Mechanistic Investigation of the Oxidation of Glyoxylic and Pyruvic Acids by Tris(biguanide)manganese(IV) in Weakly Acidic Aqueous Media

Abstract

In this study the kinetics of two electron transactions between the reducing substrates glyoxylic and pyruvic acids and the oxidising complex cation, tris(biguanide)manganese(IV) in aqueous acidic media have been examined. Under the experimental conditions (pH = 1.50−3.78) only the fully protonated form of the oxidant reacts with the anions of the substrates whereas the parent acids, viz. glyoxylic and pyruvic acids, were found to be unreactive. The second-order

Kinetic and Mechanistic Studies on the Oxidation of Nitrogen(III) (HNO2/$\rm{{{NO}}_2^ - }$) by the Tris(biguanide)manganese(IV) Ion in Aqueous Acidic Media

“…Electrostriction [27][28][29][30], or some kind of it, aided by Hbonding [31] [32] involving the free amino or imino groups of the ligand with the reducing agents, is thus a finite possibility. Such interactions were previously proposed for this Mn IV oxidant with substrates like Fe 2+ [3], In I [5], and glyoxylic and pyruvic acids [4]. It may be noted here that a strong pre-equilibrium adduct formation between Mn …”

“…The paucity of available aqueous-chemistry data of mononuclear Mn IV complexes, which are key species in photosystem II (PS II), is alarming; until recently, only a few kinetic studies on the reduction of the tris(biguanide)manganese(IV) ion ([Mn(bigH) 3 ] 4+ , bigH = biguanide, C 2 N 5 H 7 ; see Fig. 1) in aqueous solution have been published [3][4][5][6]. Solution studies of mononuclear Mn IV complexes, the basic unit of the tetrameric manganese cluster of the oxygen-evolving complex (OEC) of PS II [7], are of potential importance for a better understanding of the rather unexplored polynuclear higher-valent manganese chemistry.…”

“…For reactions in D 2 O, the pD was calculated as pD = pH + 0.40 [47]. Nitric acid (99 atom-% D; Aldrich) and solid NaOH were dissolved in D 2 4 ) was maintained in all the kinetic runs. Electrochemical measurements were recorded with an electrochemical analyzer (model 600A; CH Instruments, USA) by using a three-component electrode system consisting of Pt-wire working and counter electrodes and an Ag/AgCl reference electrode.…”

Mechanistic Studies on the Oxidation of Hydrazine by Tris(biguanide)manganese(IV) in Aqueous Acidic Media

“…Yet, a purely outer-sphere ( À 1.17 V) 2 ) is thermodynamically very unfavorable. This highly unfavorable potential is the stumbling block for N 2 H 5 entering into an outer-sphere reaction, particularly when the oxidant is not so powerful, like the present Mn IV oxidant [4]. Another barrier for an outer-sphere electron transfer is the small selfexchange rate ( 0.3m s À1 ) [15] for the one-electron hydrazine redox couple.…”

“…In aqueous solution, complex 1 behaves as a weak dibasic acid (pK a1 5.30 AE 0.20, pK a2 7.60 AE 0.30), and the deprotonations arise from the protons bound to the sp 2 N-atoms of the ligand [2]. The aqueous chemistry of mononuclear Mn IV remained uncovered until recently oxidation kinetics of metal ions like Fe 2 [2] [3], In [3], and a-keto acids like glyoxylic and pyruvic acids [4] were published.…”