Effect of the Ligand Backbone on the Reactivity and Mechanistic Paradigm of Non-Heme Iron(IV)-Oxo during Olefin Epoxidation

Article abstract of DOI:10.1002/anie.202102484

The oxygen atom transfer (OAT) reactivity of the non-heme [Fe^IV(2PyN2Q)(O)]²⁺ (2) containing the sterically bulky quinoline-pyridine pentadentate ligand (2PyN2Q) has been thoroughly studied with different olefins. The ferryl-oxo complex 2 shows excellent OAT reactivity during epoxidations. The steric encumbrance and electronic effect of the ligand influence the mechanistic shuttle between OAT pathway I and isomerization pathway II (during the reaction stereo pure olefins), resulting in a mixture of cis-trans epoxide products. In contrast, the sterically less hindered and electronically different [Fe^IV(N4Py)(O)]²⁺ (1) provides only cis-stilbene epoxide. A Hammett study suggests the role of dominant inductive electronic along with minor resonance effect during electron transfer from olefin to 2 in the rate-limiting step. Additionally, a computational study supports the involvement of stepwise pathways during olefin epoxidation. The ferryl bend due to the bulkier ligand incorporation leads to destabilization of both (Formula presented.) and (Formula presented.) orbitals, leading to a very small quintet–triplet gap and enhanced reactivity for 2 compared to 1. Thus, the present study unveils the role of steric and electronic effects of the ligand towards mechanistic modification during olefin epoxidation.

Full text of DOI:10.1002/anie.202102484

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International Edition: doi.org/10.1002/anie.202102484
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Oxygen Atom Transfer
Effect of the Ligand Backbone on the Reactivity and Mechanistic
Paradigm of Non-Heme Iron(IV)-Oxo during Olefin Epoxidation
Jyoti Prasad Biswas+, Mursaleem Ansari+, Aniruddha Paik, Sheuli Sasmal, Sabarni Paul,
Sujoy Rana,* Gopalan Rajaraman,* and Debabrata Maiti*
Abstract: The oxygen atom transfer (OAT) reactivity of the
non-heme [FeIV(2PyN2Q)(O)]2+ (2) containing the sterically
bulky quinoline-pyridine pentadentate ligand (2PyN2Q) has
been thoroughly studied with different olefins. The ferryl-oxo
complex 2 shows excellent OAT reactivity during epoxidations.
The steric encumbrance and electronic effect of the ligand
influence the mechanistic shuttle between OAT pathway I and
isomerization pathway II (during the reaction stereo pure
olefins), resulting in a mixture of cis-trans epoxide products. In
contrast, the sterically less hindered and electronically different
[FeIV(N4Py)(O)]2+ (1) provides only cis-stilbene epoxide. A
Hammett study suggests the role of dominant inductive
electronic along with minor resonance effect during electron
transfer from olefin to 2 in the rate-limiting step. Additionally,
a computational study supports the involvement of stepwise
pathways during olefin epoxidation. The ferryl bend due to the
bulkier ligand incorporation leads to destabilization of both dz2
and dx2Ày2 orbitals, leading to a very small quintet–triplet gap
and enhanced reactivity for 2 compared to 1. Thus, the present
study unveils the role of steric and electronic effects of the
ligand towards mechanistic modification during olefin epox-
idation
realm of non-heme chemistry, tetradentate ligand-based
iron-complexes were employed for olefin cis-dihydroxylation
and epoxidation reactions.[12–36] In these cases iron(V)-oxo
complexes have been proposed as the active species. Non-
heme manganese catalysts were also found to be suitable for
asymmetric epoxidations and cis-dihydroxylations.[31–45]
Surprisingly the pentadentate ligand containing iron(IV)-
oxo complexes are only a little explored for olefin epoxida-
tions. A recent study has revealed that [FeIV(N4Py)(O)]2+ can
effectively transfer oxygen atom to olefins in the presence of
excess triflic acid (TfOH).[46] The triflic acid coordinates with
iron(IV)-oxo to facilitate the proton coupled electron transfer
from olefin and subsequently favours the oxygen atom
transfer to the olefin to provide the epoxides. Therefore, we
opted to explore the oxygen atom transfer (OAT) reactivity of
2 with different olefins and investigate the mechanistic details
À
(Scheme 1). In our earlier report, we have described the C H
halogenation and oxidation chemistry with sterically bulky
pentacoordinate ligand (2PyN2Q) containing ferryl-oxo com-
plex, [FeIV(2PyN2Q)(O)]2+ (2).[47]
Introduction
Ferryl-oxo species are the key intermediates for various
À
natural processes such as olefin epoxidation, C H halogen-
ation, hydroxylation, etc.[1–7] The activity of the responsible
enzymes for the transformations like oxygenases, hydroxy-
lases, halogenases, and haloperoxidases are governed by high-
valent iron-oxo intermediates.[7–9] In the synthetic scenario,
several Cyt P450 model systems were utilized for olefin
epoxidations and mechanistic investigations.[5,10–12] In the
Scheme 1. Olefin epoxidation by [FeIV(2PyN2Q)(O)]2+ (2), quinoline-
pyridine containing ligand backbone.
[*] J. P. Biswas,[+] M. Ansari,[+] S. Sasmal, G. Rajaraman, D. Maiti
Department of Chemistry
Indian Institute of Technology Bombay
Powai, Mumbai-400076 (India)
E-mail: rajaraman@chem.iitb.ac.in
dmaiti@chem.iitb.ac.in
Results and Discussion
A. Paik, S. Paul, S. Rana
Department of Chemistry
University of North Bengal
Raja Rammohunpur, Darjeeling, West Bengal, Pin-734013 (India)
E-mail: sujoyran@gmail.com
Initially, we have synthesized the iron(IV)-oxo 2 by
reacting iron(II)-triflate complex, [FeII(2PyN2Q)](OTf)2]
(1a) and 1.5 equivalent of iodomesitylene diacetate in dry
acetonitrile at room temperature (Figure 1). Complex 2 was
spectroscopically characterized using Mossbauer, IR study,
and ESI-MS measurement.[47,48] Subsequently, complex 2 was
reacted with different internal and terminal olefins. The
reaction of cyclooctene, norbornene with 2 provided cyclo-
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202102484.
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substituted styrene (internal olefin), 2-methyl-1-phenyl-1-
propene, also provided corresponding epoxide products with
good yield (Table 1, entry 7). Further, we have synthesized
a soluble version of iodosobenzene that is, 1-(tert-butylsul-
fonyl)-2-iodosylbenzene, (SO2t-Bu)PhIO[49a] and reacted
1.0 equiv of it with 1a in acetonitrile. The resulting reaction
mixture shows a UV-vis band at 770 nm, characteristic for
iron(IV)-oxo species. Afterwards, we have performed the
epoxidation reaction with trans-b-methylstyrene and ob-
tained epoxide product albeit in low yield. Similarly, iodo-
sylbenzene (PhIO) is solubilized in 4:1 acetonitrile-methanol
and reacted with 1a, resulting the formation of iron(IV)-oxo
species (UV-vis band at 770 nm). Further, this soluble form of
PhIO affords corresponding epoxide product upon reacting
with trans-b-methylstyrene in low yield. Therefore, possibility
of in situ generation of acidic species (during reaction with
iodomesitylene diacetate) and its role in epoxidation reaction
cannot be ruled out.[49b] Later, we have carried out the
epoxidation reactions with terminal olefins. Different para-
substituted styrene derivatives were reacted with iron-oxo 2.
Simple styrene provided a good yield of styrene epoxide
(46%) (Table 1, entry 1). Electron deficient para-chloro
(-Cl), fluoro (-F), trifluoromethyl (-CF3), cyano (-CN), and
nitro (-NO2) styrene derivatives also provided epoxide
products in relatively less yields (Scheme 2, 2a–2g). How-
Figure 1. ORTEP diagram of aqua-coordinated iron(II)-triflate complex
1a, [FeII(2PyN2Q)](OTf)2 (CCDC: 1470119) and DFT-optimized struc-
ture of iron(IV)-oxo 2, [FeIV(2PyN2Q)(O)(OTf)]2+
.
Table 1: Scope of epoxidation of 2 with internal olefins.[a]
Entry
1
Substrate
Product(s)
52%
56%
2
3
16%
30%
36%
cis:trans 1:2.2
cis:trans 1:2.2
4
14%
5
6
60%
54%
Scheme 2. Scope of epoxidation of terminal olefins by complex 2.
(2.0 mM of 1a in acetonitrile, 1.5 equiv oxidant, and 100 equiv of
substrates were reacted inside the glove box). [a] Major rearrangement
product 4-methyl benzaldehyde (30%) was obtained, minor amount
(<5%) of aldehydes were obtained).
7
44%
[a] Reaction conditions: 2.0 mM of complex 1a in acetonitrile, 1.5 equiv
oxidant, and 100 equivalent of substrates were reacted inside the glove
box.
ever, electron-rich para-methyl styrene provided p-methyl
benzaldehyde (30%) as a major product with minor epoxide
formation (20%). The product analysis during epoxidation of
terminal olefins clearly indicates that electron-rich olefins
(e.g. styrene) provide comparatively better yields than the
electron-deficient olefins (e.g. 4-nitrostyrene). Thus, an elec-
trophilic nature of oxidant iron(IV)-oxo complex 2 can be
proposed. As the nucleophilicity of the olefins decreases, the
extent of reaction (between olefin and complex 2) as well as
the yield of epoxide decreases. The modified electronic effect
of weakly donating 2PyN2Q ligand compared to N4Py ligand
possibly enhances the electrophilic character of complex 2.
Furthermore, we have carried out the 18O labeling study
during the epoxidation reactions. With the 18O labeled
complex 2 (prepared by addition of H218O), when olefin
substrates like trans-b-methyl styrene and cis-stilbene were
octene oxide (52%) and norbornene oxide (56%), respec-
tively in moderate yield (Table 1, entries 1 and 2).
Thereafter, we have tested sterically hindered cis-stilbene
for epoxidation. Notably, cis-stilbene provided a mixture of
cis-trans epoxides (with a ratio of cis:trans 1:2.2) with
moderate yield (52%) (Table 1, entry 3). Similar cis-trans
epoxide product formation was observed for cis-b-methyl as
well (with a ratio of cis:trans 2.2:1) (Table 1, entry 4). But,
both trans-stilbene and trans-b-methyl styrene provided
exclusive trans-epoxides in moderate yields (60% and 54%,
respectively) (Table 1, entries 5 and 6). Importantly, the
observation of isomeric cis-trans epoxide formation from
stereo pure olefins suggests that the reaction with 2 might be
proceeding through a stepwise pathway. Further, terminally
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reacted, > 95% 18O labeled epoxide products were obtained
in both cases (Scheme 3).[50] Thus, the labeling experiment
supported that the oxygen atom of the epoxides comes from
the iron(IV)-oxo complex 2.
Scheme 3. Labeling experiment during olefin epoxidation by
[FeIV(2PyN2Q)(O)]2+, 2.
Figure 3. Hammett–Brown plot for the epoxidation of para-substituted
styrene by complex 2.
Initial reactivity study with different olefins has motivated
us to focus on the kinetics of olefin epoxidation to understand
the mechanism of the OAT reactivity of 2 with olefins. The
kinetics have been executed by monitoring the decay of the
characteristic UV-vis band of 2 at 770 nm (Figure 2a,b) by
different internal olefins. We have studied the second-order
kinetics of cis and trans-stilbene. It was found that the second-
order rate constant, (k2 ꢀ 1.12 MÀ1 SÀ1) of the trans isomer is
almost ten times of the cis-stilbene (k2 ꢀ 0.141 MÀ1 SÀ1). The
higher olefin epoxidation rate of the trans-stilbene can be
rationalized by the lower one-electron oxidation potential of
the trans-stilbene compared to cis-stilbene.[67]
Evidently, we have observed major isomerization during
epoxidation of pure cis-stilbene and cis-b-methyl styrene
(vide supra, Table 1). Now the obvious question arises,
whether the observed isomerization in the case of cis-stilbene
and cis-b-methyl styrene occurs via a well-defined reaction
pathway involving complex 2 or via a simple rearrangement
between epoxide and 1, [FeII(2PyN2Q)]2+ ? In order to
address this query, we have carried out the control experiment
with a cis-stilbene epoxide in the presence of iron(II)-
complex (with and without oxidant). Interestingly the control
reaction does not provide any isomerized trans epoxide
product and rules out the possibility of rearrangement
reactions. Moreover, the above isomerization phenomenon
suggests that epoxidation does not occur in a concerted
pathway; instead, it might proceed in a stepwise pathway upon
reaction with 2 involving a neutral radical intermediate.[68]
Thus, in the present study a longer lifetime of intermedi-
ate IMcis is expected so that C-C rotation becomes feasible to
provide isomerized product (Scheme 4).[54,58,69] The present
observation of isomerization in the stepwise mechanism is in
contrast with Cyt-P450 enzymes mediated epoxidations.
Interestingly, after the formation of intermediate IMcis, there
are two possible pathways by which it can react: firstly, if
pathway I occur preferentially over pathway II then the only
cis-epoxide product is expected (Scheme 4). Secondly, if there
is a competition between these two pathways (I and II) then
a mixture of cis and trans epoxide should be obtained. Clearly,
our findings are consistent with the second possibility, that is,
after the stepwise formation of intermediate I, the competition
between pathway I and II results in a mixture of epoxides
(Scheme 4). In contrast with the above phenomenon, the
pentadentate pyridine-pyridine ligand (N4Py) containing
iron(IV)-oxo, 1 with cis-stilbene and provides only cis-
stilbene epoxide. Possibly, the presence of quinoline moiety
in the 2PyN2Q ligand creates enough steric bulk that triggers
the mechanistic shuttle between pathway II and pathway I.
Figure 2. a) UV-vis spectra recorded during epoxidation of 4-methyl
styrene by complex 2. b) Decrease of the 770 nm band over time.
varying the concentration of the substrates under pseudo-
first-order conditions (> 10 times stoichiometry of complex 2)
with different para-substituted styrene derivatives.[51–53] Sub-
sequently, the second-order kinetics constants (k2) were
obtained from these plots. The higher values of k2 for
electron-rich styrene derivatives suggest a faster reaction rate
compared to electron-deficient styrene and thus further
support the electrophilic character of 2.[54] The second-order
constants (k2’s) obtained from kinetics study with styrene and
its para-substituted derivatives have been used to construct
the Hammett plot. The plot of log(kX/kH) against Hammett–
Brown constants (sP+) (considering polarity and resonance
effect) shows linear correlation (Figure 3)[22] with a negative
slope that is, negative reaction constant (1+ = À1) value. Thus,
it suggests the electrophilic character of iron-oxo complex 2
that possibly reacts with olefins through the transition state
involving small charge separation in the rate-limiting
step.[54–59] Moreover, the electron transfer from olefin (in
r.d.s.) can be presumed to be governed by dominant electronic
inductive effect along with minor resonance effect. Therefore,
the observed reaction constant value is quite lower than some
of the metal-oxo complexes and metalloenzymes mediated
epoxidations reactions.[13,61–66] Further, the small negative
value of the reaction constant is similar to some of the
reported high-valent iron-oxo and iron(III)-acylperoxo com-
plexes.[17,49,62] Thereafter, we have carried out kinetics of the
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distances (Figure 4a). A bond angle (N-Fe-O) of 171.48 was
observed corresponding to the ferryl oxygen atom.[48,76] This
ferryl-bend is also consistent with the X-ray structure of 2
(170.58).[48] Interestingly, unlike most of the FeIV O species,
=
which are found to be linear, the bond angle N-Fe-O is found
to be bend towards the pyridine ring owing to the presence of
a bulkier quinoline group. In species 2, we observed two
À
À
strong C H···O interactions between the C H atom of the
quinoline and ferryl-oxygen atom, consequently the distances
were noted to be 2.021 ꢀ, which is also in accord with
experiments. These interactions and ferryl-bending are ex-
pected to increase the reactivity of this species compared to
2+
[(N4Py)FeIV O] species.[50,77–79] Our computed spin density
=
value for the ground state (32(is)) on Fe and O atom is noted as
1.04 and 0.98 (see Figure S29a and S30–S32 in ESI). Both
triplet and quintet state exhibit strong ferryl bending, and this
leads to significant polarization of the spin density. Compared
to [(N4Py)FeIV O]2+ species, in 2 the equatorial donations are
=
much weaker, leading to a very small triplet-quintet gap
suggesting the possibility of two-state reactivity.[80–82] Natural
Bond Orbital (NBO) analysis was performed on species 2 at
the B3LYP/TZVP level, which explains the nature of bond-
ing, intramolecular rehybridization, and delocalization of
electron density within the molecule (see Figures S33 and S34
in ESI).
Scheme 4. Schematic mechanism for the formation of a cis and trans
epoxide from complex 2.
The computed Fe-O vibrational stretching is found to be
819 cmÀ1. This value is slightly underestimated compared to
the experimental value of 834 cmÀ1 reported for the species
2.[48] Both experimental and theoretical Fe-O vibrational
stretching are found to be lower than the value reported for
Also, the extent of isomerization (cis:trans epoxides products)
between cis-stilbene and cis-b-methyl styrene (Table 1) clear-
ly shows the dependency of the extent of isomerization
pathway (pathway II) on the steric bulk of both ligand and
reactants. Additionally, the detailed computational study is
performed to unravel the mechanistic intricacies in depth, is
discussed in the following section.
known FeIV O bond, and this is essentially due to the ferryl-
=
bend in combination with two weak CH···O interactions
present in species 2. The computed quadrupole splitting and
isomer shift values are 0.51 and 0.02 mmsÀ1, respectively, and
these are in agreement with the experimental estimates of
0.56 and 0.03 mmsÀ1, respectively. The computed zero-field
splitting parameter (D) value is 25.4 cmÀ1, and this is close to
the experimental estimate of 24.3 cmÀ1. The calculated
structural parameters offer confidence in our methodology
used for calculations.[47]
Theoretical Study
Based on earlier reported experimental and theoretical
studies, we have proposed a mechanism for the conversion of
an alkene to epoxide. DFT calculations (B3LYP-D2/6-
31G(LanL2DZ-Fe)//B3LYP-D2/TZVP setup[48,70–72]
)
have
been performed on the formation of FeIII/IV-oxo species,
which is used for the catalysis (also see ESI for the additional
Epoxidation of cis-Stilbene by 2 (R)
writeup on the formation of the FeIII/IV O species). For
=
understanding the reaction mechanism, we have started our
Based on the experimental inputs, the mechanism shown
schematically in Scheme 4 is adapted for our calcula-
tions.[66,83,84] Indeed, there is a growing recognition that the
2+
calculations on the active catalyst [(2PyN2Q)FeIV O] (2)
=
species. The computed energetics reveal that the triplet state
(32(is)) is the ground state with 52(hs), and 12(ls) states lying at 3.8
and 122.4 kJmolÀ1, respectively. Computed energetics, the
nature of the ground state, and the structural features are
consistent with previously reported experimental and theo-
reactions of FeIV O unit with olefins undergo in a stepwise
=
manner.[73] In the stepwise mechanism, the olefin approaches
the FeIV O (R) centre in an end-on fashion to form a reactant
=
complex (RC). After the formation of RC, an electrophilic
3
IV
IV
retical data.[71,73–75] For 2(is) spin state, FeIV O, Fe -Npy, Fe -
NQ, and FeIV-Namine bond lengths are computed to be 1.656 ꢀ,
1.999 ꢀ, 2.047 ꢀ, and 2.057 ꢀ, respectively. The X-ray
attack of the FeIV O unit on the carbon atom of the alkene via
=
=
transition state (TS) takes place, followed by the formation of
III
[83,84]
À
the O C bond leading to Fe -radical intermediate IMcis.
structure of 2 is reported where FeIV O, Fe -Npy, Fe -NQ,
and FeIV-Namine bond lengths are found to be 1.677 ꢀ, 2.022 ꢀ,
2.067 ꢀ, and 2.084 ꢀ, respectively. These values are consis-
tent with the computed structure. Computed bond lengths
show that Fe-Npy distances are slightly shorter than the Fe-NQ
This radical intermediate can undergo either O C bond
formation via TScis, to the form cis-epoxide or C C rotation,
leading to trans radical intermediate IMtrans, which further
IV
IV
=
À
À
À
undergoes O C bond formation via TStrans, leading to the
trans-epoxide formation (Scheme 4).
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Figure 4. Optimized structures of a) 32(is), b) 3RC(is), c) 5TS(hs) along with d) the corresponding spin density plot, e) 5IM1cis(hs), f) 5TStrans(hs)
,
g) 5IM2trans(hs), and h) 5P(hs). Some important structural parameters computed for the spin states and spin density values are given below for
species 2 (R), RC, TS, IM1cis, 5TStrans(hs), 5IM2trans(hs) and 5P(hs). For spin state 32(is): FeIV-O=1.656, FeIV-Npy =1.999, FeIV-NQ =2.047, FeIV-
3
N
N
amine =2.057, FeIV-Navg =2.029 and C H···O=2.021, /Namine-FeIV-O=1718 and /Neq-FeIV-O=938. For spin state RC(is): FeIV-O=1.655, FeIV-
À
py =1.994, FeIV-NQ =2.053, FeIV-Namine =2.058, FeIV-Navg =2.029, O-C1=3.583, O-C2=3.944 and C H···O=2.018, /Namine-FeIV-O=1728, /Neq-
À
FeIV-O=938, /FeIV-O-C1=1558 and /FeIV-O-C2=1458. For spin state 5TS(hs): FeIV-O=1.746, FeIV-Npy =2.227, FeIV-NQ =2.243, FeIV-Namine =2.283,
FeIV-Navg =2.238, O-C1=2.229, O-C2=2.719 and C H···O=2.189, /Namine-FeIV-O=1718, /Neq-FeIV-O=978, /FeIV-O-C1=1748, /FeIV-O-
À
C2=1498 and /C1-O-C2=318 and spin density FeIV =3.81, O=0.50, C1=À0.06, C2=À0.39 and C atoms of benzene=À0.05. For spin state
5IM1cis(hs): FeIII-O=1.800, FeIII-Npy =2.208, FeIII-NQ =2.234, FeIII-Namine =2.251, FeIII-Navg =2.228, O-C1=1.487, O-C2=2.460 and C H···O=2.371,
À
/Namine-FeIII-O=1708, /Neq-FeIII-O=968, /FeIII-O-C1=1798, /FeIII-O-C2=1458 and /C1-O-C2=1108. For spin state TStrans(hs): FeIII-O=2.017,
5
FeIII-Npy =2.150, FeIII-NQ =2.165, FeIII-Namine =2.233, FeIII-Navg =2.173, O-C1=1.562, O-C2=1.731 and C H···O=2.351, /Namine-FeIII-O=1638,
À
/Neq-FeIII-O=938, /FeIII-O-C1=1698 and /FeIII-O-C2=1388. For spin state IM2trans(hs): FeII-O=2.147, FeII-Npy =2.253, FeII-NQ =2.262, FeII-
5
N
amine =2.243, FeII-Navg =2.255, O-C1=1.564, O-C2=1.561 and C H···O=2.466, /Namine-FeII-O=1678, /Neq-FeII-O=928, /FeII-O-C1=1598 and
À
5
/FeII-O-C2=1428. For spin state P(hs): FeII-Npy =2.203, FeII-NQ =2.134, FeII-Namine =2.149 and FeII-Navg =2.165. All bond lengths are given in ꢀ and
À
angles are given in 8. All hydrogen atoms (except H-C(1)=C(2)-H and C H···O are omitted for clarity.
3
Here, three possible spin states have been computed, and
triplet 3RC(is) is found to be the ground state while other spin
states are high lying in energy at 13.9 and 125.2 kJmolÀ1, for
quintet and singlet states, respectively. The computed ground
state energy for the reactant complex (3RC(is)) is estimated to
stilbene in case of the TS(is). However, this is not the lowest
energy pathway as the 5TS(hs) transition state is found to have
a barrier height of 88.5 kJmolÀ1 from the R(is) surface. This
3
certainly demands
a minimum energy crossing point
(MECP)[85] between the two spin surfaces, the spin-orbit
coupling and large anisotropy are estimated for these states.
A facile spin crossover is expected.[85] Although different
substrates are used, our computed barrier heights are
consistent with earlier reported values for the epoxidation
3
be 56.1 kJmolÀ1 from R(is) (Figure 5). Computed bond
IV
lengths for ground state in the FeIV O, Fe -Navg, O-C1, and
=
O-C2 are 1.671 ꢀ, 2.131 ꢀ, 3.230 ꢀ, and 3.402 ꢀ (Figure 4b),
respectively. We have also computed spin density values on
FeIV, O, C1, and C2 for the ground state, and the values came
out to be 1.03, 0.99, 0.01, and 0.00, respectively (see Fig-
ure S29b in ESI). In the reactant complex (RC), both the
3RC(is) and the 5RC(hs) states are energetically accessible,
suggesting a possibility of two-state reactivity (Figure 5). For
the transition state (TS), all three possible spin states have
been computed, 5TS(hs), 3TS(is), and 1TS(ls). The 5TS(hs) is found
to be the ground state, and with the calculated barrier height
by different FeIV O complexes.
With respect to the
[85–87]
=
À
=
reactant Fe O, as well as the C C bonds were elongated.
Partial delocalization of the spin densities on the iron and
oxygen atoms to the adjacent olefinic carbon atom was found.
In the transition state, FeIV O bond length elongates to
=
1.746 ꢀ, compared to the bond length in the reactant
(1.656 ꢀ), and simultaneously, the FeIV Navg bond length also
À
elongates. The length of the newly forming O···C1 bond in the
transition state was computed to be 2.229 ꢀ (Figure 4c), and
of 146.5 kJmolÀ1 (Figure 5) for C C bond activation of cis-
=
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IV
Figure 5. B3LYP-D2 computed energy profile diagram for C C bond activation of cis-2-stilbene by species [Fe (2PyN2Q) O]2+: a) cis-epoxidation
and b) trans-epoxidation pathways (energies are given in kJmolÀ1). The red vertical line represents single point calculations performed on the
corresponding ferromagnetic state, while the magenta vertical line represents barrier heights estimated from a relaxed scan performed for the
corresponding coordinate (see S36 and S37 in ESI).
=
=
the C1···C2 bond is nearly a single bond (1.404 ꢀ). The
computed energy profile diagram is illustrated in Figure 5. In
the transition state, the O···C1 bond formation spurs the
generation of a radical character on the carbon atom (C2),
suggesting a radical type intermediate and not cationic or
anionic intermediate. Computed optimized geometries and
spin density plots for ground spin state obtained are shown in
Figure 4c, d. The spin density value on the FeIV of 5TS(hs) state
is noted as 3.81, and for O atom, C1, C2 atoms of the benzene
ring are noted as 0.50, À0.06, À0.39, and À0.05, respectively,
with small overall charge (group charge QC2 = 0.03 and
Figure 4d). Spin density values indicate that the one a-
electron transfer from cis-stilbene to the FeIV centre, but b-
electron remains present on the carbon centre (C2) of the cis-
stilbene (Figure S35 and S36 in ESI). Spin density values also
suggest that the radical intermediate is formed during the
course of the reaction and is corroborated with experimental
data. The orbital evolution diagram[85,88] and Eigen-value plot
from the corresponding ferromagnetic state. Here, the
5
5IM1cis(hs) and IM1trans(hs) are found to be the ground state,
while other intermediate species are higher in energy (Fig-
5
ure 5). For IM1cis/trans(hs) spin state, FeIII-O, FeIII-Navg, O-C1,
and O-C2 bond lengths are computed to be 1.800/1.805 ꢀ,
2.228/2.222 ꢀ, 1.487/1.491 ꢀ, and 2.461/2.448 ꢀ, respectively
(Figure 4e). Computed spin density values for 5/7IM1cis/trans(hs)
-
state on FeIII, O, C1, C2, and benzene ring are noted as 3.95/
3.93, 0.45/0.43, 0.06/0.06, À0.65/À0.59, and À0.22/À0.20 (see
Figure S29c in ESI). The spin density values clearly suggest
that the b-electron is delocalized over the benzene as well as
present on the carbon centre (C2), which is connected to the
carbon centre (C1) (see Figure S29c in ESI).
In the ring-closing steps, the O-C2 bond elongates to make
a bond at the same time the FeIII O bond is also elongated.
The TScis/trans(hs) is found to be the ground state, while other
À
5
spin states are high in energy (Figure 5). The computed bond
5
lengths for FeIII-O and O-C2 of TScis/trans(hs) are 2.015/2.018
are shown in Figures S35 and S36. This emphasizes the species
and 1.739/1.731 ꢀ, respectively (Figure 4 f). The computed
3
5
emerging from the doubly degenerate ground state, R(is)
,
spin density for the TScis/trans(hs) on the FeIII, O, and C2 are
À
=
during C C epoxidation. During C C bond activation, an
electron shifts from pC-C orbital of olefin to one of the d-based
orbitals in 5R(hs), leading to six different spin states for the cis
3.71/3.71, 0.02/0.03, and 0.03/0.02, respectively (see Fig-
ure S39a in ESI). The product selectivity is decided by the
transition state of O-C2 bond formation, and this recombi-
nation of the O-C2 bond is found to be 33.9 kJmolÀ1 for trans
product and 49.4 kJmolÀ1 for a cis product from the reactant.
The energy difference between 5TStrans(hs) and 5TScis(hs) is
estimated to be 15.6 kJmolÀ1, and energetic data also suggest
that the trans epoxide is expected to be the dominant product,
as revealed in the experiments. When we compared the
=
intermediates. A ferryl oxygen atom attack on the C C bond
leads to the formation of a radical intermediate that is found
to be delocalized on the benzylic carbon atoms. The cis radical
intermediate is converted into trans intermediate by rotation
of the C-C bond leading to a mixture of products. As
À
expected, an elongation of the Fe O bond occurs along with
=
O-C1 bond closures, and the (IM1cis/trans). Among six possible
barrier heights of C C bond activation with the O-C rebound
spin states, IM1cis/trans(hs) and 3IM1cis/trans(ls) intermediates were
step, the former is substantially larger and hence is the rate-
determining step.
7
successfully optimized, and the energies of antiferromagneti-
cally coupled state between the FeIII and carbon radical centre
were obtained by performing single point (SP) calculations
À
In the consequent step, the O C bond formation leads to
the generation of FeII-epoxide adduct (IM2). We have
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5
3
calculated all possible spin states IM2cis/trans(hs), IM2cis/trans(is)
,
more electrophilic than 1 (Fe; O charges are 0.35; À0.28 and
0.28; À0.25 for the former and the latter, respectively).
The potential energy surface constructed for species
1 promoting epoxidation of cis-stilbene is shown in Figure 6
(see Figure S42 in ESI for optimized structures and spin
density plots of ground state). Clearly, the reactivity here as
well originates from the quintet state with the estimated
barrier height of 97.7 kJmolÀ1, which is 9.1 kJmolÀ1 higher
compared to the same for species 2.
and 1IM2cis/trans(ls). The 5IM2cis/trans(hs) is found to be the ground
state, while other spin states are high in energy (Figure 5). The
bond lengths for Fe-O of 5IM2cis/trans(hs) are 2.178/2.147 ꢀ (see
Figure 4g and S39b in ESI). The trans FeII-epoxide adduct is
found to be more stable than the cis adduct (Figure 5). The
energy difference between trans and cis intermediate is
estimated to be 5.1 kJmolÀ1.
II
À
In the last step, the Fe O bond cleaves from the Fe -
epoxide adduct leading to the generation of the final product
with the overall thermodynamic stabilization of
À103.7 kJmolÀ1 (Figure 5, Figure 4h and S39c in ESI).
5Ptrans(hs) isomer is thermodynamically more stable than the
5Pcis(hs) by 10.7 kJmolÀ1. All the energetics computed reveal
that a racemic mixture of trans- and cis-product is expected as
observed in experiments. We have also calculated the ratio of
the product formation using the ring-closing transition state
energy barrier. The ratio of product formation is found to be
1:2.2 for cis:trans isomer, which is consistent with exper-
imental data. The single turnover epoxidation of cis-stilbene
using species 2 yields a mixture of 16% cis and 36% trans-
stilbene oxides, revealing that the trans- epoxide is dominant
over cis-epoxide product. Due to this reason, we have also
performed calculations on the trans- stilbene using 2, and
calculated barrier height on the quintet surface of this
reaction is found to be 83.2 kJmolÀ1 from the reactant
(3R(is)). The computed barrier height suggested that the
formation of trans-epoxide (60%) product is more preferred
than the cis-epoxide product, which is also corroborated with
the experimental data (Table 1, entry 5 and see Figure S40 in
ESI).
=
Figure 6. B3LYP-D2 computed energy profile diagram for C C bond
activation of cis-2-stilbene by [(N4Py)FeIV O] (1) species (energies
2+
=
are given in kJmolÀ1).
While the epoxidation and the racemization of the
To underpin the origin of this difference in the barrier, we
have looked at carefully the HOMO–LUMO gap between the
substrate and the catalyst.[91] For species 2, the energy
stereochemistry are understood, we turn to understand the
electronic origin behind the reactivity of [(2PyN2Q)FeIV
=
O]2+ (2) compared to [(N4Py)FeIV O]2+ (1) species,[89] which
difference between the LUMO of the Fe O s* and C C
orbital of cis-stilbene is 1.06 eV at the quintet surface, while
for species 1 is found to be 1.19 eV at the same spin surface.
These HOMO–LUMO gaps are suggesting that species 2 is
=
=
=
does not perform epoxidation without additives. We have
chosen cis- stilbene as a substrate computed the energetics
associated with the epoxidation reaction for species 2. Firstly,
we have observed that both complexes have an intermediate
spin state (S = 1) as a ground state. But, our calculations
=
more reactive towards C C activation of cis-stilbene than
species 2. Interestingly, we have found comparable dipole
moment values for 6.86 D of species 2 and 4.70 D for 1. These
dipole moment values are also suggesting that the species 2 is
suggest that the quintet-triplet gap is very small (3.8 kJmolÀ1
)
in the case of species 2, while the quintet-triplet gap is
relatively large (19.6 kJmolÀ1) for species 1.[90] The quintet-
triplet gap is higher for species 1 due to the presence of strong
nitrogen atoms donation at the equatorial as well as axial
=
more reactive towards C C activation of cis-stilbene than
species 1.
Additionally, we have also performed an energy decom-
position analysis (EDA) for 1 and 2 by treating N4Py/
2PyN2Q, Fe, and O as separate fragments ({N4Py}/{2PyN2Q}
+ {Fe} + {O}) (see Table 5 in ESI). The calculated EDA
À
positions compared to species 2, as affirmed by Fe N bond
lengths for the former (FeIV-Navg = 2.029 ꢀ for species 1, and
FeIV-Navg = 2.000 ꢀ for species 2).[14,90] Two different donor
nitrogen atoms present in the equatorial position accompa-
results for
1
and
2
are found to be À4180.1 and
nied by the ferryl bend wherein N-Fe O angle in species 2
À3930.8 kJmolÀ1, respectively, which shows that the DEint
=
IV
for [(2PyN2Q)FeIV O] is smaller than the [(N4Py)Fe O].
These DEint values are suggesting that species 2 is less stable
than species 1 and account for the fact that 2 is more reactive
=
=
leads to stabilization of the corresponding dz2 and dx2Ày2
orbitals compared to the species 1 (see Figure S41 in
ESI).[47,48,90] This leads to the smaller quintet-triplet gap. In
fact, the gap in 2 is so small that a mean-energy crossing point
from the triplet to quintet is not required for the reactivity,
while for species 1, this is mandatory suggesting additional
energy penalties which tend to slow down the reaction. We
have also computed the Natural Population Analysis (NPA)
charges for both complexes, which suggest species 2 to be
=
towards C C bond activation than 1. A significant contribu-
tion to the interaction energy for 1 arises from orbital
stabilization, indicating a stronger metal-ligand (Fe-L) bond
for 1 compared to 2. NBO second-order perturbation analysis
reveals a significant difference present in the s-bonding
interactions between these two species. The s-bonding
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Angewandte
Research Articles
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interaction is found to be composed of Fe(dz2 )-O(pz) orbitals,
differences, however minor, lead to a dramatic difference in
and the stabilization energies for 2 is 151.8 kJmolÀ1, which is
the reactivity between the two species. This comparative
study has interesting implications in biomimetic/enzymatic
chemistry. Various amino acid residues coordinate to Fe could
play a role in fine-tuning the reactivity by inducing a “ferryl”
bend also could alter the electronic structure and enhance the
reactivity. This aspect has not been fully explored yet in both
smaller compared to species 1 (161.9 kJmolÀ1). Besides, the
À
Fe O bond in species 2 is found to be less covalent in nature
À
compared to the Fe O bond in species 1 (37.03% Fe (dz2 ) and
62.97% of O(pz) for 1 and 35.06% Fe(dz2 ) and 64.94% of
O(pz) for 2) (see Figure S41 and S42 in ESI).
Additionally, the {(N4Py)FeII-epoxide} product is also
found to be not as stable as the one corresponding to
[(2PyN2Q)FeII-epoxide] species. The cleavage of the epoxide
from the {FeII-epoxide} product was found to be less
exothermic for N4Py ligand architecture compared to
heme, and non-heme FeIV O catalyzed enzymatic reactions.
=
Acknowledgements
2PyN2Q
ligand
architecture
(À46.9 kJmolÀ1
vs.
This activity is supported by SERB, India (CRG/2018/
003915). G.R. would like to thank CSIR and SERB
(01(2980)/19/EMR-II, CRG/2018/000430; DST/SJF/CSA-03/
2018-10; SB/SJF/2019-20/12) New Delhi, for financial sup-
port. S.R. acknowledges SERB for Start-Up Grant (SRG/
2019/000310). Financial support has been received from
CSIR-India (fellowship to J.P.B. and S.S.). M.A. would like
to thank IITB for funding. A.P. acknowledges SERB for the
Project Assistant Fellowship in Start-Up Grant (SRG/2019/
000310).
À68.7 kJmolÀ1 for N4Py and 2PyN2Q, respectively). This is
essentially due to the shorter Fe-O(epoxide) distance found
for the N4Py suggesting a product inhibition in action, even if
epoxide formation results.[14,85]
Further, we have also probed the origin of a large
difference in the barrier height of the quintet transition state
between two-ligand architecture by calculating the deforma-
tion energies. Calculations reveal that while steric factor
contributes nearly 68.8 Æ 1.6% to the barrier height for both
the ligands, the interaction energy between the fragments
found to add a significant penalty for the N4Py compared to
2PyN2Q ligand architecture (29.6 kJmolÀ1 vs. 28.6 kJmolÀ1
for N4Py and 2PyN2Q, respectively). Thus, lesser steric
repulsion and less favourable orbital stabilization are the
main reason for N4Py to be unreactive towards olefin under
normal reaction conditions. All these collectively lead to
extremely slow or no reactivity at all for the N4Py ligand
architecture, while the addition of two quinolines was found
to promote a significant change with respect to steric and
electronic behaviour. A similar correlation on the epoxida-
tion of olefin is also observed earlier in different ligand
architecture.[14,73,85]
Conflict of interest
The authors declare no conflict of interest.
Keywords: biomimetic synthesis · epoxidation · ligand effects ·
non-heme iron-oxo · oxygen atom transfer
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Manuscript received: February 18, 2021
Accepted manuscript online: April 9, 2021
Version of record online: && &&
,
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Angew. Chem. Int. Ed. 2021, 60, 2 – 12
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Research Articles
Oxygen Atom Transfer
The steric and electronic effects exerted
by the bulky quinoline-pyridine penta-
dentate ligand (2PyN2Q) in the non-
heme complex [FeIV(2PyN2Q)(O)]2+ play
a key role in olefin epoxidation by oxygen
atom transfer and trigger mechanistic
diversification. In the present study, the
mechanism of olefin epoxidation was
thoroughly investigated and rationalized
with a detailed computational study.
J. P. Biswas, M. Ansari, A. Paik, S. Sasmal,
S. Paul, S. Rana,* G. Rajaraman,*
D. Maiti*
&&&& — &&&&
Effect of the Ligand Backbone on the
Reactivity and Mechanistic Paradigm of
Non-Heme Iron(IV)-Oxo during Olefin
Epoxidation
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ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 12
These are not the final page numbers!