Clusters on surfaces: Adsorption of {M6Cl12}(n+) (M = Nb, Ta) to gold and silver surfaces via thiocyanate bridges

The redox-active metal clusters {Ta6Cl12}(n+) and {Nb6Cl12}(n+) were immobi lized on gold and silver surfaces with thiocyanate ligands as the linking a gents. Gold surfaces treated with [Bu4N]NCS, Au/SCN, adsorb [Bu4N](2)[Ta6Cl 12(OSO2CF3)(6)] from methylene chloride with a concomitant loss of OSO2CF3- as determined by XPS. Cyclic voltammetry on Au/SCN electrodes exposed to [ Bu4N](2)[Ta6Cl12(OSO2CF3)(6)] reveal a single reduction wave at -0.56 V vs Ag/AgCl and a surface coverage of 2.0 x 10(-10) mol/cm(2). Upon reduction, the cluster desorbs from the surface leaving a Au/SCN film. QCM measurement s on gold electrodes exposed to the clusters [Bu4N](z)[M6Cl12(NCS)(6)] (M = Nb, z = 4; M Ta, z = 3) reveal that both compounds adsorb in a monolayer f ashion; however, only [Bu4N](4)[Nb6Cl12(NCS)(6)] is retained on the surface after a rinse with pure CH2Cl2. Redox couples associated with [Nb6Cl12(NCS )(6)](4-) immobilized on a gold electrode are virtually undetectable by cyc lic voltammetry, these films also block the oxidation of ferrocene in solut ion. Both properties are attributed to a shielding layer of the organic cou ntercations that cover the cluster anions directly adsorbed to the gold sur face. XP spectra of silver surfaces treated with [Bu4N](4)[Nb6Cl12(NCS)(6)] reveal that niobium is present on the surface in a higher oxidation state (4+ or 5+) and provide evidence for decomposition of the {Nb6Cl12}(n+) core . By contrast [Bu4N](3)[Ta6Cl12(NCS)(6)] adsorbs on silver.