Synthesis and structural characterization of the racemic and meso forms of 2,2′′-dibromo-1,1′′-biferrocenes and their mixed-valence salts

Abstract

The relationships between the molecular structures of the racemic and meso isomers of 2,2′′-disubstituted-1,1′′-biferrocenes and their crystal packings and electrochemical properties are of interest. However, there are few studies on their structure–property relationships. In this study, we prepared and characterized the racemic (1a) and meso (1b) forms of 2,2′′-dibromo-1,1′′-biferrocene. Single-crystal X-ray structural analysis revealed that 1a possesses a twisted molecular structure with axial chirality and forms racemic crystals. In the crystal, 1a exhibited a pseudo-three-dimensional network structure constructed via Br⋯HC hydrogen bonding interactions, whereas 1b exhibited a pseudo-two-dimensional sheet structure owing to Br⋯Cp halogen bonding interactions (Cp = cyclopentadienyl). The monocations of 1a and 1b (1a⁺ and 1b⁺) showed broad intervalence charge-transfer bands at approximately 1900 nm. The reaction of 1a with iodine produced a monocation salt ([1a]I₇), and its crystal structure was determined. In the crystal, the cation moiety was contained in a cube-like polyiodide cage composed of I₃⁻ and I₂ molecules. The positive charge in the mixed-valence cation was localized to one of the two ferrocenyl units because of electrostatic interactions. Additionally, 1-bromo-2-cyanoferrocene, a byproduct of the reaction, was crystallographically characterized, revealing a helically assembled structure via intermolecular Br⋯NC halogen bonding in the crystal. The intramolecular and intermolecular interactions in these crystals are discussed based on Hirshfeld analyses and interaction energy calculations.