Fluorinated Antimony(V) Tetraarylporphyrins as High-Valent Electron Acceptors with Unparalleled Reduction Potentials

A series of fluorinated antimony(V) porphyrins, SbTPP(OMe)2 center dot PF6, SbTPP(OTFE)2 center dot PF6, SbT(4F)PP(OMe)2 center dot PF6, SbT(35F)PP(OMe)2 center dot PF6, SbT(345F)PP(OMe)2 center dot PF6, SbT(4CF3)PP(OMe)2 center dot PF6, SbT(35CF3)PP(OMe)2 center dot PF6, and SbT(35CF3)PP(OTFE)2 center dot PF6, have been synthesized with phenyl [P], 4-fluorophenyl [(4F)P], 3,5-difluorophenyl [(35F)P], 3,4,5-difluorophenyl [(345F)P], 4-trifluoromethylphenyl [(4CF3)P], and 3,5-bis(trifluoromethyl)phenyl [(35CF3)P], in the mesopositions. Additionally, the SbTPP(OTFE)2 center dot PF6 and SbT(35CF3)PP-(OTFE)2 center dot PF6 carry trifluoroethoxy units in their axial-positions. The fluorination on the porphyrin peripherals ranges from zero fluorine atoms in SbTPP(OMe)2 center dot PF6 to 30 fluorine atoms in SbT(35CF3)PP(OTFE)2 center dot PF6. X-ray crystallography confirmed the structures of the investigated antimony(V) porphyrins. The absorption spectra depend on the number of fluorine atoms as it is blue-shifted with increasing fluorination. The series also exhibited rich redox chemistry with two reduction processes and one oxidation process. Remarkably, these porphyrins manifested the lowest reduction potentials reported among the main-group porphyrins, which are as low as -0.08 V vs SCE for SbT(35CF3)PP(OTFE)2 center dot PF6. On the contrary, the oxidation potentials were found to be very large, that is equal to 2.20 V vs SCE or even higher for SbT(4CF3)PP(OMe)2 center dot PF6 or SbT(35CF3)PP(OMe)2 center dot PF6 and SbT(35CF3)PP(OTFE)2 center dot PF6, respectively. These unprecedented potentials are due to a combination of two factors: (i) the +5-oxidation state of antimony in the porphyrin cavity and (ii) the presence of the strong electron-withdrawing fluorine atoms on the porphyrin peripherals. Density functional theory (DFT) calculations were used to support the experimental results. The systematic study of antimony(V) porphyrins, especially their high potentials, make them ideal for the construction of photoelectrodes and excellent electron acceptors for photoelectrochemical cells and artificial photosynthetic systems, respectively, for solar energy conversion and storage applications.