Kinetic Patterns of Ligand Reactivity - Advances in Chemistry (ACS

7 Kinetic Patterns of Ligand Reactivity

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 12, 2014 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0049.ch007

MARK M. JONES Vanderbilt University, Nashville,

Tenn.

In m a n y cases, the rate behavior of l i g a n d r e a c t i o n s is g o v e r n e d b y t h e s a m e gross mechanistic f e a t u r e s as those for the free ligands. When t h e electrophilic a t t a c k of a n a r o m a t i c s p e c i e s , L, is e x a m i n e d , t h e o r d e r of the rate constants f o r t h e r e a c t i o n s is: the free ligand > metal c o m plex

>

protonated

ligand. A n electrostatic

m o d e l of such r e a c t i o n s is d e v e l o p e d t o e s t i m a t e t h e r e l a t i v e r a t e constants of t h e c o m plex a n d t h e p a r e n t l i g a n d . For o t h e r systems, such a s r e d o x r e a c t i o n s o f a l i p h a t i c l i g a n d s , coordination may

result i n a m a s k i n g

which

simply stops the r e a c t i o n f o r steric r e a s o n s . In all cases t h e k i n e t i c b e h a v i o r is d e s c r i b e d i n t e r m s of t h e s t a b i l i t y constants of t h e c o m plexes concerned a n d the rate constant char acteristic o f e a c h such s u b s t r a t e species w h i c h is p r e s e n t .

Λ ne of t h e u n s o l v e d p r o b l e m s of m o d e r n c o o r d i n a t i o n c h e m i s t r y i s t h e of changes i n l i g a n d r e a c t i v i t y w h i c h a r i s e o n c o o r d i n a t i o n .

prediction

Ideally, this pre

d i c t i o n s h o u l d be i n t e r m s of b o t h t h e k i n e t i c a n d t h e r m o d y n a m i c b e h a v i o r of t h e ligand reaction.

W h i l e t h e r e are u n d o u b t e d l y

r e a c t i o n s of c o o r d i n a t e d

ligands

w h i c h are n o t f o u n d for t h e free species, i t i s m o r e p r o f i t a b l e a t p r e s e n t t o e x a m i n e s y s t e m s w h e r e t h e changes i n r e a c t i v i t y a r e m o r e r e g u l a r .

I t is assumed t h a t the

r e a c t i v i t y p a t t e r n s of t h e l i g a n d a r e m e r e l y m o d i f i e d (or m a s k e d ) b y rather than completely altered.

coordination,

W i t h this starting point, we can examine

some

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

W e can then divide such

known

r e a c t i o n s i n t o three general classes: 1. R e a c t i o n s of the l i g a n d w h i c h are so r e t a r d e d b y c o o r d i n a t i o n t h a t t h e r a t e c o n s t a n t s are v e r y different for t h e free a n d the c o m p l e x e d species. D i a z o c o u p l i n g of a r o m a t i c p h e n o l a t e s falls i n t o t h i s class.

153 In Mechanisms of Inorganic Reactions; Kleinberg, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.

154

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

2. R e a c t i o n s of the l i g a n d w h i c h are v e r y s l o w i n the a b s e n c e of a n e l e c t r o n pair acceptor or m e t a l l i c coordination center. B a s i c h y d r o l y s i s of a m i n o a c i d esters i s a n e x a m p l e of s u c h a r e a c t i o n . 3. R e a c t i o n s w h o s e r a t e is n o t g r e a t l y affected b y c o o r d i n a t i o n . H e r e t h e r e a c t i v e site of t h e l i g a n d is u s u a l l y q u i t e d i s t a n t f r o m t h e c o o r d i n a t i o n c e n t e r , though a v e r y active a t t a c k i n g reagent can lead to m u c h the same result if i t is sufficiently unselective. C o o r d i n a t i o n m a y c h a n g e t h e c h a r a c t e r of a r e a c t i o n b y a f f e c t i n g e i t h e r t h e r m o


d y n a m i c or k i n e t i c factors, or b o t h .

O n l y k i n e t i c f a c t o r s w i l l be c o n s i d e r e d here.

K i n e t i c effects m a y be p r o d u c e d b y changes i n ΔΗ%, àS%, o r b o t h , a n d i t is a l m o s t impossible to preclude

small thermodynamic

changes

i n such reactions.

The

l a t t e r a r e o f t e n less i m p o r t a n t a n d w i l l be t e n t a t i v e l y i g n o r e d . S i n c e c o o r d i n a t i o n c a n a l t e r t h e b e h a v i o r of a r e a c t i n g s y s t e m i n m a n y w a y s , (IS),

we w i l l l i m i t our consideration to relatively simple systems—those i n w h i c h

a l i g a n d f o r m s a σ - t y p e c o o r d i n a t e b o n d w i t h a n a c c e p t o r species. m i x e d complexes

While many

are c a t a l y t i c a l l y i m p o r t a n t , we w i l l assume t h a t t h e i r k i n e t i c

b e h a v i o r i s t h e s a m e a s s i m p l e c o m p l e x e s t o a first a p p r o x i m a t i o n . Ligand Polarization.

P o l a r i z a t i o n is the most i m p o r t a n t i m m e d i a t e

sequence of l i g a n d c o o r d i n a t i o n .

con

T h e fact t h a t ligand p o l a r i z a t i o n can assist some

of t h e l i g a n d r e a c t i o n s w a s first e x p o u n d e d i n d e t a i l a n d used b y M e e r w e i n (14,

24).

M e e r w e i n a l s o p r o v i d e d ( b u t n e v e r p u b l i s h e d ) t h e first i n t e r p r e t a t i o n of t h e w a y i n w h i c h c o o r d i n a t i o n m a y affect t h e p a t h of t h e a c t i v a t e d c o m p l e x a l o n g t h e r e a c tion coordinate. a b l e (11, IS).

F a i r l y c o m p r e h e n s i v e r e v i e w s of t h i s t y p e of r e a c t i o n a r e a v a i l H o w e v e r , t h e k i n e t i c aspects h a v e r a r e l y been d i s c u s s e d i n d e t a i l

(except for b i o c h e m i c a l cases (23, meager.

Therefore,

8)), because c o m p a r a t i v e e x p e r i m e n t a l d a t a are

the examples

cited below

fall i n t o several loosely

related

groups. M e t a l Ions a n d Protons.

B e c a u s e of the s i m i l a r i t i e s of t h e r e a c t i o n s of p r o

t o n s a n d m e t a l i o n s w i t h L e w i s bases, there is m u c h t o r e c o m m e n d a c o m p a r i s o n of t h e t w o processes t o f u r n i s h a g u i d e t o t h e b e h a v i o r of m e t a l c o m p l e x e s .

This

c o m p a r i s o n s h o w s t h e e n o r m o u s l y g r e a t e r effect of t h e p r o t o n d u e t o i t s g r e a t e r polarizing power.

A l t h o u g h m e t a l ions have been called " s u p e r - a c i d " catalysts b y

W e s t h e i m e r (38),

t h e y are n o r m a l l y m u c h less effective t h a n t h e p r o t o n o n a m o l e -

f o r - m o l e basis.

However, they do permit essentially acid catalyzed reactions i n

neutral or basic m e d i a .

T w o f u n d a m e n t a l reasons for t h e g r e a t e r effect of

the

p r o t o n a r e : (a) t h e p r o t o n - d o n o r a t o m d i s t a n c e is u s u a l l y m u c h s h o r t e r t h a n t h e m e t a l - i o n - d o n o r a t o m i n t e r n u c l e a r d i s t a n c e i n c o m p a r a b l e c o m p o u n d s (the e l e c t r i c field i n t e n s i t y f a l l s off as q/r ) 2

a n d (b) t h e effect of t h e c h a r g e of t h e m e t a l i o n is

u s u a l l y s h a r e d b y f o u r t o s i x d o n o r a t o m s ; so for a n y one a t o m , i t s f u l l effect is p a r t i a l l y r e d u c e d b y t h e i n t e r a c t i o n w i t h these o t h e r l i g a n d s .

T h e f u l l c h a r g e of t h e

p r o t o n is u s u a l l y e m b e d d e d i n t h e e l e c t r o n i c c h a r g e c l o u d of a single d o n o r a t o m . B a c k b o n d i n g w i l l r e d u c e t h e effect of s u c h i o n i c c h a r g e o n the m e t a l e v e n f u r t h e r a n d m u s t be i m p o r t a n t i n d i f f e r e n t i a t i n g t h e effect of t r a n s i t i o n a n d n o n - t r a n s i t i o n e l e m e n t c o o r d i n a t i o n centers. A c r i t i c a l p r o b l e m r e l a t e d t o t h i s is p r e d i c t i n g t h e r e l a t i v e c h a r g e d i s p l a c e m e n t s w h i c h a r i s e o n c o o r d i n a t i o n t o a series of m e t a l i o n s o r of one m e t a l i o n i n a series of different c o o r d i n a t i o n e n v i r o n m e n t s . Nyholm

(26)

w h i c h i s a c o r o l l a r y of

I n t h i s l a t t e r case t h e r e is a g u i d e g i v e n b y P a u l i n g ' s p r i n c i p l e of e l e c t r o n e u t r a l i t y .

In Mechanisms of Inorganic Reactions; Kleinberg, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.

7.

JONES

Kinetic

Patterns

155

N y h o l m states t h a t t h e d i s p l a c e m e n t of charge t o w a r d t h e c e n t r a l i o n decreases a s t h e n u m b e r of l i g a n d s increases.

T h u s , the ligands should e x h i b i t chemical be

h a v i o r s i m i l a r t o t h a t of t h e free l i g a n d , as t h e c o o r d i n a t i o n n u m b e r of t h e c e n t r a l i o n increases.

Since most metal ions tend to a t t a i n a constant coordination n u m b e r

b y i n t e r a c t i o n w i t h t h e s o l v e n t w h e n r e q u i r e d , t h i s g u i d e m u s t be u s e d w i t h c i r c u m s p e c t i o n w h e n a p o l a r s o l v e n t is u s e d .

R e p l a c i n g the other ligands b y more

p o l a r i z a b l e ones a l s o decreases t h e p o l a r i z a t i o n of a g i v e n l i g a n d .

A related problem


w h i c h i s m o r e d i f f i c u l t t o f o r m u l a t e i n specific t e r m s i s t h e r e l a t i v e e l e c t r o n e g a t i v i t y of v a r i o u s l i g a n d s .

P a r t of t h i s p r o b l e m is t o r e l a t e t h e e l e c t r o n e g a t i v i t y of t h e

ligand t o the orbitals from w h i c h the shared electrons w i l l come.

A n o t h e r part is

t o assign some q u a n t i t a t i v e measure t o the v a r i o u s electronegativities w h i c h a l i g a n d m a y e x h i b i t (15,

41). T h e ligand react on rate can v a r y

K i n e t i c - T h e r m o d y n a m i c Correlations.

:

g r e a t l y as t h e m e t a l i o n is c h a n g e d , e s p e c i a l l y for r e d o x r e a c t i o n s o r t h o s e w i t h v e r y specific s t e r e o c h e m i c a l d e m a n d s .

I n cases w h e r e o n l y t h e a c i d i t y of t h e m e t a l i o n

is i m p o r t a n t one m i g h t e x p e c t m o r e r e g u l a r i t y .

F o r very regular systems a " B r o n -

s t e d C a t a l y s i s L a w " for m e t a l i o n s s h o u l d be v a l i d . hi

=

Thus,

GMKM

7

w h e r e ku is the r a t e c o n s t a n t for m e t a l i o n c a t a l y s i s b y t h e i o n M, KM is t h e s t a b i l i t y c o n s t a n t of t h e c o m p l e x i n v o l v e d , a n d GM a n d y a r e c o n s t a n t s c h a r a c t e r i s t i c of t h e r e a c t i o n , s o l v e n t , a n d t e m p e r a t u r e .

T h e r e a r e r e l a t i v e l y few r e a c t i o n s for

w h i c h b o t h r a t e a n d e q u i l i b r i u m c o n s t a n t s are a v a i l a b l e for t h e s i m p l e r e a s o n t h a t s t a b i l i t y constants are difficult to measure accurately w i t h a system undergoing r e a c t i o n r a p i d l y e n o u g h t o a l l o w g o o d r a t e d a t a t o be o b t a i n e d .

I n s o m e cases

w h e r e d a t a a r e a v a i l a b l e s u c h a r e l a t i o n s h i p does n o t h o l d , e.g. t h e m e t a l - i o n c a t a l y z e d h y d r o l y s i s of A T P a t p H 9 (36).

I n c l o s e l y r e l a t e d r e a c t i o n s , s u c h as

t h e same r e a c t i o n a t p H 5, t h e q u a l i t a t i v e d a t a a v a i l a b l e i n d i c a t e t h a t i t m i g h t hold.

I n t h e r a n g e of p H 7-8, m a n y of the m e t a l i o n s m u s t be p r e s e n t a s p a r t i a l l y

o r c o m p l e t e l y h y d r o l y z e d species if free, so t h e c a t a l y t i c a l l y effective species are p r o b a b l y n e v e r t h e s i m p l e m e t a l ions, w h e r e t r a n s i t i o n m e t a l i o n s a r e i n v o l v e d . A n o t h e r r e a c t i o n w h e r e a r e g u l a r t r e n d w o u l d be e x p e c t e d is t h e m e t a l - i o n c a t a l y z e d h y d r o l y s i s of a n a m i n o a c i d ester.

F o r t h i s r e a c t i o n the effectiveness of t h e

first

r o w t r a n s i t i o n m e t a l i o n s f o l l o w s the o r d e r of t h e s t a b i l i t y c o n s t a n t s as p r e d i c t e d b y the I r v i n g - W i l l i a m s order

(16).

Generalized Rate E q u a t i o n s .

T h i s last r e a c t i o n is t y p i c a l of a large class of

reactions w h i c h m a y proceed simultaneously b y several parallel paths.

T h e rate

c a n be expressed as t h e s u m of s e v e r a l c o n t r i b u t i o n s f r o m t h e u n c a t a l y z e d a n d catalyzed paths.

T h e o v e r a l l r a t e f o r t h e h y d r o l y s i s of a n a m i n o a c i d e s t e r i n t h e

presence of a m e t a l s a l t m a y be e x p r e s s e d as Ν

Rate «

Ν

Σ HML ][OH-] i=1 t

+ £ [H+][L] + *OH[OH-][L] + H

Σ h=1

H e r e b o t h a c i d a n d base c a t a l y z e d r e a c t i o n s m u s t be c o n s i d e r e d . exhibit a complicated temperature dependence.

h[ML ][U 0] h

2

Such reactions

E a c h of t h e i n d i v i d u a l r a t e c o n

s t a n t s w i l l be g o v e r n e d b y a d i f f e r e n t A r r h e n i u s e x p r e s s i o n , a n d t h e e q u i l i b r i u m c o n s t a n t s g o v e r n i n g t h e c o n c e n t r a t i o n s of the v a r i o u s MLi dependent.

are also temperature

T h i s c a n be s t a t e d e x p l i c i t l y b y u s i n g s t a b i l i t y c o n s t a n t , β.


156

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

=

o r [ML ]

[ML ]/[M][LY T

%

=

fii[M][L}\

so one m a y w r i t e Ν

Ν

Σ

Rate =

kMmimOH-]

+

fe[H+][L]

+Σ

+ *OH[OH-][L]

W

T h e t e m p e r a t u r e d e p e n d e n c e o f β{ a l l o w s i t t o b e expressed a s


T h e f r a c t i o n o f t h e m e t a l t i e d u p as M L

otm -

i=Ar ^ Σ

M

; here /3 -

m

and a

m

M

exp(—AG °/RT). t

c a n be expressed a s

1, M A f L ] «

0

T

is related t o the t o t a l concentration of

M

m e t a l , CM, b y t h e e x p r e s s i o n [ML ] = a CM, M

O

m

A M * -

i'=0

C

M

N

^

Σ

AfL]*-"

i=

1

t'=0

T h i s c a n be c o m b i n e d w i t h t h e r a t e e x p r e s s i o n a n d t h e t e m p e r a t u r e d e p e n d e n c e o f t h e β / s t o g i v e (where t h e a t t a c k o f w a t e r o n t h e c o m p l e x e s i s i g n o r e d , i.e. kh = 0 for a l l h), i=N Rate

g

-

-àGplRT

e

φ -

ά

Ε

Λ *

τ

[ 0 Η - Κ

Μ

î=j

+

*H[H+][L] +

^ H[OH"][L] 0

t =o

T h i s w i l l g i v e t h e r a t e o f a base c a t a l y z e d r e a c t i o n i n v o l v i n g s i m p l e

complexes.

O b v i o u s l y i f h y d r o x y c o m p l e x e s a r e i n v o l v e d , t h e y m u s t be used i n the r a t e e x p r e s s i o n i n a d d i t i o n t o , o r i n place of, t h e s i m p l e c o m p l e x e s .

If the a t t a c k of the c o m

plexes b y w a t e r i s i m p o r t a n t , t h e t e r m s i n kh c a n b e i n c l u d e d t o o b t a i n a t e r m analogous i n form t o t h a t resulting from the a t t a c k of h y d r o x i d e i o n .

One result

of t h e t e m p e r a t u r e d e p e n d e n c e is t h a t gross r a t e s o f s u c h r e a c t i o n s m a y b e e x p e c t e d to exhibit temperature extrema.

Both

a n d &OH a l s o s h o w a n e x p o n e n t i a l t e m

perature dependence. Aromatic

Systems

Electrophilic Reactions.

T h e r e i s sufficient i n f o r m a t i o n a v a i l a b l e o n e l e c t r o -

p h i l i c s u b s t i t u t i o n o f a r o m a t i c l i g a n d s t o e x a m i n e t h e u t i l i t y o f t h e general e q u a t i o n i n t h e l i m i t o f one c o m p l e x .

T h e r a t e of t h e d i a z o c o u p l i n g r e a c t i o n o f 8 - h y d r o x y -

q u i n o l i n e - 5 - s u l f o n i c a c i d a n d i t s z i n c c h e l a t e h a s been d e t e r m i n e d i n a n a c e t a t e buffer s y s t e m a t p H 5 (22). fanilic acid.

T h e d i a z o n i u m salt used was t h a t derived from s u l -

T h i s a t t a c k s the 7-position of the quinoline ring.

I n a system con

t a i n i n g t h i s l i g a n d a n d z i n c i o n , t h e r e are f o u r possible s u b s t r a t e s for t h i s r e a c t i o n : t h e p h e n o l a t e i o n , t h e free p h e n o l , t h e 1:1 z i n c c o m p l e x a n d t h e 2:1 c o m p l e x . T h e p h e n o l does n o t u n d e r g o t h e d i a z o c o u p l i n g r e a c t i o n i n s i m p l e s y s t e m s since this is a reaction of the phenolate i o n exclusively under such conditions.

If the

rate of formation of the diazo-coupled product is determined i n a system where v a r y i n g a m o u n t s of z i n c are a d d e d , one obtains v e r y interesting results.

If only

t h e i n i t i a l rates a r e c o n s i d e r e d , t h e p s e u d o first-order r a t e c o n s t a n t s o b t a i n e d f o r t h e coupling reaction are as follows: Zn+ /Hgand 2

k X 10 /min. x

3

0.00 39.3

1:2

1:1

5:1

10:1

32.9

25.6

12.9

10.3

50:1 7.89


100:1 7.87

7.

JONES

Kinetic

Patterns

157

T h e i n i t i a l decrease i n t h e p s e u d o

first-order

r a t e c o n s t a n t as t h e m o l e r a t i o ( z i n c /

l i g a n d ) increases i n d i c a t e s t h a t t h e c o m p l e x e s f o r m e d a r e less r e a d i l y a t t a c k e d t h a n the phenolate o r i g i n a l l y present i n solution.

T h e final l e v e l l i n g off of t h e r a t e a t

h i g h r a t i o s s h o w s t h a t these s o l u t i o n s u l t i m a t e l y c o n t a i n a l l of t h e l i g a n d i n t h e s a m e f o r m a n d t h a t t h e c o m p l e x is a l s o s u b j e c t t o t h e d i a z o c o u p l i n g r e a c t i o n . If t h e s t a b i l i t y c o n s t a n t s of a l l t h e r e l e v a n t c o m p l e x e s were k n o w n w i t h s o m e degree of p r e c i s i o n , i t w o u l d be possible t o d e t e r m i n e t h e r a t e c o n s t a n t f o r e a c h


complex.

separate

In this particular system the aromatic ligand must compete w i t h the

a c e t a t e of the buffer t o f o r m c o m p l e x e s .

T h e r e f o r e , the c o n s t a n t s d e t e r m i n e d for

use here m u s t be for m i x e d c o m p l e x e s .

F r o m the reported e q u i l i b r i u m constants

for t h i s s y s t e m i n t h e absence of a c e t a t e i t c a n be e s t i m a t e d t h a t t h e r a t i o of t h e c o n c e n t r a t i o n of Z n ( O R ) t o t h a t of Z n ( O R ) is a b o u t 1 ( T .

T h e r a t e i n t e r m s of t h e

4

2

p r i n c i p a l r e a c t i n g species i s : Rate = [RN +] { ^ [ R C T ] + £ [ROZn] + 2

2 2

M2n(0R)j]}

B e c a u s e of the s m a l l c o n c e n t r a t i o n of t h e 2:1 c o m p l e x t h e l a s t t e r m c a n be i g n o r e d . F r o m t h e e x t r e m e r a t e v a l u e s i n t h e a b s e n c e of z i n c a n d w i t h a n excess of z i n c , & i 2

a n d & 2 a r e d e t e r m i n e d as 2.4 X 1 0 m i n . 4

2

- 1

a n d 1.57 m i n . " " r e s p e c t i v e l y . 1

These

v a l u e s c a n be c o m b i n e d w i t h t h e t r e n d i n t h e r a t e c o n s t a n t s t o g i v e t h e s t a b i l i t y c o n s t a n t of the r e a c t i v e c o m p l e x , p r e s u m a b l y Z n ( O R ) ( O A c ) , a s 3 X 1 0 . 7

F o r the

s i m p l e z i n c c o m p l e x i n w a t e r t h e l i t e r a t u r e v a l u e s of t h e s t a b i l i t y c o n s t a n t for t h e 1:1 c o m p l e x v a r y f r o m 2.5 X

1 0 t o 6.3 8

X

10 .

T h e d i a z o c o u p l i n g r e a c t i o n of

8

t h e c o m p l e x i n d i c a t e s t h e s m a l l e r effect of c o o r d i n a t i o n vis a vis p r o t o n a t i o n since t h i s r e a c t i o n is v e r y s e n s i t i v e t o s u c h effects a n d does n o t p r o c e e d w i t h p h e n o l s . U n f o r t u n a t e l y t h e c h o i c e of c a t i o n s for s u c h a r e a c t i o n i s r e s t r i c t e d since t h e c a t i o n should not interfere w i t h the a n a l y t i c a l methods used to o b t a i n the k i n e t i c d a t a ; nor should i t introduce a d d i t i o n a l reactions such as occur w i t h t r a n s i t i o n m e t a l c a t i o n s w h i c h c a n c a t a l y z e t h e d e c o m p o s i t i o n of t h e d i a z o n i u m s a l t v i a a r e d o x process. A n o t h e r e l e c t r o p h i l i c s u b s t i t u t i o n r e a c t i o n w h i c h has been e x a m i n e d f o r b o t h a free l i g a n d a n d i t s m e t a l c o m p l e x e s is m e r c u r a t i o n . aromatic compounds

T h e r a t e of m e r c u r a t i o n of

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

type: Rate =

& [Aromatic][Hg(OAc) ] 2

2

T h e s a m e k i n d of a r a t e l a w has been f o u n d for t h e m e r c u r a t i o n of c o p p e r o x i n a t e in glacial acetic acid.

H e r e t h e r a t e of f o r m a t i o n of t h e m e r c u r a t e d c o m p l e x i s

given b y the expression : Rate = & -2[Cu(8 2

HQ) ][Hg(OAc) ] 2

2

w h e r e C u ( 8 — H Q ) is c o p p e r o x i n a t e a n d t h e f a c t o r t w o a r i s e s f r o m t h e f a c t t h a t 2

e a c h m o l e c u l e of t h e c o p p e r c o m p l e x c o n t a i n s t w o r e a c t i v e a r o m a t i c s y s t e m s .

In

t h e e x a m p l e s t u d i e d i t w a s f o u n d t h a t t h e c o p p e r c o m p l e x undergoes m e r c u r a t i o n m u c h m o r e r a p i d l y t h a n t h e free p h e n o l i n t h e s a m e e n v i r o n m e n t (5).

The activa

t i o n e n e r g y for t h e m e r c u r a t i o n of t h e c o m p l e x w a s f o u n d t o be 19 k c a l . a n d t h e f r e q u e n c y f a c t o r h a d t h e u n u s u a l l y h i g h v a l u e of 9.6 X 10 . 12

T h i s c a n be c o m p a r e d

w i t h t h e f r e q u e n c y f a c t o r r e p o r t e d b y D . H . B u s c h a n d h i s c o - w o r k e r s for the


158


a l k y l a t i o n of t h i o l c o m p l e x e s of n i c k e l ( I I ) (3). factor i n the range 5 Χ

10 to 2 Χ 6

These workers found a frequency

10 as expected for a r e a c t i o n o c c u r r i n g a t 8

l o c a l i z e d p o r t i o n s of a large m o l e c u l e .

In the mercuration reaction the a t t a c k i n g

reagent m a y u n d e r g o s o m e v a r i a t i o n i n s t r u c t u r e w i t h t e m p e r a t u r e . I n t h e e l e c t r o p h i l i c s u b s t i t u t i o n r e a c t i o n s of c o o r d i n a t e d a r o m a t i c l i g a n d s f o r w h i c h rate d a t a are presently available, there is every i n d i c a t i o n t h a t the m e c h a n i s m of t h e r e a c t i o n is u n c h a n g e d i n i t s essentials.

F o l l o w i n g t h e l e a d of t h e p h y s i c a l


o r g a n i c c h e m i s t s t h e course of t h e r e a c t i o n of t h e c o m p l e x a n d t h e d i a z o n i u m i o n c a n be d e p i c t e d a s : R N + + H — A r — M -> R N — A r — M 2

R—N —A—M 2

step 1

2

+ Β -> R N — A r — M + H B +

step 2

2

T h e r e i s e v e r y r e a s o n t o believe t h a t t h e first s t e p is r a t e d e t e r m i n i n g i n t h e c o u p l i n g r e a c t i o n of b o t h t h e free a n d c o o r d i n a t e d species. i n b o t h cases.

T h e o v e r a l l r e a c t i o n i s t h u s S#2

T h e d i a z o c o u p l i n g s t u d i e s s h o w h o w different t h e r e s u l t s of c o o r

d i n a t i o n t o a p r o t o n a n d t o a m e t a l i o n m a y be. W i t h o u t s u p p o r t i n g r a t e d a t a , i t i s n o t possible t o e x c l u d e a m e c h a n i s m c h a n g e of a g i v e n r e a c t i o n w h e n a t t a c k i n g a g e n t s a r e c h a n g e d .

T h u s , the b e n z o y l a t i o n of

c o p p e r o x i n a t e c a n be e a s i l y effected w h e n a l u m i n u m c h l o r i d e is u s e d as a c a t a l y s t . T h e a c e t y l a t i o n of t h i s same c o m p l e x has n o t y e t been s u c c e s s f u l l y c a r r i e d o u t e v e n u n d e r f o r c i n g c o n d i t i o n s (5).

I t seems p o s s i b l e t h a t c o o r d i n a t i o n c a n s e r i o u s l y

d i s r u p t t h e n o r m a l p a t h of a r e a c t i o n w h e n t h a t p a t h i n v o l v e s w c o m p l e x f o r m a t i o n . General Order of R a t e C o n s t a n t s .

T h e r a t e c o n s t a n t s of e l e c t r o p h i l i c r e a c

t i o n s of a r o m a t i c l i g a n d s a n d t h e i r m e t a l c o m p l e x e s f a l l i n t h e o r d e r kh > ^ML > km,.

T h e difference b e t w e e n these r a t e c o n s t a n t s b e c o m e s g r e a t e r as t h e a c t i v i t y

of t h e a t t a c k i n g r e a g e n t decreases.

W h e n L is a p h e n o l a t e , HL

w h e n L i s a n a m i n e , HL is t h e c o r r e s p o n d i n g a m m o n i u m d e r i v a t i v e .

is t h e p h e n o l ; T h e possible

s y n t h e t i c a p p l i c a t i o n s of t h i s sequence c a n be a p p r e c i a t e d f r o m t h e f a c t t h a t 8h y d r o x y q u i n o l i n e i s u s u a l l y s u l f o n a t e d w i t h 15 t o 3 0 % o l e u m , w h i l e i t s c o p p e r ( I I ) c o m p l e x c a n be r e a d i l y s u l f o n a t e d i n 7 0 % s u l f u r i c a c i d (5). S i n c e e l e c t r o p h i l i c s u b s t i t u t i o n s of ML

are s l o w e r t h a n t h o s e of L t h e r e v e r s e

s h o u l d be t h e case for n u c l e o p h i l i c r e a c t i o n s .

A t present there is no evidence c o n

cerning homogeneous systems w i t h metal ions on this point, although the hetero geneous s y s t e m s c o n s i s t i n g of p y r i d i n e o r i t s d e r i v a t i v e s a n d a c o p p e r s a l t i n d i c a t e t h a t t h i s m a y be t r u e .

A n u n u s u a l k i n d of c o o r d i n a t e b o n d is f o u n d i n i V - o x i d e s .

T h e b e h a v i o r of p y r i d i n e - i V - o x i d e a n d i t s d e r i v a t i v e s has been s t u d i e d i n d e t a i l a n d p r o v i d e s i n f o r m a t i o n c o n c e r n i n g t h e r e a c t i v i t y of s y s t e m s c o n t a i n i n g t h i s k i n d of bond.

T h e p y r i d i n e r i n g of p y r i d i n e - i V - o x i d e s h o w s a l l t h e signs one e x p e c t s f r o m

t h e f o r m a t i o n of a n e l e c t r o n w i t h d r a w i n g c o o r d i n a t e b o n d .

Thus, electrophilic

s u b s t i t u t i o n i s r e n d e r e d m o r e d i f f i c u l t a n d t h e r e l a t i v e r e a c t i v i t y of t h e r i n g p o s i t i o n s m a y be a l t e r e d . facilitated.

Correspondingly, nucleophilic substitutions are generally

T h e o x y g e n a t o m b e h a v e s as a m o r e e l e c t r o n e g a t i v e species t h a n t h e

metals w h i c h form complexes w i t h pyridine.

T h u s , t h e o r i e n t a t i o n of n i t r a t i o n i n


7.

JONES

Kinetic

Patterns

159

p y r i d i n e - i V - o x i d e is t o t h e 4 - p o s i t i o n w h i l e i n p y r i d i n e i t s e l f a n d t h e r u t h e n i u m c o m p l e x , RUPV3CI3, t h e o r i e n t a t i o n is t o t h e 3 - p o s i t i o n (34).

T h e electron w i t h

d r a w i n g p o w e r of t h e o x y g e n is a l s o g r e a t e r t h a n t h a t of t y p i c a l L e w i s a c i d s s u c h as a l u m i n u m c h l o r i d e since p y r i d i n e c o m p l e x e s of these species e x h i b i t the same o r i e n t a t i o n for h a l o g e n a t i o n as t h e free l i g a n d (28). Some Related Examples.

A c l o s e l y r e l a t e d p r o b l e m is t h e r a t e b e h a v i o r of

a r o m a t i c donors i n F r i e d e l - C r a f t s acylation a n d analogous reactions.


d i n a t i o n p l a y s a d u a l role.

H e r e coor

T h e i n i t i a l L e w i s a c i d w h i c h is a d d e d is t a k e n u p b y

t h e best d o n o r species, f r e q u e n t l y t h e s u b s t r a t e -

O n c e t h i s r e a c t i o n is a t e q u i

l i b r i u m , a d d i t i o n a l a m o u n t s of L e w i s a c i d c a n r e a c t w i t h t h e o t h e r species p r e s e n t t o generate t h e effective e l e c t r o p h i l e . d e l i n e a t e d b y O l i v i e r i n 1914.

T h e k i n e t i c b e h a v i o r of s u c h s y s t e m s w a s first

H e studied the reactions:

B r C H S 0 C l + A1C1 6

4

2

BrC H S0 Cl-AlCl 6

4

2

B r C H S 0 C l · A1CI

3

= BrC H S0 CeH -A1C1

3

8

3

+ C H 6

6

6

6

4

4

2

2

6

+ HC!

T h e r a t e of the s e c o n d r e a c t i o n is q u i t e l o w as l o n g as there is e n o u g h free s u l f o n y l c h l o r i d e t o r e a c t w i t h a d d i t i o n a l a m o u n t s of a l u m i n u m c h l o r i d e .

O n c e t h i s is n o

l o n g e r t r u e , f u r t h e r a d d i t i o n s of c a t a l y s t e n o r m o u s l y increase t h e r e a c t i o n r a t e (27).

T h e r e c e n t l y d i s c o v e r e d s w a m p i n g c a t a l y s t effect i n t h e h a l o g e n a t i o n

a r o m a t i c d o n o r species (35) p r o b a b l y e x h i b i t s a n a l o g o u s k i n e t i c b e h a v i o r .

of

The

b a s i c r a t e e x p r e s s i o n f o u n d b y O l i v i e r c o n s i s t e d of o n l y one t e r m f o r r e a c t i o n w h e n a r e l a t i v e l y s m a l l a m o u n t of a l u m i n u m c h l o r i d e w a s p r e s e n t : dx/dt where K

x

JfefZ-AlCla] -

^[AlClsXZ]

is t h e d i s s o c i a t i o n c o n s t a n t of t h e a l u m i n u m c h l o r i d e c o m p l e x a n d t h e

sulfonyl chloride, Z.

W h e n e n o u g h a l u m i n u m c h l o r i d e w a s p r e s e n t i n the r e a c t i o n

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

T h i s r a t e increase i s

g r e a t e r t h a n t h a t p r o j e c t e d b y t h e a b o v e r a t e e x p r e s s i o n for v a r i a t i o n s i n [AICI3], t h a t O l i v i e r felt t h a t u n d e r t h e c o n d i t i o n s u s e d b y h i m , o n l y t h e c o m p l e x e n t e r e d i n t o the r e a c t i o n .

ΖΆΙΟ3

F u r t h e r i n t e r a c t i o n of benzene a n d a l u m i n u m c h l o r i d e

is possible. T h e r e a c t i o n of b e n z o y l c h l o r i d e a n d benzene i s c a t a l y z e d b y a l u m i n u m c h l o r i d e a n d a l s o proceeds v i a a c o m p l e x , C e H C O C l - A I C I 3 (37). 6

p l e x i t s e l f w i t h benzene is v e r y s l o w .

T h e r e a c t i o n of t h e c o m

W h e n a d d i t i o n a l a l u m i n u m chloride is a d d e d

t o t h e c o m p l e x a n d benzene m i x t u r e , t h e r a t e is i n c r e a s e d e n o r m o u s l y .

A c t i o n of

t h e a d d i t i o n a l a l u m i n u m c h l o r i d e m a y f o l l o w a t least t w o s e p a r a t e r o u t e s .

The

first i n v o l v e s a n a c t i v a t i o n of one o r t h e o t h e r benzene r i n g b y f o r m i n g a π c o m p l e x . T h e s e c o n d i n v o l v e s f o r m a t i o n of a m u c h m o r e r e a c t i v e c o m p l e x i n w h i c h t w o a l u m i n u m c h l o r i d e m o l e c u l e s are c o o r d i n a t e d t o t h e b e n z o y l c h l o r i d e , one t h r o u g h t h e c h l o r i n e a n d the o t h e r t h r o u g h t h e o x y g e n .

T h i s would then facilitate the forma

t i o n of t h e r e a c t i v e C6H5CO " i o n o r i t s e q u i v a l e n t .

However, the literature con

t a i n s n o e v i d e n c e for a c o m p l e x , C e H e C O C l ' 2AICI3.

T h e reaction most probably

4

proceeds v i a a n a c t i v a t i o n w h i c h does n o t i n v o l v e t h e f o r m a t i o n of a s e c o n d σ t y p e coordinate bond. W h e n t h e s u b s t r a t e f o r m s a c o m p l e x w i t h t h e a t t a c k i n g species ( b u t i n a w a y n o t c o n d u c i v e t o f u r t h e r r e a c t i o n ) t h e n e t r e s u l t m a y be m e r e l y t o increase t h e a m o u n t



160

of a t t a c k i n g species r e q u i r e d f o r a g i v e n degree of r e a c t i o n .

T h i s s i t u a t i o n is

a p p a r e n t l y f o u n d i n s o m e m e t a l a t i o n r e a c t i o n s i n v o l v i n g a l k y l l i t h i u m (32). k i n e t i c i n v e s t i g a t i o n of t h e r e a c t i o n of e t h y l l i t h i u m a n d a n i s o l e s h o w e d i t t o s e c o n d o r d e r i n e t h y l l i t h i u m a n d a p p a r e n t l y first o r d e r i n a n i s o l e (12).

A be

T h i s is


consistent w i t h the reaction scheme:

C HÔ—Ô: 6

LiCtHi +

C H —O—CH3 -

-> L i C H

+

2

6

5

6

LiC H 2

6

-+

C H — Ô : -> L i C H 6

—Q:

CH3

-> L i C H 2

5

6

+

C H 2

E

CH3

T h e r a t e of e t h a n e f o r m a t i o n w a s u s e d t o o b t a i n k i n e t i c d a t a o n t h i s r e a c t i o n .

The

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

the

ortho position. Electrostatic Models. static model.

F o r p o l a r reactions i t is possible t o develop a n electro

T h i s i s n e c e s s a r i l y a n o v e r s i m p l i f i c a t i o n , t h o u g h i t does a l l o w t h e

gross features of s u c h s y s t e m s t o be d e l i n e a t e d .

T h u s , w h e n a c o m p l e x a n d t h e free

l i g a n d b o t h r e a c t w i t h t h e s a m e gross m e c h a n i s m , i t i s p o s s i b l e t o e s t i m a t e t h e e l e c t r o s t a t i c c o n t r i b u t i o n t o t h e c h a n g e i n e n t r o p y of a c t i v a t i o n w h e n t h e r e a c t i o n i s p o l a r or ionic.

F o l l o w i n g L a i d l e r (21) w e c a n e s t i m a t e t h e e l e c t r o s t a t i c c o n t r i b u t i o n t o

t h e e n t r o p y of a c t i v a t i o n a s : ASte.8.

=

(for

-10Z ZB A

water)

If Ζ A i s t h e c h a r g e o n t h e a t t a c k i n g r e a g e n t , ZBX t h a t o n t h e l i g a n d a n d Z #

2

is the

charge o n the m e t a l i o n p r i o r t o coordination, t h e n ASte.a.,

ligand »

A ^ î e . s . , complex ~

—

IQZAZBI

— IOZA'ZBI

+

ZBI)

a n d the change i n going to the c o m p l e x is t h e n AASje.*. ~

-10Z Z 2 A

B

A n a l t e r n a t i v e e x p r e s s i o n b a s e d o n t h e s i n g l e s p h e r e a c t i v a t e d c o m p l e x c a n a l s o be d e v e l o p e d , b u t i t s use r e q u i r e s a k n o w l e d g e of t h e r a d i i of t h e a c t i v a t e d c o m p l e x as w e l l as t h e r e a c t a n t s .

T h e c h a n g e i n t h e e n t r o p y of a c t i v a t i o n p r e d i c t e d d e p e n d s

o n t h e s o l v e n t a n d t h e c h a r g e o n t h e species a t t a c k i n g t h e s u b s t r a t e .

If the solvent

is w a t e r a n d t h e a t t a c k i n g species i s p o s i t i v e l y c h a r g e d , c o o r d i n a t i o n w i l l m a k e t h e e n t r o p y v a l u e s m o r e n e g a t i v e a n d m a k e t h e a t t a i n m e n t of t h e t r a n s i t i o n s t a t e m o r e difficult.

I f t h e a t t a c k i n g species i s n e g a t i v e l y c h a r g e d , t h e o p p o s i t e is t r u e .

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


7.

JONES

Kinetic

l i g a n d a n d the c o m p l e x .

161

Patterns

F o r t h e s i m p l e t w o - s p h e r e m o d e l the r e s u l t m a y be g i v e n

as: ZMZB^

tdABkT w h e r e kc is t h e r a t e of t h e r e a c t i o n for the c o m p l e x , k? t h a t of t h e free l i g a n d , ZM is t h e c h a r g e o n t h e m e t a l , Ζ Β is t h e charge o n t h e a t t a c k i n g species, € is t h e d i e l e c


t r i c c o n s t a n t of t h e r e a c t i o n m e d i u m , CIAB i s t h e s e p a r a t i o n d i s t a n c e of t h e s u b s t r a t e a n d a t t a c k i n g species i n t h e a c t i v a t e d c o m p l e x ( a s s u m e d t o be t h e same i n t h e r e a c t i o n of b o t h t h e c o m p l e x a n d free l i g a n d ) , k is B o l t z m a n n ' s c o n s t a n t , a n d Τ is t h e absolute temperature.

A n a n a l o g o u s e q u a t i o n c a n be d e r i v e d for a n i o n d i p o l e

reaction. T h e s e e q u a t i o n s s h o w the g e n e r a l t h e o r e t i c a l basis for t h e e m p i r i c a l o r d e r of r a t e c o n s t a n t s g i v e n e a r l i e r ; for e l e c t r o p h i l i c a t t a c k o n a n a r o m a t i c l i g a n d L, i t s m e t a l c o m p l e x ML, a n d i t s p r o t o n a t e d f o r m H L , one finds kh > fan > & H L -

Conflicting

reports i n the literature state t h a t coordination c a n b o t h accelerate electrophilic a r o m a t i c s u b s t i t u t i o n (30) a n d s l o w i t d o w n e n o r m o u s l y (2).

I n t h e first case t h e

r a t e s of n i t r a t i o n of t h e d i p r o t o n a t e d f o r m of o - p h e n a n t h r o l i n e a n d i t s C o ( I I I ) a n d F e ( I I I ) complexes were compared.

Here coordination prevents protonation i n the

m i x e d a c i d m e d i u m u s e d for n i t r a t i o n a n d kML >

I

n

t h e s e c o n d case t h e

p h e n o l a t e f o r m of 8 - h y d r o x y q u i n o l i n e - 5 - s u l f o n i c a c i d a n d i t s m e t a l c h e l a t e s were compared.

T h e complexes underwent i o d i n a t i o n m u c h more slowly, if a t a l l , a n d

kh > kjtfhAliphatic

Systems

T h e i n f o r m a t i o n available o n a l i p h a t i c systems is m u c h more diversified t h a n t h a t o n a r o m a t i c s y s t e m s t h o u g h t h e effects of l i g a n d p o l a r i z a t i o n a r e s i m i l a r .

In

g e n e r a l , m e t a l i o n s a l l o w us t o generate t h e l i g a n d p o l a r i z a t i o n w h i c h a s s i s t s n u c l e o philic attack.

F o r s u i t a b l e l i g a n d s t h i s c a n be effected i n a b a s i c e n v i r o n m e n t w h e r e

p r o t o n s c a n n o t f u r n i s h t h e s a m e g e n e r a l effect. A m i n o Acid Ester Hydrolyses.

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

a c i d esters w a s first d i s c o v e r e d b y K r o l l (20) a n d l a t e r s t u d i e d b y o t h e r i n v e s t i g a t o r s (39, 1, 6).

T h e m e c h a n i s m has been t h e s u b j e c t of s o m e d i s p u t e , b u t i t is n o w

o b v i o u s t h a t t h e r e a r e a t least t w o r e a c t i o n s i n s u c h s y s t e m s .

T h e first is f o u n d a t

p H 2 - 8 , h i g h m e t a l t o ester r a t i o s a n d c o r r e s p o n d s t o a s e c o n d o r d e r r e a c t i o n i n w h i c h t h e h y d r o x i d e i o n o r w a t e r a t t a c k s t h e c h e l a t e d 1:1 c o m p l e x . m u s t be e x a m i n e d u n d e r c o n d i t i o n s w h i c h d o n o t p r e c i p i t a t e t h e m e t a l .

T h i s reaction T h e second

r e a c t i o n i n these s y s t e m s is a s e c o n d o r d e r r e a c t i o n w h e r e t h e h y d r o x i d e i o n a t t a c k s a 2:1 o r h i g h e r c o m p l e x i n w h i c h t h e a m i n o a c i d e s t e r f u n c t i o n i s n o t n e c e s s a r i l y chelated.

T h i s r e a c t i o n is m u c h s l o w e r because t h e p o l a r i z i n g p o w e r of t h e m e t a l

c a t i o n is s p r e a d o v e r t w o o r m o r e l i g a n d s .

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

p H w h i c h have high ligand to metal ratios.

T h e r e l a t i v e c o n t r i b u t i o n s these t w o

r e a c t i o n s m a k e t o a g i v e n s y s t e m m a y be s e p a r a t e d since t h e i r r a t e s differ w i d e l y ; t h e c o n c e n t r a t i o n s of t h e c o m p l e x e s

m a y be a d j u s t e d i n t h e r e a c t i o n m i x t u r e s .

H o w e v e r , t h i s h a s n o t been d o n e c o m p l e t e l y f o r a n y s u c h s y s t e m .

Bender and

T u r n q u e s t (1) c a r r i e d o u t a s t u d y i n a buffer s y s t e m c o m p o s e d of g l y c i n e a n d i t s hydrochloride.

T h e y postulated t h a t the reaction proceeds t h r o u g h a m i x e d c o m -


162

MECHANISMS

plex c o n t a i n i n g b o t h ester a n d g l y c i n e .

O F I N O R G A N I C REACTIONS

U n d e r comparable

circumstances

the

m i x e d c o m p l e x i n v o l v i n g g l y c i n e w a s a m u c h m o r e effective c a t a l y s t t h a n t h a t w i t h t h e buffer used b y K r o l l ( t r i s ( h y d r o x y m e t h y l ) a m i n o m e t h a n e ) .

Bender and T u r n -

q u e s t present e v i d e n c e s u p p o r t i n g a m e c h a n i s m i n w h i c h t h e ester first c o o r d i n a t e s t o the c o p p e r t h r o u g h i t s a m i n o g r o u p .

I t t h e n undergoes a t r a n s i e n t i n t e r a c t i o n

w i t h c o p p e r v i a i t s c a r b o n y l g r o u p w h i c h t h e n a d d s o n w a t e r a n d undergoes a p r o t o n shift to give: g l y c i n e : C u : N H — C H — C ( O H ) ( O C H ) . 2

2

2

I t t h e n s p l i t s off w a t e r t o

3


g i v e a n exchange r e a c t i o n , o r m e t h a n o l t o g i v e g l y c i n e i n a h y d r o l y t i c r e a c t i o n . T h e p a r t i c i p a t i o n of t h e buffer s y s t e m i n t h i s r e a c t i o n m a k e s t h e m e c h a n i s m s of s u c h r e a c t i o n s s o m e w h a t different f r o m t h e same s y s t e m w h e n n o buffer i s p r e s e n t . B r o m i n a t i o n of Beta K e t o Esters.

The

w o r k of

K. A.

Pedersen

on

the

b r o m i n a t i o n of b e t a k e t o esters i n t h e presence of c o p p e r ( I I ) a n d o t h e r d i v a l e n t m e t a l i o n s p r o v i d e s s e v e r a l e x a m p l e s of r e a c t i o n s p r o c e e d i n g v i a c o m p l e x e s

(29).

T h e c o m p l e x e s p r o v i d e a n a l t e r n a t i v e a n d m u c h m o r e r a p i d r o u t e for t h e b r o m i n a tion reaction.

T h e s e r e a c t i o n s a r e a c c e l e r a t e d b y bases w h i c h t a k e u p a p r o t o n

f r o m t h e b e t a k e t o esters.

F o r s u c h s u b s t r a t e s , e.g. e t h y l a c e t o a c e t a t e , t h e g e n e r a l

e x p r e s s i o n for t h e p s e u d o first o r d e r r a t e c o n s t a n t i n t h e presence of c o p p e r has t h e form: k where k is the pseudo

k

0

+ kfCufll)] + kC B

first-order

+

B

/b'[Cu(II)]C

B

r a t e c o n s t a n t a n d b o t h t h e n o r m a l a n d t h e base

c a t a l y z e d r e a c t i o n s are s u b j e c t t o m e t a l i o n a c c e l e r a t i o n .

W h e n copper(II) is

t h e m e t a l i o n p r e s e n t , w i t h e t h y l a c e t o a c e t a t e p r e s e n t as s u b s t r a t e a n d a c e t a t e i o n as base, t h e r a t e c o n s t a n t has t h e e x p l i c i t f o r m : k* = 0.4343& = 0.01855 + 2.69[Cu(II)] + 8 . 2 l [ A c " ] +

1143[Cu(II)][Ac-].

T h e r a t e d e t e r m i n i n g s t e p i n t h i s r e a c t i o n is t h e t r a n s f e r of a p r o t o n f r o m t h e free o r c o o r d i n a t e d e t h y l a c e t o a c e t a t e t o a base w h i c h m a y be w a t e r (k a n d k ' Q

o r a n o t h e r base (ks a n d kn

terms).

0

terms)

C o o r d i n a t i o n m a k e s t h i s process easier t o

effect t h r o u g h t h e g e n e r a l a c i d s t r e n g t h e n i n g c h a r a c t e r of t h e process f o r l i g a n d s . T h e m e c h a n i s m of t h i s r e a c t i o n is a p p a r e n t l y t h e same for b o t h t h e c o m p l e x a n d t h e free l i g a n d .

P e d e r s e n a l s o c a r r i e d o u t s t u d i e s o n t h e b r o m i n a t i o n of 2 - c a r b -

ethoxycyclopentanone

i n t h e presence of s e v e r a l different m e t a l i o n s .

c o o r d i n a t i o n a l s o has a d i s t i n c t a c c e l e r a t i n g effect.

I n t h i s case

T h e rate constant at

18°C.

as before, c a n be expressed as k* -

0.0350 +

Σ&ι*€ι

w h e r e Cj is t h e c o n c e n t r a t i o n of t h e m e t a l i o n / , a n d ki* i s t h e r a t e c o n s t a n t for t h e reaction due to this added i o n . 3.17;

N i ( I I ) , 2.15; Z n ( I I ) , 0.55;

F o r t h e i o n s e x a m i n e d kr* v a l u e s w e r e : C u ( I I ) , P b ( I I ) , 0.28; a n d M n ( I I ) , 0.17.

Here the order

seems t o be t h e same as t h a t of t h e s t a b i l i t y c o n s t a n t s , so a B r ^ n s t e d t y p e r e l a t i o n may hold. Hydrolyses of Schiff Bases.

T h e b e h a v i o r of Schiff bases a n d t h e i r c o m p l e x e s

has been s u f f i c i e n t l y s t u d i e d so t h a t t h e s i m i l a r i t i e s of t h e k i n e t i c p a t h s of t h e i r h y d r o l y s e s c a n be c l e a r l y d e l i n e a t e d .

T h e r e a c t i o n is i n t e r p r e t e d i n t e r m s of a

change i n the r a t e d e t e r m i n i n g s t e p as the p H i s c h a n g e d .

I n neutral solutions the

r a t e d e t e r m i n i n g step i s c o n s i d e r e d (7) t h e h y d r a t i o n s t e p :


7.

JONES

Kinetic Patterns

163 OH

\

\ C=NR

+ H 0

f i

2

/

/ C

/

\

H + or O H -

N H R

U n d e r a c i d i c conditions the rate d e t e r m i n i n g step is the subsequent s p l i t t i n g :


OH \ /

C

/

2). T h e r e a c t i o n is t h o u g h t t o i n v o l v e e l e c t r o p h i l i c s u b s t i t u t i o n . fluence

T h i s substantial i n

of s u b s t i t u e n t s o n t h e r e a c t i o n r a t e is u n d o u b t e d l y a n e l e c t r o n i c r a t h e r t h a n

a s t e r i c effect. C

Γ

O \

"

\

o—c

C—O H—C

C

Cr /

Π

C—Χ

\

C—ο

I

/ •

/

Br—C

Cr

* /

C—o

\

c T h e effect of

o—c

\ / _ \

O

\

o—c

1

c—o

L e . t h e o v e r a l l c h a r g e of

/

o—c

1

c

C — X

O

\

x

L

e

.

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

b y bis(ethylenediamine)-acetylacetonatocobalt(III).

T h e acetylacetonate ring i n

t h i s c h a r g e d c o m p l e x fails t o r e a c t w i t h i V - b r o m o s u c c i n i m i d e — e v e n u n d e r f o r c i n g conditions.

B y c o n t r a s t , n e u t r a l chelates s u c h as t r i s ( a c e t y l a c e t o n a t e ) c o b a l t

act r a p i d l y w i t h N B S .

re

T h i s e n o r m o u s difference i n l i g a n d r e a c t i v i t y m u s t be a s

c r i b e d t o t h e charge o n t h e c o m p l e x . t e r a t i o n i n s t r o n g l y a c i d i f i e d D2O.

T h e c a t i o n i c c o m p l e x does u n d e r g o s l o w d e u T h i s r e a c t i o n c a n be f o l l o w e d b y n u c l e a r m a g

netic resonance.

\ C—Ο

/

\

\

/

H—C

Co(en)

2

C—Ο

/

.

c

F i n a l l y , I w o u l d l i k e t o c o m m e n t b r i e f l y o n t h e effect of t h e m e t a l o n l i g a n d reactivity.

I n a n u m b e r of i n s t a n c e s c h e l a t e r i n g s i n t h e t r i s a c e t y l a c e t o n a t e s of

r h o d i u m ( I I I ) , c o b a l t (111), a n d c h r o m i u m (111) u n d e r g o e l e c t r o p h i l i c s u b s t i t u t i o n s a t different r a t e s . c o b a l t ^> r h o d i u m .

T h e r e l a t i v e r e a c t i v i t y of these c o m p l e x e s

is c h r o m i u m

T h u s f a r o u r d a t a are o n l y q u a l i t a t i v e .


>

172

MECHANISMS O F INORGANIC

REACTIONS


10

PH

•

• Ag -poly A Ag -poly C

0

2

ο

Ag--poly I

•

Ag--poly u

4 6 8 MOLES OH"/ MOLE A g

10

+

Figure E. Titration curves for 1:1 mixtures of silver ion and polynucleotides R a y m o n d Dessy:

W e h a v e been l e d t o o b s e r v a t i o n s w h i c h I w o u l d l i k e t o

sketch briefly. T h e s e i n v o l v e , n o t t h e t r a n s i t i o n m e t a l s w h i c h h a v e b e e n stressed here b u t r a t h e r n o n t r a n s i t i o n m e t a l s , w h i c h c o u l d be d e s c r i b e d a s du, do, a n d no-d cases. T h e first o b s e r v a t i o n w a s t h a t s u b s t a n c e s c o n t a i n i n g m e r c u r y b o n d e d t o t h e unusual carbomethoxyl group, Ο H+

PhHg—C

\ OCH3

->

PhHg+

+

CO

+

CH OH 3


7.

JONES

Discussion

173

i n d i m e t h y l s u l f o x i d e s o l v e n t (a p o o r c o o r d i n a t o r for a n i o n s ) u n d e r g o c l e a v a g e b y acetic acid to liberate m e t h y l alcohol, carbon monoxide, and a phenylmercury anion compound.

I n t h e a b s e n c e of h a l i d e i o n , t h e r e a c t i o n goes a t a n u n o b s e r v a b l e r a t e .

A s a m a t t e r of fact we generate t h i s p a r t i c u l a r c o m p o u n d i n t h e presence of a c e t i c acid.

O n l y w h e n h a l i d e i o n is p r e s e n t does t h i s r e a c t i o n p r o c e e d .

c o n s t a n t s we h a v e o b s e r v e d , t h e r a t i o k*~/k° is g r e a t e r t h a n 10 .

W i t h the rate T h e sequence of

4

catalytic a c t i v i t y observed i s : iodide ion > bromide ion > chloride i o n .

Strangely


e n o u g h t h i s same s o r t of effect c a n be seen i n a do case, t h e d e c o m p o s i t i o n of t r i b u t y l t i n hydride, again i n dimethylsulfoxide solvent; acetic acid provides a proton source w h i c h w i l l d i s c h a r g e a g a i n s t t h e i n c i p i e n t h y d r i d e i o n t o g i v e h y d r o g e n gas. W hen halide ion is added the reaction is t r u l y c a t a l y t i c i n halide i o n . T

F r o m the

r a t e d a t a a n d k i n e t i c s i t is p o s s i b l e n o w t o o b t a i n a n e x a c t e x p r e s s i o n for t h e r a t i o , T h i s i s t h e r a t e a t w h i c h t h i s r e a c t i o n proceeds i n t h e presence of s t o i c h i o

k ~/k°. Ql

m e t r i c a m o u n t s of c h l o r i d e i o n t o t h e r a t e of t h e r e a c t i o n w h e n c h l o r i d e i o n is a b s e n t . A g a i n , i t i s a b o u t 10 . 4

The

h a l i d e sequence is c o m p l e t e l y r e v e r s e d : c h l o r i d e i o n >

iodide ion. for i t .

bromide ion

>

T h i s h a s c a u s e d us s o m e p r o b l e m , b u t as y e t w e h a v e n o e x p l a n a t i o n

I n b o t h cases t h e r e is e n o u g h e v i d e n c e f r o m o t h e r k i n e t i c i n v e s t i g a t i o n s a n d

d e u t e r i u m labelling to realize t h a t the m e t a l w i t h the a t t a c h e d organic group ι

»

1

1

*

R

1

POLYI ρ H 4.0

22Ô

Figure F.

24Ô

260 280 WAVELENGTH IN m μ

30Ô

320

Ultraviolet spectra of silver complexes of Poly I and Poly A compared with the spectra of the uncomplexed polynucleotides


174

MECHANISMS OF INORGANIC

REACTIONS

b e c o m e s c o o r d i n a t e d w i t h h a l i d e i o n , a n d t h a t t h i s c o o r d i n a t i o n s t e p affects t h e r e a c t i v i t y of R . T h i s w o u l d i n d i c a t e t h a t a t l e a s t i n these t w o cases, dio a n d do, t h e r e a c t i v i t y of a l i g a n d , a n a l k y l g r o u p , o r a h y d r o g e n a t t a c h e d t o a m e t a l c a n be affected b y p l a c i n g another ligand on the m e t a l . W e h a v e a l s o l o o k e d a t a no-d case w h e r e we h a v e c l e a v e d R3B c o m p o u n d s , triethylboron, w i t h carboxylic acids i n an ethereal environment.

Once again we


h a v e t h e p o s s i b i l i t y of c o o r d i n a t i n g a s u b s t a n c e l i k e a c a r b o x y l i c a c i d w i t h o u r b o r o n compound i n a six-membered ring, R

\ / R B 3

RCOOH ether

\

Β A

Η

I

ο ο W c

W h e n the boron c o m p o u n d coordinates w i t h the carboxylic a c i d i n this w a y , two things result.

F i r s t it labilizes the potential carbanion.

b y infrared observations where, i n going from B H weakens the B H linkage.

3

One can prove this

t o BH4, s u c h c o o r d i n a t i o n

A t t h e s a m e t i m e , t h i s s h o u l d increase t h e a c i d i t y of t h e

ADENOSINE pH 4.0 pH 6.0 pH 9.0

0.6

Ag ADENOSINE pH4.0 pH6.0 ρ H 9.0

0.4

Q2

00

220

260 280 WAVELENGTH IN mp

300

320

Figure G. Ultraviolet spectra of silver complexes of Poly I and Poly compared with the spectra of the uncomplexed polynucleotides


A


7.

JONES

0

Discussion

5

175

10

15

20

25

30

HOURS AT 6 4 C e

Figure H. Degradation of the polynucleotides by zinc in the presence of silver ion p r o t o n a n d l e a d , as w e c a n o b s e r v e v e r y n i c e l y , t o t h e p r o d u c t i o n of R H a n d R2BO2CR.

Interestingly the kinetic observations that result from v a r y i n g the

a c i d s t r e n g t h are j u s t w h a t m i g h t be e x p e c t e d f r o m s u c h a process.

A p l o t of t h e

l o g of the s e c o n d - o r d e r r a t e c o n s t a n t for t h i s r e a c t i o n vs. the p K a ' s of the a c i d s u s e d , gives a c o r r e l a t i o n — a s a m a t t e r of f a c t ( F i g u r e I) a l i n e a r one ( b u t I t h i n k t h a t i s m o r e c o i n c i d e n c e t h a n a n y t h i n g e l s e ) — a n d t h e slope is n o t w h a t y o u m i g h t e x p e c t f r o m e l e c t r o p h i l i c a t t a c k o n t h e c a r b o n of t h e R3B c o m p o u n d

by proton.

The

s t r o n g e r t h e a c i d , t h e s l o w e r t h e r e a c t i o n . T h e o n l y e x p l a n a t i o n I c a n t h i n k of i s t h a t t h e n u c l e o p h i l i c c h a r a c t e r of t h e o x y g e n is t h e i m p o r t a n t f a c t o r . T h e s e three s y s t e m s t h e n i n d i c a t e t h a t a t t a c h i n g l i g a n d s t o s i m p l e m e t a l l i c c o m p o u n d s c a n change t h e i r r e a c t i v i t y m a r k e d l y .

organo-

I t is not necessary t o

l o o k a t t h i s t y p e of d i r e c t c o o r d i n a t i o n ; e v e n changes i n t h e e n v i r o n m e n t c a n o b v i o u s l y m o d i f y organometallic reaction rates, a n d a p p a r e n t l y the chemist is m u c h more familiar w i t h this.

coordination

B u t t h i s t y p e of k n o w l e d g e h a s n o t been

transferred over v e r y well to organometallic chemistry. is r e a c t i o n s of n i t r i l e s w i t h G r i g n a r d reagents.

O n e case w e h a v e o b s e r v e d

T h i s leads t o a n a d d i t i o n across t h e

—C=N. Ph EttO

PhC=N

+

"PhMgBr"

\

—> Ph

/

C=N—MgBr



176

-

EîgB + RCOOH

-

O.IM

O.IM

Diglyme 31°

$» EtH + e î e : ;

..0.

2

Jcr

Me-CCOOH,


MeCCH^COOH,, EtCH(#C00H/, J «CH C00H|i//"l Ο V__T/ / EtCOOH MeCOOH « CHC00Hj. 2

f

2

CICH C00H 2

CIpCHCOOH Τ ι 1 1.7

2.3

2.9

35

4.7

4.1

pKd Figure I. Log of second-order rate constant vs. pK

a

I n ether solvents this reaction is v e r y slow.

A s a m a t t e r of f a c t , i t m a y t a k e 24 t o

30 h o u r s t o a c c o m p l i s h a t r e a s o n a b l e c o n c e n t r a t i o n .

B u t adding materials like

l i t h i u m p e r c h l o r a t e accelerates t h e r e a c t i o n b y a f a c t o r of a b o u t 100, a n d i t i s o v e r i n 15 m i n u t e s .

P r e s u m a b l y , some t y p e of i o n q u a d r a p o l e a s s o c i a t i o n i s o c c u r r i n g ,


7.

JONES

Discussion

177

let's say, between the G r i g n a r d reagent a n d the l i t h i u m perchlorate.

N o w we a r e

a b l e t o affect t h e r e a c t i v i t y of a n o r g a n o m e t a l l i c c o m p o u n d n o t o n l y b y d i r e c t c o o r dination but by changing its solvent environment. Leonard K a t z i n :

I c a n ' t c o m m e n t o n a l l these e x a m p l e s t h a t D r . D e s s y h a s

p r e s e n t e d , b u t c e r t a i n l y these c a n be v i e w e d as a n i o n r e p l a c e m e n t r e a c t i o n s of a c o n v e n t i o n a l i n o r g a n i c t y p e , w i t h i n v e r s i o n of t h e o r d e r of t h e a n i o n s .

I a m not

sure i f i t is t y p i c a l of t h e h y d r i d e s o r i f i t is t y p i c a l of the q u a d r i v a l e n t m e t a l s , b e


cause t h e one e x a m p l e I a m t h i n k i n g of is a t h o r i u m c o m p o u n d , i n o r g a n i c .

T h i s is

solid material prepared b y treating thorium metal with hydrochloric acid, w i t h w h i c h i t r e a c t s v i o l e n t l y l e a v i n g l a r g e a m o u n t s of s o l i d T h H ( O H ) 0 .

I t is d e c o m

p o s e d b y a d d i n g a l i t t l e fluoride, w h i c h is the c o n t i n u a t i o n of t h e series here. things are duplicated. tetravalent metal.

Two

One is the fact y o u have the hydride, but also y o u have a

A t t h i s p o i n t I w o u l d n ' t s a y w h i c h is the d e t e r m i n i n g f a c t o r i n

h o w t h i s series r u n s , b u t c e r t a i n l y t h a t is the s a m e s o r t of t h i n g ; a n d here i t w o u l d d e p e n d o n t h e a c t u a l s t r u c t u r e of t h i s c o m p l e x .

B u t i f the e t h e r is c o o r d i n a t e d t o

the m a g n e s i u m , i t is possible t h a t e v e n t h o u g h p e r c h l o r a t e is g e n e r a l l y a p o o r d o n o r , i t m i g h t be b e t t e r t h a n e t h e r a n d d i s p l a c e t h e e t h e r a t t h i s t i m e . I w a n t to ask D r . Dessy two questions.

Jack Halpern:

S o m e t i m e ago

we

l o o k e d a t s u b s t a n t i a l l y t h e same r e a c t i o n (i.e., d e c o m p o s i t i o n of m e t h o x y c a r b o n y l m e r c u r i c a c e t a t e ) i n a q u e o u s s o l u t i o n , a n d we f o u n d a l s o t h a t t h i s d e c o m p o s i t i o n i s catalyzed b y hydrogen ion a n d by chloride i o n . b u t we f o u n d t h a t i n a d d i t i o n t o a p a t h

first-order

T h e rate is

first-order

in ( H ) , +

i n (Cl~~) there were a l s o a p p r e

ciable contributions from higher order paths, c e r t a i n l y second-order a n d possibly also t h i r d .

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

t h a n one h a l i d e t o t h e m e t a l .

I w o n d e r e d w h e t h e r there were a n y c o r r e s p o n d i n g

i n d i c a t i o n s i n these s o l v e n t s y s t e m s . T h e other point concerns the persistent controversy about the detailed m e c h a n i s m s of g e n e r a l r e a c t i o n s of t h i s t y p e — i . e . , the d e o x y m e r c u r a t i o n r e a c t i o n s , n o t o n l y of m e t h o x y c a r b o n y l

compounds

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

olefin

adducts.

T h e s e are c l e a r l y c o n c e r t e d processes t h a t are a i d e d i n some w a y b y c o o r d i n a t i o n of X ~ o n t h e m e r c u r y a n d c o o r d i n a t i o n of H

+

on the oxygen.

T h e r e h a s been c o n

t r o v e r s y , i n w h i c h W r i g h t a n d o t h e r s h a v e p a r t i c i p a t e d , as t o w h e t h e r t h e d e t a i l e d mechanism involves a cyclic intermediate, i n other words whether undissociated H X participates i n the reaction or whether the assistance a t the m e t a l a n d oxygen c e n t e r s a r e i n d e p e n d e n t processes.

H a v e you a n y comments on this?

F i r s t , I t h i n k M a u r i c e K r e e v o y feels t h a t t h e d e o x y m e r c u r a t i o n

D r . Dessy: reaction \

/

H

C—C

/I Ο CH

+

\

-

l\

x

"

/

C = C

+

/

CH3OH

+

H g A X

\

Hg 3

\ A (ί)

c a n i n v o l v e p r o t o n p a r t i c i p a t i o n , o f t e n w i t h s o m e t y p e of h a l i d e a s s i s t a n c e .

Dr.

K r e e v o y h a s s t a t e d — a n d I agree w i t h h i m — t h a t i n t h e d e c o m p o s i t i o n we l o o k e d a t we m a y h a v e c h o s e n r e a c t a n t c o n c e n t r a t i o n s w h i c h w o u l d n o t r e v e a l , f r o m


the


178

k i n e t i c d a t a , higher orders i n halide i o n .

REACTIONS

A s a m a t t e r of f a c t , i n t h e d e o x y m e r c u r a -

t i o n r e a c t i o n t h a t he s t u d i e d , w h e r e t h e a n i o n i s i o d i d e i o n , t h e k i n e t i c s i n d i c a t e t h a i t h e r e a c t i o n i s first-order i n s u b s t r a t e , a n d m a y be z e r o - o r first-order i n p r o t o n , a n d zero-,

first-,

or second-order i n iodide i o n .

U n l e s s we w e n t b a c k a n d l o o k e d

b e t t e r a t t h i s s y s t e m , w h i c h I h a v e n o i n t e n t i o n of d o i n g , I w o u l d s a y i t i s p o s s i b l e t h a t m o r e t h a n one h a l i d e i o n c o u l d be t h e r e .

I h a v e a l w a y s felt t h a t t h e o d d o r d e r

i n h a l i d e , 1.6, t h a t y o u h a d —


Dr. Halpern: D r . Dessy: kinetics.

T h a t was a c t u a l l y a composite

order.

Y e s , a n d i t indicates w h y y o u h a d such a difficult time w i t h the

T h e s o l v e n t s y s t e m t h a t w a s used h a d h y d r o g e n b o n d i n g p o s s i b i l i t i e s

w i t h the halide i o n .

H e n r y K u i v i l a has d e c o m p o s e d t r i b u t y l t i n h y d r i d e i n a l c o h o l

s o l v e n t s w i t h c a r b o x y l i c a c i d s , a n d there is n o effect of h a l i d e i o n .

A p p a r e n t l y the

t i n u n d e r those c o n d i t i o n s c a n n o t c o m p e t e w i t h t h e s o l v e n t for t h e h a l i d e i o n . H a l i d e ion is a l l hydrogen

bonded.

W i t h respect to the W r i g h t controversy, I w o u l d rather not John Bailar:

comment.

W e h a v e o b s e r v e d a case o r t w o i n w h i c h S c h i f f ' s base i s f o r m e d

directly from ligands i n solution. free a l d e h y d e a n d t h e free a m i n e .

I d o n ' t t h i n k i t is a l w a y s n e c e s s a r y t o h a v e t h e I f t h e y are i n s o l u b l e , t h a t is a n o t h e r m a t t e r .

Y e s t e r d a y I a t t r i b u t e d t o P r o f . D w y e r a v e r y n i c e piece of w o r k r e a r r a n g e m e n t of t h e d i c h l o r o t r i e t h y l e n e t e t r a m i n e c o m p l e x .

I have

on

the

discovered

since t h a t t h i s w o r k w a s d o n e b y D r . S a r g e s o n , a n d I w a n t t o give h i m c r e d i t for i t . Y o u m i g h t be i n t e r e s t e d i n s o m e l i g a n d r e a c t i o n w o r k we a r e d o i n g .

Someone

r e m a r k e d y e s t e r d a y t h a t t h e d i s c u s s i o n d o e s n ' t s e e m t o be r e l e v a n t t o t h e p a p e r s presented except t h a t t h e y are on the same general topic, I have t a k e n t h a t as j u s t i f i c a t i o n t o c o m m e n t o n a r e a c t i o n we a r e s t u d y i n g w h i c h is b i o l o g i c a l a n d t h u s ties i n w i t h G u n t h e r E i c h h o r n ' s p a p e r , t h o u g h t h e w o r k i s v a s t l y different. T h i s goes b a c k t o t h e t i m e w h e n P a s t e u r d i s c o v e r e d t h r e e m e t h o d s of r e s o l v i n g organic compounds.

O n e c a n p i c k t h e e n a n t i o m o r p h s o u t w i t h tweezers, o r f o r m

diastereoisomers, or let bacteria eat t h e m .

P e o p l e h a v e used t h e first t w o m e t h o d s

t o resolve i n o r g a n i c c o m p l e x e s , b u t t h e y h a v e n ' t used t h e t h i r d .

It occurred

to

m e s o m e t i m e a g o t h a t p e r h a p s b a c t e r i a w o u l d e a t s o m e of these c o m p l e x e s a n d m a y b e t h e y w o u l d be stereospecific

about it.

I have some p r e l i m i n a r y results

which m a y interest you. J o h n G i l d a r d has r e s o l v e d t h e c o m p l e x t r i s ( e t h y l e n e d i a m i n e ) c o b a l t ( I I I )

into

t h e D a n d L f o r m , a n d b y f o r m i n g d i a s t e r e o i s o m e r s he h a s t r e a t e d these s e p a r a t e l y with pseudonoma bacterium.

T h e r e s u l t s h a v e been v e r y , i n t e r e s t i n g i n d e e d .

T h e s e b a c t e r i a w i l l eat t h e D - f o r m of t h i s c o m p o u n d , o r a t least t h e y c o n s u m e t h e n i t r o g e n of t h e e t h y l e n e d i a m i n e .

T h e y c a n ' t get a t t h e c a r b o n , a p p a r e n t l y ;

we h a v e t o a d d s o m e o t h e r source of c a r b o n o r t h e y w o n ' t g r o w , a n d of course, t h e b a c t e r i a need e n z y m e s a n d s m a l l a m o u n t s of m i n e r a l m a t t e r . W h e n t h e b a c t e r i a are e x p o s e d t o t h e £ - c o m p l e x , a v e r y i n t e r e s t i n g p i c t u r e appears.

N o t o n l y w i l l t h e y n o t e a t t h e L - i s o m e r , b u t i t i n h i b i t s t h e i r g r o w t h ; so

o b v i o u s l y t h i s b a c t e r i a l m e t h o d c a n n o t be u s e d t o resolve t h e t r i s e t h y l e n e d i a m i n e complex.

T h e r e a r e , of c o u r s e , t h o u s a n d s of b a c t e r i a a n d h u n d r e d s of c o m p l e x e s

w h i c h we can s t u d y . I believed t h a t w h e n the bacteria ate a w a y a n ethylenediamine molecule t h a t n i t r o g e n a n d c a r b o n d i o x i d e w o u l d escape a n d t h a t w o u l d be t h a t .

B u t the bac

t e r i a are m u c h m o r e c o m p l e x , a n d t h e y c o n v e r t t h e e t h y l e n e d i a m i n e i n t o a n a m i n o


7.

JONES

179

Discussion

a c i d o r s o m e t h i n g of t h a t s o r t . picture is b u i l t up.

T h i s complexes again, a n d a rather complicated

H o w e v e r , I a m h i g h l y e n c o u r a g e d , because t h e b a c t e r i a d o

b e h a v e d i f f e r e n t l y t o w a r d these t w o stereoisomers, a n d w h e t h e r we resolve t h e c o m plex or not, w h i c h was the original goal, doesn't really matter. Gilbert Haight:

F o r m a n y y e a r s , I h a v e been s t u d y i n g m o l y b d a t e - c a t a l y z e d

r e d u c t i o n s of s u c h w e a k c o m p l e x i n g agents as p e r c h l o r a t e s a n d n i t r a t e s , u s i n g t h e B r a y t e c h n i q u e of s t u d y i n g r a t e l a w s a n d s t o i c h i o m e t r y a n d d e d u c i n g t h e r e s t .


I n o r d e r t o i n t e r p r e t these s t u d i e s we h a d t o p o s t u l a t e a c o m p l e x b e t w e e n t h e inert nitrate or perchlorate a n d the molybdate.

I w o u l d like to show y o u some

a n a l a g o u s s t u d i e s we h a v e d o n e i n o r d e r t o j u s t i f y these c o m p l e x e s a n d some of the r e a c t i o n s we h a v e o b t a i n e d w i t h c h r o m a t e . W e have found t h a t H C r O - T i n dilute solutions w i l l undergo condensation w i t h h y d r o g e n p h o s p h a t e , b i s u l f a t e , a n d even h y d r o c h l o r i c a c i d i n w a t e r t o f o r m these mixed anhydrides like C r S 0 7 ~ . 2

W e h a v e been a b l e t o measure e q u i l i b r i u m c o n

s t a n t s , a n d I w o n d e r e d i f t h i s w o u l d n ' t p r o v e f r u i t f u l i n s t u d y i n g s u c h r e a c t i o n s as t h e o x i d a t i o n of sulfite w i t h c h r o m a t e .

S u l f i t e is o x i d i z e d t o a m i x t u r e of d i t h i o n a t e

and sulfate. T h e r e has been a great d e a l of w o r k s h o w i n g s y n e r g i s t i c effects w h e n c h r o m â t e s are reduced.

T h e very active intermediates—chromium(V) and c h r o m i u m ( I V ) —

seem t o o x i d i z e r e d u c i n g agents t h e c h r o m a t e i t s e l f w i l l n o t .

I t h i n k m o s t of y o u are

f a m i l i a r w i t h the o x i d a t i o n of manganese (I I) t o m a n g a n e s e d i o x i d e i n t h e presence of r e d u c i n g a g e n t s w h e n c h r o m a t e is u s e d . I n sulfite we h a v e , a p p a r e n t l y , a r e d u c i n g a g e n t w h i c h c a n f u n c t i o n b o t h a s a n o x y g e n a c c e p t o r a n d a n e l e c t r o n a c c e p t o r a n d c a n serve as i t s o w n t r a p for these active C r ( I V ) and C r ( V ) intermediates.

W e h a v e c a r r i e d o u t t h e k i n e t i c s of t h i s

r e a c t i o n a t p H 5 i n a n acetate buffer, w h e r e , a c c o r d i n g t o the l i t e r a t u r e , t h e species present i n s o l u t i o n are H C r O ^ for the c h r o m a t e a n d HSO3" for t h e b i s u l f i t e .

The

kinetics follow the stoichiometry Cr(VI)

+

Cr(III)

2HSO3-

+

HS 0 2

6

2

+

SOr

since t h e r a t e l a w i s first-order i n c h r o m a t e a n d s e c o n d - o r d e r i n s u l f i t e . t i o n is a l s o

first-order

i n hydrogen ion.

p r e f o r m a t i o n of C r S O e .

2

T h e reac

T h i s leads t o a m e c h a n i s m i n v o l v i n g t h e

W e t h i n k t h i s is a p r e - e q u i l i b r i u m because s o m e fast

- 2

reaction w o r k done i n G i l G o r d o n ' s laboratory at M a r y l a n d recently has shown t h a t t h e r e a c t i o n is i n i t i a l l y s l o w e r d u r i n g t h e first few m i l l i s e c o n d s t h a n i n the e v e n t u a l steady state. W l i e n a second sulfite o r SO2, i f y o u l i k e , o r H2SO3 a t t a c k s 0 3 C r O S 0 2 ~ , i t 2

goes a l l i n one fell-swoop t o c h r o m i c i o n , t o w h i c h s u l f a t e is a t t a c h e d a t t h e e n d of the reaction, a n d a n S 0 ~ r a d i c a l . 3

i m m e d i a t e l y after the reaction.

One can't precipitate sulfate w i t h b a r i u m

C o o r d i n a t i o n of t h e c h r o m i u m ( I I I ) s t a b i l i z e s t h e

r a d i c a l u n t i l i t c a n r e a c t w i t h a n o t h e r as w a s m e n t i o n e d b y D r . H a l p e r n .

This

m e c h a n i s m a c c o u n t s for b o t h t h e s t o i c h i o m e t r y a n d k i n e t i c s . A p o i n t t h a t seems t o h a v e been c o m i n g o u t of a l l of o u r w o r k is t h a t i n i n t e r a c t i o n s , e s p e c i a l l y o x i d a t i o n - r e d u c t i o n r e a c t i o n s i n v o l v i n g o x y g e n a t e d species, we h a v e t o c o n s i d e r s u c h c o n d e n s a t i o n s as t h i s .

I s h o u l d n ' t be s u r p r i s e d i f t h e y were i n

v o l v e d i n a l o t of the r e a c t i o n s i n v o l v i n g s i m p l e m e t a l ions w h i c h are h y d r a t e d .

A

recent a r t i c l e (2) states t h a t b i c h r o m a t e also condenses w i t h a n a q u o c o m p l e x of c o b a l t w i t h a m u c h h i g h e r f o r m a t i o n c o n s t a n t t h a n t h a t for C r S 0 7 ~ a n d w i t h 2



180

REACTIONS

t h e e l i m i n a t i o n of a p r o t o n , w h i c h s h o u l d m a k e t h a t m o r e d i f f i c u l t t h a n t h e f o r m a t i o n of C r S 0 7 ~ . 2

I t h i n k w e are g o i n g t o see m o r e of t h i s s o r t of a n h y d r i d e o c c u r i n g

in redox reactions. I d o n ' t k n o w i f t h i s i s t h e influence of t h e c e n t r a l a t o m s u l f u r o n t h e l i g a n d chromate, or the central a t o m c h r o m i u m on the ligand sulfate.


Literature

Cited

(1) B u s c h , D a r y l e , B a i l a r , Jr., John, J. Am. Chem. Soc. 78, 1137 (1956). (2) S u l l i v a n , J. C., French, J. E., Inorg. Chem. 3, 832 (1964).


Kinetic Patterns of Ligand Reactivity - Advances in Chemistry (ACS

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