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

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.