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

Abstract

The photoinduced addition of 0, onto the unsaturated cluster Pd,(dppm),C~~+ (as a CF,CO,-salt) in acetonitrile is reported. The final product ~d , ( d~~m ) , ( O , )~ (~( 0 , ) = 838 and 866 cm-') is formed in a multistep fashion in which two photochemical intermediates are observed (presumably ~d,(dppm),(0,)(~0)~+ and ~d , ( d~~m ) , ( 0 , )~+ ) .No X-ray structure could be obtained, but numerous spectroscopic findings demonstrate that 0, binds the Pd, center as a peroxo-02, 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 0, molecule must be preassembled in the excited state prior to any photoinduced transformation. This ~d , ( d~~m ) , C 0~+ . . . 0 , 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 (0) = 0.033 ? 0.004) is explained by the very short excited state lifetime at 298 K, and the competition of 0, with solvent molecules to occupy the unsaturated site of the empty cavity in ~d , ( d~~m ) ,~0~+ (i.e., ground state guest-host chemistry). The binding constant for 0, with ~d , ( d~~m ) ,~0~+ is roughly estimated to range between 1 and 730 M-I in the ground state and is considered to be weak.

Photoinduced oxidative degradation of unsaturated M3(dppm)3CO2+ clusters (M = Pd, Pt) by chlorocarbons and chloride ion

“…This decomposition mechanism is complex since high molecular weight fragments are observed (higher than dppm in some cases), indicating that the processes involved formation of free radicals. The very low photochemical quantum yields ( a ) have been commented on in one of our studies on 0, activation (6). There are many possible explanations for the low values: (i) the excited state lifetimes are found to be extremely short (7, < 40 ps, M = Pd; 7, < 40 ps, 70 < 7, < 400 ns, intensity ratio for T~/ T~ = 9515, M = Pt; (ref.…”

Geometry Optimizations of the Ground and Excited Triplet State Structures of the Low-Valent Metal−Metal Bonded Isocyanide and Carbonyl Di- and Trinuclear Palladium Complexes Using Density Functional Theory

“…The change in M−M bond length , in the excited states strongly depends upon the nature of the complexes and the extent of excited state mixing 7a or photoadds O 2 using appropriate irradiation wavelengths and temperature conditions 7b. On the basis of theoretical calculations (extended Hückel molecular orbital, EHMO), the M−M interactions are antibonding in the excited states, which subsequently were confirmed experimentally .…”

Thermodynamic and Kinetic Control over the Reduction Mechanism of the Pd3(dppm)3(CO)(I)+ Cluster

“…Since the first report on the synthesis and characterization of the Pd 3 (dppm) 3 (CO) 2+ cluster (dppm = bis(diphenylphosphino)methane) in the 1980s, by Puddephatt et al, , and its Pt analogue, numerous and exhaustive works on its reactivity and properties have appeared in the literature. − This cluster exhibits a triangular Pd 3 frame with three dppm-supported M−M single bonds. While one M 3 face is capped by a CO group, the other one is unsaturated, giving rise to rich coordination chemistry. − The dppm-phenyl groups form a cavity above this M 3 plane, limiting access to smaller substrates only. , Of particular interest in this work, thebinding of halide ions onto this Pd 3 2+ species leads to very stable architectures, , and the approximated binding constants spectroscopically evaluated by UV−visible methods indicate that the stability varies as I ≫ Br ≫ Cl. −

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