Syntheses, structures, and redox properties of 1,4- bis(dimesityiphosphino)-2,3,5,6-tetrafluorobenzene and the corresponding bis(phosphoryl) and bis(phosphonio) derivatives

Sterically hindered 1,4-bis(dimesitylphosphino)-2,3,5,6- tetrafluorobenzene (1) was synthesized by the aromatic nucleophilic substitution of lithium dimesitylphosphide with hexafluorobenzene. Two phosphorus atoms of I were oxidized and methylated to give the corresponding bis(phosphoryl)benzene 2 and bis(phosphonio)benzene 3²⁺·2TfO^-(TfO^- = trifluoromethanesulfonyl), respectively. On the other hand, reaction of 1 with butyllithium and phenyllithium gave more crowded 2,5-dibutyl-1,4- bis(dimesitylphosphino)-3,6-difluorobenzene (4) and 1,4- bis(dimesitylphosphino)2,5-difluoro-3,6-diphenylbenzene (5), respectively. Structures of l, 2, 3²⁺·2TfO^-, 4, and 5 were confirmed by conventional spectroscopic methods; particularly ¹⁹F NMR spectroscopy reflected their crowded structures. Molecular structures of 1, 2, and 3²⁺·2TfO^-, were further investigated by X-ray crystallography, where unusually large bond angles around phosphorus atoms were observed. Electrochemical measurements were carried out to investigate redox properties of 1, 3²⁺·2TFO^-, 4, and 5. Although the cyclic voltammogram of diphosphine 1 showed irreversible oxidation waves above 0.8 V, in spite of substitution at all the ortho positions to the phosphorus atoms in 1, displacement of the two fluorine atoms of 1 by the butyl or the phenyl groups lowered the oxidation potential and improved the stability of the corresponding radical cations. Particularly, 5 clearly showed two-step oxidation waves. On the other hand, bis(phosphonium salt) 3²⁺·2TfO^- showed two steps of quasi-reversible redox waves, which suggested reduction to the cation radical and the neutral species, although their stability was limited.