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27Al NMR measurements and mixed ligand complex formation of Al(III) at low concentrations
JOURNAL
OF MAGNETIC
RESONANCE
16,362-364
(1974)
27Al NMR Measurementsand Mixed Ligand Complex Formation of Al(III) at Low Concentrations It is well-known that in NMR spectra of electrolyte solutions two different signals for the solvent molecules bound to the cation and for the bulk solvent can be observed if the lifetime of the molecules in the solvation sphere is long enough. This is the case with small highly charged cations dissolved in solvents which are strong Lewis bases, such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), hexamethylphosphoramide (HMPT), etc. (1). Fewer examples are known in which the chemical shift of bound molecules depends on the composition of the solvation shell in such a way that different resonance lines can be observed (2). We used 27Al resonance measurements to determine the distribution of DMF and DMSO around Al(II1) in a solution of Al(ClO.J, in nitromethane (NM). In pure DMF and in pure DMSO, the solvation number of Al(111) is 6 (I). Since NM is a very weak Lewis base compared with DMF and DMSO, we used NM as a matrix to obtain signals of similar height for the bound and bulk DMF and DMSO in ‘H NMR spectra. r Al(ClO,),*8H,O (Merck) was used in the preparation of Al(C10J3*6 DMF and Al(ClO,),*6 DMSO (3). Since the solubility of Al(ClO,),* 6 DMSO in NM is 0.013 moles/kg solution, this concentration was used in the preparation of the stock solutions of Al(C10,),*6 DMF and of Al(ClO,),*6 DMSO in NM. All samples were prepared gravimetrically. We measured the 27Al spectra on a HFX 90 Bruker spectrometer using the 13C resonance frequency (22.63 MHz). The magnetic field was lowered down to 2.04 T to find the 27A1 resonance corresponding to a resonance frequency of 86.85 MHz for protons. The field must be locked because of very weak 27A1 signals at this concentration. The spectra were taken in the cw mode using signal accumulation. With a minimal and inexpensive modification (4), we obtained the field stabilization using the Schomandel ND 100 M frequency synthesizer which is already available with the spectrometer for the decoupling technique. The operation of the stabilization lock channel of the spectrometer is in no way affected which allows us to use this technique for the measurement of other nuclei as well. Thus, the 13C equipment was used in the calibrated frequency sweep mode in our measurements. The spectra were accumulated up to 64 times. The last three traces in Fig. I show single-scan spectra as a comparison. In Fig. 1 the single resonance line of a solution with composition XnMF = 0.00 refers to the species Al(DMS0),3+. In the spectrum of mole fraction 0.1, the second line, located 1.255 ppm at higher magnetic field, belongs to Al(DMSO),DMF3+. With increasing concentration of DMF up to xDMF = 1.O, a total of seven lines of the solvates Al(DMSO),-,(DMF), (z = 0, 1, . . ., 6) can be observed. The chemical shift between neighboring lines gets smaller with increasing z. The relative 1 If the concentration of DMF and DMSO is high enough to fill the primary solvation shell of Al(III), NM will not participate in this process. Copyright 0 1974 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
362
363
COMMUNICATIONS
,
mole Allaq), a013
1OOOg solution
X DMF=1-x DMSO 0.0 0. I 0.2 0.3 0.1 0.5 0.6 0.7 0.6
0.9
1.0
IZ7~~~~~~~),~~~~~~,191
r=o’
1’
‘I2 ”3 L56 ‘I I ’
0.0
I. 0 FIG. 1. 27A1 NMK nitromethane.
spectra of 0.013 M Al(ClO&
solutions in mixtures of DMSO and DMF in
concentrations of the cations with a different composition of the solvation shell were determined by cutting out the spectra and weighing them. The equilibrium constants of Al(DMSO),-,(DMF):Z,
+ Al(DMSO),-,(DMF):=,
+ 2 A1(DMSO),-z(DMF)”
are I:, = 2.08 + 0.09(2.4); Kz = 1.57 + O.lO(1.87); K3 = 1.51 & 0.06(1.78); K4 = 1.50 + 0.06(1.87); and K5 = 1.77 f 0.13(2.40). The values in parentheses are calculated for random distribution. The position of the resonance lines cannot be explained with a linear model
364
COMMUNICATIONS
in which the chemical shift 6(z) of Al(DMSO),-,(DMF), d(z) =
is proportional
to z:
6(z = 0) - 6(z = 6) z 6
Taking into account the interaction of neighboring solvent molecules, however, a pairwise additivity model can be used successfully to calculate the chemical shifts of the solvates including the geometrical isomers for z = 3, 4, 5 (5). The calculated shifts given schematically in Fig. 1 are in good agreement with the experimental results. The widths of lines z = 3,4,5 are broader than the widths of the other resonances due to extensive overlap of the pairs of geometrical isomers. Quadrupole broadening by the unsymmetrical environment of the Al(II1) ions (spin 5/2) in these solvates can be neglected. The ratio of the cis (lower magnetic field) to truns (higher magneticfield)configuration in Al(DMS0)4(DMF),3+and Al(DMSO),(DMF)i’ is 4: 1 in a purely random distribution. The unsymmetrical lineshape of these solvates reflects such a behavior. The line of Al(DMSO),(DMF), is also broader than those of the solvates z = 0, 1, 5, and 6, but the symmetry of the shape is in agreement with the ratio cis: trans = 3:2. The smallest linewidth observed (4.0 Hz) is comparable with that of Al(HzO),3’ (6). REFERENCES 1. A. FRATIELLO, “Progress in Inorganic Chemistry,” Vol. 17, II, p. 57, Interscience Publ., New York, 1972; S. F. LINCOLN, Coord. Chum. Rev. 6,309 (1971). 2. J. F. HON, Mol. Phys. 15,57 (1968); J. J. DELPIJECH, A. PEGNY, AND M. R. KI%DDAR, J. Mugn. Resonunce6,325 (1972); R. D. GREEN AND N. SHEPPARD, J.C.S. Faraday ZZ821(1972); F. TOMA, M. VILLEMIN, AND J. M. THIERY, J. Phys. Chem. 77,1294 (1973). 3. R. S. DRAGO, D. W. MEEK, M. D. JOESTEN, L. LA ROCHE, Znorg. Chem. 2,124 (1963). 4. D. GIJDLIN, to be published. 5. R. G. KIDD AND H. G. SPINNEY, Znorg. Chem. 12,1967 (1973). 6. B. W. EPPERLEIN UND 0. LUTZ, Z. Nuturforsch. 23a, 1413 (1968).
D. GUDLIN H.
Max-Planck Institut fiir Biophysikalische Chemie Gdttingen Germany Received July 26,1974
SCHNEIDER
OF MAGNETIC
RESONANCE
16,362-364
(1974)
27Al NMR Measurementsand Mixed Ligand Complex Formation of Al(III) at Low Concentrations It is well-known that in NMR spectra of electrolyte solutions two different signals for the solvent molecules bound to the cation and for the bulk solvent can be observed if the lifetime of the molecules in the solvation sphere is long enough. This is the case with small highly charged cations dissolved in solvents which are strong Lewis bases, such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), hexamethylphosphoramide (HMPT), etc. (1). Fewer examples are known in which the chemical shift of bound molecules depends on the composition of the solvation shell in such a way that different resonance lines can be observed (2). We used 27Al resonance measurements to determine the distribution of DMF and DMSO around Al(II1) in a solution of Al(ClO.J, in nitromethane (NM). In pure DMF and in pure DMSO, the solvation number of Al(111) is 6 (I). Since NM is a very weak Lewis base compared with DMF and DMSO, we used NM as a matrix to obtain signals of similar height for the bound and bulk DMF and DMSO in ‘H NMR spectra. r Al(ClO,),*8H,O (Merck) was used in the preparation of Al(C10J3*6 DMF and Al(ClO,),*6 DMSO (3). Since the solubility of Al(ClO,),* 6 DMSO in NM is 0.013 moles/kg solution, this concentration was used in the preparation of the stock solutions of Al(C10,),*6 DMF and of Al(ClO,),*6 DMSO in NM. All samples were prepared gravimetrically. We measured the 27Al spectra on a HFX 90 Bruker spectrometer using the 13C resonance frequency (22.63 MHz). The magnetic field was lowered down to 2.04 T to find the 27A1 resonance corresponding to a resonance frequency of 86.85 MHz for protons. The field must be locked because of very weak 27A1 signals at this concentration. The spectra were taken in the cw mode using signal accumulation. With a minimal and inexpensive modification (4), we obtained the field stabilization using the Schomandel ND 100 M frequency synthesizer which is already available with the spectrometer for the decoupling technique. The operation of the stabilization lock channel of the spectrometer is in no way affected which allows us to use this technique for the measurement of other nuclei as well. Thus, the 13C equipment was used in the calibrated frequency sweep mode in our measurements. The spectra were accumulated up to 64 times. The last three traces in Fig. I show single-scan spectra as a comparison. In Fig. 1 the single resonance line of a solution with composition XnMF = 0.00 refers to the species Al(DMS0),3+. In the spectrum of mole fraction 0.1, the second line, located 1.255 ppm at higher magnetic field, belongs to Al(DMSO),DMF3+. With increasing concentration of DMF up to xDMF = 1.O, a total of seven lines of the solvates Al(DMSO),-,(DMF), (z = 0, 1, . . ., 6) can be observed. The chemical shift between neighboring lines gets smaller with increasing z. The relative 1 If the concentration of DMF and DMSO is high enough to fill the primary solvation shell of Al(III), NM will not participate in this process. Copyright 0 1974 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
362
363
COMMUNICATIONS
,
mole Allaq), a013
1OOOg solution
X DMF=1-x DMSO 0.0 0. I 0.2 0.3 0.1 0.5 0.6 0.7 0.6
0.9
1.0
IZ7~~~~~~~),~~~~~~,191
r=o’
1’
‘I2 ”3 L56 ‘I I ’
0.0
I. 0 FIG. 1. 27A1 NMK nitromethane.
spectra of 0.013 M Al(ClO&
solutions in mixtures of DMSO and DMF in
concentrations of the cations with a different composition of the solvation shell were determined by cutting out the spectra and weighing them. The equilibrium constants of Al(DMSO),-,(DMF):Z,
+ Al(DMSO),-,(DMF):=,
+ 2 A1(DMSO),-z(DMF)”
are I:, = 2.08 + 0.09(2.4); Kz = 1.57 + O.lO(1.87); K3 = 1.51 & 0.06(1.78); K4 = 1.50 + 0.06(1.87); and K5 = 1.77 f 0.13(2.40). The values in parentheses are calculated for random distribution. The position of the resonance lines cannot be explained with a linear model
364
COMMUNICATIONS
in which the chemical shift 6(z) of Al(DMSO),-,(DMF), d(z) =
is proportional
to z:
6(z = 0) - 6(z = 6) z 6
Taking into account the interaction of neighboring solvent molecules, however, a pairwise additivity model can be used successfully to calculate the chemical shifts of the solvates including the geometrical isomers for z = 3, 4, 5 (5). The calculated shifts given schematically in Fig. 1 are in good agreement with the experimental results. The widths of lines z = 3,4,5 are broader than the widths of the other resonances due to extensive overlap of the pairs of geometrical isomers. Quadrupole broadening by the unsymmetrical environment of the Al(II1) ions (spin 5/2) in these solvates can be neglected. The ratio of the cis (lower magnetic field) to truns (higher magneticfield)configuration in Al(DMS0)4(DMF),3+and Al(DMSO),(DMF)i’ is 4: 1 in a purely random distribution. The unsymmetrical lineshape of these solvates reflects such a behavior. The line of Al(DMSO),(DMF), is also broader than those of the solvates z = 0, 1, 5, and 6, but the symmetry of the shape is in agreement with the ratio cis: trans = 3:2. The smallest linewidth observed (4.0 Hz) is comparable with that of Al(HzO),3’ (6). REFERENCES 1. A. FRATIELLO, “Progress in Inorganic Chemistry,” Vol. 17, II, p. 57, Interscience Publ., New York, 1972; S. F. LINCOLN, Coord. Chum. Rev. 6,309 (1971). 2. J. F. HON, Mol. Phys. 15,57 (1968); J. J. DELPIJECH, A. PEGNY, AND M. R. KI%DDAR, J. Mugn. Resonunce6,325 (1972); R. D. GREEN AND N. SHEPPARD, J.C.S. Faraday ZZ821(1972); F. TOMA, M. VILLEMIN, AND J. M. THIERY, J. Phys. Chem. 77,1294 (1973). 3. R. S. DRAGO, D. W. MEEK, M. D. JOESTEN, L. LA ROCHE, Znorg. Chem. 2,124 (1963). 4. D. GIJDLIN, to be published. 5. R. G. KIDD AND H. G. SPINNEY, Znorg. Chem. 12,1967 (1973). 6. B. W. EPPERLEIN UND 0. LUTZ, Z. Nuturforsch. 23a, 1413 (1968).
D. GUDLIN H.
Max-Planck Institut fiir Biophysikalische Chemie Gdttingen Germany Received July 26,1974
SCHNEIDER