Substituent effects and the stability of chromarenes

381

Journal of Orgonometallic 0 Elsevier Sequoia

Chemistry.

S.A., Lausanne -

SUBSTITUENT

EFFECTS

Nguyen

Hao

161 (1978) 381-390 Printed in The Netherlands

AND

THE

and

Michael

Department

STABILITY

of

McMaster

CHROMARENES

J. McGlinchey*

Chemistry

University

Hamilton,

Ontario

Canada (Received

OF

L8S July

4Ml 18th,

1978)

Summary The and

the

with

chemistry unsymmetric

the

proposed

sum

of

that

stability

Hammett

the

is supported

to that

the

an

chromium

polarographic

_ ____ _.__--_ ____~_

can

(Ar2Cr)

be correlated

substituents.

exhibit

of the

and

chromarenes

ArCr(C6H6)

of

chromarenes

by NMR

symmetric

type

am parameters

analogous

_____._____~~_

of the

of

unsymmetrical

effect

postulate

air

chromarenes

the

the

stabilisation This

and

It is

internally

compensating

tricarbonyl

moiety.

evidence.

_______.-_

.__

Introduction

The widespread among

other

complexes substituents

things,

into new

chromarenes

have

are

now

metal

vapors

to a resurgence

(chromarenes)

fascinating

system

use of

this

hitherto

chemistry. been

becoming

The

(5).

Over

prepared, apparent.

of

as synthetic interest

ready

almost the and

reagents

in the

incorporation unexplored

past some

five general

has led,

chrcmium

- arene

sandwicl

of a wide

variety

of

system

years,

(l-4)

has

more

chemical

revealed

than trends

much

60 substituted within

the

382 Discussion (a)

Symmetric chromarenes The existence

Pr2Cr

,

of this (a)

series

of two effects,

viz.,

is a reflection

of the strength

barrier

of compounds depends upon the subtle

the thermodynamic stability

to decomposition

The thermodynamic stability by methods such as calorimetry from crystal lographic although

announced this

will

with molecular

actually

been studied

of vibrational poration

to conclude

vibrations

(8).

Valuable

substituents

predictions

for

available

in symmetrical shifts

was less

substituents,

by the enhanced thermal stability

when n exceeds

(11) and by the failure In essence,

2.

electron-withdrawing

have is that

that the incorchromarenes of in-plane

He was thus able pronounced

and consequently

the arene-chromium bond was weaker than in (C6H6)*Cr itself.

substitution

available

technique

of the aryl

benzene itself.

that the chromium to arene back-donation

case of chromarenes with electronegative

supported

obtained

are clearly

has demonstrated

complexation

than is observed

of bond lengths

but again few molecules

and Kiabunde (10)

to smaller

experimentally

change - indeed some workers have already

calculations,

- withdrawing

the kinetic

few data are currently

Perhaps the most readily

(9).

which

of the chromium atom.

be investigated

Unfortunately,

goal

orbital

the type (C6H4X2)2Cr leads

in principle

undoubtedly

spectroscopy,

of electron

stretching

(7).

as a long-range

possible

could

oxidation

(6) or by a comparison

studies

the situation

of the molecule,.

of the arene - chromium bond, and(b)

via the one-electron

interplay

in the

that

This View is

of chromarenes with increasing

to isolate

complexes

of the type (C6FnH6_n)2Cr

the -i-basici ty of arenes with a mu1titude

substituents

is apparently

alkyl

of

too low to lead to isolable

sandwich compounds of the type Ar2Cr. In contrast, rings barrier

the presence

should retard

of electronegative

the one-electron

should correlate

well with the oxidation

measured pol arographi cal ly (10) . of al kyl groups into

oxidation

to Ar$r+ potential

the arene rings

leads

in the arene

and the activation

energy

1

which is perhaps best

Russian workers have shown

that

the

to a shif t of the half-wave

(E ) towards negative values of the potentials. 1 be directly proportional to the number of alkyl per al kyl group( 12).

substituents

incorporation

potential

The change in ES was found to groups introduced

and was 0.3V

/ 1 I I

383

Another

physical

distribution

of electron

'H (13,14)

study

factors

carbon

considerably

was

also

realised

of

chromarenes

these

data

It appears that

that

reduced,

with

from

a somewhat

the

facts

and

19

of the

range

kinetically

in their

documented identity

of the

isomerism

is less

important.

by a number

to of

Thus

the

stabilities

the

sum

the

Hammett

As

can

of the

inductive

from

indicator

simple

incorporate

not

an unreasonable

should

correlated

be available

(10).

(but

Figure

of

and

are

still

can

now

with

Zo,,

(Xi),

was

the

the

that

Hammett

the

of

realistic desired

and we trust

the

(5,10,13,15-18).

the

and

is the

ortho,

of the

given

also

seemed

purpose work

with

we

chose

well).

- .X:0,- is an Using

substituents

of the

complex.

one It is

polarographically

appropriate

4, 4

meta

chromarene.

on the

1, l',

chromarenes

this

values

obtained

be

and

is crucial

For

stability

that

oxidation,

parameters

the

only

inertness

It therefore

limits

es values

will

so also

of

(20)3

stability

place

but

substituents.

of the

chromarenes

3, 3' - tetrakis

stability

Swain-Lupton

sum

maintain

prediction

well

the

1, the

prediction,

symmetrical

which

(15,16).

oxidative

of

the oxidative

one

comparable the

parameters

(19)

concept,

can

oxidative

to correlate

urn values

be seen

excellent this

to attempt

susceptibility

kinetic

I, l',

isomer

reasonable

was

shifts

aerial

substituents

air-stable

complexes

density

laboratories

is remarkably

parafluorotoluene

to the

complexes

The

(trifluoromethylj-chromarene

and

examined

effects

This

the

inert

resistance

it is the

manner,

most

of symmetrical

enhanced

F NMR

of

of substituents.

that

in like

that

to be expected.

a picture

been

(10);

number

v-electron

and

the

carefully

substituent

arene

was

have

positional

well

free

(16) who

vein,

and

(17).

stable

is reflected

In this

of

as to the

spectroscopy

in a large

that

an analysis

these

limited

is NMR

Lagowski shifts

ring

information

molecule

transmission

the

some

reported.

and

of the

on

thermodynamically for

been

experimentally

emerges

formed the

from

in agreement

there

the

give

the

C chemical

that

substitution

subsequently

in which

I3

drastically

modified

to nucleophilic

Thus,

was

have

of Graves

the

They showed

C-4 ring

which

is that

might

within

spectra

influencing

chromarenes.

which

density

lg F (15)

and

comprehensive the

technique

data will

soon

384 (b)

Unsymmetrical While

it is clear

behaviour systems

of a given of the

negative Thus,

groups

(see

Figure

(C6H3F3)2Cr

there

arene

the

presumably

has

by the

x-C6H6

rings

for

ring.

in the molecules

has

also

been

the

shown

molecules

moiety

pushes

electron

(C6F6)Cr(C6H6) Thus, also

kinetic

Cr vapor,

must,

Cr(N0)4

and

showed

always

work

is heavily

the

a nice

K - C6H6

substituted

when example has

relative with

inter-ring

between JHF

effect

(as

is

(5);

constan

bases

in the

that

ring

two

Furthermore,

by strong bands

buffe

the

coupling

the

this

to nucleophilic

to three

(18),whereas Indeed

a more

chromium

n = 4, 5 or 6 (5,17).

comparable

made

full

(17).

infrared n-(C6H6)C

is also attack

of

(17).

mixture

the

the

complex

was

(18)

phenomenon

electronegative

analogous

the

groups,

can

of thermodynam reaction

where

favored

of

n = 0 through product.

that

the

cocondensation

route

has

been

One meth

successful;

(18,23).

to donate

deficiency

which

molecule

interplay

C6FnH6_,,

conventional

electron

ligands

cocondensation

of arenes

capacity

PF3

subtle

by the

by Timms

of this the

interaction

fluorinated

exemplified

point

group

the

be

the

the

electronically

(C6F5M(CO)x)Cr(C6H6)

difluorobenzene

the

even

C6F6

unknown.

equimolar

extent,

susceptibility

moiety

is nicely the

and

chemistry

unsymmetrical

by the

vCO

molecules

their

whereby

deprotonated

electro-

sandwich

and

in the

of corresponding

into

must

some

unsymmetrical

(C6FnH6_,)Cr(C6H6),

21)

solely

large

is readily

reduced

a-(C6F6)Cr

recall

to satisfy

ring

that,

electronic

where

18,

to the

mere

ArpCr

mechanism

to

surprisingly

to free ring

be,

in the

synthesized,

tolerated

is clearly

somewhat

that

provides If the

demand

the

however,

not

the

effects PF3

6, which

can

is currently

(C6F6)Cr(CO)3 and

but

density

F-C~H~

stabilise

ArpCr)

C6F5M(CO)x

relative

the

be

that

incorporate

(5, 17,

It seems

is crucial

in the

been

available

by a comparison

of the

with

so far

not

C6H6

spectra

consistent

not

need

by the

to

is possible

(C6FnH6_n)Cr(C6H6),

in (C6F6)Cr(C6H6),

noticable

a self-compensating

There

as is evidenced

substituents

it is possible

(5,22).

exists

case

of the

it is also

readily

substituents

the

role

1) than

beenextensivelystudied

chromarenes.

may

the

ArCr(C6H6),

n = 4, 5 or 6, are

has

of

that

ArCrAr'

chromarene,

type

although

when

chromarenes,

electron of

its

this

density

arene

should

partner

on which

be reflected

in

385

the

NMR

spectroscopic

withdrawal the

of electron

aromatic

Such

effects

ring

have

course,

2.0-

=0--

in which

that

all

the

a complicating

this

%FsPh - _ _

C6F4HCO2R C6F5H---

_ 1 _ :=

- -__ - -

C6F4Ph2 - - -

_

-

spectra

CSHS

ring

might

the

case

(see

(24)

the

Thus,

Table

in a series alkyl

shifts

increasing

be expected 1 and

are

in one

quite

recorded

under

identical

here

is that

changes

in the

the

to deshield Figure

2).

of ethyi-substituted

groups

are

small

ring and

shields it is

conditions. CsHs

Of

to chromium

&,Cr

_

-_--

-

_A--

- C6 H4(C F3j2

x

---

-

__----

C6H3F3

- - C6H4CiCF3 C6e4FCF3 - - - C6k?4Ci2

- - - - C6H4FCf -----C6H4F2

_ - _‘=

C6F4(SiMe3h

- - - -

C6F4RUt2 - -

-

C&J”~

ring.

of extra

However,

other.

C6F4HSiMe3

-

TT - C&

previously

introduction

factor

the

is in fact

C6F&O2Et)2 C6FgCHO. .‘\ C6F5COMe _ J:,, _ C6F5CO$t- - 2‘ C6F5C025 I’-_- C6F6 - -

c6F4H2

IO-

the

of the from

noticed

in the

C6F55iMe3 C6~5C02-

IS-

and

been

protons

essential

density

protons

chromarenes the

properties

-

_ _ ____ -c&CF3

__--

- C6 ti5C1 ~:=~-C6H5CO2R -_--C6H5F ------_@&fMe

0.5-

&CdCgHg)

__--

-C6K50Ph

----

-C6tt5oMe _C6H4(AlejO

_/-_/--

o.o-

F_;g 1.

H C6H6 ,CgHgSiMe: - - -C6H5RUt - C6u5~f2

--_:-----

The

sum

of the

chromarenes.

am

values

of

a series

of

symmetrical

and

unsymmetrical

bonding

80

85

9OF

4.2

4.3

4.4ppm

75

20t

1.5 1

ztr

1

7.0

0.5 E t

0.0 -

Fig 2.

Plots

of the sum of the om values ‘H and l3 C NMRchemical

versus

could

also

alter

of directly density

the ring

anisotropic

(25).

In particular,

although

the protons,

to some extent A much clearer

in

the

series

the I3 C shifts good,

leading picture

a diminution

and so these to a less

a withdrawal

of the ring current

two effects

than unequivocal

in which the correlation

and the number of fluorine

(1” = 0.9997).

contributions

tend to nullify result

emerges from the 13C NMRshifts

(C6F,H6_,)Cr(C6H6)

Although it

of ArCr(C6H6)

of the C6H6 ring.

and the local

lead to an apparent shielding other

shifts

current

bonded protons

should deshield

of the substituents

to the shiof electron

would each

(26).

of the C6H6 moiety (see

atoms in the other

Figure 2) between

ring

is remarkably

is true that the use of 13C NMRspectroscopy

1 :

387

to interpret

changes

some

(27),

caution

it can lists and

in bonding in organometallic nevertheless,

be an excellent the

I3

'H and

it is noticable

1.

Table

NMR

probe

C NMR

for

the

shifts

shifts,

Shiftsa

the

reservations

a series

chemical

that

Chemical

with

for

systems

of closely for

a series

in genera?,

the

C6H6

must

discussed related

with

by Lagowski

with

(16)

Table

molecules.

of molecules

correlate

ring

be treated

ArCr(C6H6)

"urn and with

the

of ArCr(C6H6) ______-

___-

-Ar

-lH

______~_._

1% ._~~~

____-___

C6F4(C02W2

4.64

88.6

C6F5CO-fury1

4.71

87.6

C6F5C02Et

4.54

87.0

4.41

85.8

4.37

84.0

4.33

82.2

4.12

74.2

c6F6 'gFgH 'gFqH2 b CsH6

-__

____~__ a All

spectra

recorded

as 5% solutions quoted b

Data

oxidative

stability

electronic the

Refs.

inductive

that

effects

parameters

with

13C

of the of the

ways,

this w co

TMS

spectrometer; as

the

all

internal

samples

standard

run and

standard.

16.

complex

effect

In many

this

14 and

apparent

electronic

chromarene.

of

of the

buffering

NH-90

containing

downfield

from

It is thus

for

in C D 66

in ppm taken

on a Bruker

(see NMR

Figure

unsubstituted substituents

effect

in arene

1).

spectroscopy

parallels chromium

provides ring

in the the

a measure

as it tries other

ring

well-known

tricarbonyl

tc

of the compensate

of the correlation

systems

(28).

Epilogue These

relatively

simple

ideas

using

substituent

parameters

1

to correlate

of

388 molecular

reactivity

of obtaining

certain

it is now

apparent

cbromarene

(22),

using

not

with

that with

in the

a view

acyl

the

but

hydroxyl

the

a separate

of

stability

it not

this

to the

However,

although

involving

the

be favored.

chromarenoic

acid

a-complexed

tetrafl

by elimination Thus,

other

elsewhere

the

of routes

for

have

1, 2, 4,

a polyamide,

chromarene-containing

Progress

value.

be

the electron

-

amino groups,

possess

in this

copolymers must

monomer.

should

Thus,

5 - tetrafluoro-

substituents

electron-donating

monomer

or difficulty

organometallic

the

to prepare

ease

predictive

of

air-stable

the

the

some

monomers,

than

substituents.

area

Likewise

the

wiil

acid

for

or ester

be reported

in

(29).

for

pentafluorochromarenoic value

rather

make predictions

Thus,

.

of

chromarene-containing

We can also

also

to producing

in the

publication

chromarenes

may

Hence,

linkages,

polyesters,

a rationale

difunctionalisation

as one

be incorporated

not

but

some forethought.

withdrawing would

provide

compounds

a fluorochromarene

chosen

‘Orn

only

about years

several

acid

molecule,

with

it

the

2.055,

the

desired

intent:

would

strongly

molecule

base

yields

not

the

uorobenzyne

the

acid

carboxylate This ion

as yet

unsynthesised

it

to

pK, value.

be of

comparable

and hence

probably

stable

the ethyl

from the

enough,

routes

of

need to be explored

to would

from the

presumably

produced

and these

5

pentafluoro-

derivable

intermediate

4, The

isolable.

ester

initially

3,

readily

functionality

but products latter

2,

its

carboxylate of

(30).

CO2 and fluoride to

(22)

is

hydrolysis

of measuring

indicate

electron-releasing

Indeed

of

has been our aim to obtain

known (C6F5C02Et)Cr(C6H6) the

stabilities

are

arise

carboxyl

ate.

discussed

(22).

In conclusion, stability

and

substituent

chemistry

parameters

provide

a useful

probe

as to the

of chromarenes.

Acknowledgements We thank

Council

of

Mr.

B.G.

Canada for

Sayer financial

for

the

13

support.

C .NMR Spectra

and the National

Research

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G-A.

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le