Coordination Nanoparticles Stabilized by Alkyl Surface Coordination

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J. Phys. Chem. C 2008, 112, 1953-1962

1953

Metal(II) Hexacyanochromate(III) MCr (M ) Co, Cu, Fe) Coordination Nanoparticles Stabilized by Alkyl Surface Coordination Ligand: Downsizing Effect on Their Crystal Structure and Magnetic Properties Masaya Arai,† Mikio Miyake,*,† and Mami Yamada*,‡,§ School of Materials Science, Japan AdVanced Institute of Science and Technology (JAIST), 1-1 Asahidai, Nomi-shi, Ishikawa 923-1292, Japan, Department of Applied Chemistry, Tokyo UniVersity of Agriculture and Technology (TUAT), 2-24-16 Nakamachi, Koganei, Tokyo 184-8588, Japan, and PRESTO/Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan ReceiVed: October 10, 2007; In Final Form: NoVember 11, 2007

We have reported the reversed micelle synthesis and isolation of metal(II) hexacyanochromate(III) MCr (M ) Co, Cu, Fe) coordination nanoparticles, which are protected by alkyl surface coordination ligand of 4-(dioctadecylamino)pyridine (OPy). The dependence of their physicochemical (shape, size, crystal structure, and electronic state) and magnetic properties on the metal constituent and the downsizing effect was investigated by comparing the MCr (M ) Co, Cu, Fe) nanoparticles with their bulk crystals. These compounds were precisely characterized by transmission electron microscopy observation, UV-vis, elementary analysis, Fourier transform infrared, X-ray diffraction, and direct current and alternating current (ac) magnetic studies. The shape and diameter of MCr (M ) Co, Cu, Fe) nanoparticles, which differ in metal constituents, are cubic particles of 9.06 ( 0.68 (M ) Co), subround particles of 15.71 ( 1.81 (M ) Cu), and cubic particles of 10.01 ( 0.84 (M ) Fe) nm. The MCr (M ) Co, Fe) nanoparticles show typical face-centered cubic structure as their bulk crystals; however, the CuCr nanoparticle shows unexpected crystal structure, different from the CuCr bulk crystal. In the case of FeCr compounds, linkage isomerism is observed not only in bulk crystal but also in nanoparticles. Additionally, the magnetic properties of FeCr nanoparticles are influenced by the additive amount of OPy. The MCr (M ) Co, Cu, Fe) nanoparticles show strong frequency dependence in both real and imaginary components of the ac magnetic susceptibility.

Introduction There has been much attention focused on molecule-based magnets exhibiting spontaneous magnetization in the past decades.1-3 The main synthetic strategy for this research area is usually to connect two or more paramagnetic building blocks to form multidimensional complexes with versatile crystal structures. The large variety of combinations among the building units has been extensively investigated to construct highly ordered complexes, which may exhibit high-ordering temperature due to the increased number of interacting neighbors. Among molecule-based magnets, Prussian blue analogues (PBAs) are one of the most widely explored metal coordination complexes. PBAs consist of octahedral [M(CN)6]n- (M ) transition metal) complexes bridged through metal ions to form a three-dimensional cubic structure. They exhibit several interesting phenomena, including magnetic properties such as high Curie temperature (Tc)4-9 and photoinduced magnetization,10-14 pressure tuning,15-18 linkage isomerism,17,19-23 and gas absorption,24-26 by tuning their constituents of transition metal ions in the structure. A lot of research has been devoted to investigate bulk crystals of PBA for a long time, however, studies focusing on making these crystals nanometric scale have been explored only for the * Authors to whom correspondence should be addressed. (M.M.) Tel: +81-761-51-1540. Fax: +81-761-51-1149. E-mail: [email protected]. (M.Y.) Tel and Fax: +81-42-388-7379. E-mail: [email protected]. † JAIST. ‡ TUAT. § PRESTO/JST.

past several years. The research on synthesis of PBA nanoparticles was pioneered independently by Das et al. and Mann et al., who both applied the reversed micelle technique using di2-ethylhexylsulfosuccinate sodium salt.27,28 After that, there have been reported several studies related to the preparation of PBA nanoparticles using reversed micelle technique,29-33 polymers and biopolymers,34-40 ion exchange membrane,41 and porous materials as matrixes.42,43 To prevent particle aggregation, a chemical stabilizer to protect the particle surface is necessary. This is a well-known technique for other nanomaterials, for example, metallic nanoparticles.44-49 As a stabilizer for PBA nanoparticles, a organic polymer such as poly(vinylpyrrolidone) is used.34-38 However, we have decided to introduce an alkyl molecular ligand with a functional group, which coordinates to transition metal ions on the surface of PBA nanoparticles. Until now, research on alkyl ligand-stabilized metal coordination nanoparticles has been rarely reported. The advantage of this method is that the stable coordination protection of alkyl ligand enables the removal of excessive stabilizer after the isolation process of the PBA nanoparticles. Practically, we have previously reported the synthesis and precise isolation of cobalt hexacyanoferrate/chromate (CoFe/Cr) nanoparticles protected by stearylamine.31 From our experimental data, the amino group of stearylamine is attached to the cobalt ions located on the surface of CoFe/Cr nanoparticle with almost 1:1 complexation. In this article, we have demonstrated the isolation and characterization of metal(II) hexacyanochromate(III) MCr (M ) Co, Cu, Fe) coordination nanoparticles protected by alkyl

10.1021/jp709889v CCC: $40.75 © 2008 American Chemical Society Published on Web 01/23/2008

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SCHEME 1: Synthesis of the MCr-OPyn (M ) Co, Cu, Fe; n ) 1-5) by Reversed Micelle Technique and Isolation by Addition of Surface Coordination Ligand Figure 1. Surface coordination ligand (OPy) applied in this research as a stabilizer.

surface coordination ligand, using reversed micelle technique with polyethylene glycol mono 4-nonylphenyl ether (NP-5, HO(CH2CH2O)nC6H4C9H19, n ) 5) (Scheme 1). As a surface coordination ligand, we employed 4-(dioctadecylamino)pyridine (OPy) with a reactive functional group of pyridine, which is able to coordinate to metal ions on the surface of MCr (M ) Co, Cu, Fe) nanoparticles (Figure 1). A hydroxy group of NP-5 molecules attached on the MCr nanoparticle surface in reversed micelle can be easily replaced by alkyl pyridine derivatives, because the coordination ability of a hydroxy group is lower than that of a pyridine group. Second, to understand how the physicochemical and magnetic properties of PBAs change when they are confined on a nanometer scale is not only fundamentally interesting but also nanotechnologically important to develop novel nanomaterials with effective functions. Several studies have been devoted to synthesis of PBA nanoparticles, however, works on the comparison of physicochemical and magnetic properties between nanoparticle and bulk crystal in PBAs are relatively few. Therefore, we have presented the physicochemical (shape, size, crystal structure, and electronic state) and magnetic properties of the isolated MCr (M ) Co, Cu, Fe) nanoparticles in comparison with their bulk crystals. Experimental Section Materials. All starting materials were commercially available, reagent grade, and were used as purchased without further purification. K3[CrIII(CN)6] was purchased from Aldrich. CoIICl2‚6H2O, CuIICl2‚2H2O, FeIICl2‚4H2O, cyclohexane, methanol, acetone, acetonitrile, chloroform, diethyl ether, potassium carbonate, n-pentane, pyridine hydrochloride, 1 M ammonium hydroxide, anhydrous MgSO4, KBr, and standard solution for inductively coupled plasma (ICP) measurement were purchased from Wako Pure Chemical Industry. Polyethylene glycol mono 4-nonylphenyl ether (NP-5: HO(CH2CH2O)nC6H4C9H19, n ) 5) and 1-bromooctadecane were purchased from Tokyo Chemical Industry. Ultrapure water was prepared over 18.3 MΩ cm delivered with a Millipore Milli-Q SP. Synthesis of 4-(Dioctadecylamino)pyridine (OPy). A 21.24 g (0.09 mol) portion of 4-amino-1-methylpyridinium iodide,50 75.24 g (0.225 mol) of 1-bromooctadecane, and over 33.17 g of potassium carbonate were refluxed in 450 mL of acetonitrile for 3 days. The solvent was removed under reduced pressure. The crude product was dissolved in 120 mL of chloroform and was filtered off to remove the potassium carbonate. The solution was mixed with 1000 mL of n-pentane to precipitate the

powdery solid. The solid was filtered and washed well with diethyl ether and acetonitrile and dried under vacuum. A 55.58 g (0.075 mol) portion of the obtained solid and 216.68 g (1.875 mol) of pyridine hydrochloride were refluxed under nitrogen in the absence of solvent. After 4 days, the mixture was cooled, and 410 mL of water was added to dissolve the unreacted pyridine hydrochloride. The crude product was filtered off and redissolved in 580 mL of chloroform. The chloroform solution was extracted three times with 290 mL portions of 1 M ammonium hydroxide and dried over anhydrous MgSO4 before removal of the solvent under reduced pressure. Acetonitrile (580 mL) was added to the oil that remained, and the mixture was vigorously stirred in an ice bath. The precipitated solid was redissolved in chloroform (580 mL), treated with 2 g of activated carbon, and filtered off. The solution was mixed with 470 mL of acetonitrile to precipitate the product as solid. The solid was filtered and washed well with diethyl ether and acetonitrile and recrystallized several times and dried under vacuum (29.90 g, 66.50%). 1H NMR (CD3Cl, ppm): 8.07, d, 2H; 6.34, d, 2H; 3.40, t, 4H; 1.54, m, 4H; 1.25, m, 60H; 0.87, t, 6H. Elemental analysis calcd for C41H78N2: C, 82.20; H, 13.12; N, 4.68. Found: C, 81.96; H, 12.99; N, 4.48. MS calcd: 599.07. Found: 599.6. Synthesis of Cyanide-Bridged MCr (M ) Co, Cu, Fe) Nanoparticles Stabilized by OPy (MCr-OPyn; n ) 1-5). All the operations for the syntheses were carried out in the dark as much as possible to avoid decomposition of the potassium hexacyanochromate(III). The MCr (M ) Co, Cu, Fe) nanoparticles were synthesized by mixing two kinds of water-in-oil microemulsions, each of which was prepared from 24 mL of 0.4 M NP-5/cyclohexane with the addition of 0.84 mL of 0.1 M aqueous potassium hexacyanochromate (K3[CrIII(CN)6]) or 0.1 M aqueous metal chloride (MIICl2, M ) Co, Cu, Fe). After 24 h of stirring, surface coordination ligand of OPyn, which was n equivalents relative to all of the included transition metal ions (n ) 1, 2, 3, 4, and 5), was added to the reaction mixture. After vigorous stirring for another 1 h, the slurry product was deposited by a centrifuge with a sufficient amount of methanol. The crude specimen was suspended in a small amount of cyclohexane (ca. 0.5 mL) and again dispersed in excess methanol. The resulting precipitate was filtered and washed with a sufficient amount of methanol and water. The final filtered precipitate was dried until constant weight in air at room temperature to give MCr-OPyn (M ) Co, Cu, Fe; n ) 1-5). In this research, the additive amount of OPy was up to 5 equiv, because OPy with more than 5 equiv do not dissolve homogeneously in the reversed micelle solution in our synthetic condition. Synthesis of Cyanide-Bridged MCr (M ) Co, Cu, Fe) Bulk Crystals (MCr-Bulk). Furthermore, the MCr (M ) Co, Cu, Fe) bulk crystals, as reference, were adjusted by mixing in aqueous solution. Aqueous potassium hexacyanochromate (K3[CrIII(CN)6]) (0.1 M) was added to a vigorously stirred 0.1 M aqueous metal chloride (MIICl2, M ) Co, Cu, Fe). After 24 h of stirring, the resulting precipitate was filtered and washed with a sufficient amount of water. The final filtered precipitate was dried until constant weight in air at room temperature to give MCr-Bulk (M ) Co, Cu, Fe). Measurements. All the measurements were carried out as soon as possible after synthesis of compounds. The ultraviolet-

MCr (M ) Co, Cu, Fe) Coordination Nanoparticles visible (UV-vis) absorbance spectra were recorded on a Shimadzu UV-3600 spectrophotometer. The Fourier transform infrared spectra (FT-IR) with samples prepared as KBr disk were recorded on a HORIBA FT-720 spectrophotometer in the 4000400 cm-1 region. X-ray powder diffraction (XRD) patterns were measured with a Rigaku RINT 2000 diffractometer with Cu KR (50 kV, 200 mA) radiation from 10 to 60 degrees (2θ value). During the XRD measurements, the powder samples were mounted on an amorphous silicon plate. Transmission electron microscopy (TEM) studies were performed with a Hitachi H-9000NAR at an accelerating voltage of 100 kV. A drop of dispersed chloroform solution of the obtained MCr-OPyn (M ) Co, Cu, Fe: n ) 1-5) was placed on a 200 mesh copper grid covered with a carbon film supplied by Okenshoji Co., Ltd. The grid was rinsed with ethanol and dried under ambient conditions. The mean diameters were calculated by counting 100 particles from the TEM photographs. Elemental analyses of metal element were obtained by a Shimadzu ICPS-7000 ICP apparatus, and analyses of carbon, hydrogen, and nitrogen were carried out with a SiberHegner Elementar Vario EL III. The elemental analyses were carried out several times, and the average values were used. X-ray photoelectron spectra (XPS) were obtained using a ULVAC-PHI ESCA5600 spectrometer equipped with a monochromatic Al KR (300 W) radiation. The binding energies of each element were calibrated based on the C (1s) peak of the hydrocarbon chain at 284.5 eV. The direct current (dc) and alternating current (ac) magnetic properties were investigated using a 5 T Quantum Design superconducting quantum interference device magnetometer and a 9 T Quantum Design physical property measurement system. A powder sample was wrapped in a commercial transparent film, mounted inside a transparent straw supplied by Quantum Design, and then supported at the end of the sample-transport rod. The zero field-cooled (ZFC) dc magnetization measurements were obtained by cooling in zero field to 2 K, and then data were collected on warming in 50 Oe external magnetic field. The field-cooled (FC) dc magnetization measurements were performed by cooling the sample to 2 K in the presence of an external field of 50 Oe. The field dependence dc magnetization measurements were obtained by cooling in zero field, and data were collected. For the ac magnetic susceptibility, χac, all measurements of the in-phase (real) susceptibility, χac′, and outof-phase (imaginary) susceptibility, χac′′, were made after zero field to 2 K and measured upon warming in zero direct current field with a 5 Oe ac field at driving frequencies from 10 to 1000 Hz. NMR spectra were obtained on a Varian FT-NMR GEMINI 2000 spectrometer. Mass spectrometry (MS) analyses were performed with Bruker Daltonics BioAPEXII 7.0e. Results and Discussion TEM Observation of MCr-OPyn (M ) Co, Cu, Fe; n ) 1-5). The MCr-Bulk (M ) Co, Cu, Fe) are insoluble in any solvent; however, the obtained MCr-OPyn (M ) Co, Cu, Fe; n ) 1-5) can be redispersed in low-polar solvents such as chloroform and tetrahydrofuran, implying the hydrophobic nature of an alkyl coordination ligand that exists on the surface of nanoparticles.31 TEM images of MCr-OPyn (M ) Co, Cu; n ) 5) are shown in Figure 2. The CoCr-OPy5 displayed cubic particles, which might originate in the face-centered cubic (fcc) structure of PBA with an average diameter (Dav) of 9.06 ( 0.68 nm (Supporting Information, Figure S1 left). On the other hand, the CuCr-OPy5 displayed subround particles with Dav of 15.71 ( 1.81 nm (Supporting Information, Figure S1 right). The reason for the large difference of Dav between CoCr-OPy5 and

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Figure 2. TEM images of (a) CoCr-OPy5 and (b) CuCr-OPy5. Scale bars in all figures are 50 nm.

Figure 3. TEM images of FeCr-OPyn (n ) 1-5); n ) (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5. Scale bars in all figures are 50 nm.

CuCr-OPy5 may originate from the difference of complexation constant. Similar results have been reported by Hashimoto et al. in the quasi nanometer scale MCr (M ) Co, Cu) PBA particles on an ion-exchange membrane.51,52 It is interesting to note that the size and shape of FeCrOPyn (n ) 1-5) were dependent on the number of n, namely, the additive amount of OPy in synthetic solution. As the additive amount of OPy was increased, the relative number of cubic particles to nonuniform small particles decreased (Figure 3). The FeCr-OPyn (n ) 1-4) possessed cubic particles with Dav ) 10.01 ( 0.84, 9.33 ( 0.94, 9.20 ( 0.85, and 8.73 ( 0.96 nm, respectively (Supporting Information, Figure S2). However, the FeCr-OPy5 did not show cubic particles. In addition, obvious UV-vis spectral changes dependent on the number of n were observed only for the FeCr-OPyn (n ) 1-5). Figure 4 shows the UV-vis absorption spectra of the FeCr-OPyn (n ) 1-5) during syntheses before and after the addition of OPy.

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Arai et al. TABLE 1: Formulas of the Obtained Compounds

Figure 4. The UV-vis absorption spectra of FeCr-OPyn (n ) 1-5) during syntheses.

Figure 5. Unit cell structure of Prussian blue analogue depending on metal composition. M/Cr ) 1.5 (left) and M/Cr ) 1.0 (right).

Without OPy, the spectrum showed an absorption peak around 440 nm, which can be assigned to the intervalence charge transfer (IVCT) band of FeII and CrIII.53,54 When OPy was added, the IVCT band decreased gradually in proportion to the additive amount of OPy and finally disappeared at n ) 5. It is suggested that the structural components of CrIII-CN-FeII should be eliminated by increasing the amount of OPy during syntheses, which may affect the physicochemical and magnetic properties of isolated FeCr-OPyn (n ) 1-5). These phenomena depending on the additive amount of OPy in solution were not observed for the MCr-OPyn (M ) Co, Cu; n ) 1-5), even when reaction time was increased. On the basis of these results, we have investigated the relation between the additive amount of OPy and the properties of isolated FeCr nanoparticles. Elementary Analysis. For the unit cell structure of PBA by using a divalent transition metal cation (MII) and a hexacyanochromate ([CrIII(CN)6]3-) anion bearing three negative charges, it is known that two extreme stoichiometries may be presented, according to the electroneutrality conditions (Figure 5).12,55,56 In the case of M/Cr ) 1.5 (Figure 5 left), no potassium cations are present due to the electroneutrality condition. In this structure, the 33% of [CrIII(CN)6]3- vacancies appear, the MII cations are linked by 4 nitrogen atoms of [CrIII(CN)6]3- and 2 oxygen atoms of H2O in octahedral symmetry, resulting in a fcc structure with lattice defects. In the case of M/Cr ) 1.0 (Figure 5 right), potassium cations are present in half of the tetrahedral sites of the lattice to compensate for the charge balance. The [CrIII(CN)6]3- vacancies disappear, and MII cations are linked by 6 nitrogen atoms of the [CrIII(CN)6]3- in octahedral symmetry, resulting in a perfect fcc structure without lattice defects. In the intermediate cases, the [CrIII(CN)6]3- vacancies varies from 33% (M/Cr ) 1.5) to 0% (M/Cr ) 1.0) as a function of the amount of potassium cations in the lattice to compensate

compounds

formulas

CoCr-OPy5 CoCr-Bulk CuCr-OPy5 CuCr-Bulk FeCr-OPyn (n ) 1-5) FeCr-Bulk

K0.14Co1.43[Cr(CN)6]‚(OPy)0.24‚5.2H2O K0.20Co1.40[Cr(CN)6]‚6.7H2O Cu1.5[Cr(CN)6]‚(OPy)0.44‚2.7H2O K0.1Cu1.45[Cr(CN)6]‚6.3H2O Table S1 K0.20Fe1.40[Cr(CN)6]‚7.1H2O

for the charge balance. Therefore, the chemical formula of MCr PBA bulk crystal is generally expressed as KIxMIIy[CrIII(CN)6]z‚ wH2O (1.0 e y/z e 1.5, x + 2y ) 3z). It should be noted that MII cations located on a surface of the particle (including an edge and a corner) are naked. Therefore, the surface MII ions usually contain coordinated water molecules irrespective of a metal composition. The chemical formula of MCr-OPyn (M ) Co, Cu, Fe; n ) 1-5) with MCr nanocore/OPy-shell hybrid structure can be presented as KIxMIIy[CrIII(CN)6]z‚(OPy)V‚wH2O.31,37 The resulting formulas normalized by hexacyanochromate are listed in Table 1. The y/z value of CoCr-OPy5 was regarded as the same as that of CoCr-Bulk. The molar ratio of OPy was ca. 0.17 relative to the Co sites from the obtained formula of CoCrOPy5 (i.e., OPy 0.24/Co 1.43). This value agrees with the calculated value of 0.15, assuming that the OPy is attached to the Co sites located on a CoCr nanocore surface in the proportion of one part to two, considering that the cubic particle size of CoCr nanocore is 9 nm determined by the TEM image with the lattice constant of ca. 10 Å (see below). From our previous report,31 the amino group of stearylamine as a surface coordination ligand is found to form 1:1 complexation to the Co sites located on a cobalt hexacyanoferrate/chromate (CoFe/ Cr) nanocore surface. The ratio of OPy to Co sites decreased compared to stearylamine (almost a half value in CrCo-OPy5), because the size of OPy is larger than that of stearylamine. For the CuCr-OPy5, no potassium cations incorporated into the lattice were detected at all, and the y/z value of CuCr-OPy5 was slightly different compared to that of CuCr-Bulk. Because CuCr-OPy5 showed an unexpected XRD pattern (see below), the complexation state between the Cu sites located on a CuCr nanocore surface and OPy-shell was not able to be detected. For the FeCr-OPyn (n ) 1-5), appropriate composition formulas were not obtained, because the present FeCr-OPyn (n ) 1-5) contained impurities with pure FeCr PBA nanoparticles (see below) and the y/z values were over 1.5 (Supporting Information, Table S1). This may be due to the complexation reaction between FeII ions and OPy by the etching reaction of surface coordination ligand, and a large amount of iron complexes was generated as impurities with the cubic shape particles derived from FeCr PBA nanoparticle. Therefore, the nonuniform small particles observed by TEM images might be the above-mentioned impurities. IR Spectra and XRD Patterns. IR spectra of the MCrOPyn (M ) Co, Cu, Fe; n ) 1-5) contained common characteristic peaks (not shown in figures): CH3 antisymmetric stretching vibration at 2958 cm-1, CH2 antisymmetric stretching vibration at 2922 cm-1, CH2 symmetric stretching vibration at 2852 cm-1, and cyanide groups stretching vibration from 2200 to 2000 cm-1 were observed, independent of the metal constituents. Above all, the cyanide groups stretching vibration can be utilized as an electronic probe, because they are affected by the electronic state of coordinated metal and the coordination environment. 1. CoCr Compounds. The IR spectra and XRD patterns of the CoCr-OPy5 and CoCr-Bulk are shown in Figure 6. The

MCr (M ) Co, Cu, Fe) Coordination Nanoparticles

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Figure 6. IR spectra of the cyanide group stretching region (left) and XRD patterns (right) for the CoCr compounds: (a) CoCr-OPy5 and (b) CoCr-Bulk.

Figure 7. IR spectra of the cyanide group stretching region (left) and XRD patterns (right) for the CuCr compounds: (a) CuCr-OPy5 and (b) CuCr-Bulk.

IR spectra exhibited a sharp absorption peak at 2180 cm-1 for the CoCr-OPy5 and at 2170 cm-1 for the CoCr-Bulk. These peaks were assigned to CrIII-CN-CoIIHS.31,57-59 It is plausible that the shoulder peak around 2140 cm-1 was free cyanide groups located on the crystal surface, considering that the relative intensity of that shoulder peak to the main sharp peak of CoCr-OPy5 was larger than that of CoCr-Bulk. Similar combination peaks (sharp and shoulder) were observed in NiCr compounds.37,60 From the XRD patterns, these diffraction patterns represented the fcc structure with lattice constants of 10.40 and 10.52 Å for the CoCr-OPy5 and CoCr-Bulk, respectively. Especially, the XRD pattern of CoCr-OPy5 showed broadened peaks due to the downsizing effect.61 From the Scherrer formula,62

d ) 0.94λ/B cos θB

(1)

where d is the particle size in Å, λ is the X-ray wavelength in Å, θB is the Bragg angle in radians, and B is the peak width at half-height. The particle size (d) of CoCr-OPy5 was calculated as 6.3 nm, which was smaller than the result of TEM observation. 2. CuCr Compounds. The IR spectra of CuCr-OPy5 and CuCr-Bulk showed two similar sharp absorption peaks (Figure 7). The higher wavenumber peak at 2182 cm-1 for both CuCr compounds were assigned to CrIII-CN-CuIIHS component,63 and the lower one (at 2120 cm-1 for CuCr-OPy5, at 2110 cm-1 for CuCr-Bulk) might indicate the existence of another CtN bridges such as site/bond disorder components in the lattice.64,65 The presence of lattice disorder in the CuCr-Bulk was confirmed by the ac susceptibility study (see below). Anderson et al. have also reported similar IR spectrum in CuCr PBA bulk crystal.66 From the XRD patterns, although the CuCr-Bulk represented the typical fcc structure with lattice constant of 10.38 Å the CuCr-OPy5 showed an unexpected diffraction pattern, different from the fcc structure. The crystal structure of CuCr-

Figure 8. IR spectra of the cyanide group stretching region (left) and XRD patterns (right) for the FeCr-OPy1 and FeCr-Bulk: (a) as-prepared FeCr-OPy1, (b) FeCr-OPy1 maintained for several months, (c) as-prepared FeCr-Bulk, and (d) FeCr-Bulk maintained for several months.

OPy5 cannot be determined; however, it is clear that CuCrOPy5 was comprised of the crystal mixture of the basic fcc structure with the octahedral Cu sites and the other crystalline phases (maybe with non-octahedral Cu sites such as tetrahedral Cu), considering that several peaks appeared at positions close to the peaks of CuCr-Bulk. The particle size (d) from the Scherrer formula of CuCr-OPy5 was calculated as 15.2 nm, and it was almost the same value of the result of TEM observation. As a proposed mechanism of lattice disorder to be considered, the lattice disorder may be first induced by the strong JahnTeller distortion of octahedrally coordinated CuII ion in the CuCr PBA structure.63 After that distortion, the lattice disorder may be expedited by the gradual release of the coordinated water molecules from the CuCr lattice, which plays the essential role in stabilizing the fcc structure.67 As a basis of this mechanism, the IR spectra and XRD patterns of CuCr compounds maintained at ambient environments for several months are shown in Supporting Information, Figure S3. From the IR spectra (Figure S3 left), the relative intensity of a higher wavenumber peak to a lower one decreased both in CuCr-OPy5 and in CuCr-Bulk. On the other hand, the XRD pattern of CuCr-OPy5 was almost same, however, that of CrCu-Bulk did not show fcc structure. It is thought that coordinated water molecules were released from the CuCr lattice as mentioned above. This kind of lattice instability has been reported for some molecule-based magnets prepared from nonaqueous solvents.19,68,69 Therefore, we believe that the lower wavenumber peaks can be assigned to the site/ bond disorder components in the lattice. Generally, the specific coordination number of the CuII ion that exhibits great stereochemical plasticity by applying appropriate capping ligands contributes to the formation of the unexpected crystal structure. There are a lot of studies of Cu(II)-containing cyanide-bridged molecule-based magnets based on hexacyanometalates building blocks in the construction of numerous one-, two-, and three-dimensional structures.64,65,70-74 In particular, it is known that CuII ion prefers two- or fourcoordinate geometry to six-coordinate octahedral geometry in the presence of large size coordination molecules.75-77 Therefore, during the synthesis of CuCr-OPy5, the OPy might act not only as a stabilizer but also as a capping coordination molecule for the Cu ion, leading to the unexpected crystal structure. 3. FeCr Compounds. The IR spectra of as-prepared FeCrBulk showed two absorption peaks at 2161 and 2088 cm-1, which were assigned to CrIII-CN-FeIIHS and CrIII-NC-FeIILS of the cyano flip, respectively (Figure 8c left).78,79 This cyano flip phenomenon is explained by the linkage isomerism that

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Figure 9. IR spectra of the cyanide group stretching region (left) and XRD patterns (right) for the as-prepared FeCr-OPyn (n ) 1-5): n ) (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5.

iron(II) hexacyanochromate(III) is transformed into the more stable chromium(III) hexacyanoferrate(II) on heating or standing at room temperature for several months with an obvious color change from brick red to green (i.e., CrIII-CN-FeIIHS f CrIIINC-FeIILS).78,79 The linkage isomerism and the subsequent spin change accompany the shift of the ν(CN) value to a lower wavenumber region, because the cyanide group is a better σ-donor and a poor π-acceptor, relatively.80 In addition, this transformation follows the decrease of the lattice constant due to the loss of the antibonding eg electrons.20,78 Actually, from the IR spectra of FeCr-Bulk (Figure 8c,d left), the relative intensity of the CN stretching peak of CrIII-CN-FeIIHS to the peak of CrIII-NC-FeIILS decreased. In addition, the XRD pattern of FeCr-Bulk shifted to the wider 2θ region, and the lattice constant decreased from 10.62 to 10.16 Å with the obvious color change from brick red to dark green (Figure 8c,d right). This linkage isomerism was observed even in FeCrOPy1 (Figure 8a,b). Namely, in the IR spectra of FeCr-OPy1 (Figure 8a,b left), the relative intensity of the higher wavenumber peak to the lower one decreased. In addition, the lattice constant also changed from 10.58 to 10.21 Å while maintaining the fcc structure and the broadened diffraction peaks of the XRD pattern because of the downsizing effect (Figure 8a,b right). When as-prepared FeCr-OPy1 and as-prepared FeCr-Bulk are compared, linkage isomerism of FeCr-OPy1 is easier than that of FeCr-Cr-Bulk, because the lattice energy effect of former is lower than that of latter. The IR spectra and XRD patterns of as-prepared FeCr-OPyn (n ) 1-5) are shown in Figure 9. From the IR spectra of asprepared FeCr-OPyn (n ) 1-5), the relative intensity of the higher wavenumber peak to the lower one decreased with increasing the number of n (equal to the additive amount of OPy) (Figure 9 left). The results of these IR spectra agree with those of the UV-vis spectra in Figure 4, where the intensity of the IVCT band of CrIII-CN-FeII decreased with increasing n. Only from these results, it might be thought that the linkage isomerism of FeCr-OPyn (n ) 1-5) is induced by the coordination of pyridine group of OPy onto the FeCr nanoparticle surface. However, this behavior cannot be assigned to typical linkage isomerism, because the XRD diffraction patterns of FeCr-OPyn (n ) 1-5) became more and more broad with increasing n. (Figure 9 right). The particle sizes (d) from the Scherrer formula of FeCr-OPyn (n ) 1-3) were calculated as 11.1, 11.7, and 12.1 nm, respectively. An opposite result was obtained with the result of TEM observation for particle size. In general, the broadening of the X-ray diffraction peaks can be related to the decrease of crystal size and the lattice disorder. Remembering the TEM images, the nonuniform small particles of ca. 2 nm could be observed more frequently in addition to the cubic particles of ca.10 nm when the additive amount of

Figure 10. The ZFC and FC magnetization curves at an external magnetic field of 50 Oe (top) and temperature dependence of in-phase (χ′) (middle) and out-of-phase (χ′′) (bottom) components at Hdc ) 0 Oe and Hac ) 5 Oe for the CoCr-OPy5. (Inset: Arrhenius plots).

OPy was increased. Therefore, it is natural to understand that the nonuniform small particles were produced as some impurities from the etching reaction between the FeCr nanocore and OPy. From this interpretation, it might be explained that the fcc structure of FeCr nanoparticles begins to collapse by etching reaction of OPy. Then, the chemical units (Fe, Cr, CN) are released from the PBA lattice, and the released units then might become amorphous small particles by reacting with OPy. Therefore, the broadening of the XRD patterns is attributed to both the size decrease of FeCr nanoparticles and the increase of the amorphous small particles by the etching reaction of OPy. In addition, although XPS spectra were measured to investigate the chemical-bonding state and the electronic state, no clear difference was observed between the FeCr-OPyn (n ) 1-5) probably due to the presence of a large amount of impurities. Magnetic Properties. 1. CoCr Compounds. The ZFC and FC dc magnetization versus temperature curves at an external magnetic field of 50 Oe and the temperature dependence of ac magnetic susceptibility data at different frequencies (f) between 10 and 1000 Hz in zero dc field (Hdc ) 0 Oe) and 5 Oe ac field (Hac ) 5 Oe) for the CoCr-OPy5 are shown in Figure 10. For CoCr-OPy5, the Curie temperature (Tc), the blocking temperature (Tb), and coercive force (Hc) at 5 K were 24 K, 14 K, and 394 Oe, respectively. On the other hand, the Tc, Tb, and Hc for the CoCr-Bulk were 27 K, 24.5 K, and 706 Oe, respectively. In PBAs, it is recognized that Tc is shifted to lower temperature as particle size becomes smaller, assuming that the parameters of the material such as exchange coupling constant (J) and spin states (S) are the same. The Tc value in this system is expressed as57

Tc ) {2(ZCoCrZCrCo)1/2 |JCoCr|/(3kB)} × {SCo(SCo + 1)SCr(SCr + 1)}1/2 (2)

MCr (M ) Co, Cu, Fe) Coordination Nanoparticles

J. Phys. Chem. C, Vol. 112, No. 6, 2008 1959

where ZCoCr and ZCrCo are the numbers of the nearest neighbors of the CrIII and CoII ions, respectively, JCoCr is the exchange coupling constant between the CoII and CrIII ions, and kB is the Boltzmann constant. SCo and SCr are spin states of the CoII and CrIII ions, respectively. In present CoCr compounds, the decrease of Tc is observed. Because it is thought that spin states (SCo and SCr) and JCoCr of CoCr-OPy5 are almost the same as the CoCr-Bulk from the IR spectra and XRD patterns,57 this size dependent feature of Tc results from a decrease of the numbers of the nearest neighboring ions (here ZCoCr, ZCrCo) as the particle size becomes smaller. On the other hand, the Hc of CoCr-OPy5 was lower than that of CoCr-Bulk due to superparamagnetism (Supporting Information, Figure S4) (see below). Generally, Hc gradually rises as particle size becomes smaller because a single magnetic domain is formed, and the Hc takes the maximum value at a certain size. Thereafter, the Hc comes to decrease inversely and finally is not shown due to superparamagnetism as the particle size becomes even smaller.81 From ac magnetic susceptibility measurement, although the CoCr-Bulk did not demonstrate frequency dependence (Supporting Information, Figure S5) the CoCr-OPy5 showed strong frequency dependence in both the in-phase (χ′(T)) and the outof-phase (χ′′(T)) components, all of the χ′(T) and χ′′(T) curves for CoCr-OPy5 exhibited the peak maximums, and the peak positions were shifted gradually to the higher-temperature region with increasing frequency due to spin glass and/or superparamagnetism.36,82 The spin glass can be estimated by quantifying the frequency dependence in the χ′(T) data through the parameter φ, eq 3,19,68,69,82 which is written as

φ ) ∆Tf′/[Tf′∆(log f)]

(3)

where Tf′ is the temperature of the peak maximum in the χ′(T) curve measured at the initial frequency, ∆Tf′ is the difference of the peak maximum temperatures in the χ′(T) curve between the initial and the final frequency, and ∆(log f) is the difference between the logarithms of the initial and the final frequencies in hertz. Generally, spin glass material possesses the φ value in the range of 0.01 < φ < 0.1.36,69 Regarding superparamagnetism, the fitting of the thermal variation of the relaxation time (τ) with the Arrhenius law should be done by eq 482,83

τ ) 1/2πf ) τ0 exp(∆Ε/kBT)

(4)

where τ0 is the pre-exponential factor, f is the frequency, ∆E/ kB is energy barrier to magnetic reversal in an isolated particle, and T is the absolute temperature. The plot of ln τ vs 1/T′′max, where T′′max represents the peak maximum temperature in χ′′(T), gives the linear relation by the slope of ∆E/kB and the intercept of τ0. When the material shows pure superparamagnetism, the value of τ0 should be in the range of 1 × 10-11 < τ0 < 1 × 10-9 s.82,83 As noted above, the magnetic materials begin to show superparamagnetic behavior when their size becomes smaller. Considering how the τ0 value is different from the range of 1 × 10-11 < τ0 < 1 × 10-9 s, we can know the presence of interparticle interaction. That is, τ0 tends to decrease with increasing particle interaction.82,83 The value of φ for CoCr-OPy5 was calculated as 0.037. This φ value should be due to superparamagnetism and not to the spin glass, because CoCr-Bulk did not show frequency dependence in χ′(T) and χ′′(T). ∆E/kB and τ0 were given as (∆E/kB, τ0) ) (713 K, 2.70 × 10-26 s) for CoCr-OPy5. Here, it is recognized that the obtained τ0 value is significantly smaller than the appropriate range of 1 × 10-11 < τ0 < 1 × 10-9 s for

Figure 11. The ZFC) and FC magnetization curves at an external magnetic field of 50 Oe (top) and temperature dependence of in-phase (χ′) (middle) and out-of-phase (χ′′) (bottom) components at Hdc ) 0 Oe and Hac ) 5 Oe for the CuCr-OPy5. (Inset: Arrhenius plots for the LT and HT regions of the CuCr-OPy5).

superparamagnetism, indicating the presence of interparticle interaction.82 2. CuCr Compounds. The ZFC and FC magnetization curves at 50 Oe and the ac magnetic susceptibility data at different frequencies (f) between 10 and 1000 Hz in zero dc field (Hdc ) 0 Oe) and 5 Oe ac field (Hac ) 5 Oe) for the CuCr-OPy5 are shown in Figure 11. For the CuCr-Bulk, the Tc, Tb, and Hc were 68 K, 35.5 K, and 13 Oe, respectively. On the other hand, the CuCr-OPy5 exhibited almost the same Tc of 68 K; this result means that the size does not have a significant effect on Tc for the present CuCr compounds. This might be understood that the target compounds synthesized here contain many lattice defects of [CrIII(CN)6]3- (Cu/Cr ) ca. 1.5, see above), meaning that the number of the nearest neighboring ions in an inner crystal is almost the same, of which the value does not differ from that of surface atoms. In general, ZFC and FC curves of ferromagnetic materials show a characteristic “λ” shape with one blocking temperature like the curves of CuCr-Bulk.43,82 However, CuCr-OPy5 showed unusual magnetic behavior. That is, the ZFC curve of CuCr-OPy5 possessed two separated blocking temperatures, TbLT in the lower temperature (LT) region and TbHT in the higher temperature (HT), because different crystal structures existed in the CuCr-OPy5 as seen from the XRD pattern. These values of CuCr-OPy5 were measured as (TbLT, TbHT) ) (8.5 K, 50 K). The magnetic hysteresis loop at 5 K for the CuCr-OPy5 showed an unusual constricted hysteresis loop with Hc of 118 Oe, and the Hc of CuCr-OPy5 was higher than that of CoCr-Bulk (Supporting Information, Figure S6). A clear cause has not been found because not only size but also crystal structure are different from CuCr-Bulk. For the ac magnetic susceptibility data, not only CuCr-OPy5 but also CuCr-Bulk showed frequency dependence in both χ′(T) and χ′′(T) components (Supporting Information, Figure S7). The MCr-Bulk (M ) Co, Fe) did not show spin glass behavior;

1960 J. Phys. Chem. C, Vol. 112, No. 6, 2008

Figure 12. FC magnetization curves at an external magnetic field of 50 Oe (top) and field dependence of magnetization at 5 K (bottom) for the as-prepared FeCr-OPyn (n ) 1-5) and FeCr-Bulk: n ) (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) FeCr-Bulk. Inset in top panel: FC magnetization curve for the FeCr-Bulk.

only CuCr-Bulk displayed spin glass behavior. Because the CuCr-Bulk is bulk crystal, this frequency dependence will originate in the spin glass. In general, the spin glass behavior is induced by contributions of structural randomness such as site/bond disorder within crystal19 as described in the IR spectra section, and the degree of spin glass (φ) was 0.011. The χ′(T) and χ′′(T) curves of the CuCr-OPy5 demonstrated two peak maximums derived from TbLT and TbHT (Figure 11), of which the positions were dependent on frequency. For the values of φ, ∆E/kB, and τ0 from the LT and HT regions, the LT region possessed (φ, ∆E/kB, τ0) ) (0.110, 138 K, 6.10 × 10-11 s) and the HT region showed (φ, ∆E/kB, τ0) ) (0.027, 2977 K, 5.12 × 10-28 s) for the CuCr-OPy5. These values of the former are in line expected for pure superparamagnetism. Some Mn12 clusters are known to exhibit two maximums in χ′′(T) curves that originate in two crystal forms of the sample.84,85 For the present CuCr-OPy5, we believe that the presence of two crystal forms is the cause, as described in the XRD pattern section. 3. FeCr Compounds. The FC magnetization curves at an external magnetic field of 50 Oe, and the magnetic hysteresis loops at 5 K for the FeCr compounds, are shown in Figure 12. For FeCr-Bulk, the Tc, Tb, and Hc were 21 K, 17.5 K, and 286 Oe, respectively. The FeCr-OPy1 possessed almost the same Tc ) 21 K as the FeCr-Bulk, and the value was slightly shifted (∼2 K) to lower temperature region as the additive amount of OPy was increased. In addition, Tb and Hc were greatly changed dependent on the additive amount of OPy, ranging from 14.5 K (CrFe-OPy1) to 9.5 K (CrFe-OPy5) and from 354 Oe (FeCr-OPy1) to 64 Oe (FeCr-OPy5), respectively. One of the notable points is that the Hc of the FeCr-OPyn (n ) 1, 2) was larger than that of FeCr-Bulk. This is attributed to the fact that a single magnetic domain might be formed because the particle size becomes smaller. As the additive amount of OPy was increased, the decrease of Tc and Hc was attributed to the increase of diamagnetic [FeII(CN)6] components and the collapse of fcc structure, as evidenced by IR spectra and XRD patterns. In general, the linkage isomerism of CrIII-CN-FeIIHS (i.e.,

Arai et al. CrIII-NC-FeIILS) allows superexchange interactions between the second nearest neighboring CrIII ions (S ) 3/2) through the NC-FeII-CN linkages, because FeII (S ) 0) is low spin, leading to low Curie temperature like Prussian blue. Furthermore, the occurrence of lattice disorder introduces the breakdown of longrange superexchange interaction and tends to decrease Tc and Hc values. For the magnetic properties of the FeCr-OPy1 and FeCrBulk maintained several months, the Tc and Hc values were greatly changed due to the increase of diamagnetic [FeII(CN)6] components, as expected by IR spectra and XRD patterns. The Tc and Hc at 5 K values were changed from 21 to 7.5 K and from 354 to almost 0 Oe for the FeCr-OPy1 and from 21 to 5.4 K and from 286 to almost 0 Oe for the FeCr-Bulk, respectively (Supporting Information, Figures S8 and S9). The ac magnetic susceptibility data at different frequencies (f) between 10 and 1000 Hz in zero dc field (Hdc ) 0 Oe) and 5 Oe ac field (Hac ) 5 Oe) showed strong frequency dependence in both the in-phase, χ′(T), and the out-of-phase, χ′′(T), components for the FeCr-OPyn (n ) 1-5) (Supporting Information, Figures S10-S14). FeCr-Bulk did not show frequency dependence (Supporting Information, Figure S15). From FeCr-OPyn (n ) 1-5), the strength of frequency dependence became larger gradually as the additive amount of OPy was increased; the degree of spin glass (φ) ranged from 0.022 (CrFe-OPy1) to 0.043 (CrFeOPy5), the anisotropy energy barrier (∆E/kB) ranged from 1187 K (FeCr-OPy1) to 232 K (FeCr-OPy5), and the relaxation time (τ0) ranged from 8.7 × 10-36 s (CrFe-OPy1) to 5.0 × 10-20 s (CrFe-OPy5). As the additive amount of OPy was increased, the ac susceptibility peaks were shifted to lower temperature region, and the values of τ0 and φ became larger. The τ0 values follow the Arrhenius law, however, below the predicted superparamagnetic region, and the φ values fall within the typical spin glass range, indicating not pure superparamagnetism but the presence of interparticle interaction in the FeCr-OPyn (n ) 1-5). The dc and ac magnetic properties of the MCr-OPyn (M ) Co, Cu, Fe; n ) 1-5) are summarized in Supporting Information, Table S2. Comparison with Other Prussian Blue Nanoparticles in ac Magnetic Susceptibility Data. Until now, there have been some reports related to the ac magnetic susceptibility data of PBA nanoparticles. For example, Ohkoshi et al. have reported the spherical nanoparticle (8 ( 2 nm) of cobalt(II) hexacyanochromate(III) embedded into a nafion membrance film with τ0 ) 5 × 10-14 s.41 Mallah et al. have showed the nickel(II) hexacyanochromate(III) nanoparticle (6 nm) protected by PVP polymer with τ0 ) 1.1 × 10-13 s.37 The nickel(II) hexacyanoferrate(III) nanoparticle (3 nm) in a chitosan bead has been reported by Guerin et al. with τ0 ) 1.05 × 10-28 s.39 The τ0 values of our samples are relatively small compared with other reports, and the reason is not clear at present. However, a possible reason is that we use alkyl monomer ligand as a stabilizer, therefore the interparticle interaction might be strong due to the presence of space not covered with the stabilizer between each particle. Conclusion We have succeeded in the synthesis and isolation of metal(II)hexacyanochromate(III) MCr (M ) Co, Cu, Fe) coordination nanoparticles, which are protected by an alkyl surface coordination ligand of 4-(dioctadecylamino)pyridine (OPy). From the comparison of MCr (M ) Co, Cu, Fe) nanoparticles and their bulk crystals, we see that the physicochemical and magnetic

MCr (M ) Co, Cu, Fe) Coordination Nanoparticles properties are different depending on the metal constituents. In the CoCr compounds, while keeping almost the same electronic state and structure, only the magnetic properties are changed due to superparamagnetism. In constant, in the CuCr compounds not only the magnetic properties but also the crystal structure are changed. Although an exact cause of this behavior has not been clarified, we believe that OPy might act not only as a stabilizer but also as a capping coordination molecule, leading to the formation of the unexpected crystal structure due to the specific coordination number of the CuII ion. The linkage isomerism is observed not only FeCr-Bulk but also FeCrOPyn (n ) 1-5). When as-prepared FeCr-OPy1 and asprepared FeCr-Bulk are compared, linkage isomerism of FeCrOPy1 is easier than that of FeCr-Bulk because the lattice energy effect of former is lower than that of latter. In addition, the physicochemical (shape, size, crystallinity, and electronic state) and magnetic properties of FeCr-OPyn (n ) 1-5) are greatly influenced by the additive amount of OPy due to the increase of diamagnetic [FeII(CN)6] components and the collapse of fcc structure. Finally, because we have previously succeeded in the isolation of PBA nanoparticles protected by stearylamine,31 we are willing to report surface effect with experiment and theoretical calculation in NiCr PBA nanoparticles protected by stearylamine or OPy.60 Acknowledgment. We thank Dr. T. Kawamoto (AIST), Dr. M. Kurihara (Yamagata University), and Dr. M. Sakamoto (Yamagata University) for their helpful discussion and Mary Ann Mooradian (JAIST) for reading the manuscript. This work was supported by Grants-in-Aid for Scientific Research (No. 16710069) Center of Excellence (COE) Program from the Ministry of Culture, Education, Science, Sports, and Technology, Japan. Supporting Information Available: Size distribution histograms, elementary analysis, IR spectra, XRD patterns, dc magnetization, and ac susceptibility. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kahn, O. Molecular Magnetism; VCH: New York, 1993. (2) Manriquez, J. M.; Yee, G. T.; McLean, R. S.; Epstein, A. J.; Miller, J. S. Science 1991, 252, 1415. (3) Gatteschi, D.; Caneschi, A.; Pardi, L.; Sessoli, R. Science 1994, 265, 1054. (4) Mallah, T.; Thie´baut, S.; Verdaguer, M.; Veillet, P. Science 1993, 262, 1554. (5) Entley, W. R.; Girolami, G. S. Science 1995, 268, 397. (6) Ferlay, S.; Mallah, T.; Ouahe`s, R.; Veillet, P.; Verdaguer, M. Nature 1995, 378, 701. (7) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 271, 49. (8) Dujardin, E.; Ferlay, S.; Phan, X.; Desplanches, C.; Cartier dit Moulin, C.; Sainctavit, P.; Baudelet, F.; Dartyge, E.; Veillet, P.; Verdaguer, M. J. Am. Chem. Soc. 1998, 120, 11347. (9) Holmes, S. M.; Girolami, G. S. J. Am. Chem. Soc. 1999, 121, 5593. (10) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 272, 704. (11) Sato, O.; Einaga, Y.; Fujishima, A.; Hashimoto, K. Inorg. Chem. 1999, 38, 4405. (12) Bleuzen, A.; Lomenech, C.; Escax, V.; Villain, F.; Varret, F.; Moulin, C. C. D.; Verdaguer, M. J. Am. Chem. Soc. 2000, 122, 6648. (13) Escax, V.; Bleuzen, A.; Cartier dit Moulin, C.; Villain, F.; Goujon, A.; Varret, F.; Verdaguer, M. J. Am. Chem. Soc. 2001, 123, 12536. (14) Shimamoto, N.; Ohkoshi, S.; Sato, O.; Hashimoto, K. Inorg. Chem. 2002, 41, 678. (15) Moritomo, Y.; Hanawa, M.; Ohishi, Y.; Kato, K.; Takata, M.; Kuriki, A.; Nishibori, E.; Sakata, M.; Ohkoshi, S.; Tokoro, H.; Hashimoto, K. Phys. ReV. B 2003, 68, 144106. (16) Ksenofontov, V.; Levchenko, G.; Reiman, S.; Gutlich, P.; Bleuzen, A.; Escax, V.; Verdaguer, M. Phys. ReV. B 2003, 68, 024415.

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