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]X-2, and [4A]X-4 with one, two, and four cationic [B(R)bipy] (+) acceptors (R = Me, Br, OEt, NC4H8; X = Br, PF6; bipy = 2,2'-bipyri dine) covalently attached to a ferrocene donor is described. Apart fro m 2,2'-bipyridine, 1,10-phenanthroline and 2,2'-bipyrimidine have also been used as chelating amines. With the exception of R = Br, water-st able 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 r eversible three-step redox systems, capable of storing three, five, an d nine electrons, respectively. In their cationic state, the complexes possess an intense purple color, which can be attributed to charge-tr ansfer 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 co nsiderable admixture of the ligand and metal orbitals.