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

Structural, spectroscopic, and DFT analysis of the first molecular complexes of Th2+ indicate they have a 6d2 electron configuration of the type expected for the transactinide ions Rf2+ and Db3+.

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 Ln 2+ ions of Eu, Yb, Sm, Tm, Dy, and Nd were obtainable in solution on the basis of calculated reduction potentials 2 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 rst fully characterizable U 2+ complex, [K(2.2.2-cryptand)][Cp 0 3 U] (Cp 0 ¼ C 5 H 4 SiMe 3 ), was isolated according to eqn (2). 4 Synthesis of a Th 2+ complex via eqn (1) or (2) seemed even more unlikely for several reasons. Complexes of Th 3+ are already difficult to obtain. The Th 4+ /Th 3+ redox potential is estimated to be À3.0 and À3.8 V vs. NHE 5 and a Th 3+ /Th 2+ redox potential of À4.9 V vs. NHE is in the literature. 6 Reduction to metallic thorium would be predicted to be favored before formation of a Th 2+ species. 6 Many studies have been reported to nd oxidation states lower than +4 for thorium, 7 but only ve Th 3+ complexes have ever been structurally characterized. 7k-o An analog of eqn (2) was not possible since Cp 0 3 Th has not yet been synthesized. Despite these issues, thorium reduction chemistry was examined using Cp 00 3 Th [Cp 00 ¼ C 5 H 3 (SiMe 3 ) 2 -1,3], 7k prepared by Lappert et al. in 1986, and  3 Th], 3, which was also crystallographically characterized [see (ESI †)], eqn (3). Elemental analysis was consistent with the structures determined crystallographically. The 1 H and 13 C NMR spectra of 2 and 3 gave resonances in the diamagnetic region with a Me 3 Si 1 H NMR resonance shied about 0.4 ppm from that of KCp 00 . A resonance was observed in the 29 Si NMR spectrum of 3 at À6 ppm in the region close to the À8 and À15.5 ppm signals of diamagnetic Cp 00 3 ThBr and KCp 00 , respectively. Evans method measurements 8 on both 2 and 3 and SQUID analysis 9 at low temperature suggest the [Cp 00 3 Th] 1À anion is diamagnetic. No EPR spectra were observed for 2 and 3. Decomposed samples showed the EPR spectrum of Cp 00 3 Th. 7m (3) The structures of the anions in 2 and 3 are very similar to the structure of Cp 00 3 Th. All three structures have a trigonal planar arrangement of the three Cp 00 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 00 3 La]. 1a The average Th-(Cp 00 ring centroid) distances of 2.521Å in 2 and 2.525Å in 3 are equivalent to the 2.520Å distance in Cp 00 3 Th. The negligible differences in the Th-(ring centroid) distances between the Th 3+ precursor and the formally Th 2+ complexes 2 and 3 are similar to the small differences between the Cp 0 3 Ln and Cp 00 3 Ln Ln 3+ complexes and the (Cp 0 3 Ln) 1À and (Cp 00 3 Ln) 1À complexes, respectively, of all the new Ln 2+ ions that have 4f n 5d 1 ground states 1 instead of the 4f n+1 congurations expected by reduction of a 4f n Ln 3+ ion. Similarly, the 2.521Å U-(ring 3 U], which appears to have a 5f 3 6d 1 ground state, is only slightly larger than the 2.508Å value in the U 3+ analog, Cp 0 3 U. 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 0.10-0.20Å differences generally seen for complexes of 4f n+1 Ln 2+ complexes compared to their 4f n Ln 3+ 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 À1 cm À1 , that are signicantly larger than those of Cp 00 3 Th, 5000 M À1 cm À1 . This is similar to the larger intensities observed for the +2 complexes, 3 U], 4 compared to their +3 analogs, Cp 0 3 Ln and Cp 0 3 U, respectively. However, the absorptions of the Th 2+ complexes are even more intense and the solutions look like ink.
Density functional theory (DFT) using the TPSSh functional 12 was used to examine the (Cp 00 3 Th) 1À anion in 2 and 3. Calculations using scalar-relativistic effective core potentials 13 and triple-zeta valence basis sets, def-TZVP, for thorium 14 predicted trigonal planar structures for Cp   3 Th], 3 (blue) and calculated (dotted) UV-Vis spectra of (Cp 00 is interesting to note that the calculations for the Th 3+ complex show a slightly longer metal ligand distance than for the Th 2+ complex. The calculations indicate a spin-paired ground state of 6d 2 for (Cp 00 3 Th) 1À and a 6d 1 ground state for Cp 00 3 Th; the latter is consistent with previous analyses of Cp 00 3 Th, 7g,m (C 5 Me 5 ) 2 [ i PrNC(Me)N i Pr]Th 7n and [K(DME) 2 ]{[C 8 H 6 (Si t BuMe 2 ) 2 ] 2 Th}. 7l Gas-phase studies of Th 2+ indicate a ground state of 5f 1 6d 1 , but the 6d 2 conguration is just 63 cm À1 higher and the 5f 1 7s 1 is 2527 cm À1 higher than the ground state. 15 For (Cp 3 Th) 1À the triplet 5f 1 6d 1 state is computed to be 9-14 kcal mol À1 higher in energy than the singlet 6d 2 ground state.
The 6d 2 singlet ground state can arise in this case due to stabilization of a d z 2 orbital by the trigonal ligand environment as found in DFT calculations on (Cp 0 3 Ln) 1À and (Cp 0 3 U) 1À 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 00 3 Th and the highest occupied molecular orbital (HOMO) of (Cp 00 3 Th) 1À have d z 2 character, Fig. 3. Complexes 2 and 3 provide the rst examples of the 6d 2 conguration since stable transition metal ions are only known with the 5d n congurations of the third row transition metals. The 6d 2 conguration is that predicted for ions like Rf 2+ and Db 3+ . 17 Time-dependent density functional theory was used to simulate the UV-Vis spectra for the (Cp 00 3 Th) 1À 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 oen 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 0 3 Ln) 1À 1b-d and (Cp 0 3 U) 1À . 4 However, the d / f transitions found for (Cp 00 3 Th) 1À were not apparent in the analysis of the spectra of (Cp 0 3 Ln) 1b-d and (Cp 0 3 U) 1À . 4 The rate of decomposition of [K (18-crown-6)

00
3 Th], 3, at room temperature was studied by 1 H NMR spectroscopy since monitoring by UV-Vis spectroscopy is complicated by the formation of highly colored Cp 00 3 Th, as identied by X-ray crystallography. 7k The rate of decomposition of 3 is much slower than that of the U 2+ complex, [K(2.2.2-cryptand)][Cp 0 3 U], which has a half-life of 1.5 h in THF at room temperature. 4 Complex 3 decomposed only 8% aer 8 days at 298 K and a sample kept in the dark showed even less decomposition. This suggests that the formally Th 2+ species are signicantly more stable than the other newly discovered +2 ions. 1d, 4 Complexes 2 and 3 were treated with H 2 to determine if a Th 3+ hydride complex such as "[K (2.2.2-cryptand) 3 U] with H 2 . 4 Analogous chemistry is not observed with either H 2 or KH. Complexes 2 and 3 react in solution within minutes with 1 atm of H 2 and also over several hours at 60 psi in the solid state 19 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 H 2 contrasts with that of the Th 3+ complex, Cp 00 3 Th, which does not react under analogous conditions. The (Cp 00 3 Th) 1À anion displays net two-electron reduction chemistry in its reaction with 1,3,5,7-cyclooctatetraene (C 8 H 8 ). The Th 4+ complex Cp 00 2 Th(C 8 H 8 ), 4, is formed as shown in eqn (4) and was characterized by X-ray crystallography, Fig. 4. The (C 8 H 8 ) 2À ring in 4, like that of (C 5 Me 4 H) 2 U(C 8 H 8 ), 20 displays considerable distortion from the normal planar geometry with several atoms 0.095Å out of the best plane of the eight carbon atoms. This is reected by a large range of Th-C(C 8 H 8 ) distances: 2.736(4) to 2.841(4)Å. This 0.105Å range is similar to the 0.123Å range in (C 5 Me 4 H) 2 U(C 8 H 8 ). 20 (4) The isolation of the formally Th 2+ ion in (Cp 00 3 Th) 1À 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 U 2+ in the (Cp 0 3 U) 1À anion 4 and in the (Cp 0 3 Ln) 1À complexes of the new Ln 2+ ions. 1 In the absence of these  potassium-stabilizing chelates, isolation of Th 2+ appears to be more difficult as described in a 2001 paper by Lappert and coworkers on the formation of Cp 00 3 Th by Na-K reduction of Cp 00 3 ThCl. 7m In that paper, Lappert reports that treatment of Cp 00 3 ThCl with excess Na-K alloy caused the initially blue solution (presumably Cp 00 3 Th) to change to dark green. They isolated a diamagnetic green compound they postulated to be "[K(THF) x ][ThCp 00 3 ] and/or ThCp 00 2 (THF) x " but they could not characterize it or obtain reproducible analytical results. Hence, the (Cp 00 3 Th) 1À 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 Th 3+ complexes, further reduction is still possible with thorium: the +2 formal oxidation state of this metal is accessible in soluble molecular complexes. The Th 2+ complexes provide the rst examples of an isolable ion with a 6d 2 electron conguration, the conguration possible for fourth row transition metal congeners of Hf 2+ or Ta 3+ . The synthesis of these complexes demonstrates the power of specic ligand elds to generate new ground states with actinides. The identication of Th 2+ is more evidence that the oxidation state diversity for the f elements is still increasing. Stabilization of higher-lying d orbitals by the ligand eld 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.