J. Phys. Chem. C 2009, 113, 15751–15755
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Extremely Gradual Spin-Crossover Phenomenon in a Cyano-Bridged Fe-Mo Bimetallic Assembly Wataru Kosaka,†,‡ Hiroko Tokoro,†,§ Tomoyuki Matsuda,† Kazuhito Hashimoto,‡ and Shin-ichi Ohkoshi*,† Department of Chemistry, School of Science, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Toyko 113-0033, Japan, Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and PRESTO, JST, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan ReceiVed: March 26, 2009; ReVised Manuscript ReceiVed: July 21, 2009
We report a unique type of spin-crossover phenomenon in a three-dimensional (3-D) Fe-Mo network with a cubic structure, Fe2[Mo(CN)8] · (3-pyCH2OH)8 · 3H2O (3-py ) 3-pyridyl). This compound exhibits an extremely gradual FeII spin-crossover over a wide temperature range, which is more gradual than the crossover according to the Boltzmann distribution. The electronic states at 320 and 50 K are represented as (FeIIhs)2[MoIV(CN)8] · (3-pyCH2OH)8 · 3H2O and (FeIIhs)0.48(FeIIls)1.52[MoIV(CN)8] · (3-pyCH2OH)8 · 3H2O, respectively, where hs and ls denote high spin (S ) 2) and low spin (S ) 0), respectively. The model calculation based on Slichter-Drickamer’s model suggests this extremely gradual spin-crossover can be explained by the contribution of 3-D alternating alignment of hs and ls sites, i.e., -hs-ls-hs-ls-. This system is a strongly correlated system of spin-crossover sites because the spin-crossover FeII sites are directly linked by -NC-Mo-CN- with a high symmetry (FeII sites have one type of symmetry). The elastic interaction due to the volume change in a spin-crossover site isotropically propagates in the whole crystal. Since the CN bridges cannot be disconnected during spin-crossover, a 3-D alternating order of hs and ls sites is considered to be preferable. 1. Introduction
2. Experimental Section
The spin-crossover phenomenon has been aggressively studied in the field of physical, inorganic, and solid state chemistries.1-6 This phenomenon occurs in metal complexes where the energy difference between the high-spin (hs) and lowspin (ls) states is close to the thermal energy. Generally, this type of spin-crossover is classified as a gradual change (according to the Boltzmann distribution), an abrupt transition, and a phase transition with thermal hysteresis loop (Figure 1). The classification depends on the cooperativity between the spincrossover site, which is understood by electron-phonon coupling, Jahn-Teller distortions, elastic interactions, etc.7-12 In cyano-bridged metal assemblies, which have received attention from the viewpoint of ferromagnetism,13-22 several cyanobridged FeII spin-crossover complexes have been reported recently.23-30 In the present work, we prepared an FeII spincrossover compound with a three-dimensional (3-D) cyanobridged Fe-Mo network, Fe2[Mo(CN)8] · (3-pyCH2OH)8 · 3H2O (3-py ) 3-pyridyl). This compound showed an extremely gradual FeII spin-crossover over a wide temperature range. Thermodynamic analysis suggests that this spin-crossover is attributed to the 3-D alternating alignment of hs and ls. Herein, we report the temperature changes in the magnetic susceptibilities, crystal structure, and optical absorption of the FeII spincrossover in Fe2[Mo(CN)8] · (3-pyCH2OH)8 · 3H2O as well as discuss the mechanism of the extremely gradual spin-crossover.
The target compound was prepared by reacting a mixed aqueous solution of FeCl2 · 4H2O (0.1 mol dm-3) and 3-pyCH2OH (0.5 mol dm-3) with an aqueous solution of K4[Mo(CN)8] · 2H2O (0.05 mol dm-3) under an argon atmosphere. The mixed solution was stirred for 5 min at room temperature, and the resulting precipitate was filtered to yield a powder (yield: 84%). Elemental analyses of Fe and Mo for the prepared materials were measured by HP4500 inductively coupled plasma mass spectroscopy, while those of C, H, and N were determined by standard microanalytical methods. The morphologies of the compounds were measured by a KEYENSE YE-9800 scanning electron microscope (SEM) with a 0.5 kV accelerating voltage. Infrared spectra were recorded on a Shimadzu FT-IR 8200PC spectrometer with samples as KBr pellets in the 4000-400 cm-1 region. X-ray powder diffraction (XRD) measurements were conducted on a Rigaku RINT2100 with Cu KR radiation (λ ) 1.5406 Å) within the range 10° e 2θ e 60°. Rietveld analyses were performed by the RIETAN-FP program.31 The magnetic measurements were performed on polycrystalline samples using a Quantum Design MPMS superconducting quantum interference device (SQUID) magnetometer. Pascal’s constants were used to determine the diamagnetic contribution. The UV-vis reflectance spectra were measured by a Shimadzu UV-3100 spectrometer.
* To whom correspondence should be addressed. E-mail: ohkoshi@ chem.s.u-tokyo.ac.jp. † School of Science, The University of Tokyo. ‡ School of Engineering, The University of Tokyo. § JST.
3. Results The obtained sample was a yellow powder, and elemental analyses indicated the composition was Fe2[Mo(CN)8] · (3pyCH2OH)8 · 3H2O. Calcd: Fe, 8.3; Mo, 7.1; C, 50.1; H, 4.7;
10.1021/jp902735v CCC: $40.75 2009 American Chemical Society Published on Web 08/11/2009
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Figure 1. Temperature dependence of spin-crossover (high spin fraction vs temperature): (a) gradual change (Boltzmann distribution); (b) abrupt transition; (c) phase transition with a thermal hysteresis loop.
Figure 2. SEM image of the obtained sample.
N, 16.7. Found: Fe, 8.3; Mo, 7.1; C, 50.3; H, 4.7; N, 16.9. The prepared powder sample consisted of microcrystals measuring 3 ( 1 µm (SEM image, Figure 2). In the IR spectrum, one CN stretching peak was observed at 2134 cm-1,32 whereas peaks due to 3-pyCH2OH were observed at 646, 708, 800, 1035, 1431, 1483, and 1601 cm-1. X-ray single crystal structural analysis showed that the crystallinity was poor, but the analysis did provide rough structural information. Using this structural data, Rietveld analysis of the observed XRD pattern was carried out (Figure 3a), and indicated that this compound had a cubic crystal structure in the Ia3jd space group (a ) 34.6716(5) Å, Z ) 24). Tables 1, 2, and 3 list the crystallographic data, fractional atomic coordinates used in the Rietveld refinement, and selected bond lengths and angles, respectively. Each antisymmetric unit consisted of one Fe, one Mo, two cyano groups, two 3-pyCH2OH molecules, and two oxygen atoms from the lattice water (Figure 3b). Fe, Mo, and O3 were located on a 2-fold axis, 4-fold screw axis, and 3-fold screw axis, respectively. Disordered oxygen atoms were present in the 3-pyCH2OH molecule (O2a and O2b). The coordination geometries of the Fe and Mo sites were pseudo-octahedron (D4h) and dodecahedron (D2d), respectively (Figure 3c). The two axial positions of Fe were occupied by the cyanide nitrogen atoms of [Mo(CN)8], whereas the equatorial positions were occupied by the four nitrogen atoms of 3-pyCH2OH. The four equatorial CN groups
Figure 3. (a) XRD pattern at 293 K and Rietveld analysis. The red dots, black line, and blue line are the observed plots, calculated pattern, and their difference, respectively. The green bars represent the calculated positions of the Bragg reflections. (b) Antisymmetric unit with the atomic numbering scheme. (c) Coordination environments around Fe and Mo. (d) Cyano-bridged Fe-Mo 3-D framework. 3-pyCH2OH, terminal CN ligands, and water molecules are omitted for clarity.
TABLE 1: Crystallographic Data by Rietveld Analysis of the Powder XRD Pattern empirical formula M crystal system space group a/Å V/Å3 dcalcd/g cm-3 T/K Z Rwp/Rp
C56Fe2H62MoN16O11 1342.84 cubic Ia3jd 34.6716(5) 41679.4(25) 1.25 293 24 0.0913/ 0.0677
of [Mo(CN)8] were bridged to four Fe, but the four axial CN groups were free. Fe and Mo were alternatively bridged by cyano groups, which formed a 3-D bimetallic framework (Figure 3d).33 The XRD peaks shifted continuously as the temperature decreased, but the cubic structure was maintained (Figure 4a). The lattice constant decreased from 34.67 Å (293 K) to 34.22 Å (20 K). Figure 5 shows the product of the molar magnetic susceptibility (χM) and temperature (T) vs T plots at a cooling (or warming) rate of 1 K min-1 in an external magnetic field of 5000 Oe. At 320 K, the χMT value was 7.49 K cm3 mol-1. As the temperature decreased, the χMT value slowly decreased over a wide temperature range, and the χMT value at 50 K was 1.81 K cm3 mol-1. Below 10 K, the χMT value sharply decreased and reached 0.81 K cm3 mol-1 at 2 K. In the warming process, the
Cyano-Bridged Fe-Mo Bimetallic Assembly
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TABLE 2: Fractional Atomic Coordinates Used in the Rietveld Refinement atom
site
x
y
z
Fe Mo C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 N1 N2 N3 N4 O1 O2a O2b O3 O4
48g 24d 96h 96h 96h 96h 96h 96h 96h 96h 96h 96h 96h 96h 96h 96h 96h 96h 96h 96h 96h 96h 96h 16a 96h
0.125 0.375 0.046 0.029 0.019 0.050 0.054 0.028 0.057 0.078 0.143 0.157 0.172 0.150 0.076 0.145 0.072 0.045 0.059 0.151 0.078 0.016 0.113 0.000 0.041
0.682 0.000 0.222 0.234 0.124 0.106 0.068 0.044 0.310 0.119 0.202 0.203 0.301 0.331 0.117 0.227 0.205 0.227 0.349 0.173 0.153 0.024 0.231 0.000 0.116
0.568 0.250 0.389 0.325 0.296 0.293 0.291 0.306 0.256 0.297 0.342 0.307 0.180 0.190 0.396 0.284 0.399 0.302 0.260 0.373 0.264 0.087 0.252 0.000 0.122
g
Figure 5. χMT-T plots measured while cooling (blue) and warming (red) in an external field of 5000 Oe.
0.562 0.438
TABLE 3: Selected Bond Lengths (Å) and Angles (deg) Fe-N1 Fe-N4 Mo-C2 C2-N2 Fe-N1-C1 N1-Fe-N4
2.32 2.27 2.07 1.00 165.8 93.7
Fe-N3 Mo-C1 C1-N1
2.24 1.95 1.10
N1-Fe-N3 N3-Fe-N4
85.4 95.1
Figure 6. Optical reflectance spectra in the visible and near-infrared region measured at 293 K (dashed line) and 77 K (solid line).
χMT value corresponded to that in the cooling process at each temperature; i.e., thermal hysteresis was not observed with this change. The optical reflectance spectra in the visible and near-infrared region were measured at 293 and 77 K (Figure 6). At 293 K, strong absorption was observed below 480 nm, and a small absorption peak was observed at 850 nm (peak A). As the
Figure 7. (a) Schematic illustration of the strain induced by spincrossover. White and gray circles represent FeIIhs and FeIIls, respectively. For the hs-ls alternating arrangement, strain arising in the entire structure is small (left), but when the domains are formed, large strain is induced (right) because all FeII sites are bridged by a cyano group in 3-D. (b) Calculated temperature dependence of spin-crossover in the case of hs-ls alternating alignment (γ < 0) based on Slichter-Drickamer’s model.
temperature decreased, the intensity of peak A decreased, and a new peak (peak B) appeared at 550 nm. 4. Discussions
Figure 4. (a) Temperature dependence of XRD patterns for 293 K (red), 250 K (orange), 200 K (green), 150 K (cyan), and 100 K (blue). (b) Temperature dependence of the lattice constant (a).
In the optical reflectance spectra, the strong absorption below 480 nm was assigned to the d-d transition of MoIV.34,35 Peak A at 293 K was assigned to the d-d transition of FeIIhs (5T2 f 5 E), whereas peak B at 77 K was assigned to the d-d transition of FeIIls (1A1 f 1T1), since the wavelength of these peaks correspond to that of the FeII d-d transition.3 Hence, the observed changes in the χMT-T plots are due to the spincrossover at the FeII sites. The observed χMT value (7.49 K cm3
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Kosaka et al. during spin-crossover. In fact, free CN bonds are not observed in the variable temperature IR spectra (Figure S2 in the Supporting Information). Under these conditions, a 3-D alternating order of hs and ls sites is likely, i.e., -hs-ls-hs-ls(Figure 7a). If this hypothesis is correct, then half of FeIIhs remains, even at low temperature. The model calculation along this scenario was performed using Slichter-Drickamer’s model.37 The Gibbs free energy (G) in this model can be described as G ) R∆H + γR(1 - R) + T{R[R ln R + (1-R) ln(1-R)] - R∆S}, where R is the fraction of hs, ∆H is the transition enthalpy, ∆S is the transition entropy, R is the gas constant, and γ is the interaction parameter. In this analysis, γ depends on temperature, γ ()γa + γbT).38 The observed χMT-T plots are well reproduced when the parameters of ∆H, ∆S, γa, and γb are 8.2 × 103 J mol-1, 46 J K-1 mol-1, -12.1 × 103 J mol-1, and 55 J K-1 mol-1, respectively (Figure 8a).39 In this simulation, γ is negative at T < 219 K. γ < 0 in this model corresponds to a hs-ls alternating alignment (Figure 7b).40 This is the first system to display a negative γ spincrossover. At T ) 219 K, γ is zero, but above 219 K, γ becomes positive. Figure 8b shows the calculated temperature dependence of G vs the hs fraction plots and schematically illustrates the distribution of hs and ls sites at each temperature. 5. Conclusions
Figure 8. (a) Observed (circles) and calculated (line) temperature dependence of the hs fraction. The observed plot is derived from magnetic data. Calculation is carried out using Slichter-Drickamer’s model; the parameters ∆H, ∆S, γa, and γb are set to 8.2 × 103 J mol-1, 46 J K-1 mol-1, -12.1 × 103 J mol-1, and 55 J K-1 mol-1, respectively, and schematic illustrations of the distribution of hs and ls with the temperature change (black squares, ls sites; white squares, hs sites). The dashed line represents the Boltzmann distribution with the same ∆H and ∆S. (b) Temperature dependence of the calculated G vs R curves between 0 and 300 at 50 K intervals (circles indicate the thermal population).
mol-1) at 320 K almost corresponds to the χMT value of the spin-only values of FeIIhs (7.26-7.59 K cm3 mol-1), assuming that the g-value of FeIIhs is in the range 2.20-2.25.36 In contrast, the observed χMT value at 50 K (1.81 K cm3 mol-1) corresponds to the value where 76% of FeIIhs transits to FeIIls. Therefore, the formulas at 320 and 50 K can be described as (FeIIhs)2[MoIV(CN)8] · (3-pyCH2OH)8 · 3H2O and (FeIIhs)0.48(FeIIls)1.52[MoIV(CN)8] · (3-pyCH2OH)8 · 3H2O, respectively. The decrease in the χMT value in the very low temperature region (
