Intramolecular C–H and C–F Bond Oxygenation by Site-Differentiated Tetranuclear Manganese Models of the OEC

Abstract

The dangler manganese center in the oxygen-evolving complex (OEC) of photosystem II plays an important role in the oxidation of water to dioxygen. Inspired by the structure of the OEC, we synthesized a series of site-differentiated tetra-manganese clusters [LMn_3(PhPz)_3OMn][OTf]_x (2: x= 2; 3: x = 1) that features an apical manganese ion—distinct from the others—that is appended to a trinuclear manganese core through an μ4-oxygen atom bridge. This cluster design was targeted to facilitate studies of high-valent Mn-oxo formation, which is a proposed step in the mechanism for water oxidation by the OEC. Terminal Mn-oxo species—supported by a multinuclear motif—were targeted by treating 2 and 3 with iodosobenzene. Akin to our previously reported iron complexes, intramolecular arene hydroxylation was observed to yield the C–H bond oxygenated complexes [LMn3(PhPz)_2(OArPz)OMn][OTf]x (5: x = 2; 6: x = 1). The fluorinated series [LMn_3(F_2ArPz)_3OMn][OTf]_x (8: x = 2; 9: x = 1) was also synthesized to mitigate the observed intramolecular hydroxylation. Treatment of 8 and 9 with iodosobenzene results in intramolecular arene C–F bond oxygenation as judged by electrospray ionization mass spectrometry. The observed aromatic C–H and C–F hydroxylation is suggestive of a putative high-valent terminal metal-oxo species, and it is one of the very few examples capable of oxygenating C–F bonds.

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