Pseudo-Encapsulation: Open Prison Ligands for Metal Ions

Abstract

Abstract Rapid growth in the range of polydentate ligands prepared in recent decades has produced firstly macromonocyclic systems and subsequently macropolycyclic encapsulating ligands. Intermediate between these can be identified a set of topologically distinctive ligands whose collective grouping is based on their capacity to satisfy fully the coordination demands of metals ions and bind sets of three adjacent donors to polyhedral faces. This family of ligands offer pseudo-encapsulation of metal ions; whereas a macrobicycle may act as a “cage” or “prison” for a metal ion by trapping it in the cavity, the ligands described here present themselves as “open prisons” because of their less complete framework. The concept and types of pseudo-encapsulating ligands extant is examined here for saturated hexaamines which act as sexidentate ligands and commonly form octahedral complexes. Examples of the various topologies assigned as pseudo-encapsulating hexaamine ligands are described. In particular, the differing shape-directed coordination chemistry of syn− and anti−diammac (6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine) is examined in some detail. The latter isomer is a sterically efficient ligand which forms complexes with “compressed” coordination spheres and extreme physical properties, whereas the former isomer displays no such effects.