Download Spontaneous Resolution of CiS-BIS (ethylendiamine) dinitrocobalt Salts | CHEM 4100 and more Lab Reports Inorganic Chemistry in PDF only on Docsity!
The Spontaneous Resolution
of cis-Bis(ethylenediamine)dinitrocobalt(lll]Salts
Alfred Werner's Overlooked Opportunity
Ivan Bernal University of Houston. Houston, TX 77004 George 6. Kauffman California State University. Fresno, CA 93740
Around the turn of the present century, when women in science were still rarities and "Women's Lib" was undreamt of, a young Englishwoman journeyed from her home in Lon- don to carry out research in Ziirich, the beautiful old city of Zwingli and one of the leading cultural, scientific, and intel- lectual centers of Europe. The woman, Edith Humphrey, had chosen to work on her doctorate in chemistry under a still comparatively unknown 34-year-old Ausserordenti- licher Professor named Alfred Werner (1866-1919) (Fig. 1) (1,2). Humphrey undoubtedly had been attracted by Wer- ner's growing international reputation, for the research fa-
cilities in the old huildine a t Ramistrasse 85 (Fie.. .. 2) were
woefully inadequate even as early as during Victor Meyer's trnure (1871-ldSj,(31. Werner's arowine UOUD of students was forced to workin what they aptly nicinamed the "Kata- komhen" (Catacombs) (Fig. 3)-unfinished cellars and stor- age rooms for wood, so poorly illuminated that artificial lighting was required even a t noon (4). Yet Helmholtz's dictum that "the best works come out of the worst lahorator- ies" may have some degree of validity, for i t was in the Catacombs that the major portion of Werner's life work was performed. Humphrey's role in the history of coordination chemistry is twofold. First, she was Werner's first female doctoral can- didate (Doktorandin) as well as his first female assistant.' ~ecund,shenssthefirstofhisstudents tosucceed in prepar- ing M'rrner's first new series uf geometrically isomeric cobalt col~~plexes (51, a class ot mmpounds that were crucial in the develo~menrand woof of his coordination theurv (2.6). It is this second aspect of her role and a recent ironic develop- ment in the spontaneous optical resolution of her com- pounds (7,8) with which we are concerned in this article.
Geometric isomerism and Proof of the Octahedral Configuration
The eenesis of Werner's coordination theorv- (9).. .,in which
the young 26-year-old Priuat-Dozent with virtually no expe- rience in inoreanic chemistrvawoke in 1892 one mornine a t 2 a.m. and wrote uninterruptedly until 5 p.m. the following day (I, lo), ranks with August KekulB's alleged dream of the
' Paradoxically, although it was not until 1981 that all Swiss can-
tons except Appenzell had granted women the right to vote in canton- al elections (they had been allowed to vote in federal elections in 1971). the liberal and progressive Universitat Zurich has the distinc- tion of being the first European university to admit women students (1840).During the 1870's the term "Zurcher Studentin" became a famous mark of distinction throughout the continent. Therefore it is not surprising that many of Werner's students were women. No biographical facts about Humphrey could be found,perhaps because most of Werner's students did not make any great contributions to chemistry after receiving their degrees, the cases of Paul Karrer, John Read, Yuji Shibata, and a few others notwithstanding. Furthermore, she probably married, resulting in a change in her last name.
Figure 1. Werner as an atom tamer (watercolorcaricature probably by one of his students, who apparently knew nothing of the mechanism of reactions).
Figure 2. The old chemical laboratory on the Ramistrasse
benzene ring as a classic case of the flash of genius (11). An unidentified "northern colleague" of Werner's had called the coordination theory "an ingenious impudence" (eine geniale Frechheit) (12), for a t the time of its inception it was
804 Journal of Chemical Education
Experiment 3 Reference 2
Figure 3. The "Catacombs"
largely without experimental verification. The data that Werner cited in support of his ideas had been obtained bv the painstaking labors of others, especially of the ~ a n i s h chemist Sophus Mads JBrgensen (1837-1914) (Fig. 4) (13). Within the next few years, together with his friend and former fellow student Arturo Miolati (1869-1956) (Fig. 5) (14). Werner uuhlished his first exnerimental work in suo- portof his new theory-several studies of electrical condu& ances of complexes intended to establish their constitution, i.e., the manner of bonding of the constituent atoms and groups (15-17). Werner attempted to determine the configuration (the spatial arrangement of these atoms and groups) of these complexes by the method of "isomer counting", a method considered as early as 1875 by Jacobus Henricus van't Hoff (18) and best known to most chemists throueh Wilhelm
number and type of isomers theoretically predicted for vari- ous configurations with the number and type of isomers actually prepared. By this means Werner was able to adduce strong evidence that the configuration of hexacoordinate cobalt(II1) is octahedral rather than any of the other sym- metrical oossibilities (hexaeonal ulanar. hexaeonal uvrami- dal, or trigonal 56, 99-100; 17rpp 165-121). Since the octahedral configuration for cobalt(1II) and other transition metal cations was an integral part of the coordina- tion theorv from its inceotion (9). oroof of this confieuration was one oI ~ e r n e r ' s goals. The geometric isomers cited by Werner on behalf of the octahedral configuration for cobalt(II1) had been prepared by others, viz., violeo (cis) and praseo (trans) [CoC12en2]CI (en = ethylenediamine) by JBrgensen (20) and flavo (cis) and croceo (trans) [CO(NOZ)Z(NH~)~]Xby JBrgensen (21) and Gibbs (22).. ., resuectivelv. ., (23).. In 1901 Humohrev. , , , (5.24) , first described two new geometrically stereoisomeric series of cobalt(II1) salts, [ C O ( N O ~ ) ~ ~ ~ ~ ] X(X = NOz, NO3, C1, Br, I, 'IzPtCL, and SPtCls; also AuC14 (cis only)),both yellowish brown and comoletelv analoeous in constitution. confieura- tlon, and chemical be.hawor Lo the p i t m e n t ~ m e dflav;) and
croceo l C o W h ( N H .),I?( salrs Humohrev's.. isomers u,ere
only the first ofthe dozens of series of isomers which Werner and his students were to prepare in support of the octahedral configuration for robal&lll~(2.51. \tii;h Humphrey's cum- pounds Werner abandoned his '20-paper series .'Heitrag zur I<onsritution anorganischer Verbindunpen" (Contrit~urion to the Constitution 01 lnorganic Compoundsl, which he had oublished in theZeiractrrift fur onorronrschr, Chwnie begin- ning with his coordinatiod theory ini893 (9), and he he& a
new four-oaoer.. series "Uber Isomerien bei anoreauischen
Verbindungen" (On Isomerisms among Inorganic Com- pounds) in the Berichte der Deutschen Chemischen Gesell-
i(&ner2s work on disuhstituted and trisubstituied benzene derivatives (19). The method consists of comparing the
Figure 4. Sophus Mads J#rgensen (1837-1914). Werner's primary scientific adversav.
Figure 5. Anuro Miaiati (left) and Alfred Werner (right) on the steps of the Eidgenassisches Poiytechnikum, Jan. or Feb. 1893. Fellow students Franr Feist and Roland Schoii are peeking out the door.
Volume 64 Number 7 July 1987 605
Figure 8.Werner's models of enantiomorphs of octahedral trisbidentate com- plexes.
Complexes containing chelate groups, on the other hand, are easier to resolve, and it was in 1911 with coordination compounds of the type cis-[CoCI(NH3)enz]Xzthat Werner, together with Victor L. King, finally carried out the first successful resolution of a coordination compound (30,36) by use of the silver salt of (+)-3-bromocamphor-9-sulfonic acid (37) and the improved Schmidt and Haensch (Landolt type) polarimeter (Fig. 7) and Nernst lamp, which had become available at about that time. This efficient and elegant reso- lution has been adapted for use in the undergraduate inor- ganic laboratory (38), and resolutions of similar trisbiden- tate complexes (Fig. 8), e.g., [Coens]Xs, by use of R,R-tar- trates (39)... anoear.. in standard laboratorv manuals (40). "whenever Werner opened up a new field, he expanded it with unbelievable soeed" (41). This statement bv Werner's former student, the ~ o b e ' llaureate chemist ~ a u lKarrer, applies to Werner's investigations of optically active com- plexes, for once Werner and King had found the key to the resolution process (which, incidentally, led directly to the Nobel Prize in chemistry for Werner in 1913) (Fig. 9), a large number of articles describing additional resolutions (more than 40 series of complexes within eight years) appeared from Werner's institute with great rapidity. However, al- thoueh the comoounds that Werner had resolved un to 1914 represented a remarkable variety of compound tybes, they all contained carbon. Because of the then-prevalent view that optical activity was almost always connected with carbon atoms. Werner's contemooraries could araue that the- nptical uctwity oi all these compounds a,ai somehou, due to the ethvlenediamineur biovridvl molc.culesor ro rheoxaiate ions contained in them. 1" i 9 1 4 however, Werner unequivo- cally proved his concept of the octahedral configuration for cohalt(II1) by a resolution carried out by Sophie Matissen, who, like Edith Humphrey, was one of his female Doktoran- drnnerr (421. lronicaliy, the compound resol\d, n n~mplete- Iy carbun.iree roordinarion compound of the wishidenrate tvne-.. I C ~ I I O H I , C ~ I I. U H. ~ I ~ I I I B ~ ~ - ~ ~ ~. ,. .-. " been discovered 16 years earlier by J$rgeusen (43). In Werner's own words, the investigation proved that "carbon-free inorganic com- pounds can also exist as mirror image isomers" and that therefore "the difference still existine between carbon com- pounds and purely inorganic compo;nds disappears" (42). At last. he had confirmed his lona-held view of the unitv of all chemistry.
In his last works. Werner stood on the threshold of an extremely complicated research area-the investigation of optically active coordination compounds containing ligands that are-themselvesoptically active (44). As early a s ~ o v e m - her 15,1907, Werner had written to the Russian chemist Lev Aleksandrovich Chugaev (1873-1922) (45):
Figure 9. "The Journey to Stockholm:' student canoon (the "Seehof" was one of Werner's drinking and eating places).
I see from your beautiful paper in Berichte that you have been more successful in resolvine". oro~vlenediamine .. than we have. Now I wish to ask you whether you would permit me to use the active propylenediamine in the investigation of compounds
ofwhich we have already obtained five inactive series
A number of studies were carried out by Werner and his co-workers with this ligand, of which one (46) concerned the stereochemistry of [Co(NOz)z(en)(pn)]Br,for which 10 opti- cally active isomers are possible-all of which were isolated. However, the fact that all these molecules contain a chiral carbon atom denied Werner the opportunity of assigning rotatory power to the octahedral metal center-the long- sought Holy Grail.
Spontaneous Resolution of Coordination Compounds It is clear that Werner intended to effect chiral resolution a t the dissymmetric Co(II1) center via the chiral carbon of 1,2-diaminopropane-an early example of intramolecular chiral induction. It is equally clear that he must have at- tempted the resolution of Humphrey's [Co(NOz)z(en)2]+ cation (6, see table) (5,24).In 1911 he succeeded in resolving it by adding silver (+)-3-bromocamphor-9-sulfonate to a solution of the chloride (47), and subsequently, his Swiss Doktorand Jakob Bosshart (48,49) resolved a series of salts by "seeding" solutions with crystals of resolved (+I-(& see table)X (X = CI, Br, etc.). I t is ironic to note that these time- consumine efforts. lastine a t least 14 vears. were lareelv- unnecessary since t w o f the salts-used, cis-[Co(N- 07)9(en)71X.. ., (X = CI. Br). undereo spontaneous resolution.. intt, conyl~mem~esof antipodal crystals, a conglwnrmtr heinr defined as.'a merhani,,al mixtnre c~fcrsstnlsof the rwo enankomers" (50). Bernal has reported recently (7,8)the structure and ahso-
Volume 64 Number 7 July 1987 607
lute configuration of 6 (table), as well as those of some other derivatives of anion 4 and of related species, all of which spontaneously undergo conglomerate crystallization from aqueous solutions, some of the crystals attaining massive sizes-large enough to allow the preparation of solutions of pure chiral species suitable for chiroptical measurements during Werner's time. Details are given below of the work done on 6 while details of the studies carried out on the other salts alluded to above are given elsewhere (7,s). Large prismatic crystals were obtained by the slow evapo- ration of aqueous solutions of 6 a t room temperature (- K). One crystal attained a weight of 0.637 g, which, given its density of 1.77 g/mL, had a volume of about 0.36 m G a fairlv substantial crystal. Smaller specimens were used for
Partlal List of Compounds of Co(lll) Available to Werner that Undergo Spontaneous Conglomerate Crystalllzation
Geometry Space Group Reference cisdinitnr P2,2,2, 52 fransdinltra P2,2,2, 5 3 mer P212121 5 4 fransdi-NHa P2,2,2, 7 hmsdi-NHs P2,2,2, 55 asdinitro P2, 7
... Unknown' lP2.2.2.1?1 (Enb
Thefactmaftheracemic ssltx = Br is moresolubleihantheenantiomerrwas known to Werner by 1914(60. A more detailed study of the soivbilihldiagramr of this system n: the x-ray study, which revealed that the substance had x^ =^ CI.^ B ~ Ias^ we11as^ that^ of^^6 is given in^ re;^ 62. spontaneonsly resolved into chiral each^ of^ which^^ dSViClly^ speaking, the^ *pace^ group of this^ oubrtance^ is^ unknown^ for^ any^ of^ its^ halides as obtained from conglomeratecrystailization experiments quoted in ref. 81. Aoki et al. contained exclusively a single enantiomer. This became evi- (^) ,627 determined the snuctureand of me bromide: however,their dent when the space group was found (7) to be P21 and the cation had been preresolved with an external agent and convened to the bmmide. absolute configuration, as determined from the refinement inasmuchasthespscegravpaf^ me^ crystals^ obtained from^ amnglomeratecrynsllize.tion and the Bijvoet test ( 5 1 ) , revealed the don" the space group may not be^ have^^10 be^ lsomaphour P2,2,2,^ withthose in me^ obtained from former case.^ a^ preresaivedpursenantiomsr, shown in the two stereo lots (Fies. 10 and 11). The cation is shown in Figure TO,which depicts it along a line of sight emphasizing the following stereochemical facts: atoms and the axial^ -NHz^ ligands so that each nitro group (1) the upper ethylenediamine (Nl-C1-C2-N2) is lel, while has a short and a long bond (i.e., 0.1..H8 = 2.483; 02..Hl =
the lower one (N3-C3-C4-N4) is ob; (2) the proper chiropti- 2.319;^ 03..H15^ =^ 2.340;^ 04..Hl^ =^ 2.789^ A).^ Thus the^ confor-
cal description of this cation is A(6h); and (3) there are mation of the nitro groups can he viewed as that of two flat intramolecular hydrogen bonds between the nitro oxygen paddles having distinct orientations for a pair of^ enantio- mers, since, as the orienting -NH2 groups are mirrored, the nitro ligands follow them, guided by the hydrogen bonds. Consequently, these paddlelike nitro groups contribute a specific form of dissymetry to cis-dinitro complexes which is lacking, for example, in their cis-dihalo analogues. The packing of the molecules in the unit cell is shown in Figure 11,which depicts the cation in a variety of orientations, each suitable for the ob- m servation of stereochemical details of its differ- ent fragments. I t also shows the relationship be- tween cations and anions, whose closest contacts are, as expected, between CI- ions and amino hydrogen atoms.
Conclusion Figure 10. A stereo view of A(6Ak(+k[Co(N0,)2(en)2]CI.Parameters of infer- est: Ca-N1 = 1.962(3), C s N 2 = 1.960(3)A, Co-N3 = 1.976(4). C o - ~ 4 = I t is historically significant and ironic that the selected 1.959(3)A. co-NS = t.924(4). ~ 0. ~ = 6 l.s25(4)A. ~ o ~~ 1. ~ 1 - ~ group of cobalt compounds listed in the table, all of which i ~ ~ ~ ~ C2-N2 = 42.59'. N3-C3-C4-N4 = -40.30'. (^) were available to Werner, undergo spontaneous resolution
Y
Figure 11. Packing of (f )ssr[Co(N02)(en)21CI.
608 Journal of Chemical Education
