Hetergeneous Catalysis Using Molecular Self-Assembly And Electrochemical Polymerization-Self-Assembly-Doping Of Pd (Ⅱ)-Schiff Base Complexes

Palladium-catalyzed cross-coupling reactions are most powerful and utilized tools for the construction of a C-C bond. The properties of such palladium catalysts can be tuned by ligands or additives. Proper ligand design has led to catalysts that exhibit higher conversions(TONs) and frequency conversion(TOFs), have improved lifetimes, and are suitably stable to run the reactions in the mild conditions. Over the past few decades, various forms of homogeneous catalytic systems based PdII or Pd0 been extensively studied. Many products could be synthesized by this methodlogy for the first time or in a much more efficient way than before. In recent years, the development of safe, stable, green and efficient novel catalysts has become more and more important, because of the lack of energy and environmental pollution in the world. Heterogeneous catalysts, which have obvious advantage of good recycleability, would be a promising "green" choice.Self-assembled monolayers(SAMs) have also attracted considerable attention because they present the possibility of the precise tailoring of surface properties, which will have significant potential application in drug delivery,sensor, and heterogeneous catalysis. The Self-assembled palladium catalysts are present as a high stability and recycling capabilities because most palladium complexes are covalently attached on a solid supports surface. However, solid-supported catalysts are complex assemblies. Minor changes of their preparation conditions can significantly influence the delicate balance of conflicting demands: high activity, high selectivity, and long lifetime. Often, heterogeneous catalysts are still chosen on an empirical basis without understanding why a given catalyst is superior to another one. Thus, the preparation of heterogeneous catalysts is a challenging task.In this paper, Pd-Schiff base complex self-assembly films and Pd-Schiff base complex electrochemical polymerization-self-assembly-doping(ECP-SA-D) films have been prepared and characterized. The prepared catalysts, as true heterogeneous catalysts were applied for catalyzing Suzuki coupling reaction. The main results are as follows:1. A self-assembled monolayer of palladium(II)-ferrocenyl Schiff base complex(Pd(FcL)–Si) has been prepared and characterized by water-drop contact angle(WDCA), atomic force microscopy(AFM), Raman spectroscopy(RS), cyclic voltammetry(CV) and X-ray photoelectron spectroscopy(XPS). Pd(FcL)–Si showed the higher catalytic efficiency compared with homogeneous catalyst. Recycling of Pd(FcL)-Si-catalysed Suzuki coupling reaction could be performed for five run without a significant loss of catalytic activity. These results of WDCA, AFM and RS, confirmed that surface sites on Pd(FcL)-Si were crucial to the catalysis of the coupling reaction. The precatalyst hybrid was used for the in situ reduction of the catalytically active Pd0 before reaction using cyclic voltammetry. The first reaction step on the substrate, i.e., the oxidative addition of aryl halide to Pd0(FcL)-Si, was established by XPS. In addition, evidence of catalyst restoration to its initial state after catalytic reaction was obtained. Pd0/ PdII redox interplay on the surface was also clearly detected.2. A series of self-assembly palladium(II)-alkyl Schiff base complexes [PdLn-Si](1-5) [n=0, 1,3,6,10; behaving as different carbon chain length], have been synthesized and characterized by WDCA, AFM, UV-vis, RS, and XPS. Cat.3 as example, having the moderate alkyl chain among the five complexes, behaves as a highly active and recyclable catalyst towards Suzuki coupling reaction for the synthesis of biaryl organics. Three-phase test using CV and UV-vis indicated that the self-assembled palladium(II)-alkyl Schiff base complex acts as a true heterogeneous catalyst in coupling reaction.The length of the alkyl chain of the catalysts significently affected the catalytic activity. Alkyl substituent would promot oxidative addition of aryl halides to the Pd intermediate by making palladium more electron rich and increase the activity consequently. The other reason was that the longer alkyl chain could benefit the dispersion of Pd nanoparciles generated in situ during the catalytic process. However, the catalytic active of Cat.4 and Cat.5 decreased under similar reaction conditions. The reason may be that the catalysts are two-dimensional surface, in which wrapping between long alkyl chains resulted in burying of the reactive center, which makes the substrates difficult to contact with active center. Thus the catalytic activity and recycle ability decreased. RS and XPS analysis were employed to understand the surface catalysis process of the Cat.3, in which the catalytic mechanism was heterogenerous via absorption/desorption at interface and recycle of Pd0/PdII to realizing catalysis.3. Pd-thienyl Schiff base complex self-assembly film(PdThi-ITO) was prepared by self-assembly, and Pd-thienyl Schiff base complexes electrochemical polymerization self-assembly-doping thin film(Pd PThi-ITO, PdTT-ITO, PdTF-ITO and PdTM-ITO) has been prepared by electrochemical polymerization-self-assembly-doping(ECP-SA-D) through adjusting the concentration of monomer, scaning run of CV, condition of electropolymerization and characterized by CV, AFM and XPS. PdTT-ITO and PdTF-ITO showed the higher catalytic efficiency compared with PdThi-ITO. Recycling of PdTF-ITO used for catalyzing Suzuki coupling reaction could be performed at seven runs without a significant loss of catalytic activity, indicating that improving catalytic activity and stability of the catalyst could be achieved through electrochemical polymerization-self-assembly-doping(ECP-SA-D). Thus we propose an efficient method for fabricating a high activity, high stability of electrochemical polymerization-self-assembly-doping(ECP-SA-D) catalysis film.