Electron- and hydride-transfer reactivity of an isolable manganese(V)-Oxo complex

Article abstract of DOI:10.1021/ja108395g

The electron-transfer and hydride-transfer properties of an isolated manganese(V)-oxo complex, (TBP₈Cz)Mn^V(O) (1) (TBP ₈Cz = octa-tert-butylphenylcorrolazinato) were determined by spectroscopic and kinetic methods. The manganese(V)-oxo complex 1 reacts rapidly with a series of ferrocene derivatives ([Fe(C₅H₄Me) ₂], [Fe(C₅HMe₄)₂], and ([Fe(C ₅Me₅)₂] = Fc*) to give the direct formation of [(TBP₈Cz)Mn^III(OH)]^- ([2-OH] ^-), a two-electron-reduced product. The stoichiometry of these electron-transfer reactions was found to be (Fc derivative)/1 = 2:1 by spectral titration. The rate constants of electron transfer from ferrocene derivatives to 1 at room temperature in benzonitrile were obtained, and the successful application of Marcus theory allowed for the determination of the reorganization energies (λ) of electron transfer. The λ values of electron transfer from the ferrocene derivatives to 1 are lower than those reported for a manganese(IV)-oxo porphyrin. The presumed one-electron-reduced intermediate, a Mn^IV complex, was not observed during the reduction of 1. However, a Mn^IV complex was successfully generated via one-electron oxidation of the Mn^III precursor complex 2 to give [(TBP₈Cz)Mn ^IV]⁺ (3). Complex 3 exhibits a characteristic absorption band at λ_max = 722 nm and an EPR spectrum at 15 K with gmax_′ = 4.68, gmid_′ = 3.28, and gmin _′ = 1.94, with well-resolved ⁵⁵Mn hyperfine coupling, indicative of a d³ Mn^IVS = ³/ ₂ ground state. Although electron transfer from [Fe(C ₅H₄Me)₂] to 1 is endergonic (uphill), two-electron reduction of 1 is made possible in the presence of proton donors (e.g., CH₃CO₂H, CF₃CH₂OH, and CH₃OH). In the case of CH₃CO₂H, saturation behavior for the rate constants of electron transfer (k_et) versus acid concentration was observed, providing insight into the critical involvement of H⁺ in the mechanism of electron transfer. Complex 1 was also shown to be competent to oxidize a series of dihydronicotinamide adenine dinucleotide (NADH) analogues via formal hydride transfer to produce the corresponding NAD⁺ analogues and [2-OH]^-. The logarithms of the observed second-order rate constants of hydride transfer (k_H) from NADH analogues to 1 are linearly correlated with those of hydride transfer from the same series of NADH analogues to p-chloranil.

Full text of DOI:10.1021/ja108395g

ARTICLE
pubs.acs.org/JACS
Electron- and Hydride-Transfer Reactivity of an Isolable
Manganese(V)-Oxo Complex
Shunichi Fukuzumi,*,†,‡ Hiroaki Kotani,† Katharine A. Prokop,§ and David P. Goldberg*,§
†Department of Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
‡Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
§Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States
S Supporting Information
b
ABSTRACT: The electron-transfer and hydride-transfer
properties of an isolated manganese(V)-oxo complex,
(TBP8Cz)MnV(O) (1) (TBP8Cz=octa-tert-butylphenyl-
corrolazinato) were determined by spectroscopic and
kinetic methods. The manganese(V)-oxo complex 1 reacts
rapidly with a series of ferrocene derivatives ([Fe-
(C5H4Me)2], [Fe(C5HMe4)2], and ([Fe(C5Me5)2] = Fc*)
to give the direct formation of [(TBP8Cz)MnIII(OH)]-
([2-OH]-), a two-electron-reduced product. The stoichi-
ometry of these electron-transfer reactions was found to
be (Fc derivative)/1 = 2:1 by spectral titration. The rate
constants of electron transfer from ferrocene derivatives to 1
at room temperature in benzonitrile were obtained, and the
successful application of Marcus theory allowed for the determination of the reorganization energies (λ) of electron transfer. The λ
values of electron transfer from the ferrocene derivatives to 1 are lower than those reported for a manganese(IV)-oxo porphyrin.
The presumed one-electron-reduced intermediate, a MnIV complex, was not observed during the reduction of 1. However, a MnIV
complex was successfully generated via one-electron oxidation of the MnIII precursor complex 2 to give [(TBP8Cz)MnIV]þ (3).
Complex 3 exhibits a characteristic absorption band at λmax = 722 nm and an EPR spectrum at 15 K with g0max = 4.68, gm0 id = 3.28,
and g0min = 1.94, with well-resolved 55Mn hyperfine coupling, indicative of a d3 MnIV S=3/2 ground state. Although electron transfer
from [Fe(C5H4Me)2] to 1 is endergonic (uphill), two-electron reduction of 1 is made possible in the presence of proton donors
(e.g., CH3CO2H, CF3CH2OH, and CH3OH). In the case of CH3CO2H, saturation behavior for the rate constants of electron
transfer (ket) versus acid concentration was observed, providing insight into the critical involvement of Hþ in the mechanism of
electron transfer. Complex 1 was also shown to be competent to oxidize a series of dihydronicotinamide adenine dinucleotide
(NADH) analogues via formal hydride transfer to produce the corresponding NADþ analogues and [2-OH]-. The logarithms of
the observed second-order rate constants of hydride transfer (kH) from NADH analogues to 1 are linearly correlated with those of
hydride transfer from the same series of NADH analogues to p-chloranil.
’ INTRODUCTION
A number of studies involving high-valent manganese-oxo
porphyrins have focused on their oxygen-atom-transfer reactivity
with alkenes, halides, sulfides, PR3, and NR3 substrates as well as
their C-H activation and hydride-transfer chemistry, and this
work has included theoretical calculations.8-13 In this regard, the
higher-valent MnV-oxo porphyrins are more challenging to
study than the MnIV-oxo complexes because of their inherent
instability. In contrast, the MnV-oxo complexes of the ring-con-
tracted porphyrinoids known as corrolazine and corrole show much
better stability.14,15 In particular, the MnV(O) corrolazine (TBP8Cz)-
MnV(O) (1) (TBP8Cz = octa-tert-butylphenylcorrolazinato) is
remarkably stable at room temperature and can be isolated as a
solid following conventional benchtop chromatography.16,17
High-valent metal-oxo species are frequently invoked as the
key oxidizing intermediates in various oxidation reactions of
heme and non-heme iron enzymes.1,2 For example, the cyto-
chrome P450 family of enzymes is believed to depend on the
formation of a transient iron(IV)-oxo porphyrin π-radical
cation, [(Porp)•þFeIV(O)]þ (Porp = porphinato dianion), as
the critical active oxidizing intermediate in the catalytic cycle.3,4
Significant efforts have been expended in generating the corre-
sponding manganese-oxo porphyrins [(Porp)MnV(O)]þ and
(Porp)MnIV(O) in order to study their fundamental role in
biomimetic oxidation reactions and synthetic catalytic cycles of
practical significance.5,6 High-valent manganese-oxo intermedi-
ates have also been implicated in the oxygen-evolving center of
photosystem II.7
Received: September 16, 2010
Published: January 10, 2011
r
2011 American Chemical Society
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Scheme 1
reaction with one of the ferrocene derivatives were also examined
to clarify the proton-coupled electron transfer (PCET) mecha-
nism. Earlier studies examined the ET properties of the oxo-
manganese(IV) porphyrin complex (TMP)MnIV(O) (TMP =
5,10,15,20-tetrakis(2,4,6-trimethylphenyl)porphinato dianion),13
which are compared with those of 1 in this study. The hydride-transfer
reactivity is also compared with those of the dioxomanganese(V)
porphyrin complex [(TPFPP)MnV(O)2]- (TPFPP = meso-tetrakis-
(pentafluorophenyl)porphinato dianion)9b and an organic
p-benzoquinone derivative.21 The accessibility of the stable
MnV(O) corrolazine 1 provides a rare opportunity to examine
the electron-transfer and hydride-transfer reactivities of a MnV-
(O) complex, and comparison with the MnIV(O) and MnV(O)2
porphyrin complexes provides valuable insights into the mecha-
nism of high-valent metal-oxo-mediated oxidations. The com-
pounds investigated in this study are presented in Chart 1.
The mechanism of the C-H bond-cleaving step in oxidations
mediated by high-valent metal-oxo complexes can involve
hydrogen-atom transfer (HAT), hydride transfer, or electron
transfer (ET). Determining the factors that control these differ-
ent reaction pathways and favor one pathway over another is
critical to our understanding of biomimetic oxidations.18 How-
ever, these factors remain poorly understood, especially for
transient species such as manganese(V)-oxo porphyrins. Pre-
viously, the mechanism of HAT with the manganese(V)-oxo
corrolazine complex 1 was examined in some detail for both
O-H substrates (2,6-t-Bu-4-X-phenols) and C-H substrates
(1,4-cyclohexadiene and 9,10-dihydroanthracene).19 Electrochem-
ical measurements indicated that 1 is a relatively weak oxidant
(0.02 V vs SCE), suggesting that the thermodynamic driving
force for the oxidation of O-H and C-H substrates, which
proceeds through a HAT mechanism, is significantly enhanced
by the basicity of the incipient [MnIV(O)]- species. The
thermodynamics of the HAT process can be analyzed in terms
of the ET and proton-transfer (PT) steps shown in Scheme 1. It
was suggested that the proton affinity of the Mn-oxo unit,
represented by the thermodynamic favorability of the PT step on
the right side of the square, allows 1 to overcome a low oxidation
potential, represented by the ET step at the top. In more recent
work, these findings were supported by the observation of
unprecedented rate enhancements for C-H activation by the
addition of anionic axial ligands to 1.19c Theoretical calculations
indicated that the addition of anionic donors should decrease the
oxidation potential of 1 but also increase the proton affinity to an
even larger extent, thereby providing the dramatic rate enhance-
ments found experimentally for HAT. It should also be noted
that the initial intermediate following HAT to 1 is postulated to
be the MnIV(OH) complex, but this species has not been
observed during HAT to 1. The data indicate that a second fast
HAT step occurs, giving the MnIII(Cz) product. These results
imply that the MnIV(OH) complex may be a highly efficient oxidant
in its own right.
’ EXPERIMENTAL SECTION
Materials. The starting material (TBP8Cz)MnIII was synthesized
according to published procedures.16 The commercially available re-
agents octamethylferrocene, 1,10-dimethylferrocene, decamethylferro-
cene (Fc*), 10-methylacridone, acridine, 1-benzyl-1,4-dihydronicoti-
namide (BNAH), (p-BrC6H4)3Nþ•SbCl6-, methyl iodide (MeI),
NaBH4, LiAlD4 (CIL, Inc.), CH3CO2H, CF3CH2OH, and CH3OH
were of the best available purity and used without further purification
unless otherwise noted. Toluene and dichloromethane were purified via
a Pure-solv solvent purification system from Innovative Technologies,
Inc. Benzonitrile (PhCN) and diethyl ether were dried according to
literature procedures and distilled under Ar prior to use.22 9,10-Dihydro-
10-methylacridine (AcrH2) was prepared from 10-methylacridinium
iodide (AcrHþI-) by reduction with NaBH4 in methanol and purified
by recrystallization from ethanol. AcrHþI- was prepared by the reaction
of acridine with methyl iodide in acetone and purified by recrystallization
from methanol. The dideuterated compound [9,90-2H2]-10-methylacri-
dine (AcrD2) was prepared from 10-methylacridone by reduction with
LiAlD4 in ether.23 9-Ethyl-9,10-dihydro-10-methylacridine was pre-
pared by the reduction of AcrHþI- with Grignard reagent
(EtMgBr).23 The dideuterated compound 1-benzyl-1,4-dihydro-
[4,40-2H2]nicotinamide (BNAH-4,40-d2) was prepared from the mono-
deuterated compound (BNAH-4-d1) by three cycles of oxidation with
p-chloranil in dimethylformamide and reduction with dithionite in
deuterium oxide.24
(TBP8Cz)MnV(O) (1). The original synthesis of this complex was
reported previously,17 and the preparation is repeated here for conve-
nience. Excess PhIO (0.763 g, 3.471 mmol) was added to a solution of
(TBP8Cz)MnIII (0.50 g, 0.347 mmol) in CH2Cl2 (25 mL), and the
reaction mixture was stirred at room temperature for 30 min. PhIO is
only sparingly soluble in CH2Cl2, and thus, an excess was added to
ensure the complete oxidation of (TBP8Cz)MnIII. The solution changed
from dark-green-brown to deep-green. The reaction was monitored by
UV-vis spectroscopy to confirm the complete conversion of
(TBP8Cz)MnIII to 1. The solvent was then removed on a rotary
evaporator, and the residue was purified by column chromatography
(CH2Cl2, silica gel) to give 1 as a dark-green solid. Any unreacted PhIO,
which is mostly insoluble, was easily removed by the column purification
In this study, we examined the electron-transfer properties of
the MnV(O) complex 1 with a series of one-electron reductants
(ferrocene derivatives) and its hydride-transfer properties with a
series of NADH analogues [10-methyl-9,10-dihydroacridine
(AcrH2), its 9-ethyl-substituted derivative (AcrHEt), 1-benzyl-
1,4-dihydronicotinamide (BNAH), and their deuterated de-
rivatives]. This work provides the first fundamental information
on the ET properties of an isolated MnV(O) complex as well as
the first direct measurements of hydride-transfer kinetics with a
MnV(O) complex.
1
of 1. Complex 1 is diamagnetic, and its H NMR spectrum and other
analytical data have been reported previously.16b
The driving-force dependence of the electron-transfer rate
constant was analyzed in light of the Marcus theory of electron
transfer,20 leading to the evaluation of the reorganization energy
of electron transfer for 1. The effects of proton donors (e.g.,
CH3CO2H, CF3CH2OH, and CH3OH) on an endergonic ET
Spectral and Kinetic Measurements. Hydride-transfer reac-
tions involving the NADH analogues and 1 were carried out by the
following representative procedure for AcrH2. The reaction of AcrH2
and 1 (1.0 ꢀ 10-6 M) was examined by monitoring spectral changes
in the presence of an appropriate amount of AcrH2 (6.5 ꢀ 10-5 to
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Chart 1
Figure 1. (a) UV-vis spectral changes in electron transfer from Fc* to 1 (1.9 ꢀ 10-5 M) in PhCN at 298 K. (b) Spectral titration to determine the
stoichiometry of the electron-transfer reaction.
2.6 ꢀ 10-4 M) at 298 K using a Hewlett-Packard 8453 photodiode-array
spectrophotometer and a quartz cuvette (path length = 10 mm).
Typically, a deaerated PhCN solution of AcrH2 was added with a
microsyringe to a deaerated PhCN solution containing 1. Kinetic
measurements were performed on a UNISOKU RSP-601 stopped-flow
spectrometer equipped with a MOS-type highly sensitive photodiode
array or a Hewlett-Packard 8453 photodiode-array spectrophotometer
at 298 K. Rates of electron transfer from Fc*, octamethylferrocene, and
1,10-dimethylferrocene to 1 were monitored by either the increase in the
throughCeliteandtransferredtoaWilmadquartzEPRtube(3mmi.d.). The
sample was deaerated and remained under an argon atmosphere as 1 equiv of
(p-BrC6H4)3Nþ•SbCl6- (1 mM) was added. The contents of the tube were
mixed, and a color change from green-brown to brown was observed. After
1 h, the sample was frozen and stored at 77 K. Electron paramagnetic reso-
nance (EPR) spectroscopy experiments were performed on a Bruker EMX
spectrometer controlled with a Bruker ER 041 X G microwave bridge and
equipped with a continuous-flow liquid helium cryostat (ESR900) coupled to
an Oxford Instruments TC503 temperature controller. The spectra were
obtained at 15 K under nonsaturating microwave power conditions (ν =
9.475 GHz, microwave power = 15.98 mW, modulation amplitude = 10 G,
modulation frequency = 100 kHz). Spectral simulation was performed using
Bruker WINEPR SimFonia software (version 1.26). UV-vis spectral titra-
tions showing the interconversion of the MnIII and MnIV complexes were
performed by adding 0.25-2 equiv of (p-BrC6H4)3N•þSbCl6- (from a
1.8 mM stock solution in CH2Cl2) to 2 (18 μM) in CH2Cl2 (3 mL). A 10
min equilibration time was allotted between aliquots, until there were no
further changes in absorbance. Isosbestic conversion was observed from 0.25
to 1 equiv, and no further changes in absorption were seen beyond 1 equiv.
Addition of 0.25-1 equiv of cobaltocene (from a 1.8 mM stock solution in
CH2Cl2) resulted in the regeneration of 2.
absorption band due to [(TBP8Cz)MnIII(OH)]- ([2-OH]-) (λmax
=
688 nm, εmax = 3.5 ꢀ 104 M-1 cm-1) or by the decrease in absorbance
due to 1 (λmax = 634 nm, εmax = 2.0 ꢀ 104 M-1 cm-1). The concen-
tration of ferrocene derivatives were maintained in more than 10-fold
excess with respect to 1 for all kinetic measurements in order to ensure
pseudo-first-order conditions.
Electrochemistry. High-purity N2 was used to deoxygenate the
PhCN solution before the electrochemical experiments were performed.
Tetra-n-butylammonium hexafluorophosphate (TBAPF6) was purchased
from Sigma-Aldrich, recrystallized from ethyl alcohol, and dried under
vacuum prior to use. Cyclic voltammetry was conducted with an EG&G
model 263A potentiostat. A three-electrode system consisting of a glassy
carbon working electrode, a platinum counter electrode, and a Ag/
AgNO3 0.01 M reference electrode was used. All of the redox potentials
(vs Ag/AgNO3) were converted to values vs the saturated calomel
electrode (SCE) by adding 0.29 V. Redox potentials were determined
using the relation E1/2 = (Epa þ Epc)/2.
’ RESULTS AND DISCUSSION
Electron Transfer from Ferrocene Derivatives to
(TBP8Cz)MnV(O). When a one-electron reductant such as deca-
methylferrocene (Fc*) was employed, electron transfer
from Fc* to (TBP8Cz)MnV(O) (1) occurred, producing the
Generation of (TBP8Cz)MnIV(OH). To a solution of (TBP8Cz)-
MnIII (1 mM) in toluene was added excess Me4NþOH- 5H2O (solid,
3
partially soluble). The solution was stirred for 30 min and then filtered
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Figure 2. (a, b) UV-vis spectral changes observed in the electron-transfer reactions of (a) (TBP8Cz)MnIII with (p-BrC6H4)3N•þSbCl6- and (b)
[(TBP8Cz)MnIV]þ with Cp2Co in CH2Cl2. (c, d) Spectral titrations to determine the stoichiometry of the electron-transfer reactions in (a) and (b),
respectively.
Figure 3. X-band EPR spectrum at 15 K of 3-OH produced by the
addition of (p-BrC6H4)3N•þSbCl6- to a solution of [2-OH]- in toluene.
The experimental spectrum is shown in black and the spectral simulation
1
in red. Parameters for simulation: S0 = /2; g0 = [4.68, 3.28, 1.94]; A0 =
Figure 4. Cyclic voltammogram of [2-OH]- (2.0 ꢀ 10-3 M) after the
[84, 86, 56] G; Lorentzian line widths W = 23, 55, and 42 G.
addition of Et4NOH (2.0 ꢀ 10-3 M) to a solution of 2 (2.0 ꢀ 10-3 M)
in PhCN containing 0.1 M TBAPF6 with a glassy carbon working
corresponding ferrocenium ion (Fc*þ) and [(TBP8Cz)MnIII-
electrode at 298 K. The sweep rate was 25 mV s-1
.
(OH)]- ([2-OH]-) as a two-electron reduced product (eq 1):
from addition of Et4NOH to (TBP8Cz)MnIII in PhCN (see
Figure S1 in the Supporting Information). A Job's plot analysis of
the binding of OH- to (TBP8Cz)MnIII revealed a 1:1 stoichi-
ometry, confirming the formulation of [2-OH]- (Figure S1).
The stoichiometry of the electron-transfer reaction (eq 1) was
confirmed by performing a spectral titration at 482 nm
(Figure 1b). Because the spectral change in going from 1 to
[2-OH]- in Figure 1a exhibited clean isosbestic points, the two-
electron reduction of 1 occurred without any significant buildup
of the one-electron-reduced species [(TBP8Cz)MnIV(O)]-.
V
ꢁ
ðTBP8CzÞMn ðOÞ þ 2Fc
PhCN
s
f
-
ꢁ
þ H2O
½ðTBP CzÞMnIIIðOHÞꢂ þ 2Fc þ þ OH-
8
ð1Þ
As shown in Figure 1a, the absorption band at 634 nm associated
with 1 decreased smoothly, accompanied by an increase in the
absorption bands at 482 and 688 nm. The latter bands are
assigned to [2-OH]-, in agreement with the spectrum obtained
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Figure 5. (a) UV-vis spectral changes observed in electron transfer from [Fe(C5HMe4)2] (2.4 ꢀ 10-4 M) to 1 (1.0 ꢀ 10-6 M) in PhCN at 298 K. It
should be noted that the difference spectra were obtained with respect to the spectrum after the reaction. (b) Time profile of the absorbance at 688 nm
due to [2-OH]- along with a nonlinear fit to a single-exponential kinetic equation.
Table 1. Oxidation Potentials (Eox) of Ferrocene Derivatives and Rate Constants (ket) and Reorganization Energies (λ) for
Electron Transfer from Ferrocene Derivatives to 1 in PhCN and (TMP)MnIV(O) in MeCN at 298 K
(TBP8Cz)MnV(O)
(TMP)MnIV(O)c
ket (M-1 s-1
ferrocene derivative
Fc*
Eox (V vs SCE)a
ket (M-1 s-1
)
λ (eV)
)
λ (eV)
n.d.
-0.08
-0.04
0.26
(8.3 ( 0.5) ꢀ 105
(7.5 ( 0.4) ꢀ 104
(1.1 ( 0.2) ꢀ 102 b
n.d.
1.46 ( 0.04
1.56 ( 0.07
1.58 ( 0.07
n.d.
n.d.
n.d.
[Fe(C5HMe4)2]
[Fe(C5H4Me)2]
[Fe(C5H5)2]
n.d.
(9.2 ( 0.9) ꢀ 105
(3.2 ( 0.2) ꢀ 105
(4.3 ( 0.3) ꢀ 104
1.80 ( 0.10
1.72 ( 0.09
1.62 ( 0.07
0.37
[Fe(C5H5)(C5BrH4)]
0.54
n.d.
n.d.
a Data taken from refs 13 and 30. b The ket value was determined as the saturation value in the plot of ket vs [CH3CO2H] in Figure 7. c Data taken from
ref 13.
product. The strong influence of proton donors on the ET
reaction was confirmed (see below).
The absence of any spectral evidence for the one-electron-
reduced intermediates [(TBP8Cz)MnIV(O)]- and (TBP8Cz)-
MnIV(OH) upon reduction of 1 by Fc* motivated us to find
alternative methods for preparing such species. We considered
the possibility of generating a MnIV complex in the absence of
reductants. We speculated that perhaps addition of an appro-
priate oxidant to the MnIII complex (2) would provide access to a
MnIV species. The reaction of 2 with the strong one-electron
-
oxidant (p-BrC6H4)3N•þSbCl6 resulted in the formation of a
new species characterized by a sharp Q band at 722 nm
(Figure 2a), as shown in the forward step of eq 2:
-
ðp-BrC6H4Þ3Nþ•SbCl6
ðTBP8CzÞMnIII
½ðTBP8CzÞMnIVꢂþ ð2Þ
f
r
Cp2Co=CH2Cl2
Figure 6. Dependence of log(ket/M-1 s-1) on -ΔGet for electron
transfer from ferrocene derivatives to 1 in PhCN (9) and to
(TMP)MnIV(O) in MeCN (O) at 298 K.
This Q-band was well-separated from bands due to either the
MnIII complex 2 or the MnV(O) complex 1 and did not match
those of any other Mn corrolazines,17,19 consistent with the
conclusion that it arose from a new (Cz)Mn oxidation state. The
722 nm species immediately reacted with the one-electron
reductant Cp2CoII to regenerate 2 (eq 2 and Figure 2b). Both
the oxidation and reduction reactions occurred with good
isosbestic behavior, and the 1:1 stoichiometry of these reactions
These data are consistent with the notion that [(TBP8Cz)-
MnIV(O)]- is strongly basic and likely abstracts a proton from
residual water to afford (TBP8Cz)MnIV(OH). This complex is
then readily reduced by Fc* to yield [2-OH]- as the final
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Figure 7. Plots of the rate constants (kobs) of electron transfer from [Fe(C5H4Me)2] to 1 (1.0 ꢀ 10-6 M) as functions of (a) [CH3CO2H] and (b)
[CF3CH2OH] (b) or [CH3OH] (9) in PhCN at 298 K.
was confirmed by spectral titrations (Figure 2c,d). These data
indicate that a one-electron-oxidized species, formally the
manganese(IV) complex [(TBP8Cz)MnIV]þ (3), is generated
upon treatment of 2 with (p-BrC6H4)3N•þSbCl6-. Attempts to
isolate the Mn(IV) complex as a solid were not successful,
leading instead to recovery of Mn(III). Thus, it appears that
Mn(IV) corrolazine is not stable at high concentrations or in the
solid state.
Scheme 2
Confirmation that 3 contains a manganese ion in the þ4
oxidation state was obtained by low-temperature EPR spectros-
copy, as shown in Figure 3. The oxidation of 2 for EPR
measurements was done in the presence of excess Et4NOH.
Under these conditions, 2 is quantitatively bound by OH- to
give [2-OH]-, and one-electron oxidation was expected to
generate (TBP8Cz)MnIV(OH) (3-OH), although direct evi-
dence for the presence of the axial OH- in the oxidized product
was not obtained. The well-resolved rhombic spectrum in
Figure 3 provides conclusive evidence for the formation of a
tetrahedral CoII, it is appropriate here because of the equivalent
ground states [(t2)3] for tetrahedral CoII and square-pyramidal
(distorted octahedral) MnIV ions. With a relatively large, positive
D value (D . 0.5 cm-1) and an intrinsic, isotropic g value of 2.00
for MnIV, use of the perturbation equations with E/D = 0.115 led
to the calculated g0 values gy0 = 4.64, gx0 =3.28, and g0z=1.92 for the
lower Kramers doublet (MS = (1/2). These g values are in good
agreement with those obtained in Figure 3. In comparison, a five-
coordinate MnIV complex with trigonal-bipyramidal symmetry
prepared by Borovik and co-workers27 gave an EPR spectrum
that was simulated with E/D = 0.26. The larger rhombicity in this
case may not be surprising in view of the lower symmetry of this
complex. There is also an additional six-line feature at the low-
field edge of the spectrum in Figure 3 that can be assigned to the
upper Kramers doublet (MS = (3/2). A similar feature was
derived from the simulation of the spectrum by Borovik and co-
workers. More detailed studies, such as high-field and high-
frequency EPR spectroscopy measurements,28 are needed to
definitively obtain the ZFS parameters, particularly the magni-
tude of D, but it is clear that 3-OH is an S = 3/2 MnIV complex.
The one-electron oxidation potential of [(TBP8Cz)-
MnIII(OH)]- in PhCN was determined to be 0.69 V (vs
SCE), as shown by the cyclic voltammogram in Figure 4, which
reveals a reversible redox wave for the MnIV/MnIII couple. The
value of E1/2 was found to be independent of the scan rate (ν),
while the current was shown to be proportional to ν1/2. Thus,
electron transfer from the MnIII complex 2 to the strong
3
high-spin MnIV (d3, S = /2) complex.25 Similar EPR spectra
were also obtained when 2 was oxidized in the absence of added
OH-, but the Mn hyperfine coupling was less apparent. These
results suggest that intermolecular interactions may become
more significant in the absence of a strong axial donor such as
OH-, perhaps through π-stacking effects. Simulation of the
spectrum in Figure 3 assuming a fictitious spin of S0 = 1/2 gave
effective g0 values of gm0 ax = 4.68, gm0 id = 3.28, 0and gm0 in = 1.94 and
55Mn hyperfine coupling constants of Ag = 84, Ag = 86,
0
max
mid
0
and Ag = 56 G. These parameters can be compared to
min
those of the MnIV corrole [(tpfc)MnIV]þ [tpfc = 5,10,15-tris-
(pentafluorophenyl)corrole], which were reported to be gm0 ax
=
4.57, g0mid = 3.60, and gm0 in = 1.995 (the A values were not
given),25a and those of the MnIV porphyrin (TPP)MnIV-
(OCH3)2 (TPP = tetraphenylporphyrin), which0 were reported
0
mid
to be g0max = 5.43, gm0 id = 2.4, g0min = 1.6, and Ag = 72 G (Ag
max
25b
0
and Ag were not resolved).
min
A perturbation theory approach, as described by Banci et al.,26
was applied to the spectrum of 3-OH in order to analyze the
effective g0 values in terms of the zero-field splitting (ZFS)
parameters D and E for an S = 3/2 ion. Although this approach
was developed by Banci et al. for EPR spectra arising from
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Journal of the American Chemical Society
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Figure 8. (a) Spectral changes observed in the reaction of 1 (1.0 ꢀ 10-6 M) and AcrH2 (2.5 ꢀ 10-4 M) in deaerated PhCN at 298 K. (b) Time profiles
of absorption changes at λ = 688 and 634 nm for the formation of [2-OH]- and decay of 1, respectively.
one-electron oxidant (p-BrC6H4)3N•þSbCl6- (1.05 V vs SCE)29
as shown in eq 2 is thermodynamically feasible. On the basis of
the one-electron reduction potential of 3-OH, which is equivalent to
the one-electron oxidation potential of [2-OH]- (0.69 V vs SCE),
the second electron transfer from the ferrocene derivatives to 3-OH
must be much faster than the first electron transfer from Fc to 1.
Further insights into the mechanism of electron transfer
between the ferrocene derivatives and 1 were obtained from
kinetic measurements. The rate of formation of [2-OH]- via
ET from [Fe(C5HMe4)2] to 1 was determined from the
increase in absorbance at 688 nm using a stopped-flow
technique, which coincided with the decay rate of 1 deter-
mined from a decrease in absorbance at 634 nm (Figure 5).
The formation rate of [2-OH]- obeyed first-order kinetics,
and the pseudo-first-order rate constant (kobs) increased linearly
with increasing [Fe(C5HMe4)2] concentration (Figure S2 in the
Supporting Information).
The observed single-exponential kinetics indicates that the
first electron transfer from [Fe(C5HMe4)2] to 1 is the rate-
Figure 9. Plots of the observed pseudo-first-order rate constant (kobs
)
of hydride transfer from NADH analogues to 1 (1.0 ꢀ 10-6 M) in the
presence of (0) AcrH2 (6.5 ꢀ 10-5 to 2.6 ꢀ 10-4 M), (9) AcrD2 (5.0
ꢀ 10-4 to 2.0 ꢀ 10-3 M), and (b) AcrEt (5.0 ꢀ 10-4 to 3.0 ꢀ 10-3 M)
in deaerated PhCN at 298 K.
determining step and that this is followed by facile proton-
ation and the second electron transfer from [Fe(C5HMe4)2]
to (TBP8Cz)MnIV(OH), as expected on the basis of the
thermodynamics mentioned above. The second-order rate
constant (ket) of electron transfer from [Fe(C5HMe4)2] to 1
was determined to be (7.5 ( 0.4) ꢀ 104 M-1 s-1. The kinetics
of electron transfer were similarly examined for the ferrocene
derivatives Fc* and [Fe(C5H4Me)2] to determine the second-
order rate constants of electron transfer (ket) in reaction with
1 (Table 1).
electron transfer (eq 4):
"
#
2
λ ð1 þ ΔGet=λÞ
ket ¼ Z exp -
ð4Þ
4
kBT
The driving-force dependence of the rate constants of electron
transfer from ferrocene derivatives to 1 in PhCN at 298 K is
shown in Figure 6, where the log ket values are plotted against
the -ΔGet values. The driving force of electron transfer from
ferrocene derivatives to 1 was determined using eq 3:
where λ is the reorganization energy of electron transfer, kB is the
Boltzmann constant, Z is the collision frequency (which was taken
as 1 ꢀ 1011 M-1 s-1), and T is the absolute temperature.20 The λ
value can be determined using eq 5, which is derived from eq 4:
ꢀ
ꢁ
ket
Z
λ ¼ - ΔGet - 2RT ln
- ΔGet ¼ eðEred - EoxÞ
ð3Þ
(
)
1=2
ꢂ
ꢀ
ꢁꢃ
2
ket
Z
2
where Eox is the one-electron oxidation potential of the ferrocene
derivative and Ered is the one-electron reduction potential of
1.13,30 The Ered value of 1 was already determined to be 0.02 V vs
SCE in PhCN. Each curve was well-fitted by the solid line
obtained using the Marcus theory of adiabatic outer-sphere
þ
ΔGet þ 2kBT ln
- ðΔGetÞ
ð5Þ
Very similar λ values were obtained irrespective of the
driving force as shown in Table 1, because ferrocene
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Table 2. Oxidation Potentials (Eox) of NADH Analogues and Rate Constants (kH) for the Reactions of Hydride Transfer from
NADH Analogues to Cl4Q, [(TBP8Cz)MnV(O)] (1), and [(TPFPP)MnV(O)2]- in Deaerated PhCN at 298 K
kH (M-1 s-1
1b
)
entry
NADH analogue
E
ox (V vs SCE)a
Cl4Qa
[(TPFPP)MnV(O)2]- c
1
2
3
4
5
6
7
BNAH
BNAH-4,40-d2
0.57
0.57
0.81
0.81
0.84
0.88
0.84
1.0 ꢀ 103
1.9 ꢀ 102
1.5 ꢀ 10
1.7
9.4 ꢀ 10-1
6.6 ꢀ 10-1
4.6 ꢀ 10-1
2.0 ꢀ 104
1.4 ꢀ 103
9.6 ꢀ 10
1.4 ꢀ 10
-
1.3 ꢀ 103
1.4 ꢀ 102
1.6 ꢀ 10
1.3
3.4 ꢀ 10-1
2.0 ꢀ 10-1
1.2 ꢀ 10-1
AcrH2
AcrD2
AcrHMe
AcrHPh
AcrHEt
-
1.3
a Data taken from refs 21 and 37. b The experimental errors are within (5%. c Data taken from ref 9b.
derivatives have similar reorganization energies of electron
self-exchange.31
The ket and λ values for the porphyrin complex (TMP)-
MnIV(O) are listed in Table 1 for comparison. The λ value for the
electron-transfer reduction of 1 (av 1.53 ( 0.05 eV) is signifi-
cantly smaller than the value found for (TMP)MnIV(O) (av 1.71 (
0.09 eV).13 An even greater contrast can be seen when one
compares the values for 1 with the recently obtained λ values for
non-heme iron(IV)-oxo complexes, which range between 1.97
and 2.74 eV.30 The relatively small reorganization energy for 1
may be a reflection of the particularly flat and rigid corrolazine
macrocycle, which suggests that synthetic modifications to
increase the driving force of ET in metal-oxo corrolazines
may be a good strategy for yielding exceptionally powerful
oxidation catalysts.
Effect of Protons on Electron Transfer. As shown in eq 1,
the presumed source of Hþ for the facile protonation of the
reduced [(TBP8Cz)MnIV(O)]- is residual H2O. To further
Figure 10. Plots of log kH for hydride transfer from NADH analogues to
1 and [(TPFPP)MnV(O)2]- vs log kH for hydride transfer from the
same series of NADH analogues to Cl4Q in PhCN at 298 K.
investigate the importance of protons in the reduction of the
MnV(O) complex, the rate of reaction between [Fe(C5H4Me)2]
and 1 was determined in the presence of a series of proton
donors. [Fe(C5H4Me)2] was selected as an electron donor for
these experiments because its reaction with 1 is relatively slow
and easily monitored by conventional UV-vis spectroscopy
(Figure S3 in the Supporting Information). In addition, the
driving force for the reaction between 1 and [Fe(C5H4Me)2] is
slightly uphill (ΔGet = 0.21 eV), so we anticipated that the effect
of adding Hþ may be significant and easily observed as an
increase in the reaction rate.
in Figure 7. This saturation behavior is consistent with the
mechanism shown in Scheme 2. In this case, the uphill nature
of the ET reaction (ket) implies that the ion-pair complex
[MnIV(O)- þ Fcþ] undergoes back-ET (kbet) in competition
with protonation of [MnIV(O)]- (kp) followed by rapid electron
transfer from Fc to 3-OH (MnIV) to yield 2-OH and Fcþ. A
kinetic model based on Scheme 2 leads to the dependence of kobs
on [Hþ] as shown in eq 6:
The two-electron reduction of 1 by [Fe(C5H4Me)2] was
monitored in the presence of various amounts of three potential
proton donors: CH3CO2H, CF3CH2OH, and CH3OH. These
three donors were selected because they span a large range of pKa
values, allowing us to address the influence of proton donation in
some detail.32 Addition of 10 equiv of CH3CO2H resulted in a
30-fold increase in the second-order rate constant for the
conversion of 1 to [2-OH]-. Clean isosbestic behavior was
maintained, as shown in Figure S4 in the Supporting Informa-
tion, indicating that the reaction proceeds in the same manner as
in the absence of CH3CO2H with no evidence for the buildup of
an intermediate. Addition of CH3CO2H to 1 in the absence of a
reductant caused no UV-vis spectral change, indicating that 1 is
not protonated under these conditions. The kobs values for the
reaction between 1 and excess [Fe(C5H4Me)2] increased with
increasing concentration of CH3CO2H and exhibited saturation
behavior, reaching a constant value (ket = 110 M-1 s-1) as shown
kpket½Hþꢂ
kobs
¼
ð6Þ
kbet þ kp½Hþꢂ
This indicates that the kobs value should increase with increas-
ing [Hþ] to reach a constant value equal to ket (see the detailed
derivation on p S5 in the Supporting Information). The ket
value of electron transfer from [Fe(C5H4Me)2] to 1 in
Scheme 2 was obtained as the saturation value in Figure 7. The
ket value fits well with the driving-force dependence of ket in
Figure 6.
The addition of the weaker proton donors CF3CH2OH and
CH3OH also had an impact on the rate of reduction of 1 by
[Fe(C5H4Me)2]. In both cases, the reaction rates were linearly
dependent on the concentration of the added proton donor at a
fixed concentration of [Fe(C5H4Me)2] (Figure 7b). Second-
order rate constants in the presence of these proton donors were
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Scheme 3
derived from plots of kobs versus the concentration of [Fe-
(C5H4Me)2] (Figure S5b,c in the Supporting Information),
which show a 7-fold increase in the rate constant using excess
CF3CH2OH (10000 equiv) but only a 2-fold increase using
excess CH3OH (10000 equiv). There is a clear correlation
between the pKa of the proton source and the reaction rate,
although a possible increase in the reactivity due to a change in
the Mn(V/IV) reduction potential in the presence of a large
amount of CH3OH or CF3CH2OH has yet to be ruled out.
These results support the proposed mechanism in Scheme 2, in
which protonation of the [MnIV(O)]- species is critical to the
initial rate-determining step for the reduction of 1 to [2-OH]-.
These data also indicate that the reduction of 1 by ferrocene
derivatives is formally a PCET process,33-35 similar to the HAT
reactions previously observed for 1.19
Comparison of the Hydride-Transfer Reactivities of
[(TBP8Cz)MnV(O)] and p-Chloranil. The rate constants of
hydride transfer from NADH analogues to 1 were com-
pared with those of hydride transfer from the same series
of NADH analogues to the organic oxidant p-chloranil (Cl4Q)
(Table 2).21a A good linear correlation between the log kH
values for 1 and the corresponding values for Cl4Q was
found, as shown by the black line in Figure 10, suggesting
similar mechanisms of hydride transfer for 1 and Cl4Q. For
both 1 and Cl4Q, there is a significant decrease in the ET rate
upon substitution of an ethyl (Et) group at the C9 position of
AcrH2 (compare the log kH values for AcrHEt vs AcrH2). The
Et group is electron-donating and would be expected to
stabilize the carbocation intermediate arising from a con-
certed hydride-transfer mechanism, which in turn should
lead to an increase in the reaction rate. It is also positioned in
a boat axial conformation, which forces the hydrogen to be
located in an equatorial position and minimizes the steric
hindrance of the Et group on hydride transfer. Thus, the
remarkable decrease in the H- transfer rate for 1 induced by
the addition of an Et group to C9 of AcrH2 indicates that the
rate-determining step for this reaction does not involve
concerted H- transfer.
There is a good linear correlation between the log kH values for
the trans-dioxomanganese(V) porphyrin [(TPFPP)MnV(O)2]-
and the corresponding values for Cl4Q, as shown by the red
line in Figure 10.9b The log kH values for 1 are 10 times larger
than the corresponding values for [(TPFPP)MnV(O)2]-. The
lower reactivity for [(TPFPP)MnV(O)2]- than for 1 may be
a direct result of the additional oxo group in [(TPFPP)-
MnV(O)2]-. The hydride-transfer reactions for Cl4Q and
[(TPFPP)MnV(O)2]- with NADH analogues have been shown
to proceed through a rate-determining step involving PCET that
is followed by fast ET from the resulting NAD radical analogue
intermediates.23,36-38 The parallel reactivity of 1 with both
[(TPFPP)MnV(O)2]- and Cl4Q in Figure 10 suggests that the
same mechanism is operative, i.e., that a slow PCET step is
followed by fast ET from the resulting NAD• analogues to
Hydride Transfer from NADH Analogues to [(TBP8Cz)-
MnV(O)]. Upon addition of an NADH analogue (AcrH2) to the
solution of 1, UV-vis spectral changes were observed as shown
in Figure 8. The spectrum after the reaction agreed well with the
superposition of the AcrHþ and [2-OH]- spectra (Figure 8a).
Thus, the stoichiometry of the reaction is given by eq 7:
ðTBP8CzÞMnVðOÞ þ AcrH2
-
f ½ðTBP8CzÞMnIIIðOHÞꢂ þ AcrHþ
ð7Þ
The rate of formation of [2-OH]- obeyed first-order kinetics,
and the pseudo-first-order rate constant (kobs) increased linearly
with increasing AcrH2 concentration (Figure 9). The second-
order rate constant (kH) for hydride transfer from AcrH2 to 1 was
determined to be 9.6 ꢀ 10 M-1 s-1 from the linear plot of kobs
versus AcrH2 concentration. When AcrH2 was replaced by the
dideuterated compound AcrD2, a kinetic deuterium isotope
(KIE) value of 6.9 was observed. Hydride-transfer reactions
using the NADH analogue BNAH and its deuterated analogue
as well as an ethyl-substituted acridine (AcrHEt) were also investi-
gated, and the kH values are listed in Table 2.
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(TBP8Cz)MnIV(OH) to afford NADþ analogues and [2-
OH]-, as shown in Scheme 3. The large kinetic isotope
effects observed for both AcrH2 and BNAH are consistent
with a rate-determining step that involves C-H bond
cleavage. Together with the previously described influence
of the addition of the Et group on the acridine derivative, the
kinetic data provide strong evidence for the PCET mecha-
nism proposed in Scheme 3.
’ ASSOCIATED CONTENT
S
Supporting Information. Figures S1-S5 and the deriva-
b
tion of eq 6. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
fukuzumi@chem.eng.osaka-u.ac.jp; dpg@jhu.edu
’ CONCLUSION
’ ACKNOWLEDGMENT
The electron-transfer properties of an isolated MnV-oxo
complex have been measured for the first time. The ET from
ferrocene derivatives to (TBP8Cz)MnV(O) (1) is followed
by faster ET to (TBP8Cz)MnIV(OH), producing the two-
electron-reduced species [(TBP8Cz)MnIII(OH)]- after fac-
ile protonation of [(TBP8Cz)MnIV(O)]-. Thus, (TBP8Cz)-
MnIV(OH) is much easier to reduce than (TBP8Cz)MnV(O),
which suggests that the MnIV(OH) complex is a more potent
oxidant than the starting MnV(O) complex. A Marcus
analysis of the ET rates yielded reorganization energies (λ)
for the electron-transfer reduction of 1 by three different
ferrocene derivatives that are in close agreement with each
other, with λav = 1.53 ( 0.05 eV. This value is significantly
smaller than the values found for the heme-related complex
(TMP)MnIV(O) (av 1.71 ( 0.09 eV) and a series of non-
heme FeIV(O) complexes (2.05-2.74 eV), suggesting that
designing new metal-oxo corrolazines with greater oxida-
tion potentials may lead to powerful oxidation catalysts.
Despite the lack of direct spectroscopic evidence for
(TBP8Cz)MnIV(OH) from the one-electron reduction of 1,
a manganese corrolazine in the þ4 oxidation state was
independently synthesized by the stoichiometric oxidation
of [2-OH]- with a one-electron oxidant. This oxidation
yielded a new, distinct UV-vis absorbance at 722 nm and
This work was supported by the NSF (CHE0909587 to D.
P.G.), by the NIH (GM62309 to D.P.G.), by a Grant-in-Aid
(20108010) and a Global COE Program (“The Global
Education and Research Center for Bio-Environmental Chemi-
stry”) from the Ministry of Education, Culture, Sports, Science,
and Technology, Japan, and by KOSEF/MEST through the WCU
Project (R31-2008-000-10010-0). K.A.P. is grateful for a Harry
and Cleio Greer Fellowship. We thank Prof. Joshua Telser
(Roosevelt University) for helpful comments.
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’ NOTE ADDED AFTER ASAP PUBLICATION
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