Redox-Switchable Chiral Anions and Cations Based on Heteroatom-Fused Biferrocenes

Abstract

A new class of planar chiral heteroatom-bridged biferrocene species was prepared using (pR,pR,SS,SS)-2,2″-bis(p-tolylsulfinyl)-1,1″-biferrocene (pR,pR,SS,SS)-2 as a readily accessible precursor. Initially, one of the sulfinate groups in compound 2 is replaced with a trimethylstannyl or dimesitylboryl group. Replacement of the second sulfinate group with t-BuLi then results in cyclization to give the Me2Sn-bridged species (pS,pS)-4Sn and the Mes2B-bridged borate anion [(pS,pS)-4B]−. The products are redox active, as cyclic and square wave voltammetry data of (pS,pS)-4Sn and [Li(12-c-4)2][(pS,pS)-4B] reveal two separate reversible oxidation events. The mixed-valent species [(pS,pS)-4Sn]+ and (pS,pS)-4B can also be obtained by chemical oxidation with AgPF6 or by simple exposure to air, respectively. A comparison of X-ray crystal structures reveals that the oxidized (mixed-valent) species contain distinct Fe(II) and Fe(III) sites, but UV–visible–NIR spectra display IVCT bands at ca. 4000 and 5000 cm–1, indica...