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10.1021/ed100188m Published on Web 05/03/2010

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Synthesis and Characterization of Europium(III)and Terbium(III) Complexes: An AdvancedUndergraduate Inorganic Chemistry ExperimentShawn SwaveyDepartment of Chemistry, University of Dayton, Dayton, Ohio 45469-2357Shawn.swavey@notes.udayton.edu

Descriptions of the f-block metals is typically reserved forthe end of most inorganic courses and often only briefly coveredin a typical one-semester advanced course. There are a limitednumber of undergraduate experiments involving lanthanidecoordination complexes (1). This is unfortunate given the easewith whichmany of these complexes can bemade, the interestingand instructive photophysical properties, and the relativelyinexpensive starting materials. In the classroom, the term sym-bols and their use in describing spectroscopic properties is usuallyrelegated to the d-block metals; however, laboratory illustrationsof the link between theory and experiment are often difficult tosolidify in the minds of the students. Lanthanide coordinationcomplexes make this link with little need to introduce newconcepts; students can take what they have learned in theclassroom with respect to microstates and term symbols andapply these principles to the f orbitals associated with lanthanidecomplexes.

Compared to transition-metal coordination complexes, thearea of lanthanide coordination chemistry is a relatively young field.A number of advances in this area have been made in recent yearswith applications in catalysis (2), biosensors (3), and light-emittingmaterials (4) to name just a few. Stabilization of the diffuse 4forbitals leads to an open-shell electronic configuration in which thelanthanides favor the þ3 oxidation state. Lanthanide(III) cationsdisplay typical a-class (hard) properties; therefore, coordination isaccomplished by strong Lewis bases and in particular bidentateligands containing oxygen or nitrogen (5). Charged oxygen atomsfavor ionic bonding to lanthanides. Metal-metal charge-transfertransitions of the 4f electrons into the 4f* excited state areforbidden by the selection rules. To circumvent these forbiddentransitions, “antenna” ligands are used to absorb energy in the UVregion of the spectrum transferring that energy to the lanthanidemetal inducing a 4f-4f* (ligand field) transition. Relaxation of theexcited 4f electrons to the ground state leads to emission spectra inthe visible and near-infrared regions of the spectrum. The highdegree of spin-orbit coupling associated with lanthanides leads tointense ligand-field emissions. Excellent coordination and energy-transfer ligands, for example, β-diketonates, have been identi-fied (6). Europium(III) and terbium(III) are instructively interest-ing owing to their sharp long-lived emission lines lying in the visibleregion of the spectrum. For example, terbium(III) complexesexcited with UV light emit green light, whereas europium(III)complexes excitedwithUV light emit yellow or red light (Figure 1).It should be noted that emissions of the complexes in solution aresimilar to those in the solid state.

We have identified a series of inexpensive and readilyavailable reagents that can be used to synthesize Eu(III) and

Tb(III) complexes. The synthesis of Tb(III) and Eu(III) com-plexes (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, highercoordination numbers are typically observed; for example,Tb(tmh)3bpy and Eu(tmh)3bpy (Scheme 1) are eight coordi-nate with six oxygen atoms from three tmh ligands and twonitrogen donors from the 2,20-bpy ligand.

The addition of 2,20-bipyridine displaces coordinated watermolecules, which deactivate the metal emission through OHvibrations. The presence of C-H bonds leads to vibrationalquenching of the lanthanide emission. To avoid this mechanismof deactivation, some of the β-diketones chosen contain heavierC-F bonds. The β-diketones are stirred in a basic ethanolicsolution to ensure the enol form before addition of the lantha-nide salt. In this experiment, the synthesis of two terbium(III)and two europium(III) complexes incorporating 2,20-bipyridine

Figure 1. Illustrations of the effect of irradiation, using a UV light box ofTb(tdh)3bpy and Eu(tdh)3bpy complexes.

Scheme 1. Synthetic Scheme of 2,20 -Bipyridine-bis{tris[2,2,6,6-tetra-methyl-3,5-heptanedione]}-lanthanide(III): Tb(tmh)3bpy and Eu-(tmh)3bpy

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and either europium(III) chloride or terbium(III) chloride andthree β-diketones [1,1,1,-trifluoro-5,5-dimethyl-2,4-hexanedione(tdh), 2,2,6,6-tetramethyl-3,5-heptanedione (tmh), and thenoyl-trifluoroacetone (tta)] are described. Their spectroscopic andfluorescence properties are measured and used to create energydiagrams 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] isaccomplished in one 3-h laboratory by the addition of an aqueoussolution of the respective lanthanide salt to a basic ethanolic solu-tion containing the respective β-diketone and 2,20-bipyridine.(Note: If desired, all three ligands can be used for bothTb(III) andEu(III) giving a total of six complexes.) Precipitation of theproducts occurs immediately. After vacuum filtering and air dryinguntil the next laboratory period, the europium complexes can beisolated as a pale-yellow powder, whereas the terbium complexesare isolated as a green powder. The π-π* transitions of theβ-diketonate ligands aremeasured by dissolving a small quantity ofeach complex inmethanol and scanning from 200 to 400 nmwiththe UV-vis spectrophotometer. The 4f-4f* (ligand field) transi-tions of the metals are forbidden and therefore not observedin these measurements; however, the single intense absorptionbands observed in theUV region of the spectrum for each complexcan be associated with the S0 f S1 (π-π*) transition of theβ-diketonate. The peak absorption wavelength measured is alsothe wavelength at which the associated complex will be excited inthe fluorescence experiments.

Solid-state fluorescence studies of the four metal com-plexes is accomplished by placing the powder of the complex ofinterest into a quartz fluorescence cuvette and photoexcitingthe complex at the wavelength that corresponds to the π-π*transition of the β-diketonate (determined by the UV-visexperiments). Absorption of the 2,20-bipyridyl ligand occurs ata higher energy than the β-diketonate ligand and with muchlower intensity considering there is only one bipyridine andthree β-diketones per complex. For this reason, energy transfercomes primarily from the β-diketonate ligand. Scanning thevisible region reveals an emission spectrum associated withthe lanthanide metal. For example, Tb(III) has a ground-stateelectronic configuration of [Xe]4f8 with six unpaired electrons.This configuration leads to a 7FJ ground state with spin-orbitcoupling J = 0, 1, 2, 3, 4, 5, 6 corresponding to seven possible

transitions, with 6 lines distinguishable in the emission spec-trum as illustrated in Figure 2 for a Tb(III)-β-diketonatecomplex. For Tb(III), the first excited state can be described as a5D4 state and its energy corresponds to the transition to thelowest ground state, 5D4 f

7F6 (Figure 2). The difference inenergy between the 5D4 f

7F6 and the 5D4 f7F5 allows for

calculation of the energy of the 7F5 state. The energies of theother various states can be calculated in a similar manner. Oncethe electronic states of the terbium(III) center along with the S0and S1 states of the respective β-diketonates have been deter-mined, the triplet T1 states of the β-diketonates are determinedby fluorescence measurements of the lone ligands in solution.An energy diagram for each complex, similar to the one inFigure 3, can be constructed from the information gathered inthe spectroscopic analyses. It may be instructive for students inthe class to pool their results for comparative purposes.

To determine the term symbols associated with a particularmetal ion one must consider its electronic configuration. Forexample, europium(III) has an [Xe]4f 6 ground-state electronconfiguration that, similar to Tb(III), has six unpaired electronsin the 4f orbitals.

The total orbital angular momentum quantum number L andtherefore the ground-state term for this configuration is the sumof the ml values,L ¼ fðþ 3Þþ ðþ 2Þþ ðþ 1Þþ ð0Þþ ð- 1Þþ ð- 2Þg ¼ 3

and L = 3 corresponds to an F state. The total spin-angular-momentum quantum number S and therefore the spin state forthe ground term are equal to the sum of the ms values,S ¼ fð1=2Þþ ð1=2Þþ ð1=2Þþ ð1=2Þþ ð1=2Þþ ð1=2Þg ¼ 3

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

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

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and S = 3 gives a spin state 2S þ 1 = 7. Spin-orbit couplingdetermines how the ground term splits into different energiesand is determined by:

J ¼ Lþ S,Lþ S- 1,Lþ S- 2, :::, jL- SjFor 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 asfollows:

This splitting indicates that the Eu(III) complex should haveseven emission lines in the fluorescence spectrum. The firstexcited state for the Eu(III) configuration is the 5D0 state withthe 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 theground-state energies is the reverse of the Tb(III) ground-stateF term (i.e., 7F0 is the lowest energy in the Eu(III) case, whereas7F6 is the lowest energy for Tb(III), Figure 3). FromHund's thirdrule, for subshells that are less than half-filled, the state havingthe lowest J value has the lowest energy; for subshells that aremore than half-filled, the state having the highest J value has thelowest energy (8). The S0, S1, and T1 states will depend on theβ-diketonate used. It is important for the students to recognizeany difference in emission intensities and see if they can relate theintensity differences to the energy differences between the tripletstate of the ligand and the excited state of the metal. The 2,20-bipyridyl ligand is not included in the energy diagram because theπ-π* transitions associated with this ligand are too high inenergy to be involved with energy transfer to the metal; again, thestudents should be reminded that the purpose of the 2,20-bipyridyl ligand is to enhance the luminescence intensity bypreventing OH quenching from coordinated water molecules.

Hazards

Methanol is flammable and an irritant. 2,20-Bipyridine andthenoyltrifluoroacetone are harmful if swallowed and cause irrita-tion to skin, eyes, and respiratory tract. They also have strong odorsand should be handled in the fume hood. Although the otherreagents may be handled outside of the hood, gloves and eye

protection should be worn at all times. 2,2,6,6-Tetramethyl-3,5-heptanedione is a combustible liquid and vapor and causes eye,skin, and respiratory tract irritation. 1,1,1,-Trifluoro-5,5-dimethyl-2,4-hexanedione is flammable and is an irritant. Europium chlorideand terbium chloride are hygroscopic and irritants. Sodium hydro-xide is corrosive. Ethyl alcohol is flammable.

Conclusions

The experiments described here are ideally suited to accom-pany an advanced undergraduate inorganic chemistry course andwould be best performed after microstates, spin-orbit coupling,and term symbols are discussed in the lecture. From purely asynthetic point, this laboratory can be easily adapted for thegeneral chemistry laboratory; however, with very large sections, itcould become expensive. The use of different β-diketonates withvaried absorption maxima gives the student examples of the rolethat the ligand plays in the energy-transfer process. It is expectedthat this will be discussed in the student's final report. I requirethat the students' reports mirror those of an ACS journal withtitle, abstract, experimental procedures, results and discussion,conclusions, and reference sections. Obviously, this example doesnot need to be followed for this laboratory to be successful. Thetime required for the students to analyze their data is sufficient toreap the benefits of this laboratory.

Literature Cited

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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. Polyhedron2009, 28, 188. (e) Irfanullah, M.; Iftikhar, K. Inorg. Chem. Commun.2009, 12, 296.

8. Miessler, G. L.; Tarr, D. A. Inorganic Chemistry, 3rd ed.; PearsonPrentice 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 theInternet at http://pubs.acs.org.