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

Abstract

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 degenerate isomers RuH2[η2-HB(OCH2)2](η2-H2)(PMe3)2 (5Bpin_a) (analogous to 2Bpin) and RuH[(μ-H)2B(OCH2)2](η2-H2)(PMe3)2 (5Bpin_b), demonstrating that σ-H2 rotation and σ-borane versus dihydridoborate ligation are intimately correlated. In contrast, the 9-BBN reagent is a strong Lewis acid and leads to a dihydridoborate complex. The theoretical study on RuH[(μ-Η)2Bpin](η2-HBpin)(PCy3)2 (3Bpin) shows that the bonding is also dependent on the hydride basicity: the RuH[(μ-H)2B(OCH2)2](PMe3)2 fragment used as a model for RuH[(μ-Η)2Bpin](PCy3)2 is not basic enough to contain a second ligand bound in a dihydridoborate mode, despite the stabilization that should be gained from the resulting symmetrical structure.