Catalytic activities and properties of Au(III)/Schiff-base complexes in methanol oxidative carbonylation

Article abstract of DOI:10.1016/j.molcata.2011.03.008

Several Au(III)/Schiff-base complexes were studied and proven to be effective catalysts for oxidative carbonylation of methanol to generate dimethyl carbonate (DMC). Effects of Schiff-base ligands, promoters, and promoter mole ratio to Au(III)/Schiff-base complexes on catalytic activity were studied. When the reaction was carried out at a methanol/Au(III) molar ratio of 5060:1, an [AuCl₂(phen)]Cl/KI (phen = 1,10-phenanthroline) ratio of 1:4, a CO/O₂ pressure of 2:1, and a temperature of 120 °C for 3 h, the conversion, selectivity, and TOF value of the model reaction were 10.8%, 98%, and 138.9 h^-1, respectively. The catalyst was characterized by FTIR, UV-vis, ¹H NMR, and cyclic voltammetry. The oxidation state of gold during the reaction and the role of KI were discussed using data on electrochemical experiments and ESI-MS. [AuI₂(phen)]⁺ was considered an intermediate in the reaction. A plausible Au(III)/Au(I) catalytic cycle mechanism was proposed.

Full text of DOI:10.1016/j.molcata.2011.03.008

Journal of Molecular Catalysis A: Chemical 340 (2011) 53–59
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Catalytic activities and properties of Au(III)/Schiff-base complexes in methanol
oxidative carbonylation
a
a
a
a
a
a,b,∗
Jinjin Li , Jianglin Hu , Yanlong Gu , Fuming Mei , Tao Li , Guangxing Li
a
School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 1037 Wuhan, Hubei 430074, PR China
Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan 430074, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Several Au(III)/Schiff-base complexes were studied and proven to be effective catalysts for oxidative car-
bonylation of methanol to generate dimethyl carbonate (DMC). Effects of Schiff-base ligands, promoters,
and promoter mole ratio to Au(III)/Schiff-base complexes on catalytic activity were studied. When the
reaction was carried out at a methanol/Au(III) molar ratio of 5060:1, an [AuCl2(phen)]Cl/KI (phen = 1,10-
Received 22 October 2010
Received in revised form 15 March 2011
Accepted 16 March 2011
Available online 23 March 2011
◦
phenanthroline) ratio of 1:4, a CO/O2 pressure of 2:1, and a temperature of 120 C for 3 h, the conversion,
−
1
selectivity, and TOF value of the model reaction were 10.8%, 98%, and 138.9 h , respectively. The catalyst
Keywords:
was characterized by FTIR, UV–vis, 1H NMR, and cyclic voltammetry. The oxidation state of gold during
Homogeneous gold catalysis
Schiff base complex
Oxidative carbonylation
Methanol
the reaction and the role of KI were discussed using data on electrochemical experiments and ESI-MS.
+
[
AuI2(phen)] was considered an intermediate in the reaction. A plausible Au(III)/Au(I) catalytic cycle
mechanism was proposed.
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
The application of gold in catalysis has long been neglected
Recently, Corma reported Au-catalyzed Suzuki and Sonogashira
reactions of arylhalide where the oxidation of Au(I) to Au(III)
was involved [13,14]. Alternatively, external oxidants have been
employedtooxidizeAu(I) toAu(III), leading to dimerizations. Zhang
developed the first example of C–C bond forming cross-coupling
reactions, leading to the one-step synthesis of R-arylenones in com-
bination with Au(I) oxidation. These examples not only incorporate
Au(III)/Au(I) catalytic cycles into contemporaneous gold chemistry
but also promise a new area of gold chemistry by merging pow-
erful gold catalysis and oxidative metal-catalyzed cross-coupling
reactions [15,16]. These promising results also demonstrate the
feasibility of designing a homogeneous system with Au(III)/Au(I)
catalytic cycles that facilitate reaction mechanism studies and per-
mit systematic modification of gold catalysts to improve reaction
scope and efficiency [17].
Oxidative carbonylation of methanol catalyzed by a metal com-
plex is a typical model reaction in the field of homogeneous
catalysis. Recently, some promising results have been achieved in
the oxidative carbonylation of alkyl alcohol [18–20], aniline [21],
and phenol [22] using Cu, Co, and Pd complexes as catalysts. We
considered the Au(III) complexes to be catalytically active in the
oxidative carbonylation of methanol due to the following reasons:
due to the preconceived notion that gold is chemically inert.
Due to the pioneering studies of Haruta and other researchers,
extraordinarily good catalytic activities were observed with gold
for low-temperature CO oxidation [1] and hydrochlorination of
ethyne [2] in the 1980s. Since then, catalysis with gold has gained
much attention [3], and many successful examples have shown
that gold catalysts can indeed be applied to several fields of het-
erogeneous catalysis, such as oxidation reactions [4,5] water-gas
shift reactions [6], and in many fields of homogeneous cataly-
sis (i.e., e.g., carbon–carbon bond forming reactions [7], hydration
of alkynes [8], hydrogenation [9], and carbonylation of olefins or
aliphatic diamines [10,11]). However, despite the fact that explo-
ration of gold in catalysis has recently surged to unprecedented
levels compared with other commonly used noble metal catalysts,
the generality and applicability of gold in catalysis remain relatively
limited. In particular, while the effectiveness of gold in heteroge-
neous catalysis has been well- recognized, far less efforts has have
been spent on studies regarding homogeneous gold complexes
[
12].
(1) gold is located in the same group as Cu in the periodic table of
elements, allowing homogeneous Au complexes to exhibit similar
properties as Cu; and (2) the electronic configuration of Au(III) is
very similar to that of Pd(II). In this study, we presented the success-
ful outcome of this endeavor where Au(III)/Shiff-base complexes
were used as catalysts for the oxidative carbonylation of methanol
to DMC with the aid of KI (Scheme 1). Some insights into this
∗ Corresponding author at: School of Chemistry and Chemical Engineering,
Huazhong University of Science and Technology, 1037 Wuhan, Hubei 430074, PR
China.
E-mail address: ligxabc@163.com (J. Li).
1
381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.03.008

5
4
J. Li et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 53–59
853 (m), 703 (s) cm 1; and UV–vis: ꢀmax(CH3OH)/nm: 230, 280,
−
O
Au(III)/L
n
/KI
2
CH OH + CO + 1/2O2
CH
3
OCOCH
3
+ H
2
O
317.
.3. Oxidative carbonylation of methanol
All reactions were conducted in a 100 mL stainless steel auto-
3
T, P
3 2
L = en, py, bipy, (CH ) N-py, salen, phen
2
n = 1 or 2
N
clave equipped with a mechanical stirrer, polytetra-fluoroethylene
(PTFE) liner, and an automatic temperature controller accord-
ing to the procedure reported in the literature [18,19]. Typically,
H
2
N
NH
2
N
N
N
N
py
(CH
3
)
2
N-py
en
bipy
0
.15 mmol Au(III) complex and 30 mL anhydrous methanol
R1
(0.74 mol) were charged into the autoclave. The autoclave was then
N
N
pressurized with CO/O2 at room temperature. During the reaction,
the total pressure was maintained at 3.0 MPa by the regular addi-
tion of CO. After the reaction, the liquid mixture was evaporated,
and the obtained distillate was analyzed by gas chromatography
OH HO
salen
N
N
R2
R3
phen: R1 = R2 = R3 = H;
-phen: R = NO
1
2
3
NO
CH
2
2
, R = R = H;
(GC) with a flame ionization detector and an HP-5 capillary column.
1
2
3
(
3
)
2
-phen: R = H, R = R = CH
3
.
Safety advice: Although oxidative carbonylation of methanol
has been industrialized by ENI chem for 30 years [25] and no
security incident has been reported up to date, high-pressure
carbonylation experiments with compressed gases represent a sig-
nificant safety risk and should only be conducted in conjunction
with the use of suitable equipment and special care.
Scheme 1. Oxidative carbonylation of methanol and gold catalysts.
catalytic reaction (i.e., shift of oxidation state of gold species dur-
ing the reaction and the role of KI) were discussed. A plausible
Au(III)/Au(I) catalytic cycle mechanism was also proposed.
2.4. Catalyst characterization
2
. Experimental
FTIR spectra in KBr pellets were recorded with a Bruker Equinox
−
1
2
.1. Materials and characterization
55 FTIR spectrophotometer in the range of 4000–400 cm . The
UV–vis spectra were measured with a Shimadzu UV-2550 PC
UV–vis spectrophotometer in the range of 800–200 nm. The C, H,
and N elemental analyses were carried out on a Vario ELIII Elemen-
tal Analyzer. 1H NMR spectra were obtained using a Bruker AV 400
The 1,10-phenanthroline monohydrate (phen·H O), 2,2-
2
bipyridine, pyridine, HAuCl ·4H O, KI, tetrabutylammonium
4
2
tetrafluoroborate (Bu NBF ), acetonitrile, anhydrous methanol,
4
4
and anhydrous ethanol were analytically pure reagents. They were
used as received.
spectrometer in DMSO-d , with tetramethylsilane as the internal
standard. ESI-MS spectra were recorded using Agilent 1100 LC/MSD
Trap XCT.
6
Cyclic voltammetry measurement was performed in ace-
2
.2. Synthesis of Au(III)/Schiff-base complexes
tonitrile using tetrabutylammonium tetrafluoroborate (Bu NBF ,
4
4
−
1
0
.1 M = 0.1 mol L ) as supporting electrolyte using the Electro-
In the general catalyst synthesis [23,24], an ethanol solution
chemical Workstation CS300 at room temperature. A conventional
three-electrode system was employed. A Pt coil served as a work-
ing electrode, and a large Pt foil was used as a counter electrode.
A commercial saturated calomel electrode (SCE) was utilized as
the reference electrode. Current–voltage curves were measured at
of phen (10 mL, 0.96 M) was added to an ethanolic solution of
1
4
.0 g HAuCl4 (10 mL) under stirring. The mixture was stirred for
h under reflux conditions. At the end of the reaction, a large
amount of yellow or orange solid was formed. After filtration, wash-
ing with ethanol (3× 5 mL), and drying in air, AuCl (phen)]Cl was
2
−
1
5
0 mV s . All potentials were reported in volts versus SCE. The
obtained with 95% yield, showing the following elemental com-
gold complex concentration was 1–3 mM. All measurements were
performed in well-deaerated solutions under nitrogen atmosphere.
position: C, 29.62%; H, 1.72%; N, 5.68%; AuCl C H N requires
3
12
8
2
C, 29.79%; H, 1.67%; N, 5.79%; 1H NMR (DMSO-d , ı, ppm): 9.70
6
−
1
Cyclic voltammetry on the blank solution of Bu NBF4 showed no
(
1
d, 2H), 9.34 (d, 2H), 8.52 (s, 2H), 8.44 (m, 2H); IR (KBr, cm ):
637 (s), 1582 (s), 1513 (m), 1485 (m), 1413 (s), 1218 (m),
4
electrochemical activity between −1.5 and 1.5 V.
Table 1
a
Catalytic performance of different catalysts in oxidative carbonylation.
Conversion (%)b
Selectivity (%)c
TOF (h )d
−1
Entry
Catalysts
Promoter
Substrate/catalyst (mol)
1
2
3
4
5
6
7
8
9
HAuCl4
HAuCl4
AuCl3(py)
AuCl3(py)
–
–
–
5060
2500
5060
5060
5060
5060
5060
5060
5060
5060
5060
–
–
–
–
–
–
7.4
23.2
60.5
59.2
70.8
138.9
113.2
–
0.6
1.8
4.7
4.6
5.5
10.8
8.8
Trace
6.3
93.2
95.1
92.9
91.6
95.9
98.5
97.5
–
KI
KI
KI
KI
KI
KI
KI
KI
[Au(en)2]Cl3
[AuCl2(salen)]Cl
[AuCl2(bipy)]Cl
[AuCl2(phen)]Cl
AuCl3((CH3)2N-py)
[AuCl2(NO2-phen)]Cl
[AuCl2((CH3)2-phen)]Cl
1
1
0
1
96.4
81.1
a
b
c
Reaction conditions: methanol, 30 ml; KI, 0.586 mmol; Ptotal = 3.0 MPa (PCO/PO2 = 2); 120 ◦C; 4 h; stirring speed, 850 rpm.
Conversion of methanol.
Selectivity to DMC.
TOF (mol converted methanol/mol catal. h).
d

J. Li et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 53–59
55
Table 2
Catalytic performance of [AuCl2(phen)]Cl with different halide promoters.
a
TOF (h 1
−
)
Entry
Promoter (mmol)
Promoter/Au (mol ratio)
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
8
No
KCl
KBr
CH3I
I2
KI
KI
KI
No
4
4
4
2
2
4
8
Trace
Trace
1.8
4.9
5.2
5.2
10.8
2.3
–
–
–
–
92.3
94.7
96
97.2
98.5
96.7
23.2
63.1
66.9
66.9
138.9
29.6
a
Unless otherwise specified, all reactions were carried out under the following conditions: Au(III) complexes, 0.02 mol % of methanol; methanol, 30 mL; Ptotal = 3.0 MPa
◦
(
PCO/PO2 = 2 : 1); 120 C; 4 h; stirring speed, 850 rpm.
3
. Results
the nitrogen atom in the pyridyl ring, consequently enhancing
the coordination ability of this ligand to the Au(III) center. The
significant effects of the ligand substituent can be demonstrated
by the fact that only trace amounts of the product was obtained
using a 1,10-phenanthroline derivative containing an electron-
withdrawing group, 5-nitro-1,10-phenanthroline (Table 1, entry
10). In this case, the poor activity might be resulted from an elec-
tronic effect. Although presence of an electron-donating group is
beneficial for enhancing the catalytic activity of Au(III) complexes,
at the same time, steric effect has also to be considered in order
to improve the catalytic performance of the catalyst. For exam-
ple, in the case of 2,9-dimethyl-1,10-phenanthroline, although the
methyl here is an electron-donating group, however, with this lig-
and, a decreased TOF value (81.1 h ) was obtained (Table 1, entry
11). In our case, steric hindrances of the two methyl groups might
be responsible for the decreased activity. This is probably due to
steric hindrances between the two methyl groups. In light of this
discussion, at this stage, the best ligand in this system is 1,10-
phenanthroline.
3.1. Catalytic studies
Initially, different gold complexes were used as catalysts for
the oxidative carbonylation of methanol. The results are summa-
rized in Table 1. First, the gold species, HAuCl , was employed
for this reaction; however, its activity was poor, and no DMC was
detected (Table 1, entries 1 and 2). In this case, tiny gold particles
were formed in the reaction, indicating the susceptibility of HAuCl4
under these reaction conditions. Recently, Hashmi and colleagues
4
[
26] used some gold complexes to catalyze the transformation of
alkynes to phenols, and they observed that pyridine gold complexes
showed better results than the others. Cinellu et al. [27] reported
the use of an Au(III)/bipyridine complex for performing the oxi-
dation of cyclic alkenes. Inspired by these reports, we attempted
to use Shiff-base ligands to stabilize gold ion. In the first instance,
AuCl (py) was used (Table 1, entry 3), obtaining 0.6% of methanol
conversion and 93.2% selectivity to DMC. More importantly, the TOF
value reached 7.4 h , which was higher than that in our previous
research on the oxidative carbolylation of methanol using Cu-based
catalysts [18,19]. Au(III) is unstable and can be easily reduced to
gold metal in the presence of carbon monoxide. However, in the
−
1
3
−1
AuCl(PPh ), an Au(I) complex proven to be an active catalyst
3
for the carbonylation of amines by Deng et al. [28], was also
examined in this model reaction. Unfortunately, under identical
conditions, only a small amount of DMC was obtained at a TOF
−
1
oxidative carbonylation of methanol catalyzed by AuCl (py), a clear
value of 5.1 h . A well-known homogeneous Pd catalyst system,
3
solution was obtained at the end of the reaction. This result implies
that no Au(0) was formed in the presence of nitrogen-containing
ligands. Based on the literature survey, nitrogen-containing ligand
provides a very good means of stabilizing Cu(I), and it has been
used as a multi-functionalized promoter for carbonylation reac-
tions [18].
PdCl (phen)/KI, was also examined. However, in this case, only
2
−
1
about 1% methanol conversion and 11.6 h
TOF were obtained.
GC/MS analysis revealed that the main by-product of the reaction
is methyl acetate.
3.2. Effect of promoter on oxidative carbonylation
Despite the low conversion of methanol at this stage, it is
undoubtedly an important hint that gives us impetus to inves-
tigate further the performance of the gold complex in oxidative
carbonylation of methanol. When KI was used as a promoter in
The effect of promoter on this catalytic system was also
investigated. Promoters are very important for enhancing the per-
formance of catalysts and understanding the reaction mechanisms
[29–31]. As shown in Table 2, the best catalytic performance was
the AuCl (py)-catalyzed reaction, a considerable improvement in
3
terms of reaction rate and TOF value was observed under the same
reaction conditions (Table 1, entry 4). In order to improve car-
bonylation performance, the other components involved in this
system were then optimized. The screening of ligands revealed
achieved by the combination of [AuCl (phen)]Cl and KI (Table 2,
2
entry 7). When KBr was used as a promoter, only 1.8% methanol
conversion and 92.3% selectivity to DMC were obtained (Table 2,
entry 3). No reaction occurred when KCl was used alone (Table 2,
that [AuCl (phen)]Cl is the best catalyst. Under an optimal con-
entry 2). In the absence of a promoter, [AuCl (phen)]Cl produced
2
2
dition, the methanol conversion and selectivity to DMC reached
only trace amounts of DMC (Table 2, entry 1). These results indi-
cate that the use of iodide as a promoter is very important for the
progress of the model reaction. As such, other iodine-containing
1
1
0.8% and 98.5%, respectively. TOF value reached as high as
−1
38.9 h
when the reaction was performed under a very high
molar ratio of methanol to catalyst (Table 1, entry 8). This value
is 19 times higher than that obtained using the initial catalyst
reagents, such as I2 and CH I, were applied in combination with
3
[AuCl (phen)]Cl. Similar results (i.e., 5% methanol conversion and
2
AuCl (py). In particular, the effects of the ligands on the gold
>94% selectivity to DMC) were obtained in both cases (Table 2,
entries 4 and 5). According to previously published results, the sig-
3
catalysts were studied. As shown in Table 1, the promotional
effect of various ligands on the activity increases in the follow-
ing order: py < en ≈ salen < bipy < phen (Table 1, entries 4–8). The
complex AuCl ((CH ) N-py) exhibits better activity than AuCl (py)
nificant promoting effect of KI on the [AuCl (phen)]Cl catalyzed
2
oxidative carbonylation of methanol can be ascribed mainly to an
easy dissociation of the iodide anion in the reaction [32].
The effect of KI dosage on the catalytic performance of
3
3
2
3
(
Table 1, entry 9). This can be ascribed to the nucleophilicity of
the dimethylamino group that increases the electron density of
[AuCl (phen)]Cl in the oxidative carbonylation of methanol was
2

5
6
J. Li et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 53–59
1
1
1
1
1
1
1
60
50
40
30
20
10
00
100
100
95
1
1
1
1
1
1
1
60
50
40
30
20
10
00
95
90
85
80
90
TOF
85
TOF
Selectivity
Selectivity
80
9
8
7
0
0
0
9
0
7
5
75
80
70
70
70
2.0
2.5
3.0
3.5
4.0
4.5
5.0
100
110
120
130
140
Total Pressure (MPa)
Temperature (ºC)
Fig. 2. Effect of reaction total pressure on the oxidative carbonylation. Reaction con-
Fig. 1. Effect of reaction temperature on the oxidative carbonylation. Reaction con-
ditions: Au(III) complexes, 0.02 mol % of methanol; methanol, 30 ml; KI, 0.586 mmol;
Ptotal = 3.0 MPa (PCO/PO2 = 2); 4 h;stirring speed, 850 rpm.
ditions: Au(III) complexes, 0.02 mol % of methanol; methanol, 30 ml; KI, 0.586 mmol;
120 C; 4 h;stirring speed, 850 rpm.
◦
3.4. Effect of pressure on the oxidative carbonylation
also considered. The best results were obtained when an 4 mol
equivalent of KI was used. When large excess amounts of KI was
The influence of total O2 and CO pressure on the oxidative car-
bonylation was also examined, the results of which are presented
in Fig. 2. The selectivity to DMC seemed to be insusceptible to
the increase in the total pressure from 2.0 MPa to 5.0 MPa. How-
used, the catalytic activity of [AuCl (phen)]Cl decreased signifi-
2
cantly (Table 2, entry 8). This could have resulted from a preferential
coordination of the iodide anion, compared with methanol, to
Au(III) species, restricting the interaction between the substrate
and active centers [33].
−
1
ever, TOF of the catalyst was sensitive, increasing from 79.7 h to
−
1
1
55.6 h with the increase in total pressure. Despite the fact that
high pressure favors the generation of DMC, our model reaction
was performed at 3.0 MPa considering the safety factor.
3.3. Effect of temperature on oxidative carbonylation
4. Discussion
Fig.
1
shows the effect of reaction temperature on
[
AuCl (phen)]Cl/KI-catalyzed synthesis of DMC at 3.0 MPa and a
4.1. Electrochemical properties of [AuCl (phen)]Cl
2
2
CO/O2 molar ratio of 2. Only a small amount of DMC was obtained
◦
below 100 C. With the increase of reaction temperature from
1
The electrochemical behavior of [AuCl (phen)]Cl was studied
2
◦
◦
00 C to 120 C, methanol conversion increased significantly;
as a result, the TOF value also increased rapidly. However, the
selectivity to DMC decreased from 99.2% to 93.2% as the reaction
temperature elevated from 100 C to 140 C. Performing the reac-
tion >120 C, the by-product DME was detected. Subsequently, the
in acetonitrile solution by cyclic voltammetry. The obtained result
was compared with that of a reference Au(III) inorganic compound
(HAuCl ·4H O) (Fig. 3). Both complexes showed a very similar elec-
4
2
◦
◦
trochemical behavior, and two well-defined cathodic peaks were
observed in both the voltammograms (I and II). Particularly, the
height of the first peak in the voltammograms was approximately
◦
◦
reaction was performed at 120 C.
b 120
a
II
II
120
I
I
8
4
0
0
0
8
4
0
0
0
0.8
0.4
0
-0.4 -0.8 -1.2 -1.6
0.8
0.4
0
-0.4
E / V vs. SCE
E / V vs. SCE
Fig. 3. Cyclic voltammograms: (a) 2 mM HAuCl4·4H2O and (b) 2 mM [AuCl2(phen)]Cl in 0.1 M. Bu4NBF4/CH3CN solutions, scan rate 50 mV s−1.

J. Li et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 53–59
57
two times higher than that of the second one. These peaks could
140
120
100
be ascribed to transformation of gold species in the following
sequence: (i) a two-electron reduction of Au(III) species to Au(I)
at approximately 0 V versus SCE (for HAuCl ·4H O [34]) or 0.5 V
I
4
2
versus SCE (for [AuCl (phen)]Cl); and (ii) the subsequent reduc-
2
tion of Au(I) to Au(0) at more negative potentials (approximately
−
1.0 V in the case of HAuCl versus SCE [35] and 0.1 V versus SCE in
4
8
6
4
2
0
0
0
0
0
the case of [AuCl (phen)]Cl). These results clearly verify the (i) exis-
II
2
+
tence of a 3 oxidation state of gold in the complex [AuCl (phen)]Cl;
2
and (ii) in the presence of the 1,10-phenanthroline ligand, Au(III)
species seemed more potent for resisting reduction than without
the ligand. This tendency can be also confirmed by a previous obser-
vation. Although a shining golden metal particle was formed after a
reaction with the HAuCl catalyst, [AuCl (phen)]Cl-catalyzed reac-
4
2
tion yielded a very clear solution, and no Au(0) was observed in
the presence of this nitrogen-containing ligand under the same
reaction conditions. Therefore, both electrochemistry investigation
1
.2
1
0.8
0.6
0.4
0.2
0
E / V vs. SCE
and catalytic test using HAuCl4 and [AuCl (phen)]Cl demonstrate
2
undoubtedly that the enhancing effect of the nitrogen-containing
ligand 1,10-phenanthroline on the stability of Au(III) is the key to
render Au(III) species efficient for the model catalytic reaction.
Fig. 4. Cyclic voltammograms: 1 mM [AuCl2(phen)]Cl/KI (1:4 mol/mol) in 0.1 M
−
1
Bu4NBF4/. CH3CN solutions, scan rate 50 mV s
.
4.2. Role of KI
observed at 0.96 V vs SCE (peak I) and 0.40 V vs SCE (peak II),
respectively (Fig. 4). However, compared with [AuCl (phen)]Cl,
2
Cyclic voltammetry was also utilized to shed light on the role
the reduction of Au species in [AuCl (phen)]Cl/KI system occurred
2
of KI in Au-catalyzed methanol oxidative carbonylation. The cyclic
voltammetric profiles of [AuCl (phen)]Cl and [AuCl (phen)]Cl/KI
in a relatively positive potential range under the same condi-
tions (Fig. 3b): 0.50 V vs SCE (peak I) and 0.10 V vs SCE (peak
II). Similar results were also obtained in two Pd systems includ-
ing PdX (phen) (X = Cl, I) [36] and PdX (PPh ) (X = Cl, I) [37].
2
2
(
1:4 mol/mol) in acetonitrile solutions are shown in Figs. 3b and 4,
respectively.
2
2
3 2
The electrochemical behavior of [AuCl (phen)]Cl and the cyclic
These results could be partially responsible for the high activ-
2
voltammetric response of [AuCl (phen)]Cl/KI also showed a two-
ity obtained using [AuCl (phen)]Cl/KI as a catalyst. In addition,
2
2
step reduction sequence in view of the fact that two peaks were
when KI was added to a methanol solution of [AuCl (phen)]Cl,
2
Fig. 5. ESI-MS analyses of [AuCl2(phen)]Cl/KI.

5
8
J. Li et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 53–59
Fig. 6. A proposed mechanism of homogeneous gold-catalyzed oxidative carbonylation.
the color of the solution immediately changed to brown from the
original orange; a few minutes later, a deep brown solid precipi-
tated from the solution. Following the ESI-MS analysis (Fig. 5) of
the collected solid, some molecular fragment peaks (m/z) were
4.3. Mechanistic considerations
Xu et al. [10] synthesized [Au(CO)n]+ (n = 1, 2) using a facile
method from commercial Au(III) oxide Au O3 in concentrated
2
+
+
detected in 630.7 ([AuI (phen)] ), 539.8 ([AuClI(phen)] ), 504.0
H SO that exhibits high catalytic activity for carbonylation of
2
2
4
+
+
(
[AuI(phen)] ), and 412.0 ([AuCl(phen)] ), indicating the fact that
Cl in [AuCl (phen)]Cl was exchanged with I after adding KI into
olefins under atmospheric pressure and room temperature to
produce tert-carboxylic acids in high yields. The properties and
reactions of some simple gold carbonyls such as AuCl(CO) have
been described; however, the application of these gold carbonyl
complexes in catalysis are rarely investigated [45,46]. Based on the
results from both electrochemical experiments and data of the ESI-
MS analysis, a plausible mechanism of homogeneous Au-catalyzed
oxidative carbonylation of methanol was proposed (Fig. 6). In
the beginning of the catalytic cycle, the formation of an inter-
mediate species B occurred through a halide anion exchange of
−
−
2
the solution.
Iodide is one of the “softest” ligands [38]. Due to the unique
properties of iodide including low oxidation state, polarizable, and
electron rich compared with other halides, iodide can interact with
soft metals, such as latter transition metals, more preferably and
strongly [39,40]. Consequently, in a catalytic cycle that involve
−
these species, especially metals in low oxidation states, I was con-
sidered an appropriate reagent that enables metal species to keep
from being precipitated, resulting in the removal of metal from the
cycle and consequently in an inevitable deactivation of the catalyst.
This unique ability also makes iodide distinguishable from most
other ligands [39,40]. Duckworth et al. [41] reported strong interac-
tions between the gold and iodine atoms in [AuI (diars) ]I. Indeed,
+
−
[AuCl (phen)] with I . CO was subsequently activated by the coor-
2
dination with the Au(III) center ion in the iodide complex to form
gold carbonyl species C. The nucleophile methanol was bonded
with Au(III) center ions to form Au(III)-OCH3 species D through
a nucleophilic displacement by eliminating HI. After the insertion
of CO to Au(III)-OCH3 bond and the subsequent nucleophilic dis-
placement of the next methanol molecule, a new Au species G
was formed. This Au species G further underwent an elimination
of DMC to generate Au(I) complex H. In the presence of oxygen,
2
2
−
Au(I) is one of the “soft” acids, and I is an appropriate ligand to
stabilize the Au(I) species. This finding is also consistent with the
results obtained from electrochemical experiments. Iodide can also
be a good nucleophile and can be easily and reversibly oxidized to I2
[
42–44]. These properties of iodide are highly different with those
HI could be oxidized to I . Thus, the generated H could be fur-
2
of chloride and bromide and allow a facile proceeding of oxidative
carbonylation reactions (Table 2, entries 2 and 3).
ther reoxidized to B, where an oxidative addition of I2 to H was
involved.

J. Li et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 53–59
59
5
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be effective catalysts for the carbonylation of methanol to gener-
ate dimethyl carbonate. Relatively high activities were achieved by
very low concentration of [AuCl (phen)]Cl/KI catalyst. Both ligand
[
[
[
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conditions. These studies not only provided strong evidence for
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disclosed some inside information on homogeneous carbonylation
using gold complex catalysts, where the active intermediate could
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that [AuI (phen)] might be a key intermediate for activating both
2
[
[
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(
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[
[
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[
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The authors thank the Research Foundation of Science,
Huazhong University of Science and Technology for the financial
support. We also thank the Analytical and Testing Center, Huazhong
University of Science and Technology for the spectroscopic analysis
of the catalysts.
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