Classical and non-classical phosphine-Ru(ii)-hydrides in aqueous solutions: many, various, and useful

Abstract

Hydrogenation of the water-soluble [{RuCl(2)(mtppms)(2)}(2)] (mtppms = monosulfonated triphenylphosphine) was studied in aqueous solutions in the presence of excess mtppms both with H(2) and with aqueous HCOONa. Depending on the reductant, the pH and H(2) pressure altogether nine hydride species were identified. In acidic solutions at 1 bar H(2) pressure the known [RuHCl(mtppms)(3)] (1) and [{RuHCl(mtppms)(2)}(2)] (3) were formed, however, elevated pressure led to the formation of trans-[RuH(2)(mtppms)(4)] (11). In basic solutions at atmospheric pressure cis-fac-[RuH(2)(H(2)O)(mtppms)(3)] (12) was observed which was readily replaced by [RuH(2)(η(2)-H(2))(mtppms)(3)] (13) at higher H(2) pressures. 13 is the first water-soluble and stable η(2)-H(2) Ru(II)-complex stabilized only by monodentate phosphine ligands. [RuHBr(mtppms)(3)] (9) and [RuHI(mtppms)(3)] (10) were obtained analogously to 1. In concentrated aqueous HCOONa solutions (often used in H-transfer hydrogenations) the major species was trans-[RuH(2)(HCOO)(mtppms)](-) (14) while in dilute solutions trans-[RuH(2)(H(2)O)(mtppms)(3)] (15) could be observed. Formation of these various hydride species offers an explanation for the earlier observed pH and pressure dependence of the rates and selectivities in hydrogenation of unsaturated aldehydes catalyzed by [{RuCl(2)(mtppms)(2)}(2)] + mtppms.

Hydrogenation of Cinnamaldehyde by Water‐Soluble Ruthenium(II) Phosphine Complexes: A DFT Study on the Selectivity and Viability of trans‐Dihydride Pathways

“…Extensive studies on classic organic phase Ru(II)‐PPh 3 systems implicate that “tetrahydride” or cis ‐dihydride complexes are the most likely forms of the catalyst [17] . They also appear in the Ru‐ m tppms system, and the predominance of either one depends on many factors like the pH or external H 2 pressure, as indicated in Scheme 1 [10] . In the following, note that our model neglects the sulfo groups, but uses water as solvent and can account for different H 2 pressures through the RTln(p/p 0 ) energy correction.…”

“…Our group has developed numerous water‐soluble hydrogenation catalysts by utilizing sulfonated phosphine ligands [6a,9] . One example is the Ru(II)‐PPh 3 derived Ru(II)‐ m tppms ( m tppms=monosulfonated triphenylphosphine) system which – depending on the experimental conditions – may yield many individual complexes, as thoroughly detailed in a previous work [10] . These were synthesized from the m tppms‐containing Ru(II)‐precursor A , providing the novel (pre‐)catalysts shown in Scheme 1.…”

Unexpectedly fast catalytic transfer hydrogenation of aldehydes by formate in 2-propanol–water mixtures under mild conditions

“…4) and this again does not support the assumption that the large rate increase is simply caused by the higher concentration of cinnamaldehyde in the water-2-propanol solvent mixture. [50]. Selective 31 P decoupling unambiguously showed the trans-mer coordination in the 'RuH 2 (mtppms) 3 fragment of octahedral complexes.…”

“…Water may influence the actual molecular form of the catalyst by promoting hydrolysis (formation of hydroxocomplexes) [50]; preferring heterolytic activation of H 2 [50]; allowing formation of several hydrido-and molecular hydrogen complexes from the same catalyst precursor depending on the pH of the aqueous phase and on H 2 pressure [23, 24,50]; protonation/deprotonation equilibria, etc. Concerning phase transfer and solubility effects the chemical reaction may proceed either in the catalyst-containing bulk aqueous phase, or at the interphase of the two bulk phases.…”

A novel carbohydrate labeling method utilizing transfer hydrogenation-mediated reductive amination

“…Please note that the complex is also capable of catalyzing hydrogenation with gaseous H 2 [16], albeit, this side reaction does not hinder reduction of the unsaturated substrate, and may require the use of larger amounts of HCOOH due to the escape of a part of H 2 into the gas phase.…”