Synthesis, Structures, and Redox Properties of Tetracyano-Bridged Diferrocene Donor-Acceptor-Donor Systems

Abstract

A set of tetracyanobutadiene (TCBD)- and dicyanoquinodimethane (DCNQ)-bridged ferrocenyl dimers 5-8 were designed and synthesized by the [2 + 2] cycloaddition-retroelectrocyclization reaction of diferrocenyl complexes 3 and 4 with tetracyanoethylene (TCNE) and 7,7,8,8 tetracyanoquinodimethane (TCNQ), respectively. The effect of constitutional isomers (para vs meta) and different acceptors on their donor-acceptor interactions and photophysical and redox properties as well as electronic structures was evaluated using a variety of experimental and theoretical methods. The single-crystal X-ray structures of TCBD- and DCNQ-bridged ferrocenyl dimers 6 and 7 are reported. The DCNQ-bridged ferrocenyl dimers 7 and 8 have lower HOMO-LUMO gap values with red-shifted absorption bands in comparison to those of TCBD-bridged ferrocenyl dimers 5 and 6. Mössbauer spectra of 3-8 are suggestive of very similar isomer shifts and quadrupole splittings in all diferrocene complexes despite their different proximities to the electron-withdrawing fragment. Spectroelectrochemical data on 5-8 are suggestive of the presence of ferrocene-centered HOMOs in these compounds as well as a lack of electronic coupling between ferrocene groups.