The Resolution of a Completely Inorganic Coordination Compound A Modified Classic Experiment for the Inorganic Laboratory Takaji Yasui and Tomoham Ama Kochi University, Akebono-cho, Kochi 780, Japan George 6. Kauifman California State University, Fresno, Fresno, CA 93740 Today the classics of chemistry are still more often quoted than read, but during the last few decades systematic efforts have been carried out to make classic papers available in English translation ( 1 4 ) . Classic experiments, modified or simplified for use by undergraduate students, however, are still virtually nonexistent. In the field of coordination chemistry two classic resolutions can be adapted for the undergraduate laboratory. The first, which led directly to Alfred Werner's Nobel Prize in chemistry (1913), has already appeared in this Journal (7). The second is described in the present article. Werner had used the technique of "isomer countingcomparing the number and type of isomers actually prepared with the number and type theoretically predicted for various configurations-not only to confirm his own coordination theory (8)and to discredit the rival BlomstrandJdrgensen chain theory (9-11) but also to prove that tripositive cobalt(II1) possesses an octahedral configuration rather than another possible symmetrical arrangement such as hexagonal pyramidal, hexagonal planar, or trigonal prismatic. In 1907, when Werner finally sent Jdrgensen a sample of the long-sought, unstable, highly crucial violeo tetraammines, C ~ S - [ C O C ~ ~ ( N H (12) ~)~ (Fig. ] X 1) that were a necessary consequence of his theory but not of the chain theory, his Danish opponent immediately conceded defeat. Although this discovery convinced Jdrgensen that his views could not be correct, Werner's success in preparing t w o and only two-isomers for compounds of type MA4B2 was insufficient to prove conclusively the octahedral configuration. Despite such "negative" evidence, i t could still be logically argued that failure to isolate a third isomer of these
compounds did not necessarily prove their nonexistence since i t is logically impossible to prove a negative (negatiua non sunt probanda). Clearly a more "positive" proof was required.
Fiaure 1. Sam~iesof ComDwnds of crucial ImDortance In the hlstorv of coordination theory from the Alfred Werner Coliectlon, Unlvenitat Zorich. (left to right): C ~ ~ [ C ~ C I ~ ( N Hcis[CaCI2(NH&]NO3 ~)~]CI, (video salts, 1907); (*I. (+b, and (-t[CaCl(en),(NH,)]Clg (the first coordination compound to be resolved. 1911); K3(+b[Cr(C20&)], (Strychnini~m)~ (-t[Cr(C20&] (mefirst coordination wmpound with an optically active anion to be resolved. 1912). Photcgraphby Richard Taubenest.
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Two Classlc Resolutions This "positive" proof involved the resolution into optical isomers bf certain-types of asymmetric coordination-compounds containing chelate moups (13,14). In 1911 Werner ;eported the first &ccessf&res&tion of coordination compounds, a true milestone in stereochemistry. The new instrumentation then available-the Schmidt & Haensch (Landolt-type) polarimeter (Fig. 2) and Nernst lamp-may have been factors in Werner's success. With his American Doktorand Victor L. King (15) he resolved the cis-amminechlorobis(l.2-ethanediamine)cohalt(III) series. cis-ICoCI(~~)Z(N&)]X (Fig. Z 11, a n d with his assistant E. ~ c h o l z e he resolved the cis-amminebromobis(l,2-ethanediamine)cobalt(II1) series, cis-[CoBr(en)z(NH3)]Xz (16). Since, of the twoseries, the bromo series is easier to resolve because of the greater difference in solubility between the diastereomers that are formed with the resolving agent, ammonium (+)-3bromocamphor-9-sulfonate (In,Kauffman and Lindley (7) adapted Werner's classic resolution as a reproducible laboratory experiment. Once Werner had found the key to the resolution of complexes, a large number of artirksdescribing further resolutions appeared from his institute with great rapidity. Within eieht ;ears. ~, he and his students had resolved more than 40 series of cationic and anionic complexes, not only of cobalt but of other hexacoordinate transition metals as well (Fia. 1). However, although the compounds that Werner had resolved UD t o 1914 reoresented a remarkable variety of compound Gpes, they all possessed one common ch&acteristic-thev all contained carbon. Because of the then-prevalent view that optical activity was almost always connected with carbon atoms, a number of Werner's contemporaries could argue that the optiral activity of all these romponnds was somehow due to the 1.2-ethanediamine (ethylenediamine) or biovridvl - ~ ~ ~ " molecules ~ ~ , or to the oxalate ions contained in them, even though these symmetrical ligands are themselves onticallv inactive. Bv such devious reasonina, they were abie to ;ast doubt o d the validity of the oct&edral configuration. ~
~
~
~
~
~
D
~
~
~
s
~
."
Figure 2. The polarimeter used by Werner to measure the rotation of optically active coordination compounds (Franr Schmidt and Haensch No. 8142). Photograph by RichardTaubanest.
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Journal of Chemical Education
In 1914 Werner was able t o silence even the most skeptical of his opponents and to prove unequivocally his octahedral conceot. In that vear. with the aid of Sophie Matissen, he resolGed a compl~tel~a&on-freetetranuclear coordination compound of the M(AA)3 type, viz., tris[di-p-hydroxotetraamminecobalt(III)]cobalt(III)bromide (18):
I1
the so-called ammine-hexol, a compound that, ironically, had been first discovered 16 years previously by J#rgensen, Werner's primary scientific adversary (19) (Fig. 3). Since this compound is structurally similar to [M(enh]X, salts with three
r~
~
-I+
~
ions in place of the three 1,2-ethanediamine molecules, i t should be caoable of existing in nonsuperimposable mirrorimage formsand hence shouid be r e s o i v a b l e ; ~the ~ present day, with the sole exceptions of the resolutions of Na cis[RhIS0z(NHz)z1~1(20), [CozMo~0034(0H)a16-(21), [Pt(Ss)3]2- (22), and cis-cis-cis-[Co(NO~)~(HzO)z(NH3)~]+ (23), Werner's resolution of [ C O [ ( O H ) ~ C O ( N H ~ ) re~)~]B~~ mains the only example of the resolution of a completely carbon-free coordination compound and constitutes his crowning achievement in coordination chemistry. In Werner's own words. the investination moved that "carhon-free inorganic compounds can also exist as mirrnrimaee isomers" and that therefore "the difference still existing ibetween carbon compounds and purely inorganic compounds disappears" (18). At last he had confirmed his longheld view of the unity of all chemistry. The structural theory of organic chemistry was only a special case of coordination chemistry, in which the carbon atom happened to have its valence equal to its coordination number. The last brick in the crumbling wall of separation between inorganic and organic chemistry had been razed. The demolition begun 86 years earlier by Friedrich Wohler with his artificial synthesis of urea from ammonium cyanate in 1828 had been completed by Werner (24).
Figure 3. Salts of me [CO((OH)~CO(NH.)~]~]~+ cation (the first completely inorenic complex ion to be resolved. 1914) hom the A l h d Wemer Collection. Universitat ZiMch. (len to right): (*)-Bromide; (+).(+tBCS; W,(-bBC3 (+bbromid* . (-).(+)BCS; (-)bromide (BCS = 3-bromocamphor-9-sulfonate). Photogaph by Richard Taubenee.
The Experiment Werner's classic resolution involves a complicated procedure that often gives isomers of low optical purity, as he himself admitted (18). The process was extremely tedious because of the small differences in solubility between the diastereomers. Optical measurements were hindered by rapid racemization and the deep color of the solutions. Rotations were measured in 1:l acetone-water solutions to miuimize racemization. Mason and Wood (25)also resolved amminehexolsalts, but they did not give a detailed procedure. Kudo and Shimura (26) modified Werner's resolution method by using sodium bis(w-d-tartrato)diantimonate(III) as a resolving agent. They obtained the diastereomer as four fractions, hut only the first and second crops indicated relatively high optical purity. T h e convenient procedure described below, which is a modification of Kudo and Shimura's method, gives a n optically pure product of high optical activity. After conversion of the racemic ammine-hexol chloride t o the ~erchlorateon an anion-exchanee column. the (+)diastereomer is precipitated with sodium bis(p-d-tartratojdiantimonate(II1) and converted to the (+)-bromide with sodium bromide. The (-)-enantiomer is isolated as the sparingly soluble sulfate by adding ammonium sulfate t o the filtrate from the (+)-diastereomer precipitation, then converted into the chloride hy adding harium chloride, and the (-benantiomer is finally isolatedm the hromide by metathesis with sodium bromide.
Procedure Caution: Perchlorate salts ore potentially erplosiue. The promdures given should not be carried out on a scale larger than given below. Also filter paper rather than sintered-glass filters should he used. Raeemic ammine-hexol chloride hexahydrate (26) (5.0 g; 0.0058 mol) is dissolved in 100mL of water.' The solution is passed through a column (3.2 em X 20 em) of Dowel: 1-X8 anion-exchange resin (200-400 mesh, CIO1- form) in order to convert the chloride ion to the perchlorate ion and is then eluted with water (about 200 mL). The colored hand on the column is eluted into a glass beaker at an elution rate of 12 mL min-'. and the volume of the eluant is increased to 300 mL by addition of water. A solut~oncontaming sodium bi~(~-d-tartrato)diantimonate~IIIJ pentahydrate (3.8 g; 0.0037 moll in 100 mLofwater (thissolution iaaridio O h ) isadded to this eluant. A white-brown precipitate appears immediately in the solution, hut it is not the desired diastereomer. (This precipitate 'The corresponding sparingly soluble hexol sulfate may be preDared accordinn to the orocedure of Kauffman and Pinnell 1271 and converted in s c k o n td the oerchlorate -~~- bv -,metathesis - -~ - - - wim' barium perchlorate solution. Conversion to the perchiorate on an anionexchange resin column requires a large volume of solution oecause of the limited solubility of the sulfate. If the resolution is carried out on a larger scale, ttm optical purity of both antipcdes is decreased. In 50% acetone-water the optical rotatory power of the optically active arnmine-hexol bromide decreases with time. The half-life period of the intensitv loss at 22% is about 55 min. 'This precipitate is probably a mixture of (f)diastereomer with (-)-enant#omer n excess. an0 (-)-hex01 oromldeobtainedfrom it has low opt cal p.rlly If the procedure in these two sentences is omined. the (-)-hem bromide obtained is of low optical purity. ~~
~
~
~
7~
~
~
may contain another type of diastereomer, e.g., (i)-[Col(OH)sCo(NH3)4)d[Sbz(d-C4H206)2]z). The desired diastereomer in the form of a dark brown crystalline powder is farmed by stirring the solution at 50 'C for about 20 min and is accompanied by disappearance of the white-brown precipitates. After the solution has been cooled to room temperature, the crystalline powder is collected by suction filtration on filter paper and is washed with 5 mL of cold 0.001 M perchloric acid. This powder is used in the neat section without being dried completely. Therefore a yield is not calculated. Anal. (for the air-dried powder)-calcd. for [Col(OH)zCo(NH3)413][Sbz(C1H~06)2]2(C104)2 . 9H20: C, 9.73; H, 3.47; N, 8.51%.Found: C, 9.69; H, 3.60; N, 8.76%. The filtrate is stored in an ice bath to obtain the (-)s3s-enantiomer of the hexol hromide.
(+)-Trls[dl-~-hydroxo-t&aammkecobalt(lll)]cobalt(lll) Bromlde Dlhydrate
Procedure The diastereomer powder obtained above is suspended in 20 mL of cold 0.001 M perchlorie acid, saturated with sodium bromide, and the suspension is vigorously stirred for 3 min in an ice bath. The resulting brown crystals of the crude (+)-hex01bromide are collected by suction filtration, washed with 5 mL of cold water containing 1.0 g of sodium bromide, and then washed with 10 mL of cold methanol. The crude crystals are purified by reprecipitation as follows. They are placed in 50 mL of cold 0.001 M perchloric acid, the mixture is stirred for 2 min, and insoluble materials are removed by suction filtration. To the filtrate 20 mL of cold 0.001 M perchloric acid, saturated with sodium bromide, is added. The resulting crystalline deposit is collected hy suction filtration, washed with 20 mL of methanol, and air-dried. The yield is 2.5 g (84%, based on the assumption of equal amounts of each antipode in the raeemie hexol chloride hexahydrate).2 The specific rotation, [alsss, = +2,580°, and the molecular rotation, [h4Jsm,= +27,230° (c = 0.05%ina1:1acetone-watermixtureat 25 DC).2(For 0.05%solutions Wernerreparted [ o ] ~= .+2,620° ~ and = +27.713Oor [uIm = +4,446' and [MJSEO = +47,03E0 in 1:l [MJ5s9.3 aqueous acetone) (18).Aesl4= -13.10 and Aeso7 = +11.08 in 0.01 M hydrochloric acid at 25 "C (Kudo and Shimura reported Aesld = -13.5 and Acsos = +11.3) (26). An additional recrystallization does not increase the optical purity.
(-)-Trls[dl-p-hydroxo-tetrammlnecobaR(II(lll) Bromlde Dlhydrate (-)-[COI(OH),CO(NH,)~I~](SO~)~ + 3BaC1,. 2H,O
-
Procedure To the filtrate from the (+)-diastereomerprecipitationa solution containing 2.2 g (0.0033 mol) of sodium bis(rr-d-tartrato)diantimonate(II1)pentahydrate in 20 mL of water is added, and the solution is stirred at 50 OC for 10 min. The resulting white-brown precipitate4 is removed by suction filtration, and a solution containing 2.0 g of ammonium sulfate in 10 mL of water is added to the filtrate with stirring. Violet-brown crystalline flakes of the (-)-hepol sulfate appear immediately. After the solution is allowed to stand at room temperature (25'C) for 30 min, the crystalsare collected by suction filtration, washed with 10mL of cold water, and air-dried. Thevield is 1.1 g. Inasmuch as the hexol sulfate is sparingly soluble in water, it is converted into the chloride hy metathesis with barium chloride Volume 66
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solution and then isolated as the bromide as follows. The sulfate (1.1 g) is added t o a solution containing 1.2 g of barium chloride dihydrate in 35 mL of cold 0.01 M perchloric acid, and the suspension is stirred for about 3 min. The white precipitate of barium sulfate is removed by suction filtration, and 12 mL of cold 0.01 M perchlarie acid, saturated with sodium bromide, is added to the filtrate. The fine, dark hrown crystalline deposit of (-)-hex01 bromide dihydrate is filtered, washed with 1 0 m L of methanol, and airdried. The yield is 0.98 g (32%. hased on the assumption of equal amounts of each antipode in the racemie hex01 chloride hexahydrate). Anal. Calcd. for [CO{(OH)~CO(NH~)$&.2H20: C, 0.W; H, 4.38; N, 15.89%.Found: C, 0.04; H, 4.53; N, 15.96%.The specific rotation, [a]sg, = -3,821°. The molecular rotation, [MIsss,= -40,410'. = +19.35, and ACW = -16.90. Thesevalues were obtainediu 0.01 M hydrochloric acid a t 25 "C.
Properties The structure of the tris[di-F-hydmxo-tetr~amminecobalt(m)]cobalt(II1) ion has been established by an X-ray crystal structure analysis of the racemie chloride (28), and several investigations of the optical activity of the resolved isomers and related h e ~ a - ~ hydroxo-tetraeohalt(II1)-typecomplex ions have been made (18,25,
-"
.7&.7!2\ --,.
Bc,th enantiomera of trinldi-N-hydroxo-retraamminecobalt~III~]cohalttIIIJ bromide are ohtarned as dihydraten. They are very s o h I& in water and raddlv undergo raremwatron. The hwheat sweifir rotation values that Werner (18) reported for 0.05%~olutior1sare [a]m9.3 = +2,620° 01[ a ] m = +4.446- for the (+)-bromide and [ a ] m = -4,500° for the (-)-bromide in 1:1 aqueous acetone. The active bromides exhibit strongly anomalous rotatory dispersion (18,33,34). Kudo and Shimura (26) reported that the circular dichraism intensity of the optically active ammine-hexol is affected by the anion coexisting in the solution, that the antipodes racemize readily in neutral and basic aqueous solutions, and that the racemization rate in acidic aqueous solution is slaw enough to measure the CD spectra. On the basis of their detailed CD measurements they determined the absolute confieuration of the I + h antioode as A. based upon a negative sign ol t k E , vwnponentofthe cod, chromo: phorr. I'reviously, two differeut aasignmentu fur the absulure ronfiguration of [his type of cumplex had been made 125.321.
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Journal of Chemical Education
Literature Cited 1. Alembic ClubReprint Series: Livingstone: Edinburgh, 19321963; Vob. 1-22. 2. Leicester, H. M.: Kliekstein, H. S. A Source Book in Chemistry, I4W-19W: Hsrvard Uniuemity: Camhidge, MA, 1952; Leieater, H. M. A Soures Book in Chrmrrtv, 1900-1950: Hsrvard University: Cambridge, MA, 1968. 3. Holton, G., Ed. Clossie of Science Series; Dover: New York, 1963-1978:Vols. 1-8. I . Hart. H., Ed. CIossicReaeamherin Gen~rolChomislrySeriesandClos.sicRaaearch i n Orponic Chemistry Series; Houghtan Miinin: Boaton. MA, 19W. 5. Farber. E. Milestones o/Modern Chemirtry; B d e : New York. 1966. 6. Knight. D. M., Ed. Cloaairol Scientific Papers: Chemistry; American Elsevier: New York, 1963; Second Series; American Eiepvier: New York. 1970. 7. Ksuffman, G . B.; Lindley, E. V., Jr. J. Chem. Educ. 1914.51.424. 8. Werner, A. 2. ~ M T P . Chem. 1893. 3. 267: for an annotated Enelish translation see
