Ferrocenyl Maleimides – Synthesis, (Spectro‐)Electrochemistry, and Solvatochromism

Abstract

AbstractFerrocenylmaleimides have been synthesized from 2,5‐dibromo‐N‐methyl‐1H‐pyrrole. Bromine shift and oxidation of the pyrrole core with subsequent ferrocenylation using the Negishi C–C cross‐coupling protocol led to the formation of 3‐ferrocenyl‐N‐methylmaleimide (3), 3‐bromo‐4‐ferrocenyl‐N‐methylmaleimide (4), and 3,4‐diferrocenyl‐N‐methylmaleimide (5). The structural properties of 4 and 5 were investigated by single‐crystal X‐ray diffraction. Cyclic and square‐wave voltammetry, in situ UV/Vis/NIR and IR spectroelectrochemistry (5) highlight the electrochemical properties of these compounds. Compounds 3 and 4 exhibit one reversible ferrocenyl‐based redox event, whereas 5 shows two separate electrochemically reversible one‐electron transfer processes with remarkably high ΔE°′ values and reduction potentials of E1°′ = 50 and E2°′ = 380 mV (ΔE°′ = 330 mV), respectively, with [NBu4][B(C6F5)4] as the supporting electrolyte. The NIR measurements confirm the electronic communication between the iron centers (FeII/FeIII) as intervalence charge transfer absorptions were observed in 5+. Compound 5 was classified as a weakly coupled class II system, according to Robin and Day. UV/Vis investigations of the solvatochromic behavior of 3–5 revealed the complex solvation of these push–pull systems, which reflects that three important solvent properties (hydrogen bond formation ability, polarizability, and solvation of the carbonyl group and the C=C bond) affect $\tilde {\nu}$max.