Project Overview

1 Project Objectives

The constant increase in chemical production and associated increase in energy consumption and waste generation obstruct the achievement of protection of our environment. Water is cheap and non toxic solvent, making it attractive in terms of economy and safety. The overall purpose of this project is to bring together top European laboratories who collectively possess all the required skills to develop and implement the use of catalytic processes compatible with aqueous media, providing a unique training ground for early-stage and experienced researchers. The main project objectives will be:

2 Overall approach and methodology


1. New hydrosoluble ligands and their coordination chemistry

The design, synthesis and characterization of new hydrosoluble ligands will be accomplished using an "appropriate modification of established ligand" approach. General classes of ligands will include: (a) modified monodentate and bidentate phosphines bearing hydrophilic substituents such as aminoacids, (oligo)peptides, nucleosides, sulphonates, and carbohydrates; (b) functionalised mono- and polysubstituted cyclopentadienes with hydrophilic substituents as above; (c) modified water-soluble porphyrin, azacrown, and bipyridyl; (d) functionalised dendrimers bearing a variety of different donor groups (phosphines, phosphonites, phosphinites, phosphites, amines, imines, etc) to be placed either on the surface or within the cavities of the dendrimers.

2. Aqueous organometallic chemistry with aqua, hydroxo, and oxo ligands

This will include: investigations of known high-valent organometallic compounds [e.g. CpReO3, (CH3)ReO3, (CH3)3ReO2, (CH3)4OsO, Cp2M2O5 (M = Mo, W)] and new related derivatives in water. A reductive approach to new middle-valent organometallics containing hydroxo and/or aqua ligands will be examined. Specific aqueous characterization methodologies will be applied such as (a) speciation studies as a function of pH by using a variety of analytical tools (ESI-MS, EPR, NMR spectrometry) and kinetics analyses, (b) studies of the redox behaviour and associated chemical transformations by classical electrochemical techniques and by in-situ techniques, such as coupled ESI-MS electrochemistry and spectroelectrochemical methods.


3. Substrate activation and stoichiometric reactivity

Study of the interaction between the two above classes of compounds and different substrates will be accomplished though the isolation or in situ studies of the substrate-complex adducts. Specific examples will include the activation of: (a) water, of relevance to the photo-induced water splitting process. In model compounds, O-H activation may lead to hydrido-hydroxo species or oxo species and dihydrogen; (b) dioxygen and hydrogen peroxide of relevance to the oxidation and hydroxylation of organic compounds based on enzymatic processes; (c) C-F bonds for the purpose of synthesis of fluoro-organic molecules that are otherwise inaccessible; (d) nitriles, for a variety of nucleophilic addition processes and 1,3-dipolar cycloadditions; (e) carbon dioxide, bicarbonate and carbonate in stoichiometric reactions with pre-formed transition metal hydrides; (f) other substrates (alkynes, alkenes, alcohols, thiols, H2, CO, CO2, SO2, etc.). The systematic study of the influence of pH and metal oxidation state (if redox active) will be performed.

4. Physico-chemical studies of complex-solvent interactions

These studies will be carried out based on spectroscopic, thermodynamic (equilibrium), kinetic and mechanistic investigations of the interaction between transition metal complexes and solvent molecules. These will include: (a) water coordination and exchange; (b) proton transfer to and from a metal centre (lone pair/hydride ligand equilibrium) and ligands (e.g. oxo/hydroxo, hydroxo/aqua, sulphide/hydrosulphide, amide/amine, alkylidene/alkyl, alkyl/alkane, hydride/dihydrogen, etc.) as a function of pH; (c) hydrogen bonding interactions between water as a proton donor and transition metals (lone pairs) or ligands (oxo, hydroxo, nitrido, sulphide, hydride, alkylidene, alkyls, etc.) as proton acceptors. The studies will be carried out in pure water (whenever possible) and in water-containing media (e.g. water-alcohol, water-acetonitrile, etc.). In addition, study of the effect of solvent composition on pKa, rate constants, reaction enthalpies, and. spectroelectrochemical study of the effect of oxidation state on solvent-ligand and -metal interactions (e.g. hydrogen bonds) will be performed.

5. In-situ analytical techniques.

The development or improvement of analytical instrumentation for on–line analysis of reaction pathways will be pursued as follows: (a) Evolution of on-line flow-through electrochemical reactors and ESI-MS in order to: (i) widen the detection range, (ii) identify short-life intermediates, (iii) increase the sensitivity (e.g., by introduction of derivatisation agents), (iv) develop appropriate mathematical models. (b) Development of an electrochemical flow cell coupled on–line with ESI-MS detection and capable of performing intermediate electrophoresis or chromatographic separations. This will allow increased selectivity and will provide a way to distinguish speciation induced by the electrospray interface. (c) Expansion of the scope of on-line, flow through ESI-MS investigations to photocatalytic processes. The photocatalyst will be either coated on the inside surface of a quartz capillary or continuously fed in the form of a colloidal suspension. (d) Development of new spectroelectro­chemical cells for work under cryostatic conditions in order to study the effect of the oxidation state on solvent-ligand and -metal interactions. (e) Development of a probe for low temperature laser photolysis within the NMR probe.

6. Theoretical studies

Computational studies, by ab initio and/or density functional methods, in parallel with the experimental studies will be done as follows: (a) Modelling of catalytic cycles by fully quantum mechanical or QM/MM methods, including the geometry optimisation of reaction intermediates and transition states and the comparison with available experimental results (geometries from structural studies, energetics from thermodynamics and kinetics data). (b) Analysis of the hydrogen bonded interactions. The polarity of these interactions may be modified or even reversed upon changing the metal oxidation state. (c) Modelling of solvent effects by both continuum models (solute inside a cavity with a polarisable dielectric), and by discrete solvent models (several solvent molecules are included in the quantum mechanical calculations). This will be necessary in particular when the solvent participates in hydrogen-bonding interactions.

7. Catalytic studies in aqueous or biphasic media.

The new water-soluble metal complexes will be used as catalyst precursors for a variety of catalytic processes under biphasic conditions at controlled pH. Typical spectroscopic, kinetic and analytical techniques will be employed (vide infra). Illustrative examples are: (a) CO2 hydrogenation aimed at exploiting this feedstock as a C1 building block. This may become a model sink for CO2 and for chemical removal of this greenhouse gas. (b) Oxidations of various substrates (e.g. alcohols, sulfides, aromatic hydrocarbons) with dioxygen or H2O2 or the powerful “green” oxidants hydroxyl and hydroperoxyl radicals (electrogenerated during high potential catalytic splitting of water), thereby circumventing halogenations. (c) Hydrodehalogenation of fluorinated and chlorinated hydrocarbons. (d) Extension of the processes catalysed by natural vanadium catalysts (Amavadine) [e.g. peroxidative hydroxylation, oxygenation, carboxylation and halogenation of alkanes] to a wider variety of substrates and catalysts (e.g. oxo-or hydroxo-3d-metal systems). (e) Alkene metathesis in water (RCM, ROM and ROMP), opening the way to biologically active compounds such as aminoacids, azasugars, and peptidomimetics that are otherwise difficult to prepare. (f) Catalytic aqueous detoxication of poisonous nitriles, especially those from industrial waste waters. (g) Hydroxylation of white phosphorus in water to phosphorus acids, as an alternative to the currently adopted chlorination of P4 to PCl3 followed by substrate phosphorylation. The catalytic activity of the water-soluble metalladendrimers will be compared with that of the corresponding water-soluble monomers. The catalytic activity and selectivity (chemo-, regio-, stereo-, and enantioselectivities) for all processes will be compared with those of similar systems operating in non aqueous media. The feedback from these comparisons will lead to the development of second generation of ligands and complexes.

8. Electrocatalytic studies in aqueous/biphasic media

New electrocatalytic processes of two main different types will be developed: (1) Electron transfer chain (ETC) catalysed processes (overall non redox processes, catalytic in electrons, occurring in the presence of a redox active complex), e.g. ligand exchange, isomerizations, disproportionation, migratory insertions and extrusions, etc. (2) Redox processes using a stoichiometric amount of electrons and the catalytic action of an electron-transfer mediator which may be either free in solution or anchored on the electrode surface. These processes could be related to regular catalytic processes detailed in the previous section: (a) reduction processes using proton and a cathode in place of H2 (e.g. hydrodehalogenation, hydrodesulfurisation, etc.); (b) oxidation processes using water and an anode in place of an oxygen atom donor (e.g. alcohols to aldehydes, sulfide to sulfoxides, etc.). Studies of these processes with on-line plug flow electrochemical cells coupled with on-line diode array spectrometric or mass spectrometry analyses will allow the study of both product distribution and catalyst transformation in the electrochemical cell.

9. Photochemistry and photocatalysis in aqueous/biphasic media.

The development of new photocatalysts in water and biphasic media will be pursued by extending the principles of known photochemical and photocatalytic processes. For instance, YoK will investigate the photochemistry of the [Cp*RuH2(PTA)2]+ system, recently reported to be an excellent catalyst in aqueous media by CNR. The photochemical activity of known systems in organic solvents will be compared with that of water soluble analogues [e.g. Cp*Rh(PMe3)H2 and Cp*Rh(PTA)H2]. Whenever necessary, these studies will also be carried out by using on-line plug flow photoreactors coupled on-line to a mass spectrometer. In this latter device the photocatalyst could either be introduced in the liquid phase along with the reactants or immobilized on sol-gel silicates or organically modified silicate films (Ormosil) spread onto the walls of a fused silica capillary photoreactor. Study of solar-induced reactions catalysed by water soluble metal complexes in aqueous and biphasic conditions will be pursued, first by using a home-built micromolar photoreactor for parameters optimisation (radiation density, temperature, reaction time, etc.), then scaled up to the molar level by using the solar flow reactor (SOLFIN) available at the “Plataforma Solar de Almería”. The influence of the direct solar radiation on the efficiency and product distribution will be evaluated. The solar approach will be focused to the synthesis of added-value molecules (drug precursors or fine materials for industrial applications) or will be targeted to selectively eliminate waste molecules from industrial process.

3 Work Plan

The overall project may be broken down into six tasks, labelled from T1 to T6. The relationship between the tasks and expertises, and between the expertises and the teams, are shown in the flow diagram below.


Further details on each task are as follows:

Task 1: Hydrosoluble ligands and their complexes. Synthesis of new hydrosoluble ligands: (i) new water-soluble complexes with phosphines and carbene-type ligands; (ii) functionalised phosphines and cyclopentadienes bearing highly polar water soluble substituents; (iii) modified porphyrin, azacrown, and bipyridines; (iv) functionalized dendrons, bis dendrons and dendrimers. Co-ordination chemistry of the new ligands on catalytically active metal centres and model studies with heavier congeners of the above metals. Hydrosoluble organometallic complexes containing these ligands. Characterization by MS analysis and IR, UV-vis, NMR, and EPR spectroscopic techniques. Structural studies by single crystal X-ray diffraction. Electrochemical and photochemical investigations of the redox-active complexes. Computational investigations. Task coordinator: YoK. Participants: YoK, CNR-ICCOM, UD, UAL, CNRS-LCC/a, UAB, IST.

Task 2: High oxidation state organometallic aqua ions. Synthesis of new ionic aqua complexes of low-to-intermediate oxidation states transition metals (V, Ru, Rh, Pd) and hydroxo and oxo complexes of middle-to-high oxidation state early transition metals (Nb, Ta, Mo, W, Re). Thermodynamics and kinetics of ligand binding studies. Binding on dendrimers. Spectroscopic studies of hydrogen bonding interactions. Structural studies by single crystal X-ray diffraction. Electrochemical investigations. Computational investigations. Task coordinator: CNRS-LCC/b. Participants: CNRS-LCC/a, CNRS-LCC/b, uab, UEN, INEOS, IST.

Task 3: Solvent-complex interactions. Hydrogen-bonding between solvent molecules and ligands or metal centres. Hydrogen-bonding between solvent molecules and dendrimers and metalladendrimers. Mechanistic studies of water exchange and proton transfer processes to and from ligands or metal centres, detection and characterization of reaction intermediates. Hydrogen bonding to metal fluorides, interactions of pendant groups to metal hydrides, hydrogen bonding of water to metal hydrides. Computational studies by PCM or discrete solvent models. Task coordinator: UAB. Participants: UAB, INEOS, CNRS-LCC/a, YoK, UEN, CNRS-LCC/b.

Task 4: Speciation studies. Investigation of the nature of aqua/hydroxo/oxo ions or other hydrosoluble complexes in water or aqueous solvents at different pH by potentiometry, electrospray ionisation mass spectrometry, high-pressure and stopped-flow. Effect of the oxidation state on ligand binding properties. Flow-through electrochemistry and on-line ESI-MS studies. Task coordinator: HUJI. Participants: HUJI, UD, UEN, CNRS-LCC/b, YoK, IST.

Task 5: Substrate activation and mechanistic studies. Stoichiometric reactivity of transition metal complexes containing hydrosoluble ligands and/or aqua/hydroxo/oxo ligands with inorganic and organic substrates, including: dihydrogen, dioxygen, nitriles, saturated and fluorinated hydrocarbons, terminal alkynes and propargylic alcohols. Mechanistic studies using rapid-mixing techniques, high-pressure techniques including T-jump, flash photolysis, pulse radiolysis, etc., low-temperature spectroscopy, parahydrogen NMR, pH-potentiometry, and electrochemistry. Task coordinator: UEN. Participants: CNR-ICCOM, UD, UAB, YoK, CNRS-LCC/b, IST, UAL, INEOS, UEN.

Task 6: Catalysis, photocatalysis and electrocatalysis in water and biphasic media. Catalytic studies in aqueous and biphasic media at both atmospheric and high pressure. In situ monitoring by high-pressure NMR and IR. Photocatalytic investigations. Solar photocatalytic process mediated by water soluble metal complexes. Electrocatalytic investigations. Focused microwave irradiation studies. Task coordinator: UD. Participants: UD, IST, CNR-ICCOM, YoK, UAL, CNRS-LCC/a, HUJI.