Gold trifluoromethyl complexes as efficient regioselective catalysts in alkyne hydration.

Gold (III) complexes containing trifluromethyl ligands are efficient catalyst in the hydration of alkynes, operating at low catalyst loadings, without additives, using environmentally friendly solvents and at mild conditions (60 ºC). Hydration of terminal and internal alkynes provide the corresponding ketones in quantitative yields without special precautions as dry solvents or inert atmospheres. Remarkably, hydration of asymmetric internal alkynes proceeds with moderate to notable regioselectivities, providing mixtures of the two possible isomers with ratios up to 90:10.

Ion mobility mass spectrometry uncovers regioselectivity in the carboxylate-assisted C-H activation of palladium N-heterocyclic carbene complexes.

Carboxylate-assisted Pd-catalyzed C-H bond activation constitutes a mild and versatile synthetic tool to efficiently and selectively cleave inert C-H bonds. Herein, we demonstrate a simple method to experimentally evaluate both reactivity and selectivity in such systems using mass spectrometry (MS) methods. The N-heterocyclic carbene (NHC) cations [(NHC)PdX]+, bearing as X- ligand bases commonly used to promote the C-H activation (carboxylates and bicarbonate), are generated in the gas-phase by ESI-MS. Their C-H bond activation at the N-bound groups of the NHC is then studied using Collision Induced Dissociation (CID) experiments. Ion Mobility Spectrometry (IM)-MS is exploited to identify a number of regioisomers associated with the distinctive site selective C-H activations. It is demonstrated that such C-H activation concomitant with acetic acid release occurs from a mixture of activated [(NHC-H)Pd(CH3CO2H)]+ and non-activated [(NHC)Pd(CH3CO2)]+ complexes. The identity of the X-type ligands (X = Cl-, carboxylates and bicarbonate) has a significant impact on the regioisomer branching ratio upon CID conditions. IM-MS in conjunction with a DFT mechanistic study is presented for the acetate-assisted C-H activation of the [(NHC)Pd(CH3CO2)]+ cation featuring butyl and aryl as N-donor groups.

Introducing Ion Mobility Mass Spectrometry to Identify Site-Selective C-H Bond Activation in N-Heterocyclic Carbene Metal Complexes.

The activation of C-H bonds in a selective manner still constitutes a major challenge from a synthetic point of view; thus, it remains an active area of fundamental and applied research. Herein, we introduce ion mobility spectrometry mass spectrometry-based (IM-MS) approaches to uncover site-selective C-H bond activation in a series of metal complexes of general formula [(NHC)LMCl]+ (NHC = N-heterocyclic carbene; L = pentamethylcyclopentadiene (Cp*) or p-cymene; M = Pd, Ru, and Ir). The C-H bond activation at the N-bound groups of the NHC ligand is promoted upon collision induced dissociation (CID). The identification of the resulting [(NHC-H)LM]+ isomers relies on the distinctive topology that such cyclometalated isomers adopt upon site-selective C-H bond activation. Such topological differences can be reliably evidenced as different mobility peaks in their respective CID-IM mass spectra. Alternative isomers are also identified via dehydrogenation at the Cp*/p-cymene (L) ligands to afford [(NHC)(L-H)M]+. The fragmentation of the ion mobility-resolved peaks is also investigated by CID-IM-CID. It enables the assignment of mobility peaks to the specific isomers formed from C(sp2)-H or C(sp3)-H bond activation and distinguishes them from the Cp*/p-cymene (L) dehydrogenation isomers. The conformational change of the NHC ligands upon C-H bond activation, concomitant with cyclometalation, is also discussed on the basis of the estimated collision cross section (CCS). A unique conformation change of the pyrene-tagged NHC members is identified that involves the reorientation of the NHC ring accompanied by a folding of the pyrene moiety.

Visible-Light-Promoted Iridium(III)-Catalyzed Acceptorless Dehydrogenation of N-Heterocycles at Room Temperature.

An effective visible-light-promoted iridium(III)-catalyzed hydrogen production from N-heterocycles is described. A single iridium complex constitutes the photocatalytic system playing a dual task, harvesting visible-light and facilitating C-H cleavage and H2 formation at room temperature and without additives. The presence of a chelating C-N ligand combining a mesoionic carbene ligand along with an amido functionality in the IrIII complex is essential to attain the photocatalytic transformation. Furthermore, the IrIII complex is also an efficient catalyst for the thermal reverse process under mild conditions, positioning itself as a proficient candidate for liquid organic hydrogen carrier technologies (LOHCs). Mechanistic studies support a light-induced formation of H2 from the Ir-H intermediate as the operating mode of the iridium complex.

Gold nanoparticle-catalysed functionalization of carbon-hydrogen bonds by carbene transfer reactions.

Gold nanoparticles stabilized by NHC ligands and supported onto reduced graphene oxide (rGO) catalyse the functionalization of cyclohexane and benzene C-H bonds upon insertion of carbene CHCO2Et (from N2CHCO2Et) groups. This is the first example in which such Csp3-H or Csp2-H bonds are functionalized with this strategy with nanoparticulated gold. This Au-NP@rGO material shows an exceptional activity, providing TON values 5-10 times higher than those already reported for molecular gold catalysts. Recyclability is also effective, reaching an accumulated TON value of 1400 after six consecutive uses.

Reduced Graphene Oxides as Carbocatalysts in Acceptorless Dehydrogenation of N-Heterocycles.

The catalytic properties of graphene-derived materials are evaluated in acceptorless dehydrogenation of N-heterocycles. Among them, reduced graphene oxides (rGOs) are active (quantitative yields in 23 h) under mild conditions (130 °C) and act as efficient heterogeneous carbocatalysts. rGO exhibits reusability and stability at least during eight consecutive runs. Mechanistic investigations supported by experimental evidence (i.e., organic molecules as model compounds, purposely addition of metal impurities and selective functional group masking experiments) suggest a preferential contribution of ketone carbonyl groups as active sites for this transformation.

Improving Catalyst Activity in Hydrocarbon Functionalization by Remote Pyrene-Graphene Stacking.

A copper complex bearing an N-heterocyclic carbene ligand with a pyrene "tail" attached to the backbone has been prepared and supported on reduced graphene oxide (rGO). The free and supported copper materials have been employed as homogeneous and heterogeneous catalysts in the functionalization of hydrocarbons such as n-hexane, cyclohexane, and benzene through incorporation of the CHCO2 Et unit from ethyl diazoacetate. The graphene-anchored complex displays higher reaction rates and induces higher yields than its soluble counterpart, features that can be rationalized in terms of a decrease in electron density at the metal center due to a remote net electronic flux from the supported copper complex to the graphene surface.

Stabilization of Nanoparticles Produced by Hydrogenation of Palladium-N-Heterocyclic Carbene Complexes on the Surface of Graphene and Implications in Catalysis.

Palladium nanoparticles (NPs) have been obtained by decomposition of well-defined palladium complexes noncovalently anchored onto the surface of reduced graphene oxide. Morphological analysis by microscopy showed the presence of small palladium NPs homogeneously distributed on the support. Characterization by X-ray photoelectron spectroscopy confirmed that palladium NPs contain Pd(2+) and Pd(0) oxidation states and the presence of N-heterocyclic carbene and bromo ligands. The catalytic properties of the NPs with and without the support have been evaluated in the hydrogenation of alkynes. Supported palladium NPs showed increased activity versus the nonsupported ones and could be recycled up to 10 times without the loss of catalytic activity. The composition of the palladium NPs is different for each catalytic cycle indicating a dynamic process and the formation of different catalytic active species. On the contrary, the unsupported palladium NPs showed limited activity caused by decomposition and could not be recycled. The role of the support has been investigated. The results indicate that the support influences the stability of palladium NPs.

Catalytic Dehydrogenative Coupling of Hydrosilanes with Alcohols for the Production of Hydrogen On-demand: Application of a Silane/Alcohol Pair as a Liquid Organic Hydrogen Carrier.

The compound [Ru(p-cym)(Cl)2 (NHC)] is an effective catalyst for the room-temperature coupling of silanes and alcohols with the concomitant formation of molecular hydrogen. High catalyst activity is observed for a variety of substrates affording quantitative yields in minutes at room temperature and with a catalyst loading as low as 0.1 mol %. The coupling reaction is thermodynamically and, in the presence of a Ru complex, kinetically favourable and allows rapid molecular hydrogen generation on-demand at room temperature, under air, and without any additive. The pair silane/alcohol is a potential liquid organic hydrogen carrier (LOHC) for energy storage over long periods in a safe and secure way. Silanes and alcohols are non-toxic compounds and do not require special handling precautions such as high pressure or an inert atmosphere. These properties enhance the practical applications of the pair silane/alcohol as a good LOHC in the automotive industry. The variety and availability of silanes and alcohols permits a pair combination that fulfils the requirements for developing an efficient LOHC.

Catalytic Hydrogen Production by Ruthenium Complexes from the Conversion of Primary Amines to Nitriles: Potential Application as a Liquid Organic Hydrogen Carrier.

The potential application of the primary amine/nitrile pair as a liquid organic hydrogen carrier (LOHC) has been evaluated. Ruthenium complexes of formula [(p-cym)Ru(NHC)Cl2 ] (NHC=N-heterocyclic carbene) catalyze the acceptorless dehydrogenation of primary amines to nitriles with the formation of molecular hydrogen. Notably, the reaction proceeds without any external additive, under air, and under mild reaction conditions. The catalytic properties of a ruthenium complex supported on the surface of graphene have been explored for reutilization purposes. The ruthenium-supported catalyst is active for at least 10 runs without any apparent loss of activity. The results obtained in terms of catalytic activity, stability, and recyclability are encouraging for the potential application of the amine/nitrile pair as a LOHC. The main challenge in the dehydrogenation of benzylamines is the selectivity control, such as avoiding the formation of imine byproducts due to transamination reactions. Herein, selectivity has been achieved by using long-chain primary amines such as dodecylamine. Mechanistic studies have been performed to rationalize the key factors involved in the activity and selectivity of the catalysts in the dehydrogenation of amines. The experimental results suggest that the catalyst resting state contains a coordinated amine.

In situ decoration of graphene sheets with gold nanoparticles synthetized by pulsed laser ablation in liquids.

The demand for nanocomposites of graphene and carbonaceous materials decorated with metallic nanoparticles is increasing on account of their applications in science and technology. Traditionally, the production of graphene-metal assemblies is achieved by the non-environmentally friendly reduction of metallic salts in carbonaceous suspensions. However, precursor residues during nanoparticle growth may reduce their surface activity and promote cross-chemical undesired effects. In this work we present a laser-based alternative to synthesize ligand-free gold nanoparticles that are anchored onto the graphene surface in a single reaction step. Laser radiation is used to generate highly pure nanoparticles from a gold disk surrounded by a graphene oxide suspension. The produced gold nanoparticles are directly immobilized onto the graphene surface. Moreover, the presence of graphene oxide influences the size of the nanoparticles and its interaction with the laser, causes only a slight reduction of the material. This work constitutes a green alternative synthesis of graphene-metal assemblies and a practical methodology that may inspire future developments.

Hydrodefluorination of carbon-fluorine bonds by the synergistic action of a ruthenium-palladium catalyst.

Catalytic hydrodefluorination of organic molecules is a major organometallic challenge, owing to the strength of C-F sigma bonds, and it is a process with multiple industrial applications. Here we report a new heterodimetallic ruthenium-palladium complex based on a triazolyl-di-ylidene ligand. The complex is remarkably active in the hydrodefluorination of aromatic and aliphatic carbon-fluorine bonds under mild reaction conditions. We observe that both metals are required to promote the reaction process. The overall process implies that the palladium fragment facilitates the C-F activation, whereas the ruthenium centre allows the reduction of the substrate via transfer hydrogenation from isopropanol/sodium t-butoxide. The activity of this heterodimetallic complex is higher than that shown by a mixture of the related homodimetallic complexes of ruthenium and palladium, demonstrating the catalytic benefits of the heterodimetallic complex linked by a single-frame ligand.

Dual catalysis with an Ir(III)-Au(I) heterodimetallic complex: reduction of nitroarenes by transfer hydrogenation using primary alcohols.

A triazolyl-di-ylidene ligand has been used for the preparation of a homodimetallic complex of gold, and a heterodimetallic compound of gold and iridium. Both complexes have been fully characterized and their molecular structures have been determined by means of X-ray diffraction. The catalytic properties of these two complexes have been evaluated in the reduction of nitroarenes by transfer hydrogenation using primary alcohols. The two complexes afford different reaction products; whereas the Au(I)-Au(I) catalyst yields a hydroxylamine, the Ir(III)-Au(I) complex facilitates the formation of an imine.

Catalytic 1,3-difunctionalisation of organic backbones through a highly stereoselective, one-pot, boron conjugate-addition/reduction/oxidation process.

A simple one-pot, three-step synthetic route to chiral 1,3-amino alcohols and 1,3-diols has been established. Considering the overall stereocontrol of the synthetic protocol, the first and key step is an enantioselective ß-boration of α,ß-unsaturated imines and ketones, respectively. The enantioselectivity provided by the Cu(I) catalyst modified with Josiphos- and Mandyphos-type ligands has been examined. The oxidative substitution of the boryl unit with a hydroxyl group proceeds with complete retention of configuration at the C(ß)-atom. In parallel, the stoichiometric reduction of the imino or carbonyl group provides a second stereogenic centre. Depending on the nature of the reducing reagent, exceptionally high diastereoselectivity is achieved, especially for syn-1,3-amino alcohols and 1,3-diols.

Intramolecular oxidation of the alcohol functionalities in hydroxyalkyl-N-heterocyclic carbene complexes of iridium and rhodium.

A series of hydroxyalkyl-functionalized imidazolium salts have been coordinated to Rh and Ir to afford the corresponding MCp*-(NHC) (Cp*=pentamethylcyclopentadienyl) complexes. The reactivity of the new complexes has been studied with special attention to the transformations that deal with the alcohol functionality. The metal-mediated intramolecular transformations allowed the formation of several products that resulted from the oxidation of the alcohols to aldehydes and esters. All the new complexes have been fully characterized, and the crystal structures of the most representative complexes have been resolved.

An Ir-Pt catalyst for the multistep preparation of functionalized indoles from the reaction of amino alcohols and alkynyl alcohols.

The 1,2,4-trimethyltriazolylidene (ditz) ligand allows the preparation of homo- and heterodimetallic complexes of Pt(2) and Ir-Pt. These two complexes have been characterized by means of spectroscopic and diffractommetric techniques. The catalytic activity of these complexes, together with that of other Pt-based compounds, has been explored in the cyclization-addition of alkynyl alcohols and indoles. The Ir-Pt complex [{PtI(2)(py)}(µ-ditz){IrI(2)(Cp*)}] (py=pyridine; Cp*=pentamethylcyclopentadienyl) allows the combination of an iridium-mediated oxidative cyclization of 2-(ortho-aminophenyl)ethanol to form indoles, with a further step employing a Pt-based multistep reaction that functionalizes indoles. Our results show that the Ir-Pt complex is a very active catalyst in this new multistep preparation of functionalized indoles from the reaction of an amino alcohol with alkynyl alcohols.

One-pot preparation of imines from nitroarenes by a tandem process with an Ir-Pd heterodimetallic catalyst.

A new tandem catalytic process has been studied for a heterodimetallic complex containing both iridium and palladium fragments connected by a 1,2,4-trimethyltriazolyldiylidene ligand. The process implies the unprecedented preparation of imines from the direct reaction of nitroarenes and primary alcohols. The global process comprises the following steps: 1) reduction of the nitroarene to an amine, 2) oxidation of the alcohol to aldehyde, and 3) condensation of the aldehyde and the amine to form the corresponding imine. The oxidation of the alcohol to aldehyde is promoted by the iridium fragment, while the reduction of the nitro group to amine is facilitated by palladium. A wide set of different catalytic systems has been studied, showing that the Ir/Pd complex 1 is a highly active and stable catalyst in the preparation of imines.