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) (<b>2</b>), [PBP]­Ru­(CO)­(η²-BH₄) (<b>3</b>), [PBP]­(μ-H)₂Ru­(OAc-κ²<i>O</i>) (<b>4</b>), and [PBP]­(μ-H)₂Ru­(η²-BH₄) (<b>5</b>) 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 <b>3</b>–<b>5</b> 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 <b>4</b> and <b>5</b> were proven to be different from that previously reported. The obtained complexes <b>3</b>–<b>5</b> were applied as catalysts for the hydrogenation of aldehyde. Complex <b>5</b> 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