C-H bond activation of external substrates with a bis(μ-oxo)dicopper(III) complex [2]

Full text of DOI:10.1021/ja015721s

J. Am. Chem. Soc. 2001, 123, 6203-6204
6203
monooxygenases.6,7 However, little is known about mechanistic
details on the reactions of bis(µ-oxo)dicopper(III) complexes and
external substrates.8
C-H Bond Activation of External Substrates with a
Bis(µ-oxo)dicopper(III) Complex
Masayasu Taki,‡ Shinobu Itoh,†,* and Shunichi Fukuzumi‡,*
Department of Chemistry
Graduate School of Science
Osaka City UniVersity, 3-3-138, Sugimoto
Sumiyoshi-ku, Osaka, 558-8585, Japan
Department of Material and Life Science
Graduate School of Engineering
We report herein the mechanistic studies on the C-H bond
activation of external substrates such as 10-methyl-9,10-dihy-
droacridine (AcrH2) and 1,4-cyclohexadiene (CHD) with a bis-
(µ-oxo)dicopper(III) complex supported by bidentate ligand LPy1Bz
(LPy1Bz ) N-ethyl-N-[2-(2-pyridyl)ethyl]-R,R-dideuterio-benzy-
lamine).9 An unusual kinetic behavior observed in the present
reactions implicates the existence of a new copper-active oxygen
intermediate that is responsible for the C-H bond activation of
the external substrates.
Osaka UniVersity, CREST
Japan Science and Technology Corporation
2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
ReceiVed February 26, 2001
ReVised Manuscript ReceiVed May 22, 2001
C-H bond activation by metal-oxo species has long been one
of the most important and attractive research objectives not only
in bioinorganic chemistry but also in numerous and diverse array
of catalytic oxidation reactions for organic synthesis.1 Reactivity
of iron-oxo complexes of both heme and non-heme systems in
various oxidation states has so far been studied extensively to
provide valuable insight into the catalytic mechanisms of iron-
dependent oxygenases as well as of Fenton-type and Gif-type
reactions.1-3 Much attention has recently been focused on the
dinuclear copper-oxo species such as bis(µ-oxo)dicopper(III),
since it is considered as a possible active oxygen intermediate in
the aliphatic hydroxylation by dinuclear copper monooxygenases.4
Structural and spectroscopic characterizations of the bis(µ-oxo)
species have been well-documented,5 and their decomposition
processes leading to intramolecular aromatic and aliphatic ligand
hydroxylation as well as oxidative N-dealkylation of the ligand
sidearm have been investigated as functional models for copper
Introduction of dry O2 gas into an acetone solution of [CuI-
(LPy1Bz)(CH3CN)]PF6 at -94 °C resulted in a spectral change,
where a characteristic absorption band at 400 nm (ꢀ ) 17400
M-1 cm-1) readily developed (see Supporting Information, S1).
The resulting acetone solution was ESR silent, and the resonance
Raman spectrum of the oxygenated intermediate in frozen acetone-
d6 (λex ) 457.9 nm) exhibited a characteristic peak at 604 cm-1
(S2).10 These spectral features together with the observed stoi-
chiometry of Cu:O2 ) 2:1 demonstrate the formation of a bis-
(µ-oxo)dicopper(III) intermediate (1).10 The bis(µ-oxo)dicopper-
(III) complex supported by the deuterated ligand LPy1Bz was stable
enough to examine the reactivity toward external substrates at
the low temperature.
Addition of the substrates into an acetone solution of 1 (5.0 ×
10-5 M) at -94 °C under Ar atmosphere resulted in a color
change from dark brown to green. Figure 1A shows the spectral
change for the reaction with AcrH2 as a typical example, where
the characteristic absorption band at 400 nm due to 1 decreases
with a concomitant increase in the absorption bands at 358, 395,
415, and 440 nm due to the oxidation product AcrH+ (N-
methylacridinium ion). From the absorption intensity at 440 nm
† Osaka City University.
‡ Osaka University.
(1) (a) Valentine, J. S., Foote, C. S., Greenberg, A., Liebman, J. F., Eds.
ActiVe Oxygen in Biochemistry; Chapman and Hall: London, 1995. (b) Foote,
C. S., Valentine, J. S., Greenberg, A., Liebman, J. F., Eds. ActiVe Oxygen in
Chemistry; Chapman and Hall: London, 1995. (c) Meunier, B., Ed. Biomimetic
Oxidations Catalyzed by Transition Metal Complexes; Imperial College
Press: London, 1999. (d) Meunier, B., Ed. Metal-Oxo and Metal-Peroxo
Species in Catalytic Oxidations; Springer: Berlin, 2000.
(ꢀ ) 2150 M-1 cm-1 11 is determined the yield of AcrH+ as 100%
)
based on 1. Oxidation of CHD also proceeded smoothly to
produce the corresponding oxidation products, i.e., benzene.12 The
nearly quantitative formation was confirmed by GC-MS. Thus,
(2) (a) Que, L., Jr.; Ho, R. Y. N. Chem. ReV. 1996, 96, 2607-2624. (b)
Wallar, B. J.; Lipscomb, J. D. Chem. ReV. 1996, 96, 2625-2657. (c) Feig, A.
L.; Lippard, S. J. Chem. ReV. 1994, 94, 759-805.
(3) (a) Barton, D. H. R.; Doller, D. Acc. Chem. Res. 1992, 25, 504-512.
(b) Sawyer, D. T.; Sobkowiak, A.; Matsushita, T. Acc. Chem. Res. 1996, 29,
409-416. (c) FacFaul, P. A.; Wayner, D. D. M.; Ingold, K. U. Acc. Chem.
Res. 1998, 31, 159-162.
(6) (a) Mahapatra, S.; Halfen, J. A.; Tolman, W. B. J. Am. Chem. Soc.
1996, 118, 11575-11586. (b) Holland, P. L.; Rodgers, K. R.; Tolman, W. B.
Angew. Chem., Int. Ed. 1999, 38, 1139-1142.
(7) Itoh, S.; Taki, M.; Nakao, H.; Holland, P. L.; Tolman, W. B.; Que, L.,
Jr.; Fukuzumi, S. Angew. Chem., Int. Ed. 2000, 39, 398-400.
(4) (a) Karlin, K. D., Tyekla´r, Z. Eds. Bioinorganic Chemistry of Copper;
Chapman & Hall: New York, 1993. (b) Kitajima, N.; Moro-oka, Y. Chem.
ReV. 1994, 94, 737-757. (c) Liang, H. C.; Dahan, M.; Karlin, K. D. Curr.
Opin. Chem. Biol. 1999, 3, 168-175. (d) Tolman, W. B. Acc. Chem. Res.
1997, 30, 227-237. (e) Holland, P. L.; Tolman, W. B. Coord. Chem. ReV.
1999, 190-192, 855-869. (f) Mahadevan, V.; Gebbink, R. K.; Stack, T. D.
P. Curr. Opin. Chem. Biol. 2000, 4, 228-234. (g) Schindler, S. Eur. J. Inorg.
Chem. 2000, 2311-2326.
(5) (a) Halfen, J. A.; Mahapatra, S.; Wilkinson, E. C.; Kaderli, S.; Young,
V. G., Jr.; Que, L., Jr.; Zuberbu¨hler, A. D.; Tolman, W. B. Science 1996,
271, 1397-1400. (b) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. Z.; Pan, G.;
Wang, X.; Young, V. G., Jr.; Cramer, C. J.; Que, L., Jr.; Tolman, W. B. J.
Am. Chem. Soc. 1996, 118, 11555. (c) Mahapatra, S.; Young, V. G., Jr.;
Kaderli, S.; Zuberbu¨hler, A. D.; Tolman, W. B. Angew. Chem., Int. Ed. Engl.
1997, 36, 130-133. (d) Holland, P. L.; Cramer, C. J.; Wilkinson, E. C.;
Mahapatra, S.; Rodgers, K. R.; Itoh, S.; Taki, M.; Fukuzumi, S.; Que, L., Jr.;
Tolman, W. B. J. Am. Chem. Soc. 2000, 122, 792-802. (e) Mahadevan, V.;
Hou, Z.; Cole, A. P.; Root, D. E.; Lal, T. K.; Solomon, E. I.; Stack, T. D. P.
J. Am. Chem. Soc. 1997, 119, 11996-11997. (f) Henson, M. J.; Mukherjee,
P.; Root, D. E.; Stack, T. D. P.; Solomon, E. I. J. Am. Chem. Soc. 1999, 121,
10332-10345. (g) Hayashi, H.; Fujinami, S.; Nagatomo, S.; Ogo, S.; Suzuki,
M.; Uehara, A.; Watanabe, Y.; Kitagawa, T. J. Am. Chem. Soc. 2000, 122,
2124-2125. (h) Straub, B. F.; Rominger, F.; Hofmann, P. Chem. Commun.
2000, 1611-1612.
(8) (a) Berreua, L. M.; Mahapatra, S.; Halfen, J. A.; Houser, R. P.; Young,
V. G., Jr.; Tolman, W. B. Angew. Chem., Int. Ed. 1999, 38, 207-210. (b)
Mahadevan, V.; DuBois, J. L.; Hedman, B.; Hogson, K. O.; Stack, T. D. P.
J. Am. Chem. Soc. 1999, 121, 5583-5584. (c) Mahadevan, V.; Henson, M.
J.; Solomon, E. I.; Stack, T. D. P. J. Am. Chem. Soc. 2000, 122, 10249-
10250. (d) Obias, H. V.; Lin, Y.; Murthy, N. N.; Pidcock, E.; Solomon, E. I.;
Ralle, M.; Blackburn, N. J.; Neuhold, Y.-M.; Zuberbu¨hler, A. D.; Karlin, K.
D. J. Am. Chem. Soc. 1998, 120, 12960-12961.
(9) For a recent report on the C-H bond activation by distinct copper(III)
complexes, see: Lockwood, M. A.; Blubaugh, T. J.; Collier, A. M.; Lovell,
S.; Mayer, J. M. Angew. Chem., Int. Ed. 1999, 38, 225-227.
(10) Because of low intensity of the Raman bands of the 18O-derivative,
the amount of the isotope shift could not be determined accurately. Although
the Raman evidence for the bis(µ-oxo)dicopper(III) core is not as strong as
in earlier instances,9 the structural similarity of this molecule to others in the
series7 and great similarities of the UV-vis and the 16O-Raman data to those
of the other bis(µ-oxo)dicopper(III) complexes5 are strong enough arguments
for the presence of the diamond core intermediate.
(11) Itoh, S.; Kumei, H.; Nagatomo, S.; Kitagawa, T.; Fukuzumi, S. J.
Am. Chem. Soc. 2001, 123, 2167-2175.
(12) Stack and co-workers reported that the bis(µ-oxo)dicopper(III) complex
supported by N,N,N′,N′-tetramethyl-1,3-propanediamine did not react with
1,4-cyclohexadine and 9,10-dihydroanthracene.8b
10.1021/ja015721s CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/02/2001

6204 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Communications to the Editor
Scheme 1
Figure 1. (A) Spectral change (20 s interval) observed upon addition of
10-methyl-9,10-dihydroacridine (5.0 × 10-4 M) into an acetone solution
of 1 (5.0 × 10-5 M) at -94 °C under Ar. (Inset) Second-order plot based
on the decrease of the absorption at 400 nm. (B) Plots of kobs vs substrate
concentration for the reaction of 1 with AcrH2 (O) and AcrD2 (0) under
the same experimental conditions.
strates (Scheme 1).15 In such a case, the reaction is second-order
with respect to 1 as experimentally observed.16 Unfortunately,
however, we could not obtain any direct evidence for such an
intermediate, since the equilibrium of the disproportionation
reaction may lie far to the left, that is, to the starting material
[bis(µ-oxo)dicopper(III) complex].
Alternatively two molecules of the bis(µ-oxo)dicopper(III)
intermediate may function in unison (e.g., via a tetranuclear
copper-oxo complex) to oxidize the substrate. At present,
however, it is extremely difficult to distinguish between the
disproportionation complex in Scheme 1 and a tetranuclear
copper-oxo complex, since such a complex could not be detected
because of the facile dissociation of the complex (k-1 . k1 in
Scheme 1).17
In conclusion, we present the first example of the C-H bond
activation of external noncoordinative substrates such as AcrH2
and CHD with a bis(µ-oxo)dicopper(III) complex, where partici-
pation of a new copper-active oxygen species such as a (µ-oxo)-
(µ-oxyl radical)dicopper(III) or a tetranuclear copper-oxo com-
plex is suggested by the kinetic data. The present results will
provide important insight into the C-H bond activation mech-
anism of copper monooxygenases, since such a (µ-oxo)(µ-oxyl
radical)dicopper unit has been suggested as an actual active
oxygen intermediate in the reaction of particulate methane
monooxygenase.18
the present system gives us an excellent opportunity to examine
the C-H bond activation mechanism of external substrates with
the bis(µ-oxo)dicopper(III) species.
To our surprise, the decrease of the absorption band at 400
nm due to 1 obeys second-order kinetics even in the presence of
a large excess substrate as shown in the inset of Figure 1A, where
the second-order rate constant kobs was obtained as a slope of the
linear second-order plot [(A0 - A)/{(A - A∞)[1]0} vs time]. The
second-order dependence on the bis(µ-oxo)dicopper(III) inter-
mediate (1) has also been confirmed by the result that the kobs
values obtained at various initial concentrations of 1 were
inversely proportional to the initial concentration of 1 as expected
from the equation of the second-order plot: (A0 - A)/{(A -
A∞)‚[1]0} ) kobst. The second-order rate constant kobs was then
plotted against the substrate concentration to demonstrate the first-
order dependence of kobs on [substrate] (Figure 1B, line AcrD2),
from which the third-order rate constant k3 was determined as
1.2 × 106 M-2 s-1 from the slope. The same kinetic behavior
(the second-order dependence on 1 and the first-order dependence
on [substrate]) was observed in the reaction with CHD, and the
k3 values at -94 °C was determined as 1.5 × 104 M-2 s-1. In
H
addition, a large primary kinetic deuterium isotope effect (k3 /
Acknowledgment. This work was partially supported by Grants-in-
Aid for Scientific Research on Priority Area (Nos. 11228205, 11228206)
and Grants-in-Aid for Scientific Research (Nos. 11440197, 12874082,
and 13480189) from the Ministry of Education, Culture, Sports, Science,
and Technology, Japan. We also acknowledge Dr. P. L. Holland, Professor
W. B. Tolman, and Professor L. Que, Jr. of University of Minnesota for
their help in obtaining the Raman data and also for their helpful
discussions.
D
k3 ) 22.7) was obtained at -94 °C when AcrD2 (AcrD2 ) the
9,9-dideuterated analogue of AcrH2) was used in place of AcrH2
(Figure 1B, line AcrD2). The overall activation energies were
determined from their temperature dependence as E3(H) ) 8.9 (
0.6 kcal mol-1 and E3(D) ) 10.0 ( 0.2 kcal mol-1 (S3).13 The
H
D
k3 /k3 value is significantly larger than the primary kinetic
deuterium isotope effect (3.0) reported for the hydrogen abstrac-
tion from AcrH2 by hydroperoxyl radical (HO2 ).14 Such a large
•
Supporting Information Available: Experimental details including
synthetic procedures, resonance Raman spectra (S2), product analysis,
manometry, and kinetic measurements (S1 and S3) (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
primary kinetic deuterium isotope effect indicates that a tunneling
hydrogen-transfer process is involved in the rate-determining step
of the C-H bond activation by 1. However, the direct reaction
between 1 and AcrH2 would afford a first-order dependence of
the rate with respect to the concentration of each reactant, contrary
to the present experimental observation.
One of the possible explanations for such an unusual kinetic
behavior (the second-order dependence on 1) is following.
Disproportionation of two molecules of the bis(µ-oxo)dicopper-
(III) complexes may afford one molecule of (µ-oxo)(µ-oxyl
radical)dicopper(III) (A) and one molecule of bis(µ-oxo)Cu(II)-
Cu(III) (B), the former of which is a real active species for the
C-H bond activation (hydrogen atom abstraction) of the sub-
JA015721S
(15) The reaction rate was not influenced even when it was carried out in
the presence of excess O2. This may indicate that the subsequent hydrogen-
•
atom transfer and the electron transfer from the radical intermediates (C6H7
and AcrH•) are much faster than the initial hydrogen abstraction from the
substrates.
(16) According to Scheme 1, the observed rate constant k3 is given by kH-
(k1/k-1), when the overall activation energy (E3) consists of the activation
enthalpy of each process: ∆HH + ∆H1 - ∆H-1q. The overall activation
q
q
entropies (∆S3 ) ∆SH + ∆S1 - ∆S-1q) were also determined from the
temperature dependence of k3 as -35, -35, and -13 cal K-1 mol-1 for AcrH2,
AcrD2, and CHD, respectively (see Supporting Information S3).
(17) The observed negative activation entropies16 are consistent with
formation of an associative complex (a disproportionation complex or a
tetranuclear complex).
q
q
q
(13) The activation energy for the reaction with CHD was also determined
from the temperature dependence of k3 as E3 ) 6.4 ( 0.1 kcal mol-1 (S3).
(14) Fukuzumi, S.; Ishikawa, M.; Tanaka, T. J. Chem. Soc., Perkin Trans.
2 1989, 1037-1045.
(18) Elliott, S. J.; Zhu, M.; Tso, L.; Nguyen, H.-H. T.; Yip, J. H.-K.; Chan,
S. I. J. Am. Chem. Soc. 1997, 119, 9949-9955.