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Thc role o f metalloenzymes in important biological transformations has attracted incrcasing attention over the past sever-;II decades. Of the many chemical transformations mediated by enzymes, few are a s challenging ;is multielectron redox reactions. Kcccnt studies have revealed ;I partial structiirnl and mechanistic dcscription o f these redox-active metallocn/ymes. but thcrc is much still to be Icarned regarding the mechanisms of substrate transformation. Due to the complexity of the metalloenzyme systems, simplified model systems are employed to mimic structural or functional features of the enzyme. In multielectron redox enzymes, several metals are probably involved in both substrate binding and the subsequent redox reactions. Thus, functional mimics of multielectron redox enzymes might also need two or more metal centers to be efficacious. The roles of multiple metal centers are to 1) increase the substrate's affinity for the catalyst, 2) increase the rate of electron transfer to the bound substrate, 3 ) increase the reactivity of the bound substrate, and 4) inhibit deleterious side reactions. Determining the importance of each factor may help in the development of these catalysts. Cofacial metallodiporphyrins, because of the control they provide over the geometric and electronic properties of the synthetic reaction center, are ideal bimetallic model complexes. The knowledge gained from model studies will help in understanding the mechanisms of metalloenzymes and can be used to design new homogeneous catalysts to effect multielectron transformations.

Ligand exchange reactions of 1.5-nm triphenylphosphine-stabilized nanoparticles with omega-functionalized thiols provides a versatile approach to functionalized, 1.5-nm gold nanoparticles from a single precursor. We describe the broad scope of this method and the first mechanistic investigation of thiol-for-phosphine ligand exchanges. The method is convenient and practical and tolerates a surprisingly wide variety of technologically important functional groups while producing very stable nanoparticles that essentially preserve the small core size and size dispersity of the precursor particle. The mechanistic studies reveal a novel three-stage mechanism that can be used to control the extent of ligand exchange. During the first stage of the exchange, AuCl(PPh3) is liberated, followed by replacement of the remaining phosphine ligands as PPh3 (assisted by gold complexes in solution). The final stage involves completion and reorganization of the thiol-based ligand shell.

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