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

Abstract

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.