Overlooked Opportunities in Stereochemistry Part 11. The Neglected Connection between Metal-Ammines (Alfred Werner) and Organic Onium Compounds (William Jackson Pope) George B. Kauffman California State University, Fresno, Fresno, CA 93740 Ivan Bernal University of Houston, Houston. TX 77004 One of the intriguing questions in the history of coordination chemistry is why i t took so long for Alfred Werner (18661919) ( I , 2) and his students to resolve coordination compounds, thus proving the correctness hoth of his concept of the octahedral configuration for cohalt(II1) and, more eenerallv. ..of his coordination theorv (3). In view of Werner's expertise in organic stereochemistry, especially that of nitroeen comnounds. and his documented familiaritv with the work of i i r ~ i l i i a mJackson Pope (1870-1939) 14-6), i t is surprising that he did not immediately avail himself of the new and versatile resolving agents introduced by that famous English stereochemist. Similarly, i t is surprising that Pope did not make use of Werner's theoretical views to interpret the configurations of the unusual compounds that he had synthesized and resolved for the first time. In this article we shall examine the contrihutions of hoth chemists t o the field of stereochemistry and speculate as to the reasons why they failed to use each other's work. In doing so we mav shed some light on the relations between scientists of different nationalities working in the same area and on the blind suots that thev sometimes exhihit in dealine with and evaluaiing the work; of rivals.
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Werner's Interest In Organic Chemistry Alfred Werner is, of course, best known as the founder of coordination chemistry and the great systematizer of structural inorganic chemistry. Yet many chemists fail to realize that he was trained primarily as an organic chemist at the Eidgenossisches Polytechnikum (now the Eidgenossische Technische Hochschule, ETH) in Zarich. Despite his later preoccupation with coordination chemistry, his interest in organic chemistry continued throughout the 30 years of his scientific career (7). He made important contrihutions to theoretical and experimental knowledge of oximes, hydroxamic and hydroximic acids, phenanthrenes, oxonium salts, dyestuffs, and optical activity. Of his first 30 publications (1890-1896). organic articles outnumber inorganic ones by
two to one. Of his 174 publications, 45-roughly one-quarter-deal with organic topics, and of the more than 200 doctoral dissertations sunervised hv him. over 60 deal with organic chemistry (8). Both Werner's doctoral disseratiou (1890) and his Habilitationsschrift (1891)' dealt exclusively with organic compounds. In the latter, titled "Beitrage zur Theorie der Affinitat und Valenz" (9),Werner attempted to replace Keknlgs concept of rigid, directed valencies with a more flexihle approach in which he viewed affinity as "an attractive force acting equally from the center of the atom toward all parts of its soherical surface." Bv such simule reasonine and without introducing any ad hoc hypotheses such as directed valence, Werner loeicallv deduced and exnlained the confieurations of organic molecules deduced by van't Hoff, ~ i s l i c e n u sand , others hv more arbitraw. areuments. He also a n ~ l i e dhis .. theory of nondirected valence to the benzene mc,i&le. He accounted not onlv tor the Crum Brown-Gihsun substitution rules hut also for the previously unexplained 1.4- addition to coniuaated double bonds. This phenomenon was later explained more satisfactorily by ~ h i ~ l etheory 's of partial valencies. This early also contained the seeds that ~-paper later were to flower in the primary valence (Hauptualenz) and secondary valence (Nebenualenz) of the coordination theory.
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For Part 1 of this series see Bernal, I.: Kauffrnan. G. B. J. Chem. Educ. 1987, 64, 604. An abbreviated version of the present article was presented at the Symposium on Historical and Educational Aspects of Coordination Chemistry, XXV International Conference on CoordinationChemistry.Nanjing, People's Republic of China, July 28, 1987. A Privat-Dorentur(unsaiaried lectureship) required the venia le-
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gendior venia docendi, the privilege of lecturing at a university, which was awarded only upon acceptance by the faculty of a Habilite tionsschrift, a paper embodying the results of original and independent research.
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Werner and Hantzsch's Theory According toHantzsch, this first correct view of the stereochemistry of nitrogen was ( l l a , p 30; l l b , p 165) ementially the intellectual property of Herr Werner. Only he himself clearly grasped the basic idea with its most important consequences when others only vaguely expressed the thought that perhaps nitrogen could also give rise to geometric isomerism, in a manner similar to carbon. In contrast t o the more conventional views of their predecessors, Werner and Hantzsch attributed the stereoisomerism not to the carbon atom hut to the nitrogen atom. After critically reviewing the theoretical and experimental research on the subject, they state ( l l a , p 17; l l b , p 159): The three valences of the triuolent nitrogen atom (perhaps the valences of other multivalent atoms also) do not always lie in a plane with the nitrogen atom itself.
Werner's first hook, Lehrbuch der Stereochemie (10) (1904), is predominantly an organic text. He contemplated writing additional organic texw. In his notebook of 1909 we find the entry "2 Bucher schreiben: Die L'mlagerungsreaktionen in der organischen Chemie; Die Cartumium- und Oxoniumverhinduneen" .(Write two books: Rearraneement Heactions in Organic Chemistry; Carbonium and Oxonium Compounds). Werner was called t o the Universitat Ziirich in 1893 to teach organic chemistry. I t was not until the winter semester 1902103 that he was assigned the main lecture course in inorganic chemistry, which he continued to teach along with orzanic chemistrv throughout the remainder of his career. Organic Trivalent Nitrogen Compounds Werner's first publication, "Uber raumliche Anordnung der Atome in stickstoffhaltiwn Molekiilen" (On the Snatial Arrangement of Atoms in h o g e n - ~ o n t a i n h g~ o l e c u l e s ) ( 1 0 , based upon his doctoral dissertation, remains his most widely read and important organic paper. With this paper, coauthored with his mentor Arthur Hantzsch (1857-1935),2 one of the most important organic chemists of the time (12) (Fig. I ) , the 23-year-old Werner entered into one of the most exciting disputes in the history of organic chemistry. The case centered on the oximes (13)and constitutes the classic example of geometric isomerism among trivalent nitrogen compounds (Fig. 2). In this controversy Werner locked horns with Victor Meyer, who had coined the word "Stereochemie". Thus Werner played a central role in the elucidation of the stereochemistrv of nitroeen iust as he later did in that of tripositive cobalt.
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Despite Hantzsch's more than 500 publications, his greatest discovery was probably Alfred Werner, who was not only his most outstanding pupil but also his life-long friend. According to Werner's former student, colleague, and "chiefof-staff. Paul Pfeiffer,Werner clearly surpassed his teacher in originality and creative talent, a fact which Hantzsch readily admitted (Pfeiffer.P. Mein Lebenslauf; Bonn. 1947; unpublished typescript; p 23; courtesy of Fraulein Crescentia Roder, Bonn, Bundesrepublik Deutschland). According to Ernst Berl 1.1 Educ. 1942. ~-~Chem - -. 19.~. 1531. ~ ~Hantzsch . . at first did not acceot Werner s proposal of a tetraheoral config~rationfor n trogen. out on the following oay he told Werner that ne believed his idea lo be correct. According to modern oribital theory, which limits the elements of thesecond period toacovalency of four. "quinquevalent" nitrogen is, of course, impossible, but to evaluate the work of Werner and others in the light of modern knowledge is to belittle their achievements. Hindsight is always easier than foresight, and we must preserve a sense of historical perspective. ~
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They then proceeded to the basic hypothesis of the paper, a direct transfer of van't Hoff and Le Bel's spatial concepts to the nitrogen atom ( l l a , p 18; l l b , p 159): In certain compounds, the three valences of the nitrogen atom are directed toward the corners of a (inany case irregular) tetrahedron whose fourth corner is occupied by the nitrogen atom itself. On the whole, Werner and Hantzsch developed a surprisingly simple explanation for the isomerism of aldoximes, ketoximes, diazo compounds, hydroxamic acids, and other nitrogen derivatives. Their views later proved compatible with the early electronic theory of valence and have required only slight modification during almost a century. Today, with only slight modification, the theory has assumed its rightful place alongside the Le Bel and van't Hoff concept of the tetrahedral carbon atom as one of the foundations of stereochemistry. "Quinquevalent" Nitrogen Compounds The stereochemistrv of what was then called auinaueva. . lent (liinfwertig)' n i t k e n and is now known as tetrarovalent nitrogen (14.15) was an area of research to which both Werner &d Pope made valuahle contributions yet apparently did not make use of each other's results. Metal-ammonia salts (now called metal-ammines) are prototypical coordination compounds and the ones most extensivelv investieated hv Werner. His first . Daoer . on the coordination theory, "Heitrag zur Konstitution anorganischer Verbindungen" (1893),openswith thestatement 116): By metal-monia salts are meant compounds which result from metal salts by the insertion of ammonia molecules into their
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Flgun, 2. Alfred Werner (1866-1919). work In hi6
laboratory.
The only known photograph of him at
molecules, or hetter yet: metd-ammonia salts are compounds which are formed from ammonia and metal salts according to the same reaction by which ammonium chloride is farmed from hydrochloric acid (the halide salt of hydrogen) and ammonia. Just as Werner first approached metal-ammines in terms of their analogy with the hetter known ammonium salts, he later (1903) applied his coordination theory t o the ammonium salts in his paper "Die Ammoniumsalze als einfachste Metallammoniake" (17). Thus ammonium salts, like metalammines. occunied a central ~ o s i t i o nin Werner's work. Three contending constitutional formulas (shown here for ammonium chloride) were orooosed: 1. the molecular formula (MolekiLlformel) defended by ~ u & s tKekuli. (1864) and adherents to his principle of constant valence (18), 2, the valence formula (Valenzformel) proposed by Edward Frankland (1852) (IS), and 3, the auxiliary valence formula (Nebenualenzformel) first suggested by Werner (1893) (17) and universally accepted today:
1
Constant valence
Variable valence
Auxiliary valence
The main reason for considering ammonium salts'as valence compounds (2) rather than molecular compounds (1)-namely, the direct bonding of all alkyl radicals to nitrogen in tetraalkylammonium salts-is also consistent with their formulation as auxiliary valence compounds (3). The Kekuli. constitution of ammonium salts as addition compounds of trivalent nitrogen (1) was therefore effectively eliminated in favor of the other two formulations. For the quinquevalent formula, three configurational models (4, 5, and 6) had been proposed (Fig. 3). The last proposed model for ammonium salts, Werner's tetrahedron (71, differs from the three previous models not only in structure but also in constitution. The cubic (19), trigonal bipyramidal(20), and tetragonal pyramidal (21) models all assume five valence bonds; Werner's tetrahedral model (16) assumes only four, all of them equivalent.
The nroof for the tetrahedral confieuration (7) of the nitroge;l atom in ammonium salts and sikilar com&unds as well as the exclusion of the other three theoretically possible configurations (cubic, 4; trigonal bipyramidal, 5; and tetragonal pyramidal, 6) came from the same three main lines of evidence as were used to prove Werner's coordination theory and the octahedral configuration of cobalt(III), viz., (1) chemical evidence such as "isomer counting" and transformation reactions (23); (2) resolution of selected compounds (23); and (3) X-ray diffraction studies. The first techniquecomparing the number and type of known isomers with theoretical predictions for various configurations--is prohably most familiar through Wilhelm KBrner's work on disubstituted and trisubstituted benzene derivatives (24). In the case of "quinquevalent" nitrogen compounds the numbers and types of isomers prepared for various compounds were less than the numbers and types predicted for configurations 4,5, and 6 (Table By the 1920's, after a clear distinction had been made between covalence and electrovalence and after it had been recognized that many strong electrolytes are ionized even in the solid state, it was generally ameed, in accordance with Werner's suggestion, that nitrogen could form a t most four bonds. The controversy over the configuration of ammonium salts thus became a struggle for supremacy between the tetragonal pyramid deprived of its fifth valence bond (atom X in 6) and Werner's tetrahedral model (7). Werner postulated a complete analogy between ammonium compounds and methane derivatives, a view that excluded geometric as well as optical isomerism among the compounds [N&]X, [NA%B]X,[NAzBzIX, and [NAzBCIX and is in exact agreement with the experimental facts. Compounds of the type [NABCDIX, however, contain what would be called, in exact analogy to carbon compounds, an asymmetric nitrogen atom. However, with very few exceptions the successful resolutions of such compounds, while providing strong evidence in favor of a tetrahedral configuration, are also explainable hv the tetraeonal ovramidal model.5 Thus.. desoiG , . the numerous resolutions of ammonium derivatives accomplished durine his lifetime. Werner did not live to see the definitive triumph of his views. The first eenerallv a c c e ~ t e dresolution of a substituted ammonium compound wa; accomplished by William Jackson Pope, who, by use of the silver salt of d-bromocamohorsulfonic acid, succeeded in resolving methylallylb~nzylphenylammonium iodide (25). This was soon followed by numerous similar resolutions (26). Definitive evidence f(ir the tetrahedral configuration to the exclusion of the retragonal nvramidal modelwas not forthcomine until 1925. when ~ i l i s - a n dWarren (27) resolved 4-phenyl'14'-carbeth~xybispiperidinium-l,l'-spirane bromide:
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~
~
Figure 3. Configurational models of ammonium salts.
Type of compound
PTBdicted number of stereoisomers Trigonal Tetragonal Cube bipyramld pyramids
AIBNX
2 geometric
2 geometric
t form
AzBflX
2 geometric
2 geometric
2 geometric
AIBCNX
2 geometric I enamiomeric
2 geometric
2 geometric
ABCDNX
pair 4 enamiomeri~ #?airs
I enantiomeric
pair 4 enantiomeric oairs
3 enantiomeric mirs
If the nitrogen atom in this compound possesses a pyramidal configuration, the cation would exist in cis and trans isomeric forms, each of which would have a plane of symmetry containing the central nitrogen atom and the four termi-
'For details on the results of "isomer wunting" see refs 14 and 15.
The common misconception that resolution of a compound of type MABCD proves that M has a tetrahedral configuration is found in a number of books, e.g., Sidgwick, N. V. The ChemicalElements and Their Compounds; Clarendon: Oxford, 1950: Vol. I, p 801: as well as many modern organic chemistry texts. See Kauffman. G. B. J. Chem. Educ. 1983,60,402. Volume 66
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that each nitroeen atom was surrounded tetrahedrallv hv four methyl o r e t h y l groups (30). Werner's prediction 02 almost 40 vears earlier had a t last received its most direct confirmation. Pope's Stereochemical Research
Figure 4. Sir William Jackson Pope (1870-1939).
nal groups and hence would not he resolvahle. On the other hand, if the nitrogen atom is tetrahedral, geometric isomerism would be impossihle, hut the cation would he asymmetric and therefore resolvahle. Two years later Mills provided an independent proof for the tetrahedral configuration (28): Together with J. D. Parkin and W. J. V. ward, he synthesizedcis and trans isomers of a numher of 4-hydroxy-4-phenyl-piperidiuium salts of the type
If the nitrogen atom possessed a pyramidal configuration, geometric isomers should exist even if H'and H" wrre identical, and, if R' and R" were different, each geometric isomer should be resolvahle. If nitrogen possessed a tetrahedral configuration, on the other hand, geometric isomers would he found only when R' and R" were different, and neither isomer would be resolvahle. Mills prepared five salts of the type shown above. Three of them with R' different from R" were separated into cis and trans isomers; the remaining two in which R' and R" were identical could not he so separated-a neeative t . w.e of proof. t o he sure. but one stronelv .. . aupporti& the tetrahedial coniiguratiun.. In 1921 the iinal and unassailable proof of Werner's tetrahedral configuration for tetracovaledt nitrogen was obtained hv the third line of evidence-X-ray diffraction-with wyckoff and Posnjak's determination of the crystal structure of ammonium hexachloroplatinate(IV), (NH4)2[PtCl6] (29). In 1928 Wyckoff examined the crystal structure of tetramethyl- and tetraethylammonium halides and found 296
Journal of Chemical Education
William Jackson Pope (1870-1939) (4-6) (Fig. 4) attended Finshury Technical College and the Central Institution (now the City and Guilds' College of the Imperial College of Science and Technology), hoth in London, where he was one of Henry Edward Armstrong's (1848-1937) earliest students. Armstrong believed that the teaching of chemistry should follow the order of original discovery (the so-called heuristic method). Since he forbade his students to take anv examinations, they, Pope among them, ultimately departed without their demees. Pone studied crvstalloera~hvunder Henry ~lexande;Miers (i858-1942) and, while kiil a student. began his own crvstalloera~hicinvestieations of oreanic compounds. He coliaboraced h i r h ~ r m s c o n g ' sassisiant, Frederic Stanley Kippina (l86:l-1949). on important studies on camphor &d the c o k i t u t i o n and charicterization of externally compensated compounds. Pope was successively Head of the Chemistry Department a t the Goldsmiths' Company, New Cross, London, and Lecturer on Crvstalloeranhv a t the Central Institution (18971901), ~ r o f i s s o r2 hemis is try and Head of the chemistry Department a t the Municinal School of Technoloev. Mancheswr (1901-1908). and professor of chemistry'% Cambridge Universitv (1908-1939). When theSchoolofTechnology became the center of the Faculty of Technology a t the University of Manchester in 1908, Pope received his first university degree, six years alter his elertion to fellowship in the Hoyal Society. His work during World War I, especially that on the manufacture of photographic sensitizers and of mustard gas, was recognized in 1919 by the award of a Knighthood of the Order of the British Empire (K.B.E.). Pope received many honors, was a memher and officer of a numher of British chemical societies. and was a memher of many foreign scientific societies. He worked for several years t o promote the formation of the International Union of Pure and Applied Chemistry, of which he became the first president (1922-1925). Of Pope's numerous contributions to the resolution of optically active isomers only those directly relevant to Werner's work will be considered. Camphor Derivatives. In 1891, as Pope began his scientific career, Marsh and Cousins prepared sulfonic acid derivatives of chloro- and b r o m ~ c & ~ h o hy r the action of chlorosulfonic acid (31).Armstrong, who had long been interested in the action of sulfuric acid on camphor, asked Pope to repeat Marsh and Cousins's work. Pope's first attempts were not successful. However, in what wasthe beginning of a most fruitful collaboration with Kipping he succeeded in preparine the desired derivatives in a nure state. Although camp ( ~ cannot ~ be sulfonated by ordinary concentrated-sulfuric acid (32), in 1893 Kipping and Pope confirmed Marsh and Cousins's work and found that camphor can he sulfonated by fuming sulfuric acid as well ashy chlorosulfonic acid (33). They also found that the chlorosulfonates, hromosulfonates, and their derivatives crystallized exceptionally well. A further study of these compounds reveaied relationships that Kipping and Pope called pseudoracemism (34). One of Pope's first investigations a t the Goldsmiths' Institute was the attempted resolution of what was called "tetrahvdronanaverine"6. Natural or d-tartaric acid. the onlv acid hkheko'employed {or resolving externally 'compensated bases, had proven ineffective since i t yielded a partial racemate with "tetrahydropapaverine". Pope recalled that the camphorsulfonic acids, which he had investigated earlier This compound was later found to be dlhydropapaverine (Fyman. F. L. J. Chem. SOC.1909, 95, 1610).
with Kipping (33), were strong acids, forming stable salts even with weak bases. that thev were monoorotic and thus free of the disadvantages connected with the diproticity of tartaric acid. and that their salts crvstallized extremelv well (56, p 703). Thus, together with s t h e y John Peache; (35), he easilv resolved "tetrahvdrooaoaverine" with d-ol-bromocamphor-T-sulfonic acid imodein (+)-3-hromocamphor-9sulfonic acid) (36) or less convenientlv with (+)-3-chlorocamphor-9-sulfonic acid. According to Werner (10, p 68) and to his American Ihktorond Virtor L. Kina (18861938), this seems to be the earliest resolution using bromocamphorsulfonic acid (37, pp 13-30). Pone ouicklv followed this work with resolutions of several otier basi/compounds and dissymmetric cations using camphorsulfonates and bromocamphorsulfonates (38). He also improved resolution techniques, e.g., by using nonionizine solvents to diminish the risk of racemization. Later, a t cambridge University, with John Read, who had been a Doktorand with Werner a t Ziirich from 1905 to 1907; he began the investigation of asymmetric compounds of simple structure in order to determine the degree of molecular asymmetry needed for optical activity. Before 1914 no one had synthesized and resolved a compound having only one carbon atom, which must necessarilybeasymmetrir. In that year Pope and Read synthesized and resolved chloroiodoðan&ulfonic acid (39),
Chiral Centers Other Than Carbon (14, Chapter 10). Until 1899 compounds exhibiting optical activity all contained one or more asymmetric carhon atoms, and it was widely believed that the nresence of an asvmmetric carhon atom was necessary for optical activity. After recognizing the suoerioritv of the cam~horsulfonatesand bromocamohorsulfonates over other resolving agents, Pope "lost no time in applying them to one of the outstanding problems of the time-the resolution of asymmetrically substituted ammonium salts" (5a, p 260). As we have seen, in 1899Pope and Peachey (25) succeeded in resolving methylallylbenzylphenylammonium iodide by the This ~ - use - ~ of silver (+l-3-bromocamohor-9-sulfonate. . , ~ ~ compound had first been prepared b y Wedekind (26) but found to be unresolvable. This resolution not onlv furnished evidence for Werner's postulated tetrahedral configuration for nitroeen. but for the first time ~rovidedan ooticallv active compound that owed its actiGity to an asymmetrfc atom other than carbon. This oroved that the valence honds of an element other than carbon had sufficient stability to eive rise to ootical activity in an asvmmetric confirmration. ~nfonunateiy,Pope inteipreted h b resolution i n ~ e r m sof "auinauevalent" nitrogen rather than Werner's tetrahedral nkrog;n, stating that % proved (25) ~~~
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Once he had shown that the carbon atom had no monopoly on optical activity, Pope followed his resolution of an asymmetric nitrogen compound with resolutions of asymmetric compounds of other elements. The following year he and Peachey resolved an asymmetric compound of sulfur, methylethylthetine bromide,
which they formulated as
with (+)-3-bromocamphor-9-sulfonate(40). Although Pope and Peachey are generally given credit for the first resolution of an optically active sulfonium compound, according to Derek Davenport (41), as reported by Ramsav -~~~ ~ (14. . ...o 244). the credit riehtfullv belongs to Samuel resolved a Smiles (1877-1953); who earlier in the same similar compound, methylethylphenacylthetine bromide.
using the same resolving agent (42). Frederick George Mann of Cambridge University, who was Pope's student. told Joseph Chatt of the University of Sussex that (43) Pope wss writing up his paper when Smiles's paper. . .was sent to him to referee. He sent in his paper and delayed Smiles's, so getting his published first.. . . Dates of receipt of papers by the Chemical Society were only introduced in 1915. Whenever Smiles spoke of optically active sulfonium compounds, he would state, in a unique example of a stereochemical spoonerism (14, p 244; 42). "These were discovered by Peep and Poachey-I beg your pardon-by Pope and Peachey." Apparently, Pope was not always the English gentleman. That same year Pope and Peachey resolved an asymmetric tin compound, methylethyl-n-propyltin(1V)iodide,
that quaternary ammonium derivatives in which the five substituringgroups are different,contain an asymmetricnitrogen atom which gives rise u, antipodal relatiunshipr of the same kind as those correlated with an asymmetric carbon atom. This discovery attracted wide attention. It was immediately recognized as a stereochemical contribution of the first magnitude Read was Werner's IOlst Doktorand. As we have emphasized in this article, Werner was involved in organic research throughout his career: 63 of his 214 Doktoranden worked on organic problems. According to Read, "my thesis, entitled Untersuchungen in der Ccmar- und Cumarinsburereihe("Researches in the Coumaric and Coumarinic Acid Series"), was one of an organic-stereochemicalnature, without any bearing on the Coordination Theory" (Read, J. Humour andHumanism in Chemistry; Bell: London, 1947; p 281).
which they formulated (44) as
and in 1902Pope and A. H. D. Neville resolved an asymmetric selenium compound, methylphenylselenetine bromide, Volume 66 Number 4
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readily and became proficient in bothGerman and French in his teens (5b, p 698; 6, p 87), and "his fluent French and German oroved invaluable in later vears and sineled him out to represent [England] with significant success at international conferences" (56, p 698). Charles S. Gihson reported that "Pope was a most graceful speaker in English, French, German and Italian,"and heomany times heard Pope translate the spoken word directly from French to German" (46, p 325). At about the time that he had recognized the advantages of the hromocnmphorsulfnnates as resolving agents, Pope translated Andreas Ludwig Fock's Einleilung in die chemische Kystallographie (Engelmann: Leipzig. 1888) into English (48). Also, although there seems to have been some mutual rivalry and jealousy between Hritish and continental chemists,'" Pope was internationally oriented and became the first president of the ILl'AC. Anv possible rivalrs did not prev'nt him from collaborating wLhthe German chemist Otto Wallach on a classic paper, published simultaneously in Englishand ~ e r m a n , o ~ t redo~ution he ofa centroasymmetric compound, 1-methylcyclohexrlidene-4-aceticacid, which contains no f o r m a ~ l ~ a s ~ m e atom t r i c (49).11 Even if Pope had not been fluent in German, most of Werner's articles were almost immediately abstracted in the Journal of the Chemical Society (London), and a few, including his hook Neuere Anschauuneen auf dem Gebiete der anorgonischen Chemie (SO),were made available in Endish translation (51). Bv the closine vears of the 19th centuWerner was i e l i kn& in ~ n & n d (32) and attracted nraduate students from that c o u n t r ~ . Direct '~ evidence of Pope's acquaintance with Werner's work and of the manner in which i t was then viewed by British chemists is provided hy John Read. Read returned to England in the Autumn of 1907 after receiving his doctorate under Werner and began a fruitful eight-year collaboration with Pope (footnote 7, p 284):
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which they formulated (45) as
in both cases through the precipitation of the (+)-3-hromocamphor-9-sulfonaks. Wkh charlea Stanley Gihson, Pope later (1912) unsuccessfully attempted to resolve asymmetric phosphoniumcompounds and with'I'. F. WinmilLasymmetric arsonium compounds. According to Gihson (4b, p 307), The experimentalproofs that nitrogen, sulphur, selenium and tin could, like carhan, be centres of asymmetry and give rise to optically active compounds constituted the greatest advance in stereochemistry since the work of Pasteur, van't Hoff and Le Bel. However, as in his interpretation of "quinquevalent" nitrogen compounds, Pope was incorrect in his assignment of constitution to the optically active compounds of tetravalent sulfur and selenium, which he regarded as formally analogous to compounds of asymmetric tetravalent carbon. Yet, even a t a time when the distinction between what we now call electrovalence and covalence was imperfectly understood, when the role of electron pairs and empty orbitals in bonding was unknown, and before W. H. and W. L. Bragg had used X-ray diffraction to demonstrate the existence of ions in the solid state, Pope pointed out that the compounds in question were salts, which ionized in solution, and that in the optically active ions only three groups were bonded t o the asymmetric atom (5a, p 260; 46). The Neglected Connection We have shown that the results of Pope's research should have been of great interest to Werner. We shall now briefly examine evidence for each stereochemist's awareness of the other's work. Two coordination chemists who have been active in the field since the 1930's, K. A. Jensen of the University of Copenhagen and Joseph Chatt, who was a student a t Cambridge University during the latter years of Pope's tenure there. are fairlv certain that there was no direct contact between Pope a n d Werner. They base their supposition largelv on the fact that Pope was a "classical ornanic chemiit"(Jensen), while " ~ e r n e rwas an inorganiE chemist, and .. the gulf between the disciplines was wider in the early part of this century even than i t is now" (Chatt). Also, Chatt's research supervisor, Frederick G. Mann, never mentioned any such contact. An examination of Werner's correspondence for the period October 8,1896, to April 6,1909, (47) reveals a few letters to English chemists hut none to Pope. Among the few surviving letters of Pope's none t o Werner could he found.8 However, we have demonstrated that Werner was active in organic chemistry throughout his career and did not believe in the customary divisions hetween the different branches of chemistry, while much of Pope's research dealt with inorganic or organometallic compounds. Therefore, as productive, internationally known stereochemists conversant with the literature of both organic and inorganic chemistry, they were presumahly well acquainted with each other's work. Pooe's Awareness of Werner's Work. Althoueh most of ~ e r i e r ' swork appearkd in German, this shouli have presented no obstacle for Pope? who learned foreign languages
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ry
When he pope] had shaken hands he asked: "Well, and what do you think of Werner's Co-ordination Theory?" Evidently he was unable at that time to share to the full the opinion of an ardent diseiole: . . hut British chemists in eeneral were not disoosed to regard Werner'a v i e w wnh the attention they deserved until he schrewd the first resoiutimuf acwordinationcompound in 1911. For this information we wish to acknowledge the assistance of Miss S. P. Anderson. Assistant Keeper, The Royal Commission on
Historical Manuscripts, London: David Beasiey, Assistant Librarian, Goldsmiths' Hall. London; Prof. Sir Jack Lewis. F. R. S.. University Chemical Laboratory, Cambridge: Peter McNiven. Archivist. The John Rylands University Library of Manchester; A. E. 8. Owen, Keeper of Manuscripts. Cambridge University Library: Miss B. M. S. Rees, Senior Assistant Registrar (Establishments),Goldsmiths' College. Universitv of London: Dr. J. K. Sutherland. Professor of Oraanic Chemistrv. -
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That P@followed the German literature carefully and rapidly is shown by his resolution (ref2301 methylallyloenzylphenylammonlum iodide in tne same year that Wedekind first synthes~redit (Ber 1899, 32, 517). lo For example, a Brltish author argued that the Englishman William Hyde Woilaston should be credited with the discovery of isomorphism rather than the German Eilhard Mitscherlich (Tunon, A. E. H. Crystab lography and Practical Crystal Measurement; Macmlllan: London, 1972: .---. n 12211.. " ~ccordingto Gibson. "This resolution constituted the greatest advance in stereochemistry since Pope's own classical discoveries of the synthesis and resolution of compounds containing asymmetric atoms other than carbon. It may be described as the basis of all modem stereochemical investigation" (ref 4b. pp 314-315). '2 In addition to Edith Humphrey (dissertation, 1901),other British doctoral students who completed their dissertations under Werner (dates in parentheses) were: Frederick Beddow (1895,the first Doktorand to work on coordination compounds with Werner). Alfred Rudolph Klien (1897),Hermann Schwabacher (1901). James L. Klien (1902). William John Bowis (1905). Herbert Henstack (1906). John Read (1907). and Hugh Edmund Wans (1912): American students were Emii Grebe (1898), Walter Peters (1901),Jenny Kruh (1911), Victor L. King (1912),and Helen Somersby French (1913).
Ironically, the advantages of the resolving agent, (+)-3-hromocamphor-9-sulfonic acid, that would bring Werner this attention had heen pointed out by Kipping and Pope (33) 18 years earlier. British chemists finallv became convinced of the validitv of Werner's views, and, paradoxically, research in coordination chemistry outside of Zurich suffered (53, pp 120-121): In the years immediately following the First World War, interest in coordination chemistry in Britain had fallen to a law ebb. . . probably [because], apart from a very few chemists who almwt fanaticallyspurned Werner's work, most chemists accepted Werner's theories and conclusions as affording a very thorough and virtually complete interpretation of coordination compounds. According t o F. G. Mann, who had received his doctorate a t Cambridge University under Pope's supervision (53, p 121). [Pope] had been consideringvarious aspects of Werner's work and its possible extension [and] . ..suggested that I should synthesize 1,2,3-triaminopropane,the use of which as a coordinating agent tometals might open up a field comparableto that which ethylenediamine had provided in Werner's hands. Mann and Pone went on to oreoare and investieate metallic coordination 'compounds fdrmkd by this tridentate ligand 154)and otheraliohatic nolvamines (55). Mann later became the'second chemist (afier ~ e r n e r ) resolve a completely (Na[Rhinorganic coordination compound (HzO)Z((HN)~SO~)~]) into its optically active enantiomers (56). A year earlier, in the Sixth Messel Memorial Lecture, titled "Forty Years of Stereochemistry", Pope paid tribute t o Werner hoth for his coordination theory and for his postulation of a tetrahedral configuration of the nitrogen atom in oximes, which "have given a stereochemical aspect to the whole of chemistry" (57). Werner's Awareness of Pope's Work. Since lack of definitive oroofs for hoth his ammonium theom and his coordination'theory prevented their universal acceptance by the chemical community, i t is surprising that Werner did not make use of Pope's work. Werner's first paper devoted specificallv to ammonium salts (58) encountered difficulties in puhlictkion. Werner wrote to his former mentor Arthur Hantzsch on July 2,1902 (49): The publication of the ammonium theory, by the way, has a history which I do not wish to keep from you. I first sent the paper in much shorter form to Berichte, which returned it to me after some time with the note that it was "unsuitahle". . ..Therefore I sent the paper, which in the meantime had been expanded in many respects, to Annalen, hut after some time received from [the editor] Prof. [Jacob] Volhard the news that the paper could he accepted only if some experimental data were added. This led me to add the second part, the Oxonium Salts [59]. Thus I was finally successful in finding a place for the children of my chemical fantasv in one of the better means of oublication: at least thev did ~ not hate toshare the fate of m y "~eiiragelur Theorie der V und Affinitat [sir1 (91.How [hey will fare on their future life's journey, I cannot, of course, foresee; hut I fear that they shall travel a path just as wearisome as did the coordination theory. In response to Werner's postulation of a tetrahedral configuration for nitrogen (16a, p 329; 16b, p 87), an English chemist scornfully objected (60): The absurditv of the tetrahedron conceotian becomes more ,~ glaring when we pass from triad M pentad nitrogen compounds. The nitrogen atom is promptly taken away from the comer, and replaced in the middle of the tetrahedron; but as this still leaves one hydrogen too many, thrs hydrogen is crammed inta the middle to keep the nitrogen company. (x.N)obcd being considered a perfect analopue of Chbcd. Such . playing . . fast and loose with the atoms, and making them into a four-corneredfigure at any cost, is scarcely a scientific mode of dealing with the question. ~~
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Yet, today, Werner's remarkably modern viewpoint is recognized as an adumbration of what was later to become the
Z
Lewis octet theory and a precursor of Lewis's generalized theory of acids and hases. Werner was no narrowly sectarian inoreanic or o r ~ a n i cchemist: he viewed the formation of a fourth bond bythe nitrogen atom of the ammonia molecule by reaction with methvl iodide, hvdrochloric acid. or a cobalt s a t as essentially the same type of reaction. 1n modern terms, the unshared pair of electrons on the nitrogen atom forms a coordinate covalent bond. Lewis paid tribute t o Werner as follows (61): It seems orohable that the nitroeen atom is never attached to other atoms hy more than four honds. The new symmewiral formula for ammonium ion, and the corresponding formulae fur the substituted ammonium compounds, are in complete accord with the stereochemical and other properties of nitrogen compounds discussed by Werner. Werner's coordination theory initially fared no better than his ammonium theory and found little appreciation or acceotance hv chemists. esoeciallv those in Eneland. For exaAple, " ~ h t i s h~ b s t i a c t kconhensed ern& monumental 64-page paper (16) to eight lines, describing it merely as "a theoretical paper devoted to discussion of the so-called ammonio-metallic comoounds" (62). Even as late as the 1930's the English cheinist H. D. K. Drew attempted to prove that cis- and trans-[PtClz{(CzHs)zS)z] were structural rather than geometric isomers according to the discredited BlomstrandJ$rgensen chain theory (63). Although Werner was not proficient in English, he undoubtedly followed Pope's research, either in the original nuhlications or in German or French abstracts. In his Lehrbuch derstereochemie (1904) (101, hut not inany editions of his Neuere Anschauungen (64), he cited Pope's resolutions of various organic compounds (pp 68,81) and of asymmetric nitroeen (o 304). sulfur (oo ... 7.. 312313). selenium (D 314). and ;in (pp 7,'31&315) compounds i d interpretkd the constitution of the sulfur. selenium. and tin c o m ~ o u n d (DD s 316-317) in terms of exknsions of his ammonium the& (58). However, he made no reference to the fact that Pooe and Peachey's resolution of methylallylbenzylphenylatk monium iodide (2.5) was evidence in favor of his own tetrahedral configuration for nitrogen. In the previous paper in this series (52) we pointed out that Werner recoeuized that ootical resolution of asvmmetric coordination Ebmpounds Gould provide a n elegant and defmitive proof of his postulated octahedral configuration for cohalt(II1) and that he was actively seeking suitable candidates for resolution as early as February 1897 (65). Yet although he was aware of Pope's use of the (+)-3-hromocamohor-9-sulfonates, as evidenced bv his citation of the resoluiions mentioned in the last par&aph, and although he corresponded with John Read and followed his work while Read was working with Pope a t Cambridge (footnote 7, p 29% nevertheless, he did not use them until his classic resolution of cis-hromo- and chloroamminebis(ethylenediamine)cohalt(II1) salts in 1911 (37, 66). This work led to his being awarded the Nobel Prize in Chemistrv in 1913 (67). . . Beein.. ning in January 1910, King spent a year attempting to resolve lCoCO?en?lHr before switchine to ICoCltNHden~lCI~ (68). it was &p&ently he, rather t i a n werner, who ci;ose the successful resolving agent (69): I shall never forget the day that the o p t i d y active isomers were first attained. In connection with this work, I had been carrying out some 2000 fractional crystallizations and had heen studying Madame Curie's work on radium far that purpose. After having made these 2WOseparate fractional crystallizationswhich proved that the oooositeends of the svstem were oreciselv alike and that .. we had to do wmething more drastic, I proposed increasing the ] dissimilarity of the diavtomera laic1 by using brom [ s ~ rcamphor sulfonic acid as a salt-formingcondtituent having extremely high optical activity. When this was tried, the isomers in the form of these salts literally fell apart. Adozen yearsearlier, with his resolution of the first asymmetric nitrogen compound (25), Pope had forced the carbon Volume 66
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tmy through tho Time of Werner;:-audiotapeand book AmariesnChemiesl Society: Washington. DC, 19'77: Inorganic Cwrdimfion Compounds; Heyden: London,
atom to relinquish its apparent monopoly on optical isomerism. Now.,with the first resolution of an octahedral com~lex. . . accomplished with the aid of Pope's resolving agent, Werner had similarlv forced the tetrahedron to relinuuivh its aooar.. ent monopoly on optical isomerism. ~
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1981; pp 105-136,151-155. 2 4 Kamer, W. Gorr.chim. i t a l 1814.4.305; Obar dieBe9fimmungdoa chemiaehen Ortea bpi den ommorurhensubatnnren; Oatwald's Klasaiker der orakten Wissenschaften No. 17%Wilhelm Engelmsnn: Leipzig,1910.Theideaolanoetahedralmnfiguration and itng~metricconsequencesaithrespedto tho number of isomersexpeefed was considered in 1875 by J. H. uan't Hoff (MoondblndNoluurm. 1875.6,371. 25 Pope. W. J.;Peachey,S. J. J. Cham.Soc. 1899.75.1127. 26. Jones,H. 0. J. Chem. Soe. 1904,85,223:Thomas, M. B.: Jones, H. 0. J. Chem. Soc. I906,89, 280; Jones, H. 0.: Hill, J . R. J. Chem. Sor. 1908,93, 29% Wedckind, E.; FMh1ich.E. Bey 1905.36,6438: 1907.40, llW1;Frdhlieh, E.: Wedekind.E.Ber. 1907. 40,1646: Wedekind,B.: Wedekind.0. Bar. 19W,40,MSO; Wedekind, E.; Mayer, W. Ber. 1909.42.303: Everatt, R. W. J. Cham. See. 1908.93.1225. M~IIS.w. H.: warren, E. H.J. cham. sot. 1925. 127,2507; M~II=, W, H.; ~ a i n sL, , J. Chsm. Sor. 1925.127.2502. 28. Milla, W. H.; Psrkin, J. D.; Ward, W. J. V. J. Chom. Soe. 1927,2613. 29. Wyckoff, a. W. G.; Pasnjek. E. J. Am. Chem. Sor. 1921. 43. 2292: reprinted with annotations in Kauffman, C. B. Cloasics in Coordimtion Chami-fry,P.rf 3: Tmentirth-Century Papers (1W-1935): Dover: New York. 1978: pp8%111. 30 Wyckoff,R. W. C.Z. K&1.1928,67,91,550. 31. Marsh, J.E.:Cousins,H. H. J Chom.Soc. 1891,59,966. 32. Armstrong, H. E.: Kipping, F. S.J. Chem. Sac. 1893,63,75. 33. Kiooinz.F. S.:Pooe.W. J. J. Chem. Soc. 1893.63.548: Pmr.Chem. Soc. . . .1896.67.3W . . i a 9 4 h 1'63. ki. 34. Kimine. F. S.: Pow. W.J. J. Chom. Soc. 1897.71.956.962.989. 35. PO&, W. J.;peachey,S. J. J. Chem.Soc. 1898.73,893,902. 36. For methods of prepration of this compound superior to Kipping and Pope's o r i s i d
~~
Conclusion
We have shown how the work of each of two great stereochemists could have mutually enriched, reinforced, and accelerated the contributions of the other. Both men were familiar with each other's work but apparently did not make use of it for reasons unknown. Werner failed to recognize in Pope's (+)-3-bromocamphor-9-sulfonicacid the longsought resolving agent that would provide the key to resolving optically active coordination compounds needed to prove the octahedral configuration of cobalt(II1) and his in Pooe's coordination theom. He also failed to recoanize resolution of an asymmetric nitrogen compound strong evidence for his assignment of a tetrahedral configuration to the nitrogen atom. Pope likewise failed to recognize in Werner's views a simple explanation for the constitution and configuration of the unusual compounds that he was the first to resolve. Pope eventually accepted Werner's views, but by this time Werner had already died. Had they chosen to collaborate during the 1890's or even the first 10 or 15 years of the 20th century, they would have made a formidahle team. All that we can do is to lament with John Greenleaf Whittier (70):
n.
rich, 1912. 38. Pope. W. J. J Chem. Soe. 1893,75,1108: Pme. Chom. Sac. 1899,15,170: Pope, W. J.: Harvey,A. W. J. Chem. Soc. 1899.75,1110; 1901, 79, W,Pme. Chem. Soe. 19W, 16, 74: Pope, W. J.: Peaehey, S. J. J Chem Soc. 1899,75,1066. 39. Pope, W. J.;Read, J. J.Chom.Soc. 1914,105,811. 40. Pope, W. J.; Peechay,S. J. J. Cham. Soc. 19W. 77,1072. 41. Davenport, D. A. J. Chem. Edue. L981,58,682. 42. Smi1es.S. J. Chem.Soe. 19W.77.1174. 43. Chatt, Joseph, letter of January23.1985, t o h r g e B. Kauffman. 44. Pope, W. J.:Peachcy,S. J P m . Chem.Soe. 1900,16,42,116. 45. Pope, W. J.; Neville, A. J. Chem. Soc. 1902,81,1552. 46. Pope, W. J.; Harvey, A. W. J. Chsm. S o c 1901, 7 9 , 8 W 0 , p. S O . 47. This~ne.po"deneeiaprea.wdatUlehorga"iach-ChemiachesInstit"t,Univeraitst Zilrich. 48 F a k , A. An lntmduclon to ChemieolCry.tallagrophy; Pope. W. J., Tmnsl. and Ed.: Clarendon: Orford. 1895. 49. Perkin, W. H.;Pope, W. J.; Wallsch,O. J. Chem. Soc. 1909,95,1789:Ann, Chem. 1909.
Far of all sad words of tongue or pen, The saddest are these: "It might have been!"
Llterature,Cited 1. Kauffman. G. B. Alfred Wernec Founder of Caordiwtion Chemi*fry; Springer.Verh e : Berlin. 1966. .ry 0fS&"ti/ie Bbgmphy: GiU'ia~i..C.
Stareochemi~dar korboeykl>ehon Verbindungen; Kaiaerl. und Konigl. Hof-Buchdruekerei und Hof-Verlsga-Buchhandlung: Vienna. 1903. 9. The firat and theoretical part of this work was published in an o h r e journal ((a1 Werner, A. Vie,toljohrsachrift der Zicrcher Nofurf0,schendon Gsasllarhaft IWl. 36.129l.Por ansnnofafedEnelinh tranalationaee (bl Kauffman. G.B. Chymb 1967. ~
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..,.... 13. 14. 15. 16.
17. 18. 19. 20. 21.
Kauffman, G. B. Ambil LW2,19,129. Ramsay, 0. B. Stemoch~miafry:Heyden: London, 1981; pp 133-140. Keuffman, G. B. Isia 1974 64.78. (a) Werna,A. 2. onorb. Chem. 1893,3,267: (b) trsmhtedintoEnglmhraith~omme~taryas"Cont"butio"t0 thhC~~~tit"titi"tifIno~anieCompounda"i"Ka"ffmti",G. B. Classics in Caordinofion Chemlafrv. Part 1: The Selected Pomrs of Alfred WemecDmer: New York, 1966: pp 9-88. Werner, A.Bw 1903.36.147. Kauffman, G. B. J. Chem. Educ. 1972.49.813. vsn't H0ff.J. H.MaondbloduoorNotuunuefansehopppn 1877,7,1W:Ansichtonirber die orgonimhs Chomio; Vimeg: Braunschwek, 1876:p 80. Wilkerdt, C. J.prokf. Chrm., 1888,[2]37,449: 1890,12!41,291. Bureh,G. J.;Msmh, J.E. J. Chem.Soc. 18W55.656.
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11 Ris?hnff C b. Re, 1111071 ..... . .. .. . ...,. . , 1967. .....
23. For detaila on Werner.8 work to establish the configuration of c o b a l t - m i n e s ase KauNman. G. B. Coord. Chem.Rw 1974, I& 105; in Vm't Hoff-LeBd Cenlsnniol; Ramsay.0.B.. Ed.;American ChemieslSaeiety: Washington,DC. 1975;pp126142; laia 1975. 68. 38: Classics in Coordination Chemidrv. Port 2: Selected Pomm 11758-18&1;'~~er:Near York, 1976: pp 9 b 1 0 2 ~ ~ ~ ~ Chemistry: d i Ifa~ &~
300
Journal of Chemical Education
t
.,..-.... ,
68 King. \ I . ' N a c h w r ~tdpr Okuhcdcr Furmeh dcr Kubelt8nks Arhr8l fur P h D . Z ~ r k tISIL l I'J12"ppZ. 16 '1 h89 h~.b~em~~l,labnrnlnlyno!~bnuk 8s p r r a w e d m ih? Chrm#slryl a l n r a ~ mL t h r w a r llanmrmh Cdlesr. iron, u h r h Kmqrmavcd hl3 baehclor'.degree in 1907. 69. King.V. L. J.Chem. Edue. 1942,19,345. 3. G. "Maud Muller"; 1856:afanu, 53. i 70.~Whittier. ~
