Further Theoretical Studies of the Aquation of Chromium(III) Chloride Nutritional Supplement: Effect of pH and Solvation

Abstract

We have explored speciation of halochromium(III) complexes under physiological conditions involving both conjugate-base formation and halide-water exchange using DFT. Conjugate base formation is spontaneous under neutral and alkaline conditions. Several different pathways were explored for the water exchange reaction of cis- or trans-Cr(III) complexes in the series trans-[Cr(H2O)3(OH)X2]0 (X=Cl− or Br−) and cis- or trans-[Cr(H2O)4(OH)X]+, (X= F−, Cl−, Br−, or I−) including, associative interchange (Ia), dissociative interchange (Id) and dissociative (D) mechanisms. We have also investigated nucelophilic attack by OH− on [Cr(H2O)5X]+ species, (X=Cl− or Br−). The lowest overall activation enthalpies (ΔH≠) obtained for the F and Cl systems of [Cr(H2O)4(OH)X]+, are from the Ia pathway, whereas for the Br and I systems the Id pathway was found, consistent with experiment. The importance of outer sphere solvation was investigated with the addition of up to five explicit water molecules in the second coordination sphere. The ΔH≠ values for the aquation of cis-[Cr(H2O)4(OH)X]+…3H2O, are lower than the corresponding values for [Cr(H2O)5X]2+. This indicates that there will be greater speciation of chromium(III) halides under physiological conditions. These values also imply that chromium(III) derivatives that may be active in the treatment of physiological disorders such as Type(II) diabetes will be formed at different rates depending on the identity of the starting halide.