Synthesis of Azoles with Two Pyridine Substituents at Carbon Atoms and Their Use in Coordination Chemistry. (Review)

Abstract

Published data on the synthesis of azoles containing two pyridine substituents are analyzed in relation to the type of central five-membered ring. The applications of such molecules as ligands in coordination chemistry are discussed.The design of coordination polymers having unique structures and valuable functional characteristics is one of the most rapidly developing regions of modern chemistry and materials science [1,2]. Most of the employed exo-dentate ligands belong to the bipyridine class with the general formula Py-X-Py, where X is a conjugated disubstituted fragment. The parent of such ligands is 4,4'-bipyridine (X is absent), which has been widely used for a long time in the construction of coordination polymers [3]. The most widely used bridges X include double and triple carbon-carbon bonds, azo and imino groups, and benzene, pyridine, triazine, and tetrazine rings or combinations of these fragments [4].One modern trend in the synthesis of coordination polymers is to use ligands of the Py-X-Py type, where X is a meta-or ortho-substituted azole. The interest in such compounds is due, first, to their electronic characteristics and, second, to the ability of the pyridine nitrogen atom of the azole ring to coordinate with the metal ion. This fact significantly extends the possibilities of the ligands in coordination, enhancing the practical characteristics of the complexes. At the moment the coordination characteristics of a small group of dipyridylazoles have been studied. It is important to emphasize that at this time there has been no systematic treatment of data on the design and synthesis of such compounds. In view of the foregoing it seemed to us important and timely to clarify some of the problems that arise in the synthesis of dipyridylazoles for the systems that are finding use in coordination chemistry.It should be noted that in this paper we restrict ourselves to examination of the synthesis and coordination characteristics of azoles containing not less than one nitrogen atom of the pyridine type. Consequently, we do not consider five-membered donating heterocycles with one heteroatom. The second restriction is the fact that we decided to examine only azoles substituted by pyridine groups at carbon atoms, and the range of compounds studied does not therefore include tetrazoles.The review is divided up according to the types of central five-membered heterocycles. In each section the synthetic aspects and questions concerning the use of specific ligands in coordination chemistry are examined. _______ * Dedicated to Academician Nikolai Serafimovich Zefirov in honor of his seventieth birthday. __________________________________________________________________________________________