Oxidation of hydrocarbons by mono- and dinuclear ruthenium quinone complexes via hydrogen atom abstraction

Article abstract of DOI:10.1246/cl.2000.910

Deprotonation and two-electron oxidation of [Ru^II₂(OH)₂(Q)₂(btpyan)]²⁺ (Q = 3,6-di(tert-butyl)-1,2-quinone, btpyan = 1,8-bis(2,2′:6′,2″-terpyridyl)anthracene) was converted to bis(ruthenium-oxo) complex [Ru¹¹₂(O)₂(Q)₂(btpyan)]²⁺ , which oxidized 1,3-cychrohexadiene and 1,2-dihydronaphtalene to corresponding aromatic compounds in the presence of AgClO₄ and t-BuOK. On the other hand, mononuclear complex [Ru^II(OH₂)(Q)(Ph-terpy)]²⁺ (Ph-terpy = 4′-phenyl-2,2′: 6′,2″-terpyridine, [2]²⁺) was converted to [Ru^II(OH)(Q)(Phterpy)]²⁺ under the similar conditions, but displayed the low activity for the oxidation compared with the dinuclear complex [1]²⁺.

Full text of DOI:10.1246/cl.2000.910

9
10
Chemistry Letters 2000
Oxidation of Hydrocarbons by Mono- and Dinuclear Ruthenium Quinone Complexes via
Hydrogen Atom Abstraction
Tohru Wada, Kiyoshi Tsuge, and Koji Tanaka*
Department of Structural Molecular Science, Graduate University for Advanced Studies. 38 Nishigonaka, Myodaiji, Okazaki, Aichi
44-8585
Institute for Molecular Science. 38 Nishigonaka, Myodaiji, Okazaki, Aichi 444-8585
4
(Received May 12, 2000; CL-000464)
Deprotonation and two-electron oxidation of
and AgClO4 (6.0 µmol) at room temperature in the air. The
reaction was completed in one minute, and benzene was pro-
duced in a 90% yield with silver powder (Eq 1). After the reac-
II
2+
[
Ru (OH) (Q) (btpyan)] (Q = 3,6-di(tert-butyl)-1,2-quinone,
2 2 2
btpyan = 1,8-bis(2,2':6',2"-terpyridyl)anthracene) was converted
II
2+
to bis(ruthenium–oxo) complex [Ru (O) (Q) (btpyan)] ,
2
2
2
which oxidized 1,3-cychrohexadiene and 1,2-dihydronaphtal-
ene to corresponding aromatic compounds in the presence of
AgClO and t-BuOK. On the other hand, mononuclear com-
4
II
2+
tion, regeneration of [1]2+ was confirmed by the ESI-MS spec-
tra of the reaction mixture. Benzene was produced again by
the further addition of cyclohexadiene, AgClO4 and t-BuOK to
the solution, though the yield of benzene decreased to an about
80% in the second reaction. Similarly, [1] showed the high
reactivity of the oxidation of 1,2-dihydronaphthalene to naph-
plex [Ru (OH )(Q)(Ph-terpy)] (Ph-terpy = 4'-phenyl-2,2':
2
2
+
II
6
',2"-terpyridine, [2] ) was converted to [Ru (OH)(Q)(Ph-
terpy)] under the similar conditions, but displayed the low
activity for the oxidation compared with the dinuclear complex
2
+
2
+
2+
[1] .
thalene under the same reaction conditions (Table 1).
High valent transition metal–oxo complexes work as active
species in biological and chemical oxidation of various organic
substrates.1 Several ruthenium–oxo complexes have been
reported as functional models for enzymatic reactions, and
some of them have proven to hydroxylize and epoxidize hydro-
,2
3
carbons in the presence of dioxygen, peroxides such as H O
2
2
4
5
and t-BuOOH, and pyridine-N-oxides as oxygen sources.
Oxo complexes are also generated by deprotonation of
metal–aqua or metal–hydroxo complexes coupled with oxida-
6
tion. We have reported that the dinuclear ruthenium–hydroxo
complex [1]2+ with two Ru(Q)(OH) units bridged by btpyan lig-
Dehydrogenation of 1,3-cyclohexadiene and 1,2-dihydronaph-
thalene also proceeded in the presence of the mononuclear com-
2+
plex [2] under the similar reaction conditions, but the yields
of benzene and naphthalene were quite low compared with
2
+
and reversibly dissociates protons and the oxidized form of
those in the reactions catalyzed by [1] (Runs 1 and 2). On
2+
resultant bis(ruthenium–oxo) complex worked as a good elec-
the contrary, 9,10-dihydroanthracene was not oxidized by [1]
7
2+
trode catalyst of the water-oxidation to dioxygen. We describe
at all, and [2] showed relatively high activity for the oxidation
to give anthracene (42% yield, Run 3). The dehydrogenation
reactions of the substrates in Table 1 did not proceed at all in
case of the absence of either AgClO4, t-BuOK or [1] (or
[2] ). The active species of the oxidation reactions, therefore,
is the oxidized form of [1] or [2] under basic conditions.
The violet MeOH solution of [1](SbF6)2 showed the char-
acteristic Ru(II) to quinone charge-transfer (MLCT) band at
576 nm. An addition of 2.0 equiv of t-BuOK to the solution
2
+
here that the dinuclear oxo complex derived from [1] has abil-
ities of oxidation of not only water but also hydrocarbons by
+
2+
C–H bond cleavage in the presence of Ag as a mild co-oxi-
2+
2+
dant, along with the comparison of the activity of [1] and the
2
+
2+
2+
corresponding mononuclear complex [2] toward the oxida-
tion.
t-BuOK (6.0 µmol) was added to the violet acetone solu-
tion of [1](SbF ) (3.0 µmol), 1,3-cyclohexadiene (3.0 µmol)
6
2
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
911
molecules of [2]2+ would be able to participate in the abstrac-
tion of two hydrogen atoms of 9,10-dihydroanthracene (Run 3).
The active species in the present study were derived from
[1]2+ and [2] by taking advantage of the quinone/semiquinone
redox reaction coupled with the acid–base equilibrium but not
by the redox reactions of the central Ru(II)/Ru(III) redox cou-
ples. Especially, it is worthy to note from the viewpoint of bio-
resulted in the complete loss of the band 576 nm and an appear-
ance of a new band at 850 nm assigned to Ru(II) to semi-
8
,9
quinone charge-transfer (MLCT) band.
Acidification of the
2+
solution by an addition of 2.0 equiv of HClO to the solution
4
completely restored the 576 nm band and disappeared the 850
nm band. Such the reversible change of the MLCT band of
2
+
[
1] from 576 nm to 850 nm is explained by the reduction of
2+
quinone to semiquinone coupled with the deprotonation/protona-
tion equilibrium of the hydroxo ligand of the dinuclear complex.
chemistry that [1] showed activities for oxidation reactions of
both water and hydrocarbons.
2+
II
II
0
Thus, deprotonation of [1] produced [Ru (O)(SQ)Ru (O)(SQ)]
SQ = 3,6-di(tert-butyl)-1,2-semiquinone) (Eq 2). Cyclic
(
This work was partly supported by the Grant-in-Aid for
Scientific Research on Priority Areas from the Ministry of
Education, Science, Sports, and Culture of Japan (No.
1
0149259).
II
II
0
voltammetry of [Ru (O)(SQ)Ru (O)(SQ)] in MeOH revealed
II
II
0
that the redox potentials of the [Ru (O)(SQ)Ru (O)(SQ)] /
Ru (O)(Q)Ru (O)(SQ)] couple and the [Ru (O)-
Q)Ru (O)(SQ)] /[Ru (O)(Q)Ru (O)(Q)] one were +0.30 V
and +0.40 V (vs Ag/AgCl), respectively, and no other oxidation
References and Notes
II
II
+
II
[
(
1
2
a) B. J. Wallar and J. D. Lipscomb, Chem. Rev., 96, 2625
(1996). b) M. Sono, M. P. Roach, E. D. Coulter, and J. H.
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L. Casella, “Metalloporphyrins Catalyzed Oxidations,”
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a) R. A. Sheldon and J. K. Kochi, “Metal-Catalyzed
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(1998).
II
+
II
II
2+
II
wave appeared up to 1.0 V in MeOH. Thus, [Ru (O)(Q)-
Ru (O)(Q)] must be formed in the treatment of [1] with Ag+
II
2+
2+
10
under basic conditions (Eq. 3).
The mononuclear complex [2]2+ also displayed the Ru to
quinone MLCT band at 576 nm in MeOH, which shifted to 869
nm due to formation of the [Ru (OH)(SQ)] moiety upon the
II
II
+
treatment of the solution with t-BuOK. The pK value of the
3
4
a) J. T. Groves and R. Quinn, J. Am. Chem. Soc., 107, 5790
(1985). b) A. S. Goldstein, R. H. Beer, and R. S. Drago, J.
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Matsudsa, Chem. Lett., 1999, 81.
a) S. Murahashi, Y. Oda, T. Naota, and T. Kuwabara,
Tetrahedron Lett., 34, 1299 (1993). b) C.-M. Che, C. Ho,
and T.-C. Lau, J. Chem. Soc., Dalton Trans., 1991, 1901.
H. Ohtake, T. Higuchi, and M. Hirobe, J. Am. Chem. Soc.,
114, 10660 (1992).
a) B. A. Moyer and T. J. Meyer, J. Am. Chem. Soc., 100,
3601 (1978). b) K. J. Takeuchi, M. S. Thompson, D. W.
Pipes, and T. J. Meyer, Inorg. Chem., 23, 1845 (1984). c)
S. A. Adeyemi, A. Dovletoglou, A. R. Guadalupe, and T. J.
Meyer, Inorg. Chem., 31, 1375 (1992). d) A. Gerli, J.
Reedijk, M. T. Lakin, and A. L. Spek, Inorg. Chem., 34,
1836 (1995). e) L. G. Muller, J. H. Acquaye, and K. J.
Takeuchi, Inorg. Chem., 31, 4552 (1992). f) K.-Y. Wong,
V. W.-W. Yam, and W. W.-S. Lee, Electrochim. Acta, 37,
2645 (1992).
a
II
+
resultant [Ru (OH)(SQ)] was too large to form the oxo-com-
8
plex in acetone.
[
Based on the observation that
II
+
II
2+
Ru (OH)(SQ)] was oxidized to [Ru (OH)(Q)] at +0.07 V
and the latter was not further oxidized in acetone, [2] was
converted to [Ru (OH)(Q)] under the experimental conditions
2
+
II
2+
of Table 1.
The active species in the dehydrogenation reactions by
[
[
1]2+ and [2] (Table 1) are [Ru (O)(Q)Ru (O)(Q)] and
2+
II
II
2+
5
6
II
2+
Ru (OH)(Q)] , respectively, which reasonably explain the
2+
2+
regeneration of [1] and [2] after the reactions because of the
abstraction of hydrogen atoms of the substrates.
The striking characteristic of the reactivity of [1] is the
high activity for cleavage of the vicinal two C–H bonds (Table
2
+
1
9
, Runs 1 and 2) and no ability to abstract of hydrogen atoms of
,10-dihydroanthracene (Run 3). On the other hand, [2]
2
+
showed the reverse reactivity: low activity for the abstraction of
the vicinal hydrogen atoms and high ability to oxidize 9,10-
2+
dihydroanthracene. The difference in the reactivity of [1] and
2
+
[
2] for these substrates, therefore, is explained by the view
II
II
2+
that the dimeric [Ru (O)(Q)Ru (O)(Q)] has an ability to
7
8
9
T. Wada, K. Tsuge, and K. Tanaka, Angew. Chem., 112,
1542 (2000); Angew. Chem., Int. Ed. Engl., 39, 1479
(2000).
a) K. Tsuge, M. Kurihara, and K. Tanaka, Chem. Lett.,
1998, 1069. b) K. Tsuge, M. Kurihara, and K. Tanaka,
Bull. Chem. Soc. Jpn., 73, 607 (2000).
cleave the vicinal two C–H bonds simultaneously with the
2
+
regeneration of [1] , while the abstraction of H atom from
II
2+
these substrates by monomeric [Ru (OH)(Q)] inevitably pro-
duces free radical species unless two molecules of
II
2+
[
Ru (OH)(Q)] participate in the cleavage of the vicinal two
C–H bonds at the same time. The abstraction of the vicinal
M. Kurihara, S. Daniele, K. Tsuge, H. Sugimoto, and K.
Tanaka, Bull. Chem. Soc. Jpn., 71, 867 (1998).
2+
hydrogen atoms by [1] , therefore, is kinetically advantageous
2
+
2+
compared with that by [2] . Two hydroxo groups of [1]
10 Controlled-potential electrolysis at +0.55 V (vs Ag/AgCl)
0
must be located in the cavity of the dimeric linkage. Inability
of [1]2+ for the oxidation of 9,10-dihydroanthracene apparently
results from the steric hindrance for the approach to the oxo
group in the cavity of dimeric linkage. On the other hand, two
of a methanolic solution of [Ru(O)(SQ)Ru(O)(SQ)] (λ
max
2+
= 850 nm) resulted in formation of [Ru(O)(Q)Ru(O)(Q)]
(λmax = 582 nm) .