Synthesis, characterization and coordination chemistry of the new tetraazamacrocycle 4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-bis(methanephosphonic acid monoethyl ester) dipotassium salt

A new potentially hexadentate tetraazamacrocycle based on the cyclen skeleton has been synthesized and fully characterized. The macrocycle 4,10-dimethyl-1,4,7, 10-tetraazacyclododecane- 1,7-bis(methanephosphonic acid monoethyl ester) dipotassium salt (Me2DO2PME) contains mutually trans monoethyl ester phosphonate acid substituents on two nitrogen atoms, and trans methyl substituents on the other two nitrogen atoms. The protonation constants of this macrocycle and the stability constants of its complexes with Cu2+, Zn2+, Gd3+ and Ca2+ ions have been determined by pH potentiometric titrations. The protonation sequence of the macrocycle has been studied by H-1, P-31{H-1} and C-13{H-1} NMR spectroscopy: the first and second protonation steps take place at the methyl-substituted nitrogen atoms, while the third protonation involves one oxygen from a phosphonate group. Upon protonation, all the CH2 ring protons become magnetically inequivalent on the NMR time scale due to a slow conformational rearrangement, most likely occasioned by the formation of multiple hydrogen bonds within the macrocyclic ring. Me2DOPM forms neutral, mononuclear complexes with all the metals investigated. The presence of hydroxo complexes was observed for Ca2+ and Zn2+ at high pH values. Structural information on the neutral complex [Cu(Me(2)DO2PME)] has been obtained by a solution X-Band EPR study. It is proposed that Me(2)DO2PME binds Cu2+ in a distorted octahedral structure using all of its donor atoms, i.e. the four nitrogen atoms and the two phosphonate oxygen atoms. The redox chemistry of [Cu(Me(2)DO2PME)] in dimethyl sulfoxide and water has been studied by electrochemical measurements. Cyclic voltarnmetry in DMSO shows the complex to undergo a quasireversible one-electron reduction step leading to an unstable Cu-1 species.