In the Laboratory
Probing Halide Distribution in [AlX4]- Anions via 27 Al NMR: Application of the Pairwise-Additivity Model Craig M. Davis* and Bridget M. Dixon Department of Chemistry, Xavier University, Cincinnati, Ohio 45207-4221, United States *
[email protected] Many laboratory exercises published in this Journal have used NMR spectrometry to study coordination compounds. However, the ligand was the focus of the NMR analysis for the vast majority. Recently published exercises in which the central atom itself was probed by NMR spectrometry include the resolution of entiomeric [Co(en)3]3þ ions (studied with 59Co NMR) (1), the complexation of Naþ by 18-crown-6 (23Na NMR) (2), and the hydrolysis of octahedral aluminum ions (27Al NMR) (3). In this exercise, the tetrahedral tetrahaloaluminate anions are studied with 27Al NMR spectrometry.1 The students acquire spectra of the homoleptic complexes [AlX4]- and the dihalide complexes [AlX4Y4-n]- (X, Y= Cl, Br, I). The chemical shifts of these anions allow students to obtain pairwise-additivity parameters (see below) that then are used to predict the chemical shifts of the trihalide complexes [AlClmBrnI4-m-n]-. The relationship between the 27Al NMR chemical shift and the charge on the central aluminum atom (calculated with molecular modeling) is investigated, as well as the relationship between the symmetry of the anions and the line width of their respective resonances. Pairwise-Additivity Model A pedagogical bonus of probing the central atom with NMR spectrometry is the students are introduced to the pairwise-additivity model developed for tetrahedral complexes by Vladimiroff and Malinowski (4) and extended to octahedral complexes by Kidd and Spinney (5). The pairwise-additivity model recognizes that each substituent on the central atom changes the wave function of each of its neighboring substituents. Parameters are determined for each type of adjacent pair (Br-Br, Br-Cl, etc.), and the chemical shift of the central atom can be calculated by summing the pairwise-additivity parameters for all nearest-neighbor pairs: six for a tetrahedron, 12 for an octahedron. An illustration of the extraction and application of the pairwise-additivity parameters is as follows (6). The [AlCl4]anion has six Cl-Cl interactions; its 27Al NMR chemical shift is 100.1 ppm, so each Cl-Cl pairwise parameter is 16.7 ppm. The [AlCl3I]- anion (three Cl-Cl and three Cl-I interactions) has a chemical shift of 84.6 ppm; because each Cl-Cl pairwise parameter is 16.7 ppm, each Cl-I pairwise parameter must be 11.5 ppm. By obtaining all possible parameters, the chemical shift of anions with all three halides can be predicted. For example, the [AlCl2BrI]- anion has one Cl-Cl, two Cl-I, two Br-Cl (15.4 ppm), and one Br-I (6.3 ppm) interactions; hence, the
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Table 1. Aluminum Halide, Salt, and Solvent Combinations System
Aluminum Halide
Salt
Solvent
AlBr3
Pr4NCl
CH2Br2
Cl/I
AlI3
Bu4NI
CH2Cl2
I/Br
AlI3
Bu4NI
CH2Br2
Br/Cl/I
AlI3
Pr4NCl
CH2Br2
Br/Cl
calculated chemical shift is (16.7) þ (2 11.5) þ (2 15.4) þ (6.3) = 76.8 ppm, whereas the observed chemical shift is 76.6 ppm. Consistently, the pairwise-additivity model predicts chemical shifts within a few tenths of the observed shifts. Experiment Following the procedure developed by Kidd and Traux (6a), students prepare three solutions containing two of the three halogens (Cl, Br, I) (“dihalide systems”) and a fourth solution containing all three (“trihalide system”). Each solution contains an aluminum halide, a halide salt, and dihalomethane (solvent). The recommended combinations are given in Table 1.2 The aluminum halides must be opened and handled in a nitrogenfilled glove bag, but once dissolved in the appropriate solvent, they may be handled outside the glove bag. The halides exchange rapidly, and complete distribution is achieved within minutes. The exchange rate is noteworthy, because for three of the four systems the solvent is the sole source of one halogen! Pairwise parameters obtained from the three dihalide systems will permit the students to predict the chemical shifts of the [AlClmBrnI4-m-n]- anions that arise in the trihalide system. Samples are analyzed using a 300 MHz Varian Mercury spectrometer (27Al at 78.2 MHz). The number of acquisition scans is 128 for the dihalide systems and 256 for the trihalide systems; the acquisition time is 0.160 s with a delay of 1 s.3 Hazards Students should wear gloves and work in a fume hood. All compounds in this exercise are skin, eye, and lung irritants, and the volatile solvents are toxic. Results and Discussion The spectra of the three dihalide systems allow the students to accomplish four tasks. First, the students are able to assign a formula to each peak by recognizing that the homoleptic anions are common to two spectra (Table 2). Second, once the peaks are
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 88 No. 3 March 2011 10.1021/ed100655x Published on Web 12/17/2010
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Journal of Chemical Education
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In the Laboratory Table 2. Chemical Shifts for Selected [AlXnY4-n]- Anions Anion [AlCl4][AlCl3Br]
-
[AlCl2Br2][AlClBr3]
-
[AlBr4]-
Notes
δ (ppm)
Anion
δ (ppm)
100.1
[AlBr4]-
77.4
96.4
[AlBr3I]-
57.6
91.4
[AlBr2I2]-
33.3
85.0
[AlBrI3]
77.3
[AlI4]-
-
4.6 -28.3
identified, the students are asked to find a trend between line widths and the symmetry of the anions. Third, the relationship between the 27Al NMR chemical shift and the charge on the central aluminum atom (calculated with molecular modeling) is investigated. Fourth, the students obtain the pairwise-additivity parameters and then predict the chemical shifts of the [AlClmBrnI4-m-n]- anions that arise in the trihalide system, as discussed above. It should be noted that the trihalide system would be expected to display a total of 15 peaks, assuming complete exchange; however, only 13 lines are observed, due to accidental overlap. (For example, resonance of the [AlCl2BrI]anion overlaps with that of the homoleptic [AlBr4]- anion, resulting in a single asymmetric peak.) Students are asked to account for this observation. Acknowledgment The authors thank Raees Ismail and Christopher Rizik for their contributions. C.M.D. thanks an anonymous reviewer for helpful comments.
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Vol. 88 No. 3 March 2011
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1. Presented in part at the 237th ACS National Meeting, Salt Lake City, UT, March 22-26, 2009 (Poster CHED-460). 2. All possible combinations were attempted, but those not in Table 1 had solubility issues. In hindsight, this was not surprising, as AlCl3 forms three halide bridges in the solid state and consequently is polymeric, and CH2I2 is effectively nonpolar. In fact, the four recommended combinations were the four used originally to obtain the 27Al NMR spectra of the mixed tetrahaloaluminate anions (6a), although the cation for their iodide salt was Pr4Nþ. 3. These parameters are based on those reported in ref (3).
Literature Cited 1. Borer, L. L.; Russell, J. G.; Settlage, R. E.; Bryant, R. G. J. Chem. Educ. 2002, 79, 494–497. 2. Peters, S. J.; Stevenson, C. D. J. Chem. Educ. 2004, 81, 715–717. 3. Curtin, M. A.; Ingalls, L .R.; Campbell, A.; James-Peterson, M. J. Chem. Educ. 2008, 85, 291–293. 4. Vladimiroff, T.; Malinowski, E. R. J. Chem. Phys. 1967, 46, 1830– 1841. 5. Kidd, R. G.; Spinney, H. G. Inorg. Chem. 1973, 12, 1967–1971. 6. (a) Kidd, R. G.; Traux, D. R. J. Am. Chem. Soc. 1968, 90, 6867–6869. (b) Malinowski, E. R. J. Am. Chem. Soc. 1969, 91, 4701.
Supporting Information Available Student handout and notes for the instructor. This material is available via the Internet at http://pubs.acs.org.
pubs.acs.org/jchemeduc
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r 2010 American Chemical Society and Division of Chemical Education, Inc.
