American Chemical Society
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Control of Charge Transfer in a Series of Ru2II,II/TCNQ Two-Dimensional Networks by Tuning the Electron Affinity of TCNQ Units: A Route to Synergistic Magnetic/Conducting Materials

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posted on 2010-02-10, 00:00 authored by Hitoshi Miyasaka, Natsuko Motokawa, Satoshi Matsunaga, Masahiro Yamashita, Kunihisa Sugimoto, Tatsuya Mori, Naoki Toyota, Kim R. Dunbar
The isostructural series of two-dimensional (2-D) fishnet-type network compounds, [{Ru2(O2CCF3)4}2(TCNQRx)]·n(solv) (Rx = H4, 1; Br2, 2; Cl2, 3; F2, 4; F4, 5), has been synthesized from the reactions of a paddlewheel diruthenium(II, II) complex, [Ru2II,II(O2CCF3)4], and neutral TCNQ derivatives (TCNQRx = 2,3,5,6- or 2,5-halogen-substituted 7,7,8,8-tetracyanoquinodimethane) under anaerobic conditions. Corresponding Rh compounds 1-Rh5-Rh, which are diamagnetic and redox-inactive, were also synthesized for the purpose of comparison with 15. According to the electron affinity of TCNQRx, which is related to its first reduction potential, the Ru2 series (15) has the requisite driving force for charge transfer from [Ru2II,II(O2CCF3)4] to TCNQRx, which can lead to a mixed-valence state of [{Ru24.5+}-(TCNQRx• −)-{Ru24.5+}] for the 2-D network. Such a charge (or electron) transfer results in magnetic exchange interactions between [Ru2] units (S = 1 for [Ru2II,II] and S = 3/2 for [Ru2II,III]+) via TCNQRx• − S = 1/2 radicals that lead to long-range magnetic ordering in the layer. In the present series, only 5 demonstrated the full electron transfer (1-e transfer) to the mixed-valence state, whereas other members are essentially in the state [{Ru24+}-(TCNQRx0)-{Ru24+}]. Whereas 14 are paramagnetic, 5 is a metamagnet undergoing 3-D long-range antiferromagnetic ordering at 95 K (= TN) and reverts to a magnetic-field-induced ferromagnetic state exhibiting coercivity up to 60 K. This result is consistent with the fact that TCNQF4 has the strongest electron affinity among the TCNQRx molecules. Even in neutral forms, however, 14 can be observed to undergo thermally and/or field-activated charge transfers from [Ru2II,II] to TCNQRx to give semiconductors with an activation energy of 200−300 meV, which is a driving force to transport electrons over the lattice. As determined by their conducting properties, the ease of thermally and/or field-activated charge transfers is on the order of 1 < 4 < 235, which is in agreement with the order of electron affinity of TCNQRx. Indeed, a magnetic anomaly with short-range order associated with the localization of charge-transferred electrons was revealed in the low-temperature susceptibility data for 2 and 3. Finally, 5 was subjected to terahertz time-domain spectroscopy, the data from which revealed that transport hopping electrons scattered at high temperatures interact with magnetically ordered spins with the scattering being suppressed at TN, at which temperature the real part of the complex electronic conductivity (σ1) and dielectric permeability (ε1) are dramatically altered. From these collective data, we conclude that molecular design based on an interunit charge transfer in a paramagnetic lattice is an efficient route to the design of materials with synergism between magnetic and conducting properties.