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. 2017 Aug 7;56(15):9044-9054.
doi: 10.1021/acs.inorgchem.7b01022. Epub 2017 Jul 21.

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

Kurtis M Carsch  1 , Graham de Ruiter  1 , Theodor Agapie  1
Affiliations

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

Kurtis M Carsch et al. Inorg Chem. .

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 [LMn3(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 [LMn3(F2ArPz)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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Figures

Figure 1
Figure 1
Structural similarity of tetranuclear manganese clusters [LMn3(PhPz) 3OMn(O)]n+ (n = 1 or 2; top right) reported here to the oxygen evolving complex (OEC) in photosystem II (top left). The metal centers are supported by pyrazolates (inset) and a 1,3,5-triarylbenzene-based ligand (L).
Figure 2
Figure 2
Solid-state structure of (A) [LMn3(PhPz)3OMn][OTf]2 (2), (B) [LMn3(PhPz)3OMn][OTf]1 (3), and (C) [LMn3(PhPz)2(OArPz)OMn][BPh4]1 (6′). Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms, outer sphere counter ions, and co-crystallized solvent molecules are omitted for clarity. See Table 1 for selected bond angles and distances.
Figure 3
Figure 3
Cyclic voltammogram of [LMn3(PhPz)3OMn] [OTf]2 (2) recorded at 100 mV s −1 in dichloromethane at a 2 mM concentration. The recorded potentials are referenced against the Fc/Fc+ redox couple.
Figure 4
Figure 4
Reactivity of complex 2 with PhIO, and independent syntheses of complexes 5 and 6.
Figure 5
Figure 5
Solid-state structure of [LMn3(F2ArPz)3OMn] [OTf]1 (9), showing a weak interaction between Mn4 and the ortho fluorine (F4) of the pyrazolate ligand. Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms, outer sphere counter ions, and co-crystallized solvent molecules are omitted for clarity
Figure 6
Figure 6
Proposed mechanisms for arene hydroxylation observed with complex 2 via: (a) direct C–H insertion by an iodosobenzene adduct, (b) a high-valent Mn-oxo. Both paths require the loss of a hydrogen atom to access the final observed product.
Scheme 1
Scheme 1
Synthesis of complexes 1–3 and 7–9.
See this image and copyright information in PMC

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