TEMPO functionalized C60 fullerene deposited on gold surface for catalytic oxidation of selected alcohols
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C60TEMPO10 catalytic system linked to a microspherical gold support through a covalent S-Au bond was developed. The C60TEMPO10@Au composite catalyst had a particle size of 0.5–0.8 μm and was covered with the fullerenes derivative of 2.3 nm diameter bearing ten nitroxyl groups; the organic film showed up to 50 nm thickness. The catalytic composite allowed for the oxidation under mild conditions of various primary and secondary alcohols to the corresponding aldehyde and ketone analogues with efficiencies as high as 79–98%, thus giving values typical for homogeneous catalysis, while retaining at the same time all the advantages of heterogeneous catalysis, e.g., easy separation by filtration from the reaction mixture. The catalytic activity of the resulting system was studied by means of high pressure liquid chromatography. A redox mechanism was proposed for the process. In the catalytic cycle of the oxidation process, the TEMPO moiety was continuously regenerated in situ with an applied primary oxidant, for example, O2/Fe3+ system. The new intermediate composite components and the final catalyst were characterized by various spectroscopic methods and thermogravimetry.
KeywordsFullerene Self-assembly TEMPO Catalyst Alcohol oxidation Heterogeneous nanostructured catalysts
A variety of investigations have revealed that 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and its derivatives are some of the most efficient catalysts for the selective aerobic oxidation of alcohols to the corresponding carbonyl compounds (de Nooy et al. 1996). The reported catalytic systems are exemplified by both homogeneous (Semmelhack et al. 1984; Greene et al. 2015) and heterogeneous procedures (Bolm and Fey 1999; Yang et al. 2006). It is well-known that both of these procedures have their advantages and drawbacks. High reaction rates, easy removal from the reaction mixture, efficient recycling, low catalyst loading, and simple procedures for the restoration of catalytic activity as part of the reaction’s work-up are among the desirable features required.
Recently, fullerene C60TEMPO n (n = 2, 4, 12) derivatives (Beejapur et al. 2013, 2014) were applied as recyclable catalysts for the oxidation of alcohols through the “release and catch” approach (Gruttadauria et al. 2013). In this strategy, the catalytic system is initially immobilized on a silica multilayer support, but the catalytic moiety is released into the solution over the course of the reaction, and then it is recaptured at the end of the reaction. According to the authors, in this way, valuable combination of the advantages from homogeneous (high catalytic activity and reaction rates) and heterogeneous catalysis (easy separation by filtration) can be achieved. Nonetheless, although the catalytic systems turned out to be highly effective for the oxidation of alcohols, the proposed catalyst recycling procedure still suffered from several restitution steps, such as filtration, solvent removal, and readsorption onto the support.
In contrast, C60TEMPO n catalytic systems covalently linked to some kind of insoluble support should allow for at least some of the operations to be omitted. We recently reported a procedure for the deposition of in situ deprotected thioacetyl-functionalized C60 fullerene derivatives onto gold surface through Au-S bonds. The resulting C60 fullerene nanostructured films were then employed as a catalyst and also as an initiator in electrochemical polymerization (Piotrowski et al. 2014, 2015). In the present contribution, we describe the synthesis, characterization and catalytic activity of a novel C60 fullerene malonate adduct, functionalized with ten TEMPO radicals and one 8-(acetylthio)octyl substituent, that was subsequently used to decorate the surface of fine gold microspheres. The resulting C60TEMPO10@Au composite system was then used for the selective oxidation of several classes of alcohols.
Materials and methods
C60 fullerene, 8-bromo-1-octanol, malonyl chloride, 4-hydroxy-TEMPO, iodine, 9,10-dimethylanthracene (DMA), tetrabromomethane, benzyl alcohol, 4-methoxybenzyl alcohol, cyclohexanol, allyl alcohol, diphenylmethanol, 1-phenylethanol, toluene, and n-hexane were purchased from Sigma-Aldrich. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU), ethyl malonyl chloride, potassium thioacetate, silica gel 70–230 mesh, and gold powder spherical 0.5–0.8 μm S.A. 0.45–0.7 m2/g were obtained from Alfa Aesar. Sodium sulfate, acetonitrile, triethylamine, methylene chloride, dimethylformamide, and ethyl acetate were purchased from POCh (Poland). Methylene chloride, dimethylformamide, and toluene were dried and purified before use according to standard procedures. Other solvents were HPLC or analytical grade reagents and were used as received.
ESI-MS spectra were acquired on a Micromass LCT ESI-TOF mass spectrometer equipped with an orthogonal electrospray ionization source.
1H and 13C NMR spectra were recorded on Varian Unity Plus 300- and 500-MHz spectrometers using CDCl3 as solvent.
The infrared experiments on fullerene malonates were carried out using a Nicolet 8700 spectrometer while those on C60 fullerene derivatives used a Shimadzu FTIR-8400S.
The electron spin resonance (ESR) spectra were recorded under aerobic conditions using a Bruker EMXplus system equipped with an ER 4131 VTM temperature control system. The measurements were performed using ESR quartz tubes with ϕ = 4 mm for both the solid state at RT and the 3.24 × 10−3 M toluene solution. In the latter case, a 10 K temperature step in the temperature range of 230–360 K was employed; the temperature was stepped up starting from the frozen state at 100 K (larger steps were applied initially). A modulation frequency of 100 kHz and a modulation amplitude of 1 × 10−4 mT were applied.
Atomic force microscopy (AFM) measurements were carried out with a nanoScience Instruments Nanosurf easyScan 2 AFM.
XPS measurements were carried out using a VG ESCALAB 210 electron spectrometer equipped with an Al Kα source (1486.6 eV). XPS data were calibrated using the binding energy of Au 4f7/2 = 84.0 eV as the internal standard.
Thermogravimetric analysis was performed under a high purity nitrogen atmosphere using TA Instruments Q50 Thermal Gravimetric Analyzer with a heating rate of 5 K/min.
Cyclic voltammetry (CV) experiments were carried out using an Autolab potentiostat (ECO Chemie, Netherlands), with a silver/silver chloride (Ag/AgCl) electrode as the reference electrode, platinum foil as the counter electrode, and a the glassy carbon electrode (GCE, BASi, 3 mm diameter) or a modified gold electrode (Gold Arrandee, GmbH) as the working electrode. 0.1 M TBAHFP/toluene/acetonitrile was used as the supporting electrolyte solution. An argon blanket was used to deaerate the solution during the experiments.
High-performance liquid chromatography (HPLC) was performed using a Waters 600E pump, equipped with a Waters 486 tunable absorbance detector or a Waters 410 differential refractometer RI detector, a Gilson FC 203B fraction collector, and a BaseLine Chromtech data system. For isolation of the C60 hexakis adduct a Phenogel GPC column (22.5 × 250 mm, 10 μm, 100 Å) with a 5-ml/min flow of toluene was employed. The analysis of the products obtained from the C60TEMPO10-catalyzed oxidation of selected alcohols was performed using a Phenomenex Luna C18 column (4.6 × 250 mm, 5 μm, 50 Å) with a water/acetonitrile mixture as eluent.
8-Bromooctyl ethyl malonate
A solution of ethyl malonyl chloride (301 mg, 2 mmol) in dry CH2Cl2 (5 ml) was added dropwise over a period of 15 min to a solution of 8-bromo-1-octanol (418 mg, 2 mmol) and triethylamine (202 mg, 2 mmol) in anhydrous methylene chloride (20 ml) at 0 °C under a nitrogen atmosphere. The resulting mixture was allowed to heat to room temperature and was stirred for additional 4 h. After evaporation of the solvent under reduced pressure, the residue was chromatographed using column chromatography (silica gel 70–230 mesh, ethyl acetate/n-hexane 1:6).
Yield, 92%; the mass spectrum (ESI-MS) showed an [M,79Br + Na]+ peak at 345.2 and an [M,81Br + Na]+ peak at 347.1 (Fig. S1, Electronic Supplementary Material (ESM)); IR (neat) ν max(cm−1) 2931.6, 2856.4, 1748.2, 1730.1, 1464.8, 1368.4, 1329.3, 1267.5, 1184.4, 1146.1, 1032.1, 726.2, 682.4, 642.4, 604.5, see (Fig. S2, ESM); δ1H (500 MHz; CDCl3; TMS) 4.18 (q, J = 7.1 Hz, 2H), 4.11 (t, J = 6.7 Hz, 2H), 3.38 (t, J = 6.8 Hz, 2H), 3.34 (s, 2H), 1.88–1.77 (m, 2H), 1.67–1.56 (m, 2H), 1.45–1.37 (m, 2H), 1.31 (m, 6H), and 1.25 (t, J = 7.2 Hz, 3H) ppm (Fig. S3, ESM); δ13C (125 MHz; CDCl3) 166.51, 65.40, 61.42, 41.55, 33.71, 32.50, 28.85, 28.47, 28.28, 27.91, 25.54, and 13.87 ppm (Fig. S4, ESM).
8-(Acetylthio)octyl ethyl malonate
Potassium thioacetate (171 mg, 1.5 mmol) was added to a stirred solution of 8-bromooctyl ethyl malonate (345 mg, 1 mmol) in dry DMF (15 ml) and stirred for 16 h at room temperature under nitrogen. The reaction mixture was then poured into diethyl ether (100 ml), and a white precipitate formed. After filtration, the solution obtained was washed with water (3 × 10 ml) and subsequently dried over anhydrous sodium sulfate. After filtration and solvent evaporation, the crude product was purified by the means of column chromatography (silica gel 70–230 mesh, ethyl acetate/n-hexane 1:4).
Yield, 97%; the mass spectrum (ESI-MS) showed an [M + Na]+ peak at 341.2 (Fig. S5, ESM); IR (neat) ν max(cm−1) 2929.9, 2855.8, 1748.2, 1730.1, 1689.2, 1464.8, 1412.0, 1368.3, 1329.6, 1268.2, 1184.3, 1136.1, 1032.3, 954.4, 730.0, 674.2, 624.8, see (Fig. S6, ESM); δ1H (500 MHz; CDCl3; TMS) 4.21 (q, J = 7.1 Hz, 2H), 4.14 (t, J = 6.7 Hz, 2H), 3.37 (s, 2H), 2.85 (t, J = 6.8 Hz, 2H), 2.32 (s, 3H), 1.67–1.61 (m, 2H), 1.59–1.53 (m, 2H), 1.39–1.25 (m, 8H), and 1.28 (t, J = 7.2 Hz, 3H) ppm (Fig. S7, ESM); δ13C (125 MHz; CDCl3) 195.64, 166.51, 65.40, 61.42, 41.55, 36.29, 33.71, 32.50, 28.85, 28.47, 28.28, 27.91, 25.54, and 13.87 ppm (Fig. S8, ESM).
The synthesis was performed according to the modified procedure proposed by Bingel (1993). To a solution of C60 (144 mg, 0.2 mmol) in freshly distilled toluene (120 ml), a solution of 8-(acetylthio)octyl ethyl malonate (34 mg, 0.1 mmol) in toluene (5 ml), a solution of iodine (25 mg) in toluene (10 ml) and DBU (31 μl, 0.2 mmol) were added. The resulting mixture was stirred at room temperature for 16 h under a nitrogen atmosphere. After concentration using a rotary evaporator, the mixture obtained was chromatographed (silica gel 70–230 mesh, toluene/n-hexane 1:1).
Yield, 12%, (the yield was lowered by a side reaction of DBU with the S-acetyl group (Singh et al. 2010)); the mass spectrum (ESI-MS) showed an [M + Na]+ peak at 1059.4 (Fig. S9, ESM); IR (KBr disk) ν max(cm−1) 2921.9, 2850.5, 1743.3, 1688.4, 1652.6 1458.7, 1427.5, 1266.6, 1253.2, 1233.4, 1205.1, 1186.7, 1132.1, 1094.7, 1060.1, 711.2, 703.0, 668.5, 626.4, 580.3, 552.0, 527.0, see (Fig. S10, ESM); δ1H (500 MHz; CDCl3; TMS) 4.56 (q, J = 7.1 Hz, 2H), 4.50 (t, J = 6.6 Hz, 2H), 2.86 (t, J = 6.8 Hz, 2H), 2.32 (s, 3H), 1.88–1.78 (m, 2H), 1.54–1.58 (m, 4H), 1.49 (t, J = 7.2 Hz, 3H), and 1.30–1.40 (m, 6H) ppm (Fig. S11, ESM); δ13C (125 MHz; CDCl3) 195.97, 163.57, 145.32, 145.22, 145.19, 145.3, 145.10, 145.09, 144.81, 144.61, 144.60, 144.58, 144.53, 143.81, 143.02, 143.01, 142.94, 142.92, 142.91, 142.14, 141.84, 140.88, 139.01, 138.86, 71.56, 67.33, 63.36, 30.60, 29.42, 29.02, 28.98, 28.97, 28.65, 28.48, 25.83, and 14.19 ppm (Fig. S12, ESM).
Hexakis adduct (II)
Yield, 53%; the mass spectrum (ESI-MS) showed an [M + Na]+ peak at 3112.8 (Fig. S13, ESM); IR (KBr disk) ν max(cm−1) 2974.1, 2936.1, 1747.0, 1689.5, 1464.5, 1379.1, 1364.5, 1220.7, 1178.3, 1115.5, 1078.8, 1004.1, 960.3, 715.2, 528.5, see (Fig. S14, ESM); δ1H (500 MHz; CDCl3; TMS; after reduction with phenylhydrazine) 5.19–5.24 (m, 20H), 4.30 (q, J = 7.0 Hz, 2H), 4.22 (t, J = 6.7 Hz, 2H), 2.83 (t, J = 6.8 Hz, 2H), 2.32 (s, 3H), 2.00–2.03 (m, 4H), 1.62–1.66 (m, 8H), 1.13–1.31 (m, 120H), and 0.88 (t, J = 7.2 Hz, 3H) ppm (Fig. S15, ESM);
Self-assembly of the derivative on the gold surface
Experiments without deprotection of S-acetyl group did not allow for the production of the desired assembly. For this purpose, the fullerene thioacetate was converted to its thiol analogue using the previously reported in situ procedure (Piotrowski et al. 2014). Self-assembly of the deprotected C60TEMPO10 was then achieved by dipping the gold substrates into the obtained solution. The resulting films were washed with toluene and dried under a stream of argon.
Oxidation of alcohols
For oxidation of the selected alcohols, we decided to use the protocol recently reported by Ma et al. (2011), but replacing the TEMPO with the gold microspheres modified with the synthesized nitroxyl C60 fullerene derivative. In a typical procedure, to a vigorously stirred mixture of Fe(NO3)3·9H20 (0.05 mmol), acetonitrile (5 ml), C60TEMPO10@Au (15 mg), and NaCl (0.1 mmol), the corresponding alcohol (1 mmol) in MeCN (1 ml) was added. The resulting mixture was stirred under aerobic conditions overnight. It was then filtered to remove the catalyst and passed through a short plug of silica gel to remove inorganic salts. The resulting solution was examined using reverse phase HPLC to determine the concentration of the oxidation products.
Results and discussion
XPS analysis was used to confirm the chemisorption of TEMPO functionalized fullerenes on the gold surface, as well as to investigate the composition of the resulting catalyst. The x-ray photoelectron spectroscopy (XPS) spectrum of the examined nanomaterial showed the presence of gold, sulfur, nitrogen, oxygen, and carbon atoms.
The Au 4f region revealed a double-peak centered at 84.0 and 87.7 eV, corresponding to the Au 4f7/2 and Au 4f5/2 gold atoms, respectively, typical for the presence of the Au0 state (Sashuk and Rogaczewski 2016).
The strongest peak, which was observed at 400.2 eV, can be assigned to nitrogen atoms from nitroxyl radicals according to literature data (Shen et al. 1987). The presence of this signal confirmed successful grafting of the synthesized TEMPO-C60 derivative onto gold surface and proved that functionalized fullerenes were the main component of the obtained nanostructured material.
The intensities of the estimated S 2p contributions cannot be taken quantitatively. Nevertheless, they allowed us to conclude that only a small number of sulfur atoms was involved in S-Au bonds, while the main part of registered S 2p spectrum could be attributed to the physisorbed free thiol groups , which were incorporated into the multilayer structure of the obtained film. This could be expected, since the AFM results showed that the deposited film was not a single monolayer. Additionally, the absence of signals at higher binding energies (Canitez et al. 2011) suggested that the obtained catalytic film had not undergone any oxidation during handling.
Catalytic oxidation of selected alcohols
During the process, the TEMPO oxoammonium ion was responsible for the oxidation of alcohols, while being reduced to the hydroxylamine form. To close the catalytic cycle, the TEMPO moiety was continuously regenerated in situ with a primary oxidant, for example, the O2/Fe3+ system.
In conclusion, we have designed and synthesized a novel bifunctional C60 fullerene derivative, bearing a sulfur anchoring group together with ten nitroxyl radicals. The title compound can be easily self-assembled onto the surface of a microspherical gold support through a covalent S-Au bond, leading to the formation of the efficient alcohol oxidation catalyst C60/TEMPO10@Au. The catalytic activity of resulting system was evaluated in the oxidation of a variety of primary and secondary alcohols to their corresponding aldehyde and ketone derivatives. The aerobic and room temperature protocol, along with the lack of over-oxidation of primary alcohols to the corresponding carboxylic acids, shows that the presented approach allows the synthesis of aldehyde compounds in excellent yields under mild conditions. Similarly, secondary alcohols are converted into ketone analogues with high yields, even for sterically hindered molecules. The immobilization of the multifunctional C60 fullerene derivative on the Au surface allows the removal of catalytic material by simple filtration, while enhancing activity by the simultaneous reduction of the barrier to TEMPO oxidation.
The scientific work was financed from the budget for science in the years 2016–2019, project no. IP 2015 061874, received from the Ministry of Science and Higher Education of Poland.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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