Synthesis and characterization of new heterodinuclear (Eu, Tb) lanthanide pivalates

Polyhedron 50 (2013) 297–305

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Synthesis and characterization of new heterodinuclear (Eu, Tb) lanthanide pivalates Irina G. Fomina a,⇑, Zhanna V. Dobrokhotova a, Grygory G. Aleksandrov a, Valery I. Zhilov a, Irina P. Malkerova a, Andrei S. Alikhanyan a, Denis M. Zhigunov b, Artem S. Bogomyakov c, Vasilisa I. Gerasimova d, Vladimir M. Novotortsev a, Igor L. Eremenko a a

N.S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninsky Prosp. 31, 119991 Moscow, GSP-1, Russian Federation Faculty of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory 1/3, 119991 Moscow, GSP-1, Russian Federation International Tomography Center, Siberian Branch of the Russian Academy of Sciences, Institutskaya Str. 3a, 630090 Novosibirsk, Russian Federation d Scobel’tsyn Research Institute of Nuclear Physics, M.V. Lomonosov Moscow State University, Leninskie Gory 1/2, 119991 Moscow, GSP-1, Russian Federation b c

a r t i c l e

i n f o

Article history: Received 20 September 2012 Accepted 26 October 2012 Available online 16 November 2012 Keywords: Heterodinuclear lanthanide pivalates Synthesis X-ray diffraction study Magnetic and luminescence properties Thermal behavior

a b s t r a c t New heterodinuclear pivalates (HPiv)6EuTb(Piv)6 (5) and (Phen)2EuTb(Piv)6 (6), where Piv = (CH3)3CCO2, the 2,2-dimethylpropanoate anion, and Phen = 1,10-phenanthroline, were synthesized. The structures and compositions of the heterodinuclear molecules were determined by X-ray diffraction and based on magnetic measurements, inductively coupled plasma atomic emission spectrometry (ICP-AES) and the results of the research on the thermal decomposition and vaporization of the complexes. Under UV excitation, complex 6 exhibits luminescence and complete intramolecular excitation energy transfer to the Eu3+ ions involving singlet levels of the Phen ligand and f–f levels of the Tb3+ ions. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction The chemistry of molecular lanthanide complexes with organic ligands currently attracts great interest because of the unique physicochemical properties of these complexes and their potential in applications such as molecule-based functional materials in many fields; for example, they can be used as magnetic, luminescent or absorbing materials, sensors, and so on [1–12]. In spite of the progress in the chemical design of such complexes, the relationships between the composition and structure of lanthanide coordination compounds, on the one hand, and their physical properties, on the other, including magnetic and luminescence properties, which originate from the nature of the electronic structure of lanthanide ions, are still poorly known. The establishment of these relationships is a key problem, and its solution will enable one to vary, as desired, the molecular architecture of complex precursors to prepare materials with particular functional properties. In this respect, interest in carboxylate derivatives of lanthanides stems from the fact that the methods for their syntheses are quite simple and these compounds, which generally have dinuclear [13–17] or polymeric [1,18–20] structures, can be modified as desired by changing the carboxylate anion or introducing neutral organic Nand O-donor ligands, thus making it possible to vary the physical ⇑ Corresponding author. Tel.: +7 495 955 4835; fax: +7 495 952 1279. E-mail address: [email protected] (I.G. Fomina).

and chemical properties of the newly designed molecules, as well as their thermal behavior. In addition, lanthanide ions possess very similar chemical and structural properties but exhibit very different physical properties. Hence, it must be possible to design diverse types of heterometallic structures [21–25] exhibiting specific physical properties via the choice of the rare earth ions. Earlier, we started a systematic research on the thermal behavior and the magnetic and photoluminescence properties of pivalate (2,2-dimethylpropanoate) derivatives of lanthanides(III) [26–28]. Pivalate anions belong to hard Lewis bases and strongly bind to lanthanide(III) ions exhibiting pronounced hard Lewis acid properties. The presence of the branched electron-donating tert-butyl group in the carboxylate anion provides the solubility of these compounds in organic solvents, which makes it possible to prepare crystals of lanthanide pivalates suitable for studies by physicochemical methods, including X-ray diffraction analysis. In particular, it was shown that exchange reactions of aqueous lanthanide acetates with pivalic acid afford dinuclear pivalates (HPiv)6M2 (Piv)6 (M = Eu (1) or Tb (2)) [26,27]. In these complexes, the metal atoms readily bind to heterocyclic chelating N-donors in ethanol to form thermally stable highly luminescent dinuclear complexes (Phen)2M2(Piv)6 (M = Eu (3) or Tb (4)), in which Phen acts as the efficient antenna chromophore transferring the excitation energy to the central trivalent lanthanide ion [27,28]. A magnetic investigation of these dinuclear pivalates showed that these compounds exhibit different magnetic behavior [27].

0277-5387/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2012.10.051

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In the present study, we examined the possibility of the synthesis of heterodinuclear pivalates containing Eu(III) and Tb(III) atoms in a ratio of 1:1 and neutral HPiv or Phen molecules. These compounds were characterized by X-ray diffraction. The solid-state thermolysis and vaporization, as well as magnetic and photoluminescence properties, of these complexes were studied. 2. Experimental 2.1. Synthesis The known compounds 1–4 and {Tb(Piv)3}n (7) were synthesized according to the procedures described earlier [26–28]. Reagents and solvents were commercially available; HPiv (Acros Organics), M(OOCCH3)3xH2O (M = Eu, Tb), 1,10-phenanthroline monohydrate (Alfa Aesar) and ethanol (Aldrich) were analyticalgrade reagents and were used without further purification. 2.1.1. Synthesis of (HPiv)6EuTb(Piv)6 (5) To Eu(OOCCH3)34H2O (0.25 g, 0.62 mmol) and Tb(OOCCH3)3 4H2O (0.25 g, 0.62 mmol), HPiv (1.0 g, 9.79 mmol) was added. The reaction mixture was heated in air at 100 °C until the starting reagents completely dissolved. The solution was kept at 20 °C. The crystals of 5 that precipitated within 24 h were separated by decantation and dried in air. The yield of compound 5 was 0.9 g (95% with respect to the starting europium compound). The evaluated stoichiometry of the Eu and Tb atoms in the crystals of 5 is 1.04 ± 0.05:1.02 ± 0.05. Found (%): C, 47.09; H, 7.53. Calculated for C60H114Eu1.0O24Tb1.0 (%): C, 47.09; H, 7.51. IR (KBr), m, cm1: 1687 (m) for m(COOH); 1597 (m), 1560 (m), 1551 (m) for mas (COO); 1414 (s) for ms(COO). 2.1.2. Synthesis of (Phen)2EuTb(Piv)6 (6) To compound 5 (0.5 g, 0.33 mmol) and PhenH2O (0.39 g, 1.96 mmol), ethanol (50 ml) was added. The reaction mixture was heated in air at 80 °C until the starting reagents completely dissolved. The solution was filtered, concentrated at 0.1 Torr and 20 °C to 30 ml, and kept at 20 °C. The crystals of 6 that precipitated within 24 h were separated by decantation, washed with cold ethanol, and dried in air. The yield of compound 6 was 0.4 g (94% with respect to the starting compound 5). The evaluated stoichiometry of the Eu and Tb atoms in the crystal of 6 is 1.02 ± 0.05:1.03 ± 0.05. Found (%): C, 50.75; H, 5.53; N, 4.39. Calculated for C54H70Eu1.0N4O12Tb1.0 (%): C, 50.75; H, 5.52; N, 4.38. IR (KBr), m, cm1: 1581 (m), 1552 (m) for mas(COO); 1426 (s) for ms(COO); 1624 (w) for m(CN); 1570 (m), 1515 (s) for m(CC) of Phen molecules. 2.2. Methods The water content in M(OOCCH3)3xH2O (M = Eu or Tb) was determined by thermogravimetric analysis. The crystals of the compounds obtained in the synthesis were suitable for X-ray diffraction. The elemental analyses were carried out on an Euro Vector Element Analyser CHN elemental analyzer (Model EA 3000). The diffuse reflectance IR spectra were recorded on a NEXUS Infrared Fourier-transform spectrometer (4000–400 cm 1) in the Center of Collaborative Research of the N.S. Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences. The stoichiometry of europium and terbium in 5 and 6 was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) on an IRIS Advantage spectrometer. The magnetic susceptibility was measured on a MPMSXL SQUID magnetometer (Quantum Design) in the temperature range 2–300 K in a magnetic field of up to 0.5 T. The paramagnetic terms of the magnetic susceptibility v were determined taking into account

the diamagnetic contribution evaluated from Pascal’s constants. The temperature-dependent effective magnetic moment was calculated by the equation leff = [(3k/Nb2)  vT]1/2  (8vT)1/2, where N is Avogadro’s number, k is the Boltzmann constant and b is the Bohr magneton. Photoluminescence (PL) measurements were carried out in 1 mm quartz capillaries under pulsed N2-laser excitation (wavelength 337 nm, pulse duration 10 ns, pulse energy 1 lJ, repetition rate 100 Hz, 390–900 nm range). The PL signal was dispersed by a 300 mm monochromator spectrograph (spectral resolution 0.1 nm) and recorded using a Hamamatsu CCD camera. The PL excitation (PLE) spectra were measured on a Perkin Elmer LS-55 fluorescence spectrometer. All spectra were taken at room temperature and were corrected for the spectral response of the detection systems. 2.3. X-ray diffraction data collection The X-ray diffraction data for 5 and 6 were collected by a standard procedure [29] on a Bruker APEX-II CCD diffractometer equipped with a CCD detector (Mo Ka, k = 0.71073 Å). A semiempirical absorption correction was applied (min/max transmissions were 0.5949/0.9218 for 5 and 0.5276/0.9139 for 6) [30]. The structures were solved by direct methods and using Fourier techniques, and were refined by full-matrix least squares against F2 with anisotropic thermal parameters for all non-hydrogen atoms. The hydrogen atoms of the carbon-containing ligands were positioned geometrically and refined using the riding model. In the structures of complexes 5 and 6, the metal centers were refined as {Eu2}; the refinement using the {Tb2} model gave similar results. In the crystal structures of 5 and 6, there is a rotational disorder of the tert-butyl groups of the pivalate ligands (formally due to the rotation about the C–CMe3 single bond), which is manifested in large anisotropic displacement parameters for the C atoms of the tertbutyl groups. The calculations were carried out with the use of the SHELX97 program package [31]. For 5: C60H114O24Eu2, fw 1523.47, T = 296(2) K, monoclinic crystal system, space group P21/n, a = 16.3178(8) Å, b = 12.3669(7) Å, c = 20.6548(11) Å, a = 90°, b = 109.1442°, c = 90°, V = 3937.6(4) Å3, Z = 2, Dcalc = 1.286 g cm3, l = 1.656 mm1, total number of reflections/unique reflections were 36 628/11 477, Rint = 0.0351, hmax = 30.03, S = 1.002, R1 (I > 2r(I)) = 0.0541, wR2 (I > 2r(I)) = 0.1496, R1 (all data) = 0.1252, wR2 (all data) = 0.1979. For 6: C54H70N4O12Eu2, fw  1271.06, T = 120(2) K, triclinic crystal system, space group P 1, a = 10.2896(14) Å, b = 11.9710(15) Å, c = 12.7466(17) Å, a= 113.070(2)°, b = 99.720(2)°, c = 96.865(2)°, V = 1393.6(3) Å3, Z = 1, Dcalc = 1.515 g cm3, l = 2.291 mm1, total number of reflections/ unique reflections were 16 252/5643, Rint = 0.0741, hmax = 26.38, S = 1.000, R1 (I > 2a(I)) = 0.0578, wR2 (I > 2a(I)) = 0.1457, R1 (all data) = 0.0827, wR2 (all data) = 0.1552. 2.4. Thermal decomposition The thermal behavior was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Thermogravimetric analysis was performed under an argon flow (Ar > 99.998%; O2 < 0.0002%; N2 < 0.001%; water vapor <0.0003%; CH4 < 0.0001%) (20 ml/min) in alundum crucibles on a NETZSCH TG 209 F1 instrument at a heating rate of 10°/min. The composition of the gas phase at temperatures below 300 °C was studied on a QMS 403C Aëolos mass spectrometric unit under TGA conditions. The ionizing electron energy was 70 eV, the maximum measured mass number (the ratio of the mass of the ion to its charge Z) was 300 amu. The weights of the samples used in the thermogravimetric experiments were 0.5–3 mg. Differential scanning calorimetry studies were carried out under an argon flow on a NETZSCH DSC 204 F1 calorimeter in aluminum cells at a heating rate of

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10°/min. The weights of the samples were 4–10 mg. Each experiment was repeated at least three times. The temperature calibration of the thermobalance was performed based on the phase transition points of reference compounds. The calorimeter was calibrated with the ISO standard 11357-1 for the temperature and heat flow. The samples used for the thermal analysis were weighed on a SARTORIUS RESEARCH R 160P analytical balance with an accuracy of 1  10–2 mg. The thermal analysis data were processed with the use of the NETZSCH Proteus Thermal Analysis software according to ISO 11357-1, ISO 11357-2 and ISO 11358. The X-ray powder diffraction analysis of the decomposition products was carried out with a FR-552 monochromator chamber (CuKa1 radiation) using germanium as the internal standard (the X-ray diffraction patterns were processed with an IZA-2 comparator with an accuracy of ±0.01 mm) and the STOE Powder Diffraction System.

Table 2 Mass spectrum of the saturated vapor over complex 6 (T = 620 K, Uioniz = 65 V). Ion

Irel +

[EuTb] [Tb(Piv)]+ [Tb(Piv)2]+ [Tb2(Piv)CO]+ [Eu(Piv)]+ [EuTb(Piv)2]+ [Eu2(Piv)3]+

Ion

Irel +

0.17 0.73 0.41 0.25 0.33 0.16 0.23

[Tb2(Piv)3O] [Eu2(Piv)4]+ [EuTb(Piv)4]+ [Eu2(Piv)4O]+ [Eu2(Piv)5]+ [Tb2(Piv)5]+ [EuTb(Piv)5]+

0.57 0.23 0.75 0.41 0.42 1.35 1.0

2.5. Mass spectrometry The vaporization behavior of the homonuclear complexes 4 and 7 and the heterodinuclear product 6 was studied by the Knudsen effusion method combined with mass-spectrometric analysis of the composition of the gas phase in the temperature range 400– 660 K (127–387 °C) on a MC 1301 instrument. The experiments were carried out with the use of a standard molybdenum cell with a ratio of the vaporization area to the effusion orifice area of 600. The temperature was measured with a Pt–Pt(Rh) thermocouple and was maintained constant with an accuracy of ±1°. The mass spectra of the gas phase over compounds 7 and 6 are given in Tables 1 and 2, respectively. The presence of oxo ions in the mass spectrum of 7 may be attributed to the partial high-temperature hydrolysis of the polycrystalline sample, containing no more than 1–3 wt.% of absorbed water. 3. Results and discussion 3.1. Synthesis and X-ray diffraction study The heterodinuclear pivalate (HPiv)6EuTb(Piv)6 (5) was synthesized by the exchange of acetate ligands in the homonuclear compounds Eu(OOCCH3)34H2O and Tb(OOCCH3)34H2O by pivalate anions (Piv) during heating (100 °C) in PivH. Crystals of 5 readily react with an excess of 1,10-phenanthroline monohydrate (PhenH2O) in boiling EtOH to form (Phen)2EuTb(Piv)6 (6). According to the ICP-AES data, the Eu to Tb ratio in the crystals of 5 and 6 is 1.04(5):1.02(5) and 1.02(5):1.03(5), respectively. According to the X-ray diffraction data, the metal atoms in heteronuclear molecule 5 (M. . .M, 4.506(1) Å), like the metal atoms in the related homonuclear europium complex 1 and terbium complex 2 [26,27], are linked together by four bidentate bridging Piv anions (M–O, 2.336(6)–2.362(7) Å) to form a dinuclear fragment (Fig. 1a). Each metal center is linked to eight O atoms of four bridging carboxylate groups (M–O, 2.324(5) Å), one terminal deproto-

Fig. 1. Molecular structures of 5 (a) and 6. (b) (M = Eu or Tb). Hydrogen atoms are omitted for clarity.

nated carboxylate group and three neutral HPiv molecules (M–O, 2.333(6)–2.536(7) Å). The heterodinuclear pivalate 6, with phenanthroline ligands, is isostructural with the homodinuclear terbium complex (Phen)2Tb2(Piv)6 (4) [27]. In molecule 6, the metal atoms

Table 1 Mass spectrum of the saturated vapor over complex 7 (Uioniz = 65 V) at T = 613 K and T = 638 K. Ion

Irel (T = 613 K)

[TbO]+ [Tb(Piv)O]+ [Tb(Piv)]+ [Tb(Piv)2]+ [Tb2(Piv)OCH2]+ [Tb2(Piv)2CO]+ [Tb2(Piv)3O]+ [Tb2(Piv)5-CH3]+ [Tb2(Piv)5]+

1.6  102 6.9  102 9.1  102 1.1  101 1.9  101 6.0  102 1.4  101 6.0  102 9.0  102

Irel (T = 638 K)

3.4  102 4.6  101 1.9  101 6.0  102 5.3  101 6.0  102 10  101

Ion

Irel (T = 613 K)

[Tb4(Piv)2CO]+ [Tb4(Piv)3OH]+ [Tb4(Piv)5-3CH3]+ [Tb4(Piv)6]+ [Tb4(Piv)7]+ [Tb4(Piv)8O]2+ [Tb4(Piv)8]+ [Tb4(Piv)8O]+

3.5  102 6.4  103 1.1  101 1.3  101 2.2  101 1.5  101

Irel (T = 638 K)

<2.0  103 <2.0  103 <2.0  103

1.6  101

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11 20 16

−1

12

9 -1

σ / Gs cm mol

10000

8

T=2K

8

-3

20000

3

30000

0 -10000

4

-20000 -30000 -40000 -30000 -20000 -10000

0

10000 20000 30000 40000

H / Oe

7

0

50

100

150

T/K

200

250

16

10

12 -1

10000

σ / Gs cm mol

20000

3

30000

8

8

T=2K

0

4

-10000 -20000 -30000 -40000 -30000 -20000 -10000

0

10000 20000 30000 40000

H / Oe

6

0

50

100

150

200

250

0 300

T/K Fig. 3. Magnetic properties of pivalate 6: the temperature dependences of leff (d) and 1/v (j) the plot of the magnetization vs. the field (–h–), the calculated data represented by a red line. (Color online.)

the Curie constant C and the Weiss constant H are 13.81 cm3 K/ mol and 2.0 K for 5 and 14.06 cm3 K/mol and 5.1 K for 6. It should be noted that the Weiss constant for 2 is positive (10.4 K), whereas this constant for complex 4 is negative (5 K) [27]. The Curie constants C for 5 and 6 are somewhat larger than the theoretical value for one Tb3+ ion (11.81 cm3 K/mol) (the ground-state term 7F6 and gJ = 3/2), because the paramagnetism of the complexes is determined not only by Tb3+ ions but also by Eu3+ ions. The contribution of Eu3+ ions to the magnetic susceptibility decreases with decreasing temperature because the ground state of the Eu3+ ion is 7F0 (non-magnetic), and the population of the closely spaced levels with different J states, which are responsible for paramagnetism, decreases upon cooling. The values of leff at 300 K are 10.52 and 10.56 lB for 5 and 6, respectively, and they gradually decrease with decreasing temperature due to a decrease in the contribution of Eu3+ ions. The high-temperature values of leff agree well with the value of 10.37 lB obtained by the summation of the equal contributions of Eu3+ and Tb3+ ions to the magnetic susceptibility. It should be noted that, as opposed to (Phen)2Tb2(Piv)6 (4), the dependence of the magnetization on the external magnetic field for complex 6 at 2 K is non-linear (Fig. 3, insert) and it is well described by the Brillouin function with the parameters J and g being 5.02 and 0.99, respectively. At 2 K, the dependence r(H) for pivalate 5, like that for (HPiv)6Tb2(Piv)6 (2), is non-linear (Fig. 2, insert), and a transition to the magnetically ordered state is observed. The spontaneous magnetization r0 for complex 5 at 2 K, which was evaluated from the analysis of the dependence r = r0 + vH, is 28 400 G cm3/mol. 3.3. Thermal decomposition

χ / mol cm

μeff / μB

10

20

-3

The magnetic properties of the homodinuclear europium and terbium complexes 1–4 were reported in the study [27]. The temperature dependences of the effective magnetic moment and the inverse magnetic susceptibility for pivalates (HPiv)6EuTb(Piv)6 (5) and (Phen)2EuTb(Piv)6 (6) are shown in Figs. 2 and 3. In the temperature range 70–300 K, the plots 1/v(T) for pivalates 5 and 6, like those of the corresponding homodinuclear terbium complexes (HPiv)6Tb2(Piv)6 (2) and (Phen)2Tb2(Piv)6 (4) [27], are linear and are well described by the Curie–Weiss law. This is attributed to the fact that the Tb3+ ions make a major contribution to the magnetic susceptibility of pivalates 5 and 6. The optimal values of

24

−1

3.2. Magnetic properties

12

χ / mol cm

are at a non-bonded distance (M. . .M, 5.393(1) Å) and are linked together to form a dinuclear fragment only by two bidentate bridging Piv anions (M–O, 2.250(7) and 2.336(6) Å) (Fig. 1b). The coordination environment of each metal atom is completed by four O atoms of two chelating Piv anions (M–O, 2.250(7) and 2.336(6) Å) and two N atoms of the Phen molecule bound in a chelating fashion (M–N, 2.542(7) and 2.558(7) Å). As a result, the coordination number of each metal atom in 5 and 6 is 8. It should be noted that the structures of the dinuclear fragments in the heteronuclear pivalate 6 and the known homonuclear pivalate 4 differ from the structure of the dinuclear fragment in the homonuclear europium phenanthroline complex (Phen)2Eu2(Piv)6 (3) [27], containing two bidentate bridging and two chelating bridging Piv anions. In the crystal structure, dinuclear molecules of 6 are arranged in stacks stabilized by strong stacking interactions between the Phen ligands of adjacent molecules (Fig. S1, the shortest intermolecular C...C contacts (3.31–3.35 Å) are shown), as in the crystals of the homonuclear dimers 3 and 4 [27]. Since it is impossible to distinguish between the positions of Eu and Tb atoms in heterodinuclear molecules 5 and 6 based on the Xray diffraction data, the structures of 5 and 6 were refined using a model with two europium atoms {Eu2} (the refinement of the structures with the use of the model {Tb2} gave similar results). Formally, the crystals of 5 may either be composed of individual heterodinuclear molecules or consist of two corresponding homodinuclear europium and terbium complexes in a ratio of 1:1 (co-crystals). To determine the real composition of the molecules, we studied various physicochemical properties of the dinuclear derivatives 5 and 6.

μeff / μB

300

0 300

Fig. 2. Magnetic properties of pivalate 5: the temperature dependences of leff (d) and 1/v (j) the plot of the magnetization vs. the field (–h–), the calculated data are represented by a red line. (Color online.)

Earlier [26–28], we have studied the solid-state thermolysis of pairs of the dinuclear complexes (HPiv)6Eu2(Piv)6 (1), (HPiv)6Tb2 (Piv)6 (2) and (Phen)2Eu2(Piv)6 (3), (Phen)2Tb2(Piv)6 (4). These data appeared to be useful in studying the thermal decomposition of the heterodinuclear complexes (HPiv)6EuTb(Piv)6 (5) and (Phen)2EuTb(Piv)6 (6). Thus, the heating of dinuclear pivalate 5 under an inert atmosphere leads to its stepwise destructive changes similar to those of complexes 1 and 2. For complex 5, the weight loss of 38.9 ± 1.5 wt.% (the HPiv content in the complex calculated from the molecular formula is 39.63 wt.%) accompanied by an endothermic effect (Q = 567.2 ± 9.5 kJ/mol) of a complex shape (Fig. S2a and b) is observed in the temperature range of 70–74 °C. The mass spectrum of the gas phase over 5 in the range of 60–150 °C corresponds to the spectrum of HPiv [32]. Actually, the first step of the thermolysis

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involves the elimination and removal of coordinated HPiv and the formation of tris-pivalate with the composition {(Eux, Tb1x)(Piv)3}n. At temperatures above 350 °C, this tris-pivalate undergoes thermal decomposition to form the solid decomposition product (a mixture of oxides Eu2O3 and Tb2O3). In the temperature range of 25–600 °C, the total weight loss is 77.2 ± 1.5 wt.%. The study of the behavior of pivalate 6 during heating under an inert atmosphere showed that this complex also undergoes stepwise destructive changes (Fig. S2c and d). The weight loss starts at 259 ± 2 °C. In the temperature range of 259–343 °C, the mass spectrum shows ion peaks corresponding to the ionization of Phen [32]. The weight loss in this temperature range is 28.5 ± 1.5 wt.% (the Phen content calculated from the molecular formula is 28.21 wt.%). The first step of thermolysis of 6 is endothermic: Q = 297.7 ± 6.5 kJ/mol (the enthalpy of sublimation of the Phen ligand is 98.3 kJ/mol). Based on these results, it can be stated that the first step of the thermolysis of 6 involves the elimination and removal of the chelating Phen molecules and the formation of the intermediate tris-pivalate with the composition {EuxTb1x (Piv)3}n thermally stable in a wide temperature range. The last step of the thermolysis involves the decomposition of the tris-pivalate and the formation of the solid decomposition product. The total weight loss is 72.6 ± 1.5 wt.%. A comparison of the results obtained in the first step of the thermolysis of complexes 1, 2, and 5 shows that these processes have similar characteristics (Fig. S2a and b). This can be attributed to the fact that the metal atoms in heteronuclear product 5, like the metal atoms in related homonuclear complexes 1 and 2, are linked together by four bidentate bridging pivalate anions to form a dinuclear fragment. The structure of the metal core in product 6 is similar to that in homodinuclear complex 4 (see above), whereas the structure of the dinuclear fragment of homonuclear complex 3 differs from those of complexes 6 and 4. In complex 3, there are two bidentate bridging and two chelating bridging pivalate anions. Apparently, the difference in the structure of the metal core leads to a difference in the character of the first step of the thermolysis of these complexes (Fig. S2c and d). The behavior of a mechanical mixture of homodinuclear complexes 3 and 4 in the {Eu2}:{Tb2} ratio of 1:1 (the results of the investigation are presented in Fig. S2) also differs from that of 3, 4, and 6. A comparison of these results indicates that product 6 is an individual compound. These results (Table 3) suggest that the thermal stability of heterodinuclear lanthanide pivalates 5 and 6, like that of the homodinuclear complexes (M = Sm, Eu, Gd, Tb, Er) [26–28] of a similar composition, increases when coordinated HPiv is replaced by Phen. The common feature of the thermal decomposition of this class of compounds is the formation of homoand heterobimetallic tris-pivalates as stable intermediates.

molecular compounds on various substrates. To obtain these data, we investigated the vaporization of the homonuclear complexes (Phen)2Tb2(Piv)6 (4) and {Tb(Piv)3}n (7) [27], which we have synthesized earlier, and of the new heterodinuclear product (Phen)2EuTb(Piv)6 (6). It was found that the saturated vapor over tris-pivalate 7 is substantially oligomerized or polymerized (Table 1). The molecular composition of the gas phase and the character of sublimation were determined from experiments on the complete isothermal sublimation of a known weighed sample of pivalate 7. The results of one experiment are presented in Fig. 4. In the initial vaporization step at 613 K, the drop in the ion currents I[Tb2(Piv)3O]+, I[Tb4(Piv)8O]2+ and I[Tb4(Piv)8O]+ is associated with the disappearance of the possible small amount of the terbium oxopivalate phase. The current intensities I[Tb2(Piv)5]+, I[Tb2(Piv)3O]+ and I[Tb(Piv)2]+ that remain constant characterize the composition of the saturated vapor over individual terbium tris-pivalate. The further decrease in the intensities of these ion currents (T = 638 K) is due to the burning out of the sample of terbium tris-pivalate. Taking into account the composition of the gas phase, the vaporization of individual terbium tris-pivalate can be described by reactions (1) and (2) (Scheme 1). The pressure of the saturated vapor over 7 was calculated using the key equation of the isothermal evaporation method [33]. In this step, in the absence of terbium oxopivalate, the vapor pressure of the [Tb2(Piv)6] molecules was 8.6  102 Pa (T = 638 K). Based on the temperature dependences of the ion current intensities I[Tb2(Piv)5]+, I[Tb2(Piv)3O]+ and I[Tb(Piv)2]+, the enthalpy of sublimation was evaluated by the least-squares method using the Clausius– Clapeyron equation and the results of six independent experiments; DSH0[{Tb(Piv)3}n, solid, T] = 194.5 ± 8.9 kJ/mol. The error of the calculation is the root-mean-square error of the arithmetic mean of the set of measurements. The vaporization behavior of (Phen)2Tb2(Piv)6 (4) was studied in the temperature range 400–660 K. The mass spectrum of the saturated vapor over compound 4 at 400–530 K shows ion peaks corresponding to the ionization of only Phen molecules: [C12H8N2]+, [C12H7N2]+ and [C11H8N]+. In the case of evaporation in an isothermal mode in this temperature range, the intensities of all ion peaks corresponding to the ionization of Phen molecules remain unchanged during a particular period of time and then monotonically decrease until complete disappearance. At higher temperatures (>550 K), the mass spectrum shows ion peaks assigned to metal cations. The mass spectrum of the gas phase over complex 4, which was recorded in this evaporation step, is almost completely identical to that presented in Table 1. Further investigation showed that the sublimation of complex 4 is identical to

3.4. Mass spectrometry

Table 3 Temperature range for the removal of neutral ligands from dinuclear lanthanide pivalates. Complex

L2Eu2(Piv)6 L2Tb2(Piv)6 L2(EuxTb2x)(Piv)6 Equimolar mechanical mixture of L2Eu2(Piv)6 and L2Tb2(Piv)6

Temperature range for removal of the neutral ligand (°C) Ligand L = HPiv

L = Phen

82–150 [26] 65–165 [27] 70–174

263–354 [28] 260–330 [27] 259–343 260–336

12

T = 613 K

[Tb4(Piv)8O]

2+

[Tb4(Piv)8O]

+

[Tb2(Piv)3O]

+

[Tb2(Piv)5]

10

I / rel. u.

Knowledge of the vaporization processes is required for the development of technologies for the production of thin films of

T = 638 K

[Tb(Piv)2]

8

+

6 4 2 0

0

5

10

15

+

20

Time / rel. u. Fig. 4. Isotherm of the complete sublimation of compound 7.

302

I.G. Fomina et al. / Polyhedron 50 (2013) 297–305

Scheme 1.

Scheme 2.

that of {Tb(Piv)3}n (solid). Therefore, it can be concluded that the vaporization of 4 is accompanied by reactions (1)–(3) (Schemes 1 and 2), reaction (3) being the major one at lower temperatures in the initial evaporation step. In the higher temperature range (550–660 K), the sublimation of compound 4 occurs by reactions (1) and (2), like the sublimation of {Tb(Piv)3}n (solid). The partial pressures of the components of the saturated vapor over complex 4 were calculated from the Hertz–Knudsen equation: P[Phen] = 1.3  102 Pa (T = 428 K); P[Tb2(Piv)6] = 2.4  102 Pa 2 (T = 595 K) and P[Tb2(Piv)6] = 8.6  10 Pa (T = 638 K). The enthalpy of reaction (3) was evaluated from the second law of thermodynamics based on the temperature dependences of the ion current intensity I[Phen]+ in the temperature range 383–487 K. The arithmetic mean calculated by the least-squares method based on six independent measurements was DrH0[3, T] = 330.2 ± 4.2 kJ/mol. The thermodynamic characteristics of the vaporization of 4 at temperatures above 550 K are equal within the experimental error of those of the individual {Tb(Piv)3}n (solid). The vaporization of the product (Phen)2EuTb(Piv)6 (6) was studied in the temperature range 400–660 K. The mass spectrum of the saturated vapor over compound 6 at 400–530 K, like that of complex (Phen)2Tb2(Piv)6 (4), shows only ion peaks corresponding to the ionization of Phen molecules: [C12H8N2]+, [C12H7N2]+ and [C11H8N]+. At higher temperatures (>550 K), the mass spectrum exhibits ion peaks assigned to metal cations. The analysis of the mass spectrum, presented in Table 2, showed that the heterobimetallic molecules [EuTb(Piv)6] and the homobimetallic molecules [Tb2(Piv)6] and [Eu2(Piv)6] are the main components of the saturated vapor. The study with the use of the standard procedure of the complete isothermal sublimation showed that, in the temperature range under consideration, the vaporization of the product [EuTb(Piv)6] obtained after the removal of the Phen ligand proceeds congruently and can be described by reactions (4)–(6) (Scheme 3). Based on the results of experiments on the complete isothermal sublimation, we calculated the partial pressures over complex 6 (T = 638 K): P[EuTb(Piv)6] = 9.7  102 Pa, P[Tb2(Piv)6] = 3.4  102 Pa and P[Eu2(Piv)6] = 3.4  102 Pa. The enthalpy of sublimation of the heterobimetallic product and the enthalpy of reaction (6) were calculated from the second law of thermodynamics: DSH0 [(EuxTb1x (Piv)6), solid, T] = 211.7 ± 6.9 kJ/mol (605 6 T 6 649 K) and Dr.H0 (6, T) = 5.6 ± 5.9 kJ/mol. The partial pressures and the activities of the components in the quasibinary system [Tb(Piv)3]–[Eu(Piv)3] with a 1:1 composition were estimated from the data on the vaporization of individual tris-pivalates {Tb(Piv)3}n (7) and {Eu(Piv)3}n [34] and the quantitative composition of the saturated vapor over heterobimetallic product 6. These data are given in Table 4. However, it should be

Scheme 3.

noted that when performing the calculations, we had to correct the quantitative composition of the gas phase over {Eu(Piv)3}n determined earlier [34]. This led to changes in the results of the calculations of the partial pressures for the molecules [Eu2(Piv)6] and [Eu4(Piv)12]. The data presented in Table 4 were obtained taking into account the correction applied to P[Eu2(Piv)6]. As can be seen from Table 4, the total pressure of the gas phase over the system [Tb(Piv)3]–[Eu(Piv)3] is characterized by the minimum for the composition corresponding to the heterobimetallic complex (a singular point) (see Scheme 4). The value DrG0(7, 638 K) = 8.5 ± 2 kJ/mol for reaction (7) was calculated from the data presented in Table 4. By assuming an entropy change DrS0(9, T) = 12 ± 9 J/mol K (based on the published data for related reactions) [35,36], we estimated DfH0[[Eu0.5Tb05 (Piv)3], solid, 638 K] = 20.5 ± 7 kJ/mol. These data suggest that the individual molecular compound 6 containing metals in a ratio of 1:1 is formed in the solid phase, even at elevated temperatures. 3.5. Luminescence properties The PL spectra of heterodinuclear complexes 5 and 6, as well as of the known homodinuclear complexes (HPiv)6Eu2(Piv)6 (1), (HPiv)6Tb2(Piv)6 (2), (Phen)2Eu2(Piv)6 (3) and (Phen)2Tb2(Piv)6 (4) [26–28], and the mechanical mixtures 1 + 2 and 3 + 4 of the corresponding compounds with a {Eu2}:{Tb2} ratio of 1:1, are presented in Figs. 5 and 6. Under UV excitation, the europium complexes 1 and 3 and terbium complexes 2 and 4 exhibit characteristic Eu3+ (5D0 ? 7FJ, J = 0–4) and Tb3+ (5D4 ? 7FJ, J = 3–6) luminescence, respectively. The spectrum of 6 does not show electron transitions of the Tb3+ ion and resembles the spectrum of europium complex 3. The absence of a singlet PL of the Phen ligand (S1 ? S0, kmax = 360 nm [37]) in complexes 3, 4 and 6 attests to the efficient excitation energy transfer to the Ln3+ ions. The spectrum of 5, unlike the spectrum of 6, exhibits electron transitions of both the Tb3+ and Eu3+ ions, the PL intensity of the Eu3+ ions being more than an order of magnitude higher than that of 1. The spectrum of the mixture 3 + 4 shows transitions of both Eu3+ and Tb3+ ions and can be considered as a superimposition of the PL spectra of 3 and 4. These data are well consistent with the procedure for the preparation of the sample, which is a mechanical mixture of complexes 3 and 4. A similar pattern is observed for the mechanical mixture 1 + 2. It should be noted that the PL intensity for the transitions of the Eu3+ ions versus the PL intensity for the transitions of the Tb3+ ions is much higher in complex 5 compared to the mixture 1 + 2. The PLE spectra of the complexes (Phen)2Eu2(Piv)6 (3), (Phen)2Tb2(Piv)6 (4) [27,28] and (Phen)2EuTb(Piv)6 (6), for which the PL band maxima of the Tb3+ (4) and Eu3+ (3, 6) ions were chosen as the observation wavelength kobs, are presented in Fig. 7. The spectra of complexes 3 and 4 show narrow f–f transition lines of the Eu3+ and Tb3+ ions, respectively, and an UV wing (the region shorter than 370 nm) assigned to singlet–singlet transitions of the Phen ligand [27]. The spectrum of complex 6 was recorded at kobs = 700 nm, i.e. at the maximum PL wavelength of Eu3+ ions, and is associated with the absence of photoluminescence of Tb3+ ions (see Fig. 5c). It should be noted that in this case, the excitation spectrum of 6 exhibits narrow lines of Eu3+ ions along with a band at a wavelength maximum of 376 nm (7F6 ? 5D3 transition of Tb3+ ions). The PLE spectra of the complexes (HPiv)6Eu2(Piv)6 (1), (HPiv)6Tb2(Piv)6 (2) [26–28] and (HPiv)6EuTb(Piv)6 (5) are presented in Fig. 8. The pattern observed for complexes 1 and 2 is similar to that of complexes 3, 4 and 6, with the only difference being that the spectra of 1 and 2 do not show a UV wing associated with singlet–singlet transitions of the Phen ligand, which is absent in these complexes. These data are indicative of the direct PL excitation with narrow f–f transition lines of the lanthanides. The absence of f–f transitions of Eu3+ ions at the excitation wavelength

303

I.G. Fomina et al. / Polyhedron 50 (2013) 297–305 Table 4 Pressures of the components of the saturated vapor (P) and the activities (a) of the condensed compounds in the system [Tb(Piv)3]–[Eu(Piv)3] (T = 638 K). [Tb(Piv)3]–[Eu0.5Tb0.5(Piv)3]

[Eu(Piv)3]–[Eu0.5Tb0.5(Piv)3]

P (Pa)

A

P (Pa)

a

P (Pa)

a

[Tb(Piv)3] [Eu(Piv)3] [Eu0.5Tb0.5(Piv)3]

8.6  102 2.1  102 9.7  102

1 0.15 1

3.4  102 3.4  102 9.7  102

0.5 0.31 1

1.3  102 1.4  101 9.7  102

0.15 1 1

7

D0- F4

5

5

7

7

D0- F0

7

D0- F4

5

7

7

7

D4- F3

D4- F4

5

5

b

7

D4- F3

5

5

5

I / a.u.

7

7

D4- F6

D4- F4

5

7

D4- F5

5

7

D4- F6

a I / a.u.

7

5

D0- F3

5

a

D4- F5

5

7

D0- F2

7

5

D0- F1

5 7

5

D0- F0

D0- F3

5

7

Scheme 4.

5

D0- F1

7

[Eu0.5Tb0.5(Piv)3]

D0- F2

Component

b

c

c

d 500

550

600

650

700

λ / nm

d 500

550

600

λ / nm

650

700

Fig. 6. Normalized PL spectra of complexes 1 (a), 2 (b) and 5 (c), and the mixture 1 + 2 (d), kex = 337 nm.

1000

Fig. 5. Normalized PL spectra of complexes 3 (a), 4 (b) and 6 (c), and the mixture 3 + 4 (d), kex = 337 nm.

a

Phen

5

F6- D4

b

5

F0- Gj

1

7

5

0.1

7

F0- D3

5

F0- D2

7

c

7

5

F0- L6

7

10

I / a.u.

kex = 337 nm accounts for a weak PL intensity of complex 1 compared to complex 3. It should be noted that the excitation spectrum of 5 (Fig. 8c), like the spectrum of 6, exhibits narrow f–f transition lines of Tb3+ ions, for example, a band at a wavelength maximum of 351 nm (7F6 ? 5D2 transition) in the spectrum obtained at kobs = 700 nm. This is indicative of a partial excitation energy transfer directly from the f–f levels of the Tb3+ ions to the f–f levels of the Eu3+ ions in 5, because the PL spectrum shows bands of both Eu3+ and Tb3+ ions. It should be noted that this excitation energy transfer is possible because the luminescence 5D0 level (17 300 cm1) of the Eu3+ ions lies lower than the luminescence 5D4 level (20 500 cm1) of the Tb3+ ions. Based on the data obtained from the PL and the PLE spectra for the complexes (Phen)2Eu2(Piv)6 (3), (Phen)2Tb2(Piv)6 (4) and (Phen)2EuTb(Piv)6 (6), a simplified diagram of the intramolecular energy transfer in the complexes can be constructed (Fig. 9). The energies of the singlet (S1 345 nm/28 986 cm1) and triplet (T 452 nm/22 100 cm1) levels of the Phen ligand in complexes 3 and 4 were determined in the study [27]. One of the main energy-transfer paths implies Laporte- and spin-allowed ligandcentered absorptions followed by intersystem crossing (S0 ? S1 ? T, T ? Ln⁄) and a metal-centered emission [11,38]. A similar energy-transfer path (S0 ? S1 ? T ? 5D1 ? 5D0) is observed in 3 upon PL excitation to the absorption bands of the Phen

7

5

F6- D3

100

0.01

300

350

400

450

500

λ / nm Fig. 7. Normalized PL excitation spectra of 4 (a), kobs = 544 nm, 3 (b) and 6 (c), kobs = 700 nm. For better presentation of important details, a semi-logarithmic scale is used in each plot. The vertical arrow indicates the N2-laser wavelength.

ligand (kex = 337 nm) [27]. Taking into account that direct energy transfer from the S1 level of the ligand to the 5Dj level of the Tb3+

I.G. Fomina et al. / Polyhedron 50 (2013) 297–305

5

F0- D3

7

5 7

b

F0- D2

5

F0- Gj

7

7

5

F0- D4

I / a.u.

7

5

F0- L6

7

5

7

F6- D4

5

a

7

F6- L8,7

7

5

5

F6- D3

F6- D2

304

c 300

350

400

450

500

λ / nm Fig. 8. Normalized PL excitation spectra of 2 (a), kobs = 544 nm, 1 (b) and 5 (c), kobs = 700 nm. The vertical arrow indicates the N2-laser wavelength.

35000 30000

Phen

3+

Tb

3+

Eu

Phen

S1

S1 5

D3

25000

5

D3

E / cm

-1

T

5

D2

5

20000

D4

T

5

D1 D0

5

15000

λex=337 nm

λex=337 nm

10000 5000 0

S0

7

Fj

7

Fj

S0

Fig. 9. Simplified diagram of the intramolecular energy transfer in 3, 4 and 6.

ions is also known for terbium complexes [39–41], the energy transfer in compounds 4 and 6 can be represented in terms of the analysis of the energy gap DE(T ? 5D4) between the triplet level T of the Phen ligand and the emitting level of the Tb3+ ion. For efficient energy transfer, the optimum range of DE is 2500– 3500 cm1 [11]. For 4 and 6, the energy gap DE(T ? 5D4) is 1650 cm1. Hence, the back energy transfer to the triplet level of Phen is highly probable (see Fig. 9). Since a singlet PL of the Phen ligand is not observed in the spectra of complexes 4 and 6, the energy transfer directly from the singlet level S1 of Phen to the 5D3 level of the Tb3+ ions is the most probable one. The absence of electron transitions of the Tb3+ ions in the PL spectrum of complex 6 attests to an efficient excitation energy transfer from the f–f levels of the Tb3+ ions to the f–f levels of the Eu3+ ions. It should be noted

that the band corresponding to the 7F6 ? 5D4 transition of the Tb3+ ions is not observed in the PLE spectrum of complex 6. This provides an additional support for our hypothesis that there is an efficient energy transfer from the S1 level of Phen to the f–f levels of Tb3+, because this is the only way of transferring energy to the Eu3+ ions involving the Tb3+ ions without consideration of its 5D4 level. It is commonly accepted that intermetallic optical interactions become inefficient when the involved metal centers are at a distance larger than 10 Å from each other. These results are in agreement with the results of the single-crystal X-ray diffraction analysis. According to the XRD study, the intramolecular metal–metal distance in the heterodinuclear complex 6 is short (5.393(1) Å) and facilitates such an intramolecular Tb3+ to Eu3+ energy transfer (the intermolecular metal–metal distance is 9.55 Å). In complex 4, isostructural with complex 6, the excitation energy transfer follows the path S0 ? S1 ? 5D3 ? 5D4. Under UV radiation, excitation energy transfer between lanthanide ions in heterodinuclear f–f0 lanthanide coordination compounds is known to occur, and it is considered in the studies [22,42–45,21,46,47]. Based on the above results, it can be concluded that the efficiency of PL sensitization of Eu3+ ions in complex 6 is higher than in complex 5. 4. Conclusions New heterodinuclear pivalates (HPiv)6EuTb(Piv)6 (5) and (Phen)2EuTb(Piv)6 (6) containing HPiv molecules and chelating N-donor Phen molecules as neutral ligands were synthesized. Studies using physicochemical methods (X-ray diffraction, ICPAES, magnetic measurements, DSC, TGA, vaporization processes) and the method of the synthesis showed that 5 and 6 are individual compounds. The thermal stability of heterodinuclear complexes 5 and 6, like that of homodinuclear europium(III) and terbium(III) complexes with a similar composition, is determined by the nature of the neutral ligand and is enhanced after the replacement of the coordinated HPiv by Phen. The vaporization behavior of the complexes (Phen)2Tb2(Piv)6 (4), 6 and {Tb(Piv)3}n (7) was studied. These results will be useful for preparing thin films of these compounds. The dependence of the magnetization of complexes 5 and 6 on the external magnetic field at 2 K is non-linear. In the case of complex 6, this dependence is described by the Brillouin function, characterized by the parameters J and g which are equal to 5.02 and 0.99, respectively. For pivalate 5, a transition to a magnetically ordered state is observed at 2 K, and the spontaneous magnetization is 28 400 G cm3/mol. The direct energy transfer from the singlet level S1 of Phen to the 5D3 level of the Tb3+ ions is the most probable process in the complexes (Phen)2Tb2(Piv)6 (4) and (Phen)2EuTb(Piv)6 (6) upon PL excitation by UV light. In complex 6, the complete intramolecular excitation energy transfer to the Eu3+ ions, involving the singlet levels of the Phen ligand and the f–f levels of the Tb3+ ions, takes places. The efficiency of PL sensitization of the Eu3+ ions is higher in complex 6 than in the complex (HPiv)6EuTb(Piv)6 (5). Acknowledgments This study was financially supported by the Russian Foundation for Basic Research (Project Nos. 10-03-00515, 12-03-00627 and 1103-12053), the Council on Grants of the President of the Russian Federation (Grants NSh-2357.2012.3 and NSh-1670.2012.3), the Ministry of Education and Science of the Russian Federation (SC02.740.11.0546 and SC-8437), the Target Programs for Basic Research of the Presidium of the Russian Academy of Sciences, and the Division of Chemistry and Materials Science of the Russian Academy of Sciences.

I.G. Fomina et al. / Polyhedron 50 (2013) 297–305

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