| Abstract
| - The presence of cadmium in the environment undoubtedly contributes to an increased risk of exposure and ultimatetoxic influence on humans. In an effort to comprehend the chemical and biological interactions of Cd(II) withphysiological ligands, like citric acid, we explored the requisite aqueous chemistry, which afforded the first aqueousCd(II)−citrate complex [Cd(C6H6O7)(H2O)]n (1). Compound 1 was characterized by elemental analysis, andspectroscopically 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 1reveals the presence of octahedral Cd(II) ions bound to citrate ligands in a molecular crystal lattice. Citrate acts asa tridentate binder promoting coordination to one Cd(II) through the central alcoholic moiety, one terminal carboxylategroup, 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 otheroxygen binds to a third Cd(II). A bound water molecule completes the coordination requirements of Cd(II). 113CdMAS 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. Thelatter predict that other similar low molecular mass Cd(II)−citrate complexes may exist in the acidic pH region, thusinfluencing the uptake of cadmium by living (micro)organisms, their ability to metabolize organic substrates, andpossibly Cd(II) toxicity.
- A reaction of Cd(NO3)2·4H2O with citrate in a stoichiometric 1:2 ratio in water, at acidic pH, afforded the first aqueous Cd(II)−citrate complex, [Cd(C6H6O7)(H2O)]n(1). Compound 1 was characterized analytically, spectroscopically, and structurally. The X-ray structure of complex 1 in corroboration with 113Cd MAS NMR spectroscopy reveals a distinct coordination mode of citrate binding around octahedral Cd(II). The coordination properties of the tricarboxylate citrate in synergy with the geometry it imposes on Cd(II) in 1 may indicate variably dictated structural requirements for cadmium−citrate species potentially related to Cd(II) speciation in biological media and its toxic manifestations.
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