Synthesis of Aminophosphonic Acid Extractants and the Effect of the Alkyl Chain on Their Extraction Selectivities for Indium(III), Gallium(III), and Zinc(II)

New extractants with different alkyl chains, α-(octylamino)-4-tert-butylbenzylphosphonic acid monobutyl ester (OATPM), α-(2-ethylhexyl-amino)-4-tert-butylbenzylphosphonic acid monobutyl ester (EATPM), and α-1-(dodecylamino)octyl-phosphonic acid monobutyl ester (DAOPM) were synthesized by the Mannich reaction. The extraction selectivities of α-dodecylaminobenzylphosphonic acid monobutyl ester (DABPM), EATPM, OATPM and DAOPM were examined using In(III), Ga(III), Cu(II) and Zn(II) from a 1 M aqueous NH4NO3 solution. The extraction order for these metals using DABPM, EATPM, and OATPM was In(III) > Ga(III) > Zn(II) > Cu(II), whereas the order for DAOPM was In(III) > Ga(III) > Cu(II) > Zn(II). The separation ability of DABPM and OATPM is higher than that of EATPM and DAOPM. These extractants were analyzed by 1H-NMR. As a result, DABPM and OATPM were found to form intramolecular hydrogen bonds between P=OꞏꞏꞏH–N and P–OHꞏꞏꞏN-C. These hydrogen bonds allow to exist in the monomer form of DABPM and OATPM in toluene, which lead to the lower extraction ability for Zn(II).