Supported transition metal complex catalysts specificity

Some general reviews on hydrogenation using transition metal complexes that have appeared within the last five years are listed (4-7), as well as general reviews on asymmetric hydrogenation (8-10) and some dealing specifically with chiral rhodium-phosphine catalysts (11-13). The topic of catalysis by supported transition metal complexes has also been well reviewed (6, 14-29), and reviews on molecular metal cluster systems, that include aspects of catalytic hydrogenations, have appeared (30-34). 

The earliest Ziegler-Natta catalysts were insoluble bimetallic complexes of titanium and aluminum. Other combinations of transition and Group I-III metals have been used. Most of the current processes for production of high-density polyethene in the United States employ chromium complexes bound to silica supports. Soluble Ziegler-Natta catalysts have been prepared, but have so far not found their way into industrial processes. With respect to stereo-specificity they cannot match their solid counterparts. 

Transition metal catalysts, specifically those composed of iron nanoparticles, are widely employed in industrial chemical production and pollution abatement applications [67], Iron also plays a cracial role in many important biological processes. Iron oxides are economical alternatives to more costly catalysts and show activity for the oxidation of methane [68], conversion of carbon monoxide to carbon dioxide [58], and the transformation of various hydrocarbons [69,70]. In addition, iron oxides have good catalytic lifetimes and are resistant to high concentrations of moisture and CO which often poison other catalysts [71]. Li et al. have observed that nanosized iron oxides are highly active for CO oxidation at low tanperatures [58]. Iron is unique and more active than other catalyst and support materials because it is easily reduced and provides a large number of potential active sites because of its highly disordered and defect rich structure [72, 73]. Previous gas-phase smdies of cationic iron clusters have included determination of the thermochemistry and bond energies of iron cluster oxides and iron carbonyl complexes by Armentrout and co-workers [74, 75], and a classification of the dissociation patterns of small iron oxide cluster cations by Schwarz et al. [76]. 

Recent work (20, 21) in our laboratory has focused upon the use of transition metal compounds to sensitize the energy-storing valence isomerization of norbornadiene, NBD, to quadricyclene, Q (Reaction 3). In particular we have found that a catalytic amount of CuCl functions as an effective and quite specific sensitizer for this transformation. Conversions of greater than 90% have been achieved since Cu(I) is ineffective as a catalyst for the energy-releasing reverse reaction. Spectral and photochemical evidence support a mechanism which features a 1 1 ClCu—NBD complex as the photoactive species. As illustrated in Figure 3, an obvious consequence of complexation is a shift of the absorption spectrum of the system into a region accessible to the 313-nm irradiation used. Possible pathways by which the photo-excited complex relaxes to Q have been discussed (12). 

Metal oxides represent one of the most important and widely employed classes of solid catalysts, either as active phases or as supports. Metal oxides are used for both their acid-base and redox properties and constitute the largest family of catalysts in heterogeneous catalysis [1-6]. The three key features of metal oxides, which are essential for their application in catalysis, are (i) coordination environment of the surface atoms, (ii) redox properties of the oxide, and (iii) oxidation state of the surface. Surface coordination environment can be controlled by the choice of crystal plane exposed and by the preparation procedures employed however, specification of redox properties is largely a matter of choice of the oxide. The majority of oxide catalysts correspond to more or less complex transition metal oxides containing cations of variable oxidation state. These cations introduce redox properties and, in addition, acid-base properties. The acid-base properties of the oxides are usually interrelated to their redox behavior. Many attempts were made... 

Solar energy conversion functions often depend on dynamic structures in solution or on a supporting matrix where a transiently appearing dynamic structure could evolve into a precursor for catalytic intermediates. Such dynamic structures are implicitly depicted by the Debye-Weller factor in the conventional XAS data analysis in Equation (12.1), without specific description of the structural origin. In many homogeneous photochemical reactions, metal complexes interact with solvent molecules to form transient dynamic solvated structures, such as dynamic bonding between the catalyst molecule and the solvent or substrate molecules. These dynamic structures may well be the precursor or transition states in catalytic reactions, but were unfortunately obscured in the conventional data analysis. 

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