1,10-Dimethyl-1,4,7,10,13,16-hexaazacyclooctadecane L and 1,4,7-trimethyl-1,4,7,10,13,16,19-heptaazacyclohenicosane L1: two new macrocyclic receptors for ATP binding. Synthesis, solution equilibria and the crystal structure of (H4L)(ClO4)4