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UV and ESR Studies of p-Bis(trimethylsilyl)- and p-Dialkylbenzenes.

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The I R spectrum measured in the NaCl and KBr regions
shows strong absorptions at 1208, 1298, and 1972 cm-1 and
bands of medium strength at 511, 843, 930, 956, 2043, 2395,
2500, 2633, and 2930 cm-1.
The 19F-NMR spectrum at 30 "C contains one very broad
signal, which becomes narrower with decrease in the temperature. At -60°C the chemical shift amounts to -55.1 ppm
referred to CC13F as internal standard.
The mass spectrum of (3) contains the typical fragments
SC-NCS ( m / e = 102), CNCS (70), SCF (63), CS (44), and
F C (31), as well as the molecule ion at m / e = 121.
An equimolar mixture of (3) and chlorine reacts at -70 "C
in a bomb tube in an extremely short time, the color becoming
paler and the chlorine being consumed quantitatively. Elemental analysis and the IR, 19F-NMR, and mass spectra of
the yellow liquid product showed that FCI(SCN)C-SCI ( 4 )
is formed quantitatively in this reaction; this compound boils
at 53 "CjlO mm and has the typical odor of sulfenyl chlorides.
The IR spectrum contains a strong absorption band at 1958
cm-1 and bands of medium strength at 530, 575, 811, 1040,
and 1165cm-1. The 19F-NMR spectrum of ( 4 ) at 3OoC
contains a single signal with chemical shift +44.7 ppm referred to CC13F as internal standard. The mass spectrum shows,
inter aliu, fragments FCl(S)C-NCS ( m / e = 156), FClC-NCS
(124), F(S)C-NCS (121), and SCClF (98).
Received: May 26th and June 5th, 1967
[Z 540b IE]
German version: Angew. Chem. 79,978 (1967)
Publication delayed at author's request
*! Dozent Dr. A. Haas and Dip1.-Chem. W. Klug
Anorganisch-Chemisches Institut der Universitat
Hospitalstr. 8-9
34 Gottingen (Germany)
[l] A. Huas and H. Reinke, Angew. Chem. 79, 687 (1967);
Angew. Chem. internat. Edit. 6,705 (1967).
Half-wave Reduction Potentials of
2,5-Bis(trimethylsilyl)- and
2,5-Di-t-butyl-l,Cbenzoquinone[**I
By H. Bock and H. A f t [*I
The interpretation of d-orbital effects
is made difficult in
the case of phosphorus-nitrogen compounds according to the
oxidation state and coordination number of phosphorus by
the presence of free electron pairs, positive charges, or partial
double bonds. However, the d-x and d-x* interactions and
their differences can be investigated by comparing siliconsubstituted x-electron systems with the corresponding alkyl
derivatives using suitable methods of measurement [21. Thus
the half-wave reduction potentials Ell, are related to the
eigenvalues of the lowest unoccupied levels of n-electron
systems into which the electron is inserted on reduction [31.
The prerequisite of a reversible redox equilibrium is fulfilled
for alkylated p-benzoquinones 141.
The 2,5-bis(trimethylsilyl)-l,4-benzoquinone (5) needed for
comparison with the 2,Sdi-t-butyl derivative was obtained
as follows: ( I ) is converted with trimethylchlorosilane/
qBr
- &rBr RQSiO
Br
OH
(1)
R,SiO
R3si@siR3
OSiR3
R
H
C(CH,),
Si(CH3),
+0.04
-0.15
+0.04
-0.47
-0.71
-0.51
The results show that the silicon-substituted quinone and
semiquinone have higher oxidation potentials than the alkyl
derivatives, i.e. they are stronger electron-acceptors. This is
inexplicable if one assumes exclusively an inductive effect
(+ISiR3 > + I c R ~ ) [ ~ Jand indicates an additional d-x*
interaction between the vacant 3d levels of the silicon and
the lowest unoccupied or singly occupied molecular orbitals
of the p-benzoquinone x-system.
This type of model, already postulated several times171, for
the Si-C,,z
bond, which requires Sid c C, back-donation
of electrons alongside the 8% + Cs- inductively polarized
5 skeleton, is in accord with the results of further polarographic measurements on silyl- and alkyl-butadienes, -styrenes, -naphthalenes, and -phenyl ketones 121.
Received: July 24th. 1967
[Z 591a IE]
German version: Angew. Chem. 79, 932 (1967)
[*I Priv.-Doz. Dr. H. Bock and Dip1.-Chem. H. Alt
Institut fur Anorganische Chemie der Universitat
Meiserstr. 1
8 Miinchen 2 (Germany)
[**I Part I of d-Orbital Effects in Silicon-substituted x-Electron
Systems.
[I] Cf. H. Bock, Chimia 21, 35 (1967).
[2] H. Bock, H. Alt, [J. Krjnitz, and H. Xidl, unpublished.
131 Cf. A. Sfreifwieserj r . : Molecular Orbital Theory for Organic
Chemists. Wiiey, New York 1961, p. 173f.
[4] M. E. Peover, J. chem. SOC.(London) 1962, 4540.
[ 5 ] P. H. Given and M. E. Peover, J. chem. SOC.(London) 1959,
1602.
[6] H. Bock and H . AIt, Angew. Chem., in press.
I71 Cf.,e.g., M . D . Curtis and A. C. Alfred, 3. Amer. chem. SOC.
87, 2554 (1965).
OSiRS
131
(2)
pyridine in boiling toluene into the ether (2) (m.p. 58-59 " C )
which reacts with sodiumjtrimethylchlorosilane in boiling
toluene to form (3) (m.p. 121 "C). Cleavage of the O-SiR3
bonds with 4 M sodium methoxide in methanol at 20 "C leads
after neutralization with diluted acetic acid to ( 4 ) (m.p. 183
to 186'C), which can be oxidized with silverli) oxide in
benzene at 20 "C to (5).
The 2,5-bis(trimethylsilyl)-l,4-benzoquinone (n + x * : vm =
19999 cm-1; E~ = 25 1 mole-1 cm-1) is orange in contrast
t o the yellow 2,Sdi-t-butyl compound (n +x* : vn, = 21 900
cm-1;
= 36 1 mole-1 cm-1); it melts a 188 OC and gives
two N M R singlets at T = 3.24 and 9.77 in the required ratio
2: 18. The half-wave reduction potentials for the two steps (I)
and (11) in acetonitrile were measured with tetra-n-butylammonium iodide as conducting electrolyte against the
constant 151 mercury anode.
U V and ESR Studies of p-Bis(trimethyIsily1)- and
p-Dialkylbenzenes[**I
By H. A h , H. Bock, F. Gerson, and J. Heinzer[*]
A notable feature of the U V spectrum of p-bis(trimethy1sily1)benzene ( I n ) , when compared with the UV spectrum
of p-di-t-butylbenzene, is that the lLa band (lBlu -+ lAlg)
Angew. Chem. internat. Edit.
Vol. 6 (1967)
1 No. I 1
94 1
which is polarized along the substituent axis is shifted bathochromically and considerably intensified.
I
lLb
I
'La
Em
vm (cni-
~
(la)
(Ib)
_
_
value of 21.2 gauss and Hiickel spin densities ( C J ~ )of~
1/12 and 1/4, respectively; this proves the occupation of
(Js in (2.) as required by the -M effect of the R3Si group,
and the occupation of (Jas of (26) as required by the +I
effect of the H$ group. The d-x* splitting thus demonstrated
for the radical anion (2n) follows - although different electron interactions have to be considered - for the neutral p bis(trimeihylsily1)benzene ( l a ) from the electron spectra too.
(1 mole-' cm-1)
_
36250
37050
37 900
38750
390
480
460
340
43 500
44250
36900
38 000
39050
40000
310
340
240
I60
45 850
46950
14700 (shoulder)
16 850
8 850
9 400
I
The difference in 'La transition energies amounts to 2700cm-1
= 7.7 kcal/mole and can be interpreted as the difference
between the d-n* and d-x interactions in ( l a ) if the varying
inductive effects are neglected.
In general "1, electron spin resonance measurements for
radical anions permit conclusions to be drawn about the
unoccupied non-bonding or antibonding states of the original
neutral molecule and thus about the electronic effects
of substituents. For instance, from a comparison of the ESR
spectra of silyl- and alkyl-substituted benzene [21 and biphenyl
radical anionsr31 it was concluded that a Sid + C, backdonation of electrons counteracts the inductive 8% +C*polarization 141. I n the case of p-bis(trimethylsily1)benzene
( l a ) such a -M effect of the R3Si groups, which is already
indicated by differences in the electron spectra of ( l a ) and
(Ib), should be demonstrable for the monoanion (2a) by
ESR spectroscopy. For biphenyl derivatives [31 interpretation
is made difficult by twisting effects, and for monosubstituted
benzene derivatives [21 splitting of the initially degenerate
antibonding eZu orbitals $s and $as may be too small to
exclude vibrationally induced mixing with energy-richer
states; but p-disubstituted benzene derivatives are planar and
show a considerable energy difference between CS and $as
orbitals [5J. Owing to participation of unoccupied Si-3d states
in the x-system of the benzene radical anions the ( J ~ molecular orbital that takes up the unpaired electron should thus
be more strongly depressed, in this way increasing the stability
of the radical. In accordance with expectation, a solution of
p-di-t-butylbenzene (Ib) in potassium/l,2-dimethoxyethane
at -70 "C shows the ESR signal of only the solvated electron,
whereas the ESR spectrum of the p-bis(trimethylsily1)benzene
radical anion (2a) (Fig. 1) can be observed up to more than
0 OC.
H,c-$~-cH,
H-C-H
H , c - ~ -CH,
H-
I
-
CH,
ha)
t
LI - H
H
I
I
-sir
-C-
-si-
-C-
I
I
Fig. 2. MO scheme of inductive and mesomeric substituent effects on
the antibonding molecular orbitals (ezuf of p-disubstituted benzene
derivatives. Hiickel coefficients cJ2 of the symmetrical (s) and the
antisymmetrical (as) molecular orbitals.
However, experimental verification of a d-n interaction in
p-bis(trimethylsily1)benzene encounters difficulties. Attempts
to determine approximately the ionization potentials of ( l a )
and (Ib) from the longer-wavelength band of the chargetransfer complexes failed because this band was always overlapped by the shorter-wavelength band of the charge transfer
complex to give a single band of large half-width. The -M
effect of R3Si groups in the ground state can, however, be
proved by means of charge-transfer complexes of mono- or
di-substituted benzene and naphthalene derivatives 171.
Received: July 24th, 1967
[Z S91b IE]
German version: Angew. Chem. 79, 933 (1967)
[*] Dip1.-Chem. H. Alt and Priv.-Doz. Dr. H. Bock
Institut fiir Anorganische Chemie der Universitat
Meiserstr. 1
8 Munchen 2 (Germany)
Priv.-Doz. Dr. F. Gerson and Dip1.-Chern. J. Heinzer
Laboratorium fur Organische Chemie der Eidgenossischen
Technischen Hochschule
Universitatsstr. 6
Ziirich 6 (Switzerland)
[**I Part I1 of d-Orbital Effects in Silicon-substituted x-Electron
Systems. - Part I: H . Bock and H. Alt, Angew. Chem. 79, 932
(1967); Angew. Chem. internat. Edit. 6, 941 (1967).
[I] A. Carrington, Quart. Re\. 17, 67 (1963).
121 .
I
A.
.Bedford, J . R. Bolton, A. Carrington, and R . H . Prince,
Trans. Faraday SOC.59, 53 (1963).
[3] M . D . Curtis and A . C. AIlred, 1. Amer. chern. SOC.87,2554
(1965).
[4] Cf. Part I, [**I.
[5] E. de Boer and 1. P. Co[pa, J. phys. Chern. 71, 21 (1967).
161 T. R. Tuttle and S. I. Weissrnan, J. Amer. chern. SOC.80, 5342
(1958).
17) H. Bock and H. AI;, unpublished.
126)
a,=176G
Fig. 1. ESR spectrum of p-bis(trimethylsilyl)benzene, measured in
K-l,2-dimethoxyethane at -70 "C. Coupling constants of the ring
protons of p-disilyl- and p-dialkyl-benzene.
As can be caIculated from the simple 5-group spectrum the
coupling constants of the four equivalent nuclear protons of
(
2
.
)
amount to 1.76 gauss, whereas those of the p-xylene
radical anion (2b) [61 selected as reference substance amount
to 5.34 gauss. These values can be reproduced quantitatively
by use of the McConnell equation U H = Q . ~ H with a Q
942
Substituent Effects of (3-Trimethylsilyl Groups in
Bis-, Tetrakis-, and Hexakis-(trimethylsily1)-pxylenes I**]
By H . Bock and H. Alt [*I
An interaction between unoccupied Si-3d orbitals and antibonding states of neighboring x-electron systems can be
demonstrated by comparing the UV and ESR spectra of
Angew. Chem. internat. Edit. VoI. 6 (1967) 1 No. I 1
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