Manganese(v)-oxo corroles in hydride-transfer reactions

Article abstract of DOI:10.1039/c0cc03373a

Hydride transfer from dihydronicotinamide adenine dinucleotide (NADH) analogues to manganese(v)-oxo corroles proceeds via proton-coupled electron transfer, followed by rapid electron transfer. The redox potentials (E _red) of manganese(v)-oxo corroles exhibit a good correlation with their reactivity in hydride-transfer reactions.

Full text of DOI:10.1039/c0cc03373a

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Manganese(V)–oxo corroles in hydride-transfer reactionsw
a
a
Yejee Han,z Yong-Min Lee,z Mariappan Mariappan,a Shunichi Fukuzumi*b and
Wonwoo Nam*a
Received 20th August 2010, Accepted 20th September 2010
DOI: 10.1039/c0cc03373a
Hydride transfer from dihydronicotinamide adenine dinucleotide
(NADH) analogues to manganese(V)–oxo corroles proceeds via
proton-coupled electron transfer, followed by rapid electron
transfer. The redox potentials (Ered) of manganese(V)–oxo
corroles exhibit a good correlation with their reactivity in
hydride-transfer reactions.
and redox potential of the Mn(V)–oxo species has been
discussed as well.
Manganese(V)–oxo corroles, Mn(V)(O)(TNPC) (1) (TNPC =
5,10,15-tris(4-nitrophenyl)corrolato trianion), Mn(V)(O)(TFMPC)
(2) (TFMPC = 5,10,15-tris(3,5-trifluoromethylphenyl)corrolato
trianion), and Mn(V)(O)(TPFC) (3) (TPFC = 5,10,15-tris-
(pentafluorophenyl)corrolato trianion) (see structures in
Chart 1, Mn(V)(O)(Cor) Complexes), were prepared by the
published procedures;7 manganese(III) corroles were reacted
with 2.0 equiv. iodosylbenzene (PhIO) in CH3CN at 10 1C.
The UV-vis spectrum of 1 exhibits a strong band at 405 nm,
characteristic of Mn(V)–oxo corroles (Fig. 1a for 1; ESI1, Fig.
S1 for 2 and 3). 1 was stable enough to be used in reactivity
studies at 10 1C (t1/2 = B2 h). First, the reaction of 1 with
NADH analogues, 10-methyl-9,10-dihydroacridine (AcrH2)
and its derivatives, was investigated (see Chart 1, Hydride
Donors). Upon addition of substrates to a solution of 1, the
intermediate reverted back to the starting manganese(III)
corrole, showing UV-vis spectral changes with a clear isosbestic
point at 438 nm (Fig. 1a). First-order rate constants, determined
by the pseudo-first-order fitting of the kinetic data for the
decay of 1, increased linearly with increasing the substrate
concentration, leading us to determine the second-order rate
constant of 8.2(3) ꢀ 10 Mꢁ1 sꢁ1 for AcrH2 (Fig. 1b). By using
the dideuterated compound, AcrD2 (see Chart 1, Hydride
Donors), a large kinetic isotope effect (KIE) value of 10(2)
was obtained (Fig. 1b and c; ESIw, Table S1). Such a large KIE
value indicates that the C–H bond activation of AcrH2 by 1 is
the rate-determining step in the hydride-transfer reaction.5c,8
The hydride-transfer reaction was also investigated with
other AcrH2 derivatives bearing a substituent R at the C-9
position, such as AcrHMe and AcrHEt (see the structure of
AcrHR in Chart 1, Hydride Donors), and the reaction rates
were found to vary depending on the substituent R of AcrHR
(Fig. 1b). This result is similar to those observed in the
hydride-transfer reactions by heme and non-heme Fe(IV)–oxo
High-valent metal–oxo species have been implicated as the key
intermediates in the catalytic oxidation of organic substrates
by metalloenzymes.1 By synthesizing biomimetic metal–oxo
complexes and investigating their structural, spectroscopic, and
chemical properties, our understanding of the nature of the
metal–oxo intermediates and the mechanistic details of oxygen
atom transfer in oxidation reactions has been advanced greatly.2
Especially, reactivities of heme and nonheme iron–oxo complexes
are well investigated in biomimetic oxidation reactions.2
High-valent manganese(V)–oxo porphyrins have also been
proposed as reactive intermediates in the oxidation of organic
substrates by manganese(III) porphyrin catalysts.3 Very
recently, manganese(V)–oxo porphyrins were characterized
1
by various spectroscopic methods,4–6 and the H NMR and
resonance Raman data revealed that the manganese(V)–oxo
porphyrins are trans-dioxomanganese(V) species.4e Reactivities
of the manganese(V)–oxo porphyrins have also been reported
recently in hydride-transfer reactions using dihydronicotinamide
adenine dinucleotide (NADH) analogues and in the oxidation
of organic substrates.5,6a
Manganese(V)–oxo corroles with triply bonded MnVRO
moiety were successfully characterized with various spectro-
scopic techniques, especially resonance Raman spectroscopy.7
However, the reactivities of the manganese(V)–oxo corroles
have been poorly investigated in oxidation reactions due to the
low oxidizing power. In addition, fundamental redox potentials
and hydride-transfer properties of manganese(V)–oxo corroles,
which would provide valuable mechanistic insight into the
chemical properties, have yet to be reported. In this
communication, we report the first example of hydride transfer
from NADH analogues to manganese(V)–oxo corroles
together with the determination of redox potentials of the
manganese(V)–oxo corroles. An acid effect on the reactivity
a Department of Bioinspired Science, Department of Chemistry and
Nano Science, Ewha Womans University, Seoul 120-750, Korea.
E-mail: wwnam@ewha.ac.kr; Fax: +82 2 3277 4441;
Tel: +82 2 3277 2392
b Department of Material and Life Science,
Graduate School of Engineering, Osaka University, Suita,
Osaka 565-0871, Japan. E-mail: fukuzumi@chem.eng.osaka-u.ac.jp;
Fax: +81 6 6879 7370; Tel: +81 6 6879 7368
w Electronic supplementary information (ESI) available: Experimental
and reactivity details. See DOI: 10.1039/c0cc03373a
z These authors contributed equally to this work.
Chart 1
c
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with AcrH2 and its dideuterated compound, AcrD2 (Fig. 1c;
ESIw, Table S1).
It has been proposed previously that hydride transfer from
NADH analogues to Mn(V)–oxo porphyrins occurs via
proton-coupled electron transfer (PCET), followed by rapid
electron transfer.5c We therefore compared k2 values obtained
in the hydride-transfer reactions by Mn(V)–oxo corrole and
Mn(V)–oxo porphyrin complexes (see Fig. 1c). The observation
of a linear correlation in the reactivity comparisons implies
that hydride transfer from AcrH2 and its derivatives to
MnV(O)(Cor) follows the mechanism proposed in the hydride-
transfer reactions by [MnV(O)2(Porp)]ꢁ (see Scheme 1).5c,8–10
Further, the reactivity of MnV(O)(Cor) turned out to be
greater than that of [MnV(O)2(Porp)]ꢁ. For example, the
reactivity of 3 determined at 10 1C is B60 times greater
than that of [Mn(V)(O)2(TPFPP)]ꢁ (TPFPP=meso-tetrakis-
(pentafluorophenyl)porphinato dianion) determined at 25 1C
(compare k2 values of MnV(O)(Cor) and [MnV(O)2(TPFPP)]ꢁ
in ESIw, Table S1).
We then investigated electron-transfer (ET) reactions with
MnV(O)(Cor) and ferrocene derivatives to determine the
redox potentials of Mn(V)–oxo corroles. Upon addition of
1,10-dimethylferrocene (DiMeFc) to the solution of 1 at 10 1C,
the absorption band at 405 nm due to 1 decreased, accom-
panied by an increase in the absorption bands at 481 and
659 nm due to MnIII(TNPC) (ESIw, Fig. S2). This result
indicates that two-electron reduction of 1 by DiMeFc took
place. Interestingly, the ET between 1 and the reductant was in
equilibrium [eqn (1)],11 where the final concentration of
MnIII(TNPC) produced in the ET reduction of 1 increased
with an increase in the initial concentration of DiMeFc,
[DiMeFc]0.
Ket
Ð
2DiMeFcþMnVðOÞðCorÞ
2DiMeFcþ þMnIIIðCorÞ
ð1Þ
Fig. 1 (a) UV-vis spectral changes of 1 (3 ꢀ 10ꢁ5 M, blue line) upon
addition of AcrH2 (6 ꢀ 10ꢁ4 M). Inset shows time course of the decay
of 1 monitored at 405 nm (blue) and the formation of [MnIII(TNPC)]
monitored at 659 nm (red). (b) Plots of kobs versus concentrations of
AcrH2 (black circles), AcrD2 (red circles), AcrHMe (blue circles), and
AcrHEt (green circles) to determine the second-order rate constants.
(c) Plots of log k2 of 1 (black circles), 2 (red circles), and 3 (blue circles)
in the reactions of NADH analogues at 10 1C versus log k2 of
[MnV(O)2(TPFPP)]ꢁ in the reactions of NADH analogues at 25 1C.5c
The equilibrium constant (Ket) in eqn (1) was determined to be
5.0 ꢀ 10ꢁ2 at 10 1C by fitting the plot (ESIw, Fig. S3a). The
apparent reduction potential, Ered, of 1 was then determined
with Ket value and the Eox value of DiMeFc (0.26 V versus
SCE)12 using eqn (2), where n is the number of electrons
transferred. The Ered
Ered = Eox + (RT/nF)ln Ket
(2)
and Mn(V)–oxo porphyrins;5c,8b,c the reactivity of AcrHR
bearing an electron-donating R group is lower than that of
AcrH2, suggesting that the hydride-transfer reaction does not
occur via a one-step hydride-transfer mechanism.9
of 1 was calculated to be 0.22 V versus SCE. The ET reactions
from DiMeFc to other MnV(O)Cor species were also in
The effect of corrole ligands on the reactivity of Mn(v)–oxo
corroles was investigated in hydride-transfer reactions, and the
second-order rate constants of 82(3), 49(3), and 8.8(3) ꢀ
102 Mꢁ1 sꢁ1 were obtained in the reactions of AcrH2 with
1, 2, and 3, respectively (Fig. 1c; ESIw, Table S1). The
reactivity order of 3 4 1 4 2 suggests that a Mn(V)–oxo
complex bearing an electron-deficient corrole ligand is more
reactive in hydride-transfer reactions. The reactivity order of
3 4 1 4 2 was observed in the oxidation of AcrH2 derivatives
as well (Fig. 1c). In addition, large KIE values of 11(2) and
7.5(4) were obtained in the reactions of 2 and 3, respectively,
Scheme 1 Proposed mechanism of the hydride transfer from AcrH2
to a manganese(V)–oxo corrole complex.
c
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equilibrium (ESIw, Fig. S4), and the equilibrium constants (Ket)
for 2 and 3 were 5.0 ꢀ 10ꢁ3 and 1.5 at 10 1C, respectively.
Consequently, the Ered values of 2 and 3 were determined to be
0.19 and 0.27 V versus SCE, respectively. The Ered values are in a
good correlation with their reactivities, suggesting that
Mn(V)–oxo corroles with a high Ered is a more powerful oxidant.
Interestingly, in the presence of an acid (i.e., HClO4;
10 equiv., 3.0 ꢀ 10ꢁ4 M), 1 showed a reactivity with bromo-
ferrocene (BrFc; Eox = 0.54 V versus SCE); 1 did not react
with BrFc in the absence of the acid. In addition, we found
that there was an equilibrium between 1 and BrFc, where the
final concentration of MnIII(TNPC) produced in the ET
reduction of 1 increased with an increase in the initial
concentration of BrFc, [BrFc]0. The equilibrium constant
(Ket) of 3.0 ꢀ 10ꢁ1 was determined at 10 1C by fitting the plot
(ESIw, Fig. S3b). The apparent reduction potential, Ered, of 1
in the presence of acid is then calculated with Ket value and the
Eox value of BrFc by eqn (2). The Ered of 1 in the presence of
HClO4 is 0.52 V versus SCE, which is 0.30 V higher than that
of 1 in the absence of HClO4. Similar trends were actually
reported in the non-heme FeIV(O) system.13
program from the Ministry of Education, Culture, Sports,
Science and Technology, Japan (to S.F.).
Notes and references
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The hydride transfer from NADH analogues to Mn(V)–oxo
corroles was also dependent significantly on the presence of
HClO4. Upon addition of 20 equiv. AcrHEt (6.0 ꢀ 10ꢁ4 M)
to a solution of 1 (3.0 ꢀ 10ꢁ5 M) containing 10 equiv. HClO4
(3.0 ꢀ 10ꢁ4 M), 1 reverted back to the starting MnIII(TNPC)
with the concomitant formation of AcrEt+ ion from AcrHEt
(ESIw, Fig. S5a). In the absence of HClO4, AcrEt(OH) was the
final product (see ESIw, Experimental section). First-order rate
constant (kobs), determined by the pseudo-first-order fitting of the
kinetic data for the decay of 1 or the formation of AcrEt+, was
5.8 ꢀ 10ꢁ2
s
ꢁ1; the rate constant is 100 times greater than that
determined in the absence of HClO4 (kobs = 5.6 ꢀ 10ꢁ4 sꢁ1
,
Fig. 1b and ESIw, Fig. S5b). This result is in line with the
observation that the presence of an acid increased the redox
potential of 1 (vide supra). We also found that the first-order rate
constant increased proportionally with increasing the acid
concentration (ESIw, S5b). Such acceleration of the rate results
from the enhancement of the ET process in Scheme 1 due to the
protonation of [MnIV(O)(Cor)]ꢁ (PCET).
In conclusion, we have reported the first example of the
oxidation of NADH analogues by manganese(V)–oxo corroles.
We have also demonstrated that hydride transfer from AcrH2
and its derivatives to MnV(O)(Cor) occurs via PCET, followed by
rapid electron transfer, as reported previously in the reaction of
[MnV(O)2(Prop)]ꢁ.5c The redox potentials of manganese(V)–oxo
corroles were determined for the first time using 1,10-dimethyl-
ferrocene as a one-electron reductant in the redox titration
experiments, and the Ered values were in a good correlation with
their reactivities in hydride-transfer reactions; the manganese-
(V)–oxo corroles with a high Ered showed a greater reactivity.
Finally, we have demonstrated that the presence of acid in
reaction solutions increases the redox potential of manganese-
(V)–oxo corroles and the rate of the hydride transfer from
hydride donors to manganese(V)–oxo corroles.
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residual water. The Ket value in eqn (1) is defined including water.
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This Research was supported by KRF/MEST of Korea
through the CRI (to W.N.), GRL (2010-00353) (to S.F. and
W.N.), and WCU (R31-2008-000-10010-0) (to S.F. and W.N.)
and a Grant-in-Aid (No. 20108010 to S.F.) and a Global COE
13 S. Fukuzumi, H. Kotani, T. Suenobu, S. Hong, Y.-M. Lee and
W. Nam, Chem.–Eur. J., 2010, 16, 354.
c
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This journal is The Royal Society of Chemistry 2010