Infrared Study of A Case of Cis-Trans Isomerism David Tudela Departamento de Quimica, Universidad Autonoma de Madrid. 2804+Madrid, Spain
Linkage and geometrical isomerisms are important topics in coordination chemistry, and the synthesis and structural study of coordination compounds should be carried out in the advanced inorganic chemistry laboratory. IR spectroscopy can be used to determine both the coordination mode of a ligand and the geometrical arrangement of ligands around the metal atom (I). In this way, undergraduate students can see the usefulness of group theory and vibrational spectroscopy in inorganic chemistry. Indeed, the process can be particularly instructive if a couple of geometrical isomers can he isolated. Avery good example is provided by tetrachlorobis(Nfl-dimethylformamide)tin(IV) (abbreviated a s SnC14(DMF)d whose cistrans isomerism was first put forward by Davanzo and Gushikem (2). This experiment contains some synthetic work and exposes the advanced inorganic chemistry student to the idea of isomerism and the usefulness of IR spectroscopy a s a structural tool. Furthermore, the students elucidate the structures following the same procedures used in research work, so they perceive the task as relevant and meaningful. Such work is one part of the principle of "learning through doing" that underlies laboratory work (3). Experimental The SnC14and S n B q used were prepared and purified previously by the students (4),but the DMF, CHC18, n-hexane, and MeCN were distilled before use. Infrared spectra were recorded on a Perkin-Elmer 1600 FT-IR instrument, using Nujol mulls between CsI windows. Preparation of SnBm(DMF)z Asolution of DMF (2.55 g, 34.9 mmol) in CHC13 (20 mL) is added dropwise to a magnetically stirred solution of SnBr4 (7.42 g, 16.9 mmol) in n-bexane (20 mL), contained in a 100-mL, two-necked, round-bottom flask equipped with dropping funnel and calcium chloride tube. After stirring a t room temperature for 1h, the white solid formed is
Figure 1. Resonance structures for N,Ndirnethylforrnarnide.
filtered off, washed twice with n-hexane, and dried in vacuo or in a stream of air. Yield is 9.30 g (94%). Preparation of cis- and trans-SnCln(DMF)z Caution: Tin(IV)chloride fumes in moist air, and the fumes are very corrosiue. Furthermore, MeCN is toxic andflammable. Therefore manipulations involving these compounds should he carried out in a fume hood.
A 100-mL, two-necked, round-bottom flask equipped with a dry Nz inlet and magnetic stirring bar, is charged with DMF (1.31 g, 17.9 mmol) and CHC13 (40 mL). On the stirred solution, S n C 4 (1mL, 2.23 g, 8.5 mmol) is added dropwise with a syringe, and a white solid forms immediately. After stirring a t room temperature for 30 min, the solid is filtered off, washed twice with CHCL, and dried in vacuo or in a stream of air. The yield is typically above 95%, and the solid is often a mixture with varying amounts of the cis and trans isomers. Either vacuum sublimation or recrystallization from MeCN yields the pure cis isomer. The cis compound is transformed into the trans isomer by heating overnight a t 140 'C in a sealed glass tube or capillary.
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Relevant IR Bands (cm-') for cis- and transSnC14(DMF)z
Cis
Trans
Assignment
1651 vs
1655 vs
VC.0
402 s
403 s
Ligand band
344 s 329 s 313s 291 rn
337 vs
VSMI
Results and Discussion Coordination Mode
DMF is potentially a n ambidentate ligand with oxygen and nitrogen a s possible donor atoms. The resonance structures for DMF are shown in Figure 1, with 1being favored for the free lieand because no formal charge is given to any atom (5).Coordination through the nitrogen atom would further decrease the contribution of 2, thus increasing the C-0 bond order and the vc.0 stretching frequency. I n contrast, coordination through the oxygen atom would increase the contribution of 2, and vc.0 should decrease with r e s ~ e cto t the free lieand. The IR s ~ e c t r u mof CC14solutions df DMF is a good a>proximation to the spectrum of isolated DMF molecules. and vc.0 a t 1687 - - amears -. cm-' (6). Therefore, the positions of vc.0 in the IR spectra of cis- a n d t r n n s - S ~ C l ~ ( D M F(see )~ table) and SnBr4(DMFjz(1651 cm-') indicate that the ligand coordinates through the oxygen atom.
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Figure 2. IR spectra of (A) trans- and (B)cis-SnCI,(DMF),.
isomers or a mixture of them. When Raman spectra are available, they can also provide a valuable experience (2). If Mossbauer spectroscopy is taught a s part of a course in physical methods (9, 101,then Sr&(DMF)2 (X = C1, Brj provides a beautiful example of the usefulness of the technique (11).Although SnBr4(DMFj2also presents cis-trans isomerism (21, tin-bromine stretching vibrations appear below 250 cm-', and the structure of the complex formed cannot be elucidated unless far-IR or Raman data are available. An interesting feature of this experiment is that cisand t r a n ~ - S n C l ~ ( D Mcan F ) ~be easily interconverted. The C ~ S - S ~ C M D Misomerizes F)~ on heating, whereas transSnCla(DMF)7 transforms into the cis isomer bv vacuum -. sublimation"or recrystallization from MeCN. +herefore, geometric isomerism is clearly demonstrated. Furthermore, the potentially ambidentate character of the DMF lieand vrovides the mound for a discussion on linkaee isomerism. The experiment illustrates infrared interpretation, applications of group theory, and the usefulness of IR spectroscopy to elucidate the coordination mode of a ligand and the molecular structure of a coordination compound. ~
Geometrical Isomerism Once the coordination mode of the ligand is established, the question centers on the cis or trans structure of the complexes. According to group theory, four v s a and two vsnLare IR-active for cis-S&Lz (idealized Czu symmetry) complexes, whereas only one v s a and one v s , ~are expected in the IR spectrum of the trans isomer (idealized D4h symmetry) (7). T h e IR spectra of cis- a n d transSnC14(DMF)2in the range 500-250 cm-' are shown in Figure 2. Assignment of the vs,cl modes is facilitated by comparison with the spectrum of SnBr4(DMFI2,which shows bands a t 421 and 399 cn-I but no band in the range 390250 cm-'. After a brief discussion on the effect of mass on the frequencies of vibration, students are able to assign the v ~ m modes (see table) and ideutifv the isomers formed. In order to assign the vs"o modes, the students are told that thev annear around 400 em-' and that a lieand band has been reported s t 399 em.' for SnUr,! u . V ~ t 2 a n dat 401 cm Tor SnC12Rr?~I)M1.'.2 nnd SnC14cDMF.DMSO 18,.In this way, the assipvnents shown in Table 1 are completed. After the soectr;] of ns- and trans-SnClacl)MF, a r e known (Fig. 2); the students can look a t thespect&m of the sample just prepared, to check whether it is any of the A
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Literature Cited I. Nakarnoto. K infmmd ond Ramon S p d m of Inorgonlc and Coordination Composnds. 4th ed.; Wky: New York, 1986. 2. Davanro, C. U.:Gushikern, Y. J Chem Soc Dollon k n s 1981,843446. 3. Bman, G.; Atkina, M. Efecfiv. %aching in H k a r Educdion; Methue and Ltd.:
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London. 1988: o 94. 4. Brauer. G. Hondbuch do? Proooroliuen Anormnischm Chemie: Ferdinand Enks ~
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7. Ref I ,p 464. 8.Tomem. J.V.; FernBndel,V;l!~dela,D. J Moi. Stmet 1986,143,171-174. 9. Lkaga, R. S. Physical MethaLs in Chemistry:Saunden: Philsdelphis. 1971: Chapter 16
