Control of Charge Transfer in a Series of Ru₂^II,II/TCNQ Two-Dimensional Networks by Tuning the Electron Affinity of TCNQ Units: A Route to Synergistic Magnetic/Conducting Materials

The isostructural series of two-dimensional (2-D) fishnet-type network compounds, [{Ru₂(O₂CCF₃)₄}₂(TCNQR_x)]·n(solv) (R_x = H₄, 1; Br₂, 2; Cl₂, 3; F₂, 4; F₄, 5), has been synthesized from the reactions of a paddlewheel diruthenium(II, II) complex, [Ru₂^II,II(O₂CCF₃)₄], and neutral TCNQ derivatives (TCNQR_x = 2,3,5,6- or 2,5-halogen-substituted 7,7,8,8-tetracyanoquinodimethane) under anaerobic conditions. Corresponding Rh compounds 1-Rh−5-Rh, which are diamagnetic and redox-inactive, were also synthesized for the purpose of comparison with 1−5. According to the electron affinity of TCNQR_x, which is related to its first reduction potential, the Ru₂ series (1−5) has the requisite driving force for charge transfer from [Ru₂^II,II(O₂CCF₃)₄] to TCNQR_x, which can lead to a mixed-valence state of [{Ru₂^4.5+}-(TCNQR_x^• −)-{Ru₂^4.5+}] for the 2-D network. Such a charge (or electron) transfer results in magnetic exchange interactions between [Ru₂] units (S = 1 for [Ru₂^II,II] and S = 3/2 for [Ru₂^II,III]⁺) via TCNQR_x^• − 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⁺}-(TCNQR_x⁰)-{Ru₂⁴⁺}]. Whereas 1−4 are paramagnetic, 5 is a metamagnet undergoing 3-D long-range antiferromagnetic ordering at 95 K (= T_N) 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 TCNQR_x molecules. Even in neutral forms, however, 1−4 can be observed to undergo thermally and/or field-activated charge transfers from [Ru₂^II,II] to TCNQR_x 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 < 2 ≈ 3 ≪ 5, which is in agreement with the order of electron affinity of TCNQR_x. 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 T_N, 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.