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

A wide range of ruthenium complexes stabilized by the POP-pincer ligand xant((PPr2)-Pr-i)(2) (9,9-dimethyl-4,5-bis(diisopropylphosphino)-xanthene) were prepared starting from cis-RuCl2{kappa-S-(DMSO)(4)} (1; DMSO = dimethyl sulfoxide). Treatment of toluene solutions of this adduct with the diphosphine under reflux leads to RuCl2{xant((PPr2)-Pr-i)(2)}-(kappa-S-DMSO) (2), which reacts with H-2 in the presence of a Bronsted base. The reaction in the presence of Et3N affords RuHCl{xant((PPr2)-Pr-i)(2)}-(kappa-S-DMSO) (3), whereas NaH removes both chloride ligands to give RuH2{xant((PPr2)-Pr-i)(2)}(kappa-S-DMSO) (4). The stirring of 3 in 2-propanol under 3 atm of H, for a long time produces the elimination of DMSO and the coordination of H-2 to yield the dihydrogen derivative, RuHCl(eta(2)-H-2)-{xant((PPr2)-Pr-i)(2)} (5). In contrast to H-2, PPh3 easily displaces DMSO from the metal center of 3 to afford RuHCl{xant((PPr2)-Pr-i)(2)}(PPh3) (6), which can be also obtained starting from RuHCl(PPh3)(3) (7) and xant((PPr2)-Pr-i)(2). In contrast to 3, complex 4 does not undergo DMSO elimination to give RuH2(eta(2)-H-2){xant((PPr2)-Pr-i)(2)} (8) under a H-2 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((PPr2)-Pr-i)(2). A more efficient procedure to obtain 8 involves the sequential hydrogenation with ammonia borane of the allenylidene derivative RuCl2(=C=C=CPh2){xant((PPr2)-Pr-i)(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((PPr2)-Pr-i)(2)} (11), which undergoes the subsequent reduction of the Ru-C double bond to yield the hydride-tetrahydroborate complex, RuH(eta(2)-H2BH2){xant((PPr2)-Pr-i)(2)} (12). The osmium complex, OsCl2{xant((PPr2)-Pr-i)(2)}(kappa-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(PiPr(2))(2)} (14). Ammonia borane also reduces the C-beta-C-gamma double bond of the allenylidene of 14. However, the resulting vinylidene species, OsCl2(=C=CHCHPh2){xant((PPr2)-Pr-i)(2)} (15), is inert. Complex 12 is an efficient catalyst precursor for the hydrogen transfer from 2-propanol to ketones, the alpha-alkylations of phenylacetonitrile and acetophenone with alcohols, and the regio- and stereoselective head-to-head (Z) dimerization of terminal alkynes.