Synthesis, Crystal Structure, and Modelling of a New Tetramer

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J. Phys. Chem. B 2007, 111, 9228-9238

Synthesis, Crystal Structure, and Modelling of a New Tetramer Complex of Europium Ana Paula Souza,† Filipe A. Almeida Paz,‡ Ricardo O. Freire,† Luı´s D. Carlos,§ Oscar L. Malta,*,† S. Alves, Jr.,† and Gilberto F. de Sa´ † Departamento de Quı´mica Fundamental and Centro de Cieˆ ncias Exatas e da Natureza, UniVersidade Federal de Pernambuco, Cidade UniVersita´ ria, 50590-470 Recife-PE, Brazil, and Departamentos de Quı´mica e Fı´sica and Centro de InVestigaç a˜ o em Materiais Ceraˆ micos e Compo´ sitos, UniVersidade de AVeiro, 3810-193 AVeiro, Portugal ReceiVed: January 15, 2007; In Final Form: April 19, 2007

A new compound with the formula [Eu4(ETA)9(OH)3(H2O)3)], where ETA is ethyl 4,4,4-trifluoroacetoacetate, has been synthesized and investigated by photoluminescence spectroscopy. The compound was characterized by means of chemical analysis, vibrational (IR), UV-vis absorption, and luminescence spectroscopies, and X-ray crystallography. The crystal structure of the [Eu4(ETA)9(OH)3(H2O)3)] complex in the solid state, determined by X-ray diffraction analysis, revealed that it crystallizes in the triclinic crystal system, space group P1h, with four crystallographically independent europium centers. From these structural data, the groundstate geometry of the tetramer has been calculated by using the Sparkle/AM1 model. The emission spectrum shows the characteristic transitions of the Eu3+ ion. The features displayed by the5D0 f 7F0 transition in the emission spectrum are consistent with the Eu3+ ion occupying four different sites in chemical environments of low symmetries, in agreement with the X-ray data and the optimized geometry obtained from the Sparkle/ AM1 model. These structural results have allowed the theoretical calculation of 4f-4f intensity parameters, including the forced electric dipole and dynamic coupling mechanisms as well as ligand singlet and triplet states, in good agreement with experiment.

1. Introduction Interest in the synthesis of lanthanide complexes, particularly of trivalent europium and terbium with organic ligands, has been greatly intensified due to their potential use in photonic devices such as organic light-emitting diodes (OLEDs), optical markers, sensors, and dosimeters.1-4 Highly luminescent Eu3+ and Tb3+ complexes with β-diketones units and heterobiaryl ligands have been synthesized and strongly suggested as excellent lightconversion molecular devices.5,6 Europium complexes are particularly interesting due to the fact that the main emitting level (5D0) is non-degenerate. The lines in the emission spectrum are generally sharp and dependent on the chemical enviroment around the Eu3+ ion. These lines correspond to the5D0 f 7FJ transitions with J ) 0, 1, 2, 3, and 4. The 5D0 f 7F0 transition is non-degenerate and can indicate if the Eu3+ occupies one or more than one site symmetry. The 5D f 7F transition is allowed by the magnetic dipole 0 1 mechanism, not much influenced by a change in the chemical environment, and can be used as a reference transition. The theoretical study of lanthanide complexes has been an important tool in the search for new molecular systems presenting special structural and optical properties that might be attractive from a technological point of view. For the prediction of emission quantum efficiencies, the determination of the ground-state geometries of the complexes is necessary. * Author to whom correspondence should be addressed. Phone: +55 81 2126-7459. Fax: +55 81 2126-8442. E-mail: [email protected]. † Universidade Federal de Pernambuco, Cidade Universita ´ ria. ‡ Departamentos de Quı´mica and Centro de Investigac ¸ aõ em Materiais Ceraˆmicos e Compo´sitos, Universidade de Aveiro. § Departamento de Fı´sica Centro de Investigac ¸ aõ em Materiais Ceraˆmicos e Compo´sitos, Universidade de Aveiro.

Actually, two quantum chemical methodologies are available: the ab inito/effective core potential (ECP) methodology7-9 and the Sparkle/AM1 model.10,11 In a recent work12 we have shown that the Sparkle/AM1 model leads to a geometry prediction accuracy of lanthanide complexes that is quite competitive with present day ab initio/ECP calculations, with the advantage that it is hundreds of times faster. Once the ground-state geometry of the lanthanide complex has been calculated one can proceed with the design of the highly luminescent complexes based on energy transfer rates and on emission quantum yield calculations.13 The geometries thus calculated have also been successfully applied to predict spectroscopic properties such as ligand field and 4f-4f intensity parameters as well as ligand singlet and triplet energy levels.14,15 This theoretical procedure is better achieved once detailed X-ray diffraction data are available, particularly when the compound is not found to be in its monomeric form. In the present work we report on the synthesis, characterization, crystal structure, modeling, and photoluminescence properties of a new tetramer europium complex with the ethyl 4,4,4trifluoroacetoacetate (ETA) ligand. 2. Experimental Details 2.1. Synthesis of the Lanthanide Complex. The ETA and europium chloride were purchased from Aldrich Chemical Co. and used as received. The compound [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] was prepared by dissolving 2 mmol of europium chloride in triethylorthoformate, then it was dried and dissolved in anhydrous tetrahydrofuran (THF). Subsequently, to the solution was added ethyl 4,4,4-trifluoroacetoacetate previously deprotonated with the help of strong alkali in dried THF in an inert

10.1021/jp070336w CCC: $37.00 © 2007 American Chemical Society Published on Web 07/12/2007

Europium Tetramer Complex atmosphere. The compound was dried, neutralized with NH4Cl, filtered, and washed with water and solvent to eliminate the lanthanide chloride and ligand excesses. The solid compound was dried under vacuum over silica in a desiccator. 2.2. Measurements. The complex was characterized by means of chemical analysis, IR vibrational, UV-vis absorption, and luminescence spectroscopies, and X-ray diffraction. The IR spectrum was recorded using a KBr pellet and a spectrophotometer (4000-400 cm-1), Bruker model IFS 66. The absorption spectrum was recorded on a Perkin-Elmer Lambda 6 UV-vis spectrophotometer. For the luminescence spectrum the sample was excited using a 150 W xenon lamp. The appropriate wavelength was selected by a 0.25 monochromator (Jobin Yvon model H-10). The emission spectrum was analyzed using a Jobin Yvon double monochromator, model U-1000, and the fluorescence signal, detected by a water-cooled RCA C31034-02 photomultiplier, was processed by a Jobin Yvon Spectralink system. The signal excitation spectrum was recorded by an EG&G Princeton Applied Research boxcar and a gated integration model 4422 and processed by a model 4402. The excitation spectrum of the solid sample was obtained with an ISS K2 multifrequency cross-correlation phase and modulation fluorometer using a 300 W continuous xenon arc lamp as an excitation light source. Single crystals of the complex were grown from the solution of the solid complex in cyclohexane/ethyl acetate at room temperature, over a period of 5 days. A suitable single crystal of [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3]‚(CH3CO2CH2CH3) was mounted on a glass fiber using FOMBLIN Y perfluoropolyether vacuum oil (LVAC 25/6) purchased from Aldrich.16 Data were collected at 100(2) K (at the Unidade de Raios-X, RIAIDT, University of Santiago de Compostela, Spain) on a Bruker SMART 1000 charge-coupled device (CCD) areadetector diffractrometer (Mo KR graphite-monochromated radiation, λ ) 0.7107 Å), controlled by the SMART software package.17 Images were processed using the SAINTPlus software package,18 and data were corrected for absorption by using the semiempirical method implemented in SADABS.19 The structure was solved by the direct methods of SHELXS-9720 and refined by full-matrix least squares on F2 using SHELXL97.21 All non-hydrogen atoms were directly located from difference Fourier maps and refined with anisotropic displacement parameters. Two terminal CH3 methyl groups (C(6) and C(12)) belonging to two peripherally coordinated ETA- residues to Eu(1) were found to be severely affected by thermal disorder, and their position was included in the final structural model by assuming statistical structural disorder (calculated from unrestrained refinements) over two distinct crystallographic positions: C(6) and C(12) were given fixed rates of occupancy of 2/3, while C(6′) and C(12′) were 1/3. The third ETA- external residue coordinated to Eu(1) also shows the entire terminal -CH2CH3 moiety affected by structural disorder. The involved atoms were refined with fixed rates of occupancy (80% and 20%, respectively), which were calculated by employing an unrestrained refinement for this variable. To ensure chemically reasonable environments for these disordered structural moieties, the corresponding C-C bonds were also restrained to a common distance, which ultimately refined to 1.47(1) Å. Even though many of the hydrogen atoms bound to carbon were visible from the last difference Fourier maps, these were instead located at their idealized positions using appropriate HFIX instructions in SHELXL (43 for the aromatic and the conjugated carbon atom belonging to the β-diketonate groups,

J. Phys. Chem. B, Vol. 111, No. 31, 2007 9229

Figure 1. Experimental absorption spectrum of [Eu4(η3-ETA)3(η2ETA)6(µ3-OH)3(H2O)3] and the calculated eletronic absorption spectrum from the optimized geometry by the Sparkle/AM1 model.

23 for the -CH2 moieties, and 137 for the -CH3 methyl groups) and included in subsequent refinement cycles in the ridingmotion approximation with isotropic thermal displacements parameters (Uiso) fixed at 1.2 (for the former groups of hydrogen atoms) or 1.5 (only for the -CH3 moieties) times Ueq of the carbon atom to which they are attached. Hydrogen atoms associated with the µ3-bridging hydroxyl groups (O(1), O(2), and O(3); Figure 2) and coordinated water molecules (O(1W), O(2W), and O(3W)) were successfully located from difference Fourier maps and were ultimately included in the final structural model with the O-H and H‚‚‚H distances restrained to 0.95(1) and 1.55(1) Å, respectively, using a riding model with an isotropic displacement parameter fixed at 1.5 times Ueq of the oxygen atom to which they are attached. Such a procedure envisages, on the one hand, to ensure a chemically reasonable geometry for the water molecules and, on the other hand, to maximize the hydrogen-bonding interactions so to better describe the chemical environment of the involved hydrogen atoms. The last difference Fourier map synthesis showed the highest peak (2.078e Å-3) and deepest hole (-1.146e Å-3) located at 1.59 Å from O(24) and 0.46 Å from C(42), respectively. Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as Supplementary Publication No. CCDC-629091. 3. Theoretical Details 3.1. Ground-State Geometry Optimization and ExcitedStates Calculation. From the X-ray data the ground-state geometry of the complex was calculated with the Sparkle/AM1 model12 implemented in the MOPAC93r2 package.22 The MOPAC keywords used in all Sparkle/AM1 calculations were: GNORM ) 0.25, SCFCRT ) 1.D-10 (to increase the SCF convergence criterion) and XYZ (the geometry optimizations were performed in Cartesian coordinates). For the calculated ground-state geometry we have predicted their singlet and triplet excited states using the intermediate neglect of differential overlap/spectroscopic configuration interaction (INDO/S-CIS) method23-24 implemented in the ZINDO program.25 We have used a point charge of +3e to represent the trivalent europium ion. The singlet excited-state energies and oscillator strengths for each ETA ligand molecule were used

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Souza et al.

Figure 2. (a and b) Top and side views of the [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] complex, emphasizing the inner tetrametallic core surrounded by the ETA- residues (represented as thin sticks). (c) Inner {Eu4O18(OH)3(H2O)3} core of the [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] complex showing the labeling scheme for all non-hydrogen atoms. Hydrogen atoms have been omitted for clarity purposes, and thermal ellipsoids are drawn at the 50% probability level. For bond lengths and angles, see Tables 2-4.

to predict the electronic absorption spectrum of the organic part of the compound. Absorption bands were adjusted to a Lorentzian-shaped curve, compatible with the bandwidth experimentally obtained (20 nm). 3.2. Intensity Parameters. The spontaneous emission coefficients A02 and A04 of the 5D0 f 7F2 and 5D0 f 7F4 transitions, respectively, were calculated by taking the magnetic dipole transition 5D0 f 7F1 as the reference due to the fact that this transition is practically insensitive to the chemical environment around the europium ion. The following expression was used26

( )( )

A0J ) A01

S0J ν01 S01 ν0J

(1)

where S01 and S0J are the areas under the curves of the 5D0 f 7F and 5D f 7F (J ) 2 and 4) transitions, with ν 1 0 J 01 and ν0J being their energy barycenters, respectively. The coefficient of spontaneous emission, A01, in eq 1 is given by the relation A01 ) 0.31 × 10-11(n)3(ν01)3, where n is the index of refraction of the compound (assumed equal to 1.5), and its value is estimated to be around 50 s-1.27 These coefficients of spontaneous emission were used to determine the experimental intensity parameters, Ωλ (λ ) 2 and 4), according to the following expression13

Ωλ )

3ηc3A0λ 4e ω χ〈 Fλ||U || D0〉 2

3

7

(λ) 5

2

(2)

where χ is the Lorentz local field correction term that is given

by χ ) n(n2 + 2)2/9, and 〈7Fλ||U(λ)||5D0〉2 is a squared reduced matrix element whose value is 0.0032 for the5D0 f 7F2 transition and 0.0023 for the 5D0 f 7F4 one.28 From the theory of 4f-4f intensities the theoretical Ωλ intensity parameters are given by29

Ωλ ) (2λ + 1)

|Bλtp|2

∑ t,p 2t + 1

(3)

The quantities Bλtp (t ) 1, 3, 5, and 7) contain the contributions from the forced electric dipole (ed) and dynamic coupling (dc) mechanisms ed dc + Bλtp Bλtp ) Bλtp

(4)

ed ) θ(t,λ)γtp Bλtp

(5)

where

and dc )Bλtp

[

]

(λ + 1)(2λ + 3) (2λ + 1)

1/2

〈4f|rλ|4f〉(1 - σl) 〈f||C(λ)||f〉Γtp δt,λ+1 (6)

In eqs 5 and 6 both γtp (odd-rank ligand field parameters) and Γtp (ligand polarizability dependent term) contain a sum over the ligating atoms involving a spherical harmonic of rank t (Yt,p). The structural aspects and the nature of the chemical environ-

Europium Tetramer Complex ment in the first coordination sphere of the lanthanide ion are precisely taken into account in this sum. In eq 5, θ(t,λ) is a function of the lanthanide ion, and in eq 6 the quantities 〈4f|rλ|4f〉, (1 - σl), and 〈f||C(λ)||f〉 are a radial integral, a shielding factor, and a one-electron reduced matrix element, respectively.30 Once the ground-state geometry has been obtained, one may calculate the theoretical values of the Ωλ parameters from eqs 3-6 by performing the sums over ligating atoms in spherical coordinates with respect to a reference frame centered at the lanthanide ion. In this procedure the charge factors (g) that appear in the ligand field parameters γtp and the ligating atom polarizabilities (R) have been treated as variables within ranges of physically acceptable values.31 In the present case both g and R were separated into three groups, one belonging to the oxygen of the β-ketoesters, one belonging to the oxygen of the OH ligand, and the other one belonging to the oxygen of the water molecule for each Eu3+ ion in the tetramer. The usefulness of this approach is that it allows one to distinguish between the forced electric dipole and dynamic coupling mechanisms, and as a consequence one may in principle obtain important information on the chemical environment around the lanthanide ion. The intensity parameters were calculated for each individual europium ion, and arithmetic average values were obtained for the tetramer.

J. Phys. Chem. B, Vol. 111, No. 31, 2007 9231 TABLE 1: Crystal and Structure Refinement Data for [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3]‚(CH3CO2CH2CH3) formula formula weight crystal system space group a/Å b/Å c/Å R/deg β/deg γ/deg volume/Å3 Z Dc/g cm-3 µ(Mo-KR)/mm-1 F(000) crystal size/mm crystal type θ range index ranges reflections collected independent reflections completeness to θ ) 28.41° final R indices [I > 2σ(I)]a,b final R indices (all data)a,b weighting schemec

4. Results and Discussion 4.1. Characterization. The C and H contents found/ calculated for the compound [Ln4(ETA)9(OH)3(H2O)3] are: C: 27.00/27.40; H: 3.10/2.70. These data are consistent with the formula [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] obtained from the X-ray diffraction results. The absorption spectrum of the compound shows a strong intensification of the n f π* transitions characteristic of the ETA ligand, suggesting that it has been deprotonated. Figure 1 shows that the calculated electronic absorption spectrum agrees qualitatively with the one obtained experimentally, including the number of bands. The IR spectrum of the compound shows bands around 3400 cm-1 assigned to the O-H stretchings from the water and hydroxyl ligands. The bands in the 1600-1700 cm-1 region are assigned to the CdO (ketonic) and CdO (ester) stretches, and a band around 1300 cm-1 can be assigned to the C-O species in the ester. The results indicate that the β-ketoester ligand is coordinated to the lanthanide ion by the oxygen atom of the CdO (ketonic) and CdO (ester) groups, in agreement with the crystal structure. 4.2. Crystal Structure of [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3]‚(CH3CO2CH2CH3). The reaction in dried THF between ethyl 4,4,4-trifluoroacetoacetate and an inorganic source of Eu3+ metallic cations led to the isolation of a white and highly crystalline phase whose crystal structure was elucidated from single-crystal X-ray diffraction studies and ultimately formulated as [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3]‚(CH3CO2CH2CH3), where CH3CO2CH2CH3 is ethyl acetate. Phase purity was further confirmed using data from CHN elemental analysis (see above). Information concerning crystallographic data collection and structure refinement details are summarized in Table 1. Selected bond lengths and angles for the Eu3+ coordination environments are given in Tables 2 -4. Hydrogen-bonding geometries for the most relevant and structurally important O-H‚‚‚O interactions are summarized in Table 5.

largest diff. peak and hole c

C58H71Eu4F27O35 2448.99 triclinic P h1 14.603(3) 14.635(3) 22.884(5) 78.13(3) 81.29(3) 62.48(3) 4236(2) 2 1.920 3.062 2388 0.28 × 0.26 × 0.22 colorless prisms 3.51 to 28.41 -19 e h e 19 -19 e k e 19 -30 e l e 30 77 800 20 919 (Rint ) 0.0544) 98.2% R1 ) 0.0341 wR2 ) 0.0630 R1 ) 0.0619 wR2 ) 0.0727 m ) 0.0159 n ) 10.1139 2.078 and -1.146 eÅ-3

a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) x∑[w(Fo2-Fc2)2]/∑[w(Fo2)2]. w ) 1/[σ2(Fo2) + (mP)2 + nP] where P ) (Fo2 + 2Fc2)/3.

TABLE 2: Selected Bond Lengths (in Å) for the Four Eu3+ Coordination Environments Present in [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3]‚(CH3CO2CH2CH3) Eu(1)-O(1) Eu(1)-O(2) Eu(1)-O(3) Eu(1)-O(4) Eu(1)-O(5) Eu(1)-O(7) Eu(1)-O(8) Eu(1)-O(10) Eu(1)-O(11)

2.400(3) 2.427(3) 2.411(3) 2.491(3) 2.433(3) 2.496(3) 2.420(3) 2.479(3) 2.415(3)

Eu(2)-O(1W) Eu(2)-O(1) Eu(2)-O(3) Eu(2)-O(4) Eu(2)-O(13) Eu(2)-O(14) Eu(2)-O(16) Eu(2)-O(17)

2.374(3) 2.431(3) 2.369(3) 2.453(3) 2.349(3) 2.382(3) 2.396(3) 2.398(3)


2.404(3) 2.383(3) 2.413(3) 2.463(3) 2.399(3) 2.408(3) 2.358(3) 2.377(3)


2.428(3) 2.393(3) 2.420(3) 2.461(3) 2.383(3) 2.416(3) 2.350(3) 2.389(3)

The structure contains an unprecedented and remarkable discrete neutral complex in which the Eu3+ cations are joined together by three µ3-bridging hydroxyl groups (O(1), O(2), and O(3)) plus nine anionic ETA- chelating residues (Figures 2a and 2b). A search in the literature and in the Cambridge Structural Database (CSD, version 5.27, November 2005)32,33 reveals that despite isolated tetramers34-42 or aggregates34,41,43-51 of lanthanide cations being already known the vast majority shares striking topological resemblance to a cubane-type cluster34-39,41-45 (i.e., the core of the complex structures have four lanthanide centers joined together by another four hydroxyl groups placed in opposite vertices of the cuboidal arrangement; Scheme 1a) and only a handful contain Eu3+ cations.34,41,44,45 Thus, to the best of our knowledge, the tetrametallic [Eu4(η3ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] complex represents the first example of a discrete moiety in which the core is best described


Souza et al.

TABLE 3: Bond Angles (in deg) for the Eu(1) and Eu(2) Coordination Environments Present in [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3]‚(CH3CO2CH2CH3) O(1)-Eu(1)-O(2) O(1)-Eu(1)-O(3) O(1)-Eu(1)-O(4) O(1)-Eu(1)-O(5) O(1)-Eu(1)-O(7) O(1)-Eu(1)-O(8) O(1)-Eu(1)-O(10) O(1)-Eu(1)-O(11) O(2)-Eu(1)-O(4) O(2)-Eu(1)-O(5) O(2)-Eu(1)-O(7) O(2)-Eu(1)-O(10) O(3)-Eu(1)-O(2) O(3)-Eu(1)-O(4) O(3)-Eu(1)-O(5) O(3)-Eu(1)-O(7) O(3)-Eu(1)-O(8) O(3)-Eu(1)-O(10)

69.02(10) 69.07(10) 67.01(9) 137.02(10) 69.94(10) 94.31(10) 127.88(9) 141.60(9) 128.18(9) 142.84(10) 67.04(9) 70.31(9) 69.17(10) 70.55(10) 93.91(10) 127.89(9) 142.68(10) 66.90(9)

O(3)-Eu(1)-O(11) O(4)-Eu(1)-O(7) O(5)-Eu(1)-O(4) O(5)-Eu(1)-O(7) O(5)-Eu(1)-O(10) O(8)-Eu(1)-O(2) O(8)-Eu(1)-O(4) O(8)-Eu(1)-O(5) O(8)-Eu(1)-O(7) O(8)-Eu(1)-O(10) O(10)-Eu(1)-O(4) O(10)-Eu(1)-O(7) O(11)-Eu(1)-O(2) O(11)-Eu(1)-O(4) O(11)-Eu(1)-O(5) O(11)-Eu(1)-O(7) O(11)-Eu(1)-O(8) O(11)-Eu(1)-O(10)

137.46(10) 119.13(10) 70.14(10) 138.20(10) 72.63(10) 137.34(9) 72.20(10) 75.47(10) 70.37(9) 137.81(10) 119.96(10) 120.17(10) 93.14(10) 138.66(10) 77.09(10) 71.80(10) 75.78(11) 70.76(10)

O(1W)-Eu(2)-O(1) O(1W)-Eu(2)-O(4) O(1W)-Eu(2)-O(14) O(1W)-Eu(2)-O(16) O(1W)-Eu(2)-O(17) O(1)-Eu(2)-O(4) O(3)-Eu(2)-O(1W) O(3)-Eu(2)-O(1) O(3)-Eu(2)-O(4) O(3)-Eu(2)-O(14) O(3)-Eu(2)-O(16) O(3)-Eu(2)-O(17) O(13)-Eu(2)-O(1W) O(13)-Eu(2)-O(1)

76.45(10) 143.24(10) 85.00(11) 70.96(10) 141.85(10) 67.14(9) 100.33(10) 69.24(10) 71.91(10) 149.45(10) 78.59(10) 79.08(10) 81.07(11) 70.26(10)

O(13)-Eu(2)-O(3) O(13)-Eu(2)-O(4) O(13)-Eu(2)-O(14) O(13)-Eu(2)-O(16) O(13)-Eu(2)-O(17) O(14)-Eu(2)-O(1) O(14)-Eu(2)-O(4) O(14)-Eu(2)-O(16) O(14)-Eu(2)-O(17) O(16)-Eu(2)-O(1) O(16)-Eu(2)-O(4) O(16)-Eu(2)-O(17) O(17)-Eu(2)-O(1) O(17)-Eu(2)-O(4)

137.83(10) 82.08(10) 72.61(11) 138.41(10) 124.82(11) 140.49(10) 120.31(10) 74.80(10) 78.51(10) 128.50(10) 137.62(10) 71.58(10) 135.16(9) 73.45(10)

TABLE 4: Bond Angles (in deg) for the Eu(3) and Eu(4) Coordination Environments Present in [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3]‚(CH3CO2CH2CH3) O(2W)-Eu(3)-O(2) O(2W)-Eu(3)-O(7) O(2W)-Eu(3)-O(20) O(1)-Eu(3)-O(2W) O(1)-Eu(3)-O(2) O(1)-Eu(3)-O(7) O(1)-Eu(3)-O(19) O(1)-Eu(3)-O(20) O(2)-Eu(3)-O(7) O(19)-Eu(3)-O(2W) O(19)-Eu(3)-O(2) O(19)-Eu(3)-O(7) O(19)-Eu(3)-O(20) O(20)-Eu(3)-O(2)

77.00(10) 144.05(10) 141.35(10) 103.94(10) 69.54(10) 70.77(10) 77.16(10) 79.48(10) 67.77(9) 71.68(11) 126.38(10) 136.42(10) 71.71(11) 136.75(10)

O(20)-Eu(3)-O(7) O(22)-Eu(3)-O(2W) O(22)-Eu(3)-O(1) O(22)-Eu(3)-O(2) O(22)-Eu(3)-O(7) O(22)-Eu(3)-O(19) O(22)-Eu(3)-O(20) O(22)-Eu(3)-O(23) O(23)-Eu(3)-O(2W) O(23)-Eu(3)-O(1) O(23)-Eu(3)-O(2) O(23)-Eu(3)-O(7) O(23)-Eu(3)-O(19) O(23)-Eu(3)-O(20)

74.04(10) 78.51(11) 140.69(10) 73.02(10) 84.46(10) 137.39(10) 123.16(11) 72.55(11) 84.71(11) 146.42(10) 143.52(10) 119.96(10) 74.98(10) 74.27(10)

O(3W)-Eu(4)-O(10) O(2)-Eu(4)-O(3W) O(2)-Eu(4)-O(3) O(2)-Eu(4)-O(10) O(2)-Eu(4)-O(26) O(3)-Eu(4)-O(3W) O(3)-Eu(4)-O(10) O(25)-Eu(4)-O(3W) O(25)-Eu(4)-O(2) O(25)-Eu(4)-O(3) O(25)-Eu(4)-O(10) O(25)-Eu(4)-O(26) O(25)-Eu(4)-O(29) O(26)-Eu(4)-O(3W)

143.49(10) 102.76(10) 69.57(10) 71.18(9) 77.61(10) 77.01(10) 67.06(9) 71.52(10) 78.21(10) 127.96(9) 137.28(10) 71.89(10) 74.36(10) 142.46(10)

O(26)-Eu(4)-O(3) O(26)-Eu(4)-O(10) O(28)-Eu(4)-O(3W) O(28)-Eu(4)-O(2) O(28)-Eu(4)-O(3) O(28)-Eu(4)-O(10) O(28)-Eu(4)-O(25) O(28)-Eu(4)-O(26) O(28)-Eu(4)-O(29) O(29)-Eu(4)-O(3W) O(29)-Eu(4)-O(2) O(29)-Eu(4)-O(3) O(29)-Eu(4)-O(10) O(29)-Eu(4)-O(26)

134.38(10) 72.96(10) 79.36(11) 138.84(10) 71.03(10) 83.00(10) 137.97(10) 125.17(10) 72.79(10) 83.52(11) 148.23(10) 141.38(10) 121.11(10) 78.87(10)

as an open-cubane-type cluster structure (Scheme 1b) with the position of the fourth µ3-bridging hydroxyl group being instead occupied by three coordinated water molecules, which are further involved in strong hydrogen bonds (Figure 2c and the text in the following paragraphs). The [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] complex comprises four crystallographically independent Eu3+ cations. Eu-

TABLE 5: Hydrogen-Bonding Geometry (Distances in Å and Angles in deg) for [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3]‚(CH3CO2CH2CH3)a D-H···A

d(D···A)

∠(DHA)

O(1)-H(1)···O(13) O(1)-H(1)···O(20) O(2)-H(2)···O(26) O(3)-H(3)···O(28) O(3)-H(3)···O(17) O(1W)-H(1A)···O(19) O(1W)-H(1B)···O(31)a O(2W)-H(2A)···O(25) O(2W)-H(2B)···O(31)a O(3W)-H(3A)···O(31)a O(3W)-H(3B)···O(16)

2.752(4) 3.063(4) 3.014(4) 2.772(4) 3.034(4) 2.653(4) 2.814(5) 2.698(4) 2.853(5) 3.091(5) 2.709(4)

112(3) 110(3) 111(3) 114(3) 114(3) 164(5) 143(3) 172(4) 152(3) 145(3) 161(4)

a Symmetry transformation used to generate equivalent atoms: (i) -1+x, y, z.

SCHEME 1: Schematic Drawings of the: (a) Known Cubane-like {Ln4(µ3-OH)4} Clusters and (b) {Eu4(µ3OH)3(H2O)3} Core of the [Eu4(η3-ETA)3(η2-ETA)6(µ3OH)3(H2O)3] Complex, Emphasizing the Replacement of a µ3-Bridging Hydroxyl Group by Three Coordinated Water Molecules

SCHEME 2: Coordination Modes of the ETA- Organic Residue (via the β-Diketonate Groups) in the [Eu4(η3ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] Complex: (a) O,O-Chelate and (b) O,O-Chelate Associated with Bridging Anti-Unidentate

(2), Eu(3), and Eu(4) are each coordinated to three ETAresidues (one bound in a unidentate fashion, and two forming O,O-chelates via the β-diketonate groups; Scheme 2), one water molecule, and two µ3-bridging hydroxyl groups, describing {EuO8} coordination environments that resemble distorted square antiprisms as depicted in Figure 3a (see Tables 3 and 4 for bond angles). For these three coordination environments, the Eu-O bond lengths are found to be in the 2.349(3)-2.463(3) Å range (Figures 2c and 3a; Table 2), which is in good agreement with the expected values for chelating β-diketonate groups, as revealed by a search of the CSD (from 90 entries, a range of 2.24-2.53 Å with a median of 2.37 Å). Remarkably, Eu(1) has a markedly distinct coordination environment: On the one hand, it is coordinated to three ETA- residues, which are all O,O-chelated to this lanthanide center (O(4,5), O(7,8), and O(10,11); Figure 2c) and further establish unidentate bridges

Europium Tetramer Complex

Figure 3. Polyhedral schematic representations of the Eu3+ coordination environments present in the [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] complex, {EuOx} (where x ) 8 for Eu(2), Eu(3), and Eu(4); x ) 9 for Eu(1)): (a) distorted square antiprisms of Eu(2-4) and (b) distorted tricapped trigonal prism of Eu(1). Hydrogen atoms associated with the coordinated water molecules and the µ3-bridging hydroxyl groups have been omitted for clarity purposes. For bond lengths and angles in the {EuOx} coordination environments, see Tables 2-4.

with each one of the formerly described metallic centers (via O(4), O(7), and O(10); Figure 2c and Scheme 2b); on the other hand, Eu(1) is also coordinated to the three crystallographically independent µ3-bridging hydroxyl groups. Therefore, Eu(1) appears in the nine-coordinated crystal structure, {EuO9}, with the coordination sphere resembling a highly distorted tricapped trigonal prism (Figure 3b), in which all the Eu-O bond distances are within the 2.400(3)-2.496(3) Å range (Table 2). It is of considerable interest to note that the longer bond lengths for the Eu(1) coordination environment are precisely those with the oxygen atoms of the chelating β-diketonate residues that establish the anti-unidentate connections with the eightcoordinated lanthanide centers (O(4), O(7), and O(10)). Noteworthy is also the fact that these oxygen atoms correspond to those occupying the capping positions of the distorted tricapped trigonal prism (Figure 2c). Thus, due to the large coordination number of Eu(1) (which creates further steric repulsion between coordinated oxygen atoms), it is reasonable to assume that these

J. Phys. Chem. B, Vol. 111, No. 31, 2007 9233 interactions with Eu(1) are weaker than the remaining ones, being ultimately reflected in the registered long interatomic distances. The organic primary building blocks play a pivotal role in the formation of this unprecedented open-cubane-type lanthanide core (Scheme 1b and Figure 2). Indeed, the presence of a β-diketonate chelating group in the ETA- residues allows the strong coordination of a single molecule to two vacant coordinative positions of each Eu3+ cation via a typical O,Ochelating fashion as described above (η2-ETA-; Scheme 2a). This O,O-chelating coordination mode is substantially different from that described for related structures containing carboxylate groups, as observed for the vast majority of related cubanetype clusters,34-39,41-45 in the sense that these latter moieties preferentially establish syn,syn-bridges between neighboring cations belonging to the core of the clusters. The exceptions are for the structures reported by Serre et al.41 and Zheng et al.43 Nonetheless, in these structures the dominant coordination mode is still the syn,syn-bridging one, and the lanthanides tend to organize into aggregates rather than into isolated tetramers.41,43 To the best of our knowledge, the only crystallographic detailed report of a tetrameric lanthanide cubane-like structure containing β-diketonate chelating groups is that of Plakatouras et al.40 for [Gd4(µ3-OH)4(µ2-H2O)2(H2O)4(hfpd)8] (where hfpd-H is 1,1,1,5,5,5-hexafluoropentane-2,4-dione). In this material, hfpd- residues are also O,O-chelated to the lanthanide centers, but surprisingly, and in a very different way as for [Eu4(η3ETA)3(η2-ETA)6(µ3-OH)3(H2O)3],the cubane-like {Gd4(µ3OH)4} core remains intact. It is feasible to assume that the presence of the extra -CF3 external moiety in the hfpd- residue (when compared with ETA-) creates additional steric repulsion between chelated ligands, thus leading to a smaller degree of substitution in the lanthanide coordination sphere, which must be simultaneously accompanied by coordination to more water molecules and the inclusion of charge-balancing hydroxyl groups. This does not occur for [Eu4(η3-ETA)3(η2-ETA)6(µ3OH)3(H2O)3], and we attribute such a fact to the structural presence of the -O-CH2-CH3 external arm in ETA-, which seems to induce the occurrence of a considerable number of weak C-H‚‚‚F hydrogen-bonding interactions (not shown). These cooperative interactions further stabilize the structure, thus allowing a more effective close packing of the organic residues around the lanthanide centers (Figure 2). This latter structural evidence is directly reflected in the presence of a second coordination mode for the ETA- organic molecules (only for those mainly coordinated to Eu(1)) in which, apart from the aforementioned O,O-chelate, the β-keto-oxygen atom is further engaged in the already mentioned anti-unidentate coordination fashion with the closest lanthanide center (η3-ETA-; Scheme 2b and Figure 2). It is worth mentioning that this mode does not occur for the material reported by Plakatouras et al.40 The presence of two coordination modes for the organic ETAresidues also leads to two bite angles with the Eu3+ cations: While η2-ETA- subtends an average angle of ∼72.2°, for the η3-ETA- moieties the angle decreases to ∼70.4° (Figures 2c and 3; Tables 3 and 4), with the latter tendency having a geometrical explanation based on the longer Eu-O bond lengths (as described above). Nevertheless, both values are found within the expected range for related chemical environments as revealed by a search of the CSD (102 entries, a range of 65.9-76.2° with a median value for the bite angle of 71.4°). The tetrameric core of [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] is thus composed of four Eu3+ cations coupled together by only three µ3-bridging hydroxyl groups that cap three faces


Souza et al.

Figure 4. Schematic representation of the tetrametallic core of the [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] complex, emphasizing the various intermetallic distances.

of the distorted metallic tetrahedron (Figures 2c and 4). The Eu-Ohydroxyl distances are found to be in the 2.369(3)-2.431(3) Å range (Table 1), which agree well with the expected values reported for the only three known related structures (a range of 2.40-2.47 Å with a median of 2.43 Å).34,41 Notably, the shortest distances are statistically smaller than their counterparts in related materials. Indeed, the absence of the fourth µ3-bridging hydroxyl group in the tetrahedral face composed of square antiprismatic Eu3+ centers (Figures 2c and 3) leads to a reduction in steric hindrance between the remaining hydroxyl moieties that are, in comparison with the geometry for the more commonly encountered {Ln4(µ3-OH)4} cluster, raised toward the Eu(2)‚‚‚Eu(3)‚‚‚Eu(4) face. This structural modification results, on the one hand, in significantly shorter Eu-Ohydroxyl bond lengths and, on the other hand, leads to an average intermetallic Eu‚‚‚Eu distance (for this tetrahedron face) of ∼4.53 Å (Figure 4), which is significantly longer than those reported by Serre et al.41 and Wang et al.36 for the intra-tetramer analogous separations. (All intermetallic distances are shorter than ∼4 Å.) The second family of Eu‚‚‚Eu distances registered for the {Eu4(µ3-OH)3(H2O)3} core (average value of about 3.67 Å) is comparable with the expected values. Apart from the neutral [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] complex, the asymmetric unit also comprises an ethyl acetate molecule (CH3CO2CH2CH3) that interacts directly with the complex via strong and highly directional hydrogen-bonding interactions with the coordinated water molecules, namely, with those bound to Eu(2) and Eu(3) (Figure 5 and Table 5): Since the CdO oxygen atom of CH3CO2CH2CH3 is only capable of being the acceptor for two simultaneous hydrogen-bonding interactions, the more energetically favorable are those with O(1W) and O(2W) due to the closer interatomic D‚‚‚A distances (Table 5); the interaction with O(3W), despite its directionality (∠(DHA) of ∼152°), is also not very strong due to a D‚‚‚A distance greater than 3 Å (Table 5). Interestingly, the relationship between these molecular entities can be envisaged as a hostguest system, in which the ethyl acetate plays the role of a guest included in a hydrophilic cavity of a centered optically active calyx-type distribution of Eu3+ metallic centers (hereafter designated as calyx[4]Eu). The coordinated water molecules are further engaged in very strong (average D‚‚‚A distance of 2.69 Å) and highly directional (average ∠(DHA) of 166°) intra{Eu4(µ3-OH)3(H2O)3} core O-H‚‚‚O hydrogen bonds, forming three S(6) graph set motifs (Figure 5).52 Due to their internal location in the {Eu4(µ3-OH)3(H2O)3} core, the µ3-bridging hydroxyl groups interact only very weakly (i.e., not very

Figure 5. Schematic representation of the strongest and more directional (i.e., ∠(DHA) greater than 150°) hydrogen-bonding interactions involving the coordinated water molecules, which occur within the {Eu4(µ3-OH)3(H2O)3} core of the [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] complex and between this core and the encapsulated ethyl acetate molecule. Hydrogen atoms that are not directly involved in a hydrogen bond have been omitted for clarity purposes. Hydrogen bonds are represented as dashed green lines. For hydrogen-bonding geometry, see Table 5. Symmetry transformation used to generate equivalent atoms: (i) -1 + x, y, z.

directional) via hydrogen bonds with neighboring β-keto-oxygen atoms (Figure 2c and Table 5). The close packing in the solid state of the host-guest [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3]‚(CH3CO2CH2CH3) inclusion entities is essentially mediated by the combined need to effectively fill the space in conjunction with a significant (and energetically cooperative) number of weak intercomplex C-H‚‚‚F hydrogen bonds involving the two terminal groups of ETA- residues (not shown). As mentioned in the Experimental Section dedicated to the details of the single-crystal X-ray diffraction studies, the three terminal -CH2-CH3 moieties associated with the η3-ETA- residues coordinated to Eu(1) were found to be disordered in the structure, as also depicted in Figure 2b (at the bottom), showing the two crystallographic positions for each -CH3. Indeed, for each one of these positions, a number of the aforementioned C-H‚‚‚F interactions between neighboring complexes are structurally (and energetically) possible, thus being most likely the main reason for such welldefined statistical disorder of each group. Individual [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] complexes pack in a parallel fashion, thus being distributed in layers placed on the ab-plane of the unit cell. These layers are then packed along the third crystallographic direction in a typical ABAB... fashion (Figure 6). The minimum spatial separation between the centers of gravity of {Eu4(µ3-OH)3(H2O)3} cores



Figure 6. Mixed stick (for the ETA- residues) and polyhedral representation (for the Eu3+ coordination environments) of the crystal packing of [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3]‚(CH3CO2CH2CH3) viewed in perspective along the (100) crystallographic direction. Hydrogen atoms and hydrogen-bonding interactions have been omitted for clarity purposes.

Figure 7. (a) Emission spectrum of the [Eu4(η3-ETA)3(η2-ETA)6(µ3OH)3(H2O)3] complex at 77 K. (b) 5D0 f 7F0 region magnified in 900 times.

belonging to neighboring complexes (from adjacent layers) is 12.912(1) Å. 4.3. Photoluminescence Results. Figure 7 shows the emission spectrum of the complex recorded in the 570-720 nm spectral range at 77K for the solid sample. The five emission bands corresponding to the 5D0 f 7F0,1,2,3,4 transitions, typical of the Eu3+ ion, can be clearly identified. A feature that may be noticed is the presence of lines in a number higher than the maximum 2J + 1 in the cases of the 5D0 f 7F0,1,2 transitions. In the region of the 5D0 f 7F0 transition (inset) four peaks (17 226, 17 238, 17 247, and 17 256 cm-1) could be identified within a width at half-height of 37 cm-1, in agreement with the tetrameric form of the complex as given by the X-ray data and

Figure 8. Excitation spectrum at 298 K of the complex [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3], monitored at 612 nm.

ground-state geometry calculations. This indicates the existence of at least four different Eu3+ coordination environments. Even though the average distance between the Eu3+ ions in the tetrameric moiety is rather short (3.67 Å), we have found neither evidence of cooperative ion-ion effects nor vibronic interaction leading to clearly distinguishable additional peaks in the emission and excitation spectra. These effects might indeed be operative in organolanthanide compounds and have been observed and studied, particularly in the cases of Pr3+ and Nd3+ in dimeric and polymeric forms with peptides and amino acid ligands.53-56 In our case all peaks have been assigned to emissions from the four different Eu3+ environments in the tetramer. Vibronic interactions with the ligand water molecules and bridging hydroxyl groups are quite possibly attenuated due


Souza et al.

TABLE 6: Spherical Coordinates from the Sparkle/AM1 Coordination Polyhedron of the [Eu4(ETA)9(OH)3.(H2O)3] Complex [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] site 1

atom Eu3+ O (OH) O (ketoester)

site 2

Eu3+ O (OH) O (water) O (ketoester)

site 3


site 4


to the strong hydrogen bonding involving these species, as discussed in subsection 4.2. Figure 8 shows the excitation spectrum of the Eu3+ complex obtained at 298 K in the spectral range from 200 to 600 nm and monitored at 613 nm. The spectrum displays a broad band with a maximum at 357 nm, which is attributed to the ETAcentered absorption and subsequent intramolecular energy transfer to the Eu3+ ion. It is also noted that this broad band partially overlaps the sharp peaks assigned to intraconfigurational 4f-4f transitions from the ground state 7F0 to the following excited levels: 5L6 (397 nm), 5D3 (417 nm), 5D2 (467 nm), 5D1 (534 nm), and 5D0 (588 nm). Both the broad band at 357 nm and the intraconfigurational 4f-4f transitions in the excitation spectrum have roughly the same intensities. When compared with the absorption spectrum shown in Figure 1, this is a rather puzzling result since at 357 nm there is practically no absorption by the ETA ligand. Moreover, the ligand triplet position (19 000 cm-1), as calculated from the Sparkle model, is in a favorable relative energy position for intramolecular energy transfer. For the moment we have no satisfactory explanation for this fact. One possibility is that the ligand internal conversion S1 f S0 exceeds by far the intersystem crossing S1 f T and that the low-energy tail of the ETA absorption strongly overlaps with the excited Eu3+ level above 5L6. Emission quantum yield measurements are being planned to help elucidate this problem. 4.4. Optimized Geometries. The ground-state geometry of the complex was obtained via the Sparkle/AM1 model as described previously. Table 6 presents the atomic coordinates

R (Å)

θ (deg)

φ (deg)

0.000 2.380 2.325 2.380 2.443 2.400 2.447 2.399 2.446 2.398

0.000 102.248 53.158 48.590 108.963 85.147 117.614 166.530 22.513 88.208

0.000 36.540 81.687 355.018 324.618 260.758 105.300 216.706 195.416 175.070

3.645 2.380 2.380 4.997 4.625 5.948 5.267 5.082 2.443

87.248 102.248 48590 77.918 116.613 93.982 64.170 79.892 108.963

0.346 35.540 355.018 25.595 9.296 4.398 0.411 335.612 324.618

3.667 2.380 2.380 5.031 5.295 5.118 4.558 5.949 2.447

90.219 102.248 48.590 68.390 94.506 115.382 76.513 84.744 117.614

73.473 36.540 355.018 58.213 50.944 70.574 101.888 80.154 105.300

3.666 2.325 2.380 5.000 5.365 5.199 4.542 5.933 2.446

20.175 53.178 48.590 45.545 39.518 23.842 25.631 19.558 22.513

46.955 81.687 355.018 29.711 68.613 114.577 319.500 19.438 195.416

for the coordination polyhedron of the complex. As shown in Figure 9, the complex possesses one center where the Eu3+ ion is coordinated with three ETA ligands and three OH bridging moieties. The other three Eu3+ ions are coordinated to five

Figure 9. Calculated ground-state geometry, using the Sparkle/AM1 model, for the [Eu4(ETA)9(OH)3·3H2O] complex.



TABLE 7: Experimental and Theoretical Values of the Intensity Parameters, Ω2 and Ω4 (in Units of 10-20 cm2), and Coefficients of Spontaneous Emission A0λ (in Units of s-1)a [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3]

Ωed 2

Ωdc 2

Ωtotal 2

Ωed 4

Ωdc 4

Ωtotal 4

A02

A04

experimental theoretical (X-ray) theoretical (Sparkle)

1.13 1.12

8.30 7.85

11.14 10.80 10.82

0.20 0.22

2.08 2.54

6.40 3.40 3.90

336.0 325.0 326.0

81.60 43.0 49.0

a The forced electric dipole (ed) and dynamic coupling (dc) contributions were calculated by considering each mechanism separately without interference effects.

TABLE 8: Charge Factors and Polarizabilities Used in the Calculation of the Theoretical Intensity Parameters for [Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3] Complex X-ray

Sparkle

[Eu4(η3-ETA)3(η2-ETA)6(µ3-OH)3(H2O)3]

atom

R (Å3)

g

R (Å3)

g

site 1

O (OH) O (ketoester)

1.0 3.5

1.0 1.5

1.0 3.4

1.0 1.5

site 2

O (water) O (OH) O (ketoester)

2.5 4.0 1.0

0.8 1.6 1.5

4.0 4.5 1.8

1.0 1.5 1.6

site 3


3.6 4.0 1.0

1.0 1.6 1.6

3.0 3.1 2.0

1.0 1.6 1.4

site 4


4.0 4.5 1.0

0.5 1.6 1.2

3.0 4.5 1.0

0.5 1.5 1.4

β-diketonate groups of the ETA ligands, two OH moieties, and one water molecule. The site symmetry of the tetramer complex is C1, consistent with the luminescence spectrum and with the X-ray structural data. 4.5. Theoretical Intensity Parameters. Table 7 presents the values of the theoretical intensity parameters, calculated from eqs 3-6, with the odd-rank ligand field parameters given by the simple overlap model.57 The corresponding values of the coefficients of spontaneous emission are also presented. Since the charge factors, g, and ligating atoms polarizabilities, R, were treated as varying parameters (Table 8), the good agreement with respect to the experimental values, determined from the emission spectrum at 77 K and eq 2, not only corroborates the 4f-4f intensity model but also reflects the correctness of the structural data. The theoretical calculations clearly indicate a much larger contribution from the dynamic coupling mechanism ed (Ωdc λ . Ωλ ), in agreement with similar previous calculations 3+ for Eu complexes with organic ligands.58 5. Concluding Remarks A new compound of trivalent europium with β-ketoester (ETA) ligands has been synthesized and characterized. Detailed single-crystal X-ray diffraction data revealed that the compound is a tetrameric complex of trivalent europium. The results of elemental analysis and IR vibrational and UV-vis absorption spectroscopies are in accordance with the crystal structure. The characteristic transitions of the Eu3+ ion presented a number of peaks higher than the maximum (2J + 1) for each 7F manifold. In the particular case of the 5D f 7F transition J 0 0 at least four peaks have been observed, in agreement with the tetrameric form of the complex. From the X-ray data, the Sparkle/AM1 model has been used to optimize the coordination geometry, allowing the modeling of the complex and the calculation of some of its main photoluminescence parameters. The theoretical 4f-4f intensity parameters (an average over the four metallic centers), calculated on the basis of the structural data, are in good agreement with the experimental values. The calculations have shown that the

dynamic coupling mechanism (Ωdc λ ) is largely dominant. This indicates that the europium ion is in a highly polarizable medium. A systematic study of this complex as a potential candidate for an optical marker in fluoroimmunoassays and an electroluminescent material for OLEDs is in progress. Finally, the tetrameric form of the complex suggests that it might be a useful system in studies of inter-ion energy transfer processes such as up-conversion and cross-relaxation. Acknowledgment. The authors acknowledge the financial support from CAPES and CNPq (Brazilian agencies), Instituto do Mileˆnio de Materiais Complexos, and Rede de Nanotecnologia Molecular e de Interfaces, Brazil. The Centro Nacional de Processamento de Alto Desempenho at Campinas, Brazil, is also acknowledged for having made available to us their computational facilities. The authors are also grateful to Professor H. F. Brito (Instituto de Quı´mica, Universidade de Saõ Paulo) for the help with the characterization of the complex. References and Notes (1) Reyes, R.; Cremona, M.; Teotonio, E. E. S.; Brito, H. F.; Malta, O. L. Thin Solid Films 2004, 469-470, 59. (2) Pietraszkiewicz, M.; Karpiuk, J.; Rout, A. K. Pure Appl. Chem. 1993, 65, 563. (3) Lanthanide Probes in Life, Chemical, and Earth Sciences: Theory and Practice; Bu¨nzli, J.-C. G., Choppin, G. R., Eds.; Elsevier: Amsterdam, 1989. (4) Gawryszewska, P.; Sokolnichi, J.; Legendziewicz, J. Coord. Chem. ReV. 2005, 249, 2489-2509. (5) Gawryszewska, P.; Malta, O. L.; Longo, R. L.; Silva, F. R. G.; Alves, S.; Mierzwicki, K.; Latajka, Z.; Pietraszkiewicz, M.; Legendziewicz, J. ChemPhysChem 2004, 5, 1577-1584. (6) Freeman, J.; Crosby, G. H.; Lawson, K. E. J. Mol. Spectrosc. 1964, 13, 399. (7) Dolg, M.; Stoll, H.; Savin, A.; Preuss, H. Theor. Chim. Acta 1989, 75, 173. (8) Cundari, T. R.; Stevens, W. J. Chem. Phys. 1993, 98, 5555. (9) Dolg, M. In Modern Methods and Algorithms of Quantum Chemistry; Grotendorst, J. Ed.; John von Neumann Institute for Computing, Ju¨lich, Germany, 2000; p 479. (10) Rocha, G. B.; Freire, R. O.; da Costa, N. B., Jr.; de Sa´, G. F.; Simas, A. M. Inorg. Chem. 2004, 43, 2346.

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Synthesis, Crystal Structure, and Modelling of a New Tetramer

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