Stereochemistry of Optically Active Transition Metal Compounds

12

T h e N a t u r e of the E q u i l i b r i u m Displacement M e c h a n i s m for the P feiffer Effect i n Inorganic Chemistry

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 2, 2015 | http://pubs.acs.org Publication Date: May 27, 1980 | doi: 10.1021/bk-1980-0119.ch012

STANLEY KIRSCHNER and PAUL SERDIUK Department of Chemistry, Wayne State University, Detroit, MI 48202

The P f e i f f e r Effect (1) i s defined as the change i n o p t i c a l r o t a t i o n of an o p t i c a l l y a c t i v e system (usually a solution of one enantiomer of an o p t i c a l l y active compound, c a l l e d the "environment substance", dissolved i n an o p t i c a l l y i n a c t i v e solvent) upon the addition of a racemic mixture of a dissymmetric, o p t i c a l l y l a b i l e coordination compound. Much work has been done on t h i s Effect (2 - 8) and several mechanisms have been proposed to explain it, which are described i n a review by Schipper (9). It i s of i n t e r e s t to note that the Effect can occur with racemic mixtures of c e r t a i n o p t i c a l l y l a b i l e complex cations ( e . g . , D , L [ Z n ( o - p h e n ) ] 2 + ) whether the environment substance i s anionic (dα-bromo-camphor-π-sulfonate), neutral ( l e v o - n i c o t i n e ) , or c a t i o n i c (d-cinchoninium). The most frequently used solvent for the P f e i f f e r Effect is water (10), although the Effect i s known to occur i n other solvents as well ( 3 , 4 , 6 ) . Since the magnitude of the Effect i s proportional to the concentrations of both the environment substance and the complex, a s e r i e s of equations have been developed for observed P f e i f f e r r o t a t i o n , s p e c i f i c P f e i f f e r r o t a t i o n , and molar P f e i f f e r r o t a t i o n which are analagous to those for observed o p t i c a l r o t a t i o n , s p e c i f i c o p t i c a l r o t a t i o n , and molar o p t i c a l r o t a t i o n 3

(3,4,6,10).

These are

0-8412-0538-8/80/47-119-239$05.00/0 © 1980 American Chemical Society

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

240

where i s the observed P f e i f f e r r o t a t i o n ( i n degrees;, " a i s the observed r o t a t i o n of the s o l u t i o n c o n t a i n i n g t h e r a c e m i c complex and t h e environment substance, a i s the observed r o t a t i o n of the s o l u t i o n c o n t a i n i n g the environment substance only, [P] i s t h e s p e c i f i c P f e i f f e r r o t a t i o n , ( c ) i s t h e c o n c e n t r a t i o n o f t h e c o m p l e x i n g/ml ( e ) i s t h e c o n c e n t r a t i o n o f t h e environment s u b s t a n c e i n g/ml, d i s t h e p a t h l e n g t h i n dm, [PM] i- ^ roc-lar P f e i f f e r r o t a t i o n , [ c ] i s the molar c o n c e n t r a t i o n of the complex, [ e ] i s the molar c o n c e n t r a t i o n of the environment s u b s t a n c e , and d i s the path length i n meters. e

+

c

Q


s

e

m

The

E q u i l i b r i u m Displacement

Mechanism

Dwyer and c o - w o r k e r s (2) and K i r s c h n e r and c o - w o r k e r s (JO have p r o p o s e d t h a t t h e e n a n t i o m e r s o f racemic mixtures of o p t i c a l l y l a b i l e , dissymmetric complexes are i n e q u i l i b r i u m i n s o l u t i o n s c o n t a i n i n g no e n v i r o n m e n t s u b s t a n c e , a n d t h a t t h e e q u i l i b r i u m c o n s t a n t i n s u c h s y s t e m s i s e q u a l t o 1. However, i n t h e p r e s e n c e o f one e n a n t i o m e r o f a n o p t i c a l l y a c t i v e environment substance, t h i s e q u i l i b r i u m i s s h i f t e d , w i t h a c o n s e q u e n t e n r i c h m e n t o f one o f t h e enantiomers of the complex, thereby changing the e q u i l i b r i u m constant to something g r e a t e r or l e s s than 1. An e q u a t i o n w h i c h r e p r e s e n t s a t y p i c a l P f e i f f e r Effect equilibrium i s : +

A( )

D

[Ni(o-phen)^]

2 +

J

Δ(-)ρ [ N i ( o - p h e n ) ^

2 +

(

-

}

F i g u r e 1 shows t h e P f e i f f e r E f f e c t f o r t h e r a c e m i c c o m p l e x [ C r ( C 2 0 ^ ) 3]·^" i n t h e p r e s e n c e o f d - c i n c h o n i n i u m c h l o r i d e , and t h i s f i g u r e a l s o p r o v i d e s s t r o n g s u p p o r t f o r t h e e q u i l i b r i u m d i s p l a c e m e n t mechanism d e s c r i b e d above. I t s h o u l d be n o t i c e d t h a t t h e o p t i c a l r o t a t o r y d i s p e r s i o n (ORD) o f t h e c o m p l e x i n t h e P f e i f f e r E f f e c t ( F i g u r e 1) i s e s s e n t i a l l y t h e same a s t h a t o f t h e p u r e e n a n t i o m e r r e s o l v e d by c o n v e n t i o n a l means, and i t shows a m a r k e d C o t t o n E f f e c t . Since the environment s u b s t a n c e i t s e l f shows o n l y a p l a i n o r n o r m a l o p t i c a l r o t a t o r y d i s p e r s i o n i n t h e v i s i b l e r e g i o n , t h e ORD i n t h e P f e i f f e r e x p e r i m e n t m u s t be due t o a n e x c e s s o f one e n a n t i o m e r o f t h e c o m p l e x o v e r t h e o t h e r - w h i c h i s what i s p o s t u l a t e d t o o c c u r i n t h e e q u i l i b r i u m d i s p l a c e m e n t mechanism. I t s h o u l d be m e n t i o n e d a t t h i s p o i n t t h a t Yoneda and c o - w o r k e r s (11) p r e s e n t e v i d e n c e i n f a v o r



KiRscHNER A N D sERDiuK

Pfeiffer

Effect

241

1 0 . 0 rο I

7.5 5.0

z: ο

2.5

< Ι Ο

0.0

LU LL

UJ UL

< _J Ο Σ

-2.5 -5.0

-

-7.5

-

»·· · Β

•10.0 3

(-)

D

(+)

D

with

Ni(o-phen)

2+ 2+

Observed Absolute Configuation Δ

Δ

Δ

2+

Δ

Δ

2+

Δ

Δ

3

Fe (o-phen)3

Predicted Absolute Configuation Δ

Systems




246

T h i s o b s e r v a t i o n a p p e a r s t o be g e n e r a l f o r t h e P f e i f f e r E f f e c t w i t h t r i s ( b i d e n t a t e ) complexes and e n v i r o n m e n t s u b s t a n c e s w h i c h a r e o r g a n i c compounds c a p a b l e o f u n d e r g o i n g hydrogen bonding ( 1 2 ) , and i t provides a technique f o r p r e d i c t i n g the absolute c o n f i g u r a t i o n s of dissymmetric, o p t i c a l l y l a b i l e complexes, as w e l l as o f o r g a n i c a c i d s c a p a b l e o f a c t i n g as environment s u b s t a n c e s i n t h e P f e i f f e r Effect. Yoneda and c o - w o r k e r s (11) have p o i n t e d out t h a t t h i s o b s e r v a t i o n d o e s n o t h o l d when t h e e n v i r o n m e n t substance i s d-cinchoninium c a t i o n , i n support of a d i f f e r e n t mechanism f o r t h e P f e i f f e r E f f e c t i n t h i s c a s e . Hydrogen

B o n d i n g and t h e P f e i f f e r

Effect

S e v e r a l papers have appeared i n t h e l i t e r a t u r e (i-iii^t) d e s c r i b i n g hydrogen bonding to the -electron c l o u d s o f a r o m a t i c systems, and t h e y p o i n t out t h a t t h e hydrogen bond i s i n a l i n e p e r p e n d i c u l a r t o the p l a n e of the 7r-electron cloud. I t i s proposed that the fundamental n a t u r e of the P f e i f f e r I n t e r a c t i o n between t h e environment s u b s t a n c e and t h e complex ( w h i c h i s o p e r a t i v e i n t h e e q u i l i b r i u m d i s p l a c e m e n t mechanism) i s h y d r o g e n b o n d i n g b e t w e e n OH g r o u p s o f t h e e n v i r o n m e n t c o m p o u n d a n d t h e π-electron c l o u d s o f t h e l i g a n d s o f the complex. F i g u r e 4 shows s c h e m a t i c a l l y t h e " h e a d - o n " type of hydrogen bonding which occurs w i t h aromatic systems and w h i c h i s p r o p o s e d t o o c c u r f o r t h e P f e i f f e r systems d e s c r i b e d h e r e i n . S u p p o r t f o r t h i s p r o p o s a l comes f r o m a s t u d y o f t h e pH d e p e n d e n c e o f t h e P f e i f f e r E f f e c t . I t c a n be s e e n f r o m T a b l e I I I t h a t a n i n c r e a s e i n pH r e s u l t s i n a marked d i m i n u t i o n o f t h e magnitude o f the P f e i f f e r E f f e c t a r e s u l t w h i c h w o u l d n o t be e x p e c t e d on t h e b a s i s o f a n ionic a t t r a c t i o n between t h e complex and t h e environment substance. T h i s i s due t o t h e f a c t t h a t a n i n c r e a s e i n pH o f a s y s t e m w h e r e m a l i c a c i d i s t h e e n v i r o n m e n t s u b s t a n c e , f o r example, would r e s u l t i n t h e f o r m a t i o n o f t h e h y d r o g e n m a l a t e o r m a l a t e a n i o n s w h i c h m i g h t be e x p e c t e d t o be more s t r o n g l y a t t r a c t e d t.Q t h e complex cation than malic acid i t s e l f . This i s c l e a r l y not t h e c a s e , a s c a n be s e e n f r o m T a b l e I I I , s i n c e t h e P f e i f f e r E f f e c t decreases i n magnitude w i t h i n c r e a s i n g pH. R a t h e r , t h i s decrease i n magnitude of the P f e i f f e r E f f e c t w i t h i n c r e a s i n g pH may now be e x p l a i n e d a s a r e s u l t of the markedly decreased a b i l i t y of the malic a c i d t o undergo hydrogen bonding t o the a r o m a t i c e l e c t r o n c l o u d , s i n c e the hydrogens necessary f o r t h i s h y d r o g e n b o n d i n g a r e r e m o v e d a s t h e pH o f t h e system i n c r e a s e s . t


12.

K I R S C H N E R A N D SERDIUK

Pfeiffer

Effect

247

Table I I I . T h e P f e i f f e r E f f e c t a n d pH f o r t h e System Levo-Malic A c i d and D.L-[Ni (o-phen) C l £


P

H

[

P

5

M

] D ° (O)

2680 2660 1980 820 380 320 340 80 120

1.0 2.0 3-0 4.0 5.0 6.0 7.0 8.0 9.0

E n v i r o n m e n t Compounds W i t h No A v a i l a b l e f o r Hydrogen Bonding

Hydrogen

In order t op r o v i d e a d d i t i o n a l t e s t s f o r the hydrogen bonding p r o p o s a l , i t was d e c i d e d t o s t u d y the P f e i f f e r E f f e c t w i t h environment substances h a v i n g reduced (and zero) c a p a c i t y f o r hydrogen b o n d i n g t o the aromatic electron clouds o fthe ligands o f the complexes. F i g u r e 5 shows t h e f o r m u l a e o f t a r t a r i c a c i d a n d s e v e r a l o f i t s d e r i v a t i v e s which have been u t i l i z e d i n t h i s s t u d y . I t s h o u l d be n o t e d t h a t t h e 2- m e t h o x y - 2 - h y d r o x y d e r i v a t i v e p r o v i d e s some o p p o r t u n i t y f o r hydrogen b o n d i n g , whereas t h e o t h e r two s u c c i n a t e d e r i v a t i v e s p r o v i d e no such o p p o r t u n i t y . T a b l e s I V - V I I I d e s c r i b e the P f e i f f e r E f f e c t on t h e D . L - T N i ( o - p h e n ) o] ^* w i t h (+ ) - t a r t a r i c a c i d , sodium hydrogen ( + ) - t a r t r a t e , the d i e t h y l - 2 - m e t h o x y 3- h y d r o x y d e r i v a t i v e , t h e d i e t h y l - 2 , 3 - d i m e t h o x y d e r i v a t i v e , and the dimethyl-2-3-dimethoxy derivative. As c a n be seen from t h e i r s t r u c t u r e s , t h e f i r s t t h r e e o f t h e s e compounds have t h e a b i l i t y t o u n d e r g o h y d r o g e n b o n d i n g , w h i l e t h e l a s t t w o do n o t . T a b l e s I V , V , a n d VI c l e a r l y i n d i c a t e that s i g n i f i c a n t P f e i f f e r E f f e c t s occur i n the systems c o n t a i n i n g those environment substances capable o f undergoing hydrogen bonding, w h e r e a s T a b l e s V I I a n d V I I I show t h a t n o s i g n i f i c a n t P f e i f f e r E f f e c t occurs i n those systems c o n t a i n i n g environment substances which are i n c a p a b l e o f undergoing hydrogen bonding. This provides additional evidence i n support o f the proposed hydrogen bonding m e c h a n i s m m e n t i o n e d . a b o v e . -ι « ι D

Λ

American Chemical Society Library 1155 St. Metal N. W.Compounds; Douglas, B., et al.; In Stereochemistry of Optically Active16th Transition ACS Symposium Series; American Chemical Washington, DC, 1980. Washington, D. C. Society: 20038


248


UNSATURATED PI-ELECTRON CLOUD SYSTEM OF THE 0 PHΕNANTHRO L I N E RINGS

Figure 4. Hydrogen bonding mechanism. Proposed for the Pfeiffer Effect. "Head-on" hydrogen bonding to π-electron clouds of ligand ring systems; Δ(—)[Ni(o-phen)s] with S(—) -malic acid. 2+

0

D

-H

/ 0

/

Η - C - 0-H

Λ

-C H 2

5

Η - Ç - O-CHj

H-0 - C - Η

0

0

H-0 - Ç - Η

Λ

0-H

Ο

0-C H 2

5

DIETHYL-(+)2-METH0XY-3~HYDR0XYSUCCINATE

0

/

0

-C H 2

Ο

5

Ç 3

CH3-O - C - H

À

-CH

3

H - C - O-CH^ 1

CH3-O - Ç - H

0-C H 2

5

DIETHYL-(+)-2,3-DIMETHOXYSUCCI NATE

Figure 5.

/ 0

Î

H - C - 0-CH

0

χ Ν

0

A O-CH3

DIMETHYL-(+)~2/3-DIMETHOXYSUCCINATE

Tartaric acid and substituted tartaric acids


12. KiRscHNER A N D sERDiuK

Pfeiffer

Effect

249


T A B L E IV. THE P F E I F F E R E F F E C T WITH (+) ^-TARTARIC A C I D OBSERVED O P T I C A L ROTATION I N DEGREES

589

528

546

4 MIN.

0.101

0.120

0 .140

0.209

1 HOUR

0.127

0.139

0 .159

0.252

2 HOURS

0.140

0.152

0 .175

0.286

3.66

"

0.159

0.172

0 .196

0.328

5.33

"

0.181

0.191

0 .220

0.379

7-33

"

0.198

0.211

0 .242

0.428

2?.0

"

0.352

0.372

0 .431

Ο.83Ο

53-3

"

0.440

0.465

0 • 538

1.055

75.2

"

0.475

0.500

0 • 579

I.I36

98.0

"

0.488

0.513

0 • 593

1.165

λ(NM.) — >

436

TIME

COMPLEX:

TRI(ORTHO-PHENANTHROLINE)NICKEL(II)

PATH LENGTH: 1 DECIMETER SOLVENT: WATER ; TEMPERATURE: CONCENTRATIONS:

COMPLEX

2 3 ° C.

- 0 . 0 2 MOLAR

ENVIRONMENT - 0 . 0 4 MOLAR




T A B L E V. THE P F E I F F E R E F F E C T WITH SODIUM HYDROGEN (+) ^-TARTRATEOBSERVED O P T I C A L ROTATION I N DEGREES 589

λ ( NM. ) — •

578

546

436

0.221

O.340

0.226

0.348

TIME 2 MINUTES

0 .185

1 . 3 3 HOURS

0 .189

0.195 0.201

3.33

"

0 .194

0.205

0.232

Ο.362

5.17

"

0 .205

0.215

0.242

Ο.386

25.0

0 .261

0.275

0.313

0.536

47.8

0 .288

O.3OI

0.3^5

0.595

73.0

0 .300

0.314

0.357

0.632

0 .303

0.318

Ο.36Ο

0.641

99.7

"

COMPLEX: T R I ( O R T H O - P H E N A N T H R O L I N E ) N I C K E L ( I I ) PATH LENGTH: 1 DECIMETER SOLVENT: WATER; TEMPERATURE: 23° CONCENTRATIONS:

COMPLEX

C.

- 0.02 MOLAR

ENVIRONMENT

- 0.04 MOLAR



KIRSCHNER A N D sERDiuK

Pfeiffer

Effect

TABLE V I . THE P F E I F F E R EFFECT WITH DIETHYL-(+)-2-METHOXY-3-HYDROXYSUCCINATΕ OBSERVED O P T I C A L ROTATION I N DEGREES λ (NM.)—•

589

578

546

436

0.497 0.501

0.513 0.521

0 • 578

0.909

0 .586

0.919

0.505 O.510

0.531 0.532 O.540

0 • 594

0.938

0 • 599

0.950

0 .609

0.967

TIME 5 MINUTES 1 HOUR 2 HOURS

3 4

"

Ο.518

COMPLEX: T R I ( O R T H O - P H E N A N T H R O L I N E ) N I C K E L ( I I ) PATH LENGTH: 1 DECIMETER SOLVENT: WATER; TEMPERATURE: 23° C. CONCENTRATIONS: COMPLEX - 0.02 MOLAR ENVIRONMENT - 0.04 MOLAR



TABLE

VII.

THE P F E I F F E R E F F E C T WITH DIETHYL-(+)-2,3-DIMETHOXYSUCCINATE


OBSERVED O P T I C A L ROTATION I N DEGREES λ (NM. ) —

582

528

546

436

5 MINUTES

0.854

0.887

1.000

1.608

4 HOURS

0.861

0.894

1.008

1.608

TIME

COMPLEX ; T R I ( O R T H O - P H E N A N T H R O L I N E ) N I C K E L ( I I ) PATH LENGTH: 1 DECIMETER SOLVENT : WATER; TEMPERATURE: 23° C. CONCENTRATIONS:

COMPLEX

- 0 . 0 2 MOLAR

ENVIRONMENT TABLE

- 0 . 0 4 MOLAR

VIII.

THE P F E I F F E R E F F E C T WITH DIMETHYL-(+)-2,3-DIMETHOXYSUCCINATE OBSERVED O P T I C A L ROTATION I N DEGREES λ (NM. )

— -

582

528

546

436

TIME 5 MINUTES

0.219

0.226

0.256

0.400

1.5

0.218

0.226

0.256

0.401

HOURS

3-3

"

0.217

0.224

0.252

0.399

4.2

"

0.215

0.223

0.248

0.396

COMPLEX: T R I ( O R T H O - P H E N A N T H R O L I N E ) N I C K E L ( I I ) PATH LENGTH: 1 DECIMETER SOLVENT: WATER; TEMPERATURE: 23° C. CONCENTRATIONS: COMPLEX

- 0 . 0 2 MOLAR

ENVIRONMENT

- 0 . 0 4 MOLAR


12.

KiRSCHNER A N D sERDiuK

Pfeiffer

Effect

253


Experimental All optical r o t a t i o n measurements, o p t i c a l r o t a t o r y d i s p e r s i o n s , a n d c i r c u l a r d i c h r o i s m s p e c t r a were determined on a C a r y - 6 0 s p e c t r o p o l a r i m e t e r w i t h c i r c u l a r d i c h r o i s m attachment. I n a d d i t i o n , some o p t i c a l r o t a t i o n s w e r e d e t e r m i n e d on a P e r k i n - E l m e r Model 141 p h o t o e l e c t r i c polarimeter. Only r e a g e n t grade c h e m i c a l s were used i n the syntheses mentioned below. The s u c c i n a t e d e r i v a t i v e s d e s c r i b e d i n F i g u r e 5 were p r e p a r e d by t h e methods o f P u r d i e , I r v i n e , and G i l l i s ( 1 5 ) . A l l P f e i f f e r E f f e c t s t u d i e s w i t h these environment compounds were c a r r i e d o u t i n w a t e r u n d e r t h e c o n d i t i o n s described i n Tables IV-VIII. Acknowledgement The a u t h o r s w i s h t o e x p r e s s t h e i r s i n c e r e appreciation t o theNational Science Foundation f o r a research grant which contributed s i g n i f i c a n t l y t o t h e progress o f t h i s i n v e s t i g a t i o n .

Literature Cited 1.

P f e i f f e r , P. and Quehl, K., Ber.(1931) 64, 2667; (1932) 65, 560.

2.

Dwyer, F. P., Gyarfas, E. G., and O'Dwyer, M. F., Nature (1951) 167, 1036; J. Proc. Roy. Soc. N.S.W. (1955) 89, 146.

3.

Kirschner, S. and Ahmad, N., J. Am. Chem. Soc. (1968) 90, 1910; Inorg. Chim. Acta. (1975) 14, 215; Pollock, R. J., Kirschner, S., and P o l i c e c , S . , Inorg. Chem. (1977) 16, 522.

4.

Kirschner, S. and Ahmad, N. i n "Coordination Chemistry", Proceedings of the John C. B a i l a r , J r . Symposium, Kirschner, S., Ed., Plenum Publishing C o r p . , New York, (1969).

5.

Gyarfas, E. C., Rev. Pure and Applied Chem. (1954) 4, 73; Gunter, J. D. and Schreiner, A. F., Inorg. Chim. Acta, (1975) 15, 117.

6.

Kirschner, S. and Ahmad, N., i n "Progress i n Coordination Chemistry", C a i s , M., e d . , E l s e v i e r (1968); Rec. Chem. Progr. (1971) 32, 29; Ahmad, N., Ph.D. D i s s e r t a t i o n , Wayne State U n i v e r s i t y , 1969.



254


7.

Turner, Ε . Ε . and H a r r i s , M. M., Quart. Revs. (1948) 1, 299.

8.

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Stereochemistry of Optically Active Transition Metal Compounds

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