Published on the web June 22, 2013
1197
Catalytic Alkane Oxidation by Homogeneous and Silica-supported Cobalt(II) Complex Catalysts
with a Triazolyl Group-containing Tetradentate Ligand
Jun Nakazawa,*1 Akinori Yata,1 Tomoaki Hori,1 T. Daniel P. Stack,2 Yoshinori Naruta,3 and Shiro Hikichi*1
1Department of Material and Life Chemistry, Kanagawa University,
3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama, Kanagawa 221-8686
2Department of Chemistry, Stanford University, 333 Campus Drive, Stanford, California, 94305-5080, USA
3Institute for Materials Chemistry and Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581
(Received May 17, 2013; CL-130464; E-mail: jnaka@kanagawa-u.ac.jp)
Homogeneous and heterogeneous cobalt complex catalysts
(a)
Cl
O
O
OH
+
O
+
with bis(pyridylmethyl)(triazolylmethyl)amine ligand were pre-
pared. Although simple cobalt salts catalyze cyclohexane
oxidation with mCPBA, the cyclohexane selectivity increases
upon coordination of tetradentate ligands. In the heterogeneous
catalysis, the yield and selectivity of cyclohexanol are improved
with an increase in the ligand density on the silica support due to
the suppression of metal leaching.
A
K
L
Co cat.
+
Cl
Cl
CH2Cl2
MeCN
35°C, Ar
O
O
C
+
+
OH
mCBA
OOH
mCPBA
(b)
N
Partial and selective oxidation of alkane remains a chal-
lenging reaction. To overcome this issue, the development and
understanding of metal-based oxidants are considered impor-
tant.1-3 Accordingly, Itoh and co-workers have reported the
catalytic oxidation of alkanes (e.g., cyclohexane) by metal
complexes with a tpa (= tris(2-pyridylmethyl)amine) ligand and
acetate coligands (Figure 1).4,5 Among the tpa complexes of
divalent nickel, cobalt, iron, and manganese, the nickel(II)
complex shows the highest conversion, with good selectivity
of alcohol over ketone. The cobalt(II)-tpa complex exhibits the
highest alcohol selectivity with moderate conversion, although
a longer reaction time is needed. In terms of nickel(II)-tpa
complexes, the roles of the tpa ligand and the acetate coligand
have been investigated.6,7 Other nickel(II) complexes have
also been reported as mCPBA-activating catalysts, and related
nickel-oxygen complexes such as nickel(II)-acylperoxo and
nickel(III)-oxygen species have been characterized by some
research groups, including ourselves.8-10 In contrast, examples
of mCPBA-activating cobalt complexes remain scarce, and the
role of the ligands as well as the reaction mechanism of the
cobalt catalysts remains to be clarified.11-17 Meanwhile, we have
been investigating heterogenized metal complex catalysts.18-22
We have recently prepared triazolyl group-containing poly(pyr-
idyl)amine ligands, N,N-bis(2-pyridylmethyl)-N-[(1-R-1H-1,2,3-
triazol-4-yl)methyl]amine, of which R is tert-butyl (= Lt-Bu) for
a homogeneous complex catalyst, while it is a linker to silica gel
(= SBA*-L-x, x denotes the content of a ligand tether in mol %
of silicon atom) for a heterogenized complex catalyst.23 The
nickel catalysts with these ligands exhibit cyclohexane oxygen-
ation activity with mCPBA, and the catalytic performances of
the heterogenized catalysts depend on the loading density of
L on the surface of the silica support. In this work, we have
prepared homogeneous and heterogeneous cobalt(II) complex
catalysts in order to clarify the role of L and the trend of
catalyses depending on the central metal ion.
N
N
Co2+
N
N
N
N
O
N
N
N
N
O
Co2+
Co2+
O
N
O
N
N
AcO
N
O
OEt
O
Ph4B
N
t-Bu
Si
SiMe3
Ph4B
O O
O
Lt-Bu
tpa/Co
SBA*-L-x/Co
/Co
Figure 1. Cyclohexane oxidation with mCPBA catalyzed
by cobalt(II) complex catalysts (a). Molecular structures of
homogeneous and silica-immobilized complexes having tetra-
dentate ligands (b).
procedure of [Co(OAc)(tpa)](BPh4) (= tpa/Co).4,24 The mo-
lecular structure of Lt-Bu/Co was revealed by X-ray crystallog-
raphy (Figure 2).25 In addition to the bidentate coordination of
an acetate anion, the tetradentate coordination structure of Lt-Bu
is similar to the reported tpa/Co complex.26 In addition, the
strongly coordinating acetate O1 atom (Co1-O1 2.001, Co1-O2
2.438 ¡) is located in a trans position to the tertiary amine
N3 (Co1-N3 2.292 ¡), as observed in tpa/Co. We have
previously reported the molecular structures of the related
nickel(II) complexes, [NiII(OAc)2(Lt-Bu)(H2O)] (= Lt-Bu/Ni) and
[Ni(Lt-Bu)2](BF4)2 (= Lt-Bu2/Ni).23 In both nickel complexes, the
Lt-Bu ligand coordinates to the nickel centers not by the triazole
nitrogen but by the three nitrogen donors of the bis(pyridyl-
methyl)amine unit at facial positions. This tridentate coordina-
tion induces the formation of Lt-Bu2/Ni.
Heterogenized cobalt complex catalyst was prepared by the
procedure for the corresponding nickel catalyst.23,24 The ligand-
immobilized mesoporous silica gels, SBA*-L-x (x = 0.5 and
4 mol % of silicon atoms), were treated with a MeOH solution of
Co(OAc)2¢4H2O to obtain the SBA*-L-x/Co catalysts. Cobalt
loadings on the catalysts were determined by atomic absorption
A homogeneous catalyst [CoII(OAc)Lt-Bu](BPh4) (= Lt-Bu
/
Co) was prepared from Co(OAc)2¢4H2O and Lt-Bu with the
treatment of NaBPh4 in methanol according to the reported
Chem. Lett. 2013, 42, 1197-1199
© 2013 The Chemical Society of Japan