Oxidation of 2,6-di-tert-butylphenol by tetrapyridyl oxoiron(IV) complex

Article abstract of DOI:10.1016/j.poly.2018.02.015

The reactivity of the previously reported pentadentate low-spin (S = 1) oxoiron(IV) complex, [Fe^IV(O)(asN4Py)] (2) (asN4Py = N,N-bis(2-pyridylmethyl)-1,2-di(2-pyridyl)ethylamine), has been investigated in the oxidation reaction of 2,6-di-tert-butylphenol derivatives. Detailed kinetic, and mechanistic studies (kinetic isotope effect (KIE) of 4.52, and Hammett correlation with ρ = ?1.83), lead to the conclusion that the rate-determining step in this reaction involves direct hydrogen-atom transfer (HAT) from the phenol by the oxoiron(IV) species, in contrast to the heme-type horseradish peroxidase (HRP) system.

Full text of DOI:10.1016/j.poly.2018.02.015

Polyhedron 145 (2018) 227–230
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Oxidation of 2,6-di-tert-butylphenol by tetrapyridyl oxoiron(IV) complex
⇑
Dóra Lakk-Bogáth, Gábor Speier, József Kaizer
Department of Chemistry, University of Pannonia, 8200 Veszprém, Hungary
a r t i c l e i n f o
a b s t r a c t
The reactivity of the previously reported pentadentate low-spin (S = 1) oxoiron(IV) complex, [FeIV(O)
Article history:
Received 17 January 2018
Accepted 14 February 2018
(
asN4Py)] (2) (asN4Py = N,N-bis(2-pyridylmethyl)-1,2-di(2-pyridyl)ethylamine), has been investigated
in the oxidation reaction of 2,6-di-tert-butylphenol derivatives. Detailed kinetic, and mechanistic studies
kinetic isotope effect (KIE) of 4.52, and Hammett correlation with
(
q
= ꢀ1.83), lead to the conclusion that
the rate-determining step in this reaction involves direct hydrogen-atom transfer (HAT) from the phenol
by the oxoiron(IV) species, in contrast to the heme-type horseradish peroxidase (HRP) system.
Ó 2018 Elsevier Ltd. All rights reserved.
Keywords:
2
,6-Di-tert-butylphenol
Oxidation
Iron(IV)
Hydrogen-atom abstraction
Kinetics
1
. Introduction
Phenols, which are typical substrates for the horseradish perox-
(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) ligand [8], has
been presented to oxidize various substrates by a high variety of
mechanisms, including electron transfer (ET) [9a], electron
transfer-proton transfer (ET-PT) [9b], hydrogen atom abstraction
(HAT) [9c,d], and oxygen atom transfer (OAT) [9e,f]. We have
idase (HRP), are oxidized to phenoxyl radicals. Mechanistic studies
suggested that the oxidation process involves a rate determining
electron transfer from the phenol to the oxo–heme species [1], in
previously reported the synthesis and structure of iron(II)
IV
II
contrast to the biomimic Fe (T2MPyP)(O) (T2MPyP = tetra(2-N-
precursor complex [Fe (asN4Py)](ClO
4
)
2
(1) supported by
methylpyridyl)porphyrin) containing system where oxidation
occurs by hydrogen atom abstraction via simultaneous removal
of a proton and an electron [2]. Since phenols are one of the major
water and soil pollutants, in addition to enzymatic systems, transi-
tion metal complexes [3], metalloporphyrins [4], Schiff bases [5],
and metallophthalocyanines have also been investigated as cata-
lysts for phenol oxidations or degradations [6]. On the other hand,
the 2,6-di-tert-butylphenols are widely used as antioxidants and
models of vitamin E in medicine. Despite the widespread interest
in metalloporphyrin-catalyzed oxidations, there is relatively little
information on the mechanism of phenol oxidations by nonheme
pentadentate ligand asN4Py (asN4Py = N,N-bis(2-pyridylmethyl)-
1,2-di(2-pyridyl)ethylamine), the spectroscopic characterization
IV
of one of the most stable high-valent complex [Fe (asN4Py)
2
+
(O)] (2) derived from the reaction of 1 with PhIO, and its
reactivity in OAT reaction towards PhSMe derivatives [10]. In this
study, we report the stoichiometric oxidation of 2,6-di-tert-
butylphenol (DTBP) by 2 in order to get more insight into the
mechanism of O–H activation process (Scheme 1).
2. Experimental
oxoiron(IV) complexes. The oxidation of phenols and hydro-
IV
quinones (H
2
Q) by nonheme oxoiron(IV) complex, [Fe (TMC)
2.1. Materials and methods
2
+
(
O)]
(TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetrade-
cane), has been extensively studied, and a hydrogen-atom abstrac-
tion mechanism was proposed [7].
The coordination chemistry of nitrogen-rich pentadentate
ligands has received much attention. These ligands have demon-
All syntheses were done under an argon atmosphere unless sta-
ted otherwise. Solvents used for the synthesis and reactions were
purified by standard methods and stored under argon. The starting
materials for the ligand are commercially available and they were
II
strated the ability to stabilize high-valent metal centers. Oxoiron
purchased from Sigma–Aldrich. [Fe (asN4Py)(CH
3
CN)](ClO
4
)
2
and
0
(
IV) complex supported by pentadentate N4Py (N4Py = N,N -bis
asN4Py (asN4Py = N,N-bis(2-pyridylmethyl)-1,2-di(2-pyridyl)ethy-
lamine) were prepared as previously described [10]. Microanalyses
were done by the Microanalytical Service of the University of
Pannonia. The UV–vis spectra were recorded on an Agilent 8453
diode-array spectrophotometer using quartz cells. GC analyses
⇑
Corresponding author. Fax: +36 88 624 469.
E-mail address: kaizer@almos.vein.hu (J. Kaizer).
https://doi.org/10.1016/j.poly.2018.02.015
277-5387/Ó 2018 Elsevier Ltd. All rights reserved.
0

2
28
D. Lakk-Bogáth et al. / Polyhedron 145 (2018) 227–230
3
. Results and discussion
IV
2+
The reactivity of [Fe (asN4Py)(O)] (2) has been studied at 25
C in the stoichiometric oxidation reaction of DTBP and several of
°
its 4-substituted derivatives in dry acetonitrile. Complex 2 was
generated by the reaction of 1 with 1.2 equiv. of PhIO (ꢂ40 min),
and the rate of its rapid decomposition at 705 nm (Fig. 1a) was
measured as a function of the concentration of added DTBP. The
0
0
0
coupled product 4,4 -dihydroxy-3,3 ,5,5 -tetra-tert-butylbiphenyl
(
(
2 2
H DPQ) was characterized by GC/MS (H DPQ: MS, m/e: 410
M ), 395, 190, 162, 57.) Its yield (ꢂ90% based on [2]) was almost
+
quantitative, indicating that the UV–vis spectral change corre-
sponds to the oxidation process.
The rates in the presence of a large excess of DTBP obeyed
pseudo-first-order kinetics, and the pseudo-first-order rate con-
stants (kobs) increased proportionally with the initial concentration
of the phenol (entries 4–5, 10–12 in Table 1, and Fig. 1b). From this
Scheme 1. Oxoiron(IV)-mediated oxidation of 2,6-di-tert-butylphenol derivatives.
linear plotting, the second-order rate constant (k
2
) was determined
were performed on an Agilent 6850 gas chromatograph equipped
with a flame ionization detector and a 30 m CHIRASIL-L-VAL col-
umn. GC–MS analyses were carried out on Shimadzu QP2010SE
equipped with a secondary electron multiplier detector with con-
version dynode and a 30 m HP-5MS column.
1
ꢀ1 ꢀ1
to be 1.93 ꢁ 10 M
s
at 25 °C. At constant DTBP concentration
the kobs values were shown to be independent of the initial concen-
tration of the oxoiron(IV) species. The linear plot of the reaction
rate values (V) against the initial concentration of 2 states that
the reaction is first-order with respect to the oxoiron(IV) concen-
tration (entries 13–18 in Table 1, and Fig. 1c). The above results
establish a rate law of ꢀd[2]/dt = k
2
[2][DTBP]. Activation parame-
2
.2. Stoichiometric oxidations
–
ꢀ1
–
ꢀ1 ꢀ1
ters of
D
H
= 21 kJ mol , and
D
S
= ꢀ74 J mol
K
(at 298 K)
II
ꢀ3
were calculated from plot of log(k /T) versus 1/T in CH CN over
the temperature range 283–308 K (entries 15, 19–23 in Table 1,
and Fig. 1d). The activation enthalpy of 21 kJ mol is relatively
2
3
[
Fe (asN
4
Py)(CH
3
CN)](ClO
4
)
2
complex (2.00 ꢁ 10 M) was dis-
ꢀ3
solved in acetonitrile (1.5 mL), then iodosobenzene (2.4 ꢁ 10 M)
was added to the solution. The mixture was stirred for 40 min then
excess iodosobenzene was removed by filtration. Substrate
ꢀ1
IV
higher than those obtained in the oxidation of H
2
Q by [Fe (TMC)
2
+
ꢀ1
ꢀ2
ꢀ2
(O)] (6.9 kJ mol ) [7b] and in the oxidation of phenol by tert-
(
1.00 ꢁ 10 –4.00 ꢁ 10 M) was added to the solution and the
ꢀ
1
butoxy radical (11.7 kJ mol in PhCH
3
) [11], but lower than that
reaction was monitored with UV–vis spectrophotometer at
IV
ꢀ1
ꢀ1
in the oxidation of 3-CN-C H OH by Fe (T2MPyP)(O) 57.4 kJ
6
4
7
05 nm (e = 400 M cm ) (Table 1). The characterization of the
ꢀ1
0
0
0
mol ) [2]. The relatively large negative entropy is typical of asso-
ciative processes. Three alternative mechanisms, namely electron
2
product 4,4 -dihydroxy-3,3 ,5,5 -tetra-tert-butylbiphenyl (H DPQ)
based on previously described methods [6c].
Table 1
Kinetic data for the stoichiometric oxidation of DTBP with [FeIV(asN4Py)(O)](ClO
4 2
) .
DTBP (10 2 M)
ꢀ
[FeIV(O)]2+ (10 M)
ꢀ3
k
(M
ꢀ1 ꢀ1
s )
No.
T (K)
2
1
2
3
4
5
6
7
8
9
298
298
298
298
298
298
298
298
298
298
298
298
298
298
298
298
298
298
283
288
293
303
308
1
1
1
1.5
2
2
2
2
2
2.5
3
4
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
1.5
2
2.5
3
4
20.8 ± 0.8
7.11 ± 0.4
32.1 ± 1.6
22.5 ± 1.1
18.4 ± 0.7
27.9 ± 1.1
45.2 ± 2.3
0.96 ± 0.03
4.35 ± 0.5
20.6 ± 0.5
19.5 ± 0.8
16.6 ± 0.4
17.7 ± 0.6
17.8 ± 0.7
18.4 ± 0.6
18.2 ± 0.5
16.5 ± 0.5
19.3 ± 0.7
12.1 ± 0.4
15.8 ± 0.5
17.2 ± 0.8
24.1 ± 1.2
28.9 ± 1.4
a
b
c
d
e
f
1
1
1
1
1
1
1
1
1
1
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
2
2
2
2
2
2
2
2
2
2
a
b
c
d
e
f
DTBP:D
DTBP:H
2
O = 1:100.
O = 1:100
2
4
4
4
4
-Me-DTBP.
- Bu-DTBP.
-CN-DTBP.
-Br-DTBP.
t

D. Lakk-Bogáth et al. / Polyhedron 145 (2018) 227–230
229
Fig. 1. Reactions of [FeIV(asN4Py)(O)]2+ (2) with 2,6-di-tert-butyl-phenols in CH
shows time course of the decay of 2 monitored at 705 nm. (b) Plot of kobs versus [substrate] for reactions of 2 (2 mM) with DTBP. (c) Plot of reaction rate (V) versus [2] for
3
CN at 298 K. (a) UV–vis spectral change of 2 (1.5 mM) upon addition of 13 equiv. DTBP. Inset
reactions of 2 with DTBP (20 mM). (d) Eyring plot of log k/T versus 1/T. (e) Hammett plot of log krel against
r
p
of para-substituted 2,6-di-tert-butyl-phenols. (f) Plot of log k
2
versus homolytic bond dissociation energies (BDEO-H) [7a].
IV
2+
transfer from the phenol or its phenolate ion, and hydrogen atom
abstraction from the phenol can be proposed for the oxidation of
phenols. The nucleophilic character of the oxoiron(IV) can be
excluded since deprotonation leads to phenoxide coordination,
phenols by [Fe (TMC)(O)(X)]
complexes [7a].
(X = N
3
, CF
3
COO, and CH
3
CN)
The involvement of the phenolic O–H bond in the rate-deter-
mining step is indicated by the magnitude of the kO-H/kO-D kinetic
(solvent) isotope effect of 4.52 in CH
0
instead of phenoxyl radical, which can lead to a 2,2 -biphenol-cou-
3 2 3 2
CN/H O and CH CN/D O
pled product.
(entries 1–3 in Table 1). This is in contrast to the oxidation of
IV
2+
There are several lines of evidence that support a direct hydro-
gen-atom transfer (HAT) mechanism. Upon using para-substituted
2
H Q
by cis-[Ru (bpy)
2
(py)(O)]
(kH2O/kD2O 29), where the
reaction occurs by ‘‘proton-coupled electron transfer (PCET)” or
2
,6-di-tert-butylphenols with electron donating groups the rate of
‘‘electron–proton transfer (EPT)” within a H-bonded association
IV
the decay processes were increased remarkably. When pseudo-
first-order rate constants determined in the oxidation reactions
of various para-substituted 2,6-di-tert-butylphenols were plotted
complex between H
2
Q and Ru (O) to give semiquinone and
III
[Ru (OH) [12]. The mechanism above differs from the H-atom
transfer proposed for DTBP. As shown in Fig. 1f, there is a linear
against
r
p
, a good correlation was observed with Hammett
q
value
2
correlation between log k and O–H homolytic bond dissociation
of ꢀ1.83, which suggests that the metal-based oxidant 2 is elec-
energies (BDEs), which suggests that the oxidant is selective in
nature. The slope of -0.48 is comparable to those obtained with
trophilic (entries 5–9 in Table 1, Fig. 1e). This relatively small neg-
IV
2+
ative
q
value suggests that there is a small development of positive
[Fe (TMC)(O)(X)] (ꢀ0.36 to ꢀ0.55, X = N
3 3 3
, CF COO, and CH CN)
charge on the substrate in the transition state. If the phenolate ion
is the assumed substrate, oxidation must occur by electron transfer
complexes [7a].
In summary, the reaction of [Fe (asN4Py)(O)]2+, generated
from iron(II) and PhIO, with DTBP in MeCN shows first-order
dependence on the concentration of the phenol and the oxidant.
Substituent effect on the second-order rate constant was obtained
IV
where a more negative
q value would be expected. Similar values,
in a range ꢀ1.5 to ꢀ3.2, were reported for a concerted hydrogen
abstraction mechanism in the oxidation of para-substituted

2
30
D. Lakk-Bogáth et al. / Polyhedron 145 (2018) 227–230
from the oxidation of DTBP and four mono-substituted derivatives
and these data were analyzed by Hammett and modified Hammett
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8
[
equations. The
q value obtained, in conjunction with the kinetic
(
isotope effect, suggest that the rate-determining step in these reac-
tions involves hydrogen atom abstraction from the phenol by the
oxoiron(IV) species in contrast to the heme-type horseradish per-
oxidase (HRP) system.
(
(
c) M. Hassanein, F.I. Abdel-Hay, T. El-Hefnawy, El-Esawy, Eur. Polym. J. 335
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This work was supported by a grant from The Hungarian
Research Fund (OTKA) K108489, GINOP-2.3.2-15-2016-00049,
and the New National Excellence Program of the Ministry of
Human Capacities-2016-3-IV. (D. Lakk-Bogáth).
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