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

The photoinduced addition of O 2 onto the unsaturated cluster Pd 3 (dppm) 3 CO 2+ (as a CF 3 CO 2 − salt) in acetonitrile is reported. The final product Pd 3 (dppm) 3 (O 2 ) 2 2+ (v(O 2 ) = 838 and 866 cm −1 ) is formed in a multi-step 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 [Formula: see text] 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 O 2 with solvent molecules to occupy the unsaturated site of the empty cavity in Pd 3 (dppm) 3 CO 2+ (i.e., ground state guest–host chemistry). The binding constant for O 2 with Pd 3 (dppm) 3 CO 2+ is roughly estimated to range between 1 and 730 M −1 in the ground state and is considered to be weak.