Covalent attachment of nickel clusters to gold electrode surfaces. Formation of rectifying molecular layers

Attachment of electroactive triangular nickel clusters to gold electrodes was accomplished with 4-isocyanophenyl thiolate linkages. The disulfide linked ''dimer of trimers'' cluster [{Ni-3(mu(3)-I)(mu(2)-dppm)(3)(mu(3),eta(1)-C=NC6H4S-)}(2)](2+), 5, was prepared by reaction of 2 equiv of [Ni-3(mu(3)-I)(2)(mu(2)-dppm)(3)], 2, with 1 equiv of 4,4'-bis(isocyano)diphenyl disulfide, 4. Films of [Ni-3(mu(3)-I)(mu(2)-dppm)(3)(mu(3),eta(1)-C=NC6H4S-)] 6, On gold electrodes were formed by electrochemical deposition or by immersing the electrode in a methylene chloride solution of 5. After deposition, persistent oxidation and reduction of the adsorbed cluster was observed via cyclic voltammetry with peak current (i(p)) proportional to the scan rate. The electron accepters methyl viologen (MV(2+)) and the methyl ester of cobaltocenium carboxylate [CoCpCpCOOMe][PF6] were used to demonstrate rectification in the interfacial electron transfer from nickel cluster modified gold electrodes to the electron accepters. Surface coverage was estimated by both electrochemistry and reflection-absorption infrared spectroscopy(RAIR). Both RAIR and electrochemical measurements indicated surface coverages of cluster 6 of approximately one monolayer or 2 x 10(14) molecules/cm(2).