Donor-extended meso-Disubstituted Dipyrromethane-Based Dipodal Ligands and Their Metal Complexes: An Overview

The meso-disubstituted dipyrromethanes are an important class of ligand system which coordinate metal-ions across the periodic table as they feature two σ-donor anionic pyrrolic-N-atoms and offer pyrrole-π-electrons to the metal centres. They have the active 1,9-positions for preparing their variants that may contain additional donor atoms in the side arms. The introduction of the arms to the dipyrromethane moieties not only make them polyfunctional but also enhances flexibility of the system. In this review article, we highlight the syntheses and metal-ion coordination of various donor-extended meso-disubstituted dipyrromethane-based ligands that include the scaffolds decorated with diamine, diimine, bis-azo, diphosphine, or bis-pyrazole functions on both sides. The resulting NNNN and PNNP types of tetradentate dipodal ligands yield a large number of transition or alkali metal complexes with attractive structural and coordination properties, expressing the main focus of this article. We also emphasize the structural variations of the complexes dictated by the steric-bulk of the substituents at the meso-position or at the termini of the dipods. In addition, certain metal complexes bearing the extended ligand(s) participate in distinct catalytic transformations and also show interesting magnetic behaviors that are minutely discussed. Finally, a few hints of research endeavours are outlined for future purview.