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

Abstract

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.

Effect of substituents and anions on the phase behavior of Ru(ii) sandwich complexes: exploring the boundaries between ionic liquids and ionic plastic crystals

“…In the case of ferrocenium salts having cyano-substituted anions, such as tetracyanoquinodimethane (TCNQ), the cyano groups of the anion are often close to the Fe atom of the cation (Fe•••N = 4.1-4.7 Å). 22 Although these distances are larger than the sum of the respective van der Waals radii, such a close arrangement is accompanied by electrostatic interactions. 22 In the case of [C0] Cations and anions often form alternating arrangements in the crystals of sandwich complex salts.…”

“…22 Although these distances are larger than the sum of the respective van der Waals radii, such a close arrangement is accompanied by electrostatic interactions. 22 In the case of [C0] Cations and anions often form alternating arrangements in the crystals of sandwich complex salts. For example, metallocenium salts containing tetracyanoethylene anions exhibit a mixedstack structure, 23 whereas those with tetracyanoquinodimethane anions exhibit both mixedand segregated-stack structures.…”

Paramagnetic ionic plastic crystals containing the octamethylferrocenium cation: counteranion dependence of phase transitions and crystal structures

“…atom of the cation. This close arrangement causes cation-anion electrostatic interactions, although these distances are greater than the vdW contact distance 15. In[1]X (X = B(CN)4, C(CN)3, N(CN)2), two CN groups are located close to the Fe atom.…”

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

“…For 1 – 7 , the cations were located at the inversion centers, exhibiting symmetrical structures. The Fe–Cp (Cp = C 5 H 5 , centroid) distances were approximately 1.68 Å (Table 1) and were typical of biferrocenium cations undergoing rapid valence tautomerization 9a,12. The negative charges of the acceptors were estimated from the bond lengths13 and were determined to be approximately –0.5 (Table 1); therefore the dimer units have a negative charge of –1.…”

“…Several biferrocenium salts with organic acceptors have also been synthesized 5a,7. Previously, we reported the structures and electronic properties of alkyl‐ and thioalkyl‐biferrocenium salts containing F n –TCNQ (TCNQ = tetracyanoquinodimethane; n = 0, 1, 2, 4)8,9a,10 and [M(mnt) 2 ] (mnt = maleonitriledithiolate; M = Ni, Pt) 9,11. It was revealed that the valence state of the cations in these salts were dependent on the donor/acceptor (D/A) ratio.…”