Singly Unified Driving Force Dependence of Outer-Sphere Electron-Transfer Pathways of Nonheme Manganese(IV)-Oxo Complexes in the Absence and Presence of Lewis Acids

Article abstract of DOI:10.1021/acs.inorgchem.9b02403

Epoxidation of styrene derivatives, sulfoxidation of thioanisole derivatives, and hydroxylation of toluene derivatives by a nonheme manganese(IV)-oxo complex binding triflic acid, [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂ [1-(H⁺

Full text of DOI:10.1021/acs.inorgchem.9b02403

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Singly Unified Driving Force Dependence of Outer-Sphere Electron-
Transfer Pathways of Nonheme Manganese(IV)−Oxo Complexes in
the Absence and Presence of Lewis Acids
Namita Sharma,†,‡ Yong-Min Lee,†,‡ Xiao-Xi Li,† Wonwoo Nam,*,†,§
and Shunichi Fukuzumi*,†,⊥
†Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
§School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, China
⊥Graduate School of Science and Engineering, Meijo University, Nagoya, Aichi 468-8502, Japan
S
* Supporting Information
theory of ET, which allows us to predict the absolute rate
ABSTRACT: Epoxidation of styrene derivatives, sulfox-
idation of thioanisole derivatives, and hydroxylation of
toluene derivatives by a nonheme manganese(IV)−oxo
c o m p l e x b i n d i n g t r i fl i c a c i d , [ ( N 4 P y ) -
MnIV(O)]2+−(HOTf)2 [1-(H+)2], and scandium triflate,
[(N4Py)MnIV(O)]2+−(Sc(OTf)3)2 [1-(Sc3+)2], occur via
outer-sphere electron-transfer (OSET) pathways, exhibit-
ing singly unified driving force dependence, enabling one
to predict absolute values of the second-order rate
constants of these three types of substrate oxidations by
the manganese(IV)−oxo complex, using the Marcus
theory of electron transfer. When [(N4Py)MnIV(O)]2+
(1) was replaced by [(N4Py)FeIV(O)]2+ (2), OSET
pathways were changed to inner-sphere electron-transfer
(ISET) pathways. The difference in the OSET versus
ISET pathways is clarified based on the difference in the
Lewis basicity of the oxo moieties in 1 and 2.
constants by knowing the one-electron oxidation potentials of
substrates.6 However, there has so far been no direct
comparison of the PCET and MCET pathways of different
metal−oxo species in terms of the ET driving force
dependence of the PCET and MCET rate constants.22−28
We report herein the singly unified ET driving force
dependence of the logarithm of the rate constants of MCET
and PCET reactions of [(N4Py)MnIV(O)]2+−(Sc(OTf)3)2 [1-
(Sc3+)2] and 1-(H+)2,7,15 respectively, as OSET reactions in
light of the Marcus theory of ET (Scheme 1).28 The singly
Scheme 1. OSET versus ISET Pathways for 1 and 2 in the
Presence of Lewis Acids, Respectively
ater oxidation to dioxygen and the reverse reaction, i.e.,
Wdioxygen reduction to water, are critical redox reactions
for biological energy conversions, in which proton-coupled
electron transfer (PCET) plays a pivotal role.1−4 The binding
or release of redox-inactive metal ions, such as scandium ion
(Sc3+), acting as Lewis acids as well as protons can also couple
with electron transfer (ET), referred to as a metal-ion-coupled
electron transfer (MCET).5−13 Ever since Sc3+ was shown to
be bound to the oxo moiety of an iron−oxo complex,14
extensive efforts have been devoted to investigating the MCET
pathways of not only metal−oxo species but also metal−-
peroxo and −superoxo species in relation with the correspond-
ing PCET pathways.14−21 We have recently reported that
different types of redox reactions, such as epoxidation,
sulfoxidation, and hydroxylation of styrene, thioanisole, and
toluene derivatives by [(N4Py)MnIV(O)]2+−(HOTf)2 [1-
(H+)2; N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)-
methylamine], proceed via rate-determining outer-sphere
electron transfer (OSET) from substrates to 1-(H+)2.6 log k2
(where k2 is the observed second-order rate constant) by 1-
(H+)2, and PCET from one-electron reductants to [(N4Py)-
MnIV(O)]2+ (1) in the presence of HOTf shows a remarkably
unified ET driving force dependence in light of the Marcus
unified ET driving force dependence of the logarithm of the
rate constants of MCET and PCET reactions of 1-(Sc3+)2 and
1-(H+)2 is compared with the ET driving force dependence of
the logarithm of the rate constants of PCET reactions of
[(N4Py)FeIV(O)]2+ (2) to clarify the difference in the Lewis
basicity of the oxo moieties in 1 and 2.
1-(Sc3+)2 was synthesized by the addition of Sc(OTf)3 to a
1:1 mixed solution of trifluoroethanol (TFE) and acetonitrile
(MeCN) containing 1 at 273 K according to the literature
procedure.15,16 One Sc(OTf)3 molecule binds with the oxo
moiety, and the other Sc(OTf)3 molecule binds with the OTf
moiety of Sc(OTf)3 binding to the oxo moiety.15 The binding
of two Sc(OTf)3 molecules in 1-(Sc3+)2 results in a weak
interaction with electron-donor substrates, providing an
Received: August 8, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02403
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
excellent opportunity to investigate OSET pathways, which
require only weak interaction between an electron donor (one-
electron reductant) and acceptor (one-electron oxidant).
Epoxidation of trans-stilbene with 1 was reported to proceed
slowly with the second-order rate constant of 1.6 × 10−2 M−1
s−1 in TFE/MeCN (1:1) at 273 K.6 When 1 was replaced by 1-
(Sc3+)2, the epoxidation rate of trans-stilbene became 280
times faster to afford the rate constant of 4.4 M−1 s−1 (Figure
S1). The rate constants of oxidation of other styrene
derivatives with 1-(Sc3+)2 were also determined (Table S1).
The observed second-order rate constants of oxidation of
styrenes with 1-(Sc3+)2 are compared with those of OSET from
one-electron reductants to 1, 1-(Sc3+)2, and 1-(H+)2 by the
Marcus theory of adiabatic OSET (eq 1).28 The driving forces
(−ΔGet) of OSET are obtained from the one-electron-
reduction potentials (Ered vs SCE) of one-electron oxidants,
1 (0.80 V),16 1-(Sc3+)2 (1.42 V),16 and 1-(H+)2 (1.65 V),7 and
the one-electron-oxidation potentials (Eox vs SCE) of one-
electron reductants using eq 2.
Figure 1. Unified ET driving force (−ΔGet) dependence of the
logarithm of the observed second-order rate constants (log kox) for
the oxidation of styrenes6 [(1) 4-methoxystyrene, (2) trans-stilbene,
(3) cis-stilbene, (4) 4-methylstyrene, and (5) styrene] by 1-(Sc3+)2
(black open circles) and 1-(H+)2 (black closed circles), sulfoxidation
of p-X-substituted thioanisoles,7,15 [X = (6) MeO, (7) H, (8) F, (9)
Br, and (10) CN] with 1-(Sc3+)2 (red open triangles) and 1-(H+)2
(red closed triangles), and C−H bond activation of toluene
derivatives8,30 [(11) hexamethylbenzene, (12) pentamethylbenzene,
(13) durene, and (14) mesitylene] with 1-(H+)2 (green open circles)
and log ket for ET from various one-electron reductants16 [(15)
[FeII(Me2phen)3]2+, (16) [FeII(Ph2phen)3]2+, (17) [FeII(bpy)3]2+,
ket = Z exp[−(λ/4)(1 + ΔGet/λ)2/kBT]
(1)
−ΔGet = −e(Eox − Ered
)
(2)
The ET driving force dependence of log ket of OSET from
ferrocene derivatives to 1 and from coordinatively saturated
metal complexes to 1-(Sc3+)2 and 1-(H+)2 is well fitted by the
Marcus equation of OSET (eq 1)28 using the identical value of
the reorganization energy of ET (λ = 2.16 eV), as shown in
Figure 1.7,16 The Z value (normally 1.0 × 1011 M−1 s−1)29 of
OSET indicates that the formation constant (K) value of the
OSET reactions is ca. 0.02 M−1 because of little interaction in
the precursor complexes for OSET reactions of electron-donor
complexes with 1-(Sc3+)2 and 1-(H+)2.
The log kox values of the oxidation of styrenes and
thioanisoles with 1-(Sc3+)2 [black open circles (nos. 1−4)
and red open triangles (nos. 6−10) in Figure 1, respectively]
agree remarkably well with the log ket values of OSET from
one-electron donors [i.e., ferrocene derivatives (pink closed
circles, nos. 24−27) and coordinatively saturated metal
complexes (blue open and closed squares, nos. 15−23)] to
1, 1-(Sc3+)2, and 1-(H+)2 (black line in Figure 1)]. Thus, the
oxidation of styrenes and thioanisoles by 1-(Sc3+)2 and 1-(H+)2
as well as the hydroxylation of toluene derivatives30 by 1-(H+)2
(green open circles, nos. 11−14) proceed via OSET from
electron-donor substrates (i.e., styrenes, thioanisoles, and
toluene derivatives) to 1-(Sc3+)2 and 1-(H+)2, as shown in
Scheme 2. OSET from an organic substrate (S) to 1-(Sc3+)2
occurs as the rate-determining step, following the precursor
complex formation, followed by oxygenation of S•+ to produce
the oxidized organic product (SO) with manganese(II)
species, accompanied by the release of two Sc3+ ions. In the
case of 1-(H+)2 as well, OSET from S including toluene
derivatives to 1-(H+)2 is the rate-determining step. The OSET
pathway of 1-(Sc3+)2 and 1-(H+)2 may result from the steric
effects of two Sc(OTf)3 and two HOTf molecules, respectively,
which bind to 1, precluding inner-sphere interaction between 1
and electron-donor substrates.
(18) [FeI I (5-Clphen)3 ]2 +
,
(19) [RuI I (bpy)3 ]2 + (20)
[RuII(Me2bpy)3]2+, (21) [RuII(5-Clphen)3]2+, (22) [RuII(5-
Brbpy)3]2+, and (23) [RuII(5-NO2phen)3]2+] to 1-(Sc3+)2 (blue
open squares) and 1-(H+)2 (blue closed squares) in TFE/MeCN (v/v
1:1) at 273 K. The pink closed circles exhibit the ET driving force
dependence of log ket for ET from one-electron reductants [(24)
ferrocene, (25) bromoferrocene, (26) acetylferrocene, and (27) 1,1′-
dibromoferrocene] to 1 in TFE/MeCN (v/v 1:1) at 273 K.16
Scheme 2. Oxygenation of Substrates by 1-(Sc3+)2 via OSET
kox values for sulfoxidation of thioanisoles and C−H bond
activation of toluene derivatives with 2 in the presence of
Sc(OTf)3 and HOTf (10 mM; nos. 1−5, black open and
closed circles in Figure 2, respectively) are ∼3 orders
magnitude higher than the log ket values of OSET with the
same ET driving force. The larger kox values than the ket values
with the same ET driving force result from the inner-sphere
The ET driving force dependence of log ket of OSET from
one-electron reductants to 2 in the absence and presence of
Sc(OTf)3 and HOTf is shown in Figure 2, where the
reorganization energy of ET (λ = 2.74 eV, blue line) to fit
all of the OSET data is larger than that of 1 in the absence and
presence of HOTf. In contrast to the case of 1-(H+)2, the log
B
DOI: 10.1021/acs.inorgchem.9b02403
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
In conclusion, the epoxidation of styrenes, sulfoxidation of
thioanisoles, and C−H bond activation of toluene derivatives
with 1-(Sc3+)2 and 1-(H+)2 proceed via the rate-determining
OSET reactions, which exhibit the singly unified ET driving
force dependence of the logarithm of the observed second-
order rate constants, which is fitted well by using the Marcus
theory of OSET (Figure 1), enabling us to predict the absolute
values of the rate constants by knowing only the one-electron
redox potentials of the reactants. In contrast to 1, the oxidation
of thioanisoles and C−H bond activation of toluene derivatives
with 2 in the presence of Sc(OTf)3 and HOTf proceed via the
rate-limiting inner-sphere electron-transfer (ISET) reactions,
in which the formation constant (K) of the precursor complex
between the organic substrates and 2 in the presence of HOTf
is significantly larger than that between the same substrates and
1-(H+)2. Such a difference in the PCET reactivity between 1
and 2 results from the higher basicity of the oxo moiety in 1
than in 2. This study provides an important step forward to fill
the gap between the OSET pathways and the reactions of
cationic electrophiles and neutral nucleophiles through ISET
pathways.33,34
Figure 2. Plots of the logarithm of the observed second-order rate
constants (log kox) for sulfoxidation of p-X-substituted thioanisole
derivatives [X = (1) Me, (2) H, (3) Cl, (4) Br, and (5) CN] by 2 in
the presence of Sc(OTf)3 (10 mM; black open circles) and HOTf (10
mM; black closed circles), C−H bond activation of toluene
derivatives [(6) hexamethylbenzene, (7) pentamethylbenzene, (8)
durene, (9) 1,2,4-trimethylbenzene, (10) p-xylene, and (11)
mesitylene] by 2 in the presence of Sc(OTf)3 (10 mM; red open
circles) and HOTf (10 mM; red closed circles) versus the ET driving
force (−ΔGet),17,30 with the ET driving force dependence of log ket
for OSET from various one-electron reductants [(12)
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02403.
[FeII(Ph2phen)3]2+, (13) [FeII(bpy)3]2+
,
(14) [RuII(4,4′-
Me2phen)3]2+, (15) [RuII(5,5′-Me2-phen)3]2+, (16) [FeII(5-
Clphen)3]2+, and (17) [RuII(bpy)3]2+] to 2 in the presence of
HOTf (10 mM; blue open circles) in MeCN at 298 K.17 The blue
closed circles show the driving force dependence of log ket for ET
from one-electron reductants [(18) decamethylferrocene, (19)
octamethylferrocene, (20) 1.1′-dimethylferrocene, (21) n-amylferro-
cene, and (22) ferrocene] to 2 in the absence of HOTf in MeCN at
298 K.9
Experimental section, Tables S1 and S2, and Figures S1
and S2 (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: wwnam@ewha.ac.kr.
*E-mail: fukuzumi@chem.eng.osaka-u.ac.jp,
■
ORCID
nature of the PCET pathway, in which the interaction between
organic electron-donor substrates and 2 in the presence of
HOTf (10 mM) is significantly larger than that between the
same substrates and 1-(H+)2.31 In fact, the formation constants
of the precursor complex between durene and 2 in the
presence of HOTf (10 mM) were determined to be 1.2 × 102
M−1, which is much larger than that of typical OSET reactions
(ca. 0.02 M−1).17 It should be noted that two Sc(OTf)3 or two
HOTf molecules are bound to 1 to produce 1-(Sc3+)2 or 1-
(H+)2, respectively, but no binding of Sc(OTf)3 or HOTf to 2
was observed prior to ET.
Namita Sharma: 0000-0001-9372-0880
Yong-Min Lee: 0000-0002-5553-1453
Xiao-Xi Li: 0000-0002-3593-7536
Wonwoo Nam: 0000-0001-8592-4867
Shunichi Fukuzumi: 0000-0002-3559-4107
Author Contributions
‡These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
The strong binding of Sc(OTf)3 or HOTf to the oxo moiety
in 1, which shows sharp contrast to the very weak binding of
Sc(OTf)3 or HOTf to that in 2, indicates that the oxo moiety
in 1 should be more basic than that in 2, which may result
from the smaller electronegativity of manganese (2.05 mdyn)
than that of iron (2.31 mdyn),32a as well as the smaller
electrophilicity index of manganese (0.93) than that of iron
(1.05).32b Density functional theory calculations on both 1 and
2 also revealed that MnIV(O) species with a higher negative
charge of −0.81 on the oxo moiety show higher basicity than
FeIV(O) species with a lower negative charge of −0.52 on the
oxo moiety (see the Experimental Section and Table S2).
Furthermore, the binding energy of a proton to these two
metal oxo species has also been calculated, indicating that the
proton affinity of 1 is stronger in energy than that of 2 by 13.22
kcal mol−1 (Table S2).
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI (Grant
16H02268 to S.F.) from MEXT, Japan, and the NRF of
Korea through the CRI (Grant NRF-2012R1A3A2048842 to
W.N.), GRL (Grant NRF-2010-00353 to W.N.), and Basic
Science Research Program (Grant 2017R1D1A1B03029982 to
Y.-M.L. and Grant 2017R1D1A1B03032615 to S.F.).
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