Cd(II) complexes with phthalic acids: solution study and crystal structure of cadmium(II) phthalate hydrate

Abstract

The acid–base properties of phthalic acid (H2L) have been determined by simultaneous spectroscopic, potentiometric, and polarographic measurements. Stability constants of the complexes CdHL+, CdL, CdL22−, CdL34−, CdH−2L24− and their distribution versus pH were determined at 25 °C and ionic strength in 0.1 M NaNO3. The advantages of each method are pointed out in such a way as to describe more precisely the interactions between cadmium and phthalic acid over a wide range of ligand and metal concentrations. The crystal structure of the [CdL] complex has been established by single-crystal X-ray diffraction. Cd(C8H4O4)•H2O is orthorhombic, space group Pbca, a = 23.463(5), b = 9.293(2), c = 7.730(1) Å, Z = 8, and ρ = 2.323 g cm−3. The structure was refined to a final R value of 0.054 for 3231 unique reflections with I > 3σ(I). The crystal is a three-dimensional polymeric network in which the Cd atom is seven-coordinated. The first ligand coordinates in a bidentate carboxylate (O,O′) mode (Cd—O = 2.372(5) and 2.445(5) Å); a second ligand is part of a seven-membered ring formed with its two carboxylate groups (Cd—O = 2.317(5) and 2.341(5) Å); two other phthalic ligands make one coordination bond (Cd—O = 2.255(5) and 2.449(5) Å) while the seventh bond is a water molecule (Cd—O = 2.333(6) Å). Key words: crystal structure, cadmium complexes, phthalic acid complexes.