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

Abstract

Ferrocenium salts [(C 5 H 5 ) 2 Fe]X (X = BF 4 , PF 6 ) ( I + · ) react with tertiary phosphines PR 3 (PMe 3 , PEt 3 , P n Bu 3 , PMe 2 Ph, and PMePh 2 ) in dichloromethane at room temperature to form a mixture of the corresponding ferrocenylphosphonium salts [(C 5 H 5 )FeC 5 H 4 PR 3 ]X ( III ), ferrocene ( I ), and phosphonium salts [HPR 3 ]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 [(η 5 ‐C 5 H 5 )Fe · (η 4 ‐C 5 H 5 – + PR 3 )]X ( II +· ); b) redox reaction of the initial adduct II +· with the starting ferrocenium salt I + · to result in ferrocene (C 5 H 5 ) 2 Fe ( I ) and iron phosphoniocyclopentadiene dication [(η 5 ‐C 5 H 5 )Fe + (η 4 ‐C 5 H 5 – + PR 3 )](X) 2 ( II 2+ ) where the C sp3 –H bond is involved in the agostic interaction with the metal atom; and c) deprotonation of the C sp3 –H bond in II 2+ 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.