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)2Mn(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 °C. The decomposition of the Mn(III/IV)-oxo dimer in aqueous solution at ∼60 °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.