Metal reaction centres

In principle, reactions which are subject to electrophilic catalysis by protons can be catalysed by metal ions also (e.g. Tee and Iyengar, 1988 Suh, 1992). However, metal ions may function in other ways, such as to deliver a hydroxide ion nucleophile to the reaction centre (e.g. Dugas, 1989 Chin, 1991), and it is often difficult to decide between kinetically equivalent mechanisms without resorting to extensive (and intensive) model studies. Use of the Kurz approach may help to resolve such ambiguities, as shown below. 

All these observations point to the occurrence of a 8 2 rather than an outer sphere, dissociative electron-transfer mechanism in cases where steric constraints at the carbon or metal reacting centres are not too severe. It is, however, worth examining two other mechanistic possibilities. One of these is an electrocatalytic process of the Sg -type that would involve the following reaction sequence. If, in the reaction of the electron donor (nucleophile), the bonded interactions in the transition state are vanishingly small, the alkyl radical is formed together with the oxidized form of the electron donor, D . Cage coupling (144) may then occur, if their mutual affinity is... 

Nature uses the transition-metal elements iron and nickel, rather than noble metals, and in their ionic form rather than the metals. As will be seen in this book, for the simplest chemical reaction, the metal-ion centres in hydrogenases are some of the most complex catalysts known. Their structures, which have just been elucidated, have proved to be an elegant and totally unexpected solution to the problem. The construction of these catalysts is in itself a molecular assembly line of extraordinary sophistication. 

The role of transition metals in the chemical synthesis of corrins is more than just adding a touch of inorganic elegance to organic synthesis- leave elegance to tailors and cobblers , a physicist once said — it is a vital role in the sense that perhaps no synthetic corrin would as yet exist without recourse to metal templates. Beside the purely topological function of arranging the proximity of reaction centres, metal ions have served this purpose as follows ... 

EPR is the method of choice to study the paramagnetic intermediates occurring in the light-induced charge separation process in the reaction centres of plants and photosynthetic bacteria2. Furthermore, other paramagnetic species in particular metal centres - that participate in the photosynthetic processes can be detected and characterized by EPR. Examples are various cytochromes,... 

Jean-Pierre Sauvage is a CNRS director of research and is located at the Universite Louis Pasteur in Strasbourg, France. His current research interests include the development of models of the photosynthetic reaction centre using transition metals and porphyrins [5], topology (synthetic catenanes and knots) [6], and molecular machines [7]. 

Abstract The photochemical properties of transition metal complexes, such as those of iridium(III) or ruthenium(II), can be exploited in various ways to generate charge-separated (CS) states, in relation to the mimicry of the natural photosynthetic reaction centres, or to set multicomponent compounds or assemblies in motion. The first part of the present chapter summarizes the work carried out in our groups (Bologna and Strasbourg) in recent years with iridium(III)-terpy complexes (terpy 2,2,6,6"-terpyridine). The synthesis of multicomponent iridium(III) complexes in reasonable yields has been... 

The visible absorption and luminescence properties of the [Ru(bpy)3]2+-like centre in the N-ethylated polymer closely resemble those of [Ru(bpy)3]2+ and depend only slightly on the loading of the metal complex centre on the polymer (Ennis et al, 1986). The electron transfer reactions of a 1 11 copolymer have been studied in most detail. 

A schematic view of bacterial photosynthetic reaction centre with a chain of electron-carrying metal prosthetic groups is shown in Figure 11.4. The metal ions involved are magnesium (bacteriochlorophylls) and iron (haem and non-haem iron species). 

Even a relatively simple bacterial photosynthetic system is very complex and its synthetic imitation is a challenging task. Mimicking of the natural photosynthetic process requires synthetic models of all the crucial components and linking them together into a working molecular assembly. All the elements (antenna, charge separation, and reaction centres) may involve transition metals. Application of metal complexes facilitates mimicking of this complex chemical system due to rich and versatile photochemical processes typical for transition metal complexes (see section 6.4 in Chapter 6) [48]. 

A separate problem arises when the surface of the support bears non-metallic active centres. Such bifunctional catalysts are widely used in naphtha reforming and it has been suggested that a bifunctional mechanism may also operate in syngas reactions [35]. 

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