Homogeneous water oxidation by a di-μ-oxo dimanganese complex in the presence of Ce4+

O2 evolution was observed upon mixing aqueous [(terpy)(H2O)Mn(O)2Mn(H2O)(terpy)](NO3)3 (1, terpy = 2,2′:6′,6″-terpyridine) with aqueous solutions of Ce4+. However, when the solution of 1 was incubated at pH 1 (by dissolving in dilute HNO3) before mixing with Ce4+, very small amounts of O2 were observed. This observation of acid-induced deactivation suggests an explanation, both for the previously reported lack of O2 evolution from aqueous solutions of 1 with Ce4+ as oxidant, and the present observation of low amounts of O2 production with the very acidic Ce4+ reagent. Evidence is provided for water being the source of evolved O2, and for the requirement of a high valent multinuclear Mn species for O2 evolution. We test the possibility of complications in the use of ceric ammonium nitrate (CAN) in oxidation chemistry due to the presence of the oxidizable NH4+ ion.

The title compound 1 is shown to oxidize water in the presence of Ce4+ as the primary electron acceptor. Evidence is provided for higher oxidation states of 1 as the water-oxidizing species, and low-valent mononuclear manganese species formed from 1 under acidic conditions are shown to be inactive. The potential for catalytic water oxidation by 1 in the presence of electron acceptor oxidants is discussed.Figure optionsDownload as PowerPoint slide