Direct Phosphination of Ferrocenium Ion with Tertiary Phosphines by the Mechanism of Oxidative Nucleophilic Substitution

Ferrocenium salts [(C5H5)2Fe]X (X = BF4, PF6) (I+·) react with tertiary phosphines PR3 (PMe3, PEt3, PnBu3, PMe2Ph, and PMePh2) in dichloromethane at room temperature to form a mixture of the corresponding ferrocenylphosphonium salts [(C5H5)FeC5H4PR3]X (III), ferrocene (I), and phosphonium salts [HPR3]X. The same reaction was carried out in an electrochemical cell as the electrolysis of a solution of ferrocene and phosphine in dichloromethane at the oxidation potential of ferrocene. Possible reaction pathways for phosphination of ferrocenium ion were studied by DFT at the M06‐L/6‐311++G(d,p) theory level. The experimental and theoretical studies of this reaction suggest it to proceed according to the Scheme of oxidative nucleophilic substitution of hydrogen in I+· including the following steps: a) nucleophilic exo addition of a phosphine to the cyclopentadienyl ring of I+· to form the radical cation adduct [(η5‐C5H5)Fe·(η4‐C5H5–+PR3)]X (II+·); b) redox reaction of the initial adduct II+· with the starting ferrocenium salt I+· to result in ferrocene (C5H5)2Fe (I) and iron phosphoniocyclopentadiene dication [(η5‐C5H5)Fe+(η4‐C5H5–+PR3)](X)2 (II2+) where the Csp3–H bond is involved in the agostic interaction with the metal atom; and c) deprotonation of the Csp3–H bond in II2+ by the starting phosphine to form III. The relation between the data obtained and current concepts of C–H functionalization in transition metal arene and cyclopentadienyl complexes is discussed. Ferrocenium salts I+· react with tertiary phosphines to form a corresponding ferrocenylphosphonium salts III. Our studies suggest the Scheme of oxidative nucleophilic substitution: a) nucleophilic exo addition of a phosphine to the cyclopentadienyl ring of I+· to form a radical cation adduct II+·; b) the redox reaction of II+· with I+· to result in dication II2+; and c) deprotonation of the Csp3–H bond in II2+ to form III.