The cluster dication [H6Ru4(C6H6)4]2+ revisited: the first cluster complex containing an intact dihydrogen ligand?

A low-temperature 1H-NMR study suggests the tetranuclear cluster dication [H6Ru4(C6H6)4]2+ (1) to contain an H2 ligand that undergoes, upon warming of the solution, an intramolecular exchange with the four hydride ligands at the Ru4 framework. Whereas two of the three NMR signals at −120°C in the hydride region show T1 values in the range 200–300 ms, the least deshielded resonance at δ=−17.33 ppm exhibits a T1 value of only 34 ms, characteristic of an H2 ligand. A re-examination of the single-crystal X-ray structure analysis of the chloride salt of 1 supports this interpretation by a short distance of 1.14(0.15) Å between two hydrogen atoms coordinated as a HH ligand in a side-on fashion to one of the triangular faces of the Ru4 tetrahedron. The distance between one of the two hydrogen atoms of the H2 ligand and one of the four hydride ligands is also very short [1.33(0.15) Å], suggesting an additional H2⋯H interaction. The presence of this H3 unit over one of the three Ru3 faces in 1 may explain the deformation of the Ru4 skeleton from the expected tetrahedral symmetry. Density functional theory (DFT) calculations on 1 indicate a very soft potential energy surface associated with the respective displacement of the three interacting cofacial hydrogen atoms. In accordance with these results, the cluster dication 1 tends to loose molecular hydrogen to form the cluster dication [H4Ru4(C6H6)4]2+ (2). The equilibrium between 1 and 2 can be used for catalytic hydrogenation reactions.