Synthesis, spectroscopic, and structural characterization of the first aqueous cobalt(II)-citrate complex: toward a potentially bioavailable form of cobalt in biologically relevant fluids

Citric acid represents a class of carboxylic acids present in biological fl uids and playing key roles in biochemical processes in bacteria and humans. Its ability to promote diverse coordination chemistries in aqueous media, in the presence of metal ions known to act as trace elements in human metab olism, earmarks its involvement in a number of physiological functions. Cob alt is known to be a central element of metabolically important biomolecule s, such as B-12, and therefore its biospeciation in biological fluids const itutes a theme worthy of chemical and biological perusal. In an effort to u nravel the aqueous chemistry of cobalt in the presence of a physiologically relevant ligand, citrate, the first aqueous, soluble, mononuclear complex has been synthesized and isolated from reaction mixtures containing Co(II) and citrate in a 1:2 molar ratio at pH similar to 8. The crystalline compou nd (NH4)(4)[CO(C6H5O7)(2)] (1) has been characterized spectroscopically (UV /vis, EPR) and crystallographically. Its X-ray structure consists of a dist orted octahedral anion with two citrate ligands fulfilling the coordination requirements of the Co(II) ion. The magnetic susceptibility measurements o f 1 in the range from 6 to 295 K are consistent with a high-spin complex co ntaining Co(II) with a ground state S=3/2. Corroborating this result is the EPR spectrum of 1, which shows a signal consistent with the presence of a Co(II) system. The spectroscopic and structural properties of the complex s ignify its potential biological relevance and participation in speciation p atterns arising under conditions consistent with those employed for its syn thesis and isolation.