Co-ordination polymers

Abstract

This paper is a review of the metal complex forming coordination polymers. A polymer–metal complex is composed of synthetic polymer and metal ions bound to the polymer ligand by a coordinate bond. A polymer ligand contains anchoring sites like nitrogen, oxygen or sulphur obtained either by the polymerization of monomer possessing the coordinating site or by a chemical reaction between a polymer and a low molecular weight compound having the coordinating ability. The polymer–metal complexes may be classified into different groups according to the position occupied by the metal, which is decided by the method of preparation. The method include the complexation of polymeric ligand with various metal ions, cross-linked polymers with pendent, ligands forming either intramolecular and/or intermolecular chelating functions are highlighted in the first part. The various works on the coordination complexes has revealed that the heterogeneous systems possess more economical potentials and advantages over homogeneous systems. The co-ordination polymers belong to the former case. The high molecular weight polymer–metal complexes work as storage houses for solar energy. Efficient chemical conversion in the storage of solar energy will be difficult with the homogeneous systems. The synthesis results in an organic polymer with inorganic functions. The metal atoms attached to the polymer backbone are bound to exhibit characteristic catalytic behaviour, which are distinctly different from their low molecular weight analogue. Many synthetic polymer–metal complexes, found to possess high catalytic efficiency, in addition to ion selectivity in waste water treatment, recovery of trace metal ions, and hydrometallurgy are enlightened in the final part.

Introduction

Polymer science has emerged as an active discipline of material science. This field impinges on areas of commodity, engineering and speciality polymers, thereby stimulating interest all over the globe in exploiting newer domains. One such branch that has emerged is polymer–metal complexes comprising an organic polymer containing coordinating sites, complexed with metals. This is of relatively recent origin and an interdisciplinary approach taking into its fold areas, viz. chemistry, metallurgy, environmental and material sciences.

A coordination compound may be defined as a compound containing a central atom or ion which are attached with molecules or ions whose number usually exceeds the number corresponding to the oxidation number or valency of the central atom or ion. The groups that are bound to the central metal or ion in a symmetrically oriented fashion through coordinate or coordinate-covalent bond are called “ligands”. For a long time, the coordination compounds were considered as a rare and special class but subsequently have become versatile.

The chemistry of coordination compounds is at present undergoing rapid development in diverse disciplines. The impetus for progress in this area has resulted from its many biological applications. Metal chelates play an essential role in the chemistry of living matter, viz. chlorophyll's (Mg(II) complex) and haemoglobin (Fe(II) complex) [1]. A large number of metal proteins and other metal complexes of biological importance have been studied.

Apart from the biological field, coordination compounds play an essential role in chemical industries. For instance in 1963, the Nobel prize in chemistry was awarded jointly to K. Ziegler of the Max Plank Institute, in Germany and G. Natta of the University of Milan in Italy for developing a new metal complex catalyst containing aluminium and titanium. This catalyst revolutionized polymer synthesis.

Work on coordination complexes has revealed that heterogeneous systems possess more economical potentials and advantages over homogeneous systems. Polymer–metal complexes belong to the former case. The high molecular weight polymer–metal complexes work as storage houses for solar energy. Efficient chemical conversion in storage of solar energy will be difficult with the homogeneous systems. Molecular design using a heterogeneous system would therefore be important. Fundamental research on the photoreaction in the microheterogeneous environment provided by the polymer has been reported [2].

Metalloenzyme is a kind of polymer–metal complex present in nature, where metal ions are surrounded by a giant protein molecule of definite three-dimensional structure. A typical example of such a metalloenzyme whose structure has been determined is plastocyanin (a kind of blue-copper protein) [3]. In plastocyanin the copper ion shows a distorted tetrahedral structure and is coordinated by methionine's sulphur atom, which is not normal in usual low molecular weight metal complexes. This abnormal coordination behaviour and the hydrophobic environment around the copper ion brought by giant protein molecule cause unusual redox behaviour of the copper ion. Generally, the protein in metalloenzyme not only decides the chemical structure but also causes an allosteric effect through conformational change of its polymer chain. In order to throw light on the effect of the protein surrounding metal ion, intensive studies on the structure and catalytic activity of synthetic polymer–metal complexes were initiated.

Polymer–metal complexes have been of interest to many researchers during the past three decades in the light of their potential applications in diversified fields like, organic synthesis [4], waste water treatment [5], hydrometallurgy [6], polymer drug grafts [7], recovery of trace metal ions [8] and nuclear chemistry [9]. In addition, they are also used as models for enzymes [10], [11].

A polymer–metal complex is composed of synthetic polymer and metal ions, wherein the metal ions are bound to the polymer ligand by a coordinate bond. A polymer ligand contains anchoring sites like nitrogen, oxygen or sulphur obtained either by polymerization of monomer possessing the coordinating site or by a chemical reaction between a polymer and a low molecular weight compound having coordinating ability. The synthesis results in an organic polymer with inorganic functions. The metal atoms attached to polymer backbone are bound to exhibit characteristic catalytic behaviour, which are distinctly different from their low molecular weight analogue. Indeed, many synthetic polymer–metal complexes have been found to possess high catalytic efficiency, in addition to semiconductivity, heat resistance and biomedical potentials.