Proton assisted two-electron transfer from hydroxylammonium ion to chromium(VI) through oxygen bridges

Abstract

The reduction of Cr(VI) by NH3OH+ has been studied in the pH range 0.0-4.7 and has been found to proceed by two equivalent steps following formation of an O-bonded chromate ester. The ester undergoes internal oxidation and reduction involving acid catalysis. A striking inductive effect due to protonation of H2N0Cr03_ is observed. Reactive,chromium intermediates include Cr(IV) which disproportionates and Cr(V) which is reduced by NH3OH+ to Cr(III). A reactive nitrogen intermediate, HNO, is oxidized primarily to HN02. NH3OH+ and (8) D. A.

New Mononuclear Manganese(III) Complexes with Hexadentate (N4O2) Schiff Base Ligands: Synthesis, Crystal Structures, Electrochemistry, and Electron‐Transfer Reactivity towards Hydroxylamine

“…[6,12,13,17] The bands in the 510Ϫ540 nm range have previously been attributed to phenolato-to-manganese() LMCT for similar Mn III complexes. [8,14,15,17] The EHMO calculations, however, indicate that all the frontier MOs are predominantly metalbased with significant contribution from the ligands, so the above assignment is not unambiguous. From the X-ray crystal data, the observed Mn III ϪO(phenolate) average bond lengths are 1.873 Å for 1 and 1.871 Å for 4, which is quite short in comparison with the reported values for manganese() complexes.…”

“…[8,22,23] The stoichiometry was therefore examined very carefully in the presence both of excess reductant and of excess oxidant. In the presence of excess hydroxylamine over the complexes, the stoichiometry was determined by bromatometric estimation of the unchanged reductant, [25] whereas in the presence of excess oxidant, the stoichiometry was measured spectrophotometrically with the aid of the known extinction coefficient of the complex (λ max ϭ 517 nm, ε ϭ 820 dm Table 5 also imply a variable stoichiometry in our system.…”

“…The mechanistic versatility noted in the reaction behavior of hydroxylamine is attractive in that it can act both as an oxidant or a reductant, [7] and can coordinate through either the N-or the O-end. [8] The estimated bond hydroxylamine hydrochloride have been followed spectrophotometrically in the 5.5−8.0 pH range. Similar kinetic reactivities of all the complexes towards a particular species of the reductant were noted.…”

Kinetic and mechanistic studies on the oxidation of hydroxylamine by a tri-bridged manganese(iv,iv) dimer in weakly acidic media

“…Hydroxylamine, the presently chosen reductant, is useful for probing reaction mechanisms of inorganic complexes, and the mechanistic versatility noted in its reactions is attractive in the way that it acts both as an oxidant or a reductant 7 and can coordinate through either the N-or the O-end. 8 Furthermore, the estimated dissociation energy for the N-H bond of hydroxylamine (92 kcal mol Ϫ1 ) 9 and that of O-H bond of tyrosine (86.5 kcal mol Ϫ1 ) 10 are similar and it is interesting to examine whether hydroxylamine may take part in a proton coupled electron transfer, as tyrosine does in PS II.…”

Kinetics and mechanism of the oxidation of hydroxylamine by a {Mn3O4}4+ core in aqueous acidic media

“…Stepwise oneelectron transfer and retention of nuclearity as proposed above are supported by the fact that different oxidation levels of the binuclear catalase enzyme models preserve nuclearity, 41 the various oxidation levels (S states) of the manganese cluster in PS II also apparently retain bond connectivity 2,3b,5b and high valent manganese has a propensity to form (m-oxo) complexes in weakly acidic media. 42 55 EPR spectroscopy has established the formation of H 2 NO ∑ radical in the oxidation of NH 2 OH by one electron oxidants. 52 With strong oxidants such as Mn 3+ (aq) or Ag 2+ (aq), the HNO ∑ or H 2 NO ∑ radical is further oxidized to NO 3 -.…”