Direct NMR evidence for the presence of mobile surface hydrides on ruthenium nanoparticles.

Metal colloids consist of an inner core of metal atoms stabilised by surface ligands such as aliphatic alcohols, amines or thiols. The particles may catalyse interesting organic transformations, including asymmetric hydrogenation and C C coupling reactions. 3] The mode of coordination and the strength of binding of ancillary organic surface ligands has been studied by infrared (IR) and liquid-state NMR spectroscopy. 5] However, little is known about ligands such as hydrogen or hydride species which constitute active partners of the catalytic behaviour of metal nanoparticles. In the past, the interaction of hydrogen with defined metal surfaces such as ruthenium has been studied using established methods of surface science. However, although these methods provide the best possible information concerning the type of binding of H atoms to the metal, they are difficult to apply to nanoparticles. If the latter are charged and paramagnetic, electron paramagnetic resonance (EPR) methods are useful if hyperfine interactions between the unpaired electrons and surface hydrogen species can be observed as shown recently for Pt clusters in zeolites. Herein, we use a combination of various NMR methods, in particular solid-state H NMR spectroscopy, for a better understanding of the interactions of hydrogen with ruthenium nanoparticles. This method has been proven to be useful in the study of the motions of hydrogen in the coordination sphere of transition metal complexes. The reactivity of the hydrogen detected on the nanoparticles is demonstrated by the finding of hydrogen–deuterium exchange between surface and ligand sites (Scheme 1). The Ru nanoparticles studied are stabilised by hexadecylamine (HDA) as a protecting ligand (Coll-1) ; they were prepared and characterised as reported previously. 9] The H NMR spectra of Coll-1 in [D8]THF, recorded directly after sample preparation, suggested the presence of hydrogen in this material because a signal at 4.6 ppm was detected, corresponding to dissolved H2 released by the nanoparticles. A direct detection of hydrogen bound to ruthenium failed, probably because of line broadening due to slow tumbling of the nanoparticles in solution. In the first part of this study we, therefore, tried to detect the assumed Ru–H species directly by high-resolution H magic-angle spinning (MAS) NMR spectroscopy of the solid particles in the absence of solvents. Unfortunately, only the signals of the ligand could be observed, which were also characterised by C cross-polarisation (CP) MAS NMR spectroscopy. However, an indirect detection was obtained. Coll-1 was introduced into an empty NMR tube equipped with a teflon needle valve and evacuated at 10 6 bar for two days at 298 K. After this procedure the valve was closed and a gas-phase H NMR spectrum was recorded. Only a signal at 0.77 ppm was observed which corresponds to gaseous water outside the tube (Figure 1 a). This signal could be removed by flushing the probe with gaseous nitrogen; however, we preferred to keep this signal as an external intensity standard. No signal arising from thermally desorbed H2 could be observed by H NMR spectroscopy, even after heating additionally the sample outside the spectrometer to 360 K for one day. However, when the NMR tube was filled with D2 gas (800 mbar, room temperature), a new gas-phase peak appeared at 4.5 ppm, typical for gaseous H2 and HD. The signal intensity increased with time and remained constant after three h (Figures 1 b–1 d). The peak exhibited a linewidth of 230 Hz. In order to distinguish between H2 and HD, two separately sealed 800-mbar samples of both gases were prepared and their gas-phase H NMR spectra measured at room temperature. A line width of 1200 Hz was found for H2 and of 200 Hz for HD. This difference is in agreement with previous findings. Thus, we assign the major part of the new peak to HD, where the H atom in HD has to come from Coll-1. In other words, D2 is activated by the nanoparticle and recombination with bound H atoms leads to desorption as HD as indicated in Scheme 1, and hence to a deuteration of Coll-1. In the following, we label the sample deuterated in this way as Coll-2. In order to prove the deuteration process, we recorded a broad band 46.1 MHz H NMR spectrum of Coll-2 (Figure 2 a). Indeed, a strong signal for the deuterons introduced was obScheme 1. Ruthenium nanoparticles showing hydrogen–deuterium exchange between surface and ligand sites.