Characterization of the O2-Evolving Reaction Catalyzed by [(terpy)(H2O)MnIII(O)2MnIV(OH2)(terpy)](NO3)3 (terpy = 2,2‘:6,2‘ ‘-Terpyridine)

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Abstract
The complex [(terpy)(H2O)MnIII(O)2MnIV(OH2)(terpy)](NO3)3 (terpy = 2,2‘:6,2‘ ‘-terpyridine) (1) catalyzes O2 evolution from either KHSO5 (potassium oxone) or NaOCl. The reactions follow Michaelis−Menten kinetics where Vmax = 2420 ± 490 mol O2 (mol 1)-1 hr-1 and KM = 53 ± 5 mM for oxone ([1] = 7.5 μM), and Vmax = 6.5 ± 0.3 mol O2 (mol 1)-1 hr-1 and KM = 39 ± 4 mM for hypochlorite ([1] = 70 μM), with first-order kinetics observed in 1 for both oxidants. A mechanism is proposed having a preequilibrium between 1 and HSO5- or OCl-, supported by the isolation and structural characterization of [(terpy)(SO4)MnIV(O)2MnIV(O4S)(terpy)] (2). Isotope-labeling studies using H218O and KHS16O5 show that O2 evolution proceeds via an intermediate that can exchange with water, where Raman spectroscopy has been used to confirm that the active oxygen of HSO5- is nonexchanging (t1/2 ≫ 1 h). The amount of label incorporated into O2 is dependent on the relative concentrations of oxone and 1. 32O2:34O2:36O2 is 91.9 ± 0.3:7.6 ± 0.3:0.51 ± 0.48, when [HSO5-] = 50 mM (0.5 mM 1), and 49 ± 21:39 ± 15:12 ± 6 when [HSO5-] = 15 mM (0.75 mM 1). The rate-limiting step of O2 evolution is proposed to be formation of a formally MnVO moiety which could then competitively react with either oxone or water/hydroxide to produce O2. These results show that 1 serves as a functional model for photosynthetic water oxidation.