Iodine activation: a general method for catalytic enhancement of thiolate monolayer-protected metal clusters.

To enhance catalytic activity, the present study details a general approach for partial thiolate ligand removal from monolayer-protected clusters (MPCs) by straightforward in situ addition of iodine. Two model reactions are examined to illustrate the effects on the catalytic activity of glutathione (SG)-capped Au MPCs serving as a catalyst for the NaBH4 reduction of 4-nitrophenol to 4-aminophenol and SG-capped Pd MPCs serving as a catalyst for the hydrogenation/isomerization of allyl alcohol. Iodine addition promoted partial thiolate ligand removal from both MPCs and improved the catalytic properties, presumably due to greater surface exposure of the metal cores as a result of ligand dissociation. The rate of 4-nitrophenol reduction increased from 0.066 min-1 in the absence of I2 to 0.505 min-1 in the presence of 2.0 equivalents I2 (equivalents based on total ligated glutathione). The reaction of allyl alcohol to produce 1-propanol and propanal was similarly accelerated as indicated by the increase in turnover frequency from 131 to 230 moles products per moles catalyst per h by addition of 0.2 equivalents I2. In both reactions, as the amount of I2 added increases the catalyst recyclability decreases due to catalyst instability. Low equivalents of I2 are optimal when considering both reaction rate and catalyst recyclability.

Tunable Aminooxy-Functionalized Monolayer-Protected Gold Clusters for Non-Polar or Aqueous Oximation Reactions.

Aminooxy (-ONH2) groups are well known for their chemoselective reactions with carbonyl compounds, specifically aldehydes and ketones. The versatility of aminooxy chemistry has proven to be an attractive feature that continues to stimulate new applications. This work describes application of aminooxy 'click chemistry' on the surface of gold nanoparticles. We present here a trifunctional amine-containing aminooxy alkane thiol ligand for use in the functionalization of gold monolayer protected clusters (Au MPCs). Diethanolamine is readily transformed into an organic-soluble aminooxy thiol (AOT) ligand using a short synthetic path. The synthesized AOT ligand was coated on ≤ 2 nm diameter hexanethiolate (C6S)-capped Au MPCs using a ligand exchange protocol to afford organic-soluble AOT/C6S (1:1 ratio) Au mixed monolayer protected clusters (MMPCs). This work describes the synthesis of Au(C6S)(AOT) MMPCs and representative oximation reactions with various types of aldehyde-containing molecules, highlighting the ease and versatility of the chemistry and how amine protonation can be used to switch solubility characteristics.