Formation and characterization of a reactive chromium(V)-oxo complex: Mechanistic insight into hydrogen-atom transfer reactions

Article abstract of DOI:10.1039/c4sc02285h

A mononuclear Cr(V)-oxo complex, [Cr^V(O)(6-COO^--tpa)](BF₄)₂ (1; 6-COO^--tpa = N,N-bis(2-pyridylmethyl)-N-(6-carboxylato-2-pyridylmethyl)amine) was prepared through the reaction of a Cr(III) precursor complex with iodosylbenzene as an oxidant. Characterization of 1 was achieved using ESI-MS spectrometry, electron paramagnetic resonance, UV-vis, and resonance Raman spectroscopies. The reduction potential (E_red) of 1 was determined to be 1.23 V vs. SCE in acetonitrile based on analysis of the electron-transfer (ET) equilibrium between 1 and a one-electron donor, [Ru^II(bpy)₃]²⁺ (bpy = 2,2′-bipyridine). The reorganization energy (λ) of 1 was also determined to be 1.03 eV in ET reactions from phenol derivatives to 1 on the basis of the Marcus theory of ET. The smaller λ value in comparison with that of an Fe(IV)-oxo complex (2.37 eV) is caused by the small structural change during ET due to the dπ character of the electron-accepting LUMO of 1. When benzyl alcohol derivatives (R-BA) with different oxidation potentials were employed as substrates, corresponding aldehydes were obtained as the 2e^--oxidized products in moderate yields as determined from ¹H NMR and GC-MS measurements. One-step UV-vis spectral changes were observed in the course of the oxidation reactions of BA derivatives by 1 and a kinetic isotope effect (KIE) was observed in the oxidation reactions for deuterated BA derivatives at the benzylic position as substrates. These results indicate that the rate-limiting step is a concerted proton-coupled electron transfer (PCET) from substrate to 1. In sharp contrast, in the oxidation of trimethoxy-BA (E_ox = 1.22 V) by 1, trimethoxy-BA radical cation was observed by UV-vis spectroscopy. Thus, it was revealed that the mechanism of the oxidation reaction changed from one-step PCET to stepwise ET-proton transfer (ET/PT), depending on the redox potentials of R-BA.

Full text of DOI:10.1039/c4sc02285h

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1
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Chemical Science
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DOI: 10.1039/C4SC02285H
ARTICLEꢀ
Formation and Characterization of a Reactive
Chromium(V)-Oxo Complex: A Mechanistic Insight
into Hydrogen-Atom Transfer Reactions
Cite this: DOI: 10.1039/x0xx00000x
a
a
a
b
Hiroaki Kotani,* Suzue Kaida, Tomoya Ishizuka, Miyuki Sakaguchi, Takashi
b
c
c,d
a
Received 00th January 2012,
Accepted 00th January 2012
Ogura, Yoshihito Shiota, Kazunari Yoshizawa and Takahiko Kojima*
V
–
–
DOI: 10.1039/x0xx00000x
4 2
A mononuclear Cr(V)-oxo complex, [Cr (O)(6-COO -tpa)](BF ) (1; 6-COO -tpa = N,N-bis(2-
pyridylmethyl)-N-(6-carboxylato-2-pyridylmethyl)amine) was prepared through the reaction of
a Cr(III) precursor complex with iodosylbenzene as an oxidant. Characterization of 1 was
made by ESI-MS spectrometry, electron paramagnetic resonance, UV-vis, and resonance
Raman spectroscopies. The reduction potential (Ered) of 1 was determined to be 1.23 V vs. SCE
in acetonitrile based on the analysis of electron-transfer (ET) equilibrium between 1 and a one-
www.rsc.org/
II
2+
electron donor, [Ru (bpy)
3
]
(bpy = 2,2’-bipyridine). Reorganization energy (λ) of 1 was also
determined to be 1.03 eV in ET reactions from phenol derivatives to 1 on the basis of the
Marcus theory of ET. The smaller λ value in comparison with that of an Fe(IV)-oxo complex
(2.37 eV) is caused by the small structural change during ET due to the dπ character of the
electron-accepting LUMO of 1. When benzyl alcohol derivatives (R-BA) with different
oxidation potentials were employed as substrates, corresponding aldehydes were obtained as
–
1
the 2e -oxidized products in moderate yields as determined by H NMR and GC-MS
measurements. One-step UV-vis spectral changes were observed in the course of the oxidation
reactions of BA derivatives by 1 and kinetic isotope effect (KIE) was observed in the oxidation
reactions for deuterated BA derivatives at the benzylic position as substrates. These results
indicate that the rate-limiting step is a concerted proton-coupled electron transfer (PCET) from
substrate to 1. In sharp contrast, in the oxidation of trimethoxy-BA (Eox = 1.22 V) by 1,
trimethoxy-BA radical cation was observed by UV-vis spectroscopy. Thus, it was revealed that
the mechanism of the oxidation reaction changed from one-step PCET to stepwise ET–proton
transfer (ET/PT), depending on the redox potentials of R-BA.
Introduction
by eqn (1), i.e, hydrogen-atom transfer (HAT).
Mechanistic insights into HAT from a substrate to a high-
Extensive efforts have been devoted to preparation of high- valent metal-oxo species in oxidative reactions have been
valent metal-oxo complexes in order to understand their gained by “radical clock” substrates, which usually involve a
1-3
reactivity in oxidative conversion of organic substrates. Non- cyclopropane framework such as bicyclo[2.1.0]pentane and
7
heme high-valent iron-oxo species have been identified as key methylcyclopropane for several decades. These radical-clock
intermediates in various enzymatic oxidations involving experiments have contributed to discriminate mechanisms of
oxidative C–H bond cleavage, such as those of taurine:α- oxidation reactions by scrutinizing reaction products: Whether
4
-6
ketoglutarate dioxygenase and halogenase Cytc3.
These radical-clock compounds are oxidized via concerted, radical, or
7
enzymatic reactions have been usually triggered by transferring cationic mechanisms. Once a radical intermediate is formed by
formally a hydrogen atom (H•) from organic substrates (R–H) a HAT reaction from such a radical-clock compound to high-
n
to metal-oxo species ([M (O)]) as the initial step as expressed
valent metal-oxo species, radical rearrangements or a ring-
opening reaction occurs in competition with oxygen rebound to
7
produce hydroxylated products. Although such arguments
HAT
n
[Mn–1(OH)] + R•
[
M (O)] + R–H
(1)
should be valid only for specific substrates, further details of
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1
Cr(V)-oxo species in the d configuration. In addition, the spin
Scheme 1
ET
state is fixed to be S = 1/2, regardless of ligands used.
Cr(V)-oxo complexes have been synthesized and
n
[Mn–1(O)]– + R–H•+
[
M (O)] + R–H
View Article Online
DOI: 10.1039/C4SC02285H
characterized not only in relevance to high-valent Fe- and Mn-
18
PT
PCET
ET
PT
oxo complexes, which are mostly unstable, but also in the
light of many examples in which they have been proposed as
19
important reactive intermediates in oxidation reactions.
n
M (OH)]+ + R–
[Mn–1(OH)] + R•
[
Efforts have been rather devoted to elucidating the electronic
structure and determining crystal structures of Cr(V)-oxo
complexes, which are stabilized using highly electron-donating
HAT require a more general protocol to elucidate the
mechanism for a wide range of substrates.
18a,19a,g
ancillary ligands such as salen derivatives
18b,c,19b,e
and
porphyrinoids. The stabilization inevitably makes such
Cr(V)-oxo complexes less reactive toward external organic
n
HAT reactions performed by [M (O)] have been
categorized into stepwise electron/proton transfer (ET/PT) as
well as proton/electron transfer (PT/ET), and concerted proton-
18c,d
substrates.
Thereby, mechanistic investigation on the
8
-10
coupled electron transfer (PCET), as shown in Scheme 1.
reactivity of those stabilized Cr(V)-oxo complexes has been
limited to oxygen-atom transfer reactions including epoxidation
High-valent metal-oxo species have been recognized to oxidize
a C–H bond of a substrate by accepting an electron at the metal
centre and a proton at the oxo ligand, respectively, in a
18a,19a,b
18d,19d,e
of alkenes,
19g
oxygenation of phosphines
and
sulfides. In contrast, the lack of a characterizable but highly
reactive Cr(V)-oxo complex, which is capable of HAT
reactions from a variety of substrates, limits understanding of
9
concerted manner with showing certain kinetic isotope effect.
This concerted pathway can be recognized as a “PCET”
mechanism in Scheme 1. The one-step PCET pathway is
kinetically discriminated from stepwise ET/PT pathway
18e,20
mechanisms of the reactions by Cr(V)-oxo complexes.
In
order to gain mechanistic insights into HAT reactions by a
Cr(V)-oxo complex, the regulation of the electron density at a
Cr(V) center should be important for balancing its stabilization
and its reactivity by employing a multi-dentate ligand with
moderate electron-donating ability.
(
electron transfer process from substrates to metal-oxo species is
Scheme 2). Thus, PCET reactions can occur, even if the
8
a,10b
thermodynamically uphill. It has been suggested that
whether a net hydrogen-atom transfer reaction proceeds via a
one-step concerted pathway (PCET) or a stepwise pathway
We report herein preparation, characterization and reactivity
(
ET/PT or PT/ET) depends on underlying parameters for both
V
–
2+
–
of a Cr(V)-oxo complex, [Cr (O)(6-COO -tpa)] (6-COO -
oxidants and substrates, including C–H bond dissociation
energies of substrates, redox potentials and reorganization
energy (λ) of metal-oxo complexes, pKa of metal-oxo and
21
tpa
=
N,N-bis(2-pyridylmethyl)-N-(6-carboxylato-2-
pyridylmethyl)amine; 1), having a monoanionic pentadentate
ligand. The Cr(V)-oxo complex 1 not only exhibits moderate
stability to be spectroscopically characterized but also a high
reduction potential enough to perform HAT reactions from a
series of organic substrates, allowing us to discuss in detail on
the reactivity of Cr(V)-oxo complexes in HAT reactions for the
first time.
11-15
metal-hydroxo species.
Scheme 2. Schematic energy diagrams of (a) stepwise ET/PT
and (b) one-step PCET.
Experimental
General.
UV-vis absorption spectra were measured in acetonitrile
(
CH3CN) on Shimadzu UV-3600 and Agilent 8453
spectrometers at various temperatures. ESI-TOF-MS spectra
were obtained on an Applied Biosystems QSTAR Pulsar i-mass
1
spectrometer. H NMR spectra were recorded on a JEOL EX-
2
70 spectrometer. ESR measurements were performed on a
Bruker Bio SpinEMXPlus9.5/2.7 spectrometer in CH3CN. GC-
MS data were obtained on JEOL JMS-T100GCV
spectrometer, equipped with a capillary gas chromatograph
16
17
The λ values of Fe(IV)-oxo and Mn(IV)-oxo species
a
have been determined to be 2.37–2.74 eV and 2.27 eV,
respectively. The relatively large λ values are interpreted as the
structural change during ET due to the dσ character of the
LUMO. When the smaller λ value of high-valent metal-oxo
species is achieved, ET and PCET reactions would be
accelerated. In order to reduce the structural change, a
dπ character of the LUMO should be required as is realized in
18
(Agilent 7890A, HP-5 (19091J-413) capillary column). O-
22
labeled PhIO (PhI O)
18
and deuterated benzyl alcohol
23
derivatives were synthesized as described in the literature.
CH3CN was distilled over CaH2 under Ar prior to use. THF was
distilled from Na/benzophenone under Ar before use.
Chemicals were used as received unless otherwise noted.
2
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Synthesis of N,N-bis(2-pyridylmethyl)-N- (6-ethoxycarbonyl-2-
pyridylmethyl)amine (6-COOEt-tpa).ꢀ
through a membrane filter to remove insoluble solids. The
filtrate was evaporated to dryness and the residual solids were
dissolved into CH3CN. Vapor diffusion of ethyl acetate to the
View Article Online
DOI: 10.1039/C4SC02285H
Bis(2-pyridylmethyl)amine (2.38 g, 12.0 mmol) in CH3CN (40
mL) was added to a solution of 6-(ethoxycarbonyl)-2-
solution allowed us to obtain pink crystals. The crystals
obtained were washed with diethyl ether and then dried in
vacuo. The target compound was obtained as pink crystals (31
mg, 0.055 mmol) in 69% yield. UV-Vis (CH3CN): λmax (nm) =
24
chloromethyl- pyridine (2.20 g, 11.0 mmol) and Na2CO3 (6.36
g, 60.0 mmol) in CH3CN (60 mL) and the mixture was refluxed
for 24 h. After cooling, the mixture was filtered and CH3CN
was removed by a rotary evaporator to afford a deep brown oil.
This crude material was purified on an alumina column eluted
with EtOAc/hexane (4/1 v/v) to give the ligand as a brown oil.
–1
–1
–1
–1
3
70 (ε = 120 M cm ), 550 (ε = 180 M cm ). Anal. Calcd
for B2C20F8H21N4O3.5Cr: C, 40.10; H, 3.53; N, 9.35. Found: C,
0.30; H, 3.47; N, 9.16.
4
1
The yield was 72% (2.88 g). H NMR (CD3CN): 1.34 (t, J = 7
Hz, 3H, -CH2CH3), 3.80 (s, 4H, -CH2-py), 3.86 (s, 2H, -CH2-
X-ray crystallography on 2 and 3.
py-COOEt), 4.34 (q, J = 7 Hz, 2H, -CH2CH3), 7.13 (dd, J = 5 A purple single crystal of 2 was grown by vapor diffusion of
Hz, 1 Hz, 2H, H4 of py), 7.56 (d, J = 8 Hz, 2H, H3 of py), 7.66 THF into an CH3CN solution of 2. A pink single crystal of 3
(
and H5 of py-COOEt), 8.45 (d, J = 6 Hz, 2H, H6 of py).
dd, J = 5 Hz, 6 Hz, 2H, H5 of py), 7.8–7.9 (m, 3H, H3 and H4 was obtained by recrystallization from an CH3CN solution of 3
with vapor diffusion of ethyl acetate as a poor solvent. All
measurements were performed at 120 K on a Bruker APEXII
Ultra diffractometer. The structures were solved by a direct
method (SIR-97) and expanded with differential Fourier
technique. All non-hydrogen atoms were refined anisotropically
and the refinement was carried out with full matrix least
Synthesis of Bis(2-pyridylmethyl) (6-carboxyl-2-pyridyl
2
1
methyl)amine (6-COOH-tpa).
NaOH (2.00 g, 50 mmol) in H2O (75 mL) was added into a
solution of 6-(COOEt)-tpa (2.88 g, 8.0 mmol) in ethanol (75
mmol) and the mixture solution was refluxed for 20 h. After
cooling, the solution was neutralized by 70% HClO4 to be pH ~
squares on F. All calculations were performed using the
25
Yadokari-XG
Crystallographic details are available in the cif format as ESI†.
crystallographic
software
package.
4. Ethanol was removed by a rotatory evaporator and the
aqueous solution was extracted by CHCl3 (3 times) and then
dried over MgSO4. By removing CHCl3, 6-COOH-TPA was
Formation of a Cr(V)-oxo complex, 1.
1
V
–
obtained as a light brown liquid in 99% yield. H NMR [Cr (O)(6-COO -tpa)] (1) was prepared in situ by the reaction
2+
(
CD3CN): 3.78 (s, 4H, CH2-py), 3.83 (s, 2H, -CH2-py-COOH), of 3 (0.50 mM, 2.5 µmol) with iodosylbenzene (PhIO; 2.5 mM,
.15 (dd, J = 8 Hz, 6 Hz, 2H, H4 of py), 7.41 (m, 3H, H3 of py 12.5 µmol) in CH3CN (5 mL) at 298 K under air. While the
and H5 of py-COOH), 7.68 (t, J = 8 Hz, 2H, H5 of py), 7.79 (t, resulting suspension was stirred for 60 min, the colour change
7
26
J = 8 Hz, 1H, H3 of py-COOH), 7.94 (d, J = 8 Hz, 1H, H6 of from pink to yellowish brown was observed. The yellowish
py-COOH), 8.52 (d, J = 6 Hz, 2H, H6 of py). ESI-MS (m/z): brown solution was filtered to remove remaining PhIO. The
+
33.1 ({M – H } ).
–
3
concentration of 1 was determined to be 25 ± 5% (0.13 ± 0.03
II
2+
mM) by chemical titration with [Fe (bpy)3] and double
integration of the signal due to 1 against that of a standard
radical (TEMPO radical) using ESR measurements.
III
–
4
Synthesis of [Cr (6-COO -tpa)(Cl)](BF ) (2).
6
-COOH-tpa (1.86 g, 5.59 mmol) was dissolved in distilled
THF (40 mL) and to the solution was added CrCl2 (482 mg,
.92 mmol). The mixture was stirred overnight under Ar at 298
Kinetic measurements.
3
K. NH4BF4 (472 mg, 4.5 mmol) was added and the mixture was Kinetic measurements were performed on a UNISOKU RSP-
stirred for further 1 hour under air. The precipitate was filtered 2000 stopped-flow spectrometer equipped with a multi-channel
and washed with THF and diethyl ether. Dark purple powder of photodiode array or an Agilent 8453 photodiode-array
the crude product was reprecipitated from CH3CN/diethyl ether. spectrophotometer or a Shimadzu UV-3600 spectrophotometer
The target compound was obtained as a purple powder (641 mg, at 298 K. To a solution of the complex 1 (0.1 mM) in CH3CN,
1
.16 mmol) in 30% yield. UV-Vis (CH3CN): λmax (nm) = 393 was added a substrate (benzyl alcohol and the deuterated
–1
–1
–1
–1
(ε = 130 M cm ), 554 (ε = 190 M cm ). Anal. Calcd for derivatives) with various concentrations in CH3CN at various
BC19F4H19N4O3ClCr: C, 43.41; H, 3.64; N, 10.66. Found: C, temperatures. The reactions were monitored by the decay of the
4
3.18; H, 3.57; N, 10.66.
absorption assigned to that of 1 at λ = 330 nm.
III
–
Synthesis of [Cr (6-COO -tpa)(BF
4
)](BF
4
) (3).
ESR measurements.
III
A solution containing [Cr (6-COO -tpa)Cl](BF4) (40 mg, ESR spectra were taken on a Bruker X-band spectrometer
–
0
.080 mmol) and AgBF4 (22 mg, 0.12 mmol) in H2O (20 mL) (EMXPlus9.5/2.7) with a liquid nitrogen or a liquid helium
was stirred at room temperature and then heated to 373 K. The transfer system under nonsaturating microwave power
temperature was kept for 6 h. The pink solution was filtered conditions (1.0 mW). The magnitude of the modulation was
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chosen to optimize the resolution and the signal to noise ratio carboxyl group to the Cr(III) centre and two successive five-
(
G; modulation frequency, 100 kHz).
S/N) of the observed spectrum (modulation amplitude, 3 - 15 membered chelate rings in the meridional geometry. Note the
III
2+
View Article Online
bond lengths of Cr-Nx (x = 1–4) in [Cr (Cl)2(tpa)] have been
DOI: 10.1039/C4SC02285H
32
reported to fall in the range of 2.05–2.08 Å.
Treatment of complex 2 with AgBF4 in H2O resulted in the
Resonance Raman spectroscopy on complex 1.
III
–
formation of [Cr (6-COO -tpa)(BF4)](BF4) (3) via removing
the chloro ligand. The structure of 3 was unambiguously
determined by X-ray crystallography. As shown in Fig. 1b, the
Samples were prepared by the following procedures. For
[
V 16
Cr ( O)(6-COO -tpa)] , PhI O (5.5 mg, 25 µmol) was
–
2+
16
added to 2 mL of an CD3CN solution containing 3 (2.8 mg, 4.9
µmol) and stirred for 35 min at 298 K under Ar. For
–
coordinated anionic ligand was identified as BF4 . The crystal
structure suggests that the oxo ligand should be formed at the
trans position to the pyridine moiety having the carboxyl group.
In contrast, in the ESI-TOF-MS spectrum, the complex 3
unexpectedly exhibited a peak cluster at m/z = 404.14 (calcd.
V 18
–
2+
18
[
Cr ( O)(6-COO -tpa)] , PhI O (5.5 mg, 25 µmol) was
added to 2 mL of an CD3CN solution containing 3 (2.8 mg, 4.9
18
µmol) and H2 O (5 µL) and stirred for 35 min at 298 K under
Ar. Resonance Raman scattering was excited at 441.6 nm with
a He-Cd Laser (KIMMON KOHA CO., LTD.). The scattered
light was dispersed with a polychromator (MC-100DG, Ritsu
Oyo Kogaku) and detected with a CCD detector (Symphony,
HORIBA Jobin Yvon). The measurements were performed at
III
–
+
for [Cr (6-COO -tpa)(F)] : 404.07) without any peak clusters
–
due to the BF4 -bound Cr(III) complex as shown in Fig. S1b in
–
ESI†. The coordinated fluoride anion (F ) was presumably
Scheme 3
2
36 K using a spinning NMR tube at 135° scattering geometry.
+
+
2+
Cl
BF4
O
Electrochemical measurements.
N
N
AgBF4
H2O
N
N
PhIO
CH3CN
N
N
CrIII
N
CrIII
CrV
N
N
O
N
O
N
O
N
O
O
O
Second harmonic AC voltammetry (SHACV) and differential
pulse voltammetry (DPV) measurements were carried out in
CH3CN containing 0.1 M TBAPF6 as an electrolyte at 298 K
under Ar with a platinum working electrode, a platinum wire as
a counter electrode, and Ag/AgNO3 as a reference electrode. An
AUTOLAB PGSTAT12 potentiometer was used for SHACV
measurements and a BAS ALS-710D electrochemical analyzer
for DPV measurements, respectively.
CrIII(L)(Cl)](BF
[CrIII(L)(BF
V
[
4
)
4
)](BF
4
)
[Cr (O)(L)](BF
–
4 2
)
(
L = 6-COO -TPA)
Computational methods.
V
–
2+
IV
–
The structures of [Cr (O) (6-COO -tpa)] , [Cr (O)(6-COO -
+
tpa)] , [Fe (O)(TMC)] and [Fe (O)(TMC)] were optimized
IV
2+
III
+
27
by using the hybrid B3LYP functional without solvent effects.
III
–
28,29
Fig. 1 ORTEP drawings of the cation moieties of (a) [Cr (6-COO -
The Wachters-Hay basis set
was used for Fe and the 6-
III
–
tpa)(Cl)](BF
4 4 4
) (2) and (b) [Cr (6-COO -tpa)(BF )](BF ) (3) using
30
311+G** basis set for H, C, N and O atoms. The program
used is Gaussian 09.
5
0% probability thermal ellipsoids with numbering schemes for the
31
heteroatoms. Hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) for 2: Cr–Cl 2.2874(6), Cr–O1 1.959(2), Cr–N1
2
.088(2), Cr–N2 2.048(2), Cr–N3 2.066(2), Cr–N4 1.978(2).
Results and discussion
Selected bond lengths (Å) for 3: Cr–F 1.986(2), Cr–O1 1.958(2),
Cr–N1 2.079(2), Cr–N2 2.044(2), Cr–N3 2.046(2), Cr–N4 1.968(2).
Preparation and characterization of a Cr(V)-oxo complex.
(a)
(b)
200.54
V
The synthesis of a mononuclear Cr(V)-oxo complex, [Cr (O)(6-
–
COO -tpa)](BF4)2 (1) was accomplished by the procedure shown
sim.
[CrIII(L)(BF
)](BF
4 4
) (3)
200.59
V
[
Cr (O)(L)](BF
4
)
2
(1)
in Scheme 3. A synthetic method for a Cr(III) precursor
1.0
exp.
sim.
exp.
III
complex, [Cr (6-COO -tpa)(Cl)](BF4) (2) was described in the
–
s
201.54
201.59
Ab
experimental section. In the electrospray ionization TOF mass
ESI-TOF-MS) spectrum, the complex 2 exhibited a peak
0
.5
0
(
III
–
+
cluster at m/z = 420.10 (calcd. for [Cr (6-COO -tpa)(Cl)] :
20.04) as shown in Fig. S1a in ESI†. The crystal structure of 2
was determined by X-ray crystallography. Its ORTEP drawing
4
3
00 400 500 600 700 800 900
Wavelength, nm
198
200
202
204
m/z
is depicted in Fig. 1a and selected bond lengths are given in the Fig. 2 (a) UV-vis spectral change observed upon addition of PhIO to
Fig. caption. The bond length of Cr-N4 was 1.978(2) Å, which 3 (0.5 mM) in CH CN at 298 K. (b) Positive-ion ESI-TOF-MS of 1
upper) and O-labeled 1 (lower) in CH CN. The black lines are
simulated isotopic patterns.
3
1
8
(
3
is shorter than those of Cr-N bonds for other pyridine rings.
This result should be induced by a strong binding of the anionic
4
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–
derived from decomposition of the BF4 anion in the ionization Reduction potential of complex 1.
33
process of ESI-TOF-MS measurements.
Reaction of 3 with iodosylbenzene (PhIO) in acetonitrile
View Article Online
In order to determine the Ered value of 1 in the light of ET
DOI: 10.1039/C4SC02285H
equilibrium, [Fe (bpy)3] was employed as an electron donor
II
2+
(
CH3CN) at 298 K resulted in a colour change from pink to
39
(
[
Eox = 1.06 V vs. SCE) in CH3CN. Upon addition of
II 2+
Fe (bpy)3] to an CH3CN solution containing 1 (0.15 mM),
yellowish brown, accompanying the spectral change as shown
in Fig. 2a. This spectral feature is similar to that of a previously
18d
UV-vis spectral change was observed at 298 K (Fig. S5 in
reported Cr(V)-oxo complex described in the literature. The
stability of 1 in CH3CN was evaluated by measuring the half-
lifetime (t1/2) at different temperatures (t1/2 ~ 20 min at 298 K
and t1/2 > 24 hours at 243 K) (Fig. S2 in ESI†). The ESI-TOF-
MS spectrum of 1 exhibited a peak cluster at m/z = 200.59
III
ESI†). The final concentration of [Fe (bpy)3] was 0.15 mM
3+
–1
–
on the basis of the absorption coefficient (ε650 = 300 M cm
1
40a
)
from [Fe (bpy)3] to 1. ESR measurements clearly exhibited
, indicating that a stoichiometric ET reaction proceeded
II
2+
II
2+
V
–
2+
3
ET from [Fe (bpy) ] to 1, where the signal at g = 1.98 due to
(
calcd. for [Cr (O)(6-COO -tpa)] : 401.08), which was in
good agreement with the calculated isotopic pattern (Fig. 2b).
1
decreases, accompanied by an increase a new signal at g = 2.6
III
3+
41
16
18
When PhI O was replaced by isotopically labeled PhI O with
18
a small amount of H2 O, the peak cluster corresponding to O-
3
due to [Fe (bpy) ] (Fig. S6a in ESI†). In this case, one-way
II 2+
ET from [Fe (bpy) ] to 1 occurs to indicate that the reduction
18
3
34
labeled 1 shifted to m/z = 201.59 (Fig. 2b). Electron spin
resonance (ESR) measurements on 1 in CH3CN at 243 K and
Scheme 4
2
+
3+
2
+
+
O–
O
1
of a Cr(V) species (S = 1/2),
00 K afforded a strong signal at g = 1.9756, assignable to that
which was different from that
N
KET
N
N
N
N
N
N
N
N
N
N
N
N
N
18,19
CrV
N
RuII
CrIV
N
RuII
N
O
+
N
O
+
35
N
N
O
O
of complex 3 (S = 3/2) in CH3CN at 10 K (see Fig. S3 in
ESI†).
V
2+
II
2+
[CrIV(O)(L)]+
[RuIII(bpy)3]3+
[
Cr (O)(L)]
[Ru (bpy) ]
3
The formation yield of Cr(V)-oxo complex was calculated
to be 20 ± 3% on the basis of the spin amount obtained by
double integration of the ESR signal against a standard
(a)
(b)
1
.2
0.10
36
(
Cr(V)-oxo complex in an electron-transfer (ET) reaction from
TEMPO radical) and 25 ± 5% based on the stoichiometry of the
0
0
.8
.4
0
0
.05
0
II
2+
Fe (bpy)3] (bpy = 2,2’-bipyridine) (vide infra).
[
× 20
S
S
4
00
500
600
700
0
0.1 0.2 0.3 0.4 0.5 0.6
[[Ru (bpy)3]2+]0, mM
II
Wavelength, nm
16O
951
Fig. 4 (a) UV-vis spectral change observed upon addition of
II
2+
[
Ru (bpy)
3
]
3
(0.1 mM) to an CH CN solution of 1 (0.1 mM) at 243
III
3+
18O
918
K. (b) Plot of concentration of [Ru (bpy)
3
]
produced in electron
II
2+
transfer from [Ru (bpy)
3
]
to 1 in CH CN at 243 K vs. initial
3
II
2+
II
2+
S
concentration of [Ru (bpy)
] , [[Ru (bpy)
] ]
.
3
3
0
potential of 1 is much higher than 1.06 V.
In sharp contrast to the case of [Fe (bpy)3] , the ET
II
2+
42
II
2+
1
200 1100 1000
900
800
700
reaction between 1 and [Ru (bpy)3] (Eox = 1.24 V) is found
Wavenumber, cm–1
to be in ET equilibrium (Scheme 4), where the observed
–1 –1 40b
= 420 M cm )
III 3+
concentration of [Ru (bpy) ] (ε
V 16
–
2+
3
675nm
II
Fig. 3 Resonance Raman spectra of [Cr ( O)(6-COO -TPA)] (red
2+
V 18
–
2+
produced in the ET reaction from [Ru (bpy)3] to 1 increases
line), [Cr ( O)(6-COO -TPA)] (blue line), and their differential
spectrum (black line); measured at 236 K in CD
CN with 441.6 nm with the increase in the initial concentration of [Ru (bpy)3]
excitation. The peaks marked with ‘S’ are ascribed to the bands due ([[Ru (bpy) ] ] ) as shown in Fig. 4.
II
2+
3
II
III
2+
16,43
3
0
Formation of
3+
to the solvent.
In addition, the strong evidence to support the formation of
as a Cr(V)-oxo complex was obtained by resonance Raman
[
Ru (bpy)3] was also confirmed by the detection of ESR
41
signal at g = 2.6 as shown in Fig. S6b in ESI†. The ET
II
equilibrium between complex 1 and [Ru (bpy)3] indicates that
2+
1
II
2+
the redox potential of 1 is close to that of [Ru (bpy)3]
according to the Nernst equation (eqn 2), where F is the
spectroscopy (at 236 K, excitation at 441.6 nm in CD3CN). As
shown in Fig. 3, a Raman scattering due to the Cr(V)-oxo
16,43
–1
Faraday constant and K is an ET-equilibrium constant.
et
The
moiety was observed at 951 cm , which was comparable to
that observed for a reported Cr(V)-oxo complex with a corrole
Ket value was determined to be 0.57 ± 0.13 at 243 K by fitting
16
–1
37
derivative as a supporting ligand (986 cm ). The peak of 1-
the plot according to a equation described in the literature (red
line) as shown in Fig. 4b. The apparent one-electron reduction
potential (E ) of 1 (E (1)) was then determined to be 1.23 ±
18
18
O, which was formed by using PhI O with a small amount of
18
–1
H2 O, shifted to 918 cm ; the isotopic shift (33 cm ) is fairly
–1
red
red
–1
0.01 V using eqn (2).
consistent with the calculated value (Δν = 41 cm ) as shown in
Fig. 3.
38
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(a)
E
red = Eox + (RT/F)lnKet
(2)
0
0
.4
.2
0
V
The Ered(1) value is much higher than those of Cr (O)
0.3
.2
0.1
View Article Online
DOI: 10.1039/C4SC02285H
18,19
V
complexes reported so far,
0
corrolato) with a trianionic ligand and [Cr (O)(TMP)] (Ered
such as [Cr (O)(TpFPC)] (Ered =
.11 V vs. Ag/AgCl; TpFPC = tris(pentafluorophenyl)-
(c)
0
100
18c
V
+
8
0
0
50
Time, ms
100
= 0.76 V vs. Ag/AgCl; TMP = tetramesitylporphinato) with a
dianionic ligand,
60
18b
V
although a Cr (O) complex with a
300
400
4
500
600
700
(b)
Wavelength, nm
4
2
0
0
macrocyclic ligand (1,4,8,11-tetraazacyclotetradecane) has been
proposed to exhibit a higher Ered value (> 1.34 V vs. SCE) in
0
.10
00 nm
44
the presence of HClO4. In the case of 1, the addition of proton
showed not so much influence (~ +0.1 V) on the reduction
0
0
5
10
15
20
25
0.05
[
4-Ph], mM
45
potential as observed in DPV measurements.
When bromoferrocene (BrFc; E1/2 = 0.54 V) was employed
as a one-electron donor, complex 1 (0.17 mM) consumed 2 eq
of BrFc in CH3CN at 243 K on the basis of the absorption due
0
300
400
500
600
700
Wavelength, nm
+
–1
–1 46
Fig. 5 (a) UV-vis spectral change upon addition of 4-Ph (10 mM) to
(0.1 mM) in CH CN at 233 K. Inset: The time profile at 390 nm
due to 4-Ph . (b) UV-vis spectrum of 4-Ph produced by oxidizing
-Ph with CAN in CH CN at 233 K (c) Plots of kobs vs. [4-Ph].
to BrFc (ε630 = 330 M cm ). This result indicated that two-
electron reduction of 1 occurred to form a Cr(III) species (Fig.
S7 in ESI†). On the contrary, upon addition of 0.5 mM
1
3
•
+
•+
4
3
47
triphenylamine (Ph3N) as a one-electron donor (Eox = 0.85 V)
to an CH3CN solution containing 1 (0.04 mM) in the absence of derivatives (Eox) determined by SHACV measurements and
acid at 243 K, ET from Ph3N to 1 occurred to form one driving forces of ET (–ΔGet = –e(Eox – Ered(1))). Judging from
•+
equivalent of the one-electron oxidized product (Ph3N ), which the kinetic isotope effect values (KIE = 1.0–1.1), the reactions
showed an absorption band at 650 nm observed by UV-vis between 1 and phenol derivatives proceed via ET followed by
49,50
spectroscopy (Fig. S8 in ESI†). Subsequently, addition of PT rather than one-step PCET.
HClO4 (2 mM) to the reaction solution including Ph3N resulted
The driving-force dependence of logket for phenol
•+
in additional formation of one more equivalent of Ph3N , derivatives is shown in Fig. 6, where the logket values are
indicating that the two-electron reduction of 1 by Ph3N plotted relative to the driving force of ET (–ΔGet). The plot was
+
48
occurred in the presence of H . The formation of two analysed in light of the Marcus theory of adiabatic outer-sphere
•+
equivalents of Ph3N relative to 1 clearly indicates that 1 is the electron transfer (eqn (3)), where kdiff is the diffusion rate
sole oxidant in the solution. In addition, the protonation of one- constant, kB is the Boltzmann constant and Z [= (kBT/h)(kdiff/k–
1
1
–1 –1 51
M s .
electron reduced Cr(IV)-oxo complex leads to positive shift of diff)] is the collision frequency that is taken as 1 × 10
10
–1 –1 52
Ered of Cr(III/IV) beyond the Eox value of Ph3N. Thus two- The kdiff value in CH3CN is taken as 2.0 × 10
electron oxidation of a substrate should be possible for 1 via the
M
s
.
IV
formation of [Cr (6-COO -TPA)(OH)] , which is
–
2+
a
protonated species of the one-electron reduced species of 1, in a
PCET or ET/PT process.
(
3)
The reorganization energy of ET (λ) of 1 was thus
determined to be 1.03 ± 0.05 eV in CH3CN at 233 K on the
basis of the Marcus plot in Fig 6. The λ value of 1 is much
smaller than that (2.37 ± 0.04 eV) of a non-heme Fe(IV)-oxo
Determination of λ value of complex 1.
To gain kinetic insight into the ET reduction of 1 in CH3CN,
phenol derivatives (R-PhOH and naphtols) were employed as
electron donors. In the case of 4-phenylphenol (4-Ph), ET rates
were determined on the basis of the increase of the absorption
IV
2+ 16
complex, [Fe (O)(TMC)(CH3CN)] . This indicates that the
structural change upon the ET reduction is much smaller for 1
IV
than that for the Fe -oxo complex. In order to argue the
•+
band at 400 nm due to 4-Ph as shown in Fig. 5a. The
structural change during the ET reaction, DFT calculations
were performed to estimate the structural difference between
IV
complex 1 and the corresponding Cr (O) complex by
•+
absorption band of 4-Ph agreed with that observed in the
independent experiment using a strong one-electron oxidant
such as ammonium hexanitratocerate(IV) (CAN) as shown in
Fig. 5b. The pseudo-first-order rate constants (kobs) for the
oxidation of 4-Ph by 1 increase linearly with increasing
concentrations of 4-Ph. The second-order rate constant (ket) was
comparing bond lengths around the Cr centres. As a result, the
LUMO of 1 was revealed to localize on the dxy orbital involved
in the π* orbital of the Cr≡O bond (Fig. S12 in ESI†). Thus, the
Cr-O bond (1.55 Å) was elongated to 1.63 Å upon the ET
reduction (Fig. S13a in ESI†). On the contrary, in the case of
the Fe(IV)-oxo complex (S = 1), the LUMO has been reported
3
–1 –1
determined to be 4.3 × 10 M
s from the slope of the linear
plot as depicted in Fig. 5c. Similarly, ket values were
determined for oxidation reactions of other phenol derivatives
by 1 (Fig. S9 in ESI†). The obtained ket values are listed in
Table 1, together with the oxidation potentials of phenol
53
to be the dx2–y2 orbital and the equatorial Fe-N bonds (2.12-
2
.15 Å) were elongated to 2.24-2.29 Å (Fig. S13b in ESI†). The
6
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Table 1 One-electron oxidation potentials (Eox) of phenol Scheme 5
derivatives, driving forces of ET (–ΔGet), ET rate constants (ket),
and KIE values in ET reactions from phenol derivatives to 1 at
2
+
+
O
X–
X
O
N
N
kH
N
N
View Article Online
H
CrV
N
OH
Cr
III
N
O
+
N
DOI: 10
O
.1039
+
/C4SC02285H
2
33 K.
N
O
O
R-PhOH and
E
ox
a
,
–ΔGet,
eV
H2O
–1
–1
k
et, M
s
KIE
naphtols
V
2
3
4
4
4
4
5
4
-Me
-Ph
,3-(MeO)
,4,6-Me
-MeO
1.52
1.39
1.39
1.37
1.37
1.19
1.17
–0.29
–0.16
–0.16
–0.14
–0.14
0.04
(1.5 ± 0.1) × 10
(4.3 ± 0.2) × 10
(1.4 ± 0.1) × 10
(1.5 ± 0.1) × 10
(1.2 ± 0.1) × 10
(4.5 ± 0.2) × 10
(2.5 ± 0.1) × 10
(a)
(b)
1.0
8
6
4
2
0
4
1.1
0
.6
H-BA
2
2
2
3
KIE = 5.4
0.3
2
0
100
Time, s
200
0
.5
2
-Naphthol
-Naphthol
H-BA-d2
× 5
1
0.06
1.0
a
Determined by SHACV performed in CH
under Ar in the presence of TBAPF
3
CN at room temperature
(0.1 M) as an electrolyte (vs.
0
3
6
00
400
500
600
700
0
10
20
30
40
50
SCE).
Wavelength, nm
[H-BA], mM
Fig. 7 (a) UV-vis spectral change observed upon addition of benzyl
alcohol (10 mM) to 1 (0.1 mM) in CH CN at 233 K. Inset: The
decay time profile of the absorbance at λ = 330 nm due to 1. (b)
Concentration dependence of pseudo-first-order rate constants (kobs
1
2
3
)
8
2
for the reaction of 1 with H-BA (red) and benzyl alcohol-d (blue).
1
-Naphthol
-Naphthol
2
,4,6-Me3
measurements (Fig. S14, 15 in ESI†).
2
To elucidate the reaction mechanism of HAT reactions from
H-BA derivatives to 1, the kinetic analysis was conducted on
the basis of spectroscopic measurements. The addition of an
excess amount of H-BA to an CH3CN solution of 1 resulted in
the decay of the absorption derived from 1 with an isosbestic
point at 515 nm, as shown in Fig. 7a. The decay time profile of
the absorption at 330 nm due to 1 obeyed pseudo-first-order
kinetics (inset of Fig. 7a). The pseudo-first-order rate constant
2
,3-(MeO)2
4
0
2
-MeO
4
-Me
4
-Ph
–
1.0
–0.5
0
0.5
1.0
–
ΔG , eV
et
(
kobs) increased linearly with increasing concentrations of H-BA
Fig. 7b, red line). The second-order rate constant (kH) was
Fig. 6 Plots of logket vs. –ΔGet in ET reactions from phenol
derivatives to 1 at 233 K.
(
determined to be 2.5 M
–1
–1
s
from the slope of the linear plot.
average of the change of coordination bond lengths around the When H-BA was replaced by the corresponding deuterated
IV
metal centres is smaller for 1 (0.044 Å) than that for the Fe - compound at the benzylic position (benzyl alcohol-d , H-BA-
2
oxo complex (0.090 Å). Thus, the smaller structural change of d ), a significant deceleration of the oxidation rate (blue line in
2
–
1
–1
1
in the course of ET reactions to afford the smaller λ value Fig. 7b, kD = 0.46 M s ) was observed, giving a kinetic
should be due to the fact that the LUMO of 1 is a dπ orbital as isotope effect (KIE = k /k ) of 5.4 at 233 K.
H
D
54
suggested by DFT calculations (Fig. S12 in ESI†). In addition,
Similarly, kinetic analysis was made on the oxidation
in the case of a Mn(V)(O) complex with a corrolazine reactions of BA derivatives having substituents (R) on the
55
derivative, a smaller λ value (1.53 eV) has been reported; in aromatic ring of H-BA (R-BA) to afford corresponding
this case, the Mn(V) centre also accepts an electron into a dπ benzaldehydes as the sole products. In the case of 4-methoxy-
orbital.
BA (4-MeO-BA; Eox = 1.58 V) and 3,5-dimethoxy-4-methyl-
BA (3,5-(MeO)2-4-Me-BA; Eox = 1.49 V) used as substrates,
KIE values were also determined to be 12 and 6.8, respectively,
as listed in Table 2. The observed KIE values suggest that the
oxidation reactions of R-BA should be initiated by a one-step
PCET reaction from substrates to the Cr(V)-oxo complex rather
than an ET oxidation, since ET reactions are hard to occur
under highly endothermic situations (–ΔGet < 0).
Impact of redox potentials of substrates on their oxidation by 1.
Complex 1 showing a high reduction potential is expected to be
an efficient oxidant for HAT reactions (eqn 1) because a Cr(V)-
–
oxo complex is capable of accepting not only e at the Cr(V)
+
centre but also H at the terminal oxo ligand upon the reduction
as mentioned above. We examined HAT reactions from
substrates listed in Table 2 to 1. First, in the case of benzyl
The oxidation potentials of the substrates listed in Table 2
as no. 1 - 8 are much higher than the reduction potential of 1,
however, the oxidation potential of 3,4,5-trimethoxy-BA (3,4,5-
56
alcohol (H-BA) that shows the oxidation potential (Eox) of
–
.33 V (vs. SCE) as a substrate, complex 1 worked as a 2e -
2
oxidant to afford benzaldehyde as the sole product (Scheme 5)
(MeO)3-BA, Eox = 1.22 V) is comparable to Ered of 1. In the
course of the oxidation of 3,4,5-(MeO)3-BA with 1, a new
1
as identified and quantified by H NMR and GC-MS
absorption band appeared at 450 nm, which was assigned to
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(
a)
(b)
•+
,4,5-(MeO)3-BA (Fig. 9a). The decay time profile obeyed
3
second-order kinetics as shown in Fig. 9b and thus we assumed
0
0
0
.6
.4
.2
0
200
0
0
.4
.3
at 450 nm
at 330 nm
KIE = 1.1
View Article Online
1
50
that this process should be a proton transfer (PT) process from
•+
DOI: 10.1039/C4SC02285H
0.2
.1
IV
3
,4,5-(MeO)3-BA to a Cr (O) complex derived from one-
electron reduction of 1. The second-order rate constant (kPT)
0
0
50
100 150
100
Time, ms
2
–1 –1
was determined to be 2.5 × 10 M s . It should be noted that
the kPT values show no dependence on the concentration of
5
0
3
,4,5-(MeO)3-BA (Fig. 9c). Therefore, we conclude that the
second step is accounted for intermolecular PT from 3,4,5-
0
3
00
400
500
600
700
800
0
40
80
120
•+
IV
Wavelength, nm
[3,4,5-(MeO) -BA], mM
3
(MeO)3-BA to the Cr (O) complex to form 3,4,5-(MeO)3-
ꢀ
BA and a Cr (OH) complex.
•
IV
Fig. 8 (a) Spectral changes observed in the oxidation of 3,4,5-
MeO) -BA (10 mM) by 1 (0.1 mM) in CH CN at 233 K. Inset:
(
3
3
All kinetic parameters obtained for PCET or ET reactions
Time profiles of the absorbance at λ = 330 nm due to 1 and the from R-BA to 1 at 233 K are summarized in Table 2. When the
•
+
3
et
absorbance at λ = 450 nm due to 3,4,5-(MeO) -BA . (b) Plots of kobs rate constants were plotted against –ΔG as shown in Fig. 10, a
vs. [3,4,5-(MeO)
3
3 2
-BA (red) or 3,4,5-(MeO) -BA-d (blue)].
boundary was found around –ΔGet = –0.2 eV. It should be noted
(
a)
(b)
0.4
that KIE was still observed to be 6.8 in the case of 3,5-(MeO)2-
4
0
0
0
0
0
.5
.4
.3
.2
.1
0
1
.6
1.2
0.8
0.4
0
-Me-BA, although the –ΔGet value (–0.26 eV) is close to the
λ = 450 nm
,4,5-(MeO) -BA
•
+
0.3
mechanistic borderline. This phenomenon clearly represents the
first example of alteration of the oxidation mechanisms (one-
step PCET or stepwise ET/PT) of organic substrates by using a
metal-oxo complex without any additives to control the
3
3
0
0
.2
.1
0
0
200
400
600
Time, s
12
reactivity.
Recently, Fukuzumi and coworkers have reported a
mechanistic borderline, which discriminates between one-step
PCET and stepwise ET/PT mechanisms in the oxidation of
benzyl alcohol derivatives by non-heme Fe(IV)-oxo complexes
3
00 400
500
600
700
800
0
200
400
600
Wavelength, nm
c)
Time, s
(
4
3
2
1
0
3+ 12
in the presence and absence of Sc . In the one-step PCET
reactions, the oxidized products are also different; radical
coupling products and corresponding aldehydes in the presence
3+
and absence of Sc , respectively. In sharp contrast to the case
Table 2 One-electron oxidation potentials (Eox) of BA
derivatives, driving force for ET (–ΔGet), second-order rate
constants (kH or ket), and KIE values for oxidation of benzyl
alcohol derivatives with complex 1 in CH3CN at 233 K.
0
5
10
04 [3,4,5-(MeO) -BA], M
15
1
3
ꢀ
E
ox
a
,
–ΔGet,
eV
k or k ,
M
H
et
–1 –1
s
no.
R-BA
KIE
Fig. 9 (a) Following spectral changes observed in the oxidation of
,4,5-(MeO) -BA (1.0 mM) by 1 (0.1 mM) in CH CN at 233 K. (b)
The decay time profile at λ = 450 nm due to 3,4,5-(MeO)
Inset: Second-order plot. (c) Plots of kPT vs. [3,4,5-(MeO) -BA].
V
3
3
3
•
+
3
-BA .
1
2
3
4
5
4-NO
H
2.88
2.33
2.07
2.05
1.58
–1.65
–1.10
–0.84
–0.82
–0.35
1.4 ± 0.1
2.5 ± 0.1
5.4 ± 0.3
5.2 ± 0.2
21 ± 1
–
5.4
–
2
3
•
+
3,4,5-(MeO)3-BA radical cation (3,4,5-(MeO)3-BA ) as a new
intermediate (Fig. 8a and Fig. S16 in ESI†).
12a
4-t-Bu
4-Me
4-MeO
A time profile of the decay of the absorption at 330 nm
inset of Fig. 8a, red line) due to 1 coincides with that of the
–
(
rise of the absorption at 450 nm (inset of Fig. 8a, blue line). The
12
•
+
formation rate constant (ket) of 3,4,5-(MeO)3-BA was thus
–1
determined to be 1.8 × 10
3
,5-(MeO)
2
3
–1
6
1.49
–0.26
19 ± 1
6.8
M
s
by changing the
-4-Me
concentration of 3,4,5-(MeO)3-BA as shown in Fig. 8b (red line
with filled circles). This indicates that ET from 3,4,5-(MeO)3-
BA to 1 occurs faster than PCET because of the low oxidation
potential of 3,4,5-(MeO)3-BA. In addition, negligible KIE (1.1)
was observed for deuterated 3,4,5-(MeO)3-BA (3,4,5-(MeO)3-
BA-d2) at the benzylic position (Fig. 8b, blue line with filled
squares) to exclude a PCET pathway in the oxidation.
7
8
9
3,5-(MeO)
2
1.49
1.37
1.22
1.20
–0.26
–0.14
0.01
9.0 ± 0.5
16 ± 1
–
–
2,3,4-(MeO)
3,4,5-(MeO)
3
3
1800 ± 50
too fast
1.1
–
1
0
2,5-(MeO)
2
0.03
a
Determined by SHACV performed in CH
under Ar in the presence of TBAPF
SCE).
3
CN at room temperature
A subsequent reaction of ET from 3,4,5-(MeO)3-BA to 1
was analyzed by the decay of the absorption at 450 nm due to
6
(0.1 M) as an electrolyte (vs.
8
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–
ligand (6-COO -tpa). The Ered value of 1 was determined to be
.23 V vs. SCE on the basis of analysis of the ET equilibrium
1
with [Ru (bpy)3] . The reorganization energy of ET from
II
2+
View Article Online
DOI: 10.1039/C4SC02285H
phenols to 1 has been determined to be 1.03 ± 0.05 eV, which is
IV
much smaller than that for a non-heme Fe (O) complex, due to
the smaller structural change upon one-electron reduction.
When a series of benzyl alcohol derivatives were employed as
substrates of oxidation by 1, we have found a mechanistic
borderline between one-step PCET and stepwise ET/PT around
–
ΔGet = –0.2 eV. The present study provides a standard for the
elucidation of the reactivity of Cr(V)-oxo complexes in HAT
reactions.
Acknowledgements
Fig. 10 Plots of logk
at 233 K.
H
or logket –ΔGet in HAT reactions of R-BA by 1
This work was supported by a Grant-in-Aid (Nos. 24750052
and 24245011) from the Japan Society of Promotion of Science
(JSPS, MEXT) of Japan and financial support from The Kurata
Foundation.
•+
H
H
H
H
ET
OH
V
OH
CrIV(O)
+
Cr (O)
+
R
R
PCET
PT
Notes and references
H
OH
OH
Oxygen
Rebound
a
CrIV(OH)
OH
CrIII
+
Department of Chemistry, Faculty of Pure and Applied Sciences,
+
University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571,
Japan E-mail: kojima@chem.tsukuba.ac.jp; Fax: +81-29-853-4323
R
R
R
H
2
O
+
b
Graduate School of Life Science, University of Hyogo, Kouto, Hyogo
O
6
78-1297, Japan
H
CrIII
c
Institute for Materials Chemistry and Engineering, Kyushu University,
Motooka, Nishi-Ku, Fukuoka 819-0395, Japan
d
Elements Strategy Initiative for Catalysts & Batteries, Kyoto University,
Fig. 11 Proposed mechanism for oxidation of R-BA by 1.
of Fukuzumi and coworkers, the present study provides Nishi-ku, Kyoto 615-8520, Japan
†
Electronic
Supplementary
Information
(ESI)
available:
apparently the same net hydrogen-atom transfer reaction to
afford corresponding benzaldehydes via either PCET or ET/PT
pathway under the same conditions, without perturbation of the
reactivity of metal-oxo species by additives.
crystallographic data of 2 and 3 in CIF, ESI-TOF-MS, UV-vis, ESR, DFT
1
calculations, H NMR, and GC-MS data. CCDC 1017025 and 1017026.
See DOI: 10.1039/b000000x/
Based on these results, we propose a mechanism for the
oxidation of R-BA by 1 in CH3CN at 233 K as shown in Fig. 11.
In the case of R-BA except for 3,4,5-(MeO)3-BA, one-step
PCET occurs to yield H-atom abstracted species with showing
considerable KIE. In sharp contrast to this, the oxidation of
1
2
(a) W. Nam, Acc. Chem. Res., 2007, 40, 522–531; (b) L. Que, Jr,
Acc. Chem. Res., 2007, 40, 493–500.
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Lam, W.-L. Man and T.-C. Lau, Coord. Chem. Rev., 2007, 251,
3
3
,4,5-(MeO)3-BA by 1 allowed us to observe the formation of
2
238–2252.
•+
,4,5-(MeO)3-BA as the intermediate in the course of the
3
J.-U. Rohde, J.-H. In, M. H. Lim, W. W. Brennessel, M. R.
Bukowski, A. Stubna, E. Münck, W. Nam and L. Que, Jr,
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H. Le Tadic-Biadatti, D. L. Chestney, E. S. Roberts and P. F.
•+
reaction. Then, deprotonation from 3,4,5-(MeO)3-BA is
IV
facilitated by the more basic Cr (O) complex to form 3,4,5-
•
MeO)3-BA , which should be the same intermediate derived
(
4
5
from one-step PCET. Although such a mechanistic difference
may often result in the formation of different oxidized products,
the oxidation of R-BA by 1 provides only the corresponding
aldehydes as the two-electron oxidized products via oxygen-
57
rebound process
undergo facile dehydration.
affording α-diol intermediates, which
6
7
Conclusions
In conclusion, we have synthesized and characterized a reactive
Cr(V)-oxo complex (1) by using a monoanionic pentadentate
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1
8
2
3
O was added in 2 mL of CH CN solution in order to
(
1
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9
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III
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measurements as in the case of a non-heme Fe(IV)-oxo complex,
IV
2+
[
Fe (O)(TMC)(CH
3
CN)]
(TMC
=
1,4,8,11-tetramethyl-
1
,4,8,11-tetrazacyclotetradecane), which was reported in ref 16.
4
4
4
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8
K. Mase, K. Ohkubo and S. Fukuzumi, J. Am. Chem. Soc., 2013,
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9
0
A slope (–0.44) of the linear relationship between (RT/F)ln(ket)
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ref 11c.
We have confirmed that the reaction between 1 and 2,4,6-
–
Me
3
PhOH affords a 2e -oxidation product, 4-hydroxy-2,4,6-
trimethylcyclohexa-2,5-dienone (Fig S10 in ESI†). The product
1
was characterized by H NMR and GC-MS measurements. The
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H
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5
7
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TOC entry
View Article Online
DOI: 10.1039/C4SC02285H
Mechanistic insights were gained into hydrogen-atom-transfer reactions
from benzyl alcohol derivatives with different oxidation potentials to a
highly reactive Cr(V)-oxo complex to reveal switching of reaction
mechanisms.
1
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