A low-spin ruthenium(IV)-oxo complex: Does the spin state have an impact on the reactivity?

Article abstract of DOI:10.1002/anie.201002733

Spin doesn't matter: A ruthenium(II)-aqua complex bearing a pentadentate pyridylamine with a carboxylate group as a ligand affords a seven-coordinate low-spin (S=0) ruthenium(IV)-oxo complex (see structure) by oxidation through proton-coupled electron transfer. Comparison of the reactivity of the low-spin and an intermediate-spin (S=1) Ru^IV-oxo complexes revealed that the spin state does not affect the reactivity of catalytic oxidation of organic compounds.

Full text of DOI:10.1002/anie.201002733

Angewandte
Chemie
DOI: 10.1002/anie.201002733
Oxo Complexes
A Low-Spin Ruthenium(IV)–Oxo Complex: Does the Spin State Have
an Impact on the Reactivity?**
Takahiko Kojima,* Yuichirou Hirai, Tomoya Ishizuka, Yoshihito Shiota, Kazunari Yoshizawa,
Kenichiro Ikemura, Takashi Ogura, and Shunichi Fukuzumi*
IV
High-valent metal–oxo complexes are key reactive species for
oxidation and oxygenation of organic compounds in nature as
spin states. There have been extensive studies on Ru –oxo
[18–20]
complexes that exhibit the triplet spin state (S = 1).
However, there has been no example of Ru –oxo complexes
exhibiting the singlet spin state (S = 0) at the ground state.
Thus, comparison of the reactivity of analogous high-valent
metal–oxo species with different spin states has never been
made.
We report herein for the first time the spin state alteration
of Ru –oxo complexes with tris(2-pyridylmethyl)amine (tpa)
derivatives depending on the type of tpa derivatives. Two
Ru –aqua complexes having tpa derivatives, tetradentate tpa
and a pentadentate N,N-bis(2-pyridylmethyl)-N-(6-carboxy-
lato-2-pyridyl-methyl)amine (6-COO -tpa) monoanion, [Ru-
(tpa)(H O) ] (1) and [Ru(6-COO -tpa)(H O)] (2), were
converted into the corresponding Ru –oxo complexes by the
PCET reactions with use of (NH ) [Ce (NO ) ] (CAN) as an
oxidant. Now we have two kinds of Ru –oxo complexes,
[
1,2]
IV
well as in the laboratory.
Although iron is the most
[21]
common metal species among high-valent metal–oxo com-
[
3]
[4]
[5]
plexes, there are also manganese–oxo, ruthenium–oxo,
[6]
and other metal–oxo complexes. High-valent metal–oxo
species are produced by reductive activation of molecular
oxygen coupled with proton transfer.
[
7–9]
Peroxides such as
IV
hydrogen peroxide can provide a so-called “peroxide shunt”
[
1–3]
to produce high-valent metal–oxo species.
High-valent
II
metal–oxo species can also be produced by proton-coupled
electron transfer (PCET), in which deprotonation of a
coordinated water molecule and oxidation of the metal
ꢀ
[
10–14]
2+
[13]
ꢀ
+
center occur concertedly.
The reactivity of high-valent
2
2
2
IV
metal–oxo species varies depending on the type of metal, the
oxidation state of the metal center, ligands, and the spin state.
Theoretical studies proposed that the reactivity of high-valent
metal–oxo species may be determined by two closely lying
spin states, which have different activation barriers for the
IV
4
2
3 6
IV
2
+
[Ru(O)(tpa)(H O)] (3) in the S = 1 spin state and [Ru(O)(6-
COO -tpa)] (4) in the S = 0 spin state. Thus, analogous
Ru –oxo complexes with different spin states in hand
2
ꢀ
+
[15–17]
IV
reactions with substrates.
The most straightforward way
to clarify the effects of spin states on the reactivity of high-
valent metal–oxo species is to examine the reactivity of an
analogous series of metal–oxo complexes that have different
provide an excellent opportunity to compare the reactivity
toward substrates in light of their spin states.
II
The Ru –aqua complex 2 was prepared by the reaction of
ꢀ
a
precursor complex [Ru(6-COO -tpa)Cl]PF6 (see
[22]
Figure 1)
with AgPF6 in water by dechlorination and
[*] Prof. Dr. T. Kojima, Dr. T. Ishizuka
Department of Chemistry, Graduate School of Pure and Applied
Sciences, University of Tsukuba
1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571 (Japan)
Fax: (+81)29-853-4323
E-mail: kojima@chem.tsukuba.ac.jp
Y. Hirai, Prof. Dr. S. Fukuzumi
Department of Material and Life Science, Osaka University
2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan)
and
Department of Bioinspired Science, Ewha Womans University
Seoul, 120-750 (Korea)
Fax: (+81)6-6879-7370
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
Dr. Y. Shiota, Prof. Dr. K. Yoshizawa
Institute for Materials Chemistry and Engineering
Kyushu University, Moto-oka, Nishi-Ku, Fukuoka 819-0395 (Japan)
ꢀ
Figure 1. Crystal structure of the cationic moiety of [Ru(6-COO -
tpa)Cl]ClO with selected atom labeling. Each atom is described with
4
Dr. K. Ikemura, Prof. Dr. T. Ogura
Graduate School of Life Science, University of Hyogo
Kouto, Hyogo 678-1297 (Japan)
thermal ellipsoids at the 50% probability level. Hydrogen atoms are
omitted for clarity.
[
**] This work was supported by Grants-in-Aids (Nos. 20108010 (S.F.)
and 21350035 (T.K.)) from the Japan Society of Promotion of
Science (JSPS), MEXT, Japan (No. 20050029 to T.O. on Priority Area
spontaneous reduction. UV/Vis spectroscopic titration on 2
revealed two-step deprotonation in 0.1m Britton–Robinson
buffer with 10m NaOH solution. The first deprotonation
4
1
77), and KOSEF/MEST through WCU project (R31-2008-000-
0010-0).
process was reversible, and the pK value was determined to
be 3.5 (Figure S1 in Supporting Information).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002733.
a
[
23]
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8449

Communications
Complex 2 showed a reversible redox couple assigned to
revealed that hydrogen bonding with water molecules
should be indispensable to stabilize the singlet state over
the triplet state: Without hydrogen-bonded water mole-
cule(s), the triplet state should be dominant (Table S1 in the
Supporting Information). Thus, water molecules play an
III
IV
the Ru /Ru couple at + 0.68 V (vs. SCE), which was intact
in the pH range 2–4 in Britton–Robinson buffer at room
temperature (Figure S2 in the Supporting Information). In
[
13]
light of the redox potential (1.0 V vs. SCE at pH 0) of CAN
IV
IV
in H O, 2 should be converted into a Ru –oxo complex by the
important role to stabilize the singlet state of the Ru =O
2
oxidation with CAN.
species in water. In the optimized structure of 4, the length of
IV
IV
Oxidation of 2 by CAN in water gave rise to the formation
Ru =O bond is 1.813 ꢀ and that for the aqua ligand, Ru ꢀ
IV
of a Ru –oxo complex (4). In the course of the oxidation, we
OH , is 2.265 ꢀ. Thus, the formulation of low-spin 4 should be
2
ꢀ
2+
observed the absorption spectral change as shown in Fig-
ure S3 (see the Supporting Information). Resonance Raman
spectroscopy allowed us to observe a Raman scattering due to
[Ru(O)(6-COO -tpa)(H O)] in water.
2
II
Ru –aqua complexes 1 and 2 were applied to catalytic
oxygenation of organic substrates under conditions in which
substrates (0.1m), CAN (0.2m), and a catalyst (1 mm) (molar
ꢀ
1
the n(Ru=O) vibration at 833 cm , and this signal shifted to
ꢀ1
18
7
88 cm in the preparation in H2 O (see Figure S4 in the
ratio 100:200:1) were mixed in D O (1 mL). The reaction
2
Supporting Information). The isotopic shift is consistent with
the calculated value for the Ru=O harmonic oscillator (Dn =
mixture was stirred at room temperature for 1 h, and the
product was quantified by H NMR spectroscopy on the basis
1
ꢀ
1
IV
4
0 cm ), supporting the formation of the Ru =O species.
ESIMS analysis allowed us to observe a peak cluster at
of peak integration. To demonstrate the reactivity of catalytic
systems using those complexes toward various substrates,
three types of substrates, namely, cyclohexene (olefin), 1-
propanol (alcohol), and 4-sulfonate-1-ethylbenzene (satu-
rated C-H), were examined. Regardless of the spin states of
1
6
m/z 451.11 (calcd 451.03) assigned to 4 in H O. The reaction
2
1
8
18
of 2 with CAN in H2 O gave a mixture of 4 and O-labeled 4
1.0:0.9) owing to slow exchange of the aqua ligand (see
Figure S5 in the Supporting Information).
(
IV
the Ru =O species, the product distribution in the two
IV
1
Ru –oxo complex 4 was also characterized by H NMR
systems was identical for each substrate as summarized in
Table 1. Catalytic oxygenation of cyclohexene resulted in the
spectroscopy (Figure S6 in the Supporting Information). We
observed an AB quartet at d = 5.52 and 5.93 ppm (JAB
=
1
6 Hz), assigned to the equatorial CH moieties, and a singlet
2
at d = 5.81 ppm due to the axial CH moiety. Complex 4
showed a diamagnetic spectrum, indicating the retention of
2
Table 1: Catalytic oxidation reactions of organic substrates with 1 and 2
as catalysts.
[a]
the s symmetry in the precursor chlorido complex, which is
h
Entry Substrate
Cat. Product
Sel. Ox.
%] eff.
%]
TON
in sharp contrast to the paramagnetically shifted signals of 2
[
[
13]
(
S = 1). Thus, the spin state of 4 was determined to be S = 0,
[
and, to the best of our knowledge, this is the first example of a
IV
low-spin Ru –oxo complex.
1
2
1
2
100 100
88 88
25
22
DFT calculations at the B3LYP/LANL2DZ level of
theory suggested that the structure of 4 should be pentagonal
bipyramidal with a plane of symmetry (Cs symmetry). An
optimized structure is depicted in Figure 2. It was also
3
4
1
2
100 100
92 92
50
46
5
6
1
2
100 89
89 94
45
47
[a] [Substrate]=0.1m, [CAN]=0.2m, and [catalyst]=1 mm.
formation of adipic acid as an eight-electron oxidation
product. The selectivity of the catalytic cyclohexene oxygen-
ation with the use of 2 as a catalyst was slightly lower in the
presence of a twofold molar amount of the oxidant relative to
the substrate than that found in the reaction with 1; however,
the selectivity was much improved with the use of an eightfold
molar amount of CAN relative to cyclohexene to give adipic
acid as the sole product. Oxygenation of 1-propanol led to the
[
24]
selective formation of propionic acid.
Oxygenation of
saturated CꢀH bonds was also observed for both catalysts.
IV
All reactions should proceed via formation of Ru –oxo
species as reactive species as described above. As no
significant difference in selectivity and efficiency was
observed for the two complexes, the differences in ligands
and spin states of catalysts have no significant influence on
their catalytic activities.
Figure 2. An optimized structure of 4 with hydrogen-bonded water
molecules obtained by DFT calculations at the B3LYP/LANL2DZ level
of theory.
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To shed light on the detailed oxidation mechanism, we
conducted kinetic analysis on the oxidation of 1-propanol
with Ru –oxo complexes 3 and 4 by a spectroscopic method.
Addition of excess 1-propanol (150 mm) into aqueous sol-
utions of the oxo complexes 3 and 4 (0.1 mm) gave continuous
spectral change with isosbestic points (Figure 3a,b). In Fig-
reaction in the precursor complexes. In the oxidation of 1-
propanol with 3 at 301 K, those values were determined to be
IV
ꢀ1
ꢀ3 ꢀ1
K = 28m and k = 5.7 ꢁ 10 s . Similarly, in the case of 4,
ꢀ
1
ꢀ3 ꢀ1
they were determined to be K = 58m and k = 5.6 ꢁ 10 s .
Saturation behavior was also observed in all temperatures
regardless of difference in Ru complexes. The kinetic data are
summarized in Table 2.
Table 2: Equilibrium constants and rate constants for oxidation of 1-
propanol with 2 and 4 at various temperatures.
3
4
ꢀ
1
3
ꢀ1
ꢀ1
3
ꢀ1
T [K]
K [m
]
10 k [s
]
K [m
]
10 k [s ]
308
301
288
280
25ꢃ2
28ꢃ3
33ꢃ3
37ꢃ2
10.3ꢃ0.3
5.7ꢃ0.2
45ꢃ3
58ꢃ5
97ꢃ6
188ꢃ6
8.1ꢃ0.2
5.6ꢃ0.1
2.9ꢃ0.02
1.9ꢃ0.01
3.8ꢃ0.05
2.5ꢃ0.02
Temperature dependence of the formation constants and
the rate constants was also investigated to obtain van’t Hoff
plots and Eyring plots, respectively. These plots allowed us to
determine thermodynamic parameters to discuss on the
oxidation reaction mechanism. On the basis of the van’t
Hoff plots (Figure S9 in the Supporting Information), the
thermodynamic parameters were determined to be DH =
ꢀ
1
ꢀ1
ꢀ1
ꢀ
9.9 ꢃ 0.4 kJmol and DS = ꢀ5.0 ꢃ 1.3 JK mol (301 K)
ꢀ
1
for
3
and DH = ꢀ36 ꢃ 4 kJmol
and DS = ꢀ84 ꢃ
ꢀ
1
ꢀ1
1
4 JK mol (301 K) for 4. Thus, the formation of the
Figure 3. Spectral changes in the oxidation of 2-propanol with a) 3 and
b) 4 in water.
precursor complexes is always exothermic and stabilization of
the complex may result from the formation of hydrogen
bonding between 1-propanol and the Ru –oxo complexes.
IV
ure 3a, as the reaction proceeds, the absorption band due to 3
We made efforts to obtain a direct evidence to support the
two-step reaction including the pre-equilibrium in the oxida-
tion of 1-propanol by 4. To observe the fast first-step reaction,
stopped-flow techniques were applied to separate the two-
step reaction. As the result of measurements, we obtained
two-step spectral changes with different isosbestic points as
shown in Figure 4. The first reaction completed within 5 s,
exhibiting an isosbestic point at l = 593 nm (Figure 4, left). In
addition, initial absorption maximum at l = 542 nm shifted to
l = 539 nm in the course of the reaction (Figure 4, left; inset).
In the second step, the reaction proceeded with an isosbestic
point at l = 580 nm (Figure 4, right). Thus, we concluded that
the oxidation of 1-propanol with 4 consists of two steps, that is,
formation of the putative hydrogen-bonding complex and the
subsequent oxidation reaction in the complex.
(
l
ꢁ 450 nm) decreases and the absorption band due to
max
II
Ru (l = 624 nm) increases. The time course of the
max
absorbance at l = 624 and 450 nm obeyed first-order kinetics
and the pseudo-first-order rate constants obtained by the rise
at l = 624 nm and the decay at l = 450 nm were the same.
Similar behavior was observed for the spectra of 4 (Fig-
ure 3b). As the reaction proceeds, the absorption band due to
IV
Ru (l = 542 nm) decreases and the absorption band due
max
II
to Ru (l = 628 nm) increases.
max
Pseudo-first-order rate constants (kobs) were determined
with various concentrations of 1-propanol. The results for 3
and 4 are summarized in Figures S7 and S8 (see the
Supporting Information), respectively. In the oxidation of 1-
propanol, saturation behavior of rate constants with respect
to concentration of 1-propanol was observed for both 3 and 4.
This behavior indicates the existence of pre-equilibrium prior
to the oxidation reaction. Thus, it is suggested that there is
interaction between the substrate and the oxo complexes
prior to the oxidation reaction. Plots of kobs versus the
concentration of 1-propanol ([Sub]) were fitted by Equa-
The temperature dependence of the rate constants for the
oxidation of 1-propanol with 3 and 4 (Table 3) allowed us to
determine the activation parameters to shed some lights on
the transition state of the oxidation reaction from Eyring plots
as shown in Figure S10 (see the Supporting Information). For
IV
both Ru –oxo complexes, similar activation parameters were
[25]
°
ꢀ1
°
tion (1).
obtained: DH = 31 ꢃ 4.7 kJmol
and DS = ꢀ184 ꢃ
ꢀ
1
ꢀ1
°
ꢀ1
°
1
6 JK mol
for 3, DH = 34 ꢃ 0.8 kJmol
and DS =
ꢀ
1
ꢀ1
kobs ¼ kK½Subꢂ=ð1 þ K½SubꢂÞ
ð1Þ
ꢀ174 ꢃ 2.5 JK mol for 4. These similar activation param-
IV
eters lend credence to a similar transition state for both Ru –
The curve fitting affords equilibrium constants (K) of the
pre-equilibrium and the rate constants (k) of the oxidation
oxo complexes. Since the activation entropies are remarkably
negative, the Ru –oxo complexes are assumed to interact
IV
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8451

Communications
which is spin-forbidden for 3 to give the singlet product, is
unlikely to occur. Thus, a PCET mechanism, which is spin-
allowed irrespective of the spin state of 3 and 4, may be
operative in the oxidation of substrates with 3 and 4.
All the kinetics data including heats and entropies of
formation of the hydrogen-bonding complex, activation
parameters, and kinetic isotope effects are summarized in
Table 3. In comparative experiments between 3 and 4, no
meaningful difference was recognized, which clearly indicates
that difference in the ligand or the spin state of the metal
center of the oxo species does not affect their reactivity. This
is the first example to clarify relationship between the spin
state and the reactivity of high-valent metal–oxo species
experimentally. The a-CꢀH bond in the hydrogen-bonded
substrate is oriented to the oxo ligand to undergo PCET to
give rise to a tightly condensed transition state as reflected on
the negatively large activation entropy given in Table 3.
In summary, we have described detailed mechanistic
insights into catalytic oxygenation and oxidation reactions by
II
two Ru –aqua complexes with use of CAN as an oxidant in an
acidic aqueous solution. The two aqua complexes afford
IV
Ru –oxo complexes by the oxidation with CAN. Complex 2
ꢀ
IV
with 6-COO -tpa affords a low-spin Ru –oxo complex (S =
), which is in sharp contrast to complex 1 with tpa, which
0
Figure 4. Two-step spectral changes in the reaction of 1-propanol
IV
ꢀ1
ꢀ4
gives an intermediate-spin Ru –oxo complex (S = 1). We
(
1.5ꢀ10 m) with 4 (1.0ꢀ10 m) at 288 K obtained by stopped-flow
measurements. The bottom trace at 539 nm in the left is identical to
the top trace in the right.
have experimentally clarified for the first time that the spin
state of the Ru –oxo complexes does not affect their
IV
reactivity toward external substrates in oxygen atom transfer
and hydrogen abstraction in water. In the detailed mecha-
nistic investigation on 1-propanol oxidation, we have revealed
that the formation of hydrogen-bonding adducts between the
Ru=O complexes and the substrate occurs to gain high
selectivity and efficiency in the oxidation reactions through a
PCET mechanism. The results presented herein will provide
new fundamentals of catalytic oxidation reactions by high-
valent metal–oxo species in aqueous media.
IV
Table 3: Kinetics data for reactions of 1-propanol with Ru –oxo com-
plexes 3 and 4.
°
°
Complex DH
kJmol
DS
[JK mol
DH
[kJmol
DS
k /k
H
D
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
[
]
]
]
[JK mol
]
3
4
ꢀ9.9ꢃ0.4 ꢀ5.0ꢃ1.3
ꢀ36ꢃ4 ꢀ84ꢃ14
31ꢃ4.7
34ꢃ0.8
ꢀ184ꢃ16
ꢀ174ꢃ2.5
2.9
2.1
Experimental Section
III
strongly with the substrate to form tight transition states in
the course of the dehydrogenation reaction.
Synthesis of 2: A solution containing [Ru (6-COO-tpa)Cl](PF )
6
(50 mg, 0.081 mmol) and AgPF (25 mg, 0.099 mmol) in H O (15 mL)
6
2
was stirred until the color turned to green, and then heated to 608C
for 2 h. The deep green solution was filtered through a membrane
filter to remove insoluble solid. The filtrate was evaporated to remove
solvent. 2-Propanol was then added to the flask to obtain a green
To gain information on the hydrogen-abstraction process,
we conducted kinetic experiments using CH CH CD OH to
3
2
2
evaluate kinetic isotope effects (KIEs). A significant differ-
ence in the rate constants was observed at 301 K (Figure S11
in the Supporting Information). KIE values for the observed
maximum rate constants of 3 and 4 were determined to be 2.9
precipitate. The precipitate was filtered and washed with diethyl ether
1
and then dried in vacuo. The yield was 37% (18 mg). H NMR (D O):
2
d = 5.62 and 5.83 (ABq, JAB = 16 Hz, 4H, CH
(equatorial)), 5.70 (s,
2
[26]
2
H, CH (axial)), 7.71 (t, J = 6 Hz, 2H, H5 of py(equatorial)), 7.77 (d,
and 2.1, respectively. This result indicates that the removal
of the a-hydrogen atom is involved in the rate-determining
step. In contrast, no kinetic isotope effect was observed for
the hydrogen atom of the OH group, as indicated by
comparison of the rate constant for the oxidation of
2
J = 8 Hz, 1H, H3 of py(axial)), 7.95 (d, J = 8 Hz, 2H, H3 of
py(equatorial)), 8.2–8.3 (m, 4H, H4 of py(equatorial), H4 and H5
of py(axial)), 8.43 ppm (d, J = 6 Hz, 2H, H6 of py(equatorial)).
Absorption maxima in H O (lmax, nm): 628, 410.
2
CH CH CH OD in D O. Thus, the rate-determining step
3
2
2
2
Received: May 6, 2010
lies in the hydrogen-abstraction process at the a-position of 1-
propanol.
Revised: July 27, 2010
Published online: September 23, 2010
There are two possible mechanisms: one-step hydride
transfer or proton-coupled electron transfer (PCET).
Because there was no difference in reactivity between 3
with S = 1 and 4 with S = 0, the one-step hydride transfer,
Keywords: electronic structure · oxidation · oxo ligands ·
reaction mechanisms · ruthenium
.
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Chemie
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4
1
[
(2), b = 14.734(2), c = 14.526(2) ꢀ, b = 100.164(2)8, V=
3
1
2264.7(6) ꢀ , T= 123 K, Z = 4, R1(R ) = 0.0449(0.1029) (I >
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[
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www.angewandte.org
8453