Crucial Role of Anions on the Deprotonation of the Cationic Dihydrogen Complex trans-[FeH(2-H2)(dppe)<sub

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• 作者：Manuel G. Basallote ; Maria Besora ; C. Esther Castillo ; Marí ; a J. Fern&aacute ; ndez-Trujillo ; Agustí ; Lledó ; s ; Feliu Maseras ; M. Angeles M&aacute ; ñ ; ez
• 刊名：Journal of the American Chemical Society
• 出版年：2007
• 出版时间：May 23, 2007
• 年：2007
• 卷：129
• 期：20
• 页码：6608 - 6618
• 全文大小：430K
• 年卷期：v.129,no.20(May 23, 2007)
• ISSN：1520-5126

文摘

The kinetics of reaction of the dihydrogen complex trans-[FeH(ges/gifchars/eta.gif" BORDER=0 >²-H₂)(dppe)₂]⁺ with an excess ofNEt₃ to form cis-[FeH₂(dppe)₂] shows a first-order dependence with respect to both the metal complex andthe base. The corresponding second-order rate constant only shows minor changes when the solvent ischanged from THF to acetone. However, the presence of salts containing the BF₄^-, PF₆^-, and BPh₄^- anionscauses larger kinetic changes, the reaction being accelerated by BF₄^- and PF₆^- and decelerated in thepresence of BPh₄^-. These results can be interpreted considering that the ion pairs formed by the complexand the anion provide a reaction pathway more efficient than that going through the unpaired metal complex.From the kinetic results in acetone solution, the stability of the ion pairs and the rate constant for theirconversion to the reaction products have been derived. Theoretical calculations provide additional informationabout the reaction mechanism both in the absence and in the presence of anions. In all cases, the reactionoccurs with proton transfer from the trans-dihydride to the base through intermediate structures showingFe-H₂···N and Fe-H···H···N dihydrogen bonds, isomerization to the cis product occurring once the protontransfer step has been completed. Optimized geometries for the ion pairs show that the anions are placedclose to the H₂ ligand. In the case of BPh₄^-, the bulky phenyls hinder the approach of the base and makethe ion pairs unproductive for proton transfer. However, ion pairs with BF₄^- and PF₆^- can interact with thebase and evolve to the final products, the anion accompanying the proton through the whole proton transferprocess, which occurs with an activation barrier lower than for the unpaired metal complex.