Alkyne-Protected Ruthenium Nanoparticles†

Stable ruthenium nanoparticles protected by 1-octynyl fragments were synthesized by a wet chemical method. Transmission electron microscopic measurements showed that the resulting particles exhibited an average core diameter of 2.55 ( 0.15 nm with well-defined Ru crystalline lattice fringes. Because of the formation of RusCt bonds, the CtC vibrational stretch was found in FTIR measurements to red-shift to 1936 cm-1 from 2119 cm-1 that was observed for monomeric 1-octyne. Interestingly, the nanoparticles underwent ligand exchange reactions with alkynyl lithium (e.g., 5-phenyl-1-pentynyl lithium) for further surface functionalization, as manifested in FTIR as well as 1H and 13C NMR measurements. Optically, whereas UV-vis absorption measurements exhibited only a featureless profile, the Ru nanoparticles displayed apparent photoluminescence with an emission peak at 428 nm, which was accounted for by intraparticle charge delocalization as a consequence of the strong RusCt bonds and the conducting Ru metal cores such that the particle bound CtC moieties behaved analogously to diacetylene derivatives. The impacts of the interfacial bonding interactions on intraparticle charge delocalization were further illustrated by Ru nanoparticles functionalized with a mixed monolayer of both octyne and ethynylferrocene ligands. At a ferrocene surface coverage of ca. 13%, electrochemical measurements depicted two pairs of voltammetric peaks with a potential spacing of 265 mV. A new NIR absorption band centered around 1687 nm also started to emerge with the addition of nitrosonium tetrafluoroborate (NOBF4) as the oxidizing reagent and the peak intensity exhibited a volcanoshape dependence on the amount of NOBF4 added. These observations strongly suggested that there existed effective intervalence charge transfer between the particle-bound ferrocene groups at mixed valence, analogous to the observation where the ferrocene moieties were bound onto the particle surface by Ruscarbene π bonds.