In the presence of polyborates (detected by ¹¹B-NMR) the formation of a weak Eu(III) borate complex (lg β₁₁ ∼ 2, estimated) was observed by time-resolved laser-induced fluorescence spectroscopy (TRLFS). This complex is a precursor for the formation of a solid Eu(III) borate species. The formation of this solid in solution was investigated by TRLFS as a function of the total boron concentration: the lower the total boron concentration, the slower is the solid formation. The solid Eu(III) borate was characterized by IR spectroscopy, powder XRD and solid-state TRLFS. The determination of the europium to boron ratio portends the existence of pentaborate units in the amorphous solid.

Although 2-phosphonobutane-1,2,4,-tricarboxylic acid, PBTC, has manifold industrial applications, relevant and reliable data on the protonation of PBTC are poor. However, these data are critical parameters for ascertaining PBTC speciation, especially with regard to a sound structural and thermodynamic characterization of its metal ion complexes. A rigorous evaluation of pH-dependent ¹H, ¹³C, and ³¹P chemical shifts along with accessible scalar spin–spin coupling constants (J) was performed in order to determine the pK_a values of PBTC in 0.5 molal NaCl aqueous solution by means of nuclear magnetic resonance (NMR) spectroscopy. The phosphonate group revealed pK_a values of 0.90 ± 0.02 and 9.79 ± 0.02, and the pK_a values associated with the carboxylic groups are 3.92 ± 0.02, 4.76 ± 0.03, and 6.13 ± 0.03. Supported by DFT-calculated structures revealing strong intramolecular hydrogen bonding, the sequence of deprotonation could be unambiguously determined.

The formation equilibria of salicylatoborate, lactatoborate and 3-hydroxybutyratoborate were studied by means of ¹¹B NMR spectroscopy. The smaller the pK_a of the respective organic acid, the higher is the formation constant of the organoborate. The complexation of Eu(III) with salicylatoborate and lactatoborate was investigated by means of TRLFS (time-resolved laser-induced fluorescence spectroscopy) and ¹¹B NMR spectroscopy, yielding complexation constants lg β⁰ ₁₁ = 2.6–3.2. A Eu(III)–3-hydroxybutyrate complex was characterized by TRLFS and ¹H NMR spectroscopy (lg β⁰ ₁₁ = 2.89). DFT calculations of the investigated Eu(III)–organoborates and inorganic Eu(III)–(poly)borates provided information about the Eu(III) coordination (most likely chelate). They support the hypothesis that the complexation of Eu(III) with organic as well as inorganic borate structures containing the binding site “B(OR)₄ ⁻” (R = H, threefold coordinated boron center(s), organic moiety) is comparable.

Organophosphonates have manifold applications in the chemical industry, of which one of the most commonly used is 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC). It is widely used as a cement additive and may pose a potential risk of complexing radionuclides such as uranium in nuclear waste repositories. PBTC, in its fully deprotonated form, has four negatively charged groups, one phosphonate and three carboxylate groups, which makes it a superior ligand for metal ion complexation. In this study, for the first time, its complexation behavior towards hexavalent uranium, U(VI), in the pH range from 2 to 11, has been investigated using various spectroscopic methods. The structure-sensitive methods NMR, IR, and Raman spectroscopy were used to characterize the complex structure. The interpretation of the results was supported by density functional calculations. Over almost the entire pH range studied, U(VI) and PBTC form a chelate complex via the phosphonate and the geminal carboxylate group, highlighting the strong chelating ability of the ligand. UV-Vis spectroscopy combined with factor analysis was applied to determine the distribution of differently protonated chelate species and their stability constants. Time-resolved laser-induced luminescence spectroscopy (TRLFS) was additionally used as a complementary method.