Synthesis and luminescent properties of novel pyrazolone rare earth complexes

Synthesis and luminescent properties of novel pyrazolone rare earth complexes

JOURNAL OF RARE EARTHS, Vol. 29, No. 1, Jan. 2011, p. 15 Synthesis and luminescent properties of novel pyrazolone rare earth complexes BAO Jiqing (保积...

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JOURNAL OF RARE EARTHS, Vol. 29, No. 1, Jan. 2011, p. 15

Synthesis and luminescent properties of novel pyrazolone rare earth complexes BAO Jiqing (保积庆)1, TANG Chunhua (汤春花)2, TANG Ruiren (唐瑞仁)2 (1. Resources and Environment Technical Research Institute, Jiaxing University, Jiaxing 314001, China; 2. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China) Received 27 July 2010; revised 2 August 2010

Abstract: A novel pyrazolone pyridine-containing ligand, 2,6-bis(1-phenyl-4-ethoxycarbonyl-5-pyrazolone-3-yl)pyridine (H2L) was designed and synthesized from pyridine-2,6-dicarboxylic acid (1), and its Eu(III) and Tb(III) complexes were prepared. The ligand and complexes were characterized in detail based on FT-IR spectra, 1H NMR, elemental analysis and thermal analysis, and the formula of Ln2L3·4H2O (Ln=Eu or Tb) of rare earth complexes was confirmed. The UV-vis absorption spectra and photoluminescence properties of the complexes were investigated, which showed that the Eu(III) and Tb(III) ions could be sensitized efficiently by the ligand (H2L) and emit the photoluminescence with high intensity, narrow half-peak width, and monochromic light. The results indicated that the complexes showed potential as excellent luminescent materials. Keywords: pyrazolone ligand; pyridine-2,6-dicarboxylic acid; synthesis; rare earth complexes; luminescent properties

In recent years, more and more attentions were paid on the lanthanide trivalent cations due to their well-defined emission properties such as long lifetime, large Stokes shifts and line-like emission[1–3]. However, the low absorption coefficients of the optical transitions for these ions are the fatal drawback for their practical applications. So it is necessary to equip them with highly absorbent chelating ligands, which serve as an antenna[4] to sensitize rare earth ions. To our knowledge, pyrazolone ligand is one of the excellent chelators of rare earth ions, as it can form mononuclear, dinuclear and heterobimetallic compounds with its various coordination modes[5]. In particular, the formation of heterobimetallic complex is able to promote the enhancement and quenching of fluorescence, which can provide valuable information to the in-depth study of the intramolecular energy transfer, magnetic interaction and optical properties. To explore highly efficient luminescent materials, a novel ligand with relatively large π-conjugated system, 2,6-bis(1phenyl-4-ethoxycarbonyl-5-pyrazolone-3-yl)pyridine (H2L), was designed and synthesized starting from pyridine-2,6dicarboxylic acid (1), and its lanthanide complexes Ln2L3·4H2O (Ln=Eu or Tb) were prepared using the methods in literatures[6,7]. The ligand and complexes were characterized by FT-IR spectra, 1H NMR, elemental analysis and thermal analysis. The UV-vis absorption spectra and photoluminescence properties of the complexes indicate that the complexes are sensitive to UV excitation light and emit bright photoluminescence. The synthetic route of the ligand H2L is presented in Scheme 1.

Scheme 1 The synthetic route of the ligand H2L

1 Experimental 1.1 Materials and physical measurements 2,6-Pyridinedicarboxylic acid chloride (2) was prepared based on the method in literature[8]. LnCl3 (Ln=Eu or Tb) solution was obtained as described in Ref. [9]. Pyridine2,6-dicarboxylic acid, Ln2O3 (Ln=Eu or Tb, purity was up to 99.99%) as well as other chemicals were of A. R. grade and used without further purification. The solvents in the reaction were dried prior to use. Melting points were measured on an XR-4 apparatus (thermometer uncorrected). Infrared spectra (4 000–400 cm−1) were recorded with KBr discs on a Nicolet NEXUS 670

Foundation item: Project supported by the National Natural Science Foundation of China (21071152) Corresponding author: TANG Ruiren (E-mail: [email protected]; Tel.: +86-731-88836961) DOI: 10.1016/S1002-0721(10)60398-5

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FT-IR spectrophotometer. 1H-NMR was determined on a Bruker-400MHz nuclear magnetic resonance spectrometer with CDCl3 or DMSO as solvent and TMS as internal reference. Elemental analysis was carried out by a PerkinElmer 2400 elemental analyzer. The excitation and emission spectra were recorded on a Hitich F-4500 luminescence spectrophotometer. The luminescence data for each rare earth metal complex were determined in the solid state at room temperature. The width of emission and excitation slit was 5 nm and the voltage of photomultiplier tube was 700 V. Thermal gravimetric (TG) and differential thermal analyses (DTA) were performed in the nitrogen atmosphere using a Netzsch TG 209 thermogravimetric analyzer at a heating rate of 10 °C/min from 20 to 700 °C. All reactions were monitored by TLC on silica-gel F-254 plates (Merck) with detection under UV light. 1.2 Synthesis 1.2.1 Synthesis of 2,6-pyridinedipropanoic acid(α,α’bis (ethoxycarbonyl)-β,β’-dioxo)diethyl ester (3) 20 ml THF and 10 ml (66.0 mmol) diethyl malonate were added into a 100 ml round-bottomed flask in turn with stirring in an ice bath. Then 1.5 g fresh sodium was added in three portions with continuous stirring at 0 °C. 2,6-Pyridinedicarboxylic acid chloride was added in many portions after the sodium being molten. After stirring for another 1 h at 0 °C with a drying pipe on the condenser tube, an orange suspension was formed. TLC was used to track the reaction with petroleum ether and ethyl acetate mixture (7:3) as developer. After the mixture was kept in the refrigerator overnight, 50 ml ice water mixture was added. The pH value was adjusted to <2.0 with diluted HCl solution. The organic layer was then separated, dried over anhydrous Na2SO4. The solvent and diethyl malonate were evaporated under reduced pressure and a jacinth suspension was obtained. Consequently, the resulting mixture was dissolved in 40 ml acetic ether, washed with water, dried on anhydrous Na2SO4 overnight, evaporated and the crude product was purified by silica gel column chromatography using ethyl acetate and petroleum ether (1:1) as eluent, then the crude product was purified by recrystallization from 95% ethanol to give the white flaky crystal 3 (5.2 g, 76% yield). M.p. 68–69 ºC; IR: 3 088, 2 984, 1 755, 1 731, 1 370, 1 307, 1 264, 1 169, 1 143, 1 034, 857, 615 cm−1; 1H NMR (CDCl3, 400 MHz): δ 8.10–8.34 (m, 3H, PyH), 5.65 (s, 2H, CH), 4.24–4.30 (q, 8H, J=8 Hz, CH2), 1.18–1.21 (t, 12H, J=6 Hz, CH3); Anal. Calcd for C21H25NO10: C, 55.87; H, 5.58; N, 3.10; Found: C, 55.65; H, 5.73; N, 3.02%. 1.2.2 Synthesis of 2,6-bis(1-phenyl-4-ethoxycarbonyl-5pyrazolone-3-yl)pyridine (H2L) 0.45 g (1.0 mmol) compound 3 was added into a 50 ml round-bottomed flask. 5.0 ml glacial acetic acid containing 0.6 ml (6.0 mmol) phenylhydrazine was added dropwise in succession. After stirring for 2 h, a great deal of brown-yellow precipitate appeared. The reaction mixture was continued to react overnight, the resulting precipitate was collected by filtration and washed with ab-

JOURNAL OF RARE EARTHS, Vol. 29, No. 1, Jan. 2011

solute ethanol. The crude product was purified by recrystallization from chloroform and methanol mixture (1:2) to give the yellow brown powder 0.11 g in 20% yield. M.p. 177–179 ºC; IR: 3 060, 2 982, 2 570, 1 673, 1 593, 1 531, 1 494, 1 349, 1 216, 1 157, 783, 757, 688 cm−1; 1H NMR (DMSO-d6, 400 MHz): δ 8.32 (br, 2H, OH), 7.98 (m, 3H, PyH), 7.31– 7.52 (m, 10H, PhH), 4.15–4.20 (q, 4H, J=6.7 Hz, CH2), 1.15–1.18 (t, 6H, J=6 Hz, CH3); Anal. Calcd for C29H25N5O6: C, 64.56; H, 4.67; N, 12.98; Found: C, 64.54; H, 4.81; N, 12.91%. 1.2.3 Synthesis of the complexes The solution of LnCl3 (Ln=Tb or Eu) (0.17 mmol) in ethanol (5 ml) was added dropwise into the solution of the ligand H2L (0.24 mmol) in ethanol (5 ml). The pH value of the solution was adjusted to 7.0 with aqueous NaOH (5%), then the mixture was stirred at 60 °C for 24 h. The resulting precipitate was collected by filtration, washed three times with mixture of ethanol and chloroform (1:1) and dried in vacuum to give a yellow powder in 65% yield. Eu2L3·4H2O: m.p.>300 ºC; IR: 3 396, 2 980, 1 634, 1 597, 1 490, 1 422, 1 342, 1 219, 1 091, 761, 715, 690, 422 cm−1; Tb2L3·4H2O: m.p.>300 ºC; IR: 3 387, 2 979, 1 638, 1 597, 1 490, 1 422, 1 341, 1 218, 1 091, 760, 715, 691, 425 cm−1.

2 Results and discussion 2.1 Composition of the complexes The results of elemental analysis of C. H. N are expressed in Table 1, which are in accordance with the theoretical values calculated, indicating that the composition of the compounds is in agreement with the formulae of Eu2L3·4H2O and Tb2L3·4H2O. Table 1 Composition analytical data for the complexes/% Complexes

Ln Found (calc.)

Found (calc.)

Found (calc.)

Found (calc.)

Eu2L3·4H2O

15.12 (15.28)

52.49 (52.55)

3.98 (3.90)

10.62 (10.57)

Tb2L3·4H2O

15.97 (15.87)

52.12 (52.18)

3.77 (3.88)

10.64 (10.49)

2.2

C

H

N

1

H NMR and FT-IR spectra

For the free ligand, no absorption peak of C–H of 4-position of the keto pyrazolone was observed in 1H NMR spectra, while a weak broad band at 8.32 ppm was observed and it disappeared when exchanged with deuteroxide. The IR spectra of the uncoordinated ligand shows a weak broad band at 2 570 cm−1, assignable to ν(OH) of enol-isomer of the ligand. It is shifted downfield as a result of intramolecular hydrogen bonds formed between the enol hydroxyl and carbonyl[10]. These results indicate that the ligand exists in the form of enol-isomer in solid state. The FT-IR spectra of the two complexes are similar to each other, promising that they are structural homology. The FT-IR spectra of the two complexes were different from that of the ligand. The bands ν(OH) centered at about 3 390 cm−1 are relatively intense and can be attributed to solvated water. The ργ(H2O) and ρω(H2O) bands usually at approximately 850 and 650 cm−1 respectively were not observed in the spectra of the com-

BAO Jiqing et al., Synthesis and luminescent properties of novel pyrazolone rare earth complexes

plexes, which suggests that there are no coordinated water in the complexes. The band at 2 570 cm−1 due to the enol hydroxyl is absent, implying that the ligand is chelated with Ln(III) ion in a mode of deprotonated enolform (−C=C−O−). Compared with the bands ν(C=O) of the free ligand, the ν(C=O) peaks of complexes are shifted downfield, which demonstrates that the carbonyl group may participate in coordination reaction. Furthermore, the bands assigned to ν(Ln−O) appear at about 425 cm–1 for both complexes. The main FT-IR spectral bands of the free ligand and complexes along with their tentative assignments are given in Table 2. Facts above confirm that the Ln(III) ion is coordinated to the ligand and it can be speculated that the structure of complexes may be as follows:

Scheme 2 The chemical structure of the complexes Table 2 Characteristic IR bands (cm−1) of the ligand and its complexes Sample

ν(OH)

ν(C=O)

ν(Ln-O)

H2L

2570

1673

Eu2L3·4H2O

3396

1634

422

Tb2L3·4H2O

3387

1638

425

2.3 Thermal analysis The results of thermal gravimetric (TG) and differential thermal analyses (DTA) of the two complexes Eu2L3·4H2O and Tb2L3·4H2O are shown in Figs. 1–2. There are several successive mass loss stages in the TG-DTA curves. For Eu(III) complexes, the first loss in the range of 40–110 °C corresponds to the release of solvated water content (Mass loss: 3.7%). While for Tb(III) complexes, the loss of solvated water occurred within 30–115 °C (Mass loss: 3.55%). When the temperature was over 300 °C,

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Fig. 2 TG and DTA curves for original Tb(III) complex

both complexes showed severe mass loss, attributed to the decomposition of L. A further increase in temperature was approximately 700 °C, the mass loss was up to 82.5% and 81.15% for Eu2L3·4H2O and Tb2L3·4H2O respectively, resulting from the formation of Ln2O3 (Ln=Eu or Tb). These results were confirmed by comparing the observed/ estimated and the calculated mass of the complexes. The results prove that the complexes have the relatively high thermal stability, and that the water in the complexes is solvated rather than coordinated[11,12], which is in agreement with the FT-IR study. 2.4 UV-Vis spectra The UV-Vis spectra for each rare earth metal complex and the ligand H2L were determined in the solid state at room temperature. Results are depicted in Fig. 3. There are two absorption peaks at about 340 and 408 nm, attributed to π→π* transition of the free ligand. The spectra of the two complexes are similar to each other, but remarkably different to that of the ligand. Both complexes show strong sharp absorption bands in the vicinity of 235 nm, assigned to π→π* transition of aromatic ring. Meanwhile, the strong broad bands appear near 300 and 403 nm, corresponding to π→π* transition of the large conjugated system. 2.5 Luminescence properties The excitation and emission spectra of the two complexes

Fig. 1 TG and DTA curves for original Eu(III) complex Fig. 3 UV-vis absorption spectra of ligand and complexes

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at room temperature are given in Figs. 4 and 5. And the fluorescence characteristics data of the two complexes are listed in Table 3. The maximum excitation wavelengths of the Eu(III) and Tb(III) complexes can be observed at 303 nm, due to the π→π* transition centered onto the ligand. Two sharp characteristic emission bands at 591 and 613 nm for Eu(III) complex were corresponding to 5D0→7F1 (magnetic dipole transition) and 5D0→7F2 (induced electric-dipole transition) respectively, and the latter was much stronger. While the Tb(III) complex displayed four characteristic metal bands at 490 nm (5D4→7F6), 542 nm (5D4→7F5), 581 nm (5D4→7F4) and 618 nm (5D4→7F3), respectively. Among them, the bond at 542 nm

(5D4→7F5) was the strongest. Compared the two complexes with each other, the relative fluorescence intensity of Tb(III) complex was stronger than that of Eu(III) complex, which may depend on the energy difference between the ligand triplet state and the singlet excited state of rare-earth ions. The results indicated energy could transfer from the ligand to Ln(III) (Ln = Eu and Tb) effectively, i.e., the ligand prepared has an excellent “Antenna” effect.

3 Conclusions In this paper, a novel ligand, 2,6-bis(1-phenyl-4-ethoxycarbonyl-5-pyrazolone-3-yl)pyridine (H2L) and its corresponding Eu(III) and Tb(III) complexes were designed and synthesized from pyridine-2,6-dicarboxylic acid (1). Results of 1H NMR and FT-IR spectra indicated that the ligand existed in the form of enol-isomer in solid state. The coordination of the metal ions to the ligand occurred at the oxygen atoms and water molecules did not take part in complexation. The results of thermal analysis demonstrated that both of the complexes had relatively high thermal stability. The study of the luminescence properties showed that the Eu(III) and Tb(III) could be sensitized efficiently by the ligand, that is, H2L could be used as promising candidate luminescent materials.

References: Fig. 4 Emission spectra of the solid rare earth complex Eu2L3·4H2O

Fig. 5 Emission spectra of the solid rare earth complex Tb2L3·4H2O Table 3 Luminescence data for the complexes Complexes

λex/nm

λem/nm

RFI*

Assignment

Eu2L3·4H2O

303

591

443

5

613

1281

5

490

897

5

2458

5

456

5

131

5

Tb2L3·4H2O

303

542 581 618 * RFI: relative fluorescence intensity

D0–7F1 D0–7F2 D4–7F6 D4–7F5 D4–7F4 D4–7F3

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