Mixed‐Valence Biferrocenium Salts of X_n–TCNQ (X = F, Cl; n = 1, 2): Correlation between Molecular Structures and Assembled Structures

Abstract

Charge‐transfer salts composed of biferrocene derivatives and Xn–TCNQ (X = F, Cl; n = 1, 2; TCNQ = tetracyanoquinodimethane) were prepared and crystallographically characterized. All nine salts exhibited a 1:2 donor/acceptor (D/A) ratio consisting of mixed‐valence biferrocenium cations and anions with acceptor dimers. The packing patterns were correlated to the donor substituents. On the basis of the π–π interactions of the cations, the observed structures were classified into three categories: segregated‐stack structures for the biferrocenium and dibromo‐biferrocenium salts, mixed‐stack structures [···(DA2)n···] for the diiodo‐biferrocenium salts, and intermediate structures [···(DDA2A2)n···] for the bromo‐biferrocenium salts.

Valence engineering of ionic molecular crystals: monovalent–divalent phase diagram for biferrocene–tetracyanoquinodimethane salts

“…Among them, the crystal structures and valence states of 1b, 1c, and 1d were reported previously. 9 The structure determination revealed that all of the salts were isomorphous with a 1:2 D/A ratio. 10 The packing diagrams of 3a and 3c are shown in Fig.…”

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

“…Salt 6 was a dichloromethane solvate, which is the sole example of a solvate crystal for biferrocenium–TCNQ salts. The donor–acceptor ratios and valence states (I, monovalent; II, divalent) of 1 – 6 and related salts ,, are summarized in Table . Half of the salts are newly synthesized in the current study.…”

“…We have previously investigated the crystal structures and valence states of biferrocenium salts with tetracyanoquinodimethane (TCNQ) and other organic acceptors, and found that these salts exhibit intriguing electronic properties and phase transition phenomena. − These salts generally exist in 1:1–1:3 D/A ratios, and their crystal structures can be classified into mixed-stack structures with alternating donor/acceptor stacking, segregated-stack structures with independent columnar π–π stacking of donors and acceptors, or an intermediate type (Figure a). ,, However, unlike organic charge-transfer complexes, there are often π–π contacts between the acceptors (or acceptor dimers) even in mixed-stack structures because of the nonplanar shape of the donor. Biferrocenium–TCNQ salts exhibit either monovalent state ([D] + [A m ] − ; m = 1–3) or divalent state ([D] 2+ [A m ] 2– ) (Figure b), depending on the balance between the redox potentials of the constituent molecules and the Coulombic (Madelung) energies of the crystal. − The Madelung energies depend on the charges, volumes, and arrangements of the constituent molecules. There are also a few biferrocenium–TCNQ salts that contain both [D] + and [D] 2+ . , A phase boundary between the monovalent and divalent salts has been determined for the mixed-stack 1:2 D/A structures (Figure ).…”

Anion-Ordering Phase Transitions in Biferrocenium and Ferrocenium Salts with the Bis(trifluoromethanesulfonyl)amide Anion

“…The large neopentyl substituent in 1 prevents columnar stacking of the cations, whereas the tendency of unsubstituted biferrocene to form columnar structures has also been observed in other salts. 16,17 The packing structures of 3 and 4 were complicated.…”