Structural Evolution of Iridium Oxide Cluster Anions Ir<i>_n</i>O<i>_m</i>^– (<i>n</i> = 5–8) with Sequential Oxidation: Binding Mode of O Atoms and Ir Framework

Geometric structures of small cluster anions of iridium oxide IrnOm– (n = 5–8, m = 0–14) were investigated by ion mobility mass spectrometry (IMMS) and theoretical calculation to clarify the evolutional behavior of the binding modes of the O atoms and the motif of the Ir frameworks as a function of m. The collision cross sections (CCSs) for n = 5–7 determined by IMMS showed a monotonic increase with m, whereas those for n = 8 dropped abruptly by ∼10% at m = 11, which otherwise gradually increased with m. For n = 5, detailed comparison between the experimental CCS values and those calculated for various structural isomers revealed that the O atoms in Ir5Om– are preferentially bonded to terminal sites of the Ir5– cores throughout m = 1–8: the Ir5– cores retained compact motifs up to m = 5 but distorted significantly for m = 6–8. The experimental CCS values for Ir6O6– and Ir7O7– were also explained by on-top binding of O atoms while retaining the motifs of the Ir frameworks. The CCS values for n = 8 suggest that the O atoms are sequentially bonded to terminal sites of the cubic Ir8 frameworks for m = 1–6 and of the deformed Ir8 frameworks for m = 7–11. These on-top binding modes of the O atoms on small Ir clusters are in sharp contrast to the μ2 mode of the O atoms at bridge sites on the clusters of other transition metals and are ascribed to non-face-centered cubic (fcc) Ir frameworks. The sudden drop in the CCS values of Ir8Om– at m = 11 was associated with bond breakage in the Ir framework induced by transition of the binding mode of O atoms from an on-top motif to a μ2 bridging motif. These results suggest that the small Ir clusters exhibit novel oxidation catalysis by taking advantage of the unique chemical properties of O atoms adsorbed on the on-top sites on non-fcc Ir clusters