Switching Ca2+/Ba2+ to Ba2+/Ca2+ potentiometric selectivities of podands with phosphoryl-containing terminal groups:: A molecular modelling study

It has been shown experimentally that the Ca2+/Ba2+ potentiometric selectivity of phosphoryl-containing podand R-O-(CH2-CH2-O)(n)-R, R = -C6H4-P(O)Ph-2, n = 3 (I), switches to Ba2+/Ca2+ when the ligand contains the longer polyether chain, n = 5 (II). Here, we report molecular dynamics and free energy perturbation simulations performed using the amber 4.1 program on the complexes (LM2+)-M-. (M2+ = Ca2+, Sr2+ and Ba2+, L = I and II) in the gas phase in order to gain a microscopic insight into structural and energy binding properties of podands as a function of n. Mixed QM/MM (PM3/AMBER) calculations were performed to analyse the role of polarisation effects on the complexation selectivity of podands. It is shown that an increase of n does not affect the interactions of M2+ with phosphine oxide groups, but leads to less efficient interactions of small cations with the polyether chain. Calculated potentiometric selectivities of I (Ca2+ > Ba2+) and II (Ba2+ > Ca2+) are in agreement with the experimental data.