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

Abstract

The trivalent lanthanide complex formation constants (log K_f) 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 (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-[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.