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Spectroscopic studies of aluminum and gallium complexes with oxalate and malonate in aqueous solution
Journal of Molecular Structure 648 (2003) 225–235 www.elsevier.com/locate/molstruc
Spectroscopic studies of aluminum and gallium complexes with oxalate and malonate in aqueous solution ¨ hman, Kristina Axe, Per Persson* Maria Clause´n, Lars-Olof O Department of Chemistry, Inorganic Chemistry, Umea˚ University, SE-901 87 Umea˚, Sweden Received 25 October 2002; revised 19 December 2002; accepted 19 December 2002
Abstract The local structures of Ga(III) in aqueous oxalate and malonate complexes were studied by means of Ga K-edge EXAFS spectroscopy. Irrespective of the number and type of coordinated ligands, the EXAFS results showed very regular first coordination shells consisting of six oxygen atoms. Scattering paths from more distant atoms revealed that both oxalate and malonate form mononuclear chelate structures where one oxygen from each carboxylate group binds to Ga(III). Again, very little variation in bond distances and no changes in coordination modes were detected as the number of ligands coordinated to Ga(III) was varied. Based on the very close resemblance of IR spectra of oxalate and malonate complexes of Al(III), and the corresponding complexes of Ga(III), it is believed that the local structures of the Al(III) complexes are similar to those of the Ga(III) complexes in terms of ligand coordination modes and distortions. This conclusion was corroborated by results from theoretical frequency calculations. q 2003 Elsevier Science B.V. All rights reserved. Keywords: Gallium; Aluminum; Oxalate; Malonate; Aqueous solution
1. Introduction A detailed knowledge of the chemical speciation of metal – ligand interactions in aqueous solution requires information from a wide range of techniques. Potentiometry and solubility measurements provide means to determine the stoichiometries and thermodynamic stabilities of the species formed. These methods can be complemented by various spectroscopic techniques, such as NMR and IR/Raman, to characterize certain molecular properties of the * Corresponding author. Tel.: þ46-90-7865573; fax: þ 46-907869195. E-mail address: [email protected] (P. Persson).
complexes. In addition LAXS and EXAFS methods are in favorable cases available for in situ determination of structural parameters, such as bond distances between the metal ion and the surrounding ligands [1]. Recently, studies of aqueous metal complexes have been complemented by theoretical quantum chemical methods to gain an even deeper understanding of structural details, spectroscopic properties, and relative stabilities. Examples are the publications discussing the aqueous complexes between Al(III) and carboxylic acids [2 – 4]. The results of these theoretical calculations do, however, sometimes significantly deviate from experimentally derived information in the literature. These can be discrepancies in the number and composition of the most dominant
0022-2860/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2860(03)00026-7
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complex(es), or differences in the suggested coordination modes of the ligands. Whether such disagreements are due to problems with the interpretation of the experimental data, or to inadequacies of the theoretical models, in particular with respect to how the hydration of the complex(es) were treated, is not always easy to assess. In any case, with the rapid increase of theoretical publications, it has become obvious that there is a definitive need for high-quality experimental data for comparisons and corroboration of the theoretical models. In a recent theoretical study on Al(III)-oxalate complexes, it was found that there were large differences between the results obtained from calculations performed on the gas phase complexes as compared to calculations where hydration was simulated by the polarized continuum model [4]. In gas phase, a remarkable difference in the calculated Al –O distances was observed for the [Al(C2O4)(H2O)4]þ ˚ for Al –O(oxalate) and 1.984 A ˚ for complex; 1.805 A Al –O(water). This difference was diminished when solvation was simulated, but still a strongly distorted first coordination shell was predicted with Al – O ˚ for oxalate and water distances of 1.861 and 1.950 A coordination, respectively. Unfortunately, the validity of these predictions cannot be verified with conventional LAXS and EXAFS techniques because of the low K edge energy of Al (1.56 keV) and because of the extremely high concentrations needed for the LAXS measurements.
Instead, a different experimental strategy must be used. As a neighbor to Al in group 13 of the periodic table, Ga has a closely related chemistry and Ga(III) is often considered an Al(III) analogue [5]. An advantage with Ga is the K edge energy of 10.367 keV, which makes it suitable for EXAFS investigations of dilute aqueous solutions. In this study we analyze the local structure of Ga(III) in aqueous oxalate (ox22) and malonate (mal22) complexes by means of EXAFS. This will provide information about the dominating coordination mode(s) of the ligands, and about the degree of first shell distortion as a function of the number of ligands bound to the central Ga(III) ion. We then explore/exploit the Al(III) – Ga(III) analogy by analyzing and comparing IR spectra of oxalate and malonate complexes of both metal ions. These spectra are interpreted and assigned by comparison with theoretical frequencies obtained from molecular orbital calculations.
2. Experimental section 2.1. Chemicals and solutions Solutions of dilute HCl were prepared from concentrated hydrochloric acid (Fisher Chemicals, p.a.) and were standardized using tris(hydroxymethyl)aminomethane, ‘trisma-base’ (Sigma p.a.), dried at 80 8C. Dilute, carbonate free,
Table 1 Composition of the Ga(III) solutions analyzed by EXAFS and IR spectroscopy Solution
[Ga]tot (mM)
[H2L]tot (mM)
[H]tot (mM)
2log[Hþ]
Ga3þ (%)
GaLþ (%)
GaL2 2 (%)
GaL32 3 (%)
EXAFS A, oxalatea B, oxalatea C, oxalatea D, malonateb E, malonateb
40 40 70 40 40
120 80 70 240 80
2234 232 2123 2397 2156
3.2 1.6 1.9 4.6 3.5
0 4 29 0 0
0 24 46 0 13
4 44 22 8 84
96 28 3 92 3
IR Oxalatea Oxalatea Malonateb Malonateb Malonateb
60 10 50 20 10
20 30 25 40 60
60 258 245 277 2113
1.2 3.3 2.5 3.6 5.7
70 0 53 0 0
27 0 46 16 0
3 5 1 81 12
0 95 0 2 88
a b
Formation constants from Ref. [6]. Based on analogy with the Al(III)-malonate system, formation constants from Ref. [7].
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Table 2 Composition of the Al(III) solutions analyzed by IR spectroscopy Solution
[Al]tot (mM)
[H2L]tot (mM)
[H]tot (mM)
2log[Hþ]
Al3þ (%)
AlLþ (%)
AlL2 2 (%)
AlL32 3 (%)
Oxalate Oxalate Malonate Malonate Malonate
75 30 30 30 30
20 100 15 60 85
239.9 2200 229.3 2117.7 2165.4
3.2 5.5 3.2 4.0 5.1
74 0 51 0 0
25 0 47 11 0
1 1 1 85 45
0 99 0 4 55
The compositions of the Al(III)-malonate solutions are based on formation constants from Ref. [6] and the compositions of the Al(III)oxalate solutions are based on formation constants from Ref. [8].
sodium hydroxide solutions were prepared from filtrated 50% NaOH (Merck, p.a.) and standardized against hydrochloric acid prepared as above. The composition of the solutions used for the EXAFS and IR experiments are summarized in Tables 1 and 2. They were all prepared from weighed amounts of sodium oxalate or malonate (Riedel-de Hae¨n, p.a.) and dried NaCl (Riedel-de Hae¨n, p.a.). The metal ions were added from a stock solution of GaCl3 (Aldrich, 99.999%) or as weighed amounts of AlCl3·6H2O (Riedel-de Hae¨n, p.a.), respectively. The compositions of the solutions were chosen on the basis of extensive chemical modelling using thermodynamic constants from the literature [6 –8] and a modernized version of the chemical speciation program WinSGW1. To reach different 2 log[Hþ], appropriate amounts of dilute HCl or NaOH was added. All solutions were prepared to contain 0.6 M Na(Cl), and were left to equilibrate for at least 24 h before analysis. Beyond that time, 2 log[Hþ] and IR spectra did not change and the solutions were considered stable. A combination glass electrode, calibrated in solutions of known HCl concentrations in 0.6 M NaCl, was used to measure 2 log[Hþ].
3. Methods and measurements 3.1. EXAFS measurements Ga K-edge EXAFS data were measured at the Stanford Synchrotron Radiation Laboratory, California on beam line 4– 1. The ring energy was 3.0 GeV with ring currents between 60 and 100 mA. A Si(220) 1
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double crystal monochromator was used and detuned 50% to eliminate higher order harmonics. The incoming beam was registered using an ion chamber filled with N2. The data were collected at room temperature in the fluorescence mode, with a Lytle detector filled with Ar gas. A Zn-6 filter and Soller slit set-up were used to reduce Kb fluorescence and scattering contributions to the signal. Internal calibration was performed by simultaneously measuring spectra from a Ga(OH)3 (s) reference sample in transmission mode, throughout the duration of all scans. Three or four scans were collected per sample. 3.2. EXAFS data treatment The EXAFS data were energy-calibrated and averaged with EXAFSPAK [9], and further analyzed using WinXAS [10]. Standard procedures were used for pre-edge subtraction, data normalization, and spline removal. The k3 -weighted EXAFS oscillations were Fourier transformed over the k-range 3.3– ˚ 21 using a Bessel window function. The 14.5 A resulting Fourier transforms were used in the leastsquares refinements, thus all results presented herein were obtained from R-space fits. Theoretical phase and amplitude functions, used in the refinements, for single and multiple scattering paths within assumed molecular models were calculated with the ab initio code FEFF7 [11]. The input structures were those obtained from the DFT geometry optimizations of and Ga(mal)32 Ga(ox)32 3 3 , see below. Since the EXAFS amplitude is proportional to the coordination number and the amplitude reduction factor, S20 ; the EXAFS spectrum of an acidic solution of gallium nitrate, where the Ga – O coordination number of Ga(H2O)3þ 6 is known to be 6 [12], was used to refine
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S20 : This value was found to be 1.25, with the phase and amplitude functions obtained from the FEFF calculation of Ga(ox)32 3 , and was kept constant during the refinements of the EXAFS spectra. 3.3. IR spectroscopy Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were collected using a Perkin – Elmer 2000 FTIR spectrometer fitted with a deuterated triglycine sulfate (DTGS) detector. The spectra were recorded with a horizontal ATR accessory and a diamond crystal as the reflection element (SensIR Technologies). The sample cell was purged with nitrogen gas throughout data collection to exclude carbon dioxide and water vapor. The angle of incidence for the set-up is approximately 458, which is far from the critical angle. This and the fact that the bands analyzed are weak (, 0.05 absorbance units), and do not overlap with stronger bands (except for the asymmetric C –O stretch which overlaps with the intense H2O bend), indicate that the effects of possible distortions known to occur in ATR spectra are minimized [13]. The solutions were applied to the diamond crystal surface directly and a quartz lid was placed over the sample and pressed tightly against a rubber gasket. This sealed the sample from the atmosphere during data collection. 100 scans were collected for each sample in the 370 –7800 cm21
range with a resolution of 4 cm21. The sample spectra were interpreted after subtracting spectra of both the empty cell and 0.6 M NaCl. When necessary, interfering solution species were also subtracted from the spectra. These subtractions were guided by the predicted compositions given in Tables 1 and 2. 3.4. Molecular orbital calculations Theoretical vibration frequencies of the geometry 32 32 optimized Al(ox)32 3 , Al(mal) 3 , Ga(ox) 3 , and 32 Ga(mal)3 complexes were calculated with the density functional theory (DFT) using the hybrid functionals B3LYP. The standard 6-31G* basis set was used in all calculations. The calculations were performed with the program GAUSSIAN 94W [14] by Gaussian Inc., Pittsburgh, USA and visualization of the calculated vibrational modes was accomplished with HyperChem v. 5 by Hypercube, Gainesville, USA. The potential energy minimum structures of the complexes were obtained without applying symmetry restrictions, i.e. all bond lengths, angles and dihedral angles were allowed to vary. The subsequently calculated vibrational frequencies were scaled by the recommended scale factor 0.9613 (B3LYP/6-31G*) to account for systematic errors [15]. Only real frequencies were obtained which show that the optimized structures represent minima at the potential energy surface.
Fig. 1. k3 -weighted EXAFS and the corresponding Fourier transforms of solutions with (a) [Ga]tot ¼ 0.040 M, [H2ox]tot ¼ 0.120 M and 2log[Hþ] ¼ 3.2 (solution A), (b) [Ga]tot ¼ 0.040 M, [H2ox]tot ¼ 0.080 M and 2log[Hþ] ¼ 1.6 (solution B) and (c) [Ga]tot ¼ 0.070 M, [H2ox]tot ¼ 0.070 M and 2log[Hþ] ¼ 1.9 (solution C). The spectrum of an acidic aqueous solution of Ga(NO3)3 (d) is included for comparison. The Fourier transforms are uncorrected for phase shift. Broken lines represent the model presented in Table 3.
M. Clause´n et al. / Journal of Molecular Structure 648 (2003) 225–235
Fig. 2. Structure of Ga(ox)32 obtained from the DFT geometry 3 ˚ , and optimization. The Ga–O distances are (1.9895 ^ 0.0005) A the a angles are (81.51 ^ 0.01) 8.
4. Results and discussion 4.1. Structures in the aqueous Ga(III)-oxalate system from EXAFS spectroscopy As mentioned in Section 1, recent theoretical work concerning the related Al(III)-oxalate system [4], has predicted pronounced distortions in the first coordination sphere of the Al(ox)(H2O)þ 4 complex. Also, the commonly accepted chelating coordination of oxalate to Al(III) has recently been challenged by theoretical work [3]. The objective of this part of the study has therefore been to investigate whether these theoretical predictions could be experimentally observed for Ga(III)-oxalate complexes. To maximize the fraction of Ga(III) in each of the target complexes, a series of WinSGW1 calculations with the formation constants presented by Kulba et al. [6], were performed. On the basis of these calculations, the compositions of the solutions summarized in Table 1, were selected. As seen, only for the Ga(ox)32 3 complex, a solution with virtually all Ga(III) in one dominating species was thermodynamically possible to prepare. This implies
229
that the EXAFS results from the other Ga(III)-oxalate solutions will yield trends in the evaluated fit parameters, rather than absolute numbers related to one individual complex. The resulting EXAFS spectra, with the corresponding Fourier transforms, are shown in Fig. 1. An acidic solution of Ga(H2O)3þ 6 , where gallium is known to coordinate six water ˚ [12], is included for comparison. molecules at 1.96 A From the Fourier transforms, it is clear that the first coordination sphere in solution A through C is similar to that of the hydrated metal ion (Fig. 1). With increasing oxalate to gallium ratio, higher shells become increasingly more pronounced and the positions of these shells appear to be more or less unchanged in the different spectra. The structure of Ga(ox)32 (Fig. 2), obtained 3 from the DFT geometry optimization, was used as input to FEFF7 in order to calculate the dominating scattering paths, and to obtain phase and amplitude functions for fitting the experimental data. The paths to be included in the fit were selected from the most important single scattering contributions and the three-legged multiple scattering contributions (see Fig. 3 and Table 3). Thus, the fit results presented below will reveal structural similarities and differences between the optimized structure and the solution complex(es). To keep the number of fit parameters as low as possible no higher order (. 3) multiple scattering paths, and no multiple scattering paths from the first coordination shell only, were included. The coordination numbers, distances and Debye – Waller factors for the single scattering contributions were free variables in the fits, whereas the coordination numbers and the distances of the multiple scattering paths were correlated to the corresponding single scattering
Fig. 3. Schematic description of the dominating single (A) and three-legged multiple scattering (B and C) paths for the fivemembered ring chelate structure of oxalate. The single scattering path contributing to the first shell has been omitted.
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Table 3 EXAFS fit results of gallium-oxalate complexes Solution A
Solution B
Path
N
˚) R (A
s2
DE0
N
Ga –O Ga –C Ga –O–Cb Ga –Odistal Ga –C–Ob Ga –O–Ob
5.6 5.4 (5.9) 10.9 (11.8) 5.4 (5.9) 10.9 (11.8) 10.9 (11.8)
1.97 2.75 3.01 4.01 4.02 4.14
0.0049 0.0039 0.0025 0.0034 0.0060 0.0014
3.3 3.3 3.3 3.3 3.3 3.3
5.8 3.2 6.5 3.2 6.5 6.5
(3.9) (7.8) (3.9) (7.8) (7.8)
Solution C ˚) R (A
s2
DE0
N
1.96 2.74 3.00 4.01 4.01 4.14
0.0055 0.0035 0.0011 0.0032 0.0064 0.0032
3.2 3.2 3.2 3.2 3.2 3.2
5.5 1.8 3.7 1.8 3.7 3.7
(2.0) (4.0) (2.0) (4.0) (4.0)
˚) R (A
s2
DE0
1.96 2.73 2.99 4.01 4.02 4.14
0.0052 0.0035a 0.0011a 0.0032a 0.0064a 0.0032a
2.7 2.7 2.7 2.7 2.7 2.7
S20 fixed at 1.25. Energy shift, DE0 ; was allowed to vary but kept internally constant for all shells within a particular complex. Uncertainties ˚ and in the higher order shells to ^0.04 A ˚ . Uncertainties in coordination numbers are estimated to in R are estimated in the first shell to ^0.02 A at least ^20%. Values in parentheses are the coordination numbers expected from the compositions given in Table 1, assuming a chelate structure involving one oxygen from each carboxylate group. a DW-factor fixed to the values obtained in fit of solution B. b MS paths correlated to single scattering coordination numbers and distances, and angles fixed to values of the structures used in the FEFF calculations.
contributions. This fitting-approach provided a very good fit to data, and the models are presented in Table 3 and in Fig. 1. First shell fits revealed close to six oxygen back˚ for all solutions. No scatterers at a distance of 1.96 A trends in distances, coordination numbers or Debye – Waller factors were detected. Thus, the theoretically predicted distortion of Al(ox)(H2O)þ 4 as compared to cannot find experimental support for the Al(ox)32 3 equivalent Ga(III) complexes. Instead, the consistent values of the first shell fit variables indicate differences in average distances in the three solutions
˚ , which is the estimated experimental of # 0.02 A error. The higher order shells are satisfactorily modeled by single-scattering paths from the carbon atoms and the distal oxygens, in combination with multiple scattering contributions involving the coordinated oxygens, the distal oxygens, and the carbon atoms of the ligand (cf. Fig. 3 and Table 3). From the fitting results, it is clear that also the Ga – C and Ga –Odistal bond lengths and Debye – Waller factors are more or less equal for all three solutions, indicating that the coordination mode of oxalate to Ga(III) indeed is very
Fig. 4. k3 -weighted EXAFS and the corresponding Fourier transforms of solutions containing (a) [Ga]tot ¼ 0.040 M and [H2mal]tot ¼ 0.240 M with 2log[Hþ] ¼ 4.6 (solution D) and (b) [Ga]tot ¼ 0.040 M and [H2mal]tot ¼ 0.080 M with 2log[Hþ] ¼ 3.5 (solution E). The spectrum of an acidic aqueous solution of Ga(NO3)3 (c) is included for comparison. The Fourier transforms are uncorrected for phase shift. Broken lines represent the model presented in Table 4.
M. Clause´n et al. / Journal of Molecular Structure 648 (2003) 225–235
Fig. 5. Structure of Ga(mal)32 obtained from the DFT geometry 3 ˚ , and the a optimization. The Ga –O distances are (1.984 ^ 0.015) A angles are (89.20 ^ 1.00) 8.
similar in all GaðoxÞn ðH2 OÞ322n 622n ðn ¼ 1 – 3Þ complexes. This information together with the coordination numbers of the Ga– C and Ga –Odistal scattering paths as compared to the predicted composition of the solutions, strongly indicate a chelating coordination mode of oxalate in all predominating Ga(III)-oxalate complexes. 4.2. Structures in the aqueous Ga(III)-malonate system from EXAFS spectroscopy No complete thermodynamic model for the complex formation reactions in the Ga(III)-malonate system is available in the literature. We have therefore used the formation constants valid for Al(III)malonate complexes to model the speciation [7], and prepared Ga(III)-malonate solutions at the total concentrations of metal ion, ligand and protons, 32 known to generate Al(mal)2 2 and Al(mal)3 . The compositions of these solutions are given in Table 1 and the resulting EXAFS, with the corresponding Fourier transforms, are shown in Fig. 4, with an acidic solution of Ga(H2O)3þ 6 included for comparison. The position and shape of the first shell peaks of both Ga(III)-malonate solutions, relative to that of Ga(H2O)3þ 6 , indicate that also in this system the first coordination shells are very similar to that of
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the hydrated metal ion. Furthermore, two additional peaks are clearly visible in the Fourier transforms (Fig. 4). As the number of coordinated malonate increases, so does the magnitude of these second and third shell peaks. These shells are accordingly ascribed to single and multiple scattering involving the coordinated malonate ligands. The DFT optimized structure of Ga(mal)32 3 , where the malonate anions are coordinated in a chelating manner (Fig. 5), was used as input to FEFF. Similar to the procedure for oxalate complexes described above, the single and three-legged multiple scattering paths, predicted to contribute most to the EXAFS spectrum of Ga(mal)32 3 , were then selected for refinements of the experimental data (Fig. 6 and Table 4). The overall fitting procedure was also identical to that used for the Ga(III)-oxalate samples, and the results obtained using this approach are presented in Table 4 and in Fig. 4. The R-space data from both solutions were satisfactorily modeled with four single scattering paths and three three-legged multiple scattering paths. The first shell fits revealed close to six ˚ , as oxygen back-scatterers at a distance of 1.96 A expected from the appearance of the Fourier transforms. No significant trends in the first shell distances or in the coordination numbers could be detected as the number of malonate ligands increase from two to three. The first shell data of the malonate complexes are thus closely similar to those of the oxalate complexes, and indicate no significant distortions of the mixed-ligand Ga(mal)2(H2O)2 2 complex. Fits of the higher order shells revealed the presence of two different carbon back-scatterers and the distal oxygen, in combination with multiple scattering contributions involving the ligand (cf. Fig. 6 and
Fig. 6. Schematic description of the dominating single (A) and three-legged multiple scattering (B and C) paths for the sixmembered ring chelate structure of malonate. The single scattering path contributing to the first shell has been omitted.
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Table 4 EXAFS fit results of gallium-malonate complexes Solution D
Solution E
Path
N
˚) R (A
s2
DE0
N
Ga –O Ga –C Ga –O–Ca Ga –C Ga –Odistal Ga –C–Oa Ga –O–Oa
6.1 6.7 (5.8) 13.5 (11.7) 3.4 (2.9) 6.7 (5.8) 13.5 (11.7) 13.5 (11.7)
1.96 2.92 3.04 3.55 4.13 4.15 4.16
0.0059 0.0041 0.0092 0.0064 0.0169 0.0168 0.0096
3.5 3.5 3.5 3.5 3.5 3.5 3.5
5.6 4.3 8.7 2.2 4.3 8.7 8.7
(3.8) (7.6) (1.9) (3.8) (7.6) (7.6)
˚) R (A
s2
DE0
1.95 2.91 3.03 3.53 4.08 4.10 4.13
0.0057 0.0035 0.0046 0.0046 0.0191 0.0198 0.0097
2.6 2.6 2.6 2.6 2.6 2.6 2.6
S20 fixed at 1.25. Energy shift, DE0 ; was allowed to vary but kept internally constant for all shells within a particular complex. Uncertainties ˚ and in the higher order shells to ^ 0.04 A ˚ . Uncertainties in coordination numbers are estimated to in R are estimated in the first shell to ^0.02 A at least ^20%. Values in parentheses are the coordination numbers expected from the compositions given in Table 1, assuming a chelate structure involving one oxygen atom from each carboxylate group. a MS paths correlated to single scattering coordination numbers and distances, and angles fixed to values of the structures used in the FEFF calculations.
Table 4). From the fitting results, it is clear that also the Ga –C and Ga – Odistal bond lengths and Debye – Waller factors are more or less equal for both solutions, indicating that the coordination mode of 32 malonate is very similar in Ga(mal)2 2 and Ga(mal)3 . The obtained distances and coordination numbers for the two carbon shells, in comparison with the estimated composition of the solutions, support a chelating coordination of malonate in both complexes. Thus, the results from these refinements are in agreement with the results obtained for the Ga(III)oxalate system in that no significant distortions of the first coordination shell are observed, and in that the ligands in the predominating complexes have a chelating structure involving one oxygen atom from each of the two carboxylate groups. 4.3. Comparison between Al(III) and Ga(III) oxalate and malonate complexes based on IR spectroscopy The EXAFS results indicated a chelate structure for all predominating Ga(III)-oxalate and -malonate complexes, and little or no distortions were detected as the number of ligands coordinated to Ga(III) was increased. In line with these results, the IR spectra of 32 GaLþ, GaL2 (L ¼ ox22 or mal22) 2 , and GaL3 display only very minor differences (Figs. 7 and 8 and Tables 5 and 6). The chelate structure is furthermore corroborated by the fact that theoretical frequencies of the two GaL32 3 structures presented in
Fig. 7. Attenuated total reflectance FTIR spectra of aqueous solutions containing (a) [Al3þ]tot ¼ 0.075 M and [H2ox]tot ¼ 0.020 M at 2 log[Hþ] ¼ 3.2, (b) [Al3þ ]tot ¼ 0.030 M and [H2ox]tot ¼ 0.100 M at 2log[Hþ] ¼ 5.5, (c) [Ga3þ]tot ¼ 0.060 M and [H2ox]tot ¼ 0.020 M at 2log[Hþ] ¼ 1.2 and (d) [Ga3þ]tot ¼ 0.010 M and [H2ox]tot ¼ 0.030 M at 2log[Hþ] ¼ 3.3. All the solutions were prepared in 0.6 M NaCl. The ordinate scale is in absorbance units and is arbitrary.
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Table 6 Main experimental and theoretical IR frequencies of Ga(III)- and Al(III)-malonate complexes
Ga(mal)þ (e) Ga(mal)2 2 (e) Ga(mal)32 3 (e) Ga(mal)32 3 (t) Al(mal)þ (e) Al(mal)2 2 (e) Al(mal)32 3 (e) Al(mal)32 3 (t)
a nnb C–O
nbC – O þ nC – C þ dCH2
1599 1597 1597 1655, 1647 1602 1604 1605 1682, 1657
1430, 1389, 1286 1423, 1389, 1287 1423, 1388, 1287 1379, 1373, 1269, 1244 1412, 1383, 1283 1435, 1381, 1282 1433, 1380, 1281 1420, 1370, 1272
(e) Experimental frequencies; (t) theoretical frequencies. The assignments of the experimental frequencies are based on the results from the theoretical frequency calculations. Scaling factor for the theoretical frequencies was 0.9613. a
Fig. 8. Attenuated total reflectance FTIR spectra of aqueous solutions containing (a) [Al3þ]tot ¼ 0.030 M and [H2mal]tot ¼ 0.015 M at 2 log[Hþ ] ¼ 3.2, (b) [Al 3þ]tot ¼ 0.030 M and [H2mal]tot ¼ 0.060 M at 2log[Hþ] ¼ 4.0, (c) [Al3þ]tot ¼ 0.030 M and [H2mal]tot ¼ 0.085 M at 2log[Hþ] ¼ 5.1, (d) [Ga3þ]tot ¼ 0.050 M and [H2mal]tot ¼ 0.025 M at 2log[Hþ] ¼ 2.5, (e) [Ga3þ]tot ¼ 0.020 M and [H2mal]tot ¼ 0.040 M at 2log[Hþ] ¼ 3.6, and (f) [Ga3þ]þ tot ¼ 0.010 M and [H2mal]tot ¼ 0.060 M at 2log[H ] ¼ 5.7. All the solutions were prepared in 0.6 M NaCl. The ordinate scale is in absorbance units and is arbitrary.
Table 5 Main experimental and theoretical IR frequencies of Ga(III)- and Al(III)-oxalate complexes
Ga(ox)þ (e) Ga(ox)2 2 (e) Ga(ox)32 3 (e) Ga(ox)32 3 (t) Al(ox)þ (e) Al(ox)32 3 (e) b Al(ox)32 3 (t)
a nnb C–O
nnb C–O
nbC – O þ nC – C
nbC – O þ dO – C – O
1717 1715 1714 1674,1673 1725 1720 1680,1678
1697 1691 1690 1666 1706 1697 1670
1405 1399 1399 1355 1412 1405 1355
1272 1279 1287,1266 1269,1244 1281 1308,1272 1272,1250
(e) Experimental frequencies; (t) theoretical frequencies. The assignments of the experimental frequencies are based on the results from theoretical frequency calculations. Scaling factor for the theoretical frequencies was 0.9613. b Results from Ref. [16]. a
Figs. 2 and 5 are in good agreement with the experimental data (cf. Figs. 9 and 10). Band assignments based on these results are presented in Tables 5 and 6. The separation between the C – O stretching vibration of the non-bonded ðnnb C – O Þ and the bonded oxygen ðnbC – O Þ has been previously shown for oxalate to be sensitive to the strength of interaction between the ligand and the metal ion [16]. The fact that the b nnb C – O – nC – O separation, in the present systems, show little variation with the number of coordinated ligands (cf. Figs. 7 and 8 and Tables 5 and 6) give further support to the similarity in Ga(III) – ligand interaction of the complexes studied. The IR spectra also provide means to compare Ga(III) and Al(III) complexes of oxalate and malonate. As seen in Figs. 7 and 8 and Tables 5 and 6, these systems behave strikingly similar. This suggests that also in the predominating Al(III) complexes, oxalate and malonate form chelate structures by bonding to the metal ion via one oxygen atom from each carboxylate group. The chelate structure is consolidated by the fair agreement between the theoretical frequencies, calculated for the analogous optimized structures as in Figs. 2 and 5, and the experimental data (Figs. 9 and 10). The b similar nnb C – O – nC – O separation for all Al(III)-oxalate and-malonate complexes, respectively, indicate that also in the Al(III) systems the bonding between the ligand and the central metal ion is practically independent of the number of coordinated ligands. Taken together with the overall similarity with
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32 Fig. 9. Experimental (bottom row) and theoretical (top row) IR spectra of (A) Al(ox)32 3 and (B) Ga(ox)3 .
32 Fig. 10. Experimental (bottom row) and theoretical (top row) IR spectra of (A) Al(mal)32 3 and (B) Ga(mal)3 .
M. Clause´n et al. / Journal of Molecular Structure 648 (2003) 225–235
the Ga(III) systems, this information leads to the conclusion that only very small relative distortions of the first shells occur in the mixed-ligand-aquo complexes as compared to the AlL32 complexes. 3 Hence, in the presented results we find no support for the indication from theoretical calculations that Al(ox)(H2O)þ 4 is strongly distorted in aqueous solution [4].
Acknowledgements We thank the staff of Stanford Synchrotron Radiation Laboratory (SSRL), particularly Prof. Britt Hedman and Dr John Bargar, for their help and advise. SSRL is operated by the Department of Energy, Office of Basic Energy Sciences. We also acknowledge the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program, and the Department of Energy Office of Biological and Environmental Research, which support the SSRL Structural Molecular Biology Program whose instrumentation was used for the measurements. This work was supported by the Swedish Research Council and The Swedish Foundation for International Cooperation in Research and Higher Education (STINT).
References [1] D.T. Richens, The Chemistry of Aqua Ions, Wiley, New York, 1997.
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[2] J.D. Kubicki, G.A. Blake, S.E. Apitz, Geochim. Cosmochim. Acta 60 (1996) 4897. [3] J.D. Kubicki, D. Sykes, S.E. Apitz, J. Phys. Chem. A 103 (1999) 903. [4] A.J.A. Aquino, D. Tunega, G. Haberhauer, M. Gerzabek, H. Lischka, PCCP 2 (2000) 2845. [5] D.J. Clevette, C. Orvig, Polyhedron 9 (1990) 151. [6] F.Ya. Kulba, N.A. Babkina, A.P. Zharkov, Russ. J. Inorg. Chem. 19 (1974) 365. [7] T. Kiss, I. So´va´go´, R.B. Martin, J. Pursiainen, J. Inorg. Biochem. 55 (1994) 53. ¨ hman, J. Chem. Soc. Dalton Trans. (1985) [8] S. Sjo¨berg, L.-O. O 2665. [9] G.N. George, I.J. Pickering, EXAFSPAK—a suite of computer programs for analysis of X-ray absorption spectra, SSRL, Stanford, CA, 1993. [10] T. Ressler, J. Synch. Rad. 5 (1998) 118. [11] S.I. Zabinsky, J.J. Rehr, A. Ankudinov, R.C. Albers, M.J. Eller, Phys. Rev. B 52 (1995) 2995. [12] P. Lindqvist-Reis, A. Mun˜oz-Pa´ez, S. Dı´az-Moreno, S. Pattanaik, I. Persson, M. Sandstro¨m, Inorg. Chem. 37 (1998) 6675. [13] M.W. Urban, Attenuated Total Reflectance of Polymers, American Chemical Society, Washington, DC, 1996. [14] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. AlLaham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. HeadGordon, C. Gonzalez, J.A. Pople, GAUSSIAN 94W, Gaussian Inc., Pittsburgh, PA, 1995. [15] J.B. Foresman, A. Frisch, Exploring Chemistry with Electronic Structure Methods, Gaussian Inc, Pittsburgh, PA, 1996. [16] K. Axe, P. Persson, Geochim. Cosmochim. Acta 65 (2001) 4481.
Spectroscopic studies of aluminum and gallium complexes with oxalate and malonate in aqueous solution ¨ hman, Kristina Axe, Per Persson* Maria Clause´n, Lars-Olof O Department of Chemistry, Inorganic Chemistry, Umea˚ University, SE-901 87 Umea˚, Sweden Received 25 October 2002; revised 19 December 2002; accepted 19 December 2002
Abstract The local structures of Ga(III) in aqueous oxalate and malonate complexes were studied by means of Ga K-edge EXAFS spectroscopy. Irrespective of the number and type of coordinated ligands, the EXAFS results showed very regular first coordination shells consisting of six oxygen atoms. Scattering paths from more distant atoms revealed that both oxalate and malonate form mononuclear chelate structures where one oxygen from each carboxylate group binds to Ga(III). Again, very little variation in bond distances and no changes in coordination modes were detected as the number of ligands coordinated to Ga(III) was varied. Based on the very close resemblance of IR spectra of oxalate and malonate complexes of Al(III), and the corresponding complexes of Ga(III), it is believed that the local structures of the Al(III) complexes are similar to those of the Ga(III) complexes in terms of ligand coordination modes and distortions. This conclusion was corroborated by results from theoretical frequency calculations. q 2003 Elsevier Science B.V. All rights reserved. Keywords: Gallium; Aluminum; Oxalate; Malonate; Aqueous solution
1. Introduction A detailed knowledge of the chemical speciation of metal – ligand interactions in aqueous solution requires information from a wide range of techniques. Potentiometry and solubility measurements provide means to determine the stoichiometries and thermodynamic stabilities of the species formed. These methods can be complemented by various spectroscopic techniques, such as NMR and IR/Raman, to characterize certain molecular properties of the * Corresponding author. Tel.: þ46-90-7865573; fax: þ 46-907869195. E-mail address: [email protected] (P. Persson).
complexes. In addition LAXS and EXAFS methods are in favorable cases available for in situ determination of structural parameters, such as bond distances between the metal ion and the surrounding ligands [1]. Recently, studies of aqueous metal complexes have been complemented by theoretical quantum chemical methods to gain an even deeper understanding of structural details, spectroscopic properties, and relative stabilities. Examples are the publications discussing the aqueous complexes between Al(III) and carboxylic acids [2 – 4]. The results of these theoretical calculations do, however, sometimes significantly deviate from experimentally derived information in the literature. These can be discrepancies in the number and composition of the most dominant
0022-2860/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2860(03)00026-7
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complex(es), or differences in the suggested coordination modes of the ligands. Whether such disagreements are due to problems with the interpretation of the experimental data, or to inadequacies of the theoretical models, in particular with respect to how the hydration of the complex(es) were treated, is not always easy to assess. In any case, with the rapid increase of theoretical publications, it has become obvious that there is a definitive need for high-quality experimental data for comparisons and corroboration of the theoretical models. In a recent theoretical study on Al(III)-oxalate complexes, it was found that there were large differences between the results obtained from calculations performed on the gas phase complexes as compared to calculations where hydration was simulated by the polarized continuum model [4]. In gas phase, a remarkable difference in the calculated Al –O distances was observed for the [Al(C2O4)(H2O)4]þ ˚ for Al –O(oxalate) and 1.984 A ˚ for complex; 1.805 A Al –O(water). This difference was diminished when solvation was simulated, but still a strongly distorted first coordination shell was predicted with Al – O ˚ for oxalate and water distances of 1.861 and 1.950 A coordination, respectively. Unfortunately, the validity of these predictions cannot be verified with conventional LAXS and EXAFS techniques because of the low K edge energy of Al (1.56 keV) and because of the extremely high concentrations needed for the LAXS measurements.
Instead, a different experimental strategy must be used. As a neighbor to Al in group 13 of the periodic table, Ga has a closely related chemistry and Ga(III) is often considered an Al(III) analogue [5]. An advantage with Ga is the K edge energy of 10.367 keV, which makes it suitable for EXAFS investigations of dilute aqueous solutions. In this study we analyze the local structure of Ga(III) in aqueous oxalate (ox22) and malonate (mal22) complexes by means of EXAFS. This will provide information about the dominating coordination mode(s) of the ligands, and about the degree of first shell distortion as a function of the number of ligands bound to the central Ga(III) ion. We then explore/exploit the Al(III) – Ga(III) analogy by analyzing and comparing IR spectra of oxalate and malonate complexes of both metal ions. These spectra are interpreted and assigned by comparison with theoretical frequencies obtained from molecular orbital calculations.
2. Experimental section 2.1. Chemicals and solutions Solutions of dilute HCl were prepared from concentrated hydrochloric acid (Fisher Chemicals, p.a.) and were standardized using tris(hydroxymethyl)aminomethane, ‘trisma-base’ (Sigma p.a.), dried at 80 8C. Dilute, carbonate free,
Table 1 Composition of the Ga(III) solutions analyzed by EXAFS and IR spectroscopy Solution
[Ga]tot (mM)
[H2L]tot (mM)
[H]tot (mM)
2log[Hþ]
Ga3þ (%)
GaLþ (%)
GaL2 2 (%)
GaL32 3 (%)
EXAFS A, oxalatea B, oxalatea C, oxalatea D, malonateb E, malonateb
40 40 70 40 40
120 80 70 240 80
2234 232 2123 2397 2156
3.2 1.6 1.9 4.6 3.5
0 4 29 0 0
0 24 46 0 13
4 44 22 8 84
96 28 3 92 3
IR Oxalatea Oxalatea Malonateb Malonateb Malonateb
60 10 50 20 10
20 30 25 40 60
60 258 245 277 2113
1.2 3.3 2.5 3.6 5.7
70 0 53 0 0
27 0 46 16 0
3 5 1 81 12
0 95 0 2 88
a b
Formation constants from Ref. [6]. Based on analogy with the Al(III)-malonate system, formation constants from Ref. [7].
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Table 2 Composition of the Al(III) solutions analyzed by IR spectroscopy Solution
[Al]tot (mM)
[H2L]tot (mM)
[H]tot (mM)
2log[Hþ]
Al3þ (%)
AlLþ (%)
AlL2 2 (%)
AlL32 3 (%)
Oxalate Oxalate Malonate Malonate Malonate
75 30 30 30 30
20 100 15 60 85
239.9 2200 229.3 2117.7 2165.4
3.2 5.5 3.2 4.0 5.1
74 0 51 0 0
25 0 47 11 0
1 1 1 85 45
0 99 0 4 55
The compositions of the Al(III)-malonate solutions are based on formation constants from Ref. [6] and the compositions of the Al(III)oxalate solutions are based on formation constants from Ref. [8].
sodium hydroxide solutions were prepared from filtrated 50% NaOH (Merck, p.a.) and standardized against hydrochloric acid prepared as above. The composition of the solutions used for the EXAFS and IR experiments are summarized in Tables 1 and 2. They were all prepared from weighed amounts of sodium oxalate or malonate (Riedel-de Hae¨n, p.a.) and dried NaCl (Riedel-de Hae¨n, p.a.). The metal ions were added from a stock solution of GaCl3 (Aldrich, 99.999%) or as weighed amounts of AlCl3·6H2O (Riedel-de Hae¨n, p.a.), respectively. The compositions of the solutions were chosen on the basis of extensive chemical modelling using thermodynamic constants from the literature [6 –8] and a modernized version of the chemical speciation program WinSGW1. To reach different 2 log[Hþ], appropriate amounts of dilute HCl or NaOH was added. All solutions were prepared to contain 0.6 M Na(Cl), and were left to equilibrate for at least 24 h before analysis. Beyond that time, 2 log[Hþ] and IR spectra did not change and the solutions were considered stable. A combination glass electrode, calibrated in solutions of known HCl concentrations in 0.6 M NaCl, was used to measure 2 log[Hþ].
3. Methods and measurements 3.1. EXAFS measurements Ga K-edge EXAFS data were measured at the Stanford Synchrotron Radiation Laboratory, California on beam line 4– 1. The ring energy was 3.0 GeV with ring currents between 60 and 100 mA. A Si(220) 1
http://www.chem.umu.se/dep/inorgchem/.
double crystal monochromator was used and detuned 50% to eliminate higher order harmonics. The incoming beam was registered using an ion chamber filled with N2. The data were collected at room temperature in the fluorescence mode, with a Lytle detector filled with Ar gas. A Zn-6 filter and Soller slit set-up were used to reduce Kb fluorescence and scattering contributions to the signal. Internal calibration was performed by simultaneously measuring spectra from a Ga(OH)3 (s) reference sample in transmission mode, throughout the duration of all scans. Three or four scans were collected per sample. 3.2. EXAFS data treatment The EXAFS data were energy-calibrated and averaged with EXAFSPAK [9], and further analyzed using WinXAS [10]. Standard procedures were used for pre-edge subtraction, data normalization, and spline removal. The k3 -weighted EXAFS oscillations were Fourier transformed over the k-range 3.3– ˚ 21 using a Bessel window function. The 14.5 A resulting Fourier transforms were used in the leastsquares refinements, thus all results presented herein were obtained from R-space fits. Theoretical phase and amplitude functions, used in the refinements, for single and multiple scattering paths within assumed molecular models were calculated with the ab initio code FEFF7 [11]. The input structures were those obtained from the DFT geometry optimizations of and Ga(mal)32 Ga(ox)32 3 3 , see below. Since the EXAFS amplitude is proportional to the coordination number and the amplitude reduction factor, S20 ; the EXAFS spectrum of an acidic solution of gallium nitrate, where the Ga – O coordination number of Ga(H2O)3þ 6 is known to be 6 [12], was used to refine
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S20 : This value was found to be 1.25, with the phase and amplitude functions obtained from the FEFF calculation of Ga(ox)32 3 , and was kept constant during the refinements of the EXAFS spectra. 3.3. IR spectroscopy Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were collected using a Perkin – Elmer 2000 FTIR spectrometer fitted with a deuterated triglycine sulfate (DTGS) detector. The spectra were recorded with a horizontal ATR accessory and a diamond crystal as the reflection element (SensIR Technologies). The sample cell was purged with nitrogen gas throughout data collection to exclude carbon dioxide and water vapor. The angle of incidence for the set-up is approximately 458, which is far from the critical angle. This and the fact that the bands analyzed are weak (, 0.05 absorbance units), and do not overlap with stronger bands (except for the asymmetric C –O stretch which overlaps with the intense H2O bend), indicate that the effects of possible distortions known to occur in ATR spectra are minimized [13]. The solutions were applied to the diamond crystal surface directly and a quartz lid was placed over the sample and pressed tightly against a rubber gasket. This sealed the sample from the atmosphere during data collection. 100 scans were collected for each sample in the 370 –7800 cm21
range with a resolution of 4 cm21. The sample spectra were interpreted after subtracting spectra of both the empty cell and 0.6 M NaCl. When necessary, interfering solution species were also subtracted from the spectra. These subtractions were guided by the predicted compositions given in Tables 1 and 2. 3.4. Molecular orbital calculations Theoretical vibration frequencies of the geometry 32 32 optimized Al(ox)32 3 , Al(mal) 3 , Ga(ox) 3 , and 32 Ga(mal)3 complexes were calculated with the density functional theory (DFT) using the hybrid functionals B3LYP. The standard 6-31G* basis set was used in all calculations. The calculations were performed with the program GAUSSIAN 94W [14] by Gaussian Inc., Pittsburgh, USA and visualization of the calculated vibrational modes was accomplished with HyperChem v. 5 by Hypercube, Gainesville, USA. The potential energy minimum structures of the complexes were obtained without applying symmetry restrictions, i.e. all bond lengths, angles and dihedral angles were allowed to vary. The subsequently calculated vibrational frequencies were scaled by the recommended scale factor 0.9613 (B3LYP/6-31G*) to account for systematic errors [15]. Only real frequencies were obtained which show that the optimized structures represent minima at the potential energy surface.
Fig. 1. k3 -weighted EXAFS and the corresponding Fourier transforms of solutions with (a) [Ga]tot ¼ 0.040 M, [H2ox]tot ¼ 0.120 M and 2log[Hþ] ¼ 3.2 (solution A), (b) [Ga]tot ¼ 0.040 M, [H2ox]tot ¼ 0.080 M and 2log[Hþ] ¼ 1.6 (solution B) and (c) [Ga]tot ¼ 0.070 M, [H2ox]tot ¼ 0.070 M and 2log[Hþ] ¼ 1.9 (solution C). The spectrum of an acidic aqueous solution of Ga(NO3)3 (d) is included for comparison. The Fourier transforms are uncorrected for phase shift. Broken lines represent the model presented in Table 3.
M. Clause´n et al. / Journal of Molecular Structure 648 (2003) 225–235
Fig. 2. Structure of Ga(ox)32 obtained from the DFT geometry 3 ˚ , and optimization. The Ga–O distances are (1.9895 ^ 0.0005) A the a angles are (81.51 ^ 0.01) 8.
4. Results and discussion 4.1. Structures in the aqueous Ga(III)-oxalate system from EXAFS spectroscopy As mentioned in Section 1, recent theoretical work concerning the related Al(III)-oxalate system [4], has predicted pronounced distortions in the first coordination sphere of the Al(ox)(H2O)þ 4 complex. Also, the commonly accepted chelating coordination of oxalate to Al(III) has recently been challenged by theoretical work [3]. The objective of this part of the study has therefore been to investigate whether these theoretical predictions could be experimentally observed for Ga(III)-oxalate complexes. To maximize the fraction of Ga(III) in each of the target complexes, a series of WinSGW1 calculations with the formation constants presented by Kulba et al. [6], were performed. On the basis of these calculations, the compositions of the solutions summarized in Table 1, were selected. As seen, only for the Ga(ox)32 3 complex, a solution with virtually all Ga(III) in one dominating species was thermodynamically possible to prepare. This implies
229
that the EXAFS results from the other Ga(III)-oxalate solutions will yield trends in the evaluated fit parameters, rather than absolute numbers related to one individual complex. The resulting EXAFS spectra, with the corresponding Fourier transforms, are shown in Fig. 1. An acidic solution of Ga(H2O)3þ 6 , where gallium is known to coordinate six water ˚ [12], is included for comparison. molecules at 1.96 A From the Fourier transforms, it is clear that the first coordination sphere in solution A through C is similar to that of the hydrated metal ion (Fig. 1). With increasing oxalate to gallium ratio, higher shells become increasingly more pronounced and the positions of these shells appear to be more or less unchanged in the different spectra. The structure of Ga(ox)32 (Fig. 2), obtained 3 from the DFT geometry optimization, was used as input to FEFF7 in order to calculate the dominating scattering paths, and to obtain phase and amplitude functions for fitting the experimental data. The paths to be included in the fit were selected from the most important single scattering contributions and the three-legged multiple scattering contributions (see Fig. 3 and Table 3). Thus, the fit results presented below will reveal structural similarities and differences between the optimized structure and the solution complex(es). To keep the number of fit parameters as low as possible no higher order (. 3) multiple scattering paths, and no multiple scattering paths from the first coordination shell only, were included. The coordination numbers, distances and Debye – Waller factors for the single scattering contributions were free variables in the fits, whereas the coordination numbers and the distances of the multiple scattering paths were correlated to the corresponding single scattering
Fig. 3. Schematic description of the dominating single (A) and three-legged multiple scattering (B and C) paths for the fivemembered ring chelate structure of oxalate. The single scattering path contributing to the first shell has been omitted.
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Table 3 EXAFS fit results of gallium-oxalate complexes Solution A
Solution B
Path
N
˚) R (A
s2
DE0
N
Ga –O Ga –C Ga –O–Cb Ga –Odistal Ga –C–Ob Ga –O–Ob
5.6 5.4 (5.9) 10.9 (11.8) 5.4 (5.9) 10.9 (11.8) 10.9 (11.8)
1.97 2.75 3.01 4.01 4.02 4.14
0.0049 0.0039 0.0025 0.0034 0.0060 0.0014
3.3 3.3 3.3 3.3 3.3 3.3
5.8 3.2 6.5 3.2 6.5 6.5
(3.9) (7.8) (3.9) (7.8) (7.8)
Solution C ˚) R (A
s2
DE0
N
1.96 2.74 3.00 4.01 4.01 4.14
0.0055 0.0035 0.0011 0.0032 0.0064 0.0032
3.2 3.2 3.2 3.2 3.2 3.2
5.5 1.8 3.7 1.8 3.7 3.7
(2.0) (4.0) (2.0) (4.0) (4.0)
˚) R (A
s2
DE0
1.96 2.73 2.99 4.01 4.02 4.14
0.0052 0.0035a 0.0011a 0.0032a 0.0064a 0.0032a
2.7 2.7 2.7 2.7 2.7 2.7
S20 fixed at 1.25. Energy shift, DE0 ; was allowed to vary but kept internally constant for all shells within a particular complex. Uncertainties ˚ and in the higher order shells to ^0.04 A ˚ . Uncertainties in coordination numbers are estimated to in R are estimated in the first shell to ^0.02 A at least ^20%. Values in parentheses are the coordination numbers expected from the compositions given in Table 1, assuming a chelate structure involving one oxygen from each carboxylate group. a DW-factor fixed to the values obtained in fit of solution B. b MS paths correlated to single scattering coordination numbers and distances, and angles fixed to values of the structures used in the FEFF calculations.
contributions. This fitting-approach provided a very good fit to data, and the models are presented in Table 3 and in Fig. 1. First shell fits revealed close to six oxygen back˚ for all solutions. No scatterers at a distance of 1.96 A trends in distances, coordination numbers or Debye – Waller factors were detected. Thus, the theoretically predicted distortion of Al(ox)(H2O)þ 4 as compared to cannot find experimental support for the Al(ox)32 3 equivalent Ga(III) complexes. Instead, the consistent values of the first shell fit variables indicate differences in average distances in the three solutions
˚ , which is the estimated experimental of # 0.02 A error. The higher order shells are satisfactorily modeled by single-scattering paths from the carbon atoms and the distal oxygens, in combination with multiple scattering contributions involving the coordinated oxygens, the distal oxygens, and the carbon atoms of the ligand (cf. Fig. 3 and Table 3). From the fitting results, it is clear that also the Ga – C and Ga –Odistal bond lengths and Debye – Waller factors are more or less equal for all three solutions, indicating that the coordination mode of oxalate to Ga(III) indeed is very
Fig. 4. k3 -weighted EXAFS and the corresponding Fourier transforms of solutions containing (a) [Ga]tot ¼ 0.040 M and [H2mal]tot ¼ 0.240 M with 2log[Hþ] ¼ 4.6 (solution D) and (b) [Ga]tot ¼ 0.040 M and [H2mal]tot ¼ 0.080 M with 2log[Hþ] ¼ 3.5 (solution E). The spectrum of an acidic aqueous solution of Ga(NO3)3 (c) is included for comparison. The Fourier transforms are uncorrected for phase shift. Broken lines represent the model presented in Table 4.
M. Clause´n et al. / Journal of Molecular Structure 648 (2003) 225–235
Fig. 5. Structure of Ga(mal)32 obtained from the DFT geometry 3 ˚ , and the a optimization. The Ga –O distances are (1.984 ^ 0.015) A angles are (89.20 ^ 1.00) 8.
similar in all GaðoxÞn ðH2 OÞ322n 622n ðn ¼ 1 – 3Þ complexes. This information together with the coordination numbers of the Ga– C and Ga –Odistal scattering paths as compared to the predicted composition of the solutions, strongly indicate a chelating coordination mode of oxalate in all predominating Ga(III)-oxalate complexes. 4.2. Structures in the aqueous Ga(III)-malonate system from EXAFS spectroscopy No complete thermodynamic model for the complex formation reactions in the Ga(III)-malonate system is available in the literature. We have therefore used the formation constants valid for Al(III)malonate complexes to model the speciation [7], and prepared Ga(III)-malonate solutions at the total concentrations of metal ion, ligand and protons, 32 known to generate Al(mal)2 2 and Al(mal)3 . The compositions of these solutions are given in Table 1 and the resulting EXAFS, with the corresponding Fourier transforms, are shown in Fig. 4, with an acidic solution of Ga(H2O)3þ 6 included for comparison. The position and shape of the first shell peaks of both Ga(III)-malonate solutions, relative to that of Ga(H2O)3þ 6 , indicate that also in this system the first coordination shells are very similar to that of
231
the hydrated metal ion. Furthermore, two additional peaks are clearly visible in the Fourier transforms (Fig. 4). As the number of coordinated malonate increases, so does the magnitude of these second and third shell peaks. These shells are accordingly ascribed to single and multiple scattering involving the coordinated malonate ligands. The DFT optimized structure of Ga(mal)32 3 , where the malonate anions are coordinated in a chelating manner (Fig. 5), was used as input to FEFF. Similar to the procedure for oxalate complexes described above, the single and three-legged multiple scattering paths, predicted to contribute most to the EXAFS spectrum of Ga(mal)32 3 , were then selected for refinements of the experimental data (Fig. 6 and Table 4). The overall fitting procedure was also identical to that used for the Ga(III)-oxalate samples, and the results obtained using this approach are presented in Table 4 and in Fig. 4. The R-space data from both solutions were satisfactorily modeled with four single scattering paths and three three-legged multiple scattering paths. The first shell fits revealed close to six ˚ , as oxygen back-scatterers at a distance of 1.96 A expected from the appearance of the Fourier transforms. No significant trends in the first shell distances or in the coordination numbers could be detected as the number of malonate ligands increase from two to three. The first shell data of the malonate complexes are thus closely similar to those of the oxalate complexes, and indicate no significant distortions of the mixed-ligand Ga(mal)2(H2O)2 2 complex. Fits of the higher order shells revealed the presence of two different carbon back-scatterers and the distal oxygen, in combination with multiple scattering contributions involving the ligand (cf. Fig. 6 and
Fig. 6. Schematic description of the dominating single (A) and three-legged multiple scattering (B and C) paths for the sixmembered ring chelate structure of malonate. The single scattering path contributing to the first shell has been omitted.
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Table 4 EXAFS fit results of gallium-malonate complexes Solution D
Solution E
Path
N
˚) R (A
s2
DE0
N
Ga –O Ga –C Ga –O–Ca Ga –C Ga –Odistal Ga –C–Oa Ga –O–Oa
6.1 6.7 (5.8) 13.5 (11.7) 3.4 (2.9) 6.7 (5.8) 13.5 (11.7) 13.5 (11.7)
1.96 2.92 3.04 3.55 4.13 4.15 4.16
0.0059 0.0041 0.0092 0.0064 0.0169 0.0168 0.0096
3.5 3.5 3.5 3.5 3.5 3.5 3.5
5.6 4.3 8.7 2.2 4.3 8.7 8.7
(3.8) (7.6) (1.9) (3.8) (7.6) (7.6)
˚) R (A
s2
DE0
1.95 2.91 3.03 3.53 4.08 4.10 4.13
0.0057 0.0035 0.0046 0.0046 0.0191 0.0198 0.0097
2.6 2.6 2.6 2.6 2.6 2.6 2.6
S20 fixed at 1.25. Energy shift, DE0 ; was allowed to vary but kept internally constant for all shells within a particular complex. Uncertainties ˚ and in the higher order shells to ^ 0.04 A ˚ . Uncertainties in coordination numbers are estimated to in R are estimated in the first shell to ^0.02 A at least ^20%. Values in parentheses are the coordination numbers expected from the compositions given in Table 1, assuming a chelate structure involving one oxygen atom from each carboxylate group. a MS paths correlated to single scattering coordination numbers and distances, and angles fixed to values of the structures used in the FEFF calculations.
Table 4). From the fitting results, it is clear that also the Ga –C and Ga – Odistal bond lengths and Debye – Waller factors are more or less equal for both solutions, indicating that the coordination mode of 32 malonate is very similar in Ga(mal)2 2 and Ga(mal)3 . The obtained distances and coordination numbers for the two carbon shells, in comparison with the estimated composition of the solutions, support a chelating coordination of malonate in both complexes. Thus, the results from these refinements are in agreement with the results obtained for the Ga(III)oxalate system in that no significant distortions of the first coordination shell are observed, and in that the ligands in the predominating complexes have a chelating structure involving one oxygen atom from each of the two carboxylate groups. 4.3. Comparison between Al(III) and Ga(III) oxalate and malonate complexes based on IR spectroscopy The EXAFS results indicated a chelate structure for all predominating Ga(III)-oxalate and -malonate complexes, and little or no distortions were detected as the number of ligands coordinated to Ga(III) was increased. In line with these results, the IR spectra of 32 GaLþ, GaL2 (L ¼ ox22 or mal22) 2 , and GaL3 display only very minor differences (Figs. 7 and 8 and Tables 5 and 6). The chelate structure is furthermore corroborated by the fact that theoretical frequencies of the two GaL32 3 structures presented in
Fig. 7. Attenuated total reflectance FTIR spectra of aqueous solutions containing (a) [Al3þ]tot ¼ 0.075 M and [H2ox]tot ¼ 0.020 M at 2 log[Hþ] ¼ 3.2, (b) [Al3þ ]tot ¼ 0.030 M and [H2ox]tot ¼ 0.100 M at 2log[Hþ] ¼ 5.5, (c) [Ga3þ]tot ¼ 0.060 M and [H2ox]tot ¼ 0.020 M at 2log[Hþ] ¼ 1.2 and (d) [Ga3þ]tot ¼ 0.010 M and [H2ox]tot ¼ 0.030 M at 2log[Hþ] ¼ 3.3. All the solutions were prepared in 0.6 M NaCl. The ordinate scale is in absorbance units and is arbitrary.
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Table 6 Main experimental and theoretical IR frequencies of Ga(III)- and Al(III)-malonate complexes
Ga(mal)þ (e) Ga(mal)2 2 (e) Ga(mal)32 3 (e) Ga(mal)32 3 (t) Al(mal)þ (e) Al(mal)2 2 (e) Al(mal)32 3 (e) Al(mal)32 3 (t)
a nnb C–O
nbC – O þ nC – C þ dCH2
1599 1597 1597 1655, 1647 1602 1604 1605 1682, 1657
1430, 1389, 1286 1423, 1389, 1287 1423, 1388, 1287 1379, 1373, 1269, 1244 1412, 1383, 1283 1435, 1381, 1282 1433, 1380, 1281 1420, 1370, 1272
(e) Experimental frequencies; (t) theoretical frequencies. The assignments of the experimental frequencies are based on the results from the theoretical frequency calculations. Scaling factor for the theoretical frequencies was 0.9613. a
Fig. 8. Attenuated total reflectance FTIR spectra of aqueous solutions containing (a) [Al3þ]tot ¼ 0.030 M and [H2mal]tot ¼ 0.015 M at 2 log[Hþ ] ¼ 3.2, (b) [Al 3þ]tot ¼ 0.030 M and [H2mal]tot ¼ 0.060 M at 2log[Hþ] ¼ 4.0, (c) [Al3þ]tot ¼ 0.030 M and [H2mal]tot ¼ 0.085 M at 2log[Hþ] ¼ 5.1, (d) [Ga3þ]tot ¼ 0.050 M and [H2mal]tot ¼ 0.025 M at 2log[Hþ] ¼ 2.5, (e) [Ga3þ]tot ¼ 0.020 M and [H2mal]tot ¼ 0.040 M at 2log[Hþ] ¼ 3.6, and (f) [Ga3þ]þ tot ¼ 0.010 M and [H2mal]tot ¼ 0.060 M at 2log[H ] ¼ 5.7. All the solutions were prepared in 0.6 M NaCl. The ordinate scale is in absorbance units and is arbitrary.
Table 5 Main experimental and theoretical IR frequencies of Ga(III)- and Al(III)-oxalate complexes
Ga(ox)þ (e) Ga(ox)2 2 (e) Ga(ox)32 3 (e) Ga(ox)32 3 (t) Al(ox)þ (e) Al(ox)32 3 (e) b Al(ox)32 3 (t)
a nnb C–O
nnb C–O
nbC – O þ nC – C
nbC – O þ dO – C – O
1717 1715 1714 1674,1673 1725 1720 1680,1678
1697 1691 1690 1666 1706 1697 1670
1405 1399 1399 1355 1412 1405 1355
1272 1279 1287,1266 1269,1244 1281 1308,1272 1272,1250
(e) Experimental frequencies; (t) theoretical frequencies. The assignments of the experimental frequencies are based on the results from theoretical frequency calculations. Scaling factor for the theoretical frequencies was 0.9613. b Results from Ref. [16]. a
Figs. 2 and 5 are in good agreement with the experimental data (cf. Figs. 9 and 10). Band assignments based on these results are presented in Tables 5 and 6. The separation between the C – O stretching vibration of the non-bonded ðnnb C – O Þ and the bonded oxygen ðnbC – O Þ has been previously shown for oxalate to be sensitive to the strength of interaction between the ligand and the metal ion [16]. The fact that the b nnb C – O – nC – O separation, in the present systems, show little variation with the number of coordinated ligands (cf. Figs. 7 and 8 and Tables 5 and 6) give further support to the similarity in Ga(III) – ligand interaction of the complexes studied. The IR spectra also provide means to compare Ga(III) and Al(III) complexes of oxalate and malonate. As seen in Figs. 7 and 8 and Tables 5 and 6, these systems behave strikingly similar. This suggests that also in the predominating Al(III) complexes, oxalate and malonate form chelate structures by bonding to the metal ion via one oxygen atom from each carboxylate group. The chelate structure is consolidated by the fair agreement between the theoretical frequencies, calculated for the analogous optimized structures as in Figs. 2 and 5, and the experimental data (Figs. 9 and 10). The b similar nnb C – O – nC – O separation for all Al(III)-oxalate and-malonate complexes, respectively, indicate that also in the Al(III) systems the bonding between the ligand and the central metal ion is practically independent of the number of coordinated ligands. Taken together with the overall similarity with
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32 Fig. 9. Experimental (bottom row) and theoretical (top row) IR spectra of (A) Al(ox)32 3 and (B) Ga(ox)3 .
32 Fig. 10. Experimental (bottom row) and theoretical (top row) IR spectra of (A) Al(mal)32 3 and (B) Ga(mal)3 .
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the Ga(III) systems, this information leads to the conclusion that only very small relative distortions of the first shells occur in the mixed-ligand-aquo complexes as compared to the AlL32 complexes. 3 Hence, in the presented results we find no support for the indication from theoretical calculations that Al(ox)(H2O)þ 4 is strongly distorted in aqueous solution [4].
Acknowledgements We thank the staff of Stanford Synchrotron Radiation Laboratory (SSRL), particularly Prof. Britt Hedman and Dr John Bargar, for their help and advise. SSRL is operated by the Department of Energy, Office of Basic Energy Sciences. We also acknowledge the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program, and the Department of Energy Office of Biological and Environmental Research, which support the SSRL Structural Molecular Biology Program whose instrumentation was used for the measurements. This work was supported by the Swedish Research Council and The Swedish Foundation for International Cooperation in Research and Higher Education (STINT).
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