Reactivity of Monolayer-Protected Gold Cluster Molecules:  Steric Effects

The steric environment of alkanethiolate ligand shells of monolayer-protected gold cluster (MPCs) molecules has been investigated in three ways. First, the SN2 reactivity of ω-bromoalkanethiolate-functionalized MPCs with primary amines has been shown to respond to the steric bulk of the incoming nucleophile (rates of n-propylamine> isopropylamine> tert-butylamine), and to the relative chain lengths of ω-bromoalkanethiolate and surrounding alkanethiolate chains (rates of C12:C12Br > C12:C8Br > C12:C3Br). Also, unlike 2D-SAMs, ω-bromo-functionalized MPCs and primary alkyl halide monomers (RBr) have comparable SN2 reactivities. These results are significant in that little previously was known about the chemical reactivities of the monolayers on MPCs, and in that the poly-functional ω-bromoalkanethiolate MPCs are shown to be highly reactive, i.e., as many as 20 SN2 displacements occur per cluster molecule. Second, steric aspects of alkanethiolate monolayers on Au clusters are shown to affect the rate of cyanide-mediated decomposition of the gold core, which slows with increased chain length (up to C10) and steric bulk. Third, solution infrared spectroscopy demonstrates that, in nonpolar solvents, the alkanethiolate ligands on Au MPCs have a disorder approaching that of liquid alkanes. These results support a model of MPC ligand environment of decreasing chain packing density as the distance from the gold core increases, a motif that likely arises from the high curvature of gold nanoparticle surfaces.