Magnetic Sponge Behavior via Electronic State Modulations - Journal

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Magnetic Sponge Behavior via Electronic State Modulations Jun Zhang, Wataru Kosaka, Kunihisa Sugimoto, and Hitoshi Miyasaka J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02428 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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Journal of the American Chemical Society

Revised version on ja-2018-024283

Magnetic Sponge Behavior via Electronic State Modulations

Jun Zhang,a,b Wataru Kosaka,a Kunihisa Sugimoto,c and Hitoshi Miyasaka*a

a

Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577,

Japan. b

Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aza-Aoba,

Aoba-ku, Sendai 980-8578, Japan. c

Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo-cho, Sayo-gun Hyogo

679-5198, Japan.

Corresponding author* Prof. Dr. Hitoshi Miyasaka Institute for Materials Research, Tohoku University 2–1–1 Katahira, Aoba-ku, Sendai 980-8577, Japan E-mail: [email protected] Tel: +81-22-215-2030 FAX: +81-22-215-2031

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Abstract A reversible magnetic change in response to external stimuli is a desired function of molecular magnetic materials. The magnetic change induced by a change in the intrinsic spin is significant because the magnetic change is inevitable and could become drastic. In this study, we demonstrate a reversible magnetic change closely associated with electronic state modulations, as well as structural modifications realized by solvation/desolvation cycles of a magnetic sponge. The compound was a D2A-type-layered magnet, [{Ru2(O2CPh-2,3,5-Cl3)4}2(TCNQMe2)]·4DCM (1; 2,3,5-Cl3PhCO2− = 2,3,5-trichlorobenzoate; TCNQMe2 = 2,5-dimethyl-7,7,8,8-tetracyanoquinodimethane, DCM = dichloromethane), where [Ru2(O2CPh-2,3,5-Cl3)4] ([Ru2II,II]) is an electron-donor (D) and TCNQMe2 is an electron-acceptor (A). Compound 1 had a one-electron-transferred, charge-ordered state with a [{Ru2II,II}–TCNQMe2•−–{Ru2II,III}+] (1e-I) formula. Strong intralayer antiferromagnetic couplings between [Ru2II,II] with S = 1 or [Ru2II,III]+ with S = 3/2 and TCNQMe2•− with S = 1/2, as well as ferromagnetic interlayer interactions, induced long-range ferrimagnetic ordering at Tc = 101 K. Interstitial DCM molecules were located between layers, and these were gradually eliminated under vacuum at 80 °C to form a solvent-free compound (1-dry) without loss of crystallinity. The electronic state of 1-dry thermally fluctuated and eventually provided a charge-disproportionate disordered state, with a [{Ru2}0.5+–TCNQMe21.5−–{Ru2II,III}+] (1.5e-I) formula as the ground state. The Tc in 1-dry was 34 K because of the presence of diamagnetic TCNQMe22− in some parts of the framework. A large Tc variation with DTc ≈ 70 K was switchable; switching was achieved by charge state modulations accompanied with subtle structural modifications in solvation/desolvation treatments.


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INTRODUCTION

Porous magnets have attracted great interest due to potential applications such as in magnetic sensors, switches, and separation media.1,2 It is still a challenge to combine magnetic ordering and porosity in a single material, because of the conflict between the necessity of short bridges for strong magnetic coupling and the advantage of long bridges for the high porosity.3 A number of porous coordination polymers (PCPs) or metal–organic frameworks (MOFs) categorized as “dynamical magnetic MOFs” have been investigated.4 Because PCPs/MOFs protrude due to the flexibility that allows structural deformation to accommodate guest molecules,5 the change in the magnetic properties in the host framework induced by guest sorption is a promising phenomenon. Indeed, reversible magnetic changes induced by guest molecule accommodations, 6,7,8,9,10,11,12 including ion accommodations induced by an electrochemical approach,13,14,15,16 have been reported so far. These changes are based on a methodology apparently different from those realized by applying external stimuli such as light17,18,19,20 and pressure.21,22,23,24 Generally, driving forces for guest-induced magnetic modulation (except for the electrochemical approach) have been reported (so far) to be (i) a change in the coordination environment (changes in lattice dimensions or modification of bonds that become magnetic pathways), (ii) malformations of the lattice (e.g., disorder, rotation, shrinkage/expansion), (iii) structural collapse and recovery (e.g., transformation between crystal and amorphous phases), and (iv) a combination of the first three processes. So far, the guest-induced magnetic response has mainly been studied for spin-crossover (SCO) phenomena25 and bulk magnets.26 A typical example for guest-driven SCO behavior was reported by Kepert and coworkers,6 who demonstrated the reversible switching of SCO phenomena through the shrinkage and expansion of a channel via, respectively, desorption and adsorption of alcohol vapor (driving force (ii)). On the other hand, 3 ACS Paragon Plus Environment


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respective groups of Ohba7 and Inoue11 reported particularly notable cases with the driving force (i). A reversible transformation between a two-dimensional (2-D) sheet and 3-D pillared-sheet framework was achieved by the generation/cleavage of a coordination bond with replaceable coordinated water via dehydration/hydration. Regarding the driving force (ii), our group9 and Kurmoo group12 reported that a small structural change associated with a solvation/desolvation is producible upon a drastic magnetic phase change between antiferromagnetic and ferromagnetic phases.

A class of electron-donor (D)/-acceptor (A) charge-transfer (CT)/electron-transfer (ET) frameworks is also a promising candidate for such guest-induced magnetic materials, because CT/ET is quite sensitive to structural modulations. 27 The electronic states of systems which are near boundaries between the neutral (N; e.g., D0N0) and ionic (I; e.g., D+A–) states is quite sensitive to coordination environments and structural modulations.27,45 In such bordering systems, small structural modulations induced by guest molecules could change the charge distribution among the lattice, which would induce a drastic change in the magnetic properties without significant lattice deformation.27

Here we report a new D2A layer magnetic system that undergoes a large Tc change (DTc ≈ 70 K) upon desolvation/solvation treatments. The pristine solvated compound was [{Ru2(O2CPh-2,3,5Cl3)4}2(TCNQMe2)]·4DCM (1) (2,3,5-Cl3PhCO2− = 2,3,5-trichlorobenzoate; TCNQMe2 = 2,5dimethyl-7,7,8,8-tetracyanoquinodimethane; DCM = dichloromethane), where [Ru2II,II(O2CPh-2,3,5Cl3)4] and TCNQMe2 are D and A, respectively. Compound 1, freshly prepared, had an intra-lattice, one-electron transferred ionic state (1e-I; D0.5+2A–) with a common charge distribution with a [{Ru2II,II}–(TCNQMe2)•−–{Ru2II,III}+] formula (Figure 1), which is a ferrimagnet with Tc = 101 K.


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Compound 1 lost the DCM molecules without loss of crystallinity to form a dried compound (1-dry), which had a charge-disproportionation disordered state (1.5e-I) with a [{Ru2II,II/II,III}0.5+–(TCNQMe2•– /TCNQMe22–)1.5––{Ru2II,III}+] formula upon thermal fluctuation to nearly be a fully charge-transferred state (2e-I; D+2A2–) with a [{Ru2II,III}+–(TCNQMe2)2−–{Ru2II,III}+] formula at high temperatures (Figure 1). Compound 1-dry was also a ferrimagnet, but Tc (= 34 K) was lower than that of 1. The solvation from 1-dry to 1 was mostly reversible, but it involved a small structural change in an isostructural form which yielded to the re-solvated compound 2 with a 1e-I state, a ferrimagnet with Tc = 97 K. The re-solvated compound 2 changed to 2-dry with a 1.5e-I state upon elimination of DCMs, which was also slightly different from 1-dry despite being almost isostructural (2-dry, Tc = 30 K). The change between 2 and 2-dry revealed complete reversibility. In this system, a subtle structural modulation induced by solvation/desolvation changed the charge distribution in the lattice, which provided a drastic change in macroscopic magnetism with a Tc variation of DTc ≈ 70 K. This is the first example of a bulk magnetic change due to guest-induced, intra-lattice electron transfer.



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Figure 1. Schematic representations of the redox reactions of paddlewheel-type [Ru2] complex (a), TCNQ moiety (b), and the electronic state in the present [{Ru2}2(TCNQ)] system (D2A system) (c), where 1e-I, 2e-I, and 1.5e-I* mean a one-electron transferred ionic state, a fully electron-transferred ionic state, and a charge-disproportionate disordered ionic state, respectively.

n

RESULTS AND DISCUSSION

Preparation of the materials. The pristine solvated compound [{Ru2(O2CPh-2,3,56 ACS Paragon Plus Environment

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Cl3)4}2(TCNQMe2)]·4DCM (1) was prepared by a slow mixture of a DCM solution containing TCNQMe2 (bottom layer) and a p-xylene solution of [Ru2II,II(O2CPh-2,3,5-Cl3)4(THF)2] (top layer) in a N2 atmosphere. The desolvation of 1, however, was activated when evacuated at 80˚C for 12 hours; the crystallization solvents (four DCM molecules) were lost, yielding a solvent-free compound [{Ru2(O2CPh-2,3,5-Cl3)4}2(TCNQMe2)] (1-dry) which retained its crystallinity (vide infra). Upon thermal gravimetric analyses (TGA) at an ambient pressure of N2 atmosphere, 1 had a weight loss of 12.5% upon heating to ca. 423 K (Figure S1); this corresponded to the release of four DCM molecules (12.3%). Compound 1-dry was thermally stable as determined from the TGA curve, which plateaued even after releasing DCM upon heating up to ca. 473 K.

Single crystals of 1-dry were immersed in a DCM solvent for six days; the crystals changed to a re-solvated compound with approximately four DCM molecules per formula unit ([{Ru2(O2CPh2,3,5-Cl3)4}2(TCNQMe2)]·4DCM; 2). In fact, 1-dry adsorbed approximately four molar amounts of DCM(gas) at 298 K, showing its permanent porous nature for DCM (Figure S2). The re-solvated compound 2 is similar to 1, but not the same as 1 (vide infra). The desolvation treatment for 2 via heating at 80˚C for 12 hours yielded another solvent-free compound ([{Ru2(O2CPh-2,3,5Cl3)4}2(TCNQMe2)]; 2-dry), which was similar to 1-dry (vide infra). The solvation/desolvation process between 2 and 2-dry is reversible, as demonstrated in magnetic measurements (vide infra).

Structural

variations

via

desolvation/solvation

processes.

Single-crystal

X-ray

crystallography was conducted at around 110 K for all compounds. Regardless of whether in solvated or desolvated form, all compounds 1, 1-dry, 2, and 2-dry had similar two-dimensional layer structures (vide infra). These crystallized in the triclinic P−1 space group (Table S1) with a formula unit comprising two crystallographically independent [Ru2] units, [Ru(1)2] and [Ru(2)2], and one 7 ACS Paragon Plus Environment


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TCNQMe2 molecule with inversion centers on the midpoints of the respective units (Z = 1) (Figure S3). Four coordination-donor CN groups of TCNQMe2 coordinated to the axial sites of the [Ru2] units, forming a typical fishnet-like layer structure laying on the (100) plane (Figures 2a, 2b and S4) with interlayer vertical (l1)/translational (l2) distances of 10.52/10.98, 10.17/11.04, 10.49/11.00, and 10.13/11.01 Å for 1, 1-dry, 2, and 2-dry, respectively. These interlayer translational distances in the range of l2 ≥ 10.3 Å suggest that ferromagnetic dipole interactions could be dominant between layers in all compounds (Figure S5).28 The solvated compounds 1 and 2 had four molecules of DCM per unit; thermal gravimetric analyses confirmed the amount of DCM (Figure S1), which are located between layers (Figures 2b and S4d). Although it seems there was no significant changes even between the solvated (1 and 2) and desolvated (1-dry and 2-dry) groups, these groups were identified from their lattice dimensions and parameters (Table S1). Indeed, powder X-ray diffraction (PXRD) patterns of 1 and 1-dry are definitely distinguishable. The evolution of 1-dry from 1 in a gradual desolvation process was traced by in situ PXRD measurements (Figure 3); a fresh sample of 1 was evacuated at 300 K for 54 hours in total, and then at 353 K for 12 hours to complete desolvation. During this desolvation process, only two phases of 1 and 1-dry were identified without any intermediate phases.


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Figure 2. Packing views of 1 projected along (a) a-axis and (b) b-axis, and (c) the visualization of the structural variations in the formula units of 1, 1-dry, 2, and 2-dry via desolvation/solvation processes, where atoms of N, O, C, Cl, Ru(1), and Ru(2) are represented in blue, red, gray, green, pink, and purple, respectively. DCM molecules as crystallization solvents in (a, b) are given in cyan. Dashed circles in (c) indicate disorder forms in the 2,3,5-Cl3Ph group of the [Ru2] moiety. Hydrogen atoms in all modes are omitted for clarity.



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Figure 3. Variation of PXRD patterns for 1 during a desolvation process, where the time scale of 54 h–12 h in the legend means total evacuating time for 54 h at 300 K and then for 12 h at 353 K, and PXRD patterns for 1 calc. and 1-dry calc. are simulated patterns obtained from single-crystal X-ray crystallography.

A clear distinction between the solvated and desolvated groups were found in local bond lengths in parts of [Ru2] and TCNQMe2 units, which clearly reflect the oxidation states of units demonstrated in previous work.9,14,15,16,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47 For the [Ru2] unit, Ru−Oeq (Oeq = carboxylate oxygen atom), bond distances were quite sensitive to the oxidation state of the [Ru2] unit: 2.06–2.07 Å for [Ru2II,II] and 2.01–2.03 Å for [Ru2II,III]+.48 The Ru−Oeq distances in 1, 1-dry, 2, and 2-dry are summarized in Table S2. The oxidation state of [Ru(1)2] and [Ru(2)2] units for 1 and 2 was clearly determined as [Ru(1)2II,II] and [Ru(2)2II,III]+, thus providing a charge-distributed 1e-I form, whereas that of [Ru(1)2] and [Ru(2)2] units for 1-dry and 2-dry was indeed indistinct; the [Ru(1)2] unit had the [Ru(1)2II,III]+ oxidation state, whereas the [Ru(2)2] unit was just at an intermediate 10 ACS Paragon Plus Environment

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oxidation state between [Ru(2)2II,III]+ and [Ru(2)2II,II]. Additionally, the electronic state of TCNQ was estimated based on the Kistenmacher relationship49 (ρ = A[c/(b + d)] + B), where b, c, and d were the respective bond distances for the 7,9-, 1,7-, and 1,2-positioned C−C sets in TCNQ moiety (figure in Table S3). and A = −41.667 and B = +19.833 were evaluated based on neutral TCNQ (r = 0)50 and Rb+TCNQ•− (r = −1).51 The estimated ρ values for 1 and 2 were −1.09 and –1.26, respectively. Considering that the group of TCNQRx with electron-donating Rx groups such as Me and MeO groups tended to provide larger r values than expected, the estimated values indicate r ≈ –1, i.e., the monoanion form (TCNQMe2•–) for 1 and 2. Meanwhile, the r values for 1-dry and 2-dry were –1.67 and –2.21, respectively, suggesting a virtual oxidation state taking an intermediate state between TCNQMe2•– and TCNQMe22– (Table S3). These evaluations based on the structures suggest that the solvated compounds 1 and 2 had a chargedistributed 1e-I state with [{Ru(1)2II,II}–(TCNQMe2)•−–{Ru(2)2II,III}+], whereas the desolvated compounds 1-dry and 2-dry had a charge-disproportionation 1.5e-I state, i.e., [{Ru(1)2II,III}+– (TCNQMe2)1.5−–{Ru(2)2II,II/II,III}0.5+] at ca. 100 K. Consequently, the solvation/desolvation treatment yielded a reversible charge variation between the 1e-I and 1.5e-I states, respectively. A notable change via a desolvation/solvation process can also be seen in the orientation of 2,3,5Cl3Ph groups of [Ru2] moieties (Figure 2c). Compound 1 had a fixed, ordered form in the 2,3,5-Cl3Ph groups of [Ru(1)2] and [Ru(2)2], but via desolvation/solvation cycles to be 1-dry, 2, and finally 2-dry stepwise, they underwent flipping, sliding, or both to form disordered positions. This disorder was probably due to the generation of void space required for the moving and thermal activation of molecules upon heating during the process. A few disordered positions remained even in the resolvated form 2, and 2-dry had a much more complicated disordered form than 1-dry. Notably, although there were some local disordered forms, the PXRD patterns (Figure S6) were basically identical in the respective groups. The structural modifications from the solvated phase (1 and 2) to the desolvated phase (1-dry and 2-dry) also allowed a structural deformation of the TCNQMe2 11 ACS Paragon Plus Environment


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moiety as a rotational deformation, which is closely associated with the charge of TCNQMe2 (vide infra). The rotational deformation is defined by the dihedral angle f between the least-square planes made respectively from the C–C–(CºN)2 group and the central C6 ring in the TCNQMe2 moiety (see figure in Table S4). The values of f were ca. 10˚ in the solvated phases, which however increased to ca. 20˚ in the desolvated phases (Table S4). In general, the f value should be close to zero in the neutral TCNQMe2, because of its quinonoid form. Meanwhile, f may increase in the anionic TCNQMe2 forms because a character of benzenoid form could be enhanced; indeed, the TCNQMe22− form has a benzenoid form, which possibly allows a free rotation on the dihedral angle f (see figure in Table S4). Consequently, thermal activation of subunits with significant void space for the structural deformation in the desolvation process could allow the further electron transfer in the desolvation process. It should also be emphasized that this structural modification given in the desolvation process contributed to a Madelung stabilization for the desolvated form, since the solvated form with the 1e-I phase was indeed located at the vicinity of a boundary between the 1e-I and 2e-I phases.52 Electronic state of the desolvated compound 1-dry. The charge-disproportionation 1.5e-I state found in the desolvated compounds comprised two local compositions of [{Ru(2)2II,II}– (TCNQMe2)•−–{Ru(1)2II,III}+] (A; 1e-I) and [{Ru(2)2II,III}+–(TCNQMe2)2−–{Ru(1)2II,III}+] (B; 2e-I) in A:B = 1:1, which could take three phases in bulk: (i) a delocalized, charge-distributed state, (ii) a randomly ordered charge state as a steady state (Figure S7a), and (iii) a charge-ordered state with a superlattice as a steady state (Figure S7b). To verify this point, synchrotron radiation X-ray diffraction (SR-XRD) measurements were carried out at BL02B1/SPring-8 with the SR X-ray energy of 17.689 keV (λ = 0.7009 Å) from 20 K to 300 K. Even at 20 K, any superlattice was not observed in 1-dry. Hence, we conclude that 1-dry was in a charge-disproportionation “disordered” state (i) or (ii) at low temperatures (Figure 1c). In the variable-temperature single-crystal X-ray crystallographic measurements (Table S5), we 12 ACS Paragon Plus Environment

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found that the charge state of 1-dry gradually changed from 1.5e-I to near 2e-I upon increasing the temperature from 20 K to 300 K (Figure S8 and Table S6, S7). These data indicate that the charge distribution between [Ru(2)2II,II] unit and TCNQMe2 in 1-dry thermally fluctuated, although the ground state was 1.5e-I. This observation was also confirmed by IR absorption spectroscopy (Figure S9). In the temperature range of 10−130 K, n(C≡N) were observed at 2102, 2169, and 2199 cm−1, which corresponded to the 1.5e-I of TCNQMe2, while upon increasing temperature to 300 K, then(C≡N) bands shifted to lower energies (2096, 2161, and 2191 cm−1), which realized to be near the 2e-I state. The shift of the n(C≡N) stretching mode of TCNQ was also in IR spectra at room temperature for 1, 2, 1-dry, and 2-dry (Figure S10), in which characteristic bands in 1 and 2 were observed at lower frequencies than in 1-dry and 2-dry. The n(C≡N) stretching mode of TCNQ moiety characteristically reflected the charge state of TCNQ as well as its coordination mode. Thus, upon desolvation, the electron transfer from [Ru2] to TCNQMe2 was promoted; 1-dry had a quasi-2e-I state at room temperature, but experienced a charge fluctuation upon decreasing temperature to be the 1.5e-I state at low temperatures. Magnetic properties of 1. The temperature dependence of field-cooled magnetization (FCM) was measured in the temperature range 1.8 – 300 K under a 1 kOe dc field (Figure 4a). The χT value of 2.66 cm3·K·mol−1 at 300 K was much higher than the spin-only value of 2.00 cm3·K·mol−1 for two isolated S = 1 [Ru2II,II] spins; g = 2.00 assuming the N state without intra-unit electron transfer, which agreed with the 1e-I state evaluated from the structural analyses. Upon decreasing the temperature, the χT value gradually then abruptly increased at around 100 K to reach the maximum of 551.3 cm3 K mol−1 at 80 K, followed by a decrease to 16.5 cm3 K mol−1 at 1.8 K. The FCM also followed the abrupt increase at around 100 K without a subsequent decrease at lower temperatures. This FCM behavior was independent of applied fields (Figure S11), indicating the onset of ferrimagnetic ordering at around 100 K. The temperature dependence of alternating-current (ac) magnetic susceptibility (χ′: in phase, χ′′: 13 ACS Paragon Plus Environment


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out of phase) was measured in the frequency range from 1 Hz to 1.5 kHz under zero dc field with a 3 Oe oscillating field (inset of Figure 4a). The χ′–T plots showed a distinct peak at 97 K, accompanying a peak of χ′′; the initial rise of χ′′ was observed at 101 K (= Tc) upon cooling. The ac peak can be seen in a wide temperature range, although only a sharp peak was observed. This is probably due to slow moving of domain walls in the present compound. This type of compounds has strong anisotropy effects resulted from intrinsic anisotropy on [Ru2II,II] and [Ru2II,III]+ species and structural anisotropy, which probably cause a “tailing feature” in ac susceptibility data, although only one sharp peak was observed.32,37 A small peak found at ∼30 K in the χ′− and χ′′−T plots could have been due to defects caused by a minimal loss of the crystallization solvents (vide infra). The magnetic field dependence of the magnetization (M−H curve) was measured at several temperatures for fields ranging from −7 to +7 T (Figure 4b). The value of coercive field (Hc) was 11.6 kOe at 1.8 K, which was the same order as Hc = 16 kOe of the previous ferrimagnetic compound [{Ru2(o-ClPhCO2)4}2TCNQ(MeO)2].9 The Hc value gradually decreased with increasing temperature and finally disappeared at around 100 K (inset of Figure 4b), corresponding to Tc. The remnant magnetization (RM) decreased at temperatures above 80 K (inset of Figure 4b).


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Figure 4. Magnetic properties of 1: (a) Temperature dependence of χ and χT measured at 1 kOe; inset: temperature dependence of the ac susceptibilities χ′ (in-phase) and χ′′ (out-of-phase) at zero dc field and in a 3 Oe ac oscillating field. (b) Field dependence of the magnetization measured at various temperatures between 1.8 and 110 K; inset: temperature dependence of coercive field (Hc) and the remnant magnetization (RM).

In situ magnetic measurements for the desolvation process from 1 to 1-dry. The magnetic variation from 1 to 1-dry via a drying process was monitored by in situ magnetic measurements in a Superconducting Quantum Interference Device (SQUID) (MPMS-7XL, Quantum Design, USA) system. A polycrystalline sample of 1 was put into a small gelatin capsule with cotton wool for fixing the sample (without a fixing agent). This was measured for ac magnetization (1 Hz; Figure 5a) from 120 K to 1.8 K and M–H curves at 1.8 K (Figure 5b) via a drying process step-by-step involving 15 ACS Paragon Plus Environment


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evacuating at 300 K or 353 K for several hours. The sample of 1 was kept at 300 K for 54 h and then at 353 K for 12 h. In the process, the original peak of χ′ and χ′′ for 1 at around 100 K gradually decreased, and it disappeared upon heating at 353 K (a broad peak in the temperature range of 60– 100 K could be due to slow moving of domain walls; vide supra). A new sharp peak appeared in the temperature range 25−35 K, indicating the formation of a new magnetic phase in 1-dry. No other peaks were observed, indicating that there is no intermediate species in the desorption process. This observation is consistent with the PXRD variation in Figure 3. The Tc for 1-dry was determined as Tc = 34 K (vide infra). The trace of the χ′ (χ′′) peak at ~30 K observed in 1 (inset of Figure 4a) could thus have arisen from a minimal loss of the crystallization solvents. In in situ M−H curves at 1.8 K (Figure 5b), the Hc was gradually increased during the first 26 hours with evacuation at 300 K, but then decreased. The saturated magnetization (Ms) at 7 T was continuously decreased during the desolvation (inset of Figure 5b); eventually, 1-dry had slight M−H hysteresis with Hc = 6.7 kOe and Ms = 1.74 NµB.

Details of the magnetic properties for 1-dry were finally confirmed in an isolated sample. The χT value at 300 K was 3.30 cm3·K·mol−1, larger than that for 1, probably due to the increase in paramagnetic [Ru2II,III]+. Upon decreasing the temperature, the χT value was gradually and then abruptly increased at around 30 K to reach a maximum of 95.9 cm3 K mol−1 at 27 K, followed by a decrease to 9.4 cm3 K mol−1 at 1.8 K (Figure 6a). The FCM curve shows an abrupt increase at ca. 34 K without a subsequent decrease regardless of applying fields, indicating the onset of ferrimagnetic ordering at around 34 K (Figure S12). The χ′–T plots showed a sharp peak at 30 K, accompanying a peak of χ′′ with an initial rise at 34 K (= Tc) upon cooling (inset of Figure 6a). The M−H curve was measured at several temperatures for fields ranging from −7 to +7 T (Figure 6b). The Hc value 16 ACS Paragon Plus Environment

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gradually decreased with increasing temperature and finally disappeared at around 34 K (inset of Figure 6b), corresponding to Tc. The remnant magnetization (RM) decreased at temperatures above 20 K (inset of Figure 6b).

Figure 5. Magnetic variation during an in situ desolvation process (1 ® 1-dry) in a SQUID apparatus: (a) Temperature dependence of 1 Hz ac susceptibilities χ′ (top) and χ′′ (bottom) at zero dc field and 3 Oe ac oscillating field. (b) Field dependence of the magnetization at 1.8 K; inset: variation of coercive field (Hc) and saturated magnetization (Ms) at 7 T during the desolvation.



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Figure 6. Magnetic properties of 1-dry: (a) Temperature dependence of χ and χT measured at 1 kOe; inset: temperature dependence of the ac susceptibilities χ′ (in-phase) and χ′′ (out-of-phase) at zero dc field and in a 3 Oe ac oscillating field. (b) Field dependence of the magnetization measured at various temperatures between 1.8 and 40 K; inset: temperature dependence of coercive field (Hc) and the remnant magnetization (RM).

The increase of paramagnetic [Ru2II,III]+ (S = 3/2) species in 1-dry was also confirmed by the increase of the Curie constant after the desolvation process; 2.72 cm3 K mol−1 for 1-dry while 1.69 cm3 K mol−1 for 1 (Table S8 and Figure S13). Meanwhile, the Weiss temperature (q) decreased from 118 K for 1 to 48 K for 1-dry, indicating that the averaged exchange coupling constant was suppressed due to the production of TCNQMe22– after the desolvation (Table S8). This result is consistent with the electronic state change in the desolvation process. Namely, a perfect magnetic conjugation via µ4-TCNQMe2•− in the 1e-I state for 1 formed a high Tc magnet (Figure 1), whereas a 18 ACS Paragon Plus Environment

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partial magnetic conjugation in the 1.5e-I state for 1-dry caused by breaking a part of pathways due to the presence of TCNQMe22– formed to a low Tc magnet. Since no superstructure was present in 1dry even down to 20 K, the observed Tc = 34 K was higher than Tc = 27 K in an analogous compound with a superlattice in the 1.5e-I state reported previously.36

Reversible change in Tc by a solvation/desolvation via 2 and 2-dry. Crystals of 1-dry gradually adsorbed four molar amounts of DCM when they were immersed in a DCM solvent for several hours at room temperature; DCM-soaked crystal samples were took out at intervals, where a completely soaked sample corresponds to 2, and were magnetically characterized (Figure 7). The Tc gradually shifted from Tc (1-dry) = 34 K to Tc (2) = 97 K after immersion for 144 hours (Figure 7a and 7c). Such a continuous shift of Tc contrasts with the change in the desolvation from 1 to 1-dry (Figure 5). The solvation process is thus described by a summation of various kinds of magnetic domains that change every moment, not any variation of individual modes of 1-dry and 2 (like in the desolvation from 1 to 1-dry), i.e., like spin-glass domains. M−H curves also changed upon resorption; the values of Hc and Ms gradually increased and almost returned to those for 1 with Hc = 16.0 kOe and Ms =2.3 NµB at 7 T (Figure 7b and 7c). These results agree well with the structural variation (Table S3) and spectroscopic results (Figure S10), indicating the rebirth of the 1e-I electronic state in 2. Notably, the Tc value of 97 K in 2 is slightly different from 101 K in 1 (Figure S14), which may have been due to small defects and/or modulations; a small broad peak in χ′ and χ″−T plots in the temperature range of 20−60 K (Figure S14b) may be relevant to these domains. In addition, 2 had much more complicated structural disorders than in 1, which could cause structural fluctuations characteristic of 2. Nonetheless, it is appropriate to consider that the magnetic properties between 1 and 2 were essentially the same. 19 ACS Paragon Plus Environment


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Figure 7. Magnetic variation during an ex-situ DCM resorption process (1-dry ® 2): (a) Temperature dependence of 1 Hz ac susceptibilities χ′ (top) and χ′′ (bottom) (zero dc field and 3 Oe ac oscillating field). (b) Field dependence of the magnetization at 1.8 K. (c) Variation of coercive field (Hc) and the remnant magnetization (RM) (left) and magnetic transition temperature (Tc) (right) during resorption of DCM. 20 ACS Paragon Plus Environment

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Figure 8. Magnetic variations in desolvation/solvation cycles: (a) Reversible Tc switching: temperature dependence of molar magnetic susceptibility for solvated phase (blue curves) and desolvated phase (red curves) in an applied dc field of 100 Oe; inset shows the Tc values for each step. (b) Magnetic field dependence of the magnetization at 1.8 K for solvated phase (blue curves) and desolvated phase (red curves) in the cycling procedure.

To gain insight into the magnetic information upon transformation of electronic states between 1e-I and 1.5e-I (Table S9), we continuously measured magnetic data with the same sample (ex situ sampling). The desolvated sample (1-dry or 2-dry) was prepared by heating at 353 K for 12 h in vacuum, whereas the solvated sample was prepared by immersing the capsulated sample in DCM at 21 ACS Paragon Plus Environment


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room temperature for several days. Upon repeating these solvation/desolvation cycles, the FCMs and RMs and M–H curves for solvated and desolvated samples were measured every time (Figure 8a). The respective solvated phases (1e-I) repeatedly revealed a ferrimagnetic order at a higher temperature, in the range of 90–100 K. Meanwhile, the desolvated phases (1.5e-I) repeatedly underwent a ferrimagnetic order at approximately 30 K (= Tc for 2-dry) (Figure S15). Notably, the same Tc with the identical magnitude of magnetization was measured from the second cycle, i.e., the magnetic change between 2 and 2-dry was reversible, although it was slightly different from 1 and 1-dry, respectively (Figure 8a). Compound 2-dry had a more complicated structural disorder than 1dry (Figure 2c), so the magnetic properties of 2-dry were somewhat affected by the structural fluctuations as observed in the relationship between 1 and 2. Besides, the RM, Hc, and Ms of the solvated phases were bigger than those of the desolvated phases (Figure 8b). Thus, the Tc, as well as the hysteretic magnetization, was reversibly switchable between 100 K and 30 K via the DCM-guest desorption/adsorption cycles. These were due to the changes in intrinsic electronic states of the magnetic framework between the 1e-I and 1.5e-I states, respectively.

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CONCLUSION

The “guest-induced electronic state modulation” is one of the most promising triggers to change magnetic and other properties of porous coordination polymers. Nevertheless, to the best of our knowledge, there have been no reports of magnetic porous coordination polymers (i.e., porous magnetic materials) where the magnetic correlation was tuned by the electronic state modulation variable depending on guest adsorption/desorption. We have demonstrated the first example of such 22 ACS Paragon Plus Environment

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materials: a magnetic sponge for which Tc changed between 100 K and 30 K (DTc = 70 K) depending on solvation/desolvation, respectively. In the present systems, both solvated (1 and 2) and desolvated species (1-dry and 2-dry) have an almost isoreticular layer structure, though, a small change in the structure via a desolvation/solvation leads to a modulation of intra-layer ET between D and A units that directly links to a magnetic change. Consequently, the solvated compounds 1 and 2 have a chargedistributed 1e-I state, whereas the solvent-free compounds 1-dry and 2-dry have a chargedisproportionate, disordered state that could experience a charge fluctuation from a 2e-I state at high temperatures to a 1.5e-I state at low temperatures. Even in the same state group, i.e., 1 and 2 or 1-dry and 2-dry, the structures were slightly modulated when the desolvation/desolvation process repeated. For example, this was seen in disordered positions of 2,3,5-Cl3Ph groups of [Ru2] units, which probably produce structural fluctuations that allow a small shift of Tc (DTc < 10 K) in the same group. Nevertheless, after one desolvation/solvation cycle from 1 to 2 via 1-dry, the following state change between 2 and 2-dry occurred reversibly.

Electronic-state-sensitive, porous materials are a new class of functional porous materials. In this work, we showed only one example of magnetic sponges, for which the magnetic properties were tuned by solvation/desolvation. Meanwhile, the electronic state modulation in a material could be relevant to the conductivity of the material. Furthermore, these functional switches could be tuned using a simple, ubiquitous gas such as O2, N2, and CO2. Thus, the guest-induced electronic state modulation is a key methodology to control physical properties of porous materials — not only magnetic properties, but also electrical and optical properties.



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EXPERIMENT SECTION

General Procedures and Materials. The starting material {[Ru2II,II(O2CPh-2,3,5-Cl3)4](THF)2} was synthesized by reported means, 53 as was TCNQMe2.54 Solvents were distilled in a N2 atmosphere using common drying agents. All synthetic procedures were carried out in an inert atmosphere. Compound 1 contained a crystallization solvent, some of which was slowly lost at room temperature, so the sample for elemental analysis reflected partial loss; thus, there was a slightly different formula compared to those determined by single-crystal X-ray crystallography. Samples aged for a few hours after removal from mother liquids were analyzed for elemental data, but fresh samples taken immediately from the mother liquids were used for magnetic measurements. Synthesis of [{Ru2(O2CPh-2,3,5-Cl3)4}2 TCNQMe2]·4DCM (1). A DCM solution (20 mL) of TCNQMe2 (4.6 mg, 0.02 mmol) was separated into 2 mL portions and placed in narrow-diameter glass tubes (f = 8 mm) (bottom layer). Then a solution (2 ml) of {[Ru2II,II(O2CPh-2,3,5-Cl3)4](THF)2} (49.8 mg, 0.04 mmol) in p-xylene (20 ml) was carefully placed on the DCM layers of each tubes (top layer). The glass tubes were left undisturbed for one week or more to yield block-shaped, dark-brown crystals of 1 (yield: 61%). Elemental analysis for 1 with four molars of DCM was somewhat inaccurate, because some of the DCM molecules were eliminated during sampling (e.g., elemental analysis

(%)

was

calculated

for

[{Ru2(O2CPh-2,3,5-Cl3)4}2

TCNQMe2]·1.5DCM,

C71.5H27Cl27N4O16Ru4: C = 33.55, H = 1.06, and N = 2.19. Measured values were found — C = 33.58, H = 1.22, N = 2.21 — but they were accurately evaluated for 1-dry (vide infra). IR (KBr): ν(C≡N), 2197, 2169, 2109 cm-1. Preparation of {Ru2(O2CPh-2,3,5-Cl3)4}2 TCNQMe2 (1-dry). Crystal samples of 1 were heated at 24 ACS Paragon Plus Environment

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353 K under vacuum for 12 h. Elemental analysis (%) were calculated for C70H24Cl24N4O16Ru4: C = 34.57, H = 1.00, N = 2.30. Measured values were found: C = 34.42, H = 1.25, N = 2.39. IR (KBr): ν(C≡N), 2188, 2089 cm−1. Physical measurements. Infrared (IR) spectra were measured on a KBr pellet using a Jasco FT-IR 620 spectrometer, and the temperature dependence of IR absorption spectra were measured using a Nujol mull in the temperature range 10−300 K by employing a Jasco IRT-5000 infrared microscope with a He cryostat. Thermogravimetric analyses (TGA) were recorded on a Shimadzu DTG-60H apparatus under a N2 atmosphere from 298 K to 673 K at a heating rate of 5 K min−1. Powder X-ray diffraction (PXRD) was measured with a Rigaku Ultima IV diffractometer

with Cu Kα radiation (λ

= 1.5418 Å) at room temperature. Magnetic susceptibility measurements were conducted with a SQUID magnetometer (MPMS-XL, Quantum Design, USA) in the temperature and dc field ranges of 1.8 to 300 K and −7 to 7 T, respectively. Ac measurements were performed at various frequencies ranging from 1 to 1488 Hz with an ac field amplitude of 3 Oe. Polycrystalline samples embedded in liquid paraffin were measured except for the in situ measurement of the sample under desolvation (1 ® 1-dry) and the cycling procedure among solvation/desolvation. Diamagnetic contributions were collected for the sample holder, Nujol, and for the sample using Pascal’s constants.55 Crystallography. Crystal data for 1, 1-dry, 2, and 2-dry were collected at 112, 112, 116, and 103 K, respectively, on a CCD diffractometer (Saturn724, Rigaku, Japan) with a multi-layered mirror for monochromated Mo-Kα radiation (λ = 0.71075 Å). A single crystal was mounted on a thin Kapton film using Nujol and cooled in a N2 gas stream. The structures were solved using direct methods: SIR201156 for 1, 1-dry and 2, and SIR201457 for 2-dry, and expanded using Fourier techniques.



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Full-matrix, least-squares refinements on F2 were based on observed reflections and variable parameters, and they converged with unweighted and weighted agreement factors of R1 = Σ ||Fo| − |Fc|| / Σ |Fo| (I > 2.00σ(I)), and wR2 = [Σ(w(Fo2 – Fc2)2 )/ Σw(Fo2)2]1/2 (all data). The non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined using a riding model. These data have been deposited as CIFs at the Cambridge Data Center as supplementary publication numbers CCDC-1827015 for 1, 1827016 for 1-dry, 1827018 for 2, and 1827017 for 2-dry. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: (+44) 1223-336-033; email: ([email protected]). Structural diagrams were prepared using VESTA software.58 Synchrotron radiation Single-crystal X-ray crystallography. Synchrotron radiation (SR) X-ray diffraction measurements of 1-dry were carried out at BL02B1/SPring-8 in Japan. SR X-rays from a bending magnet were monochromatized to an energy of 17.689 keV (λ = 0.7009 Å) by a monochromator with a double-crystal of Si(311). The beam was focused by a sagittal focusing monochromator and a bent mirror made of Si crystal coated with Pt. The focused beam size at the sample position was about 0.2 × 0.2 mm2. The number of photons was about 1010 photon/sec at the sample position. A Rigaku Mercury2 CCD detector with a camera length of 60.04 mm was adopted. The temperature dependent measurements were performed at 20, 50, 100, 200, and 300 K with a cryogenic He flow system (XR-HR10K-S, Japan Thermal Engineering Co., Japan). The structures were solved by direct methods (SHELXT) and expanded using Fourier techniques. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included and constrained to ideal positions and thermal displacement parameters using the AFIX command on SHELXL-2013. The unweighted and weighted agreement factors of R1 = R = S||Fo| – |Fc||/S|Fo| (Fo>4σ(Fo)), and wR2 = [Sw(Fo2 – 26 ACS Paragon Plus Environment

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Fc2)2/Sw(Fo2)2]1/2 were used. Crystallographic data for the measurements at 20, 50, 100, 200, and 300 K were summarized in Table S5 and selected bond lengths are summarized in Table S6 and S7. Dichloromethane adsorption measurement. The sorption isotherm measurement for DCM at 298 K was carried out by using an automatic volumetric adsorption apparatus (BELSORP max; BEL Inc.). A known weight (ca. 20 mg) of the dried sample was placed into the sample glass tube, and prior to measurements it was evacuated for 12 h at 353 K. The change in the pressure was monitored and the degree of adsorption was determined by the decrease in the pressure at the equilibrium state.

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ASSOCIATED CONTENT

Supporting Information Available Our CIF files give X-ray crystallographic data for 1, 1-dry, 2, and 2-dry. Our structural figures are for 1-dry, 2, and 2-dry. Our tables give calculated bond lengths for 1, 1-dry, 2, and 2-dry, magnetic properties for 1, 1-dry, and 2, and dichloromethane adsorption for 1-dry. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Notes 27 ACS Paragon Plus Environment


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The authors declare no competing financial interests.

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ACKNOWLEDGEMENTS

This study was supported by a Grant-in-Aid for Scientific Research (Grant No. 16H02269) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan; a Grant-in-Aid for Scientific Research on Innovative Areas (“p-System Figuration” Area 2601, No. JP17H05137) from the Japan Society for the Promotion of Science; and the E-IMR project. J. Z. is thankful for the JSPS Research Fellowship for Young Scientists (No. 17J02497).


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Magnetic Sponge Behavior via Electronic State Modulations Jun Zhang, Wataru Kosaka, Kunihisa Sugimoto, and Hitoshi Miyasaka*


Magnetic Sponge Behavior via Electronic State Modulations - Journal

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