PNS-Type Ruthenium Pincer Complexes

The PNS pincer-type ligand 1 and the novel Ru­(PNS) complexes 2–8 were synthesized and characterized. The (PNS)­RuH­(Cl)­CO complex 2 was prepared by reaction of ligand 1 with RuH­(Cl)­CO­(PPh3)3. 2 reacted with KHMDS (potassium bis­(trimethylsilyl)­amide) to form the symmetrical dimeric complex 4 via the intermediacy of the dearomatized complex (PNS*)­Ru­(H)­CO 3, in which deprotonation of the benzylic-S “arm” took place. Reaction of 2 with excess NaH gave the dimeric 4, by a formal intermolecular attack of the benzylic “arm” on a second ruthenium center. Complex 4 underwent spontaneous transformation in solution to the dinuclear complex 5 via C–S bond cleavage, resulting in the loss of a S-bound tBu group. Treatment of 2 with KHMDS in the presence of PEt3 resulted in the trapping of intermediate 3 in the form of the dearomatized complex 8. Reaction of 2 with LiHBEt3 gave the trans-dihydride complex 6, which reacted with CO2 to give the formato complex 7, in which the formato ligand is located trans to the hydride. Complexes 2, 4, and 5 were also investigated as catalysts for the dehydrogenative coupling of alcohols with amines.