| Abstract
| - Reactions of Cd(II) with citric acid and with NaOH (pH∼2.5), pyridine (pH∼3), and ammonia (pH∼7), led to new complexes [Cd3(C6H5O7)2(H2O)5]·H2O (1) and (NH4)[Cd(C6H5O7)(H2O)]·H2O (2). Both species were characterized analytically, spectroscopically, and structurally. The collective physicochemical profiles of 1 and 2 emphasize the importance of the employed pH-dependent synthetic approaches, confirm the nature of Cd(II)−citrate species projected by solution studies, and shed light onto conditions under which soluble complexes may lead to bioavailability and potential Cd(II) toxicity.
- The involvement of Cd(II) in toxic manifestations and pathological aberrations in lower and higher organisms entailsinteractions with low and high molecular mass biological targets. To understand the relevant chemistry in aqueousmedia, we have launched pH-dependent synthetic efforts targeting Cd(II) with the physiological ligand citric acid.Reactions of Cd(II) with citric acid upon the addition of NaOH at pH 2.5 and pyridine at pH 3 and the addition ofammonia at pH ∼7 led to the new complexes [Cd3(C6H5O7)2(H2O)5]·H2O (1) and (NH4)[Cd(C6H5O7)(H2O)]·H2O (2),respectively. Complexes 1 and 2 were characterized by elemental analysis, spectroscopy (FT-IR and NMR), andX-ray crystallography. Complex 1 crystallizes in the monoclinic space group P21/n, with a = 18.035(6) Å, b =10.279(4) Å, c = 12.565(4) Å, β = 109.02(1)°, V = 2202(2) Å3, and Z = 4. Complex 2 crystallizes in the monoclinicspace group P21, with a = 9.686(4) Å, b = 8.484(4) Å, c = 7.035(3) Å, β = 110.28(1)°, V = 542.3(4) Å3, andZ = 2. Complex 1 is a trinuclear assembly with the citrate ligand securing a stable metallacyclic ring around oneCd(II), with the terminal carboxylates spanning into the coordination sphere of two nearby Cd(II) ions. Complex 2contains mononuclear units of Cd(II) bound by citrate in an overall coordination number of 8. In both 1 and 2, theparticipating citrates exhibit three different modes of coordination, thus projecting a distinct yet variable aqueousstructural chemistry of Cd(II) with physiological substrates. The pH-dependent chemistry and its apparent structuraldiversity validate past solution speciation studies, projecting the existence of mononuclear species such as the onein the anion of 2. The spectroscopic and structural properties of 2 emphasize the significance of the informationemerging from synthetic studies that otherwise would not have been revealed through conventional solution studies,while concurrently shedding light onto the linkage of the requisite chemistry with the potential biological toxicity ofCd(II).
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