Mabel Santoro

Interaction of divalent metal ions withd-gluconic acid in the solid phase and aqueous solution

Abstract The interaction of d -gluconic acid with MnII, CoII, NiII, CuII, CdII, HgII and PbII has been investigated. Compounds of the type Mn( d -gluconate)2·3H2O, Co( d -gluconate)2·3H2O, Ni( d -gluconate)2·3H2O, Cu( d -gluconate)2·3H2O, Cd( d -gluconate)2, Hg( d -gluconate)OH and Pb( d -gluconate)2 have been isolated. These metal-sugar salts were characterized by elemental, thermogravimetric analyses and FT-IR, UV-vis absorption, EPR and13C NMR spectroscopies. In addition, a quantitative study of the equilibria involved in the interaction of this sugar and the above mentioned metal ions in an aqueous medium was carried out by means of potentiometric measurements of the hydrogen ion concentrations at 20°C andμ = 0.100M (NaNO3). On the basis of spectroscopic studies, possible structures of these complex species were discussed.

Kinetics and mechanism of the oxidation of D-galactono-1,4-lactone by CrVI and CrV

Abstract The oxidation of d -galactono-1,4-lactone by CrVI yields d -lyxonic acid, carbon dioxide and Cr3+ as final products when an excess of sugar acid over CrVI is used. The redox reaction occurs through CrVI → CrIII and CrVI → CrV → CrIII paths. The complete rate law for the CrVI oxidation reaction is expressed by −d [CrVI] \dt = (k0+kH [H+] ) [gal] [CrVI] , where k0 = (31±3) ×10−4 M−1 s−1 and kH = (99±5) ×10−4 M−2 s−1, at 40°C. CrV is formed in a rapid step by reaction of the CO·− 2 radical with CrVI. CrV reacts with the substrate faster than does CrVI. The CrV oxidation follows the rate law : −d [CrV] \dt = ( k ′ 0 +k ′ H [H+] ) [gal] , where k ′ 0 = (15±2) ×10−3 M−1 s−1 and k ′ H = (34±4) ×10−3 M−2 s−1, at 40°C. The EPR spectra show that several intermediate [Cr (O) (gala) 2] − linkage isomers are formed in rapid pre-equilibria before the redox steps.

Interaction of zinc(II) ion withd-aldonic acids in the crystalline solid and aqueous solution

Abstract The interaction of zinc(II) ion with d -glucoheptonic acid, d -gluconic acid, d -gulonic acid, d -galactonic acid and d -ribonic acid has been investigated and compounds of the type Zn( d -glucoheptonate)2·3H2O, Zn( d -gluconate)2·3H2O, Zn( d -gulonate)2·3H2O, Zn( d -galactonate)2·3H2O and Zn( d -ribonate)2·H2O, have been isolated. These metal-sugar salts were characterized by elemental analysis, FT IR spectroscopy, thermogravimetric analysis and13C-NMR. Spectroscopic measurements showed similar patterns between these complexes and the structurally identified Mn( d -gluconate)2·2H2O. The zinc(II) ion is binding to two ligand molecules through the car☐ylate and OH groups of each sugar, as well as to water molecules. The potentiometric measurements in aqueous solutions for the systems formed by the sugar acids investigated and the zinc(II) ion at different metal-ligand ratios showed the 1:1 complexes formation. On the basis of the13C NMR, the participation of C-1 and C-2 in this complex formation was verfiied. Due to the hydroxide precipitation, quantitative evaluation of the stability constants was not performed.

Kinetics and Mechanism of the Reduction of Chromium(VI) and Chromium(V) byD-Glucitol andD-Mannitol

The oxidation of D-glucitol and D-mannitol by CrVI yields the aldonic acid (and/or the aldonolactone) and CrIII as final products when an excess of alditol over CrVI is used. The redox reaction occurs through a CrVICrVCrIII path, the CrVICrV reduction being the slow redox step. The complete rate laws for the redox reactions are expressed by: a) −d[CrVI]/dt  {kM2 H [H+]2+kMH [H+]}[mannitol][CrVI], where kM2 H  (6.7±0.3)⋅10 M s−1 and kMH  (9±2)⋅10 M s−1; b) −d[CrVI]/dt  {kG2 H [H+]2+kGH [H+]}[glucitol][CrVI], where kG2 H  (8.5±0.2)⋅10 M s−1 and kGH  (1.8±0.1)⋅10 M s−1, at 33°. The slow redox steps are preceded by the formation of a CrVI oxy ester with λmax 371 nm, at pH 4.5. In acid medium, intermediate CrV reacts with the substrate faster than CrVI does. The EPR spectra show that five- and six-coordinate oxo-CrV intermediates are formed, with the alditol or the aldonic acid acting as bidentate ligands. Pentacoordinate oxo-CrV species are present at any [H+], whereas hexacoordinate ones are observed only at pH<2 and become the dominant species under stronger acidic conditions where rapid decomposition to the redox products occurs. At higher pH, where hexacoordinate oxo-CrV species are not observed, CrV complexes are stable enough to remain in solution for several days to months.

The EPR pattern of [CrO(cis-1,2-cyclopentanediolato)2]- and [CrO(trans-1,2-cyclopentanediolato)2]-

The addition of a large excess of 1,2-cyclopentanediol to a 1:1 mixture of glutathione and CrVI at pH 7.5 stabilises the intermediate CrV species formed by the one-electron reduction of CrVI by glutathione. The isotropic EPR parameters (giso and Aiso) of the CrV species formed with both cis- and trans-1,2-cyclopentanediol correspond to those calculated for five-coordinate oxo-CrV complexes with four alcoholato donors [Cr(O)(1,2-cyclopentanediolato)2]−. The five-coordinate oxo-CrV species formed with both 1,2-cyclopentanediol isomers show very similar EPR superhyperfine patterns, but differ in their stability and the conditions required for their formation due to the different chelation ability of the cis- vs. trans-1,2-diolato moiety.

THE INTERACTION OF D-GALACTONIC ACID WITH CRVI AND CRIII. STRUCTURE, STABILITY AND PHYSICAL PROPERTIES OF CRIII-ALDONATE COMPLEXES

The redox reaction between D-galactonic acid and potassium chromate yields ((lyxonateH−1)(galactonateH−1)Cr(OH2))K·H2On, with both aldonate molecules acting as bidentate ligands with the carboxylate and one alkoxo function as the donor sites. The shift of the CO2poststaggered− stretching vibration towards lower frequencies upon coordination and the high value of Δv indicate that the carboxylate acts as a monodentate donor site. Magnetic susceptibility data for the compound in the temperature range 3–300 K exhibit a drop in the effective magnetic moment with temperature below 70 K, which is indicative of antiferromagnetic interactions between the CrIII centres. The molar magnetic susceptibility versus temperature plot could be fitted with the Fisher Hamiltonian for the case of infinite chains, equation-modified for the presence of monomeric species. The EPR and UV-Vis spectroscopic studies reveal that, in solution, the complex retains the distorted octahedral local coordination geometry. The ((lyxonateH−1)(galactonateH−1)Cr(OH2))Kn dissociates slowly in aqueous solution but faster at high [H+], because of the rapid protonation of the alkoxo bridges linking the monomeric units. The potentiometric evaluation of the closely related binary system CrIII-d-galactonate shows that the (Cr(galactonateH−n)2)1 − 2n complexes are the major species in the 4–12 pH range, when a 1:2 CrIII:ligand ratio is used. 13C NMR reveals that theCO2poststaggered− group is one of the coordination sites of the ligand.

Redox and ligand-exchange chemistry of chromium(VI/V)-methyl glycoside systems

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 = kH [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 giso1 1.9802 and giso2 1.9800/1 with Aiso 16.5(3) × 10−4 cm−1 in a 50 ∶ 50 giso1 ∶ giso2 ratio. The EPR spectral parameters and the superhyperfine pattern of the signal are consistent with the presence of two geometric isomers of the [CrVO(cis-O3,O4-Gal1Me)2]− and [CrVO(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 [CrVO(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 [CrVO(ehba)2]− at pH 7.5 with Xil1Me or Glc1Me, a very low intensity EPR singlet is observed at giso 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 [CrVO(O,O-glycoside)(OH2)3]+ species (giso 1.9716), which are not observed at pH 5.5–7.5, where only the five-coordinate bis-chelate oxochromate(V) species are observed.