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

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.

Experimental and Theoretical Investigation of Ion Pairing in Gold(III) Catalysts

“…The same structure of the cation is observed for [(ppy)Au(NHC iPr )Cl]BF 4 and [(ppy)Au(NHC iPr )H 2 O](BF 4 ) 2 . 16,17 In addition, the same configuration is observed for [(ppy)Au(NHC iCy )Cl]BF 4 [NHC iCy = 1,3-dicyclohexyl-imidazol-2-ylidene] in solid state and predicted by DFT calculations. 16,17 The complete assignment of all 1 H resonances and subsequently the structure of the cation is mandatory in order to obtain information on relative anion−cation position (ion pairing structure, Figure 1) by the 19 F, 1 H-HOESY NMR spectrum technique.…”

“…16,17 In addition, the same configuration is observed for [(ppy)Au(NHC iCy )Cl]BF 4 [NHC iCy = 1,3-dicyclohexyl-imidazol-2-ylidene] in solid state and predicted by DFT calculations. 16,17 The complete assignment of all 1 H resonances and subsequently the structure of the cation is mandatory in order to obtain information on relative anion−cation position (ion pairing structure, Figure 1) by the 19 F, 1 H-HOESY NMR spectrum technique. Given the "complicated and unsymmetrical" intramolecular structure of the cation, since several inequivalent "reporter" nuclei may assess the relative anion− cation orientation, different noteworthy regions can be identified where the anion can locate which are near the imidazolium ring (Figure 1, NHC side) and on the side of the pyridine of phenylpyridine ligand (Figure 1 In order to understand the position of the counterion around the cationic metal fragment, we recorded a 19 F, 1 H-HOESY NMR spectrum (Figure 2).…”

“…Unfortunately, any attempts to synthesize, isolate, and characterize [(ppy)Au(NHC iPr )OTf]OTf have failed. As previously observed 16,17 [(ppy)Au(NHC iPr )OTf]OTf, generated in situ in an NMR tube, is mixed with [(ppy)Au-(NHC iPr )Cl]OTf and any attempt of purification did not lead to the desired results. Therefore, we did not analyze the ion pairing structure because the 19 F, 1 H-HOESY NMR spectrum could not be analyzed due to its difficult evaluation.…”

Mechanism of Alkyne Hydroarylation Catalyzed by (P,C)-Cyclometalated Au(III) Complexes

“…It is worthy to note that the entry point (and likely rate-determining step) of the outer-sphere mechanism is the displacement of TMB by DPA at Au(III) (pre-equilibrium). 29 In this path, the key intermediates are the π-alkyne Au(III) complexes 6/6′-iPr. In contrast with the TMB adduct, the most stable isomer is the one with DPA coordinated trans to P, 6-iPr.…”

“…It results in vinyl Au­(III) complexes with the aryl group trans to gold (in line with the formation of the experimentally obtained ANTI addition product after stereoretentive protodeauration). It is worthy to note that the entry point (and likely rate-determining step) of the outer-sphere mechanism is the displacement of TMB by DPA at Au­(III) (pre-equilibrium) …”