Orthophosphate binding to zinc(II) complexes of 1, 3-bis[bis(pyridin-2-ylmethyl)amino] propan-2-OL.

It is generally considered interesting that a dinuclear complex of Zinc(II) with 1,3-bis[bis(pyridin-2-ylmethyl)amino] propan-2-ol (= tphpn) (I-II) can bind to phosphate containing molecules (III): Although a number of equilibrium constants of the reactions of the H-1Zn2-tphpn3+ complex with various organic phosphate monoester and diester anions (ROPO32- and (RO)2PO2-) in aqueous solutions have been reported, it is peculiar that the ability of Zinc(II)-tphpn complexes to bind inorganic orthophosphate has not been considered. In order to compensate for this gap we have undertaken a systematic equilibrium investigation of the H+- Zinc(II)-tphpn-orthophosphate system by potentiometric p[H] measurements. As a matter of fact a pletora of chemical equilibria are established simultaneously in aqueous solutions containing Zinc(II), tphpn and phosphate. Only a few of these equilibria will eventually produce the Zinc(II)-tphpn-phosphate complexes which are the concern of this work. However, in order to ascertain the stoichiometry and equilibrium constants of the phosphate binding reactions, the whole net of equilibria in the Zinc(II)-tphpn-phosphate solutions must be districated. In the following we present an experimental strategy which allows, with a reduced number of self contained experiments, to collect all the equilibrium data necessary to cast light on the capture of orthophosphate by Zinc(II)-tphpn complexes. Probably this strategy can result very useful in general to alleviate the numerous problems and inconsistencies which arise during the investigation of intricate simultaneous equilibria. At the heart of our strategy are repeated scans of the p[H] of the investigated solution, which are made possible by in situ coulometric generation at a platinum electrode of strong acid and strong base. Then a solution can be titrated as many times as necessary from low to high and back from high to low p[H] while its volume keeps constant and the amount of added acid and base accurately known. The utility of repeated p[H] scans will be clear from the following.