MechanisticStudiesontheOxidationofGlyoxylicandPyruvicAcidbya [Mn 4 O 6 ] 4+ Core in Aqueous Media: Kinetics of Oxo-Bridge Protonation by Suranjana Das , Jhimli Bhattacharyya, and Subrata Mukhopadhyay* Department of Chemistry, Jadavpur University, Calcutta 700032, India (e-mail: smukhopadhyay@chemistry.jdvu.ac.in) In aqueous media (pH 2.5 – 6.0), the Mn IV tetramer [Mn 4 (m-O) 6 ACHTUNGTRENNUNG(bipy) 6 ] 4+ (1 4+ ; bipy = 2,2’-bipyridine) oxidizes both glyoxylic and pyruvic acid to formic and acetic acid, respectively, under formation of CO 2 . Kinetics studies suggest that the species 1 4+ , its oxo-bridge protonated form [1H] 5+ , i.e., [Mn 4 (m-O) 5 ACHTUNGTRENNUNG(m- OH)(bipy) 6 ] 5+ , the reducing acids (RH) and their conjugate bases (R  ) all take part in the reaction. The oxo-bridge protonated oxidant [1H] 5+ was found to react much faster than 1 4+ . Thereby, the gem- diol forms of the a-oxo acids (especially in the case of glyoxylic acid) are the possible reductants. A one-electron/one-proton electroprotic mechanism operates in the rate-determining step. Introduction. – The unique manganese-oxo aggregate (OEC) present in photosys- tem II (PS-II) catalyzes the light-driven oxidation of H 2 O to O 2 [1 – 6] resulting in the O 2 -rich atmosphere encountered on Earth. The OEC cycles through five redox states, S 0 –S 4 , the index of which refer to the number of oxidizing equivalents stored [7] [8]. The overall four-electron (4 e  ) oxidation of two H 2 O molecules leading to O 2 is asso- ciated with transfer of four H + :2H 2 O ! O 2 + 4H + + 4e  . Ligands derived from H 2 O (O 2 or OH  ) are present as bridges between Mn-atoms along with carboxylato moieties in the catalytic site [9 – 11]. Successive redox reactions at the Mn site are definitely associated with a substantial change in the oxo-bridge basicity [1] [9] [10], and it is likely that a change in the protonation state of the bridged metal cluster also occurs [12]. The observed decrease in the exchange coupling between Mn IV in a model tetranuclear system [13][14], resulting from protonation of the oxo bridges, has important implications in interpreting the changes in magnetic behavior of the OEC upon S-state advancement and changes in configuration. Oxo-bridge pro- tonations also cause a substantial increase in the MnMn distance in multinuclear Mn complexes, and an increase in reduction potential [14–18]. Besides these physical effects, investigations on chemical aspects resulting from oxo-bridge protonation have hardly been studied, expect in a few reports where it was observed that catalase activity of [Mn(salpn)(m-O)] 2 (salpn = 1,3-bis(salicylideneamineto)propane) is inhib- ited by a single protonation on the oxo-bridge [18], whereas disproportionation of a (Mn III ) 2 complex requires oxo-bridge protonation [19]. It is also of note that oxo-bridge protonation in multinuclear higher-valent Mn complexes sometimes leads to cluster breakup [16] [20], rendering their redox chemistry H + -coupled. The acid–base chemis- try resulting from oxo-bridge protonation is also well-studied, along with the kinetic stabilities of oxo bridged species [15]. # 2006 Verlag Helvetica Chimica Acta AG, Zürich Helvetica Chimica Acta – Vol. 89 (2006) 1947