Transition metal water-exchange reactions

On the basis of tt complex adsorption it is possible to formulate two new mechanisms for transition metal-catalyzed exchange reactions between aromatic molecules and water or hydrogen gas (2, 4). Similar to... 

Transition metal ion-exchanged zeolites are active catalysts for alkene oxidation but generally result in deep oxidation to carbon dioxide and water (43-45). In common with CO and alkane oxidation, the platinum metal ions are more active than the first-row transition metal ions. Mochida et al. (43) have been able to correlate the catalytic activity of ion-exchanged Y zeolites for propylene oxidation with a so-called Y parameter as shown in Fig. 9. This parameter was considered to express the tendency of the metal ion toward the formation of a dative re-bond with propylene. Further, it was shown that with increasing Y factor there was a decrease in reaction order, which was considered evidence of increased propylene adsorption. In a more recent study of CuX zeolites, Gentry et al. (45) found some evidence... 

R. Akesson, L. G. M. Pettersson, M. Sandstrom, and U. Wahlgren, Theoretical Study on Water-Exchange Reactions of the Divalent and Trivalent Metal Ions of the First Transition Period, J. Am. Chem. Soc. 116, 8705-8713 (1994). 

This review deals with the reaction mechanisms of polya-minecarboxylate complexes of different transition metals such as Fe, Mn, Ni, and Ru. Three types of chemical processes are treated, viz. water-exchange reactions, the binding of NO, and the activation of peroxides. In each case, the natiu-e of the polyaminecarboxylate chelate and its influence on the imderly-ing reaction mechanism are considered. In general, the complexes are either six- or seven-coordinate and all contain a coordinated water molecule. The lability of the latter is controlled by the nature of the polyaminecarboxylate chelate and the oxidation state of the metal ion. The binding of NO and the activation of peroxide are in turn controlled by the lability of the coordinated water molecule that is displaced during the interaction with these small molecules. 

On the basis of a semi-empirical model. Swaddle and Mak estimated the limiting activation volumes for water-exchange reactions on di- and trivalent transition metal aqua complexes to be AV - -13cm mol for a D mechanism and AV 13cm mol for an A mechanism (20,21). Values of AV within these extreme values are usually interpreted in terms of Id, I, or la mechanisms, respectively (22). With these values as a guideline, assignments which type of mechanism holds can be made with significant confidence. 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

A feature common to both ir complex mechanisms is the nature of the second reagent in the exchange reaction [Eqs. (11), (12a), (12b)], namely heavy water or deuterium gas. Water is generally preferred in exchange reactions as it does not produce hydrogenated by-products. The important aspect concerning water and deuterium gas is the rapid exchange between these compounds on transition metal catalysts, which has been explained by dissociative chemisorption. 

It has been shown that the interpretation of catalytic reactions involving group VIII transition metals in terms of n complex adsorption possesses considerable advantages over classical theories by providing a link between theoretical parameters and chemical properties of aromatic reagents and catalysts. The concept has led to the formulation of a number of reaction mechanisms. In heavy water exchange the dissociative tt complex substitution mechanism appears to predominate it could also play a major role when deuterium gas is used as the second reagent. The dissociative mechanism resolves the main difficulties of the classical associative and dissociative theories, in particular the occurrence... 

The similar rate laws for the reactions of Fe(III) with a number of aprotic ligands can be rationalized in the same manner. The rate constants for ligation of Fe and FeOH are shown in Table 2.1. The exchange of water with Fe(III) and other tervalent transition metal ... 

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