Synthesis, structure, and reactivity of crystalline molecular complexes of the {[C₅H₃(SiMe₃)₂]₃Th}¹⁻ anion containing thorium in the formal +2 oxidation state

Abstract

Reduction of the Th³⁺ complex Cp^′′₃Th, 1 [Cp′′ = C₅H₃(SiMe₃)₂], with potassium graphite in THF in the presence of 2.2.2-cryptand generates [K(2.2.2-cryptand)][Cp^′′₃Th], 2, a complex containing thorium in the formal +2 oxidation state. Reaction of 1 with KC₈ in the presence of 18-crown-6 generates the analogous Th²⁺ compound, [K(18-crown-6)(THF)₂][Cp^′′₃Th], 3. Complexes 2 and 3 form dark green solutions in THF with ε = 23 000 M⁻¹ cm⁻¹, but crystallize as dichroic dark blue/red crystals. X-ray crystallography revealed that the anions in 2 and 3 have trigonal planar coordination geometries, with 2.521 and 2.525 Å Th–(Cp′′ ring centroid) distances, respectively, equivalent to the 2.520 Å distance measured in 1. Density functional theory analysis of (Cp^′′₃Th)¹⁻ is consistent with a 6d² ground state, the first example of this transition metal electron configuration. Complex 3 reacts as a two-electron reductant with cyclooctatetraene to make Cp^′′₂Th(C₈H₈), 4, and [K(18-crown-6)]Cp′′.

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One of the fundamental characteristics of any metal is the extent to which it loses electrons to form charged species in different formal oxidation states. This ionization can occur in the gas phase to form short-lived species in a wide range of oxidation states, but the number of oxidation states available in solution in molecular metal complexes for productive chemistry is smaller. Chemists have tested the limits of oxidation states of all the elements for over 100 years and the boundaries of oxidation states accessible in solution are well established.

Nevertheless, it was recently discovered that the +2 oxidation state is accessible in soluble molecular complexes for all the elements in the lanthanide series except promethium, eqn (1).¹ Previously, it was thought that only the traditional six Ln²⁺ ions of Eu, Yb, Sm, Tm, Dy, and Nd were obtainable in solution on the basis of calculated reduction potentials² and solid state chemistry.³

Extension of this reductive chemistry to uranium was not initially tried since it is well known that the redox chemistry of uranium, which includes multiple oxidation states, +3, +4, +5, and +6, is quite different from that of the rare earths. Although it was likely that uranium would be different, an analogous synthesis was eventually attempted and the first fully characterizable U²⁺ complex, [K(2.2.2-cryptand)][Cp^′₃U] (Cp′ = C₅H₄SiMe₃), was isolated according to eqn (2).⁴

Synthesis of a Th²⁺ complex viaeqn (1) or (2) seemed even more unlikely for several reasons. Complexes of Th³⁺ are already difficult to obtain. The Th⁴⁺/Th³⁺ redox potential is estimated to be −3.0 and −3.8 V vs. NHE⁵ and a Th³⁺/Th²⁺ redox potential of −4.9 V vs. NHE is in the literature.⁶ Reduction to metallic thorium would be predicted to be favored before formation of a Th²⁺ species.⁶ Many studies have been reported to find oxidation states lower than +4 for thorium,⁷ but only five Th³⁺ complexes have ever been structurally characterized.^7k–o An analog of eqn (2) was not possible since Cp^′₃Th has not yet been synthesized. Despite these issues, thorium reduction chemistry was examined using Cp^′′₃Th [Cp′′ = C₅H₃(SiMe₃)₂-1,3],^7k prepared by Lappert et al. in 1986, and the results are described here.

Addition of potassium graphite to a dark blue solution of Cp^′′₃Th, 1, and 2.2.2-cryptand in THF immediately forms a green solution from which dichroic dark blue/red crystals of [K(2.2.2-cryptand)][Cp^′′₃Th], 2, can be isolated and crystallographically characterized, Fig. 1, eqn (3). The analogous reaction with 18-crown-6 instead of 2.2.2-cryptand as the potassium chelator provides [K(18-crown-6)(THF)₂][Cp^′′₃Th], 3, which was also crystallographically characterized [see (ESI†)], eqn (3). Elemental analysis was consistent with the structures determined crystallographically. The ¹H and ¹³C NMR spectra of 2 and 3 gave resonances in the diamagnetic region with a Me₃Si ¹H NMR resonance shifted about 0.4 ppm from that of KCp′′. A resonance was observed in the ²⁹Si NMR spectrum of 3 at −6 ppm in the region close to the −8 and −15.5 ppm signals of diamagnetic Cp^′′₃ThBr and KCp′′, respectively. Evans method measurements⁸ on both 2 and 3 and SQUID analysis⁹ at low temperature suggest the [Cp^′′₃Th]¹⁻ anion is diamagnetic. No EPR spectra were observed for 2 and 3. Decomposed samples showed the EPR spectrum of Cp^′′₃Th.^7m

Fig. 1 Molecular structure of [K(2.2.2-cryptand)][Cp^′′₃Th], 2. Thermal ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted for clarity.

The structures of the anions in 2 and 3 are very similar to the structure of Cp^′′₃Th. All three structures have a trigonal planar arrangement of the three Cp′′ rings around thorium with a sum of (ring centroid)–Th–(ring centroid) angles of 360°. The structure of 2, however, is not isomorphous with the lanthanum complex of the same formula, [K(2.2.2-cryptand)][Cp^′′₃La].^1a The average Th–(Cp′′ ring centroid) distances of 2.521 Å in 2 and 2.525 Å in 3 are equivalent to the 2.520 Å distance in Cp^′′₃Th. The negligible differences in the Th–(ring centroid) distances between the Th³⁺ precursor and the formally Th²⁺ complexes 2 and 3 are similar to the small differences between the Cp^′₃Ln and Cp^′′₃Ln Ln³⁺ complexes and the (Cp^′₃Ln)¹⁻ and (Cp^′′₃Ln)¹⁻ complexes, respectively, of all the new Ln²⁺ ions that have 4fⁿ5d¹ ground states¹ instead of the 4fⁿ⁺¹ configurations expected by reduction of a 4fⁿ Ln³⁺ ion. Similarly, the 2.521 Å U–(ring centroid) distance in the U²⁺ complex, [K(2.2.2-cryptand)][Cp^′₃U], which appears to have a 5f³6d¹ ground state, is only slightly larger than the 2.508 Å value in the U³⁺ analog, Cp^′₃U.⁴ These small changes in M–(ring centroid) distances match the small changes in radial size commonly seen in transition metal complexes,¹⁰ but contrast with the 0.10–0.20 Å differences generally seen for complexes of 4fⁿ⁺¹ Ln²⁺ complexes compared to their 4fⁿ Ln³⁺ counterparts.¹¹

The UV-Vis spectra of 2 and 3 in THF, Fig. 2, contain absorptions at 650 nm with extinction coefficients of 23 000 M⁻¹ cm⁻¹, that are significantly larger than those of Cp^′′₃Th, 5000 M⁻¹ cm⁻¹. This is similar to the larger intensities observed for the +2 complexes, [K(2.2.2-cryptand)][Cp^′₃Ln]^1c,d and [K(2.2.2-cryptand)][Cp^′₃U],⁴ compared to their +3 analogs, Cp^′₃Ln and Cp^′₃U, respectively. However, the absorptions of the Th²⁺ complexes are even more intense and the solutions look like ink.

Fig. 2 Experimental (solid lines) UV-Vis spectra in THF at 298 K for Cp^′′₃Th, 1 (green), [K(2.2.2-cryptand)][Cp^′′₃Th], 2 (red), and [K(18-crown-6)(THF)₂][Cp^′′₃Th], 3 (blue) and calculated (dotted) UV-Vis spectra of (Cp^′′₃Th)¹⁻ (blue) with theoretical extinction coefficients scaled down by a factor of 1.4.

Density functional theory (DFT) using the TPSSh functional¹² was used to examine the (Cp^′′₃Th)¹⁻ anion in 2 and 3. Calculations using scalar-relativistic effective core potentials¹³ and triple-zeta valence basis sets, def-TZVP, for thorium¹⁴ predicted trigonal planar structures for Cp^′′₃Th and (Cp^′′₃Th)¹⁻ that match the crystallographic data. The calculated Th–Cp′′(centroid) lengths of 2.538 Å for Cp^′′₃Th and 2.526 Å for (Cp^′′₃Th)¹⁻ are similar to the experimentally determined distances of 2.52 Å. It is interesting to note that the calculations for the Th³⁺ complex show a slightly longer metal ligand distance than for the Th²⁺ complex. The calculations indicate a spin-paired ground state of 6d² for (Cp^′′₃Th)¹⁻ and a 6d¹ ground state for Cp^′′₃Th; the latter is consistent with previous analyses of Cp^′′₃Th,^7g,m (C₅Me₅)₂[ⁱPrNC(Me)NⁱPr]Th⁷ⁿ and [K(DME)₂]{[C₈H₆(Si^tBuMe₂)₂]₂Th}.^7l Gas-phase studies of Th²⁺ indicate a ground state of 5f¹6d¹, but the 6d² configuration is just 63 cm⁻¹ higher and the 5f¹7s¹ is 2527 cm⁻¹ higher than the ground state.¹⁵ For (Cp₃Th)¹⁻ the triplet 5f¹6d¹ state is computed to be 9–14 kcal mol⁻¹ higher in energy than the singlet 6d² ground state.

The 6d² singlet ground state can arise in this case due to stabilization of a d_z² orbital by the trigonal ligand environment as found in DFT calculations on (Cp^′₃Ln)¹⁻ and (Cp^′₃U)¹⁻ complexes^1c,d,4 and noted earlier in the literature for tris(cyclopentadienyl) metal complexes.^7g,m,16 Indeed, both the lowest unoccupied molecular orbital (LUMO) of Cp^′′₃Th and the highest occupied molecular orbital (HOMO) of (Cp^′′₃Th)¹⁻ have d_z² character, Fig. 3. Complexes 2 and 3 provide the first examples of the 6d² configuration since stable transition metal ions are only known with the 5dⁿ configurations of the third row transition metals. The 6d² configuration is that predicted for ions like Rf²⁺ and Db³⁺.¹⁷

Fig. 3 Contour plots of (a) the LUMO of Cp^′′₃Th and (b) the HOMO of the (Cp^′′₃Th)¹⁻ anion in 3. Contour value is 0.05.

Time-dependent density functional theory was used to simulate the UV-Vis spectra for the (Cp^′′₃Th)¹⁻ anion as shown in Fig. 2 (see ESI† for a description of the predicted excitations). The maxima in the calculated spectra are lower in energy than those observed experimentally, but this is often the case with such calculations.¹⁸ Analysis of the calculated low energy peak shows that it arises from metal-to-metal transitions that have d → f and d → p character. The high energy peaks arise from metal-to-ligand charge transfer transitions similar to those found in the spectral analysis of (Cp^′₃Ln)^{1− 1b–d} and (Cp^′₃U)¹⁻.⁴ However, the d → f transitions found for (Cp^′′₃Th)¹⁻ were not apparent in the analysis of the spectra of (Cp^′₃Ln)^1b–d and (Cp^′₃U)¹⁻.⁴

The rate of decomposition of [K(18-crown-6)(THF)₂][Cp^′′₃Th], 3, at room temperature was studied by ¹H NMR spectroscopy since monitoring by UV-Vis spectroscopy is complicated by the formation of highly colored Cp^′′₃Th, as identified by X-ray crystallography.^7k The rate of decomposition of 3 is much slower than that of the U²⁺ complex, [K(2.2.2-cryptand)][Cp^′₃U], which has a half-life of 1.5 h in THF at room temperature.⁴ Complex 3 decomposed only 8% after 8 days at 298 K and a sample kept in the dark showed even less decomposition. This suggests that the formally Th²⁺ species are significantly more stable than the other newly discovered +2 ions.^1d,4

Complexes 2 and 3 were treated with H₂ to determine if a Th³⁺ hydride complex such as “[K(2.2.2-cryptand)][Cp^′′₃ThH]” would form in analogy to the complex formed by reaction of [K(2.2.2-cryptand)][Cp^′₃U] with H₂.⁴ Analogous chemistry is not observed with either H₂ or KH. Complexes 2 and 3 react in solution within minutes with 1 atm of H₂ and also over several hours at 60 psi in the solid state¹⁹ to make EPR active new crystalline complexes that appear to be bimetallic, but suitable models for the crystallographic data on the products have not been obtainable. The reactivity of 2 and 3 with H₂ contrasts with that of the Th³⁺ complex, Cp^′′₃Th, which does not react under analogous conditions.

The (Cp^′′₃Th)¹⁻ anion displays net two-electron reduction chemistry in its reaction with 1,3,5,7-cyclooctatetraene (C₈H₈). The Th⁴⁺ complex Cp^′′₂Th(C₈H₈), 4, is formed as shown in eqn (4) and was characterized by X-ray crystallography, Fig. 4. The (C₈H₈)²⁻ ring in 4, like that of (C₅Me₄H)₂U(C₈H₈),²⁰ displays considerable distortion from the normal planar geometry with several atoms 0.095 Å out of the best plane of the eight carbon atoms. This is reflected by a large range of Th–C(C₈H₈) distances: 2.736(4) to 2.841(4) Å. This 0.105 Å range is similar to the 0.123 Å range in (C₅Me₄H)₂U(C₈H₈).²⁰

Fig. 4 Molecular structure of Cp^′′₂Th(C₈H₈), 4. Thermal ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted for clarity. Th–C(C₈H₈) distances (Å): Th–C12, 2.815(4); Th–C13, 2.841(4); Th–C14, 2.769(3); Th–C15, 2.736(4).

The isolation of the formally Th²⁺ ion in (Cp^′′₃Th)¹⁻ is likely aided by the stabilization of the potassium counter-cation by the 18-crown-6 and 2.2.2-cryptand ligands. This was also observed with U²⁺ in the (Cp^′₃U)¹⁻ anion⁴ and in the (Cp^′₃Ln)¹⁻ complexes of the new Ln²⁺ ions.¹ In the absence of these potassium-stabilizing chelates, isolation of Th²⁺ appears to be more difficult as described in a 2001 paper by Lappert and co-workers on the formation of Cp^′′₃Th by Na–K reduction of Cp^′′₃ThCl.^7m In that paper, Lappert reports that treatment of Cp^′′₃ThCl with excess Na–K alloy caused the initially blue solution (presumably Cp^′′₃Th) to change to dark green. They isolated a diamagnetic green compound they postulated to be “[K(THF)_x][ThCp^′′₃] and/or ThCp^′′₂(THF)_x” but they could not characterize it or obtain reproducible analytical results. Hence, the (Cp^′′₃Th)¹⁻ anion was probably generated over 10 years ago, but could not be isolated in pure form as a simple [K(THF)_x]¹⁺ salt.

In summary, although it is difficult to obtain Th³⁺ complexes, further reduction is still possible with thorium: the +2 formal oxidation state of this metal is accessible in soluble molecular complexes. The Th²⁺ complexes provide the first examples of an isolable ion with a 6d² electron configuration, the configuration possible for fourth row transition metal congeners of Hf²⁺ or Ta³⁺. The synthesis of these complexes demonstrates the power of specific ligand fields to generate new ground states with actinides. The identification of Th²⁺ is more evidence that the oxidation state diversity for the f elements is still increasing. Stabilization of higher-lying d orbitals by the ligand field appears to be a key factor in isolating these new ions and provides a new option in expanding the oxidation state chemistry of these elements. This approach should be pursued further as attempts are made to synthesize soluble molecular complexes of +1 ions of these metals.

Acknowledgements

We thank the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Sciences of the Department of Energy (DE-SC0004739, W.J.E.) for support of the experimental studies and the U.S. National Science Foundation (CHE-1213382, F.F.) for support of the theoretical studies. We also thank Jordan F. Corbey for assistance with X-ray crystallography and K. R. Meihaus and J. R. Long for the SQUID measurements under NSF grant CHE-111190.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental and computational details; crystallographic data collection, structure solution, and refinement; and crystallographic data and complete bond distances and angles for compounds 1–4. CCDC 1018011–1018014. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4sc03033h

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