Functionalization of Single-Walled Carbon Nanotubes with 4-Benzo-9-crown-3 Ether

Abstract

Sidewall functionalization of single-walled carbon nanotubes (SWCNTs) by the covalent addition of 4-benzo-9-crown-3 ether diazonium salt has been described. The covalent grafting of crown ether has led to a considerable increase in the solubility of SWCNTs in both organic and aquatic solvents such as ethanol, di­methyl sulfoxide, dimethylformamide and water. The highest solubility was attained in DMF and DMSO. The covalently bonded 4-benzo-9-crown-3 ether allowed the hosting of Li⁺. The covalent grafting of crown ether [4-(benzo-9-crown-3)] to SWCNT was identified and confirmed by infrared spectroscopy, transmission electron microscopy, UV/Vis and thermogravimetric analysis methods.

Key words

sidewall functionalization - carbon nanotubes - 4-benzo-9-crown-3-ether - phase-transfer catalyst

References and Notes

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10

Synthesis of Benzo-9-crown-3 (3): Catechol (1; 1.1 g, 0.01 mol), LiOH (0.84 g, 0.02 mol) and distilled H₂O (58 mL) were placed in a 150-mL round bottom flask. To this mixture, the bis(2-chloroethyl) ether (2; 1.43 g, 0.01 mol) was added dropwise followed by stirring for 30 min. The mixture was then refluxed for 86 h at 100 ˚C. Addition of H₂O (5 mL) followed by steam distillation afforded the white crystals of benzo-9-crown-3 ether (3) in a yield of 0.60 g (33%).
Synthesis of 4-Nitrobenzo-9-crown-3 (4): Benzo-9-crown-3 ether (3; 1.8 g, 0.01 mol), CH₂Cl₂ (40 mL), glacial AcOH (28 mL) and few drops of H₂SO₄ were placed in a 200-mL round bottom flask. Then, concd HNO₃ (5 mL) was added dropwise to this mixture and the mixture was refluxed for 3 h. The resulting yellow reaction mixture was poured onto 35 g of crashed ice, and then sufficient concd aq NaOH was added to naturalize the mixture (pH control). This material was extracted with CH₂Cl₂ (4 × 25 mL) and the combined organic extracts were dried over anhyd Na₂SO₄ (5 g). After filtration and solvent removal in vacuum, yellow crystalline 4-nitrobenzo-9-crown-3 ether was afforded in yield of 1.6 g (71%); mp 92-94 ˚C.
Synthesis of 4-Amino Benzo-9-crown-3 (5): To a mixture of 4-nitrobenzo-9-crown-3 ether (4), (0.23 g, 0.01 mol) and tin (0.46 g, 0.004 mol) the solution of HCl (3 mL) in absolute EtOH (5 mL) was added dropwise and the mixture was refluxed for 3 h. When TLC monitoring indicated no further progress of the reaction, an excess of concd aq KOH was added to make the solution basic. The reaction mixture was then stirred for 30 min. The white precipitate was washed with CHCl₃ (4 × 5 mL) and the combined organic extracts were dried over anhyd Na₂SO₄ (1.2 g). Filtration followed by solvent removal in vacuum yielded the light brown oil (1.95 g, 85%).
Preparation of SWCNT-CE (6): The SWCNTs used in this study were purchased from Petrol Co in its pure form (closed-cap, average diameters of 2-3 nm and average length of 8 µm). To further purify the SWCNTs, concd HCl was added and the mixture was briefly sonicated to remove the metal catalyst. The resulting mixture was then filtered using a 0.45 µm polytetrafluoroethylene filter (PTFE-millipore) to separate the SWCNT filter cake. The SWCNTs (6 mg, 0.5 mmol), 4-amino benzo-9-crown-3 ether (5; 46 mg, 2.0 mmol), sodium nitrite (138 mg, 2.0 mmol) and H₂SO₄ (0.45 mL, 2.4 mmol) were mixed in a flask equipped with a reflux condenser and a magnetic stir bar. The mixture was heated at 60 ˚C followed by vigorous stirring to form a paste. Subsequently, the paste was diluted with DMF and filtered through a PTFE (0.45 µm) membrane. The collected solid was washed with DMF until the filtrate become colorless. Dispersing the solid in DMF by sonication followed by filtration afforded the pure SWCNT-CE (6). Sonication was only used here to ensure that occlusion of small organics had not occurred. The added DMF with solid was removed by washing with Et₂O, and the compound SWCNT-CE (6) was dried in a vacuum oven (65 ˚C) for overnight.
Preparation of SWCNT-CE-Li ⁺ (7): To a suspension solution of SWCNT-CE (6) in DMF was added a solution of the Li⁺ salt [LiCl standard (4 M) in H₂O]. Following a sonication for a period of time, the excess of metal salt was removed by filtration through a 0.45 µm polytetrafluoro-ethylene filter (PTFE-millipore). The SWCNT-CE-Li⁺ filter cake was separated, and washed with demineralized H₂O (5 × 20 mL). The filtered cake was then dried in vacuum oven at 50 ˚C for 24 h.