Charge-Transfer Salts of Biferrocene Derivatives with F 2 - and F 4 ‑Tetracyanoquinodimethane: Correlation Between Donor− Acceptor Ratios and Cation Valence States Tomoyuki Mochida,* ,†,‡ Yusuke Funasako, † Eri Nagabuchi, ‡ and Hatsumi Mori § † Department of Chemistry, Graduate School of Science, Kobe University, Kobe, Hyogo 657-8501, Japan ‡ Department of Chemistry, Faculty of Science, Toho University, Miyama, Funabashi, Chiba 274-8510, Japan § Institute for Solid State Physics, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan * S Supporting Information ABSTRACT: Charge-transfer salts of biferrocene derivatives bearing branched-alkyl substituents [1′-R 1 -1‴-R 2 -1,1″-biferro- cene; R 1 =R 2 = isopropylthio (D1), isopropyl (D2), isobutyl (D3), neopentyl (D4), and R 1 = isobutyl and R 2 = neopentyl (D5)] were prepared and crystallographically characterized. F 2 - and F 4 -tetracyanoquinodimethane (TCNQ) produced salts with D/A ratios of 1:3 ([D1][F 2 TCNQ] 3 ,[D2][F 2 TCNQ] 3 ), 1:2 ([D2][F 4 TCNQ] 2 , [D3][F 4 TCNQ] 2 ), 2:3 ([D1] 2 - [F 4 TCNQ] 3 ), and 1:1 ([D2][F 4 TCNQ], [D4][F 2 TCNQ], [D4][F 4 TCNQ], [D5][F 4 TCNQ]). [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. ■ INTRODUCTION The crystal engineering of supramolecular organometallic materials including metallocene-based salts has become an important area of research in recent years. 1 Many charge- transfer salts of ferrocene derivatives have been synthesized to date, from the viewpoint of magnetism and electrical conductivity. 2−4 Among ferrocene-related organometallics, biferrocene is one of the most well-known electron donors and exhibits three redox states (Figure 1): neutral, monocation, and dication. Electron transfer in mixed-valence monocation salts has attracted considerable attention. The investigation of these salts by Mö ssbauer spectroscopy and other methods has revealed that their valence states are affected by molecular symmetry and the crystalline environment. 5 Recently, we have investigated the preparation and properties of biferrocenium salts with F n TCNQ (n = 0, 1, 4; TCNQ = tetracyanoquinodimethane) and [M(mnt) 2 ] anions (mnt = maleonitriledithiolate; M = Ni, Co), which contain mixed- valence monocations. 6 These salts exhibit intriguing physical properties such as magnetism, dielectric properties, 6c electrical conduction, 6a and phase transition phenomena. 6b,c Several Mö ssbauer spectroscopy studies on biferrocene−TCNQ salts have been reported in the literature. 7 The correlation between the valence states and the assembled structures of biferroce- nium salts are especially interesting from the viewpoint of crystal engineering. In this study, we have prepared charge-transfer salts of a series of biferrocene derivatives bearing branched-alkyl substituents [1′-R 1 -1‴-R 2 -1,1″-biferrocene; R 1 =R 2 = iso- propylthio (D1), isopropyl (D2), isobutyl (D3), neopentyl (D4), and R 1 = isobutyl and R 2 = neopentyl (D5)] (Figure 2a). D5 is an unsymmetrical donor having an intermediate molecular volume between that of D3 and D4. Ten salts with F 2 TCNQ, F 4 TCNQ, and [Ni(mnt) 2 ] were obtained and Received: January 2, 2014 Revised: February 1, 2014 Published: February 13, 2014 Figure 1. Three redox states of biferrocene. Article pubs.acs.org/crystal © 2014 American Chemical Society 1459 dx.doi.org/10.1021/cg500006e | Cryst. Growth Des. 2014, 14, 1459−1466