If the Crown Fits: Sterically Demanding -Heterocyclic Carbene Promotes the Formation of AuPt Nanoclusters.

While -heterocyclic carbenes (NHCs) have recently been shown to be effective ligands for gold nanoclusters, very few examples of heterometallic clusters incorporating nongroup 11 metals are known. We present herein an Au-Pt NHC cluster featuring a crown-shaped [AuPt(NHC)] core, produced in high yield without the need for chromatographic purification. The method was largely independent of the substitution pattern of the NHC backbone; however, bulky wingtip groups were needed for clean conversion to the AuPt cluster. Clusters were characterized using single crystal X-ray diffraction, multinuclear nuclear magnetic resonance, electrospray ionization mass spectroscopy, and ultraviolet-visible spectroscopy, and electrochemical features of the cluster are also presented. A detailed analysis of the in-progress reaction mixture by ESI-MS supports the direct involvement of Au-H species as intermediates in cluster formation. These studies further demonstrate that NHC wingtip sterics play a key part in determining the nature of the initial cluster species, providing critical information for the generation of new NHC-stabilized nanoclusters.