The effects of pH values on the formations of water soluble calcium 1,3-propylenediaminetetraacetates

Abstract

A series of pH dependent water soluble calcium 1,3-propylenediaminetetraacetates [Ca(1,3-H4pdta)(NO3)2(H2O)2]n (1), [Ca(1,3-H2pdta)]n•nH2O (2), K4[Ca2(1,3-pdta)2]•2H2O•2CH3OH (3) and Ca2(H2O)6[(Ca2(1,3-pdta)2(H2O)2]•4H2O (4) have been obtained from the direct reactions of calcium salts with 1,3-propylenediaminetetraacetic acid in different pH values and solvents respectively (1,3-H4pdta = 1,3-propylenediaminetetraacetic acid). 1 is composed of one-dimensional chains bridged by 1,3-H4pdta, where only two strong coordination sites are offered by 1,3-H4pdta ligands. While complex 2 features a three-dimensional (3D) structural motif and 3 forms a 2D structure. 4 also contains one-dimensional chain constructed by dimeric calcium 1,3-propylenediaminetetraacetates. Solution 13 C NMR spectra show that dimeric units in 3 dissociate into monomeric species in solution, which indicates that monomeric chelate is a dominant species in neutral and weak basic solution. It is notable that an obvious downfield shift (Δδ = 2.09 ppm) is observed for methanol in 3 that differs from methanol in D2O solvent.