The Early History of Organotin Chemistry John W. Nicholson Laboratoty of the Government Chemist, Cornwall House, Waterloo Road, London SE1 8XY. United Kingdom The origin of organotin chemistry lay in the syntheses of examples, such syntheses being sufficient for many of the 19th century pioneers of this branch of chemistry. Nonetheless, by considering the known compounds, some of these chemists were able to make important theoretical advances. Organotin compounds are defined as those that contain at least one SnC bond, the carbon atom being Dart of an oraanic group. Such compounds today are econdiica~~y important and are used a PVC stabilizers, active inaredients for antifouling paints, wood preservatives, and as hiocides in agriculture (I). The organic chemistry of tin is essentially restricted to the +4 oxidation state (2); however, organotin(I1) are species known, either where attachedto bulky ligands (3, 4) or where the organic substituent is cyclopentadiene (5). There are two broad routes to organotin compounds, namely direct from elemental tin, or indirect, via tin compounds, such as those containing SnO, SnH, or SnCl bonds (5, 6). The route from SnCL has been used extensively, including for industrial-scale production of organotins. Direct routes may utilize tin as the only metal or may have it present as part of an alloy. In either case, the organotin compounds are prepared by reaction with an organic halide, often iodide (6). The former routes yield predominantly diorganotin dihalides, according to the following:
The latter, by contrast, tend to yield mainly tetraorganotins, thus:
Modern indirect routes utilize Grignard reagents, as follows:
By these reactions then, di- and tetraorganotin compounds can be made. To prepare the mono- or triorgano- derivatives, the following redistribution reactions are employed:
--
R,Sn + 3SnCb 4RSnC13 R4Sn RZSnClz 2R,SnCI
+
With this hackground, we can turn to the activities of the earliest explorers in the field of organotin chemistry and appreciate what they achieved as they laid the foundations for this branch of the subject. Earliesl Studles The first reports of the existence of "organic bodies of tin", as they were then known, appeared in 1852 (7,8). One was by Carl Lowig (1803-1890) (7), the other by Edward Frankland (1825-1899) (8). Lowig's paper described the use of a tinlsodium alloy in reaction with ethyl iodide. The initial product was not tetraethyltin, but the polymeric diethyltin, (EtzSn).. Hampered by inadequate knowledge of atomic masses (Lowig took thevalues Sn = 59, C = 6). he formulated thissubstance as C4H6Sn. It was a thick, oily liquid that reacted readily with air to yield a white precipitate, which we would now describe as diethyltin oxide, EtzSnO. Lowig also found that
he could prepare diethyltin dihalides, either by reaction of diethyltin with the elemental halogen in the case of bromine and iodine or by treatment of the parent tin compound with ethanolic HCI in the case of chlorine. For each of these halides, which were purified by sublimation, the formula CaH5SnXwas proposed. Lowig at the time of this work was a professor in the Industrial High School in Zurich and was a noted organic chemist. During his career, he published several papers, as well as two texthooks, on organic chemistry (91. .~. The other report of 1852, by Frankland (8), was amplified in subsequent papers published in 1853 (10) and 1854 (11). Frankland was a profoundly influential chemist; he developed the use of modem graphical formulas for compounds, gave to chemistry the words bond and organometallic, and through his work described in these papers (8,10,12), recognized that atoms had definite combiningpower, and thereby arrived at the theory of valencv (12). These papers are thus important landmarks in the history of chemistry. However, for organotin chemistry, they are distinguished hecause they describe the first dire2 synthesis usingiiu as the only metal. Frankland heated tin foil with ethyliodide in sealed tubes at 180 "C and obtained as his product crystals of diethyltin diiodide, which were found to melt at 42 "C. Such direct reactions have been eenerallv found to reouire a minimum temperature of ahour160 T e ( f i ) , a fact thai has been attributed to a phase chanee in the solid form of tin from B- t o r tin, whichoccurs at i 6 l 'C, this latter crystdine mbdificition being held to be more reactive toward alkyl halides than @tin (13). Frankland also reported that ethyl iodide would comhine with tin foil under the influence of sunlight, hut this observation has not since been repeated (14). The first use of the indirect route to organotins was by G. B. Buckton (1818-1907) in 1859 (15). Buckton was aremarkable person (16). The son of a doctor, he was crippled in childhood and remained permanently disabled. As a result he was unable to attend public school, which would have been the norm for one of his background, but instead he was educated privately at home. At the age of 30 he entered the Roval Colleee of Chemistrv. .. where he remained for seven years,acting in the latter part of his timeas privateassistant to A. W. von Hoffman. The Roval College of Chemivtrv had been established in London in-1845, an;d it was at thetime the premier college for the teaching of chemistry in England. Buckton made an important contribution to the development of organometallic chemistry through the use of diethvlzinc as an alkvlatine agent for metal chlorides and in 1857 Laselected to ~ e l l o w ~ h ithe ~ oRoyal f Society. He published his last chemical oaDer in 1863and then turned hisattention to entomology. ~k continued to work in this field for the rest of his life, supported by his own private income and by a little teaching of science. Despite the importance of his chemical discoveries, Buckton's greatest claim to fame was prohahly that he was brother-in-law to the illustrious William Odling, Professor of Chemistry at the University of Oxford. Diethylzinc had been discovered only 10 years earlier, by Frankland (17), but in the days before Grignard reagents, it had rapidly acquired a reputation for usefulness in the synVolume 66 Number 8 August 1989
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thesis of various metallic and nonmetallic ethvl com~ounds (18). Although in many ways similar to ~ r i k a r dreagents (though less convenient to prepare), organozincs differ in one important respect: they do not react with carbon dioxide. Hence reactions were often carried out under an atmosphere of this gas. Diethylzinc may be prepared by the direct reaction of ethyl iodide with zinc. The initial product is ethylzinc iodide, hut when heated, diethylzinc can be distilled from this compound, since the following equilibrium exists, and it can be readily shifted to the right-hand side:
foreign metal in sufficientquantity to give an organometallic body!' Nonetheless, it did. As W. H. Perkin once remarked "In those days there were no depots where pure products for (25). Letts and Collie went research could be obtained. on to reason that, since the amount of tin in the sample could not he excessive. a fair nronortion of it must have been converted to tetraethylti< a i d they set ahout improving the method. They showed that it was not necessary to start with a tin-zinc alloy, but that intimate mixtures of the powdered metals would also undergo the reaction. Thev also -~ r o- ~ o s e d what they called a "mec~anism"for the reaction:
. ."
ZEtZnI + EhZn + Zn12 Buckton prepared this compound and studied its action on a number of metal chlorides. In this wav, he was not only the first chemist to prepare tetraalkyltins, hut also the first to synthesize organomercurials (19): EhZn
-
+ HgCl, ~.
Attempts To Prepare DlalkyRlns There was no reason for chemists in the second half of the 19th century to suppose that simple dialkyltins should not be stable. Therefore they attempted to isolate a series of R2Sn compounds bearing the same relationship to the tetraalkyltins as stannous compounds have to stannic compounds. That their search was destined to he unsuccessful could only be estahlished by experiment, and in the course of experimental studies of this point, there were a number of erroneous renorts of such com~ounds.Thev included dieth.--.---yltin (20) and diphenyltin (21i though these are now known to he polymers of tin(IV), that is, (EtzSn), and (PhzSn), (4). The search for organotin(I1) species did, however, lead to an improvement in the indirect method of preparing tetraethyltin. In 1879 Frankland, by now installed as Professor at the Roval College of Chemistry as the successor to Hoffmann (l2), assist& by Auhrey ~awrance,decided to apply Buckton's method to the problem (22). Accordingly, theistudied the reaction between stannous chloride and diethylzinc, hoping by analogy with results ohtained by Buckton simply to displace the chlorides with ethyl groups. The product, however, was not diethyltin, EtnSn, hut tetraethvltin. " . E t.S n . As a route to tetraethvltin. this reaction proved superior to Buckton's originalmethod, which used SnCl.. The reaction was much less viaorous and more readily cokrolled. Also, the inorganic tincompound employed was more pleasant to handle. This new reaction remained the method of choice for preparing tetraalkyltins until the early years of the 20th century, when Pope and Peachey first made use of the action of a Grignard reagent on stannic chloride (23). A~~
~~
~
+ Sn = Sn(C2H5),+ Zn
and ~SII(C,H,)~ = Sn + Sn(C2H5). This, they asserted, explained why the reaction yielded about 60%of organotin compound in all cases. The assertion, though, is not tenable, since it ignores the possibility of the spent tin being re.alkylated and going through the cycle repeatedly until almost all was converted to organotin compound. Nonetheless, here were two chemists, as long ago as 1886. sneculatine about the nrecise stens that occur during the prdcess of &mica1 chan'ge. The work was almost certainly carried out at Queen's College (now University), Belfast, between 1880 and 1882. These were the years that Collie was at Belfast in a junior capacity, and Letts was already there as the first person to hold the post of professor of chemistry. The former, though the junior author in this work, subsequently had the more distinguished career, culminating in his holding the Chair of Organic Chemistry at University College, London, from 1902 until 1928 (9).
-
~
Developments In Dlrect Synthesis There was a further significant improvement in the methods for the preparation of tetraalkyltins as a result of some astute detective work of two workers, E. A. Letts and J. N. Collie. Their paper (24). which describes what began as a routine attempt to prepare diethylzinc, also gives an insight into the conditions under which chemistry had to be done in the late 19th centuw. Having set out &make diethylzinc, they were amazed to discover that themain product turnedout to bean air-stable pale yellow liquid, wh&h combustion analysis proved to be tetraethyltin. As they wrote ". . .we scarcely anticipated that the commercial zinc we employed would contain any
622
Zn(C,H,),
EhHg + ZnC1,
As a resuk Buckton's work, by the end the first decade of organotin chemistry a number of different Cornpounds had been made using both direct and indirect routes, and they had been instrumental in stimulating a crucial theoretical advance, namely the development of the doctrine of valency.
~
2Zn(C2HdI+ Sn = Sn(C,H& + Zn12+ Zn or
Journal of Chemical Education
Conclusions This survey has described the first 35 or so years of organotin chemistry, during which organotin compounds of varying degrees of substitution were identified, and a numher of synthetic routes discovered and exploited. In addition, these compounds provided an enrichment to theoretical chemistry in two ways. First, they were partly responsible for leading Frankland to his conclusions concerning.the concept of va'iency. Second, by their very existence, theycontrihuted to breakingdown the barriers that had beenerected in the first half of t h e 19th century between organic and inorganic chemistry. For, as Colin Russell has written, "the very existence of organometallic compounds testified to the arsficiality of separating organic and inorganic chemistry, for if ever hybrids were to be found, here they surely were" (26).
Acknowledgment I am grateful to my colleague, A. J. Dunsdon, for help with the translation of L6wig's paper (7)from the original German. Literature cned 1. Blunden, S. J.:C-ck, P. A,: Hill, R. The Indvarriol Uses of Tin Chsmicala:Royal SacictyofChemistry: London, 1985. 2. Gynane. M . J. S.: Lapp*, M. F.; Miles, S. J.: P o w . P. P. J. Chern. Sac.. Ckm.
Cornmun.1916,256. M. J. S.; Lapp*, M. F.; Miles. S. J.: P o w . P. P. J. Chem. Sac.. Chem. Commun. 1918,192. 4. Neumann, W. P. The Organic Ckmistryaf Tin: Wiley: London, 1970. 5. van dar Kark, G.J. M.ACS Advoncea in Chemistry 1916.157,l. 6. Murphy, J. S.; Poller. R C.J Olganomt. Cham. Lib. 1980,9,189. 7. L6wig. C. A n ~ k 1852. n 84.308. 8. hankland.E.Phi1. Trans. 1852,142,418. 9. P~utington.J. R.A Hlatoryo(Chemrrtry:Maoniuao: London, 19%Vol4. 10. Frsnklsnd, E. A n ~ l e n1853.85.329. 11. Frankland, E. J. Chem. Soc. 1854.6,51. 12. Russell, C.A. Chom. Brit. 1982,18, 737. 13. Nicholson, J. W.: DouekJ, A.; Co1lins.J.D.J. O l g o ~ m fC.hem. 198,233,113. 14. Nicholson, J. W., unpublished resulfa. 2. Gynane.
15. Buckton.G.B.Phi1. h n s . 1659,426. :
16. "George Boadler Buckton, (1818-1905)': Obituary notie. Tronr.Chsm. Sac. 1907. 663. 17. FranWend, E. A n n n h 1849,72,171. 18. Greenwwd, N. N.;Earnahaw, A. The Chambtryaf CheElemnta; Pqamon: Oxford, 19M. 19. Bucktan, G. B. J. Chem. Sor. 1861.14 17. 20. Hiondahl, M. Compt.rend. 1879.88.5840: J. Chem. Soe. 1879.A.518.
21. 22. 23. 24. 25. 26.
Aronheim, B . A n ~ l e n1879,194,145; J. Chem. Soc. 1879, A, 249. Frankland,E.:Lamance,A. Tmw. Chem.Soe. 1879,130. Pope W. J.; Poachey, S. J. Roc. Chem. Soc. 1903,290. l r t t a E . A: Collie,J. N. Phil. Mog. l886,22,41. Perkin, W. H. J , Chem.Soc. 1896.69. Russell. C . A. Amb* 1987,24(3), 169.
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