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Isolation and characterization of a californium metallocene

Abstract

Californium (Cf) is currently the heaviest element accessible above microgram quantities. Cf isotopes impose severe experimental challenges due to their scarcity and radiological hazards. Consequently, chemical secrets ranging from the accessibility of 5f/6d valence orbitals to engage in bonding, the role of spin–orbit coupling in electronic structure, and reactivity patterns compared to other f elements, remain locked. Organometallic molecules were foundational in elucidating periodicity and bonding trends across the periodic table1,2,3, with a twenty-first-century renaissance of organometallic thorium (Th) through plutonium (Pu) chemistry4,5,6,7,8,9,10,11,12, and to a smaller extent americium (Am)13, transforming chemical understanding. Yet, analogous curium (Cm) to Cf chemistry has lain dormant since the 1970s. Here, we revive air-/moisture-sensitive Cf chemistry through the synthesis and characterization of [Cf(C5Me4H)2Cl2K(OEt2)]n from two milligrams of 249Cf. This bent metallocene motif, not previously structurally authenticated beyond uranium (U)14,15, contains the first crystallographically characterized Cf–C bond. Analysis suggests the Cf–C bond is largely ionic with a small covalent contribution. Lowered Cf 5f orbital energy versus dysprosium (Dy) 4f in the colourless, isoelectronic and isostructural [Dy(C5Me4H)2Cl2K(OEt2)]n results in an orange Cf compound, contrasting with the light-green colour typically associated with Cf compounds16,17,18,19,20,21,22.

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Fig. 1: Synthesis and solid-state structure of 2-Cf.
Fig. 2: UV–vis–NIR spectra of 2-Cf.
Fig. 3: Experimental solid-state and SO-NEVPT2 calculated absorption spectra of 2-Cf averaged for both cis and trans geometries.

Data availability

The data that support the findings of this study are available within the paper and its Supplementary Information files. Cambridge Crystallographic Data Centre (CCDC) deposition numbers are 2025245 (1-241Am), 2058952 (1-Ho), 2025247 (2-Cf), 2025249 (2-Gdα), 2092534 (2-Gdβ), 2025248 (2-Dy), 2063323 (3) and 2025250 (4). These data can be obtained free of charge from the CCDC via www.ccdc.cam.ac.uk/data_request/cif.

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Acknowledgements

We thank I. May at Los Alamos National Laboratory (LANL) and G. P. Horne at Idaho National Laboratory for discussions. We acknowledge the US Department of Energy (DOE), Office of Science (SC), Basic Energy Sciences (BES) Heavy Element Chemistry Program (HEC) at LANL (E.R.B., A.J.G., S.A.K., Z.R.J., B.L.S., F.D.W., P.Y., J.S.; DE-AC52-06NA25396), at Florida State University (T.E.A.-S., A.N.G., J.M.S., C.J.W.; DE-FG02-13ER16414), and at the University of California at Irvine (W.J.E., T.F.J., J.C.W.; DE-SC0004739). C.A.P.G. was sponsored by a Distinguished J. R. Oppenheimer Postdoctoral Fellowship (LANL-LDRD, 20180703PRD1). We thank LANL-LDRD (L.M.S., A.J.G.; 20190091ER) for aspects of the 241Am experimental work. Theoretical research was performed using EMSL (grid.436923.9), a DOE-SC User Facility sponsored by the Office of Biological and Environmental Research. J.S. also thanks the National Natural Science Foundation of China (22076130), and the Fundamental Research Funds for the Central Universities (20826041D4117) and start-up funds from Sichuan University. J.N.C. and J.D.A. acknowledge the DOE-SC, Isotope Development and Production for Research and Application subprogram within the Office of Nuclear Physics for 241Am sample conditioning and dispensing. J.D.A. thanks the NNSA Plutonium Sustainment Program and the National Nuclear Security Administration (NNSA) Material Recycle and Recovery for funding. We thank E. R. Birnbaum and B. L. Ortiz for procurement of 241Am through the National Isotope Development Center, purchased with LANL DOE-SC-BES-HEC funds.

Author information

Authors and Affiliations

Authors

Contributions

E.R.B., W.J.E., A.J.G., S.A.K. and P.Y. devised the project and are corresponding authors. C.A.P.G. led lanthanide synthetic protocol development and performed Cf and Am synthetic work and characterization with oversight and help from A.J.G., along with expertise and support from L.M.S. and F.D.W. (UV–vis–NIR), M.T.J. (nuclear magnetic resonance (NMR) spectroscopy), T.F.J. (ligand preparation), B.L.S. (single-crystal XRD) and S.F.B. (radiological safety). S.A.K., N.H.A. and Z.R.J. prepared the LANL 249Cf stock solution; A.N.G., J.M.S. and C.J.W. prepared the FSU 249Cf stock solution; L.M.S., A.J.G., M.R.J. and J.N.C. prepared and characterized the 241Am stock at LANL. J.C.W. provided supporting characterization of lanthanide compounds. J.S., E.R.B. and P.Y. performed and analysed all theoretical work. Principal manuscript writing was by C.A.P.G., J.S., A.J.G., S.A.K., W.J.E., E.R.B. and P.Y., with input and editing from all authors. A.J.G. was the submitting author.

Corresponding authors

Correspondence to Enrique R. Batista, William J. Evans, Andrew J. Gaunt, Stosh A. Kozimor or Ping Yang.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature thanks Weiqun Shi 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 Outline of the complexes synthesized in this work.

a, The known 1-M (M = Sm, Gd) complexes were chosen for their ionic radii similarities to Cf3+ (Sm, 0.958 Å; Gd, 0.938 Å; Cf, 0.95 Å). Complex 1-241Am was synthesized following prior methodology for 1-243Am to assess radiolytic effects. b, Complex 1-Dy was synthesized as Dy3+ (4f9) is valence isoelectronic to Cf3+ (5f9), and 1-Ho was synthesized as the ionic radius of Ho3+ (0.901 Å) is significantly smaller than that of Cf3+. c, Complexes 2-M (M = Gd, Dy) were synthesized as comparisons to 2-Cf and chosen for the metal having a similar ionic radius to Cf3+ (Gd) or for being valence isoelectronic (Dy). d, Eu3+ (0.947 Å) is a better ionic radius match to Cf3+ than Sm or Gd; however, we were unable to isolate 1-Eu using salt-metathesis protocols which instead led to reduction to a Eu2+ complex, 3.

Extended Data Fig. 2 Geometric features of the ‘{Cf(Cptet)2Cl2}1−’ anions and polymeric chain propagation.

a, View from the top of the metallocene fragment for Cf(1) showing the trans disposition of the Cptet C–H groups and the Cl–Cf–Cl bond angle. b, View from the top of the metallocene fragment for Cf(2) showing the cis disposition of the Cptet C–H groups and the Cl–Cf–Cl bond angle. c, d, View down the crystallographic c axis, showing that the Cf(1) and Cf(2) chains propagate parallel to the a axis—note that the arrangement of the panels is not indicative of their relative spatial arrangement.

Extended Data Fig. 3 Example bonding orbitals from 2-Cf and 2-Dy.

The lowest occupied valence f orbitals of a″ symmetry of 2-Cf (left, 27a″) and 2-Dy (right, 22a″) (contour value of 0.03 a.u.; see Supplementary Table 19). This shows the greater mixing between metal 5f and C 2p orbitals in 2-Cf compared to 4f and C 2p orbitals in 2-Dy.

Extended Data Table 1 Mulliken charge (q) on metal atom, (Cptet)2 fragment and (Cl2K) fragment in the [M(Cptet)2Cl2K] (M = Cf, Dy) compounds of Cs symmetry from DFT/PBE0 calculations
Extended Data Table 2 Bond distances and bond orders in [M(Cptet)2Cl2K] (M = Cf, Dy) complexes of Cs symmetry from DFT/PBE0 calculationsa
Extended Data Table 3 QTAIM analysis on [M(Cptet)2Cl2K] (M = Cf, Dy) complexes of Cs symmetry from DFT/PBE0 calculations, and previously reported data for [An(Cp)3] (An = Th − Cm)

Supplementary information

Supplementary Information

This file includes extensive experimental details on compounds synthesized in this work, photographs of select reactions and products, crystallographic data collection descriptions and comments on radiation-induced changes in crystals of 2-Cf, visual representations of molecular structures, nuclear magnetic resonance, UV–vis–NIR, FT-IR spectra, and full computational details.

Peer Review File

Supplementary Data

Crystallographic information files for compounds 1-241Am, [241Am(Cptet)3]; 1-Ho, [Ho(Cptet)3]; 2-Cf, [Cf(Cptet)2Cl2K(OEt2)]n; 2-Gdα, α-[Gd(Cptet)2Cl2K(OEt2)]n; 2-Gdβ, β-[Gd(Cptet)2Cl2K(OEt2)]n; 2-Dy, [Dy(Cptet)2Cl2K(OEt2)]n; 3, [Eu(Cptet)2(THF)2]; and 4, [K(Et2O)(Cptet)]n.

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Goodwin, C.A.P., Su, J., Stevens, L.M. et al. Isolation and characterization of a californium metallocene. Nature 599, 421–424 (2021). https://doi.org/10.1038/s41586-021-04027-8

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