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A crystalline tri-thorium cluster with σ-aromatic metal–metal bonding


Matters Arising to this article was published on 23 March 2022


Metal–metal bonding is a widely studied area of chemistry1,2,3, and has become a mature field spanning numerous d transition metal and main group complexes4,5,6,7. By contrast, actinide–actinide bonding, which is predicted to be weak8, is currently restricted to spectroscopically detected gas-phase U2 and Th2 (refs. 9,10), U2H2 and U2H4 in frozen matrices at 6–7 K (refs. 11,12), or fullerene-encapsulated U2 (ref. 13). Furthermore, attempts to prepare thorium–thorium bonds in frozen matrices have produced only ThHn (n = 1–4)14. Thus, there are no isolable actinide–actinide bonds under normal conditions. Computational investigations have explored the probable nature of actinide–actinide bonding15, concentrating on localized σ-, π-, and δ-bonding models paralleling d transition metal analogues, but predictions in relativistic regimes are challenging and have remained experimentally unverified. Here, we report thorium–thorium bonding in a crystalline cluster, prepared and isolated under normal experimental conditions. The cluster exhibits a diamagnetic, closed-shell singlet ground state with a valence-delocalized three-centre-two-electron σ-aromatic bond16,17 that is counter to the focus of previous theoretical predictions. The experimental discovery of actinide σ-aromatic bonding adds to main group and d transition metal analogues, extending delocalized σ-aromatic bonding to the heaviest elements in the periodic table and to principal quantum number six, and constitutes a new approach to elaborate actinide–actinide bonding.

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Fig. 1: Synthesis and structure of 3.
Fig. 2: Characterization data for 3.

Data availability

X-ray data are available free of charge from the Cambridge Crystallographic Data Centre under reference 2061981. Methods (general considerations, starting materials, experimental, crystallographic, spectroscopic, magnetic, computational data and refs. 36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88), Extended Data Figs. 19, Extended Data Tables 1,2 and Supplementary Tables 17) can be found online. All other data are available from S.T.L. on reasonable request.


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We gratefully acknowledge funding and support from the UK Engineering and Physical Sciences Research Council (grants EP/K024000/1, EP/M027015/1, EP/P001386/1, EP/S033181/1 and EP/T011289/1), Natural Environment Research Council (grant NE/R011230/1) European Research Council (grant CoG612724), Royal Society (grant UF110005), Deutsche Forschungsgemeinschaft (SL104/10-1), the Landesgraduiertenförderung of the State of Baden-Württemberg and The University of Manchester (including computational resources and associated support services of the Computational Shared Facility). We also thank M. Jennings (Micro Analytical Laboratory, University of Manchester) for performing elemental microanalyses. S.T.L. thanks the Alexander von Humboldt Foundation for a Friedrich Wilhelm Bessel Research Award.

Author information

Authors and Affiliations



J.T.B. prepared and characterized the complexes. J.A.S., D.H. and J.v.S. recorded and interpreted the magnetic and EPR data. A.W.W. and L.N. collected and analysed the solid-state UV–Vis data. A.J.W. collected, solved and refined all of the crystallographic data. N.K. performed and analysed the calculations. S.T.L. assisted with data analysis and directed the research. J.T.B., N.K. and S.T.L. wrote the manuscript with input from all of the authors.

Corresponding authors

Correspondence to Nikolas Kaltsoyannis or Stephen T. Liddle.

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The authors declare no competing interests.

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

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Extended data figures and tables

Extended Data Fig. 1 Spectroscopic and structural data for 3.

a, 1H NMR spectrum in C6D6 of a crude reaction mixture producing 3 showing unreacted 1 and the {C4(SiMe3)4} by-product. b, Polymeric structure of 3 at 150 K with 30% probability ellipsoids. H-atoms and disorder components omitted for clarity. Key: Th, green; K, dark blue; Cl, violet-red; O, red; C, grey. c, ATR-IR spectrum of 3 prepared in benzene. d, ATR-IR spectrum of 3 prepared in D6-benzene. e, 1H NMR spectrum of 3 in C6D6 after treatment with CCl4. Resonances at 2.15 and ~7.00-7.40 ppm correspond to residual toluene. f, 13C{1H} NMR spectrum of 3 in C6D6 after treatment with CCl4.

Extended Data Fig. 2 The two principal, colour-determining absorptions for 3".

Both transitions originate from the HOMO (MO 199) orbital. Hydrogen atoms are omitted for clarity.

Extended Data Fig. 3 Solid-state UV–Vis spectra of 3.

a, comparison of experimental spectrum (black) to the TD-DFT predicted spectrum presented as oscillator strengths (vertical red lines). b, comparison of the experimental spectrum before (black) and after (red) being allowed to oxidise in air.

Extended Data Fig. 4 NICSzz of 3" evaluated at intervals of R = 0.1 Å.

R is the perpendicular distance from the centre of the Th3 ring and the plane of the Cl ligands is at R = ±1.8 Å.

Extended Data Fig. 5 Vibrations of 3" with Th-Th character.

a, at 70.8 cm−1. b, at 71.0 cm−1. c, at 77.2 cm−1. d, at 107.4 cm−1. H-atoms are omitted for clarity.

Extended Data Fig. 6 Magnetic and EPR Data for 3.

a, Raw magnetic moment recorded by variable-temperature SQUID magnetometry on a flame sealed borosilicate tube containing a sample of 3 in a 1T field with the data for a blank tube subtracted. b, Molar paramagnetic susceptibility χ per Th3 unit recorded by variable-temperature SQUID magnetometry (black symbols). Expected curves for a d1 (S = ½) system with g = 2 (blue,) for a non-correlated d1-d1 (2 × S = ½) system, also with g = 2 (red), and for a triplet d1-d1 (S = 1) system (green). c, X-Band EPR spectra recorded on powdered 3 in a flame-sealed quartz tube at different temperatures as indicated. d, Room temperature X-Band EPR spectra recorded on a loose powder of 3 in a flame-sealed quartz tube. Simulation of the EPR spectrum assuming two species in a 20:80 ratio with principal g-tensor values of gx1 = 1.9965, gy1 = 2.0036, gz1 = 2.010, and gx2 = 1.9375, gy2 = 1.9762, gz2 = 1.9747, respectively.

Extended Data Fig. 7 NMR spectroscopic data for the treatment of thorocene and 2 with CO2.

a, 1H NMR spectrum in C6D6 of thorocene after treatment with excess CO2, recorded after 2 h. b, 1H NMR spectrum in C6D6 of 2 after treatment with excess CO2, recorded after 2 h.

Extended Data Fig. 8 NMR and IR spectroscopic data for the reaction of 3 with CO2.

a, 1H NMR spectrum in C6D6 of the mother liquor after 3 is treated with excess CO2. Trace toluene resonances are from the preparation of 3. b, 13C{1H} NMR spectrum in C6D6 of the mother liquor after 3 is treated with excess CO2. The use of an excess of CO2 is reflected by the small resonance for CO2. c, IR spectrum of the product of the reaction of 3 with excess CO2 with key absorptions at 1540 and 1371 cm−1 indicative of carbonate and not oxalate formation.

Extended Data Fig. 9 NMR spectroscopic data for the reaction of 3 with C8H8.

a, 1H NMR spectrum in C6D6 of the mother liquor after sonicating and heating 3 with C8H8. b, 13C{1H} NMR spectrum in C6D6 of the mother liquor after sonicating and heating 3 with C8H8.

Extended Data Table 1 Key Computed Metrical Data (Å) for Alternative Model Formulations of 3a
Extended Data Table 2 Calculated adsorptions for 3 with Oscillator Strength f Above 0.01 (Singlet-Singlet Transitions from Ground State)

Supplementary information

Supplementary Table 1

Final single point energy and coordinates for geometry optimized 3'.

Supplementary Table 2

Final single point energy and coordinates for geometry optimized 3"

Supplementary Table 3

Final single point energy and coordinates for geometry optimized [{Th(η8-C8H8)(μ3-Cl)2}3K2]2+.

Supplementary Table 4

Final single point energy and coordinates for geometry optimized [{Th(η8-C8H8)(μ3-Cl)2}3].

Supplementary Table 5

Final single point energy and coordinates for geometry optimized [{Th(η8-C8H8)(μ3-Cl)2}3H].

Supplementary Table 6

Final single point energy and coordinates for geometry optimized [{Th(η8-C8H8)(μ3-Cl)2}3K2H]+.

Supplementary Table 7

Final single point energy and coordinates for geometry optimized [{Th(η8-C8H8)(μ3-Cl)2}3K2H].

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Boronski, J.T., Seed, J.A., Hunger, D. et al. A crystalline tri-thorium cluster with σ-aromatic metal–metal bonding. Nature 598, 72–75 (2021).

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