Multistep redox processes and intramolecular charge transfer in ferrocene-based 2,2'-bipyridylboronium salts

A one-step high-yield synthesis of donor−acceptor complexes [1A]X to [1D]X, [2A]X2, and [4A]X4 with one, two, and four cationic [B(R)bipy]+ acceptors (R = Me, Br, OEt, NC4H8; X = Br, PF6; bipy = 2,2‘-bipyridine) covalently attached to a ferrocene donor is described. Apart from 2,2‘-bipyridine, 1,10-phenanthroline and 2,2‘-bipyrimidine have also been used as chelating amines. With the exception of R = Br, water-stable compounds are obtained in all cases. In DMF solution and under an inert atmosphere, [1A]PF6, [2A](PF6)2, and [4A](PF6)4 behave as reversible three-step redox systems, capable of storing three, five, and nine electrons, respectively. In their cationic state, the complexes possess an intense purple color, which can be attributed to charge-transfer interactions between the ferrocene unit and the electron-poor B(R)bipy substituent(s). This is confirmed by the ESR spectrum of the monoreduced species [1A]0, which features a line shape indicating considerable admixture of the ligand and metal orbitals.