A PMR method of assigning the absolute configurations of cobalt(III) S-alanine complexes

INORG.

NUCL.

CHEM.

LETTERS

Vol.

6,

pp.

847-854,

1970.

Pergamon Press.

Printed in Great Britain.

A PMR METHOD OF ASSIGNING THE ABSOLUTE CONFIGURATIONS OF COBALT(III) S-ALANINE COMPLEXES E r n e s t A. B e r e n d s a n d J o h n G. B r u s h m i l l e r D e p a r t m e n t of C h e m i s t r y , U n i v e r s i t y of N o r t h D a k o t a G r a n d F o r k s , N o r t h D a k o t a 58201 (Received 22 June 1970)

The absolute configurations

of a n u m b e r

of c o b a l t ( I I I ) a m i n o a c i d

c o o r d i n a t i o n c o m p l e x e s h a v e b e e n a s s i g n e d b y a n a n a l y s i s of t h e i r c i r c u l a r dichroism

(CD) s p e c t r a l ' 2 o r b y a n X - r a y d e t e r m i n a t i o n

In t h i s n o t e w e w i s h t o p o i n t out t h a t t h e d i f f e r e n c e s diastereomeric

cobalt(III) S-alanine complexes

of t h e m a g n e t i c a n i s o t r o p y a s s o c i a t e d the pmr spectra configurations

in t h e p m r s p e c t r a

can be understood

of

in t e r m s

with the C -- O singie bond, and that

can be used to make an assignment of t h e c o m p l e x e s .

of t h e i r s t r u c t u r e . 3 - 5

of t h e a b s o l u t e

The diastereomers

of : N a 2 / - C o ( S - a l a n ) ( o x ) 2 ~ ,

t r a n s - N - C 2- Na/_-Co(S- a l a n ) 2 ( o x ) J , a n d t r a n s - N - C 2 - N a / _ ' C o ( N - M e - S- a t a n ) 2 ( o x ) were used for this study.

6

The pmr spectrum

of e a c h d i a s t e r e o m e r

a n d t h e e f f e c t of t h e m a g n e t i c a n i s o t r o p y a s s o c i a t e d

differs

with the C -- O bond can

be observed. The simplicity allows the accurate

of t h e p m r s p e c t r u m

measurement

of a c o o r d i n a t e d S - a l a n i n e l i g a n d

of t h e c h e m i c a l

s h i f t s of t h e m e t h y l a n d

m e t h i n e p r o t o n s of t h e l i g a n d in i t s c o o r d i n a t i o n c o m p l e x e s . the pmr parameters diastereomer

o b t a i n e d f r o m an a n a l y s i s of t h e s p e c t r u m

in t h e s e r i e s .

had been previously stereoselectivity

of e a c h

T h e a b s o l u t e c o n f i g u r a t i o n of e a c h d i a s t e r e o m e r

assigned by the circular

ratios.

The Table gives

dichroism

method 7' 8 or by

9

T h e p m r d a t a in t h e T a b l e s h e w s t h a t t h e c h e m i c a l s h i f t s of t h e m e t h y l a n d m e t h i n e p r o t o n s a r e d i f f e r e n t in t h e d i a s t e r e o m e r s

of

Na2/~Co(S-alan)(ox)2 ~.

T h e m e t h i n e p r o t o n of S - a l a n i n e i s d e s h i e l d e d in o n e of t h e d i a s t e r e o m e r s 847

848

CONFIGURATIONS O F COBALT(Ill) S- ALAN1NE COMPLEXES

Vol. 6, No. 11

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Vol. 6, No. I I

CONFIGURATIONS OF COBALT(Ill) S- ALANINE COMPLEXES

r e l a t i v e to the m e t h i n e p r o t o n in t h e o t h e r d i a s t e r e o m e r , d i f f e r e n c e can be r a t i o n a l i z e d in t e r m s

849

This chemical shift

of t h e m a g n e t i c a n i s o t r o p y s h i e l d i n g

e x p e c t e d f o r a C - - O s i n g l e b o n d w h i c h is l o c a t e d in the a d j a c e n t o x a l a t e ligand,

T h e a n i s o t r o p y s h i e l d i n g e x p e c t e d f o r a C - - O b o n d i s i l l u s t r a t e d in

F i g u r e 1. 10, 11 A p r o t o n l o c a t e d n e a r t h e b o n d a x i s b u t o u t s i d e t h e c o n e w i l l

F i g u r e 1. - - A n i s o t r o p i c

deshielding expected for a C-

O bond.

not be d e s h i e l d e d by the a n i s o t r o p y a s s o c i a t e d with the bond, but a p r o t o n located along the axis or near the center of the cone will be deshielded by the a n i s o t r o p y a s s o c i a t e d w i t h the C - -

O bond.

F i g u r e 2 s h o w s t h e l o c a t i o n of

0

0 It

O=c~~O

l--co

0 ~ "csO

/o 0 ~-

C~r~

-"~J,

% \/j/~--, C~ u

o c/

o FIGURE 2

A

the methyl and methine protons in the A and A absolute configurations of N a 2 / - C o ( S - a l a n ) ( o x ) 2 _7.

In t h e A a b s o l u t e c o n f i g u r a t i o n t h e m e t h i n e p r o t o n

850

CONFIGURATIONS OF COBALT(ill) S- ALANINE COMPLEXES

Vol. 6, No. 11

is l o c a t e d j u s t off a l i n e d r a w n a l o n g t h e C - - O b o n d of t h e a d j a c e n t c o o r d i n a t e d o x a l a t e g r o u p a n d is d e s h i e l d e d b y t h e m a g n e t i c a n i s o t r o p y associated with this bond.

In t h e A a b s o l u t e c o n f i g u r a t i o n of t h e c o m p l e x ion

t h e m e t h i n e p r o t o n of S - a l a n i n e is l o c a t e d w e l l a b o v e a l i n e d r a w n d o w n the C - - O b o n d of t h e a d j a c e n t c o o r d i n a t e d o x a l a t e g r o u p and e x t e n d i n g out t o w a r d s the S - a l a n i n e ring.

The p r o t o n in t h e A a b s o l u t e c o . n f i g u r a t i o n w i l l n o t b e

d e s h i e l d e d b y t h e m a g n e t i c a n i s o t r o p y a s s o c i a t e d w i t h the C - - O b o n d .

The

l o c a t i o n s of t h e m e t h y l g r o u p s of c o o r d i n a t e d S - a l a n i n e a r e s h o w n in F i g u r e 2. T h e m e t h y l p r o t o n s a r e l o c a t e d f u r t h e r f r o m the o x y g e n a t o m s of t h e o x a l a t e ring system,

a n d t h e r e f o r e t h e d e s h i e l d i n g e f f e c t of t h e C - - O b o n d w i l l n o t b e

a s l a r g e f o r the m e t h y l p r o t o n s a s f o r the m e t h i n e p r o t o n s w h i c h a r e l o c a t e d on the c h e l a t e r i n g ,

In t h e /x a b s o l u t e c o n f i g u r a t i o n of t h e c o m p l e x i o n t h e

m e t h y l p r o t o n s of S - a l a n i n e a r e l o c a t e d j u s t a b o v e a l i n e d r a w n down the C - - O b o n d of t h e o x a l a t e r i n g a n d w i l l b e d e s h i e l d e d b y t h e a n i s o t r o p y a s s o c i a t e d with t h e C - -

O bond.

In t h e A a b s o l u t e c o n f i g u r a t i o n of t h e

c o m p l e x ion the m e t h y l p r o t o n s a r e located well above a line d r a w n down t h e C - - O b o n d of t h e a d j a c e n t o x a l a t e g r o u p a n d w i l l n o t b e d e s h i e l d e d b y the a n i s o t r o p y a s s o c i a t e d w i t h the C - -

O bond.

In t h e A a b s o l u t e c o n f i g u r a t i o n

of t h e c o m p l e x ion t h e m e t h i n e p r o t o n s s h o u l d b e d e s h i e l d e d a n d t h e m e t h y l protons shielded relative to their counterparts

in t h e A a b s o l u t e c o n f i g u r a t i o n

U s i n g t h e p m r d a t a in t h e T a b l e a n d the a n i s o t r o p y a r g u m e n t s o u t l i n e d a b o v e , t h e (-)D i s o m e r c a n b e a s s i g n e d t o t h e A a b s o l u t e c o n f i g u r a t i o n a n d t h e (+)D i s o m e r to the A a b s o l u t e c o n f i g u r a t i o n ,

The a s s i g n m e n t s m a d e by the

p m r m e t h o d a r e in a g r e e m e n t w i t h t h o s e m a d e b y t h e o p t i c a l CD m e t h o d . The diastereomers symmetry

of t r a n s - N - C 2 - N a / - C o ( S - a l a n ) 2 ( o x ) ~

7

have C2

s o t h a t t h e m e t h i n e a n d m e t h y l p r o t o n s of S - a l a n i n e a r e s y m m e t r i c a l l y

a n d c h e m i c a l l y e q u i v a l e n t in e a c h d i a s t e r e o m e r .

12

F i g u r e 3 shows the

p o s i t i o n s of t h e m e t h y l a n d m e t h i n e p r o t o n s of S - a l a n i n e i n the A a n d A a b s o l u t e c o n f i g u r a t i o n s of t h e c o m p l e x ion.

In the A a b s o l u t e c o n f i g u r a t i o n

t h e m e t h i n e p r o t o n is n o t d e s h i e l d e d b y t h e a n i s o t r o p y a s s o c i a t e d w i t h t h e

Vol. 6, No. U

CONFIGURATIONS OF COBALT(Ill) S- ALANINE COMPLEXES

"

851

0

\

o--c,' zx 0

FIGURE 3

n31 0

d,C oA

C - - O bond, but t h e m e t h y l p r o t o n s o c c u p y a deshieLdi~g p o s i t i o n with r e s p e c t to the C - - O bond.

In the A a b s o l u t e c o n f i g u r a t i o n of the c o m p l e x t h e m e t h i n e

p r o t o n o c c u p i e s a p o s i t i o n w h i c h is d e s h i e l d e d b y t h e a n i s o t r o p y a s s o c i a t e d w i t h t h e C - - O bond of t h e o x a l a t e g r o u p , w h e r e a s the m e t h y l p r o t o n s a r e not d e s h i e l d e d b y t h e a n i s o t r o p y a s s o c i a t e d with the C - - O bond.

The p m r d a t a

in t h e T a b l e show t h a t the m e t h i n e p r o t o n of the (-)546 i s o m e r is d e s h i e l d e d r e l a t i v e to the m e t h i n e p r o t o n in the (+)546 i s o m e r with the o p p o s i t e s i t u a t i o n b e i n g o b s e r v e d f o r the m e t h y l p r o t o n s .

U s i n g t h e a n i s o t r o p y a r g u m e n t s and

t h e o b s e r v e d p m r d a t a , the (-)546 i s o m e r can be a s s i g n e d to the A a b s o l u t e c o n f i g u r a t i o n , and the (+)546 i s o m e r c a n be a s s i g n e d to t h e A a b s o l u t e c o n f i g u r a t i o n in a g r e e m e n t with the a s s i g n m e n t s m a d e by the CD m e t h o d .

8

The ligand N-methyl-S-alanineis a coordinating agent that contains two asymmetric centers in the chelate ring system, one of which is a nitrogen atom.

Steric interactions between the bulky methyl groups on the chelate

ring system insure that in the coordination form the absolute configuration around the nitrogen atom will be R when the absolute configuration at the adjacent carbon atom is S.

13

The diastereomers of trans-N-C2-Na/-Co(N-Me-S-alan)2(ox)j have C2 symmetry so that the N-methyl, methyl, and methine protons are symmetrically and chemically equivalent in each diastereomeric form.

The

852

CONFIGURATIONS OF COBALT(Ill) S- ALANINE COMPLEXES

Vol. 0, No. 11

l o c a t i o n s of t h e s e p r o t o n s a r e s h o w n in F i g u r e 4 f o r b o t h a b s o l u t e c o n f i g u r a t i o n s

© a-c/C~o

A/~\ ~CH3

CH~

?

!CHsITI / H~\~°~H

u-I---q/..,, ~

~k..CH-~

C t..O~

k.

c,,

,u%~l_ I/

o

;"S---.-----c,' " 0

o

\

\

CH5

N

H>~%2C~H 5 0

zx

o

6

A

FIGURE 4 of t h e c o m p l e x ion.

T h e m e t h y l a n d m e t h i n e p r o t o n s a r e in t h e s a m e p o s i t i o n s

in t h e d i a s t e r e o m e r s

of t r a n s - N - C 2 - N a / - C o ( N - M e - S - a l a n ) 2 ( o x ) /

in t h e a n a l o g o u s d i a s t e r e o m e r s was discussed

previously.

as they were

of t r a n s - N - C 2 - N a / - C o ( S - a l a n ) 2 ( o x ) _7 w h i c h

H e n c e , t h e (-7546 i s o m e r

with the deshielded

m e t h i n e p r o t o n and s h i e l d e d m e t h y l g r o u p s h o u l d b e a s s i g n e d t h e A a b s o l u t e configuration,

whereas

t h e (+)546 i s o m e r

w i t h t h e d e s h i e l d e d m e t h y l and

shielded methine protons should be assigned to the A absolute configuration F i g u r e 4 s h o w s t h a t t h e N - m e t h y l p r o t o n s of N - M e - S - a l a n i n e

in t h e A

a b s o l u t e c o n f i g u r a t i o n of t h e c o m p l e x ion a r e d e s h i e l d e d b y t h e m a g n e t i c anisotropy associated alanine group.

w i t h t h e C - - O b o n d of t h e o t h e r c o o r d i n a t e d N - M e - S -

In t h e A a b s o l u t e c o n f i g u r a t i o n of t h e c o m p l e x ion t h e N - m e t h y l

p r o t o n s a r e l o c a t e d a b o v e t h e c a r b o n y [ g r o u p s of t h e c o o r d i n a t e d

oxalate ring

in a v e r y s h i e l d i n g p o s i t i o n . 14 H e n c e , t h e N - m e t h y l p r o t o n s in t h e A absolute configuration should be deshielded relative to the N-methyl of t h e A a b s o l u t e c o n f i g u r a t i o n .

protons

T h e p m r d a t a in t h e T a b l e s h o w t h a t t h e

N - m e t h y l g r o u p of t h e ( - ) 5 4 6 i s o m e r

is d e s h i e l d e d r e l a t i v e t o t h e N - m e t h y l

g r o u p in t h e (+)546 i s o m e r ,

On t h e b a s i s of t h e m a g n e t i c a n i s o t r o p y

arguments

is a s s i g n e d t h e A a b s o l u t e c o n f i g u r a t i o n a n d

t h e (-754 6 i s o m e r

t h e (+7546 i s o m e r

is a s s i g n e d t h e A a b s o l u t e c o n f i g u r a t i o n

This

Vol. 6, No. 11

CONFIGURATIONS OF COBALT(Ill) S- ALANINE COMPLEXES

853

a s s i g n m e n t a g r e e s w i t h the a s s i g n m e n t m a d e f r o m the c h e m i c a l s h i f t s of the m e t h y l a n d m e t h i n e p r o t o n s i n t h e l i g a n d a n d is in a g r e e m e n t w i t h the assignment made by stereoselectivity

ratios.

9

T h e m a g n e t i c a n i s o t r o p y a s s o c i a t e d with the C - - O b o n d of o x a l a t e o r a m i n o a c i d g r o u p s p l a y an i m p o r t a n t r o l e in d e t e r m i n i n g the s h i e I d i n g o r d e s h i e l d i n g of s u b s t i t u e n t s on c o o r d i n a t e d S - a l a n i n e l i g a n d s in t h e s e c o o r d i n a t i o n c o m p o u n d s a n d p r o v i d e s a m e t h o d of a s s i g n i n g t h e a b s o l u t e c o n f i g u r a t i o n s of t h e c o m p l e x e s . ACKNOWLEDGEMENTS T h e a u t h o r s a r e g r a t e f u l to M r s . methyl-S-alanine

ligand,

This research

D o n K u b i k f o r s u p p l y i n g t h e Nw a s s u p p o r t e d iN p a r t b y a U n i v e r s i t y

of N o r t h D a k o t a F a c u l t y R e s e a r c h G r a n t .

Additional financial support from

t h e H i l l F a m i l y F o u n d a t i o n in t h e f o r m of a s u m m e r r e s e a r c h

professorship

t o J. G. B. a n d f r o m t h e M i n n e s o t a M i n i n g a n d M a n u f a c t u r i n g Co. , St. P a u l , Minnesota for summer The spectrometer

s u p p o r t of E. A. B . ,

is g r a t e f u l l y a c k n o w l e d g e d

u s e d in t h i s w o r k w a s p u r c h a s e d in p a r t b y N S F f u n d s

( G r a n t No. 6 5 - 2 1 5 9 ) .

REFERENCES 1) C. T. L i u a n d B. E. D o u g l a s , I n o r g . C h e m . , 2) B. E. D o u g l a s a n d S. Y a m a d a , 3) M. G. B. D r e w ,

ibid., 4,

3, 1356 (1964).

1561 (1965).

J. H. D u n l o p , R. D. G i l l a r d and D. R o g e r s ,

Chem,

C o m m u n . , 42 (1966). 4)

J. H. D u n l o p , R. D. G i l l a r d , N. C. P a y n e a n d G. B. R o b e r s o n ,

ibid.,

874 (1966). 5) J. F . B l o u t , H. C. F r e e m a n ,

A. M. S a r g e s o n a n d K. R. T u r n b u l [ ,

i b i d . , 324 (1967i. 6) A b b r e v i a t i o n s u s e d in t h i s n o t e a r e : and N - M e - S - a l a n

ox=oxalate, S-alan = S-alanine,

= N-Methyl-S-alanine.

7) K. Y a m a s a k i , , J. H i d a k a , and Y. S h i m u r a , B u l l . C h e m .

Soc. J a p . , 422,

854

CONFIGURATIONS OF COBALT(Ill) S- ALANINE COMPLEXES

Vol. 6, No. 11

119 (1969). 8) J. H i d a k a and Y. S h i m u r a , i b i d . , 4.0, 2312 (]967). 9) E. A. B e r e n d s and J. G. B r u s h m i H e r , 10) The a n i s o t r o p y a s s o c i a t e d with a C J

Ino.rg. Nucl. Chem. L e t t e r s ,

in p r e s s

N bond h a s b e e n d i s c u s s e d by :

L. N. S c h o e n b e r g , D. W. Cooke and C. F. L i u , Inorg. C h e m . , 7 , 11) A. A. B o t h n e r - B y and C. N a a r - C o l i n , J. A m e r . Chem. S o c . , 8~0, 1728 (1958). 12) M. van G o r k o m and G. E. H a i l , Q u a r t , Rev. (London), 2 ~ , 13) M. S a b u r i and S, Y o s h i k a w a , Inorg. C h e m . , 7 ,

14 (1968).

1890 (1968).

14) G. J. K a r a b a t o s , G. C. S o m m i c h s e n , N. H s i and D. J. F e n o g l i o , J. A m e r . C h e m . S o c , , 8~, 5067 (1967).