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Nmr study of the VixSiX4−x compounds with X = Cl, Br; x = 0, 1, 2, 3
Journal of Molecular Structure, 67 (1980) Eisevier
Scientific
Publishing
Company,
NMR STUDY OF THE VixSiX,_x x = 0, 1, 2, 3
L. DELMULLE
and G. P. VAN
DER
289-292 Amsterdam
2 January
Printed
COMPOUNDS
in The Netherlands
WITH X = Cl, Br;
KELEN*
Laboratory for General and Inorganic Chemistry-B, B-9000 Gent (Belgium) (Received
-
University of Gent, Krijgsiaan 271,
1980)
ABSTRACT ‘H, ’ W and 2gSi NMR data for the compounds Vi,SiX,_, are reported_ While the ‘I-I and “C resonances from the rr system are indicative of the electron-withdrawing inductive effect (--I) of the halogens, the ‘gSi chemical shift data reveal not only a shift contribution originating from this inductive effect but also the important influence of a {d, a*-} hyperconjugation [ 11. This back-donation originates from the vinyl ‘R system and not from the halogens. The chemical shift data and the coupling constants also show an important influence from steric interaction and even from an electric field effect caused by polarization of the silicon-halogen bond.
INTRODUCTION
As a part of a systematic physicochemical study [2] of the bonding characteristics in compounds of the type R,MRh_, (R: CH,=CH-, CH2=CHCH,--; R’: CHJ, Cl, Br, I; M: C, Si, Sn), this study deals with the results of the NMR analysis of the compounds with M = Si, R = CH2=CH-, R’ = Cl, Br. EXPERIMENTAL
The mono- and di-halogenvinylsilanes were prepared by standard Grignard synthesis; ViSiC13 was prepared by a direct synthesis from elementary silicon and ViCl gas in an oven reaction with copper as a catalyst. All spectra were recorded on a Bruker HFX-90 pulse spectrometer. For the ViSiC13 molecule, a 300-MHz NMR spectrum was necessary to allow spectral analysis. The analysis was carried out by a simulation and iteration procedure. The parameters obtained are summarized in Tables 1 and 2.
*Author
for correspondence.
0022-2860/80/000~0000/$02.25
0 1980
Elsevier
Scientific
Publishing
Company
290 TABLE ‘H,
1
“C
and z9Si chemical
shift data for the Vi,SiX,_,
compounds
in 6 ppm relative
to
X
4-x
&HA
~HB
~Hc
ac,
6G
Cl
1 2 3
6.075 6.094 6.114
5.810 5.855 6.291
6.091 6.094 6.195
135.3 134.1 132.5
136.3 137.8 140.1
-
-
-26.93 -24.79 -3.22 -18.70
133.8
134.8
-27.01
4
Br
-
1
TABLE
-
6.055
-
5.778
6.083
2
Coupling
constants
X
4-x
J
Cl
1 2 3
2.1 2.0 1.92
17.5 17.0 14.60
1
2.30
i7.a
Br
RESULTS
aSi
AND
in the Vi,SiX,-, em
compounds J
cis
(Hz) ‘JI’C--‘HA
‘J
22.5 21.7 19.98
159. 160. 161.
158. 159. 157.
21.50
157.
156.
J
trans
“C-‘HB
DISCUSSION
From Table 1 it can be seen that the vinyl-hydrogen chemical shifts show a deshielding on increasing the number of halogen substituents owing to the strong electronegativity of the latter. For the bromine-substituted molecule, the deshielding is less than for the chlorine analogue, in accordance with the electronegativity difference. On progressive halogen substitution, the steric interaction as well as the electric field effect due to the bond polarization Si+6-C1-6 increases. These effects are clearly shown by the reversal of the shielding sequence: Vi,SiCl:
6 H, < S H, < 6 H,
ViSiC13: 6 HA < SH, < SH, The *% chemical shift data confirm the influence of the inductive interaction in this series of products, This is also in agreement with the results of a study of the addition of dichlorocarbene on the vinylmethylchlorosilanes done by Koutkova and Chvalovsky [3]. In Fig. 1 the 2gSi chemical shift data for the vinylchlorosilanes (Table 1) as well as for the methylchlorosilanes [4] are plotted as a.function of chlorine
291
Fig. 1. “Si chemical shift data for the Vi,SiCl,-,(-) in 6 ppm relative to TMS.
and Me,SiCI,-,(-------)
compounds
substitution. The silicon shielding apparently is much stronger for the vinyl compounds than for the methyl compounds, see also Table 3. This can be ascribed to a {d, a*-n} hyperconjugation effect with a delocalization of the n electron density towards the silicon-halogen moiety [l]. Although it is generally accepted [ 51 that the halogens can participate in a back-donation effect, it seems that in these types of products they are active via their inductive effect only. Indeed, there is a progressive deshielding of the silicon atom with increasing number of halogen atoms. For the ViSiC13 molecule the deshielding is remarkable (see Fig. 1) and may indicate the domination of the --I effect from the halogens over the hyperconjugation effect from the 7~system. The rather abnormal position of the *‘Si chemical shift in the SiC14 molecule can be ascribed, in accordance.with Pauling’s TABLE
3
AS *‘+Si for some methykilanes
and vinylsilanes in 6 ppm relative to TMS
Me,Si-Me
0
Me,Si-Vi CL,Si-Me
-10.68 + 12.47
10.68 -
-
Cl,Si-Vi
-3.22
15.69
292
theory [6], to a resonance stabilization with the appearance of a double-bond character in the Si-Cl bond, Furthermore, the 2gSi chemical shift data suggest that the hyperconjugative interaction between the vinyl R system and silicon is important to such a degree that the ability to form a @-d)n back-donation is apparently much higher for the vinyl group than for the halogens (Table 3). The coupling constants are summarized in Table 2. From the CYparameter as defined by Briigel et al. -[ 73 (Table 4) OL= JtiOn,/lv, - I+ 1, where vB and vc are the frequencies of the two trans-interacting hydrogens, the following can be concluded. (1) There is an increase in a on increasing the number of halogen atoms, indicating an increase of the --I and/or -M interaction; (2) the value of 01for the bromine compound is lower than for the chlorine analogue, in agreement with the electronegativity differences; (3) the value of OLfor the ViSiCIJ compound is abnormally high, which confirms the steric and electric field interactions that yield higher deshielding on the H, proton than expected solely on an inductive base, and causes here a lowering of the value of Iv, - “cl_ The same phenomenon appears from the J, ac._, H coupling constants. Whereas the ‘JI 3c_ 1HA coupling constant increases with increasing groupelectronegativity, the LTI3c- I HB coupling constant shows an erratic variation (Table 2). The probable origin of this behaviour is the sterically and electricallyinduced polarization on Hz. TABLE
4
(Yparameter for the Vi,SiX,_x compounds. trarwinteracting hydrogens Hn and Hc X
4-x
o!
CI
1 2 3 1
0.89 1.00 2.29 0.78
Br
CK= Jtrans/lug
-
YCI, Y = frequency of the
REFERENCES 1 2 3 4 5 6
L. Delmulle and G. P. Van der Kelen, J. Mol. Struct., 66 (1980) 315. L. Delmulle, Doct. Thesis, Rijksuniversiteit Gent, 1978. L. Koutkova and V. Chvalovsky, Collect. Czech. Chem. Commun., 37 (1972) 2100. L. Delmulle, Lit. Thesis, Rijksuniversiteit Gent, 1971. H. Vahrenkampf and H. Noth, J. Organomet. Chem., 12 (1968) 281. L. Pauling, The Nature of the Chemical Bond, Cornell University Press, Ithaca, NY, 1945, pp. 228. 7 W. Briigel et al., Z. Electrochem., 64 (1960) 1121.
Scientific
Publishing
Company,
NMR STUDY OF THE VixSiX,_x x = 0, 1, 2, 3
L. DELMULLE
and G. P. VAN
DER
289-292 Amsterdam
2 January
Printed
COMPOUNDS
in The Netherlands
WITH X = Cl, Br;
KELEN*
Laboratory for General and Inorganic Chemistry-B, B-9000 Gent (Belgium) (Received
-
University of Gent, Krijgsiaan 271,
1980)
ABSTRACT ‘H, ’ W and 2gSi NMR data for the compounds Vi,SiX,_, are reported_ While the ‘I-I and “C resonances from the rr system are indicative of the electron-withdrawing inductive effect (--I) of the halogens, the ‘gSi chemical shift data reveal not only a shift contribution originating from this inductive effect but also the important influence of a {d, a*-} hyperconjugation [ 11. This back-donation originates from the vinyl ‘R system and not from the halogens. The chemical shift data and the coupling constants also show an important influence from steric interaction and even from an electric field effect caused by polarization of the silicon-halogen bond.
INTRODUCTION
As a part of a systematic physicochemical study [2] of the bonding characteristics in compounds of the type R,MRh_, (R: CH,=CH-, CH2=CHCH,--; R’: CHJ, Cl, Br, I; M: C, Si, Sn), this study deals with the results of the NMR analysis of the compounds with M = Si, R = CH2=CH-, R’ = Cl, Br. EXPERIMENTAL
The mono- and di-halogenvinylsilanes were prepared by standard Grignard synthesis; ViSiC13 was prepared by a direct synthesis from elementary silicon and ViCl gas in an oven reaction with copper as a catalyst. All spectra were recorded on a Bruker HFX-90 pulse spectrometer. For the ViSiC13 molecule, a 300-MHz NMR spectrum was necessary to allow spectral analysis. The analysis was carried out by a simulation and iteration procedure. The parameters obtained are summarized in Tables 1 and 2.
*Author
for correspondence.
0022-2860/80/000~0000/$02.25
0 1980
Elsevier
Scientific
Publishing
Company
290 TABLE ‘H,
1
“C
and z9Si chemical
shift data for the Vi,SiX,_,
compounds
in 6 ppm relative
to
X
4-x
&HA
~HB
~Hc
ac,
6G
Cl
1 2 3
6.075 6.094 6.114
5.810 5.855 6.291
6.091 6.094 6.195
135.3 134.1 132.5
136.3 137.8 140.1
-
-
-26.93 -24.79 -3.22 -18.70
133.8
134.8
-27.01
4
Br
-
1
TABLE
-
6.055
-
5.778
6.083
2
Coupling
constants
X
4-x
J
Cl
1 2 3
2.1 2.0 1.92
17.5 17.0 14.60
1
2.30
i7.a
Br
RESULTS
aSi
AND
in the Vi,SiX,-, em
compounds J
cis
(Hz) ‘JI’C--‘HA
‘J
22.5 21.7 19.98
159. 160. 161.
158. 159. 157.
21.50
157.
156.
J
trans
“C-‘HB
DISCUSSION
From Table 1 it can be seen that the vinyl-hydrogen chemical shifts show a deshielding on increasing the number of halogen substituents owing to the strong electronegativity of the latter. For the bromine-substituted molecule, the deshielding is less than for the chlorine analogue, in accordance with the electronegativity difference. On progressive halogen substitution, the steric interaction as well as the electric field effect due to the bond polarization Si+6-C1-6 increases. These effects are clearly shown by the reversal of the shielding sequence: Vi,SiCl:
6 H, < S H, < 6 H,
ViSiC13: 6 HA < SH, < SH, The *% chemical shift data confirm the influence of the inductive interaction in this series of products, This is also in agreement with the results of a study of the addition of dichlorocarbene on the vinylmethylchlorosilanes done by Koutkova and Chvalovsky [3]. In Fig. 1 the 2gSi chemical shift data for the vinylchlorosilanes (Table 1) as well as for the methylchlorosilanes [4] are plotted as a.function of chlorine
291
Fig. 1. “Si chemical shift data for the Vi,SiCl,-,(-) in 6 ppm relative to TMS.
and Me,SiCI,-,(-------)
compounds
substitution. The silicon shielding apparently is much stronger for the vinyl compounds than for the methyl compounds, see also Table 3. This can be ascribed to a {d, a*-n} hyperconjugation effect with a delocalization of the n electron density towards the silicon-halogen moiety [l]. Although it is generally accepted [ 51 that the halogens can participate in a back-donation effect, it seems that in these types of products they are active via their inductive effect only. Indeed, there is a progressive deshielding of the silicon atom with increasing number of halogen atoms. For the ViSiC13 molecule the deshielding is remarkable (see Fig. 1) and may indicate the domination of the --I effect from the halogens over the hyperconjugation effect from the 7~system. The rather abnormal position of the *‘Si chemical shift in the SiC14 molecule can be ascribed, in accordance.with Pauling’s TABLE
3
AS *‘+Si for some methykilanes
and vinylsilanes in 6 ppm relative to TMS
Me,Si-Me
0
Me,Si-Vi CL,Si-Me
-10.68 + 12.47
10.68 -
-
Cl,Si-Vi
-3.22
15.69
292
theory [6], to a resonance stabilization with the appearance of a double-bond character in the Si-Cl bond, Furthermore, the 2gSi chemical shift data suggest that the hyperconjugative interaction between the vinyl R system and silicon is important to such a degree that the ability to form a @-d)n back-donation is apparently much higher for the vinyl group than for the halogens (Table 3). The coupling constants are summarized in Table 2. From the CYparameter as defined by Briigel et al. -[ 73 (Table 4) OL= JtiOn,/lv, - I+ 1, where vB and vc are the frequencies of the two trans-interacting hydrogens, the following can be concluded. (1) There is an increase in a on increasing the number of halogen atoms, indicating an increase of the --I and/or -M interaction; (2) the value of 01for the bromine compound is lower than for the chlorine analogue, in agreement with the electronegativity differences; (3) the value of OLfor the ViSiCIJ compound is abnormally high, which confirms the steric and electric field interactions that yield higher deshielding on the H, proton than expected solely on an inductive base, and causes here a lowering of the value of Iv, - “cl_ The same phenomenon appears from the J, ac._, H coupling constants. Whereas the ‘JI 3c_ 1HA coupling constant increases with increasing groupelectronegativity, the LTI3c- I HB coupling constant shows an erratic variation (Table 2). The probable origin of this behaviour is the sterically and electricallyinduced polarization on Hz. TABLE
4
(Yparameter for the Vi,SiX,_x compounds. trarwinteracting hydrogens Hn and Hc X
4-x
o!
CI
1 2 3 1
0.89 1.00 2.29 0.78
Br
CK= Jtrans/lug
-
YCI, Y = frequency of the
REFERENCES 1 2 3 4 5 6
L. Delmulle and G. P. Van der Kelen, J. Mol. Struct., 66 (1980) 315. L. Delmulle, Doct. Thesis, Rijksuniversiteit Gent, 1978. L. Koutkova and V. Chvalovsky, Collect. Czech. Chem. Commun., 37 (1972) 2100. L. Delmulle, Lit. Thesis, Rijksuniversiteit Gent, 1971. H. Vahrenkampf and H. Noth, J. Organomet. Chem., 12 (1968) 281. L. Pauling, The Nature of the Chemical Bond, Cornell University Press, Ithaca, NY, 1945, pp. 228. 7 W. Briigel et al., Z. Electrochem., 64 (1960) 1121.