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Structural and spectroscopic characterization of an einsteinium complex


The transplutonium elements (atomic numbers 95–103) are a group of metals that lie at the edge of the periodic table. As a result, the patterns and trends used to predict and control the physics and chemistry for transition metals, main-group elements and lanthanides are less applicable to transplutonium elements. Furthermore, understanding the properties of these heavy elements has been restricted by their scarcity and radioactivity. This is especially true for einsteinium (Es), the heaviest element on the periodic table that can currently be generated in quantities sufficient to enable classical macroscale studies1. Here we characterize a coordination complex of einsteinium, using less than 200 nanograms of 254Es (with half-life of 275.7(5) days), with an organic hydroxypyridinone-based chelating ligand. X-ray absorption spectroscopic and structural studies are used to determine the energy of the L3-edge and a bond distance of einsteinium. Photophysical measurements show antenna sensitization of EsIII luminescence; they also reveal a hypsochromic shift on metal complexation, which had not previously been observed in lower-atomic-number actinide elements. These findings are indicative of an intermediate spin–orbit coupling scheme in which jj coupling (whereby single-electron orbital angular momentum and spin are first coupled to form a total angular momentum, j) prevails over Russell–Saunders coupling. Together with previous actinide complexation studies2, our results highlight the need to continue studying the unusual behaviour of the actinide elements, especially those that are scarce and short-lived.

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Fig. 1: The organic ligand 3,4,3-LI(1,2-HOPO) forms an EsIII coordination complex, enabling structural and spectroscopic studies.
Fig. 2: The structural features of the [EsIII(HOPO)] coordination complex were probed using element-specific L3-edge XAS at 77 K.
Fig. 3: Luminescence sensitization of Es was achieved on chelation by HOPO, confirming the +3 oxidation state for Es and revealing an unusual hypsochromic shift in metal emission.

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254Es was supplied by the Isotope Program within the US Department of Energy (DOE), Office of Science, Office of Nuclear Physics. We thank N. Edelstein for discussions, M. Fox for γ spectrometer calibration, and N. Singh, B. Fairchild, S. Hays and R. Davis for assistance in planning and implementing experiments at the Stanford Synchrotron Radiation Lightsource (SSRL) and at the Molecular Foundry. This work was supported by the DOE, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division at Lawrence Berkeley National Laboratory, under contract number DE-AC02-05CH1123, and at Los Alamos National Laboratory (LANL), an affirmative-action/equal-opportunity employer, managed by Triad National Security, LLC, for the NNSA of the DOE (contract number 89233218CNA000001). K.M.S. acknowledges support from a DOE Integrated University Program graduate research fellowship. Z.R.J. was supported by the Glenn T. Seaborg Institute at LANL. K.E.K. and J.N.W. were supported by the DOE, Office of Science, Office of Basic Energy Sciences, Early Career Research Program under award DE-SC0019190. J.N.W. was also supported by the DOE, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program, which is administered by the Oak Ridge Institute for Science and Education (ORISE) and managed by ORAU (contract number DE-SC0014664) for the DOE. Use of the SSRL, SLAC National Accelerator Laboratory, is supported by the DOE, Office of Science, Office of Basic Energy Sciences under contract number DE-AC02-76SF00515. Near-infrared luminescence spectra were collected at the Molecular Foundry, a User Facility supported by the DOE, Office of Science, Office of Basic Energy Sciences under contract number DE-AC02-05CH1123.

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Authors and Affiliations



K.P.C., K.M.S., S.A.K. and R.J.A. conceived the study and designed the experiments. K.P.C., K.M.S. and S.A.K. prepared the XAS sample. S.A.K., Z.R.J. and J.N.W. collected XAS data. K.F.S., L.M.M. and C.H.B. analysed XAS results. K.P.C. and K.M.S. prepared the photoluminescence sample. L.A.-S., K.P.C. and T.M.M. collected luminescence data and interpreted the results. All authors discussed the experimental results and contributed to the manuscript.

Corresponding authors

Correspondence to Stosh A. Kozimor, Corwin H. Booth or Rebecca J. Abergel.

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The authors declare no competing interests.

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Peer review information Nature thanks Louise Natrajan and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Comparison of XANES data for [AnIII(HOPO)] complexes.

AmIII, CmIII and CfIII spectra were reported previously2 and are compared to the EsIII data reported here, when plotted as a function of ΔE, the difference between the photon energy E and the peak in the first derivative of the data E0.

Extended Data Fig. 2 Raw EXAFS data for [EsIII(HOPO)].

Data are shown as in Fig. 2b, but extended beyond the range used in the fit. The data show the effect of a wide monochromator glitch near 7 Å−1 that limited the data range.

Extended Data Fig. 3 M–O bond distance R versus kmax for [MIII(HOPO)] actinide complexes.

These [MIII(HOPO)] actinide complexes were characterized previously via EXAFS spectroscopy2. Note the difference in the fit model from ref. 2 (Methods). The worsening of the fits below 6.5 Å−1 is at least partially related to the loss of fit degrees of freedom. For instance, the number of degrees of freedom decreases from 3.3 to 1.2 from kmax = 6.5 Å−1 to kmax = 5.5 Å−1. The data for AmIII and CmIII are of higher quality because there was more material available. Samples masses are 27.1 μg, 10.9 μg and 3.3 μg for AmIII, CmIII and CfIII data, respectively. Reported one-standard-deviation errors are obtained from a previously described profiling method28.

Extended Data Fig. 4 Comparison of the first eight Es L3-edge XANES scans collected for [EsIII(HOPO)] at 77 K.

The oldest scan is shown at the top and the newest at the bottom, as indicated by the black arrow. Each scan required about 40 min. Scans are offset for clarity.

Extended Data Fig. 5 Comparison of the averaged Es L3-edge XANES scans collected for [EsIII(HOPO)] at 77 K.

The oldest averaged scan is shown at the top and the newest at the bottom, as indicated by the black arrow. Each averaged scan is taken from 10 individual scans and represents nearly 4 h of data acquisition time. Scans are offset for clarity.

Extended Data Fig. 6 Normalized excitation spectrum of [EsIII(HOPO)] in aqueous solution.

Spectrum collected on monitoring at EsIII emission maximum (1,005 nm).

Extended Data Fig. 7 Schematics of the 3D-printed sample holder used for XAS measurements.

a, b, Top views of the sample holder. c, Computer-aided design (CAD) file for the sample holder.

Extended Data Fig. 8 Comparison of backscattering line shapes and phases for calculated [AnIII(HOPO)] complexes.

Backscattering line shapes and phases were calculated using FEFF9.6 33 (Methods). The line shapes of the An–O pairs are shown for FEFF9.6 calculations using the same structure and varying only the An species to demonstrate the lack of change with species. These amplitudes are not k3-weighted, as in Fig. 2 and Extended Data Fig. 2, so there is no decrease above 4 Å−1 in the line shapes used for fitting, but rather a change in slope.

Extended Data Fig. 9 Comparison of calculated XANES spectra for [AnIII(HOPO)] complexes.

Spectra were generated from FEFF9.6 calculations using a self-consistent field cluster of 6 Å and a full-multiple scattering cluster of 4 Å. Calculations on actinides generally overestimate the amount of 5f charge transfer when included in the valence orbitals, so these orbitals are treated as core orbitals here. The core–hole lifetime broadening used by the code increases from 8.7 eV to 10.3 eV from PuIII to EsIII, the effect of which is visible in the increased broadening of the spectra.

Extended Data Fig. 10 Comparison of local density of states (LDOS) for the d orbitals.

The LDOS is determined from the calculations in Extended Data Fig. 9. The spectra are plotted as a function of ΔE, the difference between the photon energy E and the vacuum energy as calculated by FEFF9.6. The Fermi energy in all these calculations is about −7.6 eV, above which the states are unoccupied and therefore accessible to XANES. The calculations clearly show the 6d splitting between −5 eV and 0 eV. The splitting decreases by about 0.6 eV from PuIII to EsIII, in contrast to the increase in core–hole lifetime broadening. These effects are washed out in the final calculation in Extended Data Fig. 9 by the much larger core–hole lifetime broadening.

Extended Data Table 1 Summary of [EsIII(HOPO)] XAS features
Extended Data Table 2 Summary of EXAFS fits for [EsIII(HOPO)]

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Carter, K.P., Shield, K.M., Smith, K.F. et al. Structural and spectroscopic characterization of an einsteinium complex. Nature 590, 85–88 (2021).

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