Study On The Catalytic Performance Of Au(Ⅲ)/Schiff Base Complex In The Homogeneous Carbonylation

The application of gold catalysis in reaction has long been neglected 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 catalyst for low-temperature CO oxidation and hydrochlorination of ethyne in the 1980s. Since then, catalysis with gold has gained much attention, and many successful examples have shown that gold catalysts can indeed be applied to several fields of heterogeneous catalysis, such as oxidation reaction, water-gas shift reaction, and in many fields of homogeneous catalysis, e.g., carbon-carbon bond forming reactions, hydration of alkynes and hydrogenation. However, despite the fact that exploration 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 heterogeneous catalysis has been well- recognized, far less efforts have been spent on study regarding homogeneous gold complexes.Few research in the field of carbonylation using Au complex as a catalyst was reported comparing with other gold catalysis areas. Oxidative carbonylation of methanol catalyzed by metal complex 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, aniline, and phenol using Cu, Co, and Pd complexes as catalysts. We considered that the Au(Ⅲ) complexes would be catalytically active in the oxidative carbonylation of methanol due to the following reasons:(1) gold is located in the same group as Cu in the periodic table of elements, allowing Au complex to exhibit similar properties as Cu complex catalyst; and (2) the electronic configuration of Au(Ⅲ) is very similar to that of Pd(Ⅱ). In this study, we presented the successful outcome of this endeavor where Au(Ⅲ)/Shiff-base complexes were used as catalysts for the oxidative carbonylation of methanol to dimethyl carbonate (DMC).We found that the highest activity of the catalyst emerged from [AuCl2(bipy)]Cl/CuCl2 in the Schiff base ligand screening test. The conversion and selectivity value of the model reaction were 14.7%,92.5%, and no Au(0) generated under the reaction conditions:120℃, Pco/Po2=2:1, P总=3.0MPa, t=4h, indicating that the Schiff base and CuCl2 could stabilized Au(III) in CO atmosphere. After the catalyst reused for 6 times, their activity keep almost unchanged.In the continution of the new catalyst system in the oxidation carbonylation, a new type homogeneous catalytic system was developed, Au(Ⅲ)/Schiff base/halide, which showed good activity in methanol oxidative carbonylation was found. The Schiff base ligand, reaction temperature, pressure, amount and type of halide additives on the catalytic performance was studied in detail. At an optimized condition:120℃, Pco/Po2==2:1, Ptotal=3.0MPa,3h, Metnaol/Au(Ⅲ)= 5060(mol/mol), KI/Au(Ⅲ)=4, on the [AuCl2(phen)]Cl/KI, the conversion of methanol was 10.8%, the selectivity to DMC was 98% and the TOF was 138.9h-1.The activity of this new catalytic system increased substantially comparing with conventional Cu complex catalyst.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. The ligand type, amount of halide additives were studied in detail. The catalytic esults shows that under 80℃, Pco=3.0MPa,5h, KI/Au(Ⅲ)= 4(mol/mol), [AuCl2(phen)]Cl/KI exhibited the best performance with the conversion of ethyl nitrite 78.2%, the selectivity of DEC 91.7%. This catalytic system is also capable to apply to low-carbon alkyl nitrite, which suggests good selectivity in the production of corresponding carbonate. No Au0 generated in this catalytic system. Both the ligand and promoter play crucial roles in increasing the catalytic activity of the gold ion in a homogeneous reaction. Au(Ⅲ)/Shiff-base/CuCl2 was then employed as a catalyst in the carbonylation of ethyl nitrite, under the same reaction conditions, on [AuCl2(phen)]Cl/KI/CuCl2, the conversion and the selectivity were enhanced up to 80.6% and 99%, and the catalyst could be used for several times.Based on the experimental results of UV-vis, ESI-MS and cyclic voltammetry, the oxidation state of gold during the reaction and the role of KI were discussed. [AuI2(phen)]+ was considered as an intermediate in the reaction. A plausible catalytic cycle mechanism between AuⅢand AuⅠwas proposed in methanol oxidative carbonylation. In alkyl nitrite carbonylation, the reaction mechanism was provided, which was different from the mechanism catalyzed by Pd catalyst system. The RO-NO is first interacts with Au(Ⅲ) to form the Au(Ⅲ)-(OR)(NO) species, and alkyloxycarbonylgold species 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. A dialkyl carbonate is formed with release of nitric oxide (NO) by reductive elimination, and the AuⅢcomplex was regenerated in the presence of alkyl nitrite and iodide, then followed up the catalytic cyclic.