Planar-chiral phosphino alkenylferrocenes – Synthesis, solid-state structure and electrochemistry

The synthesis and characterization of phosphino alkenylferrocenes of type [Fe(η5-C5H3–2-PR′2–(E)-CH = CHR)(η5-C5H5)] (R = C6H5, 4-Cl–C6H4, 4-OMe–C6H4, 4-Me–C6H4, 4-CN–C6H4, 4-Ac–C6H4, Fc; R′ = Ph, MeFur, Cy, o-Tol, 3-Cl–C6H4; Fc = Fe(η5-C5H5)(η5-C5H4), MeFur = 5-methylfuran-2-yl, o-Tol = 2-tolyl) and the molecular structure of representative examples in the solid state are discussed. The alkenyl unit has been built up either by Heck–Mizoroki C,C cross coupling or Horner–Wadsworth–Emmons olefination. The electrochemical characterization of the sulphur-protected phosphino (phenylvinyl)-ferrocenes revealed a linear correlation between the redox potentials of the metallocenyl units (E°′) and the σpara Hammett parameters of the phenylene-bonded substituents. In contrast, the σ-donor properties of the phosphine moieties are not affected as could be demonstrated by analysis of the 1J(31P,77Se) coupling constants of the respective selenophosphines. In diferrocenyl ethylene [Fe(η5-C5H3–2-P(S)Ph2–(E)-CHdouble bond; length as m-dashCHFc)(η5-C5H5)] both ferrocenyl units are oxidized separately. In situ spectroelectrochemical measurements revealed that inter-valence charge transfer interactions contribute to the observed redox splitting of 495 mV and that the formed mono-cation can be classified as weakly coupled class II system according to Robin and Day.