Synthesis, Potentiometry, and NMR Studies of Two New 1,7-Disubstituted Tetraazacyclododecanes and Their Complexes Formed with Lanthanide, Alkaline Earth Metal, Mn(2+), and Zn(2+) Ions.

Two new 1,7-disubstituted-1,4,7,10-tetraazacyclododecane ligands, DO2P and DO2PME, and their complexes with Mg(2+), Ca(2+), Sr(2+), Mn(2+), Zn(2+) and Ln(3+) were prepared and characterized by pH potentiometry. The pH titration data showed that DO2P and DO2PME both form 1:1 M:L complexes with all divalent and trivalent metal ions. Protonated complexes did not appear to form with the bis(phosphonate ester) ligand, DO2PME, but were evident for all of the metal ion-DO2P complexes. The alkaline earth metal ion-DO2P complexes formed both ML and MHL complexes while the lanthanide ion (Ln(3+)), Zn(2+), and Mn(2+) complexes of DO2P formed ML, MHL, and MH(2)L species. Zn(2+) formed the most stable complex with both ligands. The stability (beta(101)) of the LnDO2PME(+) complexes increased by about 2 orders of magnitude along the lanthanide series (La(3+) to Lu(3+)) while the stability of the LnDO2P(-) complexes over this same series increased by over 3 orders of magnitude. The bis(phosphonate) ligand, DO2P, and some of its complexes formed with Ln(3+) ions were further examined by NMR spectroscopy. (1)H and (31)P spectra of DO2P collected as a function of pH provided evidence that the first two protonations on the ligand take place largely at the tertiary nitrogens. The similarity of the (31)P chemical shifts of EuDO2P(-) and EuDOTP(5)(-) indicate that DO2P forms an "in-cage" complex with Eu(3+) using all four macrocyclic ring nitrogens and the two phosphonate sidearms as ligands. (17)O NMR shifts of the water signal indicated that the DyDO2P(-) complex has two inner-sphere coordinated water molecules. In the presence of excess of DO2P, a 1:2 metal:ligand, LnDO2P(HDO2P)(4)(-), complex forms with the second ligand interacting only weakly with the coordination sites left vacant by the first DO2P. Both water proton relaxivity data for GdDO2P(-) and (31)P NMR spectra of EuDO2P(-) provide evidence for formation of an "out-of-cage" LnH(2)DO2P(+) complex at low pH values (<6.5) in which the two phosphonate groups of DO2P are only involved in bonding with the lanthanide cation.