Monodisperse Iridium Clusters Protected by Phenylacetylene: Implication for Size-Dependent Evolution of Binding Sites

Terminal alkynes form a variety of interfacial structures with the clusters and nanoparticles of Au, Ag, Cu, Ru, Pd, and Pt. In order to extend the scope of modification by alkynes, we herein report the synthesis and structure characterization of Ir clusters protected by phenylacetylene (PA). Small, monodisperse PA-protected Ir clusters (1.3 ± 0.2 nm) were obtained by biphasic ligand exchange from ethylene glycol-stabilized Ir clusters (1.5 ± 0.2 nm). High-resolution transmission electron microscopy, extended X-ray absorption fine structure analysis, and powder X-ray diffraction analysis indicated that Ir clusters exhibit a face-centered cubic structure. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy indicated Ir atoms are almost in the zero valence state, which is in sharp contrast to the partial oxidation observed for Ir clusters stabilized by polymers. Absence of the terminal hydrogen of PA and red-shift of the C≡C stretching mode upon ligation observed by Fourier-transform infrared spectroscopy showed that the PA ligands are bound to Ir clusters via Ir–C bonds. Mass spectrometry revealed that the number of the Ir atoms in Ir n (PA) m has a much narrower distribution ( n = 46–53) than that of the PA ligands ( m = 19–33). This finding suggests a drastic change in the binding sites of PA ligands even with a small change of the size of Ir clusters. Surprisingly, the number of PA ligands ( m ) decreased with increase in the size of Ir clusters ( n ). Within the framework of the upright binding model of PA, this counterintuitive correlation between n and m values suggests that binding sites of PA are shifted dramatically from on-top sites to bridged sites with increase in the cluster size.