Molecular catalysts, multielectron reaction

Collman JP, Wagenknecht PS, Hutchison JE. 1994. Molecular catalysts for multielectron redox reactions of small molecules The Cofacial metaUodiporphyrin approach. Angew Chem IntEd 33 1537. 

Molecular catalysts, such as transition metal coordination complexes, are capable of mediating multielectron and proton-coupled reactions because of the ability of transition metals to accommodate multiple redox states. In addition, electronic and acid/base properties of these catalysts can be conveniently tuned through the ligand design. Very low overpotentials for transition metal-catalyzed CO2 reduction have been demonstrated. However, the main drawback of molecular catalysts is their relatively low stability resulting in low turnover numbers (TON). The benchmark for industrial application is in the range 10 -10 TON [34], which is far beyond... 

For recent comprehensive reviews see Collman, J. P., Wagenknecht, P. S. and Hutchison, J. E. (1994) Molecular catalysts for multielectron redox reactions of small molecules, Angew. Chem. Int. Ed. Engl, 33, 1537-1554 Ogoshi, H. and Mizutani, T. (1998) Multifunctional and chiral porphyrins-model receptors for chiral recognition, Acc. Chem. Res., 31, 81-89. 

As mentioned in Sect. 2, many efficient homogeneous and heterogeneous oxidation systems based on POMs have been developed. Among them, some POM catalysts with multimetallic active sites show remarkable activity and selectivity in comparison with the conventional monometalhc complexes. According to the proposed reaction mechanisms, the oxidation catalyses by multimetallic active sites of POMs are classified into the following four categories (1) cooperative activation of oxidants, (2) simultaneous activation of oxidants and substrates, (3) stabihzation of reaction intermediates, and (4) multielectron transfer (Fig. 1). In Sect. 3, we focus on the selective oxidations by multimetallic active sites of POM-based molecular catalysts. 

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