Steric effects of polymethylene chain of 4-acylbis(pyrazolones) on the solvent extraction of trivalent lanthanoids: synergistic effect with mono and bifunctional neutral organophosphorus extractants

Various 4-acylbis(pyrazolones), namely 4-adipoylbis(1-phenyl-3-methyl-5-pyrazolone) (H2AdBP = 1), 4-suberoylbis(1-phenyl-3-methyl-5-pyrazolone) (H2SuBP = 2), 4-sebacoylbis(1-phenyl-3-methyl-5-pyrazolone) (H2SbBP = 3), 4-dodecandioylbis(1-phenyl-3-methyl-5-pyrazolone) (H2DdBP = 4) have been synthesized and examined their solvent extraction behavior towards trivalent lanthanoids (Nd3+, Eu3+ and Tm3+). The extraction of Ln3+ ions was found to increase monotonically with increasing atomic number of these metal ions. 1-4 reagents showed an initial increase in the extraction efficiency of Ln3+ ion with increasing polymethylene chain length, (CH2)n, from n = 4 to 8 and thereafter a decreasing trend, for n = 10. The equilibrium constants (Kex) of the extracted complexes have been deduced by nonlinear regression analysis with the aid of suitable chemically based model developed by taking into account chemical mass action principles. The Kex values were correlated with the polymethylene chain length, by measuring the distance between the carbonyl oxygen atoms connected to the polymethylene chain with the help of semi-empirical PM3 molecular modelling calculations. The synergistic effect on the addition of various neutral organophosphorus extractants to the metal-chelate system has also been investigated. Not only enhanced extraction efficiency, but also improved selectivity has been observed among these Ln3+ ions. The equilibrium constants of the synergistically extracted complexes have been correlated with the donor ability of the phosphoryl oxygen of the neutral organophosphorus extractants in terms of their 31P NMR chemical shifts and their basicity values (KH = nitric acid uptake constant).