Steric versus Electronic Effects in the Structure of Heteroatom (S and O)-Substituted Free and Metal (Cr and W)-Complexed Carbenes

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• 作者：Diego M. Andrada ; Martí ; n E. Zoloff Michoff ; Israel Fern&aacute ; ndez ; Alejandro M. Granados ; Miguel A. Sierra
• 刊名：Organometallics
• 出版年：2007
• 出版时间：November 19, 2007
• 年：2007
• 卷：26
• 期：24
• 页码：5854 - 5858
• 全文大小：198K
• 年卷期：v.26,no.24(November 19, 2007)
• ISSN：1520-6041

文摘

DFT computational energies for free alkoxy- and thio-substitued free carbenes correlate withexperimental steric data (Taft parameters) showing the importance of the steric volume at the expense ofthe electronic interaction in determining the conformation of free carbenes 1-14. The shorter C-O bonddistances in the alkoxy derivatives render alkoxycarbene more sensitive to steric effects than thiocarbenes.This strong dependence of the conformation with the steric hindrance is not applicable to metal-complexedcarbenes. Thio-substituted carbene complexes exist exclusively as syn-isomers due to the combinationof an interaction of the sulfur lone pair with two CO ligands and the steric repulsion with the "CO-wall".The stronger C_carbene-S bond compared to C_carbene-O is responsible for the increased syn-anti rotationbarrier observed for the alkylthio-substituted metal carbene complexes compared to their oxygen analogues.The differences of polarity between syn- and anti-isomers of alkylthio-substituted metal carbene complexesexplain also the increment of the isomerization barrier with the polarity of the solvent. The effect of thesubstituent attached to the carbene carbon is not decisive in the conformation of these compounds.