REACTIVITY OF MONOLAYER-PROTECTED GOLD CLUSTER MOLECULES - STERIC EFFECTS

The steric environment of alkanethiolate ligand shells of monolayer-pr otected gold cluster (MPCs) molecules has been investigated in three w ays. First, the S(N)2 reactivity of omega-bromoalkanethiolate-function alized MPCs with primary amines has been shown to respond to the steri c bulk of the incoming nucleophile (rates of n-propylamine > isopropyl amine > tert-butylamine), and to the relative chain lengths of omega-b romoalkanethiolate and surrounding alkanethiolate chains (rates of C12 :C12Br > C12:C8Br > C12:C3Br). Also, unlike 2D-SAMs, omega-bromo-funct ionalized MPCs and primary allkyl halide monomers (RBr) have comparabl e S(N)2 reactivities. These results are significant in that little pre viously was known about the chemical reactivities of the monolayers on MPCs, and in that the poly-functional omega-bromoalkanethiolate MPCs are shown to be highly reactive. i.e., as many as 20 S(N)2 displacemen ts occur per cluster molecule. Second, steric aspects of alkanethiolat e monolayers on Au clusters are shown to affect the rate of cyanide-me diated decomposition of the gold core, which slows with increased chai n length (up to C10) and steric bulk. Third, solution infrared spectro scopy demonstrates that, in nonpolar solvents. the alkanethiolate liga nds on Au MPCs have a disorder approaching that of liquid alkanes. The se results support a model of MPC ligand environment of decreasing cha in packing density as the distance from the gold core increases, a mot if that likely arises from the high curvature of gold nanoparticle sur faces.