Mechanisms of Inorganic Reactions

5 Bridging Groups in Electron Transfer Reactions

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H E N R Y TAUBE Stanford University, Stanford,

Calif.

A

m o d e r a t e l y successful c l a s s i f i c a t i o n

of car-

b o x y l a s e b r i d g i n g g r o u p s i n the reaction of the pentaamminecobalt(III) (aq.)

complexes with

Cr

+ 2

is t h i s ; (a) a c h e l a t e f u n c t i o n i s p r e s e n t

t h a t c a n a c t with the c a r b o x y l a t e f u n c t i o n o n the

Co(III); (b) a r e m o t e p o l a r g r o u p such a s

c a r b o x y l o r carbonyl is in c o n j u g a t i o n with the c a r b o x y l g r o u p o n Co(III); (c) n e i t h e r o f t h e s e special

f u n c t i o n s is p r e s e n t . W i t h i n class (c),

a d j a c e n t attack occurs, and, but for two exceptions,

the

wide

variations

rate

is

r e m a r k a b l y insensitive t o in the carboxylate

group.

V a r i a t i o n s d o occur w i t h i n a f a c t o r o f a b o u t f o u r in rate, a n d t h e s e m a y r e s u l t f r o m i n d u c t i v e a n d / o r s t e r i c effects. G r o u p (b) a s a class shows

rates

of reaction

much

higher

than

G r o u p (c). In s o m e s y s t e m s o f t h i s class r e m o t e attack

appears

to

take

place.

g y n o w n u m e r o u s d a t a h a v e been a c c u m u l a t e d o n t h e r a t e a t w h i c h C r

a q

+ 2

reducing agents react w i t h complex ions of the type C o ( N H 3 ) C 0 2 R 5

+ 2

and other .

S e v e r a l s t r u c t u r a l features of t h e l i g a n d h a v e been r e c o g n i z e d as a f f e c t i n g i t s efficacy i n m e d i a t i n g electron transfer from the reducing agent t o the o x i d i z i n g agent.

One

i m p o r t a n t f e a t u r e is a c o n j u g a t e d b o n d s y s t e m e x t e n d i n g f r o m a r e m o t e p o l a r g r o u p t o t h e c a r b o x y l a s s o c i a t e d w i t h t h e C o ( I I I ) (5, 20).

A ligand w i t h this structure

m a k e s e l e c t r o n t r a n s f e r p o s s i b l e b y r e m o t e a t t a c k , a n d leads t o a n i n c r e a s e d r e a c t i o n rate.

W h e n t h e l i g a n d c o n t a i n s g r o u p s w h i c h lead t o s t r o n g e r a s s o c i a t i o n o f C r

a q

+ 2

w i t h the o x i d i z i n g agent, as w h e n a chelate function is b u i l t i n t o i t , the rate is also i n c r e a s e d (6). T h e s e effects are n o t t h e o n l y s i g n i f i c a n t o r i n t e r e s t i n g ones, b u t o t h e r s , t h o u g h w o r t h y of d i r e c t a t t e n t i o n , h a v e b e e n c o n s i d e r e d o n l y i n a d e s u l t o r y f a s h i o n .

It

has, therefore, seemed w o r t h w h i l e t o p r e p a r e a r e v i e w of t h e o b s e r v a t i o n s a n d t o pose some of t h e q u e s t i o n s b e a r i n g o n t h e m w h i c h a r e a c u r r e n t c o n c e r n .

This

seems a l l t h e m o r e w o r t h w h i l e because e v e n t h e t w o effects w h i c h h a v e been e x 107

108

M E C H A N I S M S O F I N O R G A N I C REACTIONS

p l i c i t l y described a n d discussed are o n l y imperfectly understood.

Thus, although

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

and

a l t h o u g h the m e c h a n i s m of e l e c t r o n t r a n s f e r t h r o u g h b r i d g i n g g r o u p s h a s been d i s c u s s e d ( i l , 13), t h e r e l a t i o n of t h e t h e o r e t i c a l t r e a t m e n t s of t h e o b s e r v a t i o n s h a s n o t been e s t a b l i s h e d .


M a n y m o r e q u e s t i o n s w i l l be a s k e d t h a n a n s w e r s s u p p l i e d .

A n u m b e r of

q u e s t i o n s w i l l u n d o u b t e d l y h a v e a n s w e r s w h i c h are o b v i o u s t o readers of t h i s p a p e r , b u t i t is l i k e l y t h a t some a t least w i l l challenge t h e i r i n v e n t i v e n e s s .

If the p r o b l e m s

w h i c h a r e o u t l i n e d p r o v o k e t h o u g h t f u l d i s c u s s i o n a n d suggest e x p e r i m e n t s w h i c h i l l u m i n a t e the issues, the p u r p o s e of t h i s p a p e r w i l l be f u l l y s e r v e d . C o m p a r i s o n s of r e a c t i o n rates p r o v i d e the basis for m a n y of the q u e s t i o n s . H o w e v e r , a n y e x p l a n a t i o n w h i c h a c c o u n t s for r a t e differences, b u t i g n o r e s t e m p e r a t u r e coefficients is a t the v e r y least i n c o m p l e t e a n d m a y a c t u a l l y be w r o n g . W h e r e t e m p e r a t u r e coefficient d a t a exist, t h e y h a v e been t a k e n i n t o a c c o u n t , b u t few o b s e r v a t i o n s are so w e l l u n d e r s t o o d t h a t b o t h r e a c t i o n r a t e s a n d t h e i r v a r i a t i o n w i t h t e m p e r a t u r e are a c c o u n t e d for. E x c e p t for a few l i g a n d s , n a m e l y c e r t a i n of those i n w h i c h a t t a c k is a t a r e m o t e c a r b o n y l g r o u p , the o r g a n i c l i g a n d is t r a n s f e r r e d t o the c h r o m i u m d u r i n g r e a c t i o n of t h e C o c o m p l e x w i t h C r

+ 2

.

T h o u g h f o r m a t i o n of a C r ( I I I ) - l i g a n d

complex

does n o t p r o v e t h a t d i r e c t t r a n s f e r of t h e l i g a n d t a k e s place, w e a s s u m e t h a t i n e v e r y i n s t a n c e w e are d e a l i n g w i t h a b r i d g e d a c t i v a t e d c o m p l e x .

I n m a n y cases C r

+ 2

is

v e r y inefficient i n t r a p p i n g t h e free l i g a n d present i n s o l u t i o n w h e n t h e r e d u c i n g a g e n t is o x i d i z e d t o C r

b y a C o ( I I I ) complex.

+ 3

T h e assumption that a bridging

m e c h a n i s m operates i n the s y s t e m s w e w i l l discuss is l i a b l e t o c r i t i c i s m i n o n l y a few cases. T h e o x i d i z i n g c o m p l e x e s d e a l t w i t h are a l l of t h e p e n t a a m m i n e c o b a l t ( I I I ) class, a n d o n l y the r e a c t i o n s w i t h C r + a r e considered s y s t e m a t i c a l l y . 2

Variations

in Rate for Adjacent

Attack

T a b l e I s u m m a r i z e s k i n e t i c p a r a m e t e r s f o r t h e r e a c t i o n of C r

4 2

with

car-

b o x y l a t o p e n t a a m m i n e c o b â l t ( I I I ) c o m p l e x e s of l i g a n d s w h i c h f a v o r n e i t h e r c h e l a t i o n of t h e r e d u c i n g a g e n t n o r r e a c t i o n b y r e m o t e a t t a c k . U s e has been m a d e of t h e f a c t t h a t t h e specific r a t e v a l u e s for a series of c o m plexes c o n t a i n i n g l i g a n d s s u c h as those i n T a b l e I are n e a r l y c o n s t a n t , a n d a n y s u b s t a n t i a l r a t e increase has been a t t r i b u t e d t o s o m e " s p e c i a l e f f e c t . "

B u t the fact

t h a t t h e specific rates r e p o r t e d i n T a b l e I v a r y so l i t t l e i t s e l f deserves a t t e n t i o n . T h e l i g a n d s d o differ m a r k e d l y i n p r o p e r t i e s , a n d t h i s difference is reflected i n a v a r i a t i o n i n the d i s s o c i a t i o n c o n s t a n t s of b e t w e e n 1 0 a n d 1 0 for the c o r r e s p o n d i n g 4

organic acids.

Since C r

+ 2

5

p r e s u m a b l y a t t a c k s a n o x y g e n of the c a r b o x y l g r o u p , the

specific r a t e s h o u l d reflect the difference i n t h e a v a i l a b i l i t y of t h e u n s h a r e d elec t r o n s o n the o x y g e n s .

A r a t e decrease is i n d e e d n o t e d i n the series C H s C 0 2 ~ ,

C I C H 2 C O 2 " " a n d C b C H C C ^ " as the effect of the e l e c t r o n w i t h d r a w i n g p o w e r of R increases.

B u t e v e n t h i s decrease is n o t r e l i a b l e .

F o r , o w i n g t o the w a y AFt

is

r e s o l v e d i n t o ΑΗ% a n d Δ 5 { , the r a t e for the c o m p l e x c o n t a i n i n g C I C H 2 C O 2 " " w i l l be e v e n greater a t a h i g h e r t e m p e r a t u r e t h a n t h a t for the a c e t a t o

complex.

T h e slow r a t e of r e d u c t i o n of the a c e t a t o c o m p l e x (0.18 Af-hecr ) for e x a m p l e , t o the a q u o c o m p l e x (0.5 M'hecr ) 1

1

compared,

is n o t e w o r t h y {24).

T h e acetato

5.

TAU BE

T a b l e I·

Bridging

109

The R a t e o f R e a c t i o n of C r with Selected Pentaamminecobalt (III) C o m p l e x e s ftq

(At 25°C. ±

k, M~ sec.~

CH C0 CICHoCOo CUCHCOj F CC0 Benzoate σ-Ch lorobe η zo a to £-Chlorobenzoato p-Hydroxybenzoato Isophthalato HC0 H NCH CO,

0.18 0.10 0.074 0.052 0.14 0.074 0.21 0.13 0.13 7 0.06.

l

2

3

2

2

+

3

f 2

1.0°, μ =s 1.0 except where otherwise indicated)

Ligand 3


Groups

2

l

AHt

ASt

Ref.

3.0 7.9 2.5

-52 -37 -55

4.9 6.0 10.0 9.6 2.1

-46 -43 -28 -30 -56

20 6 6 12 6 6 6 6 20 2 15

carries a lower positive charge than the aquo complex and, formally at least, offers an opportunity for " remote" attack at the carbonyl oxygen.

(Attack at C o — Ο — C

oxygen would correspond to attack at H 0 in the aquo complex.) 2

in the case of carboxyl acting as a bridging group, whether C r

+ 2

It is not known,

attacks the carbonyl

or the C o — Ο — C oxygen. It is possible that in one series of complexes, for example, those of simple carboxyl ions, attack is at one of these positions, but when a chelating function is introduced, the position of attack changes. In addition to inductive effects, and the possibility that the position of attack may be different for one complex than for another, steric effects are also undoubtedly a factor.

T h e acid dissociation constants shew that H C 0 2 ~ is less basic than

CH3CO2"", but the rate of reduction of the formato complex is much faster than that of the acetato.

Enough effects exist to explain almost any result qualitatively,

but they are not well enough understood to sustain even qualitative predictions. T h e way A Ft decomposes into Δ11% and ASt is as remarkable as the slow rate of reduction of the acetato complex.

Other results as well as those for the activation

parameters give rise to questions.

What is the origin of the very unfavorable

W h y are ΔH% and ASt so strongly affected by substitution

entropy of activation?

at a site well removed from the reaction site (compare entries for benzoate and £-chlorobenzoate in Table I) when the effect on the rate is so minor? the values of ASt

W h y don't

for the first three complexes in Table I change monotonously

within the series? The Effect of a Chelate Function

in the Bridging

Ligand.

W h e n the bridging ligand is malonate (22), the reaction rate is approximately twice that when it is acetate. malonato complex. the C r

+ 2

T h e reaction product in this system is the chelated

T h i s observation is consistent with the view that chelation of

by malonate takes place in the activated complex but does not prove this

to be the case.

Nothing is known about the rate at which a complex such as

(H 0)5Cr02CCH C02H)~ ~ , if it were formed, would react to form the chelate a l 2

2

,

2

though the results which follow suggest a slow rate.

W i t h glycolate as the bridg

ing group (2), the specific reaction rate at 25° C . and μ = 1.0 is 3.0.

F o r this sys

tem there is evidence indicating a chelated species as the primary reaction product. T h e extinction coefficient at λ =

411 ηιμ for the Cr(III) reaction product is 39.3.

T h i s changes with a half-life of 22 db 2 hr. at 2 5 ° C . to 30.9.

T h e final value of the

110

MECHANISMS OF INORGANIC

REACTIONS

e x t i n c t i o n coefficient i s i d e n t i c a l w i t h t h a t m e a s u r e d f o r t h e C r ( I I I ) - g l y c o l a t e c o m p l e x p r e p a r e d b y d i r e c t c o m b i n a t i o n , t h e n s e p a r a t i n g t h e species of c h a r g e from the residual C r ( H 0 ) e 2

+ 3

w i t h a cation exchange resin.

+2

T h e higher extinction

coefficient f o r t h e i n i t i a l p r o d u c t suggests a h i g h e r degree of c h e l a t i o n t h a n f o r t h e final

p r o d u c t , w h i c h m a y be a n e q u i l i b r i u m m i x t u r e of c h e l a t e d a n d n o n - c h e l a t e d


forms. C o m p l e x e s of t h e a h y d r o x y a c i d s as a class r e a c t m o r e r a p i d l y t h a n those of t h e p a r e n t c o m p l e x e s a n d i n v i e w o f t h e r e s u l t s c i t e d , i t seems r e a s o n a b l e t o a s c r i b e t h i s r a t e increase t o c h e l a t i o n of C r

+ 2

i n the activated complex.

(NH ) Co —Ο 3

m

5

\

Ο

^

Η C H

C

I

O H

F o r other ligands such as malonate, w h i c h contain a chelating function b u t have no o t h e r o b v i o u s p r o p e r t y w h i c h w o u l d f a c i l i t a t e e l e c t r o n t r a n s f e r , i t seems r e a s o n a b l e t o a s c r i b e t h e r a t e i n c r e a s e t o c h e l a t i o n of C r

+ 2

.

I n a n y event, i t is interesting to

c o n s i d e r t h e r a t e d a t a f o r these c o m p l e x e s i n v i e w of t h i s p o s s i b i l i t y , a n d a n u m b e r of s u c h d a t a a r e p r e s e n t e d i n T a b l e I I . T h e r a t e increase i n t h e g l y c o l a t o , l a c t a t o , m e t h y l l a c t a t o series agrees w i t h t h e v i e w t h a t chelation occurs i n the a c t i v a t e d complex.

T a b l e II.

T h e basicity of the O H

Effect o f C h e l a t i o n o n R a t e at 2 S ° C , μ « 1.0

Ligand

6

2

2

2

2

6

5

3

2

2

2

2

2

e

2

2

2

2

e

2

6

2

2

2

6

3

AHt

ASÎ

3.1 0.42 6.7 11.8 2.7 0.36 0.29 0.22 0.07 0.65 0.45 0.17 a t 14°C. 0.17 0.075 0.15 a t μ - 3.0 - 2 . 5 X 10* 2.7 - 2 Χ 10

9.0 9.3

-26 -23

9.1

-24

Ϊ.7

-54

l

Glycolate Methoxyacetate Lactate Methyllactate a Malate* β Malate* Malonate H0 CCHC H C0 H0 CC(CH ) C02 H0 CHC H C0 C H 0 CCH C0 H0 CCH CH C0 CH 0 CCH CH C0 Phthalate Salicylate -0 CCH C0 Phthalate ion Salicylate ion 2

k, M~ sec."

2

2

2

2

1

Re}. 2 2 2* 2 2* 2 2« 2 2 2 14 20 20 20 m 22 20 22

...

8

The specific rate reported here does not agree with that reported in (6), nor did we find evidence as re ported there for the path inverse in H+ even when (H+) was made as low as 0.02M. In agreement with results in (6), we find two forms of the malato complex, and we were apparently able to make a rather clean separation of the two forms. Our rate measurements, however, do not agree with those in (6). In reference 22, the specific rate is reported as 0.34. Since the term k\ (complex) ( C r ) is small com pared to either h (complex) (Cr+ )/(H+) or kt (complex) ( C r ) (H+) it is difficult to get a precise value of kv, further as Svatos and Taube caution, the values of ΔΗιΪ and ASit reported in (22) are not reliable. In (6) a value of 0.07 at 17° is reported. • Each of these show paths involving 1/(H+), but since Kdiu. for the complex acids has not been measured, the specific rates for the reactions of the anion ligand complexes cannot be calculated. a

b

β

+ +

+

d

+Î

5.

TAU BE

Bridging

Groups

111

should increase with increased replacement of H by CH3.

When the hydrogen of

the O H group is replaced by C H , a steric factor probably comes into play and the 3

T h e higher rate for the a

benefit from chelation is less than for the glycolate.

malate presumably means that this is the form which has the O H linked a to the carboxyl on the Co(III) ; the effect of O H in chelation should diminish as this group


becomes further removed from the adjacent carboxyl. The neutral carboxyl group is not very effective in increasing the reduction rate of the complex.

However, when the proton is removed from the carboxyl, the effect

can increase and is greatest when the carboxyl ion is in a configuration favorable to chelation.

Thus, the inverse ( H ) path is not even observable for acid succinate +

in the same acidity range as that for which this path is important in the acid malonato reaction.

T h e acid dissociation constants are known well enough so

that the behavior difference between acid malonato and acid succinato can not be entirely ascribed to different acidities of the complexes.

T h e results obtained with

the acid malonate complexes, as reported in Table II, incidentally provide no support for the hypothesis (22) that electron transfer takes place by remote attack across hydrogen bonds. H CH

Co—Ο—C II

I

Ο

C

=

O

Mechanisms of Inorganic Reactions

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