Synthesis and Redox Properties of Crowded Triarylphosphines Possessing Ferrocenyl Groups

Crowded triarylphosphines possessing ferrocenyl groups [(4-ferrocenyl-2,6-diisopropylphenyl)n(2,4,6-triisopropylphenyl)3-nP (5a, n = 1; 5b, n = 2; 5c, n = 3)] were synthesized by the reaction of the corresponding arylcopper(I)
reagents with the diarylchlorophosphines. Structures of the triarylphosphines were studied by 1H, 13C, and 31P
NMR spectroscopies, and the characteristic patterns of the proton signals of the 2,6-isopropyl groups and upfielded
31P chemical shifts suggest structural similarities of the triarylphosphine moiety to the previously reported tris(2,4,6-triisopropylphenyl)phosphine (2). X-ray crystallography of 5c also revealed that the structure around the
phosphorus is similar to that of 1, where the average bond angle and length around the phosphorus atom are
110.8° and 1.854 Å, respectively. According to the electrochemical measurements, phosphines 5a, 5b, and 5c are
reversibly oxidized in two, three, and four steps, respectively, which suggests significant electronic interaction among
the triarylphosphine and the ferrocene redox centers as well as weak interaction among the ferrocene redox centers.
The EPR spectra obtained at cryogenic temperature after oxidation of phosphines 5a, 5b, and 5c are superpositions
of those for the cation radicals of the crowded triarylphosphine and ferricinium. The solution spectra obtained at
293 K, which consist of two lines typical of the cation radical of the crowded triarylphosphines, become weaker as
the number of the ferrocenyl groups increases and the cation radical of 5c does not show EPR signals. These
findings suggest not only instability of the tetra(cation radical) of 5c but also the course of oxidation where the
ferrocenyl groups in the periphery of the molecules are oxidized at first.