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Relation between the π-Contribution to Reversed Si═C Bond Polarization and the Reaction Profile for the Thermolytic Formation of Silenes
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Biochemistry and Organic Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Biochemistry and Organic Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Biochemistry and Organic Chemistry.
2008 (English)In: Organometallics, ISSN 0276-7333, E-ISSN 1520-6041, Vol. 27, no 20, 5203-5211 p.Article in journal (Refereed) Published
Abstract [en]

A quantum chemical investigation of the reaction profiles for the thermal formation of silenes Z2Si═C(XSiH3)Y from silanes Z2(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 π-conjugative contribution to reversed Si═C bond polarization (Siδ−═Cδ+) as determined by natural resonance theory (NRT) at the B3LYP density functional theory level. Good linear correlations between the weights of π-conjugated reverse polarized resonance structures (ΣΦRP(π)) in the electronic structure and the Si═C bond lengths were found for the two classes of silenes with Z = SiH3 and Me (r2 = 0.957 and 0.955, respectively). Silenes that are strongly influenced by the π-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 ΣΦRP(π) are found (r2 = 0.685 for Z = SiH3 and r2 = 0.699 for Z = Me). Species with the silyl groups replaced by trimethylsilyl groups, e.g., the Brook-type silene (Me3Si)2Si═C(OSiMe3)t-Bu, have lower contributions of ΣΦRP(π) by 3−23% than the corresponding model silenes, a result of steric bulk. The weight ΣΦRP(π) to the electronic structure of (Me3Si)2Si═C(OSiMe3)t-Bu was calculated to be 7.4%. With Z = Me, the silenes are in general not equally influenced by ΣΦRP(π) 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.
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:uu:diva-97587DOI: 10.1021/om800477jOAI: oai:DiVA.org:uu-97587DiVA: diva2:172587
Available from: 2008-10-03 Created: 2008-10-03 Last updated: 2013-05-16Bibliographically approved
In thesis
1. Lowcoordinated Silicon and Hypercoordinated Carbon: Structure and Stability of Silicon Analogs of Alkenes and Carbon Analogs of Silicates
Open this publication in new window or tab >>Lowcoordinated Silicon and Hypercoordinated Carbon: Structure and Stability of Silicon Analogs of Alkenes and Carbon Analogs of Silicates
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Quantum chemical studies on lowcoordinated group 14-16 compounds have been performed. This thesis focuses particularly on silenes influenced by reverse Siδ-=Cδ+ bond polarization. Hypercoordinated carbon compounds are also studied.

The geometries from calculations with several common computationally inexpensive methods have been tested against high level CCSD/cc-pVTZ geometries for a series of substituted silenes. Hybrid HF/DFT methods performed best among the inexpensive methods tested for silenes.

Heavy alkenes strongly influenced by reverse polarization are found to have less exothermic dimerization energies for both head-to-head and head-to-tail dimerizations, and to have higher activation energies for water addition than naturally polarized heavy alkenes.

We also investigated solvated lithium, magnesium and potassium silenolates and found that lithium and magnesium ions coordinate preferably to O, giving their SiC bond some double bond character.

Reverse polarized 2-siloxy-, 2-thiosiloxy-, and 2-(N-sila-N-methyl)-silenes could according to calculations be formed thermolytically from the corresponding tetrasilanes as transient species. It was, however, found that silenes highly influenced by π-conjugative reverse polarization have low barriers for the back-reaction, and thus these silenes are more difficult to form as stable species than naturally polarized silenes.

It is also found that conjugated 1-siladienes, formed by electrocyclic ring-opening of 1-silacyclobut-2-enes, which are highly influenced by π-conjugative reverse polarization, have higher barriers for electrocyclization back to starting material than naturally polarized 1-siladienes.

It is found that CHe54+, CHe64+, CNe54+, and CNe64+ are the closest carbon analogs of SiH5-, SiH62-, SiF5- and SiF62-, respectively. However, due to their exothermic dissociation reaction, these very high-lying local minima will be impossible to reach experimentally.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2008. 85 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 557
Keyword
silenes, zwitterionic silenes, silenolates, reverse polarization, natural resonance theory, hypervalent carbon, hypercoordinated carbon, thermolysis, lowcoordinated silicon
National Category
Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-9298 (URN)978-91-554-7294-8 (ISBN)
Public defence
2008-10-25, C8:301, BMC, Husargatan 3 753 21, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2008-10-03 Created: 2008-10-03 Last updated: 2009-03-26Bibliographically approved

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