Structural characterization and oxidation activity of a nickel(II) alkylperoxo complex

Article abstract of DOI:10.1002/anie.200804402

(Figure Presented) Substrate-dependent reactivity: The nickel( II) alkylperoxo complex 1 (see structure), obtained by the dehydrative condensation of the nickel(II) hydroxo complex with tert-butylhydroperoxide has a unique structure; the coordination mode

Full text of DOI:10.1002/anie.200804402

Communications
DOI: 10.1002/anie.200804402
Peroxonickel Complexes
Structural Characterization and Oxidation Activity of a Nickel(II)
Alkylperoxo Complex**
Shiro Hikichi,* Hideho Okuda, Yoshiko Ohzu, and Munetaka Akita
Transition-metal alkylperoxo species (M-OOR’; R’ denotes
alkyl group) are postulated as reactive intermediates in
various oxidation reactions ranging from industrial processes
Compound 1 was synthesized by the dehydrative con-
II
iPr
densation of a nickel(II) hydroxo complex, [(Ni Tp ) (m-
2
[13]
OH) ] (2, Scheme 1), with a stoichiometric amount of tert-
2
[1]
0
to biological systems. The alkylperoxo complexes of d early
transition-metal ions (groups 4–6), in particular, have
attracted much attention, owing to their potential as oxygen-
ating reagents for organic compounds (for example, olefin
[2–5]
epoxidation).
Interest in the alkylperoxo complexes of late
transition metals is also growing because of their industrial
[6–8]
significance and biological relevance.
Nickel is recognized as one of the most useful metals in
promoting various organic transformation reactions, owing its
intrinsic redox properties. Recent developments in the
chemistry of nickel dioxygen complexes have shed light on
the applicability of nickel species as oxidizing agents. For
example, dinuclear nickel(III) bis(m-oxo) compounds exhibit
H-abstraction of aliphatic hydrocarbyl species, in a similar
[
9]
fashion to the M (m-O) species of high-valent Fe and Cu.
Furthermore, several nickel dioxygen complexes
2
2
[
9b,10]
and
peroxide-based catalytic oxidation reactions with nickel
[
11]
complexes have been reported. An nickel(II) alkylperoxo
complex, reported by Suzuki and coworkers, was formed by
O oxygenation of the alkyl radical center on the metal-
2
Scheme 1. Synthesis and reactions of 1. TBHP=tert-butylhydroperox-
ide; Tp =hydrotris(3,5-di-2-propylpyrazolyl)borate.
iPr
supporting ligand, which was itself generated through H-
[
9a,12]
abstraction by the Ni (m-O)2 species.
However, the
butylhydroperoxide (TBHP). Addition of two equivalents of
TBHP at 08C to a pentane solution of 2 resulted in an
immediate change in the color of the solution, from green to
orange, and the appearance of a strong band in the absorption
2
structure and reactivity of nickel alkylperoxo complexes
remains to be studied, to afford an insight into the interaction
between nickel compounds and peroxides, the intrinsic
reactivity of the resulting nickel peroxo species, and the
properties of catalytic intermediates. Herein we report the
molecular structure and the oxidizing activity of a nickel(II)
À1 À1
spectrum at around 370 nm (e = 1400 cm m ). The molec-
ular structure of 1 was revealed by single-crystal X-ray
diffraction (Figure 1). The nickel center of 1 is coordinated by
II
iPr
iPr
iPr
alkylperoxo complex, [Ni (OOtBu)(Tp )] (1; Tp
=
three donor nitrogen atoms from the Tp ligand. The
hydrotris(3,5-di-2-propylpyrazolyl)borate).
[*] Prof. Dr. S. Hikichi, H. Okuda
Department of Material and Life Chemistry, Faculty of Engineering,
Kanagawa University
3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686 (Japan)
Fax: (+81)45-413-9770
E-mail: hikichi@kanagawa-u.ac.jp
Dr. Y. Ohzu, Prof. Dr. M. Akita
Chemical Resources Laboratory, Tokyo Institute of Technology
4259 Nagatsuta, Midori-ku, Yokohama 226-8503 (Japan)
[
**] This work was supported in part by Grant in-Aid for Scientific
Research (Nos. 19020066, 20037063, and 20360367) and a
Scientific Frontier Research Project from the Ministry of Education,
Culture, Sports, Science, and Technology of Japan, and Takahashi
Industrial and Economic Research Foundation.
Supporting Information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804402.
Figure 1. The molecular structure of 1, with thermal ellipsoids set at
50% probability. All hydrogen atoms are omitted for clarity.
1
88
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Angew. Chem. Int. Ed. 2009, 48, 188 –191

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Chemie
nonlinear O1-Ni-B bond angle (160.58) indicates a deviation
from C -symmetrical pseudotetrahedral coordination geom-
etry, which is frequently found for four-coordinate transition
per(II) alkylperoxo complex, a reductive decomposition
I
pathway, that is, CuÀO bond homolysis giving Cu and the
3
v
[
7f,g]
alkylperoxy radical, seems to be dominant.
3
R
[14]
metal complexes with k -Tp ligands.
length (1.440(7) ꢀ) is within the typical range for peroxide
The OÀO bond
[3–8]
OÀO bond lengths.
ligand in 1 is intermediate between h (end-on binding) and h
side-on binding) coordination modes. To our knowledge,
The coordination mode of the OOtBu
1
2
(
2
purely h -alkylperoxo complexes have to date only been
reported for electron-deficient d metal complexes (Ti and
V ). For complex 1 the distance from the nickel center to the
0
IV
V
distal oxygen atom (O2) is relatively short (2.467(7) ꢀ) and
the Ni-O1-O2 bond angle (96.2(4)8) is the smallest reported
Upon the oxidation of external substrates, 1 behaved as
either an electrophile or a nucleophile, depending on the
1
M-O-O bond angle for h -OOR’ complexes. The M-O-O
2
[3a]
bond angles in h -OOtBu complexes of titanium(IV) and
substrates. Like the reported superoxo complexes of nickel-
[5]
[10d,16]
vanadium(V) are 83.2(2)8 and 73.1(2)8, respectively. The
coordinatively unsaturated nickel center, as well as the
nucleophilic character of the alkylperoxide ligand, might be
the cause of the weak interaction between Ni and O2 in 1. To
date, several examples of the same group 10 palladium(II)
and platinum(II) alkylperoxo complexes have been reported.
(II),
1 exhibited weak electrophilic oxidizing properties
toward triphenylphosphine and carbon monoxide, although
not towards dimethylsulfide and olefins. The acceleration in
consumption of 1 in the presence of excess Ph P (k = 1.82 ꢁ
3
obs
À3 À1
10
s
in the presence of 25 equivalents of Ph P in toluene at
3
298 K) indicates that the oxygenation reaction proceeded
through a bimolecular mechanism. Reaction of 1 with CO
resulted in the formation of a dinuclear nickel(II) m-carbo-
II
2
iPr
All of them, including our own [Pd (OOtBu)(k -Tp )(py)],
8
adopt low-spin (d , S = 0) square-planar coordination geo-
[
8a–c]
1
II
iPr
2
2
[13]
metries.
A paramagnetically shifted H NMR spectrum of
nato complex, [(Ni Tp ) (m-k ,k -CO )] (5): A character-
2 3
À1
1
(see the Supporting Information, Figure S1) indicated a
istic IR band at 1578 cm , attributed to n(COO), was shifted
À1
13
high-spin (S = 1) electron configuration in solution, as was
consistent with its nonplanar geometry revealed by single-
crystal X-ray diffraction.
to 1538 cm upon the reaction with CO, which clearly
indicated the oxidation of carbon monoxide. In addition, 1
showed nucleophilic reactivity. Reaction of 1 with a series of
para-substituted benzaldehydes gave the corresponding ben-
We examined the thermal stability of 1. The analogous
II
iPr
II
2
iPr
cobalt(II) derivative, [Co (OOtBu)(Tp )], decomposed
spontaneously even at À788C, to cause the oxygenation of a
proximal isopropyl substituent giving the ligand-oxygenated
zoato complexes, [Ni (k -O CC H -p-X)(Tp )] (6ÀX; X =
2
6
4
OMe, Me, H, Cl), and the pseudo-first order consumption
of 1 (in the presence of 25 equivalents of benzaldehyde) was
accelerated by introduction of an electron-withdrawing sub-
stituent. The linear correlation of the Hammett plot (lnkobs vs.
s) with a positive slope (1 = 4.3; Figure 2) indicates that
oxidation proceeds through a transition state stabilized by the
electron-withdrawing substituent and that the nucleophilic
attack of the peroxo ligand at the carbonyl carbon atom may
[
6b]
dinuclear cobalt(II) bis(m-alkoxo) complex [Eq. (1)].
In
contrast, the nickel(II) complex 1 was relatively thermally
stable in the absence of substrates (Scheme 1), and its half-life
period in toluene at 298 K was approximately 2.5 h (k
=
obs
À5 À1
5
.41 ꢁ 10 s ; also see the Supporting Information, Fig-
ure S2). Analysis of the mass spectrum revealed that the
resulting green solution contained not only 2 but also another
species 3 exhibiting the mass spectral peak at m/z = 1078,
which was tentatively assigned to the nickel analogue of the
H
be involved. In addition, a kinetic isotope effect (KIE) of k /
D
k = 1.7 was evident when C H CDO was used in place of
6
5
C H CHO (see the Supporting Information, Figure S5). The
6
5
[6b,15]
ligand-oxygenated Co species.
This tentative assignment
relatively small KIE value implies that the initial H abstrac-
tion of the formyl group, prior to the nucleophilic attack of
the peroxo ligand at the C=O group, is improbable, although
breaking of the CÀH bond is a somewhat slow process.
1
was consistent with the complicated pattern of the H NMR
spectrum of the product mixture (see the Supporting Infor-
mation, Figure S3), because the three-fold symmetry of the
tris(pyrazolyl)borate ligand moiety of 3 was lost by the partial
oxygenation. Therefore, complex 1, similar to the isostruc-
tural cobalt(II) derivative, exhibits aliphatic CÀH oxidation
We also examined the applicability of 1 as a catalyst for
alkane oxidation. Solvent-free oxidation of cyclohexane with
TBHP in the presence of a catalytic amount of 1 resulted in
the formation of a mixture of cyclohexanol and cyclohex-
anone with low alcohol/ketone (A/K) ratio (approximately
1:2). Such a low A/K ratio suggests the contribution of radical
species in this reaction, which is consistent with the thermal
decomposition pathway of 1 described above.
ability, although its thermal stability is markedly different.
The lower energy of Ni d-orbitals, compared to that for Co,
led to reduced back-donation from the nickel(II) center to the
s* orbital of the peroxide moiety, to induce OÀO bond
activation (presumably OÀO homolysis). When 1 decom-
II
iPr
[13]
posed in CH Cl , a chloride complex, [Ni ClTp ] (4), was
In summary, we have succeeded in the structural charac-
terization of the nickel alkylperoxo complex 1. Crystallog-
raphy reveals the highly distorted geometry of the nickel
center coordinated by the OOtBu ligand, the coordination
2
2
also obtained (approximately 30% yield, determined by UV/
Vis spectra) in addition to 2 and 3. Although the mechanism
for the formation of 4 is not clear, a plausible explanation is
that 1 decomposed through both OÀO and NiÀO homolysis
1
2
mode of which is intermediate between h and h . The
decomposition of 1 might proceed through OÀO and NiÀO
pathways competitively. In the case of the analogous cop-
Angew. Chem. Int. Ed. 2009, 48, 188 –191
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www.angewandte.org
189

Communications
removed by evacuation. The resulting green compound was charac-
terized by the comparison of the spectroscopic data with that of the
[
13]
authenticated carbonato complex 5. An almost quantitative yield
1
of 5 (> 95%) was confirmed by H NMR spectroscopy.
Alkane oxidation catalysis: Compound 1 (26 mmol) was dissolved
in cyclohexane (13 mmol) under an argon atmosphere and 29 mL of a
5.5 m decane solution of TBHP (160 mmol) was added in one portion.
The reaction solution was stirred at 313 K for 2 h. Analysis by gas
chromatography with an internal standard confirmed the generation
of cyclohexanol (15 mmol, 9.4% based on TBHP) and cyclohexanone
(33 mmol, 41%; because two equivalents of TBHP were consumed).
Received: September 6, 2008
Published online: November 28, 2008
Keywords: N ligands · nickel · OÀO activation · oxygenation ·
.
peroxo ligands
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