Intramolecular arene C-H and C-F activation by multimetallic tetramanganese clusters relevant to the oxygen-evolving complex of photosystem II

Abstract

A series of homometallic tetramanganese clusters of compns. [LMn_3(PhPz)_3OMn] [OTf]_x (x = 1, 2; PhPz = 3- phenylpyrazolate) and [LMn_3(F_2PhPz)_3 OMn] [OTf]_x (x = 1, 2; F_2PhPz = 3-(2, 6- difluorophenyl) - pyrazolate) was synthesized and characterized by NMR, ESI- MS, and single- crystal X- ray diffraction. Each cluster consists of a trimanganese core supported by weak- field alkoxide and pyridine donors with an apical manganese ion appended by three pyrazolate bridging ligands and an interstitial μ_4-oxygen atom. The presence of a site- differentiated apical manganese metal center provides an entry toward studying the viability of high- valent manganese metal oxo motifs (Mn^(IV)-oxo) on a multi- metallic scaffold, relevant for the O-O bond coupling mechanism in the oxygen- evolving complex of photosystem II. Exposure of our newly synthesized clusters [LMn_3(PhPz)_3OMn][OTf]_x (x = 1, 2) to oxygen-atom transfer reagents (e.g., iodosobenzene or TBAIO_4) resulted in the intramol. C-H activation of the Ph ring of the pyrazolate bridging ligand. The obsd. regioselective hydroxylation is accompanied with the loss of an H-atom, suggesting a Mn^(IV)-oxo as a potential intermediate. In order to mitigate the ortho C-H activation, the fluorinated analog [LMn_3(F_2PhPz)_3 OMn][OTf]_x (x = 1, 2) was synthesized. Interestingly, exposure of [LMn (F PhPz) OMn][OTf]_x (x = 1, 2) to oxygen- atom transfer reagents resulted in intramol. arene C-F activation. C-F activation is coupled with the transfer of a fluorine anion to unreacted starting material to provide the fluoride capped clusters [LMn_3(PhPz)_3 OMn(F)][OTf]_x. This type of behavior engendered by a putative M-oxo species is rare. The identities of the products of C-H and C-F activation were confirmed by independent synthesis. With the propensity of the metal clusters to undergo intramol. electron transfer, the various mechanistic pathways (Mn^(IV)-oxo vs. Mn^(III)-oxo) will be discussed.