Higher Denticity Ligands

Since the first edition of Comprehensive Coordination Chemistry (CCC, 1987) was published, the number of known multidentate ligands has increased dramatically. The pursuit of multidentate ligands for applications in catalysis, bioinorganic and material chemistry is a primary concern of organic and inorganic chemistry alike. Much of the current impetus is provided by the need for cheap, efficient, nontoxic, water-soluble catalysts, for models of metalloenzymes in which bi- and multimetallic centers occur in the active sites, and for efficient chelating agents toward lanthanide and actinide ions used in bioinorganic chemistry and materials science. Complexes containing multidentate ligands have been investigated to understand the role of metal ions in multielectron redox reactions and in the activation of small molecules such as O2 and N2, as well as toco ntrol possible cooperative phenomena between the two metal centers. For example, suitably designed multimetallic complexes formed by binucleating ligands possessing four- to six-coordinate sites could provide reactivity patterns distinct from those shown by analogous monometallic species. The design of appropriate ligand cores, providing coordination sites with well-defined metal– metal separations, is highly desirable. Studies on the assembly of double- and triple-helicate complexes are also actually a major area in coordination chemistry. It has been observed that the formation of architecturally complex systems is directed by the interplay between simple parameters such as the stereoelectronic preference of the metal ions and the disposition of the binding sites in the ligand. How a polydentate ligand becomes partitioned into distinct metalbinding sites is a key parameter in the assembly of helicates. Recent progress in the field of crystal engineering based on polymeric coordination has been devoted to the use of novel polydentate ligands. Much study has centered upon the use of supramolecular contacts between suitable molecules to generate multidimensional arrays or networks. The simple strategy of combining metal centers with polyhapto ligands can generate crystalline architecture, with obvious implications for the rational design of new and varied topological types. There are numerous examples of polymeric sheet or network materials involving bifunctional building blocks connected by coordination to metal centres. This chapter covers the coordination chemistry of ligands with four or more donor atoms (N, O, or S) that are referred to as high-denticity or multidentate. Particular emphasis has been devoted to N- and O-donor ligands, which present several distinct advantages: they are often largely available in pure form and are easy to synthesize; some sulfur donors are also included. A large number of organic molecules with at least four donor atoms have been shown to coordinate metal cations. Many of them contain different substituents (amine, amide, hydroxy, oxo, carboxy, etc.), which makes it impossible to classify them based on the nature of their functional groups. Here we divided them into several classes taking into consideration: (i) the arrangement of the ligand around the metal center; (ii) the nature of donor groups and heteroatoms. The multidentate donors that belong to a separate, well-defined group, such as tetraketones, polypyridines, etc., are discussed in the appropriate chapters. Systematic names are cumbersome, and a variety of trivial names have been used. Both will be avoided in this chapter, and the better-known system of abbreviations, or a code number, will be used.