Synthesis and Characterization of Europium(III) and Terbium(III

May 3, 2010 - Undergraduate laboratories rarely involve lanthanide coordination chemistry. This is unfortunate in light of the ease with which many of...
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Synthesis and Characterization of Europium(III) and Terbium(III) Complexes: An Advanced Undergraduate Inorganic Chemistry Experiment Shawn Swavey Department of Chemistry, University of Dayton, Dayton, Ohio 45469-2357 [email protected]

Descriptions of the f-block metals is typically reserved for the end of most inorganic courses and often only briefly covered in a typical one-semester advanced course. There are a limited number of undergraduate experiments involving lanthanide coordination complexes (1). This is unfortunate given the ease with which many of these complexes can be made, the interesting and instructive photophysical properties, and the relatively inexpensive starting materials. In the classroom, the term symbols and their use in describing spectroscopic properties is usually relegated to the d-block metals; however, laboratory illustrations of the link between theory and experiment are often difficult to solidify in the minds of the students. Lanthanide coordination complexes make this link with little need to introduce new concepts; students can take what they have learned in the classroom with respect to microstates and term symbols and apply these principles to the f orbitals associated with lanthanide complexes. Compared to transition-metal coordination complexes, the area of lanthanide coordination chemistry is a relatively young field. A number of advances in this area have been made in recent years with applications in catalysis (2), biosensors (3), and light-emitting materials (4) to name just a few. Stabilization of the diffuse 4f orbitals leads to an open-shell electronic configuration in which the lanthanides favor the þ3 oxidation state. Lanthanide(III) cations display typical a-class (hard) properties; therefore, coordination is accomplished by strong Lewis bases and in particular bidentate ligands containing oxygen or nitrogen (5). Charged oxygen atoms favor ionic bonding to lanthanides. Metal-metal charge-transfer transitions of the 4f electrons into the 4f* excited state are forbidden by the selection rules. To circumvent these forbidden transitions, “antenna” ligands are used to absorb energy in the UV region of the spectrum transferring that energy to the lanthanide metal inducing a 4f-4f* (ligand field) transition. Relaxation of the excited 4f electrons to the ground state leads to emission spectra in the visible and near-infrared regions of the spectrum. The high degree of spin-orbit coupling associated with lanthanides leads to intense ligand-field emissions. Excellent coordination and energytransfer ligands, for example, β-diketonates, have been identified (6). Europium(III) and terbium(III) are instructively interesting owing to their sharp long-lived emission lines lying in the visible region of the spectrum. For example, terbium(III) complexes excited with UV light emit green light, whereas europium(III) complexes excited with UV light emit yellow or red light (Figure 1). It should be noted that emissions of the complexes in solution are similar to those in the solid state. We have identified a series of inexpensive and readily available reagents that can be used to synthesize Eu(III) and

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Figure 1. Illustrations of the effect of irradiation, using a UV light box of Tb(tdh)3bpy and Eu(tdh)3bpy complexes. Scheme 1. Synthetic Scheme of 2,20 -Bipyridine-bis{tris[2,2,6,6-tetramethyl-3,5-heptanedione]}-lanthanide(III): Tb(tmh)3bpy and Eu(tmh)3bpy

Tb(III) complexes. The synthesis of Tb(III) and Eu(III) complexes (7) is illustrated in Scheme 1. In this synthetic scheme, the β-diketone 2,2,6,6-tetramethyl-3,5-heptanedione (tmh), 2,20 bipyridine (bpy), and terbium(III) chloride or europium(III) chloride are combined to make the complex 2,20 -bipyridine-bis{tris[2,2,6,6-tetramethyl-3,5-heptanedione]}-lanthanide(III), Ln(tmh)3bpy. Because of the large lanthanide cations, higher coordination numbers are typically observed; for example, Tb(tmh)3bpy and Eu(tmh)3bpy (Scheme 1) are eight coordinate with six oxygen atoms from three tmh ligands and two nitrogen donors from the 2,20 -bpy ligand. The addition of 2,20 -bipyridine displaces coordinated water molecules, which deactivate the metal emission through OH vibrations. The presence of C-H bonds leads to vibrational quenching of the lanthanide emission. To avoid this mechanism of deactivation, some of the β-diketones chosen contain heavier C-F bonds. The β-diketones are stirred in a basic ethanolic solution to ensure the enol form before addition of the lanthanide salt. In this experiment, the synthesis of two terbium(III) and two europium(III) complexes incorporating 2,20 -bipyridine

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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 7 July 2010 10.1021/ed100188m Published on Web 05/03/2010

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In the Laboratory

Figure 2. Excitation and emission spectrum of Tb(tdh)3bpy.

and either europium(III) chloride or terbium(III) chloride and three β-diketones [1,1,1,-trifluoro-5,5-dimethyl-2,4-hexanedione (tdh), 2,2,6,6-tetramethyl-3,5-heptanedione (tmh), and thenoyltrifluoroacetone (tta)] are described. Their spectroscopic and fluorescence properties are measured and used to create energy diagrams for each complex. Experiment and Discussion The preparation of the Tb(III) and Eu(III) complexes [Tb(tmh)3bpy, Eu(tmh)3bpy, Tb(tdh)3bpy, and Eu(tta)3bpy] is accomplished in one 3-h laboratory by the addition of an aqueous solution of the respective lanthanide salt to a basic ethanolic solution containing the respective β-diketone and 2,20 -bipyridine. (Note: If desired, all three ligands can be used for both Tb(III) and Eu(III) giving a total of six complexes.) Precipitation of the products occurs immediately. After vacuum filtering and air drying until the next laboratory period, the europium complexes can be isolated as a pale-yellow powder, whereas the terbium complexes are isolated as a green powder. The π-π* transitions of the β-diketonate ligands are measured by dissolving a small quantity of each complex in methanol and scanning from 200 to 400 nm with the UV-vis spectrophotometer. The 4f-4f* (ligand field) transitions of the metals are forbidden and therefore not observed in these measurements; however, the single intense absorption bands observed in the UV region of the spectrum for each complex can be associated with the S0 f S1 (π-π*) transition of the β-diketonate. The peak absorption wavelength measured is also the wavelength at which the associated complex will be excited in the fluorescence experiments. Solid-state fluorescence studies of the four metal complexes is accomplished by placing the powder of the complex of interest into a quartz fluorescence cuvette and photoexciting the complex at the wavelength that corresponds to the π-π* transition of the β-diketonate (determined by the UV-vis experiments). Absorption of the 2,20 -bipyridyl ligand occurs at a higher energy than the β-diketonate ligand and with much lower intensity considering there is only one bipyridine and three β-diketones per complex. For this reason, energy transfer comes primarily from the β-diketonate ligand. Scanning the visible region reveals an emission spectrum associated with the lanthanide metal. For example, Tb(III) has a ground-state electronic configuration of [Xe]4f8 with six unpaired electrons. This configuration leads to a 7FJ ground state with spin-orbit coupling J = 0, 1, 2, 3, 4, 5, 6 corresponding to seven possible 728

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Figure 3. Energy-level diagram for the Tb(tdh)3bpy complex in Figure 2. Arrows indicate the course of energy transfer during excitation by UV radiation and emission of visible light. The ground and excited states of the bpy ligand are not included because they play a relatively small role in the energy-transfer process.

transitions, with 6 lines distinguishable in the emission spectrum as illustrated in Figure 2 for a Tb(III)-β-diketonate complex. For Tb(III), the first excited state can be described as a 5 D4 state and its energy corresponds to the transition to the lowest ground state, 5D4 f 7F6 (Figure 2). The difference in energy between the 5D4 f 7F6 and the 5D4 f 7F5 allows for calculation of the energy of the 7F5 state. The energies of the other various states can be calculated in a similar manner. Once the electronic states of the terbium(III) center along with the S0 and S1 states of the respective β-diketonates have been determined, the triplet T1 states of the β-diketonates are determined by fluorescence measurements of the lone ligands in solution. An energy diagram for each complex, similar to the one in Figure 3, can be constructed from the information gathered in the spectroscopic analyses. It may be instructive for students in the class to pool their results for comparative purposes. To determine the term symbols associated with a particular metal ion one must consider its electronic configuration. For example, europium(III) has an [Xe]4f 6 ground-state electron configuration that, similar to Tb(III), has six unpaired electrons in the 4f orbitals.

The total orbital angular momentum quantum number L and therefore the ground-state term for this configuration is the sum of the ml values, L ¼ fð þ 3Þ þ ð þ 2Þ þ ð þ 1Þ þ ð0Þ þ ð - 1Þ þ ð - 2Þg ¼ 3 and L = 3 corresponds to an F state. The total spin-angularmomentum quantum number S and therefore the spin state for the ground term are equal to the sum of the ms values, S ¼ fð1=2Þ þ ð1=2Þ þ ð1=2Þ þ ð1=2Þ þ ð1=2Þ þ ð1=2Þg ¼ 3

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In the Laboratory

and S = 3 gives a spin state 2S þ 1 = 7. Spin-orbit coupling determines how the ground term splits into different energies and is determined by: J ¼ L þ S, L þ S - 1, L þ S - 2, :::, jL - Sj For L = 3 and S = 3, J takes on the values of 6, 5, 4, 3, 2, 1, 0. Therefore, the ground-state septet F term for Eu(III) is split as follows:

protection should be worn at all times. 2,2,6,6-Tetramethyl-3,5heptanedione is a combustible liquid and vapor and causes eye, skin, and respiratory tract irritation. 1,1,1,-Trifluoro-5,5-dimethyl2,4-hexanedione is flammable and is an irritant. Europium chloride and terbium chloride are hygroscopic and irritants. Sodium hydroxide is corrosive. Ethyl alcohol is flammable. Conclusions The experiments described here are ideally suited to accompany an advanced undergraduate inorganic chemistry course and would be best performed after microstates, spin-orbit coupling, and term symbols are discussed in the lecture. From purely a synthetic point, this laboratory can be easily adapted for the general chemistry laboratory; however, with very large sections, it could become expensive. The use of different β-diketonates with varied absorption maxima gives the student examples of the role that the ligand plays in the energy-transfer process. It is expected that this will be discussed in the student's final report. I require that the students' reports mirror those of an ACS journal with title, abstract, experimental procedures, results and discussion, conclusions, and reference sections. Obviously, this example does not need to be followed for this laboratory to be successful. The time required for the students to analyze their data is sufficient to reap the benefits of this laboratory.

This splitting indicates that the Eu(III) complex should have seven emission lines in the fluorescence spectrum. The first excited state for the Eu(III) configuration is the 5D0 state with the ground-state configuration of 7FJ (J = 0, 1, 2, 3, 4, 5, 6). Because the 4f orbitals in Eu(III) are half-filled, the order of the ground-state energies is the reverse of the Tb(III) ground-state F term (i.e., 7F0 is the lowest energy in the Eu(III) case, whereas 7 F6 is the lowest energy for Tb(III), Figure 3). From Hund's third rule, for subshells that are less than half-filled, the state having the lowest J value has the lowest energy; for subshells that are more than half-filled, the state having the highest J value has the lowest energy (8). The S0, S1, and T1 states will depend on the β-diketonate used. It is important for the students to recognize any difference in emission intensities and see if they can relate the intensity differences to the energy differences between the triplet state of the ligand and the excited state of the metal. The 2,20 bipyridyl ligand is not included in the energy diagram because the π-π* transitions associated with this ligand are too high in energy to be involved with energy transfer to the metal; again, the students should be reminded that the purpose of the 2,20 bipyridyl ligand is to enhance the luminescence intensity by preventing OH quenching from coordinated water molecules. Hazards Methanol is flammable and an irritant. 2,20 -Bipyridine and thenoyltrifluoroacetone are harmful if swallowed and cause irritation to skin, eyes, and respiratory tract. They also have strong odors and should be handled in the fume hood. Although the other reagents may be handled outside of the hood, gloves and eye

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Literature Cited 1. Jenkins, A. L.; Murray, G. M. J. Chem. Educ. 1998, 75, 227. 2. Giardello, M. A.; Yamamoto, Y.; Brard, L.; Marks, T. J. J. Am. Chem. Soc. 1995, 117, 3276. (b) Martin, E.; Dubois, P.; Jerome, R. Macromolecules 2000, 33, 1530. 3. Werts, M. H. V.; Woudenberg, R. H.; Emmerink, P. G.; van Gassel, R.; Hofstraat, J. W.; Verhoeven, J. W. Coord. Chem. Rev. 2006, 250, 2501. (b) Tsukube, H.; Shinoda, S. Chem. Rev. 2002, 102, 2389. 4. Isabelle, B. Handb. Phys. Chem. Rare Earths 2003, 33, 465. 5. Parker, D. Chem. Soc. Rev. 2004, 33, 156. 6. (a) Richardson, F. S. Chem. Rev. 1982, 82, 541. (b) Skopenko, V. V.; Amirkhanov, V. M.; Sliva, T. Yu.; Vasilchenko, I. S.; Anpilova, E. L.; Garnovskii, A. D. Russ. Chem. Rev. 2004, 73, 737. 7. (a) Richards, G.; Osterwyk, J.; Flikkema, J.; Cobb, K.; Sullivan, M.; Swavey, S. Inorg. Chem. Commun. 2008, 11, 1385. (b) Swavey, S.; Krause, J. A.; Collins, D.; D'Cunha, D.; Fratini, A. Polyhedron 2008, 27, 1061. (c) Jang, H.; Shin, C.-H.; Jung, B.-J.; Kim, D.-H.; Shim, H.-K.; Do, Y. Eur. J. Inorg. Chem. 2006, 718. (d) Baker, M. H.; Dorweiler, J. D.; Ley, A. N.; Pike, R. D.; Berry, S. M. Polyhedron 2009, 28, 188. (e) Irfanullah, M.; Iftikhar, K. Inorg. Chem. Commun. 2009, 12, 296. 8. Miessler, G. L.; Tarr, D. A. Inorganic Chemistry, 3rd ed.; Pearson Prentice Hall: Upper Saddle River, NJ, 2004; Chapter 11, p 388.

Supporting Information Available Student handout; pre- and postlab questions; instructor information; answers to the pre- and postlab questions. This material is available via the Internet at http://pubs.acs.org.

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