Metal complexes activity

The key to the successful development of homogeneous catalysts has been the exploitation of the effects that ligands exert on the properties of metal complexes by tailoring the electronic and steric properties of a catalytically active metal complex, activities and selectivities can be altered considerably. This especially holds for phosphorus based ligands, which are the most commonly encountered ligands as.sociated with organometallic compounds. 

Table 6.19 Hydrogenation catalysts based on Group V-VII transition-metal complexes activated with AI / B u 3.

Jacobsen and coworkers discovered that chiral salicylimidato transition metal complexes activate epoxides in a stereoselective manner. The published mechanism indicates that one Cr° (salen)-N3 with (/ ,/ )-cyclohexyl backbone acts as Lewis acid and coordinates to the oxygen of PO, while a second catalyst molecule transfers the azide to the activated epoxide and thus opens the ring. The coplanar arrangement of the two chromium complexes prefers one enantiomer of PO and so induces stereochemical information [99,100, 121-129]. (cf. also Sect. 8.3) (Fig. 42). 

M. M. T. Khan and A. E. Martel, Homogenous Catalysis by Metal Complex Activation of Alkenes and Alkynes" Academic Press, New York, 1974. 

A few years ago Smalley and coworkers were able to obtain detailed experimental information about the reactivity of specific transition metal clusters with hydrogen molecules (1). The results for copper and nickel clusters were essentially as expected from the known results for surface and metal complex activities. For copper no clusters were able to dissociate whereas for nickel all clusters were active with a slow, steady increase of activity with cluster size. For the other transition metals studied, cobalt, iron and niobium, a completely different picture emerged. For these metals a dramatic sensitivity of the reactivity to cluster size was detected. No convincing explanation for these surprising results has hitherto been suggested. It should be added that there are no dramatic differences in the activity towards Hg for the metal surfaces (or the metal complexes) of nickel on the one hand and iron, cobalt and niobium on the other. 

Cationic transition-metal complexes activate small molecules and serve as homogeneous catalysts, especially for selective hydrogenation of unsaturated organics. Section 1.10.4.2 deals with coordinativeiy unsaturated complexes in which the ligands remain coordinated after the oxidative addition of Hj. In 1.10.4.4 the complexes may or may not be coordinativeiy unsaturated complexes that are not must lose ligand(s) before oxidative addition of Hj. Coordinativeiy unsaturated complexes are described that exchange ligands either before or after the oxidative addition of Hj. 

The future of such transfer reactions lies in catalysis. One can envision a metal complex activating a substrate and transferring the activated species to a second metal where it reacts further. Such reactions could, in principle, be catalytic in both metals. Such reactions could increase the number of catalytic reactions in an exponential fashion. 

Several monographs [9] and many reviews [10], wholly or partly devoted to the metal complex activation of C-H and C-C bonds in hydrocarbons, appeared in recent decades. Reviews and books devoted to some more narrow topics will be cited later throughout this book. 

Thus according to our classification, the first group includes reactions involving true , organometalhc metal complex activation of the C-H bonds. We call this type of activation trae , because only in this case, the closest contact between metal ion and the C-H bond (i.e., normal (ybond between M and C) is realized. A molecule of C-H compound enters in the form of an organyl cr-ligand into the coordination sphere of the metal complex. 

Brief History of Metal-Complex Activation of C-H Bonds... 

In this chapter we will briefly survey the main types of hydrocarbon transformations that occur without the participation of solid metals or oxides and metal complexes. The alkane transformations described here should be taken into consideration for comparison when discussing the metal complex activation. 

In this chapter, we will consider only the general characteristics of these reactions, which are especially important from the point of view of their comparison with metal complex activation. 

The intimate mechanism of the reaction deserves special attention not only because it was the first example of alkane activation by a metal complex. Activation of alkanes by platinum(II) complexes remains unique in many aspects. The reaction takes place in neutral water solution with conventional chloride ligands at the metal without special ways to form coordinatively unsaturated species (e.g., by irradiation). A number of works were directed towards the elucidation of the nature of the interaction between an alkane and a platinum(II) complex. The unique feature of platinum(II) complexes is to exhibit both nucleophilic and electrophilic properties. 

An alternative pathway for the participation of transition metal complexes in photosynthetic transformations involves activation of the substrates by their direct coordination to the metal center. In this respect the metal center acts as an active site, in addition to its redox functions. Numerous transition metal complexes activating inert molecules (i.e., carbon dioxide or nitrogen) by coordination to the metal center are known [132,133]. 

Direct excitation of the transition metal complex active in COj reduction was demonstrated in a photosystem composed of tricarbonyl(2,2 -bipyridin-ium)rhenium(I), /ac-Re(bpy)(CO)3X (X = C1, Br) as a light-active compound and a homogeneous catalyst [140-142]. Photosensitized reduction of carbon dioxide to CO proceeds in nonaqueous solutions (i.e., dimethylform-amide) including the rhenium(I) complex and TEOA as the sacrificial electron donor. The quantum efficiency for CO formation in the system corresponds to

Mechanistic studies show that the primary step in... 

The proposed polymerization pathway differs fundamentally from the coordination-insertion mechanism involving metal complexes, see Fig. 3.7 [5, 38]. Indeed, the nucleophilic catalyst only activates the monomer toward ring opening, whereas the metal complex activates the monomer, initiates the polymerization, and remains bound to the growing chain. The polymerization mechanism of a superbase or thiourea-amine catalyzed ROP will be discussed in more detail below. 

A great development in this research field was the discovery of metallocene and other transition metal complexes activated by methylaluminoxane. These catalysts... 

The polymerization proceeds following the same mechanism, with metal complex activating the carbon-halogen bond and proceeding via the metal-assisted repetitive activation and deactivation. 

Nickel complexes with 1,3-dienes are important intermediates in a variety of catal5dic processes. In contrast to many classes of metal-diene complexes, such as those of iron and palladium in which metal complexation activates the 3t-system towards nucleophilic attack, (T] -diene)nickel complexes are most useful in cycloaddition processes and in couplings to polar 3t-systems such as carbonyls. Few examples of diene-nickel complexes are structurally well characterized, but they are commonly invoked in the mechanisms of many synthetic procedures. 

It has been demonstrated that the combination of metal-catalysed racemisation and enzymatic kinetic resolution is a powerful method for the synthesis of optically active compounds from racemic alcohols and amines. There are many metal complexes active for racemisation, but the conditions for enzymatic acylation often limit the application of the metal complexes to DKR. In the case of DKR of alcohols, complementary catalyst systems are now available for the synthesis of both (R)- and (5)-esters. Thus, (R)-esters can be obtained by the combination of an R-selective lipase, such as CAL-B or LPS, and a racemisation catalyst, whereas the use of an A-selective protease, such as subtilisin, at room temperature provides (5)-esters. The DKR of alcohols can be achieved not only for simple alcohols but also for those bearing various additional functional groups. The DKR of alcohols has also been applied to the synthesis of chiral polymers and coupled to tandem reactions, producing various polycyclic compounds. 

The initial quantity of NaBH4 (the hydrogen donor) also significandy affects the reaction rate. The dependence of on the quantity of NaBH4 has an extremum. This fact is also explained by the dual role of NaBH4 as the hydrogen donor and the metal complex activator. The increase of activity may be explained by the increasing number of active Ru-complex particles, and the decrease corresponds to Ru "overreduction" with the formation of Ru(0), which is not active in the reaction. Indeed, it was shown that metallic Ru, both in the form of black and deposited on silica gel, is not active in this reaction. 

Khan, M.M.T. and A.E. Martel. 1974. Homogenous Catalysis by Metal Complexes. Activation ofAlkenes andAlkynes. New York Academic Press. 

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