Catalytic alkane hydroxylation reaction with nickel(II) complexes supported by di- And triphenol ligands

Article abstract of DOI:10.1246/cl.2007.748

Ni^II-complexes supported by the tetradentate ligands containing two or three 2,4-di-tert-butylphenol groups were synthesized and structurally characterized. The Ni^II-complex of the diphenol ligand has been found to act as a very effi

Full text of DOI:10.1246/cl.2007.748

748
Chemistry Letters Vol.36, No.6 (2007)
Catalytic Alkane Hydroxylation Reaction with Nickel(II) Complexes
Supported by Di- and Triphenol Ligands
Takayuki Nagataki and Shinobu ItohÃ
Department of Chemistry, Graduate School of Science, Osaka City University,
3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585
(Received March 22, 2007; CL-070307; E-mail: shinobu@sci.osaka-cu.ac.jp)
NiII-complexes supported by the tetradentate ligands
containing two or three 2,4-di-tert-butylphenol groups were
synthesized and structurally characterized. The NiII-complex
of the diphenol ligand has been found to act as a very efficient
turnover catalyst in the alkane hydroxylation reaction with
m-chloroperbenzoic acid (m-CPBA).
N
N
N
N
N
N
N
OH
TPA
LH1
OH
N
A variety of transition-metal complexes such as Fe, Co, Mn,
Ru, and Cu-complexes have extensively been examined as the
catalysts for selective hydroxylation of alkanes (saturated hydro-
carbons) with various oxidants in synthetic organic chemistry
and industrial chemistry.1–6 Especially, a great deal of attention
has been paid to the heme and non-heme iron complexes due to
their high catalytic activity and biological relevance.2
N
N
HO
OH
HO
OH
L2H2
L3H3
Recently, we have found that a simple NiII-complex such as
[NiII(TPA)(OAc)(H2O)]BPh4 [TPA = tris(2-pyridylmethyl)-
amine] works as an efficient catalyst for alkane hydroxylation
with m-CPBA (Scheme 1).7 The overall catalytic efficiency (cat-
alytic activity, product selectivity, and stability) of NiII(TPA)
was higher than that of FeII, MnII, and CoII-complexes of the
same ligand (TPA), suggesting that nickel is an attractive
transition metal for the development of efficient catalysts for
alkane-hydroxylation.7 Furthermore, we have examined the
ligand effects on the alkane hydroxylation reaction catalyzed
by a series of NiII-complexes to demonstrate that the supporting
ligands greatly affect both the reactivity and the product-selec-
tivity (alcohol vs ketone).8 One of the intriguing results of such
study was that the NiII-complex 1 supported by monophenol li-
gand L1H (see Chart 1) gave a higher catalytic activity as com-
pared to that of NiII(TPA).8 These results stimulated us to exam-
ine further the catalytic efficiency of the NiII-catalysts supported
by diphenol and triphenol ligands such as L2H2 [N-(2-pyridyl-
methyl)-N,N-bis(2-hydroxy-3,5-di-tert-butylbenzyl)amine] and
L3H3 [tris(2-hydroxy-3,5-di-tert-butylbenzyl)amine] shown in
Chart 1. The structure and catalytic activity of the NiII-com-
plexes have been examined in detail to gain deeper insight into
the ligand effects on the catalytic activity of NiII.
Chart 1. Supporting ligands.
Figure 1. ORTEP drawings of [NiII(L2H2)(OAc)2] (2) (left)
and [NiII(L3H)(TMG)] (3) (right) showing 50% probability
thermal ellipsoids. The hydrogen atoms attached to the carbon
atoms are omitted for clarity.
a slightly distorted octahedral geometry with an N2O4 donor set
provided by the ligand and two monodentate acetate counter
anions as shown in Figure 1 (left). It should be noted that both
phenol protons in 2 are retained and involved in the strong hy-
drogen-bonding interaction with the carbonyl oxygen atom of
The NiII-complex supported by the diphenol ligand,
[NiII(L2H2)(OAc)2] (2), was synthesized in 73% by mixing
˚
II
the monodentate acetate co-ligands. (H(01)–O(6) 1.46 A and
.
L2H2 and an equimolar amount of Ni (OAc)2 4H2O in
˚
H(02)–O(4) 1.49 A).
In the case of triphenol ligand L3H3, however, just mixing
CH2Cl2/MeOH mixed solvent system.9 Complex 2 was recrys-
tallized from acetonitrile to afford pale-blue single crystals
suitable for X-ray crystallographic analysis.9 Complex 2 exhibits
II
II
.
the ligand and Ni (OAc)2 4H2O did not afford any Ni -com-
plex under the same experimental conditions. On the other hand,
addition of a slightly excess of 1,1,3,3-tetramethylguanidine
(TMG) as a base gave a NiII-complex, [NiII(L3H)(TMG)] (3), al-
though the yield was not sufficient enough (24% isolated yield).9
Single crystals suitable for X-ray crystallographic analysis were
obtained from a hexane solution of 3.9 Complex 3 shows a four-
LNiII-complex (cat.)
OH
m-CPBA
Scheme 1.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.6 (2007)
749
ly efficient NiII-catalyst for the alkane hydroxylation reaction
with m-CPBA using diphenol ligand L2H2. Optimization of
the reaction conditions as well as characterization of the active
oxygen species is now under investigation.
1000
2
800
1
600
400
200
0
This work was financially supported in part by Grants-
in-Aid for Scientific Research (Nos. 17350086, 18037062, and
18033045) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
3
References and Notes
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0
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Time / min.
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Figure 2. Time courses for the oxidation of cyclohexane
(2.5 M) with m-CPBA (0.33 M) catalyzed by 1 ( ), 2 ( ), and
3 ( ) (0.33 mM) in CH2Cl2/CH3CN (v/v = 3/1, total 6 mL)
at room temperature.
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set, in which two deprotonated phenolate oxygen atoms O(1) and
O(2), tertiary amine nitrogen N(1), and the imine nitrogen N(2)
of the TMG co-ligand are involved (Figure 1, right). Thus, one of
the phenol groups of the ligand is free from coordination.
Catalytic activity of the NiII-complexes was examined using
cyclohexane as the substrate and m-CPBA as the oxidant
(Scheme 1).9 Figure 2 shows the time courses of total turnover
number (TON) of the catalysts. In all the cases, cyclohexanol
was obtained as a major product together with a small amount
of cyclohexanone and a trace amount of over oxidation product
"-caprolactone. As reported previously, the monophenol com-
plex 1 acted as an efficient turnover catalyst to give total TON
as 719 and alcohol product selectivity (A/K; alcohol/ketone
ratio) as 7.2 after 2 h. When the diphenol complex 2 was
employed as the catalyst, the total TON and A/K values became
larger as 858 and 7.7, respectively, although the turnover rate at
the initial stage of the reaction (ca. 30 min) was little lower
(Figure 2).10 On the other hand, complex 3 exhibited relatively
low catalytic activity as compared to 1 and 2, whereas the
A/K value (65) of 3 was much higher than that of others.
Although the details of the catalytic mechanism have yet to
2
´
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Supporting Information (experimental procedures and X-ray
stractural determination) is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html. Crystallographic data of 2 and 3 have been depos-
ited with Cambridge Crystallographic Data Centre as supple-
mentary publication no. CCDC-640872 and -640873, respec-
tively.
¨
3
4
5
II
III
ꢀ
be clarified, we proposed that a nickel–oxo (Ni –O or Ni =O)
species is involved as a key reactive intermediate for the alkane-
hydroxylation.7,8 Introduction of the additional phenol group in-
to L1H to give L2H2 may enhance the durability of the catalyst.
Namely, self-degradation of the catalyst may be suppressed in 2,
allowing the high TON, even though the turnover rate became a
little lower (Figure 2). On the other hand, the triphenol ligand
L3H3 itself may not fit to the octahedral geometry, which is
required for the m-CPBA-binding to generate the nickel–oxo
species. Thus, 3 may exhibit the lower catalytic activity.
Nonetheless, significantly high alcohol selectivity (A/K = 65)
of 3 is noteworthy.
6
7
8
9
Another advantage of catalyst 2 is its high solubility to
non-polar organic solvent. Thus, the oxidation of cyclohexane
(15 mmol, 1.62 mL) with 2 (0.002 mmol) in the presence of
m-CPBA (0.4 mmol, suspended) can be carried out without
adding any other solvent to yield cyclohexanol in 93% together
with a small amount of cyclohexanone (7%) based on the
oxidant (the oxidant efficiency was 100%, A/K = 13.3).9
In summary, we have succeeded in developing a significant-
10 After a prolonged reaction time (24 h), the total TON reached
ca. 1000 and all the m-CPBA (0.33 M) was consumed. Thus,
the product yield based on the oxidant was ca. 100%, demon-
strating the high durability of the catalyst.
Published on the web (Advance View) May 12, 2007; doi:10.1246/cl.2007.748