| dc.description.abstract | Tri(amino)silanes are generally prepared by the condensation route:
HSiCl? + 6 R?NH ? HSi(NR?)? + 3 R?NH·HCl
Even though these compounds have been extensively used both in industry and chemistry, only very little is known about their chemical and spectral properties. In this chapter we have tried to look into the various factors that influence the synthesis of these compounds. It has been concluded presently that in the condensation reactions of trichlorosilane and a mixture of secondary amines, both small and bulky, only one type of product is formed and all the three chlorine atoms are substituted by these amines. Even when dicyclohexylamine was used, complete replacement of all the chlorines is observed, which is quite different from the earlier reports. The condensation reactions which are carried out in the presence of a mixture of sterically hindered and less bulky amines, least bulky tri(amino)silanes are formed.
Transamination reactions that are carried out with tri(amino)silanes of the type (R?N)?SiH, where R?N? is less bulky, a mixture of products are formed which could not be separated. When the same type of reactions were carried out using slightly more sterically hindered tri(amino)silane (e.g., tris(dicyclohexylamino)silane), at room temperature only one type of product is formed. This clearly shows that it is not the basicity alone which plays the role in deciding the type of product formed, but the bulkiness of the amine concerned also plays a major role.
On comparing the chemical shift of the silicon nucleus, it is evident that in the same series, as the steric bulkiness of the substituent amine changes, it shows marginal deviation from what is expected. This indicates that in addition to the basicity (pKa) of the amine, there are some other factors which play a major role in deciding the electronic environment around the silicon and the Si–H hydrogen. As observed in the case of alkylsilanes, the Si–H frequency in tri(amino)silanes, HSi(NR?)?, varies with the group electronegativity sum (ESR). It was observed that for the less bulky system the same equation holds good, whereas maximum deviation is observed for a more bulky system.
In tri(amino)silanes of the type (R?N)?SiH, where the nitrogen atoms are nearly planar, the Si–N bond length is found to be about 1.70 Å. It is evident from the calculations (Table IV.8) that in tri(amino)silanes of the type HSi(NH?)?, the Si–N bond length is 1.71 Å when it is in the planar form, and this agrees fairly well with the experimental results. As evident from the X?ray crystal structure for compound I, where the nitrogen is in a nearly planar state, there is a tendency to form a strong (p–d)? bond with silicon ‘d’ orbitals. This conformation seems to be ideal for the hyperconjugative interaction (Fig. 4.5) with the Si–H bond and hence results in strong interaction. Here, the effective transfer of the lone pair of electrons to the Si–H antibonding orbital can take place, thereby decreasing the positive charge on silicon and increasing the negative charge on hydrogen. This is reflected in the weakening of the Si–H bond and strengthening of the Si–N bond (Table IV.6 and IV.7). This results in a nearly planar conformation for the symmetrical derivative (OC?H?N)?SiH as compared to the slightly more pyramidal structure of ((C?H??)?N)(C?H??N)?SiH, which shows a decrease in both (p–d)? bonding and the hyperconjugative interaction with the Si–H bond.
In ((C?H??)?N)(C?H??N)?SiH, non?bonding electrons on nitrogen N1 and N3 are in ideal conformation for the hyperconjugative interaction with Si–N3 and Si–N1 bonds and hence deshield the Si–H proton and shield the silicon nucleus. The effective transfer of electrons from one nitrogen to the antibonding orbital of another Si–N bond occurs, thereby showing an increase in Si–N bond length (1.71 Å). In this compound there is a marginal shortening of the Si–N2 bond length compared to the other two.
Thus, the effective interaction of both (p–d)? and hyperconjugation, electron transfer to Si–H and Si–N bonds has been shown to play a major role in the near?planar structure for the tri(amino)silane (OC?H?N)?SiH and near?pyramidal structure for the mixed aminosilane ((C?H??)?N)(C?H??N)?SiH.
4.7 Qualitative analysis of spectral data of tri(amino)silanes
Analysis of the spectral results (Sect. 3.4.1; Table III.8, III.7 and III.4) taken in the order (1) (DCA)?SiH, (2) (DCA)?(R?N)SiH, (3) (DCA)(R?N)?SiH and (4) (R?N)?SiH, for example ?(Si–H hydrogen): (1) 5.69, (2) 5.10–5.18, (3) 4.34–4.54 and (4) 4.09–4.45 ppm respectively, reveals the deshielding of the hydrogen bonded to silicon. This shows that there is a tendency for the nitrogen to be in pyramidal geometry in bulky systems, thereby indicating a small decrease in conjugation with the Si–H bond. This bond becomes stronger progressively, indicating a decrease of conjugation with this bond and strong hyperconjugation between the nitrogen lone pairs and Si–N bonds which are anti to it. It is also noted that both the (p–d)? interaction and the hyperconjugative interaction decrease the positive charge on silicon. The Si–H bond which has the maximum hyperconjugation will have the minimum force constant. The IR frequency is found to decrease along that row, indicating the weakening of the Si–H bond.
The interactions of interest in all the tri(amino)silanes are hyperconjugative and (p–d)? interactions. These results are explained by assuming a more planar structure around nitrogen in less sterically hindered tri(amino)silanes. The increased pyramidality of the nitrogen is found to favour the negative hyperconjugative interaction with Si–N bond over the Si–H bond. In the case of (R?N)?SiH, (p–d)? interaction, hyperconjugation with Si–H bond as well as Si–N bond predominates. As we go to the higher members, the tendency of the nitrogen to become non?planar increases and hence the hyperconjugation with Si–H bond and also the (p–d)? interaction decreases. But the Si–N hyperconjugative interaction is more predominant here. This results in high vibrational frequency for the Si–H bond, shielding of the silicon nucleus and deshielding of the Si–H protons. Therefore, both these major interactions-the hyperconjugative and (p–d)? interaction-play a major role in deciding the conformation of tri(amino)silanes. Although both these effects explain most of the results, they cannot be quantified in these cases. | |
