Inorg. Chem. 1997, 36, 249-252
249
Correlation between Ligand Coordination Number and the Shift of the 7F0-5D0 Transition Frequency in Europium(III) Complexes G. R. Choppin* and Z. M. Wang Department of Chemistry, Florida State University, Tallahassee, Florida 32306-3006 ReceiVed January 10, 1995X
The relative shift of the Eu(III) 7F0-5D0 transition toward lower energies due to the formation of inner-sphere Eu(III) complexes is shown to be linearly proportional to the total ligand donor number of the ligand bound to Eu(III) for 40 organic ligand complexed species in both water and DMSO solvents. Such a correlation allows estimation of the donor number, CNL, of organic ligands coordinated to Eu(III) from the relative frequency, ∆ν, of the Eu(III) 7F0-5D0 transition by the equation of CNL ) 0.237∆ν + 0.628. The validity of the method has been confirmed by using shift data of other Eu(III) complex systems from the literature. The nature of the shift of 7F0-5D0 band upon complexation is discussed as possibly due to a small degree of covalency in the Eu-L bond of the complexes with organic ligands.
Introduction Trivalent lanthanides exhibit variable and often large coordination numbers in complexes. For example, the lanthanide coordination number in crystals is as low as 3 in complexes with such bulky ligands as N(SiCH3)31 and as high as 12 with the small nitrate ligand.1 The stereochemistries in these lanthanide complexes are determined primarily by the electrostatic and spatial requirements of the ligands and are frequently difficult to predict. This is particularly true for lanthanide complexes in solution.2 Although many techniques have been used for the determination of the coordination number of lanthanide ions in complexes (e.g., conductivity,3 measurement of entropies and enthalpies of complexation,4-7 luminescence8,9 and UV-visible spectroscopy,10,11 and X-ray diffraction12,13 ), only the last one can give definite answers. Unfortunately, this technique is generally limited to crystalline complexes. Most often, the coordination numbers for lanthanide cations complexed in solution have been deduced from spectroscopic and conductance measurements. Recently, Eu(III) luminescence spectroscopy has been applied increasingly to the study of the coordination chemistry of lanthanides.14-18 In aqueous solution, the quenching of Eu(III) luminescence is due primarily to the hydroxyl group vibrations of solvated X Abstract published in AdVance ACS Abstracts, January 1, 1997. (1) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry; John Wiley and Sons: New York, 1988; p 960. (2) Greenwood, N. N.; Earnshaw, A. Chemistry of The Elements; Pergamon Press: New York, 1989; p 1429. (3) Spedding, F. H.; Atkinson, G. The Structure of Electrolyte Solutions; Wiley: New York, 1959; Chapter 2. (4) Grenthe I. Acta Chem. Scand. 1964, 18, 293. (5) Edelin De La Praudiere, P. L.; Staveley, L. A. K. J. Inorg. Nucl. Chem. 1964, 26, 1713. (6) Choppin, G. R.; Graffeo, A. J. Inorg. Chem. 1965, 4, 1254. (7) Fuger, J.; Merciny, E.; Duyckaerts, G. Bull. Soc. Chim. Belg. 1968, 77, 455. (8) Bu¨nzli, J. C. G.; Vuckovic, M. Inorg. Chim. Acta 1984, 95, 105. (9) Bu¨nzli, J. C. G.; Vuckovic, M. Inorg. Chim. Acta 1983, 73, 53. (10) Karraker, D. E. Inorg. Chem. 1973, 6, 1863. (11) Karraker, D. E. Inorg. Chem. 1968, 7, 473. (12) Oskarsson, A. Acta Chem. Scand. 1971, 25, 1206. (13) Albertsson, J. Acta Chem. Scand. 1970, 24, 1213. (14) Choppin, G. R. In Lanthanide Probes in Life, Chemical and Earth Sciences: Theory and Practice; Bu¨nzli, J. C. G.; Choppin, G. R., Eds.; Elsevier: New York, 1989, p 1. (15) Lis, S.; Choppin, G. R. Mater. Chem. Phys. 1992, 31, 159.
S0020-1669(95)00022-X CCC: $14.00
water. It has been shown that the luminescence-quenching effect is directly proportional to the number of water molecules in the inner coordination sphere.19 Similar results were also observed for N-H vibrators.20 The ground state manifold of Eu(III) (4f6) is 7Fi (i ) 0, 1, ..., 6), and the excited state manifold is 5Dj (j ) 0, 1, ...). Though the higher excited states, such as 5D , are capable of luminescing, usually the strongest observed 1 emission is from the transitions between 5D0 and levels of the ground state manifold, 7Fi (i ) 0, 1, ..., 6) due to efficient energy transfer from higher excited states to 5D0. Both of the shapes and the luminescence intensities of the excitation spectra and emission spectra of Eu(III) are sensitive to their environment and thus informative about the coordination structure.21 An interesting aspect of Eu(III) luminescence involves the 7F -5D excitation spectra. Since the excited state and ground 0 0 state are both nondegenerate and cannot be split further by the ligand field, each peak in the spectrum must correspond to a distinct Eu(III) environment. Previous work14-17,22 has shown that for Eu(III), the frequency of the 7F0-5D0 excitation spectra shift upon complexation decreases as the coordination of the complex increases. This observation suggests that the 7F05D frequency of Eu(III) may have a correlation with the ligand 0 coordination number. This paper reports the results of a study of such a correlation in Eu(III) complexes with organic ligands. The symbols used for the ligands are listed in Table 1. Experimental Section Reagents. Samples of europium oxide (99.99%, Aldrich Chemical Co.) were dissolved in perchloric acid to prepare aqueous solutions of the europium perchlorate. The concentrations of europium were determined by complexometric titration with EDTA using 20% hexaethyltetramine as a buffer and xylenol orange as an indicator.23 (16) De Sa, G. F.; Nunes, L. H. A.; Wang, Z.-M.; Choppin, G. R. J. Alloys Compd. 1993, 196, 17. (17) Barthelemy, P. P.; Choppin, G. R. Inorg. Chem. 1989, 28, 3354. (18) Horrocks, DeW., Jr.; Albin, M. In Progress in Inorganic Chemistry; Lippard, S. J., Ed.; John Wiley & Sons: New York, 1984; Vol. 31, p 1. (19) Horrocks, DeW., Jr.; Sudnick, D. R. J. Am. Chem. Soc. 1979, 101, 334. (20) Wang, Z.-M.; Choppin, G. R.; DiBernardo, P. L.; Zanonato, P.-L; Portanova, R.; Tolazzi, M. J. Chem. Soc., Dalton Trans. 1993, 2791. (21) Richardson, F. S. Chem. ReV. 1982, 82, 541. (22) Albin, M.; Horrocks, DeW., Jr. Inorg. Chem. 1985, 24, 895.
© 1997 American Chemical Society
250 Inorganic Chemistry, Vol. 36, No. 2, 1997
Choppin and Wang
Table 1. Ligands Involved in This Work and Their Abbreviations ligand
abbreviation
acetate benzoate phthalate isophthalate terephthalate hemimellitate trimellitate trimesate pyromellitate mellitate malonate succinate glutarate adipate pyridinedicarboxylate diglycolate iminodiacetate N-methyliminodiacetate diethylenetriaminepentaacetate (diethylenetriaminepentaacetato)bis(methoxylamide) 1,4,7,10-tetraazacyclododecane-N,N′,N′′,N′′′-tetraacetate 4-hydroxypyridine-2,6-dicarboxylate ethylenediamine-N,N′,N′′,N′′′-tetraacetate nitrilotriacetate ethylene glycol bis(β-aminoethyl ether) N,N,N′,N′-tetraacetate 1,3-diphenyl-1,3-propanedione 4,4,4-trifluoro-l-(2-thienyl)-1,3-butanedione 4,4,4-trifluoro-l-phenyl-1,3-butanedione enthylenediamine 1,10-phenanthroline dimethyl sulfoxide
Ac BA PHA IPA TPA HMA TMA TSA PMA MA MAL SUC GLU ADI DPA DGA IDA MIDA DTPA DTPA-BMEA DOTA Chelid EDTA NTA EGTA DBM TTA btfa en phen DMSO
Diethylenetriaminepentaacetate-bis(methoxyethylamide) (DTPABMEA) was provided by Mallinckrodt Medical Corp. Reagent grade benzoic and acetic acids were purchased from Fisher Chemical Co. Anhydrous dimethylsulfoxide, DMSO, was purchased from Aldrich. The water content in the DMSO was less than 0.005%. All other ligands and the organic solvents were reagent grade from Aldrich and used without further purification. Distilled, deionized water was used in all experiments. Stock solutions of the ligands with concentrations between 0.05 and 0.2 M were prepared by dissolving appropriate amounts of the acid ligand with adjustment of the pH with 1.0 M NaOH solution as necessary to achieve solution. The final pH of the stock solutions was adjusted to pH 5-6 with solutions either of sodium hydroxide or perchloric acid. Working solutions of the organic acids were prepared by dilution of the stock solution using distilled and deionized water. An appropriate amount of sodium perchlorate was added to maintain the ionic strength of the solutions at 0.1 M. DMSO-solvated europium perchlorate was prepared by the published method.24 A sample of europium oxide was dissolved in reagent grade perchloric acid with excess europium oxide. The mixture was heated to 50-80 °C for 4-5 h and the undissolved oxide removed by filtration. An excess of anhydrous DMSO was added to the filtrate solution after which the water was removed by evaporation under vacuum. The solid, with a formula of Eu(ClO4)3‚7.5DMSO, was obtained by precipitation with benzene. After filtration, the crystals were dried under vacuum at 50 °C for several days. The DMSO content was obtained by elemental carbon and hydrogen analysis and the europium content by EDTA titration using xylenol orange as an indicator. Solutions of europium perchlorate in DMSO was prepared by dissolving a weighed amount of this material in anhydrous DMSO. The ethylenediamine reagent, its solutions, and DMSO were stored in the presence of powdered P2O5 in a glovebox with a dry nitrogen atmosphere. Except during manipulations, the glovebox was kept dark to prevent the reagents and solutions from possible photolytic effects. The EDTA solution was prepared by dissolving reagent grade disodium salt (Aldrich) in distilled water. Hexamethylenetetramine solution (20%) was prepared by dissolution of the reagent in distilled water. Xylenol orange was prepared as a 1:100 solid dispersion in NaCl. Solutions of the complexes were prepared by mixing the europium perchlorate solution and the corresponding ligand solution. The pH of these solutions were adjusted as needed using NaOH or perchloric
Figure 1. 7F0-5D0 selective excitation spectra of Eu(III) complexes with dipicolinic acid. [Eu(III)] ) 0.01 M; [DPA]:[Eu(III)] ) 0:1 (A), 0.9:l (B), 1.8:1 (C), and 3.5:1 (D); λem ) 618 nm. Offset to the intensity: curve A, 0; curve B, 40; curve C, 160; curve D, 280. acid. Speciation calculations using the program SPECIES17 were performed to ensure that, under the solution conditions, a significant amount of complex was formed. Apparatus. The 7F0-5D0 selective excitation spectra of Eu(III) were measured in 1 cm quartz fluorimeter cells using an instrumental setup described elsewhere.20 Spectral deconvolutions were performed for overlapping spectral peaks using the program SPECTRA written in this laboratory.16 The precision of the spectral measurement is
