Noninnocent Behavior of Ancillary Ligands: Apparent Trans Coupling of a Saturated N-Heterocyclic Carbene Unit with an Ethyl Ligand Mediated by Nickel

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• 作者：Tobias Steinke ; Bryan K. Shaw ; Howard Jong ; Brian O. Patrick ; Michael D. Fryzuk ; Jennifer C. Green
• 刊名：Journal of the American Chemical Society
• 出版年：2009
• 出版时间：August 5, 2009
• 年：2009
• 卷：131
• 期：30
• 页码：10461-10466
• 全文大小：338K
• 年卷期：v.131,no.30(August 5, 2009)
• ISSN：1520-5126

文摘

Oxidative addition of the tridentate N-heterocyclic carbene (NHC) diphosphine ligand precursor ([PCP]H)PF₆ (1) {[PCP] = o-ⁱPr₂PC₆H₄(NC₃H₄N)o-C₆H₄PⁱPr₂} to Ni(COD)₂ results in the formation of the nickel(II) hydride complex ([PCP]NiH)PF₆ (2). This hydride undergoes a rapid reaction with ethylene to generate a nickel(0) complex in which an ethyl group has been transferred to the carbene carbon of the original NHC−diphosphine ligand. If the first intermediate is the anticipated square-planar nickel(II) ethyl species, then the formation of the product would require a process that involves a trans C−C coupling of the NHC carbon and a presumed Ni−ethyl intermediate. Deuterium-labeling studies provide evidence for migratory insertion of the added ethylene into the Ni−H bond rather than into the Ni−carbene linkage; this is based on the observed deuterium scrambling, which requires reversible β-elimination, alkene rotation, and hydride readdition. However, density functional theory studies suggest that a key intermediate is an agostic ethyl species that has the Ni−C bond cis to the NHC unit. A possible transition state containing two cis-disposed carbon moieties was also identified. Such a process represents a new pathway for catalyst deactivation involving NHC-based metal complexes.