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

Ferrocenylmaleimides have been synthesized from 2,5‐dibromo‐ N ‐methyl‐1 H ‐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 Δ E° ′ values and reduction potentials of E 1 °′ = 50 and E 2 °′ = 380 mV (Δ E° ′ = 330 mV), respectively, with [NBu 4 ][B(C 6 F 5 ) 4 ] as the supporting electrolyte. The NIR measurements confirm the electronic communication between the iron centers (Fe II /Fe III ) 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 .