David W. Thompson, David E. Kranbuehl, and Melvyn D. Schiavelli Colleae - of William and Marv Williarnshurg, Virginia 23185
The Synthesis and Characterization of Tin Complexes Using Inert Atmosphere Techniques
I
An advanced laboratory experiment
The use of inert atmosphere techniques for the manipulation of air-sensitive compounds has become widespread.' It is imperative that undergraduates have at least a preliminary exposure to the vacuum line, to inert atmosphere synthetic equipmentZand to the dry-box3. A study of the tin. tetrachloride-acetylacetone system is a good introduction for upper division students to techniques used in handling air-sensitive compounds. Additionally, this experiment exposes students to characterization techniques and chemical phenomena of current interest. Among these are
request. Also, available on request are detailed instructions including methods for the treatment of data for the characterization sections 3, 4, and 5. The SnC1,(CeH802) adduct is fairly sensitive to atmospheric moisture and must be prepared with care. The cisSnC12(CsH~02)Z complex is hydrolyzed quite slowly and brief exposure to air will not affect results. This complex, however, is typical of several analogous group IV complexes which are quite sensitive to moisture.
(1) the use of ir and nmr data t o determine molecular ~ t m c t u r e (2) geometrical isomerism in coordination compounds (3) stereochemical non-rigidity in coordination compounds and ligand exchange mechanisms (4) the use of conductivity and molecular weight measurements t o establish ionic character and molecular formulas (5) methods for analysis of chlorine and tin
SnClrAcetylacetone Adduct. A 100-ml %necked round-bottom flask (14/207) is equipped with a stirring bar, dropping funnel, and a nitrogen inlet adapter. The apparatus is gently heated with a Bunsen burner under a stream of nitrogen. After cooling, 40 ml of dryWHGIl and 2.0 ml (1.7 X 10-2 mole) of SnCL6 are added t o the reaction flask; 1.8 ml (1.7 X 10-2 mole) of acetylacetone is added dropwise. The resulting white solid is filtered under nitrogen and vacuum-dried a t room temperature. An "Airless Ware" filter funnel and filter flask can be used conve niently with the reaction flask by means of a simple adapter. cis-Diehlorobis(aeetylacetonato)tin(lV). This reaction is performed in the same manner as for the preparation of the adduct using 5.3 ml(5.14 X 10-'mole) of acetylacetone with 3.0 ml(2.57 X 10-4) of SnCl., and benzene as the solvent. It is sufficient to simply distil benzene under a dry atmosphere before use. After the dropwise addition of acetylacetone is complete, the dropping funnel is relnaved under N1 and replaced with a stopper. The reaction mixture is refluxed -1 br or until all the adduct dissolves with a. slow stream of nitrogen flowing up the condenser to aid in the removal of HCI. Addition of hexane with cooling precipitates a. white product which is isolated as above.
Acetylacetone, a difunctional Lewis base, reacts a t room temperature with tin tetrachloride to give a sixcoordinate adduct (1).
However, if acetylacetone and tin tetrachloride are heated at reflux, hydrogen chloride is evolved and only cis-dichlorobis(acetylacetonato)tin(IV) is formed (24). No evidence for a trans isomer has been reported.
Experimental
NOTE: Detailed instructions with diagrams of apparatus for performing the syntheses are available on
Synthesis
Characterization
All samples far analysis should be prepared in a nitrogen-filled dry box (or glove bag). The following may be performed
Recent publications have dealt with this area: (a) S ~ R I ~ E R , D. F.. "The Maninulation of Air-Sensitive Comoounds", S., &INERT, MCG~~W- ill, Ino., ~ k York, w 1969. (b) HERZOG, J., ANDLUBDES, K., "Technique of Inorganic Chemistry',, WileyInterscience, New York, 1968, Vol. 8, p. 119. Glassware for chemical synthesis under an inert atmosphere has just become available commercially: Kontes Glass Co., Vineland, N. J.-"Airless Ware". Qophhisticated dry boxes may not be readily available. However, suitable polyethylene glove hags are available for a few dollars from Instruments for Research and Industry, 108-110 Franklin Avenue, Cheltenham, Pa. 19012. ' A convenient drying procedure is t o distil fromP20s and store over Linde Molecular Sieves Type 4A. This reactive liquid is conveniently transferred using either glass or disposable plastic syringes. We have found no need to nurifv this chemical. and it can be withdrawn by syringe from the k o & bottle in air ib a hood Volume 49, Number 8, August 1972
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lower energy. (For a fuller understanding of investigations of the low temperature product the student should consult references (1-4).) The ir spectrum of the complex obtained at reflux, SnC12(CsH70z)z, shows a strong broad band at -1540 cm-' in the 1800-1500 cm-' region. Since HC1 is evolved during the reaction, the acetylacetonate anion must be bound to tin. Reference to the literature (6) reveals that oxygen-chelated acetylacetonate ligands are unambiguously characterized by the presence of strong bonds within a range of 1600-1500 cm-'. Conductivity and molecular weight data indicate that the complex is non-ionic and monomeric. At this point two isomeric structures for SnC12(CsH702)2 are consistent with the data. These are cis- and trans-dichlorobis(acetylacetonato)tin(IV). The nmr spectrum of SnC12(CsH70~)p in Cl&D at 35" exhibits three peaks; two high field peaks ( r = 7.80, 7.89) and a low field peak ( r = 4.31) with an intensity ratio of 3:3:1, re~pectively.~Considering the symmetry of each isomer, the trans isomer (DZh) has one methyl and one ring proton environment. The cis isomer (Cz) has two distinct methyl environments (see structure 11). If the cis structure is stereochemically rigid with respect to the time scale of the nmr experiment, two methyl resonances of equal intensity are expected. Indeed, the nmr data at 35'C are consistent with the existence of only the cis isomer in solution (6, 7) (see part A of figure). However, the spectrum is temperature dependent and at higher temperatures the methyl peaks coalesce into a single peak. The temperature dependence of the nmr spectra provides a method of studying the kinetics of stereochemical rearrangement of the acetylacetonate ligand, i.e., proton exchange between the two non-equivalent methyl sites in the cis structure. As the rate of this exchange process approaches the time scale of the nmr
(1) I r spectra as mulls in Nujol. (2) Melting points in sealed capillaries. (3) Nmr spectrum of cis-SnC1dCsHiOn)a over a temperature range of 35-130% in bromoform. (4) Both compounds can be analyzed for chloride by potentiometric titration with AgNOa solution. These compounds decompose readily in basic aolution to give Cl-c,,, which can be titrated without separation from other decomposition products. A gravimetric analysis for tin may also be done. (5) Molecular weight and molar conductivity values may be determined far SnClz(CsHi0dz. This experiment may be shortened by performing only analyses (1) and (2) plus the room temperature nmr spectrum; furthermore, only selected items of (4) and (5) may be done.
Discussion
Analytical data establish the empirical formula of the low temperature product as SnC14(C5HsOz) and of the product obtained at reflux as SnCl2(CsH70&. Careful observation during the preparation of SnClr (C6Ha02)indicates that no fiC1 is evolved. Thus, acetylacetone must be bound to tin as the neutral molecule. The infrared spectrum (mull) exhibits a strong broad band in the 1800-1500 cm-' region at 1670 cm-1 consistent with coordinated carbonyl groups. No bands were observed in the 0-H stretching region indicating the absence of any en01 tautomer. Thus, these data are best interpreted as the keto tautomer of acetylacetone being coordinated to tin through the oxygen atoms, i.e., the kcto tautomer behaves as a bifunctional Lewis base. Although the keto form of acetylacetone normally has carbonyl absorptions above 1700 cm-', .coordination of an acidic center can be thought of as reducing the bond order of the carbonyl group which consequently shifts the absorption to
' In bromoform, BrGH, the low field peak is obscured.
Table 2. Temperature *U
Kinetics of Exchange In kG.'
6% H n
In k ealc sec-'
sen-1
6v0 = 5.25 war used 8s the limit of slaw exchange. The rnnvent,ion in this work is to exoress freuuencv dif---. ~~~- - ~ - used ~ ferences in Hertz. Some exoerimenter~~refe;radians per second ~
NMR spectra for SnClz(CaH70d2at 35-1 20'C.
Data for SnC11(CsH70p)aand SnCIkLHsOd
Composition Compound
(%)
MP
Molecular Weight
I r (18001500 cm-I)
SnCIdC,H,O&
C, 30.97 H, 3.64
203-2M0C
387.7
1572sh 1540s, br
SnCldC~Ha02
.'
where 6w = 2r6v.
Spectrum amplitudes vary.
Table 1.
~
C1, 18.28 Sn, 30.6 C, 16.65
3, 32; Sn, 32.91
/
lournol o f Chemicd Education
-lo-'
4CHa)r(CH) 4.31
:
Js.-ca, 6.6,S.RHa
-165°C
AM = Molar conductance in nitrobenzene (em2ohm-'
570
NMR
AM*
360.6
mole^^).
1731vw 1670 8, br
experiment, the o b s e d environment of each methyl group becomes an average of the time spent a t each site. This produces a broadening of the two methyl peaks with increasing temperature. In the limit of rapid exchange (T > 80') the observed environment of all the met,hylgroups is identical and a single methyl peak ( 7 = 7.85) is recorded. The rate of exchange k between the two sites for the methyl groups is determined from an analysis of the line shapeof the spectrum (8,9). Asimple anddirect method of analysis is to compare the difference between the resonant frequencies of the two environments in the limit of slow exchange, 6vo (Part A of figure), to the separation in frequency a t a temperature in the region of partial collapse, 6". (see figure). The approximate relation of Gutowsky and Holm (10) for exchange b e tween two equally populated sites is used to calculate k.
The coalescence of the peaks occurs over a temperature of -60-82". Several values of k are measured over this temperature range. From a plot of log k versus 1/T, the energy of ictivation and frequency factor for the stereochemical rearrangement of the complex are obtained. The data and results are recorded in Table 2 and are comparable to those obtained for similar systems (9, 11). Another interesting feature of the nmr spectrum of SnClz(CrH702)2 is the observation of long-range coupling between the tin 117 and 119 nuclei (7.67 and 8.68% abundant) and the methyl protons of the ligand (1%). In addition, averaged coupling persists well-above (120°C) the coalescence temperature (see Part H of
the figure); and in C1,CD coupling is seen to persist to the ring proton (13). These observations arc consistent with the acetylacetonate ligand being attached to the same tin atom during the exchange process and thus with an intramolecular exchange mechanism. Two general intramolecular mechanisms for exchange are feasible: (1) a dissociative mechanism giving a fivecoordinate intermediate, and (2) a non-dissociative twisting mechanism. A preference for the non-dissociative mechanism has been expressed by Faller and Davison (13). However, a dissociative mechanism cannot be ruled out (14). (For a fuller discussion of the twisting versus dissociative mechanisms the reader should consult references (9, 11, 15)). Acknowledgment
The authors would like to express their appreciation to the National Science Foundation for an Undergraduate Instructional Scientific Equipment Grant. Literature Cited (1) h ~ n r oA. . L.. AND THOUPBON. D. W.. Inow. Cham., 7. 1196 (1968). (2) Moaam, G.T.,nlin Dxew, H. D. K., J. Chem. Soe.. 373 (1924). (3) MEnsom, R. C.,AND GUPTA,V. I.. J . Indian Chcm. Soc.. 40. 911 r70c2, ,An"-,.
(4) NELSON, W. H.,Inow. Chcm., 6. 1 5 W (1967). O , "Infrared Spectra of Inorgsnio and Coordination Com(5) N A ~ A M O TK., pounds." John Wilev & Sona, New York. 1963. pp. 210-215. (6) Smirn. J. A. S.. A N D W~mmNs.E. J., Chcm. Commun.. 381 (1965). (7) SMITH, J. A. S., A N D W r ~ s n r s E. , J., J . Chcm. Soc., A , 1749 (1966). (8) ALLEREAND, A,, GUTOYBIY,H. S., J O N A ~J., , AND MEINZBR, R., J . Amw. Chem. Soc., 88. 3185 (1966). (9) FAT.R. C.. AND LOWRY,R. N., Inom. Chcm., 6. 1512 (1007). H. S.. A N D HOLY, C . M., J . Cham. Phlls.. 25. 1228 (1956). (10) GUTOWBSY. . *ND P.*RSDN, R. G., "Mech~nismsof Inorganic Reactions," (11) B n s o ~ oF. John Wilev & Sons, New York. 1967. pp. 286-291. T., J . Chcm. ~ h y s . 43. . 3396 (1965). (12) K*w*s*m. Y.,AND TANAKA. A,. Inow. Chcm.. 6. 182 (1967). (13) FhLLEn. J. W., A N D DAYISON. (14) M w m ~ m T r e s ,E. L.,J . Amsr. Chcm. Soc., 90, 5097 (1958). N., Inow. Chem., 6. 1835 (1967). (15) FAY.R. C., A N D SERPONI.
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