Scorpiands: Quinquedentate Ligands that Couple the Rigidity of Cyclam and the Flexibility of a Coordinating Side Chain

Abstract

AbstractThis review tells the story of scorpiands, polyamine ligands which, when specifically stimulated, act as scorpions, and of their metal complexes, scorpiates. Scorpiands consist of a tetramine macrocycle (typically cyclam) capable of firmly including a transition metal and of a side chain ending with a nitrogen containing coordinating group: under unperturbed conditions the coordinating group is bound to the metal (ON), but on addition of acid the nitrogen is protonated and comes off the metal (OFF). Alternating acid/base addition makes the side chain move imitating the tail of a scorpion that bites a prey firmly immobilized by the claws. Occurrence of the ON‐OFF motion is monitored by the colour change of the scorpiate complexes associated to the change of coordination or by fluorescence quenching/restoring when a light‐emitting substituent is linked to the side arm. Scorpiate complexes can be considered optical molecular switches and, from a different standpoint, molecular machines able to convert chemical energy into mechanical work.