Synthesis and structure of the [MnIV(biguanide)3]4+ ion: the simplest source for water-stable manganese(iv)

Abstract

Aqueous oxidation of alkaline biguanide sulfate with KMnO 4 followed by crystallisation from 2 M HNO 3 yields the mononuclear [Mn(bigH) 3 ] 4+ ion, which has been characterised crystallographically and provides an easy route to mononuclear, water-stable manganese(IV).

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

“…1) is the only mononuclear water-soluble Mn IV complex that is quite stable over a wide range of acidity (10 À6 to 2m) and is structurally well characterized [1]. 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].…”

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

“…-In aqueous solution, quite a few oxo-bridged multinuclear Mn IV complexes are known to be stable enough for the successful study of their solution chemistry [1]. 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.…”

“…Additional interest in this study is due to the fact that the dinuclear Mn IV complex [Mn 2 IV (m-O) 2 (m-OAc)(H 2 O) 2 (bipy) 2 ] 3+ oxidizes nitrite probably by a two-electron transfer mechanism [9], whereas the mixed-valent [(L) 2 4+ complex, and the pK a (= 3.00 AE 0.10) [11] [12]for HNO 2 in H 2 O were used in calculating rate constants. For interpreting rate data in deuterated media, these acidity constants were determined at 25.08 in 95% D 2 O: pK a1 and pK a2 were found to be unchanged within experimental uncertainty, but pK a was increased to 3.80 AE 0.10.…”

“…The proposal for the two-electron step for Mn IV 2 reduction [9] ; bigH = biguanide = imidocarbonimidic diamide = C 2 N 5 H 7 ), was prepared by a known procedure [2]. Biguanide sulfate (C 2 N 5 H 7 · H 2 SO 4 ) was prepared by a known method reported previously [41].…”

“…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.…”