The formation stability, hydrolytic behavior, mass spectrometry, DFT study, and luminescence properties of trivalent lanthanide complexes of H2ODO2A.

The trivalent lanthanide complex formation constants (log Kf) of the macrocyclic ligand H2ODO2A (4,10-dicarboxymethyl-1-oxa-4,7,10-triazacyclododecane) have been determined by pH titration techniques to be in the range 10.84–12.62 which increase with increasing lanthanide atomic number, and are smaller than those of the corresponding H2DO2A (1,7-dicarboxylmethyl-1,4,7,10-tetraazacyclododecane) complexes. The equilibrium formation of the dinuclear hydrolysis species, e.g. Ln2(ODO2A)2(μ-OH)+ and Ln2(ODO2A)2(μ-OH)2, dominates over the mononuclear species, e.g. LnODO2A(OH) and LnODO2A(OH)2−. Mass spectrometry confirmed the presence of [Eu(ODO2A)]+, [Eu(ODO2A)(OH)+H]+, [Eu2(ODO2A)2(OH2)2+H]+, [Eu(ODO2A)(OH)2]− and [Eu2(ODO2A)2(OH2)3]− species at pH > 7. Density function theory (DFT) calculated structures of the EuODO2A(H2O)3+ and EuDO2A(H2O)3+ complexes indicate that three inner-sphere coordinated water molecules are arranged in a meridional configuration, i.e. the 3 water molecules are on the same plane perpendicular to that of the basal N3O or N4 atoms. However, luminescence lifetime studies reveal that the EuODO2A+ and TbODO2A+ complexes have 4.1 and 2.9 inner-sphere coordinated water molecules, respectively, indicating that other equilibrium species are also present for the EuODO2A+ complex. The respective emission spectral intensities and lifetimes at 615 nm (λex = 395 nm) and 544 nm (λex = 369 nm) of the EuODO2A+ and TbODO2A+ complexes increase with increasing pH, consistent with the formation of μ-OH-bridged dinuclear species at higher pH. Additional DFT calculations show that each Y(III) ion is 8-coordinated in the three possible cis-[Y2(ODO2A)2(μ-OH)(H2O)2]+, trans-[Y2(ODO2A)2(μ-OH)(H2O)2]+ and [Y2(ODO2A)2(μ-OH)2] dinuclear complex structures. The first and the second include 6-coordination by the ligand ODO2A2−, one by the bridged μ-OH ion and one by a water molecule. The third includes 6-coordination by the ligand ODO2A2− and two by the bridged μ-OH ions. The two inner-sphere coordinated water molecules in the cis- and trans-[Y2(ODO2A)2(μ-OH)(H2O)2]+ dinuclear complexes are in a staggered conformation with torsional angles of 82.21° and 148.54°, respectively.