Abnormal N-Heterocyclic Carbene Gold(I) Complexes: Synthesis, Structure, and Catalysis in Hydration of Alkynes

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• 作者：Xiangya Xu ; Seung Hyo Kim ; Xi Zhang ; Atanu Kumar Das ; Hajime Hirao ; Soon Hyeok Hong
• 刊名：Organometallics
• 出版年：2013
• 出版时间：January 14, 2013
• 年：2013
• 卷：32
• 期：1
• 页码：164-171
• 全文大小：419K
• 年卷期：v.32,no.1(January 14, 2013)
• ISSN：1520-6041

文摘

Two abnormal N-heterocyclic carbene (aNHC) gold(I) complexes, [(aNHC)AuCl], were prepared from C2-protected imidazolium salts. The air-stable complexes chloro(1-isopropyl-3-methyl-2,4-diphenylimidazol-5-ylidene)gold(I) (5) and chloro(1,4-diisopropyl-3-methyl-2-phenylimidazol-5-ylidene)gold(I) (6) were synthesized via transmetalation using (SMe₂)AuCl and the corresponding silver salt such as [(aNHC)AgI] or [(aNHC)₂Ag][I] and were fully characterized by NMR and mass spectroscopy and by X-ray crystallography. To investigate the structure, bonding, and catalytic activity of the aNHC-based Au complexes in comparison with their traditional NHC analogues, the sterically similar NHC-based Au complexes chloro(1,3-diisopropylimidazol-2-ylidene)gold(I) (7) and chloro(3-isopropyl-1-phenylimidazol-2-ylidene)gold(I) (8) were prepared from 1,3-diisopropylimidazolium iodide (3) and 3-isopropyl-1-phenylimidazolium iodide (4), respectively. X-ray crystallography and density functional theory (DFT) calculations showed that the aNHC complexes have longer Au鈥揅_carbene bond distances than the NHC complexes. Furthermore, DFT calculations predicted that, despite their longer Au鈥揅_carbene distances, aNHC complexes have stronger binding energies. It is suggested on the basis of additional theoretical analyses that these counterintuitive trends can be rationalized by considering individual factors that comprise the molecular interaction. The efficient back-donation of electrons and the smaller overlap repulsion in NHC complexes render the Au鈥揅_carbene distance shorter, whereas the stronger Au鈥揅_carbene bonding in the aNHC-Au complexes is attributed to the greater electrostatic attraction and the higher electron-donating ability of the carbene lone pair orbital. Catalytic activities of the NHC-based Au complexes were also compared in the alkyne hydration. Traditional NHC-based Au complexes exhibited higher efficiency in the reaction.