Monitoring of the Pre-Equilibrium Step in the Alkyne Hydration Reaction Catalyzed by Au(III) Complexes: A Computational Study Based on Experimental Evidences.

Flavio Sabatelli,Jacopo Segato,Paola Belanzoni,Alessandro Del Zotto,Daniele Zuccaccia,Leonardo Belpassi

doi:10.3390/molecules26092445

Flavio Sabatelli, Jacopo Segato + Show 4 more

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https://doi.org/10.3390/molecules26092445

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Abstract

The coordination ability of the [(ppy)Au(IPr)]2+ fragment [ppy = 2-phenylpyridine, IPr = 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene] towards different anionic and neutral X ligands (X = Cl−, BF4−, OTf−, H2O, 2-butyne, 3-hexyne) commonly involved in the crucial pre-equilibrium step of the alkyne hydration reaction is computationally investigated to shed light on unexpected experimental observations on its catalytic activity. Experiment reveals that BF4− and OTf− have very similar coordination ability towards [(ppy)Au(IPr)]2+ and slightly less than water, whereas the alkyne complex could not be observed in solution at least at the NMR sensitivity. Due to the steric hindrance/dispersion interaction balance between X and IPr, the [(ppy)Au(IPr)]2+ fragment is computationally found to be much less selective than a model [(ppy)Au(NHC)]2+ (NHC = 1,3-dimethylimidazol-2-ylidene) fragment towards the different ligands, in particular OTf− and BF4−, in agreement with experiment. Effect of the ancillary ligand substitution demonstrates that the coordination ability of Au(III) is quantitatively strongly affected by the nature of the ligands (even more than the net charge of the complex) and that all the investigated gold fragments coordinate to alkynes more strongly than H2O. Remarkably, a stabilization of the water-coordinating species with respect to the alkyne-coordinating one can only be achieved within a microsolvation model, which reconciles theory with experiment. All the results reported here suggest that both the Au(III) fragment coordination ability and its proper computational modelling in the experimental conditions are fundamental issues for the design of efficient catalysts.

Highlights

Compared to Au(I) [1,2,3,4,5,6,7,8,9,10,11], catalysis by Au(III) is far less developed
The hydration of alkynes is an important reaction in organic chemistry, and it turns to be one of the most environmentally friendly methods to form the C=O bond [41,42]. All these findings suggest that the coordination ability of Au(III) complexes is a fundamental issue for the design of increasingly performing catalysts
The aim of this work is exactly to computationally investigate the coordination ability of catalytic Au(III) complexes and its dependence on the ancillary ligands on the basis of some unexpected experimental evidences acquired by studying the alkyne hydration reaction

Summary

Introduction

Compared to Au(I) [1,2,3,4,5,6,7,8,9,10,11], catalysis by Au(III) is far less developed. The study of Au(III) chemistry is much more challenging both experimentally and theoretically, because these complexes are usually very reactive and difficult to synthesize. In recent years, many studies on the catalytic efficiency of Au(III) have been carried out, motivated by the expected more efficient activation of double and triple C-C bonds due to the larger Lewis acidity of the metal in its +3 oxidation state [12,13,14,15,16,17,18,19,20,21,22,23,24,25]. First studies were devoted to synthesize stable Au(III) complexes. The oxidative addition product, L-AuX3, formed from

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