Heterogeneous Mechanism Of C-C Reaction Catalyzed By Ordered Cyclopalladated Ferrocenylimine Self-assembled Nano-films

Green, economical, sustainable chemical process is one of the main challengesin today’s chemistry. Heterogeneous catalyst is consistent with the requirements ofgreen chemistry because of waste reduction, atom economy and efficient recycling.However, there are still many challengs to be solved in the field of heterogeneouscatalysis. Especially for the mechanism of the catalyst system has been disputed.Rapid development of synthetic technology and novel analysis methods provide amore favorable condition for scientists, which makes the investigation of catalyticmechanism be in the ascendant.Nano-catalysis film, as a kind of heterogeneous catalyst, is considered to be oneof the most fascinating topics in green chemistry due to its easy separation, reuse andsimple characterization. Heterogeneous catalytic behavior can be strengthenedthrough immobilized catalyst molecules onto solid surfaces which were easy tocharacterize. Therefore, design and preparations of nano-catalysis film, along with thesystematic studies of interface catalytic behaviors with the help of effective, scientificmeans become a very important research topic.In this paper, nano-films of cyclopalladated complexes were prepared by usingLangmuir-Blodgett technology, self-assembled monolayers and polymer brushes.Taken Suzuki-Miyaura reaction as a template, systematic study of the interfacecatalytic behavior were carried in heterogeneous catalytic processes by effectiveresearch tools, which enable us to understand the mechanism of heterogeneouscatalysis based on the molecular level. The main contents are listed:1. The monolayer and LB films of cyclopalladated ferrocenylimine1(Figure1)were prepared at different surface pressure and applied for Suzuki–Miyaura reactionsin heterogeneous catalytic system. Morphology studies of monolayer prepared atdifferent surface pressure were investigated by atomic force microscope (AFM),which showed that the surface morphologies of the LB films at different surfacepressure were significantly different. LB films of cyclopalladated ferrocenylimine1prepared at surface pressure of22mN/m showed the highest catalytic efficiencies compared with that at14,18,26,30mN/m. These results demonstrated that thecatalytic efficiency was related to the molecular arrangement of the catalysts.Figure1The structure of cyclopalladated ferrocenylimine1.The structure changes of cyclopalladated ferrocenylimine1LB films bylow-angle X-ray diffraction (LAXD) as well as the film surface morphologicalchanges by AFM in catalytic process were also investigated. The results indicatedthat the substrate and base were contacting with the catalyst to cover the catalyst filmsurface at the beginning of catalytic process, which is the key step in whole process.The whole catalytic process included constantly contacting and covering the surfaceof the catalyst to generate Pd intermediates that could be transferred into the product,which were further identified by the X-ray photoelectron emission microscopy (XPS)analysis.Figure2Illustration of the catalytic reaction mechanism of the cross-coupling reaction of4-iodobenzoic acid with arylboronic acid catalyzed by catalyst1LB films.2. In order to investigate the heterogeneous catalysis mechanism, an efficient,reusable and stable catalyst nano-sheet film (Si-CDI-Pd) was developed, in whichcyclopalladated ferrocenylimines were grafted onto silicon, glass or quartz surfacesby covalent.(Figure3) The catalysts films were tested in Suzuki-Miyaura reactionand displayed high activity for the preparation of various biaryls at elevatedtemperatures in neat water without ligands. Good reusability and stability were presented as reused at least8times with little Pd leaching into the crude product.Figure3Preparation of cyclopalladated ferrocenylimine catalystic films (Si-CDI-Pd).The reasonable and feasible reaction mechanism based on the results of AFM,XPS characterizations and CV tests of different reaction time were deeply explored,in which a cycle of PdIIto Pd0and Pd0to PdIIon the surface was clearly detected andillustrated. Pd0on the surface of nano-sheet film as an active surface acted asheterogeneous catalyst to catalyze the coupling reaction.(Figure4) The wholereaction was cooperative process of the catalyst and substrate. The synergistic effectof catalysts enhanced the catalysts catalytic efficiency due to the order arrangementmolecules.Figure4High-resolution XPS spectra of Pd3d, B1s and Br3d from the silicon wafer surfaces ofSi-CDI-Pd with different reaction time.3. Cyclopalladated arylimine functionalized polymer brushes (Si-PHAM-Pd[I])were also obtained by reacting PHAM-brushes with N, N-Carbonyldiimidazole (CDI)and cyclopalladated arylimine.(Figure5) The surface morphology and chemistrystructure which were characterized by contact angle method, AFM and XPS showedthat cyclopalladated arylimines were modified onto solid surfaces by covalent bond. Figure5Synthesis of the cyclopalldated arylimine PHAM-brushes (Si-PHAM-Pd[I]).Si-PHAM-Pd[I] had good catalytic activity in heterogeneous compared tohomogeneous catalyst and exhibited much improved stability and recyclability overtimes in Suzuki-Miyaura reaction. In recycle experiments, high yield (~90%) of theproducts was retained after five runs. However, catalystic activities ofSi-PHAM-Pd[I] gradully decreased in six run. It was proposed that the accessibilityof substrates to the catalytic active center became weaker due to the winding andaggregation of octadecyl chain.(Figure6)Figure6Yields and TOF obtained with recycled catalyst of Si-PHAM-Pd[I].4. The microenvironment of the polymer brush surface and interface had beenchanged by graftting of the different structure of catalyst on the surface of polymerbrush film in order to improve the catalytic performation. Cyclopalladatedferrocenylimine functionalized polymer brushes film (Si-PHAM-Pd [II]) wasdeveloped onto silicon, glass and quartz surfaces by covalent bond. Si-PHAM-Pd [II]was tested in Suzuki-Miyaura reaction and displayed high activity for the preparationof various biaryls at elevated temperatures in neat water without ligands. Improvedreusability and stability are presented as that catalytic film can be reused at least8 times with little Pd leaching into the crude product.Figure7Preparation of cyclopalladated ferrocenylimines functionalized PHAM-brushes catalyticfilm (Si-PHAM-Pd[II]).The reasonable and feasible reaction mechanism of the heterogeneousSuzuki-Miyaura reaction was deeply explored by CV, AFM and XPS, in which thewhole reaction occurred on the surface of the functionalized polymer brush by thecoordination of catalyst and reactant. The bromobenzene oxidative addition withpalladium (0) had been proven in the catalytic process, and the catalytic cycle of PdIIto Pd0and Pd0to PdIIon the surface was clearly detected and illustrated. In thisapproach, Pd (0) on the surface of nano-films as an active species acted asheterogeneous catalyst to catalyze coupling reaction proceeds via a mechanism thatwas proposed for the surface-catalyzed process.Figure8High-resolution XPS spectra of Pd3d, B1s and Br3d from the silicon wafer surfacesgrafted Si-PHAM-Pd[II].5. Poly (Acroloyl cyclopalladated ferrocenylimines) brushes (Pd/PBs) wasprepared by atom transfer radical polymerization successfully, which was the firsttime of the synthesis of organometallic polymer brushes. By comparing with theprevious catalytic system, the changes of the catalytic microenvironment influence oncatalytic activity were discussed. Pd/PBs exhibits improved catalytic activity than the homogeneous catalyst in Suzuki-Miyaura reaction, but lower than that of Si-CDI-Pdand Si-PHAM-[II]. That might be due to the large steric hindrance of acroloylcyclopalladated ferrocenylimines in the polymer brushes, which exhibited lower orderarrangement of surface and reduced the flexibility of polymer brushes.Figure9Synthesis of the poly (Acroloyl cyclopalladated ferrocenylimines) brushes (Pd/PBs).Pd/PBs exhibited poor stability which was proved by ultraviolet spectroscopyand cyclic voltammetry. It was because of the existence of ester group in3a, whichhydrolyzed under the base solution in Suzuki-Miyaura reaction. Other catalystmonomer with flexible chain, stable ester group will be replaced for polymer, whichmay be improved for the stability and catalytic activity of the polymer brushes infuture.