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Luminescence study on determination of the hydration number of Sm(III) and Dy(III)
Journal of
ALLOYS ELSEVIER
Journal of Alloys and Compounds 225 (1995) 284-287
Luminescence study on determination of the hydration number of Sm(III) and Dy(III) Takaumi Kimura *, Yoshiharu Kato Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken 319-11, Japan
Abstract
A luminescence study of Ln(III) ion has revealed a linear correlation between the decay constant kobs (the reciprocal of the excited-state lifetime) and the number of water molecules nH~o in the first coordination sphere of complexes. From measurements of kobs of Ln(III) in D20-H20 solutions and of Ln(BrO3)3'9H20, the nH2o of Sm(III) and Dy(III) in H20 were calculated to be 9.0 + 0.5 and 8.4 _ 0.4 respectively. Using Ln(III) complexes of polyaminopolycarboxylate ligands, empirical formulae for the calibration of kobs (ms-~) vs. nH2owere proposed as nH2o= 0.026kobs--1.6 for Sm(III) and nH2o= 0.024kobs--1.3 for Dy(III). Keywords: Hydration number; Luminescence lifetime; Polyaminopolycarboxylate complexes
1. Introduction
For trivalent lanthanide (Ln) and actinide ions such as Eu(III), Tb(III) and Cm(III), a linear correlation has been found between the reciprocal of the excitedstate lifetime (i.e. the decay constant kobs) and the number of water molecules in the first coordination sphere of their complexes nu2o [1,2]. A similar correlation is expected for Sm(III) and Dy(III) by analogy with the spectroscopic properties of trivalent lanthanide ions. Heller [3] demonstrated the linear dependence of the luminescence intensity for a series of Ln(III) ions (Ln = Sm, Eu, Tb and Dy) at low H20 concentrations in D 2 0 - H 2 0 mixtures. His results proved that OH vibrators of coordinated water molecules act independently in the de-excitation process. Stein and coworkers [4,5] investigated the luminescence of Sm(III) and Dy(III) in water, dimethylsulfoxide, acetonitrile and their deuterated analogs. The data reveal the importance not only of OH vibration but also of CH and CN vibrations in the non-radiative de-excitation. However, a relationship to determine the hydration number from measurement of the luminescence lifetime is still not clear for Sm(III) and Dy(III). Luminescence studies of Sm(III) and Dy(III) together with those of Eu(III) and Tb(III) as reference ions * Corresponding author. 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSD1 0925-8388(94)07084-9
were performed to assess the relationship between the lifetime and the hydration number of these ions in aqueous solution. The lifetimes of Ln(III) in D 2 0 - H 2 0 mixtures, of Ln(BrO3) 3 •9H20 and of Ln(III) complexed with polyaminopolycarboxylate ligands, for which residual hydration numbers are known for the Eu(III) and Tb(III) complexes [6], were measured for calibration of kobs vs. nn~o.
2. Experimental details
Ln(III) stock solution was prepared by dissolving an appropriate amount of LnzO3 (Wako Pure Chem. Ind., Ltd.) in perchloric acid. D20 (99.9 at.%) was obtained from Merck, Canada and used to prepare the D 2 0 H20 solutions. Crystalline lanthanide bromates, Ln(BrO3)3-9H20, were prepared according to the procedure of Ref. [7]. Nitrilotriacetic acid (NTA), N-(2hydroxyethyl)ethylenediamine-N,N',N'-triacetic acid (HEDTA), ethylenediamine-N,N'-diacetic-N,N'-dipropionic acid (ENDADP), ethylenediaminetetraacetic acid (EDTA), 1,2-diaminopropane-N,N,N',N'-tetraacetic acid (PDTA), trans-l,2-diaminocyclohexaneN,N,N',N'-tetraacetic acid (CDTA), diethylenetriamine-pentaacetic acid (DTPA), glycoletherdiaminetetraacetic acid (EGTA) and triethylenetetraamineN,N,N',N",N",N"-hexaacetic acid (TTHA) were used
T. Ka'mura, Y. Kato / Journal of Alloys and Compounds 225 (1995) 284-287
as received from Tokyo Kasei Kogyo Co., Ltd. Solutions of the polyaminopolycarboxylate complexes were prepared by mixing stoichiometric amounts of Ln(III) and ligand stock solutions of known concentration. The solution pH was adjusted by the addition of standard NaOH or HC104 and was checked before and after measurement of the luminescence lifetime. The concentrations of Ln(III) were 10 - 2-10-1 M in the samples. The Ln(III) in the samples was excited to the excited states of Sm (4113/2 , 464 nm) [8], Eu (5D2, 465 nm) [9], Tb (SD4, 4 8 8 nm) [10] and Dy (4115/2, 454 nm) [8] by a pulsed laser beam. Subsequently, the emission from the lowest luminescent level to the groundstate manifold (i.e. 4G5/2--~ 6H7/2 (594 nm), 6H5/2 (559 nm) for Sm(III); 5D0--*7F2 (615 nm), 7F1 (591 rim) for Eu(III); 594---~ 7 F 5 (543 nm), 7F6 (489 nm) for Tb(III); 4Fg/2----)6H13/2 (572 nm), 6H~5/2(478 nm) for Dy(III) was measured to obtain the luminescence lifetime. The 450-480 nm laser beam was obtained with a pulsed (10 Hz) 355 nm output of a Spectron SL-803 Nd:YAG laser pumping Coumarin 460 (Exciton Chemical) in methanolic solution in a Spectron SL-4000B dye laser head. The pulse power was typically 4-8 mJ and the pulse width was in the nanosecond range. The emission light was focused on a polychrometer (HR320, ISA Jobin-Yvon) and detected by a diode array multichannel analyzer (DIDA-512, Princeton Instruments, Inc.). A gated pulse generator (PG-200, Princeton Instruments, Inc.) was used for time resolution. The spectrometer was controlled by a spectrometric multichannel analyzer system (SMA, Tokyo Instruments Inc.) installed on an NEC-9801/RX personal computer.
3. Results and discussion
The hydration numbers of Eu(III) and Tb(III) were obtained using the difference in the decay rate constants in H20 and D20 solutions [1]. For Eu(III) and Cm(III), a relationship has been proposed in which the hydration number is related directly to the decay rate constant in H 2 0 [2,11]. The luminescence decay constants kobs (ms -1) of Ln 3+ (Ln=Sm, Dy, Eu and Tb) were measured in D20-H20 solutions of various volume percentages Xri:o of H20. The results are shown in Table 1, and Eqs. (1)-(4) express the relations obtained for each Ln 3+ : kobs(Sm) = 3.53X~o + 11.5
r=0.9995
(1)
kobs(Dy) = 3.58XH~o+ 24.6
r=0.9999
(2)
kobs(Eu) = 0.0857XH2o + 0.235
r--- 0.9999
(3)
kobs(Tb) = 0.0211XH~O+ 0.203
r=0.9998
(4)
285
Table 1 Luminescence decay constants for lanthanide ions as a function of volume percentage XH2o of H20 in D20-HzO solutions and lifetime ratios ~'D20/~'H20 XH20
Decay constant ko~s (ms-I)
(%) 0 25 50 75 100
~O20/TH20
Sm 3+
Dy3 +
16.7 96.8 182 277 368
24.0 116 201 296 382
22
16
Eu 3+ 0.272 2.36 4.51 6.60 8.87 33
Tb 3+ 0.215 0.729 1.23 1.80 2.32 11
where r is a correlation coefficient. The results show that the quenching behavior of Sm3+ and Dy3+ in the DzO-H20 system is similar to that of Eu 3+ and Tb 3+ and is due mainly to energy transfer from the excited state to OH vibrators of H20 molecules bound to the metal ion. The slopes of Eqs. (1) and (2) for Sm3+ and Dy3+ are similar and are about 40 and 170 times as large as those for Eu 3+ and Tb a+ respectively. This is due to the energy gaps of Sm 3+ (7400 cm -1) and Dy3+ (7850 cm -1) (defined as the energy difference between the lowest luminescent and highest non-luminescent levels), which are smaller than those of Eu 3+ (12300 cm-1) and Tb 3+ (14800 cm-1) [12]. The lifetime ratios rD~O/ZH20of Sm 3+ and Dy3÷ are similar to those of Eu 3+ and Tb 3+ and are very close to the intensity ratios ID2O/IH2oin the literature [3,12]. To determine nH2o of Ln(III) in H 2 0 , the kob s of lanthanide bromates, Ln(BrO3)3- 9H20, were measured since the bromate has no ligand and nine water molecules in the first coordination sphere. The /Cobsof the bromates obtained for Sm(III), Dy(III), Eu(III) and Tb(III) were 366, 407, 8.80 and 2.52 ms-1 respectively. For each ion, at least two different samples were prepared and the errors of the kob s measured were estimated to be within +5% for each bromate. The bromates prepared were confirmed to have the expected composition from the kob s of the Eu(III) and Tb(III) bromates which were very similar to those of the bromates in the literature [1,11]. By using the kob s of Ln(III) in H20 and of bromate, n,2 o of Sm 3+, Dy3+, Eu 3+ and Tb 3+ in H20 were calculated to be 9.0+0.5, 8.4+0.4, 9.1+0.5 and 8.3+0.4 respectively. From Xray studies of Ln 3+ in concentrated aqueous chloride solutions, Habenschuss and Spedding [13-15] determined a hydration number of 9 for the ions La 3+ to Nd 3+ and 8 for Tb 3+ to Lu 3+ with a decrease for Nd 3+ to Tb 3+ from 9 to 8, while Horrocks and Sudnick [1], from luminescence data, proposed a coordination number from 10 to 9 in the same series. It seems that our results are closer to those of Habenschuss and Spedding than to those of Horrocks and Sudnick.
T. Kirnura, Y. Kato / Journal of Alloys and Compounds 225 (1995) 284-287
286
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Fig. 1. Plots of kob, (ms -~) vs. nn2o for (a) Sm(III), (b) Dy(III), (c) Eu(IIl) and (d) Tb(III). T h e solid lines shown are the calibration relations, Eqs. (5)-(8) respectively. T h e kob, for Ln(III) in D20, in H 2 0 and of bromate are indicated by closed circles, while those for Ln(III)-polyaminopolycarboxylate complexes are indicated by open circles: 1 T T H A , 2 E G T A , 3 D T P A , 4 CDTA, 5 P D T A , 6 E D T A , 7 E N D A D P , 8 H E D T A and 9 NTA.
The residual hydration of lanthanide complexes of polyaminopolycarboxylate ligands was determined by luminescence measurements as a function of pH for Eu(III) and Tb(III) by Brittain and coworkers [16--18]. The nH2o values observed for the Eu(III) and Tb(III) complexes agreed with the uncertainty of + 0.25 water molecules. It was therefore assumed that for a given ligand the residual hydration of the Sm(III) and Dy(III) complexes is similar to that of the Eu(III) and Tb(III) complexes. Table 2 shows the results of our measurements ofkobs for Ln(III) for a number of such complexes. The solution pH was kept constant at 5.0 + 0.5 without buffer to form 1:1 complexes. The following correlations
were derived for the hydration numbers of Ln(III) under our experimental conditions: nn2o --- 0.026kobs(Sm) - 1.6 nH:o = 0.024kobs(Dy) - 1.3 nH~o= 1.1kobs(Eu) - 0.71 nH2o = 4.0kobs(Tb) - 1.0
(5) (6) (7) (8)
where the values for Sm(III)-ENDADP and Sm(III)-CDTA complexes were omitted for better correlation. The slopes of Eqs. (7) and (8) for Eu(III) and Tb(III) respectively agreed with those in the lit-
T. Kimura, Y. Kato / Journal of Alloys and Compounds 225 (1995) 284-287 Table 2 Luminescence decay constants for lanthanide complexes of polyaminopolycarboxylate ligands with H20 hydration Ligand
NTA HEDTA ENDADP EDTA PDTA CDTA DTPA EGTA "ITHA
nmo
4.5 3.1 2.7 2.6 2.6 2.3 1.1 1.0 0.2
Decay constant
kob s
(ms -1)
Sm(III)
Dy(III)
Eu(III)
Tb(III)
232 167 211 163 180 203 101 109 62.1
231 163 166 158 170 177 91.1 103 59.0
4.42 3.55 2.80 3.04 3.10 2.98 1.60 1.66 0.910
1.31 1.04 0.954 0.860 0.884 0.831 0.518 0.536 0.431
erature [1] within +5%. Eqs. (7) and (8) should give results consistent with those obtained by the procedure of Horrocks and Sudnick [1] within the uncertainty of the luminescence method, +0.5 water molecules. For the Eu(III) and Tb(III) complexes, the calculated hydration numbers agreed well with the literature values [6], within +0.2 water molecules. This suggests that Eqs. (7) and (8) are reliable for determination of the hydration number of Eu(III) and Tb(III) in the complexes and, presumably, Eqs. (5) and (6) are reliable for Sm(III) and Dy(III) respectively. Fig. 1 shows plots of kobs (ms -1) vs. nH~o for Sm(III), Dy(III), Eu(III) and Tb(III), with the calibration relations. For Eu(III) and Tb(III), the data in DzO, in HzO and of bromate are identical with the calibration relations, which suggests almost no contribution from the polyaminopolycarboxylate ligand to de-excitation of the luminescent excited state. However, the kobs for Sm(III) and Dy(III) in D20, in HEO and of bromate deviate from the calibration relations by 42_+ 2 and 25 + 4 ms -1 respectively. This may indicate that high-frequency ligand vibrations such as CH and CN in the Sm(lII) and Dy(III) complexes give a small contribution in the non-
287
radiative de-excitation. However, the OH vibration of residual water molecules in the complexes is the primary quencher for the excited states of Sm(III) and Dy(III). We conclude that under the assumption described above, Eqs. (5) and (6) can be used to determine the residual hydration of Sm(III) and Dy(III) in the complexes with an uncertainty of +0.3 water molecules. References [1] W.D. Horrocks, Jr. and D.R. Sudnick, J. Am. Chem. Soc., 101 (1979) 334. [2] T. Kimura and G.R. Choppin, J. Alloys Comp., 213/214 (1994) 313. [3] A. Heller, J. Am. Chem. Soc., 88 (1966) 2058. [4] G. Stein and E. Wiirzberg, Z. Phys. Chem., 101 (1976) 163. [5] Y. Haas, G. Stein and E. Wfirzberg, Z Chem. Phys., 60 (1974) 258. [61 E.N. RizkaUa and G.R. Choppin, Hydration and hydrolysis of lanthanides. In K.A. Gschneidner, Jr., and L. Eyring (eds.), Handbook on Physics and Chemistry o f Rare Earths, Vol. 15, Elsevier, Amsterdam, 1991. [7] N.K. Davidenko, L.N. Lugina and K.B. Yatsimirskii, Russ. J. Inorg. Chem., 17 (1972) 55. [8] W.T. Carnall, P.R. Fields and R. Rajnak, Z Chem. Phys., 49 (1968) 4424. [9] W.T. Carnall, P.R. Fields and R. Rajnak, Z Chem. Phys., 49 (1968) 4450. [10] W.T. Carnall, P.R. Fields and R. Rajnak, J. Chem. Phys., 49 (1968) 4447. [11] P.P. Barthelemy and G.R. Choppin, Inorg. Chem., 28 (1989) 3354. [12] G. Stein and E. Wfirzberg, J. Chem. Phys., 62 (1975) 208. [13] A. Habenschuss and F.H. Spedding, J. Chem. Phys., 70 (1979) 2797. [14] A. Habenschuss and F.H. Spedding, J. Chem. Phys., 70 (1979) 3758. [15] A. Habenschuss and F.H. Spedding, J. Chem. Phys., 73 (1980) 442. [16] H.G. Brittain and J.P. Jasinski, J. Coord. Chem., 18 (1988) 279. [17] H.G. Brittain, G.R. Choppin and P.P. Barthelemy, J. Coord. Chem., 26 (1992) 143. [18] C.A. Chang, H.G. Brittain, J. Telser and M.F. Tweedle, Inorg. Chem., 29 (1990) 4468.
ALLOYS ELSEVIER
Journal of Alloys and Compounds 225 (1995) 284-287
Luminescence study on determination of the hydration number of Sm(III) and Dy(III) Takaumi Kimura *, Yoshiharu Kato Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken 319-11, Japan
Abstract
A luminescence study of Ln(III) ion has revealed a linear correlation between the decay constant kobs (the reciprocal of the excited-state lifetime) and the number of water molecules nH~o in the first coordination sphere of complexes. From measurements of kobs of Ln(III) in D20-H20 solutions and of Ln(BrO3)3'9H20, the nH2o of Sm(III) and Dy(III) in H20 were calculated to be 9.0 + 0.5 and 8.4 _ 0.4 respectively. Using Ln(III) complexes of polyaminopolycarboxylate ligands, empirical formulae for the calibration of kobs (ms-~) vs. nH2owere proposed as nH2o= 0.026kobs--1.6 for Sm(III) and nH2o= 0.024kobs--1.3 for Dy(III). Keywords: Hydration number; Luminescence lifetime; Polyaminopolycarboxylate complexes
1. Introduction
For trivalent lanthanide (Ln) and actinide ions such as Eu(III), Tb(III) and Cm(III), a linear correlation has been found between the reciprocal of the excitedstate lifetime (i.e. the decay constant kobs) and the number of water molecules in the first coordination sphere of their complexes nu2o [1,2]. A similar correlation is expected for Sm(III) and Dy(III) by analogy with the spectroscopic properties of trivalent lanthanide ions. Heller [3] demonstrated the linear dependence of the luminescence intensity for a series of Ln(III) ions (Ln = Sm, Eu, Tb and Dy) at low H20 concentrations in D 2 0 - H 2 0 mixtures. His results proved that OH vibrators of coordinated water molecules act independently in the de-excitation process. Stein and coworkers [4,5] investigated the luminescence of Sm(III) and Dy(III) in water, dimethylsulfoxide, acetonitrile and their deuterated analogs. The data reveal the importance not only of OH vibration but also of CH and CN vibrations in the non-radiative de-excitation. However, a relationship to determine the hydration number from measurement of the luminescence lifetime is still not clear for Sm(III) and Dy(III). Luminescence studies of Sm(III) and Dy(III) together with those of Eu(III) and Tb(III) as reference ions * Corresponding author. 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSD1 0925-8388(94)07084-9
were performed to assess the relationship between the lifetime and the hydration number of these ions in aqueous solution. The lifetimes of Ln(III) in D 2 0 - H 2 0 mixtures, of Ln(BrO3) 3 •9H20 and of Ln(III) complexed with polyaminopolycarboxylate ligands, for which residual hydration numbers are known for the Eu(III) and Tb(III) complexes [6], were measured for calibration of kobs vs. nn~o.
2. Experimental details
Ln(III) stock solution was prepared by dissolving an appropriate amount of LnzO3 (Wako Pure Chem. Ind., Ltd.) in perchloric acid. D20 (99.9 at.%) was obtained from Merck, Canada and used to prepare the D 2 0 H20 solutions. Crystalline lanthanide bromates, Ln(BrO3)3-9H20, were prepared according to the procedure of Ref. [7]. Nitrilotriacetic acid (NTA), N-(2hydroxyethyl)ethylenediamine-N,N',N'-triacetic acid (HEDTA), ethylenediamine-N,N'-diacetic-N,N'-dipropionic acid (ENDADP), ethylenediaminetetraacetic acid (EDTA), 1,2-diaminopropane-N,N,N',N'-tetraacetic acid (PDTA), trans-l,2-diaminocyclohexaneN,N,N',N'-tetraacetic acid (CDTA), diethylenetriamine-pentaacetic acid (DTPA), glycoletherdiaminetetraacetic acid (EGTA) and triethylenetetraamineN,N,N',N",N",N"-hexaacetic acid (TTHA) were used
T. Ka'mura, Y. Kato / Journal of Alloys and Compounds 225 (1995) 284-287
as received from Tokyo Kasei Kogyo Co., Ltd. Solutions of the polyaminopolycarboxylate complexes were prepared by mixing stoichiometric amounts of Ln(III) and ligand stock solutions of known concentration. The solution pH was adjusted by the addition of standard NaOH or HC104 and was checked before and after measurement of the luminescence lifetime. The concentrations of Ln(III) were 10 - 2-10-1 M in the samples. The Ln(III) in the samples was excited to the excited states of Sm (4113/2 , 464 nm) [8], Eu (5D2, 465 nm) [9], Tb (SD4, 4 8 8 nm) [10] and Dy (4115/2, 454 nm) [8] by a pulsed laser beam. Subsequently, the emission from the lowest luminescent level to the groundstate manifold (i.e. 4G5/2--~ 6H7/2 (594 nm), 6H5/2 (559 nm) for Sm(III); 5D0--*7F2 (615 nm), 7F1 (591 rim) for Eu(III); 594---~ 7 F 5 (543 nm), 7F6 (489 nm) for Tb(III); 4Fg/2----)6H13/2 (572 nm), 6H~5/2(478 nm) for Dy(III) was measured to obtain the luminescence lifetime. The 450-480 nm laser beam was obtained with a pulsed (10 Hz) 355 nm output of a Spectron SL-803 Nd:YAG laser pumping Coumarin 460 (Exciton Chemical) in methanolic solution in a Spectron SL-4000B dye laser head. The pulse power was typically 4-8 mJ and the pulse width was in the nanosecond range. The emission light was focused on a polychrometer (HR320, ISA Jobin-Yvon) and detected by a diode array multichannel analyzer (DIDA-512, Princeton Instruments, Inc.). A gated pulse generator (PG-200, Princeton Instruments, Inc.) was used for time resolution. The spectrometer was controlled by a spectrometric multichannel analyzer system (SMA, Tokyo Instruments Inc.) installed on an NEC-9801/RX personal computer.
3. Results and discussion
The hydration numbers of Eu(III) and Tb(III) were obtained using the difference in the decay rate constants in H20 and D20 solutions [1]. For Eu(III) and Cm(III), a relationship has been proposed in which the hydration number is related directly to the decay rate constant in H 2 0 [2,11]. The luminescence decay constants kobs (ms -1) of Ln 3+ (Ln=Sm, Dy, Eu and Tb) were measured in D20-H20 solutions of various volume percentages Xri:o of H20. The results are shown in Table 1, and Eqs. (1)-(4) express the relations obtained for each Ln 3+ : kobs(Sm) = 3.53X~o + 11.5
r=0.9995
(1)
kobs(Dy) = 3.58XH~o+ 24.6
r=0.9999
(2)
kobs(Eu) = 0.0857XH2o + 0.235
r--- 0.9999
(3)
kobs(Tb) = 0.0211XH~O+ 0.203
r=0.9998
(4)
285
Table 1 Luminescence decay constants for lanthanide ions as a function of volume percentage XH2o of H20 in D20-HzO solutions and lifetime ratios ~'D20/~'H20 XH20
Decay constant ko~s (ms-I)
(%) 0 25 50 75 100
~O20/TH20
Sm 3+
Dy3 +
16.7 96.8 182 277 368
24.0 116 201 296 382
22
16
Eu 3+ 0.272 2.36 4.51 6.60 8.87 33
Tb 3+ 0.215 0.729 1.23 1.80 2.32 11
where r is a correlation coefficient. The results show that the quenching behavior of Sm3+ and Dy3+ in the DzO-H20 system is similar to that of Eu 3+ and Tb 3+ and is due mainly to energy transfer from the excited state to OH vibrators of H20 molecules bound to the metal ion. The slopes of Eqs. (1) and (2) for Sm3+ and Dy3+ are similar and are about 40 and 170 times as large as those for Eu 3+ and Tb a+ respectively. This is due to the energy gaps of Sm 3+ (7400 cm -1) and Dy3+ (7850 cm -1) (defined as the energy difference between the lowest luminescent and highest non-luminescent levels), which are smaller than those of Eu 3+ (12300 cm-1) and Tb 3+ (14800 cm-1) [12]. The lifetime ratios rD~O/ZH20of Sm 3+ and Dy3÷ are similar to those of Eu 3+ and Tb 3+ and are very close to the intensity ratios ID2O/IH2oin the literature [3,12]. To determine nH2o of Ln(III) in H 2 0 , the kob s of lanthanide bromates, Ln(BrO3)3- 9H20, were measured since the bromate has no ligand and nine water molecules in the first coordination sphere. The /Cobsof the bromates obtained for Sm(III), Dy(III), Eu(III) and Tb(III) were 366, 407, 8.80 and 2.52 ms-1 respectively. For each ion, at least two different samples were prepared and the errors of the kob s measured were estimated to be within +5% for each bromate. The bromates prepared were confirmed to have the expected composition from the kob s of the Eu(III) and Tb(III) bromates which were very similar to those of the bromates in the literature [1,11]. By using the kob s of Ln(III) in H20 and of bromate, n,2 o of Sm 3+, Dy3+, Eu 3+ and Tb 3+ in H20 were calculated to be 9.0+0.5, 8.4+0.4, 9.1+0.5 and 8.3+0.4 respectively. From Xray studies of Ln 3+ in concentrated aqueous chloride solutions, Habenschuss and Spedding [13-15] determined a hydration number of 9 for the ions La 3+ to Nd 3+ and 8 for Tb 3+ to Lu 3+ with a decrease for Nd 3+ to Tb 3+ from 9 to 8, while Horrocks and Sudnick [1], from luminescence data, proposed a coordination number from 10 to 9 in the same series. It seems that our results are closer to those of Habenschuss and Spedding than to those of Horrocks and Sudnick.
T. Kirnura, Y. Kato / Journal of Alloys and Compounds 225 (1995) 284-287
286
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4
5 6 7 8 9 1 0 n H20
Fig. 1. Plots of kob, (ms -~) vs. nn2o for (a) Sm(III), (b) Dy(III), (c) Eu(IIl) and (d) Tb(III). T h e solid lines shown are the calibration relations, Eqs. (5)-(8) respectively. T h e kob, for Ln(III) in D20, in H 2 0 and of bromate are indicated by closed circles, while those for Ln(III)-polyaminopolycarboxylate complexes are indicated by open circles: 1 T T H A , 2 E G T A , 3 D T P A , 4 CDTA, 5 P D T A , 6 E D T A , 7 E N D A D P , 8 H E D T A and 9 NTA.
The residual hydration of lanthanide complexes of polyaminopolycarboxylate ligands was determined by luminescence measurements as a function of pH for Eu(III) and Tb(III) by Brittain and coworkers [16--18]. The nH2o values observed for the Eu(III) and Tb(III) complexes agreed with the uncertainty of + 0.25 water molecules. It was therefore assumed that for a given ligand the residual hydration of the Sm(III) and Dy(III) complexes is similar to that of the Eu(III) and Tb(III) complexes. Table 2 shows the results of our measurements ofkobs for Ln(III) for a number of such complexes. The solution pH was kept constant at 5.0 + 0.5 without buffer to form 1:1 complexes. The following correlations
were derived for the hydration numbers of Ln(III) under our experimental conditions: nn2o --- 0.026kobs(Sm) - 1.6 nH:o = 0.024kobs(Dy) - 1.3 nH~o= 1.1kobs(Eu) - 0.71 nH2o = 4.0kobs(Tb) - 1.0
(5) (6) (7) (8)
where the values for Sm(III)-ENDADP and Sm(III)-CDTA complexes were omitted for better correlation. The slopes of Eqs. (7) and (8) for Eu(III) and Tb(III) respectively agreed with those in the lit-
T. Kimura, Y. Kato / Journal of Alloys and Compounds 225 (1995) 284-287 Table 2 Luminescence decay constants for lanthanide complexes of polyaminopolycarboxylate ligands with H20 hydration Ligand
NTA HEDTA ENDADP EDTA PDTA CDTA DTPA EGTA "ITHA
nmo
4.5 3.1 2.7 2.6 2.6 2.3 1.1 1.0 0.2
Decay constant
kob s
(ms -1)
Sm(III)
Dy(III)
Eu(III)
Tb(III)
232 167 211 163 180 203 101 109 62.1
231 163 166 158 170 177 91.1 103 59.0
4.42 3.55 2.80 3.04 3.10 2.98 1.60 1.66 0.910
1.31 1.04 0.954 0.860 0.884 0.831 0.518 0.536 0.431
erature [1] within +5%. Eqs. (7) and (8) should give results consistent with those obtained by the procedure of Horrocks and Sudnick [1] within the uncertainty of the luminescence method, +0.5 water molecules. For the Eu(III) and Tb(III) complexes, the calculated hydration numbers agreed well with the literature values [6], within +0.2 water molecules. This suggests that Eqs. (7) and (8) are reliable for determination of the hydration number of Eu(III) and Tb(III) in the complexes and, presumably, Eqs. (5) and (6) are reliable for Sm(III) and Dy(III) respectively. Fig. 1 shows plots of kobs (ms -1) vs. nH~o for Sm(III), Dy(III), Eu(III) and Tb(III), with the calibration relations. For Eu(III) and Tb(III), the data in DzO, in HzO and of bromate are identical with the calibration relations, which suggests almost no contribution from the polyaminopolycarboxylate ligand to de-excitation of the luminescent excited state. However, the kobs for Sm(III) and Dy(III) in D20, in HEO and of bromate deviate from the calibration relations by 42_+ 2 and 25 + 4 ms -1 respectively. This may indicate that high-frequency ligand vibrations such as CH and CN in the Sm(lII) and Dy(III) complexes give a small contribution in the non-
287
radiative de-excitation. However, the OH vibration of residual water molecules in the complexes is the primary quencher for the excited states of Sm(III) and Dy(III). We conclude that under the assumption described above, Eqs. (5) and (6) can be used to determine the residual hydration of Sm(III) and Dy(III) in the complexes with an uncertainty of +0.3 water molecules. References [1] W.D. Horrocks, Jr. and D.R. Sudnick, J. Am. Chem. Soc., 101 (1979) 334. [2] T. Kimura and G.R. Choppin, J. Alloys Comp., 213/214 (1994) 313. [3] A. Heller, J. Am. Chem. Soc., 88 (1966) 2058. [4] G. Stein and E. Wiirzberg, Z. Phys. Chem., 101 (1976) 163. [5] Y. Haas, G. Stein and E. Wfirzberg, Z Chem. Phys., 60 (1974) 258. [61 E.N. RizkaUa and G.R. Choppin, Hydration and hydrolysis of lanthanides. In K.A. Gschneidner, Jr., and L. Eyring (eds.), Handbook on Physics and Chemistry o f Rare Earths, Vol. 15, Elsevier, Amsterdam, 1991. [7] N.K. Davidenko, L.N. Lugina and K.B. Yatsimirskii, Russ. J. Inorg. Chem., 17 (1972) 55. [8] W.T. Carnall, P.R. Fields and R. Rajnak, Z Chem. Phys., 49 (1968) 4424. [9] W.T. Carnall, P.R. Fields and R. Rajnak, Z Chem. Phys., 49 (1968) 4450. [10] W.T. Carnall, P.R. Fields and R. Rajnak, J. Chem. Phys., 49 (1968) 4447. [11] P.P. Barthelemy and G.R. Choppin, Inorg. Chem., 28 (1989) 3354. [12] G. Stein and E. Wfirzberg, J. Chem. Phys., 62 (1975) 208. [13] A. Habenschuss and F.H. Spedding, J. Chem. Phys., 70 (1979) 2797. [14] A. Habenschuss and F.H. Spedding, J. Chem. Phys., 70 (1979) 3758. [15] A. Habenschuss and F.H. Spedding, J. Chem. Phys., 73 (1980) 442. [16] H.G. Brittain and J.P. Jasinski, J. Coord. Chem., 18 (1988) 279. [17] H.G. Brittain, G.R. Choppin and P.P. Barthelemy, J. Coord. Chem., 26 (1992) 143. [18] C.A. Chang, H.G. Brittain, J. Telser and M.F. Tweedle, Inorg. Chem., 29 (1990) 4468.