Facile Solvent-Free Mechanochemical Synthesis of UI₃ and Lanthanoid Iodides

Abstract

Lewis base-free lanthanoid (Ln) and actinoid (An) iodides are difficult to obtain, as standard protocols describe syntheses in solutions of donor solvents which are ultimately hard to remove. We have now established a mechanochemical approach towards the synthesis of Lewis base-free f-block metal iodides with excellent yields. In particular, we describe herein the synthesis of EuI₂ as an example of a divalent lanthanoid iodide, of CeI₃ as an example of a trivalent lanthanoid iodide, and of UI₃ as the most important actinoid iodide. Each can be obtained in high yield with minimal work-up, presenting the most efficient and simple synthetic route to access these materials to date.

1. Introduction

The chemistry of low-valent actinoid (An) and lanthanoid (Ln) complexes has propelled in recent years, leading to seminal work concerning single molecule magnetism [1,2,3,4,5], small molecule activation [6,7,8,9], new computational approaches [10,11,12,13], and an increased interest in spectroscopic properties regarding fluorescent upconversion [14,15,16] and molecular reporters [17,18]. Material that is classified as nuclear waste has proven to be a valuable source to chemists and physicists to further our understanding of f-electrons [19], chemical bonding [20,21], and coordination [22]. The importance of f-element research has been demonstrated in breakthrough articles covering novel oxidation states, small molecule activation, catalysis and single molecule magnets, and may therefore benefit from easily accessible starting materials. [23,24,25,26,27,28,29] Oftentimes—in contrast to chloride and fluoride salts of lanthanoids and actinoids—iodides make valuable starting materials for coordination chemistry, as they combine well with potassium salts of organic ligands in salt metatheses, precipitating KI. However, the relevant starting materials, (i.e., mainly the divalent or trivalent metal iodides) for these investigations are often hard to access through solution-based or traditional solid-state syntheses, involving long reaction times or the unavoidable coordination of solvent molecules from the synthesis or work-up. These may sometimes impede reactivity, trigger side reactions or interfere with spectroscopic measurements [30]. In spite of this, the synthesis of f-block metal iodides which are coordinated to Lewis bases such as THF (e.g., [SmI₂(thf)_n], n = 2–5; [31] [LaI₃(thf)₄] [32], [UI₃(thf)₄] [33]), is well established and refined to an elegant art [34]. However, acquiring base-free derivatives remains a largely tedious and unappealing enterprise due to: the inability of removing pre-coordinating Lewis bases, their preparation in liquid ammonia (e.g., EuI₂) [35], use of toxic materials (e.g., HgI₂) [36,37], extended reaction times (e.g., seven days to one month for UI₃) [38], and attempted dehydration of readily available hydrates (LnI₃(H₂O)_x) [39]. Mechanochemical synthesis of metal iodides is a practical approach to access otherwise difficult to obtain materials [40,40,41] and several new techniques [42,43] and protocols [44,45] have been established quite recently. Although mechanochemical synthesis is used across a wide variety of metal classes, this technique is considerably underdeveloped in the realms of the f-block series [46,47,48,49,50], let alone for targeting iodides. Herein, we present a facile, quick, and high yielding mechanochemical synthesis of lanthanoid and actinoid iodides. Considering both the commercial unavailability of uranium iodides, and the high prices for purchasing base-free lanthanoid iodides—not to mention the benefits of the base-free halides over solvates [32] and difficulties associated with Lewis base co-ligands in chemical transformations [51,52], spectroscopic analysis [30], and overall complex stability [53,54]—this clean, simple method of synthesising base-free f-block metal iodides provides a new way of accessing these materials.

In this study, we present Eu and Ce as two examples for this chemistry, however, Table S1 of the Supplementary Materials provides an overview regarding syntheses involving other elements of the lanthanoid series, yet they could not be fully characterised.

2. Materials and Methods

The oscillating ball mill apparatus was purchased from Retsch (400 MM), along with the milling containers (inner layer being either tungsten carbide or steel in 15 or 25 mL volume). Powder X-ray diffraction was performed using a P-XRD Stoe Stadi-P equipped with a Mythen 1 K detector and measured from 0 to 60° in 1° steps and 120 s exposure time in Debye-Scherrer geometry using Mo-Kα₁ radiation.

There are a few general ball milling considerations to ensure high yields and short reaction times for lanthanoid and actinoid iodides. For instance, a tungsten carbide container is necessary as the analogous synthesis in a steel mill appears to give alternative products. In an instance where an excess of metal is used, metal chunks can be removed through a metal sieve after reaction occurs. In the instance when excess iodine and non-granulated lanthanoid metal (e.g., chips) are used, prior to the actual mechanochemical reaction, it is recommended to mill the mixture at 16 Hz for approximately 20-30 min to break up the metal pieces. By milling at 16 Hz for this duration, no indication of a reaction was observed when the container was opened. The size of the milling container relative to the sample size must also be considered, and it is recommended that a 15 mL container be used for a targeted yield of about 2.0 g. Avoiding the use of an overly large excess of iodine is recommended in order to avoid hindering the overall obtained yield of the product. It is recommended that the container be sealed tightly (preferably with one layer of Teflon tape around the screw).

3. Results

For the 4f elements, milling the metal with an appropriate amount of iodine for a given frequency and duration (Scheme 1) readily affords either the lanthanoid diiodide EuI₂ or the lanthanoid triiodide CeI₃.

The mixing of lanthanoid metal Ln (Ln = Ce, Eu) with an excess of iodide in an oscillating ball mill produces the respective iodide as a fine powder within less than 90 min (Scheme 2). The excess iodine can be easily removed under vacuum (0.03 mbar) with gentle heating (120 °C), affording the dry powders of CeI₃ or EuI₂ in essentially quantitative yields. Yield loss is only due to losses upon removal from the container, and the material can be directly used for further synthetic purposes.

The materials could be identified by powder X-ray crystallography, though due to the method of synthesis, the samples obtained from the milling container were naturally of poor crystallinity. For CeI₃ additional tempering was required to obtain material with a higher degree of crystallinity (Figure 1).

For EuI₂, remarkably, the material obtained contained both known phases for EuI₂ (Figure 2).

Remarkably, the mechanochemical synthesis could be extended beyond the 4f elements to the 5f elements, and the synthesis of UI₃ was performed using a similar approach (Scheme 3). Initially, the material is milled followed by brief tempering of over 225 °C overnight, with the best results in terms of crystallinity at 500 °C, giving UI₃ as large single crystals in essentially quantitative yield. This approach is remarkably quick, and supersedes the previous milestone of 7 days.

The differences for UI₃ over the 4f elements were as follows: the synthesis was carried out using clean uranium turnings and 1.5 equivalents of I₂. The material was transferred into a steel milling vessel, cooled in liquid nitrogen to −196 °C, and the material was milled for 60 min at 30 Hz. This process of cooling and milling was performed six times. The isolated jet-black powder was pressed into a pellet and transferred into a quartz ampoule, sealed under vacuum, and heated to 500 °C for 24 h, resulting in a total work-up time of two days from start to finish. The purity of the UI₃ was analysed using PXRD (Figure 3) and was further confirmed using ¹H-NMR spectroscopy on the THF adduct UI₃(thf)₄ in C₆D₆, resulting in two resonances at 10.76 ppm and 6.15 ppm for the THF H atoms.

4. Discussion

In light of the aforementioned results, we also probed the potential synthesis of a range of other divalent and trivalent lanthanoid iodides. A detailed overview of these can be found in Table S1 of the SI listing conditions and yields for the respective iodides. A detailed description and photos have been provided in the supporting information. However, as it has been difficult for us to obtain high quality powder diffraction data for these materials, they are not further discussed within this work.

5. Conclusions

In conclusion we present a fast, facile, high-yielding mechanochemical synthesis of Lewis base-free f-block metal iodides. The lanthanoid iodides can thus be synthesised in the solid state in less than 90 min with minimal work-up. Considering the importance of EuI₂ and CeI₃ as important precursors for coordination chemistry and spectroscopic investigations [57,58,59,60,61], this synthetic method provides a practical pathway to valuable starting materials. In addition, we have shown that uranium (III) iodide can be easily and quickly obtained in single crystalline form using the same approach followed by heating overnight at >225 °C. The results herein show the potential of mechanochemical techniques for f-block chemistry, which is poorly developed, and future work will entail synthetic expansion beyond precursors towards direct complex synthesis.