3.4. Catalytic Reduction of 4-Nitrophenol

Freshly prepared aqueous NaBH₄ (1 mL, 0.2 mol) was added to an aqueous solution of 4-NP (3 mL, 0.01 mol) in a quartz cuvette with 1.0 cm path length and 4 mL volume. Both solutions were prepared using distilled water. The colour of the solution changed immediately to yellow upon the addition of NaBH₄. Then, a catalyst (1 mol%) was introduced into the reacting mixture and the initial yellow colour of the mixture turned colourless as the reaction proceeded. UV-Vis spectroscopy was used to monitor the progress of the reaction and the absorbance was recorded every 5 min between 200 and 800 nm. The percentage of conversion from 4-NP to 4-AP was determined by calculation from the absorbance data using the following equation

When the conversion was completed, the product was isolated by the following procedure. Firstly, the product was diluted with diethyl ether (30 mL) and separated using liquid-liquid extraction. The extraction steps were repeated three times, and the diethyl ether fractions containing the product (4-AP) were combined and dried using anhydrous sodium sulphate (Na₂SO₄). Then, diethyl ether was evaporated using a rotary evaporator. Purification of 4-AP by silica gel column chromatography was performed by eluding the product with a solvent composed of hexane and ethyl acetate in a ratio of 1:2. Each eluent was loaded on a thin layer chromatography (TLC) plate together with the 30 mL 4-AP standard. The fractions containing 4-AP were combined, and solvents were evaporated using a rotary evaporator. The product was characterized by ¹H NMR spectroscopy. Hence, 4-Aminophenol (purified): ¹H NMR (400 MHz, DMSO): δH = 8.36 (s, 1H), 6.47 (d, J = 8.4 Hz, 2H), 6.42 (d, J = 8.4 Hz, 2H), 4.35 (s, 2H).

The catalyst was recovered by centrifuging the reaction mixture for 40 min at 4000 rpm. After this time, the catalyst was settled at the bottom of the centrifugation tube and recovered by decanting the solution. The recovered catalyst was washed with deionized water twice and dried over silica gel for subsequent runs.

The reduction of 4-NP reaction was repeated three times under the same condition as the model reactions, except that the catalyst amount was varied (0.5 mol%, 1.0 mol%, and 1.5 mol%). Percentage conversion for each catalyst amount was determined from the UV-Vis spectroscopic data. This procedure was carried out for all complexes, 1, 2, and 3.

The recyclability test of the catalyst was performed to assess the reusability of catalyst over three runs. After the first run, the catalyst was separated from the reaction mixture by centrifugation. The catalyst was washed twice with deionized water to remove any residue from the catalytic reaction mixture. The separated catalyst was dried in a vacuum desiccator and reused in second and third runs. In the subsequent run, the same amount of 4-NP and a freshly prepared NaBH₄ were used. The percentage conversions of the 4-NP were calculated, and these were compared with the first run. This procedure was repeated for all three complexes.

The reproducibility test was performed by repeating the reduction of 4-NP under the same conditions twice. For each reaction, a fresh catalyst was used to determine its reproducibility over three runs. The percentage conversions of the 4-NP were calculated and compared. This procedure was repeated for all three complexes.