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

Abstract

A wide range of ruthenium complexes stabilized by the POP-pincer ligand xant(P(i)Pr2)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(P(i)Pr2)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(P(i)Pr2)2}(κ-S-DMSO) (3), whereas NaH removes both chloride ligands to give RuH2{xant(P(i)Pr2)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(η(2)-H2){xant(P(i)Pr2)2} (5). In contrast to H2, PPh3 easily displaces DMSO from the metal center of 3 to afford RuHCl{xant(P(i)Pr2)2}(PPh3) (6), which can be also obtained starting from RuHCl(PPh3)3 (7) and xant(P(i)Pr2)2. In contrast to 3, complex 4 does not undergo DMSO elimination to give RuH2(η(2)-H2){xant(P(i)Pr2)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(P(i)Pr2)2. A more efficient procedure to obtain 8 involves the sequential hydrogenation with ammonia borane of the allenylidene derivative RuCl2(═C═C═CPh2){xant(P(i)Pr2)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(P(i)Pr2)2} (11), which undergoes the subsequent reduction of the Ru-C double bond to yield the hydride-tetrahydroborate complex, RuH(η(2)-H2BH2){xant(P(i)Pr2)2} (12). The osmium complex, OsCl2{xant(P(i)Pr2)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(P(i)Pr2)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(P(i)Pr2)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.

Osmium Complexes With POP Pincer Ligands

“…The ruthenium metal fragment Ru{xant(P i Pr 2 ) 2 } avoids the oxidation state four. As a result, the osmium d 4 -polyhydrides are d 6 -dihydrogens in the ruthenium chemistry, which require different synthetic procedures from those of osmium for their preparation [68].…”

“…The scarce development of the pincer-osmium chemistry is probably due to the rooted belief that osmium is not useful in catalysis because it is a reductant and prefers to be coordinatively saturated and forms redox isomers with greater metal-carbon multiplicity [10-13]. However, recent findings have proved that osmium can be a promising alternative to the metals classically used in catalysis [14][15][16][17][18][19][20][21][22][23].The chemistry of the pincer-osmium complexes has been mainly focused on a few PCP derivatives reported by the groups of Gusev [24-28], Jia [29][30][31] and Milstein [32,33] and PNP complexes described by the groups of Caulton [34-37], Gusev [38-41], and Jia [42], along with some CCC [43-46], CCN [47], CNC [48,49], CNN [50-52], CNO [53], and NNN [54-59] species.We synthesized the diphosphines xant(P i Pr 2 ) 2 and dbf(P i Pr 2 ) 2 in 2010 [60], and subsequently started our research program on POP complexes of platinum group metals [3,[60][61][62][63][64][65], including osmium [60,[66][67][68][69][70][71]. In this chapter, we summarize the results obtained for this element until now.…”

“…We synthesized the diphosphines xant(P i Pr 2 ) 2 and dbf(P i Pr 2 ) 2 in 2010 [60], and subsequently started our research program on POP complexes of platinum group metals [3,[60][61][62][63][64][65], including osmium [60,[66][67][68][69][70][71]. In this chapter, we summarize the results obtained for this element until now.…”

Ru(II)‐PBTNNXN complex bearing functional 2‐(pyridin‐2‐yl)benzo[d]thiazole ligand catalyzed α‐alkylation of nitriles with alcohols

“…Since 1981, when Grigg and co‐workers first used RuH 2 (PPh 3 ) 4 as a catalyst in the alkylation of nitriles with alcohol, a lot of effort has been devoted to exploring various catalysts, including Ru, Rh, Ir, Pd, Os, Fe, and Mn. Of these, Ru complexes are the most reported, for example Gunanathan's group reported that Ru(PNP) (PNP = bis(2‐(diphenylphosphino)‐ethyl)amine) pincer complex showed good activity for α ‐alkylated reaction of arylmethyl nitriles .…”

Solvent-free direct α-alkylation of ketones by alcohols catalyzed by nickel supported on silica–alumina

“…[3][4][5][6] This methodology exhibits high atom economy, as water is the sole co-product. Homogeneous catalysts with precious metals such as Ru, [7][8][9][10][11] Ir, [12][13][14][15][16] Rh, 17 Os, 18 Au, 19 and Re 20 have been widely studied for the α-alkylation of ketones with alcohols. Although used in low loadings, these organometallic complexes are expensive.…”