Metal-polymer complex, catalytic behavior

These discussions will embrace homogeneous solutions of polymer-metal complexes. Of course one of the important advantages offered by the use of a polymer ligand, especially a crosslinked polymer ligand, in catalysis is the insolubilization of the attached complexes the insolubility of the polymer catalyst makes it very easy to separate from the other components of the reaction mixture. Several polymer-metal complexes have been used for this purpose, although such applications are not covered in this article. The aim here is (1) to characterize polymer-metal complexes and their behavior in such simple but important elementary reactions as complex formation, ligand substitution, and electron transfer, and (2) to describe their catalytic activity. 

The catalytic cycle is illustrated in Scheme 1, the example used being the Cu complex catalyzed oxidation of 2,6-dimethyIphenol (3 ). In the first step, the substrate phenol coordinates to the Cu(II) complex and one electron transfers from the substrate to the Cu(II) ion. Then the activated substrate dissociates from the catalyst and the reduced Cu(I) catalyst is reoxidized to the original Cu(II) complex. Among these elementary reactions, the electron-transfer step is the most important process governing the catalytic behavior of a polymer-metal complex for the following reasons (i) The electron-transfer step is often the slowest... 

In the present paper we describe the catalytic mechanisms of synthetic polymer-Cu complexes a catalytic interaction between the metal ions which attached to a polymer chain at high concentration and an environmental effect of polymer surrounding Cu ions. In the latter half, the catalytic behavior is compared with the specific one of tyrosinase enzyme in the melanin-formation reaction which is a multi-step reaction. To the following polymers Cu ions are combined. 

This time-constant rate is proportional to the a-TiCU amount which proves that, at least formally, the over-all polymerization process is really a catal3rtic one, with regard to the a-TiCh. The catalytic behavior of -TiCU is, in any case, connected with the existence on its surface of metal-lorganic complexes which act in the polymerization only if a-TiCU is present. This makes stereospecific polymerization processes (of coordinated anionic nature) very different from the better known polymerization processes, initiated with free radicals. In the latter process, the initiator is not a true catalyst, since it decomposes during the reaction, forming radicals which are bound to the dead polymer on the contrary, in the case of stereospecific polymerization, each molecule of polymer, at the end of its growing period, can be removed from the active center on the solid surface of the catalyst which maintains its initial activity. 

Interfacial electron-transfer reactions between polymer-bonded metal complexes and the substrates in solution phase were studied to show colloid aspects of polymer catalysis. A polymer-bonded metal complex often shows a specifically catalytic behavior, because the electron-transfer reactivity is strongly affected by the pol)rmer matrix that surrounds the complex. The electron-transfer reaction of the amphiphilic block copol)rmer-bonded Cu(II) complex with Fe(II)(phenanthroline)3 proceeded due to a favorable entropic contribution, which indicated hydrophobic environmental effect of the copolymer. An electrochemical study of the electron-transfer reaction between a poly(xylylviologen) coated electrode and Fe(III) ion gave the diffusion constants of mass-transfer and electron-exchange and the rate constant of electron-transfer in the macromolecular domain. 

The metal complex bound to a polymer often shows a specific catalytic behavior compared with that of the corresponding monomeric complex, because the reactivities of the complex are strongly affected by the polymer chain that surrounds the complex. ... 

Gagn6 et al. developed polymer active sites that additionally contained a receptor (recognition sites) displayed in the outer sphere of the metal center (reactive site). The Suzuki reaction of /)-bromoanisole with phenyl boronic acid and the allylation of dimethyl malonate with allyl acetate were both chosen to assess the presence and/or effect of the crown-ether in crown-ether-molecular imprinting polymer-palladium complex. The results showed that molecular imprinting can be used to functionalize the second-coordination sphere of a transition metal complex and subsequently affect its catalytic behavior. 

It is clear from this review that it is not only M-N4 complexes containing electroactive central metal which show catalytic activity. Unmetalated complexes as well as Zn and Cu complexes, which show ring-based processes only, often show electrocatalytic behavior towards the detection of some of the pollutants discussed in this work. There has been controversy surrounding the electrocatalytic activity of NiP or NiPc complexes. It seems the Ni porphyrin complexes do exhibit the Ni /Ni couple (at high potential in solution and more readily in the polymeric state) which may be involved in the electrocatalytic reactions involving these complexes. The Ni /Ni couple has not been identified electrochemically for the NiPc complexes in solution, but has been implicated in catalysis as an adsorbed polymer. It would be of interest to determine the values of the couple on polymeric NiPc complexes. 

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