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

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 & ctdot;HC hydrogen bonding interactions, whereas 1b exhibited a pseudo-two-dimensional sheet structure owing to Br & ctdot;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 & ctdot;N 00000000000000000 00000000000000000 00000000000000000 01111111111111110 00000000000000000 01111111111111110 00000000000000000 01111111111111110 00000000000000000 00000000000000000 00000000000000000 C halogen bonding in the crystal. The intramolecular and intermolecular interactions in these crystals are discussed based on Hirshfeld analyses and interaction energy calculations.