Effect of Coordination on the Reactivity of Aromatic Ligands

7 Effect of Coordination on the Reactivity of Aromatic Ligands General Patterns of Reactivity

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MARK M. JONES Vanderbilt

University, Nashville,

Tenn.

The aromatic portion of a ligand will generally exhibit the same qualitative pattern for electrophilic substitution reactions, whether present in a coordination compound or not. The reactions of the extra-aromatic portion of such a ligand will usually show a pattern of reactivity similar to that of the free ligand but modified by the effects of masking and polarization which result from coordination. For heterocyclic ligands containing donor atoms integral with the aromatic systems, more profound changes may occur, but have been demonstrated only for the case of pyridine-N-oxide to date. When redox or autoxidation reactions are considered, coordination may result in very profound changes in the rates of these reactions. It is in this latter class of reaction that the most startling effects of coordination are to be seen.

M t the p r e s e n t t i m e t h e p a u c i t y of d a t a renders

i t i m p o s s i b l e to g i v e a

complete

p i c t u r e of the various w a y s i n w h i c h c o o r d i n a t i o n m a y affect the reactivity aromatic

ligands.

It

is p o s s i b l e

to

indicate

process m a y h a v e o n the a r o m a t i c reactivity, polarization m a y

affect reactivities,

a n d to

the

effects w h i c h

however, show

at

the

to s h o w h o w m a s k i n g

least a f e w

of

coordination

instances

and

where

c o o r d i n a t i o n m a y p r o v i d e u s e f u l s y n t h e t i c r o u t e s to s o m e a r o m a t i c c o m p o u n d s . While

it m a y

ultimately prove more

f r u i t f u l to

develop

a completely

inde-

p e n d e n t m e t h o d for p r e d i c t i n g t h e r e a c t i o n s of c o o r d i n a t e d l i g a n d s , at p r e s e n t most promising approach

is to see h o w t h e

usual methods

c h e m i s t r y c a n b e u s e d o r m o d i f i e d to c o v e r t h e s e c a s e s . aspects of o r g a n i c

reactions

(as

compared

i n w h i c h the

different parts of the

molecule

the

organic

O n e of t h e m o s t s t r i k i n g

w i t h inorganic reactions)

i n v o l v e m e n t of o n l y a s m a l l p o r t i o n of the m o l e c u l e way

of theoretical

(functional

preserve their

is t h e

group) typical

p a t t e r n s as t h e g r o s s s t r u c t u r e o f t h e m o l e c u l e is s u b j e c t e d t o c h a n g e s .

116 Busch; Reactions of Coordinated Ligands Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

usual

and

the

reaction

Put

more

JONES

117

Reactivity of Aromatic Ligands

directly,

organic

chemistry

is b u i l t

u p o n the

assumption

that

a

change

in

one

p o r t i o n of a m o l e c u l e w i l l u s u a l l y h a v e little or n o effect o n the q u a l i t a t i v e aspects of reactivity

o f o t h e r p o r t i o n s , t h o u g h it m a y

affect

t h e rates of these

reactions.

T h u s w e s h o u l d e x p e c t a b e n z e n e r i n g i n a c o o r d i n a t i o n c o m p l e x to s h o w t h e general properties

as a r e f o u n d f o r b e n z e n e r i n g s i n t y p i c a l o r g a n i c

Metal-arene complexes

must be excepted

same

compounds.

f r o m this s t a t e m e n t a n d the

discussion

w h i c h follows. F o r c o n v e n i e n c e , the reactions of a r o m a t i c l i g a n d s m a y b e d i v i d e d into three groups: I.

R e a c t i o n s w h e r e t h e d o n o r a t o m is e x t e r n a l t o t h e a r o m a t i c

II.

Reactions

i n w h i c h the

aromatic

system

contains

the

system

donor atom

as

an

integral part III.

R e d o x reactions in w h i c h the c o m p l e x participates

via changes

involving

b o t h the l i g a n d a n d central m e t a l i o n

Aromatic Systems with an External Donor Atom W h e n t h e d o n o r a t o m is a n i t r o g e n o r a n o x y g e n a t o m a d j a c e n t to a n

aromatic

system, the c o m b i n a t i o n of t h e u s u a l v a l e n c e b o n d t r e a t m e n t of a r o m a t i c

orienta

tion for e l e c t r o p h i l i c reagents a n d the v a l e n c e b o n d p i c t u r e of the c o o r d i n a t e b o n d leads The

to

the

expectation

chief reason

that

for expecting

considerable

changes

i n reactivity

that coordination w i l l result

in a

should

occur.

change

in

the

o r i e n t a t i o n o f e l e c t r o p h i l i c a t t a c k u p o n a n i l i n e s o r p h e n o l s is t h e k i n d o f c a n o n i c a l structure

used

to

explain

why

the

directing, rather than meta directing. o r o x y g e n is a t t a c h e d deactivation

toward

amino

and

hydroxy

groups

If a n e l e c t r o n e g a t i v e

are

ortho-para

a t o m s u c h as n i t r o g e n

d i r e c t l y to a b e n z e n e r i n g , o n e m i g h t e x p e c t a p r o n o u n c e d typical

electrophilic

d i r e c t i n g influence for s u c h substituents.

reagents

and

possibly

even

T h u s the chloro a n d the

are d e a c t i v a t i n g , w h i l e the nitro a n d s u l f o n i c a c i d g r o u p s are m e t a

a

meta-

fluoro

groups

directing.

e x p l a i n t h e o b s e r v e d o r i e n t a t i o n p a t t e r n i n b o t h a n i l i n e a n d p h e n o l i t is

To

customary

to e m p h a s i z e t h e i m p o r t a n c e o f c e r t a i n c a n o n i c a l f o r m s w h i c h i n v o l v e t h e s h a r i n g of the l o n e p a i r o n the n i t r o g e n or o x y g e n w i t h the b e n z e n e r i n g . u s e d i n t h e f o r m u l a t i o n o f e i t h e r t h e g r o u n d s t a t e (54) T h e resultant

c a n o n i c a l f o r m s w h i c h are

These may

or the t r a n s i t i o n state

assigned importance

are

shown

be (48).

below:

Ground State Formulation. H - N - H

4H — N - H

+

+

H - N - H

Η—Ν—H

Transition State Formulation.

(three of these)

(three of these)

para attack b y Y +

ortho attack b y Y +

In either f o r m u l a t i o n the i n v o l v e m e n t of the lone p a i r i n a coordinate

Busch; Reactions of Coordinated Ligands Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

bond

ADVANCES

118 w o u l d l e a d o n e to e x p e c t s o m e

obvious qualitative

IN CHEMISTRY SERIES

changes i n reactivity.

That

s u c h c h a n g e s a r e n o t o b s e r v e d l e a d s to t h e c o n c l u s i o n t h a t t h e v a l e n c e b o n d t h e o r y is s o m e w h a t

inconsistent here.

A t p r e s e n t it seems t h a t the v a l e n c e b o n d

o f t h e c o o r d i n a t e b o n d is p r o b a b l y t h e c u l p r i t . w o u l d not expect the

In the crystal

field

picture

treatment

l o n e p a i r t o b e so d e e p l y i n v o l v e d w i t h t h e

metal

one

that

it

w o u l d b e p r e v e n t e d f r o m p a r t i c i p a t i o n i n s u c h c a n o n i c a l f o r m s as a r e n e c e s s a r y to e x p l a i n the orientation.

T h e difference between

c o n s i d e r a b l e , as p r o t o n a t i o n d o e s l e a d to a c h a n g e

protonation

and coordination

of orientation of a n i l i n e

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

picture

of

the

charge

T h i s w h o l e p r o b l e m is, h o w e v e r , ment

redistribution

which

results

theory.

of g i v i n g a

from

is

(24).

more

coordination.

still i n a state of v e r y r a p i d theoretical

develop

(55). A l l o f t h e e v i d e n c e at p r e s e n t a v a i l a b l e i n d i c a t e s t h a t t h e a r o m a t i c p o r t i o n o f

s u c h ligands retains the same pattern

of substitution w i t h e l e c t r o p h i l i c reagents i n

t h e c o m p l e x t h a t it e x h i b i t e d i n the f r e e state.

T h i s has b e e n demonstrated

v a r y i n g degrees of c o n c l u s i v e n e s s i n the e x a m p l e s 20,

26,

35-39, 41,

42,

46,

50,

tion of s u c h reactions has tropolone.

63,

73,

ever been

81).

(6,

12,

with

15,

16,

N o e x a m p l e of a c h a n g e i n orienta

confirmed, except for the

peculiar

case

of

T h i s c o m p o u n d undergoes an anomalous bromination w i t h substitution

occurring initially.

57,

cited in T a b l e I

i n the

3

position;

other

electrophilic

reagents

attack

the

5

position

T h e c o p p e r c o m p l e x , h o w e v e r , u n d e r g o e s b r o m i n a t i o n at t h e 5 p o s i t i o n ,

as w o u l d b e e x p e c t e d f o r a n o r m a l 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 . the anomalies of tropolone m a y b e f o u n d i n the literature

Table I.

Further

Metal Ion

Orientation or Remarks

Reaction

o,p o,P ρ to — O H o,p to N H m, c o o r d i n a t i o n pre vents side c h a i n halogenation p to — O H o,p to — O H

Aniline A n i l i n e , toluidines G h r o m o t r o p i c acid m-Toluidine Acetophenone

Cr(III) Pd(II) Ca(II) Cu(II) Al(III)

V a r i o u s phenols 8-Quinolinol

M g ( l l ) , Ca(II) Diazotization Cr(III), Fe(Ill), Bromination and Cu(II), Al(III) chlorination Co(III) o,p to — O H Bromination Fe(II) o,p t o — C H — N = C Various As(V) R e i m e r - T i e m a n n ο to — O H As(V) p to — O H , r a p i d Bromination Cu(II) 5 position Bromination C r ( I I l ) , etc. Reversible reaction Sulfonation w i t h complexes also ρ to — O H Nitrosation Al(III) Ga(III) p to — O H Diazotization

Salicylic acid Catechol

on

Electrophilic Substitution Reactions of Aromatic Ligands

Ligand

Salicylaldehyde Benzylisocyanide Catechol Catechol Tropolone Naphthalene derivatives

details

(6).

Bromination Bromination Diazotization Chlorination Bromination

2

2

T h e results of these studies

Lit. (73) (26) (37) (76) (57)

(72, 75) (42) (87) (20) (63) (47) (6) (46)

(38, 50) (35, 36, 39)

a l l o w the b e h a v i o r of t h e a r o m a t i c p o r t i o n of

a

l i g a n d to b e p r e d i c t e d w i t h s o m e d e g r e e o f c o n f i d e n c e , i f t h a t o f t h e f r e e l i g a n d is known

or c a n

be surmised.

Coordination appears

to

have

a

small but

effect o n the n a t u r e of the t r a n s i t i o n state f o 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


definite

( o r its e a s e

JONES

119


of a t t a i n m e n t )

when

the

donor

atom

is e x t e r n a l t o

the

aromatic

system.

t y p i c a l c h a n g e s p r o d u c e d b y c o o r d i n a t i o n w i l l b e c h a n g e s i n the rates of From the

such meager evidence

direction predicted

as is n o w

o n the

basis

available, these changes appear

of the

p o l a r i z a t i o n of the

The

reaction. to b e

ligand by

in

an

ad-

jacent positive charge. When

the

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

e x t e r n a l to t h e in the

aromatic system,

latter p o r t i o n of the

classes:

the

most

molecule.

interesting

These

a n d reactive

portions

changes i n reactivity

changes may

be

divided

occur

into

two

m a s k i n g of r e a c t i v e g r o u p s a n d activation w h i c h results f r o m the p o l a r i -

zation arising from coordination. The

masking

nitriles are —N=C

linkage

enormously 1888

(14,

elegant

19,

often

striking.

by

resistance

coordination,

Both

aliphatic

and

aromatic

by

Heldt

on

(20)

of as

aliphatic

was

first

isonitriles

clearly

to

shown

coordination

compounds

as is a c e t o p h e n o n e to s i d e c h a i n h a l o g e n a t i o n

by

Freund

p o u n d s are m o r e r e a d i l y r e d u c e d

(85).

benzyl

coordination

but some

com-

of m e t a l l i c

ions

Studies involving reducible metal

ions

T h i s holds for complexes

w h i c h are n o t r e a d i l y o x i d i z e d or r e d u c e d .

is in

recent

containing

(57),

the

hydrolysis

P h e n o l s are also r e n d e r e d m o r e resistant to o x i d a t i o n b y

37),

iso-

a n u m b e r o f a d d i t i o n r e a c t i o n s at

T h e r e s i s t a n c e to a d d i t i o n r e a c t i o n s is s h o w n i n t h e

45).

studies

15,

are The

(84).

increased

isocyanide. (12,

effects

easily h y d r o l y z e d a n d u n d e r g o

as c o o r d i n a t i o n c e n t e r s i n d i c a t e t h a t w h e n t h e l o w e r o x i d a t i o n s t a t e is s t a b i l i z e d by

c o o r d i n a t i o n , a p o w e r f u l catalysis of the

64,

oxidation process m a y

occur

(8,

M a s k i n g e f f e c t s o f a v e r y o b v i o u s s o r t s e e m n o t to b e a g e n e r a l

79).

o f c o o r d i n a t i o n a n d i t is v e r y d i f f i c u l t t o d e t e r m i n e

w h e n coordination will

i n a m a s k i n g effect, unless i n f o r m a t i o n o n strictly a n a l o g o u s It s e e m s s a f e t o s a y t h a t c o o r d i n a t i o n w i l l g e n e r a l l y

40, result result

r e a c t i o n s is a v a i l a b l e . decrease the

reactivity

o f d o n o r a t o m s i n t h e first r o w o f t h e p e r i o d i c t a b l e t h r o u g h s t e r i c e f f e c t s . s o m e r e a c t i o n s t h e e x t e n t o f this s t e r i c h i n d r a n c e m a y b e s m a l l . transformed into chloramines w h e n coordinated c o o r d i n a t e d to A 1 C 1 o r T i C l 3

and aromatic acid

(34),

With

A m m o n i a can

be

chlorides

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

4

a h i n d e r e d o n e , as i n m e s i t y l e n e c a r b o n y l c h l o r i d e

(47).

proceed through a very reactive carbonium ion, whose

T h e s e last r e a c t i o n s

may

e x i s t e n c e is r e n d e r e d

pos-

s i b l e b y t h e p o l a r i z a t i o n o f t h e l i g a n d w h i c h i t s u f f e r s as a r e s u l t o f c o o r d i n a t i o n . The bases.

coordination process m a y

If n i c k e l (II)

salts a r e

either

stabilize or destabilize

a d d e d to a m m o n i a c a l

solutions

t h e p r e c i p i t a t e o b t a i n e d is t h e i n n e r c o m p l e x salt o f n i c k e l ( I I )

aromatic

of

and salicylaldimine

If b e r y l l i u m c h l o r i d e is a d d e d t o t h e S c h i f f b a s e d e r i v e d f r o m

(61).

naphthaldehyde and ethylamine, however, i n n e r c o m p l e x of b e r y l l i u m ( I I ) H e r e the strength

2-hydroxy-l-

t h e S c h i f f b a s e is d e c o m p o s e d

and 2-hydroxy-l-naphthaldehyde

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

Schiff

salicylaldehyde,

and

is o b t a i n e d

b o n d s f o r m e d w i t h the m e t a l

seems to

the (59).

deter-

mine w h i c h complex will be formed. The

n u m b e r of cases w h e r e

a c t i v a t i o n r e s u l t s f r o m c o o r d i n a t i o n is l a r g e

m a n y examples have been collected b y Hesse

generally a v e r y strong p o l a r i z a t i o n , w h i c h results i n a p r o f o u n d w e a k e n i n g of other b o n d s of the d o n o r a t o m . (23),

This was

first

recognized explicitly b y

the

Meerwein

w h o s t u d i e d the increase i n a c i d i t y of g r o u p s c o n t a i n i n g i o n i z a b l e h y d r o g e n

w h e n the

atom

to w h i c h t h e h y d r o g e n is b o n d e d f o r m s a c o o r d i n a t e

bond.

l a t e d r e a c t i o n s i n c l u d e t h e c a t a l y t i c s p l i t t i n g o f e t h e r s v i a c o o r d i n a t i o n to Zn(II)

and

T h e b a s i s o f t h e p r o c e s s is

(23).

(2),orSnCl

(60):

4

MgL,2(C H ) 0 + 2

5

2

2G H GOGl 6

5

2C H,I + 2

2C H COOC H 6

5

2

5

+

MgCl


Re-

Mg(II),

2

120

ADVANCES IN CHEMISTRY SERIES 0

Ο SnCl OCH

0

0 CH

Another example

4

OUC, + GH G1 3

3

k

3

SnCl

3

w h i c h illustrates b o t h the p o l a r i z a t i o n a n d the f r e e d o m

r e a c t i o n w h i c h is o f t e n f o u n d at t h e c o o r d i n a t i o n site is (29)

0

ο

of

: ο

Η I n a r o m a t i c systems, the L e w i s acids w h i c h activate v i a c o o r d i n a t i o n are c a p a b l e of a c t i v a t i n g the a r o m a t i c system b y the f o r m a t i o n of σ a n d π T h e r e are

a sufficient n u m b e r of examples

also

complexes.

a v a i l a b l e to i n d i c a t e t h a t t h e

activa

t i o n v i a t h e l a t t e r p r o c e s s e s is t h e m o r e i m p o r t a n t o f t h e s e , w h e r e a l l a r e

present.

Olivier

(52)

consists

of t w o

portions.

than

the

s h o w e d i n 1913

amount

that the k i n e t i c b e h a v i o r of s u c h reactions

W h e n the

catalyst,

r e q u i r e d to

say

complex

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

all the

functional groups,

the

in

less

reaction

is

r e l a t i v e l y s l o w a n d t h e c a t a l y t i c a c t i v i t y is d u e to t h e s m a l l a m o u n t o f L e w i s a c i d resulting

f r o m the

dissociation of the

complex.

A s soon

as

all the

functional

g r o u p s a r e c o o r d i n a t e d , a n y a d d i t i o n a l L e w i s a c i d is f o u n d t o a c c e l e r a t e t h e enormously.

I n t h e s 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 it s e e m s h i g h l y p r o b a b l e t h a t

t h e a c t i v a t i o n i n v o l v e s the p i e l e c t r o n s y s t e m of the b e n z e n e r i n g .

rate the

Olivier studied

the reaction sequence :

C H B r S 0 C l + A1G1, 6

4

6

4

C H B r S 0 C l . A1G1

2

C H BrS0 Cl.AlCl 2

3

+

G H 6

6

4

2

3

-> C H B r S 0 C H . A1C1

6

6

4

2

6

5

3

+

HG1

T h i s t y p e o f d u a l i t y o f a c t i o n is p r e s u m a b l y p r e s e n t i n o t h e r s i t u a t i o n s , as t h e F r i e s r e a r r a n g e m e n t (65)

o r a c i d a n h y d r i d e s (21),

I n these reactions catalyst. (32,

33)

the F r i e d e l - C r a f t s reaction

(78),

it a p p e a r s

with acid

a n d the catalytic c h l o r i n a t i o n of n i t r o b e n z e n e t h a t t h e u n c o o r d i n a t e d L e w i s a c i d is t h e

T h e s a m e s i t u a t i o n is i l l u s t r a t e d b y r e c e n t w o r k o n a r o m a t i c and halogenation

acid-catalyzed

(57,

58,

71)

electrophilic reactions

such

chlorides (17).

effective amination

a n d s e e m s to b e g e n e r a l f e a t u r e o f L e w i s

of aromatic

compounds containing

suitable

donor groups. W h e n rearrangements be

thermodynamically

o c c u r i n these systems, the p r o d u c t s o b t a i n e d w i l l often

rather

than

kinetically determined

stances—e.g., the G a t t e r m a n n - K o c h reaction—the

(62).

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

Heterocyclic

In

some

in

stability of the c o m p l e x p r o d u c e d (4,9).

Systems with Internal Donor Atoms

W h e n t h e d o n o r a t o m is a p a r t o f t h e a r o m a t i c s y s t e m , o n e w o u l d e x p e c t m o r e obvious differences i n reactivity.

A t present relatively little c o m p a r a t i v e

t i o n is a v a i l a b l e o n s u c h h e t e r o c y c l i c s y s t e m s . are t h e r e a n y r e a s o n a b l y extensive

data.

F o r p y r i d i n e a w i d e v a r i e t y of c o o r d i

n a t i o n p r o c e s s e s a r e a v a i l a b l e a n d p y r i d i n e - N - o x i d e as w e l l as m e t a l l i c and

complexes

tive purposes

w i t h nonmetallic L e w i s acids must be considered. the

great reluctance

informa

O n l y o n p y r i d i n e a n d its d e r i v a t i v e s

with w h i c h pyridine undergoes

For

complexes compara

electrophilic


JONES


121

reactions is a considerable disadvantage. Protonation of the nitrogen is often cited as the cause of the slowness w i t h w h i c h pyridine undergoes electrophilic substitu tions i n acidic media. W h e n this is prevented b y coordination to oxygen (86) or chromium (III) (73), or b y the use of a sterically hindered derivative and a nonaqueous solvent (3, 49), the reactions are still sluggish unless the pyridine nucleus contains activating substituents. T h e general pattern of reactivity seems to be unchanged, however, for reactions other than nitration. It has been sug gested b y D a u d e l that the electronic densities a n d hence the reactivities of pyridine and the pyridinium ion w i l l be very similar (7). T h e preferred position for electrophilic substitution i n the pyridine ring is the 3 position. Because of the sluggishness of the reactions of pyridine, these are often carried out at elevated temperatures, where a free radical mechanism m a y be operative. If these reactions are eliminated from consideration, substitution at the 3 position is found to be general for electrophilic reactions of coordinated pyridine, except for the nitration of pyridine-N-oxide (30, 51). T h e mercuration of pyridine w i t h mercuric acetate proceeds v i a the coordination complex and gives the anticipated product w i t h substitution i n the 3 position (72). T h e bromination of pyridine-N-oxide i n f u m i n g sulfuric acid goes v i a a complex w i t h sulfur trioxide a n d gives 3-bromopyridine-N-oxide as the chief product (80). I n this case the coordination presumably deactivates the pyridine nucleus i n the 2 a n d 4 positions. T h e bromination of this complex i n f u m i n g sulfuric acid proceeds more rapidly than the bromination of the pyridinium ion i n 9 0 % sulfuric acid. This behavior is reminiscent of those cases where the excess of L e w i s base activates the aromatic system. T h e use of metallic halides i n the catalytic halogenation of pyridine or pyridine derivatives leads to substitution i n the 3 position or the 3 and 5 positions. T h e halides used include those of antimony, mercury (5, 2 5 ) , a n d aluminum (56). T h e nucleophilic reactions of pyridine seem to be facilitated b y coordination to a certain extent. Thus 3-bromopyridine is transformed to the corresponding amine derivative b y treatment w i t h an amine i n the presence of cupric sulfate (43). The mercuration of thiophene, w h i c h presumably goes b y w a y of a coordina tion compound, gives an initial attack i n the 2 position as expected from the re actions of thiophene itself (67). T h e bromination (or chlorination) of indazole or its silver salt also leads to the same products (82). Redox Reactions Redox reactions involving aromatic ligands may take unusual courses because of the ease w i t h w h i c h electrons m a y be conducted through an aromatic system, though they need not do so. Thus the oxidation of free or coordinated l,10-ophenanthroline leads to S ^ ' - d i c a r b o x y ^ ^ ' - b i p y r i d i n e (69), a n d the oxidation of toluidine i n sodium pentacyanotoluidine-ferrate(III) leads to phenazines (22) w h i c h can also be obtained b y oxidation of the ligand itself (10). The work of F a l l a b and his collaborators has shown h o w the coordination act may b r i n g the reactants together i n autoxidation reactions. I n several instances coordination furnishes a catalytic path for these reactions. Specific examples i n clude the autoxidation of F e + i n the presence of sulfosalicylic acid (28), the autoxidation of 1-hydrazinophthalazine b y i r o n ( I I ) (27, 83), and the autoxida tion of a formazyl-zinc complex (11). It is probable that the importance of this k i n d of a mechanism w i l l be more w i d e l y realized as more a n d more detailed kinetic studies are made on metal-catalyzed autoxidation reactions. Some other 2


ADVANCES IN CHEMISTRY SERIES

122

reactions w h i c h f a l l i n this same c a t e g o r y are t h e c u p r o u s c h l o r i d e - c a t a l y z e d o x i d a tions

of aromatic

amines

a n d related

Terentiev a n d his coworkers

(75,

76,

compounds which a n d others

77)

have

(31,

been

studied b y

This

53).

field

is a

v e r y large one a n d was early studied f r o m the v i e w p o i n t of coordination c h e m istry

(68). O f the recent

w o r k i n v o l v i n g a r o m a t i c l i g a n d s , t h a t o f T a u b e a n d S e b e r a is

p e r h a p s t h e m o s t s t r i k i n g (13,

74).

tron c a n be transferred f r o m C r ( I I )

T h e y w e r e able to demonstrate to C o ( I I P

that a n elec

through a conjugated system i n

v o l v i n g the metals a n d terephthalic a c i d derivatives.

T h i s indicates possible syn

thetic applications of these reactions.

Theoretical

Studies

T h e n u m b e r of instances i n w h i c h theoretical values of the electron (or equivalent information) its c o m p l e x e s

are v e r y f e w .

densities

at v a r i o u s a t o m s are a v a i l a b l e f o r b o t h a l i g a n d a n d O f the molecular orbital calculations

w h i c h h a v e b e e n c a r r i e d out, t w o are of d i r e c t interest. a n d its c o m p l e x e s b y S c h l a f e r a n d K o n i g

(66)

on

complexes

T h e s t u d y of p y r i d i n e

s h o w e d t h a t t h e ττ-ττ* t r a n s i t i o n o f

the p y r i d i n e itself w a s o n l y s l i g h t l y affected b y c o o r d i n a t i o n . i n g t h e t r a n s f e r o f e l e c t r o n s f r o m t h e m e t a l t o t h e l i g a n d (t

2g

Transitions involv

—* τ τ , * o r t

ττ *) 2

2s

are r e n d e r e d possible b y the c o o r d i n a t i o n act, b u t the r e l e v a n c e of s u c h transitions t o t h e a r o m a t i c r e a c t i v i t y is n o t at a l l c l e a r . sities o f p o r p h i n Berthier phin

ring

a n d its i r o n c o m p l e x e s

T h e s e s h o w that the ττ-electron densities o n the atoms of t h e p o r

(70).

are o n l y v e r y slightly m o d i f i e d w h e n p o r p h i n

iron (II) - p o r p h i n complex. (18)

T h e calculated values of electron den

have been published b y Spanjaard and is c h a n g e d

to a n i o n i c

Related calculations have been carried out b y G o u d o t

b u t are m o r e c o n c e r n e d w i t h b o n d - b r e a k i n g processes t h a n w i t h t h e a r o m a t i c

p o r t i o n o f c a t a l y s t s s u c h as c a t a l a s e . The

above remarks

a p p l y to a r o m a t i c

p o r t i o n of a c o m p l e x u n d e r g o e s the base

(1)

ligands.

In general, the L e w i s

a m u c h greater change

upon

acid

coordination than

a n d coordination c a n change the orientation i n c o m p o u n d s such

phenylboronic acid

(44)

a n d p r e s u m a b l y other

similar organometallic

as

coordina

tion centers.

Literature Cited (1) Bauer, S. H., ADV. CHEM. SER., NO. 32, 89 (1961).

(2) (3) (4) (5)

Blaise, Ε. E., Compt. rend. 139, 1211 (1904); 140, 661 (1905). Brown, H.C.,Kanner, B.,J.Am. Chem. Soc. 75, 3865 (1953). Campbell, H., Eley, D. D., Nature 154, 85 (1944). Chemische Fabrik von Heyden, A. G., Swiss Patent 174,893; Chem. Zent. 1935, II,

2283.

(6) Cook, J. W., Gibb, A. R. M., Raphael, R. Α.,J.Chem. Soc. 1951, 2244. (7) Daudel, R., Lefebure, R., Moser, C., "Quantum Chemistry," p. 251, Interscience, New York, 1959. (8) Denisov, E. T., Emanuel, Ν. N., Russian Chem. Rev., No. 12, 645 (1960). (9) Dilke, M. H., Eley, D. D.,J.Chem. Soc. 1949, 2601. (10) Elderfield, R.C.,ed., "Heterocyclic Compounds," Vol. 6, p. 637, Wiley, New York, 1957.

(11) Fallab, S., Erlenmeyer, H., Helv. Chim. Acta 42, 1153 (1959). (12) Farbwerke vorm. Meister, Lucius, und Brüning, German Patents 174,905,175,827, 177,624, 178,304, 188,819 (1904).

(13) Fraser, R. T. M., Rev. Pure Appl. Chem. 11, 64 (1961). (14) Freund, M., Ber. 21B, 935 (1888).

(15) Friedländer, P. W., "Fortschritte der Teerfarbenfabriken und verwandte Industriezweige," Vol.III,pp. 612, 616, 619, 620, 1904. Busch; Reactions of Coordinated Ligands Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

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Effect of Coordination on the Reactivity of Aromatic Ligands

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