Thermal stability of [Mn(III)(O)2Mn(IV)(H2O)2(Terpy)2](NO3)3 (Terpy=2,2′:6′,2-terpyridine) in aqueous solution

The chemistry of oxidizing water to dioxygen in photosynthesis is catalyzed by a Mn4Ca cluster in PS II. A synthesized Mn(III/IV)-oxo dimer compound, [Mn(III)(O)(2)Mn(IV)(H2O)(2)(Terpy)(2)](NO3)(3) (Terpy = 2,2':6',2 ''-terpyridine) is able to catalyze the conversion of water to dioxygen. In this work, we investigated the thermal stability of this Mn(III/IV)-oxo dimer in the range of 20-85 degrees C. The decomposition of the Mn(III/IV)-oxo dimer in aqueous solution at similar to 60 degrees C occurred involving a change in Mn valence. Values of the activation energies for the first fast step with a lifetime of 3.5 +/- 0.5 min and in the following slow step with lifetime of 19 +/- 4 min were determined to be 68 +/- 10 kJ/mol, and 82 +/- 16 kJ/mol, respectively. We speculate that the thermal inactivation of PS II may also be associated with a Mn valence change in the Mn4Ca cluster. Unexpectedly, the thermal decomposition of the Mn(III/IV)-oxo dimer was found to generate a Mn-containing precipitate that retained catalytic oxygen-evolution activity. The solid Mn-containing material is not manganese dioxide as judged by EPR, FTIR, elemental analysis, and atomic absorption spectroscopy. The novel Mn-containing precipitate, tentatively assigned as a Mn-oxo oligomer, is thermally stable and may be a unique material for fabricating catalytic materials in solar fuel production. Published by Elsevier B.V.