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

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–Ccarbene bond distances than the NHC complexes.
Furthermore, DFT calculations predicted that, despite their longer
Au–Ccarbene 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–Ccarbene distance
shorter, whereas the stronger Au–Ccarbene 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.