Solid-State Structural Studies of Chromium(III) Nicotinato Nutritional Supplements

While Cr­(III) dietary supplements are widely consumed, some commercial supplements have yet to be structurally characterized. X-ray absorption spectroscopy and other spectroscopic methods were used to characterize Cr­(III) nicotinato nutritional supplements that have long been used in complementary medicine. Different ratios of nicotinic acid and CrCl₃·6H₂O (<i>trans</i>-[CrCl₂(OH₂)₄]­Cl·2H₂O) at different pH values gave a range of products. The local structures of Cr­(III) nicotinato complexes obtained at pH 7 and of the patented complex were characterized by performing multiple-scattering analysis of their EXAFS spectra as well as EPR, UV–vis, and IR spectroscopies. For the first time, these complexes have been definitively characterized as nicotinato-bridged polymers of dihydroxido-bridged dinuclear Cr­(III) cores. In the patented complex used in commercial preparations, each Cr is octahedral with an additional terminal O-bound nicotinato ligand, two bridging nicotinato (one O and one N bound), and an aqua ligand. The other species also have two or three bridging nicotinato ligands and an aqua and, in some cases, a terminal hydroxido ligand, which is dependent upon the stoichiometry of the reactants and the pH value of the solution in which they are prepared