Kinetics and Mechanism of Oxidation of Fe²⁺ by the Tris(biguanide)manganese(IV) Ion in Aqueous Acid Media

Abstract

Tris(biguanide)manganese(IV), [Mn(LH2)3]4+ (LH2 = biguanide, C2N5H7), quantitatively oxidises Fe2+ to Fe3+ and is itself reduced to Mn2+ with almost quantitative (> 95%) release of biguanide. The reaction rate strongly depends on added Fe3+; in the presence of externally added Fe3+, the reaction shows a clear first‐order dependence in [MnIV], whereas in the absence of any added Fe3+, an initial quick loss of MnIV is associated with a subsequent very sluggish decay. Two consecutive one‐electron transfer inner‐sphere steps are proposed for the entire redox process where [Mn(LH2)3]3+, the initial one‐electron‐reduced product of MnIV, is believed to be a steady‐state intermediate. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

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

“…[1] In aqueous solutions the title Mn IV complex behaves as a mild dibasic acid (pK a1 ϭ 5.30 Ϯ 0.20, pK a2 ϭ 7.60 Ϯ 0.30) and the deprotonations are believed to originate from the protons bound to the ligand sp 2 nitrogen atoms. [2] Though quite a few di-, tri-and tetranuclear Mn IV complexes can be stabilised in aqueous solution [3] and have interesting solution chemistry, [4] the aqueous solution chemistry of mononuclear Mn IV remained unexplored until recently when we reported [2] the oxidation of Fe 2ϩ by Mn IV . Parallel to our report, Gould et al [5] observed that the oxidation of Fe 2ϩ by this Mn ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) species is noticeably accelerated by chloride ions in what the authors viewed as the accelerating effect of the polarisable anion on the redox reaction between two positively charged metal centres.…”

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

“…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.…”

“…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. In this contribution, we report the redox interaction between the authentic two-electron Mn IV 3 ] 4+ complex [6] and its conjugate base by mechanistically different electron-transfer processes, the latter reduction being associated with a proton coupling. Whether a proton transfer is an essential prerequisite for the Mn IV reduction by nitrogenous reducing agents is now investigated in the case of the reducing agent nitrite.…”

“…2). The Mn IV complex is a mild dibasic acid (pK a1 = 5.30 AE 0.20, pK a2 = 7.60 AE 0.30) [3], and in the pH range investigated, it suffers only a little or no deprotonation, however, the acid-base equilibrium of N III is of importance (pK a of HNO 2 = 3.00 AE 0.10 at 25.08, I = 1.0M) [11] [12]. Progressive generation of more NO À 2 may be a likely cause for the increased rate at higher pH.…”

“…In contrast, only one well-characterized mononuclear Mn IV complex has recently been synthesized from aqueous media [2], being stable in aqueous solution over a wide acidity range (10 À6 to 2M). 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].…”