Coordination driven discrete metallopolygons and cages from unsymmetric bidentate ligands

Abstract

Ligand geometry plays an important role in designing metallosupramolecules. Among various types of ligands used for coordination driven self-assemblies, orientations of bidentate ligands are most simple and easy to predict. Combination of a bidentate ligand with metal component produces metallosupramolecules with varying shapes and sizes. Unlike symmetric bidentate ligand, unsymmetric bidentate ligand offers multiple binding modes and forms a library of linkage isomers upon combination with metal center. In most of the cases the linkage isomer with head-tail arrangement of ligands emerges as the thermodynamic product. In this review unsymmetrical bidentate ligands and corresponding metallosupramolecules generated by their combination with various metal centers are discussed. Unsymmetric ligands are designed either by incorporating unsymmetric spacer backbone or by appending different coordinating site to the spacer backbone. Combination of an unsymmetric bidentate ligand with cis-Pd(II), cis-Pt(II), Au(II) and Ag(I) etc produces macrocyclic triangles, squares, rhomboids. However, simple Pd(II)/Pt(II) upon combination with an unsymmetrical ligand leads to a mixture of M2L4 type linkage isomers where the cis-isomer is found to be the major product.