Absolute Configuration of Transition Metal Complexes - ACS

2

Absolute Configuration of Transition M e t a l Complexes

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on February 12, 2015 | http://pubs.acs.org Publication Date: May 27, 1980 | doi: 10.1021/bk-1980-0119.ch002

YOSHIHIKO SAITO The Institute for Solid State Physics, The University of Tokyo, Tokyo, Japan

In this paper an overview will be given on the structural studies of optically active transition metal complexes and their interaction with other fields in coordination chemistry. A quarter of a century has passed away since the first determina tion of the absolute configuration of a transition metal complex, [Co(en) ] , was carried out by anomalous scattering of X-rays (1). Since that time the number of transition metal complexes whose absolute configuration have been determined by X-ray method has been growing at an increasing rate and at this time i t has ex ceeded 130. In addition to this, numerous optically active organometallic compounds have been studied and their absolute con figurations established. At an early stage of the development, the structural informa tion was rather fragmentary. Nowadays, however, the accumulation of structural data for isomers has enabled us to understand structural principles and the optical properties of chelate com plexes in considerable detail. In this connection, column chroma tography on SP Sephadex has played an important role in the sepa ration of isomers of coordination compounds (2). In view of the large number of structures, a few basic series of structures will be taken up and discussed. Among the transition metal complexes, the tris(diamine)metal system, particularly tris(ethylenediamine)cobalt(III) and its analogues, has been studied most extensively from both experi mental and theoretical sides. 3+

3

Five-Membered Chelate Rings The cause of the isomerism exhibited by this system may be characterized briefly as a combination of configurational and conformational isomerism, the latter arising from non-planarity of metal-ethylenediamine chelate ring. According to IUPAC nomen clature (3), the designation of the configurational chirality is based upon the edges of the octahedron spanned by the chelate rings. Any two such edges form a pair of skew lines describing a 0-8412-0538-8/80/47-119-013$07.50/0 © 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


14

screw o f the same handedness because o f the presence o f a t h r e e fold axis. The c o n f i g u r a t i o n a l c h i r a l i t y i s designated by Λ ( l e f t - h a n d e d screw) or Δ (right-handed screw). The d e s i g n a t i o n o f conformation i s a l s o based upon the p r i n c i p l e o f a p a i r o f skew lines. For ethylenediamine one o f these l i n e s i s d e f i n e d by the two c h e l a t i n g n i t r o g e n atoms and the other by the two carbon atoms i n the c h e l a t e r i n g . T h i s i s used to c h a r a c t e r i z e the conforma t i o n a l c h i r a l i t y S (right-handed) or λ ( l e f t - h a n d e d ) . The con cepts symbolized by £ , Λ and S , λ are i n v a r i a n t under proper r o t a t i o n s but are converted i n t o the other under improper r o t a tions. The combination o f these symbols, however, g i v e s r i s e to a c h a r a c t e r i z a t i o n o f the c h e l a t e r i n g which i s c h i r a l i t y i n v a r i ant. T h i s c h a r a c t e r i z a t i o n which i s r e l e v a n t when d i s c u s s i n g the complex o f unknown absolute c o n f i g u r a t i o n , o r the conformational energy, i s designated by l e i and ob as proposed by Corey and B a i l a r (k) · Here, the symbols l e i and ob ( p a r a l l e l and oblique) r e f e r to the d i r e c t i o n o f the bond between the carbon atoms o f the c h e l a t e r i n g r e l a t i v e to the t h r e e f o l d a x i s d e f i n e d by the three edges o f the octahedron spanned by the l i g a n d s . For example,Δ(λ) means a λ conformation a s s o c i a t e d with a c o n f i g u r a t i o n Δ and t h i s is l e i . A l l the f o u r combinations a r e :

Δ ( λ ) o r Λ(ί ) =

Δ(ί)

lei

orA(X) = ob

There are e i g h t p o s s i b l e isomers i n the [M(en)-] system. They comprise two c a t o p t r i c s e r i e s : l e l ^ , l e l p O b , l e l o b and ob_ with Δ and Λ absolute c o n f i g u r a t i o n s , r e s p e c t i v e l y . C r y s t a l s t r u c t u r e s have been determined f o r a number o f [Co(en),] and [Cr(en)_] salts. The l e i - isomers are most f r e q u e n t l y recog n i z e d i n these s t r u c t u r e s . Table I l i s t s examples o f conformers of [Coien)-]* " and iCr(en)^} other than l e l . I t seems that 2

1

T

?

5

5

Table I Compounds c o n t a i n i n g complex c a t i o n s with conformations other than l e i , Conformation lel ob

Refs.

[Co ( en) ] [ P b C l ] C l - 3 ^ 0

lel ob

(6)

[Cr(en) ][Ni(CN) ]·1.5H 0

lel ob,

(-)

lelob

Compound [Co(en) ]CSnCl ]Cl 5

3

3

2

9

?

5 8 9

2

2

2

?

[Cr(en) ](SCN) 3

3

2

2

[Cr(en) ][Co(CN) ]·6H 0 ?

g

2

0

( +) g [ C r (en) ^Cl^ 5

9

[Cr (en) ](SCN) ?

2^0

^0.75^0

b

lelob

2

(7) (8)

2

(2)

3

lel , 3

leîoO^lel+^+OJéob)

lel (70#lel+30&>b) 2

(10) (11)

the conformers other than l e i - , appear more f r e q u e n t l y i n C r ( I I I ) complexes than i n Co(III) complexes. X-ray evidence always i n d i c a t e s that such isomers are favored because they a l l o w more hydrogen bonding i n the c r y s t a l . In ( + ) c o - A - [ C r ( e n ) - ] C l _ » 2 H 0 , Q

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

p

2.

SAITO

Absolute

15

Configuration

one of the three chelate r i n g s e x h i b i t s conformational d i s o r d e r and i t s conformation can be represented as 6($>$ + 4θ# λ. In the case o f [0Γ(βη)-](30Ν)_·0·75Η 0 the d i s o r d e r vanishes and the con formation changes to lél,on lowering the temperature to 133K, whereby the u n i t c e l l volume c o n t r a c t s by 2.7#* r e f l e c t i n g the more compact l e l _ conformers, but no change i s observed i n the packing mode ( 1 2 ^ Though to l e s s extent, s i m i l a r conformational d i s o r d e r was r e c e n t l y detected i n c r y s t a l s of (+) o - ( 3 , 3 ' - d i methyl-2,2 -bipyridine)bis(ethylenediamine)cobalt\±II) c h l o r i d e d i p e r c h l o r a t e monohydrate. The absolute c o n f i g u r a t i o n can be desiganted as /\(6»-dmbpy, 90% λ + 10#£") (13). In c r y s t a l s of (+) -gQ-[Co(enV]Cl ·Η2HpO the complex c a t i o n takes the s y n - c h a i ^ - l e l conformation [/\(apê ) and i t s enantiomer] (35)· Recent c a l c u l a t i o n s by c o n s i s t e n t f o r c e f i e l d technique i n d i c a t e d that the C - c h a i r , conformer represents the g l o b a l minimum, supporting the f l e x i b i l i t y o f the c h e l a t e r i n g . ^ The s y n - c h a i r - l e l conformer i s higher i n energy by 1 0 . 9fcJmol" · The observed shape and s i z e o f the complex i o n , [Co(tn),] can be w e l l reproduced by the s t r a i n energy minimization (36)· There a r e three isomers o f 2,4-diaminopentane: R,R, S,S, and R,S. When t h i s molecule forms a six-membered chelate r i n g , the e q u a t o r i a l preference o f the s u b s t i t u t e d methyl groups f i x e s the conformation o f the chelate r i n g as f o l l o w s : R,S-ptn : c h a i r R,R-ptn : λ-twist-boat S,S-ptn : S - t w i s t - b o a t . The absolute c o n f i g u r a t i o n o f the l e i , - and ob,-isomers o f [Co(R,R-ptn),Y* are already known [ ( - ) , ^ i l - [ C o f R , R - p t n ) , ] ^ , ( 3 7 ) ; ( + ) ^ o b -[Co(R,R-ptn) ] ^ , ( 3 8 ) 3 · There a r e two p o s s i b l e geometric isomers f o r [ C o ( R , S - p t n ) · fac-(C - c h a i r , ) and mer(C - c h a i r , ) forms. The two isomers were separated ana r e s o l v e d i n t o o p t i c a l isomers ( 3 9 ) · The c r y s t a l s t r u c t u r e o f ( + ) c Q q ~ [Co(R,S-ptn),][Co(CN)g]*5H 0, the isomer which gave c r y s t a l s s u i t a b l e f o r X-ray work, was determined (40)· Figure 3 shows the absolute c o n f i g u r a t i o n o f the complex i o n , (+)egg-CCo(R,S-ptn),] · This i s the f a c i a l isomer and the three chelate r i n g s take the c h a i r conformation with the s u b s t i t u t e d methyl groups i n e q u a t o r i a l p o s i t i o n s . No unusually l a r g e thermal motion o f the r i n g carbon atoms was observed. The c i r c u l a r dichroism s p e c t r a i n the r e g i o n o f the f i r s t a b s o r p t i o n band o f the t r i s - b i d e n t a t e complex i o n s having s i x membered chelate r i n g s are known t o be p a r t i c u l a r l y s e n s i t i v e t o experimental c o n d i t i o n s . F o r example, the CD spectrum o f Δ - l e l , [Co(R,R-ptn) ]C1, i n an aqueous s o l u t i o n shows two peaks: Δ£ = - 0 . 5 8 9 , 5 2 2 nm;A8 = +0.104, 462.5 nm, whereas that o f Δ - l e l , [Co(R,R-ptn),](CIO. ) , i n an aqueous s o l u t i o n g i v e s a negative peak ( Δ ε = - 0 . 5 8 7 ; a t 5 1 8 nm (4l_). The s o l i d s t a t e CD d i f f e r from the s o l u t i o n CD and the s o l u t i o n CD are s e n s i t i v e t o the tem perature o f measurement and are a f f e c t e d by the presence o f oxo anions (42, 4 3 , 44). Table V l i s t s the lowest frequency CD s p e c t r a o f t r i s - d i a m i n e c o b a l t ( I I I ) complexes i n the CT region* A l l the absolute c o n f i g u r a t i o n s have been e s t a b l i s h e d by the X-ray method. U n l i k e the CD s p e c t r a i n the f i r s t a b s o r p t i o n r e g i o n , those i n the CT r e g i o n are i n s e n s i t i v e t o the c o n d i t i o n s o f the measurement described above. Using a l l o f the recorded CD data f o r t r i s - d i a m i n e c o b a l t (III) complexes o f known absolute c o n f i g u r a t i o n , an e m p i r i c a l r u l e r e l a t i n g the absolute c o n f i g u r a t i o n t o CD s p e c t r a o f t r i s - d i a m i n e c o b a l t ( I I I ) complexes i n the c h a r g e - t r a n s f e r r e g i o n was estab-


2

+

e

+

5

2



20 Table V

The lowest frequency CD band o f t r i s - d i a m i n e c o b a l t ( I I I ) complexes i n the CT r e g i o n Absolute •ζ -1 configuration χΛ(Τ cm -CCo(en) ]^ 47.4 -31 (45.) Λ +

5 8 9

-[Co(R,R-chxn) ]

3 +

44.1

+48

Δ

g -[Co(S,S-chxn) ]

5 +

43.9

+17

40.0

-13

40.2

-12

Δ Λ Λ Λ Λ Δ Α

5 8 9


5

3

3

9

-CCo(tn) ]

5 8 9

5

5

3

3

g -CCo(S-bn) ] 9

3 +

3

3 +

39.4

-8.7

-CCo(R,R-ptn) ]

3+

42.0

+6.5

-CCo(R,R-ptn) ]

3+

42.2

+18

43.5

-26.2

g -[Co(R,S-ptn) ] 9

3

5if6

3

5if6

5

3 +

3

g -CCo(tmd) ] 9

3 +

3

(22) (22) (46) (42) (48) (46) (46) (49)

l i s h e d : a t r i s ( d i a m i n e ) c o b a l t ( I I I ) complex whose s i g n o f the lowest-frequency CD band i n the charge-transfer r e g i o n i s negative has absolute c o n f i g u r a t i o n Λ ; i f i t i s p o s i t i v e , the absolute c o n f i g u r a t i o n i s Δ · In the case o f (+)-^-[0ο(Κ, R-ptn) » the e m p i r i c a l r u l e i s v i o l a t e d , because the p o s i t i v e CD c o n t r i b u t i o n o f the o p t i c a l l y a c t i v e l i g a n d i t s e l f i s superposed i n "this region. Seven-Membered Chelate Rings Only a small number o f s t r u c t u r e s containing seven-membered chelate r i n g s are known ( 5 0 , 5 1 ) · Tris(l,4-diaminobutane)cobalt( I I I ) i o n i s a t h i r d member o f a b a s i c s e r i e s o f s t r u c t u r e s : [Co { H N - ( C H ) - N H j ] (n=1, 2 , 3 · · · ) · This complex i o n has seven-membered chelate r i n g s . Figure 4 shows the absolute con f i g u r a t i o n o f (+)^gg-[Co(tmd).j] · I t has D^ symmetry. Table VI 5

2

2

n

Table VI

+

f

Geometry o f the chelate r i n g i n [Co(tmd),]' obs.

Co-N N-C C-C N-Co-N Co-N-C N-C-C C-C-C Co-N-C-C N-C-C-C C-C-C-C

1.991(5) 1.506(11) 1.512(13) 89.2(2)

122.9(3) 113.6(5) 111.6(6) 96.2

75.9 56.3

+

calc. 2.004 A Q

1-515 1.527

88.4° 120.4 113.3 113.7 101.1 79.0 56.1

compares the observed and c a l c u l a t e d geometries o f the complex i o n


Absolute

Configuration

21


2. SAITO

Acta Crystallographica Figure 4. A perspective drawing of the complex ion (+) 89-[Co(tmd) ] (51) 5

s

3+



22

(29)· The chelate r i n g takes on a skew conformation and i t i s s t r a i n e d : a l l the bond angles i n the chelate r i n g are g r e a t e r than the normal t e t r a h e d r a l angle. The minimized s t r u c t u r e agrees reasonably w e l l with the observation. The chelate r i n g i s c h i r a l and the conformation can be designated as λ provided that the h e l i c i t y i s defined by the l i n e j o i n i n g nitrogen atoms and the l i n e j o i n i n g the two carbon atoms next to the n i t r o g e n atoms. The c e n t r a l C-C bond i n the chelate r i n g i s i n c l i n e d by about 0.6 with respect to the t h r e e f o l d a x i s of the complex i o n . Hence t h i s i s the l e i - isomer and the absolute c o n f i g u r a t i o n i s Δ(λλλ). A marked d i f f e r e n c e i n the geometry of t h i s complex i o n from that of [Co(en)_r i s that three of the s i x methylene groups bonded to the n i t r o g e n atoms are above the upper t r i g o n a l plane of the three n i t r o g e n atoms and the remaining three are below the lower t r i g o n a l plane of the nitrogen atoms. The average d e v i a t i o n i s 0.37 A. On the other hand, the ethylene group at the center of the chelatg r i n g i s 3·83 A d i s t a n t from the t h r e e f o l d a x i s , compared to 2.81 A i n the case of [Co(en),] . These c h a r a c t e r i s t i c features i n the arrangement of the n o n - l i g a t i n g atoms a f f e c t the magnitudes of r o t a t o r y strengths R(E) and R(A ) . This point w i l l be discussed i n the next s e c t i o n .


o

C i r c u l a r Dichroism Spectra of T r i s ( d i a m i n e ) c o b a l t ( I I I ) Complexes T r i s ( d i a m i n e ) c o b a l t ( I I I ) complexes u s u a l l y give two c i r c u l a r dichroism bands with opposite s i g n and d i f f e r e n t magnitudes i n the absorption r e g i o n around 20x10 cm" i n aqueous s o l u t i o n (the f i r s t absorption r e g i o n ) . Th^se bands are a s c r i b e d tçj the d. ^ e l e c t r o n t r a n s i t i o n from the A^ ground s t a t e to the E and e x c i t e d l e v e l s o f octahedral parentage i n a D, environment. McCaffery and Mason measured the s i n g l e c r y s t a l c i r c u l a r dichroism spectrum of (+) gg-CCoCen^J^Clg^NaClôHÔ with l i g h t propagated p a r a l l e l to the o p t i c a x i s f i n which a l l the complex ions are a r ranged with t h e i r t h r e e f o l d axes p a r a l l e l to the o p t i c a x i s (45). Under t h i s c o n d i t i o n only the Ε component i s e x c i t e d . This c r y s t a l measurement showed that the i n t r i n s i c r o t a t o r y strength o f the A j p * E t r a n s i t i o n i s p o s i t i v e and s u b s t a n t i a l l y l a r g e r than that of s o l u t i o n c i r c u l a r îchrogysm. T h i s means that the i n t r i n s i c r o t a t o r y strength of A^—> A^ must be negative and almost as l a r g e as that of the Ε component, s i n c e the t r i g o n a l s p l i t t i n g i s s m a l l . Kuroda and S a i t o showed that the r o t a t o r y strengths o f the Ε and A^ components can be separated by combining the c i r c u l a r dichroism s p e c t r a o f a s i n g l e u n i a x i a l c r y s t a l and i n i t s microc r y s t a l l i n e s t a t e (52). Even i f the t h r e e f o l d a x i s of the complex i o n i s not o r i e n t e d p a r a l l e l to the o p t i c a x i s , i t i s p o s s i b l e to r e s o l v e the observed s o l i d s t a t e CD spectra i n t o the Ε and A^ com ponents by making use of the known c r y s t a l s t r u c t u r e . Jensen and Galsbjil measured the c r y s t a l CD s p e c t r a of ( + ) g Q - [ C o ( e n ) , ] ^ i o n doped i n a host c r y s t a l of racemic [ i H e n ^ ^ C I g i N a C l - ô H ^ O with l i g h t propagated both p a r a l l e l and perpendicular to the three&

a

+

5



2.

SAiTo

Absolute

Configuration

23

f o l d a x i s of [Co(en),]3+ (53)· This was f i r s t achieved by the phase modulation technique i n p o l a r i z e d spectroscopy (5^)· Before then, measurements o f CD were r e s t r i c t e d to s o l u t i o n , g l a s s e s , f i n e powder and u n i a x i a l c r y s t a l s with l i g h t propagated along the o p t i c a x i s , since the s i g n a l was otherwise d i s t o r t e d by i n t e r f e r e n c e from l i n e a r b i r e f r i n g e n c e and l i n e a r dichroism. Table VII summarizes the observed r o t a t o r y strengths o f some t r i s diamine c o b a l t ( I I I ) complexes. As seen from the t a b l e , R(E) i s p o s i t i v e and R(A ) i s negative f o r the absolute c o n f i g u r a t i o n Λ t while R(E) i s negative and R(A ) i s p o s i t i v e f o r Δ c o n f i g u r a t i o n , i n agreement with the well-known e m p i r i c a l r u l e f o r the s o l u t i o n CD spectrum. The values l i s t e d i n Table VII of R(E) and R(A ) are c o r r e c t e d f o r random o r i e n t a t i o n f a c t o r s of 2/3 and 1/3, r e s p e c t i v e l y . Thus the net r o t a t o r y strength K(T^) = R(E) + R(A ) may be compared d i r e c t l y with the observed values f o r s o l u t i o n , which are l i s t e d i n the l a s t column of Table VII. The net r o t a t o ry s t r e n g t h RiT..) changes s i g n on going from s o l i d to s o l u t i o n i n the case o f CCo?S,S-cptn),]^ and [ C o ( S , S - p t n ) J · This observa t i o n may be a s c r i b e d to tne i o n a s s o c i a t i o n or conformational change i n s o l u t i o n (41). The absolute values of the observed R(E) and R(A ) possess nearly the same magnitudes with opposite signs. |R(E7| has the major r o t a t o r y strength f o r the complexes with five-membered chelate r i n g s l i k e [Co(en),] , [Co(pn),] , [Co(chxn),;r and [ C o ( c p t n ) _ r t whereas R(A 7 possesses tne major r o t a t o r y strength f o r ^ t h e complexes CCo(ptn),] and [Co(tmd),] · This trend appears to be r e l a t e d to the s p a c i a l arrangement of the n o n - l i g a t i n g atoms around the cobalt atom. Figure 5 shows p r o j e c t i o n s of the chelate r i n g s o f these complex es upon a plane through the t h r e e f o l d a x i s and the twofold a x i s . As seen from the f i g u r e , a l l the n o n - l i g a t i n g atoms are between the t r i g o n a l planes formed by the three nitrogen atoms f o r those complexes whose |R(E)| i s g r e a t e r than |R(A )|. On the contrary, i n the case of the complexes with |R(E)| < |R(A >|, some o f the n o n - l i g a t i n g atoms are above and below the t r i g o n a l planes and those atoms l y i n g between the t r i g o n a l planes are smaller i n number and l o c a t e d more d i s t a n t than those i n the f i r s t group. The d-d t r a n s i t i o n s are magnetic dipole-allowed but e l e c t r i c d i p o l e forbidden. I f a coulombic c o r r e l a t i o n between tê compo nents o f the e l e c t r i c hexadecapole moment o f the A ^ — » Τ d e l e c t r o n t r a n s i t i o n and a t r a n s i t i o n d i p o l e moment induced i n each l i g a n d group i s considered, the c o r r e l a t e d d i p o l e moment o f the l i g a n d group g i v e s r i s e to a non-zero s c a l a r product with a component of the magnetic t r a n s i t i o n moment (55)* Thus such a d i s p o s i t i o n as w e l l as the geometry o f the chelate r i n g system might give a greater |R(A^)I than |R(E)| f o r the complex ions of the second group. 2

2

2

2

p

2

2

These D, complexes have played a prominent r o l e as model systems i n the t h e o r e t i c a l s t u d i e s of n a t u r a l o p t i c a l a c t i v i t y , since the high symmetry of the complexes makes tedious c a l c u l a t i o n s more or l e s s f e a s i b l e and a l o t of experimental data are



3

3

+31.1

A-CCo(tmd) ]Br

2

+12.5

3

3

Λ- [Co(S,S-ptn) 3C1 '2H 0

T

+57.3

+41.5

/V-[Co(S,S-cptn)_]Cl ·4Η_0 3 3 ^

3

2

+56.5

3

3

Λ-CCo(S,S-chxn) ]CI,·5H-0

3

+38.1

+43

+2

-38.7

-2.0

-14.5

-7.6

+2.8

+5.4

+1.5

+4.2

-54.5

-51.1

-36.6

-55.7

-41

(52)

(£2)

(52)

(52)

(102)

(52)

(52)

(53)

(102)

(52)

Ref.

+50.9

+4.3

R(T.,)

(45.)

-58.6

2

B(A )

+52.6

+62.9

A-CCo(S-pn) ]Br

2

+59-9

3

Λ-[Οο(βη) ]ΒΓ ·Η 0

3

A-2CCo(en) ]Cl .NaCl»6H 0

R(E)

f a c t o r s o f 2/3and 1/3 r e s p e c t i v e l y . n

cgs)

-4.9

+1.9

-4.3

+3.9

+4.2

+4.4

R(T„) 1 soin +4.4

Values o f R(E) and R(A^) are c o r r e c t e d by the f i x e d o r i e n t a t i o n

Table V I I . Rotatory s t r e n g t h s o f t r i s ( d i a m i n e ) c o b a l t ( I I I ) complexes (10~


sAiTO

Absolute

Configuration


2.


25



26

a v a i l a b l e f o r comparison. The t h e o r e t i c a l study was c a r r i e d out by a number of workers, notably by Richardson (56-61), Evans, Schreiner and Hauser (62) and Mason and Seal (557· Two models are now employed to c a l c u l a t e the o p t i c a l r o t a t o r y s t r e n g t h : one i s the c r y s t a l f i e l d model admitting only s t a t i c coupling between a metal d - e l e c t r o n and the charge d i s t r i b u t i o n i n the p e r t u r b i n g l i g a n d i n the ground s t a t e and the other i s the dynamic coupling model t a k i n g i n t o account o f the coupling of the t r a n s i t i o n i n the chromophore with e l e c t r i c d i p o l e t r a n s i t i o n induced i n the l i g a n d by the t r a n s i t i o n charge d i s t r i b u t i o n . Both models can account f o r the observed f e a t u r e s o f the CD s p e c t r a with considerable success. For [Co(en)_] , the values f o r observed R(E)'s i n Table VII should be compared with t h e o r e t i c a l values o f 35·7 and 63.8, the former being based on the c r y s t a l f i e l d model (62) and the l a t t e r on the dynamic c o u p l i n g model (55)· The agreement i s good. Diethylenetriamine

Complexes

There are three d i f f e r e n t ways o f c o o r d i n a t i n g two d i e t h y l enetriamine molecules to a c o b a l t ( I I I ) i o n . Among three geometric isomers, the u - f a c i a l - and mer-isomers are o p t i c a l l y a c t i v e and have p a i r s o f enantiomers r e s p e c t i v e l y , whereas the s _ - f a c i a l isomer i s o p t i c a l l y i n a c t i v e . A l l geometric and o p t i c a l isomers i n t h i s system were i s o l a t e d , and the geometric c o n f i g u r a t i o n s were assigned f o r the o p t i c a l l y a c t i v e isomers from the d i f ference i n racemization behavior o f the o p t i c a l l y a c t i v e uf a c i a l - and mer-isomers (63)· A l l the c r y s t a l s t r u c t u r e s o f these isomers were determined. The s - f a c i a l isomer has approxi mately C symmetry. The conformations o f the two fused c h e l a t e r i n g s are enantiomeric (64). In c r y s t a l s of (-) ,-g^-u-fac[ C o ( d i e n ) ] [ C o ( C N ) g > 2 H 0 , there e x i s t two d i f f e r e n t conformers i n an asymmetric u n i t . They both have a twofold a x i s and the absolute c o n f i g u r a t i o n can be designated as skew chelate p a i r s ΔΛΔ. However, the conformations o f the two chelate r i n g s formed by a d i e n molecule i n one complex i o n are SX while those i n the other are λλ (65). 2 h

2

2

f

B r

1

The absolute c o n f i g u r a t i o n o f (+) .Q >-mer-[Co(dien) 2 ^ 7 * EÔ has r e c e n t l y been determined (66). Figure 6 shows a per s p e c t i v e drawing o f the complex i o n . The complex i o n can be des ignated as trans-λ-ΝΗ, p r o v i d i n g that the c h i r a l i t y i s d e f i n e d by the l i n e j o i n i n g the two Η atoms and the l i n e j o i n i n g the two secondary n i t r o g e n atoms i n t r a n s - p o s i t i o n s (3, 60). The two terdentate molecules coordinate to the c e n t r a l c o b a l t atom i n mer p o s i t i o n s with three n i t r o g e n atoms forming a d i s t o r t e d o c t a h e d r a l complex. The complex i o n has an approximate twofold sym metry. The Co-secondary Ν bond o f 1.940 A i s s i g n i f i c a n t l y s h o r t e r than the Co-terminal Ν bond o f 1.981 A. The angle sub tended a t the c e n t r a l cobalt atom i s 187 · The three l i g a t i n g n i t r o g e n atoms of the l i g a n d and the c o b a l t atom are n e a r l y c o p i a r

r



2.

Absolute

sAiTo

Configuration

27

nar and the two planes formed by the cobalt and the three n i t r o g e n atoms make an angle o f 89·6 · The five-membered chelate r i n g takes an envelope form, one o f the two methylene carbon atoms that i s bonded to the secondary n i t r o g e n atom i s s h i f t e d by 0.64 A from the plane formed by the remaining four atoms. The conformations of the two fused chelate r i n g s are S and λ, r e s p e c t i v e l y . The geometry o f the complex c a t i o n agrees w e l l with the r e s u l t o f conformational a n a l y s i s (67)t as shown i n Table V I I I . The net c h i r a l i t y o f t h i s complex i o n i s zero. The o p t i c a l a c t i v i t y o f the complex i o n a r i s e s from the dissymmetric d i s p o s i t i o n o f the methylene groups with respect to the c o o r d i n a t i o n plane and the c h i r a l arrangement o f the two trans N-H bonds. R i c h a r d s o n s s e c t o r r u l e (60) was t e s t e d on the b a s i s o f the f i n a l atomic parameters. In h i s d e r i v a t i o n the p e r t u r b a t i o n treatment 1

Table VIII

Observed and c a l c u l a t e d geometries o f mer-[Co(dien) y* o

+

obs.

calc.

1.981

1.976

Co-N(H)

1.940

1.942

C-N(H )

1.493

1.495

C-N

1.482

1.486

N-Co-N

85.1

Co-N(H ) 2

2

86.0°

116.2

114.5

Co-N(H )-C

109.3

109.3

Co-N(H)-C

109.5

107.8

N(H )-C-C

108.7

109.1

N(H)-C-C

104.7

105.9

C-N-C 2

2

I

was c a r r i e d out to the second order i n both the wave f u n c t i o n and r o t a t o r y s t r e n g t h . A negative net r o t a t o r y strength i n the r e g i o n o f the f i r s t absorption, A —> T^ was p r e d i c t e d . The CD s p e c t r a of the complex i o n i n aqueous s o l u t i o n agreed with t h i s . e x p e c t a t i o n ^ = +0.096 a t 19·5χ10^ cm , d£ = -Ο.181 a t 21.9x10^ 1

cm" ,

(68)).

Complexes with a C y c l i c Terdentate,

R-MeTACN

Mason and Peacock synthesized the c y c l i c terdentate, R - ( - ) 2-methyl-l,4,7-triazacyclononane and i t s Co(III) complex, [Co(RMeTACN) ]^ (69). Figure 7 shows a perspective drawing o f the complex i o n , T^j-gQ-CCoiR-MeTACN)^^ i n i t s i o d i d e pentahydrate c r y s t a l s . The complex i o n has D symmetry by the requirement o f 2

T




28

Figure 7.

A perspective drawing of the complex ion ( -) -[Co(R-MeTACN) ] + 589

2

3



2.

Absolute

sAiTo

Configuration

29

the space group. The methyl group i s attached to one of the three c h e l a t e r i n g s o f the c y c l i c terdentate l i g a n d , so that the complex i o n e x h i b i t s o r i e n t a t i o n a l d i s o r d e r . The observed e l e c t r o n - d e n s i t y d i s t r i b u t i o n i n d i c a t e s that the methyl group i s attached i n e q u a t o r i a l p o s i t i o n s with respect to the c h e l a t e r i n g . There are three p o s s i b l e geometrical isomers f o r the complex i o n i n respect of the p o s i t i o n s of the two methyl groups. No con c l u s i o n can be drawn concerning the isomerism, due to the o r i e n t a t i o n a l d i s o r d e r , s i n c e the X-ray a n a l y s i s only i n d i c a t e s the average s t r u c t u r e . Two molecules o f the c y c l i c terdentate co o r d i n a t e to the cobalt atom with s i x secondary n i t r o g e n atoms from above and below the metal atom to form an octahedral complex. A MeTACN molecule spans a face o f the octahedron. The s i x f i v e membered chelate r i n g s take λ conformation (70). Nonomiya sepa r a t e d the c o b a l t ( I I I ) complex i n t o f i v e components u s i n g SP Sephadex column chromatography, although there are nine p o s s i b l e isomers (71)· Figure 8 shows two modes of c o o r d i n a t i o n o f RMeTACN. In the mode " b , the l i g a n d i s coordinated to the metal atom with the nine-membered r i n g upside down compared to the mode a . The absolute c o n f i g u r a t i o n o f N(1) i s S i n the mode a while i t i s R i n the b mode. There are three ways o f combining the modes a and b : aa, ab and bb. In a d d i t i o n to t h i s there are three geometrical isomers i n respect o f the p o s i t i o n s of the methyl groups. Accordingly, nine isomers are p o s s i b l e as a whole. The c r y s t a l subjected to X-ray s t r u c t u r e a n a l y s i s i n c o r p o r a t e s three aa type isomers. Nonomiya found that aa and ab type isomers can be separated, r e s p e c t i v e l y , i n t o two components: one isomer and a mixture of the remaining two. On the other hand the bb type isomers were separated as one component by h i s method. A l l these complexes show very strong p o s i t i v e CD extremes i n the r e g i o n o f the f i r s t s p i n allowed d-d transition of o c t a h e d r a l parentage. The geometric a r r a y o f c h e l a t e groups i n t h i s complex i o n d i f f e r from that o f [Co(en) "] i n the same manner as d e s c r i b e d before. I t has the n o n - l i g a t i n g atoms above and below the t r i g o n a l planes o f the l i g a t i n g n i t r o g e n atoms and none o f the nonl i g a t i n g atoms e x i s t s i n the e q u a t o r i a l plane. The c r y s t a l s o f [CoiR-MeTACN^Dl-z^HÔ are o p t i c a l l y u n i a x i a l and each complex ion i s arranged with the t h r e e f o l d a x i s p a r a l l e l to the o p t i c a x i s . The l i g h t propagated along the o p t i c a x i s can only e x c i t e the Ε component o f the t r a n s i t i o n . The s i n g l e c r y s t a l c i r c u l a r dichroism spectrum shows a s i n g l e negative peak a t 487 nm, while that i n aqueous s o l u t i o n has a p o s i t i v e peak a t about 487 nm. The s o l u t i o n c i r c u l a r dichroism s p e c t r a a l s o r e v e a l s R(A_), g i v i n g the sum, R(T^) = R(E) + R(A^). A comparison o f the s i n g l e c r y s t a l and the s o l u t i o n CD s p e c t r a i n d i c a t e s that R(E) has minor r o t a t o r y s t r e n g t h o f -0.16 ϋ β and R(A ) +0.32 DP (72). T h i s r e s u l t sup p o r t s the observed r e l a t i o n between the r e l a t i v e magnitudes o f R(E) and R(A^) and the arrangement o f n o n - l i g a t i n g atoms i n t r i s b i d e n t a t e complexes (Table V I I ) . The p o l a r capping o f ^ " " ( " ^ c S ^ w

w

w

w

,f

w

w

Μ

w

M

w

2

M





2.

SAITO

Absolute

31

Configuration

[Co(en)-/r with the phosphate i o n by hydrogen bonding i n s o l u t i o n i s known to enhance R(A > at the expense o f R(E) ( 7 3 » 7 4 ) . The p o l a r capping of (+)cgQ-[Co(en)^] by the c o v a l e n t l y bonded t r i s (methyleneamino) g r o u p ^ r e s u l t s i n ^ - ) g g - [ C o ( l , 3 » 6 , 8 , 1 0 , 1 3 , 1 6 , 1 9 octa-aza-bicyclo[6.6.6]-eicosane] (75/· This complex i o n g i v e s R(T ) o f - 0 . 0 6 8 DP , i n c o n t r a s t to +0.047 o f A-[Co(en),] . These observations i l l u s t r a t e the general enhancement o i R(A^) a t expense o f R(E) by the a d d i t i o n o f atoms or atomic groups to the p o l a r r e g i o n o f the [CoNg] chromophore o f D^ symmetry. 2

5


>|

M

E l e c t r o n - d e n s i t y D i s t r i b u t i o n i n D^

Complexes

Recent improvements i n experimental and computational t e c h niques i n X-ray c r y s t a l l o g r a p h y have made i t p o s s i b l e to estimate atomic charge-density i n an o p t i c a l l y a c t i v e t r a n s i t i o n metal complex, based on accurate i n t e n s i t y data. Two examples w i l l be d e s c r i b e d here. The c r y s t a l s t r u c t u r e s ofA-le3.T*CCo(S,S-chxn)^](NO ) ·3ΗρΟ and Δ-ob -[Co(S,S-chxn) ](N0 ) , ^ ^ Ο have been * determines ( ^ 6 ) . The number o f e l e c t r o n s w i t h i n a sphere o f r a d i u s 1 . 2 2 A (covalent r a d i u s o f cobalt) are l i s t e d i n Table IX, together with other r e l a t e d complex i o n s . The c e n t r a l metal atom i s n e u t r a l i z e d l a r g e l y by donation o f e l e c t r o n s from the l i g a t i n g n i t r o g e n atoms, i l l u s t r a t i n g that P a u l i n g s e l e c t r o n e u t r a l i t y r u l e holds f o r these t r a n s i t i o n metal complexes. Larsson and h i s c o l Table IX. E f f e c t i v e charge on the c e n t r a l metal atom Complex C(R) E f f e c t i v e charge Ref. 1

a

(77)

(ZD (78) (76) (76)

a: Number o f e l e c t r o n s w i t h i n a sphere o f r a d i u s 1 · 2 2

X

l a b o r a t o r y estimated the e f f e c t i v e charge o f cobalt i n iCo(en)J} and iCo(W)/] by ESCA and obtained the value o f + 0 . 7 ( 3 ) and + 0 . 6 ( 4 ) , r e s p e c t i v e l y ( 7 9 ) · Non-bonding

Absolute Configuration of Transition Metal Complexes - ACS

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