Synthesis and Properties of a Thermochromic Spin Crossover FeII

Laboratory Experiment pubs.acs.org/jchemeduc

Synthesis and Properties of a Thermochromic Spin Crossover FeII Complex: An Undergraduate Coordination Chemistry Laboratory Experiment Anne Vallée,† Cyrille Train,‡,∥ and Cécile Roux*,† †

Laboratoire de Chimie de la Matière Condensée de Paris, UMR CNRS 7574, UPMC Univ Paris 06, Collège de France, 11 place Marcelin Berthelot, F-75231 Paris Cedex 05, France ‡ Laboratoire National des Champs Magnétiques Intenses, UPR CNRS 3228, Université Joseph Fourier, F-38042 Grenoble, France ∥ Institut Universitaire de France (IUF), 103, bd Saint-Michel, F-75005 Paris, France S Supporting Information *

ABSTRACT: In this third-year undergraduate experiment, a coordination complex [Fe(NH2trz)3]Br2·H2O (NH2trz is 4-amino-1,2,4-triazole, C2H4N4) was synthesized and characterized by IR and UV−visible spectroscopies. This compound exhibits a spectacular reversible purple-white thermochromic transition upon heating−cooling with a large hysteresis centered at room temperature. The thermochromic transition is related to the spin crossover (SCO) from the low-spin state (LS, purple, S = 0, diamagnetic) to the high-spin state (HS, white, S = 2, paramagnetic). During this experiment, students discovered synthetic methods, such as anion metathesis and Ostwald ripening, and physical properties, such as magnetic properties, thermochromism, and bistability phenomenon, altogether leading to possible applications in optical-data storage and display. KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Laboratory Instructions, Hands-On Learning/Manipulative, Coordination Compounds, Crystal Field/Ligand Field Theory, IR Spectroscopy, UV-Vis Spectroscopy

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SCO can also present a thermal hysteresis leading to bistable systems, making these materials of large interest for use in datastorage devices.13 To our knowledge, only one previous experiment has been reported concerning the study of SCO complexes that focused on magnetic measurement.6 Other experiments deal with thermochromic compounds;1,3,10,11 however, they do not use SCO compounds and do not discuss corresponding applications. [Fe(NH2trz)3]Br2·H2O (NH2trz is 4-amino-1,2,4-triazole; Figure 1A) was selected because of its ease of synthesis and its reversible thermochromic SCO transition with a thermal hysteresis centered at room temperature (RT). These conditions are ideally suited (i) to illustrate the general concepts of an advanced undergraduate inorganic chemistry course and (ii) to set up this experiment in an inorganic chemistry laboratory with RT characterizations by usually available techniques. Following the structure published with the nitrate as a counterion, the compound should consist of linear chains where the Fe(II) ions are triply bridged by the N(1)− N(2) atoms of the triazole ligands (Figures 1B and 1C).14 A new, easy, and inexpensive synthesis of the complex [Fe(NH2trz)3]Br2·H2O was developed that was based on Lavrenova et al. synthesis of similar complexes [Fe(NH2trz)3]-

n important goal in modern chemical education is to provide students with experiments that reinforce fundamental concepts learned in the classroom and simultaneously address potential applications. In laboratory experiments of coordination chemistry, undergraduate students synthesize complexes and characterize their properties using the general concepts of synthesis, vibrational and electronic spectroscopies, and ligand field theory.1−11 The objectives are generally to study electronic configurations, optical properties, and magnetic properties as a function of metal identity,2,4,8,9 ligand field strength, 2,8,9 and geometry of the complexes.1,3,5,7,10,11 However, the potential applications are scarcely discussed. It is worth noticing that introducing considerations related to applied science in an inorganic experimental work requires supplementary knowledge, for instance, in reactivity, kinetics, and materials science. General concepts are then prerequisites rather than reinforced notions. Spin crossover (SCO) complexes are essentially d4−d7 firstrow transition-metal octahedral complexes whose potential applications can be basically understood using the general concepts listed above. These complexes undergo a reversible spin change from high spin (HS) to low spin (LS) as a function of external stimuli, such as temperature, pressure, or light.12 The spin-state change is associated with modifications of magnetic properties and often with thermochromic properties. If there are sufficient interactions between the SCO units, the © XXXX American Chemical Society and Division of Chemical Education, Inc.

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dx.doi.org/10.1021/ed4000487 | J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

to bypass this problem. The former is a means to easily follow the SCO, whereas the latter allows both spin states to be determined at RT for its characterization. Furthermore, these approaches open the possibility to illustrate experimentally the principle of optical-data storage. The objectives of this experimental work were the following: (a) Synthetize the [Fe(NH2trz)3]Br2·H2O complex and characterize it by IR spectroscopy. (b) Highlight the thermochromic effects by visual observations upon heating−cooling treatments. (c) Evidence the bistability effects by visual observations upon heating−cooling treatments. (d) Observe qualitatively the magnetic properties of both colored forms and relate the observed colors to the spin states of the compound. (e) Understand the principle of optical-data storage by using electronic spectroscopy. (f) Put into practice general concepts of ligand field theory, electronic configurations, spectroscopic terms, and electronic transitions to interpret the electronic spectra. This experiment was developed for the advanced third-year bachelor level. It has been performed every year since 2005 with groups of 12 students (at most) working in pairs.

Figure 1. (A) Structure of 4-amino-1,2,4-triazole, NH2trz. (B) Detailed and (C) schematic structures for [Fe(NH2trz)3]Br2·H2O, adapted from ref 14.

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X2·H2O (X = I−, NCS−, Cl−).15,16 As the thermally induced SCO properties (hysteresis position and width) are sensitive to synthetic conditions, solvents, anions, and ligands,15,16 magnetic measurements were performed by the authors on this compound. The thermal variations of the χM·T product, where χM is the molar paramagnetic susceptibility corrected from diamagnetic contributions17 and T, the temperature, are shown in Figure 2 (right scale). Because the LS form is

EXPERIMENTAL DETAILS The synthetic details, typical results acquired by students, and supplementary data are provided in the Supporting Information. The entire experiment requires two 3-h lab periods, the first being devoted to synthesis and the second to product collection and characterization. This experiment can be condensed into one 4-h period beginning at the second step of the synthesis or characterizing the products previously synthesized by other students. The latter procedure was chosen so that the students can understand the relationship between the color and the spin state prior to seeing the numerous color changes during the second step of the synthesis. Students record all observations and write the answers to the questions (available in the Supporting Information) in their laboratory book.

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SYNTHESIS OF [Fe(NH2trz)3]Br2·H2O The main features of the synthesis are given below. Metathesis and Ostwald ripening are the important points to discuss with students. In the first step, an aqueous solution of iron(II) bromide was obtained starting from iron(II) sulfate, a common iron(II) salt (Figure 3), by performing an anion metathesis according to the following reaction:

Figure 2. Thermal variations of χM·T (filled squares) for [Fe(NH2trz)3]Br2·H2O, recorded by the authors at 1 K/min on a SQUID magnetometer available in research units, and calculated γHS (solid lines); T↑ and T↓ are the temperatures corresponding to γHS = 50% in heating and cooling modes, respectively.

Fe2 +(aq) + SO4 2 −(aq) + Ba 2 +(aq) + 2Br −(aq) ⇌ BaSO4 (s) + Fe 2 +(aq) + 2Br −(aq)

diamagnetic, the χM·T product gives a direct access to the HS molar percentage γHS(%)18 in the sample (Figure 2; left scale). The first-order LS ↔ HS transition occurred with a hysteresis of 35 K centered around 297 K for a sweeping rate of 1 K/min (Figure 2).15,16,19 These accurate magnetic measurements are not easily accessible with usual teaching laboratory techniques. In this experiment, two properties of an SCO complex, its thermochromic phenomenon and its bistability at RT, are used

Barium(II) sulfate precipitated as a very thin powder. It was thus necessary to perform an Ostwald ripening prior to filtration (Figure 3). In the second step, 4-amino-1,2,4-triazole was added to the iron(II) bromide filtrate. The complex was obtained by evaporation of the solution containing the iron(II) bromide filtrate and 4-amino-1,2,4-triazole using nitrogen gas flow (Figure 4): B




Figure 3. Preparation of an aqueous solution of iron(II) bromide.

Figure 4. Synthesis of the complex [Fe(NH2trz)3]Br2·H2O.

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Fe2 +(aq) + 2Br−(aq) + 3NH 2trz(aq) + H 2O

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RESULTS AND DISCUSSION

Synthesis

⇌ [Fe(NH 2trz)3 ]Br2·H 2O(s)

A small minority of the students, 5%, failed to obtain the desired SCO product because they used the solid BaSO4 instead of the FeBr2 filtrate. The other students were able to obtain the product without complication with a yield ranging between 46% and 60%. However, the SCO properties were more spectacular if the conditions described in the experimental protocol were strictly followed.20

CHARACTERIZATION The IR spectra of the [Fe(NH2trz)3]Br2·H2O complex and of the NH2trz ligand in KBr pellets (2%) were recorded between 550 and 4000 cm−1. The thermochromism and bistability phenomena were qualitatively observed by warming or cooling 500 mg of the solid complex in a test tube. The magnetic properties were qualitatively observed by bringing an NdFeB permanent magnet close to the complex in a test tube suspended to a retort stand. The electronic spectra were recorded between 800 and 400 nm in the solid state (KBr pellet 10%) at RT after different thermal treatments.

IR Characterization

The IR spectra showed typical features for aromatic cyclic amines and their coordination complexes: the stretching vibrations bands of the triazole cycles were found in the 1350−1600 cm−1 range and were shifted to higher wavenumbers with a broadening of the peaks upon coordination to iron(II).21 In parallel, the presence of two bands at 620 and 680 cm−1 on both spectra indicated the same C2v symmetry for NH2trz in its free and coordinated forms, suggesting that two iron(II) ions are linked by N(1)−N(2) triazole bridges.22 No effect of the SCO is observed in the 4000−550 cm−1 region: the white- and purple-colored solids exhibited the same IR spectra.

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HAZARDS Iron(II) sulfate heptahydrate is harmful if swallowed and can cause serious eye irritation and skin irritation. Barium(II) salts are harmful if inhaled or swallowed. 4-Amino-1,2,4-triazole, potassium bromide, and iron(II) bromide are irritating to the eyes, respiratory system, and skin. Iron(II) bromide is also harmful if inhaled. The safety instructions are included in the Instructors’ Notes in Supporting Information. In line with the precautionary principle, it is appropriate to lay down added safety requirements to cover the unknown potential hazard presented by the synthesized complex [Fe(NH2trz)3]Br2·H2O.

Thermochromism

The thermochromism was visually observed during the synthesis. At the end of the evaporation, a white powder was observed. The powder became (slightly) purple when placed in C




an ice bath and remained purple when brought back to RT (Figure 4).23 Students concluded that the purple complex was favored at low temperatures, whereas the white complex was favored at high temperatures. Bistability

The color change depended on the thermal history of the compound, that is, its bistability. Treatment at 273 K was not sufficient to convert the entire sample to a single color. Use of liquid nitrogen or a warm water bath (340−350 K) was necessary to achieve full conversion of the complex either to a purple color or to a white color, respectively. Typical observations of students are shown in Figure 5. Both colors

Figure 5. Photographs at RT of student results showing tube containing the sample [Fe(NH2trz)3]Br2·H2O suspended to a retort stand (A) after cooling at 77 K and (B) after heating at 353 K. The white form (B) is attracted to the magnet, whereas the purple form (A) is slightly repelled by the magnet.

were stable at RT and observable even after numerous thermal treatments. Students thus demonstrated the hysteretic character, the reversibility, and the cyclability of the thermochromic transition.

Figure 6. (Top) Student’s electronic spectra of [Fe(NH2trz)3]Br2·H2O in KBr pellet recorded at RT after different thermal treatments. (Bottom) Simplified Tanabe−Sugano diagram for d6 octahedral complexes (E, energy of excited terms taking the energy of ground state equal to zero; B, Racah parameter; Δ, ligand field; P, electron pairing energy).

Magnetic Properties

The formation of both states at RT made possible a qualitative magnetic characterization: students observed that the purple form was weakly repelled by a permanent magnet (Figure 5A), whereas the white form was attracted to a permanent magnet (Figure 5B).23 The purple form was then a diamagnetic compound having all of its electrons paired. A ligand field approach states that the iron(II) center is in its t2g6 low-spin state. On the contrary, a paramagnetic behavior was observed for the white form due to presence of four unpaired electrons in the t2g4eg2 high-spin state.

immersed in liquid nitrogen due to an increased amount of the purple form. Students thus understood that measuring the optical density at λ = 530 nm was a way to read the information stored in the material after an appropriate thermal treatment to encode the information. Interpretation of the Electronic Spectra

The purple color was stronger for the sample immersed in liquid nitrogen, though not very intense. A d−d transition was suggested by students for the band centered at λmax = 530 nm and a d6 Tanabe−Sugano diagram used for its interpretation.25 Assuming B/B0 = 0.75 and B0 = 1058 cm−1, the only possible transition was the first spin-allowed transition in the high-field region (right part), that is, 1A1g → 1T1g. Students obtained a value for the ligand field for the LS state, Δo(LS), of ca. 20 790 cm−1. This value was slightly higher than the typical pairing energy in iron(II) octahedral complexes (15 000 cm−1). It was in the range expected for compounds exhibiting thermal SCO properties (Table 1). The values given in Table 1 allowed evaluating the wavelength range expected for the only spin-allowed transition 5T2g(HS) → 5Eg for the HS

Optical-Data Storage

To quantify the optical response of the system, electronic spectra were recorded by students at RT after various thermal treatments (Figure 6). A RT typical spectrum of the iron(II) compound in a KBr pellet presented an absorption band with a rounded maximum at λmax = 530 nm (medium green). Because there was only one absorption band in the visible range, students quickly concluded that this band was at the origin of the observation of the purple complementary color. When the pellet was heated, the absorption at 530 nm decreased, indicating a decrease amount of the purple form.24 However, the absorption at 530 nm increased after the pellet was D




Verdaguer who has largely diffused this work in many congresses, O. Durupthy for his implementation and helpful participation, T. Coradin for his corrections, and the reviewers for their constructive comments.

Table 1. Values of the Ligand Field Expected for a Octahedral Fe(II) Compounds

a

Compound

Δo(HS)/cm−1a

Δo(LS)/cm−1a

HS SCO LS

23000

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Data from ref 25.

state: it was expected in the near-IR region.26 This explained why no d−d band was observed in the UV−visible spectra of the HS form.

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CONCLUSION This experiment was performed by 200 students. The effectiveness of the experiment in achieving its objectives was assessed in several ways. First, by direct observation of both colors at RT it was possible to assess the experimental proficiency of the students (cf. Supporting Information). Second, the students’ ability to report experimental findings in a scientifically acceptable format was directly assessed from the quality of the laboratory books that must include their experimental observations and their answers to questions given in Supporting Information. Lastly, student and instructor comments were collected at the end of each semester; in general, this experiment was well received by students. The thermochromism and the optical-data storage property was always a great astonishment. They discovered the macroscopic effect of para- and diamagnetism for coordination compounds. The instructors noticed that (1) all students were able to correlate a color to a spin state for this complex; (2) they no longer associated the SCO phenomenon with a redox reaction; (3) they progressed in the understanding of the general concepts of the course such as the existence of two spin states in octahedral iron(II) complexes, the electronic transitions, the interpretation and use of Tanabe−Sugano diagrams, and the magnetic properties. It is necessary to note that the spectacular thermochromism of this complex facilitated their understanding. Finally, the setup for this experiment is simple to implement. It can be adapted to lower levels by shortening the characterization work or to the master level by adding quantitative magnetic measurements27 or by extending the study to other related magnetic complexes.

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ASSOCIATED CONTENT

* Supporting Information S

Student handouts; instructor notes, including IR and UV spectra; CAS registry numbers of chemicals; manufacturers of equipment; IR spectra. This material is available via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Bare, W. D.; Mellon, E. K. Thermochromic behavior of cobalt(II) halides in nonaqueous solvents and on filter paper. J. Chem. Educ. 1991, 68, 779−780. (2) Battle, G. M.; Allen, F. H.; Ferrence, G. M. Teaching threedimensional structural chemistry using crystal structure databases. 3. The cambridge structural database system: information content and access software in educational applications. J. Chem. Educ. 2011, 88, 886−890. (3) Cui, A.-L.; Chen, X.; Sun, L.; Wei, J.-Z.; Yang, J.; Kou, H.-Z. Preparation and thermochromic properties of copper(II)-N,Ndiethylethylenediamine complexes. J. Chem. Educ. 2011, 88, 311−312. (4) de Berg, K. C.; Chapman, K. J. Determination of the magnetic moments of transition metal complexes using rare earth magnets. J. Chem. Educ. 2001, 78, 670−673. (5) Gray, H. B. Chemical applications of group theory (Cotton, F. Albert). J. Chem. Educ. 1964, 41, 113−114. (6) Hutchinson, B.; Hance, R. L.; Hardegree, E. L.; Russell, S. A. A simple demonstration of the Curie-Weiss law and a spin-crossover compound. J. Chem. Educ. 1980, 57, 830−831. (7) Linenberger, K.; Bretz, S. L.; Crowder, M. W.; McCarrick, R.; Lorigan, G. A.; Tierney, D. L. What is the true color of fresh meat? A biophysical undergraduate laboratory experiment investigating the effects of ligand binding on myoglobin using optical, EPR, and NMR spectroscopy. J. Chem. Educ. 2011, 88, 223−225. (8) Pernicone, N. C.; Geri, J. B.; York, J. T. Using a combination of experimental and computational methods to explore the impact of metal identity and ligand field strength on the electronic structure of metal ions. J. Chem. Educ. 2011, 88, 1323−1327. (9) Sutton, L. E. Some recent developments in the theory of bonding in complex compounds of the transition metals. J. Chem. Educ. 1960, 37, 498−505. (10) Lavabre, D.; Micheau, J. C.; Levy, G. Comparison of thermochromic equilibria of Co(II) and Ni(II) complexes. J. Chem. Educ. 1988, 65, 274−277. (11) Spears, L. G., Jr.; Spears, L. G. Chemical storage of solar energy using an old color change demonstration. J. Chem. Educ. 1984, 61, 252−254. (12) Gütlich, P.; Goodwin, H. A. Spin CrossoverAn Overall Perspective Spin Crossover in Transition Metal Compounds I; Springer: Berlin/Heidelberg, 2004; Vol. 233, pp 1−47. (13) Kahn, O.; Martinez, C. J. Spin-transition polymers: from molecular materials toward memory devices. Science 1998, 279, 44−48. (14) Grosjean, A.; Daro, N.; Kauffmann, B.; Kaiba, A.; Letard, J.-F.; Guionneau, P. The 1-D polymeric structure of the [Fe(NH2trz)3](NO3)2·nH2O (with n = 2) spin crossover compound proven by single crystal investigations. Chem. Commun. 2011, 47, 12382−12384. (15) Lavrenova, L. G.; Shakirova, O. G.; Ikorskii, V. N.; Varnek, V. A.; Sheludyakova, L. A.; Larionov, S. V. 1 A 1 ⇄ 5 T 2 spin transition in new thermochromic iron(II) complexes with 1,2,4-triazole and 4amino-1,2,4-triazole. Russ. J. Coord. Chem. 2003, 29, 22−27. (16) Lavrenova, L. G.; Yudina, N. G.; Ikorskii, V. N.; Varnek, V. A.; Oglezneva, I. M.; Larionov, S. V. Spin-crossover and thermochromism in complexes of iron(II) iodide and thiocyanate with 4-amino-1,2,4triazole. Polyhedron 1995, 14, 1333−1337. (17) Bain, G. A.; Berry, J. F. Diamagnetic Corrections and Pascal’s Constants. J. Chem. Educ. 2008, 85, 532−536. (18) γHS(%) = 100 × xHS = (χM·T)exp/(χM·T)HS with xHS the molar quantity of iron(II) ions in the HS form per iron center and (χM·T)HS = 3.3 cm3 K mol−1 with gFe = 2.1 the value of the χM·T product when all the iron(II) ions are the HS form. (19) Rotaru, A.; Varret, F.; Gindulescu, A.; Linares, J.; Stancu, A.; Létard, J. F.; Forestier, T.; Etrillard, C. Size effect in spin-crossover

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank A. Proust, for implementing this laboratory experiment in the program of her course, M. E




systems investigated by FORC measurements, for surfacted [Fe(NH2trz)3](Br)2.3H2O nanoparticles: reversible contributions and critical size. Eur. Phys. J. B 2011, 84, 439−449. (20) In the first step, the temperature and the time were the important parameters to control to avoid BaSO4 impurities in the final product. In the worst cases, a filtration (Celite) or centrifugation step was necessary to obtain a clear filtrate. In the second step, the students who were lacking time could finish the water evaporation using a rotary evaporator. However, the quality of the final product increased with the volume of water that was evaporated under N2 flow. (21) Sinditskii, V. P.; Sokol, V. I.; Fogel’zang, A. E.; Serushkin, V. V.; Porai-Koshits, M. A.; Svetlov, B. S. The vibrational Spectra and structure of coordination compounds of metals with 4-amino-1,2,4triazole as bidendate ligand. Russ. J. Inorg. Chem. 1987, 32, 1950−1955. (22) Haasnoot, J. G.; Groeneveld, W. L. 1,2,4-Triazole complexses, III complexes of transition metal(II) nitrate and fluoroborates. Z. Naturforsch. 1977, 32b, 533−536. (23) If samples of both colors were not stable at RT or a pink color rather than purple color was obtained, the sample was dehydrated and magnetic properties were difficult to observe. To rehydrate the sample, cotton soaked with a small volume of water was placed in the tube over the powder. (24) The pellet heated at 340 K was white but became at RT slightly pinkish as revealed by an absorbance different from the diffusion baseline in Figure 6. The KBr pelleting probably induces a modification of thermochromic properties (position or width of hysteresis) compared with those obtained for the pure compound in the solid state. This effect gave the opportunity to speak about complex limitations for an industrial development (25) Hauser, A.; Gütlich, P.; Goodwin, H. A. Ligand Field Theoretical Considerations Spin Crossover in Transition Metal Compounds I; Springer: Berlin/Heidelberg, 2004; Vol. 233, pp 49−58. (26) The band corresponding to this transition is very broad and not very intense. It can be observed only by recording a reflectance diffuse spectra of the solid. The spectra is provided in Supporting Information. (27) Bower, N.; Yu, A. Exploiting mass measurements in different environments: density, magnetic susceptibility, and thermogravimetry. J. Chem. Educ. 2011, 88, 536−539. This publication can be used to correlate a color to magnetic property (dia- or paramagnetic). In this case, magnetic data storage could be introduced.

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Synthesis and Properties of a Thermochromic Spin Crossover FeII

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