A simple and practical wax-encapsulation method for air-sensitive XAS samples

Abstract

To facilitate X-ray absorption spectroscopy (XAS) measurements of air-sensitive samples, we present a simple method in which materials are encased in common paraffin wax to protect them from air and moisture. We demonstrate the efficacy of this approach using a highly reducing, air- and moisture-sensitive uranium(III) complex, the tris(amide) U[N(SiMe₃)₂]₃ (1). When finely dispersed in a boron nitride matrix and subsequently encased in inert paraffin wax, samples of 1 remain stable with no visible or spectroscopic degradation after several days under ambient conditions. The viability of this method for XAS measurements was further evaluated across a series of uranium compounds, ranging from uranyl species to highly air- and moisture-sensitive molecular complexes, at the uranium L₃-edge. Edge energy determinations were highly reproducible (±0.1 eV between replicates) and, where available, showed excellent agreement with literature values. This low-cost, effective, and versatile method offers a viable solution for XAS studies of air-sensitive compounds and materials.

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Introduction

X-ray absorption spectroscopy (XAS) is a valuable tool in the fields of chemistry, homogeneous and heterogeneous catalysis, molecular and condensed matter physics, materials science, engineering, earth science, and biology.^1–4 The element specificity of XAS offers several valuable capabilities, including sensitivity to oxidation state and electronic structure, as well as the ability to probe and determine local coordination environments.^2,5–10 However, the analysis of air-sensitive compounds by XAS presents additional challenges related to sample preparation, handling, transport, and maintaining sample integrity throughout the period from preparation to data collection. Most measurements still require synchrotron radiation sources capable of delivering high-energy, tunable X-rays, which necessitate sample shipment offsite, thereby increasing exposure to ambient conditions and extending the duration samples must remain outside of inert-atmosphere containment systems. Furthermore, in-house XAS systems or experiments conducted on dilute samples, such as those at environmental or biological concentrations, may require data collection on a single sample over multiple days.

Additional significant challenges arise when preparing air-sensitive samples for XAS. Solid-state samples must be finely divided to ensure homogeneity,¹¹ which significantly increases surface area and reactivity. Furthermore, beamlines may lack accessible gloveboxes for secure sample storage upon arrival or feature setups incompatible with reactive species, as has been the case in our own experience. Some samples also present additional hazards or considerations, such as pyrophoricity, radioactivity, or unknown toxicity, necessitating specialised handling and containment measures. Sample handling requirements at the beamline, particularly when mounting within specially designed holders, are often nontrivial and can significantly limit the number of samples that can be analysed and the number of data sets that can be collected within an allocated beamtime window. These constraints call for a sample preparation method that not only preserves sample integrity but also facilitates efficient data collection, especially for large sample sets.

Current methods for collecting XAS data on air-sensitive samples are varied and include sandwiching materials between X-ray transparent supports sealed with epoxy or tape,^12–14 using specially machined steel or aluminium holders fitted with X-ray transparent windows made from materials such as Kapton®,^4,15–20 and casting samples into polymers or epoxy.^11,14,21 Even with specially designed holders, it has been noted that samples may oxidise after several hours of air exposure,²⁰ and some groups have employed so-called “canary” samples as internal indicators of oxidation.²² The wide range of sample preparation techniques underscores the lack of a standardised procedure, reflecting both the diversity of experimental setups and the logistical challenges of working with reactive samples. In this context, establishing a convenient and reliable protocol for preparing air-sensitive samples could help streamline handling and broaden access to XAS measurements of reactive materials.

Our group has previously reported a method for preparing and collecting XAS data on air-sensitive uranium compounds by sandwiching compressed pellets of boron nitride sample matrices between vacuum-sealed polyethylene envelopes (Fig. S1), which provided fully contained samples that could be shipped and mounted directly at the beamline.^23,24 This method was successful in allowing the preparation of multiple samples and efficient data collection in replicate, ensuring accuracy and reproducibility in the resulting data. However, in the course of our XAS studies, we found this approach to be admittedly cumbersome and time-consuming in terms of sample preparation. More critically, we discovered that this sample containment system is inadequate for exceptionally air sensitive compounds.

Specifically, several attempts to collect data for the highly reducing uranium tris(amide) complex U(NR₂)₃ (R = SiMe₃) (1),²⁵ often resulted in the slow bleaching of the material from dark purple to yellow prior to measurement, along with inconsistent and shifted edge energy values (Fig. S1 and S2), all indicative of adventitious oxidation. While the vacuum seal of the sample envelopes remained visibly unchanged, we discovered that polyethylene is subject to oxygen permeation which, though small, is significant in the case of 1, even over periods as short as a few hours. In our hands, 1 alone visibly degrades within a couple of hours even when standing in a controlled inert-atmosphere glovebox with O₂ levels below 1 ppm, highlighting its susceptibility to oxidation. These observations suggest that the oxygen transmission rate (OTR) of polyethylene (55 cm³ m⁻² d⁻¹ at 25 μm film thickness)²⁶ is incompatible with highly sensitive samples such as 1. Attempts to optimise our system utilising different polymers and multiple vacuum bag layers proved untenable, prompting us to develop an alternative sample containment method.

Taking inspiration from Buchwald and co-workers’ paraffin wax delivery system for reactive palladium catalysts,²⁷ as well as Davis et al.'s trapping of active Fischer–Tropsch catalyst species in wax for XAS studies,^28,29 we developed a simple encapsulation method that leverages the low melting point and X-ray transparency of paraffin wax for preparing air-sensitive samples for XAS measurements. The robustness of this method was thoroughly tested utilising 1, which was closely monitored for sample degradation over an extended period. Moreover, the practicality of this approach was empirically tested through U L₃-edge X-ray absorption near-edge structure (XANES) spectroscopy of several uranium samples and compared against reported literature values.

Results and discussion

The trivalent uranium species 1 was chosen to benchmark sample stability due to its high sensitivity to oxidants and its dark purple colouration, which undergoes a readily visible, contrasting transition to yellow upon oxidation, as observed in the controlled conversion of 1 to the bridged oxide [(R₂N)₃U]₂(μ-O).³⁰ Sample preparations were conducted in a dedicated argon glovebox with <0.5 ppm O₂ and water, equipped with an exposed, stirring pool of NaK alloy to further scrub residual reactive species.

Paraffin wax offers an excellent barrier to oxygen and moisture,³¹ while also being chemically inert and X-ray transparent, making it a compelling medium for the preparation and measurement of air-sensitive samples in XAS experiments. Additionally, the solid wax serves as a primary containment layer for hazardous and radioactive materials, helping to mitigate the risk of contamination and inadvertent exposure. This approach offers a robust, sealed layer of containment that can be readily integrated into enhanced, multi-layer safety protocols, which may be compatible with a range of beamline-specific containment requirements. For our purposes, we selected Gulf Wax® brand paraffin due to its ready availability, low cost, relatively low melting point (54–60 °C), and moderate opacity. Though, it should be noted that the wax must be thoroughly degassed prior to use as we observed vigorous gas evolution upon initial exposure of the melt to vacuum. It should also be noted that, when using paraffin wax, beam attenuation becomes a consideration for lower-energy measurements (below ∼7500 eV), which may preclude certain low-energy K-edge studies in transmission mode. However, the use of high-flux synchrotron sources and fluorescence detection can mitigate these limitations.

Initial attempts at sample preparation involved heating the wax alongside a silicon ring mould to 80 °C. Upon melting, a thin layer (∼2 mm) of paraffin wax was uniformly deposited by pipette transfer to the heated mould. After cooling, a boron nitride pellet containing 1 was placed at the centre of the mould, and a second wax layer was added by pipette to fully seal the sample. These initial attempts to seal pellets of 1 in paraffin wax extended sample stability outside of the glovebox for approximately 3 d under ambient conditions; however, they were eventually compromised by the formation of a horizontal seam at the interface between the initial layer and the overlying wax.

To address this issue, the bottom wax layer was allowed to partially cool until it formed a semi-solid gel, providing a more cohesive interface for sealing (Fig. S3b). The pellet was then placed at the centre of the mould and immediately overlaid with molten wax (Fig. S3d). During this step, the mould was gently agitated to dislodge trapped argon and ensure thorough adhesion to the pellet surface, affording a fully sealed wax coin as shown in Fig. 1. (N.B. We also discovered 1 to be highly soluble in melted paraffin, enabling its direct dispersion into the wax without the need for a boron nitride matrix, thereby offering an alternative method of sample preparation. However, as the homogeneity of the dispersion was not optimised, this approach was not used for any of the reported XAS measurements.) This procedure reliably produces uniform encapsulated samples that remain stable under ambient conditions for up to 5 d without the need for additional protective layers. Moreover, the coins are partially translucent, which facilitates visual alignment and centring of the sample on the beamline (Fig. S4).

Fig. 1 Diagram and photographs of a boron nitride sample pellet of 1 encapsulated in paraffin wax. Left: Scheme showing sample dimensions and cross-section profile. Right: Photographs of a complete wax coin and its cross-section.

The viability of the samples can be further extended by sealing the coins within layers of commercially available food grade ethylene vinyl alcohol (EVOH, 3 mil, 0.0762 mm, OTR = 0.2 cm³ m⁻² d⁻¹ at 25 μm film thickness) and co-extruded polyethylene nylon (poly-nylon, 5 mil, 0.127 mm) polymers, which together provide excellent additional resistance to oxygen and water permeation (Fig. S5).²⁶ In this fashion, the samples remain stable for 15 d, vide infra, or longer under ambient conditions. This stability window significantly exceeds the typical timeframe for sample shipment and beamline analysis, especially when supplemented with additional protective containment, such as aluminised mylar, prior to beamline arrival. A potential drawback, though, is that care must be exercised when cooling the coins below 0 °C, as this can lead to fracturing of the wax. At or above 0 °C, however, samples may be stored indefinitely under inert conditions within a glovebox without degradation.

Spectroscopic verification of the extended stability provided by this system, namely coins of 1 packed within a two-layer EVOH/poly-nylon envelope, was performed on samples stored under ambient conditions for 24 h, 3 d, 9 d, and 15 d. At each timepoint, the wax coin was bisected and the embedded pellet extracted. The boron nitride matrix was dissolved in C₆D₆, and the resulting solution analysed by ¹H NMR spectroscopy (Fig. 2 and S5). No significant changes were observed across the series, and no signal corresponding to degradation products were detected.

Fig. 2 Stacked ¹H NMR spectra (C₆D₆) of 1 extracted from boron nitride sample pellets encapsulated in wax coins. Samples were sealed in EVOH and poly-nylon vacuum film and stored under ambient conditions. Spectra were recorded over a period of days to assess chemical stability.

An additional advantage of this method is that the wax coin can be customised to a number of shapes and sizes to suit specific experimental needs and beamline requirements. Furthermore, the small footprint of the coins readily lends itself to rack mounting, allowing for the efficient data collection on multiple samples in a single, uninterrupted run, which reduces the loss of usable beamtime incurred during frequent sample exchanges. For example, we constructed a customised, 3D-printed rack that accommodated 15 samples for mounting at the U.S. National Synchrotron Light Source II (NSLS-II) beamline (7-BM, QAS). Each sample was secured with Kapton® tape, and the rack was vacuum sealed within a single layer each of EVOH and poly-nylon film (Fig. 3, S4, and S6). In this manner, exceptional stability was achieved.

Fig. 3 Top: Diagram illustrating the 3D-printed sample rack and its protective EVOH and poly-nylon film layers. Bottom: Photograph of the mounted rack positioned at the beamline (NSLS-II, 7-BM, QAS) for automated data collection of multiple samples.

To test the viability of this approach for XAS measurements, a set of samples of 1 was analysed at the uranium L₃-edge. The spectra show excellent reproducibility across replicates, with ∼0.1 eV variation in the white line and edge energies and no discernible background interference from the wax or packaging materials (Fig. 4). The three samples were each housed in separate racks and measured at different timepoints over a 24 h period. This temporal and spatial separation underscores the robustness of the method, demonstrating that the sample integrity is reliably preserved across independently prepared and handled wax coins. Encouraged by these results, we extended the method to a range of air-sensitive and air-stable uranium compounds to further evaluate its broader applicability as a general approach to XAS and to benchmark the resulting spectra against literature values, where reported.

Fig. 4 Normalised uranium L₃-edge XANES plots for three independently prepared wax coins of 1. Samples were separately rack mounted and the data collection performed over different points over a 24 h period. White line energies are denoted with diamonds and edge energies denoted with triangles.

The samples chosen for this study were UI₃(1,4-dioxane)_1.5,³²1, UCl₄, [Cp*₂Co]{U(O)[N(SiMe₃)₂]₃},³³ U(O)[N(SiMe₃)₂]₃,³⁰ metaschoepite (UO₂)₄O(OH)₆·5H₂O,³⁴ UO₂Cl₂(THF)₃,³⁵ Cs₂UO₂Cl₄,³⁶ and [Na(THF)₂{UO₂[N(SiMe₃)₂]₃}].³⁷ This series was selected based upon a variety of factors including the air-sensitivity of certain compounds (e.g., UI₃(1,4-dioxane)_1.5 and 1), broad availability (e.g., UCl₄), and span of oxidation states (U(III) → U(VI)). Additionally, compounds were included to enable benchmarking against previously reported XAS data and uranium standards (e.g., Cs₂UO₂Cl₄ ¹⁸ and (UO₂)₄O(OH)₆·5H₂O¹⁵). This is especially important when comparing spectra collected at different synchrotron sources, where systematic variations between beamlines can lead to slight shifts in energy calibration.^38,39 Moreover, internal standards such as yttrium foil (17 038.4 eV), which is typically employed in uranium L₃-edge studies, may suffer from surface oxidation that may shift reference energies. To ensure consistency across datasets, all edge energies in this study were referenced against our previously reported value for UO₂Cl₂(THF)₃, set at 17 165.1 eV (calibrated against yttrium foil).²³

The merged edge energy value for 1 (17 158.2 eV) (Table 1 and Fig. 5, S7, and S8), averaged as the set of six scans, prepared as a wax coin, compares well with the value reported by Bart and co-workers for a sample prepared and sealed in a custom aluminium holder with Kapton® windows (17 158.5 eV),^12,18,40 differing by only 0.3 eV. An even smaller shift, Δ = 0.1 eV, is observed between the closely related uranium(III) iodides UI₃(dioxane)_1.5 (17 158.8 eV) and UI₃(THF)₄ (17 158.9 eV),^12,40 the latter also as determined by Bart et al.¹⁸

Fig. 5 Normalised uranium L₃-edge XANES plots for 1 and U(III)–U(VI) reference standards, all prepared as wax coins. White line energies are denoted with diamonds and edge energies denoted with triangles.

Table 1 XANES edge and white line energies for 1 and other uranium compounds. All energies are referenced to UO₂Cl₂(THF)₃ (17 165.1 eV), which was calibrated against Y foil (17 038.4 eV)

Compounds	Oxidation state	Edge energy (eV)	Reported edge energies	Δ (eV)	White line energy (eV)
a Values originally referenced to Cs2UO2Cl4; shifted to align with the UO2Cl2(THF)3 reference. b Edge energy for UI3(THF)4.
1	III	17 158.2	17 158.5^a [ref. 18]	0.3	17 163.4
UI3(1,4-dioxane)1.5	III	17 158.8	17 158.9^a^,^b [ref. 18]	0.1	17 163.0
[Cp*2Co]{U(O)(NR2)3}	IV	17 161.6			17 168.5
UCl4	IV	17 163.1	17 161.6 [ref. 23]	−1.5	17 167.1
U(O)(NR2)3	V	17 163.3	17 163.3 [ref. 23]	0	17 171.3
[Na(THF)2]{UO2(NR2)3}	VI	17 164.4			17 170.8
Cs2UO2Cl4	VI	17 164.7	17 164.7^a [ref. 18]		17 169.1
UO2Cl2(THF)3	VI	17 165.1			17 169.0
Metaschoepite	VI	17 166.9			17 170.4

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Curiously, solvent-free UCl₄ prepared as a wax coin exhibits an edge energy of 17 163.1 eV, which is significantly higher than the 17 161.6 eV (Δ = −1.5 eV) previously measured and reported by us.^23,24 We cannot explain this discrepancy other than as an anomaly, especially as UCl₄ is not prone to oxidation in air or sensitive to water. A subsequent measurement of a separately prepared UCl₄ sample, utilizing our polyethylene envelope sample preparation method, yielded an edge energy consistent with our newer value (17 163.2 eV) (Fig. S9 and S10), suggesting that the original measurement was spurious. Indeed, given the compound's chemical stability and the reproducibility of the result, UCl₄ may serve as a reasonable internal standard for future uranium L₃-edge XAS measurements. In contrast, despite its greater sensitivity to air and moisture, UI₃(dioxane)_1.5 was found to have identical edge energies regardless of our XAS sample preparation method. Further benchmarking also shows exact agreement for the air-sensitive pentavalent complex U(O)(NR₂)₃, with both the wax coin and vacuum-sealed envelope methods giving identical values of 17 163.3 eV.^23,24 Thus, with the exception of the anomalous UCl₄ measurement obtained from our original envelope method, the consistency between reported literature values, independent of sample containment method, illustrates the reliability of our wax preparation approach for the XAS measurement of air-sensitive samples.

Conclusion

In conclusion, by utilising inexpensive and readily available paraffin wax, we report a simple but robust method for the protection of air-sensitive samples in XAS measurements, such as XANES, using a wax coin method. The wax offers high versatility as it can be moulded in a variety of shapes and sizes to best accommodate beamline sample requirements. This adaptability also allows the coins to be securely mounted in sample racks, enabling the efficient measurement of multiple samples in a single run and reducing downtime from frequent sample exchanges. Moreover, this containment method may provide an effective protective barrier against hazardous, toxic, or radioactive materials. Although synchrotron-specific rules ultimately guide what can be accepted for radioactive materials, the wax-based encapsulation method described here provides a modular layer of protection that could be adapted or built into existing containment systems.

When compared to literature values, the XANES data collected from our wax coin samples agree well, supporting the validity of our new encapsulation method for preserving sample integrity while providing reproducible and accurate results. In this manner, when combined with protective layers of commercially available, food grade EVOH and poly-nylon polymer films, the sample lifetime can be extended to at least 15 d under ambient conditions.

Experimental section

General considerations

All air and moisture-sensitive operations were performed in a Vigor glovebox under an atmosphere of ultra-high purity argon, maintained at <0.5 ppm O₂ and H₂O. A continuously stirred pool of NaK alloy (∼100 mL) was maintained within the glovebox to aid in oxygen and moisture scavenging. Benzene-d₆ (C₆D₆) was purchased from Sigma-Aldrich Inc., degassed by three freeze–pump–thaw cycles, and dried over activated 4 Å molecular sieves for at least 24 h prior to use. UCl₄,^41,42 metaschoepite (UO₂)₄O(OH)₆·5H₂O,³⁴ UO₂Cl₂(THF)₃,³⁵ Cs₂UO₂Cl₄,³⁶ [Na(THF)₂]{UO₂[N(SiMe₃)₂]₃},³⁷ U(O)[N(SiMe₃)₂]₃,³⁰ [Cp*₂Co]{U(O)[N(SiMe₃)₂]₃},³³ UI₃(1,4-dioxane)_1.5,³² and U[N(SiMe₃)₂]₃ ²⁵ were synthesised following reported procedures. Grade ZG ultra-high purity boron nitride powder with an average particle size of 7.4 μm was purchased from Amazon.com, Inc. and heated under high vacuum at 200 °C for several days and subsequently stored in a glovebox under inert atmosphere prior to use. Commercially available paraffin wax (Gulf Wax®) was purchased from Amazon.com, Inc., heated under high vacuum at 90 °C for 4 h and subsequently stored in a glovebox under inert atmosphere prior to use. The melting point of the paraffin wax was determined utilising a Digimelt MPA160 melting point apparatus. Food-grade ethylene vinyl alcohol (EVOH) vacuum sealer bags (3 mil, 0.0762 mm) were purchased from Doug Care Equipment, Inc. U.S.A. (product number D1-0709), and co-extruded polyethylene nylon (poly-nylon) vacuum sealer bags (5 mil, 0.127 mm) were purchased from Sealer Sales, Inc. U.S.A. (product number VB5-0810-1000). Silicone ring moulds (Senhai brand) and aluminised Mylar® bags (Wallaby brand, 5 mil, 0.127 mm, 10 × 14 in) were both purchased from Amazon.com, Inc. The polymer films, Mylar bags, and the silicone mould were used as received. ¹H NMR spectra were recorded on a Bruker AVANCE III 400 MHz spectrometer and are referenced to the characteristic ¹H resonances of the solvent.

Caution! Depleted uranium-238, employed in this study, is radioactive and a weak α-emitter (4.197 MeV) with a long half-life (t _1/2 ≈ 4.5 × 10 ⁹ years). Care must be exercised by utilising appropriate radiological controls to mitigate the dispersal of the material while safeguarding against inhalation or ingestion.

X-ray absorption spectroscopy

Initial XAS measurements of samples of 1, prepared using our previously described vacuum envelope method (Fig. S1 and S2),²³ were performed at sector 10-BM of the U.S. Advanced Photon Source (APS) at Argonne National Laboratory (ANL), part of the Materials Research Collaborative Access Team (MRCAT). Data was collected in the standard transmission geometry mode with an incident beam of 500 × 1000 μm @ 10¹² ph s⁻¹, with energies between 17–18 keV and a vertical beam slit size of 700 μm. All sample energies are referenced to an yttrium foil located between the second and third detectors, and all spectra are aligned to a foil value of 17 038.4 eV. A separately prepared sample of UCl₄ was also measured at 10-BM under identical conditions, and the resulting edge energy (17 163.2 eV) was consistent with the value obtained from the wax coin sample.

XAS measurements on samples prepared in wax coins and vacuum sealed within a layer each of EVOH and poly-nylon films were conducted at the 7-BM Quick X-ray Absorption and Scattering (QAS) beamline at the U.S. National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory (BNL). The X-ray was monochromatized using a channel-cut Si(111) monochromator, and the energy was calibrated with an yttrium foil at 17 038.4 eV. Measurements were performed at room temperature in standard transmission geometry mode using gridded ion chambers, with an incident beam of 500 × 500 μm @ 10¹¹ ph s⁻¹, a vertical beam slit size of 1000 μm, and energies between 17–18 keV. Typical scan lengths were approximately 30 seconds. All sample energies were similarly referenced to the yttrium foil, situated between the second and third detectors. For cross-beamline consistency, the UO₂Cl₂(THF)₃ standard was then assigned an edge energy of 17 165.1 eV, matching our previously reported value²³ and chosen to address calibration offsets that can occur between different beamlines.

Sample preparation was performed in a glovebox under an inert atmosphere of high purity argon; sample powders were prepared by mixing boron nitride (BN) with the uranium compounds to concentrations between 5–14 wt% of uranium. The sample matrices were then pulverised using a mortar and pestle to produce ∼0.2 g of a very fine powder. Approximately 0.06 g of the powder was loaded into a pellet press and compressed using a force not exceeding 27 Newton-metres to produce a pellet 7 mm in diameter and 1–2 mm in depth.

The wax coins were prepared by heating the paraffin wax and a silicone ring mould to 80 °C, into which the liquified wax was dispensed into the mould to form an initial ∼2 mm base layer. After partial cooling of the wax to a semi-solid state, the BN sample pellet was centred on the base and immediately covered with additional melt. Gentle agitation during over-pour aided gas release and promoted contact at the pellet-wax interface. The resulting coins were typically ∼17 mm in diameter and ∼5 mm in thickness. The coins were subsequently seated in a custom 3D-printed rack (160 mm × 145 mm × 7 mm) designed to hold 15 samples. The recesses were evenly spaced in a 5 × 3 grid, each measuring 20 mm in diameter and 5 mm in depth, with a centred 10 mm diameter cutout to facilitate consistent pellet positioning within the X-ray beam. Encapsulated coins were secured in place using Kapton® tape. The sample racks were vacuum sealed within a 3 mm EVOH film layer and a 5 mm poly-nylon film layer, then enclosed in an aluminised Mylar® bag for shipment. At the beamline, each rack was secured to a custom 3D-printed base that was secured to a Thorlabs, Inc. aluminium breadboard platform (Fig. 3 and S4).

All samples were prepared in triplicate, and each was measured three times, yielding a total of nine scans. The scans were compared across the series to verify consistency and assess reproducibility. The data were then merged using Demeter XAS data analysis software. Edge energies were determined as the maxima of the first derivative, and white line energies were obtained from the zero crossing point.

Attenuation lengths used to assess beam transmission through wax and boron nitride were obtained from tabulated data published by Henke and co-workers.⁴³

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: X-ray spectra and supplementary figures. See DOI: https://doi.org/10.1039/d5qi01913c.

Acknowledgements

We are grateful to the Welch Foundation (AH-1922-20230405) for supporting the synthesis and characterisation of the reported molecules, as well as for travel support. The work of A. G.-T. was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Element Chemistry Program under Award Number DE-SC0025098. This research used resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility operated by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. It also utilised resources at 7-BM of the National Synchrotron Light Source II (NSLS-II), another DOE Office of Science User Facility operated by Brookhaven National Laboratory under Contract No. DE-SC0012704.

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