Acceptor Layered Frameworks with

Jan 28, 2015 - A diagram for ΔEH−L(DA) vs |2E1/2(A) − 1E1/2(A)|, where 2E1/2(A) and 1E1/2(A) are the second and first redox potentials of TCNQRx,...
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Fully Electron-Transferred Donor/Acceptor Layered Frameworks with TCNQ2− Wataru Kosaka,†,‡ Takaumi Morita,‡ Taiga Yokoyama,‡ Jun Zhang,‡ and Hitoshi Miyasaka*,†,‡ †

Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan

‡

S Supporting Information *

ABSTRACT: In a series of two-dimensional layered frameworks constructed by two electron-donor (D) and one electronacceptor (A) units (a D2A framework), two-electron transferred systems with D+2A2− were first synthesized as [{Ru2(RPhCO2)4}2(TCNQRx)]·n(solv) (R = o-CF3, Rx = H2 (1), R = o-CF3, Rx = Me2 (2), R = o-CF3, Rx = F4 (3), R = o-Me, TCNQRx = BTDA-TCNQ (4), R = p-Me, TCNQRx = BTDA-TCNQ (5), where TCNQ is 7,7,8,8-tetracyano-p-quinodimethane and BTDA-TCNQ is bis[1,2,5]dithiazolotetracyanoquinodimethane). The D+2A2− system was synthesized by assembling D/A combinations of paddlewheel-type [Ru2II,II(R-PhCO2)4] complexes and TCNQRx that possibly caused a large gap between the HOMO of D and the LUMO of A (ΔEH−L(DA)). All compounds were paramagnetic because of quasi-isolated [Ru2II,III]+ units with weakly antiferromagnetically coupled S = 3/2 spins via diamagnetic TCNQRx2− and/or through the interlayer space. The ionic states of these compounds were determined using the HOMO/LUMO energies and redox potentials of the D and A components in the ionization diagram for ΔEH−L(DA) vs ΔE1/2(DA) (= E1/2(D) − E1/2(A); E1/2 = first redox potential) as well as by previously reported data for the D2A and DA series of [Ru2]/TCNQ, DCNQI materials. The boundary between the oneelectron and the two-electron transferred ionic regimes (1e−I and 2e−I, respectively) was not characterized. Therefore, another diagram for ΔEH−L(DA) vs |2E1/2(A) − 1E1/2(A)|, where 2E1/2(A) and 1E1/2(A) are the second and first redox potentials of TCNQRx, respectively, was used because the 2e−I regime is dependent on on-site Coulomb repulsion (U = |2E1/2(A) − 1 E1/2(A)|) of TCNQRx. This explained the oxidation states of 1−5 and the relationship between ΔEH−L(DA) and U and allowed us to determine whether the ionic regime was 1e−I or 2e−I. These diagrams confirm that a charge-oriented choice of building units is possible even when designing covalently bonded D2A framework systems.

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INTRODUCTION Charge transfer molecular systems composed of donor (D) and acceptor (A) units are attractive targets for use in functional materials such as magnets,1−4 highly conductive materials,5−7 ferroelectric materials,8 and materials in which these types of materials act synergistically.9 Controlling electron transfer from the neutral (D0A0) to the ionic state (Dδ+Aδ−) is key in producing the desired functionalities in these materials.10 The degree of charge transfer (δ) can be anticipated when a D unit with a specific ionization potential (ID) and an A unit with a specific electron affinity (EA) are used. An electrostatic energy gain in the ionic lattice (i.e., the Madelung energy; M) larger than the difference between the ID and the EA (IE = ID − EA; the energy required to ionize the D/A set) generally causes the © XXXX American Chemical Society

stabilization of the ionic (I) state in the assembled system. On the other hand, the opposite relationship leads to the stabilization of the neutral (N) state. It should be noted that when the IE is approximately the same as M, both the N and the I states have similar energy levels and either of them can be perturbed by an external stimulus (such as a change in the temperature11 or pressure12 or the application of light13), which would cause a neutral−ionic (N−I) phase transition to occur.14 A D2A system with an acceptor unit (A) that can be in either the A− or the A2− state can have three oxidation states, D02A0 (neutral state; N), D+D0A−, or D0.5+2A− (one-electron (1e−) Received: October 15, 2014

A

DOI: 10.1021/ic502513p Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry transferred ionic state; 1e−I) and D+2A2− (2e− transferred ionic state; 2e−I), as shown in Scheme 1. The tuning between the N

difficult to identify the boundary between the 1e−I and the 2e− I states in an ionic D2A system using the diagram.10,15i In fact, only one example of the 2e−I state, i.e., [{Ru 2 (mCH 3 PhCO 2 ) 4 } 2 {BTDA-TCNQ}]·1.7CH 2 Cl 2 ·2.3(p-xylene) (6), has been reported;15h therefore, further studies are required to improve our understanding of the electron transfer mechanism occurring in the [Ru2]2TCNQRx system. Here, we describe five new 2e− transferred two-dimensional [Ru2]2TCNQRx compounds that have been rationally designed using a [Ru2] unit with a relatively strong donating character and a selected TCNQRx unit. The materials produced were [{Ru2(o-CF3PhCO2)4}2(TCNQ)]·2(PhNO2) (1; PhNO2 = nitrobenzene), [{Ru 2 (o-CF 3 PhCO 2 ) 4 } 2 {TCNQ(Me) 2 }]· 0.2CH2Cl2·1.8(p-Cltoluene) (2; p-Cltoluene = p-chlorotoluene), [{Ru2(o-CF3PhCO2)4}2(TCNQF4)]·2(p-Cltoluene) (3), [{Ru2(o-CH3PhCO2)4}2{BTDA-TCNQ}]·p-xylene (4), and [{Ru2(p-CH3PhCO2)4}2{BTDA-TCNQ}]·2(p-xylene)· CH2Cl2 (5), in which y-CF3PhCO2− and y-CH3PhCO2− are ortho- or para-substituted trifluoromethylbenzoate and methylbenzoate, respectively, and TCNQ(Me)2, TCNQF4, and BTDA-TCNQ are 2,5-dimethyl-7,7,8,8-tetracyanoquinodimethane, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, and bis[1,2,5]dithiazolotetracyanoquinodimethane, respectively. These compounds when added to the ΔEH−L(DA) vs ΔE1/2(DA) ionization diagram showed a linear relationship even for mixed 1e−I and 2e−I systems. Finally, another diagram, plotting ΔEH−L(DA) vs |2E1/2(A) − 1E1/2(A)|, where 2 E1/2(A) and 1E1/2(A) are the second and first redox potentials, respectively, of TCNQRx,10 was produced using the data for compounds 1−5. This diagram showed that the boundary between the 1e−I and the 2e−I regimes was controlled by the on-site Coulomb repulsion (U) of TCNQRx.

Scheme 1. Schematic Representations of the TwoDimensional Layered D2A Frameworks Constructed Using Paddlewheel [Ru2] Units and TCNQRx, and the Charge Distributions in the D2A Unit

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RESULTS AND DISCUSSION Syntheses and Characterization. Compounds 1−5 were synthesized using a method similar to that previously used to synthesize other [Ru2]2TCNQ materials.15 The method allowed the slow mixing of two solutions by diffusion. The donor solution was the bottom layer, and the acceptor solution was the top layer. The bottom solution contained the THF adduct of the [Ru2II,II] complex in nitrobenzene for the synthesis of 1 or in CH2Cl2 for the syntheses of 2−5. The top layer contained the appropriate TCNQRx in an aromatic solvent (nitrobenzene for the synthesis of 1, p-Cltoluene for the syntheses of 2 and 3, or p-xylene for the syntheses of 4 and 5). It should be noted that 1 was synthesized using only one solvent system (PhNO2). However, using only PhNO2 as the solvent in the synthesis of 1 led to the production of a small amount of needle-type crystals of [Ru2II,II(oCF3PhCO2)4(PhNO2)2] together with the main product, block-type crystals of 1, although the impurity and main product were distinguishable by their crystal shapes. Most of the compounds were obtained quantitatively, but 3 was produced in an extremely low yield. The infrared spectra of the compounds exhibited characteristic ν(CN) stretches that were different from those observed in materials with TCNQRx0 and TCNQRx•− units (only a qualitative evaluation was meaningful because the CN groups in the TCNQRx moiety coordinated to [Ru2] units; vide infra). The observed two or three ν(CN) stretches (see Experimental Section) were shifted to lower energies than the corresponding features in the materials with the TCNQRx•− units, which were generally found at lower energies than the

state and the 1e−I state can be described using the general relationship between the ID of D and the EA of A with the boundary given by M (see above). However, the 2e−I state can only be achieved when strong D units are used with very small ID values that are higher than the on-site Coulomb repulsion energy (U) produced when paired electrons form in an orbital of A. We found these charge/electron transfer mechanisms in D/A systems of metal−organic frameworks (we call these materials D/A-MOFs) constructed from the metal complex donors of carboxylate-bridged paddlewheel diruthenium(II, II) complexes (abbreviated as [Ru2II,II] or [Ru24+]) and organic polycyano acceptors such as TCNQ derivatives with x substitutions of R moieties (later abbreviated to TCNQRx) and N,N′-dicyanoquinonediimine (DCNQI) derivatives,10,15−18 while several metal−TCNQRx high-dimensional frameworks have been known.19−24 These D/A combinations allowed the production of D2A15,18 and DA16,17 multidimensional systems based mainly on TCNQRx and DCNQIRx, respectively. Summarizing the results of these studies, we successfully obtained a linear relationship that could be used to predict charge/electron transfer in systems with different D/A combinations from an ionization plot of ΔEH−L(DA) against ΔE1/2(DA), where ΔEH−L(DA) is the energy gap between the HOMO level of the [Ru2II,II] unit and the LUMO level of the TCNQRx/DCNQIRx unit conventionally estimated by DFT studies, and ΔE1/2(DA) is the difference between the first redox potentials of the [Ru2II,II] and TCNQRx/DCNQIRx units. This diagram clarified the boundary between the N and the I states, but it is still B

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the plane. The stacking angles were 76.3° for 1, 73.9° for 2, 76.3° for 3, 54.7° for 4, and 63.9° for 5. The local distances in the structures characteristically reflected the oxidation states of the component units. For the [Ru2] units, the Ru−Oeq (Oeq = equatorial oxygen atoms) bond distance was a good indicator of the oxidation state. In general, the Ru−Oeq bond lengths were 2.06−2.08 Å for [Ru2II,II] and 2.02−2.04 Å for [Ru2II,III]+. For comparison, the crystal structures of [Ru2II,II(R−PhCO2)4(THF)2] and [Ru2II,III(R− PhCO2)4(THF)2]BF4 (R = o-Me, m-Me, or o-CF3) were determined, and the structures and bond distances are summarized in Figures S1 and S2 and Tables S1−S4, Supporting Information.26 The average Ru−Oeq bond length in 1−5 was within a narrow range of 2.018−2.038 Å (the important distances around the [Ru2] units in 1−5 are listed in Table S5, Supporting Information). Without exception, all lengths were in the range for the [Ru2II,III]+ oxidation state. The component bond lengths in the TCNQRx moieties were also good indicators of the oxidation states of the units, and it was convenient to use the Kistenmacher relationship (see the caption of Table S6, Supporting Information) to estimate the charge ρ on TCNQRx;27 the bond lengths in the TCNQRx moieties and the estimated ρ values for 1−5 and typical TCNQRx compounds are given in Table S6, Supporting Information. Compounds 1−3 were evaluated by comparing them with TCNQ0 (ρ = 0)28 and Rb+TCNQ•− (ρ = −1),29 while 3 was also compared with TCNQF40 (ρ = 0)30 and (nBu4N)+TCNQF4•− (ρ = −1).19b,31 Compounds 4 and 5 were evaluated using BTDA-TCNQ0 (ρ = 0) 32 and [NEt(Me)3]+(BTDA-TCNQ)•− (ρ = −1).33 For 1−5, all estimated ρ values were around 2, which was consistent with a set of two [Ru2II,III]+ units. The formal charge distribution for the compounds was written as D+2A2−, i.e., they were 2e− transferred D 2 A systems similar to the [{Ru 2 (mCH3PhCO2)4}2(BTDA-TCNQ)] system that has been described previously.15h Magnetic Properties of 1, 2, 4, and 5. The magnetic properties of all compounds except 3 were investigated as a function of temperature in the range 1.8−300 K. The χT products of 1, 2, 4, and 5 are plotted in Figure 4 (χ is the magnetic susceptibility; χ = M/H, in which H = 1 kOe). The magnetic properties of 3 could not be investigated because of its extremely low yield. The charge distributions in the D+2A2− systems implied that paramagnetism should have arisen from isolated [Ru2II,III]+ units with S = 3/2. Indeed, the χT−T features of all compounds were essentially identical and very similar to the features of isolated [Ru2II,III]+ species.34,35 This was consistent with the conclusion that the intralattice charge distribution was D+2A2−. Cooling the compounds from 300 K led to the χT product continually decreasing, gradually at higher temperatures and more steeply below ∼150 K. This behavior was simulated at temperatures higher than 10 K using the Curie equation with S = 3/2, taking into account the zerofield splitting (D), temperature-independent paramagnetism (χTIP), and interunit interactions (zJ) in the mean-field approximated frame.36 The parameter sets that were found to be adequate are summarized in Table S7, Supporting Information. The parameters that were obtained essentially agreed with those for [Ru2II,III]+ species (D for [Ru2II,III]+ is typically 70−115 K),34,35 but the interunit interactions (antiferromagnetic) were relatively strong (zJ = −6 to −12 K), probably because of the presence of TCNQ2− or the interlayer space.15h It should be noted that the fitting procedure

features in the materials with neutral moieties (Figure 1). Indeed, a gradual red shift of frequency depending on the

Figure 1. Infrared spectra in the range 2000−2300 cm−1 of 1−6, TCNQ0, Li+TCNQ•−, and (dabcoH+)2(dabco)TCNQ2−, measured using KBr pellets at room temperature.

oxidation state (from 0 to −2) could be seen in the TCNQ family, with TCNQ0 giving a ν of 2222 cm−1, Li+TCNQ•− giving ν of 2210 and 2183 cm−1, and (dabcoH+)2(dabco)TCNQ2− giving ν of 2152 and 2104 cm−1 (Figure 1).25 These results indicate that the TCNQRx moieties of the compounds described here were strongly reduced by more than −1. Structures of 1−5 and an Evaluation of Their Charge Distributions. All compounds crystallized in the triclinic P−1 space group with a formula of two [Ru2] units and one TCNQRx unit, each of which had an inversion center (Z = 1) (Figure 2). The axial positions of both [Ru2] units were coordinated with four TCNQRx cyano groups to form a fishnet-type two-dimensional layer with a composition of D2A (Figure 3), which is a network structure similar to that previously found in other D2A compounds.15a,b,e−g,j The layers lay on the (100) plane for 1−3, (110) plane for 4, and (10−1) plane for 5, and the interlayer vertical distances were 9.91 for 1, 9.63 for 2, 9.86 for 3, 8.77 for 4, and 10.44 Å for 5. The interlayer environment associated with the interlayer distance and the stacking mode was strongly affected by the number and orientation of the crystallization solvent molecules in the void spaces between the layers (Figure S5 for 1−3 and Figure S6 for 4 and 5, Supporting Information) and by the framework structure of the layers. From a perpendicular view of the layer planes, the planes in 1−3 were stacked almost in-phase, whereas the planes in 4 were largely staggered in an antiphase manner, and the planes in 5 were almost in between the inphase and antiphase patterns (Figure 3). Each pattern was determined from the stacking angle, which is defined as the angle between the plane and the shortest translation vector for C

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Figure 2. Thermal ellipsoid plots (using 50% probability ellipsoids) of the formula units, with atom numbering schemes, in 1−5. O, C, N, F, S, and Ru are shown in red, gray, blue, green, yellow, and purple, respectively. Crystallization solvent molecules and hydrogen atoms are omitted for clarity.

state and D+D0A− or D0.5+2A− in the 1e−I state. This indicates that the ID of the [Ru2II,II] unit was significantly lower than the EA of the TCNQRx units and that the difference was sufficiently large to overcome the on-site Coulomb repulsion energy U (= 2 E1/2(A) − 1E1/2(A)) on TCNQRx, which has been found to be 450−650 mV for the general TCNQ series.10 We recently proposed a simple method for predicting whether the electronic state is N or I in D/A-MOF and that this method involved evaluating the energy gap between the HOMO level of the D unit and the LUMO level of the A unit, ELUMO(A) −

was conducted excluding the low-temperature region (and some were simulated using a fixed χTIP) to avoid obtaining a meaningless numerical distribution between zJ and D, both of which contributed to the decrease in χT, and the overparameterization that would occur if any spin impurities were taken into account. Molecular Orbital Calculations for the [Ru2II,II] Units, and Ionization Diagrams. Five new 2e−I compounds, 1−5, were successfully isolated with similar two-dimensional D2A frameworks to those previously observed for D02A0 in the N D

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Figure 3. Vertical views of the D2A layers in (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5. C, N, S, and Ru atoms are shown in gray, blue, yellow, and purple, respectively. Equatorial carboxylate ligands of the [Ru2] units and hydrogen atoms are omitted for clarity.

It should be mentioned that the ΔEH−L(DA) vs ΔE1/2(DA) plot cannot be used to identify the boundary between the 1e−I and the 2e−I regimes because the distinction between these regimes is dependent on the used TCNQRx unit. The magnitude of U increased in the order of TCNQ(MeO)2 < TCNQ(Me)2 < BTDA-TCNQ < TCNQBr2 < TCNQ ≈ TCNQCl2 < TCNQF4 ≈ TCNQF2, such that the boundary separating the 1e−I and 2e−I regimes should be an inverse diagonal in the ΔEH−L(DA) vs |2E1/2(A) − 1E1/2(A)| (= U) plot.10 The ΔEH−L(DA) vs |2E1/2(A) − 1E1/2(A)| plot for all compounds relevant to 1−5 is shown in Figure 6. In this diagram, the boundary between N and I is given as a horizontal line, ΔEH−L(DA) = 0. The boundary between 1e−I and 2e−I stated in the ionic regime (ΔEH−L(DA) < 0) is dependent on the TCNQRx unit, so the slope of the line representing the boundary between the 1e−I and the 2e−I regimes could be defined by the linear line ELUMO(TCNQRx) = 0.0064505·U + α, where ELUMO(TCNQRx) is the LUMO energy of TCNQRx (in electronvolts), U is |2E1/2(A) − 1E1/2(A)| (mV), and α is a variable defined by the series.10 In the current diagram, the line passed through the point for 6 (ΔEH−L(DA) = −0.5981 eV and U = 544 mV for BTDA-TCNQ), which was the basis for the boundary between the 1e−I and the 2e−I states (i.e., α = 2.9110) because currently there are more examples of BTDATCNQ compounds, and indeed, the ionic state varied between 1e−I and 2e−I around the point. Although some compounds (e.g., 15 and 1 in Figure 6) were relatively distant from the boundary line, the boundary between the 1e−I and the 2e−I regimes was close to the line. From this diagram we concluded that D2A compounds with lower ΔEH−L(DA) values than the line will be expected to be in the 2e−I state.

EHOMO(D) = ΔEH−L(DA). The N and I regimes were expected when ΔEH−L(DA) > 0 and ΔEH−L(DA) < 0, respectively.10 This method is very easy to use because the HOMO and LUMO energy levels for the D and A components can be easily obtained from DFT calculations. This rough evaluation method identified the electronic state to be either N or I, even for the covalently bonded D/A-MOFs in the [Ru2II,II]/TCNQ, DCNQI series. Importantly, a plot of the difference between the first redox potentials of D and A, ΔE1/2(DA), showed a good linear relationship, which allowed the Madelung potential to be determined (M ≈ 430 mV) from the point where the line intersected ΔEH−L(DA) < 0.10,15i The calculated EHOMO(D) and ELUMO(A) values for the components used in this work are summarized in Table 1 together with the E1/2 values (computational details and cyclic voltammograms are given in Tables S8 and S9, Supporting Information). Ionization diagrams (ΔE H− L (DA) vs ΔE1/2(DA)) for the present compounds together with relevant previously reported compounds are shown in Figure 5. The compounds reported previously and the compounds presented here, even those with large negative ΔEH−L(DA) and a small ΔE1/2(DA) values, followed the linear relationship. Because the [Ru2] units used in this study have specifically electrondonating characteristics, all combinations of [Ru2] and TCNQRx in 1−5 had large negative ΔEH−L(DA) values compared to those of the previously reported [Ru2]/TCNQRx, DCNQIRx combinations. As a result, the compounds presented here (green diamonds in Figure 5) are in the lowest left part of the plot, which agrees well with the full ionization features of TCNQRx2−. E

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Figure 5. Plot of ΔEH−L(DA) vs ΔE1/2(DA) for D2A and DA materials in the [Ru2II,II]/TCNQRx, DCNQIRx series that have been described up to now. Red circles, blue squares, and green diamonds represent N, 1e−I, and 2e−I, respectively, and the purple triangle is for a DA compound that exhibits a temperature- and pressure-induced N−I phase transition: 1, [{Ru2(o-CF3PhCO2)4}2(TCNQ)]; 2, [{Ru2(oCF 3 PhCO 2 ) 4 } 2 {TCNQ(Me) 2 }]]; 3, [{Ru 2 (o-CF 3 PhCO 2 ) 4 } 2 (TCNQF 4 )]; 4, [{Ru 2 (o-CH 3 PhCO 2 ) 4 } 2 (BTDA-TCNQ)]; 5:, [{Ru2(p-CH3PhCO2)4}2(BTDA-TCNQ)]; 6, [{Ru2(mCH3PhCO2)4}2(BTDA-TCNQ)];15h 7, [{Ru2(CF3CO2)4}2{TCNQ(MeO) 2 }]; 15f,i 8, [{Ru 2 (CF 3 CO 2 ) 4 } 2 {TCNQ(Me) 2 }]; 15f,i 9, [{Ru 2 (CF 3 CO 2 ) 4 } 2 (TCNQ)]; 1 5 f , i 10, [{Ru 2 (CF 3 CO 2 ) 4 } 2 (TCNQBr 2 )]; 15f,i 11, [{Ru 2 (CF 3 CO 2 ) 4 } 2 (TCNQF 2 )]; 15f,i 12, [{Ru2(CF3CO2)4}2(TCNQCl2)];15f,i 13, [{Ru2(2,3,5,6-F4PhCO2)4}{DCNQI(Me)2}];16 14, [{Ru2(CF3CO2)4}2(TCNQF4)];15b,h,i 15, [{Ru 2 (o-ClPhCO 2 ) 4 } 2 {TCNQ(MeO) 2 }]; 1 5 g 16, [{Ru 2 (mFPhCO2)4}2(BTDA-TCNQ)];15d 17, [{Ru2(p-FPhCO2)4}2(BTDATCNQ)];15e 18, [{Ru 2(o-FPhCO 2)4 }2(BTDA-TCNQ)]; 15e 19, [{Ru2(4-Cl-2-MeOPhCO2)4}(BTDA-TCNQ)].17 Figure 4. Temperature dependence of χ (○) and χT (□) for 1, 2, 4, and 5. Solid red lines are simulated curves based on the Curie paramagnetic model with S = 3/2, taking into account zero-field splitting (D), temperature-independent paramagnetism (χTIP), and intermolecular interactions (zJ).

together with other relevant DA and D2A systems. However, we could not identify the boundary between the 1e−I and the 2e−I regimes from the ionization diagram, and thus, the diagram was modified using the on-site Coulomb repulsion energy of TCNQRx, U = |2E1/2(A) − 1E1/2(A)|, as a horizontal axis, i.e., forming another diagram, ΔEH−L(DA) vs |2E1/2(A) − 1 E1/2(A)|, in which the 1e−I and 2e−I regimes were clearly distinguishable and TCNQRx dependent. This implies that the potential window for 1e−I (ΔE1e−I in Figure 6) increases in the order of TCNQ(MeO)2 < TCNQ(Me)2 < BTDA-TCNQ < TCNQBr2 < TCNQ ≈ TCNQCl2 < TCNQF4 ≈ TCNQF2. It

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CONCLUSIONS Five new 2e− transferred two-dimensional [Ru2]2TCNQRx compounds were directionally synthesized using D/A combinations that could give a large ΔEH−L(DA) energy gap. These compounds were successfully added to the ΔEH−L(DA) vs ΔE1/2(DA) ionization diagram and formed a linear relationship

Table 1. Estimated HOMO and LUMO Energy Levels (eV) and Redox Potentialsa (mV) for the Relevant Compounds compound

HOMO

[Ru2(o-CF3PhCO2)4(THF)2] [Ru2(o-MePhCO2)4(THF)2] [Ru2(m-MePhCO2)4(THF)2] [Ru2(p-MePhCO2)4(THF)2] TCNQ TCNQMe2 TCNQF4 BTDA-TCNQ

−4.3821 −4.1617 −4.1372 −4.0401

LUMO

−5.1215 −4.9114 −5.8295 −4.7353

1

2

126 −94 −99 −116 −55 −148 363 −207

−637 −657 −266 −751

E1/2

E1/2

E1/2 − 1E1/2

2

582 509 629 544

a

Potentials were measured in THF containing 0.1 M n-Bu4NPF6 under N2 and adjusted using the ferrocene/ferrocenium couple that was used as an internal standard (Fc/Fc+ = 213 mV (ΔE = 91 mV) in THF vs Ag/Ag+). F

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Inorganic Chemistry

nitrobenzene (the middle layer) was carefully added to each tube. A nitrobenzene solution (10 mL) of TCNQ (4.1 mg, 0.02 mmol) was divided into 2 mL aliquots, and each aliquot was carefully added on the top of the middle layer in each tube. The glass tubes were left undisturbed in a glovebox for 2 weeks or more, after which triangleshaped brown green crystals of 1 were obtained. A small amount of needle-type crystals of [Ru2II,II(o-CF3PhCO2)4(PhNO2)2] were obtained along with the block-type crystals of 1, but the impurity was distinguishable from the shapes of its crystals. Anal. Calcd for [{Ru2(o-CF3PhCO2)4}2(TCNQ)]·2(nitrobenzene) (1), C88H46N6O20F24Ru4,: C, 44.64; H, 1.96; N, 3.55. Found: C, 44.98; H, 2.14; N, 4.14. FT-IR (KBr; cm−1): ν(CN) 2188, 2102. Synthesis of 2. A dichloromethane (CH2Cl2) solution (10 mL) of [Ru2(o-CF3PhCO2)4(THF)2] (44 mg, 0.04 mmol) was divided into 2 mL aliquots, and each aliquot was placed in a sealed glass tube with a small diameter (ϕ 8 mm). This was the bottom layer. A mixture of pchlorotoluene (p-Cltol) and CH2Cl2 (1:1 v/v; 1 mL), the middle layer, was carefully added to each tube. A p-Cltol solution (10 mL) of TCNQ(Me)2 (4.6 mg, 0.02 mmol) was divided into 2 mL aliquots, and each aliquot was carefully added on the top of the middle layer in each tube. The glass tubes were left undisturbed in a glovebox for 2 weeks or more, after which triangle-shaped brown green crystals of 2 were obtained. Anal. Calcd for [{Ru2(o-CF3PhCO2)4}2{TCNQ(Me)2}]·1.8(p-Cltol)·0.2(CH2Cl2) (2), C90.8H53N4O16F24Cl2.2Ru4: C, 45.55; H, 2.23; N, 2.35. Found: C, 45.35; H, 2.57; N, 2.48. FT-IR (KBr; cm−1): ν(CN) 2187, 2100. Synthesis of 3. Compound 3 was synthesized using a method similar to that used to synthesize 2 using TCNQF4 (5.5 mg, 0.02 mmol) instead of TCNQ(Me)2, but the yield was extremely low. The charge state of 3 was only identified from its structure and IR data. FTIR (KBr; cm−1): ν(CN) 2191, 2114. Synthesis of 4. A CH2Cl2 solution (10 mL) of [Ru2(oCH3PhCO2)4(THF)2] (35.5 mg, 0.04 mmol) was divided into 1 mL aliquots, and each aliquot was placed in a sealed glass tube with a small diameter (ϕ 8 mm). This was the bottom layer. A mixture of p-xylene and CH2Cl2 (1:1 v/v; 1 mL), the middle layer, was carefully added to each tube. A p-xylene solution (15 mL) of BTDA-TCNQ (9.6 mg, 0.02 mmol) was divided into 3 mL aliquots, and each aliquot was carefully added on the top of the middle layer in each tube. The glass tubes were left undisturbed in a glovebox for 2 weeks or more, after which block-shaped brown green crystals of 4 were obtained. Anal. Calcd for [{Ru2(o-CH3PhCO2)4}2(BTDA-TCNQ)]·p-xylene (4), C84H66N8O16Ru4S2: C, 52.77; H, 3.48; N, 5.86. Found: C, 52.34; H, 3.58; N, 5.90. FT-IR (KBr; cm−1): ν(CN) 2185, 2131, 2104. Synthesis of 5. Compound 5 was synthesized using a method similar to that used to synthesize 4, but [Ru2(p-CH3PhCO2)4(THF)2] (35.5 mg, 0.04 mmol) was used instead of [Ru 2 (oCH3PhCO2)4(THF)2]. Anal. Calcd for [{Ru2(pCH 3 PhCO 2 ) 4 } 2 (BTDA-TCNQ)]·p-xylene·2(CH 2 Cl 2 ) (5), C102H90N8O16Cl4Ru4S2: C, 50.55; H, 3.13; N, 6.21. Found: C, 50.22; H, 3.56; N, 5.80. FT-IR (KBr; cm−1): ν(CN) 2168, 2112, 2089. General Physical Measurements. Infrared spectra were measured using KBr disks and a Jasco FT-IR 620 spectrophotometer. Powder reflection spectra were measured using pellets of the compounds of interest diluted with BaSO4 using a Shimadzu UV3150 spectrometer. Magnetic susceptibility measurements were conducted using a Quantum Design SQUID magnetometer (MPMS-XL) in the temperature and dc field ranges from 1.8 to 300 K and −7 to 7 T, respectively. Polycrystalline samples embedded in liquid paraffin were analyzed. Experimental data were corrected for the sample holder and liquid paraffin and for the diamagnetic contribution calculated from the Pascal constants.40 Crystallography. Single crystals with dimensions of 0.101 mm × 0.031 mm × 0.019 mm for 1, 0.240 mm × 0.228 mm × 0.040 mm for 2, 0.051 mm × 0.041 mm × 0.020 mm for 3, 0.059 mm × 0.038 mm × 0.025 mm for 4, and 0.240 mm × 0.228 mm × 0.040 mm for 5 were mounted on cryo-loops using Nujol and cooled with a stream of cooled N2 gas. All measurements were made using a Rigaku Saturn CCD area detector with graphite monochromated Mo Kα radiation (λ

Figure 6. Plots of ΔEH−L(DA) vs |2E1/2(A) − 1E1/2(A)|. Indices indicate the compound used in this paper and are given in the Figure 5 caption.

can be assumed that TCNQ(MeO)2 is a relatively weak acceptor, but it is predisposed to be in the 2e−I state in its ionic form. This situation is reversed for TCNQF2 and TCNQF4. The boundary line between 1e−I and 2e−I in the second diagram currently appears to be rather arbitrary, although, if it is approximately determined, it offers a clear-cut strategy for designing D/A-MOFs with desired ionic states that are directly related to their physical properties. The D/A-MOFs at the boundary between the 1e−I and the 2e−I states are very intriguing because the electronic state in such a compound is expected to be highly sensitive to external stimuli, such as light, pressure, and electric fields, similar to the N−I phase transition materials.16 Studies of such compounds are currently in progress.

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EXPERIMENTAL SECTION

General Procedures and Materials. All synthetic procedures were performed under an inert atmosphere using standard Schlenkline techniques and a commercial glovebox. All chemicals were purchased from commercial sources and of reagent grade. Solvents were dried using common drying agents and distilled with ultrapure nitrogen prior to use. Starting materials for the [Ru2II,II] units were prepared according to previously reported methods (Supporting Information).37 [Ru2II,II(p-MePhCO2)4(THF)2] and its oxidized compound [Ru2II,III(p-MePhCO2)4(THF)2]BF4 were already reported by Chisholm et al.38 Organic acceptors, TCNQ(Me)2 and BTDATCNQ, were synthesized using a published method,39 whereas TCNQ and TCNQF4 were purchased from a company that supplies reagents. All compound crystals contained some crystallization solvent molecules, and some solvent molecules were slowly lost at room temperature, making elemental analysis data difficult to interpret; therefore, samples were dried prior to elemental analysis. The samples were aged, as described above, for a few hours after removing from their mother liquids. Magnetic measurements were performed on fresh samples immediately after they were removed from their mother liquids. Synthesis of 1. A nitrobenzene solution (10 mL) of [Ru2(oCF3PhCO2)4(THF)2] (44 mg, 0.04 mmol) was divided into 2 mL aliquots, and each aliquot was placed in a sealed glass tube with a small diameter (ϕ 8 mm). This was the bottom layer. A 1 mL aliquot of G

DOI: 10.1021/ic502513p Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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= 0.71075 Å). The structures were solved using direct methods (SHELXL 9741 for 1 and 3−5 and SIR 9242 for 2) and expanded using Fourier techniques (DIRDIF99).43 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were introduced as fixed contributors. Full-matrix least-squares refinements on F2 were based on observed reflections and variable parameters and converged with the unweighted and weighted agreement factors R1 = ∑||Fo| − |Fc||/∑| Fo| (I > 2.00σ(I)) and wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2 (all data). All calculations were performed using the CrystalStructure crystallographic software package.44 Structural diagrams were prepared using VESTA software.45 Crystallographic Data for 1. Formula: C88H46F24N6O20Ru4, Mr = 2367.60, triclinic, P−1 (#2), a = 10.196(3) Å, b = 14.230(4) Å, c = 16.164(4) Å, α = 74.925(17)°, β = 78.792(17)°, γ = 79.635(17)°, V = 2200.4(11) Å3, T = 103(1) K, Z = 1, Dcalc = 1.787 g cm−3, F000 = 1168.00, λ = 0.71075 Å, μ(Mo Kα) = 8.001 cm−1, 17 673 measured reflections, 9562 unique (Rint = 0.0607). R1 = 0.0866 (I > 2σ(I)), R1 = 0.1435 (all data), and wR2 = 0.2761 with GOF = 1.057. CCDC1029158. Crystallographic Data for 2. Formula: C90.8H53Cl2.2F24N4O16Ru4, Mr = 2394.27, triclinic, P−1 (#2), a = 10.0175(19) Å, b = 14.828(3) Å, c = 16.410(3) Å, α = 75.169(6)°, β = 79.666(7)°, γ = 75.517(8)°, V = 2264.0(8) Å3, T = 103(1) K, Z = 1, Dcalc = 1.756 g cm−3, F000 = 1183.20, λ = 0.71075 Å, μ(Mo Kα) = 8.376 cm−1, 15 070 measured reflections, 7748 unique (Rint = 0.0145). R1 = 0.0289 (I > 2σ(I)), R1 = 0.0331 (all data), and wR2 = 0.0765 with GOF = 1.069. CCDC1029160. Crystallographic Data for 3. Formula: C90H46Cl2F28N4O16Ru4, Mr = 2446.51, triclinic, P−1 (#2), a = 10.146(4) Å, b = 14.690(6) Å, c = 16.296(7) Å, α = 74.464(18)°, β = 79.95(3)°, γ = 78.36(3)°, V = 2273.0(17) Å3, T = 103(1) K, Z = 1, Dcalc = 1.787 g cm−3, F000 = 1204.00, λ = 0.71075 Å, μ(Mo Kα) = 8.368 cm−1, 14 938 measured reflections, 7785 unique (Rint = 0.0603). R1 = 0.0908 (I > 2σ(I)), R1 = 0.1381 (all data), and wR2 = 0.2853 with GOF = 1.050. CCDC1029159. Crystallographic Data for 4. Formula: C84H66N8O16Ru4S2, Mr = 1911.89, triclinic, P−1 (#2), a = 10.7395(17) Å, b = 11.920(2) Å, c = 15.796(3) Å, α = 82.817(5)°, β = 82.930(6)°, γ = 76.098(6)°, V = 1928.3(6) Å3, T = 103(1) K, Z = 1, Dcalc = 1.638 g cm−3, F000 = 962.00, λ = 0.71075 Å, μ(Mo Kα) = 8.932 cm−1, 13 075 measured reflections, 6641 unique (Rint = 0.0205). R1 = 0.0343 (I > 2σ(I)), R1 = 0.0447 (all data), and wR2 = 0.0991 with GOF = 1.047. CCDC-1029161. Crystallographic Data for 5. Formula: C102H90Cl4N8O16Ru4S2, Mr = 2294.09, triclinic, P−1 (#2), a = 11.624(4) Å, b = 15.454(4) Å, c = 15.732(5) Å, α = 96.213(3)°, β = 108.752(3)°, γ = 107.136(3)°, V = 2492.1(13) Å3, T = 103(1) K, Z = 1, Dcalc = 1.528 g cm−3, F000 = 1162.00, λ = 0.71075 Å, μ(Mo Kα) = 8.122 cm−1, 16 633 measured reflections, 8566 unique (Rint = 0.0504). R1 = 0.0676 (I > 2σ(I)), R1 = 0.1016 (all data), and wR2 = 0.1975 with GOF = 1.066. CCDC1029162. These data have been deposited as CIFs at the Cambridge Data Centre as supplementary publication nos. CCDC-1029158 for 1, CCDC-1029160 for 2, CCDC-1029159 for 3, CCDC-1029161 for 4, and CCDC-1029162 for 5. Copies of the data can be obtained free of charge by application to CCDC, 12 Union Road, Cambridge CB21EZ, U.K. (fax (+44) 1223-336-033; email [email protected]).

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Article

AUTHOR INFORMATION

Corresponding Author

*Phone: +81-22-215-2030. Fax: +81-22-215-2031. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Hiromichi Kamo (Department of Chemistry, Tohoku University) for his help in synthesizing the compounds and with computational work. This work was supported by Grants-in-Aid for Scientific Research (Grant Nos. 21350032 (H.M.) and 26810029 (W.K.)) from the Japanese Ministry of Education, Culture, Sports, Science, and Technology, the ICCIMR, the LC-IMR, and The Asahi Glass Foundation.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

CIF files containing the X-ray crystallographic data for 1−5, [Ru2(R−PhCO2)4(THF)2], and [Ru2(R−PhCO2)4(THF)2]BF4 (R = o-Me, m-Me, o-CF3); figures showing the structures of 1−5, [Ru 2 (R−PhCO 2 ) 4 (THF) 2 ], and [Ru 2 (R− PhCO2)4(THF)2]BF4; computational details. This material is available free of charge via the Internet at http://pubs.acs.org. H

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