Chiral undecagold clusters: synthesis, characterization and investigation in catalysis

Abstract

Enantiopure undecagold clusters protected by chiral atropisomeric diphosphine ligands (P^P) have been synthesized by the stoichiometric reduction of the corresponding (P^P)(AuCl)(2) complexes with NaBH(4). The molecular mono-disperse [Au(11)(P^P)(4)Cl(2)]Cl species have been thoroughly characterized using an array of analytical techniques. (31)P NMR experiments suggested the presence of a slow intramolecular ligand exchange process. Circular dichroism measurements showed that enantiomeric clusters display mirror-image chiroptical activity. Such undecagold clusters containing two chloride ligands bound to the peripheral Au(I) atoms were expected to display a carbophilic Lewis acidity similar to the well-documented molecular Au(I) complex catalysts. Chloride abstraction, performed to generate active Au(+) sites, induced the Au(11) cluster evolution to larger gold clusters and nanoparticles, together with Au(I) complexes, which, in fact, perform the catalysis. This result was corroborated by running an asymmetric tandem hydroarylation-carbocyclization reaction, for which the enantiomeric excesses obtained with Au(11) clusters are similar to those reported using Au(I) complexes.

From individual to collective chirality in metal nanoparticles

“…Interestingly, for both cases discussed above, the anisotropy factor was reported to be larger (by a factor of 3) for particles directly prepared with the respective thiols, than for those obtained via the ligand exchange method. Thus, apparently only a few chiral ligands are needed to induce significant optical activity in the metalbased electronic transitions [63,66]. All these observations are consistent with a chiral footprint model (mechanism (c)).…”

“…The use of chiral ligands in the proximity of nanoclusters and nanoparticles has already led to the synthesis of important novel nanomaterials with individual dissymmetry, such as intrinsically chiral nanoparticles [54,55], capped nanocrystals with a chiral distribution of the electron density [61,75], and footprinted nanoparticles [64,65]. Although still in their infancy, these systems have shown very high catalytic activity and enantioselectivity toward several organic transformations [66,143], and can be considered potential electrochemical [144], VCD [57], and SEROA (surface enhanced Raman optical activity) [145] sensors. However, the potential uses of nanoparticles with individual chirality as a plasmonic material [52,76] is still hindered primarily due to limitations in colloidal synthesis.…”

Phosphine-Coordinated Pure-Gold Clusters: Diverse Geometrical Structures and Unique Optical Properties/Responses

“…We have also found that the simple NaBH 4 reduction of Au 2 (dppp)Cl 2 in ethanol gives 30 as the main cluster product (f) [69]. On the other hand, the mass spectrometric analyses of the reduction system Au(PPh 3 [71] and MeO-BIPHEP [72] in ethanol (h), which are identified by mass and absorption spectra but their single-crystal X-ray structures are not available.…”

Some insights into the gold-catalysed A 3 -coupling reaction

“…12,13 Despite the ever burgeoning literature concerning gold-catalysed reactions, the majority of structurally defined chiral gold complexes are restricted to those bearing chiral phosphane ligands. [14][15][16][17][18][19] Given the fact the use of gold-NHC complexes [20][21][22][23][24][25][26] is now de rigour in synthesis, it is somewhat surprising that reports of structurally characterised, chiral, Au(I)-NHC [27][28][29][30][31][32][33][34][35][36] and Au(I)-ADC/NAC 37 complexes are still comparatively scarce. Likewise, the synthesis and characterisation and catalytic activity of cyclometallated gold(III) complexes is relatively unexplored, with only a handful of examples being cited in the literature.…”