Table 1. EXAFS fit parameters of local environment of copper
atom for Cu1L and Cu2L
Distance
Coordination
2
a
2
Catalyst
Shells
· /¡
/
¡
number
Cu1L
CuO
CuN
CuCu
CuO
CuN
CuCu
1.86
2.00
®
1.86
1.99
3.04
1.89
2.06
®
1.99
1.94
0.83
0.0014
0.018
®
0.0025
0.0045
0.0030
(
b)
Cu2L
(
a)
a·: DebyeWaller factor.
0
1
2
3
4
5
6
Distance/Å
Table 2. Yields of phenol and hydroquinone by using the
Cu1L and Cu2L copper complex catalysts
a
3
Figure 3. Radial distribution functions of the k -weighted Cu
K-edge EXAFS data for Cu1L (a) and Cu2L (b).
Yield/μmol
Ph/Cu
®]
Hq/Cu
[®]
Catalyst
Phenol
Hydroquinone
[
(Ph)
(Hq)
Cu1L
Cu2L
7.3
33.4
82.0
236.0
0.073
0.170
0.82
1.20
4
-tert-butyl-2,6-diformylphenol being a precursor for the syn-
thesis of the ligands. The spectrum of Cu1L also shows two
¹
1
sharp peaks at 1690 and 1640 cm , which were assigned to
(C=O) and ¯(C=N), respectively.12 These peaks represent the
aReaction condition is as follows; benzene (2.8 mmol), copper
catalyst (0.1 mmol), L-ascorbic acid (5.7 mmol), reaction time
¯
C=O bond derived from 4-tert-butyl-2,6-diformylphenol and
C=N bond formed by the simple condensation of the other
carbonyl group with one amino group of trimethylenediamine to
give an iminophenol ligand. On the other hand, the spectrum of
(
5 h), and reaction temperature (333 K).
The Cu1L and Cu2L catalysts with molecular oxygen were
¹
1
Cu2L exhibits a peak at 1640 cm , assigned to the C=N bond,
used for benzene hydroxylation. Phenol and hydroquinone were
produced, whereas no formation of catechol and benzoquinone
was observed. The yields of phenol and hydroquinone using
Cu1L and Cu2L are shown in Table 2. Phenol is obtained in a
yield of 33.4 ¯mol using Cu2L, which is four times higher than
Cu1L (7.3 ¯mol). Hydroquinone is also obtained in a yield of
236.0 ¯mol using Cu2L, which is three time higher than Cu1L
(82.0 ¯mol). Ph/Cu and Hq/Cu were calculated as the ratio of
product yields to copper content. It was confirmed that the Ph/
Cu and Hq/Cu values were higher than 1 at a reaction time of
20 h in this reaction. Cu2L has a higher Ph/Cu and Hq/Cu
than Cu1L. This indicates that the active sites in Cu2L are
more effectively activated for phenols production, especially for
hydroquinone, in comparison with Cu1L. It is speculated that
metalmetal interactions between the near copper atoms lead
to improvement of catalyst performance. A binuclear copper
species may also easily activate the dissociation of molecular
oxygen to two oxygen atoms in comparison with mononuclear
¹
1
and quite a small peak at 1690 cm , assigned to the C=O bond.
Therefore, Cu1L and Cu2L are similar with regards to the
structures of the ligands, as illustrated in Figure 1. The Cu K-
edge EXAFS was performed on Cu1L and Cu2L catalysts to
2+
investigate the coordination states of the Cu centers. The
3
Fourier transforms of the k -weighted EXAFS data for the
synthesized Cu1L and Cu2L are shown in Figure 3. The
parameters of the local structure of copper atoms, retrieved from
EXAFS fitting, are listed in Table 1. The Fourier-transformed
EXAFS data for Cu1L shows one major peak at ca. 1.5 ¡
(
Figure 3a), which is assigned to the first coordination shell,
1
316
consisting of oxygen (CuO) and nitrogen (CuN) atoms.
The atomic distances of CuO and CuN were 1.86 and 2.00 ¡,
respectively. The fitting data represents the coordination number
of the CuN and CuO bonds (2.06 and 1.89, respectively),
indicating two neighboring nitrogen and two neighboring
oxygen atoms around the copper. Additionally, the peak
assigned to the coppercopper bond (CuCu)1 is not observed
in the fitting data for Cu1L, indicating that copper atoms are
mononuclear and is further confirmed by the ESR measured
result. The Fourier-transformed EXAFS data for Cu2L showed
two major peaks at around 1.5 and 2.7 ¡ (Figure 3b). The fitting
data represents the coordination distance and number for CuN,
CuO, and CuCu (Table 1). The coordination number of Cu
Cu in Cu2L is calculated to be N = 0.83, indicating that a copper
atom coordinates to another and it is therefore a binuclear
complex. Additionally, it can be suggested that Cu1L and Cu2L
have similar coordination structures, except for the formation of
a binuclear structure, because no significant difference in the
parameters is obtained between Cu1L and Cu2L, except for
CuCu coordination.
2,15
13,17
copper.
Mono- and binuclear copper complexes were synthesized as
catalysts for the hydroxylation of benzene. FT-IR and EXAFS
measurements indicated that mono- and binuclear complexes
were successfully prepared. The hydroxylation of benzene to
phenol and hydroquinone was carried out using the synthesized
mono- and binuclear copper complex catalysts with molecular
oxygen, and it was obvious that the binuclear catalyst effec-
tively produced phenol and hydroquinone due to metalmetal
interactions.
The synchrotron radiation experiments were performed at
the BL01B1 of SPring-8 with the approval of the Japan
Synchrotron Radiation Research Institute (JASRI).
© 2015 The Chemical Society of Japan | 385