Relation between the pi-Contribution to Reversed Si=C Bond Polarization and the Reaction Profile for the Thermolytic Formation of Silenes
2008 (English)In: Organometallics, ISSN 0276-7333, E-ISSN 1520-6041, Vol. 27, no 20, 5203-5211 p.Article in journal (Refereed) Published
A quantum chemical investigation of the reaction profiles for the thermal formation of silenes Z(2)Si=C(XSiH3)Y from silanes Z(2)(H3Si)Si-C(=X)Y (X = O, S, NMe; Y = NMe2, OMe, SMe, Me; Z = SiH3, Me) has been performed. Focus was put on the influence of the pi-conjugative contribution to reversed Si=C bond polarization (Si delta-=C delta+) as determined by natural resonance theory (NRT) at the B3LYP density functional theory level. Good linear correlations between the weights of pi-conjugated reverse polarized resonance structures (Sigma Phi(RP)(pi)) in the electronic structure and the Si=C bond lengths were found for the two classes of silenes with Z = SiH3 and Me (r(2) = 0.957 and 0.955, respectively). Silenes that are strongly influenced by the pi-conjugative reverse polarization have low barriers for back-reaction to the silanes, making these silenes more difficult to isolate when formed through a [1,3]-silyl shift than those that are naturally polarized. Modest exponential dependencies of the activation barriers for the reverse reactions on Sigma Phi(RP)(pi) are found (r(2) = 0.685 for Z = SiH3 and r(2) = 0.699 for Z = Me). Species with the silyl groups replaced by trimethylsilyl groups, e.g., the Brook-type silene (Me3Si)(2)Si=C(OSi-Me-3)t-Bu, have lower contributions of Sigma Phi(RP)(pi) by 3-23% than the corresponding model silenes, a result of steric bulk. The weight Sigma Phi(RP)(pi) to the electronic structure of (Me3Si)(2)Si=C(OSiMe3)t-Bu was calculated to be 7.4%. With Z = Me, the silenes are in general not equally influenced by Sigma Phi(RP)(pi) as with Z = SiH3, their energies relative to the silanes are higher, and they have higher activation barriers for both forward and backward reactions.
Place, publisher, year, edition, pages
2008. Vol. 27, no 20, 5203-5211 p.
IdentifiersURN: urn:nbn:se:uu:diva-107830DOI: 10.1021/om800477jISI: 000259988800006OAI: oai:DiVA.org:uu-107830DiVA: diva2:233286