Tuning the Gold(I)-Carbon σ Bond in Gold-Alkynyl Complexes through Structural Modifications of the NHC Ancillary Ligand: Effect on Spectroscopic Observables and Reactivity

Understanding the features of the gold(I)-carbon sigma bond and its modulation induced by an ancillary ligand has become fundamental for the purposes of ligand design, due to the increasing interest towards gold(I)-alkynyl complexes and their wide range of applications. We carry out a systematic computational analysis of 16 gold(I)-acetylide complexes bearing different N-Heterocyclic Carbenes (NHCs) as ancillary ligands [NHC-Au(I)-CCH]. The results show that the strength and features of the Au-C bond can be efficiently tuned by performing specific structural modifications on the NHC, enabling a more efficient pi communication between the alkynyl and the ancillary ligand. We also demonstrate that the effect of the bond modulation can be revealed via NMR spectroscopy, as highlighted by the tight correlation between the computed nuclear shielding constants and the bonding parameters. Finally, we show that, for the dual-gold-catalyzed Bergman cyclization as case study, suitable structural modifications on the NHC ligand, which modulate the pi-acidity of the metal fragment sigma-coordinated to an enediyne substrate, could affect the reaction barrier and the thermodynamic stability of the product. All the reported results can be well rationalized in the framework of distortion/interaction analysis, which has been recently extended to the dual (sigma,pi-type) Au catalytic systems by Alabugin et al (J. Am. Chem. Soc. 2017, 137, 3406-3416).