© Copyright 1997 by the American Chemical Society
VOLUME 101, NUMBER 20, MAY 15, 1997
LETTERS Cation-Driven Electron Transfer Involving a Spin Transition at Room Temperature in a Cobalt Iron Cyanide Thin Film O. Sato,† Y. Einaga,‡ T. Iyoda,† A. Fujishima,*,‡ and K. Hashimoto*,†,‡ Kanagawa Academy of Science and Technology, Tokyo Institute of Polytechnics, 1583 Iiyama Atsugi, Kanagawa 243-02, Japan, and Department of Applied Chemistry, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan ReceiVed: January 8, 1997; In Final Form: March 17, 1997X
The electronic structure of a cobalt iron cyanide thin film, Na0.4Co1.3[Fe(CN)6]5H2O, was switched from FeIII(low spin)-CN-CoII(high spin) to FeII(low spin)-CN-CoIII(low spin) by exchanging cations in interstitial sites from Na+ to K+. The reverse process was also induced by replacing Na+ with K+. That is, the alkali cations in the interstitial sites dominate the electronic and spin states of the host compound. This type of switching phenomenon can be achieved by choosing a compound with both electronic degeneracy and zeolitic properties.
The development of molecular units and molecule-based assemblies which respond to external triggers holds considerable interest as a means of realizing molecular devices.1-6 One of the current topics in this field is the reversible control of physical properties in solid state compounds. Several compounds which exhibit bistability in their properties in a given range of external conditions (pressure, temperature, light, etc.) have been reported.7-10 Another factor with which to control the functions of molecular assemblies is the immediate chemical environment. Examples can be found in biological molecular systems, which are intrinsically sensitive to their chemical surroundings, e.g., concentrations of K+ and Na+ ions.11 Here we describe a reversible metal to metal electron transfer involving a spin transition driven by the exchange of guest cations in a cobalt iron cyanide thin film. Specifically, the exchange of Na+ (crystal diameter ) 1.9 Å) and K+ (2.7 Å) in interstitial sites results in a change in the electronic and spin states of the host †
Kanagawa Academy of Science and Technology. The University of Tokyo. X Abstract published in AdVance ACS Abstracts, April 15, 1997. ‡
S1089-5647(97)00145-4 CCC: $14.00
compound from FeIII(low spin)-CN-CoII(high spin) to FeII(low spin)-CN-CoIII(low spin) at around room temperature (Figure 1). Materials with electronic degeneracy, including those with mixed valence and valence tautomerism, are sensitive to their environmental conditions. It has been reported that the replacement of counterions in a mixed valence material during synthesis strongly influences the potential energy surface of the compound, which leads to a modification of the intramolecular electron-transfer process.12 Our strategy to control the electronic properties of molecule-based materials is to utilize compounds with such an extreme environmental sensitivity and with zeolitic character. That is, the choice of an appropriate compound with electronic degeneracy and with the ability to undergo ion exchange allows us to replace the guest ions even after the preparation of the material by exposure to different solution environments, potentially facilitating the reversible control of the electronic properties in the solid state. After a thorough exploration of existing zeolitic and electronically labile materials along these lines, we have found a prominent example in a cobalt iron cyanide. © 1997 American Chemical Society
3904 J. Phys. Chem. B, Vol. 101, No. 20, 1997
Figure 1. Schematic illustration of a cation exchange-induced electron transfer involving a spin transition and unit cell of cobalt iron cyanide (interstitial ions, water molecules, and defects have been omitted for clarity). The two states, FeIII(low spin)-CN-CoII(high spin) and FeII(low spin)-CN-CoIII(low spin), can be switched by the replacement of interstitial alkali cations (Na+ and K+) in cobalt iron cyanide. Note that a certain number of [Fe(CN)6] sites are vacant. The number of the vacancy depends on the stoichiometry.
The cobalt iron cyanide thin film (about 0.05-0.1 µm thick) was electrochemically synthesized on a Pt electrode under potentiostatic conditions at -0.4 V versus a saturated calomel electrode (SCE) in an aqueous solution containing 0.5 mmol L-1 K3FeIII(CN)6, 0.5 mmol L-1 CoII(NO3)2, and 1 mol L-1 NaNO3.13,14 This compound and its oxidized form were subjected to several types of physical characterization.15 Elemental analysis yields the formula Na1.4Co1.3[Fe(CN)6]5H2O [Anal. Calcd: Na, 7.83%; K, 0%, Co, 18.6%; Fe, 13.6%; C, 17.6%; N, 20.5%; H, 2.5%. Found: Na, 6.90%; K, 0.44%, Co, 17.7%; Fe, 12.8%; C, 18.8%; N, 20.0 %; H, 2.4% (Na:K:Co: Fe ) 1.31:0.05:1.31:1)], hereafter designated as compound 1. The X-ray powder diffraction pattern was consistent with a facecentered cubic structure (unit cell parameter ) 10.31 Å). Infrared (IR) spectral measurements in the region from 2000 cm-1 to 2200 showed a strong peak at about 2100 cm-1 (Figure 2). This peak is ascribable to CN stretching in the FeII-CNCoII structure. An 57Fe Mo¨ssbauer spectrum showed a single peak with an isomer shift of -0.07 ( 0.01 mm s-1, indicating the presence of low-spin FeII. This compound exhibits a reversible electrochemical process in 1 mol L-1 NaCl aqueous solution, with the anodic and cathodic voltammetric peaks being observed at around 0.45 V versus SCE. An 57Fe Mo¨ssbauer spectrum obtained after oxidation shows that the single peak decreased significantly and that a new doublet with an isomer shift of -0.14 ( 0.01 mm s-1 and a quadrupole splitting of 0.56 mm s-1 appeared. This means that FeII is oxidized to lowspin FeIII. The IR spectrum shows that the CN stretch at 2100 cm-1 decreased and a new peak at around 2160 cm-1 appeared (Figure 2). The new peak is ascribed to CN stretching in the FeIII-CN-CoII structure. It is reported that the CN stretching of CoII3[FeIII(CN)6]2 is observed at around 2160 cm-1,16 which is consistent with our assignment. Elemental analysis after oxidation yielded the formula Na0.4Co1.3[Fe(CN)6]5H2O [ Anal. Calcd: Na, 2.37%; K, 0%, Co, 19.8%; Fe, 14.4%; C, 18.6%; N, 21.7%; H, 2.6%; Cl, 0%. Found: Na, 2.32%; K, 0.17%, Co, 17.4%; Fe, 13.1%; C, 21.1%; N, 20.7%; H, 2.4%; Cl, 0.5% (Na:K:Co:Fe ) 0.43;0.02:1.26:1)], hereafter designated as compound 2. Thus, the electrochemical oxidation reaction is
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Figure 2. Changes in the CN stretching frequency induced by electrochemical treatment and cation exchange: (a) Na1.4Co1.3[Fe(CN)6]5H2O synthesized electrochemically (compound 1), (b) after electrochemical oxidation in 1 mol L-1 NaCl solution (compound 2), (c) after cation exchange from Na+ to K+ in 1 mol L-1 KCl solution (compound 3), (d) after cation exchange from K+ to Na+ in 1 mol L-1 NaCl solution (compound 2). The increase of the CN stretching peak around 2100 cm-1 indicates that a fraction of the compound is reduced to FeIICN-CoII.
Figure 3. UV-vis spectra before (compound 2) and after (compound 3) K+ treatment.
expressed by Na1.4CoII1.3[FeII(CN)6] f Na0.4CoII1.3[FeIII(CN)6] + Na+ + e(1) The powder X-ray diffraction pattern shows that the unit cell parameter is 10.33 Å. When compound 2 was immersed in a solution containing 1 mol L-1 K+ for about 5 s at 313 K, a drastic change in the electronic state was observed. In the UV-vis region, the peak at around 400 nm disappeared and a peak at around 550 nm increased (Figure 3). The IR spectrum shows that the CN stretching peak at about 2160 cm-1 shifts to around 2135 cm-1 (Figure 2, curve c). The X-ray powder diffraction pattern showed that the unit cell parameter decreased to 9.96 Å. The initial electronic state was substantially restored when the sample was immersed in a solution containing 1 mol L-1 Na+ for about 60 s at 313 K. Elemental analysis after the K+ treatment yielded the formula K0.4Co1.3[Fe(CN)6]5H2O [Anal. Calcd: Na, 0%; K, 3.97%, Co, 19.4%; Fe, 14.2%; C, 18.3%; N, 21.3%; H, 2.6%, Cl, 0%. Found: Na, 0.18%; K, 4.62%, Co, 17.7%; Fe, 13.5%; C, 20.6%; N, 21.0%; H, 2.0%, Cl, 0.50% (Na:K:Co:Fe ) 0.03: 0.49:1.24:1)], hereafter designated as compound 3. This shows that the Na+ ions were replaced by K+ ions. An 57Fe Mo¨ssbauer spectrum showed that a singlet with an isomer shift of -0.08 ( 0.01 mm s-1 appeared after the cation exchange, indicating the presence of low-spin FeII. Thus, the IR peak at around 2135 cm-1 can be assigned to CN stretching in the FeII-CN-CoIII structure. Hester et al. reported that the stretching of the bridging
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J. Phys. Chem. B, Vol. 101, No. 20, 1997 3905 most stable ones for compound 2 and 3, respectively. Thus, we can conclude that the change of the electronic states driven by the cation exchange from Na+ to K+ at 313 K is expressed by
CoII(t2g5eg2, S ) 3/2)-NC-FeIII(t2g5, S ) 1/2) f CoIII(t2g6eg, S ) 0)-NC-FeII(t2g6, S ) 0) CoII(t
Figure 4. χMT versus T plots in the cooling mode of compound 2 (b) and compound 3 (O). Electron transfer involving a spin transition occurs at around 260 K for compound 2. A similar effect appears to be observed at around 340 K for compound 3. However, the magnetization, which increases above 340 K in the heating mode, decreases again, probably due to dehydration or degradation of the compound.
CN in a [(NC)5FeII-CN-CoIII(CN)5]6- ion is observed at 2130 cm-1,17 which supports our assignment. The broad absorption around 550 nm observed after K+ treatment can be assigned to a charge transfer from FeII to CoIII. Note that the dimetal complexes, [(NC)5FeII-CN-CoIII(CN)5]6- and [(NC)5FeIICN-CoIII(ethylenediaminetetraacetate)]6- , have CT bands at 395 and at 565 nm, respectively.17,18 These observations indicate that the exchange of alkali cations induced an electron transfer from CoII to FeIII in the compound. It is noteworthy that when compound 1 is oxidized in 1 mol L-1 KCl solution, the IR peak continuously shifts from 2100 cm-1 toward 2135 cm-1, with no isosbestic points. The preferential oxidation of Co in the presence of K+ is consistent with the results of the cation exchange. It should also be noted that the electrochemical redox process involving the spin transition can be repeated. Magnetic properties were investigated with a superconducting quantum interference device (SQUID) magnetometer. The product of the molar magnetic susceptibility and temperature, χ T, is shown as a function of T for compounds 2 and 3 in M Figure 4. The χMT product of compound 2 decreased slightly from 3.3 cm3 mol-1 K at 340 K to 2.8 cm3 mol-1 K at 280 K, then dropped abruptly to 1.3 cm3 mol-1 K at 230 K, and reached 0.8 cm3 mol-1 K at 20 K. In the UV-vis region, the peak at around 450 nm disappeared and a peak at 550 nm significantly increased at low temperature (
