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

摘要

The 2H quadrupolar interaction is a sensitive tool for the characterization of deuterium−metal binding states. In the present study, experimental solid-state 2H MAS NMR techniques are used in the investigations of two ruthenium clusters, D₄Ru₄(CO)₁₂ (1) and D₂Ru₆(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 2H 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 [DRu₆(CO)₁₈]− (2−) and cationic [D₃Ru₆(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 2H 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.