The Role of Optical Activity in the Development of Coordination

23

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The Role of Optical Activity in the Development of Coordination Chemistry BODIE E . DOUGLAS Department of Chemistry, University of Pittsburgh, Pittsburgh, Pa.

Werner relied on classical techniques involving the syn thesis and separation of geometrical and optical isomers in establishing his intuitive coordination theory. To refute the belief that the optical activity might arise from the presence of carbon, Werner prepared and resolved [Co((OH) Co(NH ) ) ]X . Mills and Quibell resolved a Pt(II) complex which would be dissymmetric if planar, but not if tetrahedral. Bailar established the Walden type inversion for some substitution reactions of Co(III) com plexes using optical rotatory dispersion (ORD) data. Re cent studies of optical activity have concerned ORD and circular dichroism (CD) measurements which have been used for assigning absolute configurations. The CD studies are particularly useful in determining the stereo chemistry of complexes from the splitting patterns de termined by the molecular symmetry. 2

3

4

3

6

lfred Werner's coordination theory was intuitive. The substantiation of his flash of genius required many years of work. I n the absence of physical methods on which we rely, he had to devise compounds which would permit one to distinguish among the various geometrical and bind ing situations which were considered. Werner had to consider all reason able geometries, as well as a number of proposed descriptions which we would now dismiss as unreasonable, but which had to be refuted. The general approach used was to prepare complexes which could exist in isomeric forms, the number of isomers expected depending upon the assumed model. So great was Werner's ingenuity and skill that many of the isomeric complexes now known can be considered to be some variation of those isolated by him. It is significant that optical activity played an 357 In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.


358

WERNER

CENTENNIAL

important role in establishing the configurations of octahedral, tetrahedral, and even planar complexes before modern structural methods were used to settle the issues more directly. Although the term "chelate" was introduced later, i t is apparent that Werner recognized and used chelation to great advantage. A l l of the optically active complexes prepared involved chelation. The relative stabilities of five- and six-membered chelate rings were studied later by M a n n using the optical activity produced i n a ligand when coordinated. H e resolved the octahedral [Pt(tap)Cl ] (41) and planar [ P t ( t a p ) C l ] - H X (42) (tap = a, J3,7-triaminopropane) to demonstrate that tap formed a fivemembered ring giving an asymmetric carbon atom rather than a symmetri cal six-membered ring. Octahedral Complexes. Werner had shown for many examples that the number of geometrical isomers which could be isolated for com plexes with coordination number 6 was that expected for an octahedron. However, the most powerful approach was reported in 1911 when he showed that complexes of the type m - [ C o ( e n ) X ] could be resolved into optical isomers (68). H e reported the isolation of several more isomers of this type within a few months and the next year (69) reported the resolution of [Co(en) ] . Werner must have felt satisfied that the octahedral arrangement of ligands in 6-coordinate complexes was firmly established, but some critics objected because the resolved complexes contained carbon, and optically active carbon compounds were well known. Werner silenced this objec tion by preparing and resolving a completely inorganic complex (71), [ C o { ( O H ) C o ( N H ) } ] , i n which the chelate ligands around the central Co(III) are the complex ions m-[Co(NH ) (OH) ]+ W i t h this accom plishment, the major points of Werner's coordination theory for 6-coordi nate complexes were firmly established long before modern structural methods were available. Tetrahedral and Planar Complexes. Two isomers of [ P t ( N H ) C l ] were known, and Werner (67) proposed that they were cis and trans isomers with a planar configuration about Pt(II). Study of the reactions of this and other Pt(II) compounds led Werner to introduce the concept we now call the trans effect. Because isomerism of the type expected for a planar configuration was known only for complexes of Pt(II) and a few examples for Pd(II), the existence of anything other than a tetrahedral configuration for 4-coordinate species was questioned. M a n n (48) attempted to prove the configuration of 4

2

2

3

2

3

4

3

2

n+

3+

6+

3

4

2

3

In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

2

2


23.

DOUGLAS

Role of Optical

359

Activity

by looking for evidence of isomerism, but the attempt was unsuccessful. He found no cis-trans isomerism as expected for a planar complex, although we might expect the differences i n physical properties to be too small to permit separating the isomers if they formed. If the complex ion were tetrahedral, it would be dissymmetric, but attempts to resolve it were unsuccessful. A n unsuccessful resolution is never conclusive. M i l l s and Quibell (50) ruled out a tetrahedral arrangement and pro vided evidence favoring a planar arrangement for the Pt(II) complex con taining one molecule of isobutylenediamine, (CH3) 2C

CH2,

I

NH

I

NH

2

2

and one molecule of meso-stilbenediamine, H

H -C—CeHs,

CeHs—C~ NH

NH

2

2

by resolving the complex into optical isomers. The ion would contain a plane of symmetry if the N atoms are arranged tetrahedrally about the metal ion, but it is asymmetric for a planar configuration. The corre sponding Pd(II) complex was also resolved, and the resolving agents were removed completely. Although these results eliminated a tetrahedral configuration, a pyramidal configuration about the metal ion could also account for the optical activity. It remained for structural methods to remove this uncertainty. The burden of proof for planar 4-coordinate complexes was on Werner and those who supported this view for Pt(II) and Pd(II) complexes. I n other cases chemists hardly needed to be convinced that the complexes should be tetrahedral. Tetrahedral complexes containing unsymmetrical bidentate ligands are dissymmetric, and this was demonstrated by the resolution of t h e 3 e ( I I ) complex of benzoylpyruvic acid (49), CO2H

0=C

Be 0—C

\C H \

CeHs

Optical activity was also demonstrated for the corresponding complexes of Zn(II) and Cu(II), although in these cases coordination numbers higher than 4 are more likely than for Be. In all three cases the resolving agent could not be removed i n less time than required for complete racemization of the complexes. The complex bis(benzoylacetonato)beryllium(II) has


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been resolved on quartz, so that the active complex was obtained free from any resolving agent (6). The complex racemizes in a matter of hours. Modern

Reaction Mechanisms. Bailar (2, 8) studied the formation of [Co(en) C0 ]+ from optically active m - [ C o ( e n ) C l ] and found that products of opposite configurations were obtained under different condi tions. He proposed a Walden type inversion for the substitution process. The configurations of the original complex and the products were related using optical rotatory dispersion curves. Dwyer (19) studied these reac tions in detail and concluded that the inversion occurs through a trans displacement process involving both Ag+ and O H ~ Measurements of optical rotations have been used to follow the course of other substitution reactions (5, 4&, 48, 70, 72). The loss or retention of optical activity or inversion during a substitution process gives useful information concerning the mode of attack and the symmetry of inter mediates or activated complexes. Studies of racemization and isomeriza tion have led to elucidating the mechanism of stereochemical rearrange ments in the fine work of F a y and Piper (20) with metal complexes of unsymmetrical 1,3-diketones. Taube's review article (64), which provided the first basis for classify ing inert and labile complexes, made use of all available observations of rates of formation or substitution reactions. Most of the observations were qualitative, but for the purpose of the classification it was sufficient to know whether the reaction was complete within a couple of minutes or whether a complex was sufficiently inert for geometrical or optical isomers to be isolated. Important qualitative observations came from attempts to resolve complexes, and much of the quantitative data came from studies of rates of racemization or isomerization. There has long been dispute as to whether reduction processes such as in electroplating require the complex ion to dissociate first. Dissociation undoubtedly occurs in some cases, but Dwyer provided the most direct proof that oxidation and reduction processes can occur without dissociation (15). The optically active complex ion [Ru(dipy) ] + (dipy = dipyridyl) was oxidized to [Ru(dipy) ] + with Ce(IV) and then reduced back to the original complex ion with F e S 0 without loss in rotation. 2


Developments

3

2

2

3

3

+

2

3

4

Stereospecific Effects. E a r l y studies of Co (III) complexes of optically active propylenediamine (62, 65, 66), 1,2-cyelopentanediamine (28, 29, 80, 81), and 1,2-cyclohexanediamine (82) indicated that one did not get all combinations of (+) and (—) ligand configurations with the two configurations of chelate rings in the tris complexes. The composition of the cyclopentanediamine complexes reported by Jaeger have been found



23.

DOUGLAS

Role of Optical Activity

361

to be incorrect (58) so the results are questionable. It was believed that for an optically active ligand one isomer, A [ + + + ] or A[ ], was formed exclusively. Later studies have shown that such stereospecific effects are generally not complete except for cases such as the sexadentate ligands propylenediaminetetraacetate ion and cyelohexanediaminetetraacetate ion. Lifschitz (39) isolated both optical isomers of a-[Co(L-alaninate)3] (meridional), and recently both optical isomers of the 0 (facial) isomer have been isolated i n four laboratories. Bailar (8) isolated both isomers of [Co( — ) p n C 0 ] . H e and several of his students have used the selective substitution in such optically active complexes of pn and en by a racemic mixture of a ligand to resolve partially the ligand. Dwyer studied the tris(propylenediamine) complexes of Co(III) (16) and P t ( I V ) (17) and found that their formation is not absolutely stereospecific, as previously believed. A classic paper by Corey and Bailar (7) provided the basis of under standing stereospecific effects i n chelate complexes through conformational analysis of the chelate rings. Dwyer (18, 58) obtained all of the mixed en-(—)pn complexes with Co(III). The energy differences proved con sistent with predictions from the Corey-Bailar treatment. Contributions to the optical activity from the vicinal effect of the pn and from the spiral of the chelate rings have been studied and found to be additive (10). C o n formational aspects of chelate rings have been reviewed recently (23). 2

3

+

Spectroscopic Work. E a r l y studies of optically active complexes used optical rotations at one or two wavelengths to characterize the com pounds and to check for complete resolution. Later, optical rotatory dis persion ( O R D ) curves were used to relate configurations and establish whether they were retained or inverted during substitution reactions. E x tensive O R D studies were made by Mathieu, Sargeson, Burer, Hidaka, Shimura, Tsuchida, and several other Japanese workers. Kirschner (1) has studied the effects of inactive ions on O R D curves, and Woldbye (73) has studied the effects of ring size. Woldbye's excellent chapter (74) on O R D gives a comprehensive compilation of work on optically active complexes. Mathieu (44, 46, 47) first combined studies of O R D , circular dichroism ( C D ) , and absorption spectra of extensive series of complexes. During the same period K u h n (84) was also interested i n O R D and C D of complexes. Then for many years there was almost no active interest i n C D studies. The renewal of interest i n C D came about because of the surge i n O R D work which was stimulated by improvements i n instrumentation and by realizing the importance of Mathieu's and K u h n ' s earlier work. Shortly after the reawakened interest i n C D , the first recording instrument was developed (24) and was soon available.


362

WERNER

CENTENNIAL

Several C D papers have been concerned with assigning chirality to complexes.

In most cases C D curves provide a more reliable basis for these

assignments than do O R D curves because the components for individual electronic transitions are more easily separated for C D .

A n unequivocal

assignment of chirality requires a definite assignment of a C D peak to an electronic transition, for which the sign can be predicted reliably.


The shapes of C D peaks are similar to those of absorption bands, but the C D peaks can be either positive ( c i > € ) or negative (e\ < c ) and, for r

r

transition metal complexes, the C D peaks are more narrow.

T h e extent

of overlap of C D peaks is far less serious than for O R D curves of more complex shape.

Mason and co-workers (4) found that ( + ) D - [ C o ( e n ) ] 3

3+

gives two C D peaks of opposite sign in the region of the first absorption band.

Only the positive C D peak is observed for light directed along the

trigonal axis of the complex ion in a crystal.

On this basis the long wave-

length positive C D peak was assigned to the transition of E symmetry. The interpretation of the crystal C D spectrum has been questioned by Dingle (9), but Mason's assignment has been used as the basis for assigning many other complexes.

The presence of two C D peaks in the first

band region for the complex in solution reveals the true D

symmetry

3

rather than the O symmetry suggested by the absorption spectrum, where h

the small trigonal splitting is not evident. The absorption spectrum of K [ C o ( E D T A ) ] shows two intense symmetrical bands

in the

visible region

cubic symmetry for the complex ion. two

Cotton effects in the

ion

[Co(en)(malonate) ]~ was

(33,

60),

long wavelength

2

suggesting

effectively

However, the O R D curve reveals

studied as

(25).

band region

a model,

The

cts-[CoN 04]~, 2

of

[ C o ( E D T A ) ] ~ , and the C D spectrum clearly revealed three peaks in each band region (11). C . 2

This is the splitting expected for the true symmetry,

Gaussian analysis

of

the

CD

curves

for

[Co(en)(C 04) ]~ 2

[ C o ( E D T A ) ] ~ also gave three C D peaks in each band region. (21, 22)

2

and

Gillard

looked at the C D spectrum of [ C o ( E D T A ) ] - without reference

to the model compounds and concluded that only two C D peaks were present in each band region. The absorption spectra of [Fe(dipy) ] , [Fe(phen) ] 8

2+

8

2+

(phen =

1,10-

phenanthroline), and some related complexes are characterized by very intense and rather featureless charge transfer bands, which obscure the d-d transitions.

M a n y papers have dealt with the solution spectra, and a

recent paper reports polarized crystal spectra (52).

The C D spectra re-

veal well-resolved peaks which have been assigned as d-d transitions

(26).

The ligand C D bands have been used to assign (40) the same configurations to (+)-[Fe(dipy) ] 3

2+

and ( - ) - [ F e ( p h e n ) ] , even though the C D

curves appear enantiomorphic throughout.

3

2+

This was done because of the

differing symmetry of the w orbitals of the two ligands.

It would not seem

reasonable to expect the w bonding to reverse the signs of the C D peaks



23.

DOUGLAS

363


for the d-d transitions also, and the latter were used to assign the opposite configurations to these complexes {26). Mason et al. cited the solubilities of diastereoisomers as being consistent with their assignment, but the solubility rule is probably unreliable when the diastereoisomers differ i n composition, as i n this case (26). The C D spectra have also been used for metal complexes of dithiooxalate ion to observe the d-d transitions obscured i n the absorption spectra b y the intense charge transfer bands (27). The stereochemistry of complexes containing multidentate ligands can be studied using C D spectra which often reveal the molecular symmetry rather than the higher effective symmetry suggested b y the absorption spectra. Sargeson and Searle (59) studied the complexes of triethylenetetraamine of the type [Co (trien) X Y ] + . W i t h X and Y i n cis positions there are two isomers, one (a) with the terminal nitrogens trans to one another, and another (@) with them cis. The complexes were characterized and their absolute configurations assigned using C D spectra and knowledge of reaction mechanisms for the formation and reactions of the complexes. Four isomers of [Co(L-alaninate) ] have been prepared and studied b y C D (8, 12, 14, 86). Denning and Piper used N M R spectra to support their assignments of the configurations of the isomers. Their assignments for the a and a isomers (meridional) have been confirmed b y an x-ray structure investigation of the a isomer (18). Studies of O R D and C D spectra have been particularly important i n developing the understanding of coordination chemistry because they give more detailed information about electronic transitions than is available from absorption spectra. Such information is of paramount importance i n advancing bonding theories. Contributions to the theory of optical activity of complexes necessarily advance the bonding theories as well. K u h n (84, 85) was an early contributor to the theory of optical activity of metal complexes. Moffitt (51) made important contributions i n more modern terms. The recent theoretical work of Liehr (87, 88), Sugano (68), and Shinada (61) are of particular importance. Mason has been a major contributor to the experimental work i n recent years with signifi cant theoretical contributions also. Further advancement from the ex ceptionally fine experimental work and imaginative theoretical develop ments of the late Piper (54, 55, 56, 57) will be missed. n

3

1

Literature

Cited

(1) Albinak,M.J.,Bhatnagar, D. C., Kirschner, S., Sonnessa, A.J.,Can.J.Chem. 39, 2360 (1961). (2) Bailar, J. C., Jr., Auten, R. W., J. Am. Chem. Soc. 57, 774 (1934). (3) Bailar, J. C., Jr., McReynolds, J. P., J. Am. Chem. Soc. 61, 3199 (1939). (4) Ballard, R. E., McCaffery, A.J.,Mason, S. F., Proc. Chem. Soc. 1962, 331. (5) Brown, D. D., Ingold, C. K., J. Chem. Soc. 1953, 2680.



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(6) Busch, D.H.,Bailar, J. C., Jr., J. Am. Chem. Soc. 76, 5352 (1954) (7) Corey, E. J., Bailar, J. C., Jr., J. Am. Chem. Soc. 81, 2620 (1959). (8) Denning, R. G., Piper, T. S., Inorg. Chem. 5, 1056 (1965). (9) Dingle, R., Chem. Commun. 1965, 304. (10) Douglas, B. E., Inorg. Chem. 4, 1813 (1965). (11) Douglas, B. E., Haines, R. A., Brushmiller, J. G., Inorg. Chem. 2, 1194 (1963). (12) Douglas, B. E., Yamada, S., Inorg. Chem. 4, 1561 (1965). (13) Drew, M. G. B., Dunlop, J. H., Gillard, R. D., Rogers, D., Chem. Commun. 1966, 42. (14) Dunlop, J. H., Gillard, R. D., J. Chem. Soc. 1965, 6531. (15) Dwyer, F. P., Gyarfas, E. C., J. Proc. Roy. Soc. N. S. Wales 83, 174 (1950). (16) Dwyer, F. P., Garvan, F. L., Shulman, A., J. Am. Chem. Soc. 81, 290 (1959). (17) Dwyer, F. P., Sargeson, A. M., J. Am. Chem. Soc. 81, 5272 (1959). (18) Dwyer, F. P., AustralianJ.Sci. 24, 97 (1961). (19) Dwyer, F. P., Sargeson, A.M.,Reid, I. K., J. Am. Chem. Soc. 85, 1215 (1963). (20) Fay, R. C., Piper, T, S., Inorg. Chem. 3, 348 (1964). (21) Gillard, R. D., Nature 198, 580 (1963). (22) Gillard, R. D., Spectrochem. Acta 20, 1431 (1964). (23) Gillard, R. D., Irving,H.M.,Chem. Rev. 65, 467 (1965). (24) Grosjean, M., Legrand, M., Compt. Rend. 251, 2150 (1960). (25) Hidaka, J., Shimura, Y., Tsuchida, R., Bull. Chem. Soc. Japan 33, 847 (1960). (26) Hidaka, J., Douglas, B. E., Inorg. Chem. 3, 1180 (1964). (27) Ibid. 3, 1724 (1964). (28) Jaeger, F. M., Blumendal, H. B., Z. Anorg. Allgem Chem. 175, 161 (1928). (29) Ibid., p. 198. (30) Ibid., p. 200. (31) Ibid., p. 220. (32) Jaeger, F. M., Bijkerk, L., Proc. Acad. Sci. Amsterdam 40, 116 (1937). (33) Jørgensen, C. K., Acta Chem. Scand. 9, 1362 (1955). (34) Kuhn, W., Bein, K., Z. Physik. Chem. (B) 24, 335 (1934). (35) Kuhn, W., Bein, K., Anorg. Allgem. Chem. 216, 321 (1934). (36) Larsen, E., Mason, S. F., J. Chem. Soc. (A) 1966, 313. (37) Liehr, A., J. Phys. Chem. 68, 665 (1964). (38) Ibid., p. 3629. (39) Lifschitz, J., Z. Physik. Chem. 114, 493 (1925). (40) McCaffery, A. J., Mason, S. F., Norman, B.J.,Proc. Chem. Soc. 1964, 259. (41) Mann, F. G., J. Chem. Soc. 1927, 1224. (42) Ibid. 1928, 890. (43) Ibid. 1928, 1261. (44) Mathieu, J. P., Bull. Soc. Chim. France (5) 3, 476 (1936). (45) Ibid. 4, 687 (1937). (46) Ibid. 6, 873 (1939). (47) Mathieu, J. P., J. Chim. Phys. 33, 78 (1936). (48) Matoush, W. R., Basolo, F., J. Am. Chem. Soc. 78, 3972 (1956). (49) Mills, W. H., Gotts, R. A., J. Chem. Soc. 1926, 3121. (50) Mills, W. H., Quibell, T. H. H., J. Chem. Soc. 1935, 839. (51) Moffitt, W., J. Chem. Phys. 25, 1189 (1956). (52) Palmer, R. A., Piper, T. S., Inorg. Chem. 5, 864 (1966). (53) Phillips, J. F., Royer, D. J., Inorg. Chem. 4, 616 (1965). (54) Piper, T. S., J. Am. Chem. Soc. 83, 3908 (1961). (55) Piper, T. S., J. Chem. Phys. 36, 2224 (1962). (56) Piper, T. S., Karipedes, A., J. Chem. Phys. 40, 674 (1964). (57) Piper, T. S., Karipedes, A., Mol. Phys. 5, 475 (1962). (58) Sargeson, A. M., "Chelating Agents and Metal Chelates," p. 200, F. P. Dwyer and D. P. Mellor, eds., Academic Press, New York, 1964. (59) Sargeson, A. M., Searle, G. H., Inorg. Chem. 4, 45 (1965). (60) Shimura, Y., Tsuchida, R., Bull. Chem. Soc. Japan 29, 643 (1956).


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(61) Shinada, M., J. Phys. Soc. Japan 19, 1607 (1964). (62) Smirnoff, A. P., Helv. Chim. Acta 3, 177 (1920). (63) Sugano, S., J. Chem. Phys. 33, 1883 (1960). (64) Taube, H., Chem. Rev. 50, 69 (1952). (65) Tschugaeff, L., Sokoloff, V., Ber. 40, 177 (1907). (66) Ibid. 42, 55 (1909).

(67) Werner, A., Z. Anorg. Allgem. Chem. 3, 267 (1893). (68) Werner, A., Ber. 44, 1887 (1911). (69) Ibid. 45, 121 (1912).


(70) Ibid., p. 1228. (71) Ibid. 47, 3090 (1914).

(72) Werner, A., Bull. Soc. Chim. France (4) 11, 19 (1912). (73) Woldbye,F.,Rec. Chem. Progr. 24, 197 (1963). (74) Woldbye, F., "Technique of Inorganic Chemistry," p. 249, H. B. Jonassen and A. Weissberger, eds., Interscience Publishers, New York, 1965. RECEIVED July 1, 1966.


The Role of Optical Activity in the Development of Coordination

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