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A 29Si and 1H NMR study of (CH3)4-n SiXn compounds with X N(CH3)2, OCH3 and SCH3
329
Journnl of Organometallic Chemistry, 0 Ekevier Sequoia S-A., Lausanne -
122 (1976) 329-334 Printed in The Netherlands
A “Si AND ‘H mIR STUDY OF (CH,)_,_, X= N(CH& OCH, AND SCH,
Six, COMPOUNbS
WITH
E.V. VAN DEN BERGHE and G-P. VAN DER KELEN * Laboratory for General Ghent (Belgium)
and Inorganic
Chemistry-B.
University
of Ghent,
Krijgslaan
271,
B-9000
(Received June 15th, 1976)
The ‘H and “Si NMR spectra of (CH,)4_n SIX,, compounds (X = N(CH&, OCHS and SCH3) have been studied. The 2gSi resonance shows an upfield shift with X = N(CH&, OCH3, and n = 2, 3, 4. This can be accounted for by increasing Si-0 and Si-N back donation. Van der Waals interactions are proposed to esplain certain NMR parameters. A linear correlation has been found between the 13C-H coupling constants and the product of the electronegativity and Van der Waals radius in the (CH,), Six compounds. This provides an explanation of the lower 13C-H coupling constants of (CH3)$iOCH3 and (CH3)$ZGN(CH& compared with those in Si(CH,), Introduction In extending our NMR study of (CH3)4_-nMcIVBjXncompounds [l-5] we report a ‘H and “Si NMR investigation of (CH,),_, Six, compounds (X = 0CH3, N(CH,), and SCH& ‘H and *‘Si NMR parameters of (CH3)4_-nSi(OCH3)n have been reported by several authors [6,7] but we redetermined these in order to obtain results under identical conditions. For the other two series of compounds only ‘H chemical shifts have been reported [ 8,9]. Egperimental
The NMR spectra were recorded in frequency-sweep mode on a HFX-90 MHz Bruker Physik NMR spectrometer using 25% (v/v) benzene solutions of the compounds in 5 mm NMR tubes. The benz;ene signal served as the lock. The 2gSi chemical shifts were obtained from the 2gSi NMR INDOR spectra Only broadened signals could be observed for these compounds. The error in the *Towhomcorrespondenceshouldbeaddressed.
~---.-.__._.--._-.-_.-____-
--_.
-_-
_____._ ------___l.-._-.------..-.-----_-I-------
“_V,
I
:;
---Mo4SI MogSlOMe Me#I(OMe)Z MoSi(OMe)s Si(OMe)3 Mo3SLNMo2 Mc#I(NMo2)2 MoSi(NMo2)2 SI(NMe2)4 ! Mo$lSMc MqSl(SMo)2 MeSi(SMc)3 Sl(SMe)q ----......----
dW
s(29sl)
3J
J(%-_)I) dW Si-MC 29Si-X-C-H Sk-Me Si-X-Me 2%1-C-H _.-___-._._-.__________ ---.---.-._.-_ ---_--_____.___ -.-__-_I -_-.-.___.- ._.__.” ___.______-._-113.9 0 0 G.6 118.0 +0,04 t3.27 t17.7B 4,l 6.7 118.6 +0,01 t3.34 - 1,62 7.2 3-9 -C,o2 119.4 t3.38 -39.8 388 8.3 t3.42 -79,lG 3.6 to,04 t2.40 t G,G2 117.7 6.G 3,G 113.0 to.06 t2.44 - 1.8G 6.G 3,3 to.09 t2.46 -16.80 113.3 6.9 3,2 CZ.GO -28.60 3.0 to,18 Cl.77 t10.4G 120.0 6.7 42 +0,3i5 C28.14 122.3 t1.34 7.0 4,8 tom t1.87 t34.00 5,6 122.8 7.3 +3&E&J t1.94 6.4 ----.-.--_-..__ -_.--___.-_-________________-,_-._,________,_______
2J
141,o 141.8 142,7 143,4 132,8 133.0 133,5 134.0 139.8 140.2 141,o 14204
J&-H) SI-X-Mc
,,
.'
."
1
“: ,,
.’
, ,:; ', "' : ‘.’ 1 ‘,,
',',:
:
‘,’
CHEMICALSHIFTS(PPM)ANDCOUPLINGCONSTANTSIN(CH3)4~, SIX,, COMPOUNDSWlTHXm OMo,NMq ANDSMo -I_ .~--------..~_-_._-_C-,__--_._..-___._._,._._~__... -....... ._._~..______.__._._-..~..--.~-...--..--._.~.---_.--_.__ Chem@shlfts(ppm) Coupling conetnnts ,.
TABLE1
:‘.
331
2g~i-che&A shifts in these circumstances is estimated to be about 0.1 to 0.2 ppm. The 13C chemical shifts of some t-butyl compounds: C(CH3)4, (CH&COCH3 and (CH,),CN(CH,), were measured with F.T. NMR . &sults
and discussion
The NMR data are listed in Table 1: ‘H and 2gSi chemical shifts 8 in ppm vs. the ‘H and *‘Si resonance of Si(CH3),; positive values stand for high frequency shifts i.e.&o lower field. The 13C values of (CH3)&OCH3 and (CH3)3CN(CH3)2, viz. 25 and 42 ppm, respectively are given relative to C(CH3)4.
1. 2gSi and ‘H chemical shifts The 2gSi chemical shifts for (CH3)4--nSi(OCH3)n and (CH3)4--nSi(N(CH3 )&* are downfield for n = 1 and then more progressively high field for n F 2, 3, 4. This trend has been also observed for other (CH,),_, Six, systems and analoguous Sn compounds [3] and has been discussed in terms of several factors which. along with electrical and magnetic shielding effects, mainly tend to decrease the. contribution of the pammagnetic term to the shielding. One of these, increasing Si*X back-donation, has been strongly criticized as an explanation for such behaviour [ 33. For the (CH3)4--nSi(OCH3), series this effect was proposed to explain the upfield *‘Si chemical shifts on increased OCH, substitution [ 71. The 2gSi-O-C-H and 2gSi-N-C-H coupling constants discussed in section 2.2. also yield strong evidence for a substantial Si%EH3 and SieN(CH3)2 back-donation_ On the other hand the (CH3)4_-nSi(SCH3)n derivatives show a continuous 2gSi shift to lower field on increased SCH3 substitution. This was also observed for the analoguous tin [2] and carbon compounds [lo]. We believe that the contribution of two factors should be taken into consideration. First increasing Van der Waals (V.D.W.) interaction or dispersion forces may shift the 2gSi, ’ “Sn, 13C and ‘H resonances to lower field. Second, the diamagnetic anisotropy effect of the C-S bond was estimated by Spiesecke and Schneider [11] to be of similar magnitude to that of the C-I bond, and so could also contribute to the downfield shifts. The same arguments can account for the great downfield shift of the (Si-CH3) ‘H resonances of these compounds vs those of the series (CH3)4--nSi(OCH3), and (CH3)4_-nSiN(CH3)n. In Fig. 1 the “Si chemical shifts of (CH,)&liN(CH&, (CH3)3 SiOCH3 and of Si(CH3)4 (zero point) show a linear correlation with the N and 0 electronegativities. Further, if the slope of this Si correlation line is compared with that for the 13C chemical shift of the central carbon in the analoguous t-butyl compounds a pronounced highfield shift is observed. As was already discussed in the case of the methylsilicon halides [ 31 such increased shielding may originate from SiN and Si*O (p_d. )r back-donation. The 2gSi chemical shift of (CH3)3SiSCH3, however, was found to fit the halogen correlation line [ 31. 2. Coupling constants
2.1. J(2gSI-C-H)
and J(13C-H)
of the Si-CH3
moiety.
The *‘Si-C-H
Eletronegativity I%
of
X CE,l
1. Variation of 13C and *‘Si NMR chemical shifts with the electronegativity of X for (CH3)3CX and (CH313Sii (-). respectively: .:X=CH3:0tX=N
(- - - -)
coupling constants of the series (CH3)4_-nSi(OCH3)n are higher than those of compounds. This is in accord with the electronegatithe (CH&-,Si(N(CH&), vity effect. Those of the (CH3)4_-nSi(SCH3), compounds, however, are higher than those .of the (CH,),_,Si(N(CH,),), which is opposite to the electronegativiiy effect. In an earlier study on methylsilicon halides [3] higher coupling constants were observed with bulkier subsituents. J(2gSi-C-H) and J( 13C-H) values increased in going from Cl + Br + I as substituents in mono and dihalo substituted compounds_ This occurred with trimethyltin halides [S], and the explanation given there may also account for the coupling constants in the thiomethyl compounds. It is striking that the values of the 13C-H coupling constants of the CH3-Si moiety in the methoxy- and climethylamino-silanes are lower than those of Si(CH& This effect was not found for analogous carbon [lo] and tin compounds [4,5J but was reported by Engelhardt [12] for (CH3)3SiOCH3 and attributed by him to (p + d)?r back-donation from oxygen to silicon. Assuming that V-D-W. interactions could also influence this behaviour we investigated the variation of the J(13C-H) values in monosubstituted silicon compounds as a function of the V.D.W. radii of the X atom and of its electronegativity for X = Cl, Br, I, C, N, 0, S_ From Table 2 the following conclusions may be drawn: (a) The most bulky substituents such as the halogens and sulfur produce the highest coupling constants. A similar trend is observed in analoguous 13C and tin compounds. (b) With the bulky substituents the increasing trend in the coupling constant parallels the increase of the V.D.W. radii, but does not fit with the electrcnegativity effect. (c) With the smaller and more electronegative substituents N and 0 the electronegativity effect predominates despite a decrease of the V.D.W. radii. From this it is clear that there is no direct correlation between the 13C-H coupling constants and either Ex or Rv.n.w.. But a plot of the 13C-H coupling constants against ExJE (Fig. 2) yields a straight line. The N and 0 &b&tuted
333 TABLE
2.
VARIATION RADII (R)
X
OF J<‘3C-_R) WITH ELECTRONEGATIVITY OF X IN (CH&SiX COMPOUNDS J(‘3C-H)
(Ex.
V.D.W.
EX
ALLRED)
AND
VAN
DER
WAAL’S
Ex.R-
R(A) N 0 CH3 S Cl Br I
117.7 118.4 118.8 120-O 120.9 121.4 121.8
3.0 3.5 2-5 2.8 3.2. 3.0 2.7
1.5 1.4 2.0 1.85 1.80 1.95 2.15
4.5 4.9 5.0 5.2 5.7 5.8 5.8
compounds, however, exhibit J(13C-H) values below that of Si(CHs),. This suggests that the values of the r3C-H coupling constants in these compounds are determined by competition between the electronegativity and V.D.W. effect. The higher J(13C-H) value in Si(CH,), vs. this in (CH3)3SiOCH3 and (CH3)3SiN(CH3)2 would then be due to a greater V.D.W. interaction between the four CH3 groups. However in substituted methanes [ 143 or in t-butyl compounds [ 131 with N or 0 containing substituents such variation of the 13C-H coupling constants compared to the J(13C-H) value in methane or neopentane is not observed. So it must be assumed that this effect does not occur in the silicon case, in which back-donation by such substituents does not much lower their electron withdrawing capacity. 2.2. J(2gSi-X-C-H) and J(13C-H) of the Si-X-CH3 moiety (X = O,N,S). The 2gSi-O-C-H and 2gSi-N-C-H coupling constants decrease on increased substitution. This can be interpreted in terms of increased total Si%, SFN back-
122
121
9
120
E! r 3 119
Fig. 2. Dependence tiVitY).
of J(13C-H)
on
EX
. R(v_D_w_)
in (CH&SiX
compounds.
(Ex
= Allred
electronega:
334
..
::
_.
._. -.. ~dbnation. Starting-Gith the.Si(OC!IJ& &d $N(dH,)& &s&, one would argue that in these c’ompounds a maximum pol*_ation of §i&&rs;- yielding a maximum @ + d)x interaction. This -would raise the. pond e~eCtioneg&ivities of & and N= to a high level. In the less substituted~compounds with‘n = 3, .2 or 1 the. polarizatioh would be lower and accordingly also the &t&t. of (p +-;d)r participation_ The bond electronegativities of the Si? and Si-N bonds compared with those in the Si(OR)4 and Si(NR& species would therefore decrease and the 3Jcsi_n) coupling constants increase, as observed. Confirmation of this-mterpretation can be found in a recent study .ofH. Notb et al. 1151 of the -14Nresonances =msilylamines, which showed that in the series (CH,),-, Si(N(CH,),), a deshielding of the nitrogen atom paralleled increases -N(CH,), substitution. The increasing values of the 13C-H coupling constants in the Si-X-CH3 moiety for both series of compounds on increased substitution may also be explained by the above hypothesis. Contrary to the observations made for the methoxy- and dimethyhuninosilanes, J( 2gSi-S-C-H) in the thiomethylsilanes increases on progressive substitution. This suggests that, taking the monothiomethyl compound as reference, the amount of s-character of the Si + SCH3 orbitaJs increases with progressive SCH3 substitution and the consequent rise of the orbital electronegativities may be responsible for the observed increase of the J( * 3C-H) values of the thiomethyl groups. Fkferences 1 E-V. Van Den Berghe and G.P. Van Der Kden. J. Organometzd. Chem.. 6 <1966)
2 3 4 5 6 7 8 9 10
515. Den Berghe and G.P. Van Der K&en, J. Organometal. Chem. 26 (1971) 207. Den Berghe and G.P. Van der Kelen, J. Organometal. Chem.. 59 (1973) 175. Den Berghe and G-P. Van Der KeIen. J. Organometal. Chem.. 61 <1973) 197. Den Berghe and G.P. Van Der Ke1en.J. Mol. Struct., 20 (1974) 147. B.K. Hunter and L-W. Reeves. Can_ J_ Cheru. 46 (1968) 139% G. Engelhardt. H. Jancke. M. Magi. T. Pekk and E. Lippmaa. J. Organometal. Chem.. 28
11 H. Spiesecke and W_Fu. Schneider. J. Chem. Phys., 95 <1961) 722. 12 G.EngeIhardt.J_O_rg~ometal_Chem..8<1967)P27_ 13 Unpublishedresults.
14 &It Goldstein. V-S. Watts and L.S. Rattet. in J.W. Emsley. J. Feeney and L.H. Sutcliffe (Eds.) Progress in Nuclear Magnetic Resonance Spectroscopy. Vol. 8. Part 2. Perkanton Press. Oxford. 1971. 15 H Noth. W. Tinhof and B. Wrackmeyer. Chem. Ber.. 107 (1974) 518.
Journnl of Organometallic Chemistry, 0 Ekevier Sequoia S-A., Lausanne -
122 (1976) 329-334 Printed in The Netherlands
A “Si AND ‘H mIR STUDY OF (CH,)_,_, X= N(CH& OCH, AND SCH,
Six, COMPOUNbS
WITH
E.V. VAN DEN BERGHE and G-P. VAN DER KELEN * Laboratory for General Ghent (Belgium)
and Inorganic
Chemistry-B.
University
of Ghent,
Krijgslaan
271,
B-9000
(Received June 15th, 1976)
The ‘H and “Si NMR spectra of (CH,)4_n SIX,, compounds (X = N(CH&, OCHS and SCH3) have been studied. The 2gSi resonance shows an upfield shift with X = N(CH&, OCH3, and n = 2, 3, 4. This can be accounted for by increasing Si-0 and Si-N back donation. Van der Waals interactions are proposed to esplain certain NMR parameters. A linear correlation has been found between the 13C-H coupling constants and the product of the electronegativity and Van der Waals radius in the (CH,), Six compounds. This provides an explanation of the lower 13C-H coupling constants of (CH3)$iOCH3 and (CH3)$ZGN(CH& compared with those in Si(CH,), Introduction In extending our NMR study of (CH3)4_-nMcIVBjXncompounds [l-5] we report a ‘H and “Si NMR investigation of (CH,),_, Six, compounds (X = 0CH3, N(CH,), and SCH& ‘H and *‘Si NMR parameters of (CH3)4_-nSi(OCH3)n have been reported by several authors [6,7] but we redetermined these in order to obtain results under identical conditions. For the other two series of compounds only ‘H chemical shifts have been reported [ 8,9]. Egperimental
The NMR spectra were recorded in frequency-sweep mode on a HFX-90 MHz Bruker Physik NMR spectrometer using 25% (v/v) benzene solutions of the compounds in 5 mm NMR tubes. The benz;ene signal served as the lock. The 2gSi chemical shifts were obtained from the 2gSi NMR INDOR spectra Only broadened signals could be observed for these compounds. The error in the *Towhomcorrespondenceshouldbeaddressed.
~---.-.__._.--._-.-_.-____-
--_.
-_-
_____._ ------___l.-._-.------..-.-----_-I-------
“_V,
I
:;
---Mo4SI MogSlOMe Me#I(OMe)Z MoSi(OMe)s Si(OMe)3 Mo3SLNMo2 Mc#I(NMo2)2 MoSi(NMo2)2 SI(NMe2)4 ! Mo$lSMc MqSl(SMo)2 MeSi(SMc)3 Sl(SMe)q ----......----
dW
s(29sl)
3J
J(%-_)I) dW Si-MC 29Si-X-C-H Sk-Me Si-X-Me 2%1-C-H _.-___-._._-.__________ ---.---.-._.-_ ---_--_____.___ -.-__-_I -_-.-.___.- ._.__.” ___.______-._-113.9 0 0 G.6 118.0 +0,04 t3.27 t17.7B 4,l 6.7 118.6 +0,01 t3.34 - 1,62 7.2 3-9 -C,o2 119.4 t3.38 -39.8 388 8.3 t3.42 -79,lG 3.6 to,04 t2.40 t G,G2 117.7 6.G 3,G 113.0 to.06 t2.44 - 1.8G 6.G 3,3 to.09 t2.46 -16.80 113.3 6.9 3,2 CZ.GO -28.60 3.0 to,18 Cl.77 t10.4G 120.0 6.7 42 +0,3i5 C28.14 122.3 t1.34 7.0 4,8 tom t1.87 t34.00 5,6 122.8 7.3 +3&E&J t1.94 6.4 ----.-.--_-..__ -_.--___.-_-________________-,_-._,________,_______
2J
141,o 141.8 142,7 143,4 132,8 133.0 133,5 134.0 139.8 140.2 141,o 14204
J&-H) SI-X-Mc
,,
.'
."
1
“: ,,
.’
, ,:; ', "' : ‘.’ 1 ‘,,
',',:
:
‘,’
CHEMICALSHIFTS(PPM)ANDCOUPLINGCONSTANTSIN(CH3)4~, SIX,, COMPOUNDSWlTHXm OMo,NMq ANDSMo -I_ .~--------..~_-_._-_C-,__--_._..-___._._,._._~__... -....... ._._~..______.__._._-..~..--.~-...--..--._.~.---_.--_.__ Chem@shlfts(ppm) Coupling conetnnts ,.
TABLE1
:‘.
331
2g~i-che&A shifts in these circumstances is estimated to be about 0.1 to 0.2 ppm. The 13C chemical shifts of some t-butyl compounds: C(CH3)4, (CH&COCH3 and (CH,),CN(CH,), were measured with F.T. NMR . &sults
and discussion
The NMR data are listed in Table 1: ‘H and 2gSi chemical shifts 8 in ppm vs. the ‘H and *‘Si resonance of Si(CH3),; positive values stand for high frequency shifts i.e.&o lower field. The 13C values of (CH3)&OCH3 and (CH3)3CN(CH3)2, viz. 25 and 42 ppm, respectively are given relative to C(CH3)4.
1. 2gSi and ‘H chemical shifts The 2gSi chemical shifts for (CH3)4--nSi(OCH3)n and (CH3)4--nSi(N(CH3 )&* are downfield for n = 1 and then more progressively high field for n F 2, 3, 4. This trend has been also observed for other (CH,),_, Six, systems and analoguous Sn compounds [3] and has been discussed in terms of several factors which. along with electrical and magnetic shielding effects, mainly tend to decrease the. contribution of the pammagnetic term to the shielding. One of these, increasing Si*X back-donation, has been strongly criticized as an explanation for such behaviour [ 33. For the (CH3)4--nSi(OCH3), series this effect was proposed to explain the upfield *‘Si chemical shifts on increased OCH, substitution [ 71. The 2gSi-O-C-H and 2gSi-N-C-H coupling constants discussed in section 2.2. also yield strong evidence for a substantial Si%EH3 and SieN(CH3)2 back-donation_ On the other hand the (CH3)4_-nSi(SCH3)n derivatives show a continuous 2gSi shift to lower field on increased SCH3 substitution. This was also observed for the analoguous tin [2] and carbon compounds [lo]. We believe that the contribution of two factors should be taken into consideration. First increasing Van der Waals (V.D.W.) interaction or dispersion forces may shift the 2gSi, ’ “Sn, 13C and ‘H resonances to lower field. Second, the diamagnetic anisotropy effect of the C-S bond was estimated by Spiesecke and Schneider [11] to be of similar magnitude to that of the C-I bond, and so could also contribute to the downfield shifts. The same arguments can account for the great downfield shift of the (Si-CH3) ‘H resonances of these compounds vs those of the series (CH3)4--nSi(OCH3), and (CH3)4_-nSiN(CH3)n. In Fig. 1 the “Si chemical shifts of (CH,)&liN(CH&, (CH3)3 SiOCH3 and of Si(CH3)4 (zero point) show a linear correlation with the N and 0 electronegativities. Further, if the slope of this Si correlation line is compared with that for the 13C chemical shift of the central carbon in the analoguous t-butyl compounds a pronounced highfield shift is observed. As was already discussed in the case of the methylsilicon halides [ 31 such increased shielding may originate from SiN and Si*O (p_d. )r back-donation. The 2gSi chemical shift of (CH3)3SiSCH3, however, was found to fit the halogen correlation line [ 31. 2. Coupling constants
2.1. J(2gSI-C-H)
and J(13C-H)
of the Si-CH3
moiety.
The *‘Si-C-H
Eletronegativity I%
of
X CE,l
1. Variation of 13C and *‘Si NMR chemical shifts with the electronegativity of X for (CH3)3CX and (CH313Sii (-). respectively: .:X=CH3:0tX=N
(- - - -)
coupling constants of the series (CH3)4_-nSi(OCH3)n are higher than those of compounds. This is in accord with the electronegatithe (CH&-,Si(N(CH&), vity effect. Those of the (CH3)4_-nSi(SCH3), compounds, however, are higher than those .of the (CH,),_,Si(N(CH,),), which is opposite to the electronegativiiy effect. In an earlier study on methylsilicon halides [3] higher coupling constants were observed with bulkier subsituents. J(2gSi-C-H) and J( 13C-H) values increased in going from Cl + Br + I as substituents in mono and dihalo substituted compounds_ This occurred with trimethyltin halides [S], and the explanation given there may also account for the coupling constants in the thiomethyl compounds. It is striking that the values of the 13C-H coupling constants of the CH3-Si moiety in the methoxy- and climethylamino-silanes are lower than those of Si(CH& This effect was not found for analogous carbon [lo] and tin compounds [4,5J but was reported by Engelhardt [12] for (CH3)3SiOCH3 and attributed by him to (p + d)?r back-donation from oxygen to silicon. Assuming that V-D-W. interactions could also influence this behaviour we investigated the variation of the J(13C-H) values in monosubstituted silicon compounds as a function of the V.D.W. radii of the X atom and of its electronegativity for X = Cl, Br, I, C, N, 0, S_ From Table 2 the following conclusions may be drawn: (a) The most bulky substituents such as the halogens and sulfur produce the highest coupling constants. A similar trend is observed in analoguous 13C and tin compounds. (b) With the bulky substituents the increasing trend in the coupling constant parallels the increase of the V.D.W. radii, but does not fit with the electrcnegativity effect. (c) With the smaller and more electronegative substituents N and 0 the electronegativity effect predominates despite a decrease of the V.D.W. radii. From this it is clear that there is no direct correlation between the 13C-H coupling constants and either Ex or Rv.n.w.. But a plot of the 13C-H coupling constants against ExJE (Fig. 2) yields a straight line. The N and 0 &b&tuted
333 TABLE
2.
VARIATION RADII (R)
X
OF J<‘3C-_R) WITH ELECTRONEGATIVITY OF X IN (CH&SiX COMPOUNDS J(‘3C-H)
(Ex.
V.D.W.
EX
ALLRED)
AND
VAN
DER
WAAL’S
Ex.R-
R(A) N 0 CH3 S Cl Br I
117.7 118.4 118.8 120-O 120.9 121.4 121.8
3.0 3.5 2-5 2.8 3.2. 3.0 2.7
1.5 1.4 2.0 1.85 1.80 1.95 2.15
4.5 4.9 5.0 5.2 5.7 5.8 5.8
compounds, however, exhibit J(13C-H) values below that of Si(CHs),. This suggests that the values of the r3C-H coupling constants in these compounds are determined by competition between the electronegativity and V.D.W. effect. The higher J(13C-H) value in Si(CH,), vs. this in (CH3)3SiOCH3 and (CH3)3SiN(CH3)2 would then be due to a greater V.D.W. interaction between the four CH3 groups. However in substituted methanes [ 143 or in t-butyl compounds [ 131 with N or 0 containing substituents such variation of the 13C-H coupling constants compared to the J(13C-H) value in methane or neopentane is not observed. So it must be assumed that this effect does not occur in the silicon case, in which back-donation by such substituents does not much lower their electron withdrawing capacity. 2.2. J(2gSi-X-C-H) and J(13C-H) of the Si-X-CH3 moiety (X = O,N,S). The 2gSi-O-C-H and 2gSi-N-C-H coupling constants decrease on increased substitution. This can be interpreted in terms of increased total Si%, SFN back-
122
121
9
120
E! r 3 119
Fig. 2. Dependence tiVitY).
of J(13C-H)
on
EX
. R(v_D_w_)
in (CH&SiX
compounds.
(Ex
= Allred
electronega:
334
..
::
_.
._. -.. ~dbnation. Starting-Gith the.Si(OC!IJ& &d $N(dH,)& &s&, one would argue that in these c’ompounds a maximum pol*_ation of §i&&rs;- yielding a maximum @ + d)x interaction. This -would raise the. pond e~eCtioneg&ivities of & and N= to a high level. In the less substituted~compounds with‘n = 3, .2 or 1 the. polarizatioh would be lower and accordingly also the &t&t. of (p +-;d)r participation_ The bond electronegativities of the Si? and Si-N bonds compared with those in the Si(OR)4 and Si(NR& species would therefore decrease and the 3Jcsi_n) coupling constants increase, as observed. Confirmation of this-mterpretation can be found in a recent study .ofH. Notb et al. 1151 of the -14Nresonances =msilylamines, which showed that in the series (CH,),-, Si(N(CH,),), a deshielding of the nitrogen atom paralleled increases -N(CH,), substitution. The increasing values of the 13C-H coupling constants in the Si-X-CH3 moiety for both series of compounds on increased substitution may also be explained by the above hypothesis. Contrary to the observations made for the methoxy- and dimethyhuninosilanes, J( 2gSi-S-C-H) in the thiomethylsilanes increases on progressive substitution. This suggests that, taking the monothiomethyl compound as reference, the amount of s-character of the Si + SCH3 orbitaJs increases with progressive SCH3 substitution and the consequent rise of the orbital electronegativities may be responsible for the observed increase of the J( * 3C-H) values of the thiomethyl groups. Fkferences 1 E-V. Van Den Berghe and G.P. Van Der Kden. J. Organometzd. Chem.. 6 <1966)
2 3 4 5 6 7 8 9 10
515. Den Berghe and G.P. Van Der K&en, J. Organometal. Chem. 26 (1971) 207. Den Berghe and G.P. Van der Kelen, J. Organometal. Chem.. 59 (1973) 175. Den Berghe and G-P. Van Der KeIen. J. Organometal. Chem.. 61 <1973) 197. Den Berghe and G.P. Van Der Ke1en.J. Mol. Struct., 20 (1974) 147. B.K. Hunter and L-W. Reeves. Can_ J_ Cheru. 46 (1968) 139% G. Engelhardt. H. Jancke. M. Magi. T. Pekk and E. Lippmaa. J. Organometal. Chem.. 28
11 H. Spiesecke and W_Fu. Schneider. J. Chem. Phys., 95 <1961) 722. 12 G.EngeIhardt.J_O_rg~ometal_Chem..8<1967)P27_ 13 Unpublishedresults.
14 &It Goldstein. V-S. Watts and L.S. Rattet. in J.W. Emsley. J. Feeney and L.H. Sutcliffe (Eds.) Progress in Nuclear Magnetic Resonance Spectroscopy. Vol. 8. Part 2. Perkanton Press. Oxford. 1971. 15 H Noth. W. Tinhof and B. Wrackmeyer. Chem. Ber.. 107 (1974) 518.