PHOTOINDUCED ADDITION OF DIOXYGEN MOLECULES IN THE UNSATURATED SITES OF THE PD-3(DPPM)(3)CO2+ CATALYST

The photoinduced addition of O-2 onto the unsaturated cluster Pd-3(dppm)(3)CO2+ (as a CF3CO2- salt) in acetonitrile is reported. The final product Pd-3(dppm)(3)(O-2)(2)(2+) (nu(O-2) = 838 and 866 cm(-1)) is formed in a multistep fashion in which two photochemical intermediates are observed (presumably Pd-3(dppm)(3)(O-2)(CO)(2+) and Pd-3(dppm)(3)(O-2)(2+)). No X-ray structure could be obtained, but numerous spectroscopic findings demonstrate that O-2 binds the Pd-3 center as a peroxo-O-2, and acts as a two-electron donor that triply bridges the metal atoms (forming a 44-electron cluster). The very small excited state lifetimes (between 25 and 35 +/- 10 ps) obtained by picosecond flash photolysis indicate that the primary photoreaction is unimolecular, and demonstrate that the first photochemically added O-2 molecule must be preassembled in the excited state prior to any photoinduced transformation. This Pd-3(dppm)(3)CO2+...O-2 ground state complex is responsible for the photoinduced production of the bisdioxygen compound and can be observed by UV-visible spectroscopy. The low efficiency of the photoreaction (quantum yield (Phi) = 0,033 +/- 0.004) is explained by the very short excited state lifetime at 298 K, and the competition of O-2 with solvent molecules to occupy the unsaturated site of the empty cavity in Pd-3(dppm)(3)CO2+ (i.e., ground stale guest-host chemistry). The binding constant for O-2 with Pd-3(dppm)(3)CO2+ is roughly estimated to range between 1 and 730 M(-1) in the ground state and is considered to be weak.