Exploring aqueous speciation of lanthanide chloride clusters by electrospray ionization mass spectrometry and quantum chemistry

Abstract

The mobility and deposition of REE metals in hydrothermal fluids is strongly influenced by their speciation, in particular, by molecular clustering and solvation processes. The characterization of individual REE halide cluster stoichiometries and structures poses a special challenge to experimentalists, and this task is in many cases either difficult or impossible to achieve solely by solution-based techniques. On the other hand, electrospray ionization (ESI) mass spectrometry is emerging as a promising tool for the study of the solution chemistry of metal complexes, and in combination with adequate theoretical methods, can provide valuable insight into the molecular-scale speciation of aqueous REE metals. Here we report results from a combined ESI mass spectrometric and theoretical study of the aqueous NdCl3, EuCl3 and CeCl3 systems. Theoretical calculations at the MP2 and DFT theory level have been applied to examine the structures and energies of Nd, Eu and Ce chloride clusters, both water-free and microsolvated. A brief summary of the results for the NdCl3 systems is given here, while a more detailed description of our findings for CeCl3 and EuCl3 will be given in the presentation. Electrospray ionization of dilute (1 mM) aqueous solutions containing neodymium chloride results in the formation of solvated mononuclear complexes, these being [NdO]+(H2O)n, [NdClOH]+(H2O)n, [NdCl2]+(H2O)n and the dinuclear clusters [Nd2Cl4OH]+(H2O)n and [Nd2Cl5]+(H2O)n. Results from MP2 and DFT calculations for microsolvated [NdO]+, [NdClOH]+, and [NdCl2]+ support a solvation shell configuration around the complex, in which up to 5 waters are bound into the first shell. In the case of [Nd2Cl4OH]+ and [Nd2Cl5]+, both Nd centers are linked via two bridging Cl atoms and closely resemble the equilibrium structure of Nd2Cl6. MP2 generated structures for microsolvated [Nd2Cl4OH]+ and [Nd2Cl5]+ reveal that that the first four water molecules are strongly bound onto Nd centers in highly exothermic reactions steps; subsequent water molecules are located in the second shell and are loosely bound (close to the water condensation enthalpy) with first-shell water molecules. We also conducted a series of mass spectrometric experiments in which the concentration dependence (0.1-20mM) of the neodymium chloride cluster distribution and abundance was probed. Results from these concentration-dependent experiments demonstrate an increasing trend toward cluster growth with increasing NdCl3 concentration, in other words, at higher NdCl3 concentrations more neodymium chloride is present as polynuclear [Nd2Cl4OH]+ and [Nd2Cl5]+. In general, results from this ESI FT-ICR mass spectrometric and MP2 study indicate that polynuclear species are important because they provide molecular-scale insight into the dominant stoichiometries and structures present in the gas-phase, which is of particular interest for understanding the speciation and transport of REE chloride complexes in hydrothermal fluids with gas-like densities.