In the Laboratory
Synthesis, Characterization, and Lewis Acidity of SnI2 and SnI4 Richard W. Schaeffer,* Benny Chan, Michael Molinaro, Susan Morissey, and Claude H. Yoder Department of Chemistry, Franklin and Marshall College, Lancaster, PA 17604 Carolyn S. Yoder and Stephanie Shenk Department of Chemistry, Millersville University, Millersville, PA 17551
An exploration of synthetic procedures not only teaches the student about one of the principal provinces of the chemist but also provides an opportunity to learn about a variety of procedures and methodologies. In this project the student has the opportunity to learn about (i) the direct synthesis of compounds from the elements, (ii) stoichiometry and limiting reagent, (iii) isolation by recrystallization, (iv) use of inert atmosphere, (v) identification by melting point, gravimetric analysis, powder X-ray diffraction, and NMR spectroscopy, and (vi) at least one method for determination of relative Lewis acidity. Although SnI4 chemistry has been the subject of a few articles in this Journal (see below), this paper addresses a wider range of compounds and characterization techniques. The broad scope of the project makes it ideal for a multiweek project in the introductory course or in an upper-level course such as inorganic chemistry. It can also be adapted to more individual learning experiences where, for example, one student might be asked to explore a particular synthesis of SnI4 , another might synthesize SnI2 , and others might pursue methods to analyze the compounds, to convert one to the other, to determine their Lewis acidity, and so on. The SnI2 / SnI4 system is ideal for student work because the elements are easily accessible and relatively nontoxic, yet reactive. The products are related through oxidation /reduction, yet are sufficiently stable to allow isolation and analysis. The products also react as Lewis acids. Tin(IV) iodide can be prepared by direct combination of the elements, preferably in a solvent such as toluene that will dissolve elemental iodine (1, 2). The reaction in toluene, once initiated by heating, is slightly exothermic and produces product in good yield (ca. 80%). Moreover, SnI4 is sufficiently insoluble in toluene that it can be isolated from cold toluene and recrystallized from the hot solvent. A simple characterization of the compound can be accomplished by melting point and also by gravimetric analysis (3). We have developed a simple gravimetric procedure that is ideal for introductory students because it avoids the necessity of filtration. The procedure converts SnI4 to SnO2 by treatment with 3 M HNO3 . If the procedure is performed slowly, followed by heating in the Bunsen flame, the HI, water, etc. are removed by evaporation and heat, and the residual SnO2 can be weighed. This procedure works for both SnI4 and SnI2 (with slight modification) and gives results within several percent of the theoretical percent of tin. Because the products differ by 13% in the percent of tin, the method is sufficient to establish which product is present, as well as to reveal the presence of major impurities. Clearly, it is not an analytical procedure designed for high accuracy. More accurate results can be obtained by volumetric analysis of iodide after hydrolysis of the product (4). Tin(II) iodide can be obtained by first preparing (or supplying) SnCl2. The SnCl2 is easily obtained by published
procedures such as reaction of tin with HCl (5). This is then followed by treatment with I2 . Modest yields from 30 to 50% were typical for this procedure due in part to volatilization of diiodine from the solution. Because SnI2 is easily oxidized, it must be handled in an inert atmosphere such as N2. For example, when analyzing for tin, the sample should be introduced into a preweighed vial in an inert atmosphere, closed, and then weighed on an analytical balance. The two compounds differ considerably in melting point: SnI2, 320 °C; SnI4, 144 °C. In addition to their obvious difference in percent composition the compounds also differ slightly in color, the SnI2 being slightly more red than the orange-red SnI4. The compounds have significantly different powder X-ray diffraction patterns as illustrated by the 2 θ and d-spacing data in Table 1. As a result, it is an easy matter to identify the products and determine their purity (e.g., the extent of SnI4 contamination in SnI2). Although the infrared spectrum is difficult to obtain because of the low wavenumber Sn–I vibrational frequencies, the 119Sn NMR spectrum is relatively easy to obtain in deutero-DMSO. The 119Sn chemical shift for SnI is one of the lowest tin shifts 4 known, presumably a result of the usual diamagnetic influence of iodine. Because SnI2 has fewer iodines, the tin shift for this compound occurs at a higher frequency. The shifts in d6-DMSO are {779 ppm and {2024 ppm for SnI2 and SnI4, respectively, relative to tetramethyltin. The shifts for SnI 4 were also obtained in CH2Cl2 and acetone as {1755 ppm and {1827 ppm, respectively. Interestingly, samples of both compounds have 119Sn NMR spectra that show minor peaks due to the other compound. Because of the similar stabilities of the +2 and +4 oxidation states, the presence of a mixture is not surprising. Each compound is sufficiently pure, however, to provide for relatively sharp melting points (we estimate that in DMSO the minor impurity is generally not present as more than 10%).
Table 1. Major Powder X-ray Diffraction Peaks for SnI2 and SnI4 (6 ) SnI2 2θ 38.10
SnI4
Relative d -Spacing Intensity 2.360
100
Relative Intensity
2θ
d -Spacing
25.14
3.540
100 65
65.91
1.416
66
20.79
2.170
25.57
3.481
47
24.61
1.850
47
63.25
1.469
40
29.06
3.070
34
51.50
1.773
40
60.26
1.534
27 17
46.23
1.962
39
75.88
1.253
71.15
1.324
32
59.26
1.558
13
26.87
3.316
17
81.45
1.181
12
40.31
2.184
16
17.69
5.010
11
*Corresponding author.
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In the Laboratory The SnI2 is easily oxidized to SnI 4 by treatment with diiodine in toluene. Although SnI 2 is a relatively weak Lewis acid, SnI4 will react with a variety of Lewis bases, producing in most cases a solution of distinctly darker color. With some bases, such as pyridine, a precipitate also forms (DMSO and acetone do not form precipitates easily but do produce darker solutions in a solvent such as CH2Cl2). Because the 119Sn shift is difficult to observe in solutions of the adducts, it is more convenient to study the Lewis characteristics by monitoring the shift of a base that contains an NMR-sensitive nucleus. An obvious candidate for these studies is, therefore, a phosphine oxide such as triethylphosphine oxide (Et3PO). The acidity of SnI 4 relative to SnCl4, SnI2 , etc. can be obtained qualitatively from the extent to which the 31P nucleus shifts in solutions of similar concentration. In these solutions only one 31P resonance is observed because of exchange of the base, and the observed shift is, therefore, a weighted average of the shift of free base and the shift of the adduct(s). The use of the observed shift as a measure of acidity, therefore, relies upon a number of assumptions: the 31P nucleus is affected by electron withdrawal by tin; the shifts are not affected in a major way by the presence of small amounts of other adducts (such as 1:2 as well as 1:1) or impurities; and the adduct shift parallels the magnitude of the equilibrium constant (if this were not the case, one might have a large equilibrium constant but a small adduct shift and a consequent small observed shift that would be interpreted as weak Lewis acidity). Clearly, implementation of the entire project or parts thereof depends upon the course level, the instructor’s objectives and pedagogical temperament, and the amount of time available. For traditional introductory-level courses, we recommend synthesis and characterization of SnI4 , as well as qualitative observations of reactions of SnI4 with water, bases, and even salts such as KI. In upper-level courses, the addition of the preparation and characterization of SnI2 , as well as the acquisition of 119Sn NMR and the use of NMR for the determination of Lewis acidity, would be appropriate. Extensions such as isolation and analysis of adducts, acquisition and interpretation of IR spectra and/or powder X-ray diffraction data, and quantitative determination of equilibrium constants for adduct formation are all possible. Experimental Procedure
Preparation of SnI2 A 0.5-g sample of tin (Fisher certified, {20 mesh) and a 1.2-g sample of diiodine (Aldrich, 99.8%; CAUTION: highly toxic, corrosive lachrymator) were placed into a three-neck, 100-mL flask fitted with a cold-water condenser and a mineral oil bubbler. The flask was thoroughly flushed with nitrogen for fifteen minutes. Next, a 10-mL aliquot of 2 M HCl was added to the vessel. The reaction was heated to gentle reflux for an hour until all the diiodine had reacted, as evidenced by the solution becoming a pale-yellow color. The solution was gravity filtered while hot into another threeneck flask to remove any unreacted tin. The flask was kept hot and repurged with nitrogen for 10–15 minutes to remove oxygen introduced while filtering. The crystals were formed by cooling in an ice bath under nitrogen and were collected by vacuum filtration. The product was washed with 10 mL of cold water and allowed to dry in a vacuum desiccator. Yields ranged from 30 to 50% due in part to volatilization of diiodine from the solution. The product was stored in a vacuum desiccator backfilled with nitrogen gas to prevent reaction with atmospheric oxygen. A melting
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point was taken with a Fisher–Johns melting point apparatus fitted with a Pyrex chamber to maintain a nitrogen atmosphere. The target SnI2 melted at 320 ± 1 °C (melting at approximately 144 °C indicates formation of tin tetraiodide).
Preparation of SnI 4 from the Elements Approximately 1.0 g of tin and 2.5 g of diiodine (±0.01 g) were weighed into a 50-mL Erlenmeyer flask along with 15 mL of toluene. The flask was covered with a watch glass and heated gently on a hot-plate until the toluene began to boil. The heat was reduced immediately, because the reaction is exothermic and did not require much external heating once started. The flask was then gently heated and swirled occasionally for 20–30 minutes until the violet color of diiodine was no longer visible and the solution assumed an orange-brown color. The hot solution was filtered into a 50-mL beaker and cooled in an ice bath. Bright orange crystals were collected with vacuum filtration, washed with 5 mL of cold toluene, and allowed to dry in a vacuum desiccator. The product was recrystallized from a minimum amount of cold toluene and manifested a melting point of 144 ± 1 °C, indicative of the target compound. Undergraduate students typically realized yields of 50–70% for the purified product.
Preparation of SnI4 from SnI2 Samples of SnI2 (CAUTION: corrosive lachrymator) and I2 in equimolar proportions (0.20 and 0.14 g, respectively) were placed in a 100-mL, three-neck flask fitted with a coldwater condenser and mineral oil bubbler. The system was flushed with nitrogen for fifteen minutes. A 20-mL aliquot of toluene was added to the system and refluxed until the reaction was complete as indicated by the disappearance of the violet color of diiodine. The solution was filtered hot into a 150-mL Erlenmeyer flask and cooled in an ice bath. Large orange-red crystals formed readily and were collected and recrystallized with toluene as described above. Melting point determinations were used to confirm the formation of the tetraiodide.
Preparation of Na2[SnI6] In a 100-mL beaker, 1.00 g of SnI 4 ( CAUTION: may produce corrosive vapors) and 0.50 g of NaI were dissolved with gentle heating in 20 mL of acetone. A dark solution (almost black) immediately formed and, after all the reactants were dissolved, was allowed to cool. The solution was placed in a desiccator overnight and the acetone slowly evaporated to dryness. Iridescent dark-blue crystals resulted, but a melting point could not be obtained because of product decomposition. Other hexaiodostannates were produced with the same procedure using KI and NH4I, but the sodium analog provided the most reproducible results.
Gravimetric Analysis of Tin Compounds Three porcelain crucibles were heated to constant weight with Bunsen burners and weighed to ± 1 mg. Approximately 0.5 g of SnI4 (weighed to 0.1 mg) was placed into each crucible followed by 1 mL (20 drops) of 3 M HNO 3. Each crucible was covered and left undisturbed for at least 24 hours. An additional 10 drops of 3 M HNO3 were added and the crucible allowed to set for several more hours. Next, the crucibles were heated gently on a hot-plate for several hours followed by heating with a low Bunsen burner flame to remove volatiles (care must be exercised to avoid splattering). Finally, the crucibles were heated strongly over a Bunsen burner for 30 minutes, allowed to cool for several minutes, and then placed in a desiccator until at room temperature. The crucibles and contents were weighed and the
Journal of Chemical Education • Vol. 74 No. 5 May 1997
In the Laboratory composition of the original SnI 4 calculated by assuming complete conversion to SnO2. The procedure for SnI2 was similar except that longer, more gentle digestion periods with stirring seemed to be beneficial.
NMR Parameters 119Sn-NMR spectra were obtained on saturated solutions in 10-mm tubes containing a coaxial inner tube with a 0.1 M solution of tetra(n-propyl) tin in d-acetone using a Varian Unity 300 operating at 111.847 MHz with 45° flip angles, 3 s delay times, and approximately 5000 transients. The shifts are reported relative to tetramethyltin.
Powder XRD Parameters Powder diffraction patterns were collected on samples of SnI2 and SnI4 ground in an agate mortar and pestle and
dry-mounted on glass slides using a Phillips APD 3520 diffractometer equipped with a theta compensating slit and a focusing graphite monochromator. Samples were scanned from 5 to 90 degrees 2θ over 30 minutes using copper Kα radiation (1.5406 Å) . However, useful patterns could be obtained with shorter scan times and/or a more limited 2θ range. Peak positions and intensities were calibrated using a NIST certified alumina standard. Literature Cited 1. 2. 3. 4.
Moeller, T.; Edwards, D. C. Inorg. Synth. 1953, IV, 119. Hickling, G. G. J. Chem Educ. 1990, 67, 702–703. Wheatland, D. A. J. Chem. Educ. 1973, 50, 854. Skoog, D. A.; West, D. M.; Holler, F. J. Fundamentals of Analytical Chemistry, 4th ed.; Saunders: New York, 1992; pp 869–871. 5. Moser, W.; Trevena, I. C. J. Chem. Soc. D 1969, 25. 6. Powder Diffraction File; McClune, W. F., Ed.; International Centre for Diffraction Data: Newtown Square, PA, 1994.
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