POP–Pincer Ruthenium Complexes: d6 Counterparts of Osmium d4 Species

A wide range of ruthenium complexes stabilized by the POP–pincer ligand xant­(PiPr2)2 (9,9-dimethyl-4,5-bis­(diisopropylphosphino)­xanthene) were prepared starting from cis-RuCl2{κ-S-(DMSO)4} (1; DMSO = dimethyl sulfoxide). Treatment of toluene solutions of this adduct with the diphosphine under reflux leads to RuCl2{xant­(PiPr2)2}­(κ-S-DMSO) (2), which reacts with H2 in the presence of a Brønsted base. The reaction in the presence of Et3N affords RuHCl­{xant­(PiPr2)2}­(κ-S-DMSO) (3), whereas NaH removes both chloride ligands to give RuH2{xant­(PiPr2)2}­(κ-S-DMSO) (4). The stirring of 3 in 2-propanol under 3 atm of H2 for a long time produces the elimination of DMSO and the coordination of H2 to yield the dihydrogen derivative, RuHCl­(η2-H2)­{xant­(PiPr2)2} (5). In contrast to H2, PPh3 easily displaces DMSO from the metal center of 3 to afford RuHCl­{xant­(PiPr2)2}­(PPh3) (6), which can be also obtained starting from RuHCl­(PPh3)3 (7) and xant­(PiPr2)2. In contrast to 3, complex 4 does not undergo DMSO elimination to give RuH2(η2-H2)­{xant­(PiPr2)2} (8) under a H2 atmosphere. However, the latter can be prepared by hydrogenation of Ru­(COD)­(COT) (9; COD = 1,5-cyclooctadiene and COT = 1,3,5-cyclooctatriene) in the presence of xant­(PiPr2)2. A more efficient procedure to obtain 8 involves the sequential hydrogenation with ammonia borane of the allenylidene derivative RuCl2(CCCPh2)­{xant­(PiPr2)2} (10), which is formed from the reaction of 2 with 1,1-diphenyl-2-propyn-1-ol. The hydrogenation initially gives RuCl2(CCHCHPh2)­{xant­(PiPr2)2} (11), which undergoes the subsequent reduction of the Ru–C double bond to yield the hydride-tetrahydroborate complex, RuH­(η2-H2BH2)­{xant­(PiPr2)2} (12). The osmium complex, OsCl2{xant­(PiPr2)2}­(κ-S-DMSO) (13), reacts with 1,1-diphenyl-2-propyn-1-ol in a similar manner to its ruthenium counterpart 2 to yield the allenylidene derivative, OsCl2(CCCPh2)­{xant­(PiPr2)2} (14). Ammonia borane also reduces the Cβ–Cγ double bond of the allenylidene of 14. However, the resulting vinylidene species, OsCl2(CCHCHPh2)­{xant­(PiPr2)2} (15), is inert. Complex 12 is an efficient catalyst precursor for the hydrogen transfer from 2-propanol to ketones, the α-alkylations of phenylacetonitrile and acetophenone with alcohols, and the regio- and stereoselective head-to-head (Z) dimerization of terminal alkynes.