Redox and ligand-exchange chemistry of chromium(vi/v)-methyl glycoside systems

Abstract

In order to establish a general pattern for the redox and coordination chemistry of glycosides with Cr(VI) and Cr(V), reactions of a series of methyl glycosides with Cr(VI) and Cr(V) have been studied at different acidities. Oxidations of methyl α- and β-D-glucopyranoside (Glc1Me), methyl α- and β-D-mannopyranoside (Man1Me), methyl α- and β-D-galactopyranoside (Gal1Me) and methyl α- and β-D-ribofuranoside (Rib1Me) by Cr(VI) proceed rapidly at pH ≤ 1, and yield Cr(III) and methyl glycofuranuronolactone as final products when an excess of methyl glycoside over Cr(VI) is used. At constant [H⁺], the reaction follows the rate law −d[Cr(VI)]/dt = k_H [Gly1Me] [Cr(VI)]. Relative reactivities of methyl glycosides toward Cr(VI) reduction are: β-Rib1Me > α-Gal1Me > α-Rib1Me ≈ β-Gal1Me > β-Man1Me > α-Man1Me > α-Glc1Me > β-Glc1Me. This sequence is interpreted in terms of the degree of unfavorable steric interactions induced by the nonbonded 1,3-diaxial interactions in the respective Gly1Me-Cr(VI) monochelate, which is formed in rapid equilibrium that precedes the rate determining step. For all the glycosides, the oxidation rate decreases with an increase in pH value and becomes negligible at pH > 5. At pH 5.5 and 7.5, addition of an excess of α-Man1Me or α(β)-Gal1Me to an equimolar cysteine-Cr(VI) mixture, afforded two EPR triplets at g_iso1 1.9802 and g_iso2 1.9800/1 with A_iso 16.5(3) × 10⁻⁴ cm⁻¹ in a 50 ∶ 50 g_iso1 ∶ g_iso2 ratio. The EPR spectral parameters and the superhyperfine pattern of the signal are consistent with the presence of two geometric isomers of the [Cr^VO(cis-O³,O⁴-Gal1Me)₂]⁻ and [Cr^VO(cis-O²,O³-Man1Me)₂]⁻ complexes. The same final spectral pattern is observed at pH 7.5 for the ligand-exchange reaction of Man1Me and Gal1Me with [Cr^VO(ehba)₂]⁻ (ehba = 2-ethyl-2-hydroxybutanoato(2−)). No EPR signal is observed when an excess of Xil1Me or Glc1Me is added to an equimolar cysteine-Cr(VI) mixture. In the ligand-exchange reactions of [Cr^VO(ehba)₂]⁻ at pH 7.5 with Xil1Me or Glc1Me, a very low intensity EPR singlet is observed at g_iso 1.9799. These results show that only glycosides with one cis-diolato group (such as Man1Me and Gal1Me) are effective for stabilizing Cr(V) at pH 5.5 and 7.5. The high redox reactivity of methyl glycosides with Cr(V) at high [H⁺] is attributed to the formation of [Cr^VO(O,O-glycoside)(OH₂)₃]⁺ species (g_iso 1.9716), which are not observed at pH 5.5–7.5, where only the five-coordinate bis-chelate oxochromate(V) species are observed.