Molecular Rulers as in-situ Probes of Dendrimer-Encapsulated Palladium Nanoparticles

Dendrimer-encapsulated metal nanoparticles (DEMNs) are synthesized by first sequestering metal ions within dendrimer interiors and then chemically reducing the composite.  As we have previously shown, DEMNs can be used as selective catalysts for hydrogenation reactions.  For example, we have examined the catalytic activity of a series of DEMNs toward the hydrogenation of allyl alcohol and four a-substituted allylic alcohol derivatives having different sizes and shapes.  Our results showed that the dendrimer periphery acts as a size-selective filter controlling access of small molecules that undergo catalytic hydrogenation on intradendrimer Pd nanoparticles.  However, a key question raised in these studies concerns the size, shape, and location of the encapsulated nanoparticles.  Transmission electron microscopy (TEM) results along with the relatively high degree of steric crowding within the dendrimer interior, suggests that encapsulated nanoparticles have complex shapes.

Recently, graduate student Yanhui Niu and postdoc Dr. Julio Alvarez developed an in-situ method for determining the average distance between the surface of dendrimer-encapsulated palladium nanoparticles and the periphery of their fourth-generation, hydroxyl-terminated poly(amidoamine) dendrimer hosts.  The advantage of measuring the periphery-to-surface distance in-situ, is that we avoid having to make assumptions about the nanoparticle size and shape. The measurements were made using molecular rulers consisting of three parts: a catalytically active probe at the distal terminus that may undergo a hydrogenation reaction upon contact with the encapsulated catalyst; a large molecular "stopper" at the proximal end that is unable to enter the host; and intervening alkyl chains having different lengths that tether the probe to the stopper and which therefore define the maximum extension of the ruler (Figure 1).  Three molecular rulers were configured with a monosubstituted b-cyclodextrin (b-CD) stopper, which is too large to penetrate the dendrimer periphery, at one end and an alkene functional group at the other.  The time necessary to hydrogenate these molecular rulers was monitored by ¹H NMR, and this information was then used to estimate the average location of the encapsulated Pd nanoparticles.

The important findings are that the shortest ruler, mono-6-deoxy-6-(allylamino)cyclodextrin(R1, 0.5 nm in length), had the slowest reaction rate while the two longer rulers (R5 and R9, 0.9 and 1.4 nm, respectively) reacted substantially faster.  Figure 2 shows plots of the percentage completion of the hydrogenation reactions as a function of time for aqueous solutions (D₂O) containing 5 mM molecular rulers in the presence (R1A, R5A, and R9A) and absence (R1, R5, and R9) of 1-adamantanol.  The catalysts were (a) and (b) Pd supported on carbon (5% Pd/C), (c) and (d) G4-OH(Pd₄₀).  The data indicate that the surface of the encapsulated nanoparticle is situated 0.7 ± 0.2 nm from the surface of the dendrimer.  A research paper describing these results was submitted for publication in December, 2002.