Generation, Characterization, and Reactivity of a CuII-Alkylperoxide/Anilino Radical Complex: Insight into the O-O Bond Cleavage Mechanism

Article abstract of DOI:10.1021/jacs.5b04104

The reaction of [Cu^I(TIPT₃tren) (CH₃CN)]ClO₄ (1) and cumene hydroperoxide (C₆H₅C(CH₃)₂OOH, ROOH) at -60°C in CH₂Cl₂ gave a Cu^II-alkylperoxide/anilino radical complex 2, the formation of which was confirmed by UV-vis, resonance Raman, EPR, and CSI-mass spectroscopy. The mechanism of formation of 2, as well as its reactivity, has been explored.

Full text of DOI:10.1021/jacs.5b04104

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Communication
II
Generation, Characterization, and Reactivity of a Cu-alkylperoxide/
anilino-radical Complex: Insight into the O–O Bond Cleavage Mechanism
Sayantan Paria, Takehiro Ohta, Yuma Morimoto, Takashi Ogura, Hideki Sugimoto,
Nobutaka Fujieda, Kei Goto, Kaori Asano, Takeyuki Suzuki, and Shinobu Itoh
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b04104 • Publication Date (Web): 20 Aug 2015
Downloaded from http://pubs.acs.org on August 23, 2015
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Generation, Characterization, and Reactivity of a CuII-
alkylperoxide/anilino-radical Complex: Insight into the O–O Bond
Cleavage Mechanism
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†
‡
†
‡
†
†
Sayantan Paria, Takehiro Ohta, Yuma Morimoto, Takashi Ogura, Hideki Sugimoto, Nobutaka Fujieda,
¶
†
Kei Goto, Kaori Asano,§ Takeyuki Suzuki,§ and Shinobu Itoh*,
†Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka
University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
‡Picobiology Institute, Graduate School of Life Science, University of Hyogo, RSC-UH LP center, Koto 1-1-1, Sayo-cho, Sayo-gun,
Hyogo 679-5148, Japan
¶Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-
ku, Tokyo 152-8551, Japan
§ Comprehensive Analysis Center, The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki,
Osaka 567-0057, Japan
Supporting Information Placeholder
Copper(I)-complex 1 was synthesized by mixing an
ABSTRACT:
The
reaction
of
[CuI(TIPT3tren)(CH3CN)]ClO4 (1) and cumene hydrop-
eroxide (C6H5C(CH3)2OOH, ROOH) at –60°C in CH2Cl2
gave a CuII-alkylperoxide/anilino-radical complex 2, the
formation of which was confirmed by UV-vis, resonance
Raman, EPR, and CSI-mass spectroscopy. The mechanism
of formation of 2 as well as its reactivity has been explored.
equimolar
amount
of
ligand
TIPT3tren
and
[CuI(CH3CN)4]ClO4 in acetone. Single crystals of 1 suitable
for X-ray diffraction analysis were obtained as a BF4– salt (for
synthetic procedures and characterizations, see Experimental
Section and Figures S1–S8 in Supporting Information).
Copper–active-oxygen species play versatile roles in a varie-
ty of biological and chemical oxidation/oxygenation reac-
tions.1-3 So far, several types of supporting ligands have been
developed to examine copper(I)/dioxygen reactivity as well
as copper(II)/peroxide reactivity, providing important in-
formation about the structure, physical properties, and reac-
tivity of the reactive intermediates involved in those reac-
tions.4-6 However, little attention has been focused on non-
innocent supporting ligands in the oxygen-activation chemis-
try by copper complexes.
In this study, we have developed a mononuclear copper(I)
complex 1 supported by an N4-tetradentate ligand,
TIPT3tren, consisting with three bulky substituents TIPT
(3,5-bis(2,6-diisopropylphenyl)phenyl) and tren (tris(2-
aminoethyl)amine) (Figure 1). The reaction of 1 with cu-
mene hydroperoxide (ROOH) at a low temperature gave an
unprecedented CuII-alkylperoxide/anilino-radical complex 2
(Scheme 1), where the non-innocent aniline moiety of the
ligand played crucial roles. The mechanism of formation of 2
as well as its reactivity is reported in this Communication.
Figure 1. ChemDraw structure of TIPT3tren ligand (left) and
ORTEP drawing of the crystal structure of the core domain of
complex 1 (right), where only the carbon atoms connected to
the aniline nitrogen atoms, N(2), N(3), and N(4), of the TIPT
substituents are shown. All hydrogen atoms and counter anion
are also omitted for clarity. For the whole structure, see Sup-
porting Information.
Scheme 1. Reaction of Complex 1 with Cumene Hydrop-
eroxide
+
+
TIPT
NH
OH
H3C
O
O
C
2
TPIT
O
H
N
N
TIPT
TIPT
CuI
N
TIPT
H
N
CuII
N
NH
TIPT
N
N
H
OH
H2O
+
1
2
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Figure 1 shows the crystal structure of 1, where copper(I)
Page 2 of 5
to 788, 589, 558, and 531 cm–1 (∆ν = 43, 15, 11, and 10 cm–
1
2
3
4
5
6
7
8
1), respectively, upon using isotope labeled cumene hydrop-
eroxide (C6H5C(CH3)2(18O)2H, R(18O)2H) (Figures 3a and
3b). Based on the analogy of the resonance Raman features
center exhibits a distorted tetrahedral geometry ligated by
three nitrogen atoms, N(1), N(2), and N(3), of the ligand
and one nitrogen atom, N(5), of acetonitrile molecule
(τ4 = 0.73, τ4 = [(360°–(α + β))/141°], where α and β are
the two largest bond angles of four-coordinate structure. For
an ideal tetrahedral geometry, τ4 = 1, and for a perfect square
planer geometry, τ4 = 0).7 Thus, one of the ligand arms,
N(4), is detached from the copper ion. The FAB-mass spec-
trum of the complex showed a molecular ion peak cluster at
m/z = 1397.98, which matched with a molecular composi-
tion of [(TIPT3tren)Cu]+ (Figure S7).
with those of reported CuII-alkylperoxide complexes,8-11
a
similar type of alkylperoxide complex may be generated in
the present reaction. Thus, the band at 831 cm–1 is assigned
to the O–O stretching vibrations, whereas the band at 604
cm–1 is due to the Cu–O stretching vibration. Furthermore,
the bands around the lower frequency region are attributable
to the carbon skeleton deformation (C–C–C/O–C–C) to
the carbon skeleton deformation (C–C–C/O–C–C) modes.
The isotope shifts of these two bands (569 to 558 and 541 to
531 cm–1) observed by using R(18O)2H indicate the presence
of significant coupling of the C–C–C/O–C–C deformation
modes with the Cu–O stretching mode.9
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The reaction of
1 with cumene hydroperoxide
(C6H5C(CH3)2OOH = ROOH) was then examined. Addi-
tion of an excess amount of ROOH to 1 at –60 °C in CH2Cl2
under anaerobic conditions resulted in formation of a meta-
stable intermediate 2 exhibiting intense absorption bands at
396 nm (ε = 5400 M–1 cm–1), 465 nm (6000 M–1 cm–1), and
748 nm (6600 M–1 cm–1) (Figure 2). The reaction obeyed
pseudo-first-order kinetics, and from a linear plot of kobs vs
[ROOH] was obtained a second order rate constant k2
1.67 ± 0.06 M–1 s–1 (Figure S9).
=
Figure 2. UV-vis spectral changes observed upon addition of
ROOH (10 equiv) to 1 (0.25 mM) in CH2Cl2 at –60°C under
anaerobic conditions.
Figure 3. Resonance Raman spectra of (a) 2 (black) and its
18O-derivative (red), (b) a difference spectrum (16O – 18O), and
(c) 2 (black) and its ligand-deuterated derivative 2-d9 (orange).
Product analysis of organic compounds by HPLC (Figure
S10) after quenching the reaction at –60°C revealed nearly
quantitative formation of cumyl alcohol (C6H5C(CH3)2OH)
(98% based on 1) together with a trace amount of acetophe-
none (C6H5C(O)CH3). The quantitative formation of
cumyl alcohol (ROH) indicated that the reaction formally
involved a heterolytic O–O bond cleavage of the alkylperox-
ide moiety.8
The X-band EPR spectrum of the reaction solution indi-
cated that most of the species in the solution were EPR silent,
although a small amount of a CuII signal (~20 %) was ob-
served (Figure S11) probably due to a decomposition
product of copper complex intermediate.
Inspection of the resonance Raman spectrum at the higher
frequency region revealed the presence of several bands be-
tween 1300–1600 cm-1 (1588, 1578, 1555, 1405, and 1345
cm–1; Figure 3c, black line spectrum). Importantly, no 18O-
isotope shift was observed in the 18O-isotope labeled inter-
mediate 2 (Figure S12),clearly indicating that the origin of
these bands is not the CuII-alkylperoxide moiety. Notably,
these Raman bands are very similar to those reported for
metal-bound anilino radical species, where the bands have
been assigned to the C–C and C–N stretching vibrations of
the anilino radical moiety.12,13 In order to confirm this possi-
bility, we have prepared a deuterated ligand at the 2,4,6-
positions of the aniline rings of all the TIPT substituents,
referred to as TIPT3tren-d9 (Figures S5, S6, S7 and S8), and
examined the resonance Raman spectrum (Figure 3c, orange
line spectrum) of the ROOH adduct intermediate. In this
The resonance Raman spectrum obtained with 488 nm
excitation laser light gave isotope sensitive Raman bands at
831 (Fermi doublet), 604, 569, and 541 cm–1, which shifted
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case, we observed prominent peak shifts to 1562, 1546, 1529,
ROH (path i), which may be assisted by the ammonium
group (TIPT–NH2 –) as a general acid catalyst. In fact, the
1
2
3
4
5
6
7
8
and 1337 cm–1, respectively. The result confirmed that the
bands at the higher frequency region were due to the anilino
ligand radical bound to the CuII ion. Considering the reso-
nance structure of the anilino radical moiety, the band at
1588 cm–1, which shifted to 1562 cm–1 in the deuterated de-
rivative 2-d9, can be assigned to the Cortho–Cmeta (ν8a) stretch-
ing vibration. The other band at 1578 cm–1 (shifted to 1546
cm–1) was tentatively assigned to the ν19a vibration mode.
Then, the most intense band at 1555 cm–1 has been assigned
to C–N stretching vibration (ν7a), which shifted to 1529 cm–1
in the deuterated derivative. The band at 1405 cm–1 disap-
peared in the deuterated derivative, suggesting that the band
has significant contribution to C–H bending motion, thus
assigned to the Wilson ν14 mode.14 The vibration at 1345 cm–
+
reaction did not occur at all in the presence of base such as
triethylamine or 2,6-lutidine. The added base may abstract
proton from the ammonium group to prevent the intramo-
lecular hydrogen bonding interaction and the intramolecular
general acid catalysis. The generated intermediate B imme-
diately abstracts one of the hydrogen atoms of the aniline
moieties to give (L•)CuII–OH (intermediate C), where L•
denotes the ligand anilino radical (path ii). A concerted
mechanism is also possible for the generation of C from A
directly (path iii). Intermediate C undergoes ligand ex-
change reaction of the -OH group with another equivalent of
ROOH to give the final product 2. In fact, almost two equiv-
alent of ROOH is needed to get a quantitative formation of
complex 2 (Scheme 1). The reaction of an M–OH complex
with alkyl/acyl peroxide is well known in the literature.18,19
Although an iron(IV)-oxo intermediate supported by a lig-
and radical is known for the porphyrin system,20 and Peters
et al. have recently reported a mononuclear Cu(I)-radical
complex,21 compound 2 represents a unique example of a
metastable peroxide-complex supported by a non-porphyrin
organic radical.
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1
could be assigned to the non-totally symmetric ν19b mode,
which shifted to 1337 cm–1 in the deuterated
intermediate.14,15 These assignments of the Raman data were
well supported by the DFT calculations (Figure S25, Table
S3). It was also noted that the O–O/Cu–O stretching modes
at 831 and 604 cm–1 slightly shifted to 828 and 606 cm–1,
respectively, in the deuterated intermediate 2-d9, suggesting
a direct interaction between the anilino radical and CuII-
OOR moieties (Figures S13a and S13b).
Scheme 2. Proposed Mechanism for the Formation of 2
All these results are consistent with the assignment of
compound 2 as a CuII-alkylperoxide/anilino-radical complex
as illustrated in Scheme 1. The intense absorption bands in
the visible to near-IR region shown in Figure 2 can be as-
signed to π→π* transitions of the metal bound organic radi-
cal and an anilino radical to CuII as well as an alkylperoxide to
CuII charge transfer transitions. The EPR silence of 2 may be
due to antiferromagnetic coupling between CuII and anilino
radical as in the case of the oxidized-form of galactose oxi-
dase, in which there is a strong antiferromagnetic coupling
between CuII and coordinating phenoxyl radical species, thus
being EPR silent.16,17
TIPT
+
NH
R
O
ROOH
TIPT
NH
O
CuI
[CuI(L)(CH3CN)]+
1
H
N
N
H
A
TPIT
(i)
(iii)
ROH
OH
ROH
+
OR
+
+
O
O
H2O
ROOH
(L•)
(ii)
II
(L•)CuII
2
Cu
II
(L)
Cu
B
In support of this assignment, the CSI (cold electrospray
ionization)-mass spectrum of intermediate 2 measured in
CH2Cl2 at –60°C showed a peak cluster at m/z = 1550, the
isotope distribution pattern of which is consistent with the
C
Reactivity of the copper(II)-alkylperoxide/anilino radical
complex 2 toward alcohols was examined, since 2 has a simi-
lar structural motif with that of the active form of galactose
oxidase as mentioned above. Complex 2 readily reacted with
an excess amount (20 equiv) of 4-methylbenzyl alcohol at –
45°C (Figure S18a) to give 4-methylbenzaldehyde (Scheme
3-1) together with cumyl alcohol as confirmed by HPLC and
1H-HNR spectroscopy (Figure S19 and S20).
molecular component of [Cu
+
TIPT3tren
+
C6H5C(CH3)2OOH] (Figure S14). Upon using the isotope
labeled cumene hydroperoxide (R(18O)2H), the peak cluster
was shifted by four mass unit (Figure S15), clearly demon-
strating that 2 is an alkylperoxide-adduct.
A possible mechanism for the formation of 2 is presented
in Scheme 2. Addition of ROOH to complex 1 initially forms
a peroxide adduct A by replacing the coordinating CH3CN
by ROOH. In this step, the non-coordinating ligand arm of 1
(N(4) in Figure 1) may act as a base to accept proton from
ROOH to promote its coordination to CuI. The resulting
The decay of absorption bands due to 2 obeyed pseudo-
first-order kinetics as shown in Figure S18b, and plot of the
pseudo-first-order rate constants kobs against the substrate
concentration [ArCH2OH] gave a linear correlation, from
which the second-order rate constant k2 = 0.57 ± 0.03 M–1
s–1 was obtained from the slope (Figure S18c). The Ham-
+
ammonium group (TIPT–NH2 –) may stabilize the al-
+
mett analysis (plot of log k2 vs Hammett constant σp ) using
kylperoxide adduct A via an intramolecular hydrogen
bonding interaction. Then, the heterolytic O–O bond cleav-
age takes place to give (L)CuII–O• (intermediate B) and
a series of p-substituted benzyl alcohols (-OCH3, -CH3, -F, -
Cl) gave a Hammett ρ value as –0.42 0.08 as indicated in
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ASSOCIATED CONTENT
Figure S21. Notably, the Hammett ρ value of the present
reaction is very close to that of the oxidation of benzyl alco-
1
2
Supporting Information
hol derivatives by galactose oxidase (ρ = –0.09 0.32),22
suggesting that the mechanism of the present reaction is sim-
ilar to that of galactose oxidase.22 Namely, the alcohol sub-
strate initially replaces the alkylperoxide ligand (ROO) to
give an alkoxide adduct (Scheme S1). Then, inner sphere
electron transfer takes place from the coordinated alkoxide to
CuII, generating an alkoxyl radical intermediate, from which
benzylic hydrogen atom is abstracted by the anilino radical moi-
ety to give the aldehyde product. This event produces starting
copper(I) complex 1, from which 2 is regenerated according
to the reaction pathways shown in Scheme 2. In fact, the
All experimental details and additional data mentioned in the
text. This material is available free of cost via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: shinobu@mls.eng.osaka-u.ac.jp.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
reaction proceeded in
a catalytic manner to give 4-
This work was partly supported by a JSPS fellowship for over-
seas researchers (to S.P.) and Grants 22105007 (to S.I.) and
24109015 (to H.S.) for Scientific Research on Innovative Areas
from MEXT of Japan. The authors also express their gratitude
to prof. Shunichi Fukuzumi and Dr. Kei Ohkubo of Osaka Uni-
versity for their help in EPR measurement.
methylbenzaldehyde in a 350% yield based on the copper
complex after a prolonged reaction time (1 h) at 25°C.
Scheme 3. Reactivity of Complex 2 toward External Sub-
strates.
CHO
OH
2
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2
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(2)
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MeO
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N
N
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OH
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Complex 2 also reacted with typical hydrogen atom do-
nors such as 2,6-di-tert-butyl-4-methoxyphenol (Scheme 3-
2) and 2,2,6,6-tetramethylpiperidine-1-ol (TEMPOH,
Scheme 3-3) at a low temperature to give 2,6-di-tert-butyl-4-
methoxyphenoxyl radical and TEMPO•, respectively, as con-
firmed by UV-vis and/or EPR spectroscopy, respectively
(Figures S22 and S23), further demonstrating the radical
character of 2. It should be noted that the three absorption
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bands due to 2 (λmax = 396, 465, and 748 nm) decayed in the
same rate in each reaction (Eq 1-3, see Figures S18b, S22b,
and S23b). The results clearly indicate that the three absorp-
tion bands shown in Figure 2 arise from one species, that is
complex 2, but not from a mixture of multiple compounds.
In summary, we have developed
a
novel CuII-
(17) In a parallel-EPR spectrum taken at 4K, there was no signal around g ~
4. All the 1H and 2H signals appeared in the diamagnetic region for 2 and 2-d9 in
alkylperoxide/anilino-radical complex 2, which represents
the first example of a metal-alkylperoxide species supported
by a non-innocent ligand radical. For the formation of 2, the
bulky TIPT substituents prohibit undesirable dimerization
reaction and one of the alkylamine arms is dissociated from
the copper ion to dictate acid-base catalysis for the formation
of CuI–OOR and following O–O bond heterolytic cleavage
(Scheme 2). Complex 2 induces catalytic oxidation of benzyl
alcohol derivatives, presumably through a similar mechanism
of galactose oxidase. Further studies are now being under-
taken to explore the in-depth reactivity of this novel inter-
mediate.
1
2
the H- and H-NMR spectra (Figure S16 and Figures S17). All these results
were consistent with the assignment of 2 as a singlet species (S = 0). However, a
preriminaly DFT caluculation of complex 2 suggested that the triplet state is
more stable than the singlet state by 3.3 kcal mol–1 (see SI). Thus, further
detailed studies are needed to clarify the presice electronic structure of 2.
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(22) Whittaker, M. M.; Whittaker, J. W. Biochemistry 2001, 40, 7140-7148.
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52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment