Insight into the Mechanism of the CuAAC Reaction by Capturing the Crucial Au₄Cu₄–π-Alkyne Intermediate

Abstract

The classic Fokin mechanism of the CuAAC reaction of terminal alkynes using a variety of Cu(I) catalysts is wellknown to include alkyne deprotonation involving a bimetallic σ,π-alkynyl intermediate. In this study, we have designed a CNTsupported atomically precise nanocluster Au 4 Cu 4 (noted Au 4 Cu 4 /CNT) that heterogeneously catalyzes the CuAAC reaction of terminal alkynes without alkyne deprotonation to a σ,π-alkynyl intermediate. Therefore, three nanocluster−π-alkyne intermediates [Au 4 Cu 4 (π-CHC-p-C 6 H 4 R)], R = H, Cl, and CH 3 , have been captured and characterized by MALDI-MS. This Au 4 Cu 4 /CNT system efficiently catalyzed the CuAAC reaction of terminal alkynes, and internal alkynes also undergo this reaction. DFT results further confirmed that HCCPh was activated by π-complexation with Au 4 Cu 4 , unlike the classic dehydrogenation mechanism involving the bimetallic σ,π-alkynyl intermediate. On the other hand, a Cu 11 /CNT catalyst was shown to catalyze the reaction of terminal alkynes following the classic deprotonation mechanism, and both Au 11 /CNT and Cu 11 /CNT catalysts were inactive for the AAC reaction of internal alkynes under the same conditions, which shows the specificity of Au 4 Cu 4 involving synergy between Cu and Au in this precise nanocluster. This will offer important guidance for subsequent catalyst design.

N-Heterocyclic Carbene-Stabilized Gold Nanoclusters with Organometallic Motifs for Promoting Catalysis

“…Together with the well-defined structures that can be determined by X-ray crystallography, the presence of both organic ligands and metal–metal bonds makes ligand-protected atomically precise metal nanoclusters (NCs) an ideal model catalyst system to bridge the gap between organometallic catalysts and metal nanocatalysts. − For example, the surface coordination structures of ligands, e.g., phosphine, thiolate, alkynyl, and N-heterocyclic carbene (NHC), on metal NCs have now been well demonstrated. ,, Moreover, the molecular understanding of their catalytic mechanism is expected to provide insights on how surface ligands and metal–metal interactions interplay together to determine the catalytic performance of metal catalysts. − Among them, the recently emerging Au NCs protected by NHCs are expected to serve as an ideal candidate for catalytic mechanism investigations due to their outstanding stability but possible presence of exposure metal sites. − On the other hand, homogeneous gold catalysts with NHC-Au-X structures have been widely applied in chemical transformations, including, but not limited to, the activation and transformation of alkynes, alkenes, and allenes . While the NHC ligands prevent the complex decomposition, and control electronic and steric effects for modulating catalysis, the anionic (X) ligands help balance the charge and often get involved in catalytic cycles by ligand exchange with substrates .…”

Insights into mechanisms of diphosphine-mediated controlled surface construction on Au nanoclusters

“…[1][2][3][4][5][6] More importantly, evaluation of the atomically precise structure of nanoclusters has revealed that the nature of nanoclusters is closely related to their diverse structures. [7][8][9][10][11][12][13] Therefore, nanoclusters are considered ideal models for resolving questions related to various important issues that affect practical applications and fundamental scientific research, such as catalytic mechanisms, [14][15][16][17] the origin of chirality, 7,[18][19][20][21][22] and fluorescence. [23][24][25][26] On the other hand, various theoretical models have been constructed to comprehend the stability and size evolution pattern of nanoclusters.…”

Atomically Precise Cun(n=3, 6 and 11) Nanocatalysts for Alkyne‐Haloalkane‐Amine (AHA) Coupling Reaction

“…Recently, atomically precise nanoclusters have attracted a considerable attention in the area of catalysis, exhibiting remarkable catalytic performance in various chemical reactions [21–24] . Furthermore, nanoclusters(NCs) offer a new perspective for understanding the structure/activity relationships at the molecular level owning to their perfect monodispersion and well‐defined structures [25,26] .…”

“…Recently, atomically precise nanoclusters have attracted a considerable attention in the area of catalysis, exhibiting remarkable catalytic performance in various chemical reactions. [21][22][23][24] Furthermore, nanoclusters(NCs) offer a new perspective for understanding the structure/activity relationships at the molecular level owning to their perfect monodispersion and well-defined structures. [25,26] For example: Jin and co-workers reported the oxide-supported spherical Au 25 (SR) 18 and rod-shaped Au 25 (PPh 3 ) 10 (C�CPh) 5 X 2 (X=Cl, Br) NCs for the semihydrogenation process of alkynes and identified a new alkyne-activation pathway.…”