Oxyl Radical Required for O−O Bond Formation in Synthetic Mn-Catalyst

DFT calculations using the B3LYP functional support the suggestion that the [(terpy)(H₂O)Mn^IV(μ-O)₂Mn^III(H₂O)(terpy)]³⁺ (terpy=2,2‘:6,2‘ ‘-terpyridine) complex functions as a synthetic O₂ catalyst. The calculated barrier for O−O bond formation with water is 23 kcal/mol. In this complex, as well as in models of the oxygen evolving complex in PSII, the active species is a Mn^IV-oxyl radical. From comparisons with inactive Mn^V-oxo complexes, it is proposed that radical formation is actually a requirement for O₂ formation activity in Mn-complexes.