Structural requirements for rapid metal ion ingress into hydroxyethylated tetraaza macrocycles

Synthesis of two new bis(2-hydroxyethyl) substituted cyclam (1,4,8,11-tetraazacyclotetradecane) derivatives, 1,4-bis(2-hydroxyethyl)-8,11-dimethylcyclam (1,4-DMHEC-14) and 1,11-bis(2-hydroxyethyl)-4,8-dimethylcyclam (1,11-DMHEC-14), has enabled a study of the effects of partial hydroxyethylation of the nitrogen atoms in a macrocyclic ligand framework to be made. The ligands were synthesised through the reaction of ethylene oxide with the appropriately methylated cyclam precursor. Stability constants for complexes of these ligands with Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+ and Pb2+ have been measured by potentiometric titration and are generally intermediate between those shown by tetramethylcyclam (TMC-14) and tetrakis(2-hydroxyethyl)cyclam (THEC-14), consistent with a lower level of ligand stabilisation through internal hydrogen bond formation than in THEC-14. Equilibration times noted during the potentiometric titrations indicate that these bis(2-hydroxyethyl) ligands, like THEC-14 and unlike TMC-14, facilitate rapid entry of the metal ion into the macrocyclic annulus demonstrating that two pendant 2-hydroxyethyl arms, when located on adjacent nitrogen atoms, are sufficient to generate facilitated entry. The crystal structures of both [Ni(1,4-DMHEC-14)](ClO4)(2) and [Ni(1,11-DMHEC-14)](ClO4)(2) have been determined by a combined molecular mechanics and X-ray crystallography study and show that in each isomer both pendant donors are coordinated in a trans-relationship, in addition to the four nitrogen atoms, giving rise to octahedral stereochemistry. In the former case the ligand is in the trans-III configuration whereas the latter complex has a trans-II geometry.