Hydrido-Ruthenium Cluster Complexes as Models for Reactive Surface Hydrogen Species of Ruthenium Nanoparticles. Solid-State ²H NMR and Quantum Chemical Calculations

Abstract

The (2)H quadrupolar interaction is a sensitive tool for the characterization of deuterium-metal binding states. In the present study, experimental solid-state (2)H MAS NMR techniques are used in the investigations of two ruthenium clusters, D(4)Ru(4)(CO)(12) (1) and D(2)Ru(6)(CO)(18) (2), which serve as model compounds for typical two-fold, three-fold, and octahedral coordination sites on metal surfaces. By line-shape analysis of the (2)H MAS NMR measurements of sample 1, a quadrupolar coupling constant of 67 +/- 1 kHz, an asymmetry parameter of 0.67 +/- 0.1, and an isotropic chemical shift of -17.4 ppm are obtained. In addition to the neutral complex, sample 2 includes two ionic clusters, identified as anionic [DRu(6)(CO)(18)](-) (2(-)) and cationic [D(3)Ru(6)(CO)(18)](+) (2(+)). By virtue of the very weak quadrupolar interaction (<2 kHz) and the strong low-field shift (+16.8 ppm) of 2(-), it is shown that the deuteron is located in the symmetry center of the octahedron spanned by the six ruthenium atoms. For the cationic 2(+), the quadrupolar interaction is similar to that of the neutral 2. Quantum chemical DFT calculations at different model structures for these ruthenium clusters were arranged in order to help in the interpretation of the experimental results. It is shown that the (2)H nuclear quadrupolar interaction is a sensitive tool for distinguishing the binding state of the deuterons to the transition metal. Combining the data from the polynuclear complexes with the data from mononuclear complexes, a molecular ruler for quadrupolar interactions is created. This ruler now permits the solid-state NMR spectroscopic characterization of deuterium adsorbed on the surfaces of catalytically active metal nanoparticles.

Theoretical Study of Hydrogen Adsorption on Ruthenium Clusters

“…Poteau et al [38] studied spectroscopic and thermodynamic properties of surfacic hydrides on Ru (0001) model surface: the influence of the coordination modes and the coverage by density functional theory and this study partially opens the route to DFT studies of multistep hydrogenation reactions at the surface of ruthenium nanoparticles monitored by spectroscopic techniques. Torsten Gutmann et al [39] studied hydrido-ruthenium cluster complexes as models for reactive surface hydrogen species of ruthenium nanoparticles by solid-state 2 H NMR and quantum chemical calculations and found that the 2 H nuclear quadrupolar interaction is a sensitive tool for distinguishing the binding state of the deuterons to the transition metal.…”

2H NMR calculations on polynuclear transition metal complexes: on the influence of local symmetry and other factors

“…Getting knowledge of their structure would be very helpful, but their structure may often not be characterized at the same level as well-defined molecular species. [8][9][10] The coordination of hydrogen to metal nanoparticles synthesized by organometallic techniques is especially important. 7 In most cases, spectra assignment is not obvious, so that there is a need for providing reference data.…”

“…Although in all these low-energy structures hydride atoms adopt edge-bridging or face-capping positions, it is possible to study terminal positions in geometries with higher energy as well as subsurfacic hydrides in tetrahedral or octahedral sites. 10,18 Nevertheless, the wide chemical shift range observed for a given coordination mode does not allow us to make safe assignations, unless theoretical calculations are involved. 19,20 Tetrahedral and octahedral structures belong, respectively, to the nido and closo family of clusters, so that Ru 4 and Ru 6 clusters obeying the rules should exhibit 60 and 86 valence electrons.…”

Understanding the growth of RuO2 colloidal nanoparticles over a solid support: An atomic force microscopy study

“…Over Ru metal surfaces, hydrogen can be adsorbed on a single-atom site, two-fold site and also threefold hollow site. However, the lowest adsorption energy allowing hydrogen activation has been found for the three-fold hollow site, which may be of the type fcc (on the surface) or hcp (on an octahedral subsurface site) [22,23]. On the other hand, the most active sites for N 2 dissociative adsorption are the B 5 -type sites, exposing a three-fold hollow site close together with a bridge site [24][25][26].…”