Spectroscopic Characterization of μ-η¹:η¹-Peroxo Ligands Formed by Reaction of Dioxygen with Electron-Rich Iridium...

Although oxygen is a common ligand in supported metal catalysts, its coordination has been challenging to elucidate. We now characterize a diiridium complex that has been previously shown by X-ray diffraction crystallography to incorporate a μ-η¹:η¹-peroxo ligand. We observe markedly enhanced intensity at 788 cm^-1 in the Raman spectrum of this complex, which is a consequence of bonding of the peroxo ligand but does not shift upon ¹⁸O labeling. Electronic structure calculations at the density functional theory level suggest that this increase in Raman intensity results from bands associated with rocking of CH₂ substituents directly attached to P(Ph)₂ groups coupling with the O-O band. These results provide part of the foundation for understanding oxygen ligands on a silica-supported tetrairidium carbonyl cluster stabilized with bulky electron-donating phosphine ligands [p-tert-butyl-calix[4]arene(OPr)₃(OCH₂PPh₂) (Ph = phenyl; Pr = propyl)]. Reaction of the cluster with O₂ also led to the growing in of a Raman band at 788 cm^-1, similar to that in the diiridium complex and also assigned to the bonding of a bridging peroxo ligand. Infrared spectra recorded as the supported cluster reacted in sequential exposures to (i) H₂, (ii) O₂, (iii) H₂, and (iv) CO indicate that two bridging peroxo ligands were bonded irreversibly per tetrairidium cluster, replacing bridging carbonyl ligands without altering either the cluster frame or the phosphine ligands. X-ray absorption near edge and infrared spectra include isosbestic points signifying a stoichiometrically simple reaction of the cluster with O₂, and mass spectra of the effluent gas show that CO₂ formed by oxidation of one terminal CO ligand per cluster as H₂ (and not H₂O) formed, evidence that hydride ligands had been present on the cluster following treatment (i). The understanding of how O₂ reacts with the metal polyhedron provides a foundation for understanding of how oxidation catalysis may proceed on the surfaces of noble metals.