Charge-Transfer Salts of Biferrocene Derivatives with F₂- and F₄‑Tetracyanoquinodimethane: Correlation Between Donor–Acceptor Ratios and Cation Valence States

Charge-transfer salts of biferrocene derivatives bearing branched-alkyl substituents [1′-R1-1‴-R2-1,1″-biferrocene; R1 = R2 = isopropylthio (D1), isopropyl (D2), isobutyl (D3), neopentyl (D4), and R1 = isobutyl and R2 = neopentyl (D5)] were prepared and crystallographically characterized. F2- and F4-tetracyanoquinodimethane (TCNQ) produced salts with D/A ratios of 1:3 ([D1]­[F2TCNQ]3, [D2]­[F2TCNQ]3), 1:2 ([D2]­[F4TCNQ]2, [D3]­[F4TCNQ]2), 2:3 ([D1]2­[F4TCNQ]3), and 1:1 ([D2]­[F4TCNQ], [D4]­[F2TCNQ], [D4]­[F4TCNQ], [D5]­[F4TCNQ]). [Ni­(mnt)2] produced a 1:1 salt [D3]­[Ni­(mnt)2]. Although the biferrocenium salts reported to date contain only monocations, the cation valence in these salts decreases as the donor/acceptor ratio increases; the 1:3 and 1:2 salts contain biferrocenium dications, the 1:1 salts contain mixed-valence biferrocenium monocations, and the intermediate 2:3 salt contains both the dication and monocation. The packing structures of the salts differ significantly despite being composed of donors and acceptors with very similar shapes. The salts are paramagnetic, and their magnetic susceptibility values are consistent with the valence state of the cations. The cations in the 1:1 salts exhibited valence-trapped states because of the local electrostatic interactions between the cation and anion.