Spectroscopy of Eu3+ and Tb3+ pyridine- and pyrazine-2-carboxylates

Journal of Alloys and Compounds 451 (2008) 149–152

Spectroscopy of Eu3+ and Tb3+ pyridine- and pyrazine-2-carboxylates V.F. Zolin a,∗ , V.I. Tsaryuk a , V.A. Kudryashova a , K.P. Zhuravlev a , P. Gawryszewska b , J. Legendziewicz b , R. Szostak b a

Institute of Radioengineering and Electronics of RAS, 1 Vvedenskii Square, Fryazino Moscow Reg. 141190, Russia b Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie Str., Wrocław 50-383, Poland Available online 19 April 2007

Abstract The luminescence excitation, luminescence and vibrational spectra of europium and terbium pyridine- and pyrazine-2-carboxylates were investigated. A special attention was paid to spectroscopic characterization of tetrakis-compounds, MLn(Lig)4 ·H2 O, where M = NH4 , Na; Ln = Eu, Tb; Lig = pyridine-2-carboxylate and pyrazine-2-carboxylate anions. The features of the spectra indicated a decrease of the interaction of carboxylic groups and an increase of the donor–acceptor interaction of coordinated nitrogen atoms with the lanthanide ions in different tetrakis-salts in comparison with the tris-salts. This is accompanied by clear manifestation of the 4f–5d electronic transitions in the excitation spectra of terbium salts. © 2007 Elsevier B.V. All rights reserved. Keywords: Eu3+ ; Tb3+ ; Excitation spectra; Luminescence spectra; Vibrational spectra; Pyridine-2-carboxylate; Pyrazine-2-carboxylate

1. Introduction This work is a continuation of the spectroscopic studies of the lanthanide tris-pyridine-carboxylates [1,2]. It is focused on spectroscopy of ammonium- and sodium-lanthanide tetrakis-compounds, MLn(Lig)4 ·H2 O, where M = NH4 + , Na+ ; Ln = Eu3+ , Tb3+ ; Lig = pyridine-2-carboxylate (picolinate – pic) and pyrazine-2-carboxylate (pyr). An agreement between the spectra of europium and terbium tris-pyridine-carboxylates and the structure details was demonstrated in Ref. [1]. Temperature dependence of the width of lines in the luminescence spectra of nicotinates and isonicotinates was more prominent than that for picolinates. This illustrates the difference between coordination of solely carboxylic groups in the two former compounds and formation of the chelate cycle at coordination of the carboxylic group and the nitrogen atom of the ligand in the third compound. Intraligand charge transfer (ILCT) bands were observed in the luminescence excitation spectra of nicotinates and isonicotinates with long-wave edge at ∼333 and ∼367 nm, respectively. Indications of 4f–5d electronic transitions in the luminescence excitation spectra of terbium picolinate were revealed [1]. More prominent signs of these transitions

∗

Corresponding author. E-mail address: [email protected] (V.F. Zolin).

0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.125

were found in the spectra of sodium-terbium dipicolinate [1,2]. The ability of the picolinate ligands to form chelates with the lanthanide ions was conformed by the IR and Raman spectra. In this work we obtained spectroscopic characteristics of tetrakis-ammonium- and -sodium-lanthanide picolinates and pyrazine-2-carboxylates. Methods of synthesis of the tetrakis-lanthanide picolinates were described in Ref. [3]. Tris-europium pyrazine-2carboxylates were obtained and studied in Refs. [4,5], complexes of tris-europium pyrazine-2-carboxylates with phenanthroline were presented in Ref. [6]. Judging from the X-ray data, the ammonium- and sodium-lanthanide tetrakis-picolinates have a chain-like structure [3], and tetrabutylammonium–erbium tetrakis-picolinate has mononuclear molecular structure [7]. 2. Experimental Ammonium- and sodium-lanthanide tetrakis-picolinates MLn(C5 NH4 COO)4 ·H2 O (or MLn(pic)4 ·H2 O) and pyrazine-2-carboxylates MLn(C4 N2 H3 COO)4 ·H2 O (or MLn(pyr)4 ·H2 O); M = NH4 + , Na+ ; Ln = Eu3+ , Tb3+ , were synthesized and investigated. The structures of the ligands are given in Fig. 1. Sodium-lanthanide tetrakis-compounds were obtained at admixture of LnCl3 to water solution of sodium salt of picolinic or pyrazine-2-carboxylic acid (the ratio LnCl3 : the salt is equal to 1:4). The structures of the ligands are given in Fig. 1. Ammonium-lanthanide tetrakis-compounds were obtained at addition of ammonium hydroxide to water solution of LnCl3 and corresponding acid. pH of the mixture of reagents was adjusted to 6.5. The reaction product precipitated at the heating of the mixture. The compounds investigated were characterised

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V.F. Zolin et al. / Journal of Alloys and Compounds 451 (2008) 149–152

Fig. 1. The structures of the ligands. by the elemental analysis. The experimental data for ammonium-europium tetrakis-picolinate are: C—42.33; 42.12, H—3.02; 3.05, N—10.26; 10.24. The calculated data for (NH4 )Eu(pic)4 ·H2 O are: C—42.6, H—3.25, N—10.35. Some of samples of sodium-lanthanide compounds investigated had an admixture of NaCl. Identity of IR spectra of all four tetrakis-picolinates testifies to the same composition and related structures of compounds MLn(pic)4 ·H2 O. The tetrakis-picolinates and tetrakis-pyrazine-2-carboxylates have intense luminescence. The luminescence and luminescence excitation spectra were measured with SLM Aminco SPF 500 spectrofluorimeter, LOMO DFS-12 spectrometer and LOMO UM-2 monochromator at 77 and 300 K. IR spectra in the region 400–4000 cm−1 were obtained with Brucker FS 88 FTIR and Nocolet Magna 750 FTIR spectrophotometers. The method of KRr pellets was used for preparation of the samples for IR measurements. Emulsions with nujol and fluorinated oil were used also. Raman spectra were recorded with Nicolet Raman accessory attached to Nicolet Magna 860 spectrometer.

3. Results and discussion 3.1. Luminescence spectra Luminescence spectra of europium tris- and tetrakispicolinates and tetrakis-pyrazine-2-carboxylates are presented in Fig. 2. The spectra of tetrakis-picolinates and tetrakispyrazinates indicate the related structure of the Eu3+ nearest surroundings in these compounds. One can suppose that pyrazine-2-carboxylate coordination is realized through carboxylic group and the nitrogen atom at first position of the aromatic ring. At the same time, the other nitrogen atom situated at the fourth position of the hetero-ring, is left uncoordinated. The comparison of the Stark splittings of 7 FJ -states, J = 1–4, obtained from these spectra demonstrates the strongest Eu3+ ligand interaction in the Eu(pic)3 and somewhat lower crystal field strengths in the tetrakis-compounds. The conclusion about more appreciable distortions of the nearest surroundings of lanthanide ions in the tetrakis sodium–europium salts than in the ammonium–europium salts follows from the lifting of degeneracy of some components of the 7 F1,2,4 -states of Eu3+ in the spectra of former compounds in comparison with the spectra of the latter salts.

stronger donor–acceptor interaction in the tetrakis-picolinates. The signs of the 4f–5d electronic transitions in the excitation spectra of terbium tetrakis-pyrazine-2-carboxylates are masked by the ILCT band, that can be assigned to the n–␲* transition of the nonbonding electrons of the heterocyclic nitrogens. This band at 300–350 nm is seen clearly in the absorption spectrum of pyrazine-2-carboxylic acid [6]. The appearance of the ILCT band with the longwave edge at 352–357 nm in the excitation spectra of NaEu(pyr)4 ·H2 O and NaTb(pyr)4 ·H2 O (Figs. 3 and 4) can demonstrate the absence of coordination of nitrogen atom situated at the fourth position of the ring.

3.2. Luminescence excitation spectra

3.3. IR and Raman spectra

Luminescence excitation spectra of europium and terbium tris- and tetrakis-picolinates, and tetrakis-pyrazine-2carboxylates are presented in Figs. 3 and 4. The spectra of the sodium tetrakis-picolinates are similar to the spectra of analogous compounds presented in Ref. [3]. The comparison of the europium and terbium spectra points at 4f–5d electronic transitions in the region of 300–340 nm of the luminescence excitation spectra of terbium tetrakis-picolinates, much more prominent than in the spectrum of terbium tris-picolinate. This indicates the

Looking for the differences in the interaction of ligands with the lanthanide ions leading to the increase of manifestations of the 4f–5d electronic transitions in the spectra we analyzed the vibrational spectra of compounds. IR and Raman spectra are given in Fig. 5. The spectra of the ammonium-lanthanide and the sodium-lanthanide compounds are similar, so the latter are not presented in this paper. The 1410–1590 cm−1 region includes the bands of the complex vibrations: the bending δ(CH) vibrations plus the stretching vibrations of hetero-ring ν(C C)

Fig. 2. Luminescence spectra of NaEu(pyr)4 ·H2 O (a), (NH4 )Eu(pyr)4 ·H2 O (b), NaEu(pic)4 ·H2 O (c), (NH4 )Eu(pic)4 ·H2 O (d), Eu(pic)3 (e) at 77 K.

V.F. Zolin et al. / Journal of Alloys and Compounds 451 (2008) 149–152

Fig. 3. Luminescence excitation spectra of Eu(pic)3 (a), NaEu(pic)4 ·H2 O (b), NaEu(pyr)4 ·H2 O (c), (NH4 )Eu(pic)4 ·H2 O (d) at 77 K.

Fig. 4. Luminescence excitation spectra of Tb(pic)3 (a), NaTb(pic)4 ·H2 O (b), NaTb(pyr)4 ·H2 O (c), (NH4 )Tb(pic)4 ·H2 O (d) at 77 K.

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Fig. 5. Vibrational Raman (a, c, e) and IR (b, d, f) spectra of Eu(pic)3 (a, b), (NH4 )Eu(pic)4 ·H2 O (c, d), and (NH4 )Eu(pyr)4 ·H2 O (e, f) at 300 K. IR spectra presented were registered using KBr pellets.

and ν(C N) [8,9]. The bending vibration of ammonium anion ν4 (NH4 + ) (1400 cm−1 ) [10] is situated near the region of the symmetric stretching vibrations of carboxylate group νs (COO− ) (∼1350–1370 cm−1 ). The positions of the bands of antisymmetric stretching vibrations νas (COO− ) are ∼1635–1650 cm−1 . In tetrakis-compounds this band overlaps completely the feeble band of bending vibrations of water molecules. The detailed interpretation of the vibrational spectra of tris-lanthanide picolinates and dipicolinates is presented in Ref. [1]. The difference between νas (COO− ) and νs (COO− ) (∼270 cm−1 ) in the spectra of tetrakis-compounds is ∼50 cm−1 less than that in the spectra of tris-picolinate (∼320 cm−1 ) (Fig. 5). The comparison of the vibrational spectra of europium tris-picolinate with corresponding spectra of tetrakis-compounds, keeping in mind the differences of the luminescence spectra discussed above leads to conclusion of different coordination functions of carboxylic groups of tris- and tetrakis-picolinates. The crowded surroundings of the lanthanide ions and the Coulomb repulsion of carboxylic groups in tetrakis-compounds lowering their interaction with the lanthanide ion change also their orientation and coordination function. The decrease of the interaction of carboxylic groups with the lanthanide ion reminding the situation in dipicolinates should lead to increase of donor–acceptor interaction of the lanthanide ion with the nitrogen atoms of the ligands and to more prominent manifestation of 4f–5d electronic transitions in the spectra of tetrakis-salts of ammonium-terbium and sodium-terbium compounds invest igated.

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4. Conclusions The luminescence, luminescence excitation, and vibrational (IR, Raman) spectra of tetrakis-ammonium-lanthanide and sodium-lanthanide salts of picolinic and pyrazine-2-carboxylic acids were studied. Coulomb repulsion of the ionized carboxylic groups in tetrakis-salts changes orientation and lowers the interaction of these groups with the lanthanide ion. That leads to an increase of donor–acceptor interaction of the coordinated nitrogen atoms of these ligands with the lanthanide ios. It conditions a stronger manifestation of the 4f–5d electronic transitions in the excitation spectra of ammonium-terbium and sodium-terbium tetrakis-picolinate in comparison with the terbium tris-picolinates. Acknowledgements The authors are indebted to Dr. Z.S. Klemenkova and Prof. B.V. Lokshin for measurements of IR spectra and Prof. U.H. Kynast for helpful discussion. The work was supported by the Russian Foundation for Basic Research (grant no. 04-02-17303)

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