Article pubs.acs.org/IC
Tuning of Stepwise Neutral−Ionic Transitions by Acceptor Site Doping in Alternating Donor/Acceptor Chains Keita Nakabayashi,† Masaki Nishio,† and Hitoshi Miyasaka*,‡ †
Department of Chemistry, Division of Material Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan ‡ Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan S Supporting Information *
ABSTRACT: The stepwise neutral−ionic (N−I) phase transition found in the alternating donor/acceptor (DA) chain [Ru2(2,3,5,6-F4PhCO2)4(DMDCNQI)]·2(p-xylene) (0; 2,3,5,6-F4PhCO2− = 2,3,5,6-tetrafluorobenzoate; DMDCNQI = 2,5-dimethyl-N,N′-dicyanoquinonediimine) was tuned by partly substituting the acceptor DMDCNQI with 2,5dimethoxy-N,N′-dicyanoquinonediimine (DMeODCNQI), which displays a poorer electron affinity in an isostructural series. The site-doped series comprised [Ru 2 (2,3,5,6F4PhCO2)4(DMDCNQI)1−x(DMeODCNQI)x]·2(p-xylene) for doping rates (x) = 0.05 (0.05-MeO), 0.10 (0.10-MeO), 0.15 (0.15-MeO), and 0.20 (0.20-MeO). The neutral chain [Ru2(2,3,5,6-F4PhCO2)4(DMeODCNQI)]·4(p-xylene) (1), which only contained DMeODCNQI, was also characterized. All site-doped compounds were isostructural to 0 except 1 despite their identical DA chain motif. Except at an x value of 0.20, they displayed a two-step N−I transition involving an intermediate phase. This transition occurred at high temperatures in 0 but shifted to lower temperatures in a parallel manner with increasing doping rate. Simultaneously, each transition broadened with increasing doping rate, leading to a convergence of two transitions at an x value approximating 0.2. Donor/acceptor-site-doping techniques present somewhat different impacts in terms of interchain Coulomb effects.
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INTRODUCTION The neutral−ionic (N−I) phase transition in a donor (D)/ acceptor (A) pair, which involves electron transfer between distinct neutral (N: D0A0) and ionic (I: D+A−) valence states, is an intriguing switching phenomenon that drastically changes the fundamental physical properties of this pair, such as spin states, electronic transport, and electrical polarization. Since the discovery of the first organic DA pair comprising tetrathiafulvalene and chloranil by Torrance et al. in 1981,1 about 10 organic compounds2−8 and their derivatives8−10 have exhibited this transition. The first covalent DA system to undergo the N−I transition is the metal-complex chain compound [Ru2(2,3,5,6-F4PhCO2)4DMDCNQI]·2(p-xylene) (0, 2,3,5,6F4PhCO2− = 2,3,5,6-tetrafluorobenzoate, DMDCNQI = 2,5dimethyl-N,N′-dicyanoquinonediimine, Scheme 1).11 In this alternating chain, carboxylate-bridged paddlewheel-type [Ru2II,II(2,3,5,6-F4PhCO2)4] ([Ru2II,II] or [Ru2(F4)]) and DMDCNQI acted as D and A, respectively. The control of the N−I transition in DA pairs primarily depends on molecular composition, which determines the “balance” between the ionization potential of D and the electron affinity of A.12,13 Nonetheless, because it originates from a bulk phenomenon, the N−I transition also relies on crystal engineering, which modulates the alignment of DA units through intermolecular interactions and/or the Madelung stability in the I phase. The © XXXX American Chemical Society
balance between ionization potential and electron affinity is tuned by chemically modifying DA units and partly substituting these units by impurities belonging to an isostructural series.8,9b,d N−I transitions observed in 0 have previously been modulated by bottom-up site doping using [Ru2II,II(F5PhCO2)4] dummy donor units, which present a weaker ionization potential than [Ru2II,II(2,3,5,6-F4PhCO2)4] (hereafter, we use the term of “site doping” on the partial chemical modification of the A unit in this work). These stepwise transitions, wherein the intermediate (IM) phase involved an alternate arrangement of neutral and ionic chains,14−16 systematically varied as a function of the doping rate x.17 For x values less than 0.1, they abruptly occurred, concomitant with a clear shift in transition temperatures, demonstrating that, albeit tiny, partial local changes drastically affect bulk transitions similar to a domino effect. However, interchain Coulomb effects on intrachain electron transfer resulting from impurity doping remain unclear. In this study, N−I transitions in 0 were altered by replacing DMDCNQI by the dummy acceptor 2,5-dimethoxy-N,N′dicyanoquinonediimine (DMeODCNQI) as an less active impurity. The resulting series comprised [Ru2 (2,3,5,6Received: December 10, 2015
A
DOI: 10.1021/acs.inorgchem.5b02858 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Scheme 1. Schematic Representations of the Investigated Chain Compoundsa
a
Here 0 is the parent compound described in ref 11.
to the [Ru2II,II] oxidation state using previous ranges obtained for [Ru2II,II] (2.06−2.07 Å) and [Ru2II,III]+ (2.02−2.03 Å).18,19 Meanwhile, the oxidation state of the DMeODCNQI moiety was readily evaluated using the Kistenmacher relationship,20 δ = −{Aδ[c/(b + d)] + Bδ}, where Aδ = −36.900, Bδ = 17.295, and b, c, and d bonds are defined in the DMeODCNQI/ DMDCNQI chart shown in Scheme 1. This evaluation assumed that [Rh2II,II(CF3CO2)4(DMDCNQI)] corresponded to the neutral form with δ = 021 and [MnIII(TMesP) (DMDCNQI)] (TMesP = meso-tetrakis(2,4,6trimethylphenyl)porphyrinate) represented the ionic form with δ = 1.22 The estimated δ value amounted to 0.081 for 1, consistent with the neutral form and the [Ru2II,II] oxidation state. This assignment was only conducted at 103 K. However, magnetic data obtained for 1 were consistent with the paramagnetic behavior of [Ru2II,II] (S = 1), demonstrating that 1 adopted a neutral state in the entire measurement temperature range (1.8−300 K) (Figure S2). Interestingly, DMeODCNQI exhibited a higher LUMO energy level (ELUMO) (−4.322 eV) than DMDCNQI (−4.652 eV) by DFT studies, convincing the oxidation state of 1. Acceptor partially modified compounds were produced by the same synthetic procedure as 1, but DMeODCNQI was mixed into the DMDCNQI solution at ratios 1:19, 1:9, 1:5.7, and 1:4. Hereafter, the acceptor moiety will be noted DCNQI when DMeODCNQI and DMDCNQI do not need to be distinguished, while site-doped compounds will be called 0.05MeO, 0.10-MeO, 0.15-MeO, and 0.20-MeO according to mixing ratios (%). Very difficult to determine accurately, the doping rates of these compounds were evaluated from their structures using the occupancy of the MeO groups of the DCNQI moiety and elemental analysis (see Experimental
F4PhCO2)4(DMDCNQI)1−x(DMeODCNQI)x]·n(p-xylene) with x values of 0.05 (0.05-MeO), 0.10 (0.10-MeO), 0.15 (0.15-MeO), 0.20 (0.20-MeO), and 1 (1) (Scheme 1). All compounds exhibited the same DA chain motif, and all sitedoped compounds were isostructural to 0 with n = 2 except 1, which displayed a different packing mode with n = 4. Here, the site-doping effect of these DMeODCNQI less active acceptor units on N−I transitions was investigated in detail.
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RESULTS AND DISCUSSION Characterization of 1 and Site-Doped Compounds 0.05-MeO−0.20-MeO. The reaction of [Ru 2 (2,3,5,6F4PhCO2)4(THF)2] with DMeODCNQI in a dichloromethane/p-xylene mixture produced the one-dimensional chain compound [Ru2(2,3,5,6-F4PhCO2)4(DMeODCNQI)]·4(p-xylene) (1) comprising [−{Ru2}−(DMeODCNQI)−] repeating units. Compounds 0 and 1 exhibited very similar chain forms, but their number of crystallization solvents differed (e.g., 4 vs 2 mol of p-xylene was detected in 1 and 0, respectively), indicating their slightly dissimilar packing arrangement. Compound 1 crystallized in the triclinic P−1 space group (T = 103 K), and one-half of its formula unit, which displayed inversion centers at the midpoint of the Ru−Ru bond and the DMeODCNQI moiety, was crystallographically determined as an asymmetric unit (Z = 1) (Figure 1a). Packing diagrams of 1 are shown in Figure S1, while relevant local bond distances are listed in Table S1. The oxidation state of [Ru2] was determined by comparing bond lengths between metal and equatorial oxygen atoms (Oeq), while the oxidation state of DMeODCNQI was evaluated by comparing C−C bond lengths.11,17 The Ru−Oeq bond length averaged 2.071 Å in 1, which was assigned B
DOI: 10.1021/acs.inorgchem.5b02858 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 1. Structures of 1 (a) and 0.20-MeO (b−d) (50% probability ellipsoids for a and b). Packing diagrams of 0.20-MeO projected (c) along the a axis and (d) along the b axis. Hydrogen atoms were omitted for clarity. Symmetry operations: *, −x + 1, −y + 2, −z; #, −x + 1, −y + 2, −z + 1; **, −x + 1, −y, −z + 1; ##, −x + 2, −y, −z + 1.
of temperature. Both variations are related with each other in the relation between donor and acceptor (i.e., as [Ru2II,II], δ = 0, and as [Ru2II,III]+, δ = 1). Essentially, the partial substitution with DMeODCNQI stabilized the N phase. Therefore, twostep transitions shifted to lower temperatures with increasing doping rate and became undetectable at x = 0.2 by this technique that presents a cooling limit of 97 K. At low doping rates (x < 0.1), they clearly featured a plateau, indicating the presence of IM phase comprising an equimolar N-chain/I-chain mixture. At an x value of 0.1, this plateau gradually shrank to disappear, allowing structural changes to occur. At x = 0.15, the site-doped compound no longer displayed the plateau region (in the structural study, but not in the magnetic study; vide infra), gradually changing from the N phase appeared at T ≥ 200 K to the I phase probably presented at T < 100 K. Evaluation of N−I Transitions in Magnetic Variations. The investigated chain compounds adopted a paramagnetic state (S = 1) in the N phase and a short-range ferrimagnetic state with antiferromagnetic coupling (S = 3/2 for [Ru2II,III]+; S = 1/2 for DCNQI•−) in the I phase. Therefore, their N−I transitions were monitored in terms of magnetic variations. Figure 3 shows the temperature dependence of χT and
Section). These data showed that actual doping rates reflected the mixing ratios. All site-doped compounds were isostructural to 0, which crystallized in the monoclinic P21/n space group with inversion centers located at the midpoint of the Ru−Ru bond and DCNQI moiety (Z = 2) at 97 K (Figure 1b). The chains ran along the a axis of the unit cell (Figure 1c and 1d), while crystallization solvents (2 mol of p-xylene) were located between chains and interacted with DCNQI and 2,3,5,6-F4Ph groups by π-stacking. Since these structural details have been previously addressed,11,17 the discussion focuses here on oxidation states. Site-doped compound structures were characterized by single-crystal X-ray crystallography at temperatures ranging between 97 and 260 K. All unit cells were determined using an original unit cell with the monoclinic P21/n space group (Z = 2) without considering any previously discussed superlattice for the IM phase.11,17 Local bond lengths were investigated to estimate charges on [Ru2] and DCNQI moieties. Figure 2 shows variations in average Ru−Oeq bond lengths and δ value estimated using the Kistenmacher relationship and local dimensions of the DCNQI moieties (vide supra) as a function C
DOI: 10.1021/acs.inorgchem.5b02858 Inorg. Chem. XXXX, XXX, XXX−XXX
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transition (vide infra). The χT plot slightly decreased at high temperature for the N phase, which may stem from the strong anisotropic effect of [Ru2II,II] (S = 1, zero-field splitting parameter D/kB ≈ 330 K; see Figure S2 for 1).18,19,23 It increased at low temperature for the I phase, indicative of shortrange ferrimagnetic ordering (S = 3/2 for [Ru2II,III]+; S = 1/2 for DCNQI•−) through the chain. In agreement with temperature-dependent structural results, these magnetic data demonstrate a successful insertion of DMeODCNQI in the chain. Transition temperatures are shown in the doping rate− temperature phase diagram (Figure 4), wherein error bars
Figure 2. Average Ru−Oeq length (top) and degree of charge −δ on the DMDCNQI moiety estimated using the Kistenmacher relationship (bottom) of all compounds as a function of temperature. Each structure was determined using the monoclinic space group P21/n. Red and blue regions roughly indicate expected locations for N and I phases, respectively, while the purple region corresponds to an intermediate area. Temperatures T1 and T2 shown in the same color as their corresponding compounds are N−I transition temperatures estimated from magnetic measurements (Figures 3 and S3).
Figure 4. Doping rate (x)−temperature phase diagram for the oxidation states in neutral (N), ionic (I), and intermediate (IM) phases (dotted lines are only guides for the eye), where doping rate x means the site-doped ratio of acceptor in [Ru 2 (2,3,5,6F4PhCO2)4(DMDCNQI)1−x(DMeODCNQI)x]·2(p-xylene). T1 and T2 values were obtained from magnetic measurements. Error bars were estimated as standard deviations of a Gauss fitting for individual −d(χT)/dT peaks (Figure S3).
correspond to standard deviations from a Gauss fitting of individual −d(χT)/dT peaks (Figure S3). These standard deviations rose with increasing doping rate, indicating that transitions occurred in a temperature region that varied according to the environment of each D/A component within and between chains. Therefore, the DMeODCNQI dopant (A′) was randomly inserted into chains with DMDCNQI, and resulting D/AA′ configurations and interchain Coulomb interactions strongly affected transitions in local D/A parts. For x values inferior or equal to 0.15, transitions occurred stepwise and the IM temperature range shifted in a parallel fashion to lower temperatures upon doping rate increase. Only one transition, associated with a large standard deviation in a wide temperature range, was detected at an x value of 0.2. A significant increase in χT was also observed for 0.20-MeO in the I phase, consistent with the presence of relevant correlation lengths, such as a correlation length through two to three DA units with a magnetic exchange constant (J) approximating −100 K. This result also demonstrates the random insertion of dopant molecules. The convergence of two transitions, which suggests the disappearance of the IM phase, may result from the fact that the Coulomb gain reached the Madelung stabilization when the temperature decreased. Pressure-Induced Variations of Phase Diagrams. Neutral−ionic transitions were tuned by applying hydrostatic pressures to polycrystalline samples of the site-doped compounds in Apiezon-J oil, which acted as a pressure-
Figure 3. Temperature dependence of χT and −d(χT)/dT during cooling for 0, 0.05-MeO, 0.10-MeO, 0.15-MeO, and 0.20-MeO. −d(χT)/dT values were obtained from data measured at 0.1 T.
−d(χT)/dT of site-doped compounds and 0 (where χ = M/H) upon cooling. The N−I transitions were clearly observed in all site-doped compounds including 0.20-MeO, which did not show any distinct transition in structural investigations conducted between 97 and 260 K (vide supra). These transitions shifted to lower temperatures when the doping rate increased. Compounds 0, 0.05-MeO, 0.10-MeO, and 0.15MeO displayed transitions at temperatures T1 and T2, which were determined using the peaks appearing in the −d(χT)/dT plot (Figure 3). In contrast, 0.20-MeO exhibited a one-step D
DOI: 10.1021/acs.inorgchem.5b02858 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 5. Temperature dependence of the magnetization for 0.05-MeO (a), 0.10-MeO (b), 0.15-MeO (c), and 0.20-MeO (d) at 1 kOe under applied pressures. MT = M × T, where M is the raw magnetization.
Figure 6. Applied pressure−temperature phase diagrams for charge-ordered states in neutral (N, red), ionic (I, blue), and intermediate (IM, purple) phases for (a) 0.05-MeO, (b) 0.10-MeO, (c) 0.15-MeO, and (d) 0.20-MeO.
quantum interference device (SQUID) magnetometer, and the magnetization was measured as a function of temperature at
transmitting medium, using a piston−cylinder-type Cu−Be alloy cell.24,25 The cell was placed in a superconducting E
DOI: 10.1021/acs.inorgchem.5b02858 Inorg. Chem. XXXX, XXX, XXX−XXX
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transition itself as well as control over magnetic switching. Here, as demonstrated in the N−I transition works, DA systems comprising [Ru2II,II] and DCNQI/TCNQ may serve as charge mediators to control physical properties, such as magnetism and electron transport. These redox-active metal− organic frameworks may lead to multiply controllable functional molecular materials.
different applied pressures (Figure 5). The applied pressure reached a maximum of 8.96 kbar for 0.20-MeO, and the actual pressure was estimated from the superconducting transition temperature for Pb. The obtained pressure−temperature diagrams are shown in Figure 6 for 0.05-MeO, 0.10-MeO, 0.15-MeO, and 0.20-MeO. The oxidation phase strongly depended on the applied pressure. In 0.05-MeO and 0.10MeO, transition temperatures T1 and T2 quadratically rose with applied pressure, the increase of which reached approximately 100 K at about 1 kbar (Figure 6a and 6b). This quadratic increase was less pronounced for 0.15-MeO than that for 0.05MeO and 0.10-MeO (Figure 6c). The single transition temperature varied in a sigmoidal manner for 0.20-MeO (Figure 6d). Compounds 0.05, 0.10, and 0.15 retained their IM phase under applied pressure. These observations are very similar to previous findings obtained by donor partial modification.17 They indicate that applied hydrostatic pressure affected the chains isotropically as well as spherical Coulomb interactions between chains. Similarly to magnetization variations (Figure 5d), the correlation length of 0.20-MeO grew with increasing applied pressure concomitant with a rise in transition temperatures, which is comparable to trends obtained for a perfect-ionic chain at P ≈ 9.0 kbar.
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EXPERIMENTAL SECTION
Materials. All synthetic procedures were performed under N2 atmosphere using standard Shlenk 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 under N2 atmosphere before use. Starting materials including [Ru2II,II(2,3,5,6-F4PhCO2)4(MeOH)2]19a and DCNQI derivatives (2,5-dimethyl- and 2,5-dimethoxy-DCNQI)26 were synthesized according to previous methods. Crystals contained p-xylene molecules as crystallization solvents, which slowly disappeared at room temperature, making elemental analyses difficult to interpret. Therefore, samples were dried before elemental analysis. Samples were aged for a few hours after their removal from their mother liquids unlike samples destined for magnetic measurements, which were immediately used. Synthesis of DMeODCNQI-Doped Compounds. The sitedoped compounds 0.05-MeO, 0.10-MeO, 0.15-MeO, and 0.20-MeO were synthesized by following the procedure used for compound 0.11 A DMDCNQI/DMeODCNQI solution (100 mL) in dichloromethane with a molar ratio of (0.25 − x) mmol/x mmol (x = 0.0125, 0.025, 0.0375, and 0.05 for 0.05-MeO, 0.10-MeO, 0.15-MeO, and 0.20-MeO, respectively) was divided into 2 mL aliquots. Each aliquot was placed in a narrow-diameter sealed glass tube (ϕ: 8 mm) to form the bottom layer. A 1:1 p-xylene/dichloromethane mixture (v/ v; 1 mL) was carefully added to the bottom layer to give the middle layer. A 2 mL aliquot of [Ru2(2,3,5,6-F4PhCO2)4(MeOH)2] solution (260 mg, 0.25 mmol) in p-xylene (100 mL) was then carefully deposited on the middle layer in each tube. The glass tubes were turned upside down and left undisturbed for at least 2 weeks to produce the site-doped compounds as plate-shaped brown crystals. For 0.05-MeO, yield 79.2 mg (27%). Anal. Calcd for dried 0.05-MeO (C38H12F16N4O8.1Ru2): C, 39.34; H, 1.04; N, 4.83. Found: C, 39.12; H, 1.17; N, 4.86. FT-IR (KBr pellet, 4000−400 cm−1 at RT): ν(C N), 2108; ν(CC), 1538; ν(CO), 1586, 1405 cm−1. For 0.10MeO, yield: 77.8 mg (27%). Anal. Calcd for dried 0.10-MeO (C38H12F16N4O8.2Ru2): C, 39.28; H, 1.04; N, 4.82. Found: C, 39.28; H, 1.28; N, 4.79. FT-IR (KBr pellet, 4000−400 cm−1 at RT): ν(C N), 2110; ν(CC), 1539; ν(CO), 1588, 1406 cm−1. For 0.15MeO, yield: 49.9 mg (17%). Anal. Calcd for dried 0.15-MeO (C38H12F16N4O8.3Ru2): C, 39.23; H, 1.04; N, 4.82. Found: C, 39.39; H, 1.10; N, 4.66. FT-IR (KBr pellet, 4000−400 cm−1 at RT): ν(C N), 2108; ν(CC), 1539; ν(CO), 1587, 1405 cm−1. For 0.20MeO, yield: 73.8 mg (25%). Anal. Calcd for the dried 0.20-MeO (C38H12F16N4O8.4Ru2): C, 39.18; H, 1.04; N, 4.81. Found: C, 39.23; H, 1.11; N, 4.80. FT-IR (KBr pellet, 4000−400 cm−1 at RT): ν(C N), 2109; ν(CC), 1536; ν(CO), 1586, 1403 cm−1. Synthesis of 1. [Ru2(2,3,5,6-F4PhCO2)4(THF)2] (56 mg, 0.05 mmol) was dissolved in p-xylene (30 mL) and filtered. The resulting solution was mixed with a DMeODCNQI solution (10.8 mg, 0.05 mmol) in dichloromethane (30 mL) and left undisturbed overnight, affording 1 as brown crystals. Yield: 18.5 mg (23%). Anal. Calcd for dried 1 (C38H12F16N4O10Ru2): C, 38.33; H, 1.02; N, 4.71. Found: C, 38.47; H, 1.26; N, 5.01. FT-IR (KBr pellet, 4000−400 cm−1 at RT): ν(CN), 2180, 2114; ν(CC), 1588, 1540; ν(CO), 1575, 1396 cm−1. Physical Measurements. Infrared spectra were measured using KBr disks at room temperature on a JASCO FT-IR 620 spectrophotometer. Magnetic susceptibility measurements were conducted using a SQUID magnetometer (Quantum Design MPMS-XL) for temperatures ranging from 1.8 to 300 K and magnetic fields spanning from −7 to 7 T. Alternating current (ac) measure-
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CONCLUSION The N−I transition can be mainly tuned by (i) the balance between the ionization potential of D and electron affinity of A, (ii) Coulomb interactions around individual D/A unit, and (iii) the Madelung stability in the I phase. These parameters were experimentally assessed in stepwise N−I transition systems based on covalently bonded DA chains. When impurities, such as less redox-active D/A units, were inserted at a very low concentration (ca. 5%), the partial (local) modification strongly affected the N−I transition phenomenon, causing transition temperatures to significantly drop with an IM phase that reflects Coulomb effects between neighboring chains. This demonstrates that a small local part (or factor) in bulk determines an essential bulk phenomenon, similar to a domino effect. Moreover, an increase in impurity concentration enhanced the randomness of transitions because the D/A unit started to perceive the influence of next-neighboring species as well as through the entire chain. Also, the effect of interchain Coulomb interactions may also change in each D/A moiety. Therefore, transition temperatures decreased. In addition, the N−I transition mainly depended on factors (i) and (iii) at low temperatures with T1 ≈ T2 because the energy gain from the Coulomb effect should be covered by the Madelung stability, resulting in the disappearance of the IM phase. The above-mentioned mechanism is generally valid for Dand A-partially modified compounds. However, site-doped compounds exhibited slightly different shifts in the IM phase. The IM phase region tended to converge with increasing doping rate in the D-modified series17 but shifted in a parallel manner in the A-modified series (this study). However, T1 and T2 transitions converged at an x value of about 0.20 in both series. In addition, transition broadening was more prominent in A-modified compounds. Therefore, D/A-partial modification approaches produce somewhat different Coulomb effects. The stepwise N−I transition associated with Coulomb effects and the magnetic correlation growth in the ionic phase are unique to compound 0. Therefore, a detailed investigation of this type of transition is expected to provide information on the F
DOI: 10.1021/acs.inorgchem.5b02858 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
cm−1, 21 127 reflections measured, 5987 unique reflections (Rint = 0.0323). R1 = 0.0368 (I > 2σ(I)), R1 = 0.0421 (all data), wR2 = 0.0954 (all data) with GOF = 1.084. CCDC 1440597. Crystallographic Data for 0.20-MeO. C54H32F16N4O8.4Ru2, Mr = 1377.38, monoclinic, P21/n (#14), a = 14.803(2) Å, b = 10.560(2) Å, c = 17.269(3) Å, β = 96.729(2)°, V = 2680.8(7) Å3, T = 97 K, Z = 2, Dcalc = 1.706 g/cm3, F000 = 1366.4, λ = 0.71075 Å, μ(Mo Kα) = 6.787 cm−1, 17 550 reflections measured, 4654 unique reflections (Rint = 0.0193). R1 = 0.0291 (I > 2σ(I)), R1 = 0.0313 (all data), wR2 = 0.0714 (all data) with GOF = 1.052. CCDC 1440598. Computational Investigations. Theoretical ab initio calculations were performed by density functional theory (DFT), as implemented in the Gaussian 09 software,36 using the Beck’s three-parameter hybrid functional with the correlation functional of Lee, Yang, and Parr (B3LYP).37 Molecules containing [Ru2] units were optimized by unrestricted open-shell calculations using an effective core potential basis set LanL2TZ with polarization (LanL2TZ(f))38 for the Ru atom and 6-31G basis sets with polarization and diffuse functions (631+G(d))39 for C, H, F, N, and O atoms. Spin polarization was used with SZ = 1 (triplet spin multiplicity) for [Ru2] unit. Atomic coordinates determined by X-ray crystallography19a were used as input in the calculations involving the [Ru2 ] unit and geometry optimizations of organic acceptors.
ments were performed at frequencies ranging from 1 to 1488 Hz with an ac field amplitude of 3 Oe. Polycrystalline samples were embedded in liquid paraffin. Experimental data were corrected for the sample holder, liquid paraffin, and diamagnetic contribution calculated from the Pascal constants.27 Magnetic measurements under hydrostatic pressures of up to 8.96 kbar were conducted using a piston−cylindertype Cu−Be alloy cell in the SQUID magnetometer.24 The sample was dispersed in Apiezon-J oil, which acted as a pressure-transmitting medium. Next, a piece of Pb, whose superconducting transition temperature was used to estimate the actual pressure, was added. The mixture was placed in a Teflon bucket and fixed using a cell clamp. X-ray Crystallography. Single crystals of 0.05-MeO, 0.10-MeO, 0.15-MeO, 0.20-MeO, and 1, measuring 0.194 × 0.091 × 0.052, 0.297 × 0.152 × 0.043, 0.225 × 0.189 × 0.048, 0.327 × 0.144 × 0.074, and 0.118 × 0.085 × 0.020 mm3, respectively, were mounted on a thin Kapton film with Nujol and cooled to 97(1) K using a stream of cooled N2 gas. Subsequently, they were treated at several temperatures using temperature-tuned N2 gases for which individual temperatures were checked using a thermocouple. Data were collected using a Rigaku CCD diffractometer (Saturn VariMax) with graphite- (λ = 0.71075 Å) or multilayer mirror-monochromated Mo Kα radiation (λ = 0.71075 Å) for 1 and site-doped compounds, respectively. Structures were solved by direct methods (SIR200828 for site-doped compounds, SIR9229 for 1) and expanded using Fourier techniques. All nonhydrogen atoms were anisotropically refined. Hydrogen atoms were only introduced for nondisordered carbon atoms at fixed positions because their position appeared inaccurate on disordered atoms and were refined using the riding model. A full-matrix least-squares refinement on F2 was performed on the basis of observed reflections and variable parameters, and the refinement cycle was estimated from unweighted and weighted agreement factors of R1 = ∑||F0| − |Fc||/∑| F0| (I > 2σ(I) and all data) and wR2 = [∑{w(F02 − Fc2)2}/ ∑w(F02)2]1/2 (all data). A Sheldrick weighting scheme was used. Neutral atom scattering factors were taken from Cromer and Waber.30 Anomalous dispersion effects were induced in Fc;31 Δf ′ and Δf ″ values were those of Creagh and McAuley.32 Mass attenuation coefficients are those of Creagh and Hubbell.33 All calculations were performed using the CrystalStructure crystallographic software package,34 except refinements, which were conducted using SHELXL-97.35 Only sitedoped compound structures measured at 97(1) K were deposited as CIF files at the Cambridge Data Centre (CCDC-1440599, 1440595, 1440596, 1440597, and 1440598 for 1, 0.05-MeO, 0.10-MeO, 0.15MeO, and 0.20-MeO, respectively). Duplicates can be obtained free of charge by application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: (+44) 1223-336-033; e-mail: ([email protected]). Crystallographic Data for 1. C70H52F16N4O10Ru2, Mr = 1615.32, triclinic, P−1 (#2), a = 10.636(4) Å, b = 10.845(5) Å, c = 15.399(6) Å, α = 88.253(17)°, β = 70.490(13)°, γ = 88.656(17)°, V = 1673.1(12) Å3, T = 103 K, Z = 1, Dcalc = 1.603 g/cm3, F000 = 812, λ = 0.71075 Å, μ(Mo Kα) = 5.586 cm−1, 13 504 reflections measured, 7261 unique reflections (Rint = 0.0524). R1 = 0.0655 (I > 2σ(I)), R1 = 0.1055 (all data), wR2 = 0.1385 (all data) with GOF = 1.087. CCDC 1440599. Crystallographic Data for 0.05-MeO. C54H32F16N4O8.1Ru2, Mr = 1372.58, monoclinic, P21/n (#14), a = 14.757(4) Å, b = 10.557(3) Å, c = 17.121(4) Å, β = 96.953(3)°, V = 2647.6(11) Å3, T = 97 K, Z = 2, Dcalc = 1.722 g/cm3, F000 = 1361.6, λ = 0.71075 Å, μ(Mo Kα) = 6.865 cm−1, 21 256 reflections measured, 6065 unique reflections (Rint = 0.0269). R1 = 0.0290 (I > 2σ(I)), R1 = 0.0330 (all data), wR2 = 0.0716 (all data) with GOF = 1.065. CCDC 1440595. Crystallographic Data for 0.10-MeO. C54H32F16N4O8.2Ru2, Mr = 1374.18, monoclinic, P21/n (#14), a = 14.778(3) Å, b = 10.573(2) Å, c = 17.125(4) Å, β = 97.203(3)°, V = 2654.5(9) Å3, T = 97 K, Z = 2, Dcalc = 1.719 g/cm3, F000 = 1363.2, λ = 0.71075 Å, μ(Mo Kα) = 6.849 cm−1, 21 223 reflections measured, 6070 unique reflections (Rint = 0.0237). R1 = 0.0348 (I > 2σ(I)), R1 = 0.0388 (all data), wR2 = 0.0839 (all data) with GOF = 1.128. CCDC 1440596. Crystallographic Data for 0.15-MeO. C54H32F16N4O8.3Ru2, Mr = 1375.78, monoclinic, P21/n (#14), a = 14.759(3) Å, b = 10.557(2) Å, c = 17.175(3) Å, β = 96.876(3)°, V = 2656.8(7) Å3, T = 97 K, Z = 2, Dcalc = 1.720 g/cm3, F000 = 1364.8, λ = 0.71075 Å, μ(Mo Kα) = 6.846
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02858. Bond lengths and angles for 1, figure for structure of 1, magnetic data for 1, and plots of −d(χT)/dT vs T with Gaussian fitting for the series of compounds (PDF) X-ray crystallographic data for 1 (CIF) X-ray crystallographic data for 0.05-MeO (CIF) X-ray crystallographic data for 0.10-MeO (CIF) X-ray crystallographic data for 0.15-MeO (CIF) X-ray crystallographic data for 0.20-MeO (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Phone: +81-22-215-2030. Fax: +81-22-215-2031. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (No. 15K13652) and a Grant-in-Aid for Scientific Research on Innovative Areas (“π-System Figuration” Area 2601, No. 15H00983) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, E-IMR project, Asahi Glass Foundation, and Mitsubishi Foundation.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.5b02858 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.5b02858 Inorg. Chem. XXXX, XXX, XXX−XXX
Tuning of Stepwise Neutral−Ionic Transitions by Acceptor Site Doping in Alternating Donor/Acceptor Chains Keita Nakabayashi,† Masaki Nishio,† and Hitoshi Miyasaka*,‡ †
Department of Chemistry, Division of Material Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan ‡ Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan S Supporting Information *
ABSTRACT: The stepwise neutral−ionic (N−I) phase transition found in the alternating donor/acceptor (DA) chain [Ru2(2,3,5,6-F4PhCO2)4(DMDCNQI)]·2(p-xylene) (0; 2,3,5,6-F4PhCO2− = 2,3,5,6-tetrafluorobenzoate; DMDCNQI = 2,5-dimethyl-N,N′-dicyanoquinonediimine) was tuned by partly substituting the acceptor DMDCNQI with 2,5dimethoxy-N,N′-dicyanoquinonediimine (DMeODCNQI), which displays a poorer electron affinity in an isostructural series. The site-doped series comprised [Ru 2 (2,3,5,6F4PhCO2)4(DMDCNQI)1−x(DMeODCNQI)x]·2(p-xylene) for doping rates (x) = 0.05 (0.05-MeO), 0.10 (0.10-MeO), 0.15 (0.15-MeO), and 0.20 (0.20-MeO). The neutral chain [Ru2(2,3,5,6-F4PhCO2)4(DMeODCNQI)]·4(p-xylene) (1), which only contained DMeODCNQI, was also characterized. All site-doped compounds were isostructural to 0 except 1 despite their identical DA chain motif. Except at an x value of 0.20, they displayed a two-step N−I transition involving an intermediate phase. This transition occurred at high temperatures in 0 but shifted to lower temperatures in a parallel manner with increasing doping rate. Simultaneously, each transition broadened with increasing doping rate, leading to a convergence of two transitions at an x value approximating 0.2. Donor/acceptor-site-doping techniques present somewhat different impacts in terms of interchain Coulomb effects.
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INTRODUCTION The neutral−ionic (N−I) phase transition in a donor (D)/ acceptor (A) pair, which involves electron transfer between distinct neutral (N: D0A0) and ionic (I: D+A−) valence states, is an intriguing switching phenomenon that drastically changes the fundamental physical properties of this pair, such as spin states, electronic transport, and electrical polarization. Since the discovery of the first organic DA pair comprising tetrathiafulvalene and chloranil by Torrance et al. in 1981,1 about 10 organic compounds2−8 and their derivatives8−10 have exhibited this transition. The first covalent DA system to undergo the N−I transition is the metal-complex chain compound [Ru2(2,3,5,6-F4PhCO2)4DMDCNQI]·2(p-xylene) (0, 2,3,5,6F4PhCO2− = 2,3,5,6-tetrafluorobenzoate, DMDCNQI = 2,5dimethyl-N,N′-dicyanoquinonediimine, Scheme 1).11 In this alternating chain, carboxylate-bridged paddlewheel-type [Ru2II,II(2,3,5,6-F4PhCO2)4] ([Ru2II,II] or [Ru2(F4)]) and DMDCNQI acted as D and A, respectively. The control of the N−I transition in DA pairs primarily depends on molecular composition, which determines the “balance” between the ionization potential of D and the electron affinity of A.12,13 Nonetheless, because it originates from a bulk phenomenon, the N−I transition also relies on crystal engineering, which modulates the alignment of DA units through intermolecular interactions and/or the Madelung stability in the I phase. The © XXXX American Chemical Society
balance between ionization potential and electron affinity is tuned by chemically modifying DA units and partly substituting these units by impurities belonging to an isostructural series.8,9b,d N−I transitions observed in 0 have previously been modulated by bottom-up site doping using [Ru2II,II(F5PhCO2)4] dummy donor units, which present a weaker ionization potential than [Ru2II,II(2,3,5,6-F4PhCO2)4] (hereafter, we use the term of “site doping” on the partial chemical modification of the A unit in this work). These stepwise transitions, wherein the intermediate (IM) phase involved an alternate arrangement of neutral and ionic chains,14−16 systematically varied as a function of the doping rate x.17 For x values less than 0.1, they abruptly occurred, concomitant with a clear shift in transition temperatures, demonstrating that, albeit tiny, partial local changes drastically affect bulk transitions similar to a domino effect. However, interchain Coulomb effects on intrachain electron transfer resulting from impurity doping remain unclear. In this study, N−I transitions in 0 were altered by replacing DMDCNQI by the dummy acceptor 2,5-dimethoxy-N,N′dicyanoquinonediimine (DMeODCNQI) as an less active impurity. The resulting series comprised [Ru2 (2,3,5,6Received: December 10, 2015
A
DOI: 10.1021/acs.inorgchem.5b02858 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Scheme 1. Schematic Representations of the Investigated Chain Compoundsa
a
Here 0 is the parent compound described in ref 11.
to the [Ru2II,II] oxidation state using previous ranges obtained for [Ru2II,II] (2.06−2.07 Å) and [Ru2II,III]+ (2.02−2.03 Å).18,19 Meanwhile, the oxidation state of the DMeODCNQI moiety was readily evaluated using the Kistenmacher relationship,20 δ = −{Aδ[c/(b + d)] + Bδ}, where Aδ = −36.900, Bδ = 17.295, and b, c, and d bonds are defined in the DMeODCNQI/ DMDCNQI chart shown in Scheme 1. This evaluation assumed that [Rh2II,II(CF3CO2)4(DMDCNQI)] corresponded to the neutral form with δ = 021 and [MnIII(TMesP) (DMDCNQI)] (TMesP = meso-tetrakis(2,4,6trimethylphenyl)porphyrinate) represented the ionic form with δ = 1.22 The estimated δ value amounted to 0.081 for 1, consistent with the neutral form and the [Ru2II,II] oxidation state. This assignment was only conducted at 103 K. However, magnetic data obtained for 1 were consistent with the paramagnetic behavior of [Ru2II,II] (S = 1), demonstrating that 1 adopted a neutral state in the entire measurement temperature range (1.8−300 K) (Figure S2). Interestingly, DMeODCNQI exhibited a higher LUMO energy level (ELUMO) (−4.322 eV) than DMDCNQI (−4.652 eV) by DFT studies, convincing the oxidation state of 1. Acceptor partially modified compounds were produced by the same synthetic procedure as 1, but DMeODCNQI was mixed into the DMDCNQI solution at ratios 1:19, 1:9, 1:5.7, and 1:4. Hereafter, the acceptor moiety will be noted DCNQI when DMeODCNQI and DMDCNQI do not need to be distinguished, while site-doped compounds will be called 0.05MeO, 0.10-MeO, 0.15-MeO, and 0.20-MeO according to mixing ratios (%). Very difficult to determine accurately, the doping rates of these compounds were evaluated from their structures using the occupancy of the MeO groups of the DCNQI moiety and elemental analysis (see Experimental
F4PhCO2)4(DMDCNQI)1−x(DMeODCNQI)x]·n(p-xylene) with x values of 0.05 (0.05-MeO), 0.10 (0.10-MeO), 0.15 (0.15-MeO), 0.20 (0.20-MeO), and 1 (1) (Scheme 1). All compounds exhibited the same DA chain motif, and all sitedoped compounds were isostructural to 0 with n = 2 except 1, which displayed a different packing mode with n = 4. Here, the site-doping effect of these DMeODCNQI less active acceptor units on N−I transitions was investigated in detail.
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RESULTS AND DISCUSSION Characterization of 1 and Site-Doped Compounds 0.05-MeO−0.20-MeO. The reaction of [Ru 2 (2,3,5,6F4PhCO2)4(THF)2] with DMeODCNQI in a dichloromethane/p-xylene mixture produced the one-dimensional chain compound [Ru2(2,3,5,6-F4PhCO2)4(DMeODCNQI)]·4(p-xylene) (1) comprising [−{Ru2}−(DMeODCNQI)−] repeating units. Compounds 0 and 1 exhibited very similar chain forms, but their number of crystallization solvents differed (e.g., 4 vs 2 mol of p-xylene was detected in 1 and 0, respectively), indicating their slightly dissimilar packing arrangement. Compound 1 crystallized in the triclinic P−1 space group (T = 103 K), and one-half of its formula unit, which displayed inversion centers at the midpoint of the Ru−Ru bond and the DMeODCNQI moiety, was crystallographically determined as an asymmetric unit (Z = 1) (Figure 1a). Packing diagrams of 1 are shown in Figure S1, while relevant local bond distances are listed in Table S1. The oxidation state of [Ru2] was determined by comparing bond lengths between metal and equatorial oxygen atoms (Oeq), while the oxidation state of DMeODCNQI was evaluated by comparing C−C bond lengths.11,17 The Ru−Oeq bond length averaged 2.071 Å in 1, which was assigned B
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Figure 1. Structures of 1 (a) and 0.20-MeO (b−d) (50% probability ellipsoids for a and b). Packing diagrams of 0.20-MeO projected (c) along the a axis and (d) along the b axis. Hydrogen atoms were omitted for clarity. Symmetry operations: *, −x + 1, −y + 2, −z; #, −x + 1, −y + 2, −z + 1; **, −x + 1, −y, −z + 1; ##, −x + 2, −y, −z + 1.
of temperature. Both variations are related with each other in the relation between donor and acceptor (i.e., as [Ru2II,II], δ = 0, and as [Ru2II,III]+, δ = 1). Essentially, the partial substitution with DMeODCNQI stabilized the N phase. Therefore, twostep transitions shifted to lower temperatures with increasing doping rate and became undetectable at x = 0.2 by this technique that presents a cooling limit of 97 K. At low doping rates (x < 0.1), they clearly featured a plateau, indicating the presence of IM phase comprising an equimolar N-chain/I-chain mixture. At an x value of 0.1, this plateau gradually shrank to disappear, allowing structural changes to occur. At x = 0.15, the site-doped compound no longer displayed the plateau region (in the structural study, but not in the magnetic study; vide infra), gradually changing from the N phase appeared at T ≥ 200 K to the I phase probably presented at T < 100 K. Evaluation of N−I Transitions in Magnetic Variations. The investigated chain compounds adopted a paramagnetic state (S = 1) in the N phase and a short-range ferrimagnetic state with antiferromagnetic coupling (S = 3/2 for [Ru2II,III]+; S = 1/2 for DCNQI•−) in the I phase. Therefore, their N−I transitions were monitored in terms of magnetic variations. Figure 3 shows the temperature dependence of χT and
Section). These data showed that actual doping rates reflected the mixing ratios. All site-doped compounds were isostructural to 0, which crystallized in the monoclinic P21/n space group with inversion centers located at the midpoint of the Ru−Ru bond and DCNQI moiety (Z = 2) at 97 K (Figure 1b). The chains ran along the a axis of the unit cell (Figure 1c and 1d), while crystallization solvents (2 mol of p-xylene) were located between chains and interacted with DCNQI and 2,3,5,6-F4Ph groups by π-stacking. Since these structural details have been previously addressed,11,17 the discussion focuses here on oxidation states. Site-doped compound structures were characterized by single-crystal X-ray crystallography at temperatures ranging between 97 and 260 K. All unit cells were determined using an original unit cell with the monoclinic P21/n space group (Z = 2) without considering any previously discussed superlattice for the IM phase.11,17 Local bond lengths were investigated to estimate charges on [Ru2] and DCNQI moieties. Figure 2 shows variations in average Ru−Oeq bond lengths and δ value estimated using the Kistenmacher relationship and local dimensions of the DCNQI moieties (vide supra) as a function C
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transition (vide infra). The χT plot slightly decreased at high temperature for the N phase, which may stem from the strong anisotropic effect of [Ru2II,II] (S = 1, zero-field splitting parameter D/kB ≈ 330 K; see Figure S2 for 1).18,19,23 It increased at low temperature for the I phase, indicative of shortrange ferrimagnetic ordering (S = 3/2 for [Ru2II,III]+; S = 1/2 for DCNQI•−) through the chain. In agreement with temperature-dependent structural results, these magnetic data demonstrate a successful insertion of DMeODCNQI in the chain. Transition temperatures are shown in the doping rate− temperature phase diagram (Figure 4), wherein error bars
Figure 2. Average Ru−Oeq length (top) and degree of charge −δ on the DMDCNQI moiety estimated using the Kistenmacher relationship (bottom) of all compounds as a function of temperature. Each structure was determined using the monoclinic space group P21/n. Red and blue regions roughly indicate expected locations for N and I phases, respectively, while the purple region corresponds to an intermediate area. Temperatures T1 and T2 shown in the same color as their corresponding compounds are N−I transition temperatures estimated from magnetic measurements (Figures 3 and S3).
Figure 4. Doping rate (x)−temperature phase diagram for the oxidation states in neutral (N), ionic (I), and intermediate (IM) phases (dotted lines are only guides for the eye), where doping rate x means the site-doped ratio of acceptor in [Ru 2 (2,3,5,6F4PhCO2)4(DMDCNQI)1−x(DMeODCNQI)x]·2(p-xylene). T1 and T2 values were obtained from magnetic measurements. Error bars were estimated as standard deviations of a Gauss fitting for individual −d(χT)/dT peaks (Figure S3).
correspond to standard deviations from a Gauss fitting of individual −d(χT)/dT peaks (Figure S3). These standard deviations rose with increasing doping rate, indicating that transitions occurred in a temperature region that varied according to the environment of each D/A component within and between chains. Therefore, the DMeODCNQI dopant (A′) was randomly inserted into chains with DMDCNQI, and resulting D/AA′ configurations and interchain Coulomb interactions strongly affected transitions in local D/A parts. For x values inferior or equal to 0.15, transitions occurred stepwise and the IM temperature range shifted in a parallel fashion to lower temperatures upon doping rate increase. Only one transition, associated with a large standard deviation in a wide temperature range, was detected at an x value of 0.2. A significant increase in χT was also observed for 0.20-MeO in the I phase, consistent with the presence of relevant correlation lengths, such as a correlation length through two to three DA units with a magnetic exchange constant (J) approximating −100 K. This result also demonstrates the random insertion of dopant molecules. The convergence of two transitions, which suggests the disappearance of the IM phase, may result from the fact that the Coulomb gain reached the Madelung stabilization when the temperature decreased. Pressure-Induced Variations of Phase Diagrams. Neutral−ionic transitions were tuned by applying hydrostatic pressures to polycrystalline samples of the site-doped compounds in Apiezon-J oil, which acted as a pressure-
Figure 3. Temperature dependence of χT and −d(χT)/dT during cooling for 0, 0.05-MeO, 0.10-MeO, 0.15-MeO, and 0.20-MeO. −d(χT)/dT values were obtained from data measured at 0.1 T.
−d(χT)/dT of site-doped compounds and 0 (where χ = M/H) upon cooling. The N−I transitions were clearly observed in all site-doped compounds including 0.20-MeO, which did not show any distinct transition in structural investigations conducted between 97 and 260 K (vide supra). These transitions shifted to lower temperatures when the doping rate increased. Compounds 0, 0.05-MeO, 0.10-MeO, and 0.15MeO displayed transitions at temperatures T1 and T2, which were determined using the peaks appearing in the −d(χT)/dT plot (Figure 3). In contrast, 0.20-MeO exhibited a one-step D
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Figure 5. Temperature dependence of the magnetization for 0.05-MeO (a), 0.10-MeO (b), 0.15-MeO (c), and 0.20-MeO (d) at 1 kOe under applied pressures. MT = M × T, where M is the raw magnetization.
Figure 6. Applied pressure−temperature phase diagrams for charge-ordered states in neutral (N, red), ionic (I, blue), and intermediate (IM, purple) phases for (a) 0.05-MeO, (b) 0.10-MeO, (c) 0.15-MeO, and (d) 0.20-MeO.
quantum interference device (SQUID) magnetometer, and the magnetization was measured as a function of temperature at
transmitting medium, using a piston−cylinder-type Cu−Be alloy cell.24,25 The cell was placed in a superconducting E
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transition itself as well as control over magnetic switching. Here, as demonstrated in the N−I transition works, DA systems comprising [Ru2II,II] and DCNQI/TCNQ may serve as charge mediators to control physical properties, such as magnetism and electron transport. These redox-active metal− organic frameworks may lead to multiply controllable functional molecular materials.
different applied pressures (Figure 5). The applied pressure reached a maximum of 8.96 kbar for 0.20-MeO, and the actual pressure was estimated from the superconducting transition temperature for Pb. The obtained pressure−temperature diagrams are shown in Figure 6 for 0.05-MeO, 0.10-MeO, 0.15-MeO, and 0.20-MeO. The oxidation phase strongly depended on the applied pressure. In 0.05-MeO and 0.10MeO, transition temperatures T1 and T2 quadratically rose with applied pressure, the increase of which reached approximately 100 K at about 1 kbar (Figure 6a and 6b). This quadratic increase was less pronounced for 0.15-MeO than that for 0.05MeO and 0.10-MeO (Figure 6c). The single transition temperature varied in a sigmoidal manner for 0.20-MeO (Figure 6d). Compounds 0.05, 0.10, and 0.15 retained their IM phase under applied pressure. These observations are very similar to previous findings obtained by donor partial modification.17 They indicate that applied hydrostatic pressure affected the chains isotropically as well as spherical Coulomb interactions between chains. Similarly to magnetization variations (Figure 5d), the correlation length of 0.20-MeO grew with increasing applied pressure concomitant with a rise in transition temperatures, which is comparable to trends obtained for a perfect-ionic chain at P ≈ 9.0 kbar.
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EXPERIMENTAL SECTION
Materials. All synthetic procedures were performed under N2 atmosphere using standard Shlenk 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 under N2 atmosphere before use. Starting materials including [Ru2II,II(2,3,5,6-F4PhCO2)4(MeOH)2]19a and DCNQI derivatives (2,5-dimethyl- and 2,5-dimethoxy-DCNQI)26 were synthesized according to previous methods. Crystals contained p-xylene molecules as crystallization solvents, which slowly disappeared at room temperature, making elemental analyses difficult to interpret. Therefore, samples were dried before elemental analysis. Samples were aged for a few hours after their removal from their mother liquids unlike samples destined for magnetic measurements, which were immediately used. Synthesis of DMeODCNQI-Doped Compounds. The sitedoped compounds 0.05-MeO, 0.10-MeO, 0.15-MeO, and 0.20-MeO were synthesized by following the procedure used for compound 0.11 A DMDCNQI/DMeODCNQI solution (100 mL) in dichloromethane with a molar ratio of (0.25 − x) mmol/x mmol (x = 0.0125, 0.025, 0.0375, and 0.05 for 0.05-MeO, 0.10-MeO, 0.15-MeO, and 0.20-MeO, respectively) was divided into 2 mL aliquots. Each aliquot was placed in a narrow-diameter sealed glass tube (ϕ: 8 mm) to form the bottom layer. A 1:1 p-xylene/dichloromethane mixture (v/ v; 1 mL) was carefully added to the bottom layer to give the middle layer. A 2 mL aliquot of [Ru2(2,3,5,6-F4PhCO2)4(MeOH)2] solution (260 mg, 0.25 mmol) in p-xylene (100 mL) was then carefully deposited on the middle layer in each tube. The glass tubes were turned upside down and left undisturbed for at least 2 weeks to produce the site-doped compounds as plate-shaped brown crystals. For 0.05-MeO, yield 79.2 mg (27%). Anal. Calcd for dried 0.05-MeO (C38H12F16N4O8.1Ru2): C, 39.34; H, 1.04; N, 4.83. Found: C, 39.12; H, 1.17; N, 4.86. FT-IR (KBr pellet, 4000−400 cm−1 at RT): ν(C N), 2108; ν(CC), 1538; ν(CO), 1586, 1405 cm−1. For 0.10MeO, yield: 77.8 mg (27%). Anal. Calcd for dried 0.10-MeO (C38H12F16N4O8.2Ru2): C, 39.28; H, 1.04; N, 4.82. Found: C, 39.28; H, 1.28; N, 4.79. FT-IR (KBr pellet, 4000−400 cm−1 at RT): ν(C N), 2110; ν(CC), 1539; ν(CO), 1588, 1406 cm−1. For 0.15MeO, yield: 49.9 mg (17%). Anal. Calcd for dried 0.15-MeO (C38H12F16N4O8.3Ru2): C, 39.23; H, 1.04; N, 4.82. Found: C, 39.39; H, 1.10; N, 4.66. FT-IR (KBr pellet, 4000−400 cm−1 at RT): ν(C N), 2108; ν(CC), 1539; ν(CO), 1587, 1405 cm−1. For 0.20MeO, yield: 73.8 mg (25%). Anal. Calcd for the dried 0.20-MeO (C38H12F16N4O8.4Ru2): C, 39.18; H, 1.04; N, 4.81. Found: C, 39.23; H, 1.11; N, 4.80. FT-IR (KBr pellet, 4000−400 cm−1 at RT): ν(C N), 2109; ν(CC), 1536; ν(CO), 1586, 1403 cm−1. Synthesis of 1. [Ru2(2,3,5,6-F4PhCO2)4(THF)2] (56 mg, 0.05 mmol) was dissolved in p-xylene (30 mL) and filtered. The resulting solution was mixed with a DMeODCNQI solution (10.8 mg, 0.05 mmol) in dichloromethane (30 mL) and left undisturbed overnight, affording 1 as brown crystals. Yield: 18.5 mg (23%). Anal. Calcd for dried 1 (C38H12F16N4O10Ru2): C, 38.33; H, 1.02; N, 4.71. Found: C, 38.47; H, 1.26; N, 5.01. FT-IR (KBr pellet, 4000−400 cm−1 at RT): ν(CN), 2180, 2114; ν(CC), 1588, 1540; ν(CO), 1575, 1396 cm−1. Physical Measurements. Infrared spectra were measured using KBr disks at room temperature on a JASCO FT-IR 620 spectrophotometer. Magnetic susceptibility measurements were conducted using a SQUID magnetometer (Quantum Design MPMS-XL) for temperatures ranging from 1.8 to 300 K and magnetic fields spanning from −7 to 7 T. Alternating current (ac) measure-
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CONCLUSION The N−I transition can be mainly tuned by (i) the balance between the ionization potential of D and electron affinity of A, (ii) Coulomb interactions around individual D/A unit, and (iii) the Madelung stability in the I phase. These parameters were experimentally assessed in stepwise N−I transition systems based on covalently bonded DA chains. When impurities, such as less redox-active D/A units, were inserted at a very low concentration (ca. 5%), the partial (local) modification strongly affected the N−I transition phenomenon, causing transition temperatures to significantly drop with an IM phase that reflects Coulomb effects between neighboring chains. This demonstrates that a small local part (or factor) in bulk determines an essential bulk phenomenon, similar to a domino effect. Moreover, an increase in impurity concentration enhanced the randomness of transitions because the D/A unit started to perceive the influence of next-neighboring species as well as through the entire chain. Also, the effect of interchain Coulomb interactions may also change in each D/A moiety. Therefore, transition temperatures decreased. In addition, the N−I transition mainly depended on factors (i) and (iii) at low temperatures with T1 ≈ T2 because the energy gain from the Coulomb effect should be covered by the Madelung stability, resulting in the disappearance of the IM phase. The above-mentioned mechanism is generally valid for Dand A-partially modified compounds. However, site-doped compounds exhibited slightly different shifts in the IM phase. The IM phase region tended to converge with increasing doping rate in the D-modified series17 but shifted in a parallel manner in the A-modified series (this study). However, T1 and T2 transitions converged at an x value of about 0.20 in both series. In addition, transition broadening was more prominent in A-modified compounds. Therefore, D/A-partial modification approaches produce somewhat different Coulomb effects. The stepwise N−I transition associated with Coulomb effects and the magnetic correlation growth in the ionic phase are unique to compound 0. Therefore, a detailed investigation of this type of transition is expected to provide information on the F
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cm−1, 21 127 reflections measured, 5987 unique reflections (Rint = 0.0323). R1 = 0.0368 (I > 2σ(I)), R1 = 0.0421 (all data), wR2 = 0.0954 (all data) with GOF = 1.084. CCDC 1440597. Crystallographic Data for 0.20-MeO. C54H32F16N4O8.4Ru2, Mr = 1377.38, monoclinic, P21/n (#14), a = 14.803(2) Å, b = 10.560(2) Å, c = 17.269(3) Å, β = 96.729(2)°, V = 2680.8(7) Å3, T = 97 K, Z = 2, Dcalc = 1.706 g/cm3, F000 = 1366.4, λ = 0.71075 Å, μ(Mo Kα) = 6.787 cm−1, 17 550 reflections measured, 4654 unique reflections (Rint = 0.0193). R1 = 0.0291 (I > 2σ(I)), R1 = 0.0313 (all data), wR2 = 0.0714 (all data) with GOF = 1.052. CCDC 1440598. Computational Investigations. Theoretical ab initio calculations were performed by density functional theory (DFT), as implemented in the Gaussian 09 software,36 using the Beck’s three-parameter hybrid functional with the correlation functional of Lee, Yang, and Parr (B3LYP).37 Molecules containing [Ru2] units were optimized by unrestricted open-shell calculations using an effective core potential basis set LanL2TZ with polarization (LanL2TZ(f))38 for the Ru atom and 6-31G basis sets with polarization and diffuse functions (631+G(d))39 for C, H, F, N, and O atoms. Spin polarization was used with SZ = 1 (triplet spin multiplicity) for [Ru2] unit. Atomic coordinates determined by X-ray crystallography19a were used as input in the calculations involving the [Ru2 ] unit and geometry optimizations of organic acceptors.
ments were performed at frequencies ranging from 1 to 1488 Hz with an ac field amplitude of 3 Oe. Polycrystalline samples were embedded in liquid paraffin. Experimental data were corrected for the sample holder, liquid paraffin, and diamagnetic contribution calculated from the Pascal constants.27 Magnetic measurements under hydrostatic pressures of up to 8.96 kbar were conducted using a piston−cylindertype Cu−Be alloy cell in the SQUID magnetometer.24 The sample was dispersed in Apiezon-J oil, which acted as a pressure-transmitting medium. Next, a piece of Pb, whose superconducting transition temperature was used to estimate the actual pressure, was added. The mixture was placed in a Teflon bucket and fixed using a cell clamp. X-ray Crystallography. Single crystals of 0.05-MeO, 0.10-MeO, 0.15-MeO, 0.20-MeO, and 1, measuring 0.194 × 0.091 × 0.052, 0.297 × 0.152 × 0.043, 0.225 × 0.189 × 0.048, 0.327 × 0.144 × 0.074, and 0.118 × 0.085 × 0.020 mm3, respectively, were mounted on a thin Kapton film with Nujol and cooled to 97(1) K using a stream of cooled N2 gas. Subsequently, they were treated at several temperatures using temperature-tuned N2 gases for which individual temperatures were checked using a thermocouple. Data were collected using a Rigaku CCD diffractometer (Saturn VariMax) with graphite- (λ = 0.71075 Å) or multilayer mirror-monochromated Mo Kα radiation (λ = 0.71075 Å) for 1 and site-doped compounds, respectively. Structures were solved by direct methods (SIR200828 for site-doped compounds, SIR9229 for 1) and expanded using Fourier techniques. All nonhydrogen atoms were anisotropically refined. Hydrogen atoms were only introduced for nondisordered carbon atoms at fixed positions because their position appeared inaccurate on disordered atoms and were refined using the riding model. A full-matrix least-squares refinement on F2 was performed on the basis of observed reflections and variable parameters, and the refinement cycle was estimated from unweighted and weighted agreement factors of R1 = ∑||F0| − |Fc||/∑| F0| (I > 2σ(I) and all data) and wR2 = [∑{w(F02 − Fc2)2}/ ∑w(F02)2]1/2 (all data). A Sheldrick weighting scheme was used. Neutral atom scattering factors were taken from Cromer and Waber.30 Anomalous dispersion effects were induced in Fc;31 Δf ′ and Δf ″ values were those of Creagh and McAuley.32 Mass attenuation coefficients are those of Creagh and Hubbell.33 All calculations were performed using the CrystalStructure crystallographic software package,34 except refinements, which were conducted using SHELXL-97.35 Only sitedoped compound structures measured at 97(1) K were deposited as CIF files at the Cambridge Data Centre (CCDC-1440599, 1440595, 1440596, 1440597, and 1440598 for 1, 0.05-MeO, 0.10-MeO, 0.15MeO, and 0.20-MeO, respectively). Duplicates can be obtained free of charge by application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: (+44) 1223-336-033; e-mail: ([email protected]). Crystallographic Data for 1. C70H52F16N4O10Ru2, Mr = 1615.32, triclinic, P−1 (#2), a = 10.636(4) Å, b = 10.845(5) Å, c = 15.399(6) Å, α = 88.253(17)°, β = 70.490(13)°, γ = 88.656(17)°, V = 1673.1(12) Å3, T = 103 K, Z = 1, Dcalc = 1.603 g/cm3, F000 = 812, λ = 0.71075 Å, μ(Mo Kα) = 5.586 cm−1, 13 504 reflections measured, 7261 unique reflections (Rint = 0.0524). R1 = 0.0655 (I > 2σ(I)), R1 = 0.1055 (all data), wR2 = 0.1385 (all data) with GOF = 1.087. CCDC 1440599. Crystallographic Data for 0.05-MeO. C54H32F16N4O8.1Ru2, Mr = 1372.58, monoclinic, P21/n (#14), a = 14.757(4) Å, b = 10.557(3) Å, c = 17.121(4) Å, β = 96.953(3)°, V = 2647.6(11) Å3, T = 97 K, Z = 2, Dcalc = 1.722 g/cm3, F000 = 1361.6, λ = 0.71075 Å, μ(Mo Kα) = 6.865 cm−1, 21 256 reflections measured, 6065 unique reflections (Rint = 0.0269). R1 = 0.0290 (I > 2σ(I)), R1 = 0.0330 (all data), wR2 = 0.0716 (all data) with GOF = 1.065. CCDC 1440595. Crystallographic Data for 0.10-MeO. C54H32F16N4O8.2Ru2, Mr = 1374.18, monoclinic, P21/n (#14), a = 14.778(3) Å, b = 10.573(2) Å, c = 17.125(4) Å, β = 97.203(3)°, V = 2654.5(9) Å3, T = 97 K, Z = 2, Dcalc = 1.719 g/cm3, F000 = 1363.2, λ = 0.71075 Å, μ(Mo Kα) = 6.849 cm−1, 21 223 reflections measured, 6070 unique reflections (Rint = 0.0237). R1 = 0.0348 (I > 2σ(I)), R1 = 0.0388 (all data), wR2 = 0.0839 (all data) with GOF = 1.128. CCDC 1440596. Crystallographic Data for 0.15-MeO. C54H32F16N4O8.3Ru2, Mr = 1375.78, monoclinic, P21/n (#14), a = 14.759(3) Å, b = 10.557(2) Å, c = 17.175(3) Å, β = 96.876(3)°, V = 2656.8(7) Å3, T = 97 K, Z = 2, Dcalc = 1.720 g/cm3, F000 = 1364.8, λ = 0.71075 Å, μ(Mo Kα) = 6.846
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02858. Bond lengths and angles for 1, figure for structure of 1, magnetic data for 1, and plots of −d(χT)/dT vs T with Gaussian fitting for the series of compounds (PDF) X-ray crystallographic data for 1 (CIF) X-ray crystallographic data for 0.05-MeO (CIF) X-ray crystallographic data for 0.10-MeO (CIF) X-ray crystallographic data for 0.15-MeO (CIF) X-ray crystallographic data for 0.20-MeO (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Phone: +81-22-215-2030. Fax: +81-22-215-2031. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (No. 15K13652) and a Grant-in-Aid for Scientific Research on Innovative Areas (“π-System Figuration” Area 2601, No. 15H00983) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, E-IMR project, Asahi Glass Foundation, and Mitsubishi Foundation.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.5b02858 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.5b02858 Inorg. Chem. XXXX, XXX, XXX−XXX
