Metallocenyl di-carbonium ions -13C and1H NMR spectra of 1,1′-di(1-methyl-l-ethylium)ferrocene dication

C3

Journal of Organometallic Chemistry, 174 (1979) C3--C10 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

Preliminary

communication

Metallocenyl

Di-Carbonium

Ions - 13C and IH NMR Spectra

of l,l'-di(l-methyl-l-ethylium)ferrocene

Dication

C.R. Jablonski Department of chemistry Memorial University of Newfoundland St. John's, Newfoundland, Canada AIB 3X7 (Received March 27th, 1979)

Summary 13C and IH NMR data for a heteroannular ion are presented.

The results are consistent with increased

ization of positive charge on C mono-carbonium

ferrocenyl @,e-di-carbonium local-

compared to related ferrocenyl

ions.

The exceptional

stability of ~-metallocenyl

carbonium ions, l,

has posed a long standing structural puzzle which, over the last 20 years, has been approached

from a number of v i e w p o i n t s I.

More

recently 13C NMR data 2-5 have been reported for a number of examples

M

M=Fe, Ru, Os

C4 and used, in conjunction with the established

relation of 13C chemical

shift and carbon electron density 6"16, to establish the electronic origin of the stabilization.

Carbon chemical shifts are anticipated

to provide a more sensitive probe of electron deficiency

in carbonium

ions since the largest changes in electron density occur at C not H 6. We present here our preliminary d-ferrocenyl

carbonium

data relating to both mono and di

ions prepared by protonation

(isopropenyl)ferrocene,

of l,l'-bis-

3.

Addition of methylene

chloride-d 2 solutions of ~ to cold

stirred solutions of excess trifluoroacetic gave bright red solutions

(-78°C),

acid and sulfur dioxide

containing a mono-protonated

ion*

(cf. Fig. i)°

Both IH and 13C NMR spectra are in accord with mono-protonation clearly show the presence of one remaining Tables 1 and 2). decoupling

isopropenyl moiety

Chemical shift assignments

experiments

and (cf.

are supported by homonuclear

in the case of the IH spectra and by the

observation of both proton coupled and specific

13C {IH} decoupled

13C spectra. Further corroboration Tables 1 and 2 derives

of the correctness

noticeable

9

The IH chemical

, however the isopropenyl

substituted

shifts

ring shows

(0.5 - 0.6 ppm) downfield shifts compared to 3 for both ring

and olefinic protons. and ~

in

from the IH and 13C NMR spectra of the corresponding

2,2'-d 2 derivative prepared 8'9b as in Fig. ~. of 5 are unexceptional

of the assignments

A similar situation prevails

for the 13C shifts

values of 5 where the ring and terminal olefin carbon are

deshielded by i0 ppm compared to 3. In stronger acid di-carbonium

(FSO3H/SO2),

deep purple solutions of the s,~'-

ion 4 are formed at low temperature I0.

ion 4 is much more sensitive

than the mono-carbonium

degrade severely at temperatures greater than -30°C.

*The related l,l'-bis(~-hydroxyethyl)ferrocene under similar conditions,

cf. ref. 7.

The di-carbonium ion 5 and spectra Proton spectra

gives only the mono-cation

C--Me

0

/

Fe

Me Li

-

2_.~a

CMe20D

2

13ram Hg

AI203, 160°C

Ph~P- C.H~

®®

Me20H

@Me20H

Fe

~

I X.S. C4HgLi

13mm Hg

AI203,160°C

AND 5

CMe20D

4

N D4c I

D

Fe

~ ~

D

PREPARATION OF IONS

C -Me\

Fig I.

/

3_.

Fe

D

Fe

D

3o xs. FSO3H

=_

CR3CO2H

XS.

/so~,-~

5

415 Fe Me ,

4

~e

4'

3~/~Me 4 15

C6 TABLE 1 IH CHEMICAL SHIFTS FOR FERROCENYL MONO- AND DI- CARBONIUM IONS

Cation

6(ppm) a H2, 5

H3, 4

5.96

6.60

Other

4b (FSO3H/SO2/CD2CI 2)

2.58 (-Me)

-80°C Me 2.23 (--~Me)

5 (TFA/SO2/CD2CI 2)

5.05

6.13

H2',5')

4.90 -80oC

(H3,,4,

1.87 (Me) 5.30 (Hci s ) 5.45 (Htran s)

1 R 1 = R 2 = cH3C

5.00

6.33

4.88 (Cp) 2.28 (Me)

aTMS (int) = 0.0 ppm; Spectra recorded on a Bruker WP-80 operating at 80 MHz with CD2CI 2 internal lock. bchem, shifts of 7.25, 6.59 and 3.20 are reported for 4, cf. ref. i0. Ccf. ref. 9d.

(Table i) resemble that previously reported by Pittman I0, however there is a considerable discrepancy in the chemical shift values** most probably arising from different reference standards. Proton chemical shift assignments for 4 agree with those previously reported I0 and are confirmed by comparison with the 2,2'-d 2 derivative

**d ppm -- (this work - ref. 10) = -0.65 ppm, shifts in this work are relative to internal TMS = 0.0 ppm.

f 101.2

78.7

94.7

[25. i]

(28.0)

95.3

[30.8]

(30.6)

97.9

C(3.4 )

[A~, ppm] c

158.6

[16.7]

(88.9)

157.8 e

[76.7]

(148.9)

217.8

C(~)

{99.2~ d 98.9"

C(l,)

C(3,,4, )

82.9

{77.4] d 77.8"

C(2, 5, )

-

-

C ,

13C spectra of ~ and ~ were determined in CDCl 3 with internal TMS reference.

fcf. ref. 2.

eThe assignment of C

and=C__.'for ~ a r e

tentative.

ddefinitive assignments were not possible;

c[A] = ~cation - 63;

b(~] = ~cation - ~2

(H2C=) (-c'Me) e -~

27.8 (Me)

135.8

117.1

26.9 ~_e 20.4 (=C,)

31.0

Other

aTMS (int) = 0 ppm; Spectra recorded on a Bruker WP-80 operating at 20.1 MHz with CD2CI 2 internal lock, 8K data table and t a = 0.819 sec.

1 (R1 = R 2 = CH 3)

[15.5]

i1.8 [ii. 5 ]

-80°C

82.6 (16.7)

d

-i.0 (_1.3)

~99.2~ "98.9"

5

[18.2]

(19.4)

85.3~

C(2,5 )

(FEA/SO2/CD2Cl 2)

[7.2]

(-5.6)

94.6

C(I )

~(ppm) a, (A6, ppm) b

-80°C

(FSO3H/SO2/CD2CI 2)

4

CATION

13C CHEMICAL SHIFTS FOR FERROCENYL MONO- AND DI- CARBONIUM IONS

TABLE 2

-a

C8 w h i c h shows a r e d u c e d i n t e n s i t y triplet 5.96 ppm.

(M of an A , A ' M X system) at

Of i n t e r e s t is the o b s e r v a t i o n that a l t h o u g h H2, 5 and H3, 4

of 4 are less s h i e l d e d than the c o r r e s p o n d i n g p r o t o n s in 5 or 1 (R 1 = R 2 = Me) 9d, the d i f f e r e n c e in chemical s h i f t b e t w e e n them is s m a l l e r

(A94 = 51 Hz, A~ 1 = 106 Hz).

S i m i l a r effects are o b s e r v a b l e but a t t e n u a t e d in the 13C NMR s p e c t r u m o f 4.

(Table 2).

However, w h i l e C2, 5 and C3, 4 are less

s h i e l d e d in 4 c o m p a r e d to 5 o r !

(R1 = R 2 = Me), C 1 is m o r e shielded

p o i n t i n g to s u b s t a n c i a l o l e f i n i c c h a r a c t e r of the exocyclic b o n d 2.

The

m o s t s t r i k i n g d i f f e r e n c e b e t w e e n 4 and the r e l a t e d 3 ° m o n o - c a r b o n i u m ion is, nevertheless,

the extreme d e s h i e l d i n g

(217 ppm in 4, 178 p p m in

(R 1 = R 2 = Me)) and c o n c o m i t a n t large A~ values e x p e r i e n c e d by C .

Conclusion If 13C chemical shifts are i n d i c a t i v e of the extent of charge l o c a l i z a t i o n in t r a n s i t i o n m e t a l ~ - b o n d e d systems II'12, one can infer t h a t s i g n i f i c a n t l y m o r e charge l o c a l i z a t i o n occurs at C

in the

d i - c a r b o n i u m ion 4 than p r e v i o u s l y r e p o r t e d in 3 ° ~ - f e r r o c e n y l m o n o c a r b o n i u m ions.

The m e c h a n i s m o p e r a t i n g to d e s h i e l d H3, 4 and C3, 4

r e l a t i v e to H2, 5 and C2, 5 in the m o n o - c a r b o n i u m ions ! is m a n i f e s t in the d i - c a r b o n i u m ion 4. The r e s u l t s are in accord w i t h a m o d e l for 1 w h i c h i n c o r p o r a t e s c o n s i d e r a b l e i n t e r a n n u l a r transfer of e l e c t r o n density toward the s - c a r b o n f r o m a r e l a t i v e l y e l e c t r o n rich F e C p m o i e t y . * * *

In the

l i m i t the s y s t e m m a y be c o n s i d e r e d as an F e C p I+ m o i e t y ~ - b o n d e d to a 6~ fulvene.

R e c e n t e x t e n d e d Huckel SCC calculations 13'14 for 1

(R1 = R 2 = H) as w e l l as ESCA and M o s s b a u e r d a t a 15 i n d i c a t e that the p o s i t i v e charge is w e l l d i s p e r s e d and, in k e e p i n g w i t h the electro-

***The C p carbons suffer an a p p r o x i m a t e 15 p p m d o w n f i e l d s h i f t upon f o r m a t i o n of ! cf. Ref. 2.

(R 1 = R 2 = Me) from its c o r r e s p o n d i n g alcohol,

69 neutrality principle, Fe suffers a relatively modest increase in positive charge (+0.2) on going form ferrocene to !

(R1 = R 2 = H).

In the di-carbonium ion 4, Fe cannot sustain a much larger positive charge than it does in ! ization at C

(R1 = R 2 = Me) resulting in increased local-

which begins to approximate the carbinyl carbon of

PhNe2 C+ (6 = 254.3 ppm) 16 A reasonable but admittedly somewhat speculative structure for 4 which both accepts the NMR evidence presented here and follows the precedent established in recent related examples 17'18'19 contains a bent exocyclic bond with an overall anti conformation which minimizes interannular electrostatic repulsion.

Acknowledgement The author is greatful to the Natural Sciences and Engineering Council of Canada (NSERC) for a grant in support of this work. Generous financial assistance from Memorial University as well as NSERC allowing the purchase of an FT NMR spectrometer is also appreciated.

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