Structures of the conformational isomers and polymorph modifications of N-substituted 2,6-(E,E)-bis(ferrocenylidene)piperid-4-ones: photo- and electrochemically induced E/Z isomerization

Abstract

Four N-substituted 2,6-(E,E)-bis(ferrocenylidene)piperid-4-ones (NH 1, NMe 2, NEt 3, NCH₂Ph 4) were prepared by aldol condensation between ferrocenecarbaldehyde and two equivalents of N-substituted piperid-4-ones with high yields. The N-protonated compounds were obtained by reaction with HBF₄·Et₂O acid. The molecular structures of compounds 2, 3, 2·HBF₄ and 4·HBF₄ were confirmed by X-ray diffraction analysis and three types of conformational isomers were elucidated. Two polymorph modifications were found for compound 2·HBF₄. The electron transfer properties of the complexes were examined by electrochemical and spectroelectrochemical techniques. Complexes 1–4 undergo a reversible process of two-electron oxidation and partially reversible one-electron reduction. The photo- and electrochemically induced E/Z isomerisation of the complexes was monitored by UV-vis and ¹H NMR spectroscopy.