High-Spin and Low-Spin Perferryl Intermediates in Fe(PDP)-Catalyzed Epoxidations

Article abstract of DOI:10.1002/cctc.201900842

Two bioinspired hydroxo-bridged diferric complexes 6 and 7 with N₄-donor ligands of the PDP type (PDP=N,N′-bis(pyridin-2-ylmethyl)-2,2′-bipyrrolidine), differing by substituents at the pyridine rings (4-NMe₂ in 7 vs. 3,5-Me₂-4-OMe in 6), efficiently catalyze the enantioselective alkene epoxidation with H₂O₂ and peracetic acid in the presence of a carboxylic acid additive (up to 99 catalyst turnover numbers, TON, toward epoxide, up to 94 % ee). Catalyst systems based on complex 7 display the high-spin perferryl intermediate 7 a^AA (S=3/2, g₁, g₂=3.69, g₃=1.96), whereas catalyst systems based on complex 6 exhibit the low-spin perferryl intermediate 6 a^AA (S=1/2, g₁=2.07, g₂=2.01, g₃=1.96). The S=3/2 and the S=1/2 intermediates directly react with cyclohexene and cyclohexane at low temperatures (?40 °C and ?85 °C, respectively). The catalyst systems, exhibiting less reactive intermediate 7 a^AA, demonstrate higher enantioselectivity (% ee) in the epoxidation of chalcone. The origin of the unprecedented high-spin state of the perferryl intermediate is discussed.

Full text of DOI:10.1002/cctc.201900842

Accepted Article
Title: High-Spin and Low-Spin Perferryl Intermediates in Fe(PDP)
Catalyzed Epoxidations
Authors: Alexandra M. Zima, Oleg Y. Lyakin, Konstantin Petrovich
Bryliakov, and Evgenii P. Talsi
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10.1002/cctc.201900842
ChemCatChem
High-Spin and Low-Spin Perferryl Intermediates in
Fe(PDP) Catalyzed Epoxidations
Alexandra M. Zima,[a,b] Dr. Oleg Y. Lyakin,[a,b] Prof. Dr. Konstantin P. Bryliakov,*[a,b]
and Prof. Dr. Evgenii P. Talsi*[a,b]
[a] Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russian Federation.
[b] Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russian Federation
E-mail: bryliako@catalysis.ru, talsi@catalysis.ru
Abstract
Two bioinspired hydroxo-bridged diferric complexes 6 and 7 with N4-donor ligands of the PDP
type (PDP = N,N′-bis(pyridin-2-ylmethyl)-2,2′-bipyrrolidine), differing by substituents at the
pyridine rings (4-NMe2 in 7 vs. 3,5-Me2-4-OMe in 6), efficiently catalyze the enantioselective
alkene epoxidation with H2O2 and peracetic acid in the presence of a carboxylic acid additive (up
to 99 catalyst turnover numbers, TON, toward epoxide, up to 94% ee). Catalyst systems based on
complex 7 display the high-spin perferryl intermediate 7aAA (S = 3/2, g1, g2 = 3.96, g3 = 1.96),
whereas catalyst systems based on complex 6 exhibit the low-spin perferryl intermediate 6aAA (S
= 1/2, g1 = 2.07, g2 = 2.01, g3 = 1.96). The S = 3/2 and the S = 1/2 intermediates directly react
with cyclohexene and cyclohexane at low temperatures (−40 °C and −85 °C, respectively). The
catalyst systems, exhibiting less reactive intermediate 7aAA, demonstrate higher
enantioselectivity (% ee) in the epoxidation of chalcone. The origin of the unprecedented high-
spin state of the perferryl intermediate is discussed.
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Introduction
In the last 15 years, significant efforts have been invested in the investigations of the
mechanisms and the nature of active sites of biomimetic oxidations, mediated by synthetic iron
complexes modeling non-heme iron oxygenases. It is now generally accepted that the active
species of the catalyst systems based on iron complexes with N4-tetradentate ligands and
hydrogen peroxide, are oxoiron(V) intermediates.[1–5] Recently, the highly unstable iron-oxo
intermediates with relatively small g-factor anisotropy (g1 = 2.05–2.07, g2 = 2.005–2.01, g3 =
1.94–1.96), active toward selective olefin epoxidations, have been trapped in the systems based
on complexes 1,[6] 2,[3a] 2′,[5i] 3,[5c] 4,[5d] 4′,[5g] 5, 6[5e] and hydrogen peroxide or peroxycarboxylic
acids as oxidants (Figure 1).
These intermediates are further denoted as 1aAA, 2a, 2′a, 3a, 4aAA, 4′aCCA, 5aAA and 6aAA
(Figure 2, Table 1). Superscripts AA and CCA indicate that acetate anion or cyclohexane
carboxylate anion is present in the coordination sphere of the intermediate. When 2-
ethylhexanoic acid (EHA) is used as a catalytic additive, the corresponding intermediates are
denoted by superscript EHA. The assignment of intermediates 2a, 2′a, 3a, 4aAA and 4′aCCA to
the oxoiron(V) species has been corroborated by the data obtained using various spectroscopic
techniques (variable-temperature mass spectrometry, UV-visible, resonance Raman, Mössbauer
and EPR spectroscopy), and reactivity studies.[3a, 5c,d,g,i]
On the basis of EPR spectroscopic and reactivity data, the intermediates 5aAA and 6aAA
(Figure 2) were assigned to the formally oxoiron(V) intermediates.[5e] The close similarities of
the EPR spectra of 5aAA, 6aAA and those of the well spectroscopically characterized oxoiron(V)
intermediates 3a, 4aAA and 4′aCCA (Table 1) were taken in support for this assignment.[5c,d,g]
2
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Figure 1. Iron complexes with N4-donor tetradentate ligands.
It should be noted that precise electronic structure of the majority of listed formally
oxoiron(V) intermediates remains debatable. The results of the detailed bonding analysis of
complexes 2a, 3a and 4aAA on the basis of ab initio multireference CASSCF calculations suggest
that the only complex 2a is bona fide oxo-iron(V) species, while the formally perferryl
aminopyridine iron-oxo complex 4aAA (Figure 2) has a dominant character of SFe = 1 iron(IV)
center that is antiferromagnetically coupled to a σ*O–O radical. Complex 3a is best formulated as
a triplet ferryl unit, strongly interacting with the trans-acetylimidyl radical in antiferromagnetic
fashion.[5h]
Previously, we formulated 5aAA and 6aAA as (L•+)FeIV=O species rather than their (L)FeV=O
electronic isomers.[5e,7] More recently, Que, Lipscomb and coworkers published EPR
spectroscopic evidence in favor of (L)FeV=O electronic structure for the related iron-oxo
intermediate 1aAA formed in the catalyst system 1/CH3CO3H.[6] Later, an iron(V) oxidation state
of intermediate 4′aCCA (Figure 2) was suggested on the basis of Mössbauer data.[5g] In
subsequent publications, we have used the (L)FeV=O notation for 6aAA, living unambiguous
3
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10.1002/cctc.201900842
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discrimination between (L)FeV=O and (L•+)FeIV=O electronic structures to further investigations.
[5f,j]
Figure 2. Proposed structures of the formally oxoiron(V) intermediates observed in the catalyst
systems based on complexes 1-7 and H2O2 or CH3CO3H as oxidants.
Table 1. EPR spectroscopic data for S = 1/2 and S = 3/2 oxoiron species.
No.
Species
g1
g2
g3
Ref
1
2
3
4
5
6
7
8
9
[(TPA)FeV=O(OC(O)CH3)]2+ (1aAA)
[(TAML)FeV=O]− (2a)
2.07
1.99
2.01
1.97
1.96
1.74
6
3a
[(bTAML)FeV=O]− (2′a)
1.983 1.935 1.726
2.053 2.010 1.971
5i
[(TMC)FeV=O(NC(O)CH3)]+ (3a)
[(PyNMe3)FeV=O(OC(O)CH3)]2+ (4aAA)
[(MeOPyNMe3)FeV=O(OC(O)R)]2+ (4′aCCA
[(TPA*)FeV=O(OC(O)CH3)]2+ (5aAA)
[((S,S)-PDP*)FeV=O(OC(O)CH3)]2+ (6aAA)
[((S,S)-PDP*)FeV=O(OC(O)R)]2+ (6aEHA
5c
2.07
2.07
2.01
2.01
1.95
1.94
5d
)
5g
2.070 2.005 1.956
2.071 2.008 1.960
2.070 2.008 1.958
5e
5e
7
)
10 [((S,S)-PDPNMe2)FeV=O(OC(O)CH3)]2+ (7aAA) 3.96
11 [((S,S)-PDPNMe2)FeV=O(OC(O)R)]2+ (7aEHA
3.96
3.96
3.96
1.96 this work
1.96 this work
)
All intermediates considered above display EPR spectra characteristic of low-spin iron
species (S = 1/2), yet early DFT calculations predicted that the iron center of (TPA)FeV=O(OH)
4
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species should have quartet ground state.[8a] Very recently, Costas and coworkers have reported
on the same S = 3/2 ground state for the related species (5tips3TPA)FeV=O(OH) in gas phase,
using a variety of spectroscopic techniques and DFT calculations.[8b] However, so far there have
been no spectroscopic observations of S = 3/2 oxoiron(V) species in solution.
In this work we have found that the catalyst systems 7/H2O2/RCOOH (RCOOH = AA or
EHA, Figure 1) exhibit intermediates 7aAA and 7aEHA, with EPR spectra typical for high-spin (S
= 3/2) species (g1, g2 ∼ 4, g3 ∼ 2). Complex 7 is structurally similar to 6, the only difference
being the stronger electron-donating properties of the remote substituents (NMe2) at the pyridine
rings of the PDP ligand (Figure 1).
7aAA and 7aEHA can be reasonably assigned to the high-spin analogs of the low-spin
intermediates 6aAA and 6aEHA. It is interesting to compare the activity and selectivity of the S =
1/2 and S = 3/2 oxoiron(V) intermediates, because for the related oxoiron(IV) species, the spin-
state is considered as one of the key factors determining its chemical reactivity.[8] To this end,
catalyst systems 6/H2O2/RCOOH and 7/H2O2/RCOOH, as well as 6/CH3CO3H/RCOOH and
7/CH3CO3H/RCOOH, have been compared in the enantioselective epoxidation of chalcone, and
direct reaction of the intermediate 7aAA with cyclohexene has been monitored by EPR
spectroscopy at low temperature. In this study, we have returned to possible (L•+)FeIV=O
presentation of 6aAA and 7aAA for the purposes of comparing their electronic structures (see
below).
Results and discussion
The structure of the starting complexes 6 and 7 in the reaction solution
According to single-crystal X-ray data, 6 is the diferric complex [((S,S)-PDP*)FeIII(µ-
OH)2FeIII((S,S)-PDP*)](OTf)4.[5e] In agreement with its diferric structure, 6 is EPR silent in the
solid state and in solution (CH3CN/CH2Cl2 mixture). The range of 1H NMR paramagnetic shifts
of 6 (−10 ... 70 ppm, Figure S2A, Supporting Information, SI) is similar to that for the related
5
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diferric
complexes
[FeIII (µ-O)(µ-OH)(BPMEN)2](ClO4)3,
[FeIII (µ-O)(µ-
2
2
OH)(BPMCN)2](ClO4)3 and [FeIII (µ-OH)2(BPMEN*)2](ClO4)4 (BPMEN = N,N′-dimethyl-N,N′-
2
bis(2-pyridylmethyl)ethane-1,2-diamine, BPMCN = N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)-
trans-1,2-diaminocyclohexane,
BPMEN*
=
N,N′-dimethyl-N,N′-bis(3,5-dimethyl-4-
methoxypyridyl-2-methyl)ethane-1,2-diamine).[9–12] Hence, 6 retains its diferric structure in
CH3CN/CH2Cl2 mixture.
Several attempts to obtain X-ray quality single crystals of 7 were unsuccessful.
1
Nevertheless, the H NMR spectroscopic data suggest that the solution structures of 6 and 7 are
1
similar. The H NMR spectrum of 7 in CD2Cl2/CD3CN (v/v = 1.5:1) mixture at 20 °C (Figure
S2B, SI) displays two intense signals at δ 33.3 and 32.7 with total integral intensity 12H from
two NMe2 groups, and four signals 1:1:1:1 at δ 70.7, 63.9; 55.4, 49.4 with total integral intensity
8H from the β-protons of bipyrrolidine moiety. Besides, two resonances at δ −3.2 and −9.4 are
observed, that can be assigned to the α-protons of bipyrrolidine bridge. In agreement with this
1
assignment, the H NMR spectrum of 6 also contains signals at δ 67.9 and 50.8 from the β-
protons of bipyrrolidine bridge and those at δ −2.8 and −9.1 from the corresponding α-protons.
The broad signal at 25–35 ppm can belong to Me- and OMe-substituents of the pyridine rings of
1
6 (Figure S2A, SI). The similarity of the H NMR spectra of 6 and 7 clearly indicates that 7
exists in CH3CN/CH2Cl2 mixture as the diferric complex [((S,S)-PDPNMe2)FeIII(µ-
OH)2FeIII((S,S)-PDPNMe2)](OTf)4.
EPR spectroscopic trapping of the S = 3/2 iron-oxo intermediates in the catalyst systems
7/oxidant/RCOOH
As was noted, the diferric complex 7 is EPR silent in solution. The addition of acetic acid
(10 equiv) to the solution of 7 at room temperature leads to the appearance of a new stable ferric
complex 7bAA with an EPR signal at g = 4.3, characteristic of a mononuclear high-spin (S = 5/2)
ferric species in a rhombic coordination environment (Figure 3A). The addition of 10 equiv of
6
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EHA to the solution of 7 gives rise to the EPR signal of complex 7bEHA, resembling that of 7bAA
(Figure 3B).
Previously, we found that the reaction of diferric complex 6 with AA or EHA
([RCOOH]/[Fe] < 10) resulted in the formation of mononuclear low-spin (S = 1/2) complexes
[((S,S)-PDP*)FeIII(κ2-OC(O)R)]2+ (6bAA and 6bEHA), with rhombically anisotropic EPR
spectrum (g1 = 2.54, g2 = 2.41, g3 = 1.79).[5e,7] By analogy, 7bAA and 7bEHA can be assigned to
the high-spin (S = 5/2) ferric complexes [((S,S)-PDPNMe2)FeIII(κ2-OC(O)R)]2+ (RCOOH = AA or
EHA). ESI-MS data confirmed this assignment. Upon the addition of 10 equiv of acetic acid to
the solution of 7 at room temperature, a new signal from [((S,S)-PDPNMe2)FeIII(CH3COO)(OTf)]+
at m/z = 672.3 appeared; adding 50 equiv of AA converted most part of dinuclear complex 7 into
the monomeric species [((S,S)-PDPNMe2)FeIII(CH3COO)(OTf)]+ (Figure S5, Supporting
Information).
4.3
7bAA
A)
7bEHA
B)
7bAA
7aAA
1.96
7aAA
C)
7bEHA
Y
3.96
7aEHA
D)
7aEHA
1200
1800
2400
3000
3600
H / G
Figure 3. EPR spectra (−196 °C) of the samples (A) 7/AA ([Fe]:[AA] = 1:10) frozen 10 min
after mixing the reagents at 25 °C, (B) 7/EHA ([Fe]:[EHA] = 1:10) frozen 10 min after mixing
the reagents at 25 °C, (C) “A” frozen 1.5 min after mixing with H2O2 at −70 °C ([H2O2]:[Fe] =
7
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3:1), (D) “B frozen 1.5 min after mixing with H2O2 at −70 °C ([H2O2]:[Fe] = 3:1). [Fe] = 0.04 M
in all samples. CH2Cl2/CH3CN mixture (1.8:1 v/v) was used as a solvent. Y denotes resonances
(g⊥ = 1.96, g║ = 1.98) of unknown relatively stable iron complex. Blue dotted line in “C”
highlights resonance at g3 = 1.96 of the EPR spectrum of 7aAA.
After the addition of 3 equiv of H2O2 to the sample containing 7bAA and mixing the reagents
for 1.5 min at −70 °C, the EPR spectrum of a new complex 7aAA with signals at g1, g2 = 3.96,
and g3 = 1.96 appeared. The relatively broad resonance of 7aAA at g3 = 1.96 overlaps with a
sharper signal (g⊥ = 1.96, g║ = 1.98) from an unidentified low-spin ferric complex Y (Figure
3С). The maximum concentration of 7aAA does not exceed 3% of the total iron concentration.
Complex 7aAA is unstable and decays with first-order rate constant k1 = (2.0 ± 0.5) × 10−3 s−1 at
−40 °C (Figure 4A).
In the presence of 3 equiv of cyclohexene, the decay of 7aAA significantly accelerated (kobs
=
(7 ± 2) × 10−3 s−1 at −40 °C, Figure 4B), thus witnessing direct reaction of 7aAA with
cyclohexene. Moreover, it was found that 7aAA directly reacts with cyclohexane, too, the rate of
its decay at −40 °C increasing 4 times upon the addition of 20 equiv of cyclohexane into the
reaction solution (kobs = (8 ± 2) × 10−3 s−1, Figure S7, SI). We have also conducted competitive
catalytic oxidations of an excess of cyclohexene and cyclohexane with H2O2 in the presence of 7
(Table S1, Supporting Information). Despite the conditions required for the EPR measurements
and for the catalytic reaction are rather different, the observed ratio of TON values for
cyclohexene and cyclohexane under the catalytic conditions are in good qualitative agreement
with the observed ratio of rate constants for the reaction of 7aAA with cyclohexene and
cyclohexane under the EPR conditions.
8
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c /c0
1.0
1.0
0.8
0.6
0.4
0.2
k1 = 2 × 10−3 s−1
kobs = 7 × 10−3 s−1
0.8
0.6
0.4
0.2
(−40 °C)
(−40 °C)
7aAA
0
2
4
6
8
10 12 14 16
0
1
2
3
4
t / min
t / min
B
A
7bAA
7bAA
7aAA
0 min
7aAA
3.96
1.96
1.96
3.96
0 min
1 min
1 min
3 min
5 min
3 min
5 min
8 min
11 min
8 min
Y
17 min
Y
1400
2100
2800
3500
1400
2100
2800
3500
H / G
H / G
Figure 4. EPR spectra (−196 °C) (A) of the sample 7/H2O2/AA ([Fe]:[H2O2]:[AA] = 1:3:10, [Fe]
= 0.04 M) frozen after mixing the reagents during 2 min at −70 °C in a CH2Cl2/CH3CN mixture
(v/v = 1.8:1) and storing the sample at −40 °C for various times, and (B) of the same sample with
cyclohexene (0.12 M). Relative concentrations of 7aAA (c/c0) vs time calculated from the decay
of its EPR resonance at g1 = 3.96 are presented at the top. Y denotes resonances (g║ = 1.98, g⊥ =
1.96) of unknown relatively stable iron complex.
When AA was replaced with EHA as an additive, the intermediate 7aEHA with the EPR
spectrum, virtually identical to that of 7aAA, was observed (Figure 3D). The maximum observed
concentration of 7aEHA was much smaller than that of 7aAA. This complicates the evaluation of
the effect of cyclohexene on the decay rate of 7aEHA
.
The interaction of complex 7 with peracetic acid instead of H2O2/CH3COOH generated the
same species 7aAA (g1, g2 = 3.96, g3 = 1.96; upper blue spectra in Figure 5). The comparison of
the EPR spectra of the samples 7/H2O2/AA and 7/CH3CO3H/AA, recorded 15 min after storing
the samples at −40 °C, shows that in the system 7/CH3CO3H/AA (lower spectrum in Figure 5B),
9
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the starting complex 7bAA converts into unidentified complex Y (g║ = 1.98, g⊥ = 1.96), which
process may account for the catalyst degradation pathway under catalytic conditions.
A
B
oxidant: H2O2
oxidant: CH3CO3H
7aAA
3.96
7aAA
1.96
1.96
3.96
7bAA
7bAA
Y
Y
Y
4.3
4.3
1200
1800
2400
H / G
3000
3600
1200
1800
2400
3000
3600
H / G
Figure 5. EPR spectra (−196 °C) of the catalyst systems 7/oxidant/AA frozen 2 min after mixing
the reagents during 2 min at −70 °C (blue spectra) and storing the samples at −40 °C for 15 min
(black spectra). Oxidants: H2O2 (A), CH3CO3H (B). Conditions: [Fe]:[oxidant]:[AA] = 1:3:10,
[Fe] = 0.04 M, solvent CH2Cl2/CH3CN (1.8:1 v/v).
The direct reactivity of 7aAA toward cyclohexene clearly shows that it contains an oxidizing
moiety, which may be FeIII–OOH or FeV=O. The FeIV=O formulation can be ruled out, because
the corresponding FeIV=O complexes are EPR-silent.[1d] The EPR spectra of 7aAA and 7aEHA (geff
∼ 4 and 2) differ from the EPR spectra of known high-spin (S = 5/2) ferric complexes (L)FeIII–
OOH and (L)FeIII–OOtBu. The intermediates (L)FeIII–OOtBu (L = 6-MeTPA, 6-Me2TPA and 6-
Me3TPA, TPA = tris(2-pyridylmethyl)amine) display isotropic EPR signal at g = 4.3,
characteristic of a high-spin iron(III) center in a rhombic coordination environment.[13] The
intermediate
[(TMC)FeIII(OOH)]2+
(TMC
=
1,4,8,11-tetramethyl-1,4,8,11-
tetraazacyclotetradecane) exhibits EPR signals at geff = 6.8, 5.2 and 1.96, indicative of a high-
spin (S = 5/2) iron(III) species with a small rhombic distortion;[14] spectra of this type have also
been observed for high-spin (S = 5/2) intermediates [L1FeIII(OOtBu)]2+ and [L1FeIII(OOH)]2+ (L1
= bis(6-pivalamide-2-pyridylmethyl)-(2-pyridylmethyl)amine),[15] and [L2FeIII(OOH)]2+ (L2 =
(R,R)-N,N′-dimethyl-N,N′-bis(2-methyl-8-quinolyl)cyclohexane-1,2-diamine)[16] (Figure 6).
10
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Figure 6. High-spin (S = 5/2) iron hydroperoxo and iron alkylperoxo intermediates.
So, the EPR spectra of 7aAA and 7aEHA differ from those of known FeIII–OOH and FeIII–
OOR species. Moreover, so far there have been no precedents of direct reactivity of FeIII–OOH
and FeIII–OOR intermediates toward cyclohexene. Therefore, the assignment of 7aAA and 7aEHA
to the ferric hydroperoxo and ferric alkylperoxo species can be ruled out.
To date, there have been no data on the direct spectroscopic detection of high-spin (S = 3/2)
FeV=O complexes,[17] yet Meyer, Smith and co-workers predicted that high-spin oxoiron(V)
complexes of small rhombicity should display an EPR spectrum with two components at g ~ 4
and one component at g ~ 2.[18] Therefore, we believe that the assignment of 7aAA and 7aEHA to a
S = 3/2 species is feasible; discussion of this issue will follow the next section.
Asymmetric epoxidation of chalcone with the catalyst systems 7/oxidant/RCOOH and
6/oxidant/RCOOH
The catalyst systems, exhibiting the S = 1/2 (6aAA and 6aEHA) and the S = 3/2 (7aAA and
7aEHA) active species, have been compared in the asymmetric epoxidation of chalcone (Table 2).
Both catalysts are reasonably stable, demonstrating high catalytic efficiencies under the applied
conditions (up to 200 catalytic turnovers), affording enantiomerically enriched chalcone epoxide
in up to nearly quantitative yield.
The system 7/H2O2/AA is significantly more enantioselective than the system 6/H2O2/AA
(86 vs. 63% ee, Table 2, entries 6 and 1), and the system 7/H2O2/EHA is more enantioselective
than 6/H2O2/EHA (94 vs. 82 % ee, Table 2, entries 7 and 2). To be more enantioselective, the
11
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intermediate 7aAA, operating in the system 7/H2O2/AA, apparently should be less reactive than
6aAA, operating in the system 6/H2O2/AA. Indeed, 6aAA rapidly reacts with cyclohexene even at
−85 °C,[5e] whereas 7aAA displays comparable reactivity only at −40 °C. These data show that the
putative S = 3/2 iron-oxo species 7aAA is less reactive but more enantioselective in chalcone
epoxidation than its low-spin (S = 1/2) congener 6aAA. Apparently, the attenuated reactivity and
better enantioselectivity of 7aAA vs. 6aAA cannot be directly ascribed to the difference in the
spin-state (S = 3/2 vs. S = 1/2). For example, the system 6/H2O2/RCOOH displays better
chalcone epoxidation enantioselectivity than the related system Fe(PDP)/H2O2/AA, despite in
both systems, the active iron-oxygen intermediates are low-spin complexes.[7] The enhanced
enantioselectivities exhibited by catalyst 7 in this work correspond to the data of Costas and
coworkers, who attributed high enantioselectivity of the iron catalyst with NMe2-substituted PDP
ligand to the attenuation of the reactivity and electrophilicity of the respective, presumably
oxoiron(V) intermediate.[3e] In turn, the high-spin state of intermediates 7aAA and 7aEHA is of the
same origin as their attenuated electrophilicity, namely due to the strong ligand field, induced by
the strongly electron-donating NMe2 substituents.
Table 2 Asymmetric epoxidation of chalcone with various oxidants catalyzed by complexes 6
and 7.
Entry Catalyst Oxidant
Additive Epoxide yield [%] ee [%][a]
1
2
3
4
5
6
7
8
9
H2O2
H2O2
AA
EHA
–
75
89
54
49
53
20
33
99
99
63
82
67
67
67
86
94
83
83
6
6
6
6
6
7
7
7
7
CH3CO3H
CH3CO3H
CH3CO3H
H2O2
AA
EHA
AA
EHA
–
H2O2
CH3CO3H
CH3CO3H
AA
12
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10
11
CH3CO3H
CH3CO3H
EHA
–
99
0
84
–
7
–
Reaction conditions: chalcone (100 µmol), oxidants, H2O2 (200 µmol), CH3CO3H (110 µmol),
carboxylic acid (55 µmol), catalyst load 0.5 mol.% (1 mol.% Fe), oxidant was added by a syringe
pump over 30 min at 0 °C, and the mixture was stirred for an additional 2.5 h at 0 °C, followed
by workup and analysis (see Supporting Information for the details). [a] Absolute configuration
of chalcone epoxide was (2R,3S).
The catalyst systems 7/CH3CO3H, 7/CH3CO3H/AA and 7/CH3CO3H/EHA display virtually
identical enantioselectivities (83−84% ee, Table 2, entries 8-10). The same picture has been
documented for the systems 6/CH3CO3H, 6/CH3CO3H/AA and 6/CH3CO3H/EHA (67% ee,
Table 2, entries 3-5). In either case, the presence and nature of additive (either AA or EHA) does
not influence the epoxide yield and ee, suggesting that the active oxygen-transferring species
contain no carboxylic moiety derived from the additive.
By contrast, the systems 7/H2O2/AA and 7/H2O2/EHA, 6/H2O2/AA and 6/H2O2/EHA
showed substantially different enantioselectivities (86 and 94% ee, Table 2, entries 6 and 7; 63
and 82% ee, entries 1 and 2), sensitive to the nature of carboxylic acid.
Previously, we have proposed that in the catalyst systems 6/CH3CO3H/RCOOH,
acylperoxo-iron(III) complexes are principal oxidizing intermediates, whereas in catalyst
systems 6/H2O2/RCOOH, the epoxidation is conducted by the oxoiron(V) species.[19] In the
former case, κ2-coordinated acylperoxo moiety hampers the coordination of carboxylic acid to
the iron center of the oxygen-transferring species, thus making the enantioselectivity of chalcone
epoxidation insensitive to the nature of carboxylic acid additive. In the latter case, the anion of
carboxylic acid is incorporated into the structure of the active intermediate
[(L)FeV=O(OC(O)R)]2+, so the structure of carboxylic acid directly affects the enantioselectivity.
Herewith, one can also conclude that it is the high-spin (S = 5/2) iron-acylperoxo intermediate
[((S,S)-PDPNMe2)FeIII(κ2-OOC(O)CH3)]2+ (7c) which predominantly conducts chalcone
epoxidation by the catalyst systems 7/CH3CO3H/AA and 7/CH3CO3H/EHA. Unfortunately,
13
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10.1002/cctc.201900842
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direct detection of 7c by EPR spectroscopy is likely impossible: the putative high-spin reactive
species 7c is expected to display EPR resonance at g = 4.3, overlapping with the intense signal of
the [((S,S)-PDPNMe2)FeIII(κ2-OC(O)CH3)]2+ complex (7bAA),[20] which is the major EPR-visible
species in the reaction solution (Figure 3A,C).
Alternatively, Costas and coworkers concluded that the same oxygen-atom transferring
species (LFeV=O) operate in the related catalyst systems containing H2O2/RCOOH and R′CO3H
as oxidants, and Z-β-methylstyrene, 1-cyclohexanone and benzylideneacetone as substrates. This
conclusion was made on the basis of the identity of the enantioselectivity values for the systems
containing H2O2/RCOOH and R′CO3H oxidants.[21]
Discussion of the nature of the S = 3/2 intermediates
Given the similarity of the catalytic properties of the catalyst systems based on complexes 6
and 7, and of the reactivities of the S = 1/2 intermediates 6aAA and 6aEHA, on the one hand, and
the S = 3/2 intermediates 7aAA and 7aEHA, on the other hand, the origin of the high-spin state of
7aAA and 7aEHA is very intriguing. The only apparent difference between 6 and 7 is the much
higher electron-donating properties of the remote p-NMe2 substituents in 7 (Brown-Okamoto
+
+
constant σp = −1.7) compared with the remote p-OMe substituents in 6 (σp = −0.78), which
reflects the superior ability to stabilize positive charge at the NMe2-substituted aromatic rings.
The p-OMe substituted intermediates 6aAA and 6aEHA were previously assigned to [((S,S)-
PDP*)•+FeIV=O(OC(O)R)]2+ structure,[5e] with preferential localization of the unpaired spin at the
aromatic ligand, the S = 1/2 overall spin resulting from the antiferromagnetic coupling between
the S = 1/2 ligand cation radical and the S = 1 iron(IV) center, likewise in compounds I of heme
enzymes, [(TBP8Cz)•+FeIV=O] and [(Me3TACN)FeIV=O({Cl-acac}•+)]2+ (TBP8Cz = octakis(4-
tert-butylphenyl)corrolazinato, Me3TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane, Cl-acac
=3-chloro-acetylacetonate).[22]
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In 2017, Wang and co-workers performed a detailed DFT calculation of electronic structure
of iron-oxo species formed in the catalyst system [(PDPNMe2)FeII(OTf)2]/H2O2/CH3COOH, based
on mononuclear ferrous complex with the same PDPNMe2 ligand as for the dinuclear ferric
complex 7 studied in our work. According to these calculations, the quartet (S = 3/2) state of the
formally
perferryl
oxygen-transferring
species
in
the
catalyst
system
Fe(PDPNMe2)/H2O2/CH3COOH lies lower in free Gibbs energy than the doublet (S = 1/2) and
sextet states (S = 5/2) by 1.2 and 4.8 kcal/mol, respectively.[23] Theoretical results by Wang and
co-workers predict that the key epoxidizing species of the catalyst system
[(PDPNMe2)FeII(OTf)2]/H2O2/CH3COOH should have a predominant character of the
[(PDPNMe2)•+FeIV=O(OC(O)CH3)] type species, with the S = 1 FeIV=O moiety, coupled
ferromagnetically to the ligand radical cation (S = 1/2), stabilized by strong electron-donating p-
NMe2 group, thus providing the plausible explanation of the electronic structure of the EPR
spectroscopically observed highly reactive S = 3/2 iron-oxo species 7aAA and 7aEHA. It should be
noted that the early DFT calculations of the intermediate cis-HO–FeV=O formed upon O–O bond
heterolysis in the hydroperoxo complex (TPA)FeIII–OOH (TPA = tris(2-pyridylmethyl)amine)
predicted that cis-HO–FeV=O should have a quartet ground state,[8a] yet until now the S = 3/2
reactive intermediates in catalyst systems based on aminopyridine iron complexes have not been
experimentally observed.
In a simplified way, this may be described as follows. The FeIV=O (S = 1) unit and ligand
cation radical (S = 1/2) are coupled antiferromagnetically in 6aAA and ferromagnetically in the
related intermediate 7aAA. So far, there have been no examples of such different spin interactions
in structurally related nonheme iron complexes. However, for heme counterparts, such examples
have been reported. For example, in Compound I of cytochrome P450 enzyme CYP119, having
S = 1/2 overall spin, the FeIV=O (S = 1) and a ligand cation radical (S = 1/2) moieties are coupled
antiferromagnetically,[22b] whereas synthetic models of Compound I display ferromagnetic
coupling with S = 3/2 ground state.[24]
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Conclusions
Catalyst systems 7/oxidant/RCOOH (7 = Fe(PDP) complex with p-NMe2 substituents at the
pyridine rings; oxidant = H2O2 or CH3CO3H; carboxylic acid additive RCOOH = acetic acid or
2-ethylhexanoic acid) exhibit high-spin (S = 3/2) iron-oxo intermediates 7aAA and 7aEHA (g1, g2
= 3.96, g3 = 1.96). For intermediate 7aAA, direct reactivity toward cyclohexene at −40 °C has
been demonstrated.
The observation of the S = 3/2 iron-oxo intermediates is in sharp contrast to the previously
studied catalyst systems 6/oxidant/RCOOH, based on the related complex 6 with p-OCH3 and m-
CH3 substituents at the pyridine rings, featuring low-spin (S = 1/2) iron-oxo intermediates 6aAA
and 6aEHA (g1 = 2.07, g2 = 2.01, g3 = 1.96).
The catalytic patterns of the systems exhibiting the high-spin (S = 3/2) and the low-spin (S =
1/2) iron-oxo intermediates have been compared in the asymmetric epoxidation of chalcone. The
systems 7/H2O2/AA and 7/H2O2/EHA, exhibiting the high-spin (S = 3/2) intermediates,
demonstrate higher enantioselectivity (ee) in the epoxidation of chalcone. Direct reactivity
studies have shown that intermediates 7aAA and 7aEHA are less reactive toward cyclohexene than
intermediates 6aAA and 6aEHA. The attenuated epoxidation reactivity of 7aAA and 7aEHA well
conforms to the higher enantioselectivity of the catalyst systems displaying these intermediates
in chalcone epoxidation.
Based on the EPR spectroscopic and chemical reactivity data, as well as on the DFT studies
by Wang and co-workers, the origin of the unprecedented high-spin (S = 3/2) state of the
formally oxoiron(V) intermediates 7aAA and 7aEHA in solution can be reasonably explained in
terms of the [((S,S)-PDPNMe2)•+FeIV=O(OC(O)R)]2+ structure, combining PDPNMe2 ligand cation
radical (S = 1/2), ferromagnetically coupled to the S = 1 ferryl moiety. This work provides the so
far missing examples of the active iron-oxo species in the catalyst systems bearing very strong
16
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electron-donating substituents, thus summarizing a consistent picture of the nature of the
perferryl intermediates observed in Fe(PDP) based and related catalyst systems.
Experimental Section
The detailed experimental procedures and data are provided in the Supporting Information.
Acknowledgements
This work was conducted with the financial support from the Russian Science Foundation
(project 17-13-01117). We thank Dr. Dmitriy G. Sheven (Nikolaev Institute of Inorganic
Chemistry SB RAS) for the ESI-MS measurements.
Conflicts of interest
There are no conflicts to declare.
Keywords: Enzyme models, epoxidation, EPR spectroscopy, iron, reaction mechanisms
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FULL PAPER
High-spin, higher selectivity:
Bioinspired catalyst system,
based on chiral non-heme iron
complex, featuring high-spin (S =
3/2) perferryl intermediate,
stabilized by strongly electron-
donating NMe2 groups, ensures
higher enantioselectivity in olefin
epoxidations with H2O2 than
analogous catalyst systems
exhibiting low-spin (S = 1/2)
perferryl intermediates.
A. M. Zima, O. Y. Lyakin, K. P.
Bryliakov,* E. P. Talsi*
Page No. – Page No.
High-Spin and Low-Spin
Perferryl Intermediates in
Fe(PDP) Catalyzed Epoxidations
22
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