Effects of Phosphine鈥揅arbene Substitutions on the Electrochemical and Thermodynamic Properties of Nickel Complexes

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• 作者：Brandon R. Galan ; Eric S. Wiedner ; Monte L. Helm ; John C. Linehan ; Aaron M. Appel
• 刊名：Organometallics
• 出版年：2014
• 出版时间：May 12, 2014
• 年：2014
• 卷：33
• 期：9
• 页码：2287-2294
• 全文大小：354K
• 年卷期：v.33,no.9(May 12, 2014)
• ISSN：1520-6041

文摘

Nickel(II) complexes containing chelating N-heterocyclic carbene鈥損hosphine ligands ([NiL₂](BPh₄)₂, for which L = [MeIm(CH₂)₂PR₂], MeIm = 1-methylimidizolylidene, and R = Ph or Et) have been synthesized for the purpose of studying how this class of ligand affects the electrochemical and thermodynamic properties compared to the nickel bis-diphosphine analogues. The nickel complexes were synthesized and then characterized by X-ray crystallography, electrochemical methods, and thermodynamic studies, including DFT calculations. On the basis of the reduction potentials (E掳), substitution of an NHC for one of the phosphines in a diphoshine ligand resulted in negative shifts in potential by 0.6 to 1.2 V relative to the corresponding nickel bis-diphosphine complexes. From computational studies of the nickel hydride complex of the phenyl-substituted phosphine鈥揷arbene ligand, the hydride donor ability was determined to improve by 32 kcal/mol relative to the estimated hydride donor ability for the analogous nickel complex of the chelating diphosphine ligand 1,3-bis(diphenylphosphino)propane. The free energy for addition of H₂ is presented, and the implications for catalysis are discussed. These quantitative results highlight the substantial effect that NHC ligands can have on the electronic properties of the metal complexes.