Valence Control of Ionic Molecular Crystals: Effect of Substituents on the Structures and Valence States of Biferrocenium Salts with Fluoro Tetracyanoquinodimethanides

In this work, the effect of substituents on the valence states of biferrocenium salts was investigated in order to explore the valence control of ionic molecular materials. Reactions of 1 ',1 "'-disubstituted biferrocene derivatives (R-2-bifc) and fluoro tetracyanoquinodimethanes (F-n-TCNQ; n = 1, 2, 4) produced single crystals of six salts (1-6), most of which were either monovalent ([D](+)[A(m)](-); m = 1-3) or divalent ([D](+)[A(m)](-)) salts. (Et-2-bifc)(F-1-TCNQ)(2) (1) was a mixed-stack monovalent salt, whereas the corresponding F-2-TCNQ salt was a divalent salt. (MeO2-bifc)(F-1-TCNQ)(2) (2) was a monovalent salt because of the small Madelung energy of its segregated-stack structure, despite the low redox potential of the donor. (MeS2-bifc)(2,6-F-2-TCNQ)(2) (3) was a mixed-stack salt with an apparent intermediate valence state ([D](1.5+)[A(2)](1.5-)), containing [D](2+) and [D](+) in a 1:1 ratio. Its valence state, which is intermediate between those of corresponding salts with F-1-TCNQ and 2,5-F-2-TCNQ, is probably related to the unsymmetrical crystalline environment. (Bu-2-bifc)(F-1-TCNQ)(3) (4) was a divalent salt with a segregated-stack structure, whereas the corresponding TCNQ salt was monovalent. (R-2-bifc)(F-4-TCNQ) [R = Bu (5), I (6)] were monovalent salts. Their magnetic susceptibilities were found to be consistent with their valence states. These results demonstrated that the valence control of biferrocenium salts could be achieved via the fluorine substitution of acceptors and crystal engineering.