Thermal-Induced Changes in Molecular Magnets Based on Prussian

The thermal-induced changes in molecular magnets based on Prussian blue analogues, M3[Fe(CN)6]2·xH2O (M = Mn, Co, Ni, Cu, Zn, and Cd), were studied ...
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J. Phys. Chem. B 2006, 110, 7296-7303

Thermal-Induced Changes in Molecular Magnets Based on Prussian Blue Analogues R. Martı´nez-Garcia,† M. Knobel,‡ and E. Reguera*,†,§ Institute of Science and Technology of Materials, San Lazaro and L, 10400 HaVana, Cuba, Institute of Physics “Gleb Wataghin”, UNICAMP, 13083-970 Campinas SP, Brazil, and Center of Applied Science and AdVanced Technologies of IPN, Mexico D.F., Mexico ReceiVed: September 29, 2005; In Final Form: February 17, 2006

The thermal-induced changes in molecular magnets based on Prussian blue analogues, M3[Fe(CN)6]2‚xH2O (M ) Mn, Co, Ni, Cu, Zn, and Cd), were studied from infrared, X-ray diffraction, thermo-gravimetric, Mo¨ssbauer, and magnetic data. Upon being heated, these materials loose the crystalline water that enhances the interaction between the metal centers, as has been detected from Mo¨ssbauer spectroscopy data. At higher temperatures, a progressive decomposition process takes place, liberating CN- groups, which reduces the iron atom from Fe(III) to Fe(II) to form hexacyanoferrates(II). The exception corresponds to the cobalt compound that undergoes an inner charge transfer to form Co(III) hexacyanoferrate(II). In the case of zinc ferricyanide, the thermal decomposition is preceded by a structural transformation, from cubic to hexagonal. For M ) Co, Ni, Cu, and Zn the intermediate reaction product corresponds to a solid solution of M(II) ferricyanide and ferrocyanide. For M ) Mn and Cd the formation of a solid solution on heating was not detected. The crystal frameworks of the initial M(II) ferricyanide and of the formed M(II) ferrocyanide are quite different. In annealed Mn(II) ferricyanide samples, an increasing anti-ferromagnetic contribution on heating, which dominates on the initial ferrimagnetic order, was observed. Such a contribution was attributed to neighboring Mn(II) ions linked by aquo bridges. In the anhydrous annealed sample such interaction disappears. This effect was also studied in pure Mn(II) ferrocyanide. The occurrence of linkage isomerism and also the formation of Ni(III), Cu(III), and Zn(III) hexacyanoferrates(II) were discarded from the obtained experimental evidence.

I. Introduction Prussian blue (PB) analogues, probably the oldest studied family of coordination compounds, have received renewed attention as prototypes of molecular magnets in the past decade.1 Several PB analogues are included in those molecular materials with the highest temperature of magnetic ordering (Tc).2 In some compositions, unusual effects have been observed. For instance, Co(III) ferrocyanide, which is a nonmagnetic material, becomes magnetic when it is illuminated at low temperatures.3 This effect in cobalt iron cyanide has been intensively studied as a prototype of a photoinduced molecular magnet3,4 in recent years, and it has encouraged the study of other transition metal ferricyanides, particularly the heat-treatment effect probably oriented to find a similar charge-transfer process in other Prussian blue analogues.5-9 For cobalt(II), nickel(II), copper(II), and zinc(II) ferricyanides, the formation of cobalt(III), nickel(III), copper(III), and zinc (III) ferrocyanides has been reported.5-9 However, nickel(III), copper (III), and zinc(III) are unusual species not detected even in ozonized ferrocyanides.10 For cobalt(II) and copper(II) ferricyanides, the formation on heating of CoII-CNFeIII and CuII-CN-FeIII species has also been reported.5,7 This is an unexpected result since all of the known stable hexacyanide complexes are formed with transition metals where the inner metal has unoccupied the eg orbitals to favor a strong cationligand interaction with the formation of low-spin complexes. In CoII-CN-FeIII and CuII-CN-FeIII hexacyanides, the elec* Author to whom correspondence should be addressed. E-mail: ereguera@ yahoo.com. † Institute of Science and Technology of Materials. ‡ UNICAMP. § Center of Applied Science and Advanced Technologies of IPN.

tronic configurations for cobalt and copper atoms are t2g6eg1 and t2g6eg3, respectively. The formation of these species also supposes the occurrence of a linkage isomerization process up to now only observed in ferrous chromicyanide and manganicyanide.11 The behavior of Prussian blue analogues on heating has been studied;12 ferricyanides reduce to ferrocyanides releasing CNanions. In this process the liberated CN- groups play the role of the reducing agent. In the exhaust reaction gases, C2N2 has been detected through the use of mass spectrometry and IR spectroscopy. A similar mechanism is present during the reduction of ferricyanides to ferrocyanides by milling or sonication.13 In the milling, the colliding points of the microcrystals behave as “hot points” where CN- groups are liberated. From all this evidence, we believe that the reported thermalinduced changes in some molecular magnets based on PB analogues5-9 are related to a simple reduction of ferricyanides to ferrocyanides without the presence of other heat-induced effects, with the exception of the cobalt analogue. In this paper, we present experimental results that support this hypothesis. In addition, some interesting new effects were observed and are discussed. This work concerns mainly those heat-induced changes in divalent transition metal ferricyanides for heating below 200 °C. Above this temperature ferrocyanides are always formed, and they remain stable approximately up to 350 °C. Their final decomposition leads to the reduction of the transition metals involved, in some cases forming alloys of small particle size. Such metallic end products have been appropriately characterized.5-8

10.1021/jp0555551 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/18/2006

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II. Experimental Methods Mn(II), Co(II), Ni(II), Cu(II), cadmium, and zinc ferricyanides were obtained from aqueous solutions of these divalent cations and of ferricyanic acid, prepared in situ,14 to obtain a product free of the accompanying alkaline cation. The obtained precipitates were filtered, washed several times with distilled water, and then dried in air until constant weight. According to chemical analysis, the M:Fe atomic ratio obtained for all the studied compositions was 3:2. Such an atomic ratio corresponds to compounds with the following formula unit, M3[Fe(CN)6]2‚ xH2O. From now on these compounds will be labeled as M3Fe2. The same ferricyanides were also obtained from potassium ferricyanides to discard any probable effect of the accompanying alkaline metal when it remains in the obtained material. For excess of the transition metal, a complex salt practically free of potassium was obtained. As reference samples, the corresponding divalent transition metal ferrocyanides were prepared using the same procedure. The nature of all the studied compounds was established from infrared (IR), X-ray diffraction (XRD), thermo-gravimetry (TG), Mo¨ssbauer, and magnetic data. The heat treatments were carried out under an argon atmosphere (99.99%) during 3 h at 60, 80, 100, 120, 140, 160, 180, and 200 °C, allowing sample cooling until room temperature within the furnace used. The obtained annealed samples were then characterized using the mentioned techniques. The IR spectra were recorded in Nujol mulls between KBr windows. On milling and pressing processes with KBr, ferricyanides reduce to ferrocyanides.15 Mo¨ssbauer spectra were run at room temperature in a constant acceleration spectrometer operated in the transmission mode with a 57Co/Rh source. To obtain spectra of anhydrous samples a special vacuum glass cell with windows transparent to γ-rays was used. The obtained spectra were fitted using a least-squares minimization algorithm and pseudo-Lorentzian line shape to obtain the values for isomer shift (δ), quadrupole splitting (∆), line width (Γ), magnetic hyperfine field (Bh), and relative area (A). The values of δ are reported relative to sodium nitroprusside. The magnetic measurements were done at low temperature using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS XL7). The TG curves were recorded in the high-resolution mode using a TA Instrument (TG-2950 model) thermo-balance. Powder XRD patterns were recorded in Bragg-Brentano geometry in a D5000 diffractometer (Siemens) with monochromatic Cu KR radiation in the (10-110)/2θ angular range, at a step of 0.025°/2θ with a counting time of 10 s. Some powder XRD patterns were recorded at the XPD beamline of the LNLS synchrotron radiation facilities (in Campinas, Brazil). The DicVol program16 was used to assign Miller indexes to the powder XRD patterns. The cell parameters were calculated through a profile fitting using the FullProf’1998 code.17 III. Results and Discussion A. Crystal and Electronic Structure of the Studied Materials. Except for Cu(2+), the studied ferricyanides crystallize with the cubic cell (Fm3hm), which is typical of Prussian blue analogues. Cu(2+) ferricyanide appears reported as Fm3hm;7,18 however, the appearance of a series of weak peaks in its powder XRD pattern (Figure 1) that do not correspond to that space group discards such an assignment. This compound was found to be also cubic but within the Pm3hm space group. This space group has also been reported for ferric ferrocyanide.19 The crystal structure of Prussian blue analogues (Fm3hm) is known, and their cell edge values have been reported.18,20-23

Figure 1. Powder XRD patterns of Cu(II) ferricyanide and of the heatinduced decomposition product. Inset A: Dependence of the cell edge size on the annealing temperature. Inset B: Cell contraction in Cu(II) ferricyanide on the water removal (XRD patterns recorded under vacuum).

The cell edge corresponds to the Fe-CtN-M-NtC-Fe chain length. In these structures, both metal centers have octahedral coordination. The iron atom is linked to the C ends of six CN ligands. The outer metal, M, has a mixed coordination sphere formed by four N ends from the CN groups plus two coordinated water molecules, M(NC)4(OH2)2. According to their formula unit, M3[Fe(CN)6]2‚xH2O, 33% of the building unit (Fe(CN)6) sites remain vacant in the material framework, creating a network of pores of about 8.5 Å that communicate by windows of c.a. 4.5 Å. These pores are usually filled with zeolitic waters hydrogen bonded to the coordinated ones. These weakly bonded water molecules can be removed by heating at relatively low temperatures (discussed below). Zinc ferricyanide appears dimorphic, cubic (Fm3hm), and hexagonal (R3hc). In the hexagonal phase the zinc atom is found tetrahedrally coordinated to N ends of CN ligands. This structure is free of vacancies but preserves a porous feature, with pores of about 5 × 8 × 13 Å3 communicating by windows of c.a. 5 Å.24 These pores are free of water molecules because they have a practically nonpolar surface. This hexagonal phase is formed when the synthesis is carried out from hot solutions and also when the cubic phase is dehydrated by heating (discussed below). The reference compounds, divalent transition metal ferrocyanides, crystallize with a higher diversity of structures, depending on the M metal involved. For Mn and Cd, simple and mixed salts can be obtained. The simple salts, M2[Fe(CN)6]‚ 8H2O, crystallize with a monoclinic cell (P21/n),25 while the mixed ones, MA2[Fe(CN)6]‚xH2O (A, alkaline cation), adopt a cubic or pseudo-cubic structure depending on the alkaline metal involved, which is occupying interstitial sites in the material framework.23 For Co, Ni, Cu, and Zn, only mixed compositions are known, with a cubic or pseudo-cubic structure22,23 for the first three and with the R3hc hexagonal structure for Zn (space group already mentioned for zinc ferricyanide24). In mixed zinc ferrocyanide, the alkaline metal is located within the porous framework. In transition metal ferrocyanides, the alkali metal always behaves as a charge balance cation, and it is an extra framework and exchangeable species. In hexacyanoferrates, the iron atom coordinated to the C ends of the CN groups is always found in a low-spin configuration, and their Mo¨ssbauer spectra are single lines for ferrocyanides and a quadrupole splitting doublet for ferricyanides (Figure 2). Both the single lines and the quadrupole splitting doublets have relatively low isomer shift values (Table 1) due to the existence

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Figure 2. Mo¨ssbauer spectra, at room temperature, of Mn3Fe2: (a) hydrated sample; (b) anhydrous sample; (c) anhydrous thermal decomposition product from an annealing of 200 °C; (d) hydrated thermal decomposition product from an annealing of 380 °C. Inset: Mn2[Fe(CN)6]‚xH2O (Mn2Fe) in its hydrated and anhydrous forms.

of a strong π back-bonding effect from the iron atom toward the CN ligands. This effect reduces the number of 3d electrons on the metal and their shielding of the s electron density at the iron nucleus. That π back-bonding effect is more pronounced in ferrocyanides than that in ferricyanides due to the higher 3d electrons availability in the former ones. From this fact, the isomer shift values in ferricyanides and ferrocyanides have only a small difference of about 0.05 mm/s. However, that amount is sufficient to differentiate them. The quadrupole splitting in ferricyanides is mainly related to the existence of an unpaired electron in their t2g orbitals, which creates an electric field gradient around the iron nucleus. This unpaired electron is also responsible for the magnetic order at low temperature in ferricyanides. The IR spectrum is probably the simplest and most efficient sensor to differentiate ferro- from ferricyanides. Ferricyanides have their ν(CN) absorption band 80 cm-1 above the frequency of this vibration in ferrocyanides (Figures 3 and 4). Table S1 of the Supporting Information contains the observed frequency for this vibration in all of the studied ferricyanide samples, before and after the heat-induced effects. The ν(CN) band is also sensitive to the oxidation state and electronic configuration of the metal bonded to the N end. An increment in the valence of this metal increases the ν(CN) frequency by 10-25 cm-1. For instance, in manganese hexacyanoferrates, when the Mn ion goes from a valence of 4+ to 3+, a frequency shift of -24 cm-1 is observed.26 The frequency shift is -14 cm-1 when the Mn atom valence goes from 3+ to 2+.26 With regards to intensity, ferrocyanide bands are very intense and broad, whereas ferricyanide bands are narrow and of medium intensity. B. Dehydration and Thermal Stability. In ferricyanides both coordinated and zeolitic waters are weakly bonded, and under the material heating the crystal water evolves at a relatively low temperature, usually below 100 °C (Figure 5). The mass loss corresponds to up to 30% of the sample weight, which means up to 14 water molecules per formula unit; 6 of them are coordinated. This easy dehydration is also related to the

Martı´nez-Garcia et al. porous framework of these materials, which facilitates the removed water molecules release from the solid. It seems that the dehydration process is accompanied by the decomposition of a small sample fraction (particularly the sample fraction of smaller particle size). The TG curve of the anhydrous sample shows a slight but continued weight loss up to a certain temperature where a pronounced evolution of a gaseous species is observed. For copper, this temperature is about 150 °C, while for Mn, Ni, Cd, and Zn it is above 225 °C. Such rapid weight loss for the anhydrous sample is not observed for the cobalt complex. This compound remains stable up to 350 °C. That information on the stability of the anhydrous phases, derived from the TG curves, should be taken cautiously. Even for the most stable compounds, a pronounced decomposition degree with formation of ferrocyanide is observed on prolonged heating, even at 140 °C. Infrared spectra of the annealed samples provide conclusive evidence in that sense. The formed ferrocyanides remain stable up to 350 °C. C. Heat-Induced Changes as Detected from IR Data. For all the studied compounds the ν(CN) band of the ferrous species, always present in the as-synthesized samples as an impurity, gains in intensity on heating at the expense of a weakening of the ferricyanide band (Figures 3 and 4). However, each of the studied compositions shows certain peculiarities on heating. When the copper compound, the less stable of the studied ferricyanides (Figure 5), is heated at 160 °C, the ferricyanide band at 2174 cm-1 disappears, and a broad band at 2106 cm-1 due to copper ferrocyanide formation appears (Figure 3A). For the nickel compound, one of the most stable ferricyanides, an increase in the intensity of the ferrocyanide band on heating was observed but accompanied by a band of lower frequency at 2040 cm-1 (Figure 3C). That low-frequency band was attributed to a ferrocyanide with CN groups unlinked at the N end, as a decomposition byproduct. For instance, cesium ferrocyanide has its ν(CN) vibration at that frequency of 2040 cm-1. When cobalt ferricyanide is heated, an inner charge transfer takes place, forming a stable ferrocyanide without decomposition, according to

Co3[Fe(CN)6]2 f (Co2+)3-x(Co3+)x[(FeIII)2-x(FeII)x(CN)12] f (Co2+)(Co3+)2[FeII(CN)6]2 This mixed valence state system in cobalt iron cyanide is characterized by a broad ν(CN) absorption at 2114 cm-1, 32 cm-1 above the expected frequency value for Co(II) ferrocyanide (Figure 3B). In the formed mixed valence system, the Co(III) atom adopts a low-spin configuration where the eg orbitals are free of electrons. This favors a large charge subtraction from the CN group, through its weakly antibonding 5σ orbital. That broad band at 2114 cm-1 probably results from contributions of Co(III)-NC-Fe(II) and Co(II)-NC-Fe(II) chains within the same framework. Such a charge-transfer process was not observed for the remaining ferricyanides. For manganese and cadmium, the heating process leads to the formation of anhydrous M2[Fe(CN)6] (M ) Mn, Cd) as the main reaction product, with a ν(CN) absorption band of a relatively low frequency, around 2030 cm-1 (Figures 4A and 4C). In contact with humid air these anhydrous phases incorporate water to form octahydrates, M2[Fe(CN)6]‚8H2O. That process is accompanied by a ν(CN) frequency shift up to approximately 2060 cm-1 and by the appearance of new absorption bands in the 900-700 cm-1 region. These new bands are assigned to rocking, F(H2O), and wagging, w(H2O), motions

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TABLE 1: Mo1 ssbauer Parameters at Room Temperature of the Studied Ferricyanide Samples in the Hydrated and Anhydrous States and of the Resulting Ferrocyanides by Thermal-Induced Decomposition sample

δa (mm/s)

∆ (mm/s)

Γ (mm/)]

A (%)

Mn3[Fe(CN)6]2‚xH2O anhydrous decomposition product (anhydrous) decomposition product (hydrated)

0.11 0.10 0.07 0.18

0.27 0.97 0.21

0.29 0.46 0.26 0.26

100

Cd3[Fe(CN)6]2‚xH2O anhydrous decomposition product (anhydrous) decomposition product (hydrated)

0.11 0.10 0.08 0.17

0.26 0.88 0.20

0.31 0.48 0.25 0.27

100 100 100

Fe(III) LS Fe(III) LS Fe(II) LS Fe(II) LS

Co3[Fe(CN)6]2‚xH2O anhydrous decomposition product (anhydrous)

0.12 0.10 0.17 0.10 0.17 0.11

0.43 0.97

0.31 0.56 0.48 0.39 0.47 0.41

100 100 79 21 78 22

Fe(III) LS Fe(III) LS Fe(II) LS Fe(III) LS Fe(II) LS Fe(III) LS

decomposition product (hydrated)

0.91 0.44

100 100

assignmentb Fe(III) LS Fe(III) LS Fe(II) LS Fe(II) LS

Ni3[Fe(CN)6]2‚xH2O anhydrous decomposition product

0.11 0.10 0.17

0.49 0.99

0.34 0.55 0.42

100 100 100

Fe(III) LS Fe(III) LS Fe(II) LS

Cu3[Fe(CN)6]2‚xH2O anhydrous decomposition product

0.10 0.09 0.16

0.54 0.91

0.31 0.47 0.32

100 100 100

Fe(III) LS Fe(III) LS Fe(II) LS

Zn3[Fe(CN)6]2‚xH2O (cubic) anhydrous (cubic) anhydrous (hexagonal) decomposition product

0.12 0.11 0.12 0.18

0.35 0.49 0.18

0.38 0.42 0.38 0.26

100 100 100 100

Fe(III) LS Fe(III) LS Fe(III) LS Fe(II) LS

a Isomer shift values are reported relative to sodium nitroprusside. The fitting error in the values of δ, ∆, and Γ remains below 0.01 mm/s. b LS ) low spin.

Figure 3. IR spectra (CN stretching region) of (A) Cu3Fe2 and of its thermal decomposition products, indicated is the annealing temperature; (B) Co3Fe2 and of annealed samples of this compound where the formation of low-spin Co(III) ferrocyanide is detected; (C) Ni3Fe2 before and after its annealing at 160 °C where the formation of two species of ferrocyanide are observed.

of the water molecules that are forming bridges between two neighboring M(II) atoms. In Mn(II) and Cd(II) ferrocyanides the metals are found in the following sequence -Fe-M-MFe-, where neighboring M metals remain linked by water bridges.25,27 When such aquo bridges are removed, the ν(CN) frequency decreases about 30 cm-1. Zinc ferricyanide also shows an interesting behavior. Its dehydration on heating induces a structural transformation from cubic to hexagonal, observed in the corresponding IR spectra as a ν(CN) frequency shift of 26 cm-1 (Figure 4B). In the tetrahedral coordination, the zinc atom takes part in a stronger interaction with the CN ligand, probably

Figure 4. IR spectra (CN stretching region) of (A) Mn3Fe2 and of its thermal decomposition products and of the Mn2[Fe(CN)6]‚8H2O (Mn2FeII) reference sample in its hydrated and partially dehydrated states; (B) Zn3Fe2 and of annealed samples of this compound where the decomposition process appears preceded by a structural transformation from cubic and hexagonal; (C) Cd3Fe2 before and after its annealing at 160 °C where the formed Cd(II) ferrocyanide appears in its anhydrous state.

subtracting a higher electron density from the CN group, mainly from its 5σ orbital. A reduction in the coordination number leads to an increase in the positive charge on the metal, which, in turn, increases the σ donation increasing the ν(CN) frequency.28 The anhydrous hexagonal phase has a relatively high stability on heating, because in this structure the two metal centers have saturated their coordination sphere through a strong bond to CN ligands, but it also decomposes on heating, forming a Zn(II) ferrocyanide where the Zn(II) atom, according to the

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Figure 5. Thermo-gravimetric curves for the studied transition metal ferricyanide samples. Cu3Fe2 appears as the less stable ferricyanide. In the cobalt complex a thermal-induced inner charge transfer with the formation of a highly stable low-spin Co(III) ferrocyanide takes place.

observed frequency (2099 cm-1) for the ν(CN) vibration, preserves the tetrahedral coordination. For an octahedral coordination that vibration falls at 2091 cm-1, for instance, in mixed zinc-cesium ferrocyanide (Fm3hm). According to the IR results discussed above, with the exception of the cobalt complex, the remaining ferricyanides decompose on heating, forming M(II) ferrocyanides as the main intermediate products. No formation of Ni(III), Cu(III), and Zn(III) ferrocyanides and of CoII-CN-FeIII and CuII-CN-FeIII species was detected. The existence of these last isomers is highly improbable since it requires a CN ligand flipping. Such a linkage isomerization process is only favored when, in the new configuration, the metal that is bonded to the C end has filled or partially filled the t2g orbitals and emptied the eg ones. This leads to a higher stability of the new solid network through a stronger metal-ligand bond. For instance, ferrous chromicyanide and manganicyanide undergo CN ligand flipping to form Cr(II) and Mn(II) ferrocyanides11 because in the former compounds the CN ligands are interacting with only three and four t2g electrons, respectively, while in the resulting ferrocyanides such an interaction takes place with six t2g electrons. This favors a stronger M-CN bond through a higher π back-donation from the iron atom toward the CN group. Such a CN flipping process is unfavorable for Co(II) and Cu(II) because they have (t2g)5(eg)2 and (t2g)6(eg)3 as electronic configurations, respectively, without the possibility of forming stable octahedral complexes with CN groups at the C end. In the case of cobalt, the option to achieve a higher stability is in the observed charge transfer to form Co(III) ferrocyanide where the cobalt atom remains bonded to the N end but changing its configuration to low spin. This contributes to a higher stability of the complex through a stronger bond of both metal centers to the CN ligands. Once ferricyanides have decomposed and ferrocyanides are formed, the last ones remain stable up to 350 °C where evidence of their final decomposition appears. The liberated CN- groups reduce the transition metals involved to their metallic state. Such end solid byproducts have been well-identified and characterized from XRD, Mo¨ssbauer, and magnetic data.5-8 D. Mo1 ssbauer Spectra. The Mo¨ssbauer data obtained provide additional insight on the heat-induced changes in the studied molecular magnets. On the ferricyanide sample dehydration, an increase in the quadrupole splitting (∆) value was observed (Figure 2b, Table 1). In the crystal structure of the M3Fe2 complexes studied, the outer metal (M) has a mixed coordination sphere formed by four N atoms from CN ligands plus two water molecules. The CN group (at the N end) and the water molecule

Martı´nez-Garcia et al. have different bonding properties. From this fact, the coordination environment of this metal is slightly distorted and since the metal centers remain strongly linked through CN ligands, such local distortion is partially transmitted to the iron atom environment. This explains the observed quadrupole splitting value in ferricyanides. When these two coordinated water molecules are removed on dehydration, all of the interaction of the outer metal is with the CN groups, increasing the charge subtraction from the ligands through their 5σ orbitals. This induces a higher π back-donation from the iron atom toward the CN groups, creating certain charge asymmetry around the iron atom, in addition to the mentioned local deformation. This is observed as a pronounced increase in the value of ∆ on dehydration. When the anhydrous sample is heated at a higher temperature, an increasing ferrocyanide fraction appears that becomes dominant from a certain temperature where the sample decomposes as a whole (Figures 1-5). The formed ferrocyanide is usually accompanied by a new secondary high-spin Fe(II) phase, detected as a weak broad quadrupole doublet (Figure 2c). A similar doublet has been observed during the pyrolysis of Prussian blues, and it has been tentatively assigned to a decomposition byproduct or to the formation of ferrous ferrocyanide.29 When the sample is heated at a higher temperature, a partial decomposition of the previously formed ferrocyanide is favored, and metallic species appear5-8 that oxidize when they are exposed to air, forming iron oxides (Figure 2d). In the pyrolyzed samples, other decomposition byproducts could also be detected. For the particular cases of Mn(II) and Cd(II) ferricyanides, the Mo¨ssbauer spectra of the formed ferrocyanides appear as practically an unresolved doublet with an atypically low value of the isomer shift (Figure 2c, Table 1). When the obtained ferrocyanide sample rehydrates, that unresolved doublet is replaced by a single line with the isomer shift value expected for a ferrocyanide (Figure 2d). The rehydration process leads to the formation of M2[Fe(CN)6]‚8H2O (monoclinic, space group P21/n). Such an assignment agrees with the observed behavior for pure M2[Fe(CN)6]‚8H2O samples on dehydration (Figure 2, inset). For the zinc complex, the heat-induced structural change, from cubic to hexagonal, is also detected in the obtained Mo¨ssbauer spectra. In contrast to the cubic phase, a very low value of quadruple splitting (∆) indicates the formation of the hexagonal modification. In the R3hc structure the Zn atom is coordinated to only four N atoms in a tetrahedral configuration, which favors a relatively high symmetry for the charge distribution around the iron nucleus. E. Head-Induced Structural Changes. Figure 1 shows a sequence of powder XRD patterns for a Cu(II) ferricyanide sample submitted to heat treatment. On the reduction reaction with the formation of ferrocyanide, a progressive peak shift to higher values of 2θ is observed, indicating the occurrence of a cell contraction (Figure 1, inset A) but preserving the material framework. On being heated, within the material framework CN- groups are liberated, which reduce neighboring iron atoms to form Cu(II)-NtC-Fe(II) chains. Within the sample bulk, this process takes place randomly, and the net effect is appreciated as a cell contraction since in the formed ferrocyanide the C-Fe interatomic distance is smaller than that in the starting ferricyanide. Such behavior on heating was observed for Ni(II), Cu(II), and Zn(II) ferricyanides. For the Co(II) complex the cell contraction and the solid solution formation are related to the already mentioned inner charge transfer. For these four metals the formed ferrocyanide preserves the crystal framework of the parent ferricyanide, and the partially reduced material appears as a solid solution of M-NtC-Fe(III) and M-Nt

Thermal-Induced Changes in Molecular Magnets C-Fe(II) chains within the same network. Except for Co, in the formed ferrocyanides a fraction of M(II) atoms probably remains occupying interstitial sites as charge balance cations. For Mn(II) and Cd(II) ferricyanides, from a certain degree of decomposition the formation of M2[Fe(CN)6] is detected, which incorporates water to form M2[Fe(CN)6]‚8H2O (monoclinic P21/ n).25,27 It seems that the accompanied secondary byproducts during the heat-induced reduction process of ferricyanides to ferrocyanides usually have low crystalline order. No reflections attributed to such decomposition byproducts were observed in the corresponding powder XRD patterns. When the heating was carried out in a vacuum and the powder XRD pattern was recorded under this condition, a significant cell contraction on the crystal water removal was observed (Figure 1, inset B). This is equivalent to a shortening of the M-NtC-Fe-CtN-M chain length and to a stronger interaction among the metals through the ligands. This corroborates the evidence discussed above from Mo¨ssbauer spectra of anhydrous ferricyanides, observed as a large increase in the value of ∆ on the water removal process. F. Heat-Induced Changes in the Magnetic Properties. In ferricyanides, the magnetic order is established by an indirect exchange interaction between the metallic centers through the CN bridges. According to the results from IR, Mo¨ssbauer, and XRD data discussed above, once the studied ferricyanides have decomposed, ferrocyanides are obtained where poor magnetic order is expected. In ferrocyanides, all of the magnetic interaction proceeds from the M metal centers because the iron(II) remains in a low-spin state (S ) 0). However, as will be discussed below, for Mn an interesting magnetic interaction was found. For the studied ferricyanides, three different magnetic behaviors are expected. Zinc and cadmium ferricyanides may show a weak anti-ferromagnetic order. For Ni and Cu ferricyanides, a ferromagnetic order may be present, while, for Mn and Co, mixed ferro- and anti-ferromagnetic interactions are expected for a net ferrimagnetic order. The estimated values for the Curie (Tc) and Curie-Weiss (θCW) temperatures, Curie constant (C), Ne´el temperature (TNe´el), residual magnetization (MR), magnetization at 5173 kA/m (6.5 T) (M6.5T), and coercive field (HC) for the studied transition metal ferricyanide samples are reported in Table S2 of the Supporting Information. Zinc ferricyanide is an example of the first type of behavior. In this complex the superexchange is established through the overlapping of identical t2g5 orbitals from iron atoms, and it has anti-ferromagnetic nature as a result of the Pauli exclusion principle.30 As has been already discussed, the heat-induced dehydration of zinc ferricyanide induces a structural transformation to form the hexagonal R3hc phase. In both crystalline phases, cubic and hexagonal, the overlapping density is small because the superexchange interaction is established among the second nearest neighbors, an unfavorable situation for a magnetic interaction. In the cubic structure, the FeIII ions appear at a distance of 10.2 Å, and at 7.4 Å for the hexagonal one. From this fact the stronger magnetic interaction could be expected for the R3hc phase. From a linear fitting, according to the Curie-Weiss law χ-1 ) (T - θCW)/C, of the reciprocal mass susceptibility (1/χp) vs temperature curve, the values for the Curie constant C and for the Curie-Weiss temperature (θCW) in the zinc ferricyanide phases were estimated. The obtained θCW values are -5.1(1) and -3.7(1) K for cubic and hexagonal zinc ferricyanide, respectively. The negative sign of θCW corresponds to the

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Figure 6. Field-cooled magnetization curves as a temperature function, (MFC(T)), at an applied field of 0.4 kA/m (5 Oe), for Cu3Fe2 and Ni3Fe2 before and after their annealing. Inset: MFC vs T curves for the Ni(II) complex samples. The samples are (a) the as-synthesized ferricyanides, (b) annealed at 80 °C, (c) annealed at 100 °C, (d) annealed at 120 °C, (e) annealed at 140 °C, and (f) annealed at 160 °C.

presence of a net anti-ferromagnetic interaction, and its relatively low absolute value indicates that it is weak. The weaker interaction corresponds to the hexagonal phase, although the metallic centers FeIII (t2g1, S ) 1/2) are located at a shorter distance from one another. It seems other factors are contributing to a reduction in the effective interaction between the unpaired electrons on the iron atoms, probably the occurrence of certain deviation of the Fe-CtN-Zn-NtC-Fe chain from the linearity, related to the tetrahedral coordination of the Zn atom to the NC ligands. For a tetrahedral coordination the N-Zn-N angle is >90 °. This reduces the overlapping density from neighboring FeIII ions. Such deviation is absent in the cubic phase where the N-Zn-N angle is 90°. The cubic structure is formed by a three-dimensional network of Fe-CtN-Zn-Nt C-Fe linear chains. This favors an optimal overlapping of t2g orbitals from neighboring FeIII atoms and a higher θCW value. The second studied magnetic behavior corresponds to Ni and Cu ferricyanides. These compounds have ferromagnetic order because the magnetic interaction results from orthogonal orbitals. Low-spin Fe (III) has an unpaired electron in π orbitals, t2g5 (S ) 1/2), while, nickel(II) and copper(II) have only unpaired electrons in σ orbitals, eg2 and eg3, respectively. Figure 6 shows the field-cooled magnetization (MFC(T)) curves as a function of temperature, at an applied field of H ) 0.4 kA/m (5 Oe), for Cu(II) and Ni(II) ferricyanides before and after their annealing at 80, 100, 120, 140, and 160 °C. The reduction of ferricyanide to ferrocyanide on heating is accompanied by a decrease in the measured value for MFC. Below 20 K for copper and 25 K for nickel, the magnetization saturates in the characteristic way for a ferromagnetic material. From the maximum in the dMFC/dT curve, the critical (Curie) temperature (Tc) for the onset of longrange ferromagnetic order was estimated, which resulted in 17.9 and 18.9 K for Cu(II) and Ni(II) ferricyanide, respectively. As has been discussed above, for nickel and copper and for cobalt and zinc as well, the reduction of ferricyanide to ferrocyanide on heating takes place through the formation of a solid solution of ferric and ferrous species. From this fact, as the amount of Ni(II) or Cu(II) ferrocyanide in the annealed sample increases, the material shows a ferrimagnetic behavior as a net effect from combined ferro- and anti-ferromagnetic

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Figure 7. Field-cooled magnetization curves as a temperature function, (MFC(T)), at an applied field of 0.4kA/m (5 Oe), for Mn3Fe2 before and after its annealing. The samples are (a) as-synthesized Mn(II) ferricyanide, (b) annealed at 100 °C, (c) annealed at 120 °C, and (d) annealed at 140 °C (d). Inset: The corresponding temperature dependence of the reciprocal mass susceptibility (1/χp(T)).

interactions, the last ones among M(II) ions in the formed ferrocyanide. For the nickel compound, the anti-ferromagnetic contribution is detected from an annealing of 120 °C, with a Ne´el temperature of 22.9 K, which increases up to 24 K for the samples annealed at 140 and 160 °C. For the copper complex, which is a less stable compound, the anti-ferromagnetic contribution is detected from an annealing temperature of 100 °C. For the sample at 140 °C, the resulting magnetic order is very weak with a θCW value of only 2.3 K. Cobalt ferricyanide shows a behavior on heating very similar to that observed for the nickel analogue but with a lower stability and with an anti-ferromagnetic contribution that is present from the as-prepared sample. The heat-induced charge transfer favors a fast dilution effect of the Co(II)-NtC-Fe(III) chains and a progressive decrease in the resulting magnetic order. From an initial Tc value of 13.5 K, with a θCW of 11.8 K (for the nonannealed sample), these parameters have fallen to 4 and 0.4 K, respectively, for the sample heat-treated at 120 °C. Manganese ferricyanide showed a unique behavior on heating. The ferrimagnetic order observed for the as-prepared compound appears combined to a relatively strong anti-ferromagnetic contribution for samples annealed above 100 °C. Such a dominant anti-ferromagnetic contribution is observed as a relatively large increase in the zero field-cooling magnetization (MFC) (Figure 7), which reaches a maximum value and then falls on sample cooling. With a higher annealing temperature, the more pronounced is that effect. That increasing antiferromagnetic contribution was assigned to neighboring Mn(II) atoms in the formed Mn2[Fe(CN)6]‚8H2O ferrocyanide. Unlike the remaining ferrocyanides, in this compound two neighboring M atoms remain bridged by two aquo bridges,21,25,27 which allows a strong anti-ferromagnetic interaction between them through the oxygen atoms of the water molecules. These Mn(II) atoms remain at a relatively short distance (3.67 Å).27 This favors a significant electron cloud overlapping, although the Mn-O-Mn angle, close to 100°, is an unfavorable factor for a strong magnetic interaction. Such an assignment for the dominant anti-ferromagnetic contribution in annealed Mn(II) ferricyanide samples was corroborated from a parallel study in a pure Mn2[Fe(CN)6]‚ 8H2O sample. In this compound a strong anti-ferromagnetic

Martı´nez-Garcia et al.

Figure 8. Reciprocal mass susceptibility (1/χp) versus temperature (T) curves for a pure Mn(II) ferrocyanide, Mn2[Fe(CN)6]‚8H2O, sample. Inset: Coordination environment for the Fe and Mn atoms in this compound. Two neighboring Mn atoms appear linked through two aquo bridges favoring their anti-ferromagnetic interaction.

interaction between Mn(II) atoms with an estimated θCW of -3.5 K was observed (Figure 8). Such coupling disappears when the sample is dehydrated, and it is then restored when it is rehydrated. To the best of our knowledge, an analogue magnetic interaction between two manganese atoms through aquo bridges within a basically nonmagnetic matrix has not previously been reported. Conclusions On being heated divalent transition metal ferricyanides reduce to ferrocyanides. No occurrence of linkage isomerism was observed. The formation of Ni(III), Cu(III), and Zn(III) hexacyanoferrates(II) was also discarded. Only for cobalt, an inner charge transfer with formation of low-spin Co(III) ferrocyanide was detected. Except for the case of Mn(II) and Cd(II) ferricyanides, the thermal decomposition and the accompanied reduction process led to the formation of a solid solution of ferric and ferrous species within the same framework. For Mn(II) and Cd(II), the reduction process on heating leads to the formation of an anhydrous M2[Fe(CN)6] phase, which in contact with humid air incorporates water to form octahydrates, M2[Fe(CN)6]‚8H2O. The hydration process is relatively fast for the manganese complex and slow for the cadmium one. The formed Mn2[Fe(CN)6]‚8H2O shows a strong anti-ferromagnetic interaction attributed to neighboring Mn(II) atoms linked by aquo bridges. Upon being heated, cubic zinc ferricyanide dehydrates and undergoes a structural transformation to a hexagonal phase where the Zn(II) atom is found tetrahedrally coordinated to N ends of CN ligands. In this hexagonal phase the FeIII atoms are involved in a weak anti-ferromagnetic interaction due to the tetrahedral coordination for the Zn atom. The crystal water removal in the studied family of molecular magnets has a pronounced effect on the metal center interaction that is detected through the Mo¨ssbauer data. Acknowledgment. R.M. thanks the support provided by the CLAF-ICTP program for his Ph.D. studies. The access to the LNLS synchrotron radiation facilities is gratefully recognized. The support from FAPESP and CNPq (Brazilian agencies) is acknowledged. The authors thank C. Vazquez from IIM-UNAM for the TG data acquisition.

Thermal-Induced Changes in Molecular Magnets Supporting Information Available: Frequency values for the CN stretching vibration, ν(CN), in the studied transition metal ferricyanides and in their heat-induced decomposition products, the values obtained for the Curie (Tc) and CurieWeiss (θCW) temperatures, Curie constant (C), Ne´el temperature (TNe´el), residual magnetization (MR), magnetization at 5173 kA/m (6.5 T) (M6.5T), and coercive field (HC) in the studied transition metal ferricyanides samples before and after their annealing, and the pseudo-Lorentzian line shape function used for the Mo¨ssbauer spectra fitting. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Dujardin, E.; Mann, S. AdV. Mater. 2004, 16, 1125. (b) Beltran, L. M. C.; Long, J. R. Acc. Chem. Res. 2005, 38, 325. (c) Escax, V.; Bleuzen, A.; Itie´, J.; Munsch, P.; Varret, F.; Verdaguer, M. J. Phys. Chem. B 2003, 107, 4763. (2) (a) Ferlay, S.; Mallah, T.; Ouahes, R.; Veillet, P.; Verdaguer, M. Nature 1995, 378, 701. (b) Holmes, S. M.; Girolami, G. S. J. Am. Chem. Soc. 1999, 121, 5593. (3) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 272, 704. (4) (a) Yokoyama, T.; Kiguchi, M.; Ohta, T.; Sato, O.; Einaga, Y.; Hashimoto, K. Phys. ReV. B 1998, 58, 8257. (b) Yoshizawa, K.; Mohri, F.; Nuspl, G.; Yamabe, T. J. Phys. Chem. B 1998, 102, 5432. (c) Gutsev, G.; Reddy, B.; Khana, S.; Rao, B.; Jena, P. Phys. ReV. B 1998, 58, 14131. (d) Nishino, M.; Yamaguchi, K.; Miyashita, S. Phys. ReV. B 1998, 58, 9303. (e) Bleuzen, A.; Lomenech, C.; Escax, V.; Villain, F.; Varret, F.; Cartier, C.; Verdaguer, M. J. Am. Chem. Soc. 2000, 122, 6648. (f) Pejakovic, D.; Manson, J.; Miller, J.; Epstein, A. Phys. ReV. Lett. 2000, 85, 1994. (g) Kawamoto, T.; Asai, Y.; Abe, S. Phys. ReV. Lett. 2001, 86, 348. (h) Shimamoto, N.; Ohkoshi, S.; Sato, O.; Hashimoto, K. Inorg. Chem. 2002, 41, 678. (i) Sato, O. Acc. Chem. Res. 2003, 36, 692. (j) Escax, V.; Bleuzen, A.; Itie´, J.; Munsch, P.; Varret, F.; Verdaguer, M. J. Phys. Chem. B 2003, 107, 4763. (k) Sato, O. J. Photochem. Photobiol., C 2004, 5, 203. (5) Ng, C. W.; Ding, J.; Gan, L. M. J. Solid State Chem. 2001, 156, 400. (6) Ng, C. W.; Ding, J.; Wang, L.; Gan, L. M.; Quek, C. H. J. Phys. Chem. A 2000, 104, 8814.

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