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, NC 4 H 8; X = Br, PF 6; 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]PF 6, [2A](PF 6 ) 2, and [4A](PF 6 ) 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.