Reactivity of mononuclear alkylperoxo copper(II) complex. O-O bond cleavage and C-H bond activation

Article abstract of DOI:10.1021/ja800443s

A detailed reactivity study has been carried out for the first time on a new mononuclear alkylperoxo copper(II) complex, which is generated by the reaction of copper(II) complex supported by the bis(pyridylmethyl)amine tridentate ligand containing a phenyl group at the 6-position of the pyridine donor groups and cumene hydroperoxide (CmOOH) in CH₃CN. The cumylperoxo copper(II) complex thus obtained has been found to undergo homolytic cleavage of the O-O bond and induce C-H bond activation of exogenous substrates, providing important insights into the catalytic mechanism of copper monooxygenases. Copyright

Full text of DOI:10.1021/ja800443s

Published on Web 03/12/2008
Reactivity of Mononuclear Alkylperoxo Copper(II) Complex. O-O Bond
Cleavage and C-H Bond Activation
Atsushi Kunishita,† Hirohito Ishimaru,‡ Satoru Nakashima,‡ Takashi Ogura,‡ and Shinobu Itoh*,†
Department of Chemistry, Graduate School of Science, Osaka City UniVersity, 3-3-138 Sugimoto, Sumiyoshi-ku,
Osaka 558-8585, Japan, and Picobiology Research Center, Graduate School of Life Science, UniVersity of Hyogo,
3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan
Received January 19, 2008; E-mail: shinobu@sci.osaka-cu.ac.jp
Mononuclear copper-active oxygen species play important roles
as reactive intermediates in many biological and industrial catalytic
oxidation processes.1-3 For copper monooxygenases such as
peptidylglycine R-amidating monooxygenase (PAM) and dopamine
â-monooxygenase (DâM), a mononuclear hydroperoxo copper(II)
species LCu(II)-OOH, which is formally generated by the reaction
of LCu(I) and O2 and subsequent addition of H• (or H+ + e-), has
been suggested as the reactive intermediate for the aliphatic
hydroxylation of the substrates.4 More recently, a mononuclear
superoxo copper(II) species LCu(II)-OO•, initially formed inter-
mediate of the reaction between LCu(I) and O2, has also been
proposed as a possible reactive intermediate.5-8 Furthermore, a
recent QM/MM calculation study has suggested that a copper(II)-
oxyl radical species LCu(II)-O•, which can be generated by O-O
bond homolysis of LCu(II)-OOH, is the most reactive species
among the intermediates.9 To gain insight into the dioxygen
activation mechanism at the mononuclear copper active sites, a great
deal of effort has been made in model chemistry to evaluate the
structure, spectroscopic features, and reactivity of the mononuclear
copper-active oxygen species.1-3,10-18
On the other hand, alkylperoxo iron(III) complexes LFe(III)-
OOR have been studied extensively in model systems to provide
significantly important insights into the catalytic mechanism of non-
heme iron monooxygenases.19-21 In this respect, studies of mono-
nuclear alkylperoxo copper(II) complexes LCu(II)-OOR may also
afford important information about the dioxygen activation mech-
anism at the mononuclear copper reaction centers. However, little
is known about the reactivity of LCu(II)-OOR complexes.22-24
We herein report the reactivity study of a new mononuclear
alkylperoxo copper(II) complex 2, which is generated by the
reaction of copper(II) starting material 124 and cumene hydroper-
oxide (CmOOH) in CH3CN (Scheme 1). The cumylperoxo copper-
Figure 1. (A) Spectral change for the reaction of 1 (0.6 mM) with CmOOH
(1.2 mM) in the presence of NEt3 (0.6 mM) in CH3CN at -40 °C. (B)
Resonance Raman spectra of 2 generated by using Cm16O16OH (solid line,
below) and Cm18O18OH (dotted line, above) obtained with λex ) 488.0 nm
in CH3CN at -40 °C; s denotes the solvent bands.
(II) complex 2, the formation of which was confirmed by the
following experimental data. Thus, complex 2 exhibited a relatively
intense absorption band at 465 nm (ꢀ ) 1100 M-1 cm-1) due to
the peroxo-to-copper(II) charge transfer transition (LMCT)
together with a weak d-d band at 725 nm (ꢀ ) 320 M-1 cm-1) as
shown in Figure 1A. Complex 2 also gave isotope-sensitive
resonance Raman bands at 885, 841, 608, 529, and 485 cm-1, which
shifted to 855, 808, 597, 524, and 474 cm-1 upon 18O-substitution
using Cm18O18OH instead of Cm16O16OH (Figure 1B). Appearance
of the multiple resonance Raman bands and their associated isotope
shifts (∆n ) 30, 33, 11, 5, and 11 cm-1) are similar to those reported
from the resonance Raman studies of the cumylperoxo copper(II)
complex supported by the hydrotrispyrazolylborate ligand and the
cumylperoxo iron(III) complex of 6-Me3-TPA [tris(6-methyl-2-
pyridylmethyl)amine].23,25 By analogy to those detailed Raman
studies, the bands in the 800 cm-1 region of 2 can be assigned to
mixed O-O/C-O/C-C vibrations of the cumylperoxo group and
the band at 608 cm-1 to the Cu-O stretching vibration. Then, the
additional 529 and 485 cm-1 bands of 2 can be assigned to C-C-C
and C-C-O deformation modes of the alkylperoxo moiety.23
The ESR spectrum of 2 (Figure S1, g1 ) 2.250, g2 ) 2.065, g3
) 2.030, A1 ) 160, A2 ) 7, A3 ) 5 G), which is different from
that of the starting material 1 (Figure S2), reflected a distorted
tetragonal geometry of 2, and its mononuclearity was confirmed
by spin quantification using the ESR spectrum (99% spin remained).
Unfortunately, instability of 2 precluded us from getting ESI-MS
data even at the low temperature.
Scheme 1
(II) complex 2 has been found to undergo homolytic cleavage of
the O-O bond and induce C-H bond activation of exogenous
substrates, providing important insight into the catalytic mechanism
of the copper monooxygenases.
Treatment of 1 with cumene hydroperoxide in the presence of
triethylamine in acetonitrile at -40 °C gave cumylperoxo copper-
The cumylperoxo copper(II) complex 2 gradually decomposed,
obeying first-order kinetics even at -40 °C (kdec ) 2.2 × 10-3
s-1, Figure S3) to give bis(µ-hydroxo)dicopper(II) complex 4 in a
64% isolated yield, where no ligand hydroxylation took place (see
Supporting Information). Notably, acetophenone (PhC(O)CH3) was
obtained in a 92% yield from the final reaction mixture. This clearly
† Osaka City University.
‡ University of Hyogo.
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J. AM. CHEM. SOC. 2008, 130, 4244-4245
10.1021/ja800443s CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
18033045, 19020058, 19027048, and 19028055 for S.I.) and by
Global Center of Excellence (GCOE) Program (“Picobiology: Life
Science at Atomic Level” to T.O.) from MEXT, Japan.
Supporting Information Available: Experimental details for the
synthetic procedures and additional spectroscopic and kinetic data. This
material is available free of charge via the Internet at http://pubs.acs.org.
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alkylperoxo copper(II) complex has been explored for the first time
to provide important insights into the catalytic mechanism of copper
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Acknowledgment. This work was financially supported in part
by Grants-in-Aid for Scientific Research on Priority Area (Nos.
JA800443S
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