Alkyne-Protected Ruthenium Nanoparticles

Abstract

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 Ru—C≡ bonds, the C≡C 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 Ru—C≡ bonds and the conducting Ru metal cores such that the particle bound C≡C 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 volcano-shape 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 Ru—carbene π bonds.