Efficient alkane hydroxylation catalysis of nickel(ii) complexes with oxazoline donor containing tripodal tetradentate ligands

Article abstract of DOI:10.1039/d0dt00733a

Tris(oxazolynylmethyl)amine TOA^R(where R denotes the substituent groups on the fourth position of the oxazoline rings) complexes of nickel(ii) have been synthesized as catalyst precursors for alkane oxidation withmeta-chloroperoxybenzoic acid (m-CPBA). The molecular structures of acetato, nitrato,meta-chlorobenzoato and chlorido complexes with TOA^Me2have been determined using X-ray crystallography. The bulkiness of the substituent groups R affects the coordination environment of the nickel(ii) centers, as has been demonstrated by comparison of the molecular structures of chlorido complexes with TOA^Me2and TOA^tBu. The nickel(ii)-acetato complex with TOA^Me2is an efficient catalyst precursor compared with the tris(pyridylmethyl)amine (TPA) analogue. Oxazolynyl donors’ strong s-electron donating ability will enhance the catalytic activity. Catalytic reaction rates and substrate oxidizing position selectivity are controlled by the structural properties of the R of TOA^R. Reaction of the acetato complex with TOA^Me2andm-CPBA yields the corresponding acylperoxido species, which can be detected using spectroscopy. Kinetic studies of the decay process of the acylperoxido species suggest that the acylperoxido species is a precursor of an active species for alkane oxidation.

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Efficient alkane hydroxylation catalysis of nickel(II)
complexes with oxazoline donor containing
tripodal tetradentate ligands†
Cite this: DOI: 10.1039/d0dt00733a
Ikumi Terao, Sena Horii, Jun Nakazawa,
Shiro Hikichi
Masaya Okamura
and
*
Tris(oxazolynylmethyl)amine TOAR (where R denotes the substituent groups on the fourth position of the
oxazoline rings) complexes of nickel(II) have been synthesized as catalyst precursors for alkane oxidation
with meta-chloroperoxybenzoic acid (m-CPBA). The molecular structures of acetato, nitrato, meta-chlor-
obenzoato and chlorido complexes with TOAMe2 have been determined using X-ray crystallography. The
bulkiness of the substituent groups R affects the coordination environment of the nickel(II) centers, as has
been demonstrated by comparison of the molecular structures of chlorido complexes with TOAMe2 and
TOAtBu. The nickel(II)-acetato complex with TOAMe2 is an efficient catalyst precursor compared with the
tris(pyridylmethyl)amine (TPA) analogue. Oxazolynyl donors’ strong σ-electron donating ability will
enhance the catalytic activity. Catalytic reaction rates and substrate oxidizing position selectivity are con-
trolled by the structural properties of the R of TOAR. Reaction of the acetato complex with TOAMe2 and
m-CPBA yields the corresponding acylperoxido species, which can be detected using spectroscopy.
Kinetic studies of the decay process of the acylperoxido species suggest that the acylperoxido species is a
precursor of an active species for alkane oxidation.
Received 27th February 2020,
Accepted 14th April 2020
DOI: 10.1039/d0dt00733a
rsc.li/dalton
Cobalt(II) complexes with phenyltris(4,4-dimethyloxazolynyl)
borate show catalytic alkane oxygenation activity with meta-
Introduction
Oxazoline based compounds have been used as ligands of chloroperoxybenzoic acid (m-CPBA; HOOC(vO)C6H4-meta-
metal complexes catalyzing various organic transformations Cl).15,16
including oxidation reactions. For example, the oxazolynyl-
To date, various nickel(II) complexes have been reported as
phenoxide and oxazolynyl-pyridine ligand complexes of tran- efficient catalysts for alkane hydroxylation with the m-CPBA
sition metals such as Mn,1–3 Fe,4 Mo,5–7 Pd8 and Re9 catalyze oxidant.17–27 The supporting ligands of these nickel complexes
the oxygenation of alkanes and alkenes with H2O2 and tert- comprise nitrogen-donating azoles such as pyridine, imidazole
BuOOH oxidants. Poly(oxazolynyl) ligands have also been and pyrazole as well as oxygen-donating phenols. As described
employed as the supporting ligands of oxidation catalysts. above, oxazoline derivatives are also expected to be versatile
Enantioselective epoxidation of alkenes with PhIO is catalyzed supporting ligands of nickel complexes catalyzing alkane
by a chiral oxazoline ligand complex of iron.10 Stereoselective hydroxylation. However, oxazoline based ligands have not been
allylic oxidation with alkylperoxycarboxylate is mediated by used for nickel complex catalysts activating m-CPBA. Notably,
copper complexes supported by chiral oxazoline chelators.11–14 2-oxazolines exhibited π-electron delocalization on the N–C–O
segment. An imine nitrogen donor of 2-oxazoline derivatives is
expected to be a strong σ-electron donor because of its amido
character.28 Therefore, replacement of azoles by oxazolines in
Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa
University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan.
nickel-supporting ligands would change the catalytic perform-
E-mail: hikichi@kanagawa-u.ac.jp; Fax: +81-45-413-9770
ance. In this study, we selected tris(2-oxazolinylmethyl)amines,
†Electronic supplementary information (ESI) available: Data for cyclohexane-d12
namely TOAR (where R denotes the substituent groups on the
and toluene oxidation mediated by 1 and 7, crystallographic data for 1–5, UV-vis
spectra of CH2Cl2 solutions of 1–6, ESI-MS spectra of MeCN solutions of 1–6, IR
fourth position of the oxazoline rings), as nickel(II) supporting
ligands. Electron donation from the ligand would be enhanced
spectra of KBr pellets of 1–6, 1H NMR spectra of 1, 2, 4, 5 and 6, differential
pulse voltammograms of 1 and 7, time course of total TONs of 1–10 in cyclo-
by the combination of the tertiary amine nitrogen donor and
hexane oxidation, and decay of the acylperoxido species in CH2Cl2 or CD2Cl2 at
25 °C. CCDC 1983778–1983782. For ESI and crystallographic data in CIF or other
the oxazolines, which would be favorable for m-CPBA acti-
vation. TOAR can be regarded as the analogues of tris(pyridyl-
electronic format see DOI: 10.1039/D0DT00733A
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methyl)amine (TPA), in which the pyridine donors are replaced
by oxazolines. Notably, the first nickel-based alkane hydroxy-
lation catalyst is the TPA complex reported by Itoh and
coworkers.17
Results and discussion
Synthesis and characterization of Ni(II) complexes with TOAR
ligands
The oxazoline-containing tertiary amines TOAR have been used
as the supporting ligands of metal complex catalysts;11,12
however, the molecular structure of the TOAR complex is
reported only for the copper(I) complex with TOAMe2
,
[CuI(TOAMe2)]2(BF2)2.29 In that dimeric copper(I) complex,
TOAMe2 bridges two copper centers. The three-coordinated
copper(I) center is supported by two of the three oxazolynyl
nitrogen donors of one TOAMe2 and one oxazoline donor of
another TOAMe2; the amine nitrogen of TOAMe2 does not coor-
dinate. In this study, therefore, the coordination behavior of
TOAR toward nickel(II) was examined first.
Fig. 1 Crystal structures of the cationic nickel(II)–TOAMe2 complex parts
of 1 (a), 2 (b) and 3 (c) (30% probability). All hydrogen atoms except the
aqua ligand in 3 are omitted.
Reacting TOAMe2 and NiX2 in MeOH and adding NaBPh4
gave the corresponding mononuclear nickel(II) complexes,
[NiIIX(TOAMe2)](BPh4), where X = OAc (1), NO3 (2) and Cl (4)
(Scheme 1). These complexes showed paramagnetically shifted
1H NMR signals attributed to the unpaired electron configur-
ation on the nickel centers (Fig. S4†). X-ray crystallographic
analysis revealed their molecular structures. The nickel(II)
centers are supported by three oxazolynyl nitrogen donors and
the bridgehead amine nitrogen of TOAMe2. In the acetato and
nitrato complexes 1 and 2, the anionic ligands are bound to
the nickel(II) centers in bidentate mode to give the six-co-
ordinated complexes (Fig. 1(a) and (b), and Table 1). These
green-colored complexes showed similar UV-vis spectral pat-
terns involving three weak absorption bands (around 400, 600
and 800 nm; see Fig. S1(a) and (b)†). Conversely, an orange-
colored chlorido complex 4 showed absorption bands at
465 nm with medium intensity and at 732 nm with low inten-
sity (Fig. S1(c)†). The observed spectral pattern might be attrib-
uted to a distorted five-coordinated nickel(II) center, as evi-
denced by the crystal structure (Fig. 2(a) and Table 2). The
value of a normalized measure of geometry of the five-co-
ordinated center τ = 0.59 indicated that the geometry of the
chloride-bound nickel center was intermediate between a
square pyramid (τ = 0) and trigonal bipyramid (τ = 1).
A meta-chlorobenzoato (OC(vO)C6H4-meta-Cl) complex 3
was synthesized by reacting the nitrato complex 2 with the pot-
assium salt of meta-chlorobenzoic acid. In complex 3, the
nickel(II) center is coordinated by the tetradentate TOAMe2
,
monodentate carboxylate and H2O (Fig. 1(c) and Table 1).
Intra-molecular hydrogen-bonding interaction between
a
hydrogen atom of the aqua ligand and an oxygen atom of the
carbonyl moiety of meta-chlorobenzoate is formed in the
crystal structure, as found in the nickel(II)–acetato complex
with TPA, [Ni(OAc)(H2O)(TPA)]+ (7).17
To estimate the structural effects of the substituent groups
on the oxazoline rings, TOAtBu and TOAPh were also employed
as the supporting ligands of nickel(II)–chlorido complexes
(Scheme 2). The TOAtBu complex 5 was characterized by X-ray
crystallography (Fig. 2(b)). The geometry of the five–co-
Scheme 1 Synthesis of TOAMe2 complexes 1–4.
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Table 1 Structural parameters of hexa-coordinated nickel(II) complexes
with TOAMe2 (1–3)
Table 2 Structural parameters of nickel(II)–chloride complexes with
TOAMe2 (4) and TOAtBu (5)
3
Complex
4
5
Bond length/Å
Ni–N1oxazoline
Ni–N2oxazoline
Ni–N3oxazoline
Ni–N4amine
Ni–Cl
Complex
1
2
Molecule 1
Molecule 2
2.033(3)
2.052(3)
2.020(3)
2.279(3)
2.3264(12)
2.0553(16)
2.0447(17)
2.0241(17)
2.2218(16)
2.2517(5)
Bond length/Å
Ni–N1oxazoline 2.1092(19) 2.061(3)
Ni–N2oxazoline 2.1597(19) 2.071(3)
Ni–N3oxazoline 2.0561(19) 2.033(3)
Ni–N4amine
Ni–O1
2.088(2)
2.071(3)
2.095(2)
2.200(2)
2.019(2)
2.104(3)
2.184(2)
2.091(2)
2.196(2)
2.023(2)
2.180(2)
2.0526(19) 2.046(2)
2.1418(17) 2.331(2)
2.215(3)
Bond angle/°
N1–Ni–N2
140.50(12)
103.39(12)
80.23(11)
99.07(9)
104.96(12)
78.28(11)
100.05(9)
79.97(12)
104.10(10)
175.91(8)
0.59
123.61(7)
121.11(7)
77.96(6)
99.69(5)
102.03(7)
76.97(6)
102.72(5)
78.63(7)
104.58(5)
176.74(5)
0.89
Ni–O2
2.171(2) (H2O) 2.132(2) (H2O) N1–Ni–N3
N1–Ni–N4
Bond angle/°
N1–Ni–N2
N1–Ni–N3
N1–Ni–N4
N1–Ni–O1
N1–Ni–O2
N2–Ni–N3
N2–Ni–N4
N2–Ni–O1
N2–Ni–O2
N3–Ni–N4
N3–Ni–O1
N3–Ni–O2
N4–Ni–O1
N4–Ni–O2
O1–Ni–O2
154.96(8) 147.00(10) 153.19(9)
96.49(8)
80.42(7)
152.17(10)
94.72(10)
80.73(10)
93.10(9)
87.91(10)
96.40(9)
76.21(9)
107.74(9)
79.01(9)
80.03(9)
93.10(9)
174.17(9)
172.52(9)
95.29(9)
91.74(9)
N1–Ni–Cl
N2–Ni–N3
N2–Ni–N4
N2–Ni–Cl
N3–Ni–N4
N3–Ni–Cl
N4–Ni–Cl
τ value
100.35(11) 96.06(9)
80.48(10) 81.05(9)
100.21(8) 101.38(10) 102.30(9)
88.75(7)
93.10(8)
77.88(8)
98.41(8)
85.01(8)
82.94(7)
87.13(9)
100.56(11) 94.61(9)
77.94(9) 76.57(9)
100.37(9) 102.06(9)
83.29(9) 80.94(9)
80.94(10) 80.46(9)
90.85(9)
87.02(9)
109.01(8) 98.66(9)
(Fig. 2(c)). The solid angle analysis30 of the chlorido complexes
revealed the difference in the steric hindrance of TOAMe2 and
TOAtBu. The equivalent cone angle values (degree) of TOAMe2
and TOAtBu are 230.27° and 236.05°, respectively, and the per-
centages (%) of the nickel sphere shielded by TOAMe2 and
TOAtBu are 71.24 and 73.49, respectively. TOAPh complex 6 for-
mation was confirmed using electrospray ionization-mass
(ESI-MS) spectrometry analysis, although its molecular struc-
ture has not yet been determined using X-ray crystallographic
analysis. Notably, the UV-vis spectral pattern of the TOAPh
complex was more similar to those of the hexa-coordinated
TOAMe2 complexes [Ni(κ2-X)(TOAMe2)]+ (where X = OAc (1) and
NO3 (2)) and [Ni(κ1-OC(vO)C6H4-meta-Cl)(H2O)(TOAMe2)]+ (3)
than those of the penta-coordinated chlorido complexes with
TOAMe2 (4) and TOAtBu (5) (Fig. S1†). In fact, elemental analysis
data for the TOAPh complex, which was synthesized by the
mixing of TOAPh, NiCl2·6H2O and NaBPh4 in MeOH and then
extracting with CH2Cl2 (see experimental), was consistent with
the formulation [NiCl(TOAPh)(MeOH)](BPh4). A broad band at
around 3500 cm−1 in an IR spectrum might be assigned to the
νO–H of MeOH bound to the nickel center (Fig. S3(f)†). Also,
the ESI-MS spectra of an MeCN solution of the TOAPh complex
showed that the ion peaks were attributed to the formulation
[NiCl(TOAPh)(MeCN)]+ (Fig. S2(f)†). Three phenyl substituent
groups in TOAPh would make a deep pocket, but the coordi-
nation sphere of the nickel center of the TOAPh complex might
be larger than that of TOAtBu because of the planar shape of
the phenyl group. Therefore, the obtained 6 has a hexa-co-
171.10(7) 157.32(9) 174.97(9)
167.78(7) 178.14(9) 171.01(9)
105.10(7) 121.59(8) 96.14(9)
62.81(7)
58.73(8)
92.39(9)
Fig. 2 Top: crystal structures of the cationic part of the chlorido com-
plexes with TOAMe2 4 (a) and TOAtBu 5 (b) (30% probability). All hydrogen
atoms are omitted. Bottom (c): crystal structures of the cationic part of
4 (left) and 5 (right) viewed along the N–Ni axis.
ordinated nickel(II) center supported by tetradentate TOAPh
monodentate Cl− and the solvent (MeOH).
,
ordinated nickel(II) center is close to trigonal-bipyramid, as
indicated by the τ value = 0.89 (Table 2). The view along the
Catalytic behaviors of the TOAR complexes
Cl–Ni–N axis indicated that the three oxazoline rings linked to Cyclohexane oxidation with m-CPBA has been achieved using
nitrogen through methylene spacers appeared in a twisted various nickel(II) complexes. Therefore, the catalytic activities
screw-like arrangement because of the steric repulsion of the of a series of TOAR complexes and the previously reported cata-
tert-butyl groups in the pseudo C3v symmetric TOAtBu lytically active TPA and Tp* complexes (7 and 8)17,21 were
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Scheme 2 Synthesis of chlorido complexes with TOAtBu (5) and TOAPh (6).
examined (Table 3). In addition, chlorido complexes with were found for a series of nickel(II)–TPA complexes.18 In
anionic tridentate ligands [NiCl(L)] in which L = Tp* (9)31 and addition, the different activities for hydroxo and chloride com-
ToM (10),32 where ToM is a kind of scorpionate ligand compris- plexes with Tp* (8 and 9) highlighted the dependency of the
ing 4,4-dimethyloxazoline, were examined (Fig. 3).
anionic ligands on the catalytic performance (Fig. S8†).
The acetato complex with TOAMe2 1 showed a higher cata-
The order of the initial turnover frequencies (TOF) of the
lytic rate than the TPA complex 7 (Fig. S5†). The electro- chlorido complexes with TOAR was TOAMe2 (4) > TOAPh (6) ≫
chemical oxidation potential of 1 was lower than that of 7 as TOAtBu (5) as shown in Fig. S9.† This order seems to reflect
analyzed by the differential pulse voltmmetry (DPV) method that the bulkiness of TOAR depends on R, as discussed above.
(Fig. S6†). Therefore, the electron donating ability of oxazoline The initial TOF of the TOAPh complex 6 was almost half that of
seems to be higher than that of pyridine, and such a difference the TOAMe2 complex 4, although the yields of cyclohexanol (A)
in the electronic properties of the tripodal capping ligands (i.e. and alcohol selectivity (A/(K + L) values) obtained using both
TOAMe2 and TPA) might reflect on the catalytic performance. catalysts were comparable. Notably, the oxidant utilization
In the previously reported cyclohexane oxidation with the Ni/ efficiencies were very high with these catalysts; the total turn-
m-CPBA system, the catalytic reaction rate of the Tp* complex over number (TON) was ca. 970 for both catalysts and the
8 is higher than that of an analogous complex with an elec- theoretical maximum TON was 1000. The low reaction rate and
tron-withdrawing bromine containing ligand Tp*,Br (Tp*,Br
hydrotris(3,5-dimethyl-4-bromo-pyrazol-1-yl)borate).21
=
TON of the TOAtBu complex 5 might imply that the steric hin-
drance of the tert-butyl groups of TOAtBu was critical for the
The activity of the chlorido-TOAMe2 complex 4 was close to oxidant and substrate to access the nickel center.
that of the acetato derivative 1, whereas those of the nitrato (2)
The activities of the chlorido complexes with anionic tri-
and meta-chlorobenzoato (3) complexes were lower because dentate ligands Tp* (9) and ToM (10) were lower than those of
induction periods were involved at the initial stage of the reac- the TOAR complexes 4–6. Particularly, the different catalytic
tion (Fig. S7†). Similar anionic ligand-dependent activities activities of TOAMe2 and ToM complexes (with the ligands com-
Table 3 Cyclohexane oxidation
Products/μmol
Complex
Ligands
Time/min
A
K
L
Cy-Cl
Ph-Cl
TONa
A/(K + L)
1
2
3
4
5
6
7
8
9
TOAMe2/OAc
TOAMe2/NO3
TOAMe2/H2O/OC(vO)C6H4-m-Cl
TOAMe2/Cl
10
120
30
10
180
20
1481.2
1160.7
1231.1
1515.4
552.8
1498.5
1197.8
606.3
213.0
185.0
128.6
164.1
49.7
161.3
110.5
111.7
34.2
13.0
31.9
30.2
11.9
27.0
15.9
59.5
41.8
14.4
14.5
114.7
84.65
53.1
81.3
31.9
89.28
119.3
76.0
48.7
31.1
1053.4
660.7
492.6
786.0
286.6
862.8
771.1
480.7
234.7
202.4
967
840
801
974
369
971
829
495
262
254
6.6
5.4
7.8
8.6
7.2
8.5
7.0
3.9
7.8
6.9
TOAtBu/Cl
TOAPh/Cl
TPA/OAc
Tp*/OH
60
180
180
180
Tp*/Cl
378.0
370.2
10
ToM/Cl
39.3
a TON = (A + 2 × K + 2 × L + Cy-Cl)/nickel.
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Fig. 3 Complexes applied to the catalytic cyclohexane oxidation with m-CPBA.
prising 4,4-dimethyl oxazoline) suggest that an additional than those of Tp*.32 Therefore, the electronic properties
amine nitrogen donor on TOAMe2 (resulting in the tetradentate suggest that the intrinsic activity of the nickel(II)–chlorido
chelating ligand) enhances electron donation to induce the complex with ToM (10) will be higher than that of the Tp*
activation of the O–O bond of m-CPBA. Regarding the anionic derivative 9, but the steric hindrance arising from the six
tridentate ligands, however, the electron donating ability of methyl groups surrounding the nickel center in 10 prevents
ToM is higher than that of Tp* 33 and the catalytic activity of the oxidant and substrate from accessing the nickel center.
[CoII(acac)(ToM)] toward cyclohexane hydroxylation with
The substituent effect on TOAR toward the selectivity of the
m-CPBA is higher than that of [CoII(acac)(Tp*)].16 The catalytic substrate oxidizing position was investigated using the bulky
activities of the nickel(II)–chlorido complexes 9 and 10 were, substrates methylcyclohexane (Table 4) and adamantine
however, comparable. Previous reports on the comparison (Table 5), which involve primary, secondary and tertiary C–H
between the metal complexes with ToM and Tp* demonstrate groups. Regarding the oxidation of methylcyclohexane with
that the calculated equivalent cone angles of ToM are larger m-CPBA by the chlorido complexes 4–6 shown in Table 4, the
Table 4 Methylcyclohexane oxidation
Products/μmol
Complex
3°-ol
o-2°-ol
m-2°-ol
p-2°-ol
2°-ones
1°-ol
TONa
3°-ol : 2°-ols
3°/2° b
4
5
6
47.7
33.6
45.1
6.1
20.2
15.4
19.7
0.9
21.8
17.1
19.9
0
8.6
8.1
10.2
0
4.3
2.9
5.3
Trace
2.4
0.5
0.4
0.4
0
54
40
53
—
—
49 : 51
45 : 55
48 : 52
77 : 23
84 : 16
8.7
7.7
8.2
67.8
49.7
Nonec,d
Nonec,e
153.6
10.4
11.5
6.6
0.2
a TON = (3°-ol + 2°-ols + 2 × 2°-ones + 1°-ol)/nickel. b 3°/2° = (10 × 3°-ol)/(o-2°-ol + m-2°-ol + p-2°-ol + 2°-ones). c From ref. 21. d Reaction conditions:
13.0 mmol of methylcyclohexane, 0.26 mmol of m-CPBA, 1 mL of CH2Cl2, 40 °C, 60 min. e Reaction conditions: 13.0 mmol of methylcyclohexane,
0.26 mmol of m-CPBA, without solvent, 80 °C, 120 min.
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Table 5 Adamantane oxidation
Products/μmol
1-ol
Complex.
2-ol
2-one
1,3-diol
TONa
3°/2° b
4
5
6
95.44
73.62
79.64
11.48
14.58
16.53
13.81
2.07
0.90
0.86
1.14
2.07
0.57
0.46
0.54
0.19
57
46
48
—
18.7
12.9
16.2
8.6
None
a TON = (1-ol + 2 × 1,3-diol + 2-ol + 2 × 2-one)/nickel. b 3°/2° = [3 × (1-ol + 1,3-diol × 2)]/(2-ol + 2-one).
total amount of secondary C–H oxidized products was greater pounds (benzyl alcohol and benzaldehyde) but also many
than the tertiary C–H oxidized alcohol, although the tertiary coupling products arising from the benzyl radical. Notably, the
C–H oxidation occurred mainly without the catalyst.21 The ter- 1,2-diphenylethane (bibenzyl; the homo-coupling product of
tiary C–H bond of methylcyclohexane is sterically crowded. the benzyl radical) yield obtained using the TOAMe2 complex 1
Therefore, the bulky ligand catalyst favors the oxidation of the was higher than that obtained using the TPA complex 7. These
less hindered secondary C–H, although the bond dissociation observations suggest that the radical character of the active
energy (BDE) of the secondary C–H is higher than that of the species formed on the TOAMe2 catalyst is stronger than that
tertiary C–H. The order of total TON and 3°/2° values (selecti- formed on the TPA catalyst.
vity of the tertiary alcohols normalized by the number of
Detection of the acylperoxide adduct on nickel and the kinetic
study
hydrogen atoms attached on the carbon) was R = Me2 (4) > Ph
(6) > tBu (5) of TOAR.
As shown in Table 5, the same trend of the order of TON We previously characterized the acylperoxido complexes of
and d3°/2° values of the TOAR complexes (4 > 6 > 5) was nickel(II) supported by TpR.21,22 In this study, reactions of 1
observed for adamantane oxidation. The 3°/2° value of with m-CPBA in the absence of the substrate were monitored
m-CPBA itself (without the catalyst) toward adamantane was using spectroscopy.
close to that of the tert-butoxyl radical (generated via the
The cold-spray ionization-mass (CSI-MS) spectra of the
homolysis of O–O of tert-BuOOH)34 while smaller than the 3°/ mixture of the acetato complex 1 and m-CPBA showed ion
2° values in the presence of the catalysts. Upon methyl- peaks attributed to an acylperoxido adduct [Ni(OOC(vO)C6H4-
cyclohexane oxidation, however, the 3°/2° values of m-CPBA meta-Cl)(TOAMe2)]+. In addition, the ion peaks of the meta-
(without catalyst) were larger than those of the TOAR com- chlorobenzoato (3 - H2O - BPh4) and the chlorido (4 - BPh4)
plexes. Regarding adamantine oxidation, the small 3°/2° value complexes, which are putative decomposed products resulting
implies the strong oxidative power of the oxidant as supported from the nickel-acylperoxido species, were observed even in
by the small value (around 2) of the indiscriminate hydroxyl the analysis of the solution immediately after mixing 1 and
radical. Also, the 3°/2° value is correlated with the radical char- m-CPBA (Fig. 4(a)). In the TpR complexes, the formation of the
acter of the oxidant as suggested by the large 3°/2° values of chlorido complex was observed during the decomposition of
iron-oxo species with porphyrin ligands.34 The difference in the acylperoxido complex in the presence of CH2Cl2.21 The ion
the magnitude relationship of the 3°/2° values between the peaks of the acylperoxido adduct and 1 disappeared later
m-CPBA (without the catalyst) and the TOAR complexes (Fig. 4(b)).
depending on the substrates might indicate that the 3°/2°
value depends not only on the nature of the oxidant but also vis spectroscopy. Adding one equivalent of m-CPBA to the
on the steric hindrance of the substrates.
CH2Cl2 solution of 1 at 10 °C caused an increase in the absorp-
Acylperoxido adduct formation was also observed using UV-
To obtain insights into the nature of the active species, tion band at 398 nm (Fig. 5). This band decayed gradually at
C6D12 and toluene oxidation by the acetato complexes with the same temperature and was accelerated by adding
TOAMe2 (1) and TPA (7) was examined (Tables S1 and S2†). xanthene. Therefore, the absorption band at 398 nm was
Regarding C6D12 oxidation, the kinetic isotope effect (KIE) esti- assigned to the acylperoxido complex. Pseudo-first order reac-
mated using cyclohexanol yields on the TOAMe2 complex 1 tion rates ( =kobs) for acylperoxido complex decay were satu-
(2.67) was lower than that estimated using the TPA complex 7 rated by increasing the concentration of xanthene, which
(3.74). Toluene oxidation yielded not only oxygenated com- implies that the reaction proceeds through a Michaelis–
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Fig. 4 Cold-spray ionization mass (CSI-MS) spectra of the reaction mixture of 1 and m-CPBA in CH2Cl2 at room temperature. (a) Measured immedi-
ately after mixing 1 and m-CPBA. (b) Measured after one day at ambient temperature.
Fig. 5 UV-vis spectra of the reaction of 1 and m-CPBA in CH2Cl2 at
10 °C.
Menten type mechanism (Fig. 6). In the presence of cyclo-
hexane, however, m-CPBA complex decay was not accelerated.
Notably, the decay of the acylperoxido species of the Tp*-nickel
system exhibited the KIE depending on the solvents CH2Cl2
and CD2Cl2.21 In the present TOAMe2 complex, no KIE was
observed, as evidenced by the fact that the first-order decay
rates at 25 °C were 1.31 × 10−4 s−1 in CH2Cl2 and 1.03 × 10−4
s−1 in CD2Cl2 (Fig. S10†). These observations suggest that the
Fig. 6 Reaction of the acylperoxido complex and xanthene in CH2Cl2 at
10 °C. (a) Reaction monitored by UV-vis spectroscopy. (b) Plot of the
acylperoxido complex with TOAMe2 is a precursor of the active
species for the oxidation of high C–H BDE substrates, although
pseudo-first order decay rates of the acylperoxido complex versus the
the low C–H BDE substrates can be oxidized directly by the concentration of xanthene. (c) Lineweaver–Burk plot.
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acylperoxido complex. Some nickel-oxo or/and oxyl species Electrochemical Measurement System HZ-7000. Gas chromato-
formed through O–O bond activation of m-CPBA have been graphy (GC) analyses were conducted on Shimadzu GC-2010
reported.35–37 In the presented system, the O–O bond of the co- and GC-2025 instruments with a flame ionization detector
ordinated acylperoxide might be activated to obtain nickel(II) equipped with Restek Rtx-5 (on GC-2010) and Rtx-WAX (on
or nickel(III)-oxyl species as real active species for alkane GC-2025) capillary columns (Restek, length = 30 m, i.d. =
oxidation.
0.25 mm, thickness = 0.25 μm). GC-MS analyses were con-
ducted on a Shimadzu PARVUM2 system equipped with a
RESTEK Rtx-5MS capillary column (Restek, length = 30 m, i.d.
= 0.25 mm, thickness = 0.25 μm).
Conclusion
Synthesis of TOAMe2 complexes
Nickel(II) complexes with oxazoline donors containing tertiary
amine ligands, TOAR, were synthesized. X-ray crystallographic The acetato (1), nitrato (2) and chloride (4) complexes with
analysis of the TOAMe2 complexes revealed that the TOAMe2 TOAMe2 were synthesized using the same procedures. The
ligand acted as the tripodal tetradentate chelating reagent to meta-chlorobenzoato complex 3 was synthesized from 2 using
support the mononuclear nickel(II) center. The shape and a ligand exchange reaction.
bulkiness of the substituent groups R on the fourth position of
the oxazoline rings affect the coordination environment of the thetic procedure for the acetato complex 1 is described. A
nickel centers.
methanol solution (10 mL) of TOAMe2 (1.51 g; 4.30 mmol) was
[NiII(OAc)(TOAMe2)](BPh4) (1). As a typical example, the syn-
The TOAR complexes of nickel(II) efficiently catalyze the oxi- added to a methanol solution (15 mL) of Ni(OAc)2·4H2O
dation of alkane with m-CPBA. The catalytic activity of the (1.00 g; 4.02 mmol) at RT and the solution was stirred for
acetate–nickel complex with TOAMe2 was higher than that of 30 min. Addition of NaBPh4 (1.65 g; 4.83 mmol) to this blue-
the TPA analogue because of the oxazoline’s strong σ-donating green solution led to the precipitation of a blue-green powder.
nature. The steric hindrance derived from the R on TOAR The resulting powder was collected by filtration and recrystal-
affects the catalytic reaction rate and the selectivity of the pro- lized by the slow diffusion of CH2Cl2/n-hexane yielding blue-
ducts. The catalytic reaction might proceed through the nickel green crystals of 1 (1.43 g, 45% yield). FT-IR (KBr): ν = 3056,
(II)–acylperoxido species, which could be detected using spec- 3033, 2981, 2940, 1945, 1886, 1824, 1675, 1579, 1540, 1461,
troscopy. In the present TOAR-Ni system, the acylperoxido 1419, 1369, 1306, 1307, 1274, 1064, 1008, 950, 889, 842, 734,
complex might be a precursor of the active species for alkane 707, 686, 611 cm−1. ESI-MS (positive, MeCN): m/z = 467.1 [Ni +
oxidation.
TOAMe2 + OAc]+. UV-vis (CH2Cl2): λ (ε) = 411 (21), 609 (14),
799 nm (3.2). Elemental analysis: calcd (%) for [C20H33N4NiO5]
(C24H20B) C: 67.11, H: 6.78, N: 7.12; found C: 67.16, H: 6.79, N:
7.24.
Experimental section
General
[NiII(NO3)(TOAMe2)](BPh4) (2). Green crystals of 2 (1.20 g;
1.36 mmol; 47% yield) were obtained by re-crystallization of
All manipulations were performed under argon using standard the corresponding crude yellow-green powder that was syn-
Schlenk techniques. THF, toluene, CH2Cl2 and MeCN were thesized by reacting Ni(NO3)2·6H2O (0.84 g; 2.89 mmol) and
purified over a Glass Contour Solvent Dispensing System TOAMe2 (1.03 g; 2.92 mmol) in methanol and adding NaBPh4
under an Ar atmosphere. meta-Chloroperoxybenzoic acid (1.15 g; 3.36 mmol). FT-IR (KBr): ν = 3056, 2983, 1669, 1653,
(m-CPBA) was washed with KH2PO4-NaOH buffer solution (pH 1579, 1477, 1420, 1384, 1276, 1204, 1120, 1064, 1012, 955, 884,
7.4) and pure water to remove meta-chlorobenzoic acid. Other 841, 810, 734, 707, 645, 612 cm−1. ESI-MS (positive, MeCN):
reagents of the highest commercially available grade were used m/z = 470.2 [Ni + TOAMe2 + NO3]+. UV-vis (CH2Cl2): λ (ε) = 402
without further purification. The ligands TOAMe2 29
,
TOAtBu (25), 610 (20), 800 nm (4.5). Elemental analysis: calcd (%) for
and TOAPh 11
,
and the catalyst precursors [Ni(OAc)(H2O)(TPA)] [C18H30N5NiO6](C24H20B)·1.5CH2Cl2 C: 56.93, H: 5.82, N: 7.63;
(BPh4) (7),17,18 [(NiIITp*)2(μ-OH)2] (8),38 [NiCl(Tp*)] (9)31 and found C: 57.13, H: 5.48, N: 7.70.
[NiCl(ToM)] (10)32 were prepared using methods described in
[NiII(Cl)(TOAMe2)](BPh4) (4). Orange crystals of 4 (0.641 g;
0.839 mmol; 63% yield) were obtained by re-crystallizing the
the literature.
Elemental analyses were performed using an Elementar corresponding crude orange powder that was synthesized by
Vario Micro Cube. The IR measurements of KBr pellets of reacting NiCl2·6H2O (0.320 g; 1.35 mmol) and TOAMe2 (0.468 g;
solid compounds were conducted using JASCO FT/IR 4200 1.33 mmol) in methanol and adding NaBPh4 (0.468 g;
spectrometers. UV/vis spectra were recorded on a JASCO V650 1.35 mmol). FT-IR (KBr): ν = 3055, 2998, 2983, 2937, 1953,
or Agilent 8453 spectrometer equipped with a UNISOKU 1889, 1821, 1667, 1650, 1578, 1560, 1468, 1422, 1386, 1369,
CoolSpeK USP-203-A for low-temperature measurements. Mass 1312, 1276, 1206, 1183, 1125, 1064, 1014, 957, 886, 843, 733,
spectra were recorded on a JEOL JMS-T100LC using electro- 706, 643, 611 cm−1. ESI-MS (positive, MeCN): m/z = 443.1 [Ni +
1
spray ionization (ESI) and cold-spray ionization (CSI). H NMR TOAMe2 + Cl]+. UV-vis (CH2Cl2): λ (ε) = 465 (79), 732 nm (23).
spectra were recorded on a JEOL EX-500. Differential pulse vol- Elemental analysis: calcd (%) for [C18H30N4ClNiiO3](C24H20B)
tammograms were measured on
a
Hokuto Denko C: 66.04, H: 6.60, N: 7.34; found C: 65.69, H: 6.74, N: 7.59.
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[NiII(O2CC6H4-meta-Cl)(H2O)(TOAMe2)](BPh4) (3). The pot-
Data collection and processing were performed using
assium salt of meta-chlorobenzoic acid (2.38 mg; 0.121 mmol) Rigaku CrystalClear software.39 Structural solution by direct
was added to a CH2Cl2 solution (10 mL) of the nitrato complex methods (SIR-92)40 and refinement by full-matrix least squares
2 (92.4 mg; 0.117 mmol) at RT and the resulting suspension (SHELXL-2014/7)41 against F2 with all reflections were per-
was stirred for 12 h. After the removal of insoluble compounds formed using WinGX42 software. All non-hydrogen atoms were
by filtration through a Celite pad, volatiles were evaporated refined anisotropically. Hydrogen atoms adjacent to carbon
from the filtrate to give a pale blue solid. Recrystallizing the atoms were placed in calculated positions with C–H = 0.98 Å
crude product from the CH2Cl2/n-hexane mixture yielded pale for methyl groups, 0.99 Å for methylene groups and 0.95 Å for
blue needle-shaped crystals of 3 (69.5 mg, 0.0750 mmol; 64%). aromatic rings with Uiso(H) = 1.2Uiso (attached atom). The
FT-IR (KBr): ν = 3605, 2970, 1677, 1558, 1457, 1419, 1375, molecular structures were drawn using ORTEP-3 for
1274, 1201, 1010, 953, 731, 708, 611 cm−1. ESI-MS (positive, Windows.43 The Crystallographic Information Files (CIF) of the
MeCN): m/z = 563.2 [Ni + TOAMe2 + O2CC6H4-meta-Cl]+. UV-vis complexes reported in this paper have been deposited with the
(CH2Cl2): λ (ε) = 412 (26), 612 (17), 800 nm (5.2). Elemental Cambridge Crystallographic Data Centre as supplementary
analysis: calcd (%) for [C25H34N4ClNiO5](C24H20B)·0.5CH2Cl2 publications CCDC 1983778–1983782.†
C: 64.18, H: 5.98, N: 6.05; found C: 64.22, H: 6.12, N: 6.04.
Catalytic oxidation of substrates with m-CPBA
Synthesis of [NiII(Cl)(TOAtBu)](BPh4) (5). A methanol solu-
tion (10 mL) of TOAtBu (0.287 g; 1.21 mmol) was added to a All reactions were conducted at room temperature under Ar
methanol solution (15 mL) of NiCl2·6H2O (0.506 g; 1.16 mmol) and the products were analyzed quantitatively using GC with
at RT and the solution was stirred for 30 min. Adding NaBPh4 an internal standard.
(0.417 g; 1.22 mmol) to this solution precipitated a pink
Cyclohexane oxidation. A solution of the nickel(II) com-
powder. The powder was collected by filtration and recrystal- pounds (2.0 μmol of 1–7, 9 and 10 and 1.0 μmol of 8, that is,
lized by the slow diffusion of CH2Cl2/n-hexane yielding red 2.0 μmol of nickel(II)), cyclohexane (15.0 mmol) and nitro-
block-shaped crystals of 5 (0.217 g, 0.257 mmol; 22% yield). benzene (0.10 mmol; as an internal standard for quantitative
FT-IR (KBr): ν = 3056, 3031, 2962, 2871, 1941, 1880, 1818, analysis by GC) in a mixture of MeCN (1 mL) and CH2Cl2
1654, 1633, 1579, 1477, 1423, 1369, 1349, 1301, 1228, 1124, (1 mL) was placed in a Schlenk flask and degassed with Ar gas.
997, 956, 925, 877, 848, 809, 736, 703, 611 cm−1. ESI-MS (posi- Next, a CH2Cl2 (2 mL) solution of m-CPBA (2.0 mmol) was
tive, MeCN): m/z = 518.3 [Ni + TOAtBu + CN]+. UV-vis (CH2Cl2): added under an Ar atmosphere and stirred at room tempera-
λ (ε) = 470 (64), 723 (17), 829 nm (11). Elemental analysis: ture. To monitor product formation, 0.1 mL of the reaction
calcd (%) for [C24H39N4ClNiO3](C24H20B) C: 68.23, H: 7.04, N: mixture was corrected at certain times (reaction times 0, 10,
6.63; found C: 67.70, H: 7.39, N: 6.56.
20, 30, 60, 120, 180 min) and quenched with a dichloro-
Synthesis of [NiII(Cl)(TOAPh)(MeOH)](BPh4) (6). A methanol methane solution (0.5 mL) of triphenylphosphine (10 mg) and
solution (5 mL) of TOAPh (0.567 g; 1.15 mmol) was added to a then the solution was subjected to GC analysis (GC-2010
methanol solution (5 mL) of NiCl2·6H2O (0.274 g; 1.15 mmol) equipped with an Rtx-5 capillary column).
at RT and the solution was stirred for 15 min. NaBPh4 (0.401 g;
Methylcyclohexane oxidation. The chloride complexes with
1.17 mmol) was then added to this solution and stirred for TOAR 4–6 were applied as catalyst precursors. The procedure
30 min. No compounds were precipitated. After the removal of for the oxidation reaction was the same as for cyclohexane oxi-
volatiles by evaporation, the resulting crude green powder was dation except for the amount of m-CPBA (0.20 mmol applied).
suspended in CH2Cl2 and the insoluble compounds were In addition, a GC-2025 equipped with an Rtx-WAX capillary
removed by filtration through a Celite pad. CH2Cl2 evaporation column was used for product analysis.
from the filtrate yielded a yellow green powder of 6 (0.424 g;
Adamantane oxidation. The reaction conditions were essen-
0.47 mmol; 41% yield). The resulting compound was applied tially the same as for methylcyclohexane oxidation except for
to catalytic tests without further purification. FT-IR (KBr): ν = the solvent (sole CH2Cl2 instead of a mixture of CH2Cl2 and
3326, 3054, 1955, 1884, 1818, 1679, 1633, 1577, 1544, 1473, MeCN) and amount of substrate (1.5 mmol).
1452, 1411, 1307, 1268, 1232, 1122, 1029, 969, 931, 881, 842,
Toluene oxidation. Prior to GC analysis, the oxygenated pro-
734, 703, 638, 611 cm−1. ESI-MS (positive, MeCN): m/z = 587.1 ducts (benzyl alcohol and benzaldehyde) were trimethyl-
[Ni + TOAPh + Cl]+. UV-vis (CH2Cl2): λ (ε) = 401 (46), 663 nm silylated with a mixture of silylation reagents prepared by
(20). Elemental analysis: calcd (%) for [C31H31N4ClNiO4] mixing trimethylchlorosilane (0.1 mL) and hexamethyl-
(C24H20B) C: 70.50, H: 5.49, N: 5.98; found C: 70.45, H: 5.85, N: disilazane (0.3 mL) in anhydrous pyridine (0.9 mL). The
5.40.
acetato complexes were applied as catalyst precursors. A solu-
tion of 2.0 μmol of the acetato complexes with TOAMe2 (1) or
TPA (7), toluene (15.0 mmol) and nitrobenzene (0.10 mmol; as
an internal standard for quantitative analysis by GC) in a
Crystallographic analysis
Diffraction data for single crystals were collected using a mixture of MeCN (1 mL) and CH2Cl2 (1 mL) was placed in a
Rigaku Saturn 70 CDD area detector system with graphite Schlenk flask and degassed with Ar gas. Next, a CH2Cl2 (2 mL)
monochromated Mo-Kα radiation. Data collection was con- solution of m-CPBA (0.20 mmol) was added under an Ar atmo-
ducted at 293 K for 1, 113 K for 2–4 and 133 K for 5.
sphere and stirred at room temperature. After 60 min, 0.1 mL
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of the reaction mixture was collected and quenched with a di- 15 R. R. Reinig, D. Mukherjee, Z. B. Weinstein, W. Xie,
chloromethane solution (0.5 mL) of triphenylphosphine
(10 mg) containing 0.1 mL of the silylation reagent mixture.
The solution was analyzed using GC-MS for quantitative ana-
T. Albright, B. Baird, T. S. Gray, A. Ellern, G. J. Miller,
A. H. Winter, S. L. Bud’ko and A. D. Sadow, Eur. J. Inorg.
Chem., 2016, 2486–2494.
lysis of the products and GC for quantitative analysis of the tri- 16 T. Nishiura, T. Uramoto, Y. Takiyama, J. Nakazawa and
methylsilylated oxygenated products and 1,2-diphenylethane
on a GC-2010 equipped with an Rtx-5 capillary column.
S. Hikichi, Molecules, 2018, 23, 1466.
17 T. Nagataki, Y. Tachi and S. Itoh, Chem. Commun., 2006,
4016–4018.
18 T. Nagataki, K. Ishii, Y. Tachi and S. Itoh, Dalton Trans.,
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19 T. Nagataki and S. Itoh, Chem. Lett., 2007, 36, 748–
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Conflicts of interest
There are no conflicts to declare.
20 M. Balamurugan, R. Mayilmurugan, E. Suresh and
M. Palaniandavar, Dalton Trans., 2011, 40, 9413–
9424.
Acknowledgements
21 S. Hikichi, K. Hanaue, T. Fujimura, H. Okuda, J. Nakazawa,
Y. Ohzu, C. Kobayashi and M. Akita, Dalton Trans., 2013,
42, 3346–3356.
This research was funded by CREST, JST (JPMJCR16P1) and
Kanagawa University (ordinary budget: 411).
22 J. Nakazawa, S. Terada, M. Yamada and S. Hikichi, J. Am.
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