Electronic communications in (Z)-bis(ferrocenyl)ethylenes with electron-withdrawing substituents

Orange dimethyl (Z)-2,3-bis(ferrocenyl)-2-butenedioate (1) and blue (Z)-2,3-bis(ferrocenyl)maleimide (2) were prepared and characterized using NMR, IR, and UV-vis spectroscopy as well as X-ray crystallography. X-ray crystallographic studies of 1 and 2 revealed a locked anti conformation of the ferrocene substituents, which was explained on the basis of the steric properties of these groups. Electronic structures and solvatochromic properties of 1 and 2 were investigated by UV-vis spectroscopy and polarized continuum model (PCM) density functional theory (DFTPCM) as well as time-dependent DFT (TDDFT-PCM) approaches. The calculated vertical excitation energies are consistent with the experimental data and clearly suggest the dominance of metal-to-ligand charge-transfer bands in the visible region of the UV-vis spectra for 1 and 2. Redox properties of 1 and 2 were investigated using cyclic voltammetry, differential pulse voltammetry, and spectroelectrochemical and chemical oxidation approaches. In aDCM/(NBu₄)[B(C₆F ₅)₄] system, ferrocene-centered oxidation processes in 1 and 2 are separated by 300 and 345 mV, respectively. Stepwise chemical and spectroelectrochemical oxidation of 1 and 2 allowed us to obtain spectroscopic signatures of the mixed-valence 1⁺ and 2⁺ cations. Hush analysis of the intervalence charge-transfer bands in 1⁺ and 2 ⁺ is suggestive of class II (in Robin and Day classification) behavior.