Coordination Chemistry - ACS Publications - American Chemical Society

Chapter 24

Chirality in Coordination Compounds 1

Alex von Zelewsky, Pascal Hayoz , Xiao Hua, and Paul Haag

Downloaded by PENNSYLVANIA STATE UNIV on July 23, 2013 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0565.ch024

Institute of Inorganic Chemistry, University of Fribourg, Pérolles, CH-1700 Fribourg, Switzerland

Alfred Werner conjectured as early as 1899 that octahedrally coordinated metal complexes should occur in nonidentical mirror image isomers. For such objects, Lord Kelvin, in 1893, had coined the adjective "chiral", a term never used by Werner. It can be proved by examination of the original sample of [Co(NO2)2(en)2]Br, prepared by Edith Humphrey, a Ph.D. student of Werner's, that crystals of optically pure samples were obtained in Werner's laboratory as early as 1899 or 1900. However, Werner did not publish the first successful resolution of an octahedral metal complex until 1911. Presently, interest in chirality in coordination compounds is booming, mainly because of the importance of coordination compounds i n enantioselective homogeneous catalysis. Other interesting applications are enantioselective interactions of chiral coordination species with biomolecules, and the stereoselective synthesis of multicenter systems. Although the concept of chirality was introduced by Lord Kelvin (7) in the same year that Alfred Werner published his landmark paper in coordination chemistry, Le., 1893 (2), Werner never used this term. Instead, he used the German word Spiegelbildisomerie, or he spoke about "asymmetrical metal atoms". The conjecture uat Spiegelbildisomerie should occur in coordination compounds with octahedral coordination appears for the first time in Werner's publications in 1899 (3). Werner's later work of shows that he had a very clear understanding of what chirality means and how it would manifest itself experimentally. His investigations of the tetranuclear hexol complexes of cobalt are particularly elegant and enlightened (4). It is therefore surprising that Werner overlooked the occurrence of a case of spontaneous separation of a chiral complex into its enantiomers upon crystallization. This was investigated from a historical point of view in a series of publications "Overlooked Opportunities in Stereochemistry " by Bernai and Kauffman (5-6). As they point out, a British woman, Edith Humphrey, had already prepared in 1899, or at the latest, in 1900 the compound [Co(N02)2(en)2]Br, which separates spontaneously into the enantiomeric forms upon crystallization. But this went unnoticed by Alfred Werner. Current address: Department of Chemistry, Stanford University, Stanford, CA 94305

0097-6156/94/0565-0293$08.00/0 © 1994 American Chemical Society In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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COORDINATION CHEMISTRY

The Historic Sample of [Co(N0 )2(en) ]Br


2

2

The Royal Chemical Society invited learned societies from around the world to its 150th anniversary celebration in London in spring 1991. There was a special ceremony for the donation of the gifts to the president of the society, Sir Rex Richards. The present author represented Switzerland at this occasion. We had obtained from the Institute of Organic Chemistry of the University of Zurich (Prof. H.-J. Hansen), where the Werner collection is located, a sample of the cobalt complex prepared by Werner's British co-worker. The measurement of the CD-spectrum of a solution obtained from one crystal of the original sample shows clearly its optical activity (Figure 1). After 90 years on the shelves of the University of Zurich, the compound had not changed in any way. The nicely crystalline compound can now be seen, displayed in a teakwood-box with a transparent insert, in the exhibition room of die Royal Chemical Society, Burlington House, London. Today's Challenges in Chirality of Coordination Compounds Although chirality has been a subject of continuous interest in coordination chemistry since Werner's time, it has not reached the mature state of development that it has in organic chemistry. Stereochemistry of species with coordination number > 4 is inherently much more involved than the organic stereochemistry predominantly determined by the tetrahedron. It is therefore not surprising that even today some basic problems connected with the chirality of coordination compounds are not yet completely solved. Control of the Axial C and C Chirality in OC-6; Towards Chiral Building Blocks in Coordination Chemistry. Since the first successful 2+ resolution of the [CoBr(NH3)(en)2l ion by Victor L.King in Werner's laboratory, numerous racemic complexes of the type M ( L L ) 2 X 2 (C2-axial chirality) and M ( L L ) 3 have been separated into enantiomers. Although some progress has been made in the understanding of packing properties of diastereomeric salts of such chiral complex ions with chiral counter ions (7), the success of the resolution by this method is still largely a matter of trial and error. Yet chiral complexes of this type can be of great importance for several purposes. It has been shown, e.g., that the interaction of metal complexes with D N A is enantiomerically discriminative (#). The controlled synthesis of one or the other desired optical isomer of two enantiomeric forms of one complex can therefore be of central interest for biochemical purposes. The design and subsequent synthesis of polynuclear metal complexes as supramolecular species for molecular devices (9,10) has recently evoked considerable interest. Figure 2 shows an example of a trinuclear species recently investigated for intramolecular energy- and electron-transfer processes. One of the major problems in the characterization of such species is the occurrence of a great number of isomers. Nature has chosen to built up biopolymers as, e.g., proteins from EPC's (Enantiomerically Pure Compounds, i.e., the amino acids), certainly also in order to perform sophisticated tasks like chiral recognition, etc., but in zeroth order, to avoid the problem of producing up to 2 isomers from Ν chiral, racemic building blocks. Enantioselective (asymmetric) catalysis operates almost always with metal centers being the reactive sites, determining the stereochemical course of the reaction. Although considerable progress has been made in the design of catalysts of this type 2

3

A

A

N

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


VON ZELEWSKY ET AL.

Chirality in Coordination Compounds

Figure 1. Survey CD-Spectrum of a small crystal of the original sample of [Co(N0 ) (en) ]Br prepared by Edith Humphrey in 1900 2

2

2

4

1

(856 mg dissolved in 5.0 ml H 0 ; c = 4.9 · 1 0 ' m o l L " ) 2

Figure 2. Structure of a trinuclear ruthenium complex (Adapted from réf. 11)


295

296


(72), there is a strong need for chirally stable coordination units which survive many catalytic cycles. A

M ( L L ) X 2 Chiral Building Blocks. In 1913 Werner (73) reported that certain optically active metal complexes undergo substitution reactions without racemization. He described, e.g.,. the following reactions : 2

Ο


OC

>]C 1 + K C 0 = 2 K C 1 +

Coen

[a

2

3

laevorotatory

CC

β eη n

C N °° 22 J) [I sS C laevorotatory

coe

\™ :

X + N a N 0

coen

2

=

N a C 1

Co en

Cl

Ο dextrorotatory

+

[sCN ° 2] dextrorotatory C

e n

:

OH _ _ Co en .NH . dextrorotatory 2

2

laevorotatory 0 en Co Co en '.NH, laevorotatory 2

9

.N0

2

Co en .NH, dextrorotatory

He tacitly assumed that the configuration is retained in these cases. Several researchers have since studied substitution reactions in optically active OC-6 coordination species (14,15), but the subject is far from being exhaustively 2+ investigated. We have found that, e.g., [Ru(o-phen)2(py)2] and 2+ [Ru(bpy)2(py)2] ^ chiral building blocks for the diastereoselective synthesis of dinuclear complexes with various types of bridging ligands (16). Figure 3 shows formulas and NMR-spectra of two dimeric compounds synthesized stereospecifically, using chiral building blocks of this type. Although this strategy yields the desired molecules, the enantiomerically pure building blocks still have to be obtained through resolution of a racemic mixture, and, in addition, some racemization can occur under severe reaction conditions. Both limitations are essentially avoided by using another approach. Chiragen[X], a New Ligand Family, Predetermining Chirality at the M Center. The two bidentate ligands in a complex M ( L L ) 2 in OC-6 determine the Δ or A-helical chirality of the metal. If the two L L ligands are connected by a conformationally rigid, chiral bridge, one of the two configurations around the metal will be predeteimined. This strategy is schematically depicted in Figure 4. The introduction of chirality centers in the ligands leads to a strong discrimination of the A and Δ configuration, respectively, if the two halves of the ligand are connected with a chiral bridge. m

U

S

Q

A

A


Chirality in Coordination Compounds 297


24. VON ZELEWSKY ET AL.

(c)

JJUUUJLJL

9.0

8.8

8.6

8.4

8.2

8.0 7.8

7.6

7.4

7.2 ppm

Figure 3. *H-NMR spectra (300 M H z in acetonitrile-d3) 4+ (a) the mixture of ΔΔ/ΛΛ- and AA-[Ru(phen>2-bpym-Ru(phen)2] (b) AA-Ru(phen)2-bpym-Ru(phen)2]

4+

;

and (c) AA-[Ru(phen) -bpym2

4+

Ru(phen) ] . 2


298



Ε

Enantiomers Λ

Λ

Δ

Δ

Figure 4. Energy diagrams for bis(bidentate) ligands coordinated cis in OC-6. The introduction of chirality centers in the ligands leads to a strong discrimination of the Λ and Δ configuration, respectively, if the two halves of the ligand are connected with a chiral bridge.

,

Figure 5. 4,5-Pinene-2,2'-bipyridine (I) and 5,6-Pinene-2,2 -bipyridine (Π)


24. VON ZELEWSKY ET AL.


299

The realization of this concept has been achieved through the synthesis of pyridine molecules with substituents derived from pinene (77), the so called pinenepyridines (Figure 5). These molecules react easily at the indicated positions in a completely stereospecific way, in which the connection of two bipyridine moieties is easily achieved using dihalides as bridging molecules. Up to now, aliphatic bridges like - ( C H 2 ) - have been introduced, and a large variety of bridging groups can be applied. Chiragen[X], where X=n, is a ligand with three asymmetric carbon centers in each pinene moiety of the molecule. Being synthesized from enantiomerically pure chiral pool precursors, like myrthenal or pinene itself, the ligand is obtained as an enantiomerically pure compound with known absolute configuration. Pinene is obtainable in both enantiomeric forms from natural sources. Therefore both enantiomers of the chiragen molecules are accessible. Model considerations show that a given configuration of the chiragen molecule can coordinate with an OC-6 metal only in one of the two helical forms Δ or Λ. The configuration of the metal is therefore completely predetermined through the configuration of the ligand (Figure 6). Even if precursors with low enantiomeric purity are used for the synthesis of the ligand, the complex is obtained in much higher configurational purity, since a ligand with "mixed" configuration cannot bind to the metal. Therefore a chiral amplification is achieved. If all the steps of the synthesis yield statistical ratios of the various isomers, the enantiomeric excess of the final complex M(chiragen[X]) is (ee) omplex =


n

C

(ee)p/0.5(l + (ee)p^), if the enantiomeric excess of the precursor material is (ee)p. So far, chiragen[6] and chiragen[5] have been used for the synthesis of complexes of the 2+

type Ru(cnkagenPÇ])(Diimine) (18). NMR-analysis shows clearly the diastereoisomeric purity of the complex. The X-ray structure determination and the CD-spectra prove the expected absolute configuration of the complexes. These ligands will now be optimized for their use in producing metal complexes with predetermined absolute configurations.

New Forms of Chiral Metal Complexes. The tris(bidentate) OC-6 complexes are the classical representatives of three-bladed helical chirality in coordination chemistry. But even two bidentate ligands in OC-6 define a helix, provided the two remaining ligand positions are cis. As recently shown, two skew bidentate ligands can also occur in SP-4 coordination. It was often assumed (79,) that chirality can occur in SP-4 coordination only if either chiral ligands are coordinated to the central metal or if specially designed ligands are used, where the symmetry of the one ligand is broken by the other ligand, as, e.g., in the historically well known resolution of 1,2-diamino2-methvlpropane(meso-1,2-diamino-1,2-diphenylethane)-platinum (II) chloride reported by Mills and Quibell in 1935 (20).

The Two-Bladed Helix in SP-4.

Biscyclometallated complexes with

ο A

C N-Ligands of d metal ions are known only in the cis configuration (21-23) unless the ligands are specially designed (24) so that the trans configuration is forced upon the ligands. This is the case even when the two ligands make a coplanar arrangement impossible, as , e.g., in Pt(thq)2 or Pt(Hdiphpy)2 (Figure 7). Crystal structure analysis and detailed NMR-spectroscopic investigations reveal clearly die chiral nature of these complexes (25). Separation of these uncharged enantiomers proved hitherto to be extremely difficult. A method for the specific synthesis of one enantiomer was therefore sought


300



Figure 8. Pt(th-4,5ppy>2


24.

V O N ZELEWSKY ET AL.


301


The Versatility of the Pinene-Pyridines. The pinene bipyridines described above are only a special case of the larger class of the pinene pyridines. A number of new pinene pyridines with the thiophenyl or phenyl group as substituent instead of the second pyridine ring, suitable for forming cyclometallated complexes, have now been synthesized. Easy cyclometallation is obtained, e.g., with the ligands th-4,5pinenepyridine or th-5,6-pinenepyridine, where th=2-(thiophen-2-yl) (Figure 8). Pt(th-4,5ppy)2 is a crystalline compound, which shows, according to X-ray analysis, Λ chirality. Again the absolute configuration is obtained directly from the structure analysis because the absolute configuration of the pinene moiety is known. N M R - and CD-analysis shows that the A-configuration is preserved in solution. Oxidative addition reactions of cylometallated Pt(H) complexes (Suckling, Α.; von Zelewsky, A . Inorg. Chem., submitted ) with organic halides R X yield chiral Pt(IV), OC-6 complexes. Generally, both forms (the A and the Δ) of the octahedral complex Pt(th-4,5ppy>2(R)(X) are produced (only one stereoisomer, the C,C-cis; Ν,ΝΓ-cw, C(chelate), X-trans isomer is formed), but interestingly, they have very different solubilities, making separation an easy task. By this pathway, a new synthetic route to enantiomerically pure bis(bidentate) OC-6 complexes from chiral SP-4 complexes has been established. Acknowledgments: The authors thank the Swiss National Foundation for financial support of this work. They also wish to thank Dr. Liz Kohl for help in the preparation of this manuscript. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18)

Kelvin, Lord. In Baltimore Lectures ; Clay C. J. & Sons, Ed.; Cambridge University Press Warehouse, 1904. Kelvin, Lord. In Baltimore Lectures ; Clay C. J. & Sons, Ed.; Cambridge University Press Warehouse, 1904, Appendix Η, p 619. Werner, Α.; Vilmos, Α. Ζ. Anorg. Chem. 1899, 21, 145. Werner, A. Ber. 1914, 47, 3087; for a discussion and annotated English translation see Kauffman, G. B. Classics in Coordination Chemistry, Part 1: The Selected Papers of Alfred Werner; Dover: New York, 1968; pp 175-184. Bernal, I.; Kauffman, G. B. J. Chem. Educ. 1987, 64, 604. Kauffman, G. B.; Bernal, I. J. Chem. Educ. 1989, 66, 293. Yoneda, H. Book of Abstracts 205th ACS National Meeting, Denver, CO, March 20th-April 2nd, 1993, HIST. 35. Pyle, A. M.; Barton, J. K. Prog. Inorg. Chem. 1990, 38, 413. Supramolecular Chemistry ; Balzani, V.; De Cola, L., Eds.; Kluwer: Dordrecht, The Netherlands, 1992. Supramolecular Photochemistry ; Balzani, V.; Scandola, F., Eds.; Ellis Horwood: Warwick, England,1991. De Cola, L; Belser, P.; Ebmeyer, F.; Barigelletti, F.; Vögtle, F.; von Zelewsky, A.;Balzani, V. Inorg. Chem. 1990, 29, 495. Noyori, R. Science 1990, 248, 1194. Werner, A. Neuere Anschauungen auf dem Gebiete der Anorganischen Chemie ; F. Vieweg & Sohn : Braunschweig, 1913; p 368. Archer, R.D. Coord. Chem. Rev. 1969, 4, 243. Bailar, Jr., J.C. Coord. Chem. Rev. 1990, 100, 1. Hua, X.; von Zelewsky, A. Inorg.Chem.1991, 30, 3796. Hayoz, P.; von Zelewsky, A. Tetrahedron Lett. 1992, 33, 5165. Hayoz, P.; von Zelewsky, Α.; Stoeckli-Evans, H. J. Am. Chem. Soc. 1993, 115, 5111



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(19) Inorganic Chemistry ; Purcell, K. F.; Kotz, J. C., Eds.; W. B. Saunders Company: Philadelphia, London, Toronto, 1977;p627. (20) Mills, W. H.; Quibell, T. Η. H. J. Chem Soc. 1935, 839; for a discussion and annotated reprint see Kauffman, G. B. Classics in Coordination Chemistry, Part 3: Twentieth-Century Papers (1904-1935); Dover: New York, 1978; pp 196-224. (21) Chassot, L.; von Zelewsky, A. Helv. Chim. Acta 1983, 66, 2443. (22) Chassot, L.; Müller E.; von Zelewsky, A. Inorg. Chem. 1984, 23, 4249. (23) Cornioley-Deuschel, C.; Ward, T.; von Zelewsky, A. Helv.Chim.Acta 1988, 71, 130. (24) Cornioley-Deuschel, C. Dissertation 1988 No 938, University of Fribourg: Fribourg, Switzerland. (25) Cornioley-Deuschel, C.; Stoeckli-Evans, H.; von Zelewsky, A.J.Chem.Soc., Chem. Commun . 1990, 121. RECEIVED February 8, 1994


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