplitude for both.
The spectra of 3b(CPCA) are described with the S = ½ spin Hamiltonian
ꢁ
ꢀ
ꢁ
ꢁ
ꢈ
ꢁ
ꢁ
ꢊ
= ꢂꢃ ∙ ꢄ ∙ ꢅ + ꢃ ∙ ꢆ ∙ ꢇ + ꢀ + ꢀ
(1)
ꢉ
ꢋꢉꢌꢍꢍ
ꢎꢏ
ꢎꢔ
ꢏ
ꢏ
ꢏ
ꢁ
ꢒ
ꢒ
ꢒ
ꢀ
=
ꢐ3ꢑ − + ꢖꢗꢑ − ꢑ ꢚꢛ
(1a)
(1b)
ꢉ
ꢓ
ꢘ
ꢙ
ꢕ
ꢁ
ꢈ
= −ꢜ ꢂ ꢅ ∙ ꢇ
ꢝ ꢝ
ꢀ
ꢊ
5
7
57
where A is the Fe magnetic hyperfine tensor, Ĥ describes the Fe nuclear Zeeman
Z
interaction and Ĥ describes the interaction of the electric field gradient (EFG) tensor with
Q
57
the Fe nuclear quadrupole moment, Q, of the nuclear excited state; η = (V – V )/V is
xx
yy
zz
the asymmetry parameter. The x axis was chosen to be along gmid = g = 2.01; this choice
x
will be convenient for the presentation of the DFT results and for comparison with
IV
published data of Fe =O complexes for which the z axis is generally chosen to be along the
Fe=O bond. Because g is nearly isotropic the Mössbauer data do not convey information
about the orientations of the Aꢀ and EFGꢀtensors relative to g, and we thus assumed that g
and A are collinear for the simulations of the Mössbauer spectra. The line widths (FWHM)
of the g = 2.01 and g = 2.07 features are about 1.7 mT, while the width of the g = 1.94
x
y
z
feature is 4.3 mT. Most probably, the broadening of the g feature results from a
z
conformational heterogeneity that affects, for reasons presently not understood, essentially
1
4
g . DFT calculations, described below, show that unresolved N hyperfine interactions of
z
the four nitrogen donors of MeOꢀPyNMe are too small to be responsible for the broadening.
3
5
7
When 3b(CPCA) is enriched with Fe its gmid = 2.01 feature is split into a doublet
corresponding to |Ag=2.01| = 57 MHz. The 57ꢀMHz value seemed to be somewhat at odds
with the Mössbauer analysis which yielded A = ꢀ62 MHz (see below), but the differences
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could be reconciled by allowing that Ag=2.01 extracted from EPR is not a principal axis value.
By assuming that the gꢀ and Aꢀtensors have a common zꢀaxis and allowing a rotation of 20°
around z, the splitting at gmid = g could be simulated for A = 62 MHz. Approximately
x’
x
consistent with this result, the DFT solution presented below yields α = 26°. Our simulations
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
suggest that the signals at the other two g values have much smaller Aꢀvalues (A ≈ ꢀ8(4)
y
MHz and A ≈ ꢀ 9(3) MHz, with the signs determined from the Mössbauer analysis). The S
z
=
½ center of 3b(CPCA) thus exhibits quite a large Aꢀtensor anisotropy. As expected, the
gmid splitting can be simulated as well by rotating A by 30° around y.
Our aim is to determine the nature and the Fe oxidation state of 3b(CPCA). As considered
2
37,45
for other κ ꢀacylperoxoiron(III) intermediates proposed for bioinspired iron catalysts,
the
III
2
obvious candidates are the three isoelectronic S = ½ states [(MeOꢀPyNMe )Fe (κ ꢀ
3
2
+
IV
2+
V
OOC(O)cy)] , [(MeOꢀPyNMe )Fe (O)(•OC(O)cy)]
and [(MeOꢀPyNMe )Fe (O)ꢀ
3
3
2
+
IV
(
OC(O)cy)] . Of these three options, the proposed Fe =O/carboxyl radical species would
be inconsistent with the observed Aꢀtensor anisotropy observed by EPR and Mössbauer
spectroscopy (see below) as an axial Aꢀtensor with two large components and one small one
2
1
1
IV
(along the FeꢀO bond) would be expected for the d d d configuration of the Fe =O
xy xz yz
IV
46,47
unit, as observed for all S = 1 Fe =O complexes reported thus far.
On the other hand, an
V
Fe =O complex would exhibit an Aꢀtensor with only one large (along x if the αꢀHOMO is
dyz) and two small components (y and z), reflecting an electronic structure with one
V
–
unpaired electron in one of the dꢀorbitals, as reported and analyzed for [(TAML)Fe (O)]
V
+ 18,24
and [(TMC)(O)Fe (NR)] .
By obtaining UVꢀVis spectra of 3b(CPCA) solutions and quantifying the spin
concentrations of the same samples, the extinction coefficient ε520 of 3b(CPCA) can be
determined. For two samples of 3b(CPCA) containing 0.5 mM Fe (UVꢀVis spectrum shown
in Figure 1), we obtained 0.223 mM and 0.245 mM spins. Based on the average of 0.234
mM, we conclude that 3b(CPCA) in these samples represents 47% of the iron in the sample,
in excellent agreement with the Mössbauer result (48%, see Figure 3), with an ε520 value ≈
ꢀ1
ꢀ1
7
,500 M cm . The uncertainties for the quoted ε are estimated to be ± 15 %. Note that we
ꢀ1
ꢀ1
43
reported an ε490 of ≈ 4,500 M cm for 2b(PAA), a value that we have confirmed in this
study, suggesting that the introduction of a 4ꢀmethoxy substituent on the pyridine of the
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supporting ligand of 3b(CPCA) can result in a higher extinction coefficient. The
implications of this observation will be discussed in a later section.
Mössbauer Studies. Figure 3A shows a 4.2 K Mössbauer spectrum of a sample
containing 3b(CPCA). The spectrum was recorded in a parallel field of B = 7.0 T; spectra
recorded for B = 2.0 and 4.0 T are shown in Figure 4. The feature outlined in magenta
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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6
7
8
9
0
1
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9
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3
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8
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2
3
4
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7
8
9
0
III
represents highꢀspin (S = 5/2) Fe contaminants (at least four species) accounting for ca.
4
2% of the Fe in the sample. Xꢀband EPR (see Section II of SI) revealed a major component
ꢀ1
III
with D ≈ 0.7 cm and E/D ≈ 0.16 representing about 70% of the highꢀspin (HS) Fe ; D and
E/D are the commonly used zeroꢀfield splitting parameters. SI describes our understanding
of the HS ferric components (in particular see Table S4). A 7.0ꢀT Mössbauer spectrum of a
decayed sample of 2b(PNA) showing mainly HS ferric components is also included (Figure
S45).
In the Mössbauer spectra of Figures 3 and 4, species 3b(CPCA) absorbs in the velocity
range – 3 mm/s < v < +2.5 mm/s. This range contains two contaminant lines from the highꢀ
III
spin Fe species (magenta curve in Figure 3A). Because the D values are small, the B = 7.0
III
T spectra of the Fe components are independent of D and E/D. Importantly, the intensities
and positions of the two contaminant lines in question are known once the positions and
shapes of the outer lines are determined. The outermost lines in Figure 3A result from M =
S
ꢀ
5/2 levels; the weak features near – 3 mm/s and +5 mm/s belong to M = ꢀ3/2 states which
S
III
are populated at 4.2 K by ≈ 8%. We have fitted the outermost features of the highꢀspin Fe
species with various assumptions and found that the position and intensity of the inner two
lines are essentially independent of these assumptions. Thus, by subtracting the simulated
III
highꢀspin Fe components (Table S4 in SI) from the raw data we obtain quite reasonable
representations of 3b(CPCA). By comparing various preparations of 2b(PAA) and
3b(CPCA), and creating difference spectra, we deduced that ≈ 5% of the Fe of the sample of
IV
2+
IV
Figure 3A belongs to [(MeOꢀPyNMe )Fe (O)(MeCN)] ; this Fe species (we have spectra
3
of this species for PyNMe ; see SI Figure S26) contributes a quadrupole doublet for B < 0.1
3
T (the blue line in Figure 3A represents a B = 7 T simulation of this contaminant.). Figure
3
B, then, shows the desired 7.0 T spectrum of 3b(CPCA).
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A comment about the quadrupole splitting, ꢁE , and the isomer shift, δ, of 3b(CPCA) is in
Q
order. If at higher temperatures, say 150 K, the relaxation of the electronic spin would be
fast compared to the nuclear precession frequencies, 3b(CPCA) would exhibit a quadrupole
doublet from which ꢁE and δ could readily be extracted. However, the relaxation rate of
Q
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3b(CPCA) is still too slow at 150 K. Attempts to increase this rate by inducing spinꢀspin
relaxation using a 5 mM Fe concentration sample failed. Thus, δ, the most important
parameter for determining the oxidation state of 3b(CPCA), has to be extracted by
simulating the paramagnetic hyperfine structure of the 4.2 K spectra. We have previously
V
+
described a similar situation for [(TMC)Fe (O)(NC(O)CH )] and its conjugate acid
3
V
2+ 24
[
(TMC)Fe (O)(NC(OH)CH )] .
3
Figure 3. (A) Mössbauer spectrum of 3b(CPCA) obtained at 4.2 K for B = 7.0 T, applied parallel to
5
7
the observed γ rays ( Fe concentration 1 mM). The magenta curve represents a spectral simulation
III
for the HS Fe contaminants (42 % of Fe). The blue curve outlines 5% of an indicated [(MeOꢀ
IV
2+
PyNMe )Fe (O)] contaminant. (B) Spectrum of 3b(CPCA), in black, obtained by subtracting the
3
magenta and blue curves from the black line shown in the top spectrum. The red line (drawn to
represent 48% of total Fe) is a spectral simulation for 3b(CPCA) for δ = ꢀ0.08 mm/s. The horizontal
arrows mark the splitting due to A . (C) Theoretical curve for 3b(CPCA) assuming a δ = 0.26 mm/s
x
III
2+
pertinent for a lowꢀspin ferric [(MeOꢀPyNMe )Fe (OOR)] species. We estimate that the sample
3
contains 45ꢀ50% of species 3b(CPCA). This percentage agrees well with the EPR result and the
II
fraction of 2b(CPCA) generated by reaction of [(PyNMe )Fe (CF SO ) ] with CPCA described in
3
3
3 2
Section III of SI.
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0
1
2
3
4
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9
0
1
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9
0
Figure 4. 4.2 K spectra of 3b(CPCA) recorded in applied fields of B = 2.0 T (A) and 4.0 T (B). The
magenta and blue lines indicate, respectively, Fe and Fe contaminants mentioned in the caption of
Figure 3.
III
IV
The principal features of the Mössbauer spectra are readily understood by taking into
5
7
account the EPR information that the Fe Aꢀtensor of 3b(CPCA) has one large component
along x) and two small ones, and that the gꢀvalues are nearly isotropic. The largest
(
magnetic splitting, indicated in Figure 3B by the horizontal arrows (the ‘base’), reflects
molecular orientations in the frozen solution sample for which the applied magnetic field is
near the axis of A ; our simulations suggest that A ≈ ꢀ 62 ± 2 MHz (Note: A = ꢀ62 MHz
x
x
x
corresponds to A /g β = ꢀ45.3 T, the quantity most often quoted in the Mössbauer
x
n n
literature). This value yields an effective magnetic field B = B + B = (ꢀ(1/2)ꢞ45.3 + 7.0)
eff
int
5
7
T = ꢀ 15.7 T at the Fe nucleus. Simulations of the inner ‘triplet’ feature yielded A ≈ ꢀ8(4)
y
MHz and A ≈ ꢀ 9(3) MHz, i. e. components with values that are substantially smaller than
z
that of A , consistent with the EPR results.
x
The component of the EFG tensor along A is negative, while those along y and z are
x
positive. These sign choices shift the ‘base’ to more negative and the ‘triplet’ to more
positive Doppler velocities. Our simulations, as well as leastꢀsquare group fits to the 2.0, 4.0
and 7.0 T spectra, suggest that ꢁE ≈ +1.15 mm/s and η ≈ 0.6. We have also recorded
Q
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spectra for B = 45 mT applied parallel as well as perpendicular to the γꢀrays. At T = 4.2 K
III
and B = 45 mT, the Fe contaminants contribute as many as twelve Mössbauer spectra, one
for each Kramers doublet of the four identified species. These spectra depend, for each
contaminant, on D, E/D, the Aꢀtensor, ꢁE , and parameters describing the distribution of the
Q
ꢀ1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
zeroꢀfield splitting parameters. Moreover, for D ≈ 0.7 cm the middle Kramers doublets
are ≈ 35% populated at 4.2 K and would produce spectra entirely hidden under the
absorption of 3b(CPCA). The 45 mT spectra were insufficiently resolved to be useful. The
spin Hamiltonian parameters of 3b(CPCA) that give the simulated spectrum in Figure 3B
are listed in Table 2. The parameters extracted for 2b(CPCA) are essentially the same as
those of 3b(CPCA); spectra of 2b(CPCA) obtained at B =7.0 T and 4.0 T are shown in
Figure S24 and Figure S25, respectively.
V
2+
Table 2. Spin Hamiltonian Parameters for [(MeOꢀPyNMe )Fe (O)(OC(O)cy)] (3b(CPCA)),
3
V
2+
[(PyNMe )Fe (O)(OC(O)cy)] (2b(CPCA)) and related complexes. Values predicted by DFT are
3
shown in italics.
a
Complex
g(x,y,z)
A(x,y,z) (MHz)
∆EQ
(mm/s)
η
δ (mm/s)
3b(CPCA)
2.01, 2.07, 1.94
ꢀ62(2), ꢀ8(4), ꢀ9(3)
+1.15(30)
0.6
0.3
0.2
0.3
0.3
ꢀ0.08(3)
ꢀ0.01
2
.02, 2.04, 1.99
ꢀ56, ꢀ18, ꢀ5
+0.78
2b(CPCA)
2.01, 2.07, 1.94 ꢀ62(2), ꢀ7(4), ꢀ10(4) +1.00(30)
2.02, 2.04, 1.99 ꢀ57, ꢀ18, ꢀ5 +0.86
2.01, 2.07, 1.94 ꢀ62(2), ꢀ6(4), ꢀ11(4) +1.11(25)
ꢀ0.06(3)
ꢀ0.01
2
b(PNA)
ꢀ0.08(3)
1
.99, 1.97, 1.74
ꢀ67.5(14), ꢀ2.0(20),
ꢀ22.3(21)
4.25(10) 0.65(10) ꢀ0.42(3)
V
– b, c
[
(TAML)Fe (O)]
ꢀ
ꢀ59.8, ꢀ13.0, ꢀ20.6
4.51
ꢀ0.2
ꢀ
0.72
ꢀ3
ꢀ0.39
+0.10
ꢀ
V
[(TMC)Fe (O)ꢀ
2.05, 2.01, 1.97
.03, 2.00, 1.97
ꢀ47(2), ꢀ17(2), 0(5)
2
+ d
(NC(OH)CH )]
3
2
ꢀ45, ꢀ14, ꢀ6
ꢀ
a
b
c
Numbers in parentheses are estimated uncertainties in the least significant digits.
18
Reported by Tiago de Oliveira et al.
24
V
ꢀ
Table 1 of van Heuvelen et al. listed the Aꢀvalues of [(TAML)Fe (O)] erroneously in units of
1
8
Tesla (A/g β ), as originally reported, rather than MHz as stated.
n
n
d
V
2+
The parameters of [(TMC)Fe (O)(NC(OH)CH )] are different from the other three complexes in
3
24
this table due to the strong trans effect of the imido ligand on the Fe=O bond.
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The isomer shift, δ = ꢀ0.08 ± 0.03 mm/s, of 3b(CPCA) was obtained by group fitting the 2.0,
4
.0 and 7.0 T spectra. The value of δ obtained is substantially more negative than the value
IV
2+
of δ = + 0.05 mm/s found for [(PyNMe )Fe (O)(MeCN)] , 4 (see Section IV of SI),
3
demonstrating that 3b(CPCA) must represent a species in a higher oxidation state than 4.
III
0
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0
1
2
3
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5
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8
9
0
Also, 3b(CPCA) cannot be assigned to an Fe ꢀperoxo species such as [(MeOꢀ
III
2
2+
PyNMe )Fe (κ ꢀOOCR)] for which we obtained a δꢀvalue of + 0.26 mm/s by DFT. For
3
illustration, Figure 3C shows a simulation for δ = +0.26 mm/s, a value clearly inconsistent
with the data. These arguments suggest the assignment of 3b(CPCA) as [(MeOꢀ
V
2+
PyNMe )Fe (O)(OC(O)cy)] , resulting from O–O bond heterolysis of an initially formed
3
III
2
2+
but unobserved [(MeOꢀPyNMe )Fe (κ ꢀOOCR)] precursor (for the PyNMe ligand, this
3
3
precursor is the gmax = 2.20 species, 2a(PAA)). We have assigned here a pure oxidation state
V
–
to the iron in 3b(CPCA). Such an assignment is perfectly appropriate for [(TAML)Fe (O)] ,
V
which by all measures is a pure S = ½ Fe ꢀoxo complex, as the tetraanionic TAML ligand
V
stabilizes the Fe oxidation state. Complexes with neutral supporting ligands such as MeOꢀ
V
2+
PyNMe and TMC respectively found in 3b(CPCA) and [(TMC)Fe (O)(NC(OH)CH )]
3
3
2
4
(
see Figure 5 of the work by Van Heuvelen et al. ) would have electronic configurations
IV
with some Fe /radical character due to the greater electrophilicity of the highꢀvalent metal
centers in these complexes. We return to this point in the section describing the DFT results.
The obtained Aꢀvalues also exclude the possibility that 3b(CPCA) is a complex comprising
IV
an S = 1 Fe =O complex antiꢀferromagnetically coupled to a carboxyl radical. Typical
IV
(
local) aꢀvalues (in ꢃ ∙ ꢟꢠꢡꢎ ∙ ꢇ , S = 1) for S = 1 Fe =O complexes are a ≈ a = ꢀ(25ꢀ29)
x
y
MHz and a ≈ ꢀ7 MHz. When referred to the spin Hamiltonian of an antiferromagnetically
z
IV
coupled Fe =O/radical state, these aꢀvalues would change to A ≈ A = ꢀ(33 ~ 44) MHz and
x
y
ꢠꢡꢎ/ꢏ
∙ ꢇ , S = ½); the two tensors are related by ꢆꢠꢡꢎ/ꢏ = (4/3) ꢟꢠꢡꢎ
A ≈ ꢀ9 MHz (in ꢃ ∙ ꢢ
z
,
where 4/3 is a spin projection factor. For the S =1 heme/porphyrin radical complex of
4
8
horseradish peroxidase, HRP I, Debrunner and coworkers reported a Mössbauer analysis
4
9
with a = a = ꢀ26.5 MHz, while an ENDOR study by Hoffman et al gave A = A = ꢀ35
x
y
x
y
MHz (thus a /A near 4/3). To conclude, the Mössbauer data of 3b(CPCA) show that the
x
x
IV
observed Aꢀtensor and the value of δ are not compatible with its assignment to an Fe ꢀoxo
radical species.
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The values for A obtained from the EPR and Mössbauer analysis are quite accurate.
x
Although A and A have larger uncertainties, reasonable simulations can be obtained by
y
z
simultaneously increasing A and decreasing A while keeping (A +A )/2 near –9 MHz.
y
z
y
z
We estimate that ꢁE is accurate within ± 0.3 mm/s; in the simulations this parameter is
Q
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
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2
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8
9
0
1
2
3
4
5
6
7
8
9
0
strongly correlated with the asymmetry parameter η, as both determine the component of the
EFG along Ax.
Resonance Raman and XAS Studies. To provide additional insight into the nature
of 2b(CPCA) and 3b(CPCA), further spectroscopic studies were carried out on these
intermediates and related complexes (Scheme 2). Resonance Raman spectra were obtained
II
for samples prepared by reacting the Fe precursor in a (v/v 1:3) CH CN/CD COCD
3
3
3
o
solution with 5ꢀ10 equiv. peracid at ꢀ65 C. Similar spectra were obtained for the three
different peracids used in this study (Table 1). Of particular use was pernonanoic acid
1
8
(
PNA), for which a method was available for the synthesis of the Oꢀlabeled peracid (see
ꢀ1
section XII in the SI). 2(PNA) exhibits two resonantly enhanced Raman bands at 815 cm
ꢀ1
and 829 cm in an 11:1 intensity ratio (Figure 5 top, black trace), which disappear, together
with the UVꢀVis chromophore, upon warming up the sample to room temperature. These
two features are also observed in the resonance Raman spectra of 2(PAA) and 2(CPCA)
18
with a comparable intensity ratio (Table 1 and Figure S32). With Oꢀlabeled PNA, the 815ꢀ
ꢀ1
ꢀ1
ꢀ1
cm band downshifted to 783 cm (Figure 5 top, red trace), corresponding to a 32ꢀcm
decrease that is consistent with its assignment to a ν(Fe=O) mode based on a Hooke’s Law
calculation for a diatomic Fe=O bond. On the other hand, 3(PNA) exhibits a pair of peaks at
ꢀ1
ꢀ1
8
13 and 821 cm (Figure 5 bottom, black trace), which collapse into one peak at 784 cm
1
8
18
with Oꢀlabeled PNA (Figure 5 bottom, red trace). Given that both Oꢀlabeled
intermediates exhibit essentially one vibration at the same frequency, the pair of peaks
1
6
ꢀ1
observed in 3( OꢀPNA) represent a Fermi doublet with a frequency centered at 817 cm
ꢀ1
that is assigned to the ν(Fe=O) mode of 3b. These two 800 cm features with near equal
intensities are also observed in the resonance Raman spectra of 3b(PAA) and 3b(CPCA)
(Figure S32).
ꢀ1
The 829 cm peak observed in the resonance Raman spectra of samples of 2, but not of 3,
belongs to a different species. Designated as 2a, it exhibits ~10% of the intensity of the
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ꢀ
1
corresponding 815 cm peak in samples of 2(PAA), 2(CPCA) and 2(PNA) that is assigned
18
ꢀ1
to 2b. The use of Oꢀlabeled PNA results in the disappearance of the 829 cm peak. If this
peak were to arise from an Fe=O unit, it would be expected based on Hooke’s Law to shift
ꢀ1
to ~795 cm but such a shifted feature is not observed at this frequency. On the other hand,
0
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it could arise from 2a, the acylperoxoiron(III) isomer of 2b and be assigned as its O–O
ꢀ1
stretch. Such a mode would have a Hooke’s Lawꢀcalculated downshift of ~47 cm and a
ꢀ1
predicted peak position of 782 cm , which unfortunately would be obscured by the
ꢀ1
18
dominant peak at 783 cm arising from Oꢀlabeled 2b (Figure 5).
ꢀ
1
ꢀ1
Scrutiny of the 783ꢀcm region does reveal a shoulder at 771 cm with about 15% the
ꢀ1
ꢀ1
16
intensity of the 783ꢀcm band. The 771ꢀcm band is not observed in the spectra of the Oꢀ
1
8
isotopomers, but is also observed in the spectrum of 3( OꢀPNA), even though the spectrum
1
6
ꢀ1
ꢀ1
of 3( OꢀPNA) does not exhibit a peak at 829 cm , so the 771ꢀcm feature cannot be
ꢀ1
associated with the 829ꢀcm peak. It is however resonanceꢀenhanced because it is absent in
16
the decayed samples. The band corresponding to the O isotopomer is presumably obscured
ꢀ1
by the 813/815ꢀcm peak. Thus we are unable to ascertain the identity of this peak at the
present time.
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Figure 5. Resonance Raman spectra in CH CN/CD COCD (v/v 1:3). Top: 2(PNA) (2 mM Fe) (λ
ex
3
3
3
1
6
18
=
515 nm) (black, using OꢀPNA, red, using OꢀPNA, and orange, decayed. Bottom: 3(PNA) (2
16
18
mM Fe) (λ = 515 nm) (black, using OꢀPNA, red, using Oꢀ PNA), and orange, decayed). All
ex
spectra were collected at 77 K. See Figure S32 for corresponding spectra using PAA and CPCA.
1
1
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The Mössbauer analysis of 3b(CPCA) favors its description as a species with predominantly
V
Fe (O) character. However, the Raman bands observed for 2b(PNA) and 3b(PNA) fall into
ꢀ1
the low end of the relatively narrow range of frequencies (798ꢀ862 cm ) found for nonheme
50
oxoꢀiron species described thus far. The values at the extremes of this range are observed
IV
2–
V
–
ꢀ1
for [(bTAML)Fe (O)] and [(bTAML)Fe (O)] , respectively, demonstrating a 64ꢀcm
difference in frequency that presumably reflects the 1ꢀunit change in oxidation state between
51
IV
+
IV
+
the two complexes. However [(TMC)Fe (O )(O CCF )] and [(TMC)Fe (O )(OTf)]
anti
2
3
syn
ꢀ1 46,52
exhibit respective Fe=O frequencies of 854 and 856 cm ,
which approach that of
(bTAML)Fe (O)] , despite having a 1ꢀunit lower oxidation state. This comparison suggests
that the higher Fe=O frequencies of these three complexes may in fact arise from having a
V
–
[
5
1,52
very weak ligand or no ligand at all trans to the oxo group.
In contrast,
V
+
ꢀ
[
(TMC)Fe (O)(NC(O)R)] and its conjugate acid have ν(Fe=O) values of 798 and 811 cm
1
24
V
,
despite being considered to be Fe =O complexes. The lower frequencies of this
acid/base pair have been suggested to reflect the strong electron donating character of the
transꢀimido ligand that weakens the Fe=O bond. These comparisons suggest caution in
deducing the iron oxidation state based on the Fe=O frequency alone.
XAS studies were carried out on a sample containing 2(CPCA) in frozen CH CN/acetone
3
(v/v = 1:3). Table 3 compares the XAS results of 2(CPCA) with those of its iron(II)
precursor (1) and iron(III) decayed product. A Kꢀedge energy of 7124.8 eV is observed (see
Section XI of SI for details of this analysis), which is 1.6 eV higher than that found for
II
2+
[
(PyNMe )Fe (NCMe) ] (S = 0) in this study, but 0.5 eV lower than the 7125.3ꢀeV value
3 2
V
–
V
–
18,51
reported for both [(TAML)Fe (O)] and [(bTAML)Fe (O)] complexes.
The Mössbauer
analysis of a sample from the same batch is described in Section III of SI (~50% 2b(CPCA),
7% 2a(CPCA), ~45% highꢀspin ferric species). To our surprise, the Kꢀedge energy for the
sample of 2(CPCA) that had been allowed to decay at room temperature is within error
essentially unchanged from that of 2(CPCA) itself (7124.9 eV). However, the Kꢀedge
energy of 7124.9 eV found for decayed 2(CPCA) falls within the range found for highꢀspin
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ferric centers, which can be as high as 7126.3 eV in coordination compounds. Thus, given
the overlap in the ranges for highꢀspin ferric centers and higherꢀvalent complexes, the Kꢀ
edge energy for 2(CPCA) alone does not allow us to determine the iron oxidation state of
the sample.
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Analysis of the preꢀedge region of 2(CPCA) reveals a peak with an area of 15.6 units, a
50
value smaller than typically found for nonheme oxoiron(IV) complexes (20ꢀ35 units). It is
also much smaller than the values of 52 and 65 units recently reported for the
IV
2–
V
–
51
[(bTAML)Fe (O)] and [(bTAML)Fe (O)] complexes, which reflect the square
pyramidal nature of the TAMLꢀbased complexes that leads to a significant distortion away
from centrosymmetry. Thus, the iron centers in 2(CPCA) are very likely sixꢀcoordinate.
Unlike the uninformative Kꢀedge energy comparison between 2(CPCA) and decayed
2
(CPCA), the corresponding EXAFS data comparison turn out to be more enlightening
(
Figure 6 and Table 3). The EXAFS analysis of 2(CPCA) reveals the presence of an O
scatterer at 1.63 ± 0.02 Å with an n value of 0.5, which is consistent with the Mössbauerꢀ
derived result that 2b(CPCA) represents 50% of the sample. The short 1.63 Å distance falls
within the narrow range of values ranging from 1.59 to 1.70 Å that are associated with highꢀ
2
,48,49
valent terminal Fe=O complexes.
The values at the low and high ends of this range
V
IV
belong to the Fe (O) and Fe (O) complexes of TAML and bTAML ligands,
1
8,51,54
respectively.
DFT calculations presented in the following section provide an electronic
structure picture that reconciles the apparent incompatibilities between the Raman and
IV
EXAFS results, that are at first glance suggestive of an Fe =O unit, and the EPR and
V
Mössbauer results that strongly favor an Fe oxidation state.
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Figure 6. Unfiltered Fe Kꢀedge EXAFS spectrum (black dotted line) and best fit (red solid line) of a
sample containing 50% 2b(CPCA). Inset: corresponding unfiltered kꢀspace data (black dots) and
best fit (red line). See Table 3 and Section XI in SI for further details.
Table 3. XAS and EXAFS analysis of 1, 2b(CPCA) and decayed 2b(CPCA).
Kꢀedge
eV)
Preꢀedge
(eV)
Preꢀedge
area
EXAFS analysis
2
ꢀ3
(
N path
r (Å) σ (x10 )
3
3
6 C
4 C
N/O
N/O
1.9
4.8
5.4
2.8
2.6
1.4
4.5
6.2
3.4
3.5
5.3
6.5
8.5
2.05
2.85
3.14
1
7123.1
7112.8
4.9 units
0
3
.5 N/O 1.63
N/O 1.99
2
b(CPCA)
7124.8
7124.8
7114.4
7114.3
15.6 units
14.3 units
2 N/O 2.17
4 C
4
3
3 N/O 2.19
6 C 2.97
2.86
3.00
C
N/O 2.02
decayed 2b(CPCA)
DFT Studies and Discussion. To provide insight into the electronic structure of
3b(CPCA) we have carried out a series of DFT calculations. For the reported studies of
V
2+
[
(TMC)Fe (O)(NC(OH)CH )] we used the BP86 functional, because the hybrid B3LYP
3
IV
57
functional favored an Fe =O/radical state that gave an Fe Aꢀtensor inconsistent with the
2
4
experimental data. In contrast, the BP86 functional produced, for reasons understood, a
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V
solution with substantial Fe =O character that rationalized the set of experimental
observations. Consequently, we have used here the BP86 functional for all calculations on
3
b(CPCA).
We have carried out geometry optimizations for 3b(CPCA) (Figure 7) and a variety of
0
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related complexes (see details in Table 4). In order to compare the unperturbed Feꢀoxo bond
length in different oxidation states, we have also optimized the structures of the two [(MeOꢀ
n+
PyNMe )Fe(O)(NCMe)] complexes (n = 2, 3) and obtained a 1.67ꢀÅ Fe=O distance for
3
IV
V
the Fe state and a 1.63ꢀÅ Fe=O distance for the Fe state (Table 4). Mössbauer spectra for
IV
2+
[
(PyNMe )Fe (O)(NCMe)] , 4, are presented in Section IV of SI. Geometry optimization
3
attempted for the peroxo species 2a(PAA) and 3a(CPCA) with BP86 resulted in minimized
V
structures for the corresponding Fe =O isomers, whereas the B3LYP optimization yielded a
III
2
2+
solution for the peroxo isomer 3a(CPCA), [(MeOꢀPyNMe )Fe (κ ꢀOOC(O)cy)] .
3
For the PyNMe and MeOꢀPyNMe ligands, geometry optimizations of the corresponding
3
3
highꢀvalent ironꢀoxo complexes yielded two isomers with rather similar properties that
reproduce the experimental data very well (see Section VII of SI for a comparison of the
calculated properties). One isomer has the oxo group trans to the pyridine, whereas the other
assumes the cis configuration. The α HOMO of both optimized structures (Figures 8 and
Figure S29) has approximate d symmetry and the normal of the orbital plane in both
yz
isomers is directed toward the bound carboxylate oxygen, with z essentially parallel to the
Feꢀoxo direction. The main text focuses on the trans isomer of 3b(CPCA), which is
calculated to be 0.6 kcal/mol lower in energy than its cis analog. A corresponding α HOMO
and a spin density plot of the cis isomer are shown in Figure S29.
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V
2+
Figure 7. Geometryꢀoptimized structure of [(MeOꢀPyNMe )Fe (O)(OC(O)cy)]
3
(3b(CPCA), trans isomer): Yellow, iron; red, oxygen; blue, nitrogen; grey, carbon. For
clarity, hydrogen atoms are not shown. Electronic dꢀorbital diagram obtained from TDꢀDFT
ꢀ1
is shown and the ꢁDFT is about 8000 cm . Selected bond lengths: Fe=O 1.65 Å; O1–O2 =
2
.04 Å; Fe–O3 = 1.93 Å; Fe–Npyr = 1.97 Å, Fe–N ≈ 2.08 Å. The DFT coordinates x, y, z
am
match the coordinates used for the spin Hamiltonian analysis, eqs. 1, to within a few
degrees. TDꢀDFT computations were performed for the optimized structure including 50
excited states. In Figure 7, the energy of d orbital was obtained from the excitation of the
yz
beta electron from d to d . The energies of other orbitals (d , d , dx2ꢀy2) were obtained
xy
yz
xz
z2
from the excitation of the alpha electron from d orbital to these orbitals.
yz
The calculated Fe=O bond length of 1.65 Å for 3b(CPCA) agrees within the error with the
1
.63 ± 0.02 Å value obtained by EXAFS. This value is larger than the 1.59 Å distance
V
ꢀ 18
observed for the 5ꢀcoordinate [(TAML)Fe (O)] ; distances calculated by DFT using
B3LYP and BP86 yielded 1.59 and 1.60 Å, respectively. In section IX of SI, we considered
V
–
the hypothetical 6ꢀcoordinate [(TAML)Fe (O)(NCMe)] , for which a relaxed scan was
performed along the FeꢀNMeCN distance. These calculations suggest that if the TAML
V
complex were 6ꢀcoordinate, it would have had an Fe =O bond length of ≈ 1.63 Å. We have
n+
also optimized the structures of the two [(MeOꢀPyNMe )Fe(O)(NCMe)] complexes (n = 2,
3
IV
V
3
), obtaining 1.67 Å for the Fe state and 1.63 Å for the Fe state (Table 4). Thus, the 1.63
Å observed here by EXAFS is consistent with BP86 structures for 6ꢀcoordinate (MeOꢀ
V
PyNMe )Fe species. The calculated properties are hardly affected by the MeO substitution;
3
see Table S5.
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Figure 8. Spin density plot (left) and α HOMO (right) of BP86 solution of the trans isomer of
V
2+
[
(MeOꢀPyNMe )Fe (O)(OC(O)cy)] (3b(CPCA)). Majority spin α in blue; minority spin β (mainly
3
on O2) in dark green in the left cartoon.
Interestingly, in 3b(CPCA) the nonꢀironꢀbound carboxylate oxygen (O2) is close to the oxo
group, O1, with an O1–O2 distance of 2.04 Å. It is noteworthy that the spin density plot of
V
Figure 8 shows negative spin density at O2, indicating that the Fe ground state of
IV
3
b(CPCA) is admixed with an Fe /radical configuration. An indirect measure for the
2
oxidation state of iron in the DFT solution is the expectation value of the operator S . For a
V
2
pure Fe S = ½ state this expectation value would be <S=1/2|S |S=1/2> = S(S+1) = 0.75
IV
whereas the Fe /carboxyl radical brokenꢀsymmetry (BS) configuration would yield
2
<
BS|S |BS> = 1.75. If we assume that only these two configurations contribute, we would
2
infer from the calculated <ΨBP86|S | ΨBP86> = 0.85 that the ground state of 3b(CPCA) is 90%
V
III
2
Fe . This argument may be somewhat misleading as the S = 1/2 [(MeOꢀPyNMe )Fe (κ ꢀ
3
2+
OOC(O)cy)] configuration may also contribute; we comment on this point below.
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Table 4. DFT values for spin populations, n = n ꢀ n , of orbitals and atoms, Fe isomer shifts, bond
α
β
lengths and Raman frequencies of the ironꢀoxo bond for 3b(CPCA) and related complexes.
V
V
IV
V
III
2
[(L*)Fe (O)ꢀ
[Fe (O)ꢀ
[(L*)Fe (O)ꢀ [(L*)Fe (O)ꢀ [(L*)Fe ꢀ(κ ꢀ
2
+
– b
2+
3+
2+
(
OC(O)cy)]
(TAML)]
(NCMe)]
(NCMe)]
OOC(O)cy)]
a
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
3b(CPCA))
d
trans (cis)
trans
1.67
trans (cis)
1.63 (1.63)
ꢀ0.12 (ꢀ0.14)
cis
FeꢀO1 bond (Å)
1.65 (1.66)
ꢀ0.01(ꢀ0.02)
1.60
1.82
e
Isomer shift
ꢀ0.40
+0.05
+0.26
(
mm/s)
ꢀ
1
ν(FeꢀO1) (cm )
837 (815)
ꢀ
875
ꢀ
841
ꢀ
903 (875)
ꢀ
ꢀ
ꢀ1
f
ν(O1ꢀO2) (cm )
789 & 732
2
c
<S >
0.84
0.76
0.07
0.84
N/A
2.01
0.87
0.89
N/A
0.94
0.77
0.04
0.57
0.00
{
Feꢀd + O1ꢀp }
0.12
0.26
0.89
N/A
xz
x
{
Feꢀd + O1ꢀp }
0.90
yz
y
O2
ꢀ0.18
a
b
c
L* = (MeOꢀPyNMe3)
Parameters obtained with BP86/6ꢀ311G
The trans isomer (oxo group trans to the pyridine) did not optimize in the Fe state using either the B3LYP
III
or BP86 functional. The cis isomer of this column was optimized using B3LYP/ 6ꢀ311G, but did not optimize
with BP86. Calculated O1ꢀO2 bond length is 1.54 Å.
d
e
f
V
The DFT solution may be admixed with a small amount of S = 3/2 Fe .
55
The calculated isomer shifts were obtained by the method of Vrajmasu et al.
The two modes also have significant cyclohexyl character.
V
The Fe assignment of 3b(CPCA) is supported by the spin densities of the key orbitals in
Table 4. Note that the d +p and d +p populations are nearly equal for the S = 1 [(MeOꢀ
yz
y
xz
x
IV
2+
PyNMe )Fe (O)(NCMe)] complex but that the d +p population is substantially smaller
3
xz
x
V
than the d +p population for the two Fe complexes. (This statement requires a caveat.
yz
y
III
2
2+
Thus, the relatively long Fe–O1 bond in [(MeOꢀPyNMe )Fe (κ ꢀOOC(O)R)] leads to an
3
III
interchange of the d and d orbitals, which puts the unpaired electron of the lowꢀspin Fe
xz
yz
V
into d . Admixture of the latter state into the Fe state would have little effect on the Aꢀ
yz
tensor.)
V
The predominant Fe nature of 3b(CPCA) is also evident from the isomer shifts, δ. The
V
2+
experimental value for [(MeOꢀPyNMe ) Fe (O)(OC(O)cy)] is δ = ꢀ0.08 mm/s; the BP86
3
values are ꢀ0.01 mm/s and ꢀ0.02 mm/s for the trans and cis isomers, respectively. For
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1
1
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
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5
5
5
5
5
5
5
6
IV
2+
[(PyNMe )Fe (O)(NCMe)] (4), (see Section IV of SI) we experimentally obtained δ =
3
+
0.05(1) mm/s with Mössbauer spectroscopy, which is accurately reproduced (Table 4) by
V
3+
the BP86 calculation. For the hypothetical [(MeOꢀPyNMe )Fe (O)(NCMe)] complex, a
3
BP86 calculation yielded δ = ꢀ0.14 mm/s. The B3LYP functional optimizes the cis isomer of
III
2
2+
III
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
[(MeOꢀPyNMe )Fe (κ ꢀOOC(O)R)] to a peroxoꢀbound Fe state with δ = +0.26 mm/s,
3
III
2+
which is close to the experimental +0.23 mm/s of the [(TPA*)Fe (OOC(O)CH )] complex
3
37
of Oloo et al.
To explain its low isomer shift, the iron center in 3b(CPCA) must be
III
2
2+
substantially more oxidized than those in [(MeOꢀPyNMe )Fe (κ ꢀOOC(O)R)] and 4,
3
V
again giving support to the Fe assignment for 3b(CPCA). The experimental δ = ꢀ0.08 mm/s
is more negative than the BP86 value of ꢀ0.01 mm/s, suggesting that the calculation has
IV
III
perhaps overestimated the (Fe /radical)/(Fe ꢀperoxo) content of the state. (Note that
complex 4, for which we have a δ, has the PyNMe ligand. Our DFT studies suggest that the
3
methoxy substitution would not affect the value of δ, see Table S5).
III
Above we indicated that the ground state of 3b(CPCA) may contain a small Fe ꢀperoxo
admixture. A qualitative argument sheds some light on this possibility. As shown in Table 4
V
3+
the calculated isomer shift of [(MeOꢀPyNMe )Fe (O)(NCMe)] (see Figure S27) is δ = ꢀ
3
2
0
.14 mm/s. The calculated <S > = 0.96 for this complex suggests that the S = ½ ground state
V
may have a small (≈ 4%) highꢀspin (S = 3/2) Fe admixture. However, this contamination is
too small to affect the calculated δ significantly. In the following analysis we adopted for
IV
2+
III
the [(MeOꢀPyNMe )Fe (•OCOcy)] and (MeOꢀPyNMe )Fe ꢀperoxo configurations the
3
3
IV
2+
experimentally observed value δ = +0.05 mm/s for [(PyNMe )Fe (O)(NCMe)] (4) and the
3
DFT value δ = +0.26 mm/s (Table 4), respectively. If the ground state of 3b(CPCA) would
V
IV
III
comprise 90% Fe and 10% Fe /radical, but no Fe ꢀperoxo admixture, we would obtain a
δ value near ꢀ0.12 mm/s, which is lower than the experimentally observed – 0.08 mm/s.
Using the isomer shifts listed in Table 4 and assuming that the ground state wave function of
V
IV
III
3b(CPCA) is an admixture of Fe =O (75%), Fe =O/radical (15%) and Fe ꢀperoxo (10%)
a δꢀvalue close to the experimental δ = – 0.08 mm/s is obtained (δ = (0.10 × 0.26 + 0.15 ×
0
.05 + 0.75 × (−0.14)) mm/s = −0.07 mm/s). A similar qualitative estimate rationalizes the
experimentally determined νFe=O. From the perspective of bond order, we calculated the
Wiberg bond orders of Fe=O1 and O1–O2 in 3b(CPCA). These calculations show that
3b(CPCA) has an O1–O2 bond order of 0.35, indicating that the peroxo bond is not entirely
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III
V
ruptured, which supports the mixing of Fe ꢀperoxo and Fe =O components in 3b(CPCA).
For comparison, we have also calculated the Fe–O bond orders of 3a(CPCA),
IV
2ꢀ
V
ꢀ
[
Fe (O)(TAML)] ,
[Fe (O)(TAML)] ,
and
the
hypothetical
[(MeOꢀ
IV
2+
PyNMe )Fe (O)(NCMe)] complex (see Table S10).
3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The DFT calculated gꢀ and Aꢀtensors have a common zꢀaxis which is within 4° of the Feꢀ
oxo direction, and the xꢀaxis of the Aꢀtensor is along the FeꢀO3 bond. The principal axes of
the gꢀtensor are rotated by α = 26° around z relative to the frame for A. The close agreement
with the experimental (α = 20°) value for this angle is probably a bit fortuitous. The DFTꢀ
calculated gꢀvalues of 3b(CPCA) reproduce the experimental data quite well; it is
noteworthy that the largest component of the Aꢀtensor is closely aligned with the axis for
gmid = g , just as found experimentally.
x
5
7
FC
The Fe magnetic hyperfine tensor, A, has Fermi contact (A ), anisotropic spinꢀdipolar
SD
L
(
A ) and orbital (A ) contributions. From the experimental A
= (ꢀ62, ꢀ8, ꢀ9) MHz we
x,y,z
FC
obtain A = (A + A + A )/3 = ꢀ26.3 MHz. In order to extract A we should subtract the
iso
x
y
z
L
isotropic part of A from A The closeness of the gꢀvalues of 3b(CPCA) to g = 2.00
iso.
implies that this contribution is small. The DFT calculation for the trans isomer yields
L
L
A
–
= (ꢀ3.4, +2.5, ꢀ0.3) MHz from which follows that A iso = ꢀ0.4 MHz and therefore Aiso
x,y,z
L
iso
FC
A
= A = ꢀ25.9 MHz. DFT calculations often underestimate the magnitude of the
FC
Fermi contact term, requiring the DFTꢀcalculated A values to be multiplied by 1.6 to
FC 56
FC
match the experimental A . However, the A (DFT) = ꢀ18.6 MHz obtained for the trans
FC
FC
FC
conformer suggests a smaller correction factor, A /A (DFT) = 1.42. The value A = ꢀ
25.9 MHz deduced for 3b(CPCA) is substantially larger than the A = ꢀ21.7 MHz obtained
FC
V
ꢀ
for [(TAML)Fe (O)] . This increase may possibly be due to the admixture of the
IV
V
Fe /radical configuration into the Fe ground state, thereby increasing the internal magnetic
5
7
field at the Fe nucleus. As the simulations of the Mössbauer spectra are based on an S = ½
Hamiltonian, the increase in the internal field would have to be accounted for by a larger
FC
value for A .
While the methoxy substituent on the supporting ligand affects the equilibrium
V
III
concentrations between the Fe =O and Fe ꢀperoxo forms (ratio 7:1 for PyNMe and 50:1
3
for MeOꢀPyNMe ), the calculated properties evaluated above are essentially insensitive to
3
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the substitution (see Table S5). Moreover, the EPR parameters for 2b(PAA), 2b(CPCA) and
3
b(CPCA) are the same and, within the experimental errors, the Mössbauer parameters of
b(CPCA) and 3b(CPCA) are also the same. Note, however, that λmax of the UVꢀvis
2
absorption feature of 3b(CPCA) (λmax 520 nm) is redꢀshifted relative to that of 2b(CPCA)
4
3
0
1
2
3
4
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6
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0
1
2
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4
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9
0
(λmax 505 nm), presumably due to the presence of the MeO substituent in 3b(CPCA).
ꢀ1
ꢀ1
The rRaman spectrum of 2(PNA) exhibits two features at 815 cm and 829 cm (11:1
intensity ratio), which disappear when 2(PNA) decays (See Figure S32 in SI). Both features
IV
50
fall within the range of frequencies typically observed for Fe =O complexes. We note that
ꢀ1
similarly low Fe=O frequencies of 798 and 811 cm have been observed for
V
+
V
2+
V
[
(TMC)Fe (O)(NC(O)CH )] and [(TMC)Fe (O)(NC(OH)CH )] , Fe =O complexes with
3 3
an electronic structure quite similar to that of 2b(PAA), 2b(CPCA) and 3b(CPCA). The low
ν(Fe=O) values for the two TMC complexes were rationalized by the trans effect of the
2
4
imido ligand. For 2b(CPCA) and 3b(CPCA) the oxo group is perturbed by residual
bonding to O2. The effect of this perturbation can be seen by comparing the ν(Fe=O) values
V
3+
V
ꢀ
listed in Table 4. Thus [(MeOꢀPyNMe )Fe (O)(NCMe)] and [(TAML)Fe (O)] are
3
ꢀ1
calculated to have ν(Fe=O) = 903 and 875 cm , respectively, compared to an experimental
ꢀ1
V
ꢀ 51
value of 862 cm reported for [(bTAML)Fe (O)] . On the other hand, calculations for
ꢀ1
2
b(CPCA) and 3b(CPCA) yield values of 836 and 815 cm for the trans and cis isomers,
respectively, with the latter value being observed experimentally for 2b(CPCA) (Figure 5).
ꢀ1
The downshifts of these values from ≈ 903 cm are probably caused by the perturbation of
the Fe=O1 oscillator by interactions with O2. Thus, while the ν(Fe=O) of unperturbed Fe=O
5
0
oscillators are good indicators of the oxidation state of the iron, the lower rRaman
frequencies of 2b(CPCA) and 3b(CPCA) reflect the presence of interactions between the
oxo group and O2 of the partially cleaved peroxo bond.
Conclusions and Perspectives
Complex 1 is a truly remarkable nonheme iron catalyst that uses peracetic acid as oxidant to
generate a highꢀvalent intermediate that hydroxylates cyclohexane at ꢀ40 °C at a record
ꢀ1 ꢀ1 1
4
breaking rate of 2.8 M s . This rate is 10 ꢀfold faster than that found for the well
V
– 18,23,51
characterized bona fide oxoiron(V) complex [Fe (O)(TAML)] .
This large difference
in reactivity can be rationalized by the use of the neutral PyNMe as a supporting ligand
3
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compared to the tetraanionic TAML ligand in the latter that significantly mitigates the high
V
electrophilicity of the Fe =O unit (see Table S11).
This metastable highꢀvalent intermediate accumulates at lower temperature, permitting its
characterization by a variety of spectroscopic methods. By replacing the PyNMe ligand of
3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
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1
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1
2
3
4
5
6
7
8
9
0
1 with MeOꢀPyNMe3 and changing the peracetic acid oxidant to cyclohexyl
peroxycarboxylic acid, we have been able to obtain the S = 1/2 intermediate [(MeOꢀ
V
2+
PyNMe )Fe (O)(OC(O)cy)] , 3b(CPCA), in sufficient purity to allow its detailed
3
characterization by Mössbauer spectroscopy. Intermediate 3b(CPCA) has an isomer shift δ
=
ꢀ0.08 mm/s, which is 0.13 mm/s more negative than that measured for
IV
2+
[
(PyNMe )Fe (O)(NCMe)] (4). Our spectroscopic studies thus show that 3b(CPCA) has
3
V
an electronic structure with predominant Fe character.
DFT calculations on 3b(CPCA) reveal a species with a rather unusual geometric and
electronic structure that affords spectroscopic parameters reasonably matching those
obtained experimentally. A notable feature of this structure, which applies for 2b(PAA),
2
b(CPCA) and 3b(CPCA), is the O1•••O2 distance of ≈ 2.04 Å obtained from DFT
calculations. This unique structure keeps the carboxylate O2 in close proximity to the Fe=O
V
unit, enabling the complex to maintain in solution a facile equilibrium between the Fe =O
III
43
and Fe ꢀperoxo forms, as described by SerranoꢀPlana et al. for 2a/2b; for 3(CPCA) this
equilibrium is shifted heavily in favor of 3b(CPCA). Given that the ground state of
IV
III
3
b(CPCA) has some Fe (O)ꢀcarboxyl radical and perhaps also some Fe ꢀperoxo character
V
admixed into the dominant Fe (O) nature of the intermediate, it might appear that
3
b(CPCA) is a species with a nearly severed peroxo bond that is more stable than both its
limiting acylperoxoiron(III) or its oxoiron(V) isomers. However, a relaxed scan of the O1–
O2 distance shows that the increase of r(O1–O2) does not further enhance the calculated
V
IV
Fe character of 3b(CPCA) but in fact results in an increase of Fe ꢀradical character as the
O1–O2 bond becomes cleaved homolytically. This result suggests that the above described
V
electronic structure of 3b(CPCA) has as much Fe character as the system will allow at the
BP86 level of theory. Clearly, additional theoretical studies are desirable, but the present
work indicates how its oxidative power might arise in an unexpectedly subtle manner along
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the O–O bond stretching reaction coordinate. This notion may hold the key to understanding
the high reactivity of 3b(CPCA) as a C–H bond cleaving species.
The PyNMe ꢀderived intermediates 2 and 3 described in this paper are the latest variants in
3
V
a series of proposed Fe (O) oxidants that have been postulated to be the actual C–H bond
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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0
1
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4
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1
2
3
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0
1
2
3
4
5
6
7
8
9
0
leaving agents in the chemistry of a number of nonheme iron catalysts, including some that
5
7–60
are found to be useful for late stage oxidations of natural products and pharmaceuticals.
For the Fe(TPA) catalyst (TPA = tris(pyridylꢀ2ꢀmethyl)amine) used in combination with
III
H O as oxidant, an [(TPA)Fe –OOH] intermediate is observed to form at ꢀ40 °C and then
2
2
undergoes rateꢀdetermining O–O bond cleavage to unmask the proposed
V
[
(TPA)Fe (O)(OH)] oxidant responsible for alkane hydroxylation and olefin epoxidation
6
1
and cisꢀdihydroxylation. On the other hand, for the related Fe(TPA*) catalyst (TPA* =
tris(4ꢀmethoxyꢀ3,5ꢀdimethylpyridylꢀ2ꢀmethyl)amine) in combination with the H O oxidant
2
2
III
37,62
and excess HOAc additive, a [(TPA*)Fe (O CR))] intermediate is generated instead.
3
Kinetic analysis of the behavior of this intermediate at ꢀ40 °C in the presence of various
substrates suggests that this species is in equilibrium with a small concentration of the actual
V
37,62
oxidant, proposed to be a [(TPA*)Fe (O)(OAc)] species.
identified by Talsi as giving rise to a minor g = 2.07 EPR signal that decays at a rate
The latter has in fact been
3
5,63
dependent on substrate concentration.
This signal is essentially identical to that
associated with the major EPRꢀactive components of 2 and 3 reported in this paper. Thus, in
the series of polydentate ligands from TPA to MeOꢀPyNMe that support bioꢀinspired
3
V
nonheme iron catalysts discussed in this paper, the notion of an Fe =O oxidant has
progressed from mere hypothesis in Fe(TPA) catalysis to being detected as a minor
component in the Fe(TPA*)/H O /HOAc system and then now to becoming the
2
2
predominant species observed at ꢀ40 °C in the Fe(MeOꢀPyNMe )/peracid combination with
3
detailed spectroscopic characterization.
III
The S = ½ intermediates associated with the above systems, namely [(TPA)Fe (OOH)],
III
[
(TPA*)Fe (O CR)], 2b and 3b, all exhibit visible absorption features around 500 nm, but
3
they progressively increase in intensity in the series with molar extinction coefficients of
III
64
III
37
~
1000 for [(TPA)Fe (OOH)], ~2500 for [(TPA*)Fe (O CR)], ~4500 for 2b, and ~7500
3
for 3b. We do not yet understand the basis for this trend of increasing intensity, but suggest
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that the higher absorptivity reflects the increasing fraction of the g = 2.07 species present in
the reaction mixtures, which we attribute the unusual electronic structures of 2b and 3b. The
observation that there is a significant change in extinction coefficient between 2b and 3b as
a result of a 4ꢀOMe substitution on the pyridine of the supporting ligand suggests that the
pyridine must be involved in this chromophore Further investigation is required to gain
insight into this phenomenon.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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7
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9
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3
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0
1
2
3
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0
1
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5
6
7
8
9
0
What makes the highly reactive intermediates 2b and 3b reported in this paper truly
extraordinary is the fact that they represent 40ꢀ50% of the Fe in the catalytic mixture and are
found to be catalytically competent in the C–H bond cleavage, as evidenced by the
43
dependence of their decay on the nature and concentration of substrate. The DFTꢀbased
electronic structure that we describe here for 2b and 3b may at first glance appear to most
readers to be a transition state in the homolytic cleavage of the O–O bond of an
acylperoxoiron(III) complex, but our calculations find an energy minimum at an O•••O
distance of 2.04 Å with a calculated Wiberg O–O bond order of 0.35. Only such a structure
thus far reproduces the spectroscopic data collected for 2 and 3. Decreasing the O–O bond
beyond 2.04 Å leads to the acylperoxoiron(III) electromer, while stretching the O–O bond
IV
beyond 2.04 Å evolves the system towards the (N )Fe (O)(•OC(O)R) electromer (See
4
Figure S46) upon complete O–O bond homolysis. According to our DFT calculations, it is
V
at the 2.04ꢀÅ O•••O distance where the Fe (O) character of the intermediate is maximized.
Thus we speculate that this oxidationꢀstate buffering mechanism stabilizes this species and
allows it to accumulate and be observed.
EXPERIMENTAL SECTION
Materials. Reagents and solvents used were of commercially available reagent quality unless
otherwise stated. Solvents were purchased from Scharlab, Acros or SigmaꢀAldrich and used without
further purification. Peracetic acid (PAA) was purchased from Aldrich as a 32 wt% solution in acetic
acid containing less than 6% H O . Cyclohexanyl peroxycarboxylic acid (CPCA) was prepared
2
2
65
18
following previously described procedures. 95% Oꢀlabeled H O (28% w/w in H O) was
2
2
2
purchased from Gioxcat. Preparation and handling of airꢀsensitive materials were carried out in a N2
drybox (Jacomex) with O and H O concentrations < 1 ppm. The synthesis of MeOꢀPyNMe and its
2
2
3
29
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