Switchover of the Mechanism between Electron Transfer and Hydrogen-Atom Transfer for a Protonated Manganese(IV)–Oxo Complex by Changing Only the Reaction Temperature

Article abstract of DOI:10.1002/anie.201602460

Hydroxylation of mesitylene by a nonheme manganese(IV)–oxo complex, [(N4Py)Mn^IV(O)]²⁺(1), proceeds via one-step hydrogen-atom transfer (HAT) with a large deuterium kinetic isotope effect (KIE) of 3.2(3) at 293 K. In contrast, the same reaction with a triflic acid-bound manganese(IV)-oxo complex, [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂(2), proceeds via electron transfer (ET) with no KIE at 293 K. Interestingly, when the reaction temperature is lowered to less than 263 K in the reaction of 2, however, the mechanism changes again from ET to HAT with a large KIE of 2.9(3). Such a switchover of the reaction mechanism from ET to HAT is shown to occur by changing only temperature in the boundary region between ET and HAT pathways when the driving force of ET from toluene derivatives to 2 is around ?0.5 eV. The present results provide a valuable and general guide to predict a switchover of the reaction mechanism from ET to the others, including HAT.

Full text of DOI:10.1002/anie.201602460

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201602460
German Edition: DOI: 10.1002/ange.201602460
Bioinorganic Chemistry
Switchover of the Mechanism between Electron Transfer and
Hydrogen-Atom Transfer for a Protonated Manganese(IV)–Oxo
Complex by Changing Only the Reaction Temperature
Jieun Jung, Surin Kim, Yong-Min Lee, Wonwoo Nam,* and Shunichi Fukuzumi*
Abstract: Hydroxylation of mesitylene by
a
nonheme
step HAT mechanism implies that the hydrogen-atom trans-
fer reaction occurs without a transient intermediate when an
electron and a proton are transferred at the same time. In
contrast, the sequential pathway involves mechanistically
distinct ET and PT steps. In particular, there has been a long-
standing ambiguity as to the mechanistic borderline where
a HAT pathway is changed to a rate-determining ET pathway
IV
2+
manganese(IV)–oxo complex, [(N4Py)Mn (O)] (1), pro-
ceeds via one-step hydrogen-atom transfer (HAT) with a large
deuterium kinetic isotope effect (KIE) of 3.2(3) at 293 K. In
contrast, the same reaction with
manganese(IV)-oxo complex, [(N4Py)Mn (O)] -(HOTf)
a triflic acid-bound
IV
2+
2
(2), proceeds via electron transfer (ET) with no KIE at
[10–13]
2
93 K. Interestingly, when the reaction temperature is lowered
followed by a PT pathway or vice versa.
A more general
to less than 263 K in the reaction of 2, however, the mechanism
changes again from ET to HAT with a large KIE of 2.9(3).
Such a switchover of the reaction mechanism from ET to HAT
is shown to occur by changing only temperature in the
boundary region between ET and HAT pathways when the
driving force of ET from toluene derivatives to 2 is around
À0.5 eV. The present results provide a valuable and general
guide to predict a switchover of the reaction mechanism from
ET to the others, including HAT.
question is whether the rate-determining ET and the other
pathways are distinguishable and competing reactions, or
whether the mechanism is continuously changed between the
[14–19]
two extremes.
Such a general question has never been
clarified previously.
In ET reactions, the DS value is generally close to zero
owing to the structureless transition state.
°
[20–22]
In contrast,
°
HAT affords a largely negative DS value because of the
[
1]
highly structured transition state. Large differences in the
°
°
DH and DS values between ET and HAT reactions may
result in the change of the reaction mechanism depending on
temperature, particularly in the borderline region between
the two pathways. However, there has been no report on the
switchover of the mechanism between ET and HATreactions
by changing only reaction temperature.
H
ydrogen-atom transfer from hydrogen donors (RH) to
hydrogen acceptors is one of the most important and
fundamental reactions that have attracted much attention
not only in organic chemistry but also in inorganic chemis-
[1–7]
try.
There are three reaction mechanisms proposed in
hydrogen transfer, such as electron transfer (ET) followed by
proton transfer (PT), one-step hydrogen-atom transfer
We herein report for the first time that the reaction
mechanism in the hydroxylation of 1,3,5-trimethylbenzene
(mesitylene) by a triflic acid-bound manganese(IV)–oxo
[1a,8–12]
(
HAT), and PT followed by ET (Scheme 1).
The one-
IV
2+
complex, [(N4Py)Mn (O)] -(HOTf) (N4Py = N,N-bis(2-
2
[23,24]
pyridylmethyl)-N-bis(2-pyridyl)methylamine),
changes
from a sequential rate-determining ET pathway (followed
by an PT pathway) to an HAT pathway, both of which are
clearly distinguishable, only by lowering temperature. Such
a switchover of the mechanism has been confirmed by
a drastic change of the deuterium kinetic isotope effect
(
KIE) from 1.0(1) at 293 K to 2.9(3) at temperatures lower
Scheme 1. Possible reaction mechanisms in hydrogen-atom transfer
reaction.
than 263 K when mesitylene and [D ]mesitylene were used as
substrates. The ET driving force dependence of the rate
constants in the hydroxylation of toluene derivatives by
12
IV
2+
[
(N4Py)Mn (O)] -(HOTf) was examined to reveal the
2
[
*] Dr. J. Jung, S. Kim, Dr. Y.-M. Lee, Prof. Dr. W. Nam,
Prof. Dr. S. Fukuzumi
Department of Chemistry and Nano Science
Ewha Womans University, Seoul 03760 (Korea)
E-mail: wwnam@ewha.ac.kr
conditions, where switchover of the mechanism between ET
and HAT can occur only by varying reaction temperature.
IV
2+
The manganese(IV)–oxo complexes, [(N4Py)Mn (O)]
IV
2+
(
1) and [(N4Py)Mn (O)] -(HOTf) (2), were generated and
2
[23,24]
characterized spectroscopically as reported previously.
Prof. Dr. S. Fukuzumi
Mesitylene was hydroxylated by 1 in CF CH OH/CH CN
3
2
3
Faculty of Science and Engineering, ALCA
SENTAN Japan Science and Technology Agency (JST)
Meijo University, Nagoya, Aichi 468-0073 (Japan)
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
(
v/v = 1:1) to yield 3,5-dimethylbenzyl alcohol and [(N4Py)M-
III 3+
n ] [Eq. (1)].
The yield of 3,5-dimethylbenzyl alcohol was determined
to be 42(4)% with a small amount of 3,5-dimethylbenzalde-
hyde (3.9(8)%; Supporting Information, Table S1). It was
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201602460.
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confirmed that the decayed product of
1
was not
II 2+
III 3+
[
(N4Py)Mn ] but [(N4Py)Mn ] species, which was EPR
silent (Supporting Information, Figure S1a).
In the hydroxylation of mesitylene by 2, 3,5-dimethylben-
zyl alcohol was obtained in 94(5)% yield as the major product
with
a
negligible amount of 3,5-dimethylbenzaldehyde
[
Eq. (2)].
Figure 1. a) Visible spectral changes observed in the reaction of
1 (0.50 mm) with mesitylene (300 mm) in CF CH OH/CH CN (v/
II 2+
In this case, the decayed product of 2 was [(N4Py)Mn ] ,
3
2
3
III 3+
v=1:1) at 273 K. Inset: time course of the absorbance change
monitored at 940 nm for the decay of 1. b) Visible spectral changes
observed in the reaction of 2 (0.50 mm) with mesitylene (300 mm) in
the presence of HOTf (30 mm) in CF CH OH/CH CN (v/v=1:1) at
not [(N4Py)Mn ] , which is different from the reaction of
and mesitylene (Supporting Information, Figure S1b).
Similar results were obtained in the hydroxylation of hexam-
ethylbenzene (HMB) by 1 and 2 (Supporting Information,
Figure S2).
1
3
2
3
273 K. Inset: time course of the absorbance change monitored at
550 nm for the decay of 2.
Rates of the hydroxylation of mesitylene and other
toluene derivatives by 1 and 2 were monitored by the decay
of the absorption band at 940 nm for 1 and the absorption
band at 550 nm for 2 at 273 K (Figure 1). The decay of
1
obeyed pseudo first-order kinetics (Supporting Information,
Figures S3 and S4), and the pseudo-first-order rate constants
k1) were proportional to concentrations of toluene deriva-
(
tives (Supporting Information, Figures S5 and S6). The
second-order rate constant (kobs) increased with an increase
in the number of methyl group of benzene ring (Table 1).
The temperature dependence of the second-order rate
constant (kobs) in the oxidation of mesitylene by 1 and 2 was
examined to obtain the Eyring plots (Figure 2, blue and red
Figure 2. Eyring plots of ln(kobs/T) against 1/T obtained in the reac-
tions of 1 with mesitylene (blue circles) and [D ]mesitylene (green
1
2
Table 1: One-electron oxidation potentials (E ) of toluene derivatives
ox
circles) and 2 with mesitylene (red circles) and [D ]mesitylene (black
1
2
and second-order rate constants of hydroxylation of toluene derivatives
IV
2+
circles) at various temperatures (243–293 K).
by [(N4Py)Mn (O)] in the absence and presence of HOTf (30 mm) at
73 K.
2
[
a]
À1 À1
Toluene
Eox
kobs [m
s ]
circles, respectively). A linear correlation was obtained for 1,
whereas two different slopes were observed for 2. At temper-
atures higher than 273 K, the kobs values of 2 are larger than
those of 1. At temperatures lower than 273 K, the kobs values
of 2 are smaller than those of 1. At 273 K, the kobs values of
1 and 2 are about the same (see also Figure 1a and b for 1 and
derivative
[V vs. SCE]
without HOTf with HOTf
À2
À3
À3
À3
2
HMB
1.57
2.1”10
4.5”10
2.1”10
1.7”10
6.7”10
8.7”10
3.5
pentamethylbenzene (PMB) 1.69
,2,4,5-tetramethylbenzene 1.74
mesitylene 2.02
1
À3
1.8”10
2, respectively).
[
a] The one-electron oxidation potentials of substrates were determined
When mesitylene was replaced by [D ]mesitylene, no
by cyclic voltammetry in CF CH OH/CH CN (v/v=1:1) containing
12
3
2
3
0
.10m n-Bu NPF as a supporting electrolyte at 298 K.
deuterium kinetic isotope effect (KIE) was observed for 2 at
4
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In the case of hexamethylbenzene (HMB), the kobs values
of 2 are always much larger than those of 1 in the temperature
range of 263–293 K (Supporting Information, Figure S8). In
°
this case again, the DH value of 2 is larger than that of 1,
°
whereas the absolute DS value of 2 is much smaller than that
of 1, being close to zero. The KIE values of 1.0(1) was
obtained in the oxidation of HMB by 2 at both 263 and 293 K,
whereas KIEs larger than 1.0 (1.7(1)) were observed in the
oxidation of HMB by 1 (Supporting Information, Figures S9
[25]
and S10). Thus, the oxidation of HMB by 2 proceeds via
ET, whereas the oxidation by 1 proceeds via HAT in the
temperature range of 263–293 K. In this case, no switchover of
the mechanism was observed.
Figure 3. Absorbance changes monitored at 550 nm for the reactions
of 2 (0.50 mm) with mesitylene (blue, 300 mm) and [D ]mesitylene
1
2
Figure 4 shows the dependence of logk on driving force
et
(red, 300 mm) at 293 K. Inset shows the time profiles observed under
of ET from electron donors to 2 at 273 K. The rate constants
the identical reaction conditions at 253 K.
(ket) of ET from one-electron reductants to 2 (black circles in
Figure 4; see also Table 1) are well fitted in the light of the
Marcus theory of adiabatic outer-sphere electron transfer
[Eq. (3)]:
2
93 K (Figure 3), indicating that the hydroxylation of mesi-
[20]
tylene by 2 proceeds via ET from mesitylene to 2. At
temperatures lower than 263 K, however, a KIE value of
2
2
.9(3) was obtained for the reaction of 2 (Figure 3, inset),
ket ¼ Z exp½Àðl=4Þð1 þ DG =lÞ =k TŠ
ð3Þ
et
B
indicating that the hydroxylation of mesitylene by 2 proceeds
via HAT rather than ET. In the case of 1, the KIE remains
large (KIE = 3.2(3)) in the temperature range of 243–293 K
11
À1 À1
where Z is the collision frequency, taken as 1 ” 10 m s , l is
the reorganization energy of electron transfer, kB is the
Boltzmann constant, and T is the absolute temperature. From
the fitting of the ETrate constants (black line in Figure 4), the
l value of ET from one-electron reductants to 2 was
(
Supporting Information, Figure S7). The activation param-
eters of the oxidation of mesitylene by 1 and 2 are listed in
°
Table 2. The DH value of 2 is larger than that of 1, whereas
[24]
determined to be 2.20(2) eV.
The kobs values of the
hydroxylation of toluene derivatives by 2 fit well into the
Marcus line (red circles in Figure 4), whereas those by 1 are
much larger than those expected from the ET line (blue
circles in Figure 4). The borderline between ET and HAT is
Table 2: Activation parameters determined in the hydroxylation of
mesitylene and hexamethylbenzene (HMB) by 1 and 2.
°
À1
°
À1 À1
Toluene
DH [kcalmol
]
DS [calmol
K ]
derivative
[9b,12a]
located at ÀDG ꢀ À0.5 eV, as reported previously.
The
et
1
2
1
2
ÀDG value of the hydroxylation of mesitylene by 2 is located
et
[
a]
[a]
[b]
mesitylene
HMB
3.8Æ0.3
15Æ1
.8Æ0.3
13Æ1
À57Æ4
À16Æ1
À54Æ4
close to the borderline. This is the reason why the mechanism
of the hydroxylation of mesitylene by 2 is changed from ET to
HAT only by lowering temperature.
[
b]
4
11Æ1
À28Æ2
1.9Æ0.2
[
a] from 243 K to 263 K. [b] from 263 K to 283 K.
In conclusion, hydroxylation of toluene derivatives by
IV
a nonheme manganese(IV)–oxo complex, [(N4Py)Mn -
°
the absolute DS value of 2 is much smaller than that of 1. The
°
DS value of ET is close to zero or negative, being much less
negative than that of HAT; the transition state of which is
°
more ordered to afford the largely negative DS value. In such
a case, ET is energetically more favored at higher temper-
atures. This is the reason why the mechanism changes from
ET at 293 K with no KIE to HAT at temperatures lower than
2
2
63 K with a large KIE of 2.9(3). At temperatures lower than
63 K, the kobs value of 2 is smaller than that of 1, which is
probably due to the steric effect of HOTf, which is bound to
IV
2+ [24]
the oxo group of [(N4Py)Mn (O)] . The observation of
the different mechanisms by changing only temperature
indicates that the two mechanisms are distinguishable and
competing without the continuous change of the mechanism
and that either ET or HAT pathway becomes dominant by
changing only reaction temperature. It should be noted that
this is the first time to show that two mechanisms are
competing and one or the other is dominant by changing only
reaction temperature.
Figure 4. Plots of logkobs for the hydroxylation of toluene derivatives
(
1
1 HMB, 2 PMB, 3 1,2,4,5-tetramethylbenzene, and 4 mesitylene) by
(blue circles) and 2 (red circles) at 273 K. The black circles show the
driving force dependence of the rate constants (logk ) of ET from one-
et
II
II
II
electron reductants (5 [Ru (Me bpy) ](PF ) , 6 [Ru (bpy) ](PF ) , 7 [Ru -
(
2
3
6
2
3
6 2
II
II
5-Clphen) ](PF ) , 8 [Ru (5-Brbpy) ](PF ) , and 9 [Ru (5-NO phen) ]-
3 6 2 3 6 2 2 3
[24]
(PF ) ] to 2 in the presence of HOTf (30 mm) at 273 K.
6
2
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This work was supported by ALCA and SENTAN projects
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NRF of Korea through the CRI (NRF-2012R1A3A2048842)
and GRL (NRF-2010-00353) programs to W.N.
[
Keywords: bioinorganic chemistry · electron transfer ·
hydrogen atom transfer · manganese(IV)-oxo complex ·
mechanism switchover
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[23] a) J. Chen, Y.-M. Lee, K. M. Davis, X. Wu, M. S. Seo, K.-B. Cho,
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Angew. Chem. Int. Ed. 2016, 55, 7450 –7454
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7453

Angewandte
Communications
Chemie
Cho, R. Sarangi, W. Nam, S. Fukuzumi, J. Am. Chem. Soc. 2013,
35, 9186.
24] J. Chen, H. Yoon, Y.-M. Lee, M. S. Seo, R. Sarangi, S. Fukuzumi,
W. Nam, Chem. Sci. 2015, 6, 3624.
305. Furthermore, Mandal and Shaik recently reported that KIE
IV
1
of CÀH bond activation by Fe (O) species exhibits a Bell –
[
[
Evans – Polanyi correlation (a volcano type) with BDE; see: D.
Mandal, S. Shaik, J. Am. Chem. Soc. 2016, 138, 2094.
25] The smaller KIE value of HMB compared to that of mesitylene
may result from the larger driving force of HAT from HMB than
from mesitylene, since the KIE value is known to decrease with
increasing the driving force of HAT; see: a) W. A. Pryor, K. G.
Kneipp, J. Am. Chem. Soc. 1971, 93, 5584; b) S. Fukuzumi, S.
Koumitsu, K. Hironaka, T. Tanaka, J. Am. Chem. Soc. 1987, 109,
Received: March 10, 2016
Revised: March 31, 2016
Published online: May 18, 2016
7
454
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 7450 –7454