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

Abstract

A series of fluorinated antimony­(V) porphyrins, SbTPP­(OMe)2·PF6, SbTPP­(OTFE)2·PF6, SbT­(4F)­PP­(OMe)2·PF6, SbT­(35F)­PP­(OMe)2·PF6, SbT­(345F)­PP­(OMe)2·PF6, SbT­(4CF3)­PP­(OMe)2·PF6, SbT­(35CF3)­PP­(OMe)2·PF6, and SbT­(35CF3)­PP­(OTFE)2·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 meso-positions. Additionally, the SbTPP­(OTFE)2·PF6 and SbT­(35CF3)­PP­(OTFE)2·PF6 carry trifluoroethoxy units in their axial-positions. The fluorination on the porphyrin peripherals ranges from zero fluorine atoms in SbTPP­(OMe)2·PF6 to 30 fluorine atoms in SbT­(35CF3)­PP­(OTFE)2·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·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·PF6 or SbT­(35CF3)­PP­(OMe)2·PF6 and SbT­(35CF3)­PP­(OTFE)2·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.

Tuning Intramolecular Charge Transfer in Antimony(V) Porphyrin through Axial Fluorination

“…Analogously, HOMO – 1 is found to be exclusively on the meso -aryl units. The presence of HOMO and LUMO on different parts within the molecule creates the conditions necessary for push–pull type ICT character. , The calculated HOMO–LUMO gaps are 2.49 and 2.43 eV in [SbT­(DMP)­P­(OMe) 2 ] + and [SbT­(DMP)­P­(OTFE) 2 ] + , respectively. It is noteworthy to mention that the computed HOMO–LUMO gap is in a vacuum, and no solvent matrix was incorporated in the calculation.…”

“…The optimized structures are consistent with the earlier reported crystal structures where the central porphyrin exists in planarity. 28 , 29 Figure 2 illustrates the frontier orbitals of [SbT(DMP)P(OMe) 2 ] + and [SbT(DMP)P(OTFE) 2 ] + . The LUMO and LUMO + 1 are limited mainly to the porphyrin ring.…”

“…The optimized structures are consistent with the earlier reported crystal structures where the central porphyrin exists in planarity. 28,29 + . This is due to the presence of axial TFE electron-withdrawing units, which increases the electrostatic potential difference between the central ring and peripheral substitutions.…”

“…Among porphyrins, main-group porphyrins make up a special class of molecules where the structural and redox properties can also be tuned by the central element. For example, the insertion of antimony(+5) and phosphorus(+5) ions in the porphyrin cavity results in an electron-deficient porphyrin center with two functionally active axial bonds. − Recently, our group has employed the above criteria to synthesize a series of antimony(V) porphyrins and phosphorus(V) porphyrins, where the meso -positions were decorated with different degrees of methoxy phenyl substitutions. , The presence of electron-rich methoxyphenyl groups induced ICT in the molecules from the peripheral methoxyphenyl units to the central antimony(V) or phosphorus(V) porphyrin ring. This ICT character was determined to result from the large electrostatic potential difference between the core of the porphyrin and its peripheral substitutions.…”

Advances and opportunities in Group 15 porphyrin chemistry

“…A series of antimony(V) porphyrins, [SbT(P)P(OMe) 2 ]PF 6 , [SbT(4MP)P(OMe) 2 ]PF 6 , [SbT(35DMP) P(OMe) 2 ]PF 6 , and [SbT(345TMP)P(OMe) 2 ]PF 6 were reported Efforts to push the redox potentials further positive, a series of fluorinated antimony(V) porphyrins were synthesized recently. 83 The fluorination on the porphyrin peripherals ranges from 0 fluorine atoms in [SbT(P)P(OMe) 2 ]PF 6 , 12 fluorine atoms in [SbT(345FP)P(OMe) 2 ]PF 6 to 30 fluorine atoms in [SbT(35CF 3 P)P(OTFE) 2 ]PF 6 . As shown in Fig.…”