The pH-dependent binding of zinc citrate to bipy/phen (bipy = 2,2-bipyridine, phen = 1,10-phenanthroline)

The crystallizations of citrate-metal complexes with diverse structures are greatly affected by the deprotonated states of the citrate ion, counter cations and chemical nature of the metal ions. Herein we studied the ternary system Zn-cit-L2 (cit = citrate acid, L2 = 2,2-bipyridine (bipy) or 1,10-phenanthroline (phen)) and obtained four novel compounds Zn2(H2cit)2(bipy)2·H2O (1), Zn2(H2cit)2(phen)2·1.5H2O (2), Zn2(Hcit)(bipy)Cl (3), and Zn2(Hcit)(phen)Cl·H2O (4). All the four compounds were characterized by elemental analysis, FT-IR spectroscopy, and X-ray crystallography. Complexes 1 and 2 with the citrate ion doubly deprotonated are dinuclear species isolated in the Zn-cit system. Complexes 3 and 4 exhibit 1D chains in which the metal ions are connected by the triply deprotonated citrate. It shows that the final solid-state species is much dependent on the pH value of the solution. That is the dinuclear compounds of 1 and 2 are precipitated at pH 3–4 whereas the 1D chain of 3 and 4 at a lower acidity of pH 5–6.