Catalyst Precursors For Transfer Hydrogenation Research Articles

2-D Molecular Alloy Ru-M (M = Cu, Ag, and Au) Carbonyl Clusters: Synthesis, Molecular Structure, Catalysis, and Computational Studies.

The reactions of [HRu3(CO)11]− (1) with M(I) (M = Cu, Ag, and Au) compounds such as [Cu(CH3CN)4][BF4], AgNO3, and Au(Et2S)Cl afford the 2-D molecular alloy clusters [CuRu6(CO)22]− (2), [AgRu6(CO)22]− (3), and [AuRu5(CO)19]− (4), respectively. The reactions of 2–4 with PPh3 result in mixtures of products, among which [Cu2Ru8(CO)26]2– (5), Ru4(CO)12(CuPPh3)4 (6), Ru4(CO)12(AgPPh3)4 (7), Ru(CO)3(PPh3)2 (8), and HRu3(OH)(CO)7(PPh3)3 (9) have been isolated and characterized. The molecular structures of 2–6 and 9 have been determined by single-crystal X-ray diffraction. The metal–metal bonding within 2–5 has been computationally investigated by density functional theory methods. In addition, the [NEt4]+ salts of 2–4 have been tested as catalyst precursors for transfer hydrogenation on the model substrate 4-fluoroacetophenone using iPrOH as a solvent and a hydrogen source.

Cationic ruthenium(II)–NHC pincer complexes: Synthesis, characterisation and catalytic activity for transfer hydrogenation of ketones

Cationic ruthenium pincer complexes, [Ru(CNC)(CO)(PPh3)Cl]X (CNC = 2,6‐bis(1‐methylimidazol‐2‐ylidene)‐pyridine, X = Cl− [1a], PF6− [1b]), [Ru(CNC)(PPh3)2Cl]X (X = Cl− [2a], PF6− [2b]) and [Ru(CNC)(PPh3)2(H)]X (X = Cl− [3a], PF6− [3b]) with triphenylphosphine, CO and halides as coligands have been synthesised and characterised by 1H, 13C, 31P NMR, mass and single‐crystal X‐ray crystallography. The application of Ru complexes in the transfer hydrogenation of a wide range of ketones with 2‐propanol as the hydrogen source is explored. The in situ transformations observed during the synthesis help understand and suggest a plausible mechanism via the hydride complex 3b. All complexes appear to be efficient catalyst precursors for transfer hydrogenation of ketones.

Facile synthesis of a 2-(2′-pyridyl)-4-(methylcarboxy)quinoline ruthenium (II) based catalyst precursor for transfer hydrogenation of aromatic ketones

Facile synthesis of a 2-(2′-pyridyl)-4-(methylcarboxy)quinoline ruthenium (II) based catalyst precursor for transfer hydrogenation of aromatic ketones

Ru(II)‐η6‐benzene Complexes of Dibenzosuberenyl Appended Aroyl/Acylthiourea Ligands: In vitro Biomolecular Interaction Studies and Catalytic Transfer Hydrogenation

AbstractA series of novel N‐dibenzosuberene appended aroyl/acylthiourea ligands (L1‐L4) and their Ru(II)‐benzene complexes (1‐4), [RuCl2(η6‐benzene)L] (L=monodentate aroyl/acylthiourea ligand) was synthesized and well characterized. The coordination mode of aroyl/acylthiourea ligand with Ru ion through S (neutral monodentate) donor atom was determined by single crystal X‐ray diffraction analysis. In vitro Calf thymus DNA (CT‐DNA) and Bovine serum albumin (BSA) interaction of the complexes were investigated by UV‐Visible, fluorescence spectroscopic and hydrodynamic methods. The results showed the intercalative mode of binding (in the order of 1 > 4 > 3 > 2) of the Ru(II)‐benzene complexes with CT‐DNA. The DNA (pUC19) cleavage study showed that the complexes cleaved DNA through a hydrolytic pathway. All the complexes have been thoroughly screened for their cytotoxicity against human liver carcinoma (HepG‐2) and lung cancer (A549) cells under in vitro conditions. Complex 1 exhibited significant cytotoxic activity (IC50=46.1 μM) towards HepG‐2 cancer cell line. In addition to this, the prepared complexes have been utilized as catalyst precursors for transfer hydrogenation (TH) of ketones, aldehydes and nitro compounds using 2‐propanol as a hydrogen source. The TH reactions proceeded with exceptional conversions (up to 99%) and the present catalytic system was extended to substituted aromatic/heterocyclic carbonyl and nitro compounds.

Triazolylidene Iridium Complexes for Highly Efficient and Versatile Transfer Hydrogenation of C═O, C═N, and C═C Bonds and for Acceptorless Alcohol Oxidation.

A set of iridium(I) and iridium(III) complexes is reported with triazolylidene ligands that contain pendant benzoxazole, thiazole, and methyl ether groups as potentially chelating donor sites. The bonding mode of these groups was identified by NMR spectroscopy and X-ray structure analysis. The complexes were evaluated as catalyst precursors in transfer hydrogenation and in acceptorless alcohol oxidation. High-valent iridium(III) complexes were identified as the most active precursors for the oxidative alcohol dehydrogenation, while a low-valent iridium(I) complex with a methyl ether functionality was most active in reductive transfer hydrogenation. This catalyst precursor is highly versatile and efficiently hydrogenates ketones, aldehydes, imines, allylic alcohols, and most notably also unpolarized olefins, a notoriously difficult substrate for transfer hydrogenation. Turnover frequencies up to 260 h-1 were recorded for olefin hydrogenation, whereas hydrogen transfer to ketones and aldehydes reached maximum turnover frequencies greater than 2000 h-1. Mechanistic investigations using a combination of isotope labeling experiments, kinetic isotope effect measurements, and Hammett parameter correlations indicate that the turnover-limiting step is hydride transfer from the metal to the substrate in transfer hydrogenation, while in alcohol dehydrogenation, the limiting step is substrate coordination to the metal center.

Amidato complexes of ruthenium, rhodium and iridium from concise N–H bond activation: exploration in catalysis

Acceptor-substituted N–H groups as found in carbamides and sulfonamides are readily activated through suitable unsaturated metal complexes applying the concept of metal–ligand bifunctionality. This process constitutes the basis for an enantioselective intramolecular addition of N–H groups onto activated alkenes. The resulting compounds can also be employed as catalyst precursors for transferhydrogenation.

Ruthenium(0) complexes with triazolylidene spectator ligands: Oxidative activation for (de)hydrogenation catalysis

Transmetallation of silver triazolylidene intermediates with the ruthenium(0) precursor [Ru(CpO) (CO)2]2 afforded low-valent ruthenium(0) complexes containing a triazole-derived NHC ligand (CpO = 3,4-di(4-methoxyphenyl)-2,5-diphenyl-cyclopentadienone). Protonation of the carbonyl group of the CpO ligand significantly reduces the π character of the Ru–CO bond as deduced from νCO analysis. The new triazolylidene ruthenium(0) complexes were evaluated as catalyst precursors in transfer hydrogenation of 4-fluoro-acetophenone and in the acceptorless dehydrogenation of benzyl alcohol. Low activities were noted, though in both reactions, catalytic performance is markedly increased when cerium(IV) was added. Electrochemical analysis indicates that activation of the catalyst precursor proceeds via cerium-mediated oxidation of the ruthenium center, which facilitates dissociation of a CO ligand to enter the catalytic cycle. Such oxidative activation of catalyst precursors may be of more general scope.

Rhodium‐catalyzed transfer hydrogenation with aminophosphines and analysis of electrical characteristics of rhodium(I) complex/n‐Si heterojunctions

A series of novel neutral mononuclear rhodium(I) complexes of the P―NH ligands have been prepared starting from [Rh(cod)Cl]2 complex. Structural elucidation of the complexes was carried out by elemental analysis, IR and multinuclear NMR spectroscopic data. The complexes were applied to the transfer hydrogenation of acetophenone derivatives to 1‐phenylethanol derivatives in the presence of 2‐propanol as the hydrogen source. Catalytic studies showed that all complexes are also excellent catalyst precursors for transfer hydrogenation of aryl alkyl ketones in 0.1 m iso‐PrOH solution. In particular, [Rh(cod)(PPh2NH―C6H4―4‐CH(CH3)2)Cl] acts as an excellent catalyst, giving the corresponding alcohols in excellent conversion up to 99% (turnover frequency ≤ 588 h−1). Furthermore, rhodium(I) complexes have been used in the formation of organic–inorganic heterojunction by forming their thin films on n‐Si and evaporating Au on the films. It has been seen that the structures have rectifying properties. Their electrical properties have been analyzed with the help of current–voltage measurements. Finally, it has been shown that the complexes can be used in the fabrication of temperature and light sensors. Copyright © 2014 John Wiley & Sons, Ltd.

Reactivity patterns and catalytic chemistry of iridium polyhydride complexes

[IrH 5P 2] ( 1, P  PPr i 3) reacts autocatalytically with CF 3COOR (R  CH 2CF 3) in cyclo-C 6D 12 at 60°C according to: 1 + CF 3COOR → [IrH 2P 2(OR)] ( 2) + ROH ( 4) (eq. 1). The rate-law, − d[ 1] d t = k[ 1] 1 2 [CF 3COOR][ 2] 1 2 [ 4] − 1 2 ( k = 1.25 × 10 −4 M − 1 2 sec −1), is consistent with the mechanism, 1 + 2 ⇌ 2 [IrH 3P 2] ( 5) + 4 (rapid equilibrium); 5 + CF 3COOR → [IrH 2P 2{OCH(OR)CF 3}] ( 6) (rate determining); 6 → 2 + CF 3CHO; 5 + CF 3CHO → 2. 2 reacts rapidly with H 2 (25° C, 1 atm) according to: 2 + 2 H 2 → 1 + 4 (eq. 2). Although the combination of reactions 1 and 2 constitute a catalytic cycle for the hydrogenation of CF 3COOR (CF 3COOR + 2 H 2 → 2 ( 4), catalyzed by 1), such catalytic hydrogenation does not occur, presumably because H 2 suppresses reaction by rapidly converting the catalytic intermediates, 2 and 5, to 1. However, 1 was found to be effective as a catalyst or catalyst precursor for transfer hydrogenation, e.g. CH 2CHC(CH 3) 3 + (CH 3) 2CHOH → CH 3CH 2C(CH 3) 3 + (CH 3) 2CO. While not directly detected, IrH 3P 2 could be trapped at low temperatures by N 2 to yield the complexes [IrH 3P 2(N 2)] and [(IrH 3P 2) 2N 2] which are related through the labile equilibrium, [(IrH 3P 2) 2N 2] + N 2 ⇌ 2 [IrH 3P 2(N 2)] ( K eq ∼ 1.5 at 35° C).