Structural Evolution of Iridium Oxide Cluster Anions Ir _nO _m^- (n = 5-8) with Sequential Oxidation: Binding Mode of O Atoms and Ir Framework

Geometric structures of small cluster anions of iridium oxide Ir_nO_m^- (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 Ir₅O_m^- 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 Ir₈O_m^- 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.