A Hexanuclear Gold Cluster Supported by Three‐Center–Two‐Electron Bonds and Aurophilic Interactions

Abstract

Gold clusters or nanoparticles have been proposed as catalytically active species in a variety of homogeneous processes, [1-5] which might proceed by mechanisms that are very different from those occurring under homogeneous conditions. [6, 7] The preparation of new types of small gold clusters has attracted recent attention because of their structural novelty and potential for the discovery of new reaction types. [8] Herein we report the ready preparation of a robust hexagold cluster composed exclusively by Au I centers that is catalytically active in the activation of alkynes. [9] We decided to synthesize new gold(I) complexes of type 1 in which the ortho-boryl group could enhance the electro-philicity of the metal center in the catalytic activation of alkynes, allenes, and alkenes. This catalyst design was inspired both by the high reactivity displayed by cationic catalysts of type 2 with sterically hindered Buchwald phosphines [10] and by the stability of gold(I) boratranes, such as 3 developed by Bourissou, that display significant donor-acceptor Au!B interactions. [11] Complexes of type 1 could undergo bimolecular Au I /B transmetalation to form known dinuclear complexes of type 4. [12] Indeed, the reaction of [LAuX] with arylboronic acids leads to aryl gold(I) derivatives [ArAuL] in a general manner, [13, 14] while by using two equivalents of [Ph 3 PAuNTf 2 ], diaurated complexes [(m-aryl)(Ph 3 PAu) 2 ]NTf 2 (5) are obtained (Scheme 1). [15] Very similar gem-diarurated complexes have been obtained by reaction of arylboronic acids with two equivalents of [{Au(IPr)} 2 (m-OH)][BF 4 ]. [16] Complexes 6 and 7 a (Scheme 2) were easily prepared in 90-92 % yield by reaction of the corresponding o-borylphos-phines [17] with [Au(tht)Cl] (tht = tetrahydrothiophene). Inter-Scheme 1. Formation of diaurated complexes 5.

New Endeavors in Gold Catalysis—Size Matters

“…The other nanosized gold catalyst was reported by Echavarren et al 11b. For several reasons, it has been accepted that ligand‐based gold clusters were inactive in catalysis, in contrast to their mononuclear and naked analogues (gold nanoparticles).…”