The formation stability, hydrolytic behavior, mass spectrometry, DFT study, and luminescence properties of trivalent lanthanide complexes of H _2ODO_2A

摘要

The trivalent lanthanide complex formation constants (log K_f) of the macrocyclic ligand H_2ODO2A (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_2DO2A (1,7-dicarboxylmethyl-1,4,7,10-tetraazacyclododecane) complexes. The equilibrium formation of the dinuclear hydrolysis species, e.g. Ln_2(ODO2A) _2(μ-OH)~+ and Ln_2(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]~+, [Eu_2(ODO2A) _2(OH_2)_2+H]~+, [Eu(ODO2A)(OH) _2]- and [Eu_2(ODO2A)_2(OH _2)_3]- species at pH > 7. Density function theory (DFT) calculated structures of the EuODO2A(H_2O) _3 ~+ and EuDO2A(H_2O)_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 N_3O 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 (λ_(ex) = 395 nm) and 544 nm (λ_(ex) = 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-[Y_2(ODO2A)_2(μ-OH)(H_2O)_2] ~+, trans-[Y_2(ODO2A)_2(μ-OH)(H _2O)_2]~+ and [Y_2(ODO2A) _2(μ-OH)_2] dinuclear complex structures. The first and the second include 6-coordination by the ligand ODO2A~(2-), one by the bridged μ-OH ion and one by a water molecule. The third includes 6-coordination by the ligand ODO2A~(2-) and two by the bridged μ-OH ions. The two inner-sphere coordinated water molecules in the cis- and trans-[Y_2(ODO2A)_2(μ-OH)(H_2O) _2]~+ dinuclear complexes are in a staggered conformation with torsional angles of 82.21° and 148.54°, respectively.