NiII(TPA) as an efficient catalyst for alkane hydroxylation with m-CPBA

Article abstract of DOI:10.1039/b608311k

A simple Ni^II(TPA) complex [TPA = tris(2-pyridylmethyl)amine] has been demonstrated to act as an efficient turnover catalyst for alkane hydroxylation with m-CPBA (m-chloroperbenzoic acid), in which contribution of a NiO⁺ (nickel-oxo) type active oxygen species is suggested. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b608311k

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NiII(TPA) as an efficient catalyst for alkane hydroxylation with
m-CPBA{
Takayuki Nagataki, Yoshimitsu Tachi and Shinobu Itoh*
Received (in Cambridge, UK) 13th June 2006, Accepted 1st August 2006
First published as an Advance Article on the web 16th August 2006
DOI: 10.1039/b608311k
The NiII(TPA) complex containing acetate and water molecules
as the co-ligands, [NiII(TPA)(OAc)(H2O)]BPh4 (1), was obtained
by treating the ligand with an equimolar amount of
NiII(OAc)?4H2O followed by anion exchange reaction with
NaBPh4 in methanol. The complex exhibits a slightly distorted
octahedral geometry with the N4O2 donor set as shown in Fig. 1.
The complex is paramagnetic (S = 1) as demonstrated by
A simple NiII(TPA) complex [TPA = tris(2-pyridylmethyl)
amine] has been demonstrated to act as an efficient turnover
catalyst for alkane hydroxylation with m-CPBA (m-chloroper-
benzoic acid), in which contribution of a NiO+ (nickel-oxo)
type active oxygen species is suggested.
Selective hydroxylation of alkanes (saturated hydrocarbons) is an
important but still very difficult chemical transformation process.
A great deal of effort has long been made to develop efficient
catalysts for the alkane hydroxylation in synthetic organic
chemistry and industrial chemistry.1–9 The catalytic alkane
hydroxylation reaction is also worthy to be investigated in detail
in connection with the catalytic mechanisms of monooxygenase
metalloenzymes. So far, much attention has been paid to heme and
non-heme iron complexes due to their high catalytic activity and
biological relevance.10–18 Catalytic activities of some Mn, Co, Cu,
and Ru complexes in the alkane hydroxylation have also been
examined.8,19–22
1
the H NMR resonances spread over a chemical shift range of
0 y 50 ppm (see Fig. S5{).30
Catalytic activity of 1 was then examined in the oxygenation
reaction of alkanes with m-CPBA as summarized in Table 1. In the
case of cyclohexane as the substrate (entry 1), the oxygenation
reaction took place very efficiently to give cyclohexanol as the
major product together with a small amount of cyclohexanone as
the minor product, where alcohol selectivity (alcohol/ketone; A/K)
Nickel complexes have also attracted much recent attention in
active oxygen chemistry. So far, dinuclear nickel–active oxygen
complexes such as bis(m-oxo)dinickel(III), bis(m-superoxo)
dinickel(II), bis(m-alkylperoxo)dinickel(II), and (trans-m-1,2-
peroxo)dinickel(II) complexes as well as
a mononuclear
nickel(II)-superoxo complex have been characterised by X-ray
crystallographic analysis and/or several spectroscopic techni-
ques.23–26 On the other hand, Schro¨der, Schwarz and co-workers
have systematically investigated the gas-phase reactions of the first-
row metal-oxo species MO+ with methane, where they demon-
strated that NiO+ was the most preferable oxidant with respect to
the efficiency and the alcohol product selectivity.27 The mechanistic
aspect of the gas phase reaction has also been investigated in detail
by DFT calculations to support the experimental results.28
Nonetheless, little has been accomplished in nickel chemistry as
far as the catalytic alkane hydroxylation is concerned.29
We herein demonstrate that a simple NiII(TPA) complex [TPA:
tris(2-pyridylmethyl)amine] is a very efficient turnover catalyst for
alkane hydroxylation with m-CPBA (m-chloroperbenzoic acid).
The complex is robust enough to achieve high catalytic turnover
and shows a high alcohol-selectivity. Mechanistic study based on
the product analysis has suggested possible contribution of a NiO+
(nickel-oxo) type active oxygen species.
Fig. 1 The ligand structure of TPA and an ORTEP drawing of nickel(II)
complex 1 showing 50% probability thermal ellipsoid. The counter anion
(BPh42) and hydrogen atoms are omitted for clarity.
Table 1 Oxygenation of alkanes with m-CPBA catalysed by 1a
Entry
Substrate
Products
TONb
A/K
8.5
1
2
3
Cyclohexane
Cyclooctane
Adamantanec
Cyclohexanol
Cyclohexanoned
Cyclooctanol
Cyclooctanone
1-Adamantanol
2-Adamantanol
2-Adamantanone
1-Phenylethanol
Acetophenone
587
69
626
50
258
53
9
12.5
5.9
7.3
4
Ethylbenzene
225
31
a
Department of Chemistry, Graduate School of Science, Osaka City
University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka, 558-8585, Japan.
E-mail: shinobu@sci.osaka-cu.ac.jp; Fax: +81-6-6605-2564;
Tel: +81-6-6605-2564
{ Electronic supplementary information (ESI) available: Details of the
synthetic procedures, the product analysis and the X-ray structural
determinations. See DOI: 10.1039/b608311k
Reaction conditions: [1]
=
0.33 mM, [m-CPBA]
=
0.33 M,
[Substrate] = 2.5 M in CH2Cl2–CH3CN (3 : 1) at room temperature
b
for 1 h under Ar. Determined by GLC. Reaction conditions:
c
d
[1] = 0.17 mM, [m-CPBA] = 83 mM, [Substrate] = 0.33 M. A trace
amount of e-caprolactone was obtained as an over-oxidation product
of cyclohexanone by m-CPBA.
4016 | Chem. Commun., 2006, 4016–4018
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was 8.5. The total turnover number (TON = moles of product/
moles of catalyst) of the catalyst reached 656 within 1 h, and the
oxidant efficiency (moles of product/moles of m-CPBA consumed)
was 71%. Similar results were obtained in the case of cyclooctane
(entry 2). Furthermore, an appreciable amount of kinetic
deuterium isotope effect (kH/kD = 2.8) was obtained, when
perdeuterated substrate cyclohexane-d12 was employed.{ On the
other hand, other oxidants such as t-BuOOH and H2O2 were not
effective at all in the present system.
Oxidation of adamantane with m-CPBA also proceeded
efficiently with a high TON (320 within 1 h) to give the
corresponding alcohols together with a small amount of ketone
(entry 3). In this case, a high regioselectivity between the tertiary
(3u) and the secondary (2u) carbons was obtained (3u/2u = 12.5; the
value was derived from the amount of 1-adamantanol divided by
the amounts of 2-adamantanol and 2-adamantanone and multi-
plied by 3 to correct for the higher number of the secondary C–H
bonds). In the case of ethylbenzene (entry 4), the benzylic position
was selectively oxidized.
Chart 1
Table 3 Efficiencies w (%) and product branching ratios (%) for the
reactions of MO+ with methane under ion cyclotron resonance (ICR)
conditionsa
MO+
w
MOH+ + CH3
MCH2 + H2O M+ + CH3OH
N
+
Although the active-oxygen species involved in the present
alkane hydroxylation process has yet to be elucidated, the large
A/K ratio and the significantly high 3u/2u regioselectivity in the
adamantane oxidation as well as the appreciable amount of KIE
strongly suggest that the reaction involves a highly reactive metal-
based oxidant such as (L)NiO+.5
NiO+
CoO+
FeO+
20
0.5
20
100
100
41
,1
57
100
2
MnO+ 40
a
Data are taken from ref. 27.
In order to evaluate the catalytic efficiency of 1, the oxygenation
reaction of cyclohexane with m-CPBA was also examined by
using other divalent transition-metal complexes such as
[CoII(TPA)(OAc)]BPh4 (2), [FeII2(TPA)2(m-OAc)2](BPh4)2 (3)31
and [MnII2(TPA)2(m-OAc)2](BPh4)2 (4)32 under the same experi-
mental conditions (Table 2). All these complexes contain TPA as
the supporting ligand, acetate (OAc2) as the anionic co-ligand and
(alcohol product) selectivity of NiO+ and CoO+ is very high
(100%), whereas that of FeO+ is much lower (41%). Moreover, in
the case of MnO+, methyl radical was obtained as the major
product, but little amount of methanol was produced. On the
other hand, efficiency of CoO+ (0.5) is significantly lower than that
of others (20 y 40). Overall, the nickel-oxo (NiO+) species is the
most desirable oxidant from the viewpoints of efficiency and
alcohol-selectivity in the gas phase reaction. It should also be
emphasized that the tendencies of reactivity (TON) and alcohol-
selectivity (A/K) of 1 y 4 in the cyclohexane oxygenation in
solution (Table 2) apparently resemble those of efficiency and
alcohol-selectivity of MO+ in the gas phase reaction (Table 3). This
strongly suggests that the active oxygen species involved in both
reactions are very similar, that is, MO+-type oxidants.
2
BPh4 as the uncoordinating counter anion. The complexes also
exhibit a six-coordinate octahedral geometry with the N4O2 donor
set at the metal center, although complexes 3 and 4 form the
dimeric structure bridged by two m-acetate ligands (Chart 1,
Fig. S1–S4{). Notably, the total TON value was highest in
nickel(II) complex 1, whereas a high A/K selectivity was also
obtained with cobalt(II) complex 2 despite its TON being relatively
lower. It should also be noted that both the TON value and the A/
K selectivity of iron(II) complex 3 were lower than those of 1, and
those values of manganese(II) complex 4 were hopeless.
In Table 3 are presented the efficiency w and the product
blanching ratios for the gas phase reactions of methane with the
series of MO+ under ion cyclotron resonance (ICR) conditions
reported by Schro¨der and Schwarz.27 As clearly seen, the methanol
Table 2 Oxygenation of cyclohexane catalysed by 1–4a
TONb
cyclohexanol
TONb
Catalyst
cyclohexanonec
Total
A/K
1 (Ni)
2 (Co)
3 (Fe)
4 (Mn)
a
587
300
396
99
69
16
62
55
656
316
458
154
8.5
18.8
6.4
1.8
Reaction conditions; [M2+
] = 0.33 mM, all other reaction
b
conditions are the same as those described in Table 1. Determined
c
by GLC based on the metal ion concentration. A trace amount of
e-caprolactone was obtained as an over-oxidation product of
cyclohexanone by m-CPBA.
Fig. 2 Time courses of the total TON for the oxidation of cyclohexane
with m-CPBA catalyzed by 1, 2, 3 and 4 in CH2Cl2–CH3CN at room
temperature.
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The time courses of the total TON of 1 y 4 in the cyclohexane
oxidation are shown in Fig. 2, which clearly indicate that the
catalytic activity of the iron(II) and manganese(II) catalysts (3 and
4) is completely lost at the early stage of the reaction (within
15 min), whereas the catalytic reaction gradually proceeds
even after 60 min in the case of nickel(II) and cobalt(II) complexes
(1 and 2). The lower product selectivity of the iron and manganese
catalysts would produce free radical species as in the case of the gas
phase reaction (Table 3), which may destroy the catalysts,
depressing the catalytic activity.
In summary, the nickel(II)–TPA complex 1 has been demon-
strated for the first time to act as a very efficient turnover catalyst
for the alkane hydroxylation with m-CPBA, where a high alcohol-
selectivity is achieved. Mechanistic details as well as the ligand
effects on the catalytic efficiency of nickel(II) complexes are now
under investigation.
This work was financially supported in part by Grants-in-Aid
for Scientific Research (No. 17350086) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan. The
authors also thank Professor Yoshizawa of Kyushu University for
his valuable suggestions.{
Notes and references
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least-squares using SIR92. Crystal data for 1: C44H43N4O3NiB, M =
˚
745.36, orthorhombic, space group Pca21 (No. 29), a = 31.474(7) A,
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3
˚
˚
˚
b = 14.028(4) A, c = 9.3475(18) A, V = 4127.2(16) A , Z = 4, Dcalcd
=
1.199 g cm23, T = 173 K, m(MoKa) = 0.5123 mm21, Total data = 37384,
Unique data = 9070 (Rint = 0.036), Observed data [I . 1s(I)] = 5347, R =
0.063, Rw = 0.096, GOF = 1.007. CCDC 611580. Crystal data for 2:
C44H41N4O2CoB, M = 727.58, triclinic, space group P-1 (No. 2), a =
˚
˚
˚
11.103(6) A, b = 13.010(7) A, c = 13.255(10) A, a = 97.54(2)u,
3
˚
b = 104.24(2)u, c = 90.660(18)u, V = 1838.0(20) A , Z = 2, Dcalcd
=
1.315 g cm23, T = 173 K, m(MoKa) = 0.5107 mm21, Total data = 17939,
Unique data = 8149 (Rint = 0.049), Observed data [I . 1s(I)] = 5215, R =
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C88H82N8O4Fe2B2, M = 1448.98, triclinic, space group P-1 (No. 2), a =
˚
˚
˚
10.919(11) A, b = 12.417(10) A, c = 13.935(10) A, a = 105.00(3)u, b =
95.42(3)u, c = 92.60(3)u, V = 1812.0(26) A , Z = 1, Dcalcd = 1.328 g cm23
,
3
˚
T = 173 K, m(MoKa) = 0.4600 mm21, Total data = 17001, Unique data =
7848 (Rint = 0.052), Observed data [I . 2s(I)] = 5763, R = 0.046, Rw
0.054, GOF 1.018. CCDC 611582. Crystal data for 4:
C88H82N8O4Mn2B2, M = 1447.16, triclinic, space group P-1 (No. 2), a =
=
=
˚
˚
˚
10.865(7) A, b = 12.502(10) A, c = 14.047(8) A, a = 105.04(2)u, b =
95.394(20)u, c = 91.88(3)u, V = 1831.4(21) A , Z = 1, Dcalcd = 1.312 g cm23
,
3
˚
27 D. Schro¨der and H. Schwarz, Angew. Chem., Int. Ed. Engl., 1995, 34,
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T = 173 K, m(MoKa) = 0.4041 mm21, Total data = 18142, Unique data =
8276 (Rint = 0.037), Observed data [I . 1s(I)] = 5998, R = 0.037, Rw
0.041, GOF = 1.011. CCDC 611583. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b608311k.
=
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4018 | Chem. Commun., 2006, 4016–4018
This journal is ß The Royal Society of Chemistry 2006