Electrochemistry, Charge Transfer Properties, and Theoretical Investigation of a Macrocyclic Boronate Dimer of 1′,1‴-Biferrocenediboronic Acid and Related Ferrocenyl Boronate Complexes

The
target compound of this study is the macrocyclic tetraferrocenyl
boronate complex CP2C, which has two types of metal connections (i.e., FeII–CpCp–FeII and FeII–CpBO2C5H8O2BCp–FeII (Cp = cyclopentadienyl)) in the finite structure (C = 1′,1‴-biferrocenediboronic acid, P =
pentaerythritol). The electrochemical behavior of CP2C in dichloromethane was compared
with that of the related boronate complexes APA and BP2B, having FeII–CpBO2C5H8O2BCp–FeII, and Cester, having FeII–CpCp–FeII. The effects of the counteranion of the supporting electrolyte
on potential splitting revealed that CP2C exhibits an intrabiferrocenyl through-bond interaction
through the CpCp ligand, as well as an interbiferrocenyl through-space
interaction across the CpBO2C5H8O2BCp ligand. Chemical oxidation of CP2C with AgSbF6 produced the
one- and two-electron-oxidized species CP2C+ and CP2C2+, which exhibit intervalence charge transfer transition bands through
the CpCp ligand in the near-infrared region, giving one and two valence
isomers, respectively. DFT calculations revealed the charge distribution
of CP2C2+; the positive charges are localized on each biferrocenium
unit, especially on the longer diagonal, to minimize the electrostatic
repulsion over the CpBO2C5H8O2BCp ligand.