J . Phys. Chem. 1985,89, 5659-5665
5659
The Structure and NMR and Electronic Spectra of Europium(II1)-Crown Ether Complexes in Solution John D. Simon,+William R. Moomaw,* and Toni M. Ceckled Department of Chemistry, Williams College, Williamstown, Massachusetts 01 267 (Received: August 14, 1985)
Complexes were formed in acetonitrile solution between trivalent europium and ytterbium and the crown ethers 15-crown-5, benzo-15-crown-5, and naphtho-15-crown-5. Contrary to previous reports, we find that these ligands produce crystal field splittings on the order of 100 cm-' for several europium f f transitions and large shifts and broadening of the crown proton resonances in the NMR spectrum. The crystal field splitting patterns are consistent with a C,,geometry for the complex, providing evidence for the symmetrical crown structure in solution for the first time. Europium fluorescence is quenched by benzo-15-crown-5, but bands are split and shifted without quenching in the 15-crown-5complex. New, broad absorption bands are observed for Eu3+ and Yb3+ (but not Nd3+) and all three crowns which are assigned as ether oxygen to metal charge-transfer bands. Electrical conductance and spectral data show a concentration dependence which is interpreted as formation of strongly bound 1:l lanthanide-crown complexes, with some ion pairing to the perchlorate anion. At higher crown concentrations the data suggest 2:1 crown-metal complexes without ion pairing. These studies demonstrate that the choice of anion and dryness of solvent are crucial to the formation of these solution complexes.
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Introduction The study of crown ether-metal complexes has been an active area of research since Pederson first synthesized these multidentate ligands in 1967.' From a combination of conductivity,2 UV-vis spectros~opic,~ and N M R 4 studies, there have been significant advances in the elucidation of thermodynamic and kinetic properties of several alkali, alkaline earth, and transition-metal crown complexes. However, despite the vast number of crown ether complexes that have been reported, there is little information concerning the geometric structure of crown complexes in solution. X-ray crystallographic studies5 have determined that the etheral oxygens assume a planar, symmetrical configuration around the complexed cation. Although this structure has also been assumed to be the most stable conformation in solution, there has been little direct evidence which supports this conclusion. Proton and I3CN M R studies by Dale6 and by Live and Chan' suggest that the solution conformation for uni- and divalent cation complexes of several 18-crown-6 and 30-crown-10 derivatives depends upon solvent and hence may or may not agree with the crystallographic structure. Theoretical support for the ability of 18-crown-6 to adopt a conformation appropriate to its environment is provided by molecular mechanics calculations by Wipff et aL8 Finally, it has been reported that on the N M R time scale ( 2 2 0 MHz) interconversion of ether linkage rotamers is rapid even in the cation complexes of larger crowns. In order to further examine the geometric and electronic structure of solution-phase crown ether complexes, we have prepared several rare earth ion complexes of 15-crown-5, benzo- 15-crown-5, and naphtho-15-crown-5. By studying the pattern and intensity of the crystal field splittings of the f f electronic transitions of these crown complexes ions, it is possible to infer the symmetry of the site geometry. Furthermore, because of the high frequencies of optical transitions (10ls Hz) electronic spectroscopy makes possible the study of these complexes on a much shorter time scale than is possible with NMR. Rare earth ions are superior to transition metals as d structural probes. Unlike the broad and featureless d f transitions, even transitions of the transition metal^,^ the f in solution, demonstrate very narrow line widths, on the order to 10-30 cm-' and show resolvable crystal field splittings of 1 0 0 cm-'.'O In the case of europium, these transitions are formally orbitally and spin forbidden;" the observed spectral lines arise from either magnetic dipole or vibronically induced forced electric dipole m e c h a n i s m ~ . ' ~ JWe ~ have chosen Eu3+for this study in
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'NSF Presidential Young Investigator, 1985-1990. Current address: Department of Chemistry B-014,University of California San Diego, La Jolla, CA 92093. 'Current address: Department of Chemistry, University of Rochester, Rochester, NY 14627
0022-3654/85/2089-5659$01.50/0
order to simplify the interpretation of observed crystal field effects. Since the ground state of Eu3+ is nondegenerate 'Fo, only the excited states will be susceptible to crystal field splittings. Several transitions of the trivalent lanthanides demonstrate an unusual sensitivity in intensity and line width to their crystal field er~vironment.'~These "hypersensitive transitions" all obey the AJ = 2 selection rule and have been the subject of several theoretical treatment^.'^ As a result of their spectral responsiveness to the surrounding electrostatic field, extensive use has been made of these ions to probe their immediate environment in a variety of host crystals and biological systems.I6
(1) Pedersen, C. J. J. Am. Chem. SOC.1967,89,7017. (2)(a) Frensdorf, H. K. J. Am. Chem. SOC.1971,93,600.(b) Pedersen, C. J.; Frensdorf, H. K. Angew. Chem., Znt. Ed. Engl. 1972,ZZ, 16. (c) Chock, P. B. Proc. Natl. Acad. Sci. U.S.A. 1972, 69, 1939. (d) Evans, D. F.; Wellington, S. L.; Nadis, J. A,; Cussler, E. L. J . Solution Chem. 1972,1,499. (e) christensen, J. J.; Eatough, D. J.; Izarr, R. M. Chem. Reu. 1974,74,351. (f) Takeda, Y. Bull. Chem. SOC.Jpn. 1981,54, 3133. (3) (a) Pedersen, C. J. Fed. Proc., Fed. Am. SOC.Exp. Eiol. 1968,27, 1305. (b) Takaki, U.; Hogen-Esch, T. E.; Smid, J. J . Am. Chem. SOC.1971, Mandolini, L.; Musci, B. J. Am. Chem. SOC.1981, 93,6760.'(c) Erolani, G.; 103, 7484. (d) Shizuka, H.;Takada, K.; Morita, T. J . Phys. Chem. 1980, 84,994. (4)(a) Shporer, M.; Luz, Z . J. Am. Chem. SOC.1975,97,665.(b) Wong, K. H.; Konizer, G.; Smid, J. J . Am. Chem. SOC.1970,92,666. (c) Khazaeli, S.;Dye, J. L.; Popov, A. I. J . Phys. Chem. 1983,87,1830. (d) Schmidt, E.; Popov, A. I. J . Am. Chem. SOC.1983,105, 1873. ( 5 ) (a) Bright, D.; Truter, M. R. Nature (London) 1970,225, 176. (b) Bright, D.; Truter, M. R. J . Chem. SOC.E 1970,1544. (c) Bunzli, J.-C. G.; Klein, B.; Wessner, D. Znorg. Chim. Acta 1980,44, L147. (6) Dale, J. Zsr. J . Chem. 1980,20,3. (7)Live, D.;Chan, S. I. J . Am. Chem. SOC.1976,98,3769. (8) Wipff, G.;Weiner, P.; Kollman, P. J. Am. Chem. SOC.1982,104, 3249. (9)Cotton, F. A,; Wilkinson, G. "Advanced Inorganic Chemistry"; Wiley: New York, 1972. (IO) (a) Carnall, W. T.; Fields, P. R.; Rajnak, K. J . Chem. Phys. 1968, 49,4424. (b) Carnall, W. T.; Fields, P. R.; Rajnak, K. J. Chem. Phys. 1968, 49,4443. (c) Carnall, W. T.; Fields, P. R.; Rajank, K. J. Chem. Phys. 1968, 49,4447.(d) Carnall, W. T.; Fields, P. R.; Rajnak, K. J . Chem. Phys. 1968, 49, 4450. (11) EI'Yashevich. "Spectra of the Rare Earths"; State Publication House of Technical-Theoretical Literature: Moscow, 1953. (12) Wybourne, B.G."Spectroscopic Properties of Rare Earths"; Wiley: New York, 1965. (13)(a) Faulkner, T. R.; Richardson, F. S. Mol. Phys. 1978,35,1141. (b) Faulker, T. R.; Richardson, F. S. Mol. Phys. 1980,39, 75. (c) Richardson, F. S. Chem. Phys. Lett. 1982,86, 4. (14) Peacock, R. D.Struct. Bonding 1974,22, 83. (15)(a) Mason, S.F.; Peacock, R. D.; Stewart, B. Chem. Phys. Lett. 1974, 29, 149. (b) Reisfeld, R.;Jorgensen, C. "Laser and Excited States of Rare Earths"; Springer-Verlag: New York, 1977. (c) Jorgensen, C. K.; Judd, B. R. Mol. Phys. 1964,8,281. (d) Hufner, S."Optical Spectra of Transparent Rare Earth Compounds"; Academic Press: New York, 1968.
0 1985 American Chemical Society
5660 The Journal of Physical Chemistry, Vol. 89, No. 26, 1985
The trivalent lanthanide ions have also been extensively used to study energy transfer with organic molecules in solution. Geometric information as well as interactions between the lanthanide and ligand electronic states can be elucidated by examining changes in the fluorscence spectra and lifetimes of the probe ion.” As NMR shift reagents, these ions have elucidated structural and dynamical aspects of biological molecules.16 These studies suggest the potential use of the trivalent lanthanides as probes into the solution-phase geometry and electronic structure of crown ether complexes. Previous spectroscopic studies of crown ether-lanthanide complexes have not reported any effect of complexation on the f f transitions.Is One paper concluded that neither the hypersensitive transitions nor any other f f transitions are changed upon complex formation,Isb and reported only slight changes in the IH N M R spectrum of the crown in the presence of these paramagnetic ions.lgb These observations are inconsistent with analogous studies on biological systems where lanthanides are found to alter significantly the ‘H N M R spectrum of proteins.I6 Recently, significant paramagnetic shifts in the N M R spectra have been observed for Ln(C104)3 (Ln = lanthanide) complexes of 12-crown-4 and 15-crown-5.19 As will be discussed later, we observed several cases of crystal field splitting in the Eu3+ f transitions upon complex formation with both the crown f ethers studied. In addition, our studies reveal significant paramagnetic line broadening in the N M R spectrum accompanied by large changes in chemical shifts. These data combined with new spectral features observed in the presence of Eu3+ conclusively demonstrate the formation of crown ether complexes with Eu3+ ions. By examining the dependence of the europium crown complex formation on the counter anion present in solution, we will demonstrate that the failure of past researchers to observe these spectral changes probably results from a failure to make the lanthanide crown complex. The investigation of Eu3+complexes of benzo-15-crown-5 and 15-crown-5enables us to address several important questions. First f of all, through the crystal field effect observed on the f transitions, the geometry of crown lanthanide complexes in solution cab be inferred. To date, only a few lanthanide crown or cryptate complexes have been prepared, and little is known about their electronic structure.18 If we choose 15-crown-5 and its aromatic analogue, the resulting crown lanthanide complex allows for both the examination of the ligand field effects on the ion and the effects of the rare earth ion on the ligand electronic states. Although there have been numerous studies of lanthanides in host crystals, we are now able to examine lanthanides in a noncrystallographic point group, by using 15-crown-5. Finally, the equilibrium distribution between solvated and crown complexed ions and the dependence of this equilibrium on the particular counter anion used lends insight into ion pairing in these systems.20 ++
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Experimental Procedures Chemicals. 15-Crown-5 (Aldrich) and acetonitrile (Aldrich gold label) were dried over 3 A molecular sieves, and the aceto(16) (a) Dieke. “Spectra and Energy Levels of Rare Earth Ions In Crystals”; Wiley: New York, 1968. (b) Horrocks, W. D., Jr.; Schmidt, G. F.; Sudnick, D. R.; Kittrell, C.; Bernheim, R. A. J . Am. Chem. SOC.1977, 99, 2378. (c) Dobson, C. M.; Levine, B. A. “New Techniques in Biophysics and Cell Biology”; Wiley: New York, 1976; Vol. 3, Chapter 2. (d) Sherry, A. D.; Pascual, E. J . Am. Chem. SOC.1977, 99, 5861. (e) Elgavish, G. A,; Reuben, J. J. Magn. Reson. 1981, 42, 242. (17) (a) Horrocks, W. D., Jr.; Sudnick, D. R. Science 1979, 206, 1196. (b) Kirby, A. F.; Richardson, F. S. J. Phys. Chem. 1983, 87, 2544. (c) Kirby, A. F.; Richardson, F. S. J. Phys. Chem. 1983, 87, 2557. (d) Horrocks, W. D., Jr.; Arkle, V. K.; Liotts, F. J.; Sudnick, D. R. J. Am. Chem. SOC.1983, 105, 3455. (18) (a) Cassol, A.; Seminaro, A,; Paoli, G. D. Inorg. Nucl. Chem. Lett. 1973,9,1163. (b) King, R. B.; Heckley, P. J . Am. Chem. SOC.1974,96,3118. (c) Costa, S. M.; Queimado, M. M.; Silva, J. J. J. Photochem. 1980, 12, 3 1 . (d) Izatt, R. M.; Lamb, J. D.; Christensen, J. J.; Hagmore, B. L. J. Am. Chem. SOC.1977, 99, 8344. (19) Bunzli, J.-C. G.; Oanh, H. T.; Gillet, B. Inorg. Chim. Acta 1981, 53, L219. (20).Smid, J. In “Ions and Ion Pairs in Organic Reactions”; Scwarz, M., Ed.; Wiley: New York. 1972; Vol. 1 , p 85
Simon et al.
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The Journal of Physical Chemistry, Vol. 89, No, 26, 1985 5661
Spectra of Europium(II1)-Crown Ether Complexes
m I\
2 50
2? 0 WAVELENGTH
260
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290
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Figure 3. Effect of Eu(C104)3on the 'Lb transition of benzo-15-crown-5.
salt; - - - sa1t:crown 0.51:l; salt:crown 1,Ol:l; salt:crown 5.06:l. With increasing salt concentration, the absorption band decreases in intensity and shift to higher energy. -no
E u t 3 - 15- C R O W N - 5
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Figure 2. Effect of added 15-crown-5 on various f f transitions of Eu3+. The observed splitting patterns are consistent with a C,, geometry in which the 15-crown-5ring assumes the highly symmetrical crown configuration around the complexed ion in solution just as it does in the crystal.
0.0-3.8 X lo-* M. The results for benzo-15-crown-5 are shown in Figure 1. Similar results were obtained for 15-crown-5. For both crowns the measured conductance decreased monotonically until the ratio of crown to salt reached unity. At this point the conductance remained approximately constant until the ratio reached a value of approximately 2.0. In order to obtain a better understanding of the conductivity data, we repeated the above experiments using KC104. The association constant between potassium and 15-crown-5 is very large and we would expect to observe quantitative complex formation. For crown to salt ratios varying from 0.0 to 2.0, we observe similar behavior for KC104 and E ~ ( c 1 0 ~ )At ~ .higher ratios of crown to salt (>2), the conductance gradually increases with increasing crown concentration and at a ratio of 1O:l is approximately equal to that observed in the absence of crown, Figure 1. 2. Crystal Field Effects on the f f Transitions of Europium. While previous authorsIsbreport no effect of crown ether ligands on the f f transitions of rare earth ions, we have observed significant changes in both line widths and intensities of several bands. For Eu3+,the 5Do 7Fotransition at 579 nm is strongly forbidden by spin, electric dipole, quadrupole, and magnetic dipole selection rules. In acetonitrile solution, this transition has an estimated extinction coefficient of no more than 6 X L/(mol cm). The addition of equimolar crown causes this band to increase and 32 X to an apparent extinction coefficient of 11 X for 15-crown-5 and benzo-15-crown-5, respectively. The concentration dependence of the intensity of this correlates precisely with the conductance measurements up to a crown to salt ratio of 2:l. In addition, the following europium transitions are found to undergo changes in intensity, broadening, and/or splitting in the presence of crown: 5D2 7F0at 465 nm, 5L6 7Foat 395 nm, 5G4 'Fo at 375 nm, 5D4 'Fo at 362 nm, and SH6 'Fo a t 317 nm. The transitions were assigned by using the table published by Carnall.Io Several of these transitions are shown in Figure 2. While the influence of complex formation on the f f absorption bands was similar for the two crown ethers studied, the fluorescence properties of the rare earth ion were dependent on the particular crown complex. All of the Eu3+fluorescence was quenched by benzo-15-crown-5, for crownsalt ratios greater than 1, whereas bands appeared to be shifted or split with relatively little change in intensity by complex formation with 15-crown-5. A detailed study of the fluorescence properties of these complexes will be presented elsewhere. 3. Changes in the Benzene Absorption Band. The traditional method of demonstrating the formation of cation complexes of
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360
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460
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Figure 4. A, 1:l E~(CIO~)~:I5-crown-5 in acetonitrile. B, Eu(CIO,)~in
acetonitrile. Addition of benzo- 15-crown-5results in the appearance of this new broad absorption band, A,, = 370 nm. The intensity of this band increases linearly with crown concentration for crown:salt ratios up to 1.0. benzo-crowns has relied on the observation of a shift and sharpening of the benzene absorption band.'JSa For this study, the crown concentration was held fixed at 2.8 X low4M and the E u ( C I O ~ concentration )~ was varied from 0.0 to 57.1 X lo4 M. A maximum extinction coefficient for uncomplexed benzo- 15crown-5 of 2450 f 70 L/(mol cm) was measured at 278.5 nm. As can be seen from Figure 3, the extinction coefficient decreases and the band shape continues to change with excess salt concentrations. A continued change is observed for sa1t:crown ratios greater than one. This behavior contrasts with both the conf transitions. ductivity and spectral dependence of the f However, these observations are consistent with the paramagnetic properties of Eu3+,and significant changes in the benzene electronic structure could result from perturbations by uncomplexed ions. 4 . New Complex-Dependent Absorption Band in the Ultraviolet. Mixing the colorless solutions of europium perchlorate and benzo- 15-crown-5 produced a pale yellow solution. The color was found to arise from a low intensity absorption having a maximum at 370 nm (27 000 cm-I) and a long tail extending beyond 580 nm (see Figure 4). An apparent extinction coefficient of 13 L/(mol cm) is obtained at 370 nm for an equimolar mixture. The extinction Coefficient continues to increase slightly at higher crown concentrations. We obtained an excellent linear fit to Scott's equationz5resulting in an extinction coefficient of 40.6 wl/(mol cm) and an equilibrium constant of 54.7 M. No band appeared
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( 2 5 ) Tames, M. J . Phys. Chem. 1961, 65, 654.
5662
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985
S i m o n e t al. TABLE I: Proton Magnetic Resonancesa for Benzo-15-crown-5 in Acetonitrile-d*
crown:salt ratio benzylic protons (a)
Figure 5. Charge-transfer bands between trivalent lanthanides and various crown ethers. In all cases the ratio for sa1t:crown was 1:1, and the solvent was acetonitrile. A, Eu(C10,),, naphtho-15-crown-5. B, Eu(CIO,),, benzo-15-crown-5. C, Yb(C104),, benzo-15-crown-5. D, Eu(CIO,),, 15-crown-5. E, Yb(C104),, 15-crown-5. Comparison of the bands observed with Yb3+ and Eu3+ reveals a shift to higher energy of 24000 cm-' when Yb" is used. This shift is consistent with the difference in reduction potentials of Ybf+ and Eu3+ supporting the assignment of these band to charge transfer from the crown ring to the lanthanide ion. Comparison with gas-phase ionization potentials of benzyl and naphthyl ethers indicate that the absorption band results from a charge transfer from the ether oxygens adjacent to the aromatic moiety. Transitions from the other oxygens on the ring occur at higher energy as they do in 15-crown-5. For benzo-15-crown-5 complexes, these bands cannot be resolved due to the strong absorption of the aromatic moieties in this region.
methylene protons (b)
(c)
(d)
d
0:l 1:l 1871.3 2134.0 1866.4 1864.2 1862.4 1861.3 1859.8 1857.6 1851.8 1097.4 33.9 1095.2 1094.4 1093.3 1091.5 1090.8 1088.6 1024.4 33.9 1022.2 1021.5 1020.0 1018.5 1017.8 1015.6 989.2 33.9 986.7 984.0 98 1.9 980.8 979.3 977.9 976.0 974.9 973.1 970.5 968.0
2:l 3.5:l 1991.8 (2078.2) 1943.0
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1098.7
1090.4
1084.6
1085.1
625.1
870.7 785.3
865.8
931.3
203.7
311.6
292.1
245.7
OSpectrum was measured by using a 270-MHz spectrometer. All values in Hz.
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15-CROI*N-5
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Figure 6. Effect of Eu(C104), on the 'H N M R spectrum of benzo-15crown-5. Addition of crown results in substantial broadening and enormous shifts in the NMR bands. Band positions are reported in Table I.
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i n this region of t h e spectrum w h e n 15-crown-5 w a s mixed with europium, b u t a similar b a n d having a m a x i m u m a t 2 6 0 nm (38460 cm-') is observed instead (Figure 5D). Replacing Eu3+ by Yb3+ i n both benzo-15-crown-5 a n d 15crown-5 produces a blue shift in these b r o a d bands of 4 1 2 0 a n d 4 9 1 0 cm-', respectively (Figure 5). T h e addition of naphtho15-crown-5 to E u ( C 1 0 & yields a new absorption band, which is overlapped on t h e high energy side by t h e naphthalene moiety absorption, b u t is otherwise similar i n shape a n d position to t h a n of t h e benzo complex (Figure 5A). 5. NMR Results. T h e 'H NMR spectrum of these complexes was also measured. Unlike some previous workers,lSb w e find substantial broadening a n d enormous shifts in t h e NMR bands.
618
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546
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Figure 7. Effect of added crown on europium fluorescence. The crown to salt ratios for these spectra were as follows. Benzo-15-crown-5: A, no crown, B, 0.42:1, C, 1.04:l. 15-Crown-5: D, no crown, E, 0.67:1, F, 1.34:l. Addition of benzo-15-crown-5 causes the fluorescence intensity to decrease and complete quenching is observed for crown:salt ratios greater than 1:l. Upon addition of 15-crown-5, the band at 616 nm splits into two component. This behavior is consistent with a C,, geometry for the complex.
Band positions are presented in Figure 6 a n d T a b l e I. T h e large difference in t h e presence a n d absence of salt a r e consistent with paramagnetic shifts observed in other complexes, including biologically important ones.I6
Spectra of Europium(II1)-Crown Ether Complexes
6. Fluorescence Results. The Eu3+ fluorescence was monitored as a function of crownsalt ratio. The results are shown in Figure 7. In the presence of 15-crown-5, there is little change observed in the overall fluorescence intensity, although splitting is observed in the band at 616 nm. In addition, spectral broadening and the disappearance of a band are observed at 585 nm. In the presence of benzo- 15-crown-5, significant broadening and decreased intensities are observed. For equimolar crown:salt solution, the fluorescence is completely quenched. Discussion The data presented in the previous section provide strong evidence for the formation of complexes between E u ( C ~ O and ~)~ the crown ethers 15-crown-5, benzo-15-crown-5, and naphtho15-crown-5. In this section we will use the spectroscopic data to interpret the geometric and electronic structure of the 1:l complexes. In addition, the role of ion-pairing and multiple crown:salt complexes in the presence of excess crown and salt will be examined. This section will be concluded with a discussion of the effects of various anions and water on the stability of the crown-Eu3+ complex in acetonitrile. 1. f f Transitions of E d + : A Structural Probe. The f transitions in the presence observed changes in the Eu3+ f of crown provide the most convincing evidence for complex formation. In addition, these spectral changes yield detailed structural information about the complex. The observed spectral splittings, 30-200 cm-I, observed upon additions of crowns are consistent with expected crystal field effects.12 As indicated in the results section, several, but not all of the f absorption bands are found to undergo changes upon adf dition of crown. Two transitions characteristic of Eu3+ have been found to be hyper~ensitive.'~These are the ,D2 7Foband at 465 nm (21 504 cm-') and the ,G2 7F0band at 380 nm (26 309 cm-'). On the basis of the AJ = 2 selection rule, one would predict that these two bands would be sensitive to complex formation with crown ethers. In acetonitrile, the ,D2 7Fotransition consists of a single peaks, Figure 2. Upon addition of either crown, the band splits into two components separated by 42 cm-l. However, the ,G2 7Fo band shows no change upon addition of crown. Numerous other transitions undergo crystal field splittings similar to the ,D2 7Fo band. While we have no explanation for the insensitivity of the ,G2 7Foband nor for the apparent sensitivity of several of the Eu3+transitions, these results indicate that crown complexes may prove useful for testing both theories of hypersensitive t r a n ~ i t i o n s and l ~ ~ the ~ ~ general mechanism of forced electric dipole transitions in rare earth ions.*' The appearance of the strongly forbidden 5Do 7F0band upon addition of crown suggests a significant lowering of symmetry for the surrounding ligand field of the Eu3+ ion. For crown to salt ratio < 1.O, a linear relationship is observed between the intensity of the ,Do 7Foband and the conductivity (r2 = 0.9931). The observed splitting patterns for the Eu3+ f f transitions are consistent with a C,,geometry for the crown complex. This geometry would convert the forbidden ,Do 7Fotransition in acetonitrile into an orbitally allowed ,Al 7A1transition polarized along the fivefold axis.28 Here, the C,,representations refer to the corresponding spin-orbit coupled wave functions of the ion. It is interesting to note that this band is 3 times stronger in the lower symmetry benzo- 15-crown-5 than in 15-crown-5. In a CSugeometry, the ,D1state splits into A2 and E l components. Only the transition to the double degenerate El state is allowed and should be polarized in the plane of the crown oxygens. Owing to the degeneracy of the E , state, the allowed transitions should give rise to an unsplit band. This is what is observed at 19012 crn-'. The splitting of the ,D2 7Fotransition into two transitions centered around the uncomplexed band at 21 504 cm-' is also consistent with a fivefold geometry. The ,D2 state is
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+ -
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+ -
+ -
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+ -
(26) Simon, J. E.;Moomaw, W. R., manuscript in preparation. (27) (a) Ofelt, G. S. J . Chem. Phys. 1962, 37, 5 1 1 . (b) Ofelt, G. S. J . Chem. Phys. 1963, 38, 217. (28) Hamermesh, M. "GroupTheory"; Addison-Wesley: Reading, MA, 1962.
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5663 predicted to split into an A I , and two pairs of double degenerate states of E l and E2 symmetry. Transitions from the 'AI ground state to the ,A1 and ,El excited states are allowed; the ,E2 7A1 transition is forbidden. This prediction is consistent with the observed splitting. The low energy component is the more intense of the two bands upon complex formation with 15-crown-5 and is presumably the E, component. The relative intensities are somewhat altered when the lower symmetry benzo- 15-crown-5 is used. Another band that undergoes significant changes is the ,G4 'Fo transition at.26670 cm-'. In the presence of 15-crown-5 this band (unsplit in acetonitrile) broadens and splits into three components. In a C,,geometry, the ninefold degenerate ,G4 state can split into an A,, two double degenerate El, and two double degenerate E2 states. The ,Al 7A1and ,E1 7A1transitions are allowed while the 5E2 7A1transitions remain forbidden. AS shown in Figure 2, splitting into three components is observed. We assign the two components of equal intensity to the two ,El 7A1transitions, and the weaker band on the high energy side 7A1transition. of the band to the ,A1 Additional Eu3+f f transitions which undergo changes are more difficult to interpret as the crystal field splittings are similar to the bandwidths found in acetonitrile. As a result, only line broadening is observed upon complex formation. For all of the bands, the observed splittings are consistent with a C,, geometry for the 15-crown-5 complex. The observed increase in spectral intensities when the lower symmetry benzo-5-crown-5 is used provides additional support for concluding that the 15-crown-5 complex has C,,symmetry. Thus, it appears that the 15-crown-5 ring assumes the highly symmetrical crown configuration around the complexed cation in solution just as it does in the crystal. 2. Proton Magnetic Resonance. IH NMR studies of acetonitrile-d3 solutions of E U ( C I O ~and ) ~ the two crown ethers, 15crown-5 and benzo- 15-crown-5 provide convincing evidence of complex formation. The highly resolved resonances of the uncomplexed crown are easily assigned, Figure 6. The aromatic protons are found at 1862 Hz; the three sets of crown ring methylene protons are centered at 1092, 1019, and 978 Hz. For crown:salt < 1 :1, only two broad resonances are observed at 21 34 and 34 Hz. The former arise from the slightly shifted aromatic protons. All of the methylene protons have been significantly shifted (from approximately 100 to 34 Hz) and collapsed into a single, broad, upfield resonance. As additional crown is added (crowxsalt > 1:l), resonances appear for the uncomplexed crown. These resonances remain broad even for crown:salt ratios greater than 1O:l which may indicate the formation of "sandwich" complexes or else very rapid exchange. Similar large shifts in the methylene resonances were also observed in the presence of praseodymium. These large shift are similar to those reported for the methylene resonances of 15-crown-5 and 12-cr0wn-4,'~ and differ from the small shifts reported by King and Heckley.18b The data for crowxsalt ratios between zero and 1:l support the conclusion of complex formation. The appearance of a strongly shifted single methylene resonance in the 'H NMR spectra and the appearance of new resonances in the presence of excess crown support the notion that all of the crown molecules are complexes for crown:salt < 1. This conclusion is consistent with both conductivity data and the spectral data presented in the last section. In addition, the Eu3+ fluorescence is completely quenched by equimolar benzo- 15-crown-5. Therefore, we conclude that for crowxsalt ratios between 0 and 1:1, a single 1:l Eu3+:crown complex is formed. This conclusion implies that the complex formation constant is on the order to IO4 or greater. 3. Europium Fluorescence. The formation of europium complexes of 15-crown-5 and benzo- 15-crown-5 has significantly different effects on the fluorescence properties of the lanthanide cation. Complex formation with 15-crown-5produced little change in the overall fluorescence intensity, Figure 7 . However, significant splitting of the band at 616 nm is observed as well as broadening and loss of a band at 585 nm. The fluorescence band at 616 nm . is known to consist of two overlapping transitions ,Do 'F2 and
-
+-
-
-
+-
+ -
-
+ -
I
-
5664 The Journal of Physical Chemistry, Vol. 89, No. 26, 1985
-
,D, 7F4. The former obeys the hypersensitive AJ = f 2 selection rule.’, Thus, in a C,, geometry, we would expect this band to 7E1 split into two allowed transitions of 5A1 7A1and symmetry. This split band a t 616 nm (crowmalt S 1:l) is well fit by two Gaussians. The observation that the two components lie on either side of the uncomplexed band and differ in intensity by a factor of 2 supports this interpretation. It remains unclear where the ,D1 7F4transition is located or what effects complexation has on its shape and intensity. Addition of crown causes the fluorescence band at 585 nm to broaden and decrease in intensity. No fluorescence is observed at 585 nm for crown:salt 3 1. This band has been assigned to the ,D1 7F, transition. We cannot ascertain if this band is simply broadened, quenched, or shifted underneath the stronger band at 593 nm. In light of the observataion at 616 nm, it is possible that complexation with 15-crown-5 might selectively quenches fluorescence from the ,D1 state. Low-temperature and lifetime measurements are currently being pursued to elucidate these possibilities. The two remaining transitions, ,Do 7F0and ,Do 7F1,located at 579 and 593 nm, respe~tively,’~,’~ are unaffected by complex formation with 15crown-5. Upon addition of benzo-15-crown-5, all of the Eu3+ fluorescence bands decrease in intensity. At equimolar concentrations, the fluorescence is completely quenched. Examination of the spectra, Figure 7, indicate that the 616-nm band appears to shift to higher 7F3band energy with increasing crown. In addition, the ,D, decreases more slowly with crown concentration than the other Eu3+ bands. It is likely that the broad charge-transfer state described in the next section, which is degenerate with the fluorescing state of Ed+, plays and important role in the observed quenching. This continuum, whose origin lies below 580 nm (see Figure 4), could provide an efficient pathway for radiationless transitions. Whether it could selectively quench the ,Do relative to the ,D, levels requires additional investigation. 4 . Assignment of New Absorption Bands Observed upon Complex Formation. Addition of benzo-15-crown-5 to an acetonitrile solution of E u ( C ~ O results ~ ) ~ in the appearance of a new broad absorption band having a maximum near 370 nm, Figures 4 and 5. Broad, structureless bands similar to that observed in these solutions are often associated with charge-transfer bands. In our system, charge transfer between the crown and the complexed cation is likely as Eu3+with its f6 configuration is readily reduced to Eu2+with a standard reduction potential of +0.15 V for the perchlorate in a ~ e t o n i t r i l e . ~ ~ In order to test this hypothesis, we examined the crown complexes of ytterbium. Complex formation was confirmed by obf serving changes in band shape and intensity of the single f absorption band of Yb3+ in the near infrared region. Yb3+with an f13 configuration is less easily reduced than Eu3+ by about 0.7 V, the acetonitrile reduction potential for Yb(C104), being -0.57 V.29 Consistent with this electrochemistry is the observation that in many complexes the charge-transfer transition for Yb3+typically lies ~ 4 0 0 0cm-’ to higher energy than does that of The fact that new broad bands are observed in the ytterbium-crown complexes nearly 5000 cm-I to the blue of where they are for europium confirm their assignment as ligand to metal chargetransfer transitions. As expected, no such bands are observed in neodymium-crown complexes because of the instability of the +2 oxidation state of neodymium. Having established that Eu3+ and Yb3+ ions are common electron acceptors, it remains to identify the donor. The position of these bonds for complexes of 15-crown-5 (Figure 5) are similar to those observed for Eu3+and Yb3+ in ether or alcohol solvents. In these systems, it is evident that the absorption arises from a charge transfer from the ring oxygens to the lanthanide cation. However, the extinction coefficient ( ~ 1 5 of ) the band observed with benzo-15-crown-5 is significantly less than the values of 102-104 commonly associated with o x y - d o n ~ r s The . ~ ~ broad bands
-
-
Simon et al. t
l12 4
1
-
-
-
-
-
-
(29) Kolthof, I. M.; Coetzee, J. F. J . Am. Chem. Soc. 1957, 79, 1852. (30) Reisfeld, R.; Jorgensen, C. K. ‘Lasers and Excited States of Rare Earths”; Springer-Verlag: New York, 1977; pp 43-58.
i’ /*
t . , . , . , . , . , . , , ; , , 02
04
0.6 08 10 12 14 R A T I O OF CROWN TO SALT
I6
18
Figure 8. Intensity of the Eu” to benzo-15-crown-5charge-transfer band as a function of crown to salt ratio.
in the naphtho- and benzo-crown complexes are similar in energy and lie approximately 11 000 cm-’ lower in energy than do those of the 15-crown-5 complex. These band positions are consistent with the reported gas-phase ionization potentials of 7.87 eV for naphthyl methyl ether,,’ 8.20 eV for benzyl methyl ether,32and 10.04 eV for dimethyl ether.33 We therefore assign the near-UV broad bands in the benzo- and naphtho-crown to charge-transfer transitions from the ether oxygens adjacent to the aromatic moiety, and assume that transitions from the other oxygens occur at higher energy as they do in 15-crown-5. The charge-transfer band in the benzo- 15-crown-5 system was analyzed by using the Scott equation2,
where 1, CDo,CAo,A,-, e”, and K are the path length of the cell, total concentration of donor, total concentration of acceptor, absorbance a t v (the frequency at which the complex absorbs), the extinction coefficient at v, and equilibrium constant of complex formation, respectively. This resulted in a complex formation constant of 55 M. Despite the excellent linear fit (r2 = 0.999), this result is inconsistent with conductivity, NMR, and Eu3+ spectral data. This discrepancy is easily resolved by examining the assumptions made in deriving the Scott equation. It is assumed that the donor concentration (crown in our system) is always greater than the concentration of complexes. Our system clearly does not satisfy this requirement. These data demonstrate the fact that although functional relationships like the Scott equation may give excellent fits to the data, one must carefully determine the applicability of these approximations to a given molecular system. 5 . Ion-Pairing Phenomena in the Presence of Excess Crown or Salt. The discussion presented above supports the formation of 1:l complexes between Eu(C1O4), and the crown ethers 15crown-5 and benzo-15-crown-5. For crown:salt ratios less than 1.0, a decrease in conductivity is observed which reflects the lower mobility of the large crown-europium ion. However, addition of excess crown (crown:salt > 2 ) results in increasing conductance. For a solution containing 1O:l crown:salt, the conductance is significantly above that observed in the absence of crown. In addition, increased crown concentration results in an increase in intensity of both the ,Do 7Fotransition and the charge transfer band, Figure 8. These results can be explained in terms of ion pairing and the formation of sandwich complexes.34 Eu(ClO& can exist in many forms in solution. The following equilibria will be present: +-
Eu(C104),
-
Eu(C10&+
+ C104-
K,
(31) Box, H.; Wagner, G.; Croner, J. Chem. Ber. 1972, 105, 3850. (32) Maier, J. P.; Turner, D. W. J . Chem. Soc., Faraday Trans. 2 1973, 69,521. (33) Bain, A. D.; Bunzli, J.-C. G.; Frost, D. C.; Weiler, L. J . Am. Chem. SOC.1973, 95, 291. (34) Christensen, J. J.; Hill, J. 0.;Izatt, R. M. Science 1971, 174, 459.
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5665
Spectra of Europium(II1)-Crown Ether Complexes
--
E u ( C ~ O ~ )Eu(C104)*+ ~ + C104-
+ C104E u ( C ~ O ~ ) ~ Eu3+ +
K2 K3
The magnitudes of K1,K 2 ,and K3 have not been determined. In principle, complexes can be formed with any of these charges ion-pair species. Studies with single and doubly charged cations indicate that C104- is a weakly bonding anion and high degrees of dissociation have been observed.35 However, aqueous solutions of Eu3+ salts in the presence of S042- indicate an almost quantitative yield of EuS04+ ion pairs.36 In a complexed geometry, our results support the crown structure determined from X-ray s t ~ d i e s .Thus, ~ ion pairing between Eu3+ and C10, can occur perpendicular to the crown ring. Recent infrared studies of lanthanide perchlorate complexes of 12-crown-4 and 15-crown-519 reveal significant concentrations of bound C104-. In addition, for L s L ( C I O ~the ) ~ ,formation of a 2:l crown:salt complex in acetonitrile has been observed.Ig These observations support the conclusion that the Eu3+complexes of 15-crown-5 and benzo-15-crown-5 involve ion pairing with C10,. The nonlinear conductivity and spectral data indicate the formation of 2:l complexes at higher crown concentrations, liberating C104- into solution. 0O&O 0 I
E1C3
,'lip,
, I \ ,
. '\\
,I,/'
\' :'a Eu ,',I> ,,I\'\
Excess Crown
i .Cl0,-
0 0
Depending on the stoichiometry, "sandwich" complex formation could liberate one or two perchlorate anions. The degree of ion pairing between Eu3+ and C104-will be smaller in the complex than in the absence of crown. This results from the partial neutralization of the cation charge by the coordinating oxygens on the crown ring. Thus, the number of charged species in solution in the presence of excess crown would actually exceed that in the absence of crown. Although the mobility of the Eu3+ is greatly reduced in the 2:l crowxsalt complex, the conductivity is more sensitive to the number of charge carriers in solution. Ion pairing along these lines could account for the increase in conductivity observed for crown:salt ratios of 2: 1.up to 10:1, Figure 1. In the presence of excess benzo- 15-crown-5, further increases in the intensity of the charge-transfer absorption band are observed for crown:salt ratios greater than 1.0. In these sandwich structures, charge transfer from two crown rings is possible so that a higher intensity might be expected. The intensity of the 5Do 'FOband is also observed to continue to increase in the presence of excess crown. All of these spectral and conductance data are consistent with the formation of multiple crown europium complexes. Addition of excess salt results in significant perturbation of the 'Lb band of benzo-15-crown-5 at 270 nm, Figure 3. In the presence of E u ( N O ~ )up ~ , to sa1t:crown ratios of 1:1, no change in the 'Lb band is observed. This is consistent with conductivity data and examination of the f f transitions which indicate that complexes between the salt and crown ether are not formed. Addition of water to acetonitrile solution containing Eu3+-crown ether complexes causes the disappearance of the charge-transfer bands. In addition, the spectrum af Eu3+closely resembles that observed in water. However, despite the lack of complex formation, a decreased intensity and shift to higher energy is observed for the ILb band of benzo-15-crown-5. This perturbation must result from interaction between the benzene ring and the solvated
-
-
(35) (a) Erlich, R. H.; Roach, E.; Popov, A. I. J . Am. Chem. SOC.1970, 92,4989. (b) Green, R. D.; Martin, J. S.Con. J. Chem. 1972,50,3935. (c) Maciel, G. E.; Hancock, J. K.; Laferty, L. F.; Mueller, P. A.; Musker, W. K. Inorg. Chem. 1966, 5, 544. (d) Cahen, Y. M.; Hardy, P. R.; Roach, E. T.; Popov, A. I. J . Phys. Chem. 1975, 79, 80. (36) Hale, C. F.; Spedding, F. H. J . Phys. Chem. 1972, 76, 2925.
Eu3+ cation. Thus, the spectral shift observed in the 'Lb band in the presence of excess C U ( C ~ Ocould ~ ) ~ result from interactions with E U ( C ~ O ~ )ions ~ - ~in~solution. + The monotonic decrease in conductivity for sa1t:crown solutions of 5:l to 1:l indicate that ion-pairing effects are probably not responsible for these spectral perturbations. The choice of anion appears to be a clear determinant of success in forming solution crown-lanthanide complexes. We did not observe crystal field splittings, a shift in the 'Lb band of benzo15-crown-5, or a charge-transfer band when europium chloride or nitrate was used (up to ratios of 1:1), or if water was present in the acetonitrile. Conversely, by adding water, or potassium or tetramethylammonium nitrate or chloride, to the europium perchlorate crown complex, we observed the loss of those spectroscopic features indicative of the crown complex. For example, addition of nitrate or chloride alters the europium f f transitions to the splitting pattern characteristic of E u ( N O ~ or ) ~CuCl, in the absence of crown. These observations suggest that the complexes formed between trivalent lanthanides and chloride, nitrate, or water are stronger than those formed with crown ethers in acetonitrile. Solid crown ether europium complexes involving nitrate anions have been formed by slow evaporation from so1ution,'8asband small changes in benzene absorption have been reported for solutions of benzo-15-crown-5 mixed with Gd(NCS)3 and Nd(NCS)3 in acetonitrile.Isa Solution thermochemical data have been reported by Izatt for complexes of 18-crown-6 with the first seven members of the lanthanide chloride series in rnethanol.lse The explicit failure of King and Heckley to observe lanthanide crystal field splittings or large crown N M R shifts in solution,18bhowever, is probably due to their choice of hydrated lanthanide nitrates in acetone as their system of study. While they were able to form solid complexes by evaporating the solvent, we suggest that strong ion pairing and/or hydration prevented the formation of crown lanthanide complexes in solution.
-
Conclusion Spectroscopic, N M R , and conductance measurements all support the formation of stable 1:l complexes between several 15-crown-5ethers and E d + , Yb3+,and Nd3+. The solution crystal field splitting patterns of the complexes europium f f transitions are consistent with a fivefold symmetry and the C,, geometry found in the crystal. At higher crown to salt ratios, it appears that sandwich complexes involving two crown molecules may be formed and that ion pairs formed between perchlorate and the 1:l complexes are dissociated. We also report and assign a new charge-transfer transition from the ether oxygens to europium and ytterbium. Unlike previous workers, we found that complexing produces major changes in the solution phase rare earth ion electronic spectrum and on the crown IH N M R spectrum. Our studies strongly suggest that previous work failed to form complexes in solution because of their choice of solvent and/or anion.Isb While crystalline complexes have been reported using chloride and nitrate, we have demonstrated that we can free europium from the solution crown complex by adding these anions. Our study suggests that crown ether complexes of rare earth ions are useful systems for investigating crystal field effect on the rare earth ion electronic transitions. Their stability and unique geometries may provide new insights into the mechanisms of intensity borrowing for several types of highly forbidden transit i o n and ~ ~ ~may be particularly useful in elucidating the mechanism
-
of hyper~ensitivity.~'
Acknowledgment. This work was supported by funds from Williams College. W.R.M. thanks the Camille and Henry Dreyfus foundation for support through a teacher-scholar grant. J.D.S. thanks Kevin Peters for financial support during the preparation of this manuscript. We also express our thanks to the Yale University Instrumentation Laboratory for measuring our N M R spectra. Registry No. Eu, 7440-53-1.
