Mechanistic studies on the oxidation of thiols by a {Mn 4 O 6 } 4+ core in aqueous acidic media Maharudra Chakraborty, Pulak Chandra Mandal, Subrata Mukhopadhyay ⇑ Department of Chemistry, Jadavpur University, Kolkata 700 032, India article info Article history: Received 14 April 2012 Accepted 1 July 2012 Available online 15 July 2012 Keywords: Kinetics Oxidation Thiols Redox Mechanism abstract Described in this work is the kinetics of the oxidation of a series of thiols, viz. 2-mercaptoethanol (mer- cap), thioglycolic acid (tga) and L-cysteine (cys), a paradigm for aliphatic thiols, by a tetranuclear Mn oxi- dant, [Mn 4 (l-O) 6 (bipy) 6 ] 4+ (bipy = 2,2 0 -bipyridine), in weakly acidic aqueous media (pH 2.0–4.0) in the presence of added bipy (60.0–80.0 mM). The thiols were quantitatively oxidized to their respective disul- fides and the Mn IV 4 species was reduced to Mn II . Excess bipy present in the reaction mixture removes any possibility of Cu 2+ catalysis in these reactions. The reactions were acid-catalyzed and the oxo-bridged protonated Mn oxidant, [Mn 4 (l-O) 5 (l-OH)(bipy) 6 ] 5+ , was found to be a kinetically far superior oxidant that its deprotonated analog, [Mn 4 (l-O) 6 (bipy) 6 ] 4+ . The reactions show kinetic isotope effects. The tga oxidation rate was increased in media enriched with D 2 O whereas the rate of oxidation of the other two thiols was increased in H 2 O. However, the increase/decrease in rate in H 2 O–D 2 O mixed media varied linearly with the D 2 O content of the media, which might be indicative of transfer of a single proton in the rate step of the reactions, at least for cys and mercap. The increase in rate of tga oxidation in D 2 O media could be explained by the relative abundance of the reactive species in D 2 O in comparison to that in H 2 O. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Photosynthetic water splitting, resulting in oxygen evolution, is the primary source of oxygen in the atmosphere. The water oxida- tion process proceeds through a cycle of at least five steps, known as the Kok cycle [1]; each step is characterized as a S i (i = 0–4) state depending on the oxidation states of a cluster of four Mn atoms, along with related structural and electronic properties at each state [2,3]. Ligands derived from H 2 O (O 2 or HO  ) act as bridges be- tween the Mn atoms, along with carboxylato moieties, in the cata- lytic site of the oxygen evolving complex (OEC) in photosystem II (PS II) [4–6]. Successive changes in the redox states of the Mn atoms in the OEC during the cycle are associated with protonation/deprotona- tion of the oxo-bridges [5,7,8] and the nearby amino acid residues, along with simultaneous movement of protons and electrons with- in the OEC [6,9–11]. The large size and complexity of the PS II makes it difficult to prepare, store and handle. Reactivity studies of synthetic models could play a significant role in understanding the structure and mechanism of action of OEC. Effects of oxo-bridge protonation include a decrease in the exchange cou- pling between Mn IV in a model tetranuclear system, and this has an important bearing in interpreting the changes in the magnetic behavior of the OEC upon S-state advancement, and changes in the configurations [12], increase in the MnMn distance [13,14] in multinuclear Mn complexes and a substantial increase in the reduction potential [15]. Investigations on the chemical aspects of reactivity resulting from oxo-bridge protonation, however, are scarce. It was observed that protonation of an oxo-bridge affects the catalase activity of a Mn IV dimer [15], whereas disproportion- ation of a Mn III dimer requires oxo-bridge protonation [16]. It was also reported that oxo-bridge protonation in multinuclear Mn complexes sometimes leads to cluster break up, rendering their redox chemistry proton coupled [17]. Formation of monomeric, tri- meric and tetrameric Mn species resulting from the opening of oxo-bridges on protonation were also proposed [18–21]. Exchange of oxygen atoms between bulk water and l-oxo bridges is also be- lieved to involve elementary steps of bridge protonation and deprotonation [22–27]. The acid/base chemistry resulting from oxo-bridge protonations has been studied mostly in non-aqueous media [12–16,19–21,28–32]. However, how such protonations influence the kinetic pattern of oxidation of small substrates are rarely studied [33–35], despite the relevance of such knowledge to PS II. This is particularly true in water, the medium for PS II. The title complex [Mn 4 (l-O) 6 (bipy) 6 ] 4+ (1 4+ , Fig. 1) formally corresponds to the fully oxidized S 3 or S 4 state of OEC [36,37] (although involvement of a higher state (Mn V ) is proposed in the water oxidation process [38–43]), and is stable in aqueous solution 0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2012.07.011 ⇑ Corresponding author. E-mail addresses: smukhopadhyay@chemistry.jdvu.ac.in, ju_subrata@yahoo. co.in (S. Mukhopadhyay). Polyhedron 45 (2012) 213–220 Contents lists available at SciVerse ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly