Palladium Supported on Cross-Linked Imidazolium Network on Silica as Highly Sustainable Catalysts...

DOI: 10.1002/adsc.201300215

Palladium Supported on Cross-Linked Imidazolium Network onSilica as Highly Sustainable Catalysts for the Suzuki Reactionunder Flow Conditions

Cinzia Pavia,a Eleonora Ballerini,b Lucia Anna Bivona,a,c Francesco Giacalone,a

Carmela Aprile,c,* Luigi Vaccaro,b,* and Michelangelo Gruttadauriaa,*a Dipartimento di Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche (STEBICEF) Sez. di Chimica, Universit� di

Palermo, Viale delle Scienze s/n, Ed. 17, 90128 Palermo, ItalyFax: (+39)-091-596-825; phone: (+ 39)-091-2389-7534; e-mail: [email protected]

b Laboratory of Green Synthetic Organic Chemistry, CEMIN – Dipartimento di Chimica, Universit� di Perugia Via Elce diSotto 8, 06123 Perugia, ItalyE-mail: [email protected]

c Laboratory of Applied Material Chemistry (CMA), University of Namur, 61 rue de Bruxelles, 5000 Namur, BelgiumE-mail: [email protected]

Received: March 12, 2013; Published online: June 10, 2013

Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300215.

Abstract: Highly cross-linked imidazolium-based ma-terials, obtained by radical oligomerization of bis-vin ACHTUNGTRENNUNGylimidazolium salts in the presence of 3-mercap-topropyl-modified silica gel, were used as supportsfor palladium catalysts. Thanks to the high imidazoli-um loading these materials were able to supporta high amount of the metal (10 wt%). Such materialswere characterized by several techniques (13C magicangle spinning nuclear magnetic resonance, the Bru-nauer–Emmett–Teller technique, X-ray photoelec-tron spectroscopy, and transmission electron micros-copy). The palladium catalysts displayed good activi-ty allowing the synthesis of several biphenyl com-pounds in high yields working with only 0.1 mol% ofpalladium loading at 50 8C in ethanol/water under

batch condition. Moreover, a flow apparatus, to opti-mize the efficiency of the isolation of the pure prod-ucts and minimize waste (E-factor), was investigated.For the first time the palladium catalyst and base(K2CO3) were placed in two separate columns allow-ing an easy recovery of the products with very lowE-factor values (<4). Waste production was reducedby over 99% compared to classic batch conditions.Because of the high Pd loading only 42 mg of cata-lysts were employed in the Suzuki reaction between160 mmol of 4-bromotoluene and 180 mmol of phe-nylboronic acid. No loss in activity was observed.

Keywords: catalyst recycling; C�C coupling; palladi-um; Suzuki–Miyaura reaction

Introduction

Carbon-carbon bond forming reactions catalyzed bypalladium compounds is a field of great interest.Suzuki–Miyaura, Mizoroki–Heck and Sonogashira re-actions are widely employed in synthetic organicchemistry and in materials chemistry.[1] A plethora ofreaction conditions and substrates has been examined,but still there is enough room for catalyst improve-ment. Indeed, researches towards the development ofmore active, selective and recyclable catalysts arealways desirable. Many efforts have been focused onthe synthesis and application of supported palladiumcatalysts for the above reactions with the aim to getrecyclable catalytic materials. To avoid problems re-

lated to separation of homogeneous catalysts, palladi-um complexes or nanoparticles have been immobi-lized on various supports such as silica, alumina, zeo-lites, organic polymers, magnetic nanoparticles anddendrimers.[2–9]

In this context, several studies have been carriedout on the use of ionic liquids as reaction media andas stabilizers for metal nanoparticles. Imidazolium-based ionic liquids are able to stabilize metal nano-particles electronically and by coordination involvingthe cations, which protect against aggregation and/oragglomeration.[10–12]

In particular, great attention has been devoted tothe use of supported ionic liquid phase (SILP)-basedmaterials as heterogeneous recyclable palladium cata-

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lysts. Ionic liquid frameworks have been supportedusing ordered mesoporous silica,[13] zeolite,[14] spheri-cal silica nanoparticles,[15] nanosilica dendrimer,[16] di-ethylaminopropylated alumina,[17] polystyrene,[18] poly-styrene-PEG,[19] carbon nanotubes,[20] magnetic nano-particles[21] and colloidal poly-imidazolium salts.[22]

Therefore, such materials have been used in Suzuki–Miyaura reactions. Recently, we described a new kindof material constituted by a highly cross-linked imida-zolium network obtained by radical oligomerizationof bis-vinylimidazolium salts on the surface of mer-captopropyl-modified silica gel.[23] Usually, the sup-ported ionic liquid phase is present as a monolayerwhen it is covalently attached on the surface or asmultilayer when it is adsorbed. Our materials coverboth the above aspects: they are covalently attachedand multilayer. The advantage of a multilayered (orhigh loaded) ionic liquid-like material could arisefrom the possibility that such materials could be capa-ble of stabilizing a higher amount of catalyticallyactive Pd species. Indeed, it has been possible to im-mobilized up to 10 wt% of palladium on such materi-als.

In our previous work[23] we used a bis-vinylimidazo-lium salt having a linear C4 linker (5d, Scheme 1). Inthe present work we have investigated in depth im-portant features of these materials such as (i) the

nature and the length of the linker of the bis-vinylimi-dazolium salt, (ii) the solvents employed in theSuzuki–Miyaura reaction, (iii) scope and limitationsby using a large set of biaryl synthesis and (iv) recov-erability and reusability of catalysts. In this context,the investigation has proceeded towards the definitionof a flow apparatus able to optimize the use of thecatalytic materials prepared.

Several studies have been focused on the develop-ment of flow methods to perform this reaction usingheterogeneous catalysts. A broad range of solid sup-ports has been developed such as PdEnCatTM,[24] mon-olithic supports,[25] and polymer beads.[26] In somecases, gradual leaching of the catalytically active pal-ladium,[27] has been observed.

The goal of this research is the definition of a practi-cal and effective protocol to exploit the features ofthe solid catalytic system and to reach the highestlevel of sustainability. As a trivial and effective mea-sure to evaluate and compare our results, we haveused the environmental factor (E-factor) defined askg of waste produced/kg of isolated product.[28] Theuse of flow chemistry can facilitate the definition ofefficient processes featuring the reduction of the envi-ronmental cost and may offer several advantages ascompared to traditional batch reactors.[29,30]

Scheme 1. Synthesis of Pd-supported catalysts 5a–d and 6. The wavy lines in the scheme symbolize further connections,either by cross-linking to other bis-imidazolium units or by anchoring to the support.

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In accordance with our previous reports in thisfield,[31] the flow reactors and protocols have been de-fined in order to optimize the recovery and reuse ofthe catalyst, minimize waste and in particular theamount of organic solvent needed to isolate the finalproducts. We intend to fully exploit the features ofa specifically designed heterogeneous catalytic systemwith that of the flow techniques in order define a gen-erally useful and practical procedure to recover andreuse the catalytic system and the desired productwith the minimal cost in terms of time and waste dis-posal. This approach perfectly fits the current needfor the definition of more environmentally-friendlyprocedures for the transformations catalyzed by pre-cious metals, such as Pd-catalyzed Suzuki–Miyauracouplings, to maintain access to the key drugs andother products in a sustainable manner.[32]

Finally, all these materials have been characterizedby several techniques (13C magic angle spinning nucle-ar magnetic resonance, Brunauer–Emmett–Tellertechnique, X-ray photoelectron spectroscopy, trans-mission electron microscopy).

Results and Discussion

The synthesis of the catalytic materials was accom-plished in two steps (Scheme 1). The reaction be-tween the mercaptopropyl-modified silica gel SiO2-1,(SH loading 1.2 mmol g�1) and excess of bis-vinylimi-dazolium salts 2 (ca. 2.8 equiv, per SH group), in thepresence of azobisisobutyronitrile (AIBN) in reflux-ing ethanol, gave materials 3. This reaction offers allthe desirable features of a click reaction, being highlyefficient, simple to execute with no side products andproceeding relatively rapidly to high yield. Since thebis-vinylimidazolium salt was added in excess relativeto the amount of thiol groups the formation of imida-zolium cross-linked networks through self-addition re-action of the double bonds was expected.

In order to investigate the role of the silica support,material 4 was also prepared (Scheme 1). The lattermaterial was obtained after polymerization of salt 2ain the absence of SiO2-1.

The loadings of bis-imidazolium salts were in therange 1.20–1.40 mmolg�1 for materials 3a–d that cor-responds to 2.40–2.80 mmolg�1 of imidazolium units.The morphological properties of materials supports3a–d and 4 in terms of specific surface area (Brun-ACHTUNGTRENNUNGauer–Emmett–Teller, BET) were determined by N2

adsorption/desorption measurements (Table 1). Thesurface areas of these materials significantly de-creased after grafting the bis-imidazolium salt from750 m2 g�1 to about 120–150 m2 g�1. This dramatic de-crease in surface area and pore volume is ascribed tothe formation of a polymeric cross-linked networkand confirms the successful grafting of the bis-vinyli-

midazolium salts on the support. On the other hand,the absence of the silica support furnished a material(4) having a much lower surface area.

The materials 3 and 4 were used as supports for thePd nanoparticles. The formation of the supported pal-ladium nanoparticles was carried out following a two-step procedure: immobilization of PdCl4

2� on the net-work and its reduction. Modified silica gels 3a–c andmaterial 4 were stirred with an aqueous solution ofsodium tetrachloropalladate (Na2PdCl4). The materi-als were recovered by filtration and no palladium wasfound in the filtrate (ICP-OES measurements). Then,the supported PdCl4

2� species was reduced withNaBH4 in ethanol.[25d,33] Following this procedure, cat-alysts having 10 wt% of Pd loadings were prepared.

Data obtained in this work were compared to thosepreviously obtained using catalyst 5d.[23] Catalysts 5a–c and 6 were characterized by transition electron mi-croscopy (TEM) and X-ray photoelectron spectrosco-py (XPS). TEM analysis performed on the differentcatalysts shows that, in all cases, Pd nanoparticleswere obtained with a narrow particle size distribution(2–4 nm) and uniform dispersion on the polymericmatrix (Figure 1). XPS measurements were also car-ried out on recycled catalysts and are discussed laterin the text.

In order to study the catalytic behaviour of the cat-alysts, the Suzuki coupling between phenylboronicacid and three aryl bromides was selected as a modelreaction. Since it is known that an important parame-ter is represented by the reaction medium, the firststep was a short solvent study. Three reaction mediawere examined, namely EtOH/H2O 1:1, anhydrousethanol and water (Table 2).

In the case of the more reactive 4-bromobenzalde-hyde, no difference was found by changing the reac-tion medium. Indeed, yields were always excellent(entries 1–3). With 4-bromonitrobenzene and the lessreactive 4-bromotoluene, conversions decreased inthe order EtOH/H2O 1:1>ethanol>water (entries 4–9). In particular, no reaction took place when waterwas used with 4-bromotoluene. The best reaction

Table 1. BET specific surface area and cumulative porevolume of supports.[a]

Entry Support BET surface area(m2 g�1)

Cumulative porevolume (cm3 g�1)

1 SiO2-1 750 0.682 3a 129 0.233 3b 123 0.274 3c 144 0.245b) 3d 146 0.236 4 19 0.23

[a] BET: range p/p0: 0.2–0.98.[b] Data from ref.[23]

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Palladium Supported on Cross-Linked Imidazolium Network on Silica as Highly Sustainable Catalysts


medium was EtOH/H2O 1:1 probably because itbetter dissolves both organic reagents and inorganicbase. When aqueous ethanol (96%) was used as sol-vent, the yield increased from 78 to 90% (entry 10)underlining, once more, the importance of water evenif present in low amount. Yield increased up to 99%working in a more concentrated solution (entries 11and 12). Lower amounts of solvent did not allowa good stirring, with consequent decrease of the yield(entry 13). For further investigation we decided to usethe ethanol/water mixture.

With the aim to understand the role of the silicasupport, first we compared the catalytic activity ofmaterials 5a and 6. For each catalyst, two aryl bro-mides were employed (Table 3). The data collected

showed the higher catalytic activity of the silica sup-ported catalyst 5a. The lower catalytic activity of thepolymeric materials can be ascribed to the very lowsurface area of this catalyst compared with the analo-gous silica supported materials. The formation ofa more compact network in the case of 6 could limitthe reactant diffusion, then decreasing the accessibili-ty to the inner active sites of the catalyst and, subse-quently, limiting the conversion.

Having proved the importance of the support weexamined the nature of the linker, aliphatic (ethyl) oraromatic (para-xylyl) by comparing the catalytic activ-ity of materials 5a and b (Table 4). In some cases,a comparison was done with catalyst 5c having

Figure 1. Transmission electron microscopy images of the catalyst 5b recorded in bright (a, c, d) and dark field (b). Scalebar: 50 nm.




a meta-xylyl linker and with data obtained using theknown catalyst 5d.

Catalysts 5 were used in the Suzuki–Miyaura reac-tion between phenylboronic acid or 4-formylphenyl-boronic acid and a set of aryl bromides (or iodides) inethanol/water at 50 8C in the presence of K2CO3 asbase (Table 4). All the reactions were run for 19 h.Catalysts 5 were used in 0.1 mol% loading. Data ob-

tained showed an almost comparable activity of thecatalysts in the reaction conditions adopted. Conver-sions were high with yields up to 99%. Yields weresimilar to those obtained with the known catalyst 5dexcept in the case of 2-bromotoluene (entries 22–24).In some cases, bromo derivatives gave low yields; theuse of the corresponding iodides afforded biaryls inhigher yields (entries 10–13). Catalysts 5a and 5b gavesimilar results, although in selected cases 5b affordedbiaryls in higher yields. Catalysts 5b and 5c gave simi-lar results (entries 2, 3 and 13, 14).

The next step was the recycling study of the catalyt-ic materials 5a–c. To have a better understanding ofthe catalytic behavior of the catalysts 5a–c, X-rayphotoelectron spectroscopy (XPS) analysis were per-formed on the as-synthesized samples as well as onthe recovered catalyst after consecutive catalyticcycles (Table 5).

Previously we showed, by XPS analysis, an in situreduction after recycling, as evidenced by the spectraof used samples of catalyst 5d, in which the Pd(0)component increased from 32% to 52%.[23] In orderto make a comparison with recycled catalyst 5d, alsocatalysts 5a–c were used in consecutive reactioncycles and the used catalysts were investigated byXPS.

Recycling investigations were carried out using cat-alysts in 0.1 mol% in batch conditions or in a flow re-actor. The high Pd loading of catalysts 5a–c represents,in our opinion, a clear advantage because onlya small amount of heterogeneous catalytic material isemployed (1 mg of catalyst/mmol of substrate). Recy-cling under batch conditions was carried out as previ-ously reported,[23] i.e., using the reaction between phe-nylboronic acid and 4-bromobenzaldehyde at roomtemperature. Because of the very small amount ofcatalyst used, we carried out such reactions ona larger scale. In our study, 10 mg of catalysts wereused for the reaction between 1.85 g (10 mmol) of 4-bromobenzaldehyde and 1.34 g (11 mmol) of phenyl-boronic acid. Catalysts were recovered by centrifuga-tion and were reused in the same reaction in threecycles. Under the above conditions, catalyst 5a afford-ed biphenyl-4-carboxaldehyde in 97–64% yield inthree cycles, whereas catalysts 5b and 5c afforded bi-phenyl-4-carbaldehyde in 92–85% and 97–82% yields,respectively. However, recovery of only 10 mg of cata-lyst may be not quantitative. Such a drawback isavoided by working under flow conditions.

In addition, catalysts 5a and 5b employed underflow conditions were analyzed by XPS (Table 5). ThePd 3d region of the catalyst 5b, before and after use,is shown in Figure 2. The two bands at 335.0 and340.0 eV were assigned to Pd(0) (3d5/2 and 3d3/2, re-spectively), while the bands at 337.0 and 342.0 wereattributed to Pd(II). Table 5 reports the 3d5/2 bindingenergy of the catalysts under study together with the

Table 2. Solvent studies on the Suzuki–Miyaura reaction cat-alyzed by 5a.[a]

Entry R Solvent Conv. [%] Yield [%]

1 CHO EtOH/H2O 1:1 >99 982 CHO EtOH >99 983 CHO H2O >99 984 NO2 EtOH/H2O 1:1 >99 985 NO2 EtOH 78 786 NO2 H2O 35 347 CH3 EtOH/H2O 1:1 94 808 CH3 EtOH 75 759 CH3 H2O 0 –10 NO2 EtOH 96% 91 9011[b] NO2 EtOH 96% >99 9812[c] NO2 EtOH 96% >99 9813[d] NO2 EtOH 96% 87 81

[a] Reaction conditions: phenylboronic acid (1.13 mmol),aryl bromide (1 mmol), K2CO3 (1.2 mmol), 5a (0.1 mol%,1 mg), total volume 2.4 mL.

[b] Solvent 1.5 mL.[c] Solvent 1.0 mL.[d] Solvent 0.5 mL.

Table 3. Suzuki–Miyaura reactions between phenylboronicacid and 4-bromo-benzaldehyde or 4-bromo-nitrobenzenecatalyzed by 5a and 6.[a]

Entry R Catalyst Conv. [%] Yield [%]

1 NO2 5a >99 982 CHO 5a >99 983 NO2 6 53 524 CHO 6 87 81

[a] Reaction conditions: phenylboronic acid (1.13 mmol),aryl bromide (1 mmol), K2CO3 (1.2 mmol), EtOH(1.2 mL), H2O (1.2 mL), catalyst (0.1 mol%, 1 mg).




data of the previously reported 5d for comparison.The incomplete reduction of the tetrachloropalladateto Pd(0) during the synthesis of the nanoparticles canbe attributed, as already reported,[33] to the non-com-

plete accessibility of the Pd(II) sites in the polymericmatrix or to air oxidation of Pd(0).[34]

The analysis of the XPS data for the as-synthesizedmaterials show that the catalysts 5b and 5c have a sim-

Table 4. Suzuki–Miyaura reactions between phenylboronic acids and aryl halides catalyzed by 5a–d.[a]

Entry R Aryl halide Compound Catalyst Conv. [%] Yield [%]

1

H

5a 93 932 5b >99 983 5c >99 934[b] 5d 95 915

H5a 96 94

6 5b 96 91

7H

5a >99 988 5b >99 979[b] 5d 95 9510

CHO5a 27 27

11 5b 26 24

12CHO

5a 76 7613 5b >99 9714 5c >99 9815

H5a >99 98

16 5b >99 9817[b] 5d 95 9518

H5a 94 80

19 5b >99 98

20H

5a 94 9321 5b 84 83

22H

5a 89 6123 5b 88 7824[b] 5d 95 86

25

H

5a >99 8926 5b >99 8827[b] 5d 95 90

28H

5a >99 9729 5b >99 99

30CHO

5a >99 9831 5b >99 99

32CHO

5a >99 9733 5b >99 98

34

CHO

5a 67 4835 5b >99 75

36CHO

5a 78 6337 5b 96 92

[a] Reaction conditions: phenylboronic acid (1.13 mmol), aryl halide (1 mmol), K2CO3 (1.2 mmol), EtOH (1.2 mL), H2O(1.2 mL), catalyst (0.1 mol%, 1 mg).

[b] Data from ref.[23]




ilar content of Pd(0) comparable with the one report-ed for the catalyst 5d. Surprisingly the solid 5a fea-tures a lower content of Pd(0). The XPS characteriza-

tion performed on samples coming from differentpreparation (by following the same reaction proce-dure) confirms the results. The different behaviour of5a could be ascribed to the small alkyl chain used tolink the two imidazolium moieties which gave rise toa more compact structure. After use, the materialspresent a similar behaviour with an increase of thecontent of Pd(0) attributed to the in situ reductionlikely due to the solvent under basic conditions.[35] Onthe other hand, the catalytic materials display a differ-ent ability in stabilizing the palladium as Pd(0) afterreuse. Considering only catalysts 5b–d, which have analmost identical starting Pd(0) content, only catalyst5b maintained a similar level of Pd(0) after reuse. Apossible explanation could lie in the more “adapta-ble” environment possessed by materials 5c and 5dwith respect to 5b which causes a Pd(0) stabilizationby “electrosteric” factors upon reuse. Indeed, thelonger butyl linker in 5d and the two imidazoliumunits in a meta-position in 5c may be able to createa better arrangement for the electrosteric stabilizationof palladium nanoparticles.[6]

The Pd/C atomic ratios of re-used catalysts arealmost constant as compared to the fresh ones indi-cating that neither sintering of palladium particles norleaching occurred during reaction. The absence ofleaching in palladium catalysts immobilized onto gel-supported ionic liquid-like phases has been ascribedto a “release and catch” mechanism.[36] It has beenproposed that during the Suzuki coupling, palladiumis dissolved in solution (leaching), then over the com-pletion of the process palladium is redeposited ontothe support and its full and efficient recovery can beachieved if the redeposition is complete at the end ofthe reaction.[36c]

According to our project, and starting from the op-timized batch protocol we have investigated how torealize a flow procedure to optimize the efficiency ofthe isolation of the pure products and minimize thewaste associated to the process (E-factor). The flowapproach will also play a strategic role to preserve the

Figure 2. Pd 3d XPS spectra of Pd catalysts 5b fresh (A) andafter three catalytic cycles (B).

Table 5. XPS Pd 3d5/2 binding energies (eV), Pd(0) (%) and Pd/C surface atomic ratios of as prepared and used Pd catalysts.

Entry Catalyst Pd 3d5/2 [eV] Pd(0) [%] C 1s ref Pd/C

1 5a 335.3 13 385.4 0.042 5a after 3 cycles 334.9 21 385.1 0.043 5a flow reactor 335.0 18 285.0 0.044 5b 335.0 31 284.9 0.045 5b after 3 cycles 335.2 39 285.4 0.046 5b flow reactor 335.0 36 285.0 0.047 5c 335.3 32 285.5 0.048 5c after 3 cycles 334.9 56 285.1 0.049[a] 5d 334.9 32 285.1 0.0410[a] 5d after 3 cycles 335.1 52 285.2 0.04

[a] Data from ref.[23]




physical integrity of the solid catalyst during all thestages of the process and by making its recovery andreuse efficient, reproducible and simple. The schemat-ic representation of the reactor set and used is depict-ed in Figure 3 where the thermostatted chamber isnot showed for clarity.

The reactor system has been set-up with the inten-tion of exploiting the observed phenomenon relatedto the “release and catch” catalytic mechanism.[36]

Our goal is to exploit flow conditions to definea simple procedure for the recovery and reuse of thePd catalyst and keep its physical and chemical integri-ty over its use.

At this aim, the attention was initially directed to-wards the use of ethanol (aqueous azeotrope 96% wt)as reaction medium, in which K2CO3 and other saltsformed during the process are not soluble under thereaction conditions.

To realize our flow protocol we have then chargedthe base (K2CO3) and the catalyst (5a or 5b) into twoseparate low-pressure glass columns. Reactants(40 mmol of aryl bromide and 45.2 mmol of phenyl-boronic acid) in ethanol 96% were charged intoa third column used as a reservoir. The equipmentwas connected, by using the appropriate valves, toa pump and installed into a thermostatted box (notshown in Figure 3 for clarity). After setting the tem-perature (50 8C), the reaction mixture was pumpedthrough the base- and the catalyst-containing columnsat a 1.5 mL min�1 flow rate for 36 h. After this time,the air/solvent valve was opened and the pump wasset to work in order to transfer the liquid into the re-servoir and empty the other columns. To completelywash the base- and catalyst-columns some ethanol(2 �8 mL at a 1.5 mLmin�1) was cyclically pumpedfrom the air/solvent valve through the two columns

for 10 min (each fraction) and then collected into thereservoir.

This protocol was repeated for the reactions of bro-mobenzene, 4-bromotoluene and 4-bromobenzalde-hyde with phenylboronic acid in the presence of 5a or5b as catalyst to obtain the final products in very highyields (Table 6).

Although the reaction was carried out by using twoseparate columns containing the catalytic materialand the base, respectively, a very high catalytic effi-ciency was observed. This can be ascribed to a “re-lease and catch” mechanism in which the supportedpalladium catalyst serves as a reservoir for active Pdspecies[36,37] that are dissolved from the solid catalyst.The active Pd species goes through the apparatus,then it is re-captured. As a proof of this mechanism,ICP-OES analysis of the final product showed thepresence of only ca. 0.015 wt% of palladium (referredto the initial Pd content in the catalyst).

This protocol furnishes a very good proof that the“release and catch” mechanism is operative and italso offers a unique opportunity for the minimizationof waste associated to the reaction. In fact, chargingK2CO3 in a separate column makes the recovery ofPd catalyst very simple and efficient since the catalystis safely charged into the glass column and can be di-rectly reused for at least 4 consecutive runs showingan unchanged efficiency (Table 7). In addition, thefinal product is isolated pure at the end of the reac-tion without requiring any further purification.

Beside the scope of this work, the reuse of the cata-lyst was repeated for just the representative 4 runs,but still a TON of ca. 3800 has been achieved, andfurther work may clearly lead to higher values.

Figure 3. Schematic cyclic continuous-flow reactor.

Table 6. Suzuki–Miyaura reactions between phenylboronicacid and aryl halides catalyzed by 5a and 5b in a cyclic flowreactor.[a]

Entry R Biphenyl Catalyst Yield [%]

1H

5a 952 5b 96

3CH3

5a 954 5b 96

5[b]

CHO5a 96

6[b] 5b 98

[a] Reaction conditions: phenylboronic acid (45.2 mmol),aryl bromide (40 mmol), K2CO3 (48 mmol), EtOH(121.2 mL), catalyst (0.1 mol%, 42.4 mg).

[b] [EtOH]=0.25 M (160 mL).




It is interesting to note that, although 5a displayedhalf the content of Pd(0) with respect to 5b in theXPS analysis (Table 5), this did not affect the catalyticactivity under flow conditions since both materialsmaintained comparable levels of yields.

Using the flow approach, the sustainability of theprotocol has been greatly increased as proved by thereduction of the waste produced compared to batchconditions.

With the adoption of flow approach the E-factorvalues obtained are very low. In fact they are in therange 3.5–3.9 if recovery of the solvent is consideredor in the range of 18.7–21.6 if solvent recovery is notincluded in the calculations. The equivalent batch pro-tocols feature E-factor values of 3180–5100 which areaffected by the needed purification step and by theuse of a mix of solvents that cannot be easily recov-ered.

Conclusions

In summary, highly cross-linked imidazolium-basedmaterials, obtained by radical polymerization of bis-vinylimidazolium salts in the presence of 3-mercapto-propyl-modified silica gel, were prepared. Differentlinkers (ethyl, p-xylyl and m-xylyl) were used to con-nect the imidazolium units and results were comparedwith a previous material having a butyl linker. In ad-dition, the highly cross-linked imidazolium-based ma-terial 6 was obtained by polymerization of the bis-vi-nylimidazolium salt without the mercaptopropyl-modified silica gel. The synthetic approach allowed

the realization of high loaded imidazolium salt frame-works which, in turn, permitted the immobilization ofpalladium in high loading (10 wt%) after exchange ofbromide ions with tetrachloropalladate ions and re-duction with sodium borohydride. Materials werecharacterized by several techniques. Preliminary testscarried out on the catalytic materials 5a and 6 havingthe same linker (ethyl) showed a lower activity forthe latter catalyst, probably due to the much lowerspecific surface area. Solvent screening indicated, asthe best reaction medium, a 1:1 ethanol/water mixtureor aqueous ethanol (96%). Catalysts 5a–c were em-ployed in only 0.1 mol% loading in ethanol/water inthe presence of K2CO3 as base under batch conditionfor the synthesis of a large set of biaryl compounds.Yields were usually high (75–99%).

XPS analysis showed that material 5a, havinga short alkyl linker, stabilizes a lower amount ofPd(0), probably due to its more compact structure.Materials 5b–d, having longer linkers, stabilizeda higher amount of Pd(0). XPS analysis of the reusedcatalysts showed a large increase of Pd(0) contentonly in the case of catalysts 5c and 5d whereas cata-lysts 5a and 5b showed a modest increase in Pd(0)content upon reuse whichever reaction conditionswere used. The more adaptable butyl linker in 5d andthe two imidazolium units in a meta-position in 5cmay be able to create a better arrangement for theelectrosteric stabilization of palladium nanoparticles.

Thanks to the high palladium loading, only 1 mg ofcatalyst per mmol of substrate was employed. Thisaspect represents a clear advantage if compared tocatalysts that have a much lower Pd content (0.1–1 wt%) or that are used in higher amount (up to5 mol%). Recycling of catalysts was briefly investigat-ed under batch conditions in order to investigate howthe nature of palladium species changes after reuseand deeply investigated under flow conditions. To thebest of our knowledge, in this work we have reportedfor the first time a flow apparatus in which the cata-lyst and the solid base (K2CO3) were filled into twoseparate columns. Catalysts 5a and 5b afforded threedifferent biaryls in excellent yields working at0.1 mol% in ethanol. Recycling experiments with 4-bromotoluene gave the corresponding biaryl in excel-lent yields after 4 cycles achieving a TON of 3800. Byusing only 42 mg of catalyst up to 27 g of 4-bromo-ACHTUNGTRENNUNGtoluene were converted into the final product.

Waste production has been significantly reduced byadopting the flow conditions, in fact the E-factor hasbeen reduced by more than 99% without consideringsolvent recovery The results obtained prove the effi-ciency of this approach for realizing highly efficientprocesses featuring a minimal waste production.

Table 7. Reusability of catalysts 5a and 5b in cyclic flowSuzuki–Miyaura reaction.[a]

Entry Cycle Catalyst Yield [%]

1 1 5a 952 2 5a 953 3 5a 944 4 5a 925 1 5b 966 2 5b 967 3 5b 948 4 5b 95

[a] Reaction conditions: phenylboronic acid (45.2 mmol),aryl bromide (40 mmol), K2CO3 (48 mmol), EtOH(0.33 M, 121.2 mL), catalyst (0.1 mol%, 42.4 mg).




Experimental Section

General Methods

The NMR spectra were recorded on a Bruker 300 MHzspectrometer. Solid-state 13C{H} CP-MAS NMR and 29SiNMR spectra was recorded on a Agilent, 400 MHz spec-trometer with samples packed in zirconia rotors spinning at15 kHz. Carbon and nitrogen contents were determined bycombustion analysis in a Fisons EA 1108 elemental ana-lyzer. The specific surface areas were determined by apply-ing the BET method to the nitrogen adsorption isothermregistered at �196 8C using a micromeretics Tristar 3000 in-strument. The cumulative pore volume was calculated bythe BJH method[38] applied to the desorption curve in therange p/p0: 0.2–0.98. A Perkin–Elmer Optima 2100 seriesICP optical emission spectrometer was used for palladiumleaching tests. The X-ray photoelectron spectroscopy (XPS)analyses were performed with a Thermo Scientific K-Alphaspectrometer, equipped with a monochromatized Al anode(1486-6 eV): X-ray source: 12 kV, 1.8 mA; X-ray spot size:200 mm. A flood gun (electrons and Ar ions at very lowenergy) was used to avoid possible charging effects. The an-alyzer was operated at constant pass energy (CAE) toensure a constant energy resolution over the whole spec-trum. The analyzer wasoperated at 200 eV pass energy forsurvey spectra, and at 50 eV for high resolution individualspectra. Pressure in the chamber was in the range 10�8 mbar.Analyses of the peaks were performed with the softwareAvantage, based on non-linear least squares fitting programusing a weighted sum of Lorentzian and Gaussian compo-nent curves after background subtraction according to Shir-ley and Sherwood.[39,40] Atomic concentrations were calculat-ed from peak intensity using the sensitivity factors providedwith the software. TEM images were obtained using a FEITecna� 10 system with an operating voltage of 80 kV. Sam-ples were dispersed in isopropyl alcohol and deposited ona carbon coated copper grid.

Procedure for the Synthesis of Bis-vinylimidazoliumsalts 2a–c

A solution of dihalide compound (0.01 mol) and 1-vinylimi-dazole (0.021 mol) in toluene (10 mL, in the case of 1,2-di-bromoethane) and in chloroform (10 mL, in the case of ben-zylic compounds) was heated for 24 h in an oil bath at 90 8Cand 50 8C (respectively for the toluene and chloroform solu-tions) with magnetic stirring. After cooling at room temper-ature, the mixture was filtered and washed several timeswith diethyl ether. The solid product was dissolved in meth-anol and stirred overnight in the presence of activatedcarbon then filtered and dried at 40 8C under reduced pres-sure. 1H-Imidazolium, 1,1’-[1,4-phenylenebis(methylene)]-bis-(3-ethenyl)-dibromide (2b) has been already report-ed.[41,42]

1H-Imidazolium, 1,1’-(1,2-ethanediyl)bis-(3-ethenyl)-di-bromide (2a): White powder; yield: 75%; mp >300 8C;1H NMR (300 MHz, CD3OD): d=4.96 (4 H, m, CH2CH2),5.49 (2 H, dd, J= 10.4 and 3.0 Hz, cis-CH=CH2), 5.98 (2 H,dd, J=18.9 and 3.6 Hz, trans-CH=CH2), 7.31 (2 H, dd, J=10.4 and 18.8 Hz, CH=CH2), 7.85 (2 H, d, J= 2.2 Hz, 4-H or5-H), 8.09 (2 H, d, J=2.3 Hz, 4-H or 5-H), 9.54 (2 H, s, 2-H);

13C NMR (CD3OD): d=50.0, 110.8, 121.4, 124.6, 129.8,130.9, 137.4.; anal. found: C 38.3, H 4.3, Br 42.5; N 14.9%;calcd. for C12H16Br2N4: C 38.3, H 4.4, Br 42.5, N 14.9%.

1H-Imidazolium, 1,1’-[1,3-phenylenebis(methylene)]bis-(3-ethenyl)-dibromide (2c): Pale yellow oil; yield: 75%;1H NMR (300 MHz, CD3OD): d= 5.46 (2 H, dd, J=8.7 and2.7 Hz, cis-CH=CH2), 5.54 (4 H, s, NCH2Ph), 5.96 (2 H, dd,J=15.3 and 2.7 Hz, trans-CH=CH2), 7.29 (2 H, dd, J= 15.3and 8.7 Hz, CH=CH2), 7.55 (3H, m, ArH), 7.74 (1 H, m,ArH), 7.79 (2H, d, J=2.1 Hz, 4-H or 5-H), 8.05 (2 H, d, J=2.1 Hz, 4-H or 5-H), proton 2-H has exchanged with solvent;13C NMR (CD3OD): d=54.5, 111.1. 121.7, 125.1, 130.4.131.3, 131.6, 132.1, 136.7; anal. found: C 47.6, H 4.3, Br 35.6,N 12.5%; calcd. for C18H20Br2N4: C 47.8, H 4.4, Br 35.3, N12.4].

Procedure for the Synthesis of Materials 3

In a round-bottom flask were placed the 3-mercaptopropyl-modified silica gel SiO2-1 (500 mg, SH loading 1.2 mmol g�1,0.6 mmol), the bis-vinylimidazolium salt 2 (2.77 equiv.,1.66 mmol, 673 mg), ethanol (10.8 mL) and AIBN (16.3 mg).The suspension was degassed by bubbling argon for 15 min.The mixture was heated at 78 8C under argon, with stirring.After 20 h, the reaction mixture was cooled at room temper-ature, filtered under reduced pressure and washed with hotmethanol, then with diethyl ether. The obtained materialwas dried in an oven at 40 8C overnight.

Typical Procedure for the Synthesis of the PalladiumCatalysts 5

In a round-bottom flask were placed PdCl2 (95 mg,0.536 mmol), NaCl (626 mg, 10.72 mmol, 20 equiv.) andwater (13.6 mL). The flask was heated at 80 8C until thePdCl2 was dissolved. This clear reddish solution was cooledat room temperature and added to a suspension of material3 (500 mg) in water (4.5 mL). The suspension was stirred atroom temperature for 20 h, then filtered under reducedpressure, washed with water and dried overnight under re-duced pressure at room temperature. The material was sus-pended in ethanol (12 mL) and to this suspension a solutionof NaBH4 (142 mg, 3.75 mmol, 7 equiv.) in ethanol (12 mL)was added dropwise. The suspension turned black and wasstirred at room temperature for 6 h, then filtered under re-duced pressure, washed with water and dried overnightunder reduced pressure at room temperature.

Typical Procedure for the Suzuki–Miyaura Reaction

In a round-bottom flask catalyst 5 (0.1 mol%, 1 mg), phenyl-boronic acid (1.13 mmol), K2CO3 (166 mg, 1.20 mmol), thearyl halide (1 mmol), ethanol (1.2 mL) and water (1.2 mL)were placed. The reaction mixture was stirred at 50 8C.After 19 h, water was added and the mixture extracted withdichloromethane (�3). The organic phase was concentratedunder reduced pressure and the residue purified by columnchromatography (petroleum ether/diethyl ether).




Typical Recycling Procedure for the Suzuki–MiyauraReaction under Batch Conditions

In a centrifuge tube catalyst 5 (10 mg, 0.1 mol%), phenyl-boronic acid (1.34 g, 11 mmol), K2CO3 (1.67 g, 1.2 equiv.), 4-bromobenzaldehyde (1.85 g, 10 mmol), ethanol (12 mL) andwater (12 mL) were placed. The reaction was carried out atroom temperature for 19 h. After this time, diethyl etherwas added and the supernatant ethereal layer removed. Thisprocedure was repeated several times. Water was added todissolve K2CO3 and the mixture centrifuged. The aqueouslayer was removed and the procedure repeated three times.The diethyl ether phase was dried, then concentrated underreduced pressure and the residue purified by column chro-matography (petroleum ether/diethyl ether 12:1 and 9:1).Catalyst 5 was dried in an oven at 40 8C for 2 h and reused.

Typical Procedure for the Suzuki–Miyaura Reactionin Ethanol in the Flow Reactor

A pre-mixed mixture of 4-bromotoluene (40 mmol, 6.84 g),phenylboronic acid (45.2 mmol, 5.52 g) and ethanol 96%(0.33 M, 121.2 mL) was charged into a glass column func-tioning as a reservoir. Potassium carbonate (48.4 mmol,6.68 g) dispersed in 1 mm diameter solid glass beads, wascharged in a glass column; the catalyst 5b (0.1 mol%,0.04 mmol, 0.0424 g) was also charged in another glasscolumn and dispersed in 1 mm diameter solid glass beads;the equipment was installed into a thermostatted box andconnected, by using the appropriate valves, to a pump. Thereaction mixture was continuously pumped (flow rate1.5 mL min�1) through the base and catalyst columns at50 8C for 36 h to reach the complete conversion of reactantto 4-methyl-1,1’-biphenyl. At this stage, the pump was left torun in order to recover the reaction mixture into the reser-voir. To completely recover the product and clean the reac-tor, ethanol (2� 8 mL at a 1.5 mL min�1) was cyclicallypumped through the base and the catalyst columns for10 min (each fraction) and then collected into the reservoirto isolate the product in 96% yield (>99 % purity) after re-moval and recovery of the solvent (90% of solvent recov-ered) under reduced pressure.

Representative E-Factor Calculations

Continuous-flow reactor with solvent recovery: E-factor forSuzuki–Miyaura reaction of 4-bromotoluene (40 mmol) andphenylboronic acid in ethanol in the presence of 5b as cata-lyst (96% yield): E-factor = [6.84 g (4-bromotoluene)+5.52 g (phenylboronic acid)+ 6.68 g (K2CO3)+ 108.2 g(EtOH)�6.46 g (4-methyl-1,1’-biphenyl)�97.81 g (recoveredEtOH)]/6.46 g (4-methyl-1,1’-biphenyl)=3.55.

Continuous-flow reactor without solvent recovery: E-factor for Suzuki–Miyaura reaction of 4-bromotoluene(40 mmol) and phenylboronic acid in ethanol in the pres-ence of 5b as catalyst (96% yield): E-factor = [6.84 g (4-bro-motoluene)+5.52 g (phenylboronic acid) +6.68 g (K2CO3)+108.2 g (EtOH) �6.46 g (4-methyl-1,1’-biphenyl)]/6.46 g (4-methyl-1,1’-biphenyl)= 18.7.

Batch reaction: E-factor for Suzuki–Miyaura reaction of4-bromotoluene (1 mmol) and phenylboronic acid in thepresence of 5b as catalyst in batch (Table 4, entry 19), (98%yield): E-factor= [0.171 g (4-bromotoluene)+ 0.138 g (phe-

nylboronic acid)+ 0.167 g (K2CO3)+ 0.001 g (catalyst)+0.947 g (EtOH) +101.2 g (water) +332.5 (CH2Cl2)+ 118.17 g(petroleum ether)+ 12.92 g (diethyl ether)�0.165 g (4-methyl-1,1’-biphenyl)]/0.165 g (4-methyl-1,1’-biphenyl)=3431 (silica gel for column chromatography purification hasnot been considered).

Acknowledgements

We gratefully acknowledge the Ministero dell’Istruzione, del-l’Universit� e della Ricerca (MIUR) within the projects PRINand “Firb-Futuro in Ricerca”, the Universit� degli Studi diPalermo and the Universit� degli Studi di Perugia for finan-cial support. The authors thank P. Louette for support in theXPS analysis and C. Charlier for assistance in the TEM mea-surement.

References

[1] a) Handbook of Organopalladium Chemistry for Or-ganic Synthesis, (Eds.: E. Negishi, A. de Meijere),Wiley, New York, 2002 ; b) Metal-Catalyzed Cross-Cou-pling Reactions, 2nd edn., (Eds.: A. de Meijere, F. Die-derich), Wiley-VCH, Weinheim, 2004 ; c) K. C. Nico-laou, P. G. Bulger, D. Sarlah, Angew. Chem. 2005, 117,4516–4563; Angew. Chem. Int. Ed. 2005, 44, 4442–4489;d) A. Zapf, M. Beller, Chem. Commun. 2005, 431–440;e) N. T. S. Phan, M. Van Der Sluys, C. W. Jones, Adv.Synth. Catal. 2006, 348, 609–679; f) C. C. C. JohanssonSeechurn, M. O. Kitching, T. J. Colacot, V. Snieckus,Angew. Chem. 2012, 124, 5150–5174; Angew. Chem.Int. Ed. 2012, 51, 5062–5085.

[2] A. Fihri, M. Bouhrara, B. Nekoueishahraki, J.-M.Basset, V. Polshettiwar, Chem. Soc. Rev. 2011, 40,5181–5203.

[3] V. Polshettiwar, C. Len, A. Fihri, Coord. Chem. Rev.2009, 253, 259–2626.

[4] R. Andr�s, E. de Jesffls, J. C. Flores, New J. Chem. 2007,31, 1161–1191.

[5] M. Lamblin, L. Nassar-Hardy, J.-C. Hierso, E. Fouquet,F.-X. Felpin, Adv. Synth. Catal. 2010, 352, 33–79.

[6] D. Astruc, Inorg. Chem. 2007, 46, 1884–1894.[7] M. Weck, C. W. Jones, Inorg. Chem. 2007, 46, 1865–

1875.[8] D. A. Pears, S. C. Smith, Aldrichimica Acta 2005, 38,

23–33.[9] Y. M. A. Yamada, S. M. Sarkar, Y. Uozumi, J. Am.

Chem. Soc. 2012, 134, 3190–3198.[10] V. I. P�rvulescu, C. Hardacre, Chem. Rev. 2007, 107,

2615–2665.[11] F. Bellina, C. Chiappe, Molecules 2010, 15, 2211–2245.[12] Y. Liu, S.-S. Wang, W. Liu, O.-X. Wan, H.-H. Wu, G. H.

Gao, Curr. Org. Chem. 2009, 13, 1322–1346.[13] B. Karimi, D. Elhamifar, J. H. Clark, A. J. Hunt, Chem.

Eur. J. 2010, 16, 8047–8053.[14] M.-J. Jin, A. Taher, H.-J. Kang, M. Choi, R. Ryoo,

Green Chem. 2009, 11, 309–313.[15] J. Y. Shin, B. S. Lee, Y. Jung, S. J. Kim, S.-g. Lee, Chem.

Commun. 2007, 5238–5240.




[16] H. Hagiwara, H. Sasaki, N. Tsubokawa, T. Hoshi, T.Suzuki, T. Tsuda, S. Kuwabata, Synlett 2010, 1990–1996.

[17] H. Hagiwara, K. H. Ko, T. Hoshi, T. Suzuki, Chem.Commun. 2007, 2838–2840.

[18] T. Kang, Q. Feng, M. Luo, Synlett 2005, 2305–2308.[19] J.-W. Kim, J.-H. Kim, D.-H. Lee, Y.-S. Lee, Tetrahedron

Lett. 2006, 47, 4745–4748.[20] M. J. Park, S.-g. Lee, Bull. Korean Chem. Soc. 2007, 28,

1925–1926.[21] A. Taher, J.-B. Kim, J.-Y. Jung, W.-S. Ahn, M.-J. Jin,

Synlett 2009, 2477–2482.[22] Y. Zhang, L. Zhao, P. K. Patra, D. Hu, J. Y. Ying, Nano

Today 2009, 4, 13–20.[23] M. Gruttadauria, L. F. Liotta, A. M. P. Salvo, F. Giac-

alone, V. La Parola, C. Aprile, R. Noto, Adv. Synth.Catal. 2011, 353, 2119–2130.

[24] a) C. K. Y. Lee, A. B. Holmes, S. V. Ley, I. F. McCon-vey, B. Al-Duri, G. A. Leeke, R. C. D. Santos, J. P. K.Seville, Chem. Commun. 2005, 2175–2177; b) G. A.Leeke, R. C. D. Santos, B. Al-Duri, J. P. K. Seville, C. J.Smith, C. K. Y. Lee, A. B. Holmes, I. F. McConvey,Org. Process Res. Dev. 2007, 11, 144–148; c) I. R. Bax-endale, C. M. Griffiths-Jones, S. V. Ley, G. K. Tranmer,Chem. Eur. J. 2006, 12, 4407–4416.

[25] a) K. Mennecke, A. Kirschning, Synthesis 2008, 3267–3272; b) A. Gçmann, J. A. Deverell, K. F. Munting,R. C. Jones, T. Rodemann, A. J. Canty, J. A. Smith,R. M. Guijt, Tetrahedron 2009, 65, 1450–1454; c) R. C.Jones, A. J. Canty, J. A. Deverell, M. G. Gardiner,R. M. Guijt, T. Rodemann, J. A. Smith, V.-A. Tolhurst,Tetrahedron 2009, 65, 7474–7481; d) K. Mennecke, A.Kirschning, Beilstein J. Org. Chem. 2009, 5, DOI:10.3762/bjoc.5.21.

[26] N. T. S. Phan, J. Khan, P. Styring, Tetrahedron 2005, 61,12065–12073.

[27] a) S. J. Broadwater, D. T. McQuade, J. Org. Chem.2006, 71, 2131–2134; b) K. F. Bolton, A. J. Canty, J. A.Deverell, R. M. Guijt, E. F. Hilder, T. Rodemann, J. A.Smith, Tetrahedron Lett. 2006, 47, 9321–9324.

[28] a) R. A. Sheldon, Chem. Ind. (London, U.K.) 1997, 12–15; b) R. A. Sheldon, Green Chem. 2007, 9, 1273–1283;c) J. Aug�, Green Chem. 2008, 10, 225–231; d) R. A.Sheldon, Chem. Commun. 2008, 3352–3365.

[29] For a review about cross-coupling in continuous flow,see: T. Nol, S. L. Buchwald, Chem. Soc. Rev. 2011, 40,5010–5029.

[30] For some selected reviews about flow chemistry:a) R. L. Hartman, J. P. McMullen, K. F. Jensen, Angew.Chem. 2011, 123, 7642–7661; Angew. Chem. Int. Ed.2011, 50, 7502–7519; b) D. Webb, T. F. Jamison, Chem.

Sci. 2010, 1, 675–680; c) C. G. Frost, L. Mutton, GreenChem. 2010, 12, 1687–1703; d) K. Geyer, T. Gustafsson,P. H. Seeberger, Synlett 2009, 2382–2391; e) J.-I. Yoshi-da, A. Nagaki, T. Yamada, Chem. Eur. J. 2008, 14,7450–7459; f) B. P. Mason, K. E. Price, J. L. Steinbach-er, A. R. Bogdan, D. T. McQuade, Chem. Rev. 2007,107, 2300–2318.

[31] a) G. Strappaveccia, D. Lanari, D. Gelman, F. Pizzo, O.Rosati, M. Curini, L. Vaccaro, Green Chem. 2013, 15,199–204; b) T. Angelini, D. Lanari, R. Maggi, F. Pizzo,G. Sartori, L. Vaccaro, Adv. Synth. Catal. 2012, 354,908–916; c) S. Bonollo, D. Lanari, J. M. Longo, L. Vac-caro, Green Chem. 2012, 14, 164–169; d) S. Bonollo, D.Lanari, T. Angelini, F. Pizzo, L. Vaccaro, J. Catal. 2012,285, 216–222; e) A. Zvagulis, S. Bonollo, D. Lanari, F.Pizzo, L. Vaccaro, Adv. Synth. Catal. 2010, 352, 2489–2496; f) F. Fringuelli, D. Lanari, F. Pizzo, L. Vaccaro,Green Chem. 2010, 12, 1301–1305.

[32] EuCheMS,Roadmap for the Chemical Sciences titledChemistry – developing solutions in a changing world,2011, www.euchems.eu.

[33] S. Kidambi, M. L. Bruening, Chem. Mater. 2005, 17,301–307.

[34] R. W. J. Scott, H. Ye, R. R. Henriquez, R. M. Crooks,Chem. Mater. 2003, 15, 3873–3878.

[35] A. R. Siamaki, A. El Rahman S. Khder, V. Abdel-sayed, M. S. El-Shall, B. F. Gupton, J. Catal. 2011, 279,1–11.

[36] a) Z. Niu, Q. Peng, Z. Zhuang, W. He, Y. Li, Chem.Eur. J. 2012, 18, 9813–9817; b) M. I. Burguete, E.Garca-Verdugo, I. Garcia-Villar, F. Gelat, P. Licence,S. V. Luis, V. Sans, J. Catal. 2010, 269, 150–160; c) K.Kçhler, R. G. Heidenreich, S. S. Soomro, S. S. Prçckl,Adv. Synth. Catal. 2008, 350, 2930–2936; d) F. Zhao, M.Shirai, Y. Ikushima, M. Arai, J. Mol. Catal. A: Chem.2002, 180, 211–219.

[37] J. Hu, Y. Liu Langmuir 2005, 21, 2121–2123; for recentevidence, see: ref.[36a]

[38] S. Brunauer, P. H. Emmett, E. Teller, J. Am. Chem.Soc. 1938, 60, 309–319.

[39] D. A. Shirley, Phys. Rev. B 1972, 5, 4709–4714.[40] P. M. A. Sherwood, in: Practical Surface Analysis,

(Eds.: D. Briggs, M. P. Seah), Wiley, New York, 1990,p 181.

[41] C. Aprile, F. Giacalone, P. Agrigento, L. F. Liotta, J. A.Martens, P. P. Pescarmona, M. Gruttadauria, Chem-ACHTUNGTRENNUNGSusChem 2011, 4, 1830–1837.

[42] P. Agrigento, M. J. Beier, J. T. N. Knijnenburg, A.Baiker, M. Gruttadauria, J. Mater. Chem. 2012, 22,20728–20735.




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