Solvent extraction of metal picrates by phosphoryl-containing podands

Using UV spectroscopy, we have studied the thermodynamics of extraction of metal picrates M(+)Pic(-) (M+ = Li+, Na+ K+, Rb+, Cs+, Tl+ and Ag+) and M2 (Pic(-))(2) (M2+ = Ca2+, Sr2+, Ba2+, Pb2+ and UO22+) from water to a chlor oform or to a dichloromethane solution. The extractant molecules are mono-p odands R-O-(CH2-O-CH2-)(n)-O-R containing an ether chain and terminal phosp horyl groups: R = (Ph)(2)P(O)-C6H4 -; n = 3, 4 (I - II) and R = (Ph)(2)(O)- CH2-C6H4 -, n = 1, 3, 4 (III - V). We found that podand I displays a remark able Ba2+/Ca2+ extraction selectivity. All podands extract alkali and alkal ine earth picrates in dichloromethane las dissociated ion pairs) better tha n in chloroform (as non-dissociated ion pairs). Based on Molecular Dynamics simulations of the complexes I . M2+ and I . M2 +(Pic(-))(2) in the gas phase and in chloroform we suggest that the high se lectivity of I for Ba2+ is due to (i) the formation of the complex with an "optimal" pseudocavity for Ba2+, in which six donor atoms of the ligand and four oxygens of the Pic(-) counter-ion coordinate to the cation, and, (ii) the smaller dehydration energy of Ba2+ compared to other alkaline earth ca tions. The relative free energies of extraction obtained from simulations o n the I . M2+(Pic(-))(2) complexes are in good agreement with the experimen tal data.