Cobalt(II) complexes with N,N,N-scorpionates and bidentate ligands: Comparison of hydrotris(3,5-dimethylpyrazol-1-yl)borate Tp? vs. Phenyltris(4,4-dimethyloxazolin-2-yl)borate ToM to control the structural properties and reactivities of cobalt centers

Article abstract of DOI:10.3390/molecules23061466

Scorpionate ligands Tp? (hydrotris(3,5-dimethylpyrazol-1-yl)borate) and To^M (tris(4,4- dimethyloxazolin-2-yl)phenylborate) complexes of cobalt(II) with bidentate ligands were synthesized. Both Tp? and To^M coordinate to cobalt(II) in a tridentate fashion when the bidentate ligand is the less hindered acetylacetonate. In crystal structures, the geometry of cobalt(II) supported by the N3O2 donor set in the Tp? complex is a square-pyramid, whereas that in the To^M complex is close to a trigonal-bipyramid. Both Tp?- and To^M-acac complexes exhibit solvatochromic behavior, although the changing structural equilibria of these complexes in MeCN are quite different. In the bis(1-methylimidazol-2-yl)methylphenylborate (L^Ph) complexes, Tp? retains the tridentate (k³) mode, whereas To^M functions as the bidentate (k²) ligand, giving the tetrahedral cobalt(II) complex. The bowl-shaped cavity derived from the six methyl groups on To^M lead to susceptibility to the bulkiness of the opposite bidentate ligand. The entitled scorpionate compounds mediate hydrocarbon oxidation with organic peroxides. Allylic oxidation of cyclohexene occurs mainly on the reaction with tert-butyl hydroperoxide (TBHP), although the catalytic efficiency of the scorpionate ligand complexes is lower than that of Co(OAc)₂ and Co(acac)₂. On cyclohexane oxidation with meta-chloroperbenzoic acid (mCPBA), both To^M and Tp? complexes function as catalysts for hydroxylation. The higher electron-donating To^M complexes show faster initial reaction rates compared to the corresponding Tp? complexes.

Full text of DOI:10.3390/molecules23061466

molecules
Article
Cobalt(II) Complexes with N,N,N-Scorpionates
and Bidentate Ligands: Comparison of
Hydrotris(3,5-dimethylpyrazol-1-yl)borate Tp* vs.
Phenyltris(4,4-dimethyloxazolin-2-yl)borate ToM to
Control the Structural Properties and Reactivities of
Cobalt Centers
ID
Toshiki Nishiura, Takahiro Uramoto, Yuichiro Takiyama, Jun Nakazawa * and Shiro Hikichi *
Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi,
Kanagawa-ku, Yokohama 221-8686, Japan; r201670152kn@jindai.jp (T.N.); r201304066bf@jindai.jp (T.U.);
yuichiro0411@gmail.com (Y.T.)
* Correspondence: jnaka@kanagawa-u.ac.jp (J.N.); hikichi@kanagawa-u.ac.jp (S.H.);
Tel.: +81-45-481-5661 (J.N. & S.H.)
Academic Editor: Carlo Santini
Received: 25 May 2018; Accepted: 15 June 2018; Published: 16 June 2018
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Scorpionate ligands Tp* (hydrotris(3,5-dimethylpyrazol-1-yl)borate) and ToM (tris(4,4-
dimethyloxazolin-2-yl)phenylborate) complexes of cobalt(II) with bidentate ligands were synthesized.
Both Tp* and ToM coordinate to cobalt(II) in a tridentate fashion when the bidentate ligand is
the less hindered acetylacetonate. In crystal structures, the geometry of cobalt(II) supported by
the N3O2 donor set in the Tp* complex is a square-pyramid, whereas that in the ToM complex is
close to a trigonal-bipyramid. Both Tp*- and ToM-acac complexes exhibit solvatochromic behavior,
although the changing structural equilibria of these complexes in MeCN are quite different. In the
bis(1-methylimidazol-2-yl)methylphenylborate (LPh) complexes, Tp* retains the tridentate ( 3) mode,
whereas ToM functions as the bidentate ( 2) ligand, giving the tetrahedral cobalt(II) complex. The
bowl-shaped cavity derived from the six methyl groups on ToM lead to susceptibility to the bulkiness
of the opposite bidentate ligand. The entitled scorpionate compounds mediate hydrocarbon oxidation
with organic peroxides. Allylic oxidation of cyclohexene occurs mainly on the reaction with tert-butyl
hydroperoxide (TBHP), although the catalytic efficiency of the scorpionate ligand complexes is
lower than that of Co(OAc)2 and Co(acac)2. On cyclohexane oxidation with meta-chloroperbenzoic
acid (mCPBA), both ToM and Tp* complexes function as catalysts for hydroxylation. The higher
electron-donating ToM complexes show faster initial reaction rates compared to the corresponding
Tp* complexes.
Keywords: scorpionate ligand; borate; cobalt; oxidation
1. Introduction
Facially capping tridentate ligands are utilized to synthesize tetra-, penta-, and hexa-coordinated
transition metal complexes. The appropriate molecular design of the ligands controls the coordination
geometry of the metal center. For example, bulky shading substituent groups on ligands
stabilize low-coordination numbers of the metal centers of the resulting complexes. A family of
scorpionates—a nickname for chelating borate ligands—is extensively employed for a variety of
coordination compounds due to its molecular design versatility [
1,2]. A prototype of this family is
hydrotris(pyrazol-1-yl)borate, [HB(pz)3]- (Tp), which is composed of pyrazole. The third, fourth, and
Molecules 2018, 23, 1466; doi:10.3390/molecules23061466
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fifth positions of pyrazole can be modified by introducing various groups, and the resulting substituted
pyrazoles are also applied to borate ligands such as [HB(pzR)3]- (TpR; R denotes the substituent groups
on pyrazole). Hydrotris(3,5-dimethylpyrazol-1-yl)borate, called Tp* (also called TpMe2), is classified as
“the first generation of scorpionates” along with the nonsubstituted Tp. Its moderate bulkiness, derived
from the methyl groups, allows it to form both homo- and heteroleptic complexes (e.g., [M( 3-Tp*)2]
and [M( 3-Tp*)Lx]; L denotes ligands other than Tp*), in the latter of which the metal centers have
various coordination numbers. Therefore, various metal complexes with Tp* have been investigated
extensively so far [1,2].
To date, novel scorpionate-type borate ligands with various non-pyrazolyl donors such as
azoles [3–6], thioethers [7], phosphines [8], and N-heterocyclic carbenes [9] have been developed.
Among them, tris(oxazolin-2-yl)borates, called ToR (R indicates substituent groups on the fourth
and fifth positions of oxazoline), are attractive because various substituted oxazoline derivatives
are available. The first ToR is tris(4,4-dimethyloxazolin-2-yl)phenylborate (ToM), of which six
methyl groups form a bowl-shaped cavity surrounding the metal center on the resulting metal
complexes [5
]. To date, tetra-, penta-, and hexa-coordinated heteroleptic complexes with ToM have
been reported [5,10–14].
Structural and electronic properties of TpR and ToR have been compared on tricarbonyl complexes
of group VII metals (rhenium and manganese) [12,
13]. Steric bulkiness of ToM is higher than that
of Tp* estimated by solid angles derived from the crystal structures. The local structures of the
hexa-coordinated metal centers of [MI( 3-Tp*)(CO)3] and [MI( 3-ToM)(CO)3] are, however, very close,
because no steric repulsions exist between the metal-surrounding methyl groups on the scorpionates
and the liner diatomic CO ligands. The electron-donating ability of ToM is higher than that of
Tp* estimated from νCO of IR measurement. From electrochemical investigations, however, the
donating ability of Tp* is higher than that of ToM according to the order of the oxidation potentials,
[MI( 3-Tp*)(CO)3] < [MI( 3-ToM)(CO)3]. Such different interpretations arise from differences in the
oxidation state. The former estimation based on IR data is attributed to the single oxidation state of the
metal centers, while the latter results from changing the oxidation state of the metal centers on the
rigid pseudo-octahedral complexes [12].
We have also examined the structural properties of Tp* and ToM in the crystal structures of
the pseudo-tetrahedral nickel(II)-chloride complexes [NiII( 3-Tp*)Cl] and [NiII( 3-ToM)Cl]. The Tp*
complex is very close to the symmetric structure of C3v, as evidenced by an almost linear arrangement
of Cl-Ni—BTp. In the ToM complex, however, an arrangement of Cl-Ni—BTo deviates from a straight
line. Therefore, ToM does not work as a “tetrahedral enforcer,” although solid angle analysis [15
]
of the nickel-chloride complexes reveals that ToM is bulkier than Tp*. Six methyl groups of ToM
surrounding a metal center form a bowl-shaped cavity, but not a deep one. Therefore, the arrangement
of a monodentate ligand is relatively flexible [14].
In this work, we investigated coordination behaviors of Tp* and ToM toward cobalt(II) with
bidentate ligands (D–D). Cobalt(II) shows a flexible coordination structure. Varied structural and
electronic properties of the bidentate ligands reflect on the molecular structures and reactivities of the
resulting mixed ligand complexes, [CoII(scorpionate)(D–D)]. We focused on the catalytic performance
of the synthesized mixed ligand complexes toward alkene oxidation with tert-BuOOH (TBHP) and
alkane oxidation with meta-chloroperbenzoic acid (mCPBA), as the estimation of the functions depends
on the scorpionate ligands. These aspects provide useful information for the design of functional
metallocomplexes with appropriate ligands.
2. Results and Discussion
2.1. Synthesis and Characterization of Mixed Ligand Complexes
In this work, two kinds of
−
1-charged bidentate ligands were employed. One was acetylacetonate
(acac) as an oxygen-donating ligand, and the other was methylbis(1-methylimidazol-2-yl)phenylborate

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(LPh; [B(ImN-Me)2MePh]-) as a nitrogen-donating ligand [16
–
19]. These ligands exhibited different
structural properties. In acac, no steric hindrance exists around the oxygen donors. In contrast, the
nitrogen donors of LPh are incorporated in the five-member imidazolyl rings and the fourth-positioned
sp2-C-H group overhangs. Arrangements of two donor atoms of these ligands are also different.
Acac has a planar configuration, and O-M-O bite angles of the formed complexes are almost fixed
to right angles. On the other hand, two imidazolyl rings of LPh bind to the sp3-boron center and
dihedral angles of the imidazolyl rings and N-M-N bite angles are somewhat flexible [17]. Therefore,
we examined the combination of the scorpionates (Tp* or ToM) and these bidentate ligands (acac or
LPh) in order to clarify the correlations between the bulkiness and the coordination behavior of Tp* vs.
ToM (Scheme 1).
Scheme 1. Synthesis of mixed ligand complexes 1–4.
2.1.1. Acetylacetonato Complexes [CoII(Tp*)(acac)] (1) and [CoII(ToM)(acac)] (2)
The acac complexes were synthesized by ligand metathesis reactions between CoII(acac)2 and
the alkaline metal salts of the scorpionates (KTp* for
Adding the THF solution of KTp* or NaToM to the THF solution of CoII(acac)2 yielded the desired
complexes [CoII(Tp*)(acac)] ( ) and [CoII(ToM)(acac)] (
), respectively. It is known that homoleptic
1 2) under anhydrous conditions.
and NaToM for
1
2
complexes [CoII( 3-Tp*)2] [20] and [CoII( 2-ToM)2] [10] are formed as byproducts during the reaction
of scorpionate ligands and CoIIX2 (X = Cl, OAc, NO3, etc.). In the case of ToM, a protonated ligand
complex such as [CoIICl2(HToM)] is also formed in the presence of proton sources [10]. In the present
system, yields of these byproducts were small.
Both synthesized acac complexes were fully characterized by spectroscopy (IR, 1H-NMR, UV-vis)
and X-ray crystallography. Molecular structures of an MeCN adduct of
1 and 2 are shown in Figure 1.
Geometries of the five-coordinated cobalt centers supported by tridentate scorpionates and bidentate

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acac ligands are a square-pyramid in
1
(when the coordinating MeCN molecule is omitted) and a
distorted trigonal-bipyramid in
2
. In Tp* complex
1, both oxygen donors of acac were located on
equatorial positions. In ToM complex
2, however, one of the two oxygen donors from acac sat on the
trigonal plane and another was on the axial position. Such difference of locations of the acac ligands
in the static solid state arose from steric hindrance derived by cobalt-surrounding methyl groups
on the azole rings of the scorpionates. In 2, steric repulsion between two equatorial 4,4-dimethyl
oxazoline groups of ToM and a virtual equatorial acac ligand would occur. Similar structural variations
depending on the bulkiness of substituent groups of TpR are also observed in [CoII( 3-Tp3R)( 3-acac)]
(3R denotes substituent groups on the third position of the pyrazolyl rings) [21–24]. When 3R = Ph,
the geometry of the cobalt center is trigonal-bipyramid [22]. In contrast, the diphenylmethyl analogue
(i.e., 3R = CHPh2) has a square-pyramidal cobalt center [23]. Space around the metal center of the
Tp3CHPh2 complex is close to that of the Tp* complex because three tertiary sp3-C–H groups surround
the metal center.
(a)
(b)
Figure 1. Molecular structures of (
a
)
1·MeCN and (
b) 2. All hydrogen atoms except B–H moiety are
omitted for clarity; thermal ellipsoids are set at 30% probability. Selected bond lengths [Å] and angles
◦
[ ]: for 1·MeCN; Co1-N1 2.153 (1), Co1-N3 2.101 (1), Co1-N5 2.124 (1), Co1-O1 2.055 (1), Co1-O2 2.054
(1), N1-Co1-N3 88.05 (5), N1-Co1-N5 87.01 (5), N1-Co1-N7 90.76 (5), N1-Co1-O1 91.47 (5), N1-Co1-O2
176.67 (5), N3-Co1-N5 86.33 (5), N3-Co1-N7 92.80 (5), N3-Co1-O1 178.72 (5), N3-Co1-O2 93.03 (5),
N5-Co1-N7 177.63 (5), N5-Co1-O1 94.83 (5), N5-Co1-O2 96.20 (5), O1-Co1-O2 87.38 (5). For 2; Co1-N1
2.177 (1), Co1-N2 2.082 (1), Co1-N3 2.063 (1), Co1-O4 2.005 (1), Co1-O5 2.028 (1), N1-Co1-N2 84.94 (4),
N1-Co1-N3 87.60 (4), N1-Co1-O4 87.07 (4), N1-Co1-O5 172.67 (5), N2-Co1-N3 90.83 (4), N2-Co1-O4
136.48 (4), N2-Co1-O5 94.46 (5), N3-Co1-O4 131.50 (5), N3-Co1-O5 99.55 (5), O4-Co1-O5 86.98 (4).
Both
and
1
2
and
2
exhibited solvatochromic behavior. UV-vis spectra of CH2Cl2 and MeCN solutions
, its CH2Cl2 solution showed
of
1
showed different patterns (Figure 2). For Tp* complex 1
moderate-intensity (ε ≈ 80 M–1 cm–1) absorption around the 500–570 nm region, whereas the MeCN
solution revealed weakened absorption (ε ≈ 40 M–1 cm–1) around the 480–550 nm region. This
changing spectral behavior was similar to that observed for the previously reported TpMe3 ligand
complex [CoII( 3-NO3)( 3-TpMe3)] [25]. In the MeCN solution, these TpMe2,X complexes turned
to the corresponding solvated hexa-coordinated Co(II) species as found in the crystal structure
(see Figure 1 and Scheme 2). In contrast, ToM complex
2 showed different behavior. The CH2Cl2
solution of
2
exhibited weak to moderate-intensity absorption at 450 nm (ε ≈ 40 M–1 cm–1) and 555 nm
(
ε ≈ 65 M–1 cm–1). When the solvent was changed to MeCN, multiple absorption with increased
intensity could be observed at 513 nm (ε ≈ 120 M–1 cm–1), 557 nm (ε ≈ 160 M–1 cm–1), and 584 nm
(
ε ≈ 170 M–1 cm–1). This spectral pattern was similar to that attributed to four-coordinated tetrahedral
cobalt(II) species rather than the six-coordinated species as found for the above-mentioned MeCN
solutions of the Tp* complexes including
equilibrium of one of three oxazolynyl nitrogen donors of ToM induced by the action of MeCN
(Scheme 2). The spectral patterns of 1H-NMR signals of CDCl3 and d3-MeCN solutions of
, however,
1. A plausible explanation for this is coordination-dissociation
2

Molecules 2018, 23, 1466
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were not so different (see Supplementary Figures S1 and S2). Therefore, the putative tetrahedral species
resulting from partial dissociation of ToM might not be so favored.
Figure 2. UV-vis spectra of CH2Cl2 and MeCN solutions of 1 (left) and 2 (right).
Scheme 2. Solvent-dependent equilibrium of structural change of 1 (top) and 2 (bottom).
2.1.2. Bis(1-methylimidazol-2-yl)borato Complexes [CoII(Tp*)(LPh)] (3), [CoII(ToM)(LPh)] (4) and
[CoII(LPh)2] (5)
Bis(1-methylimidazol-2-yl)methylphenylborate LPh ([B(ImN-Me)2MePh]-) [16
examined as another monoanionic bidentate ligand. The reaction of LiLPh and
or
selective ligand exchange from acac to LPh to give the corresponding Tp* ( ) and ToM
respectively. In these LPh complexes, the coordination behavior of Tp* and ToM was quite different.
Tp* complex had already been reported as an analogue of a penta-coordinated iron(II)
complex [Fe(Tp*)(LPh)], which showed O2-binding capability to yield the corresponding mononuclear
iron(III)-superoxo compound [18]. In the original report, had been obtained from a cobalt(II)-acetato
complex, [Co(Tp*)( 2-OAc)] [25]. As described in the previous report, the molecular structure of
had
been revealed by X-ray crystallography. The geometry of the cobalt(II) center of is very close to a
square-pyramid. Two nitrogen donors from LPh sit on the equatorial position, with an almost right
–
19] was also
resulted in
(4) complexes,
1
2
3
3
3
3
3
◦
angle of its N–Co–N bite angle (88.4 ). Two imidazolyl rings of LPh bend each other (estimated dihedral

Molecules 2018, 23, 1466
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◦
angle was 142 ), and the resulting LPh meshed with threefold symmetrically arranged pyrazoles of Tp*
due to steric repulsion between the C–H groups of the fourth position of the imidazoles of LPh and the
methyl groups attached on the third position of the pyrazoles of Tp*.
In contrast, ToM complex
4
was a tetrahedral cobalt(II) complex with bidentate ToM and LPh
ligands, as shown in Figure 3. Two imidazolyl rings of LPh in
4 were located almost coplanar
(as estimated dihedral angle was 176◦), and an N–Co–N bite angle (97.7◦) was larger than a right
angle. Because of the steric hindrance derived from the methyl groups on the oxazoline rings of
ToM, it was impossible to form a penta-coordinated cobalt(II) complex with 3-ToM and 2-LPh
.
One of three oxazolynyl groups of ToM in
2 was left from the cobalt center through the ligand
substitution from acac to LPh such that the bidentate ToM was observed in a homoleptic complex,
[CoII( 2-ToM)2] [10]. Similarities of the geometry of the cobalt(II) centers and the coordination mode of
ToM were also reflected on spectroscopic properties. IR spectra of the solid samples of
4
(Supplementary
Figure S3c) and [CoII( 2-ToM)2] exhibited characteristic peaks around 1600 and 1575 cm−1 and 1603
and 1554 cm−1, respectively. The bands around 1600 cm−1 would be attributed to
C–N of the
ν
noncoordinating oxazolynyl groups of 2-ToM. UV-vis spectra of CH2Cl2 solutions showed intense
band characteristics to the tetrahedral cobalt(II) d–d transition at
λ
max = 525 (ε ≈ 440 M–1 cm–1)
and 554 nm (ε ≈ 580 M–1 cm–1) for
4 (Supplementary Figure S4) and λmax = 562 (ε )
= 539 M−1 cm−1
and 576 nm (ε
= 552 M−1 cm−1) for [CoII(k2-ToM)2]. The observed blue shift of the d–d transition
bands from those of [CoII(k2-ToM)2] to
4
might imply that the strength of the ligand fields is LPh > ToM
; a preliminary result of single crystal X-ray analysis
.
In fact, a homoleptic LPh complex [CoII(LPh)2] (
5
is provided as Supplementary Figure S5), which was synthesized by the reaction of Co(OAc)2 and
two equiv. of LPh (Scheme 3), exhibited the tetrahedral cobalt(II) d–d transition at
(ε ≈ 300 M–1 cm–1) and 548 nm (ε ≈ 420 M–1 cm–1) (Supplementary Figure S4).
λmax = 520
Figure 3. Molecular structure of 4. All hydrogen atoms are omitted for clarity; thermal ellipsoids are
set at 30% probability. Selected bond lengths [Å] and angles [◦]: Co1-N1 2.004 (3), Co1-N2 2.000 (3),
Co1-N4 1.989 (4), Co1-N6 1.972 (3), N1-Co1-N2 97.0 (1), N1-Co1-N4 121.9 (1), N1-Co1-N6 110.6 (1),
N2-Co1-N4 115.4 (1), N2-Co1-N6 115.3 (1), N4-Co1-N6 97.7 (1).
Scheme 3. Synthesis of [CoII(LPh)2] (5).

Molecules 2018, 23, 1466
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2.1.3. Oxidation Potentials
As we have reported previously, Tp*-LPh complex
3
exhibits reversible O2 sorption/desorption
ability at low temperature [18]. In this work, we checked the O2 binding capability of acac complexes
1
and
2
and ToM-LPh complex
4
toward dioxygen. However, these cobalt(II) complexes were inert toward
◦
dioxygen even at
−
80 C. The homoleptic LPh complex
5
was also inactive. Therefore, oxidation
potentials of the cobalt(II) centers of
previously reported cobalt(II)/(III) potential of 3 (Supplementary Figure S6).
Among the examined complexes, ToM-LPh complex
exhibited the lowest cobalt(II/III) potential.
The order of the oxidation potentials was (0.49 V) < (0.77 V) < (1.15 V) < (1.18 V). However, the
observed potential of the MeCN solution of was still higher than the reported value for (0.18 V).
Notably, a dihydrobis(pyrazolyl)borate (Bp) analogue of
, that is, [CoII(Tp*)(Bp)], showed reversible
O2 sorption/desorption ability at low temperature, although its oxidation potential was higher (0.52 V)
than that of 26]. No O2 binding ability of could be attributed to a structural reason (i.e., it was
impossible to form a desired six-coordinated cobalt(III)-O2 adduct species). In the case of acac and
3-scorpionate complexes
and , the difference of the potentials can be correlated with the extent of
1, 2, 4, and 5 were estimated by CV and compared with the
4
4
2
1
5
4
3
3
4
[
4
1
2
the electron donation from the scorpionate ligands. The trend of the electron-donating abilities of ToM
vs. Tp* in the entitled acac complexes of cobalt(II) is inconsistent with that in the previously reported
[MI( 3-scorpionate)(CO)3] [12]. In the present cobalt complexes, the observed cyclic voltammograms
were irreversible. Therefore, structural factors should be considerable.
2.2. Oxidation Catalysis
To date, some cobalt compounds, including TpR
employed as catalysts for hydrocarbon oxygenation with organic peroxides (ROOH) such as tert-butyl
hydroperoxide (TBHP) [28 36] and meta-chloroperbenzoic acid (mCPBA) [10 37 43]. Therefore,
[27, [10] complexes, have been
28] and ToM
–
, –
we explored catalysis of the entitled mixed ligand complexes toward alkene oxidation with TBHP and
alkane oxidation with mCPBA as indicated below. We also examined catalytic oxidation of alkenes and
alkanes with H2O2. But the entitled mixed ligands complexes 1–4 were inactive in the H2O2 system.
2.2.1. Oxidation of Alkene with TBHP
We reported on Tp-based immobilized cobalt complex catalysts and their activity toward
cyclohexene oxidation with TBHP [28]. Cyclohexane is a valuable substrate as a probe to gain
insights into the character of the active oxidant and reaction mechanism. When an oxidant
reacting with cyclohexene has a radical character, allylic oxidation rather epoxidation of the C=C
moiety is favored. Products derived from the allylic oxidation are allylic alcohol (
), and peroxide ( ). Formation of indicates involvement of alkylperoxyl radical (tert-BuOO
which is formed by degradation of TBHP through the Habor–Weiss mechanism and/or homolysis
of an M–O bond of a putative metal–alkylperoxo complex [36 44]. In our previous work, catalytic
A
), ketone
(K
P
P
·
),
,
activity of the cobalt(II) compounds, including a genuine cobalt(II) acetate, tetrahedral cobalt(II)
complex [CoIIBr(allyl-TpCF3)] (allyl-TpCF3 denotes allyltris(3-trifluoromethyl-1-pyrazol-1-yl)borate),
and mesoporous silica-supported TpCF3Co complexes, was examined without any additive, and the
activity of CoII(OAc)2 4H2O and [CoIIBr(allyl-TpCF3)] under homogeneous conditions was lower than
·
that of the immobilized complexes under heterogeneous reactions [28].
In this study, small amounts of triethylamine (5 equiv. of cobalt) were added. As shown in Tables 1
and 2, the major product was
TBHP in the reaction mixture was quenched by PPh3 prior to the product analysis, the yields of the
allylic oxidation products and were decreased, although the major product was (Supplementary
P in all cases, and the yields of E were very low. When the remaining
A
K
P
Figure S8). These trends suggested that the reactions proceeded through a radical mechanism. It is
known that oxidations of aliphatic C–H groups with H2O2 through radical mechanisms result in
generation of the corresponding alkylhydroperoxides, and the resulting alkylhydroperoxides are

Molecules 2018, 23, 1466
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converted to alcohols when the reaction mixtures are treated with PPh3 quencher prior to analysis [43].
In our system, the absence of an increase of suggested that cyclohexenylhydroperoxide did not
form. The activities of the entitled mixed ligand complexes
and the homoleptic complexes of LPh
) and Tp* ([CoII(Tp*)2] ( ) [20]) were lower than those of CoII(OAc)2 4H2O and CoII(acac)2
2H2O.
A
1–4
(
5
6
·
·
The order of the activities (estimated by turnover numbers (TONs)) of the mixed ligand complexes
1–
4
◦
was 2 > 4 > 1 > 3 at ambient temperature and 4 ≈ 2
>
1
>
3
at 60 C. In terms of correlation between the
activities and the scorpionates and bidentate ligands, ToM was better than Tp* and acac was better than
LPh. In the Tp* complexes, the order of the activities was
6
>
1
>
3
at both ambient temperature and
at ambient temperature and 4 > 3 > 5
(1.18 V) ≈ 1 (1.15 V) > (0.77 V) >
6 (0.36 V) > 3 (0.18 V), and that was inconsistent with the order of the activities. Coverage of
60 ◦C. In the LPh complexes, the order of activities was
4 > 5 > 3
◦
at 60 C. The order of Co(II)/(III) oxidation potentials was
(0.49 V) >
5
2
4
the cobalt centers by the scorpionate ligands hindered the access of TBHP, and that might be a reason
for lower activity compared to the less hindered cobalt(II) compounds such as CoII(OAc)2·4H2O and
CoII(acac)2·2H2O.
It is known that CoII(acac)2 reacts with excess TBHP in the presence of N-donating L ligands
such as pyridine or 1-methyimidazole to yield the corresponding cobalt(III)-OOtBu complex
[CoIII(OOtBu)(acac)2(L)] [29 30]. In contrast, relatively hindered TpR ligand hydrotris(3-alkyl-5-
,
isopropylpyrazol-1-yl)borates such as TpiPr2 (when 3-alkyl = isopropyl) and TptBu,iPr (when 3-alkyl =
tert-butyl) stabilize the corresponding alkylperoxo complexes of cobalt(II), [CoII(OOtBu)(TpR)] [45].
These results clearly indicate that redox properties of the cobalt centers depend on the supporting
ligands. The redox property of the cobalt center tuned by the ligands would affect not only the
formation of alkylperoxo complex through direct interaction between cobalt and TBHP, but also
outer-sphere electron transfer to cause TBHP degradation. Therefore, both steric and electronic
properties of the scorpionates and bidentate ligands affect the catalysis of the cobalt centers.
Table 1. Cyclohexene oxidation with tert-butyl hydroperoxide (TBHP) at room temperature.
Products/µmol
TON *1
Catalyst (Cat.)
E
A
K
P
[CoII(Tp*)(acac)] (1)
[CoII(ToM)(acac)] (2)
[CoII(Tp*)(LPh)] (3)
[CoII(ToM)(LPh)] (4)
[CoII(LPh)2] (5)
0.5
1.3
0.7
1.1
0.7
0.7
0.0
1.2
1.1
6.4
12.1
1.9
5.0
6.9
5.8
0.4
7.7
10.0
20.4
49.4
12.1
24.7
27.5
28.1
4.6
52.7
85.2
13.3
73.9
23.0
70.2
2.7
50.2
98.7
20.0
64.7
42.8
66.4
−
[CoII(Tp*)2] (6)
none
CoII(acac)2·2H2O
21.9
27.0
187.7
195.0
120.2
130.1
CoII(OAc)2·4H2O
1
*
TON = (cyclohexene oxide (
E
) + cyclohexen-1-ol (
A) + cyclohexen-1-one (
K
)
×
2 + 3-(tert-butylperoxy)-1-
cyclohexene (P))/cobalt(II) compound.

Molecules 2018, 23, 1466
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◦
Table 2. Cyclohexene oxidation with TBHP at 60 C.
Products/µmol
TON *1
Catalyst (Cat.)
E
A
K
P
[CoII(Tp*)(acac)] (1)
[CoII(ToM)(acac)] (2)
[CoII(Tp*)(LPh)] (3)
[CoII(ToM)(LPh)] (4)
[CoII(LPh)2] (5)
3.0
6.4
1.7
4.9
1.4
3.6
1.1
6.4
6.3
11.9
5.6
4.0
5.0
6.4
3.6
5.9
10.2
28.1
32.8
49.5
33.6
56.1
27.0
36.1
26.8
84.2
118.5
345.2
702.6
117.7
693.2
105.3
612.4
95.2
212.9
406.8
95.3
407.7
83.5
[CoII(Tp*)2] (6)
345.9
−
none
CoII(acac)2·2H2O
871.1
735.6
528.1
503.5
CoII(OAc)2·4H2O
1
*
TON = (cyclohexene oxide (
E
) + cyclohexen-1-ol (
A) + cyclohexen-1-one (
K
)
×
2 + 3-(tert-butylperoxy)-1-
cyclohexene (P))/cobalt(II) compound.
2.2.2. Oxidation of Alkane with mCPBA
Sadow and coworkers demonstrated that an acetate complex of cobalt(II) with ToM, [CoII(ToM)(OAc)],
functions as an efficient catalyst precursor for cyclohexane hydroxylation with mCPBA [10]. In contrast,
we reported that a hydroxo complex of cobalt(II) with Tp*, [(CoIITp*)2(
µ-OH)2], did not function as
a catalyst precursor for alkane oxidation with mCPBA when the ratio of cobalt(II) to mCPBA to
cyclohexane was 1:50:2500 [46]. However, the examined conditions (ratio of cobalt(II) to mCPBA to
cyclohexane, solvent, temperature) for the ToM and Tp* complexes were different. In this work, the
applicability of heteroleptic and homoleptic complexes
system was examined under the same conditions.
1–6 toward mCPBA-based alkane oxidation
All the examined complexes
was higher than that of the precursors (CoII(OAc)2
The order of TONs of the mixed-ligand complexes was
initial reaction rates was 4 > 2 ≈ 3 > 1. In the comparison of homoleptic complexes 5 and 6, the order
of the initial reaction rates was 6, although total TONs of and were comparable. These trends
of initial reaction rates support that the electron-donating ability of Tp* is lower than that of ToM.
Although the reaction mediated by proceeded slowly, the lifetime of the catalyst derived from
1–
6
exhibited activities, and their alcohol selectivity (
4H2O and CoII(acac)2
2H2O) as found in Table 3.
. As shown in Figure 4, the order of
A/K + L)
·
·
3
> 2 > 4 > 1
5
>
5
6
3
3
was longer than the others. At this moment, we have no information about reaction intermediates.
Formation of the chlorocyclohexane indicated involvement of the H-abstraction process, giving a
cyclohexyl radical in the Tp* and ToM-supported cobalt catalysts. In the examined conditions, however,
the efficiency of the oxidant was limited to around 50% due to concurrent nonproductive degradation
of mCPBA via radical reaction.

Molecules 2018, 23, 1466
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◦
Table 3. Cyclohexane oxidation with meta-chloroperbenzoic acid (mCPBA) at 35 C.
Products/µmol
A/(K + L) *2
TON *1
Catalyst (Cat.)
A
K
L
Cl
[CoII(Tp*)(acac)] (1)
[CoII(ToM)(acac)] (2)
[CoII(Tp*)(LPh)] (3)
[CoII(ToM)(LPh)] (4)
[CoII(LPh)2] (5)
587.2
640.0
705.2
605.4
630.3
641.8
69.4
104.1
154.8
133.6
108.8
115.9
125.4
0.5
52.4
32.5
48.1
36.7
37.0
32.9
2.1
36.2
47.8
58.1
54.8
55.5
50.3
3.8
468.1
531.2
563.3
475.6
495.8
504.3
−
3.8
3.4
3.9
4.2
4.1
4.1
26.6
2.8
2.8
[CoII(Tp*)2] (6)
none
CoII(acac)2·2H2O
456.2
609.9
146.6
181.3
18.4
33.2
26.7
33.1
406.5
535.9
CoII(OAc)2·4H2O
*1 TON = {cyclohexanol (
A
) + chlorocyclohexane (Cl) + 2
×
{cyclohexanone (K) + ε-caprolactone (L)}}/cobalt(II)
compound. *2 Alcohol selectivity = (cyclohexanol (A))/(cyclohexanone (K) + ε-caprolactone (L)).
A
K
L
Cy-Cl
A
K
L
Cy-Cl
A
K
L
Cy-Cl
Figure 4. Time course of cyclohexane oxidation with mCPBA mediated by 1–6.

Molecules 2018, 23, 1466
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3. Materials and Methods
3.1. General
Elemental analysis was performed on a Perkin-Elmer CHNS/O Analyzer 2400II (Perkin-Elmer,
Waltham, MA, USA). IR spectra were recorded on a FT/IR 4200 spectrometer (JASCO, Tokyo, Japan)
for solid samples (as KBr pellets). NMR spectra were recorded on a ECA-600 spectrometer (JEOL,
Tokyo, Japan). Magnetic susceptibilities of the cobalt(II) complexes were determined by the Evans
method using the 1H-NMR spectra of the CDCl3 solutions (measured at ambient temperature). UV-vis
spectra were measured on a V650 spectrometer (JASCO, Tokyo, Japan). Cyclic voltammetry was
performed on a Model 600C Electrochemical Analyzer (ALS, Tokyo, Japan). Gas chromatography (GC)
analysis was performed on a GC2010 gas chromatograph (Shimadzu, Kyoto, Japan) equipped with an
Rtx-5 column (length = 30 m, i.d. = 0.25 mm, thickness = 0.25 µm, Restek, Bellefonte, PA, USA).
NaToM 14] and LPh
[ [16] were prepared according to the literature. All manipulations
were performed under argon by standard Schlenk techniques. THF, toluene, CH2Cl2, and
MeCN were purified over a Glass Contour Solvent Dispensing System under Ar atmosphere.
meta-Chloroperbenzoic acid (mCPBA) was washed with KH2PO4-NaOH buffer solution (pH 7.4)
and pure water to remove meta-chlorobenzoic acid. Other reagents of the highest grade commercially
available were used without further purification.
3.2. Preparation and Characterization of the Cobalt(II) Complexes
3.2.1. [CoII(Tp*)(acac)] (1)
A THF solution (100 mL) of KTp* (1.282 g, 3.812 mmol) was slowly added to a THF solution
(50 mL) of CoII(acac)2 (0.988 g, 3.842 mmol) at room temperature, and the purple slurry solution was
stirred for 1 h. The volatile solvent of the solution was evaporated, and the residue was redissolved in
CH2Cl2. The solution was then passed through a filter with Celite to remove any inorganic salts. After
evaporation of CH2Cl2, the residue was redissolved in MeCN and the solution was passed through
a filter with Celite. Afte◦r reduction of solvent volume by evaporation, the solution was subject to
30 C to give the title complex as deep purple powder (0.731 g, 1.606 mmol,
recrystallization at
42.1%). Recrystallization from the MeCN solution at
X-ray crystallography.
−
−
30 ◦C gave a pink block crystal suitable for
−1
FT-IR (KBr): ν= 3127 (w), 2953 (w), 2924 (vs), 2727 (w), 2504 (vs, νB-H), 1591 (vs, νC=O), 1520 cm
(vs, νC=O) cm−1. UV-vis (CH2Cl2, r.t.):
λ = 567 nm (ε
= 75.7 M−1 cm−1). UV-vis (MeCN, r.t.): λ = 506 nm
(ε = 39.0 M−1 cm−1), 567 nm (ε = 75.7 M−1 cm−1). H-NMR (600 MHz, CDCl3, 25 ◦C, TMS): δ = 80.42
1
(br, 1H; acac-H), 64.82 (br, 1H; B-H), 55.91 (s, 3H; Pz-H3), 38.58 (s, 9H; Pz-CH3), 33.66 (s, 6H; acac-Me2),
1
−
60.44 (br, 9H; Pz-CH3). H-NMR (600 MHz, CD3CN, 25 ◦C, TMS):
δ
= 93.22 (br, 1H; acac-H), 52.58
(s, 3H; Pz-H3), 40.55 (s, 9H; Pz-CH3), 4.64 (s, 6H; acac-Me2),
−
69.89 (br, 9H; Pz-CH3). Magnetic
susceptibility: µeff = 4.1 µB. ESI-MS+ (MeOH): m/z = 478 [[CoII(Tp*)(acac)] + Na+]+. Elemental analysis
calcd (%) for [CoII(Tp*)(acac)] (C20H29N6BO2Co): calc. C 52.77 H 6.42, N 18.46; found: C 52.55, H 6.06,
N 18.79.
3.2.2. [CoII(ToM)(acac)] (2)
A THF solution (60 mL) of NaToM (1.329 g, 3.280 mmol) was slowly added to a THF solution
(30 mL) of CoII(acac)2 (0.999 g, 3.886 mmol) at room temperature. The purple slurry solution was
stirred for 1 h, and the reaction was heated at reflux for 2 h. The volatile solvent of the solution was
evaporated, and the residue was redissolved in CH2Cl2. The solution was then passed through a filter
with Celite to remove any inorganic salts. After evaporation of CH2Cl2, the residue was redissolved in
pentane and the solution was passed through a filter with Celite. After reduction of solvent volume by
evaporation, the solution was subject to recrystallization at
−
30 ◦C to give the title complex as red

Molecules 2018, 23, 1466
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◦
purple powder (0.550 g, 1.018 mmol, 31.0%). Recrystallization from the pentane solution at
1 day gave a purple block crystal suitable for X-ray crystallography.
−
30 C for
FT/IR (KBr):
UV-vis (CH2Cl2, r.t.):
r.t.): = 513 nm (
= 123.2 M−1 cm−1), 557 nm (
1H-NMR (600 MHz, CDCl3, 25 ◦C, TMS):
ν
= 3077 (w), 3042 (w), 2965 (vs), 2894 (w), 1589 (vs, νC=O), 1520 cm−1 (vs, νC=O).
λ
= 450 nm (
ε
= 38.4 M−1 cm−1), 555 nm (
ε
= 65.4 M−1 cm−1). UV-vis (MeCN,
= 167.6 M−1 cm−1).
λ
ε
ε
= 162.3 M−1 cm−1), 584 nm (
ε
δ
= 46.69 (br, 1H; acac-H), 43.74 (br, 2H; Ph), 27.57 (s, 6H;
60.42 (s, 18H; Ox-Me2). H-NMR
= 41.43 (br, 2H; Ph), 40.86 (br, 1H; acac-H), 23.83 (s, 6H; acac-Me2),
1
acac-Me2), 21.57 (s, 2H; Ph), 18.04 (s, 1H; Ph), 5.81 (s, 6H; Ox-H2),
−
(600 MHz, CD3CN, 25 ◦C, TMS):
δ
20.36 (s, 2H; Ph), 17.08 (s, 1H; Ph), 5.53 (s, 6H; Ox-H2),
−
56.73 (s, 18H; Ox-Me2). Magnetic susceptibility:
µeff = 4.4 µB. ESI-MS+ (MeOH): m/z = 540 [[CoII(ToM)(acac)] + H+]+. Elemental analysis calcd (%) for
[CoII(ToM)(acac)] (C26H36N3BO5Co): calc. C 57.80, H 6.72, N 7.78; found: C 57.46, H 6.68, N 8.02.
3.2.3. [CoII(ToM)(LPh)] (4)
An n-butyllithium hexane solution (1.64 M, ◦0.12 mL, 0.197 mmol) was added to a THF solution
(20 mL) of HLPh (0.051 g, 0.192 mmol) at
−
80 C to give LiLPh solution. The solution was then
gradually warmed to room temperature with stirring. This LiLPh solution was slowly added to a
THF solution (20 mL) of (0.115 g, 0.213 mmol) at
80 ◦C. The solution was then gradually warmed
2
−
to room temperature and stirred for 2 h. The volatile solvent of the solution was evaporated, and
the residue was redissolved in CH2Cl2. The solution was then passed through a filter with Celite
to remove any inorganic salts. After evaporation of CH2Cl2, the red purple solid was washed with
pentane to give the title complex as light pink powder (0.089 g, 0.126 mmol, 65.6%). Recrystallization
from the Et2O/hexane solution at room temperature gave a purple block crystal suitable for X-ray
crystallography. We were unable to obtain satisfactory elemental analysis data for this complex.
FT-IR (KBr):
ν
= 3062 (w), 2965 (w), 2927 (vs), 2889 (m), 1602 (w, νC=N), 1575 (s, νC=N), 1283 cm−1
(m, νBC). UV-vis (CH2Cl2, r.t.):
susceptibility: µeff = 4.0 µB. ESI-MS+ (MeOH): m/z = 707 [[CoII(ToM)(LPh)] + H+]+.
λ = 525 nm (ε ε
= 441 M−1 cm−1), 554 nm ( = 580 M−1 cm−1). Magnetic
3.2.4. [CoII(LPh)2] (5)
An n-butyllithium hexane solution (1.55 M, ◦1.30 mL, 2.015 mmol) was added to a THF solution
(25 mL) of HLPh (0.534 g, 2.006 mmol) at
−
80 C to give LiLPh solution. The solution was then
gradually warmed to room temperature with stirring for 1 h. To 10 mL MeOH solution of CoCl2
(0.130 g, 1.001 mmol), THF solution of LiLPh was added, and the resulting reaction mixture was
stirred for 1 h. The volatile solvent of the solution was evaporated, and the residue was redissolved
in CH2Cl2. The solution was then passed through a filter with Celite to remove any inorganic salts.
After evaporation of CH2Cl2, the resulting solid was dissolved in MeCN, and refrigeration yielded a
pink powder of [CoII(LPh)2] (0.096 g, 0.163 mmol, 16.3%). Recrystallization from the MeCN solution at
room temperature gave a red block crystal suitable for X-ray crystallography.
FT-−IR1 (KBr):
ν = 3142 (m), 3118 (s), 2917 (vs), 2905 (vs), 2890 (vs), 2836 (vs), 2828 (vs), 2760 (s),
1286 cm (vs, νBC). UV-vis (CH2Cl2, r.t.):
−1 −1
λ = 520 nm (ε ε
= 303 M cm−1), 548 nm ( = 419 M cm−1).
Magnetic susceptibility: µeff = 3.7 µB. Elemental analysis calcd (%) for [CoII(LPh)2] (C30H36N8B2Co):
calc. C 61.15, H 6.16, N 19.02; found: C 60.92, H 6.17, N 19.04.
3.2.5. [CoII(Tp*)2] (6)
The previously reported synthetic procedure for
6 [19] was modified as follows. MeOH solution
(30 mL) of [CoII(OAc)2] 4H2O (0.927 g, 3.722 mmol) was added to a THF solution (70 mL) of KTpMe2
·
(2.510 g, 7.465 mmol) at room temperature and the yellow solution was stirred for 1.5 h. The volatile
solvent of the solution was evaporated, and the residue was redissolved in CH2Cl2. The solution was
then passed through a filter with Celite to remove any inorganic salts. After evaporation of CH2Cl2,
the residue was redissolved in MeCN and the solution was filtered to give the title complex as yellow
powder (1.334 g, 2.042 mmol, 54.9%).

Molecules 2018, 23, 1466
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FT/IR (KBr):
= 470 nm (
Pz-H3), 43.97 (s, 18H; Pz-CH3),
ν
= 3123 (w), 2976 (w), 2925 (s), 2863 (w), 2733 (s), 2507 (vs, νB–H). UV-vis (CH2Cl2,
◦
ε δ = 45.16 (s, 6H;
= 130.9 M−1 cm−1). 1H-NMR (600 MHz, CDCl3, 25 C, TMS):
r.t.):
λ
−
81.56 (s, 18H; Pz-CH3). Magnetic susceptibility: µeff = 4.7 µB. ESI-MS+
(MeOH): m/z = 653 [CoIII(Tp*)2]+. Elemental analysis calcd (%) for [CoII(Tp*)2] (C30H44N12B2Co): calc.
C 55.15, H 6.79, N 25.73; found: C 54.84, H 6.73, N 25.78.
3.3. X-Ray Diffraction Study
Diffraction data for single crystals were collected using a Saturn 70 CDD area detector system
(Rigaku, Tokyo, Japan) with graphite monochromated Mo-K
were mounted on a CryoLoop with liquid paraffin and flash-cooled to 113 K (for 1, 2, and 5) or 133 K
(for ) by cold N2 gas flow on the goniometer. For complex , three sets of sweeps (at = 0, 90,
and 180◦) of data were done using 115.0 to 65.0◦ in 0.3◦ steps at = 45◦ with an
oscillations from
exposure rate of 13.3 s/◦, a detector swing angle of 25◦, and a crystal-to-detector distance of 55 mm
(total of 1800 oscillation images). For complex , two sets of sweeps (at
= 0 and 90◦) of data were
done using oscillations from
110.0 to 70.0◦ in 0.5◦ steps at = 45◦ with an exposure rate of 20 s/◦,
a detector swing angle of
20◦, and a crystal-to-detector distance of 45 mm (total of 720 oscillation
images). For complex , two sets of sweeps (at oscillations
= 0 and 90◦) of data were done using
from
110.0 to 70.0◦ in 0.5◦ steps at
α radiation. The block-shaped crystals
4
1
φ
ω
−
χ
−
2
φ
ω
−
χ
−
4
φ
ω
−
χ
= 45◦ with an exposure rate of 66.7 s/◦, a detector swing angle
of −20◦, and a crystal-to-detector distance of 45 mm (total of 1200 oscillation images). For 5, two sets
◦
◦ ◦
110.0 to 70.0 in 0.25 steps
20◦, and a crystal-to-detector
of sweeps (at
at
distance of 45 mm (total of 1440 oscillation images).
Data collection and processing were performed using Rigaku CrystalClear software (ver. SM
1.4.0 SP1, Rigaku, Tokyo, Japan) [47]. In all, 22,851 reflections ( , Rint = 0.028), 21,792 reflections
, Rint = 0.025), 13,756 reflections ( , Rint = 0.050), and 13m918 reflections (
collected. Equivalent reflections were merged. The linear absorption coefficient
was 7.02 cm−1 for , 6.05 cm−1 for , 5.00 cm−1 for , and 5.66 cm−1 for
φ
= 0 and 90 ) of data were done using
ω
oscillations from
−
χ
= 45◦ with an exposure rate of 80 s/◦, a detector swing angle of
−
1
(2
4
5
µ
, Rint = 0.139) were
for Mo-Kα radiation
1
2
4
5
. A numerical absorption
correction was applied, which resulted in transmission factors ranging from 0.7770 to 0.9077 for
0.8138 to 0.9041 for , 0.9106 to 0.9708 for , and 0.8349 to 0.9375 for . The data were corrected for
1,
2
4
5
Lorentz and polarization effects. Structural solution by a direct method (SIR-92) [48] and refinement
by full-matrix least squares (SHELXL-2014/7) [49] against F2 with all reflections were performed
on WinGX software (ver. 2014.1, School of Chemistry, University of Glasgow, Glasgow, UK) [50].
All non-hydrogen atoms were refined anisotropically. Hydrogen atoms adjacent to carbon atoms were
placed in calculated positions with C–H = 0.96 Å (for methyl groups) or 0.93Å (for aromatic rings)
with Uiso(H) = 1.2 Uiso (attached atom). Hydrogen atoms on boron atoms were located by difference
Fourier synthesis and were refined isotropically with B–H = 1.10 Å. The molecular structure was drawn
on ORTEP-3 for Windows [51]. Crystal information files (CIFs) of the complexes reported in this paper
have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications
CCDC-1843690 (1) CCDC-1843691 (2), CCDC-1843692 (4), and CCDC-1893693 (5). These data can be
obtained free of charge from the Cambridge Crystallographic Data Centre at www.ccdc.cam.ac.uk/
data_request/cif. The molecular structures, bond lengths, and angles as well as crystallographic data
and structure refinement parameters of 1, 2, 4, and 5 obtained in this work are given in Figure 1 (for 1
and 2) and Figure 3 (for 4) and Supplementary Figure S4 (5) and Supplementary Table S1.
3.4. Catalytic Reactions
3.4.1. Cyclohexene Oxidation with TBHP
A solution of the cobalt(II) compounds (2.0
µmol), Et3N (10 µmol), and cyclohexene (2.0 mmol) in
MeCN (5 mL) was placed in a Schlenk and degassed with Ar gas. Next, 70% aqueous solution of TBHP

Molecules 2018, 23, 1466
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(2.0 mmol) was added under Ar atmosphere and stirred at room temperature or 60 ◦C. The reaction
products were analyzed using GC (reaction time 0, 1, 2, 3, 6, 12, 24 h).
3.4.2. Cyclohexane Oxidation with mCPBA
A solution of cobalt(II) compounds (2.0 µmol), cyclohexane (15.0 mmol) in MeCN (1 mL), and
CH2Cl2 (1 mL) was placed in a Schlenk and degassed with Ar gas. Next, a ◦CH2Cl2 (2 mL) solution
of mCPBA (2.0 mmol) was added under Ar atmosphere and stirred at 35 C. To monitor product
formation, 0.2 mL of the reaction mixture was corrected at certain times (reaction time 0, 10, 30, 60, 90,
120, 180 min) and quenched with a dichloromethane solution (0.5 mL) of triphenylphosphine (10 mg),
and then the solution was subjected to GC analysis.
4. Conclusions
The coordination behaviors of Tp* and ToM toward cobalt(II) with bidentate ligands were
compared. Both Tp* and ToM coordinate to cobalt(II) with tridentate fashion when the bidentate
ligand is the less hindered acetylacetonate. In the bis(1-methylimidazol-2-yl)methylphenylborate
2
(LPh) complexes, Tp* retains the tridentate ( 3) mode, whereas ToM functions as the bidentate (
)
ligand, giving the tetrahedral cobalt(II) complex. The bowl-shaped cavity derived from the six methyl
groups on ToM leads to susceptibility to the bulkiness of the opposite bidentate ligand. The entitled
scorpionate compounds catalyze degradation of TBHP to induce the peroxidation of cyclohexene,
although their catalytic efficiencies are lower than those of Co(OAc)2 and Co(acac)2. On the cyclohexane
oxidation with mCPBA, both ToM and Tp* complexes function as the catalysts. The difference in the
electron-donating abilities of the scorpionates, that is, ToM > Tp*, is reflected in the initial reaction rates.
Supplementary Materials: The following are available online. Figure S1: 1H-NMR spectra of CDCl3 and CD3CN
solutions of
KBr pellets of
and . Figure S5: Molecular structure of
1
. Figure S2: 1H-NMR spectra of CDCl3 and CD3CN solutions of
2
. Figure S3: FT/IR spectra of
1
,
2,
4, and
5
measured at room temperature. Figure S4: UV-vis spectra of CH2Cl2 solutions of
4
5
5
. Figure S6: Cyclic voltammograms of
1
–
5. Figure S7: Time course of
cyclohexene oxidation with TBHP mediated by
1
–6
, CoII(acac)2 2H2O, and CoII(OAc)2
·
·
4H2O. Figure S8: Product
analysis for cyclohexene oxidation with TBHP by
2 with or without the PPh3 quencher. Table S1: Crystallographic
data of 1, 2, 4, and 5.
Author Contributions: T.N., J.N., and S.H. conceived and designed the experiments on synthesis and catalytic
application of cobalt compounds; T.N., T.U., and Y.T. performed synthesis of the complexes; T.N. performed
experiments on oxidation and analysis of products by GC; J.N. carried out x-ray experiments and their analysis;
and T.N., J.N., and S.H. wrote the paper.
Funding: This research was funded by CREST, JST (JPMJCR16P1) and Kanagawa University (ordinary budget
for 411).
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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