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 (trans-[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.