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

Abstract

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.

Investigation of the Spectroscopic, Thermal and Electrochemical Properties of Tris-(glycinato)chromium(III)

“…The weak spin‐forbidden transition 4 A 2g → 2 E g is observed at 694 nm, which is close to the value reported by Bryan et al . The ligand field splitting and Racah parameter ( B =(2 ν 1 2 + ν 2 2 −3 ν 1 ν 2 )/(15 ν 2 −27 ν 1 ))−, obtained from the spectrum in DMSO are 19.3 × 10 3 cm −1 and 575 cm −1 , respectively. Although the ligand field splitting value in solution is consistent with an (RCO 2 ) 3 (RNH 2 ) 3 environment around Cr(III), there is clearly a decrease in ▵ o from solid to solution.…”

“…This signal corresponds with the (CO 2 ) 3 (NH 2 ) 3 coordination geometry identified in the molecular structure determined from single‐crystal XRD by Bryan et al . Very similar values are obtained in the frozen state at 100 K in DMSO (1.9747) and in H 2 O (1.9787) (Figure b), which suggests that there is no change in coordination environment and is consistent with other six‐coordinate octahedral Cr(III) complexes For example, Nguyen Pham et al .…”

“…Very similar values are obtained in the frozen state at 100 K in DMSO (1.9747) and in H 2 O (1.9787) (Figure b), which suggests that there is no change in coordination environment and is consistent with other six‐coordinate octahedral Cr(III) complexes For example, Nguyen Pham et al . recently reported the EPR spectrum of the closely related Cr(III) nicotinato complex, which shows a broad signal with g ≈ 1.97−1.99, also consistent with octahedral geometry.…”

A novel ion-imprinted polymeric sorbent for separation and determination of chromium(III) species in wastewater

“…Both the formula and structure of the complexes depend on the molar ratio of the reagents (Cr to nicotinic acid) and on the conditions (temperature, acidity) used during the synthesis reaction. 41 In this paper, the Cr(III) nicotinate complex (Cr(nic) 42,43 was imprinted into the polymeric structure of IIP by noncovalent bonds.…”

“…The Cr(III)-nicotinate complex was prepared according to a procedure described elsewhere. 42,43 A hot solution of Cr(III) ions (2.5 mL, 1.16 mmol Cr(III) as CrCl 3 ) was added to a hot solution of nicotinic acid (2.5 mL, 3.51 mmol). The mixture was adjusted to pH 4 with a diluted solution of NaOH and stirred for 120 min at 80 • C. This way a grey-blue complex with a molar ratio of Cr to nicotinate equal to 1:2 was formed.…”