The formation stability, hydrolytic behavior, mass spectrometry, DFT study, and luminescence properties of trivalent lanthanide complexes of H(2)ODO2A

Abstract

The trivalent lanthanide complex formation constants (log K-f) of the macrocyclic ligand H(2)ODO2A (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 H(2)DO2A (1,7-dicarboxylmethyl-1,4,7,10-tetraazacyclododecane) complexes. The equilibrium formation of the dinuclear hydrolysis species, e.g. Ln(2)(ODO2A)(2)(mu-OH)(+) and Ln(2)(ODO2A)(2)(mu-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](+), [Eu-2(ODO2A)(2)(OH2)(2)+H](+), [Eu(ODO2A)(OH)(2)](-) and [Eu-2(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 N-4 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 (lambda(ex) = 395 nm) and 544 nm (lambda(ex) = 369 nm) of the EuODO2A(+) and TbODO2A(+) complexes increase with increasing pH, consistent with the formation of mu-OH-bridged dinuclear species at higher pH. Additional DFT calculations show that each Y(III) ion is 8-coordinated in the three possible cis-[Y-2(ODO2A)(2)(mu-OH)(H2O)(2)](+), trans-[Y-2(ODO2A)(2)(mu-OH)(H2O)(2)](+) and [Y-2(ODO2A)(2)(mu-OH)(2)] dinuclear complex structures. The first and the second include 6-coordination by the ligand ODO2A(2-), one by the bridged mu-OH ion and one by a water molecule. The third includes 6-coordination by the ligand ODO2A(2-) and two by the bridged mu-OH ions. The two inner-sphere coordinated water molecules in the cis- and trans-[Y-2(ODO2A)(2)(mu-OH)(H2O)(2)](+) dinuclear complexes are in a staggered conformation with torsional angles of 82.21 degrees and 148.54 degrees, respectively.