The reduction of CrVI to CrIII by the alpha and beta anomers of D-glucose in dimethyl sulfoxide. A comparative kinetic and mechanistic study.

The reduction of CrVI by alpha-D-glucose and beta-D-glucose was studied in dimethyl sulfoxide in the presence of pyridinium p-toluensulfonate, a medium where mutarotation is slower than the redox reaction. The two anomers reduce CrVI by formation of an intermediate CrVI ester precursor of the slow redox step. The equilibrium constant for the formation of the intermediate chromic ester and the rate of the redox steps are different for each anomer. alpha-D-Glucose forms the CrVI-Glc ester with a higher equilibrium constant than beta-D-glucose, but the electron transfer within this complex is slower than for the beta anomer. The difference is attributed to the better chelating ability of the 1,2-cis-diolate moiety of the alpha anomer. The CrV species, generated in the reaction mixture, reacts with the two anomers at a rate comparable with that of CrVI. The EPR spectra show that the alpha anomer forms several linkage isomers of the five-coordinate CrV bis-chelate, while beta-D-glucose affords a mixture of six-coordinate CrV monochelate and five-coordinate CrV bis-chelate. The conversion of the CrV mono- to bis-chelate is discussed in terms of the ability of the 1,2-cis- versus 1,2-trans-diolate moieties of the glucose anomers to bind CrV.

Catalysis and kinetics of hydrogen peroxide disproportionation by dinuclear manganese(III) complexes of 1,5-bis(salicylidenamino)pentan-3-ol and the 5-bromophenyl-substituted derivative.

The dinuclear MnIII complex [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br has been prepared and its structure and reactivity toward H2O2 studied in comparison with [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]Br (salpent-OH = 1,5-bis(salicylidenamino)pentan-3-ol and 5-Br-salpentOH = 1,5-bis(5-bromesalicylidenaminopentan-3-ol). The X-ray diffraction analysis of [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br (monoclinic, P21/n, a = 13.081(2) A, b = 13.429(2) A, c = 17.375(2) A, beta = 102.31(1) degrees, V = 2982.0 A3, Z = 4) revealed a mu-alkoxo, mu-acetatodimanganese(III) core with a Mn...Mn separation of 2.932(1) A. The ligand lies in the meridional plane, and the sixth coordination position of each manganese atom is occupied by a methanol molecule providing two substitution-labile sites in the cis position. The two complexes showed catalytic activity toward disproportionation of H2O2 in methanol and dimethylformamide in the 0-25 degrees C temperature range. The initial rate of oxygen evolution in the presence of [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br or [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]-Br is first order in catalyst concentration. The two complexes show saturation kinetics in methanol, with the higher kcat = 0.98 s-1 and kcat/KM = 70 M-1 s-1 observed for [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]Br.