Reactions of Coordinated Ligands and Homogeneous Catalysis

5 The Chemistry of Quasiaromatic Metal Chelates JAMES P.

COLLMAN

Downloaded by UNIV OF MARYLAND BALTIMORE COUNTY on January 11, 2015 | http://pubs.acs.org Publication Date: January 1, 1962 | doi: 10.1021/ba-1963-0037.ch005

The University of North Carolina,

Chapel Hill, N. C.

Nondestructive

reactions of

trisacetylacetonates

of chromium(III), cobalt(III), and rhodium(III) are reviewed.

Halogenation, nitration, thiocyanation,

acylation,

formylation,

chloromethylation,

and

aminomethylation take place at the central car bon of the chelate rings.

Trisubstituted chelates

were obtained in all cases except acylation and formylation.

Unsymmetrically and partially sub

stituted chelates have been prepared. tions

on

partially

resolved

Substitu

acetylacetonates

yielded optically active products.

NMR spectra of

unsymmetrically substituted, diamagnetic chelates were interpreted as evidence for aromatic ring currents.

Several groups were displaced from

the chelate rings under electrophilic conditions. The synthesis of the chromium(III) chelate of mal onaldehyde is outlined.

Ρ or m a n y years i n o r g a n i c c h e m i s t s h a v e s t u d i e d m e t a l chelates i n o r d e r to u n d e r stand groups

the (2,

properties

4, 17,

been investigated 1887.

21,

of

27,

metal

incessantly

S u c h studies

have

ions

under

the

perturbing

influence

F o r example, the metal acetylacetonates

82).

since the preparation

focused

on

almost

of t h e

every

first

aspect

c h e m i c a l p r o p e r t i e s of t h e c e n t r a l m e t a l i o n i n t h e s e c h e l a t e s . organic chemists have respected

of

ligand

(I),

have

acetylacetonate i n

of

the

physical

and

O n the other h a n d ,

the traditional schism b e t w e e n

organic

a n d inor

g a n i c c h e m i s t r y b y c o n s i d e r i n g t h e s e c h e l a t e s as i n o r g a n i c a n d t h u s f o r b i d d e n s u b stances.

The

remarkable

result

of these d i v e r g e n t

e v e n t h o u g h several h u n d r e d papers i n the literature

viewpoints

is t h a t

in

erties a n d p r e p a r a t i o n of m e t a l a c e t y l a c e t o n a t e s , almost n o t h i n g w a s k n o w n t h e c h e m i c a l r e a c t i o n s o f t h e c h e l a t e r i n g itself. trated b y c o m p a r i s o n of reviews (27)

a n d for organic chemists

of m e t a l

acetylacetonates written

H-C )c-o

/C-p

Η +M

+ n

=*

H_C

X

about

T h i s i n c o n s i s t e n c y is b e s t i l l u s for

(5).

;c=o

1958,

d e s c r i b e d the p h y s i c a l p r o p

N

Λ/ Μ

Λ

+

H +

f - o Γ M= Cr,Cb,Rh

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

inorganic

COLLMAN

Quasîaromatic

T h e nature

of the b o n d i n g i n these chelates

considerable controversy. pected

Calvin

stability of some

character

79

Metal Chelates

and Wilson

of these

chelates

c o n t i n u e s to b e t h e subject

suggested

might

i n 1946

be ascribed

that the

to their

of

unex-

aromatic

I n 1958, H o l m a n d C o t t o n p o i n t e d out that s u c h aromaticity was

(6).

u n r e a s o n a b l e o n the basis of the s y m m e t r y a n d e n e r g y of t h e m e t a l orbitals available for π b o n d i n g electronic

spectra

B a r n u m has recently suggested that o n the basis of the

(22).

of these chelates

there m a y b e significant π b o n d i n g

between

these ligands a n d the m e t a l i o n ( 3 ) .


In

1959

a general investigation of the reactions

initiated i n o u r laboratories.

of metal

chelate

rings was

Its t h e s i s w a s t h e c h e m i c a l e l a b o r a t i o n o f o r g a n i c

l i g a n d s b o u n d i n m e t a l chelate rings a n d t h e c o n c u r r e n t s t u d y of t h e effect o f c o o r d i n a t i o n o n the c h e m i c a l properties of the l i g a n d s .

It w a s a n t i c i p a t e d t h a t s u c h

e x p e r i m e n t s m i g h t l e a d to t h e synthesis of n e w t y p e s of c o o r d i n a t i o n c o m p o u n d s , organic heterocycles, a n d chelate polymers. T o a v o i d reactions l i k e l y to b e a t t e n d e d b y chelate r i n g o p e n i n g , t h e r e l a t i v e l y stable trisacetylacetonates were

chosen

of c h r o m i u m (III), cobalt (III), a n d r h o d i u m (III)

as s u b s t r a t e s

to explore

this

hypothesis.

These

(II)

kinetically

stable

n o n i o n i c c o m p l e x e s are a m e n a b l e to t r e a t m e n t i n o r g a n i c solvents w h e r e h y d r o l y t i c decompositions can be minimized. niques

such

as

crystallization,

Furthermore, familiar organic chemical

chromatography,

a n d infrared,

nuclear m a g n e t i c resonance spectra are u s e f u l i n the characterization tion of these substances. these chelates

would

and

a n d purifica

It w a s a l s o a n t i c i p a t e d t h a t t h e a s y m m e t r i c

be useful i n studying the mechanism

tech

ultraviolet,

nature

of

of r i n g

substitution

s e a r c h of t h e literature r e v e a l e d t w o instances i n w h i c h m e t a l

acetylaceto-

reactions.

A

nate rings h a d b e e n substituted w i t h o u t d e g r a d a t i o n of the chelate ment

of c h r o m i u m (III)

reported IV)

to y i e l d

(31).

acetylacetonate

two products—a

with bromine

tribromo-

rings.

Treat

in chloroform h a d been

a n d a hexabromochelate

(III a n d

T h e s e structures were assigned o n the basis of h a l o g e n analyses.

Later

w o r k simultaneous w i t h o u r o w n r e v e a l e d that I V w a s actually a c h l o r o f o r m sol vate of III

(24).

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

ADVANCES IN CHEMISTRY SERIES

80 A

report

ill)

i n 1959

b y Djordjevis, Lewis, a n d N y h o l m

(18)

copper acetylacetonate w i t h N ( V ) , spurred

2

our early

0

4

that reaction

afforded bis-(3-nitro-2,4-pentanediono)

efforts

i n acetylacetonate

chemistry.

of

copper-

T h e nitro-


chelate was thoroughly characterized.

Halogenation Treatment cinimides

of a

i n boiling

high yields.

series

of m e t a l

chloroform

acetylacetonates

afforded

the

metal

chelates

in

(7).

IR 1550 cm' M-Cr.Co.Rh X-I,Br,CI

IR 1570,1520,1190 cm!

The

the N - h a l o g e n suc-

T h e use of b r o m i n e or iodine m o n o c h l o r i d e i n b u f f e r e d acetic acid

also y i e l d e d t h e b r o m o - a n d i o d o c h e l a t e s

in

with

trihalogenated

distinctive

infrared

the characterization

spectra

of these

of the trihalogenated

complexes

acetylacetonates exhibit t w o intense

i n t h e 1500-

bands

chelates

Whereas

(14).

the

are useful

unsubstituted

1600-cm.

to

region,

_ 1

a c e t y l a c e t o n a t e s w i t h a g r o u p o t h e r t h a n h y d r o g e n s u b s t i t u t e d at t h e c e n t r a l b o n o f t h e c h e l a t e r i n g s h o w a s i n g l e p e a k i n t h i s r e g i o n (19). c h e l a t e e x h i b i t s a s i n g l e p e a k a t a b o u t 1550 plane bending mode of these p r o d u c t s .

c m .

-

1

car

halogenated

F u r t h e r , the characteristic i n -

o f t h e r i n g h y d r o g e n a t 1190

(32)

Each

c m .

-

1

is a b s e n t f r o m a l l

T h e p r e s e n c e o r a b s e n c e o f this b a n d w a s u s e d t h r o u g h o u t o u r

studies to check f o r u n s u b s t i t u t e d chelate rings. T h e proton magnetic g e n a t e d chelates

of the diamagnetic

halo

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

resonance

spectra

( T a b l e I)

about

7.5τ w h e r e a s t h e s t a r t i n g m a t e r i a l s p r o d u c e d s i g n a l s at 4 . 4 a n d 7.8τ o f 1 t o 6.

i n the ratio

T h e s e spectra confirm substitution of the ring h y d r o g e n .

A l t h o u g h the h a l o g e n a t e d chelates

of c h r o m i u m , cobalt, a n d r h o d i u m w o u l d

be difficult to p r e p a r e f r o m the sensitive 3-halo-2,4-pentanediones,

the c o p p e r (II)

bromochelate

and by

bromination

was

synthesized

of c o p p e r

both

from

acetylacetonate.

the

bromodiketone

T h e relatively

labile

f o r m m u c h m o r e r a p i d l y t h a n the kinetically stable chelates

copper

of c h r o m i u m , cobalt,

and rhodium.

% / «3

C-O

>

Cu++

pa

direct

chelates

NBS

c—b


COLLMAN

Quasiaromatic Metal Chelates

TABLE I.

81

Proton Magnetic Resonance Signals of Symmetrically Substituted Metal Acetylacetonates

c-o


C H /

Χ

M

Solvent

Ring H

Methyl

H H CI Br NO, SGN CH H H CI Br NO,

Rh Rh Rh Rh Rh Rh Rh Co Co Co Co Co

CG1 GHGI3 CGU GCU GGI4 CGI4 CCU C H CHC1 C H C H CHC1

4.65 4.61 .. .. .. ..

7.89 7.86 7.57 7.49 7.59 7.30 7.80 8.04 7.81 7.78 7.70 7.43

: i

4

6

3

F o r example,

R' — C H

8

, and R =

R' =

CF )

in

(VI, R

=

not

C F , 3

w e r e n o t b r o m i n a t e d u n d e r a v a r i e t y of v i g o r o u s

3

However,

and dibenzoylmethane

5

6

chelates w e r e b r o m i n a t e d w i t h difficulty or

butanedione (

6

6

the trifluoro- a n d hexafluoroacetylacetonates

conditions. C ;H ),

6

3

C e r t a i n other 1,3-dicarbonyl at a l l .

4.63 .. .. .. ..

6

the

case

of

the

(VI. R

chromium =

C H „ ( 5

reaction w i t h N - b r o m o s u c c i n i m i d e ( N B S )

r

chelates

R' =

of

C H

5

was successful.

l-phenyl-1,3-

, and R

=

T h a t the

R'

=

elec

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

bis-(ethylenediamine)-2,4-pentanedionocobalt(III)

cation

cannot

be bromin

ated even under vigorous conditions.

\

,c-o "\V,.

/ ~?\ c

NBS

NBS

• N.R. R-CF, R-CH or CF

ο

c—ο

s

R

R'

3

R=QH, R-CKorCjH, M-Cr C H

H C

3\_

\^J/

CH'

The

chemical

C

V

C—Ο

inertness

surprising a n d disappointing. nesium metal which

resulted

NBS or Br

1

°\

\ΝΗ—άΗ

• N.R.

7

2/

V

of

the

halogen

atoms

on

these

chelate

rings

T h e s e g r o u p s f a i l e d to r e a c t w i t h l i t h i u m o r

or w i t h a v a r i e t y in decomposition

of n u c l e o p h i l e s e x c e p t of

the

chelate

rings.

under The

forcing

was mag

conditions

trichlorochromium

a c e t y l a c e t o n a t e h a s b e e n c a t a l y t i c a l l y r e d u c e d to c h r o m i u m a c e t y l a c e t o n a t e .


ADVANCES IN

82

p—o

Ç—Q

-\u)#

c, c

CHEMISTRY SERIES

HCI

Pd(OH).

C—Ο

CH; T r e a t m e n t of these m e t a l a c e t y l a c e t o n a t e s w i t h l i m i t e d a m o u n t s of h a l o g e n a t i n g agents i n d i l u t e solutions a f f o r d e d c o m p l e x mixtures of starting materials mono-,

di-, and

trihalogenated


alumina chromatography

chelates.

a n d the

infrared a n d N M R spectra. chelate

mono-

or

ring

halogens,

mixtures

T h e s e m i x e d - r i n g chelates ( V I I ) these

Because

represent

chelates

the

by first

afford routes should

polymers a n d in the

of to

prove

study

of

T h e i n f r a r e d spectra of these a n d

all other m i x e d ring acetylacetonates indicate that the Martell

by

characterized

S u c h substances

synthesis of n o n - c r o s s - l i n k e d chelate

i n t r a m o l e c u l a r , i n t e r a n n u l a r e l e c t r o n i c effects. r i n g is i n d e p e n d e n t o f t h e o t h e r .

and

separated

of the inert c h e m i c a l properties

partially halogenated

difunctional coordination compounds.

useful i n the

were

partially substituted chelates

e x a m p l e s of this t y p e of c o m p o u n d . the

These

v i b r a t i o n of e a c h

chelate

has a s s u m e d this v i b r a t i o n a l i n d e -

(29)

p e n d e n c e i n a theoretical analysis of the i n f r a r e d spectra

of m e t a l

trisacetylaceto-

nates.

CH

CH

3\

ÇHr \

/CH,

3\

c

I

c-o

CH*V ft

CH;

;C-H

X-C.

N-X-

CH;

M=Cr,Co,Rh; X=l,Br,Cl m=1,n=2 m-2, n-1 ;

T h e m o n o - a n d d i b r o m i n a t e d chelates

w e r e also o b t a i n e d b y r e a c t i o n of

the

acetylacetonates w i t h c y a n o g e n b r o m i d e i n the p r e s e n c e of b o r o n trifluoride etherate o r a l u m i n u m c h l o r i d e .

I n the absence of these L e w i s a c i d catalysts

b r o m i d e does not react w i t h these acetylacetonates.

N o trace of

cyanogen

cyanogenated

p r o d u c t s was d e t e c t e d f r o m these reactions. CH,

:—o

ft

Br-CN BF -OEt/ or AICU

VII

7

CH

Nitrations T h e a c i d lability of the acetylacetonates of c h r o m i u m , cobalt, prevents their nitration i n sulfuric-nitric a c i d mixtures media. tures

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

of l a b i l e n i t r o g e n - c o n t a i n i n g c h r o m i u m chelates,

chelate.

Reaction w i t h the N

(VIII),

but in very low yield

2

0

4

. B F

(8).

3

2

0

IX,

and X)

were

rhodium

4

acidic

yielded complex mix-

b u t not the

desired nitro-

c o m p l e x afforded the c h r o m i u m nitrochelate

A mixture of c o p p e r nitrate a n d acetic a n h y -

d r i d e p r o v e d e f f e c t i v e i n n i t r a t i n g t h e s e c h e l a t e r i n g s ( 9 , 10). (VIII,

and

or i n other strongly

obtained in high

yield

T h e trinitrochelates,

b y h y d r o l y s i s of these


reaction

COLLMAN

83

Quasiaromatic Metal Chelates

mixtures.

T h e s e chelates

are of p a r t i c u l a r interest since the p a r e n t l i g a n d , n i t r o -

a c e t y l a c e t o n e , is u n k n o w n .

VIII M-Cr IX M=Co X M»Rh

C; •0 N-C( 2


c—o CH'

CH

IR 1580,1520,1190 cm.'

IR 1570,1520,820 cm'.

T h e i n f r a r e d spectra of the trinitrochelates were characterized b y strong nitro a b s o r p t i o n s at 1 5 2 0

a n d 825

cm.-

a n d b y t h e a b s e n c e o f a b s o r p t i o n at 1 1 9 0

1

T h e N M R spectra of the d i a m a g n e t i c cobalt a n d r h o d i u m nitrochelates e x h i b i t a s i n g l e s i g n a l at 7AT The low

field

c o n f i r m i n g the substitution of all three

(10),

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

cm.

(IX and

- 1

.

X)

rings.

a c o m b i n a t i o n of

electronegativity a n d a n i s o t r o p y of the nitro g r o u p . The acetone

unusual

copper

i t s e l f to f o r m

nitrate-acetic

compound prepared by Nyholm treatment

reacted

copper(II)

with

(V),

acetyl-

the

same

A n o v e l e l a b o r a t i o n of this r e a c t i o n

(18).

of h y d r a t e d c h r o m i u m (III)

d r i d e to y i e l d

a n h y d r i d e reagent

bis-(3-nitro-2,4-pentanediono)

nitrate

with

acetylacetone

i n acetic

a mixture of m o n o - a n d d i n i t r o c h r o m i u m a c e t y l a c e t o n a t e

was

anhy

(XI

and

and

XII

chelates

are

XII).

c-o, q * c Q c-o

χ

"A Ac Ο

•c-o

CH;

X

ArO

M

\^-'/ C-O

/ z

CH

CH,

XI m-1,n-2 XII m-2,n«1 I t is a l s o p o s s i b l e t o p r e p a r e m o n o - a n d d i n i t r o c h e l a t e s s u c h as X I b y use of l i m i t e d a m o u n t s of the n i t r a t i n g agent.

M i x t u r e s of these

conveniently separated b y chromatography on specially treated

Introduction The (25,

of Sulfur into Chelate

acetylacetonate For

26).

s m o o t h l y at

example,

— 1 0 ° to

ring

is

Florisil.

Ring attacked

reaction

of these

by

a

variety

chelates

give h i g h y i e l d s of the

with

of

sulfur

electrophiles

thiocyanogen

tristhiocyanochelates

proceeds

(XIII).

The

c h e m i c a l reactivity of the t h i o c y a n o g r o u p s o n these rings has n o t yet b e e n s t u d i e d . CH

5 N

M=Rh,Co,Cr c—o

NC-S-SCN CICH£H£I

-10°

CH;

Sulfur dichloride

attacks these

trissulfenyl chloride chelates

(XIV)

alcohols, a m i n e s , olefins, a n d p h e n o l .

,/ - N C - S - C' ^ ^ ; M

3

c-o CH'

chelate

rings

under mild

conditions a n d

the

h a v e b e e n r e p o r t e d to r e a c t w i t h a v a r i e t y of T h e p r o d u c t s of these reactions h a v e , for the


ADVANCES IN CHEMISTRY SERIES

84 most

part,

mixtures

not been

purified

a n d characterized,

probably

resulting f r o m the trifunctional reactant.

because

Treatment

chlorides with cyanide ion yields the thiocyano derivatives (XIII)

S C I . , / ' \\ / ^-c.s-c

Reactions of Coordinated Ligands and Homogeneous Catalysis

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