Synthesis and (spectro)electrochemistry of 1′,1′′′-disubstituted biferrocenes

Abstract

A series of 1′,1′′′-disubstituted biferrocenes was synthesized to study the influence of electron-withdrawing and electron-donating groups on the electronic interaction within the biferrocenyl unit. Therefore, biferrocenes of type R-bfc-R (bfc = 1:1″-biferrocen-1′,1′′′-diyl; R = 1,3-dioxan-2-yl (5a), H (5b), I (5c), 3,5-(CF3)2-C6H3 (5d), CHO (5e), CN (5f)) were prepared using different synthetic methodologies. Biferrocenes 5d,f have been structurally characterized by single-crystal X-ray diffraction analysis. Compound 5d shows a trans-configuration of the biferrocene moiety forming a two dimensional network based on intermolecular F···F interactions being parallel to (0–11) in the solid state. Electrochemical measurements display two separated reversible redox processes for the biferrocene unit. With an increase of the electron-withdrawing character of the substituents R, a shift of the first redox event to higher potentials is observed. For biferrocenes 5a–f the redox separations of the two individual redox processes range between 505 and 620 mV. Mixed-valent compounds [5a–5f]+ exhibit IVCT absorptions of moderate strength allowing to classify them as class II systems according to Robin and Day. The strongest interaction is found for biferrocene 5d with an IVCT intensity of 1540 L·mol–1·cm–1 and a full-width-at-half-height of 2900 cm–1. The solvatochromism of biferrocene 5a,e,f was studied using the Kamlet-Taft LSE relationship showing similar solvatochromic properties for those compounds. An increase of the dipolarity/polarizability of the solvent results in a bathochromic shift of the λmax values and increased hydrogen bond donor capacities result in a hypsochromic shift of λmax