Experimental and Computational Studies of Ruthenium Complexes Bearing Z‑Acceptor Aluminum-Based Phosphine Pincer Ligands

Reaction of [Ru­(C6H4PPh2)2(Ph2PC6H4AlMe­(THF))­H] with CO results in clean conversion to the Ru−Al heterobimetallic complex [Ru­(AlMePhos)­(CO)3] (1), where AlMePhos is the novel P–Al­(Me)–P pincer ligand (o-Ph2PC6H4)2AlMe. Under photolytic conditions, 1 reacts with H2 to give [Ru­(AlMePhos)­(CO)2(μ-H)­H] (2) that is characterized by multinuclear NMR and IR spectroscopies. DFT calculations indicate that 2 features one terminal and one bridging hydride that are respectively anti and syn to the AlMe group. Calculations also define a mechanism for H2 addition to 1 and predict facile hydride exchange in 2 that is also observed experimentally. Reaction of 1 with B­(C6F5)3 results in Me abstraction to form the ion pair [Ru­(AlPhos)­(CO)3]­[MeB­(C6F5)3] (4) featuring a cationic [(o-Ph2PC6H4)2Al]+ ligand, [AlPhos]+. The Ru–Al distance in 4 (2.5334(16) Å) is significantly shorter than that in 1 (2.6578(6) Å), consistent with an enhanced Lewis acidity of the [AlPhos]+ ligand. This is corroborated by a blue shift in both the observed and computed νCO stretching frequencies upon Me abstraction. Electronic structure analyses (QTAIM and EDA-ETS) comparing 1, 4, and the previously reported [Ru­(ZnPhos)­(CO)3] analogue (ZnPhos = (o-Ph2PC6H4)2Zn) indicate that the Lewis acidity of these pincer ligands increases along the series ZnPhos < AlMePhos < [AlPhos]+.