A Bis(μ-chlorido)-Bridged Cobalt(II) Complex with Silyl-Containing Schiff Base as a Catalyst Precursor in the Solvent-Free Oxidation of Cyclohexane

Article abstract of DOI:10.1002/ejic.201700875

A new bis(μ-chlorido)-bridged cobalt(II) complex [Co₂(μ-Cl)₂(HL²)₄][CoCl₄] (1), where HL² is a silyl-containing Schiff base, was synthesised. The structure of this compound was established

Full text of DOI:10.1002/ejic.201700875

A Journal of
Accepted Article
Title: A new bis(µ-chlorido) bridged cobalt(II) complex with silyl-
containing Schiff-base as catalyst precursor in solvent-free
oxidation of cyclohexane
Authors: Vladimir Boris Arion, Mirela Zaltariov, Veaceslav Vieru, Michal
Zalibera, Maria Cazacu, Nuno M.R. Martins, Luisa M Martins,
Peter Rapta, Ghenadie Novitchi, Sergiu Shova, and Armando
J.L. Pombeiro
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700875
Link to VoR: http://dx.doi.org/10.1002/ejic.201700875

European Journal of Inorganic Chemistry
10.1002/ejic.201700875
A new bis(µ-chlorido) bridged cobalt(II) complex with silyl-containing Schiff-base as
catalyst precursor in solvent-free oxidation of cyclohexane
a
b
c
a
d
Mirela Zaltariov, Veacheslav Vieru, Michal Zalibera, Maria Cazacu, Nuno M. R. Martins,
,d,e
c
f
a
Luísa M. D. R. S. Martins,* Peter Rapta, Ghenadie Novitchi, Sergiu Shova, Armando J.
L. Pombeiro, Vladimir B. Arion*
d
,g
a
Inorganic Polymers Department, “Petru Poni” Institute of Macromolecular Chemistry, Aleea
Gr. Ghica Voda 41 A, Iasi 700487, Romania
b
Theory of Nanomaterials Group, Katholieke Universiteit Leuven, Celestijnenlaan 200F,
3
001, Heverlee, Belgium
c
Institute of Physical Chemistry and Chemical Physics, Slovak University of Technology,
Radlinského 9, 81237 Bratislava, Slovak Republic
d
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av.
Rovisco Pais, 1049-001 Lisboa, Portugal
e
Chemical Engineering Department, Instituto Superior de Engenharia de Lisboa, Instituto
Politécnico de Lisboa, R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal; E-mail:
lmartins@deq.isel.ipl.pt; URL: isel.pt/docentes/luisa-margarida-martins
f
Laboratoire National des Champs Magnétiques Intenses, UPR CNRS 3228, 25 rue des
Martyrs, B.P. 166, 38042, Grenoble Cedex 9, France
g
University of Vienna, Institute of Inorganic Chemistry, Währinger Strasse 42, A-1090
Vienna, Austria; E-mail: vladimir.arion@univie.ac.at; URL: anorg-
chemie.univie.ac.at/magniolaPublic/Research/Bioinorganic-chemistry/Group-V-Arion.html
Keywords: cobalt(II), silyl-containing Schiff base, catalyst precursor, oxidation, cyclohexane
1
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
Abstract
2
2
A new bis(µ-chlorido) bridged cobalt(II) complex [Co
2
(µ-Cl)
2
(HL )
4
][CoCl
4
] (1), where HL
is a silyl-containing Schiff base, was synthesised. The structure of this compound was
established by X-ray crystallography revealing a zwitterionic form adopted by the organic
ligand. The temperature dependence of magnetic susceptibility and field dependence of
7
magnetisation indicate the presence of ferromagnetic interactions between paramagnetic d
‒
1
cobalt(II) centres (SCo = 3/2). The exchange coupling parameter J(Co1-Co2) = +7.0 cm
extracted from broken-symmetry (BS) DFT calculations agrees well with the value of +8.8
‒
1
cm determined from the fitting of experimental data and fitting them with the Hamiltonian
exch
ˆ
ˆ
ˆ
H  J
S
S . Electrochemical studies indicate that complex 1 is inefficient as
Co1Co2 Co1 Co2
catalyst in electrochemical reduction of protons. One of the reasons is the low stability of the
complex in solution. In contrast, 1 was found to act as an effective homogeneous (pre)catalyst
in the microwave-assisted net oxidation of cyclohexane with aqueous TBHP. The possible
mechanism of catalytic oxidation and other advantages of using 1 in oxidation of cycloalkanes
are discussed.
2
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
Introduction
The design and synthesis of earth-abundant metal complexes as efficient catalysts for water
splitting by reduction or oxidation reactions with low overpotentials and good stability
1
,2
remains a significant challenge. During the last few years cobalt complexes have received
3
,4,5,6,7,8
particular attention.
Dinuclear µ-chlorido bridged cobalt(III) complexes supported by tetradentate ligands of the
type L Co Cl were also reported to catalyse the electrolytic and photolytic reduction of
In particular, the dicobalt(III) complex with 2-pyridylamino-N,N-bis(2-
methylene-4,6-dimethyl)phenol was found to catalyse water reduction to hydrogen with a
TOF of 922 mol of H per mole of catalyst per hour at pH 7.0, while at pH 4 in the presence
of [Ru(bpy) ]Cl and ascorbic acid when irradiated with blue light hydrogen was produced
2
2
2
9
,10,11
water.
2
3
2
1
1
with a TON = 14460 mol of H per mol of catalyst. Photoinduced oxygen evolution was
2
reported for a dinuclear µ-peroxido-µ-hydroxido bridged polypyridine complex [(TPA)Co(µ-
OH)(µ-O
2
)Co(TPA)](ClO
4
)
3
(TPA
(spy)
=
tris(2-pyridylmethyl)amine) or double-helical
(spy = 2,2’:6’,2”:6”,2’’’:6’’’,2’’’’:6’’’’,2’’’’’-
dicobalt(II) complex [Co
2
2
](ClO
4
)
4
2
‒ 12,13
3 2 2 8
sexipyridine) in the presence of [Ru(bpy) ]Cl and S O .
In addition, the interest in the synthesis of cobalt complexes is fuelled by their ability to act as
catalysts or catalyst precursors in oxidation reactions including those of academic and
industrial relevance. Robust cobalt(III) sepulchrate (sep) complexes [Co(sep)]Cl
Co(sep)] (SO were found to act as homogeneous catalysts for oxidation of styrene with
benzaldehyde as the main product.
imp) Cl ]Cl ·2H O, where amp = 2-aminomethylpyridine and imp = 2-iminomethylpyridine
3
and
[
2
4 3
)
1
4
The cobalt(III) complex [Co (amp) (µ-
2
2
2
2
2
2
1
5
anion, was reported as a functional model for the phenoxazinone synthase. The compound
promotes the oxidative coupling of 2-aminophenol to the 2-aminophenoxazinone
2
chromophore in methanol saturated with O . A mononuclear macrocyclic picolinamide
complex was found to catalyse efficiently the conversion of cyclohexene into 2-cyclohexen-1-
1
6
Ph
Ph
one,
while a dinuclear cobalt(II) complex [CoCl(µ-Cl)(Hpz )
]
3 2
(Hpz
= 3-
phenylpyrazole) acts as catalyst precursor in the peroxidative oxidation of cyclohexane to
1
7
cyclohexanol and cyclohexanone under mild conditions. The dinuclear complex permitted
to achieve overall TONs up to 223 moles of products per mole of catalyst. This value was
markedly higher than that determined for related mononuclear cobalt(II) complex
3
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
Ph
[
2 4
CoCl (Hpz ) ] with total TON 105 (although similar if calculated per Co atom) or reported
1
8,19,20
for copper and vanadium complexes.
Being interested in metal complexes with ligands based on both silane and siloxane (either
open-chain or macrocyclic) we reported recently on metal complexes with Schiff bases
resulting from reactions of 1,3-bis(3-aminopropyl)tetramethyldisiloxane with substituted 2-
2
1,22
23
hydroxybenzaldehydes,
pyrrole-2-carboxaldehyde
and 2,6-diformyl-4-methylphenol
2
4,25
25
(DFF)
or trimethylsilyl methyl p-aminobenzoate with DFF. The tetranuclear copper(II)
1
2
1
complexes [Cu
4
4 4 4 4 2 4 2
(µ -O)(L )Cl ] and [Cu (µ -O)(L ) Cl ], where H L is a macrocyclic ligand
resulted from [2+2] condensation of 1,3-bis(3-aminopropyl)tetramethyldisiloxane and DFF,
2
and HL is a 1:2 condensation product of trimethylsilyl methyl p-aminobenzoate and DFF, are
effective catalyst precursors for hydrocarboxylation of a series of linear and cyclic alkanes
with overall yields up to 36% based on the alkane and peroxidative oxidation of cyclohexane
2
5
to cyclohexanol and cyclohexanone.
Herein we report on the synthesis of a dinuclear bis(µ-chlorido) bridged cobalt(II) complex
2
[Co
2
(HL )
4
(µ-Cl)
2
][CoCl
4
] (1), its magnetic properties and electrochemical behaviour, as
well as catalytic activity in the microwave-assisted solvent-free oxidation of cyclohexane to
cyclohexanol and cyclohexanone.
Results and Discussion
Synthesis. By reaction of 2,6-diformyl-4-methylphenol with trimethylsilyl-methyl-p-
2
aminobenzoate in 1:2 molar ratio in 1:1 methanol/chloroform mixture the Schiff base HL
(
Scheme 1) was generated in situ. The formation of the latter was previously confirmed by
2
5
2 2
spectroscopic and analytical data. Further addition of CoCl ·6H O and of a few drops of
triethylamine as a base afforded a crude product, which after re-crystallization from
methanol/acetonitrile 1:1 was isolated as a crystalline material 1 of X-ray diffraction quality.
The result of single crystal X-ray crystallography (see below) was also in accord with
elemental analysis of a bulk sample. The IR spectra revealed the presence of the characteristic
‒
1
bands assigned to the azomethine group at 1622 cm , the symmetric and asymmetric
‒
1
deformation vibrations of the methyl groups attached to silicon atoms at 1249 cm and 1414
‒
1
‒1
cm respectively, the methyl rocking and the Si‒C stretching vibrations at 854 cm (Figure
‒
1
‒1
S1). Strong bands at 1714 cm and 1110 cm are attributed to the C=O and C‒O stretching
4
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
‒
1
vibrations of the ester group, while weak bands in the spectral region 395‒519 cm are
2
5
associated with the new Co‒N and Co‒O coordination bonds, not present in the proligand.
N
OH
N
Si
O
O
Si
O
O
2
Scheme 1. Line drawing of the Schiff base HL .
In the mass-spectrum of 1 in acetonitrile measured in positive ion mode a doubly charged
2
+
peak with m/z 1272.3258 is seen, which was assigned to [Co
3
Cl
2 2 2
(L) (HL) ] (calcd m/z for
2
+
[
Co Cl (C31H N O Si ) (C31H N O Si ) ] is 1272.3253). Comparison of the measured and
3 2 37 2 5 2 2 38 2 5 2 2
calculated isotopic patterns is shown in Figure S2. The UV‒vis spectrum of 1 in acetonitrile
shows strong absorption bands at 262 and 314 nm assigned to π‒π* transitions involving the
benzene ring and imine (CH=N) chromophore, a ligand-to-metal charge transfer (LMCT)
band at 470 nm and four weak absorptions with maxima at 585, 631, 660 and 685 nm due to
d-d transitions (Figure S3).
Crystal structure. According to X-ray diffraction study the crystal structure of 1 consists of
2
2+
2‒
[Co
2
Cl
2
(HL )
4
]
4
dinuclear cations, [CoCl ] counteranions and co-crystallised acetonitrile
molecules, so that the final composition as well as the charge balance correspond to the
2
formulation [Co
2
Cl
2
(HL )
4
][CoCl
4
]·4.5CH
3
CN. As shown in Figure 1, the dicationic unit is
2
‒
formed by four HL ligands, two cobalt ions and two Cl ions. The observed Co–N and Co–O
bond distances (Table S1) fall in the range of 2.138(5)‒2.146(5) Å and 2.006(4)‒2.039(4) Å,
respectively. Two cobalt ions are linked by two µ-chlorido bridging ligands in cis positions to
the nitrogen atoms of Schiff base ligands, and the distance between two cobalt atoms is of
3.600(1) Å. The four CoCl bond distances for the bridging -chlorido groups are in the
range between 2.462(2) Å and 2.518(2) Å, what is in a good agreement with those found for
II
26
similar bis(-chlorido) Co complexes (2.359‒2.628 Å). Each Co atom displays a distorted
2
N O Cl octahedral cis-geometry being coordinated by two zwitterionic HL as bidentate
2
2
2
ligands and two bridging -chlorido atoms. The positional parameters of the protons attached
to C=N groups were determined from a difference Fourier map and their positional
parameters were constrained accordingly. The perspective view of the Co1 site to show the
5
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
coordination of the ligands is depicted in Figure 2. As could be seen, the non-coordinated
2
imine nitrogen atoms from both HL ligands act as proton donors in hydrogen bonding with
the phenolato groups supporting the binding mode of the ligand. It should be noted, that the
coordination Co
Mn Cl (H L)(HL)]Cl·3H
where H L is a product of macrocyclic [2+2] condensation of DFF with 1,3-bis(3-
aminopropyl)tetramethyldisiloxane.
2 2
Cl dinuclear core resembles well the similar one formed by Mn atoms in
2
4
[
2
2
2
2
O
2 2 2 2 4 3 3 2
and [Mn Cl (H L) ][MnCl ]·4CH CN·0.5CHCl ·0.4H O,
2
2
2+
2 2 4
Figure 1. X-ray structure of [Co (µ-Cl) (HL ) ] dication in 1.
6
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
Figure 2. View of the coordination environment of Co1 atom. H-bond parameters:
N2H···O1 [N2H 0.86 Å, H···O1 1.93 Å, N2···O1 2.609(6) Å, N2H···O1 134.8°];
N4H···O6 [N4H 0.86 Å, H···O6 1.89 Å, N4···O6 2.582(6) Å, N4H···O6 136.8°].
Magnetic properties. The temperature dependence of magnetic susceptibility of 1 has been
measured in the temperature range of 300-2 K under a magnetic field of 0.1 T, as shown in
3
‒1
Figure 3. The value of χ
M
T at 300 K is 9.749 cm ·K·mol , which is much larger than the
3
‒1
expected spin-only value of 5.630 cm ·K·mol for the three Co(II) ions (S = 3/2) with g =
2
.00 and suggest the presence of an important spin orbital contribution characteristic to high
II
spin Co , as well as the ferromagnetic interaction. At temperatures < 50 K, the product χ
M
T
3
‒1
increases and reaches the maximal value of 11.208 cm ·K·mol at 5 K. This implies the
presence of the ferromagnetic interaction operating between two Co(II) ions via two µ-
chlorido bridges. The field dependence of magnetisation (Figure 3 inset) supports the
presence of ferromagnetic interaction in 1. The EPR spectrum of powdered sample of 1 is
shown in Figure S4 and the recorded signals are in line with the interpretation obtained from
magnetisation measurements.
Electronic and magnetic properties of individual cobalt centers. Both CASSCF and
CASPT2 calculations show that the ground state in case of all cobalt ions corresponds to the
spin S = 3/2 (Table S2). The calculated low-lying spin-orbital levels of the cobalt(II) centres
are shown in Table 1. As can be seen, in case of cobalt(II) ions, which are in octahedral
coordination environment, the ground Kramers doublets (KD) are well separated from the
excited ones, while the cobalt(II) ion in the tetrahedral environment has a small zero-field
splitting (ZFS). This implies that in the former case the spin-orbit coupling is operative in the
first order of the perturbation theory and admixes strongly the low-lying spin-free states
7
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
(
Table S2). In the latter case the spin-orbit coupling is operative only in the second order of
the perturbation theory due to a larger energy gap between the ground and excited spin-free
states (Table S2). As a result, a small ZFS is obtained (Table 1). Different environment of
cobalt ions has also influence on the g–factors of the ground KDs. For instance, transversal
components of the g-tensor of Co1 and Co2 are smaller than the parallel component (Table 1),
which means that in this case the magnetic anisotropy is of easy axis type. On the other hand,
in case of Co3 transversal components of the g-tensor are larger than the parallel one,
indicating an easy plane type of anisotropy. Moreover, the angular momenta are also
different: L is large for Co1 and Co2 and small for Co3.
z
‒
1
Table 1. Energies of the lowest Kramers doublets (cm ) of Co centers, g-factors and orbital
B
moment (µ ) of the ground KD obtained at the CASPT2 level.
Co1
Co2
0
Co3
0
0
2
6
9
09
49
18
188
703
953
1483
1532
13
2940
3100
1
1
462
507
g
x
g
y
g
z
= 1.71
= 2.32
= 7.96
g
g
g
x
= 1.72
= 2.29
= 7.90
g
g
g
x
= 5.12
= 4.35
= 2.27
y
z
y
z
Lz = 1.31
Lz = 1.27
Lz = 0.15
Magnetic interaction between the nearest cobalt(II) centres was taken into account within
POLY_ANISO program, by employing the following Hamiltonian:
exch
ˆ
ˆ
ˆ
H  J
S
S
Co1Co2 Co1 Co2
where SCo=3/2. The exchange interaction with Co3 was set to zero, but its contribution to
χT(T) and M(H) was taken into account explicitly.
The best fitting of the magnetic susceptibility data (Figure 3) is given by J(Co -Co )=+8.8
1
2
-1
-1
cm and zJ’=+0.006 cm , where zJ’ accounts for the intermolecular interactions. Figure 3
inset) shows that there is also a good agreement between the experimental and calculated
(
magnetisation data. To note that the fitted exchange coupling parameter, J(Co
1
-Co
2
) = +8.8
‒
1
‒1
cm , matches well the one extracted from the BS-DFT calculations, J(Co
1
-Co
2
) = +7.0 cm .
8
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
Figure 3. Experimental (symbols) and calculated (solid line) magnetic susceptibility data.
Inset: Calculated (solid lines) vs. experimental (symbols) molar magnetisation curves of 1.
2
Electrochemistry. The redox properties of 1, as well as of the HL and [nBu
4
N]
2
[CoCl
4
], for
comparative purposes, have been investigated by cyclic voltammetry (CV) at a GC electrode
in a 0.1 M nBu NBF /CH CN solution at ambient temperature.
4
4
3
Cyclic voltammogram of the di-µ-chlorido-bridged di-cobalt(II) complex 1 exhibits an
red
p
+
irreversible reduction process (Figure 4a, I) at E
= –1.28 V versus the Fc /Fc couple at scan
–
1
2
rate of 100 mV s . Such reduction peak is absent in the CV of the HL ligand (Figure 4b), as
–
well as in that of the [CoCl
4
] anion (Figure 4c), and the comparison of the current intensity
+
with Fc /Fc of the same concentration suggests a two-electron process. The peak can be
II
I
tentatively assigned to the Co /Co reduction of both cobalt centres of the complex dication
that occur at similar reduction potentials. The absence of the reoxidation peak in the reverse
scan indicates irreversible chemical changes of the dimeric dication upon reduction of the di-
II
I
cobalt core. The peak potential falls into the range reported for Co /Co reductions of related
1
0,27
dinuclear Co complexes with amine-bis(phenolate) ligands,
complexes with similar Schiff
2
8
29
bases as well as cobaltoximes, supporting the assignment performed. Further reduction at
red
more negative potentials revealed additional irreverisible reduction features with E
p
= –
1.85 V, –2.1 V and –2.5 V, respectively (Figure 4a, II, III, IV). The current intensity points to
the transfer of multiple electrons in the reduction processes involved. Comparison with the
–
CVs of the proligand and the [CoCl
4
] counterion indicates that the features in the range from
I
0
–
1.5 V to –2.4 V likely represent an overlap of Co /Co reduction peaks of the transformed di-
cobalt cluster, and reduction of the ligand, while the peak at –2.5 V involves ligand reduction
9
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
II
I
2‒
processes and the Co /Co reduction of the [CoCl
4
] counterion. The anodic scan reveals an
ox
irreversible oxidation feature of multi-electron character in the CV of 1 with E
p
= 1.14 V
(
Figure 4a, V). It results from the superposition of peaks involving several oxidation
II
III
2– 30
processes, likely including the Co /Co oxidations (of the [CoCl
4
]
(Figure 4c) as well as
II
2
both Co centres of the dimeric dication), oxidation of the HL (Figure 4b), and oxidation of
–
–
4
the Cl ligands in both [CoCl ] and the dichlorido bridge.
2
Figure 4. Cyclic voltammograms of a) 1 mM 1, b) 1 mM HL and c) ~3mM [nBu
4
N]
2
[CoCl
4
]
–
1
in 0.1 M nBu N[BF ]/CH CN recorded using cylindrical GC working electrode at 100 mV s
4
4
3
at ambient temperature. Potential scan directions are indicated by the arrow. The CV peaks in
a) are labelled by roman numbers (see text).
Monomeric as well as dimeric Co complexes have been previously shown to act as efficient
+
31
catalysts for the electrochemical H reduction in the process of hydrogen production. Cyclic
voltammetry can be used as a quick test for catalytic activity estimation, since H
2
formation
+
results in an increase of the catalytic current that is proportional to the concentration of H and
related to the TON of the catalysts. We have thus investigated the electrochemical response of
1
in the presence of increasing additions of the trifluoroacetic acid (1–5 equiv) as the proton
+
donor. The presence of H in the solution resulted in a progressive increase of the first
red
reduction peak current and a minor shift of the peak potential from E
p
= –1.28 V towards
red
more positive values (E
p
3
= –1.26 V with 5 eq. of CF COOH ) (Figure S5), that could
+
indicate the H reduction. However, additional new reduction and reoxidation peaks (some
showing an adsorptive character) were found in the 0 to –1 V range indicating the interaction
of the acid with the parent complex. This makes the interpretation of the voltammograms a
1
0
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
complicated task. Moreover the only two-fold increase in the peak current even in the
presence of a five-fold excess of protons suggests a rather low TON and thus excludes 1
+
(
and/or its follow up products) as an efficient electrochemical H reduction catalyst. However,
the redox activity of 1 in the anodic part including multi-electron character indicates that this
complex could be alternatively used in catalytic oxidation of hydrocarbons as discussed in
detail below.
Catalytic Studies. The catalytic activity of 1 was tested towards the homogeneous microwave
(MW) assisted solvent-free oxidation of cyclohexane to cyclohexanol and cyclohexanone
with aqueous tert-butyl hydroperoxide (t-BuOOH, TBHP), see Scheme 2. The influence of
varying catalyst amount, time and temperature as well as of adding an acid co-catalyst was
evaluated (Table 2). No reaction was observed between cyclohexane and TBHP under MW
2
‒
2 2 4
irradiation at 100 ºC for up to 3 h, while the presence of CoCl .6H O or [CoCl ] salt led to a
maximum 3% or 5.5%, respectively, total yield of oxygenated products. The best results were
obtained when the reaction was performed at 100 ºC, during 1.5 h and using a 0.2% molar
2
ratio of [Co
2
(µ-Cl)
2
(HL )
4
][CoCl
4
] relatively to cyclohexane (Figure 5): cyclohexanol and
cyclohexanone were formed in a 53% yield (entry 3, Table 2, ca. ten times higher than those
3
2
reported for the industrial aerobic process to assure a good selectivity). No traces of by-
products were detected by GC-MS analysis of the final reaction mixtures for the above
optimised conditions.
OOH
OH
OH
O
O
Co catalyst
PPh3
+
+
+
-
OPPh3
aq. TBHP
MW, 100 ºC
KAoil
Scheme 2. Microwave-assisted neat oxidation of cyclohexane to cyclohexyl hydroperoxide,
cyclohexanol and cyclohexanone with tert-butylhydroperoxide catalysed by 1.
The reaction is believed to proceed by a radical mechanism, via formation of cyclohexyl
hydroperoxide (CyOOH) which further selectively leads to the KA oil (mixture of
cyclohexanol (A) and cyclohexanone (K), Scheme 2) final product. In fact, by using
3
3
Shul’pin’s method, the addition of PPh
a marked increase (compare entries 3 and 4 of Table 2) of the amount of cyclohexanol due to
the reduction of CyOOH to CyOH by PPh , with the formation of phosphane oxide (Scheme
3
prior to the GC analysis of the products, resulted in
3
1
1
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
2
). Moreover, the suppression of the catalytic activity upon the introduction of the radical trap
2
Ph NH to the reaction mixture also supports the hypothesis of a free-radical
1
7,18,19,20,24,34
mechanism.
a
Table 2. Selected data for the MW-assisted oxidation of cyclohexane by t-BuOOH (aq. 70%)
2
catalysed by [Co
2
(µ-Cl)
2
(HL )
4
][CoCl
4
].
b
Entry Catalyst/
Co-catalyst
Temperature
/ºC
Reaction
time /h
Yield /%
TOF
Selectivity
/%
-
1 c
substrate /% (% vs. CyH)
CyO CyOH Total /h
1
2
3
4
5
6
7
8
9
0.5
1.0
1.5
1.5
2.0
0.5
1.0
1.5
2.0
2.4
4.7
15.9
32.6
18.3
37.3
52.8
43.2
44.7
12.5
28.2
37.7
30.6
183
187
176
144
112
125
141
126
77
PCA
0.2
100
20.9 31.9
24.8 18.4
11.6 33.1
> 99
(
2.5)
d
0.0
9.9
12.5
18.3
0
.2
-
100
> 99
13.9 23.8
13.2 17.4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
0.5
1.0
1.5
2.0
0.5
1.0
1.5
2.0
0.5
1.0
1.5
2.0
0.5
1.0
1.5
2.0
0.3
4.5
2.7
3.0
30
80
101
73
20
120
233
263
3
98
97
90
89
TEMPO
(2.5)
11.4
15.9
30.4
29.3
0.2
0
0
0
0
.2
100
100
60
10.2 20.2
11.7 17.6
0.0
0.8
1.8
3.2
0.0
3.1
5.2
6.9
0.1
1.2
1.6
3.6
0.2
1.6
5.2
7.3
0.3
4.1
11.2
16.1
0.2
1.7
1.8
5.7
2.4
.02
-
-
-
> 99
7.0
10.5
0.3
98
99
96
96
7.2
36
55
56
3
.2
16.4
23.0
0.3
e
e
e
e
2.9
15
11
23
.2
100
98
4.4
9.3
a
2
Reaction conditions: cyclohexane (2.5 mmol), catalyst (0.5-5 mol), t-BuOOH (70% in H O, 5
b
mmol), 60 - 100 ºC. Moles of products [cyclohexanol (CyOH) + cyclohexanone (CyO)] per 100 mol
1
2
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
c
of cyclohexane, as determined by GC after treatment with PPh
. TOF = number of moles of products
3
d
e
per mol of catalyst per hour. GC injection before treatment with PPh
3
. Oil-bath heating instead of
MW irradiation.
The amount of catalyst, as well as the reaction temperature and time, play a significant role on
2
the catalytic activity of [Co (µ-Cl) (HL ) ][CoCl ] as depicted in Figure 5.
2
2
4
4
2
Figure 5. Effect of the reaction time, temperature, amount [Co (µ-Cl) (HL ) ][CoCl ] (0.02
2
2
4
4
or 0.2 mol% catalyst/substrate ratio) and additives on the yield of KA oil obtained by
microwave-assisted neat oxidation of cyclohexane with THBP.
2
The presence of PCA, although not essential for the catalytic activity of [Co (µ-
2
Cl)
2
(HL )
4
][CoCl
4
], enhances it in a significant way (Figure 5), allowing to reach 53% yield
of KA oil instead of 38% obtained in the additive-free system (compare entries 1-5 with 6-9
3
5
of Table 2). On the other hand, TEMPO exhibits its usual inhibiting effect (Figure 5 and
entries 1-5 and 10-13 of Table 2) in radical catalytic systems.
Moreover, the used low power MW-assisted reaction provides a much more efficient
synthetic method of KA oil than conventional heating under open atmosphere or non-
pressurised refluxing (Figure 6).
1
3
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
Figure 6. Effect of the heating mode on the yield of KA oil obtained by neat oxidation of
cyclohexane with THBP.
Conclusions
In situ performed reaction of 2,6-diformyl-4-methylphenol with trimethylsilyl-p-
aminobenzoate in 1:2 molar ratio in a protic/aprotic solvent mixture, followed by addition of
cobalt(II) chloride in the presence of a base resulted in formation of a crude product, which
2
after re-crystallization afforded the complex [Co (µ-Cl) (HL ) ][CoCl ]·4.5CH CN as
2
2
4
4
3
confirmed by single crystal X-ray diffraction analysis. Temperature dependence of magnetic
susceptibility measurements, field dependence of magnetisation along with theoretical BS-
7
DFT calculations indicate ferromagnetic interaction between the paramagnetic cobalt(II) (d ,
S = 3/2) centres in the dimeric dication and the presence of different nature of spin-orbit
2
‒
4
coupling in the cation and [CoCl ] . The electrochemical response of 1 in the presence of
increasing amounts of CF COOH (1‒5 equiv) resulted in a progressive increase of the first
3
+
reduction peak current, that could indicate the H reduction. The relatively low increase of the
first reduction peak and the presence of new reduction and reoxidation peaks implying the
interaction of the acid with the parent complex and overall its low stability in solution made
further investigation of the electrochemical reduction useless. However, complex 1 acts as
homogeneous precatalyst for the MW-assisted net oxidation of cyclohexane with aq. TBHP,
via a radical mechanism, affording KA oil in high selectivity and in a yield that is ca. ten
times higher than that of the current industrial process. Other advantages of the present
catalytic systems are the mild and environmental benign conditions such as i) solvent-free
1
4
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
protocol, ii) use of environmentally acceptable oxidant (aq. TBHP) and energy source (MW
irradiation), and iii) short reaction time. These are significant features towards the
development of a sustainable chemical process for the cyclohexane oxidation to KA oil.
Experimental Section
Materials.
Trimethylsilyl-methyl-p-aminobenzoate
was
prepared
from
(
chloromethyl)trimethylsilane (Aldrich) and p-aminobenzoic acid sodium salt by following
2
5
the published procedure.
2 2
2,6-Diformyl-4-methylphenol (Polivalent-95), CoCl ·6H O
(
(
Aldrich), triethylamine, diphenylamine, cyclohexane (Fluka), methanol, chloroform
Chimopar), acetonitrile, tert-butyl hydroperoxide (TBHP, 70% aq. sol.), nitromethane,
triphenylphosphine (Aldrich), acetonitrile (CH
CF COOH) were received from commercial sources and used without further purification.
Tetrabutylammonium tetrafluoroborate (nBu NBF , 99%, Sigma-Aldrich) was dried under
3
CN, Aldrich) and trifluoroacetic acid
(
3
4
4
reduced pressure for 24 h/70 °C and stored in exicator before use. Ferrocene (98%, Sigma-
Aldrich) was employed as internal potential standard in electrochemical measurements.
4 2 4 4 2
[nBu N] [CoCl ] was prepared from nBu NCl (Aldrich) and CoCl (Mikrochem) (2:1 molar
3
6
ratio) in a small volume of absolute ethanol as previously described.
2
Synthesis of [Co
2
(µ-Cl)
2
(HL )
4
][CoCl
4
]·4.5CH
3
CN. To a solution of 2,6-diformyl-4-methyl-
phenol (0.164 g, 1.0 mmol) in methanol/chloroform mixture (10 mL, 1:1 v/v) a solution of
trimethylsilyl-methyl-p-aminobenzoate (0.446 g, 2.0 mmol) in methanol/chloroform (10 mL,
1
:1 v/v) was added. The reaction mixture was stirred at reflux for 3 h. A solution of
CoCl ·6H O (0.476 g, 2 mmol) in methanol/chloroform (10 mL, 1:1 v/v) was slowly added to
2
2
the reaction mixture and stirred at 50 °C for 30 min. The colour changed from orange to dark
red. Then few drops of triethylamine were added and the reaction mixture was stirred at 50 °C
for other 10 min. The resulting solution was cooled to room temperature and left to stand for
crystallisation. The obtained red-brown precipitate was re-crystallised from
chloroform/acetonitrile mixture (10 mL, 1:1 v/v) to give red-brown crystals, which were
collected by filtration, washed with cold methanol/acetonitrile 1:1 (3  5 mL) and dried in air.
Yield: 196 mg, 27.0%. Anal. Calcd for C133
6 3 12.5 8
H162.5Cl Co N O20Si : C 55.65, H 5.71, N 6.10.
Found: C 55.23, H 5.30, N 5.77 (%); HR ESI‒MS (acetonitrile): positive m/z 1272.3258
2
2
2+
‒1
[
[Co Cl (L ) (HL ) ] ; IR 
max
(KBr), cm : 3425m, 3057vw, 2955m, 2904w, 1714vs,
2
3
2
2
1636vs, 1622w, 1599s, 1537vs, 1501m, 1414w, 1388, 1313vs, 1303vs, 1249vs, 1237s, 1198s,
1
172s, 1110s, 1071m, 1015m, 1000m, 854vs, 766s, 732w, 701m, 622vw, 602vw, 519w,
1
5
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
‒
1
‒1
3
95w. UV‒vis (acetonitrile), λmax (ε, M cm ): 262 (23480), 314 (18330), 470 (8315), 585
(
339), 631 (173), 660 (256), 685 (288).
Analytical and Spectroscopic Measurements. Infrared (IR) spectra were recorded on
Bruker Vertex 70 FT-IR spectrometer. Registrations were performed in transmission mode
‒
1
‒1
within 400‒4000 cm range at room temperature with a resolution of 4 cm and
accumulation of 32 scans. Elemental (C, H, N) analyses were obtained with a Perkin–Elmer
CHNS 2400 II elemental analyser. High resolution electrospray ionisation (HR ESI) mass
spectra (MS) were recorded on a maXis instrument by using acetonitrile as solvent. UV‒vis
absorption spectrum of the complex in acetonitrile was measured on Specord 200
spectrophotometer.
Magnetic measurements. Magnetic susceptibility data (2‒300K) were collected on
powdered polycrystalline sample by SQUID magnetometer on Quantum Design model
MPMS instrument and under an applied magnetic field of 0.1 T. Magnetisation isotherm was
collected at 2 K between 0 and 5 T. All data were corrected for the contribution of the sample
3
7,38
holder and diamagnetism of the samples estimated by using Pascal’s constants.
X-ray crystallography. Crystallographic measurements for 1 were carried out with an
Oxford-Diffraction XCALIBUR E CCD diffractometer equipped with graphite-
monochromated Mo Kα radiation. Single crystal was positioned at 40 mm from the detector
and 140 frames were measured each for 30 s over 1° scan width. The unit cell determination
3
9
and data integration were carried out using the CrysAlis package of Oxford Diffraction. The
4
0
structure was solved by direct methods using Olex2 software with the SHELXTL structure
4
1
solution program and refined by full-matrix least-squares on F² with SHELXL-97 using an
anisotropic model for non-hydrogen atoms. H atoms were placed at calculated positions and
refined as riding atoms in the subsequent least-squares model refinements. The positional
parameters of NH hydrogen atoms were found from difference Fourier syntheses and verified
by the geometric parameters of the corresponding hydrogen bonds. The positional parameters
of the disordered moieties were refined in combination with PART and SADI restrains using
anisotropic/isotropic model for non-H atoms. The molecular plots were obtained using the
Olex2 program. Table 3 provides a summary of the crystallographic data together with
refinement details for 1. CCDC-1561444 contains the supplementary crystallographic data for
1
6
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
this contribution. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif
Table 3. Crystallographic data, details of data collection and structure refinement parameters
for 1.
empirical formula
formula weight
temperature/K
Crystal system
space group
a/Å
133 6 3 12.5 8
C H162.5Cl Co N O20Si
2870.47
173
Monoclinic
P2 /n
1
22.3644(8)
32.0968(7)
23.2738(8)
111.132(4)
15583.1(9)
4
1.224
0.542
0.40 × 0.35 × 0.35
3.16 to 46.52
55816
b/Å
c/Å
β/°
V/Å
3
Z
D
μ/mm
3
calc/mg/mm
-1
3
Crystal size/mm
θmin, θ max( )
o
reflections collected
independent reflections
22342[Rint = 0.0542]
22342/89/1622
0.0792
0.2293
1.029
data/restraints/parameters
a
R (I>2σ(I)
1
b
wR (all data)
2
c
GOF
-3
Largest diff. peak/hole/e Å
2.50/-2.50
a
b
c
R
1
= F
wR = {[w (F
GOF = {[w(F
o
 - F
c
/F ,
o
2
2 2
2 2 1/2
2
o
- F
- F
c
) ] /[w(F
o
) ]} .
2
2 2
1/2
o
c
) ] /(n – p)} , where n is the number of reflections and p is the total
number of parameters refined.
Electrochemistry. Cyclic voltammograms were recorded in CH CN with 0.1 M n-Bu NBF
4
3
4
as the supporting electrolyte using a one-compartment electrochemical cell with glassy carbon
GC) cylindrical working electrode, Pt-coil as counter electrode, and Ag-wire
(
pseudoreference electrode. All electrochemical measurements were performed under argon
+
atmosphere using the HEKA PG 390 potentiostat. The Fc /Fc couple served as an internal
standard.
4
2
Computational details. All calculations were carried out with MOLCAS 8.0 program and
are of CASSCF/CASPT2/SO-RASSI/SINGLE_ANISO type. Structures of the calculated Co
fragments are shown in Figure S6. Each magnetic centre was calculated by replacing the
1
7
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
neighbouring Co(II) ion by nonmagnetic Zn(II) ion. Information about the employed basis set
4
3
is given in Table S3. Active space of the CASSCF method included 7 electrons in 10
2
+
44
orbitals (3d and 3d’ orbitals of Co ion) to account for the double-shell effect. The
4
5
dynamical electron correlation energy was considered within CASPT2 calculations. The
imaginary shift was set to 0.1. All obtained quartet (10) and doublet states (40) were mixed by
4
6
spin-orbit coupling within SO-RASSI program. On the basis of the resulting spin-orbital
4
7
multiplets SINGLE_ANISO program computed local magnetic properties. The exchange
4
8
49
interaction was accounted within Lines model employing POLY_ANISO program and
using the results obtained from the CASPT2 calculations. Broken-symmetry DFT calculations
have been carried out to estimate the value of the exchange coupling constant between the
closest (octahedral) Co centres, by using the structure shown in Figure S7. SVP basis set and
5
0
the B3LYP functional were used as implemented in ORCA 3.0.0 program. Scalar
relativistic effects were included within Douglas-Kroll-Hess Hamiltonian. The exchange
5
1
coupling constant was calculated by using Yamaguchi’s formula.
Catalytic Studies. The catalytic oxidations of cyclohexane were performed in sealed
cylindrical (5 mL capacity with a 1 cm internal diameter) Pyrex tubes, under focused
microwave irradiation (MW), in an Anton Paar Monowave 300 reactor fitted with a rotational
system and an IR temperature detector. Typical reaction conditions are as follows: in a tube
were introduced (by this order) 2.50 mmol of cyclohexane, 0.5 – 5 mol of the catalyst and
5
.00 mmol of 70% aqueous tert-butyl hydroperoxide (TBHP). For the assays in the presence
of a radical trap, NHPh in stoichiometric amount relative to the oxidant was added to the
reaction mixture. The tube was placed in the microwave reactor and the system was stirred
600 rpm) and irradiated (10 W) for 0.5 ‒ 24 h at 60 or 100 ºC. After the reaction, the mixture
cooled down to room temperature. Solution samples were analysed by gas chromatography
GC) after addition of nitromethane (the internal standard compound). For reactions
2
(
(
performed under conventional heating, a round-bottomed flask (5 mL) equipped with reflux
condensers in conventional oil baths, in air, were used. The reagents (see above) were
introduced in the flask and vigorously stirred at the desired temperature (60 ‒ 100 ºC) during
the desired reaction time (up to 24 h). The products were analysed as described above. GC
analyses were carried out using a FISONS Instruments GC 8000 series gas chromatograph
with a FID detector and a capillary column (DB-WAX, column length: 30 m; internal
diameter: 0.32 mm) and the Jasco-Borwin v.1.50 software. The temperature of injection was
2
40 ºC. The initial temperature was maintained at 100 ºC for 1 min, then raised 10 ºC/min to
1
8
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European Journal of Inorganic Chemistry
10.1002/ejic.201700875
1
60 ºC and held at this temperature for 1 min. Helium was used as the carrier gas. GC-MS
analyses were performed using a Perkin Elmer Clarus 600 C instrument (He as the carrier
gas). The ionisation voltage was 70 eV. Gas chromatography was conducted in the
temperature-programming mode, using a SGE BPX5 column (30 m × 0.25 mm × 0.25 µm).
Considering that the reaction mixture contains cyclohexyl hydroperoxide (primary product),
cyclohexanol and cyclohexanone, an excess of solid triphenylphosphine was added (to reduce
the cyclohexyl hydroperoxide to the corresponding alcohol, and hydrogen peroxide to water)
and the mixture was analysed again to estimate the amount of cyclohexyl hydroperoxide,
3
3
following a method developed by Shul’pin. For precise determination of the product
concentrations only data obtained after the reduction of the reaction sample with
triphenylphosphine were typically used. Reaction products were identified by comparison of
their retention times with those of known reference compounds, and by comparing their mass
spectra to fragmentation patterns obtained from the NIST spectral library stored in the
computer software of the mass spectrometer. Blank experiments, in the absence of any
catalyst, were performed under the studied reaction conditions and no significant conversion
was observed.
ACKNOWLEDGEMENTS
We would like to thank Liviu Chibotaru for useful discussions. The financial support from the
Fundação para a Ciência e a Tecnologia (FCT), Portugal, for the CATSUS fellowship
SFRH/BD/52371/2013 to N.M.R. Martins and the PTDC/QEQ-ERQ/1648/2014,
PTDC/QEQ-QIN/3967/2014 and UID/QUI/00100/2013 projects, is gratefully acknowledged.
V.V. acknowledges the postdoctoral fellowship of Fonds Wetenschappelijk Onderzoek-
Vlaanderen (FWO, Flemish Science Foundation). This work was also supported by the
Science and Technology Assistance Agency under the contract No. APVV-15-0053 and by
the Slovak Grant Agency VEGA under contract No. 1/0416/17 and a grant of the Ministry of
National Education, CNCS- UEFISCDI, project numbers PN-III-P4-ID-PCE-2016-0642.
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European Journal of Inorganic Chemistry
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Graphical Abstract
OH
O
A new bis(µ-chlorido) bridged cobalt(II) complex with silyl-containing tridentate Schiff base,
which shows ferromagnetic behaviour at low temperatures was found to act as an effective
homogeneous (pre)catalyst in the microwave-assisted net oxidation of cyclohexane with
aqueous TBHP to cyclohexanol and cyclohexanone.
2
4
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