Efficient Transfer Hydrogenation of Ketones Catalyzed by the Bis(isocyanide)−Ruthenium(II) Complexes trans,cis,cis-[RuX2(CNR)2(dppf)] (X = Cl, Br; dppf = 1,1‘-Bis(diphenylphosphino)ferrocene): Isolation of Active Mono- and Dihydride Intermediates

Dual utility of a single diphosphine-ruthenium complex: a precursor for new complexes and, a pre-catalyst for transfer-hydrogenation and Oppenauer oxidation

The diphosphine-ruthenium complex, [Ru(dppbz)(CO)₂Cl₂] (dppbz = 1,2-bis(diphenylphosphino)benzene), where the two carbonyls are mutually cis and the two chlorides are trans, has been found to serve as an efficient precursor for the synthesis of new complexes. In [Ru(dppbz)(CO)₂Cl₂] one of the two carbonyls undergoes facile displacement by neutral monodentate ligands (L) to afford complexes of the type [Ru(dppbz)(CO)(L)Cl₂] (L = acetonitrile, 4-picoline and dimethyl sulfoxide). Both the carbonyls in [Ru(dppbz)(CO)₂Cl₂] are displaced on reaction with another equivalent of dppbz to afford [Ru(dppbz)₂Cl₂]. The two carbonyls and the two chlorides in [Ru(dppbz)(CO)₂Cl₂] could be displaced together by chelating mono-anionic bidentate ligands, viz. anions derived from 8-hydroxyquinoline (Hq) and 2-picolinic acid (Hpic) via loss of a proton, to afford the mixed-tris complexes [Ru(dppbz)(q)₂] and [Ru(dppbz)(pic)₂], respectively. The molecular structures of four selected complexes, viz. [Ru(dppbz)(CO)(dmso)Cl₂], [Ru(dppbz)₂Cl₂], [Ru(dppbz)(q)₂] and [Ru(dppbz)(pic)₂], have been determined by X-ray crystallography. In dichloromethane solution, all the complexes show intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry on the complexes shows redox responses within 0.71 to -1.24 V vs. SCE. [Ru(dppbz)(CO)₂Cl₂] has been found to serve as an excellent pre-catalyst for catalytic transfer-hydrogenation and Oppenauer oxidation.

Isonitriles as supporting and non-innocent ligands in metal catalysis

Isonitriles are unique ligands for metal catalysis, owing to the possibility of their steric and electronic tuning as well as their non-innocent nature to undergo transformations with nucleophiles.

Synthesis of a ruthenium bis(diisopropylamino(isocyano)borane) complex from the activation of an amino(cyano)borane

The reactivity of the amino(cyano)borane (i)Pr(2)NBH(CN) (2) towards [RuH(2)(η(2)-H(2))(2)(PCy(3))(2)] (3) was investigated. Our study reveals that the formation of the bis(isocyanoborane) ruthenium complex [RuH(CN)(CN-BHN(i)Pr(2))(2)(PCy(3))(2)] (6) occurs via the intermediacy of the bis(σ-B-H) ruthenium complex [RuH(CN)(H(2)BN(i)Pr(2))(PCy(3))(2)] (5). This transformation involves BCN linkage isomerisation in 2. The diisopropylaminoborane ligand in 5 can be displaced to ultimately produce the bis(σ-borane) complex [RuH(2)(η(2):η(2)-H(2)BN(i)Pr(2))(PCy(3))(2)] (4) as a by-product.

31P NMR study of the stoichiometry, stability and thermodynamics of complex formation between palladium(II) acetate and bis(diphenylphosphino)ferrocene

The complexation reaction between palladium (II) acetate, and 1,1'-bis(diphenylphosphino)ferrocene, DPPF, was investigated in two different deuterated solvents CDCl(3) and DMSO at various temperatures using (31)P NMR spectroscopy. The exchange between free and complexed DPPF is slow on the NMR time scale and consequently, two (31)P NMR signals were observed. At metal ion-to-ligand mole ratio larger than 1, only one (31)P NMR signal was observed, indicating the formation of a 1:1 Pd(2+)-DPPF complex in solution. The formation constant of the resulting 1:1 complexes was determined from the integration of two (31)P signals. The values of the thermodynamic parameters (DeltaH, DeltaS and DeltaG(298)) for complexation were determined from the temperature dependence of stability constants. It was found that, in both solvents, the resulting complex is mainly entirely enthalpy stabilized and the DeltaH compensates the TDeltaS contribution.

Design and synthesis of isocyanide ligands for catalysis: application to Rh-catalyzed hydrosilylation of ketones

New isocyanide ligands with meta-terphenyl backbones were synthesized. 2,6-Bis[3,5-bis(trimethylsilyl)phenyl]-4-methylphenyl isocyanide exhibited the highest rate acceleration in rhodium-catalyzed hydrosilylation among other isocyanide and phosphine ligands tested in this study. 1H NMR spectroscopic studies on the coordination behavior of the new ligands to [Rh(cod)2]BF4 indicated that 2,6-bis[3,5-bis(trimethylsilyl)phenyl]-4-methylphenyl isocyanide exclusively forms the biscoordinated rhodium-isocyanide complex, whereas less sterically demanding isocyanide ligands predominantly form tetracoordinated rhodium-isocyanide complexes. FTIR and 13C NMR spectroscopic studies on the hydrosilylation reaction mixture with the rhodium-isocyanide catalyst showed that the major catalytic species responsible for the hydrosilylation activity is the Rh complex coordinated with the isocyanide ligand. DFT calculations of model compounds revealed the higher affinity of isocyanides for rhodium relative to phosphines. The combined effect of high ligand affinity for the rhodium atom and the bulkiness of the ligand, which facilitates the formation of a catalytically active, monoisocyanide-rhodium species, is proposed to account for the catalytic efficiency of the rhodium-bulky isocyanide system in hydrosilylation.

Catalytic Transfer Hydrogenation with Terdentate CNN Ruthenium Complexes: The Influence of the Base

The catalytic activity of the terdentate complex [RuCl(CNN)(dppb)] (A) [dppb=Ph(2)P(CH(2))(4)PPh(2); HCNN=6-(4'-methylphenyl)-2-pyridylmethylamine] in the transfer hydrogenation of acetophenone (S) with 2-propanol has been found to be dependent on the base concentration. The limit rate has been observed when NaOiPr is used in high excess (A/base molar ratio > 10). The amino-isopropoxide species [Ru(OiPr)(CNN)(dppb)] (B), which forms by reaction of A with sodium isopropoxide via displacement of the chloride, is catalytically active. The rate of conversion of acetophenone obeys second-order kinetics v=k[S][B] with the rate constants in the range 218+/-8 (40 degrees C) to 3000+/-70 M(-1) s(-1) (80 degrees C). The activation parameters, evaluated from the Eyring equation are DeltaH(++)=14.0+/-0.2 kcal mol(-1) and DeltaS(++)=-3.2 +/-0.5 eu. In a pre-equilibrium reaction with 2-propanol complex B gives the cationic species [Ru(CNN)(dppb)(HOiPr)](+)[OiPr](-) (C) with K approximately 2x10(-5) M. The hydride species [RuH(CNN)(dppb)] (H), which forms from B via beta-hydrogen elimination process, catalyzes the reduction of S and, importantly, its activity increases by addition of base. The catalytic behavior of the hydride H has been compared to that of the system A/NaOiPr (1:1 molar ratio) and indicates that the two systems are equivalent.