J. Indian Chem. Soc., Vol. 85, December 2008, pp. 1191-1198 Kinetics and mechanism of oxidation of quinol, ascorbic acid and hydrogen peroxide by [Mn 2 1 V ion t Piyali De, Amit Mondal, Dhurjati P. Kumar, S. Mukhopadhyay, R. Banerjee* and Dilip Majia Department of Chemistry, Jadavpur University, Kolkata-700 032, India E-mail : rupenju@yahoo.com aoepartment of Chemistry, Presidency College, Kolkata-700 071, India Manuscript received 2 September 2008, accepted 1 October 2008 Abstract : The oxo-bridged dimanganese(JV,IV) complex, [Mn 2 1 V(J.L-0) 2 (phen) 4 ]4+, {1} 4 + (phen = 1,10-phenanthroline) forms metastable solutions in dilute nitric acid media, where its UV-Vis spectra do not degrade for -20 min. This solution quantitatively oxidizes ascorbic acid, quinol and hydrogen peroxide to dehydroascorbic acid, quinone and oxygen respectively. In general, each mole of the oxidant consumes two moles of reducing agent (H 2 R), but stoichiometry depends on initial [H 2 0 2 ]. Under the first order conditions, maintained with excess (H 2 R), a two-step irreversible consecutive kinetics leads the oxidant itself to Mn 11 , at a rate much faster than auto-decomposition of the complex : Among all the three H 2R, hydrogen peroxide reacts slowest. When 11 2 0 2 is added in low concentrations, some of the intermediate Mn 2 111 are lost via auto-decomposition before advancing to the second (k 5 ) step and thus affects the stoi- chiometry. Protonated oxidants are more reactive than their cunj ugate bases, Mn/v is more reactive than Mn 2 111 and reactivity order of the reducing agents is : HzA H 2 Q >> H 2 0 2 • These observations are consistent with an outer sphere mechanism. The kr step for ascorbic acid exhibits significant solvent isotope effect indicating an electroprotic process to be the rate-determining step of this path. Keywords : Manganese(IV) oxidation, quinol, ascorbic acid, hydrogen peroxide, kinetics and mechanism. Introduction Manganese acts as an essential trace element 1 , that shapes life as we see it. Bacterial oxidation 2 of Mn 11 to Mn 1 v is believed to drive the oxidative segment of the global biogeochemical manganese cycle and regulates the concentration of dissolved Mn 11 in the oceanic water col- umn, where it is a critical nutrient for planktonic primary productivity. The oxidation of water by photosynthetic enzymes (photosystem II; PSII) 3 , the reduction of ribo- nucleotides in certain bacteria4, and the disproportion- ation of hydrogen peroxide by some catalases or pseudocatalases 5 have been shown to occur at bi- or poly- nuclear manganese containing active sites 6 . Syntheses of bridged binuclear manganese complexes as also their chemical and physical properties have re- ceived significant attention, largely because of the impor- toeuicated to Professor Suresh C. Ameta on his 60th birthday. J/CS-2 tance of high-oxidation-state manganese species in bio- logical systems and because of their potential use as re- dox catalysts 7 . Following the successful isolation of the 1, 10- phenanthroline complex, [Mn 2 1 V (J.L-0) 2 (phen) 4 ] 4 + ( { 1} 4 +; Fig. 1), a number ofbis(J.L-oxo)dimanganese(lV,IV) dimers fN N'\ N" I --0-............_ I /N Mn Mn N __.......... I I "' \__Y v /"'""'. N N =I, 10-phcnanthrolinc Fig. I. A graphic structure of {1} 4 +, adopted from crystal struc- ture 10 . 1191