Transition metal complexes, activation

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). 

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. 

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... 

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

DFT calculations offer a good compromise between speed and accuracy. They are well suited for problem molecules such as transition metal complexes. This feature has revolutionized computational inorganic chemistry. DFT often underestimates activation energies and many functionals reproduce hydrogen bonds poorly. Weak van der Waals interactions (dispersion) are not reproduced by DFT a weakness that is shared with current semi-empirical MO techniques. 

Metallocene (Section 14 14) A transition metal complex that bears a cyclopentadienyl ligand Metalloenzyme (Section 27 20) An enzyme in which a metal ion at the active site contributes in a chemically significant way to the catalytic activity... 

A unique method to generate the pyridine ring employed a transition metal-mediated 6-endo-dig cyclization of A-propargylamine derivative 120. The reaction proceeds in 5-12 h with yields of 22-74%. Gold (HI) salts are required to catalyze the reaction, but copper salts are sufficient with reactive ketones. A proposed reaction mechanism involves activation of the alkyne by transition metal complexation. This lowers the activation energy for the enamine addition to the alkyne that generates 121. The transition metal also behaves as a Lewis acid and facilitates formation of 120 from 118 and 119. Subsequent aromatization of 121 affords pyridine 122. 

Apart from the activation of a biphasic reaction by extraction of catalyst poisons as described above, an ionic liquid solvent can activate homogeneously dissolved transition metal complexes by chemical interaction. 

In cases in which the ionic liquid is not directly involved in creating the active catalytic species, a co-catalytic interaction between the ionic liquid solvent and the dissolved transition metal complex still often takes place and can result in significant catalyst activation. When a catalyst complex is, for example, dissolved in a slightly acidic ionic liquid, some electron-rich parts of the complex (e.g., lone pairs of electrons in the ligand) will interact with the solvent in a way that will usually result in a lower electron density at the catalytic center (for more details see Section 5.2.3). 

Many transition metal-catalyzed reactions have already been studied in ionic liquids. In several cases, significant differences in activity and selectivity from their counterparts in conventional organic media have been observed (see Section 5.2.4). However, almost all attempts so far to explain the special reactivity of catalysts in ionic liquids have been based on product analysis. Even if it is correct to argue that a catalyst is more active because it produces more product, this is not the type of explanation that can help in the development of a more general understanding of what happens to a transition metal complex under catalytic conditions in a certain ionic liquid. Clearly, much more spectroscopic and analytical work is needed to provide better understanding of the nature of an active catalytic species in ionic liquids and to explain some of the observed ionic liquid effects on a rational, molecular level. 

The purity of ionic liquids is a key parameter, especially when they are used as solvents for transition metal complexes (see Section 5.2). The presence of impurities arising from their mode of preparation can change their physical and chemical properties. Even trace amounts of impurities (e.g., Lewis bases, water, chloride anion) can poison the active catalyst, due to its generally low concentration in the solvent. The control of ionic liquid quality is thus of utmost importance. 

In comparison with catalytic reactions in compressed CO2 alone, many transition metal complexes are much more soluble in ionic liquids without the need for special ligands. Moreover, the ionic liquid catalyst phase provides the potential to activate and tune the organometallic catalyst. Furthermore, product separation from the catalyst is now possible without exposure of the catalyst to changes of temperature, pressure, or substrate concentration. 

In a catalytic asymmetric reaction, a small amount of an enantio-merically pure catalyst, either an enzyme or a synthetic, soluble transition metal complex, is used to produce large quantities of an optically active compound from a precursor that may be chiral or achiral. In recent years, synthetic chemists have developed numerous catalytic asymmetric reaction processes that transform prochiral substrates into chiral products with impressive margins of enantio-selectivity, feats that were once the exclusive domain of enzymes.56 These developments have had an enormous impact on academic and industrial organic synthesis. In the pharmaceutical industry, where there is a great emphasis on the production of enantiomeri-cally pure compounds, effective catalytic asymmetric reactions are particularly valuable because one molecule of an enantiomerically pure catalyst can, in principle, direct the stereoselective formation of millions of chiral product molecules. Such reactions are thus highly productive and economical, and, when applicable, they make the wasteful practice of racemate resolution obsolete. 

The past fifteen years have seen evidence of great interest in homogeneous catalysis, particularly by transition metal complexes in solution predictions were made that many heterogeneous processes would be replaced by more efficient homogeneous ones. There are two motives in these changes—first, intellectual curiosity and the belief that we can define the active center with... 

Heterolytic activation of hydrogen by transition metal complexes. P. J. Brothers, Prog. Inorg. Chem., 1981,28,1-61 (168). 

Dioxygen activation in transition metal complexes in the light of molecular orbital calculations. R. Boca, Coord. Chem. Rev., 1983, 50,1-72 (245). 

Reversible activation of covalent molecules by transition metal complexes. The role of the covalent molecule. L. Vaska, Acc. Chem. Res., 1968,1, 335-344 (51). 

Activation of molecular oxygen on interaction with transition metal complexes. A. V. Savitskii and V. I. Nelyubin, Russ. Chem. Rev. (Engl. Transl.), 1975,44,110-121 (124). 

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