CHELATION AND CATALYSIS

In any discussion of chelating agents or chelation chemistry the subject of catalysis must always be accorded a prominent position. The reason for this is obvious. Chelating agents interact with and modify the properties of metal ions; and it is metal ions-functioning as weak Lewis acids, as efficient electron transfer systems, as perturbing influences on the electron distributions of the molecules around them-that are the actuators in most homogeneous catalysis reactions. An excellent review of the catalytic effects of chelation and chelate compounds was presented by Martell and Calvin’ and, more recently, by Chaberek and Martell? The extensive references presented by these authors attest the importance that researchers have attached to the subject. The papers presented elsewhere in this monograph that discuss the role of chelation in enzyme systems and in organic synthesis are examples of data being collected currently on an even larger scale. For the present discussion it is useful to adopt a classification of catalytic reactions involving chelates from the earlier reference.2 Under this classification these systems are divided on the basis of the fate of the chelant during the reaction: (1) the metal chelate is not permanently altered during the course of the reaction, and (2) the metal chelate is permanently altered during the course of the reaction. Examples of reactions of the former class, in which a metal chelate, as such, functions in the role of catalyst are not numerous. That which has received most study is the hydrolysis of Auorophosphorous compounds,3 A which was found to be catalyzed by cupric chelates of such materials as ethylenediamine, the simple amino acids, a ,a-bipyridyl and o-phenanthroline. The most strongly catalytic were bidentate ligand chelates, and the mechanism would appear to involve interaction between the remaining pair of coordination sites of the copper ion and the substrate molecule to make the phosphorous atom more electrophilic for approach of the water molecule or hydroxyl ion. A similar dependence upon open adjacent coordination sites in the catalytic chelate is found in work by Wang,6 who showed that hydrogen peroxide decomposition was catalyzed by the triethylenetetramine chelate of ferric iron. The two oxygen atoms in the peroxide are thought to be drawn to the pair of coordination sites unfilled by the tetradentate ligand, aiding in the breaking of the peroxide linkage. Reactions illustrative of the second group, those in which the chelate is permanently altered, are more numerous. Perhaps the best known is work done on the autoxidation of ascorbic acid,O which serves to show that chelation of trace amounts of added cupric ion by the ascorbic acid serves to “drain” electrons from the ascorbic acid and promote its oxidation. The