Catalytically active palladium nanoparticles embedded in an organic-inorganic fluorinated hybrid material

Palladium nanoparticles were supported on a fluorous organic-inorganic hybrid material prepared by the sol-gel process without TEOS. Recycling studies on the Heck coupling reaction have shown that the catalyst can be readily recovered and reused.


Introduction
A large body of work has been dedicated to the use of palladium nanoparticles (PdNPs) as catalysts for the formation of C-C bonds. 1,2A major effort has been dedicated to supporting PdNPs on a variety of materials 3 in order to facilitate the separation, recovery and reuse of expensive palladium catalysts.Our group in particular has contributed in this work with the preparation of nanosized metallic palladium particles embedded in silica and carbon aerogels. 4e have previously reported that heavily fluorinated star-shaped compounds (A, Figure 1) favor the formation of palladium nanoparticles (PdNPs-A) that were catalytically active in C-C bond forming processes. 5Although in principle heavily fluorinated compounds do not seem ideal stabilizers for nanoparticles, we demonstrated that some compounds can indeed be used with success as a protecting shield for nanoparticles.Furthermore, we demonstrated that PdNPs-A could be embedded within a fluorous silica gel matrix and that this material showed promise as a catalyst for the Heck reaction.However, subsequent analysis of the reaction mixture revealed some leaching of both the palladium and the fluorous stabilizer. 6Most recently, we prepared a more robust hybrid material PdNPs-B with the stabilizer linked covalently to the silica gel matrix. 7In this case, the matrix was prepared through sol-gel condensation between a monosilylated fluorous precursor and a large excess of tetraethoxysilane (TEOS).The resulting material acts as an efficient stabilizer of catalytically active PdNPs (diameter 4 ± 1 nm) (PdNPs-B, Figure 1).Despite these promising results, the material was found to contain only 3.95% of palladium.With the aim of obtaining new catalytically active materials that could support larger quantities of highly dispersed nanosized palladium particles, we decided to prepare an organicinorganic polymer based on the sol-gel process of the bis-silylated compound 3b (Scheme 1).In this case, the presence of not one, but two silyl groups would allow for the synthesis of a stable sol-gel matrix without the need for addition of TEOS.The idea was that the quantity of embedded palladium would be controlled by the amount of the organic part in the hybrid material.

Results and Discussion
Our objective was to obtain an organic-inorganic hybrid material with a regular and stoichiometric distribution of the highly fluorinated organic moieties throughout the silicate network, capable of supporting metallic nanoparticles.Therefore, we proposed the synthesis of a precursor containing two trialkoxysilane groups that could condense through a sol-gel process without the presence of the TEOS. 8The idea is that increasing the number of perfluorinated chains the percentage of nanosized palladium stabilized by the material will be higher.The preparation of the bis-silylated monomer was based on successive substitutions of 2,4,6trichloro-1,3,5-triazine.The methodology was initially optimized using the non-fluorinated dodecanethiol.First, selective substitution of a single chloride was achieved trough the reaction of 1 with the dodecanethiol in THF at 0 ºC in the presence of di-isopropyl ethylamine (DIPEA).Thus, after 30 minutes, the product 2a was obtained in quantitative yield, with longer reaction times leading to the formation of some of the bis-substituted product.The remaining chlorides were substituted by the bifunctional 3-(triethoxysilyl)propan-1-amine in CH3CN at 80 ºC using DIPEA as a base, giving a 98% yield of the trisubstituted 3a.The 1 H NMR characterization of this product was complicated by the restricted rotation around C-N bonds and the presence of several interconverting tautomers at 25 ºC (Figure 2a).The spectrum, therefore, was recorded at 80 ºC, allowing for the observation of the much simplified average structure (Figure 2b). 9 With the conditions established, we repeated the steps with 1H,1H,2H,2Hperfluorodecanethiol, giving a quantitative yield of the monosubstituted product 2b and, subsequently, a 93% yield of the desired fluorinated monomer 3b (Scheme 1).Having obtained the polyfluorinated monomer, 3b, we explored the conditions necessary to carry out the sol-gel process.The standard conditions based on a mixture of ethanol, water and ammonium fluoride as catalyst led, upon the addition of water, to the rapid precipitation of the monomer.Similarly, attempted use of the formic acid led to the formation of a heterogeneous mixture. 10

ISSN 1551-7012
Page 184 Finally, a homogeneous gel was formed using trifluoroacetic acid as a gelling agent (Scheme 2) The use of this acid as both solvent and catalyst in sol-gel processes has been described by Sharp and co-workers 11 and has been successfully applied in our own work. 7The final product was characterized by the solid state 29 Si and 13 C NMR (Figure 3) and by the elemental analysis.In the 29 Si NMR spectrum only the T units at around -56 to -70 ppm are observed, indicating the hydrolysis-condensation of 3b.In the solid state 13 C NMR spectrum we were able to assign the chemical shifts of the organic part.The spectrum shows the following carbon resonances: 178.3-155.5 (1,3,5-triazina ring), 118.7-111.6 (polyfluorinated chain), 44.1 (CH2NH), 32.6 (SCH2), 23.1 (CH2CH2Si) and 10.4 (CSi) ppm, which confirms the presence the of the ligand covalently bonded to silica; elemental analysis is consistent with the formation of the desired material.Transmission electron microscopy (TEM) confirmed the presence of metal nanoparticles with an average size of 4 nm (Figure 4) and electron diffraction confirmed the existence of the face-centered cubic palladium(0).The palladium content of 12.50% determined by ICP was significantly higher than the 4% incorporation achieved using TEOS at the sol-gel stage.With respect to the phenomenon of the stabilization we propose that Nps are entrapped in the solid network, with the initial interactions probably taking place through the sulfur and nitrogen coordinating atoms.Upon reduction, the presence of long perfluorinated chains prevents the agglomeration of the nanoparticles.The resulting material was tested as catalysts in the Heck 12 reaction of iodobenzene with butyl acrylate in CH3CN at 130 ºC using tributylamine as a base with the amount of catalyst corresponding to 2% molar palladium.The reaction was complete in 24 h and the catalyst could be recovered and reused for six consecutive runs without appreciable loss of activity (Scheme 4).

Conclusions
A new fluorous organic-inorganic hybrid material has been prepared by a tetraethoxysilane-free sol-gel process.The material was used as a stabilizer in the formation of palladium nanoparticles, giving levels of palladium incorporation superior to those obtained with the TEOS-based material.We demonstrated that the phenomenon of the stabilization of the nanoparticles is not due exclusively to entrapment of metallic particles on the polymeric network, as the efficiency of the synthesis was found to directly depend on the number of the long perfluorinated alkyl chains.Moreover this allows carrying out the catalytic Heck reactions with a lower quantity of material.

Experimental Section
General Procedures.All reagents were purchased from commercial sources and were used as received.Anhydrous THF was obtained by distilling from sodium metal/benzophenone, and CH3CN was distilled from P2O5.Experiments requiring nitrogen atmosphere were carried out using standard Schlenk line techniques.All NMR measurements were carried out at the Servei de Ressonància Magnètica Nuclear at the Universitat Autònoma de Barcelona.Routine 1 H and 13 C NMR were recorded on Bruker AC250 (250 MHz for 1 H) and Avance360 (360 MHz for 1 H) instruments.Solid state Cross Polarization / Magic Angle Spinning (CP MAS) NMR spectra were recorded using a Bruker AvanceII 400 MHz WB instrument.Infrared spectra were recorded using a Bruker Tensor 27 instrument equipped with an ATR Golden Gate cell and a diamond window.Routine CHN elemental analyses were performed at the Servei de Microanàlisi del CSIC de Barcelona.ICP measurements of palladium contents were carried out at the Serveis Cientificotècnics of the Universitat de Barcelona.HR-MS measurements were performed at the Servei d'Anàlisi Química of the Universitat Autònoma de Barcelona.Melting points were determined using a Reichert brand melting point apparatus.Transmission electron microscopy (TEM) analyses were performed in the Servei de Microscòpia of the Universitat Autònoma de Barcelona, using a JEOL JEM-2010 model at 200kV.For the TEM measurement, the nanoparticulate material was subjected to sonication in perfluorooctyl bromide for several minutes.A drop of this finely dispersed suspension was placed on a specially produced structureless carbon support film having a thickness of 4-6 nm and dried before observation.