Coordination Chemistry - ACS Publications - American Chemical Society

Chapter 22

Optical Activity in Coordination Chemistry Bodie Douglas

Downloaded by PENNSYLVANIA STATE UNIV on July 23, 2013 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0565.ch022

Department of Chemistry, University of Pittsburgh, Pittsburgh, P A 15260-3995

The demonstration of the optical activity of octahedral complexes was important in confirming Alfred Werner's intuitive ideas about coordination chemistry. Early work involved the resolution of complexes characterized by optical rotations. Modern instruments for optical rotatory dispersion were developed first, but circular dichroism (CD) spectra proved to be more useful. C D has been a powerful tool for detailed studies of the stereochemistry of octahedral complexes. Contributions to rotational strength of chelate ring conformational, configurational, and vicinal contributions are additive. Chiral metal complexes are now used in enantioselective synthesis of chiral pharmaceuticals.

Alfred Werner's ideas (1) were firmly based on his vision of the stereochemistry of metal coordination compounds. He was very successful in substantiating his ideas by the isolation of the number of isomers expected for octahedral complexes. This approach did not eliminate all other possibilities. Geometrical and Optical Isomers n+

Two isomers of [CoXY(en) ] complexes (where en = H N C H N H ) were well known. Werner believed them to be cis and trans isomers, and he thought that there should be corresponding isomers (2) of [ C o X Y ( N H ) ] . Sophus Mads J0rgensen had explanations for the two isomers of the chloride of ethylenediamine complexes as shown below. J0rgensen expected only one form of [ C o C l ( N H ) ] , and only the green (trans ) isomer was known. In 1907 (2) Werner isolated the violet isomer cis [CoCl (NH ) ] . J0rgensen abandoned his criticism of Werner's proposals. 2

2

2

4

2

n+

3

4

2+

2

3

4

+

2

3

4

+

In 1897 Werner recognized that a compound such as [Co(C 0 )(en) ] should be resolvable into optical isomers, and he discussed this in a publication in 1899 (3). He made many attempts to resolve cobalt complexes. His American student Victor L. King carried out some 2000 fractional crystallizations without resolution. Finally, the resolution of cis -[CoCl(en) (NH )] and cis -[CoBr(en) (NH )] was successful in 2

2+

2

3

4

2+

2

3

0097-6156/94/0565-0275$08.00/0 © 1994 American Chemical Society In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

2


276

COORDINATION CHEMISTRY

^CHj-CH, Praseo

Co—NHj—NHy^-NHr-CI CHj—CH

2

Cl

Violeo

f

^Η ^Η

Ιο-

NHj—NH —ΝΗ -τ^ΠΗ --0

\

2

trans

Cl

2

2

2

2

CH CH 2

j0rgensen

2

Werner

In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

22.

277

Optical Activity in Coordination Chemistry

DOUGLAS

1911 using bromocamphorsulfonic acid as the resolving agent without many recrystallizations (4). Resolution of [Co(en) ] was achieved in 1912 (5). Werner must have been confident that his visions were proven valid. However, there were still skeptics. The optical activity of organic compounds was well established, and some felt that the optical activity could be caused by the organic ligands. It was not suggested as to how ethylenediamine, a symmetrical ligand, could give optical activity. Of course, nothing was known of the chiral conformations of ethylenediamine chelate rings. J0rgensen prepared (6) a compound called anhydrobasic tetraammine diaquodiammine cobalt chloride, which he believed to be a dimer involving an oxo bridge. Werner found that OH" could not be bonded to Co in the usual way since it did not react with dilute mineral acid to give an aqua complex. Decomposition of the complex by reaction with mineral acid gave three moles of c « - [ C o ( H 0 ) ( N H ) ] X , 3+


3

2

2

3

4

3

1 mole of C o X , and Vi mole of X . He concluded (5 ) that the complex cation was tetranuclear: 2

2

The wide acceptance of his ideas was shown by his receipt of the Nobel Prize in chemistry in 1913, the first Swiss chemist to be so honored. The optical resolution of [Co{(OH) Co(NH ) } ] in 1914 must have satisfied all skeptics. This was the first complex ion without carbon to be resolved (7). Examples of optically active complex ions without carbon are still rare. Werner reported die optical rotation from 478 to 675 ιημ as the first qualitative (samples racemized) optical rotatory dispersion (ORD) curve for a well characterized coordination compound. John C. Bailar, Jr., the father of American coordination chemistry, who has been eulogized at this symposium, was trained as an organic chemist. He became interested in the possibilities for isomerism in inorganic compounds, leading to his distinguished career in coordination chemistry. Bailar (8,9) observed that opposite configurations of [ C o ( C 0 ) ( e n ) ] were formed from optically active cis[CoCl (en) ] under different experimental conditions. He suggested that a Waldentype inversion can occur in coordination compounds as well as in organic compounds. He characterized compounds by optical rotations and related the configurations of reacting complexes and products using optical rotations or ORD curves. Frank Dwyer said that he had doubts about the reality of a Walden-type inversion. He followed the kinetics of substitution reactions and was pleased to find (10) that inversion does occur through a trans displacement process involving A g and OH". Bailar realized that racemization reactions could occur without the dissociation of ligands. He proposed a mechanism involving the twisting of one triangular octahedral face of Δ-[Μ(ΑΑ) ] by 120° about the C axis to form Λ-[Μ(ΑΑ) ]. This is known as the Bailar or trigonal twist. A rhomboid twist is known as the Ray-Dutt twist. In some cases racemization results from ligand dissociation. 6+

2

3

4

3

+

3

2

+

2

2

+

3

3


3

278


Optical Rotatory Dispersion and Circular Dichroism Werner used optical rotations to characterize complexes and to check for complete resolution of octahedral complexes. Sometimes more than one wavelength was needed because the rotation might be zero at a particular wavelength. The ORD curve for [Co{(OH) Co(NH ) } ] reported (7) in 1914, shows a range of molecular rotations from +47,000° (547.5 ιημ) to -22,000° (643.5 πιμ) with 0° at 617.5 and 495 πιμ. ORD curves were used in later work on complexes to relate configurations and to establish whether the configuration was retained or inverted during substitution reactions. Aimé Cotton (11,12) was the first to study ORD in detail within the region of absorption. He observed that in the region of an absorption band right and left circularly polarized light is absorbed to different extents. In this region the optical rotation changes abruptly giving a characteristic shaped curve, with a peak and a trough of opposite sign. This is known as an "anomalous" optical rotatory dispersion curve, the ORD curve. The behavior is not anomalous, but distinctly different from the "normal" dispersion curve observed outside an absorption region. These normal curves, described by the Drude equation, are really tails of the anomalous dispersion curves. The different interactions of right and left circularly polarized light is known as the Cotton effect. The difference in absorption of the components (Δ ε = E - E ) is circular dichroism (CD), and the difference in indices of refraction (n - n ) causes the rotation of the plane of linearly polarized light. 6+


2

3

4

3

t

/

r

r

Cotton constructed an apparatus to measure optical rotations and C D over a range of wavelengths. He established Cotton effects for potassium chromium tartrate and potassium copper tartrate solutions. These were the first ORD and C D studies of coordination compounds, but the complexes were not isolated. For a long period there was little activity in ORD or CD studies of complexes. Kuhn (13) was interested in metal complexes in connection with his ideas about the theory of optical activity ( 14). J. P. Mathieu (15,16) presented combined studies of ORD, C D , and absorption spectra of many complexes of Co, Cr, Pt, Rh, and Ir during the 1930s. After his work there was no activity in CD studies of complexes for many years. The development of good ORD instruments encouraged numerous studies. We began ORD studies of cobalt complexes in the late 1950s, but we had difficulties in resolving the spectral components for individual transitions. The complex, overlapping curves complicated the process. The rotation observed for a sugar or for tartaric acid in the visible region is a composite from tails (normal dispersions) of allowed transitions in the ultraviolet region. A CD peak drops to zero not far from the maximum so there is overlap only of peaks close in energy. The C D spectra give much clearer information than ORD curves, greatly simplifying the resolution of components. It was obvious that we should study CD instead of ORD. No CD instruments were available, and the construction of Cotton's apparatus was not simple. Fortunately, Grosjean and Legrand (17) developed the dichrographe in 1960, and it was soon available commercially. In a few years C D studies largely displaced ORD studies. C h i r a l Contributions of Ligands Transition metal complexes of optically active ligands display ORD and C D curves in the region of the metal d—* d absorption region. Substitution of C o for Z n in carboxypeptidase provides a "spectroscopic probe". C D studies of the C o d~+d transitions provide useful information about the environment at the metal ion site. Shimura (18) reported two Cotton effects in the region of the lower energy d—>d absorption band for [Co(L-leuc)(NH ) ] . The contribution of L-leucine was 2 +

2 +

2 +

2+

3

4


22.

279


DOUGLAS

2+

referred to as a "vicinal effect". C. T. Liu resolved [Co(glycine)(en) ] and [Co(Lpalan)(en) ] . The glycine complex and one diastereoisomer of the L-phenylalanine complex {(-) -[Co(L-palan)(en) ] ) showed very similar C D curves throughout the visible region. Both displayed one CD peak in the lower energy absorption band. The other diastereoisomer {(+) -[Co(L-palan)(en) ] } showed two C D peaks of opposite signs (Figure 1). Different intensities and shapes were expected for the two diastereoisomers but not different numbers of C D peaks. The resolutions were repeated with the same results. The CD spectrum of unresolved [Co(L-palan)(en) ] showed three C D peaks (-,+,-) in the lower energy absorption band region. The C D spectrum of the complex ion [CoL-palan(NH ) ] was very similar to that of unresolved [CoL-palan(en) ] . This, taken as the "vicinal effect" of L-palan, could be subtracted from the C D curves for the two diastereoisomers of the L-phenylalanine complex to give essentially mirror-image C D curves, corresponding to those of the enantiomers of [Co(gly)(en) ] . The contributions for C D curves are additive for the "vicinal effect" of the optically active ligand and the configurational contribution (Δ or Λ) for the complex (19). A complex such as 1,2-propanediaminetetraacetic acid (pdta) is stereospecific. The C o complex of S -pdta is A-(-) -[Co(S-pdta)]~. Most optically active ligands yield a preferred diastereoisomer but not exclusively. Dwyer and Sargeson (20) isolated all of the mixed en-(-)pn complexes with C o [(-)-pn is (-)-l,2propanediamine]. The contributions to the optical activity front the vicinal effect of pn and from the overall configurational (Δ or Λ) effect of the complex was found to be additive (21). A thorough analysis of the conformations of chelate rings was given in a classic paper by Corey and Bailar (22). The energy differences for various combinations of ligand conformations predicted from their treatment have been consistent with experimental results. 2

2+

2

2+

546

2

2+

546

2

2+

2


2+

3

4

2+

2

2+

2

m

546

1 1 1

Spectroscopy Absorption bands in the ligand field (d—>d) region are very broad because of vibronic broadening. For octahedral (Ο ) complexes of C o (d ) and Cr " " (d ) there are two allowed transitions to triply degenerate (T or T ) excited states. Lowering the symmetry gives splitting of the energy levels. For effective D or O symmetry there are two components for each of these transitions. For still lower symmetry, such as C , C , or C there are three components for each transition; the degeneracy is completely removed. The broad absorption bands obscure splittings in most cases. Usually the effect observed for lower symmetry is the appearance of a shoulder on the first (lower energy) absorption band with no effect for the second (higher energy) band. In extreme cases such as trans-[CoF (en) ] two absorption peaks appear in the first band region. 3 +

6

3

1

3

λ

lg

2g

3

2 v

2

4h

v

+

2

2

Interpretation of O R D and C D Spectra. The curve shapes for O R D and the significant contributions of transitions differing greatly in energy make the separation of components difficult. CD peaks are not broadened like absorption bands, and they overlap only with adjacent peaks. The peaks can be positive or negative, and this can help in the spectral resolution of components.


280


For [Co(en) ] the symmetry is D , but the absorption spectrum shows no more splitting than for [Co(NH ) ] (Ο ). The absorption spectrum of [Co(en) ] is consistent with effective Ο symmetry. In fact, in the absence of any splitting, the spectrum provides no basis for using lower symmetry. Mathieu (16) displayed absorption, ORD, and C D in the same figure for [Co(en) ] . In the region of the lower energy absorption band there is one well-defined O R D curve and a slight irregularity that can be noted if one knows where to expect a second ORD component. The C D spectrum displays two well-resolved peaks with opposite signs in the first band region. One C D peak appears in the second band region, as expected, since one of the two transitions expected is forbidden. CD spectra generally reveal more splitting than is observed in the absorption or ORD spectra. Three parameters, Y (energy), A £ (intensity), and half-width, describe a C D peak. The corresponding ORD curve can be calculated from these parameters. Unless the splittings are great enough to produce components that are well enough defined, curve analysis is uncertain. For three components there are nine variable parameters. The remark has been made that with five variables one can draw an elephant. There are few examples of C o complexes that reveal three components in the first absorption band region for complexes with symmetry lower than D or D ^ . The splitting of the second band is usually smaller, and these peaks commonly have low intensities. 3+

3

3

3+

3

3+

6

λ

3

λ

3+


3

m a x

m a x

3 +

3

Splitting Patterns for C D Spectra. The [Co(L-aa)(en) ] complexes containing optically active amino acids (Z^-aa) display one CD peak for one diastereoisomer or two C D peaks for the other diastereoisomer in the first band region (21). [Co(Laa)(NH ) ] has three C D peaks with alternating signs (-,+,-) and this pattern is seen for the unresolved [Co(L-aa)(en) ] (see Figure 1). This should indicate that the degeneracy of [A —*T (O )] is removed completely by the C symmetry. The dominance of some peaks can cause two or one peak to appear. For [Co(edta)]" there are two CD peaks in the first absorption band region, but three weak C D peaks in the second absorption band region (Figure 2). The complex [Co(malonate) (en)]~ was prepared (23) as a ligand field model of [Co(edta)]~. They both have cis -Co(N) (0) arrangements with C symmetry. There are three prominent C D peaks of alternating signs in the first and second absorption band regions for [Co(malonate) (en)]~. Probably this actual splitting pattern is obscured by one of the 2+

2

2+

3

4

2+

2

lg

lg

h

x

2

2

4

2

2

components being canceled or covered for [Co(edta)]". Further evidence of the presence of three components in the first absorption band region is revealed by an extensive series of C o complexes of edta-type ligands (24). The usual pattern is two C D peaks of opposite sign in the first absorption band region. Complexes of edta-type ligands with four acetate arms have one C D peak at lower energy than that of the absorption maximum, and the other C D peak has higher energy than that of emax. For complexes of ligands with acetate and propionate arms both C D peaks occur on the lower energy side of the absorption maximum, or for two of this group the higher energy CD peak is at about the same energy as that of 6 : 3 +

max


22.

DOUGLAS

281

Optical Activity in Coordination Chemistry Aim/ij

2.5

600

500

2.0

400

/ \

350

300

A-(+) .[Co(5-palan)(en) ]I 546

2

2

1.5 1.0 0.5 Δβ

0

y,

-0.5 •

\


-1.0 •

j^- Unresolved Complex

\

-1.5 -2.0 -2.5 ' 16

/ ,

1

/ M-W[Co(S-palanXen) ]!

2

1

1

\j

1 1 .1

18 20

1

1

1

I

1

1

1

1

1

1

_l

22 24 26 28 30 32 i7(cm χ I0" ) _1

'

2

_l

34

3

0

-O.I -0.2

3 (cmr»)x Ο *

3

Figure 1 Circular dichroism spectra for (+) - and (-) -[CoS-palan(en) ]I and [CoS-palan(NH ) ]I . 546

3

4

546

2

2

2

3.0 2.4 1.8 1.2 0.6 Δ€

Q -0.6 -1.2 -1.8 -2.4 -3.0

16

18

20

22

24

26

28

30

ν (kK) Figure 2. Absorption and circular dichroism spectra for (-) 5K[Co(edta)] · 2 H 0 and the circular dichroism spectrum for (-) K[Co(mal) (en)] · 2H^O. (Reproduced with permission from reference 34. Copyright 1994 Wiley.) 54

546

2


2

282


Acetate and propionate arms

Four acetate arms First abs. max.

or

I I Energy —*

I I Energy —*

C D peaks Energy —>

These patterns for the two groups should be different combinations of three components. For trans (0 )-[Co(l,3-pddadp)]" (25) and [Co(5,5 )-edds]" (26) (1,3pddadpis l,3-propanediamine-N,N '-diacetate-tyN '-di-3-propionate ion; (5,5 )-edds is 5,5 -ethylenediamine-A/,iV '-disuccinate ion) there are three well resolved C D peaks in the region of the first absorption band.


6

C h i r a l Metal Complexes i n Enantioselective Synthesis (27,28) Substitution reactions of resolved metal complexes can occur with retention or with inversion. Commonly, some activity is lost because of racemization. Since the late 1960s there have been developments in metal complexes containing chiral ligands which react catalytically in highly enantioselective organic reactions. Small amounts of the chiral complexes can produce much larger amounts of products of high enantiomeric purity. This is referred to as chemical multiplication of chirality. Diethylzinc in the presence of (-)-3-exo -(dimethylamino)isoborneol

[(-)-daib] reacts with benzaldehyde in toluene to produce (5 )-l-phenyl-1-propanol in 97% yield with 98% enatiomeric purity (29,30). The reaction can occur with only partially resolved daib (15% enrichment of the (-) isomer) to produce the product with 98% enantiomeric purity (31). This process involving a partially resolved catalyst, called amplification of chirality, results from much greater turnover efficiency of the chiral catalytic system as compared with the meso isomer formed from the racemate. Wilkinson's catalyst, [ R h C l { P C H ) } ] , modified by substitution of chiral phosphines for triphenylphosphine, can bring about enantioselective hydrogénation. The hydrogénation of the olefin shown below is reduced in the presence of [Rh(dipamp)] in the production of L-DOPA. L-DOPA is used in the treatment of 6

5

3

3

+

COOH CBbC

NHCOCH

i R h ( d i 3

P

a m

P

) I +

[COCH

C]

CBbCO< H 0

+

3

COOH

Hi

NH

Hi

[Rh(dipamp)]

+

L-DOPA


2

3

22.

DOUGLAS


283 +


Parkinson's disease. A similar hydrogénation reaction with the catalyst [Rh(pnnp)] produces L-phenylalanine used in the production of aspartame [ ( £ , £ ) aspartylphenylalanine methyl ester], the artificial sweetener.

There are many other enantioselective reactions catalyzed by chiral metal complexes. These include complexes of various chiral phosphines, tartrate ion, and others as ligands. Chiral cyclopentadienyl metal complexes (32) also are used. Chiral ligands are not limited to those containing an asymmetric carbon. [Ru(binap)] is a catalyst for enantioselective hydrogénation. In some cases such as [Rh(dipamp)] and [Rh(pnnp)] well defined complexes are used. In many cases the reaction is carried out in the presence of a chiral ligand and a metal compound. The reaction involving Z n ( C H ) above is one. Other examples include those catalyzed by T i in the presence of tartrate ion and O s 0 in the presence of chiral ligands. A l l of the Group VIII (Groups 8,9, and 10) metals, Cu, Zn, and other transition metals have been used for enantioselective syntheses. Enzymes are usually spécifie for a particular reaction and a particular substrate. Some of the chiral metal catalysts, such as [Ru(binap)] as a hydrogénation catalyst, are quite general. 2+

+

+

I V

2

5

2

4

2+

/?-binap


284


Synthesis of Chiral Pharmaceuticals. Biologically active compounds are usually chiral. Synthetic drugs produced in the past were usually racemic products. Commonly, only one enantiomer is active. Production of the active enantiomer permits the use of lower dosage with possible reduction of side effects. Applications of chiral metal complexes in enantioselective syntheses are increasing rapidly in the pharmaceutical industry (33).


Literature Cited 1. Werner, Α. Z. anorg. Chem. 1893, 3, 267. For a discussion and annotated English translation see Kauffman, G. B. Classics in Coordination Chemistry, Part 1: The Selected Papers of AlfredWerner;Dover: New York, NY, 1968; pp 5-8. 2. Werner, A. Ber. 1907, 40, 4817. For a discussion and annotated English translation see Kauffman, G. B. op. cit.; pp 141-154. 3. Werner, A. Z. anorg. Chem. 1899, 21, 145. 4. Werner, A. Ber. 1911, 44, 1887. For a discussion and annotated English translation see Kauffman, G. B. op. cit.; pp 155-173. 5. Werner, Α.; Berl, E.; Zinggeler, E.; Jantsch, G. Ber. 1907, 40, 2103. 6. Jørgensen, S. M. Z. anorg. Chem. 1898, 16, 184. 7. Werner, A. Ber. 1914, 47, 3087. For a discussion and annotated English translation see Kauffman, G. B. op. cit.; pp 175-184. 8. Bailar, Jr, J. C.; Auten, R. W. J. Am. Chem. Soc. 1934, 57, 774. 9. Bailar, Jr, J. C.; McReynolds, J. P. J. Am. Chem. Soc. 1939, 61, 3199. 10. Dwyer, F.; Sargeson, A. M.; Reid, I. K. J. Am. Chem. Soc. 1963, 85, 1215. 11. Cotton, A. Compt. Rend. 1895, 120, 989, 1044. 12. Cotton, A. Ann. Chim. Phys. 1896, 8, 347. 13. Kuhn W.; Bein, Κ. Z. Physik. Chem. (B) 1934, 24, 335. 14. Kuhn W.; Bein, Κ. Z. Anorg. Allgem. Chem. 1934, 216, 321. 15. Mathieu, J. P. Bull. Soc. Chim. France 1936, (5) 3, 476; 1937, 4, 687; 1939, 6, 873. 16. Mathieu, J. P. J. Chim. Phys. 1936, 33, 78.


22. DOUGLAS


285

17. Grosjean M.; Legrand, M. Compt. Rend. 1960, 251, 2150. 18. Shimura, Y. Bull. Chem. Soc. Japan 1958, 31, 315. 19. Liu C. T.; Douglas, Β. E. Inorg. Chem. 1964, 3, 1356. 20. Dwyer F. P.; Sargeson, A. M. J. Am. Chem. Soc. 1959, 81, 5272; Sargeson, A. M. In Chelating Agents and Metal Chelates; Dwyer, F. P.; Mellor, D. P., Eds.; Academic Press: New York, NY 1964; p 200.


21. Douglas, Β. E. Inorg. Chem. 1965, 4, 1561. 22. Corey E. J.; Bailar, Jr., J. C. J. Am. Chem. Soc. 1959, 81, 2620. 23. Douglas, Β. E.; Haines, R. Α.; Brushmiller, J. G. Inorg. Chem. 1963, 2, 1194. 24. Douglas, Β. E.; Radanovic, D. J. Coord. Chem. Rev. 1993,128,139. 25. Radanovic, D. J.;Trifunovic, S. R.; Cvijovic, M. S.; Maricondi, C.; Douglas, Β. E. Inorg. Chim. Acta 1992, 196, 161. 26. Jordan, W. T.; Legg, J. I. Inorg. Chem. 1974, 13, 2271. 27. Noyori, R. Science 1990, 248, 1194; 1992, 258, 584. 28. Scott, J. W., Topics in Stereochem. 1989, 19, 209. 29. Noyori, R.; Kitamura, M. Angew. Chem. Int. Ed. Engl. 1991, 30, 49. 30. Kitamura, M.; Suga, S.; Kawai, K.; Noyori, R. J. Am. Chem. Soc. 1986, 108, 6071. 31. Kitamura, M.; Okada, S.; Suga, S.; Noyori, R. J. Am. Chem. Soc. 1989, 111, 4028. 32. Halterman, R. L. Chem. Rev. 1992, 92, 965f. 33. Stinson, S. C., "Chiral Drugs", Chem. Engr. News 1992, September 28, 46. 34. Douglas, B.; McDaniel, D. H.; Alexander, J. J. Concepts and Models of Inorganic Chemistry; 3rd ed.; Wiley: New York, NY, 1994;p467. RECEIVED December 14, 1993


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