Electron transfer catalysis with monolayer protected Au_(25) clusters

摘要

Au_(25)L_(18) (L = S(CH2)2Ph) clusters were prepared and characterized. The resulting monodisperse clusters were reacted with bis(pentafluorobenzoyl) peroxide in dichloromethane to form Au_(25)L_(18)~+ quantitatively. The kinetics and thermodynamics of the corresponding electron transfer (ET) reactions were characterized via electrochemistry and thermochemical calculations. Au_(25)L_(18)~+ was used in homogeneous redox catalysis experiments with a series of sym-substituted benzoyl peroxides, including the above peroxide, bis(para-cyanobenzoyl) peroxide, dibenzoyl peroxide, and bis(para-methoxybenzoyl) peroxide. Peroxide dissociative ET was catalyzed using both the Au_(25)L_(18)/Au_(25)L_(18)~-and the Au_(25)L_(18)~+/Au_(25)L_(18) redox couples as redox mediators. Simulation of the CV curves led to determination of the ET rate constant (k_(ET)) values for concerted dissociative ET to the peroxides. The ET free energy ΔG° could be estimated for all donor-acceptor combinations, leading to observation of a nice activation-driving force (log k_(ET) vs. ΔG°) relationship. Comparison with the k_(ET) obtained using a ferrocene-type donor with a formal potential similar to that of Au_(25)L_(18)/Au_(25)L_(18)~- showed that the presence of the capping monolayer affects the ET rate rather significantly, which is attributed to the intrinsic nonadiabaticity of peroxide acceptors.