Synthesis and Spectroscopic and Structural Studies of a New Cadmium(II)−Citrate Aqueous Complex. Potential Relevance to Cadmium(II)−Citrate Speciation and Links to Cadmium Toxicity

The presence of cadmium in the environment undoubtedly contributes to an increased risk of exposure and ultimate toxic influence on humans. In an effort to comprehend the chemical and biological interactions of Cd(II) with physiological ligands, like citric acid, we explored the requisite aqueous chemistry, which afforded the first aqueous Cd(II)−citrate complex [Cd(C6H6O7)(H2O)]n (1). Compound 1 was characterized by elemental analysis, and spectroscopically by FT-IR and 113Cd MAS NMR. Compound 1 crystallizes in the orthorhombic space group P212121, with a = 6.166(2) Å, b = 10.508(3) Å, c = 13.599(5) Å, V = 881.2(5) Å3, and Z = 4. The X-ray structure of 1 reveals the presence of octahedral Cd(II) ions bound to citrate ligands in a molecular crystal lattice. Citrate acts as a tridentate binder promoting coordination to one Cd(II) through the central alcoholic moiety, one terminal carboxylate group, and the central carboxylate group. In addition, the central carboxylate binds to three Cd(II) ions. Specifically, one of the oxygens of the central carboxylate serves as a bridge to two neighboring Cd(II) ions, while the other oxygen binds to a third Cd(II). A bound water molecule completes the coordination requirements of Cd(II). 113Cd MAS NMR studies project the spectroscopic signature of the nature of the coordination environment around Cd(II) in 1, thus corroborating the X-ray findings. Collectively, the data at hand are in line with past solution studies. The latter predict that other similar low molecular mass Cd(II)−citrate complexes may exist in the acidic pH region, thus influencing the uptake of cadmium by living (micro)organisms, their ability to metabolize organic substrates, and possibly Cd(II) toxicity.