Characterization of nickel(ii)-acylperoxo species relevant to catalytic alkane hydroxylation by nickel complex with mCPBA

Article abstract of DOI:10.1039/c2dt32419a

Nickel complexes with hydrotris(pyrazolyl)borate (= Tp^R) ligands catalyze alkane oxidation with organic peroxide meta-Cl-C₆H ₄C(O)OOH (= mCPBA). The electronic and steric hindrance properties of Tp^R affect the catalyses. The complex with an electron-withdrawing group containing a less-hindered ligand, that is, Tp^Me2,Br, exhibits higher alcohol selectivity. Higher selectivity for secondary over tertiary alcohols upon oxidation of methylcyclohexane indicates that the oxygen atom transfer reaction proceeds within the coordination sphere of the nickel centers. A reaction of the catalyst precursor, dinuclear nickel(ii)-bis(μ-hydroxo) complexes, with mCPBA yields the corresponding nickel(ii)-acylperoxo species, as have been characterized by spectroscopy. Thermal decomposition of the nickel(ii)-acylperoxo species in CH₂Cl₂ yields the corresponding nickel(ii)-chlorido complexes through Cl atom abstraction. Employment of the brominated ligand increases the thermal stability of the acylperoxo species. Kinetic isotope effects observed on decay of the nickel(ii)-acylperoxo species indicate concerted O-O breaking of the nickel-bound acylperoxide and H-abstraction from the solvent molecule.

Full text of DOI:10.1039/c2dt32419a

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Page 1 of 35
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Characterization of Nickel(II)-Acylperoxo Species Relevant to
View Article Online
Catalytic Alkane Hydroxylation by Nickel Complex with mCPBA
Shiro Hikichi,*a Kento Hanaue,a Takako Fujimura,a Hideho Okuda,a Jun Nakazawa,a
Yoshiko Ohzu,b Chiho Kobayashib and Munetaka Akitab
a 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
b Chemical Resources Laboratory
Tokyo Institute of Technology
4259 Nagatsuta, Midori-ku, Yokohama 226-8503 (Japan)
1

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Page 2 of 35
View Article Online
Abstract
Nickel complexes with hydrotris(pyrazolyl)borate (= TpR) ligands catalyze alkane
oxidation with organic peroxide meta-Cl-C6H4C(=O)OOH (= mCPBA). The
electronic and steric hindrance properties of TpR affect the catalyses. The complex
with an electron-withdrawing group containing a less-hindered ligand, that is TpMe2,Br
,
exhibits higher alcohol selectivity. Higher selectivity for secondary over tertiary
alcohols upon the oxidation of methylcyclohexane indicates that the oxygen atom
transfer reaction proceeds within the coordination sphere of the nickel centers.
Reaction of the catalyst precursor, dinuclear nickel(II)-bis(μ-hydroxo) complexes, with
mCPBA yields the corresponding nickel(II)-acylperoxo species, as have been
characterized by spectroscopy. Thermal decomposition of the nickel(II)-acylperoxo
species in CH2Cl2 yields the corresponding nickel(II)-chlorido complexes through Cl
atom abstraction. Employment of the brominated ligand increases the thermal stability
of the acylperoxo species. Kinetic isotope effects observed on the decay of the
nickel(II)-acylperoxo species indicate concerted O–O breaking of the nickel-bound
acylperoxide and H-abstraction from the solvent molecule.
2

Page 3 of 35
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Introduction
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Selective partial oxidation of alkanes to the corresponding alcohols is an important
fundamental chemical transformation.1 In order to achieve high alcohol selectivity,
free radical-contributed reaction will give a mixture of alcohol and the corresponding
over-oxidized product (i.e. ketone or aldehyde) through the Russel termination
mechanism.2,3 It therefore remains important to design catalysts that provide a
metal-based oxidant showing alkane hydroxylation activity. The most considerable
metal-based oxidants are high-valent metal-oxo (or metal-oxyl radical) species.
Metal-peroxo species, LM-OO-Z (where L denotes a metal supporting ligand, Z = none,
H, alkyl, acyl, metal), are also possible candidates for the metal-based oxidants.
Moreover, these species are promising precursors for the metal-oxo species produced by
O–O bond activation under mild conditions.4
Recently, some experimental5–9 and theoretical10,11 investigations have revealed
the alkane hydroxylation potential of active oxygen complexes of nickel. One of the
interesting findings is the selective hydroxylation of cyclohexane catalyzed by nickel(II)
complexes with mCPBA explored by Itoh and coworkers.5a-c
In addition,
Balamurugan et al. have reported similar catalytic alkane hydroxylation by a series of
nickel(II) complexes with mCPBA.5d The correlation between the structures of the
metal-supporting ligands (shown in Figure 1) and the catalytic activities of the resulting
nickel(II) complexes has been revealed. During the catalytic process, homolytic O–O
bond rupture of a putative nickel(II)-acylperoxo intermediate occurs, as has been
evidenced by the formation of chlorobenzene from mCPBA. Although no reaction
intermediates have been detected in these catalytic systems, a related mononuclear
nickel(III)-oxygen (oxo and hydroxo) species that is formed through the reaction of the
Ni(II) precursor with mCPBA has very recently been characterized.6
In this study, hydrotris(3,5-dialkyl-4-X-pyrazolyl)borates, TpR2,X where R = Me or
iPr and X = H or Br (when X = H, H is omitted in the abbreviation shown as TpR2; the
abbreviation system follows that in the literatures12: see Fig. 1), have been employed as
3

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Page 4 of 35
the nickel-supporting scaffold. The advantage of TpR2,X is structural and electroVniiecw Article Online
controllability.12 In particular, substituent groups on the distal fourth position of the
pyrazolyl groups affect the electronic nature of the resulting TpR2,X complex, although
the structural environment around the metal center is almost same as that of the
prototype compound where X = H. Such electronic tuning leads to control of the
stability and reactivity of the nickel(II)-alkylperoxo species.13 This strategy is
expected to provide insight into the catalytic alkane hydroxylation mechanism of the
nickel(II)/mCPBA system. As reported herein, we have explored the catalyses of the
TpR2,X supported nickel(II) complexes toward cyclohexane oxygenation with mCPBA.
Notably, we have succeeded in detecting thermally unstable nickel(II)-acylperoxo
species formed by reaction of the dinuclear nickel(II)-bis(μ-hydroxo) complexes,
[(NiIITpR2,X)2(μ–OH)2] (1X where R = Me and 1’X where R = iPr, respectively),8e,13,14
with mCPBA. Products analyses and kinetic studies of the thermal decomposition
process of the nickel(II)-acylperoxo species provide insights into the alkane
hydroxylation mechanism through the O–O activation process.
[Fig. 1]
Results and Discussion
Correlation between catalytic activity and the nature of the metal-supporting
ligands
Prior to characterization of a putative nickel(II)-acylperoxo intermediate, trends in
cyclohexane oxygenation catalyses of the TpR2,X-supported nickel complexes were
explored (Table 1). The catalysts derived from 1X, of which the metal-supporting
ligands were the less hindered TpMe2,X, yielded the alcohol as the major product. The
more hindered TpiPr2,X ligand analogues 1’X did not exhibit such catalytic activity even
in the large excess substrate reaction. In contrast, the alkylperoxonickel(II) complex
[NiII(OOtBu)TpiPr2
]
can catalyze the oxygenation of cyclohexane with
4

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tert-butylhydrogenperoxide, but the major product is cyclohexanone because the
View Article Online
free-radical reaction occurs predominantly.7 As reported previously by Nam and
coworkers, a simple nickel(II) salt (= an MeCN solution of Ni(ClO4)2) cannot catalyze
cyclohexane oxidation with mCPBA.15 Therefore, the catalytic hydroxylation activity
of nickel emerges from employment of the appropriate ligands controlling the
generation of a metal-based oxidant. In the case of the tris(pyridylmethyl)amine (=
TPA) ligand system, a cobalt catalyst exhibits selective hydroxylation activity similar to
that of the nickel catalyst.5a Moreover, a cobalt-porphyrinato complex as well as
cobalt-perchlolate salt catalyze cyclohexane hydroxylation with mCPBA.15,16 In
contrast, the cobalt(II) analogues of 1X were almost inactive in the examined reaction
conditions. These findings imply that the combination of a central metal ion and its
supporting ligand is dominant.
[Table 1]
The electron-withdrawing bromine-containing TpMe2,Br ligand catalyst 1Br showed
a higher alcohol yield than that of the non-brominated ligand catalyst 1H when the
reaction proceeded in CF3C6H5 (Figure S4). In the reaction of the mixture of large
excess substrate and a small amount of CH2Cl2 as solvent for mCPBA, the extremely
high A/K value (= 53) was achieved by 1Br. In this reaction condition, TON of 1Br
reached 42 (theoretical maximum is 50). The less electron-withdrawing ligand
catalyst 1H exhibited somewhat higher TON (= 46) but lower A/K (= 32) (Fig. 2).
[Fig. 2]
Formation of chlorobenzene (= C6H5Cl) indicated that homolytic O–O bond
cleavage of a putative nickel(II)-acylperoxo intermediate occurred in the present
oxidation process.5b,d In the reactions of large excess substrate (entries 3 and 4 in
Table 1), 57 μmol of C6H5Cl formed when 1Br was used as the catalyst precursor, while
84 μmol of C6H5Cl formed when 1H used. The yield of C6H5Cl seems to be correlated
with the selectivity of the oxygenated products; a small amount of radical species such
5

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Page 6 of 35
as a phenyl radical (generated by decarboxylation of acyloxy radical) might work as a
View Article Online
mediator for a free-radical contributed reaction giving a ketone.
When methylcyclohexane was employed as a substrate, 1X/mCPBA systems
showed high secondary alcohol selectivity (Table 2). In the absence of 1X, i.e. control
reactions (entries 3 and 4 in Table2), the tertiary C–H bond was oxidized selectively.
In the case of substituted cyclohexanes, the tertiary C–H is sterically crowded.
Therefore, the bulkiness of the oxidant affects the products selectivity17, and selective
hydroxylation on secondary C–H bonds results from sterically demanding
TpMe2,X–supported nickel-based oxidant.
[Table 2]
To clarify the nature of the active oxygen species, cyclohexane oxidation in the
18
presence of H2 O was examined. No 18O-labeled cyclohexanol could be detected,
18
whereas an O-labeled cyclohexanone was obtained (Fig. S5). These findings imply
that the 18O-labeled ketone is not formed from the corresponding alcohol by its
over-oxidation, and that the formation pathways of alcohol and ketone are different in
our TpR2,X-supported nickel-mCPBA system as well as the previously reported nickel
complexes with the amine-based neutral ligands.5 In addition, the oxygen atom of the
alkane hydroxylating active species of our system (i.e. nickel-oxygen species) is not
exchangeable with the oxygen atom of the water molecule. Incorporation of the
oxygen atom of H2O into cyclohexanol occurs in the cyclohexane oxidation with
mCPBA catalyzed by iron- and cobalt catalysts (porphyrinato complexes of iron and
cobalt, and CoII(ClO4)2) reported by Nam and coworkers.15,16,18 In these iron and
cobalt systems, high-valent metal-oxo species are proposed as the active oxidant and
their oxo ligands are exchangeable. Such a difference between our nickel and the
other systems may be due to the reaction mechanism and the character of the active
species. Possible explanations for our nickel system are, (i) the active oxidant is a
putative nickel-oxygen species (such as NiIII=O or NiII–O•)5b,d,6 and its oxygen atom is
readily transferred to the substrate via hydrogen atom abstraction and radical-rebound
6

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mechanism, and (ii) a nickel(II)-mCPBA complex is the active oxidant and the reactVioienw Article Online
occurs via a concerted mechanism through concomitant O–O rupture and oxygen atom
transfer. In the iron-heme system reported by Nam and coworkers, the extent of the
18O incorporation into the product is varied depending on the nature of the catalysts as
well as the reaction conditions, including the choice of solvent.18
Characterization of nickel(II)-acylperoxo species 2X
The alkane hydroxylation catalyses of 1X motivate us to characterize a putative
nickel-acylperoxo species that may be a key intermediate formed during the mCPBA
activation process. Titration of the reaction of 1H with mCPBA at 223 K monitored by
UV/Vis spectra revealed that increase in the absorption band around 386 nm became
saturated when two equivalents of mCPBA were added to the CH2Cl2 solution of
dinuclear nickel(II)-bis(μ-hydroxo) complex 1H (Fig. 3 and Table 3). The intensity of
this band is not very high, and the spectral pattern of this pale blue solution is clearly
different from those of the dinuclear nickel(III)-bis(μ-oxo) and mononuclear
nickel(II)-alkylperoxo complexes with TpR.7,8e,13
[Fig. 3]
[Table 3]
An IR spectrum of this solution exhibited a peak at 1644 cm–1 under a
low-temperature condition, and this peak disappeared in response to increases in the
solution temperature (Fig. 4). Therefore, the thermally unstable pale blue species
could
be
assigned
as
the
nickel(II)-acylperoxo
complex,
[NiII(OOC(=O)C6H4Cl)(TpMe2)] (2H), formed through dehydrative condensation of 1H
with a stoichiometric amount of mCPBA (i.e., Ni(II):mCPBA = 1:1).19 The νC=O
bands of free mCPBA and mCBA are observed at 1738 and 1706 cm–1, respectively,
under the same condition (i.e. CH2Cl2 solution at 223 K). To date, a few
meta-chloroperbenzoate complexes have been characterized, and the wavenumbers of
their νC=O bands (observed in the solid state) are correlated to the binding mode of the
7

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acylperoxo moieties, as summarized in Table 4.20–22
As found for the
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structure-determined dinuclear copper(II) complex, the non-coordinating acyl group
shows a small blue shift of the νC=O compared to that of the free mCPBA.21 The
observed νC=O peak at 1644 cm–1 of 2H suggests coordination of the acyl group to the
nickel center (see Scheme 1), and the molecular structure of 2H is probably similar to
that of the copper(II) derivative of TpiPr2 reported by Kitajima and coworkers.22
[Fig. 4]
[Table 4]
The other nickel(II)-hydroxo complexes, not only the catalytically active TpMe2,Br
but also the inactive TpiPr2,X derivatives, gave the corresponding acylperoxo complexes
2Br and 2’X, respectively. As summarized in Tables 3 and 4, spectral patterns of
UV/Vis and wavenumbers of νC=O of 2Br and 2’X are similar to those of 2H. These
findings indicate that the reason for the catalytic inertness of the TpiPr2,X complexes is
the steric hindrance between the substrate and the isopropyl groups surrounding the
nickel center. Three isopropyl groups on the third position of the pyrazolyl rings in
TpiPr2,X work as hindered shades surrounding the nickel center, which stabilizes the
nickel(II)-acylperoxo species. In fact, the most stable complex 2’Br could be isolated,
although the C–H hydroxylation potential was retained, as evidenced by the
intra-molecular ligand oxygenation (vide infra). Unfortunately, our challenge to get
single crystals of 2’Br (and other ligands derivatives) has not met with success thus far.
Characterization of product complexes derived from thermal decomposition of 2X
and 2’X
The nickel(II)-acylperoxo species 2X and 2’X decomposed even at 253 K, and the
resulting product nickel(II) complexes were varied depending on conditions such as
solvent and temperature as well as R of TpR2,X (Scheme 1).
[Scheme 1]
(i) Products from the TpMe2,X complexes 2X. In the case of the CH2Cl2 solution
8

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of 2X having the less hindered TpMe2,X, raising the solution temperature from 223 to 253
View Article Online
K resulted in changing the solution color from pale blue-green to pale brown. The
resultant pale brown solutions exhibited absorption around 480 nm attributed to the
Cl-to-Ni charge transfer band of nickel(II)-chloride complexes, [NiII(Cl)(TpMe2,X)] (3X;
see Fig. 5(a): the formed complexes 3X were characterized by comparison to authentic
samples synthesized via another method).23 The yields of 3X were over 85 %
(determined by UV/Vis) when the CH2Cl2 solutions of 2X were stored at 253 K for 12 h
and then slowly warmed to room temperature. As we have reported previously,
thermal decomposition of the nickel(II)-tert-butylperoxo complexes with TpiPr2,X in
CH2Cl2 yields the corresponding chloride complexes [NiII(Cl)(TpiPr2,X)] (3’X) with
moderate yield (ca 30 %).7,13 The formation of 3’X may proceed through abstraction of
the Cl atom from CH2Cl2 by a putative nickel(I) species given by the Ni–O bond
homolysis of the alkylperoxo complexes.
The yielding of 3X from the
nickel(II)-acylperoxo complexes 2X suggests the formation of a nickel(I) species
through the thermal decomposition process. Itoh and coworkers have proposed
formation of the nickel(I) species after the hydroxylation of alkane in the catalytic
processes.5b In this context, we can hypothesize that nickel(I) species with TpMe2,X are
formed after the oxidation of CH2Cl2. The formation rates of 3X (3.2×10−4 for 3H and
5.2×10−5 s−1 for 3Br, respectively, at 253 K) were lower than the decay rates of the
corresponding 2X (vide infra). Therefore, the conversion from 2X to 3X might proceed
by a stepwise process through the decomposition of 2X and following reaction of the
resulting nickel(I) species with CH2Cl2 to give 3X, although we have not succeeded in
trapping any intermediates so far.
Other products in the decomposition reactions in CH2Cl2 were
nickel(II)-meta-chlorobenzoate complexes, [NiII(O2CC6H4Cl)(TpMe2,X)] (4X), having
characteristic absorption bands around 420–430 nm (see experimental section).
Decomposition of 2X in CH2Cl2 at 273 K led to an increase in the yield of 4X with
decreases in 3X, as shown in Fig. 5(b). Therefore, the formation of 3X and 4X occurred
9

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competitively.
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[Fig. 5]
Decomposition of 2X in toluene yielded 4X, with yields of 4H and 4Br of 73 and
77 % (determined by UV/Vis), respectively. A possible explanation for the formation
of 4X is coupling of the nickel(I) species with acyloxy radical resulting from homolysis
of the O–O bond of 2X (see Scheme 2). Mass spectral analyses revealed no formation
of oxidized TpMe2,X ligands compounds in the case of products derived from either
CH2Cl2 or toluene solutions. However, decomposition of 2Br in CD2Cl2 resulted in the
oxidation of TpMe2,Br. This result implies that there are competitive reactions between
the solvent and the methyl group on TpMe2,Br
.
(ii) Ligand hydroxylation on TpiPr2,X complexes 2’X. In the case of more
hindered TpiPr2,X systems, the solvent type did not affect the products. UV-vis spectra of
the products derived from both CH2Cl2 and Et2O solutions of 2’X exhibited peaks
around 430 nm, and these spectral patterns were the same as those of the TpiPr2,X
analogues of 4X, [NiII(O2CC6H4Cl)(TpiPr2,X)] (4’X). In mass spectra of the product
derived from CH2Cl2 solution, no peak of the chloride complexes 3’X existed, whereas
peaks consistent with the formula as [4’X + O] (m/z = 933) were observed. 1H NMR
spectra of the product mixture showed the existence of 4X and another species, of which
the three-fold symmetry of the tris(pyrazolyl)borate ligand moiety was lost. Moreover,
1
the H NMR spectrum derived from 2’Br contained a signal at 16.5 ppm, of which the
proton was exchangeable with external D2O. These spectral features suggest the
progress of partial oxygenation of the methine portion of the isopropyl groups of TpiPr2,X
proximal to the nickel center, and the resultants were tentatively assigned as
nickel(II)-carboxylate
complexes
with
hydroxylated
TpiPr2,X
ligands,
[NiII(O2CC6H4Cl){HB(3–Me2C(OH)–5–iPrpz)(3,5–iPr2pz)2}] (5’X; see Scheme 1).
Such intra-molecular ligand oxygenations have been observed in various transition
metal-active oxygen complexes such as M2(μ–O)2 and M–OOR, including the nickel-
and cobalt-TpiPr2 complexes.7,8d,8e,13,24
Interestingly, the selectivity for the
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ligand-hydroxylated compounds 5’X seems to be correlated with the nature of XVoienw Article Online
TpiPr2,X. The products ratio 5’X:4’X were 7:1 from 2’Br and 1:2 from 2’H, respectively
1
(determined by H NMR). Although the hindered TpiPr2,X complexes cannot catalyze
cyclohexane hydroxylation, the higher selectivity toward the intra-molecular alkyl
group hydroxylation on 2’Br is consistent with the alcohol selectivity of the TpMe2,Br
ligand catalyst being higher than that of the non-brominated TpMe2 ligand catalyst.
Kinetics of the thermal decomposition process of 2X
In the absence of external substrate, both 2X and 2’X decomposed spontaneously in
accord with first-order kinetics. The bromine-containing ligand complexes are more
stable than the parent non-brominated complex. Also, 2’Br is more stable than 2Br
(Table 5). These orders illustrate that how the electronic and steric natures of the
ligands affect the lifetime of the acylperoxo species and the substrate-oxidizing
activities. The bulky alkyl groups (i.e. 3-iPr on TpiPr2,X) wrap the O–O moiety of the
metal-bound acylperoxide to stabilize 2’X but hinder the reaction with external substrate.
The smaller methyl substituents on TpMe2,X are an appropriate size to provide
substrates-accessible space allowing for the catalyses of 2X. As well as the previously
reported nickel(II)-alkylperoxo complexes with TpiPr2,X 13
incorporation of the
,
electron-withdrawing bromine at the distal fourth position of the pyrazolyl groups in
TpR2,X results in increases in the stability of the nickel(II)-acylperoxo species.
[Table 5]
Interestingly, kinetic isotope effects were observed during the decay of 2X in
CH2Cl2/CD2Cl2. The observed kH/kD values at 253 K were 2.5 for 2H and 3.2 for 2Br,
respectively. Interestingly, the yields of the chloride complexes 3X decreased when the
deuterated solvent was used (3H: 51 % in CH2Cl2 → 12 % in CD2Cl2 at –20°C, 3Br:
36 % in CH2Cl2 → 7 % in CD2Cl2 at 0°C). Moreover, the decomposition of 2Br in
CD2Cl2 led to oxygenation of the methyl group of TpMe2,Br (Fig. S9). These findings
indicate an interaction between the solvent molecule and 2X, and hydrogen atom
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X 25
abstraction from the solvent may occur concomitant with O–O bond rupture of 2 .
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The large negative values of an activation entropy for the self decomposition of 2X in
CH2Cl2 (Fig. S11 and Table S5) indicate the reaction proceeds through an associative
transition state, and that is consistent with the interaction between the solvent molecule
and 2X. The observed KIE values are, however, not so large even at low temperatures.
Upon the decomposition of 2Br (2.0 mM) in the mixture (v/v = 1/1) of CD2Cl2 and
C6H12 or C6D12 at 273 K, the observed KIE was 1.6 (k(CD2Cl2/C6H12) and
k(CD2Cl2/C6D12) were 5.2×10−4 s−1 and 3.3×10−4 s−1, respectively). However, high
concentration of cyclohexane (v/v = 1/20 of CD2Cl2 and C6H12 or C6D12) led to
negligible KIE value (decomposition rate of 2 mM of 2Br at 283 K were
k(CD2Cl2/C6H12) = 2.8×10−3 s−1 and k(CD2Cl2/C6D12) = 2.5×10−3 s−1). Therefore, 2X
themselves have the ability of hydrogen atom abstraction from aliphatic C–H group,26
but a putative nickel–oxygen species such as NiIII=O or NiII–O• resulting from the O–O
homolysis of 2X may work as a major oxidant on catalytic cyclohexane hydroxylation
(Scheme 2). Recently, the hydrogen atom abstraction ability of the mononuclear
nickel(III)-oxo species, which is generated from the reaction of [NiII(OTf)(TMG3tren)]+
(TMG3tren denotes tris[2-(N-tetramethylguanidyl)ethyl]amine) with 1 equiv. of mCPBA,
has been reported.6 In the TpR2,XNi systems, however, we could not identify any
intermediates during decomposition of the nickel(II)-acylperoxo species 2X by
spectroscopy (UV/Vis and EPR), and there is no direct evidence for the contribution of
the nickel(III)-oxo and related species.27
[Scheme 2]
Conclusion
The nickel complexes with TpMe2,X exhibit alkane hydroxylation catalyses with mCPBA
oxidant. Introduction of electron-withdrawing group (= EWG) on the pyrazolyl
backbone of TpMe2,X leads to increases in the alcohol selectivity. Effectiveness of the
introduction of EWG on the metal supporting ligand for the selective hydroxylation of
12

Page 13 of 35
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alkanes has been demonstrated on iron-heme compounds.28 An electrophilicity of Vthieew Article Online
active oxidant may be enhanced by EWG-containing ligand, and our results indicate
that the common concept for the ligand design, that is the fine tuning of the electronic
property without changing the structure of the metal-supporting scaffold, is applicable
to non-heme catalysts. The thermal stability of the nickel(II)-mCPBA species is also
enhanced by the EWG-containing ligands TpR2,Br, which might weaken the ability of
back-donation from the metal center to the σ* orbital of the peroxide moiety.13,29
Kinetic and product analyses of the decomposition of the nickel(II)-mCPBA complexes
suggest that the nickel(II)-acylperoxo complex, NiII–OOC(=O)C6H4-meta-Cl, is an
alternative oxidant.
However, hydrogen atom abstracting potential of
nickel(II)-mCPBA complexes with TpMe2,Br is not so high and the previously proposed
O–O bond cleaved species (i.e. NiII–O• or NiIII=O) may be a major active oxidant for
the catalytic cyclohexane oxygenation.
Experimental Section
General
All manipulations were performed under argon by standard Schlenk techniques. THF,
Et2O, pentane, toluene, CH2Cl2, MeCN were purified over a Glass Contour Solvent
Dispending System under Ar atmosphere. CF3C6H5 was distilled using sodium as a
drying agent and then stored under argon. meta-Chloroperbenzoicacid (= mCPBA)
was washed with KH2PO4-NaOH buffer solution (pH 7.4) and pure water in order to
remove meta-chlorobenzoic acid. Other reagents of the highest grade commercially
available were used without further purification.
The catalyst precursors
[(NiIITpR2,X)2(μ–OH)2] (1X (R = Me) and 1’X (R = iPr); X = H, Br) and their starting
materials were prepared by the methods described in the literature.8e,13,14,30 Elemental
analyses were performed on a Perkin-Elmer CHNS/O Analyzer 2400II.
IR
measurements of KBr pellets of solid compounds were carried out by using JASCO
FT/IR-5300 or FT/IR 4200 spectrometers. IR spectra of the in situ-generated solution
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Page 14 of 35
samples were recorded using a Mettler Toledo ReactIR iC10. NMR spectra wVeireew Article Online
recorded on Bruker AC-200 (1H, 200.0 MHz) or JEOL ECA-500 (1H, 500.0 MHz)
spectrometers. Chemical shifts (δ) were reported in ppm downfield from internal
SiMe4. UV/Vis spectra were recorded on JASCO V-570, V650 or Agilent 8453
spectrometers equipped with a UNISOKU CoolSpeK USP-203-A for low-temperature
measurements. Mass spectra were measured on a JEOL JMS-700 by a field desorption
(FD) ionization method or a JEOL JMS-T100LC by an electrospray ionization (ESI)
method. Gas chromatographic (GC) analyses were carried out on a Shimadzu
GC-2010 instrument with a flame ionization detector equipped with a RESTEK
Rtx-1701 (30 m, 0.25 mmID, 0.25 μmdf) capillary column. GC-MS analyses were
carried out on a Shimadzu PARVUM2 system equipped with a RESTEK Rtx-5MS (30
m, 0.25 mmID, 0.25 μmdf) capillary column.
Catalytic cyclohexane oxidation with mCPBA
All reactions were carried at 313 K under Ar, and the products were analyzed by GC
and GC-MS with an internal standard. Cyclohexane (0.28 mL, 2.6 mmol) was added
to a 5.2 mM CF3C6H5 solution of 1X or 1’X (5.0 mL, 26 μmol). Next, 45 mg of
mCPBA (260 μmol) was added to this solution with stirring. The reactions with large
excess substrate were carried out as follows.
Due to the insolubility of the catalyst
precursors (1X and 1’H) and mCPBA toward cyclohexane, a minimal amount of CH2Cl2
was used as solvent: 1.0 mL of a 2.6 mM CH2Cl2 solution of 1X or 1’H (2.6 μmol) was
added to 1.4 mL of cyclohexane (13 mmol) under Ar. Then 45 mg of mCPBA (260
μmol) was added.
Synthesis of the nickel(II)-mCPBA complexes
[NiII(OOC(=O)C6H4Cl)(TpiPr2,Br)] (2’Br). The synthetic procedure for the
isolable TpiPr2,Br complex 2’Br is described as a typical example.
The hydroxo
complex 1’Br (250 mg, 0.16 mmol) was dissolved in Et2O (15 mL), and the solution was
14

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Dalton Transactions
cooled at 233 K. Then 5 mL of an Et O solution of mCPBA (58.2 mg, 0.33 mmol) was
View Article Online
2
added to the cold solution of 2’Br. The resulting pale blue solution was stirred for 10
min at 233 K, and the solvent was then concentrated by evaporation while maintaining a
low temperature. Refrigeration of the concentrated solution at 195 K yielded the pale
blue powder of 2’Br. Almost quantitative oxygenation of Ph3P to Ph3P=O (97 % yield
31
based on 2’Br analyzed by P NMR) in Et2O at 293 K (7 equiv. of Ph3P was applied)
indicated the isolated blue powder compound was a pure acylperoxo complex. UV/Vis
(Et2O. 233 K): λ(ε)=386 (151), 654 nm (50); IR (KBr): ν=2586 (BH), 1637 cm−1 (CO).
The other ligand complexes 2X and 2’H could be generated by similar procedures,
and spectroscopic characterization by UV/Vis and IR were performed without isolation.
Products analyses of the spontaneous decomposition of the nickel(II)-mCPBA
complexes
The product complexes derived from the spontaneous decomposition of 2X and 2’X were
characterized by comparison with the UV-vis spectral data of the nickel(II)-chloride and
meta-chlorobenzoate complexes 3X, 3’X, 4X, and 4’X. Except for the previously
reported chlorido complexes with TpMe2 (3H),23 TpiPr2 (3’H),14 and TpiPr2,Br (3’Br),13 the
authentic complexes were synthesized as follows.
[NiII(Cl)(TpMe2,Br)] (3Br). THF solution (20 mL) of NaTpMe2,Br (1.00 g, 1.80
mmol) was slowly added to a methanol solution (20 mL) of NiCl2·6H2O (0.854g 3.6
mmol) over 30 min at RT. The green solution was stirred for 30 min, and the solvent
was then evaporated. The green residue was re-dissolved into 100 mL of CH2Cl2, and
the insoluble green powder was removed by filtration through a celite pad, after which a
reddish filtrate was evaporated to dryness to give a green solid. The solid was
re-dissolved into 100 mL of MeCN, and the insoluble purple residue was removed by
filtration through a celite pad to give a bluish-green filtrate. Evaporation of the solution
gave a green solid. Crystallization from toluene gave the title complex as a green
platelet crystal (0.498 g, 0.74 mmol, 41%). UV/Vis (CH2Cl2): λ(ε)=481 (360), 801
15

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Page 16 of 35
1
(105), 900 nm (120); IR (KBr): ν=2532 cm−1 (BH); H NMR (500 MHz, CD Cl , r.t.):
View Article Online
2
2
δ=−13.16 (2H, BH), −8.52 (18H, Me), 3.74 ppm (18H, Me); elemental analysis: calcd
(%) for C18.5H23N6BBr3ClNi (3Br·1/2toluene): C 32.96, H 3.44, N 12.47; found: C 33.48,
H 3.23, N 12.65. The molecular structure was determined by X-ray crystallography,
and the details are provided as Supporting information.
[NiII(O2CC6H4Cl)(TpR2,Br)] (4X and 4’X). As a typical example, the synthetic
procedure for [NiII(O2CC6H4Cl)(TpMe2,Br)] (4Br) is described. The hydroxo complex
1Br (122 mg, 0.10 mmol) and meta-chlorobenzoic acid (mCBA, 34.4 mg, 0.22 mmol)
were dissolved in 10 mL of CH2Cl2. The green solution was stirred for 30 min at
ambient temperature, and the solvent was then evaporated. The green residue was
recrystallized from slow evaporation of EtOH to give an ethanol adduct of 4Br,
[NiII(O2CC6H4Cl)(TpMe2,Br)(C2H5OH)2], as green block crystal (127 mg, 0.15 mmol,
75 % yield). UV/Vis (CH2Cl2): λ(ε)=421 (210), 682 (47), 843 nm (39); (toluene):
λ(ε)=421 (180), 685 (38), 851 nm (31); IR (KBr): ν=2544 (BH), 1591 cm−1 (COO); 1H
NMR (500 MHz, CD2Cl2, r.t.): δ=−10.51 (1H, BH), −9.45 (9H, Me), 0.62 (9H, Me),
2.36 (6H; EtOH), 4.28 (4H; EtOH) 6.06 (1H; mCBA), 9.39 (1H; mCBA), 11.04 (1H;
mCBA), 11.47 ppm (1H; mCBA); elemental analysis: calcd (%) for
C26H35BBr3ClN6NiO4 (4Br·2EtOH): C 37.16, H 4.20, N 10.00; found: C 37.25, H 3.98,
N 10.11.
The other nickel(II)-mCBA complexes were synthesized by the same procedure.
[NiII(O2CC6H4Cl)(TpMe2)] (4H). The TpMe2 complex 4H is synthesized by the
reaction of 1H (111 mg, 0.15 mmol) and mCBA (51.6 mg, 0.33 mmol) in THF.
Recrystallization from MeCN yielded the yellow-green solid of 4H (60.1 mg, 0.12 mmol,
40 % yield). UV/Vis (toluene): λ(ε)=428 (213), 686 nm (61); IR (KBr): ν=2514 (BH),
1591 cm−1 (COO); elemental analysis: calcd(%) for C22H26N6BClNiO2 (4H): C 51.67, H
5.12, N 16.43; found C 51.20, H 4.95, N 16.47.
[NiII(O2CC6H4Cl)(TpiPr2)] (4’H). The hydroxonickel complex with TpiPr2 1’H
(75.6 mg, 0.0698 mmol) and mCBA (21.8 mg, 0.139 mmol) were stirred in 15 mL of
16

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Dalton Transactions
Et O for 10 min at room temperature. After removal of the volatiles under vacuum,
View Article Online
2
the resulting yellow-green solid of 4’H was recrystallized from pentane (76.5 mg, 0.113
mmol, 81% yield). UV/Vis (CH2Cl2): λ(ε)=429 (250), 690 (59), 854 nm (47); IR
(KBr): ν=2549 cm−1 (BH); 1H NMR (200 MHz, C6D6, r.t.): δ=−3.6 (3H, br, CH of iPr),
0.07 (18H, s, Me of iPr), 2.61 (21H, s, Me and CH of iPr), 4.80, 9.04, 11.1, 11.5 (1H×4,
s×4, Ph of mCBA), 75.4 ppm (3H, s, pz-4H); MS (FD): m/z=679 ([4’H]+); elemental
analysis: calcd (%) for C34H50N6BClNiO2 (4’H): C 60.08, H 7.41, N 12.36; found: C
59.86, H 7.43, N 12.43.
[NiII(O2CC6H4Cl)(TpiPr2,Br)] (4’Br). Recrystallization of the reaction mixture of
1’Br (352 mg, 0.226 mmol) and two equiv. of mCBA (70.8 mg, 0.452 mmol) from
hexane yielded the yellow-green powder of the TpiPr2,Br analogue 4’Br (217 mg, 0.120
mmol, 53% yield). UV/Vis (Et2O): λ(ε)=416 (240), 694 nm (52); IR (KBr): ν=2578
1
cm−1 (BH); H NMR (200 MHz, C6D6, r.t.): δ=−11.1 (1H, BH), −0.75 (br, CH of iPr),
1.78 (18H, s, Me of iPr), 2.12 (21H, br, Me and CH of iPr), 5.10, 9.10 (1H×2, s×2, Ph of
mCBA), 13.1 ppm (2H, br, Ph of mCBA); MS (FD): m/z=917 ([4’Br]+); elemental
analysis: calcd (%) for C34H47N6BBr3ClNiO2 (4’Br): C 44.56, H 5.17, N 9.17; found: C
44.85, H 5.40, N 8.89.
The molecular structures of an EtOH adduct of 4Br and 4’H were determined by
X-ray crystallography, and the details are provided as Supporting information.
Acknowledgements
This work was supported in part by a Grant in-Aid for Scientific Research (Nos.
20037063 and 20360367), a Cooperative Research Program of “Network Joint Research
Center for Materials and Devices (Nos. 2011169 and 2012216)”, a Scientific Frontier
Research Project, and a Strategic Development of Research Infrastructure for Private
Universities from the Ministry of Education, Culture, Sports, Science, and Technology
(MEXT), Japan. S. H. and J. N. also thank Kanagawa University for financial support.
17

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Page 18 of 35
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Alkane hydroxylation with mCPBA catalyzed by nickel complexes with
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F. F. Pfaff, F. Heims, S. Kundu, S. Mebs and K. Ray, Chem. Commun., 2012, 48,
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Hikichi, M. Yoshizawa, Y. Sasakura, H. Komatsuzaki, Y. Moro-oka and M. Akita,
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Roh, H. Furutachi, T. Tosha, M. Kubo, S. Fujinami, T. Ogura, T. Kitagawa and
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D. Schröder and H. Schwarz, Angew. Chem. Int. Ed. Engl., 1995, 34, 1973 –
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Y. Shiota and K. Yoshizawa, J. Am. Chem. Soc., 2000, 122, 12317 – 12326.
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Pettinari, Scorpionates II: Chelating Borate Ligans, Imperial College Press,
London, 2008.
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S. Hikichi, C. Kobayashi, M. Yoshizawa and M. Akita, Chem.–Asian J., 2010V,i5ew, Article Online
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W. Nam, I. Kim, Y. Kim and C. Kim, Chem. Commun., 2001, 1262 – 1263.
K. Kamata, K. Yonehara, Y. Nakagawa, K. Uehara and N. Mizuno, Nature
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(a) W. Nam, M. H. Lim, S. K. Moon and C. Kim, J. Am. Chem. Soc., 2000, 122,
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Nam, Chem. Commun., 2008, 1076 – 1078.
If the nickel(II) centers of 1X was oxidized to nickel(III) by mCPBA at initial
step and following ligand exchange reaction led to nickel-acylperoxo complexes,
total ratio of Ni to mCPBA would be 1:1.5 or more as found for the non-heme
iron(II) complex. K. Ray, S. M. Lee and L. Que, Jr., Inorg. Chim. Acta, 2008,
361, 1066 – 1069.
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J. T. Groves and Y. Watanabe, Inorg. Chem., 1987, 26, 785 – 786.
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N. Kitajima, K. Fujisawa and Y. Moro-oka, Inorg. Chem., 1990, 29, 357 – 358.
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S. Hikichi, H. Komatsuzaki, M. Akita and Y. Moro-oka, J. Am. Chem. Soc., 1998,
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On the basis of the observation of KIE upon the thermal decomposition of the
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20

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Yamaguchi, A. Kunishita, H. Sugimoto, N. Fujieda, T. Ogura, C. J. Cramer and
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26
27
In the heme-Fe/mCPBA system, both (por)FeIII-acylperoxo and (por•)FeIV=O
(por• denotes 1e– oxidized porphyrinate ligand) are proposed as active oxidants
for alkane hydroxylation. See ref. 18(a).
Existence of nickel(IV)-oxo species, which would generate via O–O heterolysis
of the nickel(II)-acylperoxo species or intramolecular electron transfer from the
nickel center of the to weakly binding carboxyl radical in the nickel(III)-oxo
species (O–O homolysis intermediate in Scheme 2) cannot be excluded,
although any nickel(IV) complexes with TpR have not been reported so far.
(a) W. Nam, Acc. Chem. Res., 2007, 40, 522 – 531; (b) D. Dolphin, T. G. Traylor
and L. Y. Xie, Acc. Chem. Res., 1997, 30, 251 – 259.
28
29
Stabilization of Cu2(μ-η2:η2-O2) species by TpCF3,Me has been reported. (a) Z.
Hu, R. D. Williams, D. Tran, T. G. Spiro and S. M. Gorun, J. Am. Chem. Soc.,
2000, 122, 3556 – 3557; (b) Z. Hu, G. N. George and S. M. Gorun, Inorg. Chem.,
2001, 40, 4812 – 4813; (c) G. Aullón, S. M. Gorun and S. Alvarez, Inorg. Chem.,
2006, 45, 3594 – 3601.
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S. Hikichi, Y. Sasakura, M. Yoshizawa, Y. Ohzu, Y. Moro-oka and M. Akita, Bull.
Chem. Soc. Jpn., 2002, 75, 1255 – 1262.
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Table 1 Catalytic activity of 1H for cyclohexane oxidation
View Article Online
Products / μmol a
Entry
Complex
A/K b
TON c
Solvent: C6H5CF3 (5 mL), Amount of 1X = 26 μmol, Ni: mCPBA:C6H12 = 1:5:50
1
2
1H
10.7
35.9
6.4
1.7
1.8
0.5
1.5
1Br
20.4
Solvent: CH2Cl2 (1 mL), Amount of 1X = 13 μmol, Ni: mCPBA:C6H12 = 1:50:2500
3
4
1H
227
210
7.0
4.0
32
53
46
42
1Br
a
ε-Caprolactone, which would form through the reaction of cyclohexanone and
b
mCPBA, was not obtained under the examined reaction conditions.
A/K = yield of
(cyclohexanol
c
cyclohexanol
/
yield of cyclohexanone.
TON
=
+2×cyclohexanone)/nickel.
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View Article Online

Dalton Transactions
Page 24 of 35
Table 3 UV/Vis spectral data of 2X and 2’X a
View Article Online
2H b
2Br b
2’H c
2’Br c
386 (151)
654 (50)
λ [nm]
386 (148)
633 (42)
382 (108)
628 (22)
386 (260)
654 (68)
(ε [M–1 cm–1])
a
ε values of 2X and 2’H were estimated based on the concentration of nickel in the
parent hydroxo complexes 1X and 1’H, while the ε of 2’Br was determined based on the
used 2’Br, which was isolated as a pale blue powder by refrigeration.
b
CH2Cl2
solution at 223 K. c Et2O solution at 233 K.
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Table 4 IR band of νC=O of the acyl component a
View Article Online
Compound
ν
C=O [cm–1]
1646 a
Ref.
2H
This work
This work
This work
This work
20
2Br
1661, 1643 a, c
1643 b
2’H
2’Br
1637 b
[FeIII(OOC(=O)C6H4Cl) (TTPPP)]
1744 b
[CuII (XYL-O-)(μ-OOC(=O)C6H4Cl)]2+
1745 b
21
2
[CuII(OOC(=O)C6H4Cl)(TpiPr2)]
1640 b
22
mCPBA
mCBA
1738 a, 1726 b
This work d
This work d
1706 a, 1696 b
a
b
c
CH2Cl2 solution.
Solid sample (KBr or Nujol). Two peaks appeared. See
d
supporting information (Figure S6).
Measured under the same conditions for 2X
(CH2Cl2 solution at 223 K) and for 2’X (KBr pellet at room temperature).
25

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Page 26 of 35
Table 5 First order rate for the decomposition of 2X and 2’X
View Article Online
rate / s–1
solvent
2H
2Br
2’H
2’Br
(Temp.)
a
a
CH2Cl2 (273K)
CH2Cl2 (253K)
CD2Cl2 (253K)
Toluene (273 K)
Et2O (273 K)
4.2×10–3
8.8×10–4
3.5×10–4
3.8×10–3
1.7×10–3
1.6×10–4
5.0×10–5
2.3×10–3
—
—
a
a
a
a
a
—
—
—
—
—
9.7×10–4
5.1×10–4
a
a
—
—
2.5×10–3
a not measured.
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View Article Online
List of the Legends of Schemes and Figures
Scheme 1 Formation of the nickel(II)-acylperoxo complexes and their thermal
decomposition products.
Scheme 2 Possible reaction pathways for the thermolysis of 2X.
Fig. 1
Fig. 2
Cyclohexane oxygenation by the Ni/mCPBA system and the ligands used
as the support for nickel.
Time course of cyclohexane oxidation with mCPBA mediated by 1X under
a large excess of substrate (corresponding to the entries 3 and 4 in Table
1).
Fig. 3
Fig. 4
UV/Vis spectra of in situ-generated 2H (formed by the reaction of the
dinuclear nickel(II) complex 1H with 2 equiv of mCPBA) and the parent 1H
in CH2Cl2 at 223 K.
IR spectra of the CH2Cl2 solutions of in situ-generated 2H (formed by the
reaction of the dinuclear nickel(II) complex 1H with 2 equiv of mCPBA;
top), the parent 1H (bottom), and the sample once warmed to room
temperature (recorded at 223 K).
Fig. 5
Decay of 2Br observed by time-course UV/Vis spectra. [2Br] = 1 mM.
(a) Interval of the spectrum recording was 60 min.
the spectrum recording was 20 min.
(b), (c) Interval of
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Page 28 of 35
View Article Online
Scheme 1
Formation of the nickel(II)-acylperoxo complexes and their thermal
decomposition products.
28

Page 29 of 35
Dalton Transactions
View Article Online
Concerted reaction
sub.
O
II
O
O
sub.
Ni
sub.
O
Ni I
Cl
O
II
O
O
O
Ni
O
O-O homolysis
sub.
Cl
Cl
II
Ni
III
Ni
O
O
2X
(Ni = TpMe2,XNi)
O
sub.
O
O
O
O
Cl
Cl
Cl abstraction
from CH2Cl2
re-combine
Cl
Ni II Cl
O
Ni II
3X
+
O
Cl
4X
Cl
O
+
CO2
O
Scheme 2
Possible reaction pathways for the thermolysis of 2X.
29