A novel hexaaza macrocycle with methylenephosphonate pendant arms: A potential useful chelate for biomedical applications

A new 18-membered hexaaza macrocyclic ligand with four pendant methylenephosphonates has been synthesized via the Mannich reaction. The protonation behaviour of this macrocycle has been followed by P-31 and H-1 NMR spectroscopy. The initial protonation pattern of the N-functionalised macrocycle is rather like its corresponding macrocyclic amine. The proton NMR spectrum of its lanthanum(III) complex in aqueous solution indicates that the complex adopts only one of the five possible conformations, either SSSS or RRRR. The crystal structure of the novel macrocyclic ligand shows an extended hydrogen-bonded structure in its solid state. The macrocycle has an unusual conformation with the pendants alternating 'up' and 'down' round the ring. The La-III complex has a unique ten-co-ordinate geometry with all pendant arms co-ordinated to the La-III ion which is in the plane of the six macrocyclic nitrogen donors.