1‘-(Diphenylphosphino)ferrocenecarboxylic Acid and Its P-Oxide and Methyl Ester: Synthesis, Characterization, Crystal Structure, and Electrochemistry

Abstract

The ferrocene derivative (η5-C5H4PPh2)Fe(η5-C5H4COOH) (3), a potential hybrid phosphine ligand with heteroannular soft and hard donor groups, was synthesized in 71% yield from 1-phenyl-1-phospha[1]ferrocenophane by ring opening with phenyllithium, followed by reaction with carbon dioxide and acidification. Oxidation of 3 with hydrogen peroxide and its esterification with diazomethane provided the derivatives, phosphine oxide 4 and methyl ester 5, respectively. The new compounds were characterized by IR, UV/vis, MS, and NMR spectroscopy, the degenerate 1H NMR spectra being assigned with the aid of selective 1H{1H} decoupling, 1H,1H-COSY, 13C HMQC, and nJ(P,C) from APT. The solid-state structures of 3−5 were determined by single-crystal X-ray diffraction. Voltammetric measurements of 3−5 and related monosubstituted derivatives in acetonitrile revealed that the phosphines are first oxidized to the corresponding ferrocenium derivatives which are then the subject of the intramolecular electron transfer from the phosphine group to iron, the resulting species being immediately stabilized by further electrochemical and/or chemical oxidation to give phosphine oxides. The second independent electrochemical process is the reversible oxidation of these ferrocene/phosphine oxides to ferrocenium/phosphine oxides. There are further electrochemical processes associated with the irreversible oxidation of the carboxyl group.