Heterobimetallic uranyl(VI) alkoxides of lanthanoids: formation through simple ligand exchange

Abstract

Lanthanoid and actinoid silylamides are versatile starting materials. Herein we show how a simple ligand exchange with tert-butanol leads to the formation of the first trimeric heterobimetallic uranyl(VI)–lanthanoid(III) alkoxide complexes. The μ³ coordination of the endogenous uranyl oxo atom results in a significant elongation of the bond length and a significant deviation from the linear uranyl arrangement.

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Although the chemistry of high-valent uranyl (O=U=O²⁺) accounts for 55% of all U structures reported in the CSD, not much attention has been paid to its alkoxide chemistry over the last 30 years. Only a few uranyl alkoxide compounds were structurally characterised in the 1950s by Bradley et al. and in the 1980s by Sattelberger et al.^1,2 In contrast, far more alkoxide structures of the lower-valent uranium (U(IV)–U(V))^3–6 species have been reported, because the preparation of uranyl alkoxides is accompanied by synthetic difficulties, e.g. solvolytic disproportionations, in which the cleavage of an oxo-unit takes place, while a second oxo-component is cleaved thermally to result in the formation of [U(OR)₆] and [U₂O₅(OR)₂(HOR)₂].¹ During the 1980s, coordinatively unsaturated alkoxides were reported to undergo ligand redistribution, resulting in oxo-alkoxide clusters.² Mononuclear uranyl alkoxides could only be stabilised by coordinating sterically demanding ligands like Ph₃PO by Burns et al. in 1992 and aryloxides by Barnhardt et al. in 1995.^7,8 Even though uranyl-lanthanoid assemblies have been investigated in different systems,^9–12 the heterobimetallic alkoxides have been rarely investigated, and, to the best of our knowledge, only few complexes such as [Li(THF)₂U(O^tBu)₆], [Li(Et₂O)U(O^tBu)₆] and [KU₂(O^tBu)₉] have been reported, where alkali metals are bridged via O^tBu ligands to U(IV) or U(V) centres and none of them contain uranyl (O=U=O²⁺) units.^13,14 The compound [Zr₂(OⁱPr)₉U(C₈H₈)] prepared from [Zr₂(OⁱPr)₉]UI₂(THF) by Evans in 2001 is the only known heterobimetallic alkoxide that contains U(III) and a transition metal.¹⁵ A CSD search on uranium bridged to lanthanoids via oxygen revealed only 19 examples, but none of these structures can be classified as an alkoxide. Most of them are reduced uranyl oxo-bridged complexes. In 2011 Arnold et al. first reported the reduction of a uranyl(VI) polypyrrolic Schiff-base macrocyclic complex [UO₂(THF)(H₂L)] by Sm(II) silylamide [SmN′′₂(THF)₂] (N′′ = N{Si(CH₃)₃}₂) or by sterically induced reduction with YN′′₃ resulting in [{UO₂M(py)₂(L)}₂] (M = Y, Sm) (L = “Pacman” type calix pyrrole macrocycle). The series was later extended to M = Sc, Ce, Sm, Eu, Gd, Dy, Er, Yb and Lu in 2013.^16,17 Additionally, they were also able to isolate a small series of linear, oxo-bridged trinuclear compounds e.g. [{UO₂(py)₅}₂(LnI₄)]I (Ln = Sm, Dy) containing a uranyl(V) by reduction of [UO₂Cl₂(THF)₂] with Ln(II) (Ln = Sm, Dy) halides in 2017.¹⁸ The only other known compounds are phosphate- or mellitate-bridged clusters.^19,20

Particularly important in this context is also the activation of the uranyl(VI) dication, which, due it its O=U=O bond strength with a notional bond order of three and a mean U–O bond dissociation enthalpy of 604 kJ mol⁻¹ is one of the most stable oxo-cations.^21,22 Over the last 15 years the reduction chemistry of uranyl(VI) has seen a renaissance, and particularly the coordination of hetero metal atoms to the uranyl oxo atoms has resulted in a variety of reduced uranium complexes that show catalytic activity,²³ cation-cation interactions (CCIs),²⁴ thermally, photochemically or chemically reduced uranyl(V) centres and significantly elongated U–O bonds and paramagnetism attributable to the 5f¹ state.^25–28 Many of these complexes have been stabilised with auxiliary ligands, such as macrocycles, bipyridine/phenantroline-type²⁹ or dipyrrin-type³⁰ ligands but also with just coordinating solvent molecules.³¹

We have revisited this chemistry herein, and present the first heterobimetallic uranyl-lanthanoid alkoxides with the general formula [Ln₂UO₂(O^tBu)_n(L)] (L = OAc, n = 7; L = THF, n = 8), obtained by the reaction of [UO₂(L)₂(THF)₂] (L = OAc, N′′) with the corresponding metal silylamides Ln(N′′)₃ (Ln = Nd, Sm, Gd, Dy, Ho, Er, Yb) in THF followed by adding an excess of tert-butanol (Scheme 1). The products resulting from the reaction of [UO₂(OAc)₂(THF)₂] with LnN′′₃ are oligomers by acetate-bridging of the trinuclear-monomeric unit [Ln₂UO₂(O^tBu)₇(OAc)]_x (x = 2–4) whereas the reaction of [UO₂N′′₂(THF)₂] with LnN′′₃ resulted in a merely trinuclear-monomeric complex.

Scheme 1 Synthesis of mixed uranyl-lanthanoid alkoxides by reaction of either [UO₂N′′₂(THF)₂] or [UO₂(OAc)₂(THF)₂] with the respective [LnN′′₃] (Ln = Nd, Sm, Gd, Dy, Ho, Er, Yb) to afford the monomeric cluster [Ln₂UO₂(O^tBu)₈(THF)] (a) or the acetate-bridged [Ln₂UO₂(O^tBu)₇(OAc)]_n (b).

The compounds were analysed structurally by scXRD and were investigated by Physical Properties Measurement System (PPMS). We have been able to isolate a variety of clear red-orange single crystals by careful crystallisation from saturated benzene or toluene solutions at 4 °C.

All [Ln₂UO₂(O^tBu)_n(L)] (Ln = Nd, Gd, Dy, Ho; L = OAc, n = 7; L = THF, n = 8) complexes show similar bonding motifs (Fig. 1). The molecular structure consists of two lanthanoid centres and one uranyl subunit linked through three μ²-O^tBu groups to form a triangle which is capped by one μ³-O^tBu on the one side and by one uranyl oxygen (μ³-O_endo=U=O_exo) on the other side. Moreover, the uranyl centre possesses one terminal O^tBu group to complete a distorted octahedral configuration and each Ln centre possesses one terminal O^tBu-group. To saturate the coordination sphere of the Ln centre, the acetate group is bridged to the next [Ln₂UO₂(O^tBu)₇(OAc)] unit, resulting in the formation of an oligomeric structure [Ln₂UO₂(O^tBu)₇(OAc)]_n (Scheme 1(b)). In contrast, starting from [UO₂N′′₂(THF)₂] results, due to the absence of the acetate group, in a Ln atom possessing an additional terminal O^tBu group. The coordination sphere of the second Ln centre is completed by a THF molecule resulting in the trinuclear complex [Ln₂UO₂(O^tBu)₈(THF)] (Scheme 1(a)) displaying a distorted octahedral geometry around each Ln. In addition, the endogenous uranyl O_μ³=U=O bond lengths are elongated to 1.866–1.902 Å, respectively, when compared to non-oxo-coordinated uranyl compounds (O=U=O 1.78 Å) (see Fig. S29, ESI†). This is in the range of various uranyl(V) complexes,³² even though C. J. Burns et al. have shown that significant uranyl lengthening and bending can occur in uranyl(VI) oxometallate clusters.³³ We can only hypothesise at this stage that this is due to the smaller radii of later lanthanoid ions, which are harder Lewis acids and therefore a better acceptor for the also hard uranyl oxygen. The O=U=O bond angle is also bent to below 170°. Previous studies have shown that the O=U=O unit can deviate significantly from a linear arrangement given the right ligand environment. For further details see the overview by Hayton and the references therein.³⁴ The latter study also shows that this influences the colour of the compound. As deviations from the linear arrangement are rare it is quite remarkable that our samples consistently show O=U=O bond angles below 170° (see Fig. S30, ESI†).

Fig. 1 Selected molecular structure of the monomeric 1-Sm, dimeric 2-Ho, trimeric 3-Ho, tetrameric 4-Sm, chloride and hydroxide inclusion 5-Ho and 6-Ho and alkali metal inclusion 7-Ho and 8-Ho alkoxides. Thermal ellipsoids are shown at 50% probability level and hydrogen atoms have been omitted for clarity.

The trinuclear “Ln₂U” unit resembles the molecular structure of previously reported trimeric Ln tert-butoxides [Ln₃(O^tBu)₉(THF)₂]³⁵ with one Ln component being replaced by the uranyl unit in our compounds, and consequently a tridentate O^tBu group and a coordinating THF molecule are replaced by the uranyl oxygen units (Scheme 2).

Scheme 2 Illustrations of the molecular structure of (a) trinuclear monomeric unit [Ln₂UO₂(O^tBu)₇(OAc)]_n, (b) trinuclear compound [Ln₂UO₂(O^tBu)₇(THF)] and (c) comparison with the homometallic lanthanoid tert-butoxide [Ln₃(O^tBu)₉(THF)₂].

Comparing the Ln series we observed that with increasing atomic number the Ln–O_μ³ coordination shortens from 2.626 Å (2-Nd) to 2.447 Å (3-Yb) as a consequence of the decreasing ionic radius. This results in an elongation of O_μ³=U to about 1.88 Å in 3-Yb (see Table S1, ESI†). In addition, we found different crystals of the material leading to an expanded connectivity between the acetate-bridges, depending on how long the complexes were crystallised for. When the compound was crystallised in a period of 1–2 days only crystals of monomeric constitution were isolated, but after two weeks clusters with up to four [Ln₂UO₂(O^tBu)₇(OAc)] units were found for Ln = Sm and three [Ln₂UO₂(O^tBu)₇(OAc)] units for Ln = Ho and Yb, the latter crystallising in the cubic space group Pa3̄ (see SI for details).

In addition, the reaction between [UO₂N′′₂(THF)₂] and LnN′′₃ is also prone to side reactions if unsublimed starting materials are used, as observed by the isolation of complexes with chloride or alkali metal inclusions (Fig. 1). Because these complexes contain unique connectivity of the metal centres, these were then made on purpose. To provide 1 equiv. of chloride 1/3 equiv. of the respective LnCl₃·6H₂O salt was added to a reaction mixture with sublimed LnN′′₃. Utilising the hydrated LnCl₃ salt proved to be a useful method to keep the content of water as low as possible but at the same time providing a coordinating water molecule which is a necessary feature of [Dy₃UO₂(O^tBu)₁₀Cl(H₂O)] 5-Dy and [Ho₃UO₂(O^tBu)₁₀Cl(H₂O)] 5-Ho. Both 5-Dy and 5-Ho could not be isolated when water was completely excluded even though THF is present in the reaction mixture that could saturate the coordination sphere of the Ln centre. By looking at the space filling of the tert-butanol ligands it becomes evident that a THF molecule does not have enough space in this gap. 5-Dy and 5-Ho both crystallise in the orthorhombic space group Pnma. Their structures can be described as an [UO₂(O^tBu)₂] moiety binding to a [Ln₃(O^tBu)₈Cl] trimer via coordination of the uranyl oxygen to three lanthanoid centres and two bridging μ²-O^tBu to two of the three Ln atoms. The third Ln atom shows a coordinating water molecule to saturate the coordination sphere. The bond distances of 2.481(4) Å for both 5-Dy and 5-Ho fit well with other Ln water bonds found in the literature and are in accordance with the oxidation state of the metal centres.^36,37 Alkali metal inclusions were reproduced by adding one additional equiv. of MN′′ (M = Li, K) to a reaction mixture of [UO₂N′′₂(THF)₂] and LnN′′₃. Because of the small ionic radius of Li, it fits into the small gap of the former endogenous tert-butoxides, whereas K possesses a protonated tert-butanol ligand to saturate the coordination sphere. In general, we found that complexes containing later Ln decompose rather quickly, sometimes before crystallisation can set in, identified by a colour change from orange to red, or undergo side reactions more readily than those with early Ln. The UV-vis absorption spectra (see Fig. S26, ESI†) for [Gd₂UO₂(O^tBu)₇(OAc)]₂ (2-Gd) and [Ho₂UO₂(O^tBu)₇(OAc)]₂ (2-Ho) showed an absorption at 270 nm for (2-Gd) and 275 nm for 2-Ho, respectively. Additional bands were observed at 362 nm and 450 nm. The pattern of the UV-vis absorption spectra is in accordance with the spectra of uranyl formohydroxamate reported by Albrecht-Schmitt et al. but show a hypsochromic shift to 450 nm corresponding to a low intensity charge transfer band compared to typical uranyl compounds.^21,38 The hydrolysis curve for 2-Ho was determined by the decreasing absorption of the complex after exposure to atmospheric moisture (see Fig. S27 and S28, ESI†).

PPMS studies of the susceptibility and magnetisation between 2.1 and 300 K confirmed that [Gd₂UO₂(O^tBu)₇(OAc)]₂ (2-Gd), [Dy₂UO₂(O^tBu)₇(OAc)]₂ (2-Dy) and [Ho₂UO₂(O^tBu)₇(OAc)]₂ (2-Ho) are paramagnetic according to the Curie–Weiss rule³⁹ with effective magnetic moments of 7.92(1), 10.36(1) and 10.32(1) μ_B, respectively. These values agree with the expected moments for Gd³⁺, Dy³⁺ and Ho³⁺ (Table 1), which suggests, considering charge neutrality with uranium being U(VI), that the rare-earth ions are the only active magnetic species in these compounds.

Table 1 Comparison of the measured effective magnetic moments μ^exp_eff (μ_B) with typical magneton values from the literature n^exp_eff,⁴⁰ and calculated values

Magnetic centres	μ ^expeff (μB)	n ^expeff	g J [J(J + 1)]^1/2
Gd^3+	7.92(1)	7.8–7.9	7.94
Dy^3+	10.36(1)	10.2–10.6	10.65
Ho^3+	10.32(1)	10.3–10.5	10.61

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Negative values of the Weiss constant θ (Fig. S24, ESI†) and a slight flattening of χ_mol below 4 K indicate antiferromagnetic ordering of the Dy and Ho compounds.⁴¹ In case of the Gd containing complex a negative θ value suggests antiferromagnetic ordering as well, though the transition temperature is apparently below the lower measurement limit.

Isothermal magnetisation plots (Fig. S25, ESI†) are linear with the field at 300 K. In case of [Gd₂UO₂(O^tBu)₇(OAc)]₂ the curve saturates at a value near g_JJ = 7 for Gd³⁺. The magnetisation of the Dy and Ho samples saturate against 5 μ_B, which is only half of the expected value g_JJ = 10 for Dy³⁺ and Ho³⁺, possibly due to the antiferromagnetic order.

In conclusion, we have isolated the first trimeric heterobimetallic uranyl(VI)–lanthanoid(III) alkoxide complexes via a simple and straightforward ligand exchange. The μ³ coordination of the endogenous uranyl oxo atom results in a significant elongation of the bond length and a significant deviation from the linear uranyl arrangement. This indicates that a coordination of lanthanoid atoms to the uranyl oxo-atoms may facilitate the reduction chemistry towards lower oxidation states of uranium, resulting in trimetallic systems with 5f–4f electron correlations. The magnetic susceptibilities obey the Curie–Weiss rule with effective magnetic moments compatible to Gd³⁺, Dy³⁺ and Ho³⁺, respectively, and indicate antiferromagnetic ordering below 4 K for the Dy³⁺ and Ho³⁺ compounds.

D. G.: Original draft, data curation, formal analysis, investigation, visualisation; M. W., A. L., T. G., R. N., M. H.: Data curation, formal analysis, investigation; S. M., M. W., A. K., D. J.: Funding acquisition, supervision, validation, resources, review & editing; M. Z., A. R.: conceptualisation, methodology, project administration, review & editing, supervision.

The authors thank Mario Winkler, Jonathan Wischnat and Prof. Dr Joris van Slageren, University of Stuttgart, for help with physicochemical measurements. The authors thanks Mr Dirk Pullem for carrying out the elemental analyses.

Conflicts of interest

There are no conflicts of interest to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2110472–2110482, 2111075, 2111783–2111784. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1cc05444a

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