Periodic Behavior of Lanthanide Coordination within Reverse Micelles

Abstract

Trends in lanthanide(III) (Ln(III)) coordination were investigated within nanoconfined solvation environments. Ln(III) ions were incorporated into the cores of reverse micelles (RMs) formed with malonamide amphiphiles in n-heptane by contact with aqueous phases containing nitrate and Ln(III); both insert into pre-organized RM units built up of DMDOHEMA (N,N'-dimethyl-N,N'-dioctylhexylethoxymalonamide) that are either relatively large and hydrated or small and dry, depending on whether the organic phase is acidic or neutral, respectively. Structural aspects of the Ln(III) complex formation and the RM morphology were obtained by use of XAS (X-ray absorption spectroscopy) and SAXS (small-angle X-ray scattering). The Ln(III) coordination environments were determined through use of L(3)-edge XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure), which provide metrical insights into the chemistry across the period. Hydration numbers for the Eu species were measured using TRLIFS (time-resolved laser-induced fluorescence spectroscopy). The picture that emerges from a system-wide perspective of the Ln-O interatomic distances and number of coordinating oxygen atoms for the extracted complexes of Ln(III) in the first half of the series (i.e., Nd, Eu) is that they are different from those in the second half of the series (i.e., Tb, Yb): the number of coordinating oxygen atoms decrease from 9O for early lanthanides to 8O for the late ones--a trend that is consistent with the effect of the lanthanide contraction. The environment within the RM, altered by either the presence or absence of acid, also had a pronounced influence on the nitrate coordination mode; for example, the larger, more hydrated, acidic RM core favors monodentate coordination, whereas the small, dry, neutral core favors bidentate coordination to Ln(III). These findings show that the coordination chemistry of lanthanides within nanoconfined environments is neither equivalent to the solid nor bulk solution behaviors. Herein we address atomic- and mesoscale phenomena in the under-explored field of lanthanide coordination and periodic behavior within RMs, providing a consilience of fundamental insights into the chemistry of growing importance in technologies as diverse as nanosynthesis and separations science.

Polynuclear Speciation of Trivalent Cations near the Surface of an Electrolyte Solution

“…51 , in order, 6, 7, 8, and 9). 52 In this light, the experimental result is consistent with an 8 O coordination environment of Eu. Insofar as EXAFS-determined interatomic distances are more accurately determined than coordination numbers, the experimental bond length is a strong argument for an O CN of 8.…”

Depletion of water-in-oil aggregates from poor solvents: Transition from weak aggregates towards reverse micelles

“…This has been evidenced for instance, when sodium is replaced by the trivalent aluminium for anionic extractants having flexible branched diethyl-hexyl chains [38]. This has also been observed recently in the case of trivalent lanthanides [39], as shown in Fig. 5: reverse aggregates are globular while reverse micelles containing Terbium(III) are present as cylindrical aggregates.…”

“…SAXS data of organic phases: 0.5 M DMDOHEMA in n-heptane after contact with 3 M HNO 3 containing no lanthanide (red open squares) and 0.01 M Tb(III) (red solid triangles); and after contact with 3 M LiNO 3 containing no lanthanide (blue open squares) and 0.01 M Tb(III) (blue solid triangles)[39].…”

K-edge XANES investigation of octakis(DMSO)lanthanoid(iii) complexes in DMSO solution and solid iodides

“…While the EXAFS technique has been widely employed in the past to gain structural information on ion complexes in solution, 3-24 the situation is different concerning the XANES region. This part is very sensitive to the structural details of the absorbing site and, in principle, an almost complete recovery of the three dimensional structure can be achieved from the experimental data.…”