Transition metal catalytic activity

With the advance of three-way catalysis for pollution control, used mainly in automobile catalytic conversion but also for the purification of gas exhausts from stationary sources, a need has arisen to develop a basic understanding of the reactions associated with the reduction of nitrogen oxides on transition metal catalytic surfaces [1,2]. That conversion is typically carried out by using rhodium-based catalysts [3], which makes the process quite expensive. Consequently, extensive effort has been placed on trying to minimize the amount of the metal needed and/or to replace it with an alternatively cheaper and more durable active phase. However, there is still ample room for improvement in this direction. By building a molecular-level picture of theprocesses involved,... 

The general level of catalytic activity of some metals has in the last few years been explained in terms of the electronic interaction between the metal surface and the reacting molecules. This subject has been well discussed elsewhere (16). Studies of electronic interactions, however, have been concerned with explaining the general level of catalytic activity of polycrystalline materials and not the nature of the active regions in any one catalytic material. At this point it should be emphasized that the differences in activity between different faces on one catalyst with a particular d character can be much greater than the differences in measured activity between two catalysts with appreciably different d characters. It is of interest that the greatest differences in rate with face have been found with the transition metals, the activity of which in the polycrystalline form has been interpreted in terms of the d character. 

In many homogeneous transition metal catalytic systems the more active form of the catalyst results from dissociation of a ligand in an unfavorable equilibrium like eq 1. Active catalysts... 

Various complexes of transition metals can activate and cleave unreactive C-Cl bonds via nucleophilic [1, 18-20], electrophilic [21, 22], and radical [23, 24] paths, under mild conditions. For a number of reasons [l],not all of these reactions can be utilized in a catalytic manner. In this chapter, we will discuss only those C-Cl bond cleavage reactions which can consequently lead to catalytic transformations of weakly activated or nonactivated chloroarenes. 

Because of the extraordinary strength of the carbon-fluorine bond, transition metal-mediated activation of fluoroalkanes and arenes is not easy to achieve. Nevertheless, activation of the C-F bond in highly electron-deficient compounds such as 2,4,6-trifluoropyrimidine, pentafluoropyridine, or hexafluorobenzene is possible with stoichiometric amounts of bis(triethylphosphano) nickel(O) [101] (Scheme 2.45). More recently Herrmann and coworkers [102] have described a variant of the Kumada-Corriu cross-coupling reaction [103] between fluorobenzene and aryl Grignard compounds which uses catalytic amounts of nickel carbene complexes. Hammett analysis of the relative kinetic rate constants indicated that the reaction proceeds via initial oxidative addition of the fluoroaromatic reactant to the nickel(O) species. 

The Tpx ligands can mimic the coordination environment created by three imidazolyl groups from histidine residues, which is frequently found in the active sites of metalloenzymes. Higher valent bis(ix-oxo) species, [(Tpx)M( i-0)2M(Tpx)] via 0-0 cleavage of [(Tpx)M( x-r 2 r 2-02)M(Tpx)] intermediates, but also peroxo, hydroperoxo, and alkylperoxo species, active species undergoing oxidative C-C cleavage reaction, stable hydrocarbyl complexes, and dinuclear xenophilic complexes, [(Tpx)M-M L71], are all relevant to chemical and biological processes, most of which are associated with transition metal catalytic species. 

Peroxo and alkylperoxo metal conqilexes are presumed intermediates in the homogeneous and heterogeneous catalytic epoxidations of olefins. Both solid and soluble versions containing high valent, d transition metal ions activate peroxides heterolytically, thereby facihtating oxygen atom transfer to electron-rich substrates... 

The transition metal-catalysed amination of C-H bonds via reactive metal-imide intermediates (i.e., nitrenoids) remains a powerful taetie for C-N bond formation. In that context, the intramolecular C(ip )-H amination of biaryl azides as nitrenoid sources has been computationally explored regarding the nature of the transition metal that plays the catalytic role. Four common transition metals (Ir, Rh, Ru and Zn) have thus been considered, and while the calculations have revealed similar energy profiles regardless of the nature of the metal, catalytically active Ru speeies have nevertheless been shown to be the more efficient from a kinetic viewpoint. 

Main-group examples of C-H activation, such as arene mercuration, are long known, but tend to involve stoichiometric reagents, not catalysts, and many use metals that are now avoided on toxicity grounds (Hg, Tl, and Pb). Catalytic reactions involving transition metal organometallic activation and functionalization of C-H bonds (Section 12.4) are beginning to move into the applications phase and are likely to become much more common in synthesis." " Innate selectivity can sometimes permit functionalization of one out of the many... 

Transition metal-catalyzed activation of C-H bonds has made significant progress, the applications of which have received growing efforts to the construction of various heterocycle units relevant to natural products, pharmaceuticals, and materials. However, the majority of these currently available C-H bond activation transformations require noble transition metal catalysts, typically including palladium, rhodium, or ruthenium. Therefore, the development of green, sustainable, and economical transition metal catalysts for the catalytic C-H activation is one of the major challenges. 

Sequences such as the above allow the formulation of rate laws but do not reveal molecular details such as the nature of the transition states involved. Molecular orbital analyses can help, as in Ref. 270 it is expected, for example, that increased strength of the metal—CO bond means decreased C=0 bond strength, which should facilitate process XVIII-55. The complexity of the situation is indicated in Fig. XVIII-24, however, which shows catalytic activity to go through a maximum with increasing heat of chemisorption of CO. Temperature-programmed reaction studies show the presence of more than one kind of site [99,1(K),283], and ESDIAD data show both the location and the orientation of adsorbed CO (on Pt) to vary with coverage [284]. 

Inspired by the many hydrolytically-active metallo enzymes encountered in nature, extensive studies have been performed on so-called metallo micelles. These investigations usually focus on mixed micelles of a common surfactant together with a special chelating surfactant that exhibits a high affinity for transition-metal ions. These aggregates can have remarkable catalytic effects on the hydrolysis of activated carboxylic acid esters, phosphate esters and amides. In these reactions the exact role of the metal ion is not clear and may vary from one system to another. However, there are strong indications that the major function of the metal ion is the coordination of hydroxide anion in the Stem region of the micelle where it is in the proximity of the micelle-bound substrate. The first report of catalysis of a hydrolysis reaction by me tall omi cell es stems from 1978. In the years that... 

TT-Allylpalladium chloride (36) reacts with the nucleophiles, generating Pd(0). whereas tr-allylnickel chloride (37) and allylmagnesium bromide (38) reacts with electrophiles (carbonyl), generating Ni(II) and Mg(II). Therefore, it is understandable that the Grignard reaction cannot be carried out with a catalytic amount of Mg, whereas the catalytic reaction is possible with the regeneration of an active Pd(0) catalyst, Pd is a noble metal and Pd(0) is more stable than Pd(II). The carbon-metal bonds of some transition metals such as Ni and Co react with nucleophiles and their reactions can be carried out catalytic ally, but not always. In this respect, Pd is very unique. 

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... 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

Basic oxides of metals such as Co, Mn, Fe, and Cu catalyze the decomposition of chlorate by lowering the decomposition temperature. Consequendy, less fuel is needed and the reaction continues at a lower temperature. Cobalt metal, which forms the basic oxide in situ, lowers the decomposition of pure sodium chlorate from 478 to 280°C while serving as fuel (6,7). Composition of a cobalt-fueled system, compared with an iron-fueled system, is 90 wt % NaClO, 4 wt % Co, and 6 wt % glass fiber vs 86% NaClO, 4% Fe, 6% glass fiber, and 4% BaO. Initiation of the former is at 270°C, compared to 370°C for the iron-fueled candle. Cobalt hydroxide produces a more pronounced lowering of the decomposition temperature than the metal alone, although the water produced by decomposition of the hydroxide to form the oxide is thought to increase chlorine contaminate levels. Alkaline earths and transition-metal ferrates also have catalytic activity and improve chlorine retention (8). 

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