Ruthenium-Catalyzed Reactions for Organic Synthesis.

A literature review of modern transition-metal nanoclusters, with an emphasis on those nanoclusters which are catalytically active, is presented in two parts. Part One presents background information on transition-metal nanoclusters, including an overview of common synthetic routes, a description of how nanoclusters are stabilized, and a brief summary of the multiple characterization techniques used (and the type of information that they can provide). In Part Two, five specific nanocluster case studies are presented, case studies which compare and contrast the syntheses, characterization approaches, and catalytic applications of transition-metal nanoclusters.

A review of modern transition-metal nanoclusters: their synthesis, characterization, and applications in catalysis

Ruthenium-based anticancer chemotherapies are making significant advances in clinical trials. Until recently, the focus has been on coordination complexes, and mechanisms such as "activation by reduction" and "transferrin-targeted delivery" have been proposed to account for the excellent cytotoxicity and low general toxicity of these complexes. More recently organoruthenium compounds, which to some extent appear not to follow the established rules, have started to be investigated. Despite such differences, similar activities between certain coordination and organometallic compounds suggest similar modes of action are present. DNA, the classic target, is believed to be the dominant mechanism for cytotoxicity with certain ruthenium drugs, while with others, non-classical targets are thought to be more important. In this article we describe these features and show how both ruthenium coordination complexes and organoruthenium compounds represent an ideal scaffold for further drug design and optimisation.

Classical and Non-Classical Ruthenium-Based Anticancer Drugs: Towards Targeted Chemotherapy

The chemical transformation of carbon dioxide into useful products becomes increasingly important as CO2 levels in the atmosphere continue to rise as a consequence of human activities. In this article we describe the direct hydrogenation of CO2 into formic acid using a homogeneous ruthenium catalyst, in aqueous solution and in dimethyl sulphoxide (DMSO), without any additives. In water, at 40 °C, 0.2 M formic acid can be obtained under 200 bar, however, in DMSO the same catalyst affords 1.9 M formic acid. In both solvents the catalysts can be reused multiple times without a decrease in activity. Worldwide demand for formic acid continues to grow, especially in the context of a renewable energy hydrogen carrier, and its production from CO2 without base, via the direct catalytic carbon dioxide hydrogenation, is considerably more sustainable than the existing routes.

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Direct synthesis of formic acid from carbon dioxide by hydrogenation in acidic media

Iridium half-sandwich complexes of the types Cp*Ir(N−C)X, [Cp*Ir(N−N)X]X, and [CpIr(N−N)X]X are catalyst precursors for the homogeneous oxidation of water to dioxygen. Kinetic studies with cerium(IV) ammonium nitrate as primary oxidant show that oxygen evolution is rapid and continues over many hours. In addition, [Cp*Ir(H2O)3]SO4 and [(Cp*Ir)2(μ-OH)3]OH can show even higher turnover frequencies (up to 20 min−1 at pH 0.89). Aqueous electrochemical studies on the cationic complexes having chelate ligands show catalytic oxidation at pH > 7; conversely, at low pH, there are no oxidation waves up to 1.5 V vs NHE for the complexes. H218O isotope incorporation studies demonstrate that water is the source of oxygen atoms during cerium(IV)-driven catalysis. DFT calculations and kinetic experiments, including kinetic-isotope-effect studies, suggest a mechanism for homogeneous iridium-catalyzed water oxidation and contribute to the determination of the rate-determining step. The kinetic experiments also help distin...

Half-Sandwich Iridium Complexes for Homogeneous Water-Oxidation Catalysis

Arenedi-µ-chloro-ruthenium complexes [RuCl2(arene)]2 have been prepared by dehydrogenation of the appropriate cyclohexa-1,3-diene or cyclohexa-1,4-diene with ethanolic ruthenium(III) trichloride. They react with ligands (L) such as pyridine, tertiary phosphines, or tertiary arsines to give monomeric complexes [RuCl2(arene)L], which are formally analogous to the arenetricarbonylchromium complexes. Analogous dibromo-, di-iodo-, halogeno-(methyl), and dimethyl derivatives have also been prepared; the latter are thermally unstable and air-sensitive. Reaction of the complexes [RuCl2(arene)]2(arene = benzene or p-cymene) with water gives binuclear tri-µ-chloro-cationic species [Ru2C13(arene)2]+, and reaction with silver(I) tetrafluoroborate in acetonitrile gives monomeric [Ru(arene)(MeCN)3]2+. I.r. and n.m.r. data for the new complexes are given. The complexes [RuCl2(arene)L](L = PBun3 or PPh3) undergo partial or complete arene exchange on heating or on u.v. irradiation in an aromatic solvent, co-ordinated p-cymene being the most easily displaced. The exchange method can be used to prepare hexamethylbenzene complexes, e.g.[RuCl2(C6Me6)(PBun3)]. The results are compared with available data on arene exchange in arenetricarbonylchromium complexes and discussed in terms of electronic and steric effects on metal–arene bonding.

Arene ruthenium(II) complexes formed by dehydrogenation of cyclohexadienes with ruthenium(III) trichloride

Thermolysis of [Os3(CO)10(dppm)](dppm = Ph2PCH2PPh2) in refluxing toluene affords a high yield of the unsaturated cluster [Os3(CO)8(H){Ph2PCH2P(Ph)C6H4}] which has been characterized by X-ray crystallography, and which undergoes a facile reversible reaction with CO.

The high yield synthesis of an unsaturated osmium cluster; the X-ray crystal structure of [Os3(CO)8(H){Ph2PCH2P(Ph)C6H4}]

Under mild conditions, the complex [RuHCl-(η-C6Me6)(PPh3)] is an active homogeneous catalyst for the addition of molecular hydrogen to benzene giving cyclohexane and for the hydrogenation of olefins using either molecular hydrogen or secondary alcohols.

An arene hydrido-complex of ruthenium(II) as catalyst for the homogeneous hydrogenation of benzene and olefins

Reaction of a solution of [LaL3(Ph3PO)][L = N(SiMe3)2] with Ph2PH followed by Ph3PO yields [LaL2(PPh2)(Ph3PO)2], which has been isolated as highly air-sensitive orange crystals and characterised by solution NMR spectroscopy (31P and 1H) and X-ray crystallography. The structure was refined to a final R value of 0.056 and shows the complex to have a terminal PPh2– ligand with planar co-ordination at P. Variable-temperature 31P NMR spectroscopy indicates rapid exchange between free and co-ordinated Ph3PO at room temperature but at 193 K is consistent with the solid-state structure with two Ph3PO ligands.

Synthesis and crystal structure of a lanthanum complex with a diphenylphosphide ligand

Treatment of the unsaturated clusters [Os3(µ-H)(CO)8{µ-Ph2PCH2P(Ph)C6H4}]1a and [Os3(µ-H)(CO)8{µ-Ph2PC(CH2)P(Ph)C6H4}]1b with two-electron donor ligands gave the saturated clusters [Os3(µ-H)(CO)8{µ-Ph2PCH2P(Ph)C6H4}L][L = CO 2a or PR3 or P(OR)33(R = Me, Et, Pri, Bu or Ph)], [Os3(µ-H)(CO)8{µ-Ph2PC(CH2)P(Ph)C6H4}L](L = CO 2b) and [Os3(CO)8(dppm)L2]4[dppm = Ph2PCH2PPh2; L = CO, PR3 or P(OR)3]. Two different isomers of complex 4 are observed depending on the ligand L. All the products have been characterised by IR and NMR spectroscopy. The structures of [Os3H(CO)8{Ph2PCH2P(Ph)C6H4}(PPri3)]3a, [Os3(CO)8(dppm){P(OMe)3}2]4a and [Os3(CO)8(dppm)(PPh3)2]4c have also been established by X-ray crystallography.

Reaction of the unsaturated triosmium clusters [Os3H(CO)8{Ph2PCH2P(Ph)C6H4}] and [Os3H(CO)8{Ph2PC(CH2)P(Ph)C6H4}] with phosphines and phosphites; crystal structures of [Os3H(CO)8{Ph2PCH2P(Ph)C6H4}(PPri3)], [Os3(CO)8(Ph2PCH2PPh2){P(OMe)3}2], and [Os3(CO)8(Ph2PCH2PPh2)(PPh3)2]

Anthony K. Smith

Papers

Arene ruthenium(II) complexes formed by dehydrogenation of cyclohexadienes with ruthenium(III) trichloride

The high yield synthesis of an unsaturated osmium cluster; the X-ray crystal structure of [Os3(CO)8(H){Ph2PCH2P(Ph)C6H4}]

An arene hydrido-complex of ruthenium(II) as catalyst for the homogeneous hydrogenation of benzene and olefins

Synthesis and crystal structure of a lanthanum complex with a diphenylphosphide ligand

Reaction of the unsaturated triosmium clusters [Os3H(CO)8{Ph2PCH2P(Ph)C6H4}] and [Os3H(CO)8{Ph2PC(CH2)P(Ph)C6H4}] with phosphines and phosphites; crystal structures of [Os3H(CO)8{Ph2PCH2P(Ph)C6H4}(PPri3)], [Os3(CO)8(Ph2PCH2PPh2){P(OMe)3}2], and [Os3(CO)8(Ph2PCH2PPh2)(PPh3)2]