Methane Activation with Rhenium Catalysts. 1. Bidentate Oxygenated Ligands

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• 作者：Jason M. Gonzales ; Jonas Oxgaard ; Roy A. Periana ; William A. Goddard III
• 刊名：Organometallics
• 出版年：2007
• 出版时间：March 12, 2007
• 年：2007
• 卷：26
• 期：6
• 页码：1505 - 1511
• 全文大小：141K
• 年卷期：v.26,no.6(March 12, 2007)
• ISSN：1520-6041

文摘

Trends in methane activation have been explored for rhenium-based catalysts in conjunction withbidentate oxygenated ligands of the form (L₁)(L₂)Re(OH)(OH₂) [L₁, L₂ = acac, catechol, glycol]. Whenplaced in acidic media, the equilibrium for this reference catalyst shifts to the protonated forms (L₁)(L₂)Re(OH₂)(OH₂) in almost all cases. In all cases the activation of the reference complex proceedsthrough a concerted metathesis type transition state, and only one of the 13 reference complexes proceedswith methane activation through a barrier of less than 35 kcal mol^-1. Study of the identity complexes (L₁= L₂) revealed that protonation of the ligand oxygens is unfavorable for acac and catechol, but favorablefor glycol; however in only one case is the barrier for methane activation improved by this route. Electrondensity on the central rhenium is the best predictor for the magnitude of the methane activation barrier;namely, increased electron density (obtained by considering lower oxidation states) on the metal leads tolower barriers. Lower oxidation states form weaker Re-O bonds, which increase lability of the leavinggroups and decrease the barrier to proton transfer from methane.