Synthesis, Structure, and Bonding Properties of Ruthenium Complexes Possessing a Boron-Based PBP Pincer Ligand and Their Application for Catalytic Hydrogenation

In addition to our previous works
for PBP-pincer metal complexes,
four PBP-pincer Ru complexes, [PBP]­Ru­(Cl)­(CO) (2), [PBP]­Ru­(CO)­(η2-BH4) (3), [PBP]­(μ-H)2Ru­(OAc-κ2O) (4), and
[PBP]­(μ-H)2Ru­(η2-BH4)
(5) were synthesized. All the obtained complexes were
characterized by NMR and IR spectroscopy, X-ray crystallography, elemental
analysis, and DFT calculation with AIM analysis. Through the structural
analysis, two types of interaction between boron atoms and ruthenium
atoms in 3–5 were revealed. One is
the typical two-center–two-electron bond between the boron
atom of the PBP ligand and the Ru atom, associated with bridging hydride
ligand(s) on the Ru­(IV) center. The other is an ionic interaction
between the Ru fragment and the tetrahydroborate anion. On comparison
of the structural features, vibrational analysis, NBO analysis, and
AIM analysis of the obtained compounds with those of the previously
reported complexes having “similar” interactions among
B, H, and Ru atoms, the interactions in 4 and 5 were proven to be different from that previously reported. The obtained
complexes 3–5 were applied as catalysts
for the hydrogenation of aldehyde. Complex 5 showed the
highest catalytic activity and widest range of substrate scope. Two
mechanisms for the catalytic cycle were proposed with an initial dissociation
of BH3 or anionic ligand, according to the control experiments.