Coordination Chemistry - American Chemical Society

Chapter 26

Mechanism of Optical Resolution of Octahedral Metal Complexes 1

Hayami Yoneda and Katsuhiko Miyoshi

Downloaded by PENNSYLVANIA STATE UNIV on July 23, 2013 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0565.ch026

1

2

Department of Applied Science, Faculty of Science, Okayama University of Science, 1-1 Ridai-cho, Okayama 700, Japan Department of Chemistry, Faculty of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 724, Japan 2

The study of the chiral discrimination between antimony d-tartrate and a series of [Co(N)6]3+ complexes reveals that chiral discrimination is ef fected by this anion through hydrogen-bonding association along the C2 or C 3 axis of the complex. Furthermore, it is clear that the antimonyl d -tartrate ion favors the A-complex in the C3 association, while it favors the Δ-complex in the C 2 association. A concrete association model is proposed to account for the mode of chiral discrimination between the complex and the antimonyl d-tartrate ion. The fundamental idea underly ing this association model has been applied to the interpretation of the chiral discrimination between complex cations and complex anions. The C 3 complex cation always favors the homochiral combination (Δ-Δ or Λ-Λ) with the complex anion , while the C2 complex cation always fa vors the heterochiral combination (Δ-Λ or Λ-Δ) with the complex anion. This tendency is explained by assuming that straight N-H···O hydrogen bonds are formed in such a favorable pair. This hydrogen bonding asso ciation model has made possible the highly efficient optical resolution of electrically neutral tris(aminoacidato)cobalt( III). +

+

For many years we have been interested in clarifying the mechanism of optical resolution of octahedral metal complexes. Since [Co(en)3]3+ ( en = ethylenediamine) is a prototype of chiral metal complexes and d-tartrate and antimonyl ^-tartrate ions are two main resolving agents for metal complexes, we concentrated on the system involving this complex and these and these two resolving agents. unique face-to-face

As a result, a

ion-pair structure was found as a common factor in three 2

diastereomeric salts containing A-[M(en)3]3+ and d-tart " (7,2). We believe that the origin of chiral discrimination lies in this unique ion-pair structure. Based on this C3

0097-6156/94/0565-0308$08.00/0 © 1994 American Chemical Society In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

26. YONEDA & MIYOSHI

Optical Resolution of Octahedral Metal Complexes 309 2

association model, successful chromatographic resolution by using d-tart " as eluent was achieved for several cobalt(III) complexes having a triangular face composed of three N H 2 groups. As to the mode of chiral discrimination by [Sb2(d-tart)2] ~, attention was paid to 2

the shape of the channel formed between the en chelate rings. The channel between chelate rings in the A-configuration has a shape of L , and the channel between chelate rings in the A-configuration has a shape of J. It was assumed that the [Sb2(d-tart)2] " 2

ion has a skeleton which makes a good fit to an L-shaped channel of the A-complex, Downloaded by PENNSYLVANIA STATE UNIV on July 23, 2013 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0565.ch026

but not to the J-shaped channel of the A-complex. With this L - J model, we can explain the degree of optical resolution (separation factor) for a series of [ C o ( N ) 6 l complexes having different numbers of L-shaped channels (3).

3+

Our early

investigations on the mode of chiral discrimination by these two resolving agents have been reviewed in two articles (4). While the C3 association model for the d-tart ~ ion 2

is based on the concrete ion-pair structure obtained by X-ray crystal analyses, the L-J model is a mere conceptual model of the "key-and-lock" relation applied to chiral discrimination of octahedral complexes. It does not tell anything about the concrete stereochemical features of the ion-pair composed of the complex and the chiral resolving agent. Concerning this point, we noticed that the CD spectra of cobalt(III) complexes are quite sensitive to the addition of counter ions. Analyses of CD changes caused by the resolving agent anion provided a clue to the solution of this problem. We proposed a new association model which accounts for how this chiral selector ion recognizes the chirality of [Co(en)3]3+ and related complexes in solution. In this article, we review the studies concerning this association model and the succeeding studies concerning chiral interactions between complex cations and complex anions. C h i r a l Discrimination by

2

[Sb2(rf-tart)2] "

in Solution

In order to deduce the direction of access of the resolving agent toward the complex , the effect of the resolving agent upon the C D spectra has been studied for a series of [Co(N)6]

3+

of trigonal symmetry.

These complexes have N - H bonds projecting

outward along their C3 and C 2 axes. They are classified into C 3 , C 2 and ( C 3 + +

+

+

C 2 ) complexes according to the kind of axis along which they accept the oncoming +

anion ( Figure 1). As to the effect of [Sb2(d-tart)2] " upon the C D peak in the d-d 2

transition region of the spectrum of the complex, addition of this ion enhances the intensity of the A 2 component for all the C 3 and (C3+ + C 2 ) complexes examined, +

+

while it causes enhancement of the Ea component for the C2" " complex, [Co(sep)]3+ 4

(sep = 1,3,6,8,10,13,16,19-octaazabicyclo[6,6,6]eicosane) (5). Enhancement of the A 2 and Ea components can be taken to indicate axial and equatorial perturbations,

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

COORDINATION CHEMISTRY

310

respectively, which are caused by formation of hydrogen bonding between axial or equatorial N H hydrogens of the complex and oxygen atoms of the [Sb2(tart)2] ~ ion. 2

Here,DCD spectra are defined as the CD spectra with the anion minus the CD spectra without the anion. The pattern of the DCD spectra is quite similar to the pattern of the corresponding DCD with tart - ions (6). Thus it is reasonable to assume that [Sb2(d2

tart)2] " also anchors the complex with N-H—O hydrogen bonds along the C3 or C2 2

axis. The next problem is to determine which oxygen atoms of the [Sb2(d-tart)2] " 2


ion are utilized for hydrogen bonding. Examining the structure of this anion in detail, we found that one carboxylate oxygen atom (directly linked to the Sb atom) of one tartrate group and one alcoholic oxygen atom of the other tartrate group attached to the same Sb atom have their lone-pair electrons spatially disposed to interact with two N-H bonds of the complex, whether the association takes place along the C3 or the C2 axis. The [Sb2(d-tart)2] ~ ion has four such pairs of oxygen atoms. 2

Now that the oxygen atoms of the anion for hydrogen bonding were determined, the mode of chiral discrimination by this anion can be visualized (7).

The mode of

chiral discrimination in the C3 association will be described, taking [Co(en)3] + as an 3

example (Figure 2).

Two oxygen atoms of the anion are linked to two axial N - H

bonds on a triangular face of the complex and form double hydrogen bonding. The remaining part of the anion is oriented to a position opposite to the third N - H bond on the triangular face so as to avoid steric repulsion..

In such an orientation, the

carboxylate group attached to the distal Sb atom comes upon an opening between two chelate rings of the Λ-complex., while it comes close to a chelate ring of the Δcomplex. In this way, it is clear that the d-anion forms a favorable ion-pair with the Λcomplex. Since there are two triangular faces in [Co(en)3] , each of which has three 3+

axial N - H bonds, and only two N - H bonds are required for ion-pair formation with the anion, six favorable associations are posssible for A-[Co(en)3] . 3+

Different numbers of favorable associations are possible for other [Co(N)6]

3+

complexes. The numbers of favorable associations are (6,0), (4,0), (3,0), (4,2), and (0,0) for the ( Λ , Δ ) pairs of [Co(en)3] triethylenetetramine

3

+

, a-[Co(trien)(en)]

3+

), p-[Co(trien)(en)] +, w - / a c - [ C o d i e n ) 2 ] 3

( trien = 3+

(dien =

diethylenetriamine), and /ner-[Co(dien)2] . These numbers coincide exactly with the 3+

numbers of L-shaped channels.

The separation factors obtained experimentally are

1.45, 1.37, 1.28, 1.20, and 0 in this order (5). Although the efficiency of enantiomer separation is low, the Δ-isomer is eluted earlier with [Sb2(d-tart)2] " in all cw-[CoX2(en)2] complexes. 2

+

In this case , the

anion is assumed to approach the complex along the C2 axis and to form hydrogen bonding with the two N - H bonds located trans to the negative ligands X .


In this C2


26.

YONEDA & MIYOSHI

Optical Resolution of Octahedral Metal Complexes 311

Fig. 1. Structures of A-[Co(N)6] complexes (All hydrogen atoms are omitted.) 3+

50ml

100ml

200ml

Fig. 2. Elution curve of [Co(ama)3] complexes, column : saturated with A-[Co(sep)] 3 +


312


association the distal carboxylate group of the associated anion experiences steric repulsion by the chelate ring in the Λ-complex, but no such steric repulsion is experienced by the anion for the Δ-complex.

Therefore it is not surprising that the

[Sb2(d-tart)2] " ion forms a favorable ion-pair with the Δ-complex ( Such C 3 2

association is , of course, possible also for [Co(en)3] . 3+

However, the contribution

of the C 3 association is considered to surpass the contribution of the C 2 association. This interpretation was supported by the sign of the DCD spectra.). 3


A n exception is the case of [Co(sep)] +, in which chiral discrimination takes place exclusively through the C 2 association, yet the A-complex is eluted earlier. Here the repulsion by the alkyl cap in the Δ-complex is stronger than that by the en chelate ring in the A-complex so that the A-complex is favored by the [Sb2(d-tart)2] ~ ion and 2

is eluted earlier. The cases of cis-[Co(N3)2(en)2]

+

and cis -[Co(acac)(en)2] ( acac 2+

= acetylacetonate) are also exceptions , in which the Λ-isomer is eluted earlier. It is evident that the "C3 association" ( the association corresponding to the C3 association for [Co(en)3] ) surpasses the C 2 association. 3+

Weak repulsion by the low effective

negative charge of the N3" ion and the acac-O atom is not as effective in preventing the "C3 association". The "C3 association" is assumed to occur for c/5 -a-[CoX2(trien)] , ,

+

in which the C 2 association is completely prevented. It is therefore not surprising that +

the Λ-isomer is eluted earlier for cw-a-[CoX2(trien)] . C h i r a l D i s c r i m i n a t i o n b y C o m p l e x I o n s (8) Quite a few examples of optical resolution via formation of the less soluble diastereomeric salt have been reported in which the species to be resolved and the chiral selector are both complex ions. Thus elucidation of the mode of chiral discrimination between complex cations and anions is an important factor leading to the discovery of new efficient resolving agents. As one of several approaches to this subject, we carried out very simple experiments. SP-Sephadex C-25 saturated with a Δ-complex cation was packed in a glass column. A racemic complex anion was eluted through the column with 30% aqueous ethanol. recorded.

The eluate was fractionally collected, and the A B and C D spectra were The elution curves were obtained in this way when necessary, but the

degree of resolution was generally so low that only the elution orders were determined in most cases. The complexes used as resolving agents were those having N H or N H 2 groups which accept the anion along either the C3 or the C 2 axis. resolved were those having COO" groups which accept either the C 3 or the C 2 axis.

The complexes to be

the complex cation along

They are classified as C 3 " , C 2 " , and ( C 3 - + C2") ,


26.


YONEDA & MIYOSHI

according to the mutual disposition of carboxylate groups they use in ion-pair formation. The result is shown in Table I, where the chirality of the second-eluted enantiomer is indicated. Since the second-eluted enantiomer is the one which forms a favorable pair with the chiral selector cation in the stationary phase, the result can be +

summarized as follows:

The C 3 complex cation always favors the homochiral +

combination (Δ-Δ or Λ-Λ) with the complex anion, while the C 2 complex cation always favors the heterochiral combination (Δ-Λ or Λ-Δ), irrespective of the chirality of the complex anion . This seems to be a general tendency. In contrast, the (C3 " + Downloaded by PENNSYLVANIA STATE UNIV on July 23, 2013 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0565.ch026

4

C2 ") complex represented by [Co(en)3] + favors the homochiral combination with 3

4

some complex anions and the heterochiral combination with the other complex anions, and the degree of optical resolution achieved is very low. These results can be explained by hydrogen-bonded association models. C3

+

The

complex directs its C 3 axis to the anionic complex to form double or triple

hydrogen bonds between N H 2 and COO". Inspection of the molecular model reveals that nearly straight N-H- Ο hydrogen bonds are formed in the homochiral combination of ions, while N-H- Ο hydrogen bonds are fairly bent in the heterochiral combination. On the contrary, the C 2 complex directs its C2 axis to the oxygen atoms of the anionic +

complex to form double hydrogen bonds with its N H hydrogens. hydrogen bonds are formed in the heterochiral combination.

Here nearly straight

It is natural to assume

that the straight hydrogen bonds are more stable than the bent hydrogen bonds. In the case of the (C3+ + C 2 ) complex, the C 3 and the C 2 associations take place +

simultaneously , which offsets the effect of chiral discrimination of each other. Since the contribution of the C 3 association is not equal to the contribution of the C 2 association, a small fraction of the contribution remains either for the C 3 or the C 2 association.

This explains the low efficiency of chiral discrimination by the ( C 3 + +

C 2 ) complex. +

Optical

Resolution of

Tris(aminoacidato)cobalt(III)Complexes

Although tris(aminoacidato)cobalt(III) complexes, [Co(ama)3], are electrically neutral, they can be regarded either as complex cations or as complex anions because they can anchor complex anions or complex cations with N-H- Ο type hydrogen bonds along the C3 or the C 2 axis. Therefore they may be expected to be resolvable either by chiral complex anions or by complex cations. 3

We here describe the optical resolution of 3

some [Co(ama)3] using A-[Co(sep)] + and A-[Co(chxn)3] + (chxn = cyclohexanediamine) as chiral selectors.

The procedure is almost the same as described in the

preceeding section.


314



Table I. Enantiomers Forming Favorable Pairs with Δ Cation Complexes

C2' /

[Co(sep)]

3+

[Co(acac)(en)2]

2+

[CcKoxXen)^ ciî-[Co(N02)2(en)2]

+

C3

3

I

\

/

\

1

2

3

4

5

Λ Λ

Λ Λ

Λ Λ

Λ Λ

Λ Λ

Λ Λ

Λ Λ

Λ Λ

Λ Λ

Λ Λ

Δ

Δ

Δ

Δ

Δ

Δ* Λ*

Δ Λ

Λ Λ

Λ Λ

Λ

Λ

Λ

Λ

+

lel -[Co(chxn) ] 3

3+

3

+

c +c 2

+ 3

[Co(en)3]

α

Δ Δ* Λ

3+

[CoiglyXen)^ [Co(sen)]

3+

a 1

(C2" + C " )

Ρ3"

[Co(ox) (en)r

4

2

2

C -[Co(ox)(gly)2]"

3

/oc-[Co(P-ala) ]

2

5

very low resolution

C d5(N)-[Co(ox)(gly) ]" r

2

2

[Co(ox) (gly)] 2

3


26. YONEDA & MIYOSHI


A racemic complex is loaded at the top of the column and eluted with 30% 3+

aqueous ethanol.

On the column containing A-[Co(sep)] very good resolution is

achieved for/ac-[Co(P-ala)3] and/ac-[Co(a-ala)3] ( ala = alaninate ), while only poor resolution is attained for mer-[Co(|3-ala)3] (Figure 2 )(9). On the column containing +

A-[Co(chxn)3]3 /ac-[Co(a-ala)3] and/ac-[Co(P-ala)3] are resolved to some extent but the elution order is reversed. anion.

In these cases [Co(ama)3] behaves as a complex

This reconfirms that chiral discrimination between complex cations and

complex anions is attained by hydrogen bonding along the C3 or the C2 axis and that the C 3 complex favors the homochiral ion-pair, while the C 2 complex favors the Downloaded by PENNSYLVANIA STATE UNIV on July 23, 2013 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0565.ch026

+

+

heterochiral ion-pair. [Co(chxn)3] examined so far has three N - H bonds nearly parallel to the C3 3+

axis and should be correctly designated as lel3-[Co(chxn)3] +. 3

There is another

isomer, ob3-[Co(chxn)3] , which has three N - H bonds oblique markedly from the 3

+

C3 axis. Using these two [Co(chxn)3] + complexes as a chiral selector, optical 3

2

resolution was attempted for/ac-[Co(a-ala)3],/ac-[Co(P-ala)3], [Co(ox)2(gly)] ' (ox = oxalate; gly = glycinate), Ci-ris(N)-[Co(ox)(gly)2]~, and Ci-cw(N)-[Co(ox)(Pala)2l". The elution curves obtained for fac-[Co(P-ala)3] are shown in Figure 3 (10). As seen in this figure, /ac-[Co(P-ala)3] is not as well resolved by A-lel3[Co(chxn)3] , but yet it is seen that the Δ-isomer is eluted undoubtedly later, which 3+

means that the A-lel3 complex cation favors the Δ-isomer, as expected.

In contrast to

this, the same complex is completely resolved by the A-ob3 complex ( the separation factor is extremely high, ca..7), and the Λ-isomer is eluted later, which means that the A-OD3 complex favors the Λ-isomer. Therefore the heterochiral combination (Δ-Λ) is favored in this C3 association. Why is the elution order of the resolved enantiomers reversed to that expected for the C3 association ?

Figure 4 shows the schematic structures of the complexes

involved in the present optical resolution. Three N - H bonds in a triangular face of Δob3-[Co(chxn)3]3 are markedly inclined from the C3 axis so that they assume left+

handed chirality. As a result, they form straight hydrogen bonds with the lone pairs of the three oxygen atoms of A-/ac-[Co(ama)3] (For carboxylate oxygen , s p

2

hybridization is assumed). On the contrary, the lone pairs of the three oxygen atoms of A-/ac-[Co(ama)3] have right-handed chirality so that they cannot form hydrogen bonds favorable for the stable association with left-handed A-ob3-[Co(chxn)3] . 3+

The remaining four complexes-/ac-[Co(a-ala)3] and the three complex anionswere also resolved in the same way as was/ac-[Co(P-ala)3}.

The chirality of the

second-eluted enantiomer was Δ on the column of the lel3-complex, and it was Λ on the column of the ob3-complex.

Thus a similar hydrogen bonding is also probably

operating in the optical resolution of these complexes.


316


/0C-[Co(P-ala) ] 3

3+

Column : A-lel -[Co(/-chen) ] C Association : Homochiral Pair (Δ-Δ) 3

3

3


50ml

100ml 3+

Column : A-ob -[Co(*/-chen) ] C Association : Heterochiral Pair (Δ-Λ) 3

3

3

-4h

50ml

300ml

400ml

Fig. 3. Elution curve of /ac-[Co(P-ala) ], column : saturated with A-lel -[Co(chxn) ] + and with A-ob3-[Co(chxn) ] 3

3

3

3

3+

3

— A-ob -[Co(u?-chxn) ]

3+

A-lel -[Co(/-chxn) ]

3+

3

3

3

3

N

A-/ac-[Co(P-ala) ] 3

A-/ac-[Co(P~ala) ] 3

Fig. 4. Structures of A-ob - and A-lel -[Co(chxn) ]3+ and and A-/ac-[Co(P-ala) ] 3

3

3

A-fac-

3


26.

Y O N E D A & MIYOSHI

Optical Resolution of Octahedral Metal Complexes

317

Acknowledgment We would like to express our heartfelt thanks to Professor George B. Kauffman for the invitation to speak at the Coordination Chemistry Centennial Symposium. Literature Cited 1. (a) Kushi,Y. ; Kuramoto, M . ; Yoneda, H . Chem. Lett. 1976,

135; (b)

Kushi, Y . ; Kuramoto, M . ; Yoneda, H . Chem. Lett. 1976, 3396; (c) Kushi, Y . ; Tada, T. ; Yoneda, H . Chem. Lett. 1977, 379. Downloaded by PENNSYLVANIA STATE UNIV on July 23, 2013 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0565.ch026

2. The common existence of the face-to-face close contact was verified by succeeding crystal structure analyses of complexes: (a) Δ-[Co(sen)]Cl(d-tart)·6H2O: Okazaki, H.; Sakaguchi, U . ; Yoneda, H . Inorg, Chem. 1983, 22, 1539; (b)Δ,Λ-[Ni(en)3]2(d,l-tart)·H2O: Mizuta,T.; Yoneda, H.; Kushi, Y . Inorg, Chim. Acta 1987, 132, 11; (c)Δ,Λ-[Co(en)3]2(d,l-tart)·10H2O, Λ-[Co(en)3]2(d-tart)3·19H2O, and Δ-[Co(en)3]2(d-tart)3·11.5H2O: Mizuta, T.; Tada, T.; Kushi, Y . ; Yoneda, H . Inorg. Chem. 1988, 27, 3836; (d)Λ-1e13-[Co(chxn)3]Cl(d-tart)·2H2O and Δ-1e13-[Co(chxn)3]Cl (d-tart)·2H2O: Mizuta, T.; Toshitani, K . ; Miyoshi, K.; Yoneda, H . Inorg. Chem. 1990, 29, 3020. 3. Nakazawa, H . ; Yoneda, H . J. Chromatogr. 1978, 160, 39. 4. (a)Yoneda, H . J. Liq. Chromatogr. 1979, 2, 1157; (b) Yoneda, H . J. Chromatogr. 1984, 313, 59. 5. Sakaguchi, U . ; Tsuge, Α.; Yoneda, H . Inorg. Chem. 1983, 22, 3745. 6. Sakaguchi, U . ; Tsuge, Α.; Yoneda, H . Inorg. Chem. 1983,

22, 1630.

7. Miyoshi, K . ; Izumoto, S.; Yoneda, H . Bull. Chem. Soc. Jpn. 1986, 59, 3475. 8. Miyoshi, K . ; Sakamoto, Y . ; Ohguni, Α.; Yoneda, H . Bull. Chem. Soc. Jpn.

1985, 58, 2239. 9. Miyoshi, K . ; Nakai, K . ; Yoneda, H. Presented at the Meeting of the ChugokuShikoku Branch of the Chemical Society of Japan , held in Tokushima, 1986. 10. Miyoshi, K.; Yoshinaga, M.; Yoneda, H. Presented at the Annual Meeting of the Chemical Society of Japan , held in Tokyo, 1987. R E C E I V E D December 27,

1993


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