Photoinduced addition of dioxygen molecules in the unsaturated sites of the Pd3(dppm)3CO2+ catalyst

Abstract

The photoinduced addition of O2 onto the unsaturated cluster Pd3(dppm)3CO2+ (as a CF3CO 2- salt) in acetonitrile is reported. The final product Pd3(dppm)3(O2)22+ (ν(O2) = 838 and 866 cm-1) is formed in a multi-step fashion in which two photochemical intermediates are observed (presumably Pd3(dppm)3(O2) (CO)2+ and Pd 3(dppm)3(O2)2+). No X-ray structure could be obtained, but numerous spectroscopic findings demonstrate that O 2 binds the Pd3 center as a peroxo-O2, 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 O2 molecule must be preassembled in the excited state prior to any photoinduced transformation. This Pd3(dppm) 3CO2+⋯O2 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 (Φ) = 0.033 ± 0.004) is explained by the very short excited state lifetime at 298 K, and the competition of O2 with solvent molecules to occupy the unsaturated site of the empty cavity in Pd 3(dppm)3CO2+ (i.e., ground state guest-host chemistry). The binding constant for O2 with Pd3(dppm) 3CO2+ is roughly estimated to range between 1 and 730 M-1 in the ground state and is considered to be weak.