Immobilization complexes

One of the most important parameters of an immobilized-carrier complex is stability of its activity. Catalytic activity of the complex diminishes with time because of leakage, desorption, deactivation, and the like. The half-life of the complex is often used to describe the activity stabihty. Even though there may be frequent exceptions, hn-ear decay is often assumed in treating the kinetics of activity decay of an immobilized complex. 

Fig. 10 Recycling results for the immobilized complex shown in Fig. 9 in the epoxidation of cfs-)6-methylstyrene...

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... 

Three approaches have been tested, as already described above for inorganic supports. The first attempts concern the direct reaction of transition metal carbonyls with unmodified organic polymers like poly-2-vinyl-pyridine.61 62 However, this kind of anchoring is restricted to only a few complexes. Various polymers have been functionalized with donor groups 63-72 ligand displacement reactions using these afforded the corresponding immobilized complexes. Finally, tests with modified complexes and unmodified polymers are scarce because of the low stability of these complexes under the conditions of reactions. 

Immobilization on glass surfaces can be carried out by either routes [5] or [6] in Scheme 2 both yielding the immobilized complex (1), reaction [5] being the procedure most commonly used. 

Reactions take place at the water film - organic phase interface and are catalyzed by the phase-immobilized complex catalyst which is usually [HRh(CO)(TPPTS)3]. Other... 

A. D. Pomogailo, Catalysis by Polymer-Immobilized Complexes. Gordon Breach, Australia, 1998. 

Table 14.1 Schematic View and Important Properties of Immobilized Complexes...

In spite of the theoretical interest in enzyme membranes, they have so far not been used in industry and their use in the clinic and in the laboratory is rather limited. However, as techniques for the preparation and stabilization of immobilized complex enzyme systems develop, one can expect to see an increase in the number of cases in which permeable and impermeable enzyme membranes will be used advantageously. 

In the past, an overwhelming dearth of convincing evidence for the structure assignments of immobilized complexes has permeated the literature, and the complete characterization of the immobilized species has often been substituted by speculation [202]... 

Although most of the above-mentioned rhodium-siloxy immobilized complexes have been tested in catalysis no results have been pubhshed on their efficiency in recychng tests with simultaneous analysis of the contents of rhodium to determine a metal complex leaching effect, which is a crucial point in the use of all hetero-genized complexes in catalysis. 

Generally, in conclusion, it is worth noting that the molecular and immobilized complexes show very similar catalytic activity in terms of the initial TOP in olefin metathesis. However, the supported catalyst has a longer lifetime under catalytic conditions, which indicates that the effect of active-site isolation prevents some deactivation pathways such as dimerization of reachve intermediates [30]. 

Rhodium immobilized complexes were also found to be effective catalysts of the addition of HSiMe(OSiMe3)2 and HSi(OEt)3 to various allyl ethers. The data presented in Table 7.4 confirm a high catalytic activity of catalysts 1, 3 and 5 in the conversion of allyl ethers into the corresponding silyl derivahves, but, unfortunately, only in the case of allyl phenyl ether did the catalytic achvity remained unchanged up to 10 cycles. ICP analysis of the rhodium solid catalysts after hydrosilylation tests revealed a high concentration of rhodium. Therefore, the decrease in catalytic activity of 1 does not depend only on leaching of rhodium from the silica surface. 

Similar catalytic activity of rhodium-immobilized complexes was observed in the hydrosilylation of olefins with polyhydrosiloxane (Table 7.5). 

Some examples for the application of such immobilization strategies and the application of the immobilized complexes to asymmetric hydrogenation and epoxidation as well as to ring-opening reactions of epoxides will be presented in this chapter. 

N2 sorption isotherms were measured for the pure carrier and for all the immobilized complexes, respectively. Sorption isotherms and textural characteristics for pure and loaded samples are compared in Fig. 2.1.6.1 and Table 2.1.6.1. In all cases these isotherms presented the characteristic form of mesoporous materials. It is clear that the N2 adsorbed/desorbed volume was higher in the case of pure carrier than in the case of immobilized complexes. 

Several diphosphine ligands have been applied and the corresponding complexes have been tested for the immobilization (Fig. 2.1.6.3). The activity of different free and immobilized complexes in the enantioselective hydrogenation of dimethyl itaconate and methyl a-acetamidoacrylate was investigated. In blank reactions over pure mesoporous materials no reaction took place. When rhodium supported on carriers was used as catalyst, no enantiomeric excess was observed. 

EDTA in water. Ru(bpy)2 + and its dicinnamate were immobilized in a membrane prepared from cinnamate of poly(vinyl alcohol) by photocrosslinking 51), and the immobilized complex sensitized the photoreduction of MV2 +. ... 

Immobilized complexes anchored to the surface of oxide supports may serve as active metathesis catalysts. In fact, Mo(CO)6 on alumina was reported in the first paper on metathesis by Banks and Bailey.1 Molybdenum and tungsten hexacarbo-nyls and alkylmolybdenum and alkyltungsten complexes have been studied most.6,24,26 Anchored organomolybdenum complexes are particularly effective catalysts, even at temperatures below 0°C however, they quickly lose activity.41-43... 

---