Dendrimer crown-ether tethered multi-wall carbon nanotubes support methyltrioxorhenium in the selective oxidation of ole ﬁ ns to epoxides † , 17185 –

Benzo-15-crown-5 ether supported on multi-wall carbon nanotubes (MWCNTs) by tethered poly(amidoamine) (PAMAM) dendrimers e ﬃ ciently coordinated methyltrioxorhenium in the selective oxidation of ole ﬁ ns to epoxides. Environmentally friendly hydrogen peroxide was used as a primary oxidant. Up to ﬁ rst and second generation dendrimer aggregates were prepared by applying a divergent PAMAM methodology. FT-IR, XRD and ICP-MS analyses con ﬁ rmed the e ﬀ ective coordination of methyltrioxorhenium by the benzo-15-crown-5 ether moiety after immobilization on MWCNTs. The novel catalysts converted ole ﬁ ns to the corresponding epoxides in high yield without the use of Lewis base additives, or anhydrous hydrogen peroxide, the catalyst being stable for more than six oxidative runs. In the absence of the PAMAM structure, the synthesis of diols largely prevailed.


Introduction
Multi-wall carbon nanotubes (MWCNTs) are efficient supports for the immobilization of reactive organometallic species thanks to their high surface area to volume ratio value, chemical stability, and unique redox, rheological and structural properties. 1,2 They show surface defect sites for the anchoring of spacer chains and selective recognition sites, as for example for crown ethers. 3 Different applications of MWCNTs-crown ether functionalized systems are reported, 4 including dispersive microsolid phase extraction procedures, 5 exchange reaction in quasi-solid-state processes, 6 absorption of metals and organic pollutants, 7 and ion-detection. 8 In these latter cases, MWCNT-crown ether tethered poly(amidoamine) (PAMAM) dendrimers received increasing attention. 9 These supramolecular aggregates are characterized by recognition sites on both the dendrimer core and the end-termination of the system. 10,11 The divergent methodology for the preparation of PAMAM dendrimers encompasses the graing of 1,3-diamino propane intermediates with methyl acrylate and ethylenediamine, followed by linkage of the crown ether moiety. The chemical complexity of PAMAM dendrimers (number of dendrimer sidechains and type of ramication) depends on the successive methyl acrylate/ethylenediamine treatments, and controls the distribution of the recognition sites, the leaching of the active species, [12][13][14] and the dispersibility of MWCNTs in organic solvents. 15 Crown ethers are coordination sites for methyltrioxorhenium (MTO), 16 18 The chemistry of MTO is largely reviewed, 19 including s C-H oxygen atom insertion 20 and oxy-functionalization, 21 oxidative nucleophilic substitution, 22 oxidative desulfurization (ODS), 23 synthesis of quinone derivatives, 24 Baeyer-Villiger rearrangement, 25 metathesis, 26 methanolysis of glycals, 27 and epoxidation of olens in traditional 28 and non-conventional solvents, such as chiral aliphatic amines, 24 uorinated solvent 29 and ionic liquids. 30 The formation of diols is the major drawback in the epoxidation of olens, as a consequence of the acidic property and high affinity of Re(VII) for the oxygen atom. Lewis bases such as pyridine, pyridine derivatives, 31,32 and bipyridines, 33,34 have been used as additives to inhibit this side-reaction, their efficacy depending from the stability of the corresponding MTO complex. As an alternative, anhydrous H 2 O 2 and urea-H 2 O 2 adduct (UHP) have been also applied. 35 The immobilization of MTO on solid supports can solve the problem of selectivity, stability and recyclability of the catalyst. Immobilization procedures have been reported including organic polymers bearing Lewis bases, such as chitosan, 36 poly(4-vinylpyridine), 37 poly(4-vinylpyridine-N-oxide), 38 and MTO/nitrogen ligand complexes, 39 and in alternative different inorganic supports. [40][41][42] More recently, MWCNTs wrapped on poly(4-vinylpyridine)/MTO showed high reactivity in the desulfurization of synthetic diesel fuel. 43 Lewisbase sites generally enhance the formation of epoxides, 44,45 but drawbacks relative to toxicity of the Lewis base additives (e.g. pyridine and pyridine derivatives), partial selectivity, low reactivity and leaching of rhenium have yet to be completely solved.
We report here the preparation and structural characterization of novel heterogeneous catalysts based on the immobilization of MTO on MWCNTs functionalized with benzo-15crown-5 ether (B15C5) in the presence of tethered PAMAM dendrimers. Up to rst and second generation of B15C5 PAMAM aggregates were obtained by applying the divergent methodology. The variety of catalysts was increased by the use of alkyl-diamino linkers of various length (from C-2 to C-6 atoms, respectively). FT-IR, ICP-MS and XRD analyses conrmed the coordination of MTO by the B15C5 moiety aer immobilization on MWCNTs. The novel catalysts were applied in the epoxidation of a large panel of olens using H 2 O 2 (35% water solution) as green oxidant in the absence of Lewis bases additives to yield epoxides in high yield and selectivity. As a general trend, alkyl-diamino linkers with longer carbon atom chain afforded the most reactive systems in PAMAM aggregates. Diols were obtained as the main reaction products in the absence of the dendrimer structure, highlighting the role of PAMAM in the selectivity of the reaction.

Results and discussion
Preparation of MWCNTs-B15C5/MTO catalysts IV A-D MWCNTs-B15C5/MTO catalysts IV A-D were prepared by immobilization of MTO on previously functionalized MWCNTs-B15C5 support. Briey, MWCNTs I were treated with different alkyl-diamino linkers, ethylene diamine (EDA), propylene diamine (PDA), cadaverine (CV), and hexamethylene diamine (HAD) (from C-2 to C-6 atoms, respectively), in the presence of N,N-di-isopropylcarbodiimide (DIC), 1-hydroxybenzotriazole (HOBt), and N,N-diisopropyl ethylamine (DIPEA) in DMF at 25 C for 24 h, to yield intermediates II A-D. 46 The coupling procedure was repeated with 4 0 -carboxy-benzo-15-crown-5 (B15C5) as electrophilic reagent, to afford MWCNTs-B15C5 III A-D (Scheme 1). The FT-IR analysis of intermediates II A-D and III A-D showed the shi of the peak at 1730 cm À1 (stretching vibration of the carboxylic group in native MWCNTs I) at 1637 cm À1 (SI#1, Fig. S1 and S2 †), conrming the formation of the amide bond. 47 Moreover, intermediates III A-D showed the characteristic peaks at 1120 cm À1 and 1210 cm À1 , corresponding to C-O stretching vibrational mode of B15C5 (SI#1, Fig. S2 †). 8 MWCNTs-B15C5 III A-D were successively treated with MTO in THF under gentle stirring at 25 C for 6 h, to afford MWCNTs-B15C5/MTO catalysts IV A-D. 43 Catalysts IV A-D showed the typical Re]O stretching vibrational mode at 953 cm À1 (SI#1, Fig. S3 †). Scanning Electron Microscopy (SEM) analysis of IV A-D ( Fig. 1) conrmed the integrity of MWCNTs aer the functionalization procedure. Powder X-ray analysis (XRD) was applied to evaluate the coordination geometry of MTO in representative intermediate III D and catalyst IV D. [48][49][50] The X-ray data of III D and IV D were treated by the difference method of neutron diffraction and isotopic substitution (NDIS) 37,51-53 in order to solve the contemporary presence of highly structured MWCNTs and amorphous B15C5 (generally indicated as support in the next discussion). Assuming that the general structure of III D was not signicantly altered during the MTO loading, structural data for MTO/B15C15 complex were Scheme 1 Synthesis of catalysts IV A-D. obtained by subtracting the radial distribution D(r) function of III D to that of IV D (Fig. 2, panels A and B) (see SI#2 † for a detailed description of the difference method technique). 54 The structure and distribution functions of III D and IV D are reported in SI#2 ( Fig. S5 and S6, † respectively).
Note that the oscillation pattern in the D(r) function of the two samples remain almost the same, conrming the hypothesis that the MTO loading didn't alter the structural order of the support. The D(r) function showed three peaks, set at about 1.80, 2.45, and 3.00 A, respectively (Fig. 2, panel C). The rst coordination sphere of the Re atom consists of six interactions assuming a quite octahedral conformation around the metal. The rst peak was tted by three metal-oxygen interaction lengths of exactly 1.80 A and one metal-carbon interaction of 1.75 A, respectively. The second peak at 2.45 A is due to the other two interactions Re-O necessary to complete the octahedral coordination sphere. These interactions are also responsible for the peak at 3.00 A (mutual interactions). The carbon atoms of the crown ether moiety might also contribute to this peak. The successive signals in the D(r) function were not further considered, since they derived from long-range MTO-MWCNTs interactions. In Fig. 2 (panel C) the function is compared with the theoretical peak shape function calculated for the octahedral model represented in Fig. 2 (panel D). The Re-O and Re-C bond distance values are quite in accord with the mean values previously reported for MTO/Ln complexes. 55 As shown by the model (Fig. 2, panel D), the Re atom is coordinated by two adjacent oxygen atoms of the B15C5 ring. The model refers to the couple of oxygen atoms bonded to the aromatic ring, but the possibility that another couple of adjacent oxygen atoms is involved in the interaction with the metal cannot be completely ruled-out.
Moreover, the rst Re coordination sphere analysis does not allow to determine the relative position of the methyl group with respect to the crown-ether oxygen atoms, due to the small electron differences between this group and the oxygen ones.
The distances between Re atom and ligands in the octahedral arrangement are reported in Table 1.

Preparation of MWCNTs-B15C5/PAMAM/MTO catalysts V E-H
We prepared rst and second-generation dendrimer catalysts V E-F and V G-H, respectively. As a general procedure, intermediates II A and II D were treated with methyl acrylate in MeOH at 25 C for three days, followed by addition of alkyl-diamino linkers EDA and HAD, to yield III E-F, respectively (Scheme 2, pathway A). The reaction of III E-F with B15C5 in the presence of DIC, HOBT, and DIPEA, in DMF at 25 C for 24 h, afforded IV E-F. These latter intermediates were further converted to rstgeneration MWCNTs-B15C5/PAMAM/MTO catalysts V E-F by treatment with MTO in THF at 25 C. In a similar way, two successive treatments of II A and II D with methyl acrylate and alkyl-diamino linkers afforded III G-H (Scheme 2, pathway B),   (Fig. S4 †).
Irrespective from the sample, the characteristic Re]O stretching vibrational mode at 950 cm À1 , and the CONH stretching vibrational mode at 1630 cm À1 , were always observed, besides to B15C5 C-O stretching signals at 1120, 1210, and 1250 cm À1 , respectively. The integrity of MWCNTs in catalysts V E-H aer the PAMAM functionalization was conrmed by SEM analysis (Fig. 3). Moreover, some sections of the MWCNTs in catalysts V F and V H, as representative examples, showed the presence of the PAMAM coating in TEM images (SI#6 †). The loading factor of catalysts IV A-D and V E-H was evaluated by Inductively Coupled Plasma Mass-Spectrometry (ICP-MS) analysis ( Table 2, entries 1-8).
Catalysts V E-F showed loading factors of the same order of magnitude than IV A-D ( Table 2, entries 5-6 versus entries 1-4). As a general trend, a slight decrease of the loading factor was observed for catalysts V G-H (Table 2, entries 7-8). Note that, among PAMAM catalysts, the higher value of the loading factor was obtained in the presence of the longest linker in terminal position ( Table 2, entry 6 versus entry 5, and entry 8 versus entry 7).
Examples of the decreased accessibility to coordinative sites into higher generation dendrimer systems have been reported as a consequence of conformational changes, steric hindrance and encapsulation processes. 56

Epoxidation of olens with catalysts IV A-D and V E-H
Catalysts IV A-D and V E-H have been evaluated for the epoxidation of a large panel of aliphatic and aromatic olens, including 1-hexene 1, 1-octene 2, trans-3-octene 3, vinylcyclohexane 4, styrene 5, cis-stilbene 6, and allylbenzene 7.    The reaction products have been detected by gaschromatography associated to mass-spectrometry (GC-MS) and nuclear magnetic resonance (NMR) analysis aer chromatographic purication (SI#3 and SI#4, † respectively). The oxidation with MTO alone was used as reference. Irrespective from the experimental conditions, the oxidation of 1 with MTO and catalysts IV A-D afforded the diol 9 as the only recovered product ( Table 3, entries 1-5), IV B being the most efficient catalyst (Table 3, entry 3) The corresponding epoxide was not detected in the reaction mixture, conrming the detrimental effect of the Lewis acidity of MTO in the ring-opening of the oxirane ring, even in the presence of B15C5. As a general trend, the catalytic activity was improved by increasing the number of carbon atoms in the alkyl-diamino spacer, as in the case of IV C-D (Table 3, entries 4 and 5), the lowest reactivity being observed with IV A, characterized by a linker with only two carbon atoms. MWCNTs-B15C5/PAMAM/MTO catalysts V E-H showed a different selectivity in the oxidation of compound 1, the epoxide 8 being obtained as the only recovered product in quantitative yield and high conversion of the substrate (Table 3, entries 6-9). In particular, the second-generation of PAMAM catalysts V G-H was more reactive than the rst-generation counterpart V E-F (Table 3, entries 6-7 versus entries 8-9). It is reasonable to suggest that the high selectivity toward the synthesis of the epoxide in the oxidation of 1 with PAMAM catalysts may be associated to the occurrence of additional interactions between MTO and nitrogen atoms in the dendrimer structure, acting as internal Lewis base sites. In accordance with this hypothesis, the vibrational mode at 492 cm À1 , corresponding to stretching of the Re-N group, was detected for V G, as representative example, by Far Infrared analysis (FIR) (SI#5; † MTO/1,2-cyclohexanediamine complex was used as reference). The ligand properties of tertiary amine atoms in PAMAM dendrimer are reported. 57 For example, octahedral Co(II)-PAMAM dendrimer complexes showed a slight distorted octahedral coordination of the metal atom by the polymeric matrix. 58,59 Moreover, atomistic molecular dynamics (MD) simulations on ethylenediamine (EDA) PAMAM dendrimer wrapped on MWCNTs (up to 11th generation) showed the high mobility and the low distance occurring between tertiary nitrogen atoms and the terminal amino groups of the structure, where the metal coordinative sites are expected to be located. 58 The oxidation of aliphatic olens 1-octene 2, trans-3-octene 3 and vinyl-cyclohexane 4 proceeded in a similar way. The epoxides 10, 12 and 14 were obtained in quantitative yield in the presence of catalysts V E-H (Table 3, entries 15-18, 24-27, and 33-36, respectively), while diols 11, 13 and 15 were isolated as the main recovered products with MTO alone and catalysts IV A-D (Table 3). In this latter case, only small amounts of epoxides were detected.
The second generation of PAMAM catalysts was slightly more reactive than the rst-generation counterpart in the conversion of substrate. Catalysts V E-H efficiently catalyzed the epoxidation of low reactive aromatic olens, styrene 5, cis-stilbene 6, and allylbenzene 7, affording the corresponding epoxides 16, 18, and 20, as the only recovered products in quantitative yield, from acceptable (52%) to high (89%) conversion of substrate (Table 3, (Table 4). Aer the rst run, the catalyst was recovered by ltration, washed with CH 2 Cl 2 , and used without any further purication. Catalysts V F and V H showed a slight decrease in the conversion of substrate (Table 4, entries 2-6 and entries 7-12, respectively) yielding the epoxide 8 in quantitative yield for at least six successive reactions. In accordance with these data, a low decrease of the loading factor of the catalyst was observed by ICP-MS analysis of the sample at the end of any run (Table 4).

FT-IR analysis
FT-IR analyses were performed, at room temperature, with a PerkinElmer Spectrum One and a BRUKER VERTEX 70V spectrometers, respectively, both equipped with an ATR unit. Mid Infrared spectra were recorded by averaging 32 scans, with a resolution of 4 cm À1 . In the case of FIR (Far Infrared) conguration, spectra were registered within the (600-50 cm À1 ) interval.

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM)
For the scanning electron microscopy (SEM), the sample suspensions (50 L) were let to adsorb onto carbon tape attached to aluminum stubs and air dried at 25 C. The observation was made by a JEOL JSM 6010LA electron microscope (Waltham, MA, USA) using Scanning Electron (SE) and Back Scattered Electrons (BSE) detectors. Energy Dispersive Spectroscopy (EDS) analysis was carried out to reveal the chemical elements. Transmission electron microscopy (TEM) analysis was carried out by JEM-1200 EXII Transmission Electron Microscope. For this aim one drop of nanocomposite was put on copper lace coated with carbon and dried at 25 C.

XRD analysis
The X-ray diffraction data were collected by a Bruker D8 Advance with DaVinci design located at CNIS -La Sapienza University of Rome diffractometer with a molybdenum source. The angle dispersive instrument is equipped with a Mo Ka X-ray tube (l ¼ 0.7107 A), whose radiation was focused onto the sample with Göbel mirrors. The 2q angle range available was 2.75-142 with a step of 0.25 within Bragg-Brentano parafocusing geometry. The scattered intensity was collected with the Lynxeye XE Energy-Dispersive 1-D detector.

Inductively coupled plasma mass-spectrometry (ICP-MS) analysis
The samples were weighed (from 1.0 mg to 10 mg) and transferred in uorinated ethylene propylene (FEP) vials, previously washed to avoid any kind of external contamination. Regia solution was chosen for the mineralization. In particular, 750 mL of HCl and 150 mL HNO 3 were added and the solution was heated to 80 C for 3 hours. The volume was adjusted to 5.0 mL and then diluted another 10 times before the ICP-MS analysis.
The analysis was performed with ultrahigh vacuum PHI 1257 system and an Agilent 7500 ICP-MS instrument under clean room ISO6 (Santa Clara, CA, USA), respectively.
Oxidation of olens 1-7 A 5.0 mL ask equipped with a magnetic bar was charged with CH 2 Cl 2 (5.0 mL), the olens 1-7 (0.5 mmol), the appropriate amount of selected MTO catalyst (0.0025 mmol of MTO calculated with respect to the MTO loading value) and H 2 O 2 (35% water solution, 50 mL, 0.5 mmol). The two-phase mixture was stirred vigorously at room temperature for 5 h. The progress of the reaction was monitored at half-hour intervals by gas chromatography associated to mass spectrometry (GC-MS) analysis of small aliquots of the organic phase. The conversion of olens and yield of corresponding products were determined by GC-MS using n-octane as internal standard and by comparison with commercially available standards. GC-MS was recorded on a Varian 410 GC-320 MS (Palo Alto, CA, USA) using a VF-5 ms column (30 m, 0.25 mm, 0.25 m), and an electron beam of 70 eV. Helium was used as the carrier gas at a constant ow of 1.5 mL min À1 . The GC oven temperature was started at 50 C (1 min hold) ramped at 20 C min À1 to 115 C (10 min hold), then ramped 5 C min À1 to 280 C (10 min hold).

Conclusions
MTO was efficiently supported on B15C5 and PAMAM functionalized MWCNTs. The novel catalysts showed a different selectivity in the oxidation of olens depending on the chemical complexity of the system. Epoxides were synthesized in quantitative yield and high conversion of substrate in the presence of PAMAM catalysts, while diols largely prevailed when B15C5 was directly linked to the surface of MWCNTs. XRD analysis showed that the Re atom was coordinated by an octahedral structure through interaction with a couple of oxygen atoms of the B15C5 ring. The high selectivity observed in the synthesis of epoxides with PAMAM catalysts was probably due to the Lewis base property of the nitrogen atoms present in the dendrimer aggregate in buffering the Lewis acidic character of Re(VII) atom. The reactivity of the dendrimer-based catalysts increased by increasing the order of complexity of the structure, the PAMAM of second-generation affording higher yield of epoxide with respect to the rst-generation counterpart. In the PAMAM series, terminal alkyl-diamino spacer bearing carbon atoms (cadaverine) afforded the most reactive catalysts. Note that PAMAM catalysts V E-H afforded styrene epoxide in yield higher than poly(4-vinylpyridine) and microencapsulated polystyrene MTO based systems, 37 highlighting the benecial role of the dendrimer structure in the selectivity of the reaction. In the absence of PAMAM, the Lewis acidity of MTO was responsible for the ring-opening of epoxides to diols. In this latter case, the reactivity decreased in the presence of alkyl-diamino spacer bearing more than three carbon atoms (propyl diamine). Finally, catalyst V H retained the catalytic activity for at least six successive runs without any appreciable leaching of the metal.

Conflicts of interest
There are no conicts to declare.