Coordination modes of boranes in polyhydride ruthenium complexes:: σ-Borane versus dihydridoborate

The bis(dihydrogen) complex RuH2(eta(2)-H-2)(2)(PCy3)(2) (1) reacts at room temperature with 1 equiv of either HBpin or HBcat to produce the a-borane complexes RuH2(eta(2)-HBpin)(eta(2)-H-2)(PCy3)(2) (2Bpin) and RuH2(eta(2)-HBcat)(eta(2)-H-2)(PCy3)(2) (2Bcat), respectively, by substitution of one sigma-H-2 ligand by one sigma-B-H. In contrast, when using the 9-BBN reagent, the dihydridoborate complex RuH[(mu-H)(2)BBN](eta(2)-H-2)(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 a-borane complexes presenting a small H/BH cis interaction between the boron atom and the adjacent hydride. The sigma-H-2 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[eta(2)-HB(OCH2)(2)](eta(2)-H-2)(PMe3)(2) (5Bpin_a) (analogous to 2Bpin) and RuH[(mu-H)(2)B(OCH2)(2)](eta(2)-H-2)(PMe3)(2) (5Bpin_b), demonstrating that sigma-H-2 rotation and sigma-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[(mu-H)(2)Bpin](eta(2)-HBpin)(PCy3)(2) (3Bpin) shows that the bonding is also dependent on the hydride basicity: the RuH[(mu-H)(2)B(OCH2)(2)](PMe3)(2) fragment used as a model for RuH[(mu-H)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.