Coordination Modes of Boranes in Polyhydride Ruthenium Complexes: σ-Borane versus Dihydridoborate

The bis(dihydrogen) complex RuH2(η2-H2)2(PCy3)2 (1) reacts at room temperature with 1 equiv of either HBpin or HBcat to produce the σ-borane complexes RuH2(η2-HBpin)(η2-H2)(PCy3)2 (2Bpin) and RuH2(η2-HBcat)(η2-H2)(PCy3)2 (2Bcat), respectively, by substitution of one σ-H2 ligand by one σ-B−H. In contrast, when using the 9-BBN reagent, the dihydridoborate complex RuH[(μ-Η)2BBN](η2-H2)(PCy3)2 (2BBN) is formed. The coordination modes of the borane ligands have been ascertained by NMR spectroscopy, X-ray diffraction, and theoretical studies (DFT/B3LYP). The results indicate that the dialkoxyborane ligands (HBpin and HBcat) are not acidic enough to stabilize a “true” symmetrical dihydridoborate coordination mode. They thus lead to σ-borane complexes presenting a small H/BH cis interaction between the boron atom and the adjacent hydride. The σ-H2 ligand in 2Bpin is located by X-ray diffraction at 90 K and found to be perpendicular to the equatorial plane. DFT calculations lead to the optimization of the two dege...