A three-dimensional network of two-electron-transferred [Ru₂]₂TCNQ exhibiting anomalous conductance due to charge fluctuations

Abstract

A new D₂A-type charge-transfer three-dimensional network with a charge distribution of [{Ru₂}^δ+–(BTDA-TCNQ^2δ−)–{Ru₂}^δ+] where δ ≈ 1, which exhibits a sudden decrease in electronic resistance, formed from the assembly of a paddlewheel diruthenium(II, II) complex ([Ru₂]) as a donor (D) and bis(1,2,5-thiadiazolo)tetracyanoquinodimethane (BTDA-TCNQ) as an acceptor (A).

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Assembled systems of electron-donors (D) and acceptors (A) exhibit various interesting electronic/magnetic phenomena, such as high conductance,^1,2 emerging conductance involving neutral-ionic transition,³ and magnetic ordering of paramagnetic D⁺ and A⁻.⁴ Excluding strongly correlated systems⁵ and metal-to-metal charge-transfer systems based on metal complexes,⁶ most of the examples have involved organic aggregated systems.^1–4,7 Covalent-bonded D/A systems are still rare. Nevertheless, it is thought that controlling the charge distribution in the latter systems will bring about stronger electron/spin correlations than those in aggregated materials due to mediation of charge transfer through frontier orbitals. Recently, we have reported a family of D₂A-type multi-dimensional metal–organic framework (MOF) materials composed of carboxylate-bridged paddlewheel diruthenium(II, II) complexes (abbreviated as [Ru₂^II,II]) and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) derivatives as D and A, respectively.⁸ In this system, the degree of charge transfer is closely associated with the difference between the ionization potential (I_D) of D and electron affinity (E_A) of A (IE = I_D − E_A), the stabilization energy gain for ionic lattices (i.e., Madelung energy, M), and the on-site Coulomb repulsion (U) that occurs when electrons make a pair in an orbital. Neutral TCNQ (TCNQ⁰) and ionic forms (TCNQ˙⁻ and TCNQ²⁻) occur in relation to IE > M and IE < M, respectively, in which the two-electron (2-e⁻) reduced form, TCNQ²⁻, is stabilized when M is significantly larger than U. Thus, the degree of charge transfer controls the essential nature of a compound even in an identical covalent-bonded D₂A MOF. Indeed, 1-e⁻ transferred materials, which contain TCNQ˙⁻, have been reported to be magnets with relatively high T_c values and semiconducting properties,^8b,d–f whereas materials with TCNQ⁰ have been reported to be paramagnetic due to [Ru₂^II,II] units with S = 1.^8a,c,f

The D₂A-type three-dimensional (3-D) MOF compound [{Ru₂(m-FPhCO₂)₄}₂(BTDA-TCNQ)]·3.4CH₂Cl₂·1.6(p-Cltoluene) (1; m-FPhCO₂⁻ = m-fluorobenzoate; BTDA-TCNQ = bis(1,2,5-thiadiazolo)tetracyanoquinodimethane; p-Cltoluene = p-chlorotoluene) has been shown to be a ferromagnet with T_c = 107 K owing to a 1-e⁻ transfer in the D₂A units to afford the mixed valence state [–{Ru₂^II,II}–(BTDA-TCNQ˙⁻)–{Ru₂^II,III}⁺–]_∞, where [Ru₂^II,III]⁺ is in an S = 3/2 spin state and BTDA-TCNQ˙⁻ is a radical anion with S = 1/2.^8d Following the aforementioned mechanism, using an appropriate D with a low I_D should change the charge distribution, and thus, a dramatic change in magnetism should occur. In order to demonstrate this hypothesis in a 3-D system, we changed the D unit from [Ru₂^II,II(m-FPhCO₂)₄] in 1 to [Ru₂^II,II(m-CH₃PhCO₂)₄], which has a lower I_D than the former does (the redox potential E_1/2 in THFvs. Ag/Ag⁺ for these units are 42 and −99 mV, respectively). The obtained 3-D MOF, [{Ru₂(m-CH₃PhCO₂)₄}₂(BTDA-TCNQ)]·1.4CH₂Cl₂·2.3(p-xylene) (2; m-CH₃PhCO₂⁻ = m-methylbenzoate), which is the first example of 2-e⁻ transferred system in this series of materials, has a charge distribution of [{Ru₂}^δ+–(BTDA-TCNQ^2δ−)–{Ru₂}^δ+] with δ ≈ 1. Compound 2 is paramagnetic due to [Ru₂^II,III]⁺ units but exhibits anomalous conducting behavior, involving a sudden decrease in resistance in the high temperature region. We think that a thermally-induced charge fluctuation that causes δ to vary, which enables long-range transport of charged solitons through the 3-D MOF, could be the cause.

Compound 2, which was obtained as brown prismatic crystals by a slow diffusion of a solution of BTDA-TCNQ in p-xylene into a solution of [Ru₂^II,II(m-CH₃PhCO₂)₄(THF)₂] in CH₂Cl₂, crystallized in the triclinic space groupP1̄.§ Although the formula corresponds to a D₂A type complex, three [Ru₂] units, [Ru(1)–Ru(1)*], [Ru(2)–Ru(3)], and [Ru(4)–Ru(4)^##], and two BTDA-TCNQ units (respectively labeled hereafter as D1–D3 for the [Ru₂] units and A1 and A2 for the BTDA-TCNQ units) were structurally independent (symmetry operations: (*) 2 − x, 2 − y, −z; (##) 2 − x, −y, 1 − z; Fig. 1). [Ru(1)–Ru(1)*] and [Ru(4)–Ru(4)^##] and two BTDA-TCNQ units have inversion centers through their midpoints. The m-CH₃ groups of the four m-CH₃PhCO₂⁻ ligands bridging the Ru atoms adopt a (1,3)-orientation in D2 and a cis-(2,2)-orientation in D1 and D3.⁹ All [Ru₂] units are coordinated by BTDA-TCNQs; that is, BTDA-TCNQ acts as a μ₄-bridging ligand, which has been observed for all D₂A-type compounds regardless of their structures.⁸ A1 and A2 of BTDA-TCNQ coordinate only to D1/D2 and D2/D3, respectively, forming a 3-D infinite network similar to 1 (Fig. 2). The 3-D network adopts a “bundle of helical chains” motif. Δ and Λ helical chains alternate along the <1 0 –1> direction in the bundle, whereas identical topologies are appeared along the a axis. The pores of the bundles are occupied by crystallization solvent molecules (1.4CH₂Cl₂ and 2.3(p-xylene)). Each chain has the following arrangement when projected along the b axis: [⋯D1–cis-A1–D2–cis-A2–D3–syn-A2–D2–syn-A1⋯], where syn- and cis-A designations are used for BTDA-TCNQ groups bridging through the 7,8-position (syn) and 7,7-position (cis) cyano groups, respectively (Fig. 2). The two remaining cyano groups are part of adjacent chains.

Fig. 1 ORTEP plot of 2 (50% probability ellipsoids; symmetry operations (*) 2 − x, 2 − y, −z; (**) 1 − x, 2 − y, −z; (#) 1 − x, 1 − y, 1 − z; (##) 2 − x, −y, 1 − z). Hydrogen atoms and solvent molecules are omitted for clarity.

Fig. 2 Packing diagrams for 2. The equatorial m-CH₂PhCO₂⁻ ligands located around the [Ru₂] center (brown) and solvent molecules are omitted for clarity. Δ and Λ symbols stand for helical topologies of the [⋯D1–cis-A1–D2–cis-A2–D3–syn-A2–D2–syn-A1⋯] chains.

The bond lengths in the [Ru₂] and BTDA-TCNQ units can be used to estimate the degree of charge transfer from [Ru₂] to BTDA-TCNQ. The Ru–N_ax (N_ax = cyano nitrogen of BTDA-TCNQ) distances are in the range of 2.215(2)–2.232(3) Å. They were classified as [Ru₂^II,III]⁺–N_TCNQ type bonds (N_TCNQ = cyano nitrogen of TCNQ or its anion) rather than [Ru₂^II,II]–N_TCNQ type bonds because [Ru₂^II,III]⁺–N_TCNQ and [Ru₂^II,II]–N_TCNQ bonds are usually in the range of 2.21–2.24 Å and 2.25–2.30 Å, respectively.⁸ The Ru–O_eq (O_eq = carboxylate oxygen) bond length characteristically reflects the oxidation state of [Ru₂], which is, in general, found in the range of 2.07–2.09 Å for [Ru₂^II,II] and 2.01–2.03 Å for [Ru₂^II,III]⁺.¹⁰ The average Ru–O_eq lengths for D1–D3 are: 2.021, 2.024, and 2.022 Å, respectively, meaning that each unit is in an [Ru₂^II,III]⁺ state (Table S1, ESI†). Although it is difficult to estimate accurately the oxidation state of BTDA-TCNQ from its structure, a rough trend can still be observed as in the case of TCNQ. However, the estimated values have a tendency to be −0.1 to −0.5 larger than the expected value (Table S2, ESI†). The charge on the BTDA-TCNQ moieties was estimated by using the Kistenmacher relationship,¹¹ρ = 2δ = A_ρ[c/(b+d)]+B_ρ, in relation to neutral BTDA-TCNQ (ρ = 0)¹² and [NEt(Me)₃]BTDA-TCNQ (ρ = −1)¹³ with A_ρ = −50.00 and B_ρ = 23.25. The estimated values for A1 and A2 in 2 are ρ_A1 = −2.52 and ρ_A2 = −2.51, respectively, suggesting that BTDA-TCNQ is in a −2 oxidation state. For 1, ρ = −1.45 suggesting that BTDA-TCNQ˙⁻ is present.^8d Thus, two [Ru₂^II,II] units transferred a full charge to yield BTDA-TCNQ²⁻, so the charge distribution can formally be written as [{Ru₂^II,III}⁺–BTDA-TCNQ²⁻–{Ru₂^II,III}⁺].

The difference of charge distribution between 1 and 2 is observed in their absorption spectra (Fig. S1, ESI†). While the wide absorption in continuity to lower energies can be observed in 1, a peak of absorption, which could be attributed to a charge-transfer band of [Ru₂] → BTDA-TCNQ, is found at around 0.8 eV in 2. This feature is rather similar to that of the neutral one.^8f

The charge distribution is supported by the magnetic behavior. Because BTDA-TCNQ²⁻ is diamagnetic, the paramagnetic nature of [Ru₂^II,III]⁺ should be dominant. χ and χT vs. T plots for 2 is shown in Fig. 3a. The χT value of 4.38 cm³ K mol⁻¹ at 300 K continuously decreased to 0.28 cm³ K mol⁻¹ at 1.8 K, whereas that of 1 increased with a decrease in T, owing to ferromagnetic ordering between [Ru₂] units viaBTDA-TCNQ˙⁻.^8d The χ and χT values were simulated in the temperature range of 10–300 K by using a Curie paramagnetic model with S = 3/2 involving zero-field splitting (D), temperature-independent paramagnetism (χ_TIP), and intermolecular interactions (zJ) commonly used for magnetically-isolated or weakly-interacting [Ru₂^II,III]⁺ complexes.^14,15zJ was introduced in the framework of the mean-field approximation (z = number of adjacent magnetic centers). The best fit of parameters were: g = 2.206(4), D/k_B = 82(1) K, zJ/k_B = −8.58(6) K, and χ_TIP = 538(39) × 10⁻⁶ cm³ mol⁻¹ with R = 1 − Σ[(χT_calc − χT_obs)Σ(χT_obs)]² = 0.99996 (fitted curves are in red in Fig. 3a). The obtained values of g, which are larger than 2.00, and D are typical for [Ru₂^II,III]⁺ complexes.^14,15 The large zJ value indicates the presence of antiferromagnetic exchange interactions between [Ru₂^II,III]⁺ units viaBTDA-TCNQ²⁻. We attributed the relatively large exchange to thermally-activated charge transfer between [Ru₂] and BTDA-TCNQ units (vide infra). Although these results indicate that the full charge distribution is [{Ru₂^II,III}⁺–BTDA-TCNQ²⁻–{Ru₂^II,III}⁺], analysis of the electron transport property of 2 showed that thermally-induced charge fluctuations occurred between [Ru₂^II,III]⁺ and BTDA-TCNQ²⁻.

Fig. 3 (a) Temperature dependence of χ and χT of 2 and (b) the resistivity (R) of a single crystal of 1 and 2.

Fig. 3b shows plots of electronic resistivity of single crystals of 1 and 2 as a function of T (two-probe method with a fixed voltage at 1 V). The resistivity for 2 at 300 K was 8.0 × 10⁴ Ω cm and increased gradually with a decrease in T to 290 K, like a general semiconductor. However, it suddenly decreased to 7.8 × 10² Ω cm at 277 K and then increased again with an inflection at 263 K that appeared only during the cooling process (activation energy E_a = 1.08 eV for the cooling process at 225–260 K). This anomalous feature was confirmed on eight crystals taken from different batches. On the basis of the magnitude of the resistivity, 2 (as well as 1) should be a semiconductor over the entire T range measured. However, such a decrease in the resistivity is not normal. Since no first-order phase transition was observed at the inflection T (290, 277, and 263 K), this behavior is probably due to a continuous decrease in the energy gap between the valence and conducting bands originating from transports of the partial gradient of charge distribution like charged solitons via thermally-induced charge fluctuations between [Ru₂^II,III]⁺ and BTDA-TCNQ²⁻. The generation of charged solitons concomitantly competes with structural fluctuations at high T. Hence, when the structural fluctuations become suppressed at lower T, transport of charged solitons becomes active, inducing a decrease of resistance (280–290 K). At the same time, the charge fluctuation is thermally sensitive, and the occurrence of charged solitons gradually decreases with a decrease in T because of recoupling of (±)-soliton pairs, at which point general semiconducting properties are observed (<277 K). It should be mentioned that this anomalous conducting behavior has not been observed for 1 (R = 1.1 × 10⁴ Ω cm at 300 K; E_a = 353 meV for the cooling process in the T range of 200–300 K; Fig. 3b) and already reported neutral and 1-e⁻ transferred 2-D network compounds. This can be explained by their structural symmetry (2 has a lower symmetry, containing asymmetric D1–D3 and A1 and A2 in spite of its D₂A formulation, than others) and by the fact that the competitive relationship between M and U only occurs in a 2-e⁻ transferred system, like 2.

Herein, a 2-e⁻ transferred D₂A-type 3-D compound was synthesized by using a D unit with a relatively low I_D. Although the full charge distribution [{Ru₂^II,III}⁺–BTDA-TCNQ²⁻–{Ru₂^II,III}⁺] was supported by magnetic studies, thermally-induced charge fluctuations occurred (magnetically unrelated), which caused long-range transport of charged solitons through the 3-D MOF. Precise band-filling may be efficient to increase the conductance in this series of D/A MOFs; this is an intriguing issue and now in progress.

The authors acknowledge Dr B. K. Breedlove at Tohoku University for his helpful discussion. This work was financially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (Grant No. 21350032).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Tables S1 and S2 and details of experiments. CCDC 781152. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0cc02031a

‡ This article is part of the ‘Emerging Investigators’ themed issue for ChemComm.

§ Anal. calcd for [{Ru₂(m-CH₃PhCO₂)₄}₂(BTDA-TCNQ)]·1.4CH₂Cl₂·2(p-xylene), C_93.4H_78.8Cl_2.8N₈O₁₆Ru₄S₂: C, 52.50; H, 3.72; N, 5.24%. Found: C, 52.28; H, 3.80; N, 5.26%. IR (KBr, cm⁻¹): ν 2187, 2133, 2110 (CN); details of synthesis of 2 are described in ESI†. Crystal data of 2. C_95.8H_81.8Cl_2.8N₈O₁₆Ru₄S₂, M_r = 2168.81, triclinic, P1̄ (#2), a = 15.2978(13) Å, b = 15.5927(13) Å, c = 22.148(2) Å, α = 88.841(4)°, β = 86.538(4)°, γ = 62.085(2)°, V = 4659.7(8) ų, T = 103.1 K, Z = 2, D_calc = 1.546 g·cm⁻³, F₀₀₀ = 2192.40, λ = 0.71070 Å, μ(Mo-Kα) = 8.307 cm⁻¹, 31 442 measured reflections, 15 966 unique. R₁ = 0.0388 (I > 2σ(I)), and wR₂ = 0.0978 (all data) with GOF = 1.090. CCDC 781152.

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