Biphasic systems transition metal catalysis

Transition metal catalysis in liquid/liquid biphasic systems principally requires sufficient solubility and immobilization of the catalysts in the IL phase relative to the extraction phase. Solubilization of metal ions in ILs can be separated into processes, involving the dissolution of simple metal salts (often through coordination with anions from the ionic liquid) and the dissolution of metal coordination complexes, in which the metal coordination sphere remains intact. 

As outlined in general in Section 6.3, two distinct engineering concepts can be envisaged for transition metal catalysis in scC02-based biphasic systems. In one case the compressed CO2 phase is used as the compartment for substrates and/or products, whereas the catalyst is contained in the liquid phase, in the present case water. We will refer to this scenario as H2O/SCCO2 biphasic catalysis and examples are discussed in this section. In the other case, carbon dioxide is used as the catalyst-containing phase, whereas substrates and products are retained in the polar aqueous phase. We refer to this system as inverted biphasic catalysis , denoted as SCCO2/ H2O, and it will be subject of the discussion in Section 6.4.4.4. 

The possibility of adjusting solubility properties is of particular importance for liquid-liquid biphasic catalysis. Liquid-liquid catalysis can be realised when the ionic liquid is able to dissolve the catalyst, especially if it displays partial solubility of the substrates and poor solubility of the reaction products. Under these conditions, the product phase, which also contains the unconverted reactants, is removed by simple phase decantation. The ionic liquid containing the catalyst can then be recycled. In such a scenario the ionic catalyst solution may be seen as part of the capital investment for a potential technical process (in an ideal case) or at least as a working solution (only a small amount has to be replaced after a certain time of application). A crucial aspect of this concept is the immobilisation of the transition metal catalyst in the ionic liquid. While most transition metal catalysts easily dissolve in an ionic liquid without any special ligand design, ionic ligand systems have been applied with great success to... 

Catalysis in liquid-liquid biphasic systems has developed recently into a subject of great practical interest because it provides an attractive solution to the problems of separation of catalysts from products and of catalyst recycle in homogeneous transition metal complex catalysis. Two-phase systems consist of two immiscible solvents, e.g., an aqueous phase or another polar phase containing the catalyst and an organic phase containing the products. The reaction is homogeneous, and the recovery of the catalyst is facilitated by simple phase separation. 

Scrutiny of the literature data shows that numerous transformations of diverse organic molecules are readily catalyzed by homogeneous metal complexes in two-phase systems. The joint use of transition metals and phase-transfer agents considerably expands the synthetic possibilities of both metal-complex catalysis and PTC. The combination of these two different types of catalysts in many cases leads to higher reaction rates, higher product yields, milder conditions, higher selectivities, and simpler reaction execution and product isolation. Biphasic reactions in the presence of transition-metal complexes and PT agents have been... 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

Recent research in the application of supercritical (sc) fluids and ionic liquids (IL) as solvents in homogeneous catalysis (see Sections 7.3 and 7.4), opened the way to the development of biphasic water/scCOz [171, 172] and water/IL [173] systems for the hydrogenation of various substrates, e.g., alkenes, aldehydes, etc. with water-soluble catalysts. The catalytically highly active, versatile and robust transition metal - N-heterocyclic carbene complexes [174] have also been applied for hydrogenation reactions [175], Given that water-soluble complexes with N-heterocy-clic carbene ligands are known [176], catalytic applications in aqueous systems are also foreseen. 

ILs are widely used in transition metal chemistry, such as biphasic catalytic system in 7i-acceptor ligand [41], Several reviews have been published in which ILs occupied a central theme due to their use in homogeneous and heterogeneous catalyses as well as for transition metal-mediated catalysis and organometallic reactions [42]. IL is used as a solvent in organic and bioorganic reactions [43, 44] it also acts as an organocatalyst [45]. 

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