Kinetics of oxidation of hydrogen peroxide and ascorbic acid by a tribridged manganese(IV,IV) dimer in feebly acidic media

Abstract

In weakly acidic, aqueous buffer (MeCO2-+ bipy), the complex ion [Mn2IV(μ-O)2(μ-MeCO2)(bipy)2(H2O)2]3+, 1 (bipy = 2,2prime-bipyridine), coexists in rapid equilibrium with its hydrolytic derivatives, [Mn2IV(μ-O)2(bipy)2(H2O)4]4+, 2, and [Mn2IV(μ-O)2(μ-MeCO2)(bipy)(H2O)4]3+, 3. The solution quantitatively oxidizes hydrogen peroxide to oxygen and ascorbic acid to dehydroascorbic acid, itself being reduced to MnII. In the presence of excess reductant, the reactions follow simple first-order kinetics with no evidence for the accumulation of a significant amount of any intermediate manganese complex. The ascorbate anion shows overwhelming kinetic dominance over ascorbic acid, but no evidence is available for deprotonation of hydrogen peroxide. The preferred intimate mechanism for hydrogen peroxide is inner sphere but that for ascorbic acid is uncertain. For both reductants, increased extent of aquation leads to increased kinetic activity in the order: 1 < 2 < 3.Key words: kinetics, manganese, ascorbic acid, hydrogen peroxide, 2,2prime-bipyridine.

Mechanistic Study of the Reactions of l-ascorbic Acid and Oxalic Acid with an Octahedral Manganese(IV) Complex of 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane

“…The redox reactions of Mn III L ) were slower than that of Mn IV L. This was similar to what was observed in Mn IV L+S IV [1] and in contrast with the redox reactions of trisbiguanidemanganese (IV) with glyoxalic acid and pyruvic acid [19] and a tribridged manganese (IV,IV) dimer [Mn IV 2 ðl À OÞ 2 ðl À MeCO 2 ÞðOH 2 Þ 2 (bipy) 2 ] 3+ with ascorbic acid [5] for which rate limiting one electron transfer between the redox partners with 1:1 stoichiometry was reported. The fast disproportionation of the radical, Asc AE) to Asc 2) and dehydroascorbic acid {k (dm 3 mol )1 s )1 )=2.4Â10 5 [20], 1.4Â10 5 [21] 25°C, I=0.1 mol dm )3 } and very low pK of its protonated form (HAsc AE =Asc AE) +H + , pK=)0.45±0.1) [22] in addition to the fast electron exchange for the couple HAsc ) /HAsc AE (k=10 6 dm 3 mol )1 s )1 ) [20] leaves no doubt that the reactions of the radicals HAsc AE , and Asc AE) with Mn IV/III L will have no significant effects on the rate limiting electron transfer processes in the present case.…”

“…Our objectives were to examine (i) the complexing abilities of H 2 Asc/HAsc ) and H 2 OX/HOX ) /OX 2) with Mn IV L, (ii) whether electron transfer between Mn IV L and these reductants occur in sequential steps generating the Mn III L intermediate, as was observed for S IV , or via a single step involving direct two electron transfer, and (iii) how does the Mn III L intermediate, if formed, respond to complexation/electron transfer with these two reductants. Some redox reactions of Mn IV /Mn III complexes by L-ascorbic acid have been reported [5,8]. However, the reactions of mononuclear octahedral Mn IV /Mn III complex investigated in this work have not been reported previously to the best of our knowledge.…”

“…The unreacted ascorbic acid was then determined iodimetrically using starch indicator [5]. The iodine solution was standardized against sodium thiosulphate which was standardized using potassium iodate [15].…”

“…The pK (=2.06) of (Mn IV L AE H 2 OX) (see Table 2) clearly shows that oxalic acid in the adduct is a weaker acid than in the free state (pK H 2 OX 1 ¼ 1:04) [18]. The same appears to be true for (Mn IV L AE H 2 Asc) as no reliable value of its pK ( ‡ 5) could be determined from the kinetic data (see Table 1 [23,24], [Ni III L] 2+ and [Ni IV L 2 ¢] 2+ {HL=15-amino-3-methyl-4,7,10,13-tetraazapentadec-3-en-2-one oxime, HL¢=6-amino-3-methyl-4-azahex-3-en-2-one oxime} [10], [( Mn IV 2 l À OÞ 2 (l-Me-CO 2 )(bipy) 2 [5], most of which possess potential hydrogen bonding sites, have not been reported. However, diffusion controlled inner sphere complex formation by H 2 Asc preceding slow electron transfer has been reported in its reactions with Fe III (aq) [11a], V V (aq) [25], N,N¢-ethylenebis(salicylideneiminato)M III (M=Co III [7], Mn III [8]), and [Fe III (LH)] 2+ {H 2 L=(1,2)bis(2-hydroxybenzamidoethane) [4a], (1,3)-bis(2-hydroxybenzamidopropane [4b]} in which the coordination spheres of the central metal ions have replaceable ligands (H 2 O).…”

26 Mechanisms of reactions in solution

“…Superoxide dismutation catalysed by a copper complex has also been reported. 301 The kinetics have been studied of H 2 O 2 catalase-like decomposition catalysed by dinuclear manganese(III) 302 and dinuclear tribridged manganese(IV) 303 complexes, by aqueous Fe III at pH 1^3, 304 by [(tpma)(H 2 O)Fe(m-O)Fe(OH)(tpma)] 3 in MeCN, 305 by iron complexes of macrocyclic polyaza ligands, 306 by [{Fe(H 2 O)} 2 (m-OH)(tbpo)] 4 307 and by wild-type and mutant forms of sperm-whale myoglobin. 308 Further discussion of the Fenton reaction, 309 its relevance to waste-water treatment 310,311 and comment on related processes involving alkyl hydroperoxides 312 318 and of the catalysis by Mn III , 319 V V , 320 and W VI , 321,322 325 and further studied 326^332 by Espenson who, with others, 333^337 has described studies of the conversion of SiR 3 H into SiR 3 (OH) 326 and the effect of con¢nement in the helical channels of a urea^H 2 O 2 adduct.…”