Metallocenyl Dendrimers and Their Applications in Molecular Electronics, Sensing, and Catalysis

We have investigated the movement of electrons around the peripheries of dendrimers and between their redox termini and electrodes through studies of the electrochemistry of dendrimers presenting metallocenes (and other transition metal sandwich complexes) as terminal groups. Because these compounds can be stabilized in both their oxidized and their reduced forms, their electrochemical and chemical redox processes proceed without decomposition (chemical reversibility). Most interestingly, electrochemical studies reveal that electron transfer within the dendrimers and between the dendrimers and electrodes are both very fast processes when the branches are flexible (electrochemical reversibility). When the dendrimer branches are sufficiently long, the redox events at the many termini of the metallodendrimer are independent, appearing as a single wave in the cyclic voltammogram, because of very weak electrostatic effects. As a result, these metallodendrimers have applications in the molecular recognition, sensing, and titration of anions (e.g., ATP(2-)) and cations (e.g., transition metal complexes). When the recognition properties are coupled with catalysis, the metallodendrimers function in an enzyme-like manner. For example, Pd(II) can be recognized and titrated using the dendrimer's terminal redox centers and internal coordinate ligands. Redox control over the number of Pd(II) species located within a dendrimer allows us to predetermine the number of metal atoms that end up in the form of a dendrimer-encapsulated Pd nanoparticle (PdNP). For hydrogenation of olefins, the efficiency (turnover frequency, TOF) and stability (turnover number, TON) depend on the size of the dendrimer-encapsulated PdNP catalysts, similar to the behavior of polymer-supported PdNP catalysts, suggesting a classic mechanism in which all of the steps proceed on the PdNP surface. On the other hand, Miyaura-Suzuki carbon-carbon bond-forming reactions catalyzed by dendrimer-encapsulated PdNPs proceed with TOFs and TONs that do not depend on the size of the PdNPs. Moreover these catalysts are more efficient when employed in lower (down to "homeopathic") amounts, presumably because of a leaching mechanism whereby Pd atoms escape from the PdNP surface subsequent to oxidative addition of the aryl halide. Under these conditions, the "mother" PdNPs have greater difficulty quenching the extremely active leached Pd atoms because of their low concentration. Although dendrimers presenting catalysts at their branch termini can be recovered and reused readily, their inner-sphere components can lead to steric inhibition of substrate approach. In contrast, star-shaped catalysts do not suffer from such steric problems, as has been demonstrated for water-soluble dendrimers bearing cationic iron-sandwich termini, which are redox catalysts of cathodic nitrate and nitrite reduction in water.