Ligand effects on NiII-catalysed alkane-hydroxylation with m-CPBA

Article abstract of DOI:10.1039/b615503k

Nickel(ii) complexes supported by a series of pyridylalkylamine ligands [tris(2-pyridylmethyl)amine (TPA; complexes 1a and 1b), tris[2-(2-pyridyl)ethyl] amine (TEPA; complexes 2a and 2b), 6-[N,N-bis(2-pyridylmethyl)aminomethyl]-2,4- di-tert-butylphenol (<

Full text of DOI:10.1039/b615503k

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PAPER
www.rsc.org/dalton | Dalton Transactions
Ligand effects on NiII-catalysed alkane-hydroxylation with m-CPBA†
Takayuki Nagataki, Kenta Ishii, Yoshimitsu Tachi and Shinobu Itoh*
Received 25th October 2006, Accepted 18th January 2007
First published as an Advance Article on the web 6th February 2007
DOI: 10.1039/b615503k
Nickel(II) complexes supported by a series of pyridylalkylamine ligands [tris(2-pyridylmethyl)amine
(TPA; complexes 1a and 1b), tris[2-(2-pyridyl)ethyl]amine (TEPA; complexes 2a and 2b),
6-[N,N-bis(2-pyridylmethyl)aminomethyl]-2,4-di-tert-butylphenol (DtbpPym2H; complexes 3a and 3b),
6-[N,N-bis[2-(2-pyridyl)ethyl]aminomethyl]-2,4-di-tert-butylphenol (DtbpPye2H; complexes 4a and 4b),
N-benzyl-bis(2-pyridylmethyl)amine (BzPym2; complex 5a) and N-benzyl-bis[2-(2-pyridyl)ethyl]amine
(BzPye2; complex 6a)] have been synthesized and structurally characterized by X-ray crystallographic
analysis [coordinating counter anion (co-ligand) of complexes na (n = 1–6) is AcO− and that of
complexes nb (n = 1–4) is NO3−]. All complexes, except 1b, were obtained as a mononuclear nickel(II)
complex exhibiting a distorted octahedral geometry, whereas complex 1b was isolated as a dinuclear
nickel(II) complex bridged by two nitrate anions. Catalytic activity of the nickel(II) complexes were
examined in the oxidation of cyclohexane with m-CPBA as an oxidant. In all cases, the oxygenation
reaction proceeded catalytically to give cyclohexanol as the major product together with cyclohexanone
as the minor product. The complexes containing the pyridylmethylamine (Pym) metal-binding group
(1a, 3a, 5a) showed higher turnover number (TON) than those containing the pyridylethylamine (Pye)
metal-binding group (2a, 4a, 6a), whereas the alcohol/ketone (A/K) selectivity was much higher with
−
the latter (Pye system) than the former (Pym system). On the other hand, the existence of the NO3
co-ligand (1b, 2b and 3b) caused a lag phase in the early stage of the catalytic reaction. Electronic and
steric effects of the supporting ligands as well as the chemical behavior of the co-ligands on the catalytic
activity of the nickel(II) complexes have been discussed on the basis of their X-ray structures.
The reaction exhibited a high alcohol-product selectivity and a
Introduction
high regioselectivity for tertiary carbons (3◦) against the secondary
carbons (2◦).42 An appreciable amount of kinetic deuterium
Development of an efficient catalyst for selective oxygenation
isotope effect (kH/kD = 2.8) was also obtained.42 Moreover, the
of saturated hydrocarbons is an important research objective
in synthetic and industrial organic chemistry.1–9 So far, a great
deal of attention has been focused on heme and non-heme iron
complexes as well as Mn, Co, Ru and Cu complexes.10–41 Recently,
we have found that a simple NiII(TPA) complex [TPA: tris(2-
pyridylmethyl)amine] also works as a very efficient turnover cata-
lyst for alkane hydroxylation with m-CPBA (m-chloroperbenzoic
acid) (Fig. 1).42
total TON (turnover number) of the NiII complex was higher
than that of FeII, MnII and CoII complexes of the same ligand,
and the alcohol-product selectivity of the NiII complex was much
better than that of the FeII and MnII complexes.42 Notably, orders
of the catalytic activity (TON) (NiII > FeII > CoII > MnII) and
the alcohol-product selectivity (CoII > NiII > FeII ꢀ MnII) of
the complexes in the TPA-system were similar to the orders of the
efficiency and alcohol-product selectivity in the gas phase reaction
of methane with the first-row metal-oxo species MO+ (M = Ni,
Co, Fe, Mn) reported by Schwarz and co-workers.43 On the basis
of these results, we concluded that our reaction also involves
a highly reactive metal-based oxidant such as (L)NiO+ rather
than autooxidation type free radical species.5,42 In this study, we
have investigated the ligand effects on the structure and catalytic
activity of the NiII-complexes using a series of pyridylalkylamine
derivatives shown in Chart 1. Effects of the coordinating co-
ligands (counter anion) have also been examined.
Fig. 1 NiII(TPA)-catalysed oxygenation of alkanes with m-CPBA.42
Department of Chemistry, Graduate School of Science, Osaka City Uni-
versity, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka, 558-8585, Japan. E-mail:
shinobu@sci.osaka-cu.ac.jp
Experimental
† Electronic supplementary information (ESI) available: Summary of
X-ray crystallographic data (Table S1), bond lengths and angles (Table S2),
and the time courses for cyclohexane oxidation with the Ni(II) complexes
(Fig. S1–S6). Atomic coordinates, thermal parameters, and intermolecular
bond distances and angles are provided in CIF format. See DOI:
10.1039/b615503k
General
The reagents and the solvents used in this study except the
ligand and the complexes were commercial products of the highest
available purity and were further purified by standard methods,
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and dried. Recrystallization of the crude product from CH2Cl2–
cyclohexane gave pure complex 1b as purple crystals
(295 mg, 65%). FT-IR (KBr): 1439, 1317, 1285 (NO3−), 734,
704 cm−1 (BPh4−); HRMS (FAB+): m/z 410.0759 [0.5M–
BPh4]+, calcd for C18H18N5O3Ni 410.0763; Anal. Calcd for
[Ni2(TPA)2(NO3)2](BPh4)2, C84H76N10O6Ni2B2: C, 69.08; H, 5.24;
N, 9.59. Found: C, 69.06; H, 5.20; N, 9.55.
[NiII(TEPA)(OAc)]BPh4 (2a). A methanol solution (5 mL) of
NiII(OAc)2·4H2O (62.3 mg, 0.25 mmol) was added to a methanol
solution (5 mL) of TEPA (83.1 mg, 0.25 mmol) with stirring at
room temperature. Color of the solution turned to blue. After
stirring for 2 h, NaBPh4 (86 mg, 0.25 mmol) was added to the
mixture to give a pale blue precipitate, which was collected by fil-
tration, washed with methanol and dried (159.8 mg, 83%). FT-IR
(KBr): 1536, 1456 (OAc−), 733, 707 cm−1 (BPh4−); HRMS (FAB+):
m/z 449.1481 [M–BPh4]+, calcd for C23H27N4O2Ni 449.1487; Anal.
Calcd for [Ni(TEPA)(OAc)]BPh4, C47H47N4O2NiB: C, 73.37; H,
6.16; N, 7.28. Found: C, 73.11; H, 6.28; N, 7.09.
Single crystals suitable for X-ray crystallographic analysis were
obtained by slow diffusion of cyclohexane into a CH2Cl2 solution
of the complex.
[NiII(TEPA)(NO3)]BPh4 (2b). A methanol solution (4 mL)
of NiII(NO3)2·6H2O (59.6 mg, 0.205 mmol) was added to a
methanol solution (4 mL) of TEPA (68.0 mg, 0.205 mmol) with
stirring at room temperature. The color of the solution turned
to blue. After stirring for 2 h, NaBPh4 (77 mg, 0.225 mmol)
was added to the mixture to give a pale blue precipitate, which
was collected by filtration, washed with methanol and dried
(149.1 mg, 94%). FT-IR (KBr): 1493, 1277 (NO3−), 734, 705 cm−1
(BPh4−); HRMS (FAB+): m/z 452.1241 [M–BPh4]+, calcd for
C21H24N5O3Ni 452.1232; Anal. Calcd for [Ni(TEPA)(NO3)]BPh4,
C45H44N5O3NiB: C, 69.98; H, 5.74; N, 9.07. Found: C, 70.01; H,
5.73; N, 9.10.
Chart 1
if necessary.44 Tris(2-pyridylmethyl)amine (TPA), tris[2-(2-
pyridyl)ethyl]amine (TEPA), N-benzyl-bis(2-pyridylmethyl)amine
(BzPym2), N-benzyl-bis[2-(2-pyridyl)ethyl]amine (BzPye2), 6-
[N,N-bis(2-pyridylmethyl)aminomethyl]-2,4-di-tert-butylphenol
Single crystals suitable for X-ray crystallographic analysis were
obtained by slow evaporation of an acetonitrile solution of the
complex.
[NiII(DtbpPym2H)(OAc)(MeOH)]BPh4 (3a). A methanol solu-
tion (3 mL) of NiII(OAc)2·4H2O (49.8 mg, 0.20 mmol) was
added to a methanol solution (3 mL) of DtbpPym2H (83.5 mg,
0.20 mmol) with stirring at room temperature. The color of the
solution turned to blue–green. After stirring for 2 h, NaBPh4
(69 mg, 0.20 mmol) was added. The mixture was allowed to
stand for several hours to afford blue crystals, which were
collected by filtration, washed with cold methanol and dried
(147.5 mg, 83%). FT-IR (KBr): 3356 (O–H, broad), 1608, 1480,
1428 (OAc−), 733, 705 cm−1 (BPh4−); HRMS (FAB+): m/z
534.2255 [M–BPh4–MeOH]+, calcd for C29H38N3O3Ni 534.2266;
Anal. Calcd for [Ni(DtbpPym2H)(OAc)(MeOH)]BPh4·1.5MeOH,
C55.5H68N3O5.5NiB: C, 71.32; H, 7.33; N, 4.50. Found: C, 71.26; H,
6.98; N, 4.68.
(
DtbpPym2H) and 6-[N,N-bis[2-(2-pyridyl)ethyl]aminomethyl]-2,4-
di-tert-butylphenol (DtbpPye2H) were synthesized according to
reported procedures.45–50 [NiII(TPA)(OAc)(H2O)]BPh4 (1a) was
prepared according to the reported procedure.42 FT-IR spectra
were recorded with a Jasco FT/IR-4100. Mass spectra were
recorded with a JEOL JMS-700T Tandem MS station or a PE
SCIEX API 150EX (for ESI-MS). 1H NMR spectra were recorded
on a JEOL LMN-ECP300WB or an LMX-GX400. UV-vis spectra
were measured using a Hewlett-Packard HP8453 diode array
spectrophotometer or a Jasco V-570. Elemental analyses were
performed on a Perkin-Elmer or a Fisons instruments EA 1108
Elemental analyzer.
Synthesis
[NiII2(TPA)2(l-NO3)2](BPh4)2 (1b). An ethanol solution
(10 mL) of NiII(NO3)2·6H2O (180.2 mg, 0.62 mmol) was added
to an ethanol solution (5 mL) of TPA (179.8 mg, 0.62 mmol) with
stirring at room temperature. The color of the solution turned
to brown. After the solution was stirred for 1 h, NaBPh4
(212 mg, 0.62 mmol) was added to the mixture to give a brown
precipitate, which was collected by filtration, washed with ethanol
[NiII(DtbpPym2H)(NO3)(MeCN)]NO3 (3b). An ethanol solu-
tion (3 mL) of NiII(NO3)2·6H2O (58.2 mg, 0.20 mmol) was
added to an ethanol solution (3 mL) of DtbpPym2H (83.5 mg,
0.20 mmol) with stirring at room temperature. The color of the
solution turned to blue. After stirring for 30 min, the mixture
was poured into hexane (150 mL) to give a blue precipitate,
which was collected by filtration and dried. Recrystallization of
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the crude product from acetonitrile–ether gave pure complex
3b as blue crystals (66.4 mg, 52%). FT-IR (KBr): 3370 (O–H,
broad), 1385, 1303 (NO3−); HRMS (FAB+): m/z 537.2011 [M–
NO3–MeCN]+, calcd for C27H35N4O4Ni 537.2012; Anal. Calcd for
[Ni(DtbpPym2H)(NO3)(MeCN)]NO3, C29H38N6O7Ni: C, 54.31; H,
5.97; N, 13.10. Found: C, 54.30; H, 6.01; N, 12.86.
hexane gave a pure complex of 6a as blue crystals (105.3 mg,
68%). FT-IR (KBr): 3371, 3268 (O–H, broad), 1543, 1444 (OAc−),
734, 705 cm−1 (BPh4−); HRMS (FAB+): m/z 434.1376 [M–
BPh4–H2O]+, calcd for C23H26N3O2Ni 434.1379; Anal. Calcd for
[Ni(BzPye2)(OAc)(H2O)]BPh4·C3H6O, C50H54N3O4NiB: C, 72.31;
H, 6.55; N, 5.06. Found: C, 72.18; H, 6.50; N, 5.13.
[NiII(DtbpPye2)(OAc)] (4a). A methanol solution (5 mL) of
NiII(OAc)2·4H2O (124.5 mg, 0.50 mmol) was added to a methanol
solution (5 mL) of DtbpPye2H (222.8 mg, 0.50 mmol) with stirring
at room temperature. The color of the solution turned to green.
After stirring for 2 h, the mixture was concentrated by evaporation.
The residue was re-dissolved to CH2Cl2 (5 mL), and the solution
was poured into hexane (80 mL) to give a pale blue precipitate,
which was collected by filtration and dried (233.9 mg, 83%). FT-
IR (KBr): 1606, 1552, 1483, 1445 (OAc−); HRMS (FAB+): m/z
561.2500 [M]+, calcd for C31H41N3O3Ni 561.2501; Anal. Calcd for
[Ni(DtbpPye2)(OAc)], C31H41N3O3Ni: C, 66.21; H, 7.35; N, 7.47.
Found: C, 66.17; H, 7.35; N, 7.44.
X-Ray structure determination
The single crystal was mounted on a glass-fiber. X-Ray diffraction
data were collected by a Rigaku RAXIS-RAPID imaging plate
two-dimensional area detector using graphite-monochromated
Mo Ka radiation (k = 0.71069 A) to 2hmax of 55◦. All the
˚
crystallographic calculations were performed by using Crystal
Structure software package of the Molecular Structure Corpo-
ration [Crystal Structure: Crystal Structure Analysis Package
version 3.7.0, Molecular Structure Corp. and Rigaku Corp.
(2005)]. The structures were solved with SIR92 and refined with
CRYSTALS. All non-hydrogen atoms and hydrogen atoms were
refined anisotropically and isotropically, respectively. Atomic co-
ordinates, thermal parameters, and intramolecular bond distances
and angles are given in the ESI.†
Single crystals suitable for X-ray crystallographic analysis were
obtained by slow diffusion of hexane into an acetone solution of
the complex.
CCDC reference numbers 625316–625324.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b615503k
[NiII(DtbpPye2)(NO3)] (4b). A methanol solution (5 mL) of
NiII(NO3)2·6H2O (145.3 mg, 0.50 mmol) was added to a methanol
solution (5 mL) of DtbpPye2H (222.8 mg, 0.50 mmol) with stirring
at room temperature. The color of the solution turned to dark
green. After stirring for 2 h, triethylamine (100 lL, 0.71 mmol)
was added, and the mixture was allowed to stand for several hours
to afford blue crystals, which were collected by filtration, washed
with cold methanol and dried (132.5 mg, 47%). FT-IR(KBr): 1489,
1328, 1272 (NO3−); HRMS (FAB+): m/z 564.2233 [M]+, calcd
for C29H38N4O4Ni 564.2247; Anal. Calcd for [Ni(DtbpPye2)(NO3)],
C29H38N4O4Ni: C, 61.61; H, 6.78; N, 9.91. Found: C, 61.58; H,
6.70; N, 9.93.
Catalytic oxygenation of cyclohexane
All procedures of the catalytic reaction were carried out under
anaerobic conditions (in a glove box, [O2] <1 ppm, [H2O] <1 ppm).
Typically, complex 2a (2 lmol) in CH3CN–CH2Cl2 (1 : 1 v/v, 2 ml)
was added to a CH2Cl2 (2 ml) solution containing cyclohexane
(15 mmol), m-CPBA (2 mmol), and nitrobenzene (50 lmol) as an
internal standard with vigorous stirring at room temperature. An
aliquot of the reaction mixture was taken at a certain reaction time
and quenched with PPh3 for the GLC analysis using a Shimadzu
GC-14A gas chromatograph equipped with a Restek Rtx-1701
capillary column (30 m × 0.25 mm). All peaks of interest were
identified by comparison of the retention times and co-injection
with the authentic samples. The products were quantified by
comparison against a known amount of internal standard using
a calibration curve consisting of a plot of mole ratio (moles of
organic compound/moles of internal standard) versus area ratio
(area of organic compound/area of standard).
[NiII(BzPym2)(OAc)2(H2O)] (5a). A methanol solution (3 mL)
of NiII(OAc)2·4H2O (74.7 mg, 0.30 mmol) was added to a methanol
solution (3 mL) of BzPym2 (86.8 mg, 0.30 mmol) with stirring
at room temperature. The color of the solution turned to light-
blue. After stirring for 2 h, the mixture was concentrated by
evaporation. The residue was re-dissolved in CH2Cl2 (2 mL),
and the solution was poured into hexane (30 mL) to give a
pale blue precipitate, which was collected by filtration and dried
(118.3 mg, 81%). FT-IR (KBr): 3000 (O–H, very broad), 1605,
1582, 1568, 1407 (OAc−); HRMS (FAB+): m/z 406.1055 [M–
OAc–H2O]+, calcd for C21H22N3O2Ni 406.1065; Anal. Calcd for
[Ni(BzPym2)(OAc)2(H2O)], C23H27N3O5Ni: C, 57.06; H, 5.62; N,
8.68. Found: C, 56.83; H, 5.59; N, 8.60.
Results and discussion
Structural characterization of NiII-complexes
Single crystals suitable for X-ray crystallographic analysis were
obtained by slow diffusion of ether into a CH2Cl2 solution of the
complex.
Treatment of NiII(OAc)2·4H2O and an equimolar amount of the
ligands in methanol gave the corresponding NiII–acetate (OAc)
complexes 1a–6a. In the cases of 1a,42 2a, 3a, and 6a, one of
the two acetate ions, uncoordinated to nickel(II), was replaced by
BPh4− by anion exchange reaction with NaBPh4. The NiII–nitrate
(NO3) complexes 1b–4b were also synthesized by the reaction of
[NiII(BzPye2)(OAc)(H2O)]BPh4 (6a).
A methanol solution
(3 mL) of NiII(OAc)2·4H2O (49.8 mg, 0.20 mmol) was added to
a methanol solution (3 mL) of BzPye2 (63.4 mg, 0.20 mmol) with
stirring at room temperature. The color of the solution turned
to light-blue. After stirring for 2 h, NaBPh4 (70 mg, 0.20 mmol)
was added to the mixture to give a pale blue–green precipitate,
which was collected by filtration, washed with methanol and
dried. Recrystallization of the powder sample from acetone–
−
NiII(NO3)2·6H2O and the ligands, and the uncoordinated NO3
was also replaced by BPh4− in a similar manner except in complex
3b. All the complexes were paramagnetic (S = 1) as demonstrated
1
by the H NMR resonances spread over a chemical shift range
of −20 to 70 ppm.42,51 Fig. 2 shows the ORTEP drawings of the
1122 | Dalton Trans., 2007, 1120–1128
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Fig.
2
ORTEP drawings of (A) [NiII2(TPA)2(l-NO3)2](BPh4)2 (1b), (B) [NiII(TEPA)(OAc)]BPh4 (2a), (C) [NiII(TEPA)(NO3)]BPh4 (2b),
(D) [NiII(DtbpPym2H)(OAc)(MeOH)]BPh4 (3a), (E) [NiII(DtbpPym2H)(NO3)(MeCN)]NO3 (3b), (F) [NiII(DtbpPye2)(OAc)] (4a), (G) [NiII(DtbpPye2)(NO3)]
(4b), (H) [NiII(BzPym2)(OAc)2(H2O)] (5a) and (I) [NiII(BzPye2)(OAc)(H2O)]BPh4 (6a) showing 50% probability thermal ellipsoids. The non-coordinating
counter anion and the hydrogen atoms attached to the carbon atoms are omitted for clarity.
NiII–acetate complexes 2a–6a and the NiII–nitrate complexes 1b–
4b, and the crystallographic data and the selected bond lengths
and angles are presented in Tables S1 and S2,† respectively. The
crystal structure of [NiII(TPA)(OAc)(H2O)]BPh4 (1a) was reported
in our previous communication.42
NiII–TPA complex 1a exhibits a mononuclear octahedral
structure involving an acetate and a water molecule as the co-
ligands.42 On the other hand, NiII–TPA complex 1b prepared using
NiII(NO3)2·6H2O exhibits a dimeric structure consisting of a rare
bis(l-nitrate)dinickel(II) core, at the center of which there is an
inversion center of the complex [Fig. 2(A)]. The metal center shows
a slightly distorted octahedral geometry with the N4O2 donor set,
˚
where the bond length of Ni–N(1) (alkylamine nitrogen, 2.0851 A)
is slightly longer than that of Ni–NPy [pyridine nitrogen N(2), N(3),
˚
and N(4), 2.0469, 2.0408, and 2.0547 A, respectively], and the
˚
distance between Ni and O(2) (2.1233 A) is somewhat longer than
˚
that of Ni–O(1) (2.0711 A). The overall structure of 1b is similar
to that of [M2(TPA)2(l-OAc)2](BPh4)2 (M = FeII, MnII).42,52
NiII–TEPA complexes 2a and 2b exhibit a mononuclear
structure involving a didentate acetate and nitrate co-ligand,
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respectively, [Fig. 2(B) and 2(C)]. The complexes also have a
distorted octahedral geometry with the N4O2 donor set, where
the acetate ion in 2a coordinates to Ni(II) unsymmetrically (Ni–
structure of complex 4b is similar to that of 4a. Formation of
the phenolate complexes 4a and 4b in the DtbpPye2H ligand system
further demonstrates the higher Lewis acidity of Ni(II) in the bis[2-
(2-pyridyl)ethyl]amine (Pye2) ligand system as compared with the
bis(2-pyridylmethyl)amine (Pym2) ligand system just mentioned
above.
˚
˚
O(1) = 2.0971 A; Ni–O(2) = 2.1653 A), whereas the nitrate ion in
2b coordinates to the metal center fairly symmetrically (Ni–O(1) =
˚
˚
2.1576 A; Ni–O(2) = 2.1676 A). It should be noted that the average
distance between the pyridine nitrogens and Ni(II) in 2a (2.12 A)
Tridentate ligands BzPym2 and BzPye2 also afforded mononu-
clear Ni(II)-acetate complexes 5a and 6a, when they were treated
˚
is longer than that in 1a (2.08 A).42 Similarly, the average distances
˚
II
˚
between the pyridine nitrogens and Ni(II) in 2b (2.10 A) is longer
than that in 1b (2.05 A). These results unambiguously indicate
with Ni (OAc)2·4H2O [Fig. 2(H) and 2(I)]. Both complexes exhibit
˚
octahedral geometry with N3O3 donor set. In the case of 5a, the
vacant sites are occupied by two monodentate acetate anions
and one water molecule. The coordination of water molecule is
stabilized by hydrogen bonding interactions between the hydrogen
atoms of H2O and carbonyl oxygen atoms of the acetate co-ligands
that the coordinative interaction between the metal ion and the
pyridine nitrogen in the TEPA-complexes is weaker than that in
the TPA-complexes. This may cause an increasing Lewis acidity
of Ni(II) in the TEPA-system as compared to that in the TPA-
system, resulting in the different catalytic activity in the alkane
hydroxylation reaction as reported in the following section.
The phenol ligand carrying the bis(2-pyridylmethyl)amine metal
binding unit DtbpPym2H gave the corresponding mononuclear
Ni(II)–phenol complexes 3a and 3b, in which the phenol moiety
was not deprotonated (neutral phenol form is retained) [Fig. 2(D)
and 2(E)].53 Both complexes exhibit a similar octahedral geometry
with the N3O3 donor set, in which two vacant sites are occupied
˚
˚
(H(051)–O(2) 1.80 A and H(052)–O(4) 1.73 A). On the other
hand, complex 6a involves an acetate anion acting as a didentate
co-ligand and the sixth coordination site is occupied by a water
molecule. In this case, there is no intramolecular hydrogen bonding
interaction stabilizing the coordination of H2O.
The electronic absorption spectra of complexes 1a and 6a are
shown in Fig. 3 as typical examples, and the spectral data (kmax
and e) of all complexes are summarized in Table 1. Complex 1a
exhibits two broad absorption bands at 537 nm (e = 19 M−1 cm−1)
and 917 nm (e = 22 M−1 cm−1) together with a very weak shoulder
around 800 nm [Fig. 3(A)]. The spectrum is very close to that of
by AcO− and a methanol molecule in 3a and by NO3 and an
−
acetonitrile molecule in 3b, although the relative positions of the
two pyridine rings of the ligand is different. Namely, the pyridine
donor groups occupy cis positions in 3a, whereas trans positions in
3b (3a: N(2)–Ni–N(3) = 96.35◦; 3b: N(2)–Ni–N(3) = 161.73◦). The
phenol proton in 3a and 3b forms hydrogen bonding interaction
˚
with the coordinating carboxylate oxygen (H(01)–O(3) 1.47 A)
˚
and non-coordinating nitrate oxygen atoms (H(01)–O(9) 1.65 A
˚
and H(05)–O(12) 1.87 A), respectively. Formation of the phenol
complexes clearly indicates the relatively low Lewis acidity of
Ni(II) in the bis(2-pyridylmethyl)amine (Pym2) ligand system as
mentioned above.
In contrast to the case of DtbpPym2H described above, another
phenol ligand with the bis[2-(2-pyridyl)ethyl]amine (Pye2) metal
binding moiety DtbpPye2H gave the corresponding phenolate com-
plexes 4a and 4b, containing a didentate acetate and nitrate co-
ligand, respectively [Fig. 2(F) and 2(G)]. In the case of complex 4a,
there were two structurally independent molecules (molecule 1 and
molecule 2) in the crystal lattice, both of which showed a slightly
˚
distorted octahedral geometry. The Ni–Ophenolate bond is 1.987 A
Fig. 3 UV-Vis spectra of (A) [NiII(TPA)(OAc)(H2O)]BPh4 (1a) and
(B) [NiII(BzPye2)(OAc)(H2O)]BPh4 (6a) in CH2Cl2 at 25 ◦C.
˚
in molecule 1 and 1.996 A in molecule 2, which are shorter than
˚
the Ni–Ophenol bond (2.080 A) of the phenol complex 3a. Overall
Table 1 UV-Vis data of the NiII-complexes in CH2Cl2 at 25 ◦C
Complex
Counter anion
OAc−
kmax/nm (e/M−1 cm−1
)
1a
537 (19),
540 (17),
577 (18),
574 (32),
591 (9),
579 (13),
595 (19),
595 (25),
606 (13),
796 (9),
800 (sh, 14),
783 (sh, 17),
798 (sh, 2),
800 (sh, 4),
780 (sh, 4),
782 (sh, 6),
794 (sh, 4),
800 (sh, 7),
770 (sh, 3),
831 (sh, 9),
917 (22)
868 (20)
948 (7)
1ba,b
2a
NO3
−
OAc−
2ba
3a
NO3
941 (9)
−
OAc−
410 (sh, 55),
400 (sh, 30),
314 (6300),
311 (5700),
962 (15)
964 (17)
977 (20)
975 (23)
1010 (13)
1350 (13)
−
3b
NO3
4a
OAc−
−
4b
NO3
5a
OAc−
6a
OAc−
389 (75), 634 (33),
a in CH3CN. b the coefficients per one Ni2+ ion.
1124 | Dalton Trans., 2007, 1120–1128
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(iii) the denticity (tetradentate vs. tridentate) and (iv) the counter
anion co-ligands (acetate vs. nitrate). In all the cases, the reaction
proceeded catalytically to give cyclohexanol as the major product
and cyclohexanone as the minor product as summarized in Table 2.
the octahedral nickel(II) complex of 2,6-bis[{bis(2-pyridylmethyl)-
amino}methyl]-4-methylphenol (HBPMP) reported by Que,
et al.54 On the analogy of the reported data,54 the lower energy
3
band at 917 nm may be associated with the A2g
→
3T2g(F)
transition, and the higher energy band at 537 nm can be assigned
3
3
to the A2g → T1g(F) transition. Then, the weak shoulder band
(i) Effects of alkyl linker chain (Pym vs. Pye). The time
courses of TON (turnover number) of the catalysts 1a and 2a
for the production of cyclohexanol and cyclohexanone are shown
in Fig. 4 as a typical example. In the case of 1a with the 2-
pyridylmethylamine (Pym) tetradentate ligand (TPA), the catalytic
reaction proceeded very effectively to reach the total TON of 656
(TON for cyclohexanol plus TON for cyclohexanone) after 60 min
(entry 1).42 On the other hand, the reaction of 2a with the 2-
(2-pyridyl)ethylamine (Pye) tetradentate ligand was much slower
(total TON = 265 after 60 min, entry 3), although the TON further
increased at the prolonged reaction time (Fig. 4). Similarly, the
catalytic efficiency of 3a with phenol ligand DtbpPym2H containing
the Pym metal-binding group (entry 5) is higher than that of 4a
with phenol ligand DtbpPye2H containing the Pye metal-binding
moiety (entry 7) (also see Fig. S1†), and the catalytic efficiency
of 5a with tridentate ligand BzPym2 (entry 9) is higher than that
around 800 nm can be ascribed to the spin-forbidden transition
3A2g → E1g. Thus, the spectral data suggest that complex 1a retains
1
the octahedral geometry in solution. All other complexes, except
6a, exhibit similar electronic absorption spectra due to the d–d
transitions (Table 1), indicating that those complexes also retain
the octahedral geometry in solution. In addition to the d–d bands,
complexes 3a and 3b show a weak band at ∼400 nm (e = 30–55),
which could be assigned to the phenol-to-Ni(II) charge transfer
band, and phenolate complexes 4a and 4b, exhibit a phenolate-
to-Ni(II) charge transfer band at ∼310 nm (e = ∼6000 M−1 cm−1).
Notably, the absorption spectrum of complex 6a is different
from those of the others. Namely, it shows five weak absorption
bands at 389, 634, 796, 831 and 1350 nm [Fig. 3(B)], which are
3
3
3
similar to the d–d bands of 3B1 → E, 3B1 → E, 3B1 → B2, 3B1 →
3A2 and 3B1 → E, respectively, reported for the square pyramidal
3
Ni(II) complexes.55 Thus, the results indicate that complex 6a
exhibits a five-coordinated square pyramidal geometry in a non-
coordinative solvent such as CH2Cl2. Thus, the weaklycoordinated
H2O molecule in 6a [see, Fig. 2(I)] may be dissociated in
solution.
Catalytic activity in the alkane hydroxylation reaction with
m-CPBA
In our previous communication,42 catalytic efficiency of a series of
transition-metal (NiII, CoII, FeII and MnII) complexes of TPA was
examined in the hydroxylation reaction of cyclohexane with m-
CPBA to demonstrate that the NiII(TPA) was the most preferable
catalyst with respect to the total turnover number (TON) and
the alcohol-product (A/K) selectivity. In this study, we have
examined the effects of supporting ligands on the catalytic activity
of nickel(II) complexes by changing (i) the alkyl linker chain length
connecting the pyridine donor groups and the tertiary amine
nitrogen [pyridylmethylamine (Pym) vs. pyridylethylamine (Pye)],
(ii) the ligand donor atoms (pyridine nitrogen vs. phenol oxygen),
Fig. 4 Time courses for the oxidation of cyclohexane (2.5 M) cat-
alyzed by (᭹) [NiII(TPA)(OAc)(H2O)]BPh4 (1a) (0.33 mM) and by (᭜)
[NiII(TEPA)(OAc)]BPh4 (2a) (0.33 mM) in CH2Cl2–CH3CN (v/v = 3 : 1,
totally 6 ml) at room temperature.
Table 2 Oxygenation of cyclohexane with m-CPBA catalysed by the NiII-complexesa
TONb
Entry
Complex
Counter anion
OAc−
Cyclohexanol
Cyclohexanonec
Total
A/K
1
2
3
4
5
6
7
8
9
1a
1b
2a
2b
3a
3b
4a
4b
5a
6a
587
544
257
46
657
567
332
182
455
265
69
68
8
656
612
265
47
745
628
340
186
524
272
8.5
8.0
32.1
46.0
7.5
−
NO3
OAc−
−
NO3
1
OAc−
88
61
8
4
69
7
−
NO3
9.3
OAc−
41.5
45.5
6.6
−
NO3
OAc−
10
OAc−
37.9
a Reaction conditions; [Ni2+] = 0.33 mM, [m-CPBA] = 0.33 M, [cyclohexane] = 2.5 M in CH2Cl2–CH3CN (3 : 1) at room temperature for 1 h under Ar.
b Turnover number [(moles of product)/(moles of catalyst)] determined by GLC. c A small amount of e-caprolactone was obtained as an over-oxidation
product of cyclohexanone with m-CPBA.
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of 6a supported by tridentate ligand BzPye2 (entry 10) (also see
Fig. S2†). From these results, it can be concluded that the catalytic
efficiency (total TON) of the nickel(II) complexes involving the
shorter methylene linker chain (Pym-system) is higher than that
of the nickel(II) complexes with the longer ethylene linker chain
(Pye-system).
With respect to the alcohol/ketone (A/K) selectivity, on the
other hand, the Pye-ligand system (complexes 2a, 2b, 4a, 4b,
and 6a) gave much higher selectivity as compared to the Pym-
ligand system (complexes 1a, 1b, 3a, 3b, and 5a) (see Table 2).
As seen in Fig. 4, the formations of cyclohexanol (A, solid line)
and cyclohexanone (K, dashed line) are synchronized (there is
no lag phase for the formation of K), demonstrating clearly
that cyclohexanone (K) is NOT the over-oxidation product of
cyclohexanol (A). In fact, the A/K value after 120 min (longer
reaction time) is nearly the same as that after 60 min (shorter
reaction time) in all the cases.
(iii) Effects of denticity (tetradentate vs. tridentate). The time
courses of total TON for the reactions of 5a and 6a, which are
supported by the tridentate ligand BzPym2 and BzPye2, respectively,
are shown in Fig. S2.† The reaction profiles shown in Fig. S2† are
similar to those of Fig. 4 for 1a and 2a with the tetradentate ligands
TPA and TEPA, but the TONs of the tridentate ligand system
are lower than those of the tetradentate ligand system. Thus, the
reduction of the number of donor atoms from 4 to 3 resulted in
the decrease of catalytic activity of the nickel(II) complexes.
(iv) Effects of counter anions. The counter anion effects on
the reactions are shown in Fig. S3–S6.† A distinct lag phase was
observed in the reactions catalyzed by the nitrate-complexes 1b, 2b
and 3b, whereas no such lag phase existed in the other cases. The
counter anion co-ligands may contribute to the ligand exchange
process with m-CPBA as discussed in the next section.
In our studies on the copper complexes of the same series of
ligands, we have demonstrated that the electron-donating ability of
pyridine donor group of the Pym-ligand system is higher than that
of the Pye-ligand system.56 The different electron-donor ability of
the ligands may cause the difference in catalytic activity and A/K
selectivity among the nickel(II) complexes as discussed below.
Mechanistic consideration
Although the mechanistic details of the catalytic reaction have yet
to be clarified, a possible mechanism is shown in Scheme 1. First
of all, m-CPBA reacts with the starting material [(L)NiII(X)(Xꢁ)]
to give [LNiII(X)(OOC(O)Ar)] (A) [path (a)], where L is the
supporting ligand, X and Xꢁ are the coordinated counter anion
and/or the phenolate moiety involved in L and Ar is m-
chlorophenyl group. Then, O–O bond homolysis may occur to give
(ii) Effects of ligand donor atoms (pyridine nitrogen vs. phenol
oxygen). The catalytic activity of the nickel(II) complexes with
the phenol-containing ligands was higher than that of the corre-
sponding tri-pyridine ligands [3a (745) vs. 1a (656), 3b (628) vs.
1b (612), 4a (340) vs. 2a (265), 4b (340) vs. 2b (47)]. The higher
electron-donor ability of the phenolate donor group as compared
to the pyridine donor group may also cause the enhanced reactivity
as mentioned above. In this respect, we assume that the phenol
proton of complexes 3a and 3b is also dissociated during the
catalytic cycle to give the corresponding phenolate complexes,
although the phenol moiety of those complexes in the crystal is
protonated (see Fig. 2(D) and 2(E)).
•
[LNiII(O·)(X)] (B)57 and ArCO2 [path (b)], the former of which
reacts with cyclohexane directly to give cyclohexanol product and
•
[LNiI(X)] (C) [path (c)]. On the other hand, organic radical ArCO2
readily reacts with [LNiI(X)] (C) to produce [NiII(X)(OC(O)Ar)]
(D) [path (d)] or releases CO2 to give Ar• [path (e)] which may be
reduced by [LNiI(X)] (C) [path (f)] and then protonated by another
molecule of m-CPBA to produce ArH (chlorobenzene) and the m-
CPBA adduct A [path (g)]. In fact, an appreciable amount of
chlorobenzene (∼50% based on the products) was obtained from
Scheme 1
1126 | Dalton Trans., 2007, 1120–1128
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the final reaction mixture. The ligand exchange reaction of m-CBA
adduct D with another molecule of m-CPBA also regenerates the
m-CPBA adduct A [path (h)], completing the catalytic cycle.
The higher electron-donor ability of pyridine in the Pym-ligand
system as compared to the Pye-ligand system56 may reduce Lewis
acidity of the nickel(II) metal center, enhancing the ligand exchange
processes [paths (a), (g) and (h)]. The phenolate ligand having
higher electron-donor ability may have the same effect, further
enhancing the ligand exchange processes. The higher electron-
donating effect may also enhance the O–O bond cleavage of the
m-CPBA adduct A to give reactive intermediate B [path (b)],
also accelerating the catalytic reaction. These may be the reasons
why the Pym-ligands and the phenolate ligands afforded higher
catalytic activity (higher TON). The lower catalytic activity of the
tridentate ligand system as compared to the tetradentate ligand
system could be attributed to the less electron-donating ability of
the tridentate ligands.
and the high 3◦/2◦ selectivity.5,42 We are now trying to detect
the reactive intermediate using low temperature spectroscopic
techniques.
Acknowledgements
This work was financially supported in part by Grants-in-Aid
for Scientific Research (No. 17350086, 18037062, and 18033045)
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
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