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

This 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.