The general procedure for the synthesis of 2-bromo-1,10-phenanthroline using 1-methyl-1,10-phenanthroline-2-one as starting material was as follows: phosphorus pentabromide (28 g; 65 mmol) and phosphorus tribromide oxide (50 g; 174 mmol) were added to 1-methyl-1,10-phenanthroline-2-one (10 g; 47.6 mmol) under the protection of argon gas at room temperature . The reaction system was warmed to 80 °C and the reaction mixture was stirred continuously for 6 hours. Upon completion of the reaction, the mixture was cooled in an ice bath and subsequently slowly poured into ice water. The pH of the mixture was adjusted to alkaline using concentrated ammonia. The reaction mixture was extracted using chloroform and the organic phase was collected and washed with water and subsequently dried over anhydrous sodium sulfate. The solvent was removed by concentration under reduced pressure and the resulting crude product was purified by silica gel column chromatography (250 g of silica gel, eluent ratio of toluene/ethyl acetate = 5/1 to 3/1 to 1/1, and finally pure ethyl acetate) to give 6.9 g of the target product 2-bromo-1,10-phenanthroline in 94% yield. The product structure was confirmed by 1H NMR (CDCl3): δ 7.66 (dd, J1 = 8 Hz, J2 = 4.5 Hz, 1H), 7.76-7.79 (m, 2H), 7.83 (d, J = 8.5 Hz, 1H), 8.08 (d, J = 8.5 Hz, 1H), 8.26 (dd, J1 = 8 Hz, J2 = 2 Hz, 1H). 9.24 (dd, J1 = 4.5 Hz, J2 = 2 Hz, 1H). HPLC analytical conditions: mobile phase 20-90% acetonitrile-water (containing 0.1% trifluoroacetic acid), retention time of the target peaks was 4.2 min. ESIMS (positive ionization mode) m/z 260.9, 258.2 ([M + H]+, theoretical molecular weight of C12H7BrN2 259.1).
