Metal complexes, leaching under

Reaction experiments were performed at the substrate to catalyst ratios between 250 and 5000 (Table 1). The immobilized catalyst showed a rather constant values of TOP and enantioselectivity in spite of the increase in the S/C ratio, even though these values were slightly lower than those of the homogeneous Ru-BINAP catalyst. After the reaction, the Ru content in the reaction mixture was measured by ICP-AES and was found to be under 2 ppm, the detecting limit of the instrument, indicating the at Ru metal didn t leach significantly during the reaction. These results show that the immobilized Ru-BINAP catalyst had stable activity and enantioselectivity and that the Ru metal complex formed a stable species on the alumina support. 

MAO induces leaching of some of the metal complexes from the support and this issue is currently under investigation in our laboratory. 

The procedures which allow for the immobilization of a metal complex on or in a solid are numerous and well described they result from more than thirty years of continuous and imaginative efforts. More difficult, and consequently less successful, work has been described regarding the syntheses of efficient catalysts from these precursors. An immobilized complex must show high activity and selectivity for the target reaction, must be easily recovered intact, and must be stable towards metal leaching under the reaction conditions. These two latter requirements are particularly important where asymmetric catalysis is the goal, because the metal and the ligands... 

The use by Avnir and co-workers (21) of excited-state dynamics of pyrene-doped sol-gel materials to probe the interior of the sol-gel environment has helped pave the way for similar smdies with luminescent probes such as [Ru(bpy)3] (14). From an applications standpoint, the attributes of [Ru(bpy)3] (Section lll.D.2.b) make this metal complex a particularly good O2 sensor. The matrix can have an enormous effect on O2 diffusibility to the doped sensor depending on the conditions of synthesis. The sensing ability of [Ru(bpy)3] " is probably best suited for gas-phase O2 as the molecule has been shown to leach from sol-gels under solution conditions (146). A typical optically based O2 sensing device using such a system is shown in Fig. 23 (143). 

It has generally been assumed that the bonds that link the catalyst to the polymer support are chemically stable under the reaction conditions one employs. Until recently, the literature offered little information in this regard, since lifetime studies are needed to properly evaluate stability. Recent publications have pointed out the chemical instability of the phosphorus-carbon bond of tertiary phosphine functionalized supports and the chemical reactivity of various nitrogen functionalized polymeric support materials under reaction conditions. If such chemical stability problems are present, the consequences are indeed serious. While a typical "leach" situation would necessitate a periodic reloading of the metal complex, cleavage of polymer functionality would necessitate replacement of both the metal complex and the polymer. 

By far the most complete study on elution or leaching of metal complexes under "industrial flow conditions was reported by the... 

A review article by Blumel describes classical and modern solid-state NMR methods that allow to gain insight into catalyst systems where one or two metal complexes are bound to oxide supports via bifunctional phosphine linkers, such as (EtO)3Si(CH2)3PPh2. It has been shown that many aspects of the immobilized molecular catalysts can be elucidated with the corresponding NMR technique. For example, the bulk of the support can be studied, as well as the interface of the support with the ethoxysilane. In addition, electrostatic bonding to the support via phosphonium groups can be proven by solid-state NMR. For the immobilized catalysts, leaching, and even horizontal translational mobility effects, as probed by HR MAS NMR under realistic conditions in the presence of solvents, are described. 

These elements are noble metals and, as such, can be dissolved only with great difficulty. The usual leaching agent is hydrochloric acid, with the addition of chlorine to increase the solution oxidation potential. This strong chloride medium results in the almost exclusive formation of aqueous chloroanions, with, under certain circumstances, the presence of some neutral species. Very seldom are cationic species formed in a chloride medium. However, these elements do possess a range of easily accessible oxidation states and, with the possibility of a number of different anionic complexes that are dependent on the total chloride concentration, this provides a very complicated chemistry. A summary of the most important chloro complexes found in these leach solutions is given in Table 11.6, from which the mixed aquochloro and polynuclear species have been omitted. The latter are found especially with the heavier elements. 

Ir(IV), Pt(IV), with the states from Rh(III) being termed inert. Thus, kinetic factors tend to be more important, and reactions that should be possible from thermodynamic considerations are less successful as a result. On the other hand, the presence of small amounts of a kinetically labile complex in the solution can completely alter the situation. This is made even more confusing in that the basic chemistry of some of the elements has not been fully investigated under the conditions in the leach solutions. Consequently, a solvent extraction process to separate the precious metals must cope with a wide range of complexes in different oxidation states, which vary, often in a poorly known fashion, both in kinetic and thermodynamic stability. Therefore, different approaches have been tried and different flow sheets produced. 

Two other refining processes are also frequently employed. One involves hydrometallurgical refining in which sulfide concentrates are leached with ammonia solution to convert the copper, nickel, and cobalt sulfides into their complex amines. Copper is precipitated from this solution upon heating. Under such conditions, the sulfide-amine mixture of nickel and cobalt are oxidized to their sulfates. The sulfates then are reduced to metalhc nickel and cobalt by heating with hydrogen at elevated temperatures under pressure. The metals are obtained in their powder form. 

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