Additivity of Circular Dichroism of d-d Transitions: The Vicinal Effect in

14

A d d i t i v i t y of Circular D i c h r o i s m of d-d Transitions: T h e V i c i n a l Effect i n a H o m o l o g o u s Series of

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Triethylenetetraaminecobalt(III) A m i n o Acid Complexes ROBERT JOB Department of Chemistry, Colorado State University, Fort Collins, CO 80523 Sources of dissymmetry in optically active metal complexes can be classified as: (a) inherent dissymmetry within the metal -donor atom coordination cluster, (b) configurational dissymmetry due to the chirality of the chelate system, (c) conformational dissymmetry due to the individual chelate ring conformations, and (d) vicinal dissymmetry due to asymmetric sites upon the ligands (1). For many complexes this set of contributions to the CD spectrum can be reduced to the configurational effect, the conformational effect and the vicinal effect (2,3). The most pragmatic approach to follow generally is to assume that the CD spectrum of a metal chelate complex is simply a summation of a vicinal effect and a configurational effect where the term vicinal effect retains its meaning as before and the other contributions are included in the configurational term (4,5,6,7). The independent systems/perturbation model, as carried to second order in perturbation theory in Schipper's AICD (associate-induced circular dichroism) theory, makes the following prediction (8). In the case of the complexes AB- B ... (a compos ite complex) and A'Β and A"B (substituent complexes) where A, A and A" is the same achiral chromophore and Β and B have the same configurational relationship to A in the composite complex as in their respective substituent complexes, the circular dichroism of AB- B ... is simply the summation of the CD's of A'B , A"B , etc. The purpose of this effort is to exhibit an achiral chromophore upon which exchange of chiral ligands induces negli gible perturbation in the configurational relationship of the chelating ligands, and thus to allow the utility of the predicted additivity to be demonstrated experimentally. The compounds studied are the substituted triethylenetetraaminecobalt(III) amino acid complexes depicted in Figure 1. Formally the A'B- chromophore will be the triethylenetetraaminecobalt(III) glycinato moiety (compounds 1 and 7, identified in Figure 1) (with an associated configurational effect) and the optically active (B ) chromophores will be represented by the various R substituents at the α-carbon of the chelated glycine i

i

j

j

1

i

i

j

j

i

j

i

j

2

0-8412-0538-8/80/47-119-273$05.00/0 © 1980 American Chemical Society

Douglas and Saito; Stereochemistry of Optically Active Transition Metal Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

274

(with an a s s o c i a t e d v i c i n a l

effect).

Results The compounds were synthesized a t 45° from the d i c h l o r o t r i ethylenetetraamine moiety and the appropriate amino a c i d i n aqueous s o l u t i o n with pH maintained a t 7.0 ±0.1 by pH-stat ( 9 J . In the case where R = -CH o r -CH(CH ) > only Λ - 3 and Δ-βχ produces were formed (Figure 2 ) . The c i r c u l a r dichroism spectra were obtained on the p e r c h l o r a t e s a l t s d i s s o l v e d i n 1 M HC1. Typical spectra f o r an S-amino a c i d complex ( s u b s c r i p t S) and an R-amino a c i d complex ( s u b s c r i p t R) are shown i n Figure 3. A c h a r a c t e r i s t i c of these spectra i s t h a t they can be q u i t e p r e c i s e l y deconvoluted i n t o a minimal number of Gaussian compo nents as shown by the dotted l i n e s i n Figure 3. This f a c i l i t a t e s storage and manipulation of the experimental data. This deconvol u t i o n serves to emphasize the d i f f e r e n c e between CD spectra of R & S amino a c i d complexes. I t can be seen from Figure 3 t h a t two of the Gaussian peaks have changed sign upon going from an S-amino a c i d complex to an R-amino a c i d complex. One way to o b t a i n the v i c i n a l e f f e c t of an amino a c i d i s to prepare both the Δ and Λ - t e t r a a m i n e c o b a l t ( I I I ) complexes of the S-amino a c i d and average the CD s p e c t r a . One drawback to t h i s method i s t h a t a r e s o l u t i o n procedure i s g e n e r a l l y necessary i n order to o b t a i n the pure compound (10). Our p r e f e r r e d method i s to prepare o n l y Λ complexes of the R then S-amino a c i d i n separate r e a c t i o n s then p u r i f y by simple r e c r y s t a l 1 i z a t i o n . Then, s i n c e the v i c i n a l e f f e c t of an S-amino a c i d i s i d e n t i c a l l y the negative of t h a t f o r an R-amino a c i d , the v i c i n a l e f f e c t may then be computed by the simple a l g e b r a i c manipulations o u t l i n e d i n Figure 4. The v i c i n a l e f f e c t s c a l c u l a t e d i n t h i s manner f o r a l a n i n e , v a l i n e and phenylalanine are shown i n Figure 5. The data used f o r c a l c u l a t e these v i c i n a l e f f e c t s are taken from measurements on compounds prepared as i n l i t e r a t u r e sources (11) or from Table I. Compound 10 i s the ^ - a l a n i n e complex of the t e t r a a m i n e c o b a l t ( I I I ) moiety with R = CH . Since g l y c i n e does not have an enantiomer, the v i c i n a l e f f e c t f o r \ and £ i s zero. The CD of these two complexes (shown i n Figure 6) then by d e f i n i t i o n becomes the c o n f i g u r a t i o n a l e f f e c t to which the v i c i n a l e f f e c t s of Figure 5 should be added. In Figure 7 these a d d i t i o n s are made. The dotted l i n e s represent the sum o f the c o n f i g u r a t i o n a l and v i c i n a l e f f e c t s f o r S-amino a c i d complexes and the s o l i d l i n e s represent the experimental s p e c t r a . Figure 8 shows the r e s u l t s f o r the R-alanine and R-phenylalanine complexes.


x

3

3

x

2

2

3

Discussion The summation appears to be most n e a r l y p e r f e c t f o r the l e a s t s t e r i c a l l y hindered cases [R = -CH , R = -CH , -CH(CH ) ], but does not d e v i a t e much even f o r the case where R = -CH(CH ) x

3

2

3

x


3

3

2

2


14.

JOB

Circular

Figure 2.

Dichroism

of d-d

Transitions

Reaction scheme to form the compounds depicted in Figure 1


275


276


θ

Figure S.

The CD spectra and component Gaussians of Compounds 4 which are typical of all compounds in Figure 1

R

and 4


S

14.

JOB

Circular

C D (Λ-S)

Dichroism

of d - d

Transitions

277

= C D (Λ) + C D ( S - a c i d )

C D (Λ-R) = C D (Λ) + C D (R-acid)


=

C D (Λ) - C D ( S - a c i d )

Then: Figure 4. C D ( S - a c i d ) = %[C.D.(A-S) - C D . (Λ-R)]

Figure 5.

Algebra involved in càlculating vicinal effects

The average vicinal effects calculated for Compounds 2, 3, 4, and 8 of Figure 1




Figure 6.

The CD of the glycine Complexes 1 and 7 depicted in Figure 1


14.

JOB

Circular

Dichroism

of d - d

Transitions

279

Table I . Parameters o f the Gaussian Components ( i - v i i ) For the CD Spectra o f Amino Acid Complexes.


Com plex* t

i

ii

iii

440 32.0 525

362 18..0 265

340 24..0 -685

363 17..5 315

343 24 -730

578 40 90

261 21 -3420

384 19 -235

345 21 823

560 40 -267

263 26 1390

383 17..5 -411

349 18 565

571 35 -277

235 28 15400

1

a b c

495. .7 35 5215

7

a b c

495 38 6100

10p

a b c

496 42..2 -8111

—

--

430 19 -118

a b c

513. .4 36 -6656

464 36 -4057

415 19 -236

10

s

442 32 1170

iv

V

vi

259 23.5 -1390

560 42 130

vi i

tThe constants a , b ^ a r e parameters o f the Gaussian Μ = c_ e x p [ - ( x - a ) / b T w i t h a_ the l o c a t i o n o f the peak, b^ the h a l f width a t 1/e o f the maximum peak height and c^ the peak height. *Complex 1 and 7 are defined i n Figure 1 . Complex ^ i s the A-Bx-diastereomer o f compound 2 depicted i n Figure 1. θ

2

2

and R = -CH o r where R± = -CH and R = b e n z y l . This i n t i m a t e s that these v i c i n a l e f f e c t s may be o f s i g n i f i c a n t s y n t h e t i c u t i l i t y both f o r c o n f i r m a t i o n o f s t r u c t u r e and f o r p r e d i c t i o n o f CD s p e c t r a . As examples consider the r e a c t i o n i n Figure 2 c a r r i e d out w i t h the p r o c h i r a l amino a c i d I (shown i n the i n s e r t to Figure 5). This compound may bind i n a bicfentate f a s h i o n through the amino group and one o f the carboxyl groups to give e i t h e r an R o r an S-amino a c i d complex depending on which carboxyl group binds to the metal. The CD spectrum (x 1.5) o f t h i s complex i s shown i n Figure 9 along w i t h the v i c i n a l e f f e c t gotten by s u b t r a c t i n g the CD o f ^ from i t . This e f f e c t i s c l e a r l y t h a t o f an R-amino a c i d , t h e r e f o r e the p r o c h i r a l compound has bound s t e r e o s p e c i f i c a l l y i n an R-fashion (1_2). Average CD's o f a Δ-Λ mixture have revealed a d i f f e r e n c e between the v i c i n a l e f f e c t s o f $χ and $ -amino a c i d complexes (1_0). The v i c i n a l e f f e c t o f Bx-alanine gotten by t r e a t i n g the CD spectrum o f compound 10 (the Δ-diastereomer o f compound £ w i t h the a l a n i n e bound as f ^ ) by the equation i n Figure 4 i s shown i n Figure 10. I t i s seen that the s i g n i f i c a n t d i f f e r e n c e between the $ i and $ v i c i n a l e f f e c t s allows us to peg f u r t h e r our p r o c h i r a l amino a c i d as not only bound i n an R - c o n f i g u r a t i o n but a l s o to be bound i n a 3 f a s h i o n (as confirm ed by X-ray d i f f r a c t i o n (1J_)). The same a n a l y s i s c a r r i e d out on the CD spectrum o f t h i s l i g a n d i n a s i m i l a r r e a c t i o n has revealed i t to be bound a l s o i n an R-3 f a s h i o n (T3). 2

3

3

2

2

2

2

2




WAVELENGTH

θ

WAVELENGTH Figure 7. Sum of vicinal plus configuration effects ( ) compared with experi mental CD spectra for the S-amino acid complexes of 2, 3, 4, and 8 depicted in Figure 1 ( )



14.

JOB

Circular

Dichroism

of d-d

Transitions

WAVELENGTH

θ

WAVELENGTH (nm) Figure 7 {continued)


281



282

Figure 8. Sum of vicinal plus configuration effects ( ) compared with experimental CD spectra for the R-amino acid complexes 2 and 4 depicted in Figure i(—)



14.

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Figure 9.

Circular

Dichroism

of d-d

Transitions

283

The CD spectrum of Compound 5 ( ) and the vincinal effect for the prochiral amino acid Î, shown in the insert ( )




284

Figure 10. Vicinal effect for β abound ( ) alanine derived from the CD spec tra of Compounds 10 and 1 0 which are the Δ - β , diastereomers of Compound 2 depicted in Figure 1. Vicinal effect for β abound ( ) alanine for comparison (from Figure 5). R

3

Figure 11.

s

Γ

Vicinal effect for fi -S-proline derived from Complexes 9 depicted in Figure 1 2

S

and 9


R

JOB

Circular

Dichroism

of d-d

Transitions


14.

Figure 12. Predicted CD of the β -$-ρ™Ιίηβ complex (top) compared with the experimental CD of Compound 9 depicted in Figure 1 (bottom) 2



286

In order to p r e d i c t the CD spectrum o f the Λ β product from the r e a c t i o n o f S - p r o l i n e as the amino a c i d i n Figure 2, the v i c i n a l e f f e c t i n Figure 11 f o r 3 S - p r o l i n e was determined (from measurements i n our l a b o r a t o r y upon complex 9, which had been p r e v i o u s l y synthesized (1J_) and c h a r a c t e r i z e d by X-ray d i f f r a c t i o n (14,1_5)) and added to the CD o f the g l y c i n e complex ^ to o b t a i n the CD spectrum shown i n the top h a l f o f Figure 12. This compares q u i t e f a v o r a b l y with the CD o f the p r i n c i p a l product (65%) o f the r e a c t i o n (bottom h a l f o f Figure 12). Since the p r o l i n e does not conform p r e c i s e l y to the g l y c i n a t e skeleton the a c h i r a l chromo phore i s a l t e r e d and a d d i t i v i t y i s not r e q u i r e d to hold p r e c i s e l y . However the r e s u l t s are s u f f i c i e n t l y q u a n t i t a t i v e to allow f o r un ambiguous product i d e n t i f i c a t i o n . Note that the v i c i n a l e f f e c t f o r 6 S - p r o l i n e d e p i c t s a s i t u a t i o n observed by o t h e r s , i . e . , i t s t r o n g l y resembles the v i c i n a l e f f e c t f o r the 3 i complexes o f other amino a c i d s . 2


2

2

Conclusion Although a d d i t i v i t y i n a t e t r a a m i n e c o b a l t ( I I I ) amino a c i d complexes has been p r e v i o u s l y demonstrated (6^1j6), the c u r r e n t work has demonstrated a high a d d i t i v i t y i n a c a r e f u l l y chosen system. However, treatment o f these r e s u l t s has not taken i n t o account the charge t r a n s f e r bands which, being o f proper symmetry, must have some n o n t r i v i a l e f f e c t on the d-d^ t r a n s i t i o n s . The relevance o f the s t e r i c c o n t r i b u t i o n to the d e v i a t i o n s from p e r f e c t a d d i t i v i t y w i l l be discussed i n a forthcoming p u b l i c a t i o n . Summary This r e p o r t describes a method o f e x p e r i m e n t a l l y o b t a i n i n g the v i c i n a l e f f e c t s o f amino a c i d anions bound t o a tetraaminec o b a l t ( I I I ) moiety by d e a l i n g e x c l u s i v e l y with Λ - 3 complexes o f both R and S-amino a c i d s . A d d i t i v i t y o f c i r c u l a r dichroism o f both the c o n f i g u r a t i o n a l and v i c i n a l e f f e c t s f o r c[-c[ t r a n s i t i o n s i s v e r i f i e d e x p e r i m e n t a l l y . I t i s demonstrated that the v i c i n a l e f f e c t not only contains information as to the c h i r a l i t y o f the bound amino a c i d but a l s o as to the mode o f b i n d i n g , i . e . , 3 i vs. ρ . 2

2

Literature Cited 1. 2. 3. 4. 5. 6.

Richardson, F. S., Chem. Revs., (1979) 79, 17. Hawkins, C. J., "Absolute Configuration of Metal Complexes", Wiley-Interscience: New York, N.Y. (1971) Chap. 5. Bosnich, B.; Harrowfield, J . Μ., J. Am. Chem. Soc. (1972) 94, 3425. Liu, C. T.; Douglas, Β. E . , Inorg. Chem. (1964) 3, 1356. Douglas, B. E . ; Yamada, S., ibid. (1965) 4, 1561. Douglas, Β. Ε . , ibid. (1965) 4, 1813.


14. JOB 7.


8. 9. 10. 11. 12. 13. 14. 15. 16.

Circular Dichroism of d-d Transitions

287

Yano, S.; Saburi, M.; Yoshikawa, S.; Fujita, J., Bull. Chem. Soc. Japan, (1976) 49, 101. Schipper, P. E . , J . Am. Chem. Soc., (1978) 100, 1433. Job, R.; Freeland, S., Analyt. Biochem., (1977) 79, 575. Lin, C. Y.; Douglas, Β. E . , Inorg. Chim. Acta (1970) 4, 3. Glusker, J . P.; Carrel, H. L.; Job, R.; Bruice, T. C., J . Am. Chem. Soc. (1974) 96, 5741. Job, R.; Bruice, T. C., J . Am. Chem. Soc., (1974) 96, 809. Job, R. C., J . Am. Chem. Soc., (1978) 100, 5089. Buckingham, D. Α.; Marzilli, L. G.; Maxwell, I. E . ; Sargeson, A. M.; Freeman, H. C . , Chem. Commun., (1969) 583. Freeman, H. C.; Marzilli, L. G.; Maxwell, I. E . , Inorg. Chem., (1970) 9, 2408. Hall, S. K.; Douglas, Β. E., ibid., (1969) 8, 372.

RECEIVED September 13, 1979.


Additivity of Circular Dichroism of d-d Transitions: The Vicinal Effect in

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