Abstract:
Catalytic activities of transition metal complexes are influenced by both steric and electronic
effects arising from ancillary ligands. In the case of linear gold (I) complexes, the available
evidence suggests that ligand electronic effects are more important. However, available methods
to investigate the donor abilities of ligands are not easily conducted with gold (I) complexes. In
this study, the donor abilities of selected carbene ligands, including N-heterocyclic carbenes
(NHCs), abnormal N-heterocyclic carbenes (aNHCs), and acyclic diaminocarbenes (ADCs), as
well as phosphine ligands, at a gold (I) center were studied. Methyl isocyanide adducts and 1,3-
diisopropylbenzimidazolin-2-ylidene (i Pr2-bimy) adducts of a series of gold complexes were
synthesized as IR probes and 13C NMR probes, respectively. The gold-catalyzed intramolecular
cyclization of alkynes, which is known to be accelerated by strong donor ligands, showed an
inverse relationship between the Δν of the MeNC IR probe and the reaction rate. The same trend
was observed with the 13C NMR probe, but the MeNC IR probes gave a better correlation with
catalytic rates. Explanations of the observed donicity trends in terms of bonding effects will be
discussed.
