3.1. Materials and Methods
To synthesize NiNWs, the following chemicals of analytical grade were employed: nickel chloride hexahydrate (Acros Organics, Geel, Belgium), hydrazine monohydrate (64 %wt, Acros Organics, Geel, Belgium), hydrazine monohydrate (50 %wt, Acros Organics, Geel, Belgium), formic acid (85% aqueous solution, Acros Organics, Geel, Belgium), ammonium formate (Acros Organics, Geel, Belgium), sodium borohydride (Merck, Darmstadt, Germany), 4-Nitrotoluene (Merck, Darmstadt, Germany), 4-Bromonitrobenzene (Acros Organics, Geel, Belgium), nitrobenzene (Merck, Darmstadt, Germany), 4-Nitrobenzoic acid (Acros Organics, Geel, Belgium), methanol (SureSeal®, Acros Organics, Geel, Belgium), ethyl acetate (Chempur, Piekary Slaskie, Poland), ethylene glycol (Chempur, Piekary Slaskie, Poland), sodium bicarbonate (Chempur, Piekary Slaskie, Poland), and sodium hydroxide (Chempur, Piekary Slaskie, Poland).
The standard reaction conditions were as follows. 175 mL of 0.1 M NaOH solution in ethylene glycol (EG) was combined with 50 mL of 50 %wt N
2H
4 hydrate and the mixture was heated up to 120 °C. Then, 25 mL of 0.1 M NiCl
2 solution in EG was dripped into the mixture in the presence of a neodymium magnet able to hold 55 kg of load (kept in another beaker). Immediately formed NiNWs (in the form of a dark grey suspension) were attracted to the magnet. The method is shown in
Figure 5. After the reaction, the NiNWs were separated from the liquid medium and washed thoroughly with water and isopropanol. We could not discern the presence of any other obvious sources of contamination such as precipitates of another color. Where indicated in the text, the conditions of the reaction were modified to tune the microstructure of the obtained product.
To carry out the reduction, the catalyst (10% mmol) was placed in a 10 mL glass reactor (Schlenk tube), followed by the addition of methanol (2 mL), nitroarene (1.0 mmol), reducing compound, and methanol. Then, the reaction was vigorously stirred for an appropriate time at elevated temperature (details of the synthesis are indicated in the text).
The microstructure of the material was probed by scanning electron microscopy (SEM) using a HITACHI TM3000 (Tokyo, Japan) with 5 kV acceleration voltage. The results are represented as the diameter of NiNW bundles as observed by SEM and measured using ImageJ software v. 1.52 (University of Wisconsin, UW, USA). To attain the statistical significance, tens of individual bundles were measured each time and the results are shown as mean ± standard deviation.
The X-ray diffraction (XRD, Philips PW1050, Eindhoven, Netherlands) gauged the chemical identity of the NWs (2θ = 35–100°, 0.05° step, 2 s acquisition time).
NMR spectra were recorded at 298 K on an Agilent-MR NMR (Palo Alto, CA, USA; 400 MHz for 1 h and at 100.5 MHz for 13 °C).
The progress of the reaction was monitored by TLC using silica-gel-coated aluminum plates with a fluorescence indicator (Merck, SiO2 60, F254) and visualized by UV light (254 nm and 365 nm) or dipping into an iodine chamber (well-closed chamber with a few crystals of elemental iodine and 10 g of silica gel).
Melting points (m.p.) were determined with a Boetius apparatus and are uncorrected.
Mass spectrometry were determined with an API 4000Q TRAP tandem mass spectrometer detector equipped with an electrospray ionization (ESI) source (Applied Biosystems/MDS SCIEX, Foster City, CA, USA).
3.2. Catalytic Tests
3.2.1. General Procedure of Reduction of Nitroarenes
The appropriate nitrobenzene derivative (1.000 eq.) was added to a Schlenk tube and was dissolved in methanol (each 1 mL per 1 mmol of nitrobenzene derivative). Next, the loaded nickel nanowires catalyst (0.10 eq.) and reaction mixture was stirred at 50 °C. Then, hydrazine hydrate (100%) was added slowly with a syringe and subsequently the temperature was raised to 90 °C (vigorous hydrogen gas evolution was observed). The resulting mixture was stirred for an additional 3 to 4 h and cooled to room temperature. Then, it was diluted with ethyl acetate (each 15 mL per 1 mmol of substrate) and washed with saturated sodium carbonate (each 5 mL per 1 mmol of substrate) and twice with distilled water (each 5 mL per 1 mmol of substrate). The organic solution was dried over anhydrous Na2SO4 and evaporated under reduced pressure to give pure aniline derivatives. The purity of the product was sufficient as indicated by NMR spectra analysis.
3.2.2. Large-Scale Reduction of 4-Nitrobenzoic Acid
4-Nitrobenzoic acid (3.34 g, 20.0 mmol) was added to a Schlenk tube and dissolved in anhydrous methanol (20 mL) under a N2 atmosphere (Schlenk line). Next, the loaded nickel nanowires catalyst (118 mg, 2.0 mmol) and reaction mixture was stirred at 50 °C. Then, hydrazine hydrate (64% aqueous solution, 11.6 mL, 200.0 mmol) was added slowly with a syringe and subsequently the temperature was raised to 90 °C (vigorous hydrogen gas evolution was observed). The resulting mixture was stirred for an additional 3 to 4 h and cooled to room temperature. Then, it was diluted with ethyl acetate (300 mL) and washed with saturated sodium carbonate (50 mL) and twice with distilled water (50 mL). The organic solution was dried over anhydrous Na2SO4 and evaporated under reduced pressure to give pure 4-aminobenzoic acid (2.58 g, 18.8 mmol, 94%). The purity of the product was sufficient as indicated by NMR spectra analysis.
3.2.3. Obtained Products
4-Methylaniline (2a). Following the general procedure above, the title compound (503 mg, 94%) was isolated as a white solid. Yield: 94%; m.p. 42–43 °C; (lit. mp = 42–43 °C); 1H NMR (400 MHz, DMSO-d6) δ 2.12 (s, 3H), 4.75 (bs, 2H), 6.46 (d, J = 8.2 Hz, 2H), 6.81 (d, J = 8.2 Hz, 2 H); 13C NMR (151 MHz, CDCl3) δ 20.11, 114.06, 123.96, 129.22, 146.04; MS (EI) m/z: 108.08 [M + H]+.
Aniline (2b). Following the general procedure above, the title compound (172 mg, 92%) was isolated as a colorless oil. Yield: 92%; b.p. 84 °C (lit. bp = 84 °C); 1H NMR (400 MHz, DMSO-d6) δ 4.97 (bs, 2H), 6.49 (tt, J = 7.3, 1.1 Hz, 1 H), 6.56 (dd, J = 8.5, 1.1 Hz, 2 H), 7.00 (dd, J = 8.5, 7.3 Hz, 2 H); 13C NMR (101 MHz, DMSO-d6) δ 113.87, 115.65, 128.78, 148.56; MS (EI) m/z: 93.13 [M + H]+.
4-Bromoaniline (2c). Following the general procedure above, the title compound (495 mg, 96%) was isolated as a white-yellow solid. Yield: 96%; m.p. 62–63 °C; (lit. mp = 62–64 °C); 1H NMR (400 MHz, DMSO-d6) δ 5.23 (bs, 2 H), 6.51 (d, J = 8.8 Hz, 2 H), 7.12 (d, J = 8.8 Hz, 2 H); 13C NMR (101 MHz, DMSO-d6) δ 106.00, 115.75, 131.28, 148.03; MS (EI) m/z: 172.02 [M + H]+, 174.02 [M + 2 + H]+.
4-Aminobenzoic acid (2d). Following the general procedure above, the title compound (386 mg, 94%) was isolated as a white solid. Yield: 94%; m.p. 188–189 °C; (lit. mp = 186–189 °C ); 1H NMR (400 MHz, DMSO-d6) δ 5.84 (bs, 2 H), 6.54 (d, J = 6.6 Hz, 2 H), 7.61 (d, J = 6.7 Hz, 2 H), 11.39 (vbs, 1 H); 13C NMR (101 MHz, DMSO-d6) δ 112.59, 116.93, 131.21, 153.10, 167.48; MS (EI) m/z: 159.12 [M + Na]+.