Catalysis Communications 12 (2011) 1401–1404
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Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Au (III)/N-containing ligand complex: A novel and efficient catalyst in carbonylation of alkyl nitrite Jinjin Li a, Jianglin Hu a, Guangxing Li a, b,⁎ a b
School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Luoyu Road 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
a r t i c l e
i n f o
Article history: Received 24 January 2011 Received in revised form 18 May 2011 Accepted 23 May 2011 Available online 1 June 2011 Keywords: Homogeneous gold catalysis Au(III)/N-containing ligand complex Carbonylation Alkyl nitrite
a b s t r a c t High catalytic activity of a gold N-containing ligand complex in the homogenous carbonylation of alkyl nitrite to dialkyl carbonate with KI as the promoter is reported. [AuCl2(phen)]Cl/KI (phen = 1,10-phenanthroline) complex has been used as a catalyst in the carbonylation of ethyl nitrite. The use of iodide as a promoter −1 −1 •h ) and selectivity (91.7%) for diethyl resulted in a significant increase in activity (TOF: 35.8 mol•molAu carbonate at 3.0 MPa, 80 °C and 5 h. Based on the results of ESI-MS, UV-Vis, and cyclic voltammetry (CV) experiments, a mechanism is proposed for the carbonylation of alkyl nitrite in a homogeneous system using a gold N-containing ligand complex as a catalyst. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Interest in the catalytic chemistry involving gold has undergone a marked increase in recent years [1]. Effectiveness of gold in heterogeneous catalysis has been well recognized, such as in carbon monoxide oxidation [2], water–gas shift [3], alcohol oxidation [4], and selective hydrogenation [5]. The results indicate that the potential of using gold as a catalyst warrants additional investigation. Some applications of gold in homogeneous catalysis are well defined, including carbon–carbon bond forming reactions [6], nucleophilic additions [7] and hydrogenation [8]. However, compared to other precious metal complexes used as catalysts, far less effort has been spent on studying the application of gold complexes in homogeneous catalysis [9]. Organic carbonates are an important class of compounds that can be used in many fields [10]. In recent years, the carbonylation of alkyl nitrites has been considered to be most favorable in producing organic carbonates. Although there have been efforts in exploring this reaction, only a few metal complexes are effective [11–14]. To extend the practical application of gold in homogeneous catalysis, the experimental study presented herein describes a homogeneous gold-catalyzed carbonylation of alkyl nitrite to dialkyl carbonate (Scheme 1). Several gold N-containing ligand complexes were tested and exhibited high activity. This paper also discusses how
⁎ Corresponding author at: School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan, Hubei 430074, PR China. Tel.: + 86 27 87543732; fax: + 86 27 87544532. E-mail address:
[email protected] (G. Li). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.05.025
KI promotes the catalytic reaction. A carbonylation reaction catalyzed by a gold N-containing ligand complex has not been reported previously. 2. Experimental 2.1. Catalyst preparation Based on currently available literature [15,16], the general procedure for preparing gold N-containing ligand complexes is as follows: a 10 ml ethanolic solution of 1.0 g HAuCl4 was added, with stirring, to an 10 ml ethanolic solution of 1.0 ligand, heated under reflux for several hours to give a yellow to orange precipitate, and cooled to room temperature. After filtration, the precipitate was washed with ethanol (5 mL × 3), and air-dried. A 95% yield of [AuCl2 (phen)]Cl was obtained. The elemental composition and spectra data of the precipitate are as follows: C, 29.62%; H, 1.72%; N, 5.68%; AuCl3C12H8N2 requires C, 29.79%; H, 1.67%; N, 5.79%; 1H NMR (DMSO-d6, δ, ppm): 9.70 (d, 2H), 9.34 (d, 2H), 8.52 (s, 2H), 8.44 (m, 2H); IR (KBr, cm− 1): 1637 (s), 1582 (s), 1513 (m), 1485 (m), 1413 (s), 1218 (m), 853 (m), 703 (s) cm− 1; and UV-Vis: λmax(CH3OH)/nm: 230, 280, 317. 2.2. Catalytic experiments Tests of these catalysts were carried out in a 100 mL stainless steel autoclave equipped with a mechanical stirrer and an automatic temperature controller. Consecutively, 0.144 mmol of gold N-containing ligand complex, 0.576 mmol of KI, 25 mL anhydrous ethanol, and 0.072 mol of alkyl nitrite were charged into the polytetrafluoroethylene
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3. Results and discussion 3.1. Catalytic studies
Scheme 1. Gold-catalyzed carbonylation of alkyl nitrite.
(PTFE)-lined autoclave, and the reactor was sealed and placed in an ice water bath. After purging three times with carbon monoxide, the autoclave was pressurized with CO to a total pressure of 3 MPa. The reactor was placed in the electric furnace and heated at a temperature of 80 °C for 5 h. After the reaction, the liquid mixture was analyzed qualitatively by gas chromatography (GC-Mass) (Agilent 6890/5973 with FID and a split/splitless injection system). Quantitative analysis of DEC in distillate was analyzed by gas chromatography (1790, Agilent) with a 25 m, i.d. 0.32 mm, HP-5 capillary column, and FID. The main parameters were as follows: N2 carrier gas with a flow of 2 ml/min; injector temperature, 180 °C; detector temperature, 200 °C; split ratio, 1/20; temperature programmed: initial temperature 40 °C for 2 min, and then heating to 150 °C at the ratio of 25 °C/min for 5 min.
2.3. Characterization of catalyst The Fourier-transform infrared (FTIR) spectrum in KBr pellet was recorded on Bruker Equinox 55 FTIR spectrophotometer in the range of 4000–400 cm − 1. The Diffuse reflectance UV-Vis spectra were measured on Shimadzu UV-2550 PC spectrophotometer in the range 800–200 nm. The C, H, N elemental analysis was carried out on a Vario EL III element analyzer. 1HNMR spectra were obtained on a Bruker AV 400 spectrometer in DMSO-d6, tetramethylsilane as internal standard. ESI-MS spectra were recorded with Agilent 1100 LC/MSD Trap XCT. Cyclic voltammetry measurement was performed in acetonitrile using tetrabutylammonium tetrafluoroborate (Bu4NBF4, 0.1 M = 0.1 mol·L − 1) as a supporting electrolyte using Electrochemical workstation CS300 at room temperature. A conventional threeelectrode system was employed. Current-voltage curves were measured and recorded at 50 mV·s − 1 for four cycles of each sample. All potentials were reported in volts versus SCE. All measurements were performed in well-deairated solutions under nitrogen atmosphere.
The catalytic effects of gold N-containing ligand complexes in the carbonylation of ethyl nitrite are shown in Table 1. Among all catalysts tested, [AuCl2(phen)]Cl, along with KI, exhibited the highest activity in the carbonylation of ethyl nitrite to diethyl carbonate (DEC) with an −1 activity of 35.8 mol·molAu ·h − 1 (entry 6). The promoter effect of various ligands on activity is increased according to a series of entries 1 to 6. The difference in catalytic performance of metal complexes is reportedly associated with their basicity, chelating power and rigidity [17,18]. Significant effects of substitution groups on the ligand can also be demonstrated by the fact that a lower activity was obtained by using a 1,10-phenanthroline derivative with an electron-withdrawing group, 5-nitro-1,10-phenanthroline (entry 1). The electron withdrawing nitryl decreased the electronic density of the aromatic-phen moiety, thus, causing an apparent decrease in activity. When PdCl2(phen) was employed as a catalyst, an activity of −1 20.7 mol·molPd ·h − 1 was observed. AuCl(PPh3) exhibited excellent catalytic performance in the oxidative carbonylation of amines [19], showed low catalytic activity in the carbonylation of ethyl nitrite (entry 8). CuCl/phen, which is known as an effective catalyst in the oxidative carbonylation of alkyl alcohol by molecular oxygen [20], showed only a trace amount of DEC in the carbonylation of ethyl nitrite (entry 10). These results suggest that Au(III)/N-containing ligand complexes are more efficient catalysts than the corresponding Au(I) and Pd(II) complexes in the carbonylation of ethyl nitrite to form ethyl carbonate. The electronic configuration of Au(III) is very similar to that of Pd(II), perhaps indicating that Au(III) with the same d 8 configuration as Pd(II) can catalyze the kinds of reactions typically catalyzed by palladium. In contrast to low catalytic activities in the absence of KI (entry 7), KCl or KBr promoted the carbonylation of ethyl nitrite with the −1 −1 activity of 17.9 mol·molAu ·h − 1 and 19.9 mol·molAu ·h − 1 respectively (entries 11 and 12). Matsuzaki reported that solvents are important for these carbonylation reactions, and that alcohol is the most favorable solvent for carbonylation of dialkyl carbonates [21]. We also found that reactions carried out in a non-alcohol solvent showed trace detections of dialkyl carbonate. Under the optimized conditions of adding KI and alcohol, the activity of [AuCl2(phen)]Cl/KI in the carbonylation of other alkyl nitrites is summarized in the results presented in Table 2. The corresponding products were obtained with very high activities of −1 35.6 and 35.1 mol·molAu ·h − 1, respectively. It is found that this
Table 1 Catalytic performance of gold complexes in carbonylation of ethyl nitrite.a Entry
1 2 3 4 5 6 7 8 9 10 11 12 a b c
Catalysts
[AuCl2(NO2-phen)]Cl [Au(en)2]Cl3 AuCl3(py) [AuCl2((CH3)2-phen)]Cl [AuCl2(bipy)]Cl [AuCl2(phen)]Cl [AuCl2(phen)]Cl AuCl(PPh3) PdCl2(phen) CuCl/phen [AuCl2(phen)]Cl [AuCl2(phen)]Cl
Promoter
KI KI KI KI KI KI – KI KI – KCl KBr
Product.DEC (mmol) 10.5 12.4 23.6 24.8 25.0 25.8 2.1 3.2 14.9 Trace 12.9 14.4
Selectivity (%) DEC
Acetal
Othersb
88.5 91.0 91.2 91.2 91.0 91.7 22.1 35.8 51.2 – 7.9 9.8
10.2 7.8 8.1 7.9 7.8 7.6 68.1 52.7 11.6 80.5 69.0 63.7
1.3 1.2 0.8 0.8 1.1 0.6 9.8 11.5 37.3 19.5 23.0 26.5
Yield.DEC (%)
Activity.DEC −1 (mol·molAu ·h− 1)
29.2 34.4 65.6 68.9 69.4 71.7 5.8 8.9 41.4 Trace 35.8 40.0
14.6 17.2 32.8 34.4 34.7 35.8 2.9 4.4 20.7c Trace 17.9 19.9
Reaction conditions: gold complexes, 0.144 mmol; KX, 0.576 mmol; ethyl nitrite, 0.072 mol; ethanol, 25 mL; PCO = 3.0 MPa; 80 °C; 5 h; stirring speed, 850 r/min. Ethyl acetate and diethyl oxalate. −1 mol·molPd ·h− 1.
J. Li et al. / Catalysis Communications 12 (2011) 1401–1404 Table 2 Synthesis of carbonates catalyzed by [AuCl2(phen)]Cl.a Entry
Substrates
Product.DRC (mmol)
Selectivity.DRC (%)
Yield.DRC (%)
Activity.DRC −1 (mol·molAu ·h−1)
1 2 3
ethyl nitrite n-propyl nitrite n-butyl nitrite
25.8 25.6 25.3
91.7 90.5 90.2
71.7 71.1 70.3
35.8 35.6 35.1
a
Reaction conditions: [AuCl2(phen)]Cl, 0.144 mmol; KI, 0.576 mmol; alkyl nitrite, 0.072 mol; alkyl alcohol, 25 mL; PCO = 3.0 MPa; 80 °C; 5 h; stirring speed, 850 r/min.
catalytic system is applicable to low-carbon alkyl nitrite, which suggests selectivity in the corresponding carbonate. 3.2. Effect of KI on the carbonylation of alkyl nitrite It is presumed that chloride (Cl −) in [AuCl2(phen)]Cl exchanges with iodide (I −) when KI is added to the solution. In general, iodide could be a good promoter in many carbonylation reactions [22–24], hence, attempts were made to identify the compound [AuI2(phen)] +. When KI was added to an orange methanol solution of [AuCl2(phen)] Cl, the color immediately changed to brown and a deep brown solid precipitate was formed. The solid was collected and dissolved in methanol, and analyzed by ESI-MS. Some molecular fragment peaks (m/z) were detected in 630.7 ([AuI2(phen)] +), 539.8 ([AuClI (phen)] +), 446.9 ([AuCl2(phen)] +), and 412.0 ([AuCl(phen)] +). The UV-Vis spectra of [AuCl2(phen)]Cl and [AuCl2(phen)]Cl/KI in acetonitrile solutions were recorded and shown in Fig. 1. The most distinct difference between both spectra was a new band that appeared at 360 nm in [AuCl2(phen)]Cl/KI but not in [AuCl2(phen)]Cl. The band at 360 nm was also present in the catalyst for the carbonylation of ethyl nitrite (see insert in Fig. 1). These results indicate that a halide anion exchange of [AuCl2(phen)] + with I - occurred [25], and that [AuI2(phen)] + has a strong oxidation-reduction ability in the reaction system. Duckworth estimated the distance of Au-I by the leastsquares analysis [26] and reported that there exist strong interactions between the gold and iodine atom in [AuI2(diars)2]I. This report indicates that Au (III) iodide complexes could exist in a stable state. It is well known that iodide is one of the “softest” ligands in a homogeneous catalytic system, thus, iodide could form a stronger bond with soft metals (low oxidation state, polarizable, and electron rich, such as the
Fig. 1. UV-Vis spectra of (a) [AuCl2(phen)]Cl and (b) [AuCl2(phen)]Cl/KI in acetonitrile solutions. The inset shows the UV-Vis spectra of [AuCl2(phen)]Cl/KI catalyst separated after reaction recorded in acetonitrile solutions.
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later transition metals) than other halides [27]. Differences in the properties of iodide compared to chloride and bromide could influence the rate of carbonylation reactions (Table 1, entries 11 and 12). 3.3. The oxidation state of gold Fig. 2 shows the CV response in 0.1 M Bu4NBF4/CH3CN solutions. A prominent reduction process consists of two peaks at 0.47 V (peak I) and 0 V (peak II) vs. SCE (see Fig. 2a), respectively. Peak I at 0.47 V could be assigned to a consecutive two-electron transfer process from Au (III) to form an intermediate Au (I) complex that was reduced to Au (0) at 0 V. Interestingly, when a similar electrochemical experiment was performed with the addition of ethyl nitrite, the CV response at 0.47 V disappeared and a new peak at 0.6 V vs. SCE emerged (see Fig. 2b). The disappearance of a peak at 0 V in the presence of ethyl nitrite indicated that complex Au (I) did not undergo the reduction of Au (I) to Au (0). Conversely, Au (I) was oxidized to the original Au (III), thus, constructing a normal catalytic cycle, as shown in Fig. 3. The UV-Vis spectrum (see inset in Fig. 2) shows a new band at 460 nm upon the addition of ethyl nitrite. This may be attributed to a Au-ethyl oxygen intermediate generated by the reaction between [AuCl2(phen)]Cl, KI and ethyl nitrite [28]. These experiments indicate that ethyl nitrite was nucleophilic with Au (III) ion. These experiments also confirm that Au (0) was not formed in the presence of ethyl nitrite and iodide, as reported in the literature [21].
Fig. 2. Cyclic voltammograms of 1 mM [AuCl2(phen)]Cl/KI/C2H5ONO (1:4:500 mol/mol) solution in 0.1 M Bu4NBF4/CH3CN solutions: (a) ethyl nitrite-free and (b) with ethyl nitrite, scan rate 50 mV·s-1. The inset shows the UV-vis spectra of [AuCl2(phen)]Cl recorded after addition of ethyl nitrite.
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iodide [11,21]. Incidentally, in this alkyl nitrite reaction in the presence of small amount of halogen anions, the Au (III) catalysts without the co-presence such as CuCl2 hardly deactivated, whereas it is a well-known additive in a Wacker-type reaction. Therefore, it is supposed that the smooth redox cycle of the Au catalyst is mainly due to the introduction of KI and alkyl nitrite, especially alkyl nitrite. In the case of DEC synthesis, alkyl nitrite is one of the substrates to synthesize DEC, but it is also an oxidant at the same time. 4. Conclusions The results of experimental studies described in this paper demonstrate high catalytic activity of a gold N-containing ligand complex in the carbonylation of alkyl nitrite to dialkyl carbonate with KI as the promoter. The highest activity was achieved using [AuCl2(phen)]Cl/KI as the catalyst. Both the ligand and promoter play crucial roles in increasing the catalytic activity of the gold ion in a homogeneous reaction. It is the first report of applying gold Ncontaining ligand complex as a homogeneous catalyst for carbonylation of alkyl nitrite. Acknowledgments 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. References
Fig. 3. A proposed mechanism of homogeneous gold-catalyzed carbonylation.
3.4. Proposed mechanism The carbonylation of alkyl nitrite on gold catalyst is reported for the first time in this work. Based on the spectra from ESI-MS and UVVis, as well as the results of CV experiments, the mechanism proposed for carbonylation of alkyl nitrite catalyzed by [AuCl2(phen)] Cl/KI is shown in Fig. 3. Halogen anions are indispensable in this reaction. The formation of an intermediate species B occurred through a halide anion exchange in [AuCl2(phen)] +. That species was confirmed by ESI-MS analysis after adding KI in the solution of [AuCl2(phen)]Cl. The RO-NO interacted with Au(III) in the intermediate species B to form the Au(III)-(OR)(NO) species C. Formation of this metal intermediate species explained that alkyl nitrite was more easily activated than the Au(III) intermediate with CO in this catalytic system. Moreover, UVVis spectra of [AuCl2(phen)]Cl/KI/C2H5ONO also supported this conclusion. A new band at 460 nm appeared upon the addition of ethyl nitrite. CO was subsequently activated by the formation of a coordination complex with the central atom, Au (III), to form a Aucarbonyl species D and alkyloxycarbonylgold species E. Au (III) (OCOR)(NO) was generated through the insertion of CO. The next step was a nucleophilic attack by the next alkyloxy on the ROH provided by a reactant or a solvent in the reaction. A dialkyl carbonate was formed with release of nitric oxide (NO) by reductive elimination, and the Au (III) complex was regenerated in the presence of alkyl nitrite and
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