Heterolytic Cleavage of Dihydrogen by an Iron(II) PNP Pincer Complex via Metal鈥揕igand Cooperation

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• 作者：Bernhard Bichler ; Christian Holzhacker ; Berthold St枚ger ; Michael Puchberger ; Luis F. Veiros ; Karl Kirchner
• 刊名：Organometallics
• 出版年：2013
• 出版时间：August 12, 2013
• 年：2013
• 卷：32
• 期：15
• 页码：4114-4121
• 全文大小：383K
• 年卷期：v.32,no.15(August 12, 2013)
• ISSN：1520-6041

文摘

The bis-carbonyl Fe(II) complex trans-[Fe(PNP-iPr)(CO)₂Cl]⁺ reacts with Zn as reducing agent under a dihydrogen atmosphere to give the Fe(II) hydride complex cis-[Fe(PNP-iPr)(CO)₂H]⁺ in 97% isolated yield. A crucial step in this reaction seems to be the reduction of the acidic NH protons of the PNP-iPr ligand to afford H₂ and the coordinatively unsaturated intermediate [Fe(PNP^H-iPr)(CO)₂]⁺ bearing a dearomatized pyridine moiety. This species is able to bind and heterolytically cleave H₂ to give cis-[Fe(PNP-iPr)(CO)₂H]⁺. The mechanism of this reaction has been studied by DFT calculations. The proposed mechanism was supported by deuterium labeling experiments using D₂ and the N-deuterated isotopologue of trans-[Fe(PNP-iPr)(CO)₂Cl]⁺. While in the first case deuterium was partially incorporated into both N and Fe sites, in the latter case no reaction took place. In addition, the N-methylated complex trans-[Fe(PNP^Me-iPr)(CO)₂Cl]⁺ was prepared, showing no reactions with Zn and H₂ under the same reaction conditions. An alternative synthesis of cis-[Fe(PNP-iPr)(CO)₂H]⁺ was developed utilizing the Fe(0) complex [Fe(PNP-iPr)(CO)₂]. This compound is obtained in high yield by treatment of either trans-[Fe(PNP-iPr)(CO)₂Cl]⁺ or [Fe(PNP-iPr)Cl₂] with an excess of NaHg or a stoichiometric amount of KC₈ in the presence of carbon monoxide. Protonation of [Fe(PNP-iPr)(CO)₂] with HBF₄ gave the hydride complex cis-[Fe(PNP-iPr)(CO)₂H]⁺. X-ray structures of both cis-[Fe(PNP-iPr)(CO)₂H]⁺ and [Fe(PNP-iPr)(CO)₂] are presented.