Bicyclic-guanidines, -guanidinates and -guanidinium salts: wide ranging applications from a simple family of molecules

Bicyclic guanidines have physical, electronic and chemical properties that differentiate them from their acyclic counterparts, with many of these characteristics directly imposed by the rigid framework. These distinctive features have led to this class of molecule finding practical applications in many areas of chemistry, including organocatalysis and as ligands in coordination compounds. In addition to the neutral molecules, the corresponding cationic (guanidinium) and anionic (guanidinate) species have also been widely studied. Applications of these ions range from anion recognition and supramolecular arrays involving guanidinium salts, to the utilization of bicyclic guanidinate anions as ligands for metal compounds and clusters. This article reviews the chemistry of these compounds in light of recent advances in the synthesis of new derivatives, highlighting the potential for cross-stimulation of different areas.