Advancing understanding of actinide(iii) (Ac, Am, Cm) aqueous complexation chemistry

The positive impact of having access to well-defined starting materials for applied actinide technologies – and for technologies based on other elements – cannot be overstated. Of numerous relevant 5f-element starting materials, those in complexing aqueous media find widespread use. Consider acetic acid/acetate buffered solutions as an example. These solutions provide entry into diverse technologies, from small-scale production of actinide metal to preparing radiolabeled chelates for medical applications. However, like so many aqueous solutions that contain actinides and complexing agents, 5f-element speciation in acetic acid/acetate cocktails is poorly defined. Herein, we address this problem and characterize Ac3+ and Cm3+ speciation as a function of increasing acetic acid/acetate concentrations (0.1 to 15 M, pH = 5.5). Results obtained via X-ray absorption and optical spectroscopy show the aquo ion dominated in dilute acetic acid/acetate solutions (0.1 M). Increasing acetic acid/acetate concentrations to 15 M increased complexation and revealed divergent reactivity between early and late actinides. A neutral Ac(H2O)6(1)(O2CMe)3(1) compound was the major species in solution for the large Ac3+. In contrast, smaller Cm3+ preferred forming an anion. There were approximately four bound O2CMe1− ligands and one to two inner sphere H2O ligands. The conclusion that increasing acetic acid/acetate concentrations increased acetate complexation was corroborated by characterizing (NH4)2M(O2CMe)5 (M = Eu3+, Am3+ and Cm3+) using single crystal X-ray diffraction and optical spectroscopy (absorption, emission, excitation, and excited state lifetime measurements).


Introduction
Advancing understanding of actinides in aqueous media could have a widespread impact on solving relevant problems in actinide science. Almost all aspects of technologically relevant actinide chemistry relyat some pointon aqueous actinide processing. The impact spans from large-scale plutonium metal production to actinide environmental monitoring, and attempting to achieve energy security using nuclear power. 1 Unfortunately, speciation and reactivity of actinides in aqueous solutions is oen poorly dened. One of many examples includes actinides dissolved in buffered ammonium acetate and acetic acid solutions, (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) . Beyond a few focused studies, 2-9 this stock solution is poorly characterized because of the inherent challenges associated with handling highly radioactive actinides. Interpreting spectroscopic results from actinides in this complicated aqueous environment is also challenging in comparison to well-dened organic solutions as well as when frozen in the solid-state. 10,11 Nevertheless, the reality of this intellectual void is still surprising given that actinide-containing (NH 4 )O 2 CMe (aq) :HO 2 -CMe (aq) solutions provide convenient entry into small-scale production of actinide metals, [12][13][14] as starting materials to label chelators with alpha-emitting radionuclides for therapeutic applications, 15 in aqueous synthetic efforts, 16,17 and (to a more limited extent) for actinide separations. 18 From this perspective, there is a clear need to better dene and control actinide speciation within (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) buffered stock solutions.
Having poorly characterized starting materials, like actinide (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) stock solutions, is not specic to the eld of aqueous 5f-element chemistry. It is, instead, a systemic problem. Use of poorly dened starting materials transcends many aspects of materials science, biology, organic, and inorganic chemistry, etc. Consider numerous reports published in journals focused on characterizing "simple" starting materials, those that need to be better understood because of widespread use. [19][20][21][22][23] These modest contributions are highly cited and foundational. From that perspective, we drew analogy to actinide (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) buffered stock solutions and embarked to change the current state of affairs. Additional motivation came from operational needs at Los Alamos National Laboratory (LANL) to improve actinide chelation technologies and establish a small-scale capability for actinide metal production.
To achieve our goal and advance understanding of actinide speciation in buffered (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) (pH ¼ 5.5) stock solution starting materials, we carefully selected aqueous environments where (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) was present at three different concentrations. Particular attention was paid to maintaining relevance, in that we wanted our results to be directly applicable (or easily extrapolated) to the abovementioned application space. First, actinides in solutions with dilute (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) (0.1 M) concentrations were interrogated. Here, the An 3+ and O 2 CMe 1À ions were solutes in an aqueous matrix. Second, actinides in concentrated (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) (15 M) solutions, where water was now a solute rather than a solvent, were studied. Third, we characterized actinides in a solution with an intermediate (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) (4 M) concentration. Under these conditions there was an abundance of the coordinating O 2 CMe 1À ions; meanwhile, water also remained in huge excess. A series of complementary characterization methods that were compatible with the complicated sample types (small quantities of highly radioactive actinides in aqueous media) were deployed, namely solution-phase X-ray absorption spectroscopy (XAS) and solution-phase and solid-state optical spectroscopy (absorption, emission, excitation, and excited state lifetime measurements). Results were evaluated within the context of structural data from single crystals grown from acetate solutions. Of particular relevance was characterization of Am(O 2 -CMe) 5 2À , Cm(O 2 CMe) 5 2À , and Eu(O 2 CMe) 5 2À using single crystal X-ray diffraction.

Results and discussion
Synthesis of bisammonium metal(III) pentakisacetate, (NH 4 ) 2 M(O 2 CMe) 5 , M ¼ Eu 3+ , Am 3+ , Cm 3+ Both solution and solid-state 5f-element syntheses relied on preparing chemically pure stock solutions of Ac 3+ (aq) , Am 3+ (aq) , and Cm 3+ (aq) (see Methods section for details). From these nitrate stock solutions, syntheses for bisammonium metal(III) pentakisacetate, (NH 4 ) 2 M(O 2 CMe) 5 , involved rst precipitating hydrated f-element hydroxides, M(OH) 3 $xH 2 O with ammonium hydroxide, NH 4 OH (aq) (14.5 M). Granted, this hydroxide drop was only attempted with actinide (Cm 3+ , Am 3+ ) and lanthanide (Eu 3+ ) elements that could be handled on the macroscopic scale (mg quantities), not Ac 3+ . The resulting residue was washed with waterto remove residual nitrateand dissolved in an aqueous solution of ammonium acetate, (NH 4 )O 2 CMe (aq) (10 M). Slow evaporation of these solutions at room temperature for 2.5 weeks routinely produced single crystals of (NH 4 ) 2 M(O 2 CMe) 5 that were suitable for single crystal X-ray diffraction. We estimated that the crystalline yields from Cm 3+ and Am 3+ were similar to that from Eu 3+ at $26%. Given the good success with Cm 3+ , Am 3+ , and Eu 3+ attempts to extend the procedure to trans-curium elements were made. Applying this procedure to Cf 3+ has unfortunately not yet produced single crystals that could be characterized by X-ray diffraction.
Structure of bisammonium metal(III) pentakisacetate, The (NH 4 ) 2 M(O 2 CMe) 5 compounds were isomorphous and crystallized in the monoclinic, P2 1 /n space group as onedimensional chains (Fig. 1). Each structure contained ve acetate ligands bound to M 3+ cations. Three acetates were bidentate (k 2 ) and terminal, one was monodentate and terminal (k 1 ), and one was monodentate and bridging (m 1 :k 1 ). From the perspective of the metal, local geometries were best described as having a pseudo-pentagonal plane of ve oxygen atoms, capped on one side by a single oxygen atom and on the other side by three oxygen atoms. All nine of these oxygens were associated with acetate ligands. Hence, the compounds were homoleptic and no bound H 2 O was observed. This 9-coordinate geometry approached a mono-capped square antiprism, 24 which is wellestablished for lanthanides 25 and actinides. 26 Average metal-O O2CMe distances have been provided in Table 1. These acetate structures constitute rare examples of minor actinides characterized by single crystal X-ray diffraction. [27][28][29][30] Solution-phase speciation To determine if the solid-state structures described above were maintained in solution, An 3+ L 3 -edge X-ray absorption near edge spectroscopy (XANES), An 3+ L 3 -edge extended X-ray absorption ne structure (EXAFS), ultraviolet-visible spectroscopy (UV-vis), and time resolved excited state uorescence lifetime (TRFL) measurements were carried out on the M 3+ cations dissolved in acetate solutions. Scope for XAS experiments was conned to two actinide size extremes; the largest +3 5f-element (Ac 3+ , ionic radius ¼ 1.12Å for a coordination number of 6) was compared against one of the smallest actinides (Cm 3+ , ionic radius ¼ 0.97 A for a coordination number of 6). Another constraint was being able to obtain reasonably-sized quantities of these radionuclides (mg for Ac and mg for Cm). 31 Actinide L 3 -edge XAS spectra were evaluated as a function of increasing concentrations of (NH 4  All background subtracted and normalized An 3+ L 3 -edge XANES spectra were dominated by an absorption peak superimposed on an absorption edge (Fig. 2). These absorption peaks primarily resulted from electric-dipole allowed actinide 2pelectronic excitations to nal states that contained 6dcharacter. Using Ac 3+ as an example, this involved transitions between the 2p 6 . 5f 0 6d 0 ground-state and 2p 5 . 5f 0 6d 1 excited-state electronic congurations. The inection point for Ac 3+ 34 At this stage, it is unclear why the Ac 3+ inection points were more sensitive to ligand environments than Cm 3+ ; although, it is tempting to propose that these appreciable Ac 3+ L 3 -edge inection point shis resulted from more substantial orbital mixing in Ac-ligand bonds vs. the Cmligand bonds. Room temperature k 3 -weighted An 3+ L 3 -edge EXAFS spectra and Fourier transformed data from Cm 3+ and Ac 3+ have been compared in Fig. 3-5 as a function of (NH 4 )O 2 CMe (aq) :HO 2 -CMe (aq) buffered concentration. Models of the data were constrained to maintain a reasonable number of free variables and avoided overparameterization of the ts. 37 Endpoint energies varied from 9.5 (Ac 3+ ) to 10.5 (Cm 3+ )Å À1 in k-space, which restricted the EXAFS resolution to 0.17 and 0.15Å for Ac 3+ and  5 2.50 AE 0.08 (NH 4 ) 2 Cm(O 2 CMe) 5 2.49 AE 0.08 a The standard deviation is high due to the mixture of (k 1 )-vs. (k 2 )coordination environments.  ) were xed at 0.9 and the mean-squared displacement variables (s 2 ), actinide-H 2 O distances (An-O H2O ), and H 2 O coordination numbers (N H2O ) were allowed to converge to reasonable values (Table 3 and Fig. 5). For Ac 3+ , the EXAFS data suggested that the rst coordination-sphere contained 9(1) waters at a long Ac-O H2O distance of 2.64(1)Å. For Cm 3+ , the t suggested 8(1) water molecules with a substantially shorter Cm-O H2O distance of 2.47(1)Å, consistent with the smaller Cm 3+ ionic radius.
Structural metrics from the Ac 3+ spectra in dilute (NH 4 )O 2 -CMe (aq) :HO 2 CMe (aq) buffered solutions (0.2 M) were equivalent to the two other Ac 3+ -aquo reports, namely Ac 3+ L 3 -edge EXAFS studies in dilute triic and dilute nitric acid. 34 All three of these Ac 3+ -aquo data sets also agreed (within uncertainty) to predictions from molecular dynamics simulations on Ac(H 2 O) x 3+ .
There was agreement with the original predictions we published using ab initio molecular dynamics (AIMD) calculations. That level of theory predicted 9 H 2 O ligands with an average Ac-O H2O distance of 2.69 AE 0.11Å (8 ps time interval). 35 There was also agreement with EXAFS simulations by Marcos and co-Workers using classical molecular dynamics simulations (Ac-O H2O 2.66 AE 0.02Å and a coordination number ranging from 8 to 9 over a 10 ns time interval). 38 Our results agreed serendipitously with previous XANES simulations. 36  a Inection point and the peak positions were dened graphically where the second and rst derivative of the data equaled zero, respectively.
Higher frequency contributions were modeled with the 3(1) C O2CMe atoms, 3(1) Me O2CMe atoms, and 6(1) four-component     Table 3 Parameters used to model the An 3+ (An ¼ Ac and Cm) L 3edge EXAFS spectra from Ac 3+ and Cm 3+ dissolved in (NH 4 )O 2 -CMe (aq) :HO 2 CMe (aq) buffered (pH 5.5) aqueous solutions. Note, the amplitude reduction factors (S 0 2 ) were fixed at 0.9 Scheme 1 Simulated scattering paths used to model the Ac 3+ and Cm 3+ L 3 -edge EXAFS data. Single scattering and directly bound O (yellow or blue) and C (purple) were modeled for the first shell. The higher-order features were modeled with a triangle multiple scattering path (green), and a linear scattering path from the methyl backbone (light blue).  (2)Å. There was also a second shell with 2(1) carbon atoms at 2.91(1)Å. Contributions to the EXAFS spectrum from the methyl group of the acetate ligand were too weak to detect, likely owing to a combination of the small number of coordinated O 2 CMe 1À ligands and to the long distance from Cm 3+ to the outer methyl substituent. Magnitudes and uncertainties associated with coordination  numbers, bond distances, and Debye-Waller factors (s 2 ) for the Cm-O H2O , Cm-O O2CMe , and Cm-C O2CMe scattering pathwaysas well as for the ionization energy displacement variable (E 0 ) -rened reasonably well, and the number of independent variables (9) (4 M), efforts were made to qualitatively identify acetate complexation using UV-vis ( Fig. 6 and 7), emission (Fig. 8), and excitation ( Fig. 9) spectroscopy. We anticipated that displacement of H 2 O by O 2 CMe 1À would alter the intensities for many features associated within the excitation spectra, as H 2 O binding provides non-radiative decay pathways for the optically accessed excited state species that are inaccessible in complexes containing only O 2 CMe 1À ligands.
We also expected that substituting a stronger eld and bidentate O 2 CMe 1À ligand for the weaker eld and monodentate H 2 O ligand would increase the ligand eld splitting and reduce the symmetry of the resulting acetate complexes. Taken together, these changes should systematically shi the energy for the emission peaks as a function of (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) concentration relative to the Cm 3+ -aquo ion; albeit, the exact impact of these substitutions on the intensity and energy can be difficult to predict. The UV-vis absorption spectra were anticipated to exhibit systematic changes, as well, upon increased O 2 CMe 1À complexation. Conclusions regarding substitution of O 2 CMe 1À for H 2 O based on these qualitative excitation and absorption measurements were then quantitatively substantiated using time-resolved uorescence lifetime (TRFL) spectroscopy ( Fig. 10 and 11). In this scenario, the emission lifetimes were expected to increase with increasing (NH 4  To calibrate the optical response of Cm 3+ and Eu 3+ as a function of (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) concentration, excitation spectra were initially obtained in aqueous environments that were well-established for stabilizing "true" Cm 3+ -and Eu 3+aquo ions, namely in dilute HOTf (aq) (0.11 M; Fig. 9). For the  Cm 3+ -aquo ion, excitation spectra were collected with an emission wavelength (l em ) of 598 nm (16 720 cm À1 ). The 5f / 5f transitions were labeled using conventional alphabetic designators; Z / C (454 to 458 nm), Z / D (445-449 nm), Z / E (432 to 436 nm), Z / F (396 to 399 nm), Z / G (380 to 383 nm) and Z / H (375 to 378 nm). 49 The excitation spectrum (l em ¼ 619 nm, 6170 cm À1 ) from the Eu 3+ -aquo ion also contained 4f / 4f transitions attributed previously to 7 F 0 / 5 D 2 (465 nm), 7 F 1 / 5 D 3 (415 nm), 7 F 0 / 5 L 6 (395 nm), a combination of 7 F 1 / 5 L 7 and 7 F 0 / 5 G 2 (375 to 385), and 7 F 0 / 5 D 4 (360 nm). 33 Intensity changes for subsequently obtained Cm 3+ and Eu 3+ spectra were monitored aer normalizing peak maxima for the Z / F (for Cm 3+ ) and 7 F 0 / 5 L 6 (for Eu 3+ ) transitions to unity, at 1.0.
Changing the matrix from dilute HOTf (aq) (0.11 M) to the dilute (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) buffered solution (0.1 M, pH ¼ 5.5) and then increasing the (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) concentration (to 4 M and 15 M) imparted more noticeable changes on the excitation spectra from Cm 3+ than Eu 3+ . For Cm 3+ , these changes manifested primarily as shis for the Z / F, Z / E, and Z / C peaks to lower energy. Additionally, the Z / E and Z / C features decreased in intensity. For Eu 3+ , the intensity for the 7 F 0 / 5 D 2 transitions increased while the high energy transitions (>25 000 cm À1 ) decreased. Although we were unable to exactly characterize the origin for these intensity changes and energy shis, it is qualitatively obvious that the excitation spectra reected how increasing (NH 4 )O 2 CMe (aq) :-HO 2 CMe (aq) concentration pushed the speciation prole away from the aquo ion and toward complexes with larger numbers of coordinated acetate ligands (Scheme 2). Absorption and emission spectra from Cm 3+ and Eu 3+ provided similar qualitative evidence of increasing acetate complexation ( Fig. 6-8).
Time resolved uorescence lifetime (TRFL) measurements were made to quantify hydration numbers for Cm 3+ and Eu 3+ as a function of increasing (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) buffered concentrations ( Fig. 10 and 11). This approach represents one of the most powerful techniques available for quantifying H 2 O bound by Cm 3+ and Eu 3+ in solution. In these experiments, excitation energies were xed at 393 nm (25 450 cm À1 ) for Cm 3+ and 395 nm (25 320 cm À1 ) for Eu 3+ . Subsequent analyte emission was monitored at 598 or 605 nm (16 720 or 16 530 cm À1 for Cm 3+ ) and 619 nm (16 170 cm À1 for Eu 3+ ), and the emission decay kinetics were measured. Emission decay rates were modeled with a bi-exponential function (e ÀkinstrT + e ÀkobsT ). There was initially a short decay rate (k instr ) associated with the instrument response function (IRF, >100 ms) that was followed by a longer decay rate (k obs ) associated with the Cm 3+ and Eu 3+ complexes (>100 ms, k obs being the analyte lifetime; Tables 4, 5). The number of bound water molecules (N H 2 O ) was determined using well-established eqn (1) (Cm 3+ ) and eqn (2) By convention, the uncertainty associated with the TRFL determined hydration numbers was estimated at AE0.5. 33,50,51,53 Consistent with previous studies, our analyses of the Cm 3+ -and Eu 3+ -aquo ions (HOTf, 0.11 M) showed hydration numbers at 9.1(5) and 9.0(5). 51

Outlook
The solution-phase speciation and solid-state structures from the +3 actinides (Ac, Am, Cm) have been described from the perspective of a combination of actinide L 3 -edge X-ray absorption spectroscopy, optical spectroscopy, and single crystal X-ray diffraction measurements.  3(1) . The observed reactivity differences can be rationalized by considering changes in Lewis acidity for the central actinide cations. The smaller, more Lewis acidic Cm 3+ cation had a higher effective nuclear charge and attracted more O 2 CMe 1À anionic ligands than the larger Ac 3+ cation.  These results directly refute the naïve assumption that chemistry for the +3 actinides is constant across the 5f-series and highlights a property unique to the f-elements that has been exploited for advancement of numerous technologies for decades (at least for the rare-earth elements, not actinides). For example, the rare earth elements are unique in that they represent a collection of sixteen metals (including Lu 3+ and Y 3+ and excluding Sc 3+ ) whose properties are (in general) quite similar and whose +3 ionic radii methodically and subtly decrease as a function of element identity. This enables researchers to ne-tune properties for rare earth containing materials by judicious element selection, which is not possible for the main group and d-block transition element series. Examples range from modulating electronic properties in solid-state superconducting materials 54 to tuning selectivity in diene polymerization. 55 This slight difference in Lewis acidity has also provided a foundation for successful lanthanide separation technologies, like those that were developed by Hoffman, Choppin, and Spedding and relied on systematically changing the concentration of a complexing agent (like hydroxy isobutyric acid) during ion exchange chromatography. [56][57][58][59] These same properties have provided a basis for separation of adjacent minor actinides as well (Am 3+ from Cm 3+ ). 60 Our acetate studies align with the assumption (and a limited number of experimental observations) that actinide(III) complexation is directly inuenced by actinide Lewis acidity. It provides a rare example showing how 5f-element speciation varies as actinide ionic radii contract, which is analogous to that observed for rare earth elements. Although this concept is expected, the impact from an "actinide contraction" on 5felement coordination chemistry is scarce in the literature. The absence is especially obvious for actinium and the transuranic actinides (like Am and Cm), owing to the challenges associated with obtaining and studying these rare and radioactive elements. The implications of An 3+ speciation differences in (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) buffered solutions are subtle, but important. They highlight that the dominant species present in high concentration (NH 4 )O 2 CMe (aq) :HO 2 CMe (aq) buffered solutions for the early actinides is not equivalent to that for the minor actinides. We speculated that the increased complexation tendencies for the late actinide(III) cations vs. the early actinides likely persists even when the (NH 4  Fully characterizing how small changes in Lewis acidity impact aqueous complexation chemistry in (NH 4 )O 2 CMe (aq) :-HO 2 CMe (aq) buffered solutions, and in other relevant aqueous matrixes, could have widespread impact. There is potential to substantially expand fundamental chemical understanding for actinide cations, especially those that are highly radioactive and difficult to study (like Ac 3+ and Cm 3+ ). Better characterizing this aspect of aqueous actinide coordination chemistry would arm researchers with critical information that touches on virtually every aspect of relevant actinide science and technology; spanning from advanced environmental fate and transport models to designing new technologies that selectively deliver actinide cations to diseased tissue for targeted alpha therapeutic applications. It is our considered opinion that the study herein complements inuential campaigns reported previously, those focused on better dening the fundamental landscape of actinide speciation in the presence of simple organic and inorganic complexing agents in aqueous media. 10,17,36,40,[61][62][63][64] We hope that collectively this body of work allures new research groups into the area and motivates additional study of actinides in relevant aqueous solutions.  65 isotopesand their daughterspresent serious health threats due to their neutron-, a-, b-, and g-emissions. Hence, all studies that involved manipulation of these isotopes were conducted in a radiation laboratory equipped with HEPA ltered hoods, continuous air monitors, negative pressure gloveboxes, and monitoring equipment appropriate for neutron-, a-, b-, and g-particle detection. Entrance to the laboratory space was controlled with a hand and foot monitoring instrument for a-, b-, and g-emitting isotopes and a full body personal contamination monitoring station. Free-owing solids were handled within negative pressure gloveboxes equipped with HEPA lters. The 248 Cm, 243 Am, and 227 Ac isotopes were supplied by the United States Department of Single crystal UV-vis-NIR measurements were made on single crystals mounted on a quartz slide under oil using a Craic Technologies microspectrophotometer. For absorbance measurements, data were collected from 9090.91 to 40 000 cm À1 (1100 nm to 250 nm).

General consideration
Solution-phase UV-vis-NIR measurements were recorded on a Varian Cary 6000i spectrophotometer in a screw cap quartz cuvette.
Bisammonium europium(III) pentakisacetate, (NH 4 ) 2 Eu(O 2 CMe) 5 In an open front hood and with no attempt to exclude air and moisture, europium(III) trisnitrato hexahydrate [Eu(NO 3 ) 3 -$6H 2 O, 15 mg, 0.04 mmol] was dissolved in H 2 O (18 mU, 2 mL). Europium(III) was then precipitated by adding ammonium hydroxide, NH 4 OH (aq) (14.5 M, 1 mL). The supernatant was removed by centrifugation (3000 rpm, 3 min), the pellet suspended in H 2 O (18 MU, $5 mL) using a stir rod, and the resulting supernatant removed again by centrifugation. Using this procedure, the pellet was washed a total of three times. Then the washed pellet was dissolved in (NH 4 )O 2 CMe (aq) (10 M, 3 mL). Aliquots (0.5 mL) from this stock solution were transferred to six crystallization vials and colorless single crystals (rectangular platelets) suitable for single crystal X-ray diffraction were isolated aer 2.5 weeks of slow-evaporation (0.65 mg, 25.9% yield).

Preparation of actinide stock solutions and recovery (general)
Both solution and solid-state 5f-element syntheses relied on preparing chemically pure stock solutions of Ac 3+ (aq) , Am 3+ (aq) , and Cm 3+ (aq) . Initially, these solutions were generated according to our previous reports, 36 with the exception that prior to puri-cation samples were red at 850 C to remove the organic contaminants. However, for Am 3+ and Cm 3+ the procedures evolved over the course of this study. We switched from using DOWEX (50WX8 50-100 mesh) cation exchange resin to the extraction resin (branched DGA 100-200 mesh from Eichrom) deployed in Ac 3+ recovery and purication. 36 The change was primarily driven by higher Am 3+ and Cm 3+ recoveries from DGA (>98%) vs. cation exchange resins ($80%). The modest increase in yield was of particular signicance for these isotopes, given their rarity and value. Aer using the Ac 3+ , Am 3+ , and Cm 3+ stock solutions in the coordination chemistry experiments described below, these +3 actinides were recovered using a variation of the procedure described above for preparing stock solutions. Firing at 850 C was skipped because Am 3+ and Cm 3+ could be separated from the majority of the acetate and acetic acid by precipitation with HF. More rigorous purication was achieved with the DGA column. For Ac 3+ , which was present on the microscopic level (mg), the HF precipitation was not possible. Hence, Ac 3+ purications combined the DGA extraction chromatography with anion exchange chromatography using AG-1X8 resin. 67

Preparation of curium(III) stock solution
In a HEPA ltered open front fume hood and with no attempt to exclude air and moisture, a Cm 3+ stock solution was prepared as shown in Scheme 3. Residues known to contain 248 Cm (3 to 10 mg) that had been used in previous experimental campaigns were red in a muffle furnace (a ramp rate of 1 C min À1 was used to rst heat to 110 C to dehydrate the sample for 120 minutes, before ramping at 0.5 C min À1 to 850 C where it was held for 180 minutes) within a quartz beaker. The residue was dissolved in nitric acid (HNO 3 , 8 M, 2.5 mL) and transferred into a falcon tube (50 mL A Bio-Rad column (10 mL) was charged with DGA resin (Eichorm, 50-100 mm, 3 mL). The resin was conditioned with HNO 3(aq) (4 M, 3 Â 5 mL), H 2 O (18 MU, 5 mL), and HCl (0.1 M, 3 Â 5 mL). The column was then washed once with HNO 3(aq) (4 M, 5 mL) and the Cm 3+ solution in HNO 3(aq) (4 M, 5 mL) was loaded onto the column. Under these conditions, Cm 3+ was retained on the resin. The column was washed with HNO 3(aq) (4 M, 3 Â 5 mL). Then, Cm 3+ was eluted with HCl (0.1 M, 8 Â 1 mL). The Cm 3+ elution prole was quantied by stippling small volumes (one drop) of the eluted fraction onto Pyrex slides. Aer the samples dried under air, gross 248 Cm a-activity was quantied by analyzing each slide using a Ludlum 3939E a-, b-, g-stationary survey instrument (for low activity samples) or a handheld portable a-survey meter (Ludlum 139) for high activity samples. Aerwards, stippled 248 Cm was recovered by soaking the slides in HCl (6 M) and set aside for reprocessing at a later date. The second, third, and fourth Cm 3+ elution fractions (which contained the majority of the activity) were combined and the solution was heated to a so dryness. The resulting residue was dissolved in HCl (aq) (0.1 M, 1.0 mL), giving a chemically and radiochemically pure Cm 3+ stock solution. The Cm 3+ concentration was determined by analyzing an aliquot (250 mL) of the stock solution in triuoromethanesulfonic acid (HOTf (aq) , 0.11 M, 0.75 mL) by UV-vis spectroscopy. For these measurements, we assumed the 25 220.7 cm À1 (396.5 nm) absorbance had an extinction coefficient of 52.9 L mol À1 cm À1 , as previously reported. 68,69 Bisammonium curium(III) pentakisacetate, (NH 4 ) 2 Cm(O 2 CMe) 5 In an open front hood and with no attempt to exclude air and moisture, an aliquot of the Cm stock solution (34.4 mM, 0.1 mL), described above, was transferred into a Falcon tube (50 mL). Curium(III) was precipitated by adding NH 4 OH(aq) (14.5 M, 1 mL) and the curium title compound was prepared as described above for (NH 4 ) 2 Eu(O 2 CMe) 5 . Pale yellow crystals (rectangular plates) suitable for single crystal X-ray diffraction were obtained aer 2.5 weeks by slow evaporation from an (NH 4 )O 2 CMe (aq) (10 M, 0.5 mL) solution.

Preparation of americium(III) stock solution
In an open front hood and with no attempt to exclude air and moisture, americium(IV) oxide (AmO 2 , 50 mg of Am, 0.206 mmol), acquired from Oak Ridge National Laboratory, was dissolved in HNO 3 (4 M, 2.5 mL). The solution was evaporated to a so dryness and the resulting residue dissolved with HNO 3 (4 M, 2.5 mL). This was repeated three times to ensure complete removal of Cl 1À . The resulting americium nitrate solution was used to prepare an Am 3+ stock solution in direct analogy to the procedure described above for the Cm. This was achieved by precipitating AmF 3 (using HF) followed by extraction chromatography with the DGA resin. Note, ve columns were prepared and the Am solution was split so that 10 mg of Am were run through each column. Quantication of Am 3+ eluting from the DGA column was carried out using g-spectroscopy, as opposed to monitoring activity of aliquots stippled on Pyrex slides as described for Cm above. The nal Am 3+ stock solution was isolated in 95.7% yield and contained 47.9 mg of Am 3+ dissolved in 3 mL of HCl (0.1 M), as determined by a combination of gspectroscopy and UV-vis. For these gand UV-vis measurements, we assumed the g-peak at 74.7 keV had a branching ratio of 67.2% (ref. 65)  In an open front hood and with no attempt to exclude air and moisture, an aliquot of the Am 3+ stock solution (0.165 mM, 2.5 mL), described above, was transferred into a Falcon tube (50 mL). Americium(III) was precipitated by adding NH 4 OH (aq) (14.5 M, 1 mL) and the americium title compound was prepared as described above for (NH 4 ) 2 Eu(O 2 CMe) 5 . Pale peach crystals (rectangular plates) suitable for single crystal X-ray diffraction were obtained aer 1 week by slow evaporation from an (NH 4 )O 2 CMe (aq) (10 M, 0.25 mL) solution.
Single crystal X-ray diffraction Single crystals of (NH 4 ) 2 M(O 2 CMe) 5 (M ¼ Am, Cm) were mounted with three appropriate layers of containment prior to single-crystal X-ray diffraction studies, as previously described. 66 All other single crystals were mounted on nylon loops with mineral oil (Hampton Research). Diffraction data were obtained using a D8 Bruker QUEST diffractometer. No corrections for crystal decay were necessary. Standard Apex III soware was used for determination of the unit cells and data collection control. The intensities of the reections of a sphere were collected by combining 13 sets of exposures (frames), which totaled to 2035 frames with an exposure time of 15 per frame, depending on the crystal. Apex III soware was used for data integration including Lorentz and polarization corrections. All crystal structures were solved using SHELX soware, 71

Solution-phase sample preparation
Solutions for UV-vis (Cm), uorescence (Cm), and XAS (Cm and Ac) spectroscopy measurements were prepared in an open front fume hood with no attempt to exclude air and moisture. Three buffered solutions, 0.1 M, 4 M, and 15 M, of acetate concentrations, HO 2 CMe:(NH 4 )O 2 CMe (pH ¼ 5.5) were prepared. These samples were prepared by adding aliquots of the puried Cm 3+ (0.7 mg, 2.81 mmol for XAS; 0.3 mg, 1.20 mmol for UV-vis) and Ac 3+ (28 mg, 0.123 mmol) stock solutions into conical glass vials. The aqueous solution was removed by heating the samples on a hot plate around 110 C under a ow of air until a so dryness was achieved. These residues were dissolved in (NH 4 )O 2 CMe:HO 2 CMe buffered solutions described above and the solutions were transferred to XAS holders or cuvettes (for UV-vis) for spectroscopic analyses.

Emission and time resolved uorescence lifetime measurements
The emission spectra and lifetimes were obtained using a Photon Technologies International (PTI) model QM-04 uorometer with Felix32 soware. Steady state excitation and emission spectra were collected using a 75 W Xe arc lamp as an excitation source and a thermoelectrically cooled Hamamatsu R928 photomultiplier tube to measure emission at a 90 angle. Excitation and emission slits were set to give a 1.5 nm bandpass for all samples, with the exception of the solid (NH 4 ) 2 -Eu(O 2 CMe) 5 and Eu(H 2 O) 9 (OTf) 3 . In these cases, the bandpass was set to 3 nm. Lifetimes were collected using a ms Xe ash lamp (Xenoash) operated at 5 Hz for the excitation source and a time-gated PMT detection system oriented at a 90 angle. Again, the bandpass was set to 1.5 nm for all samples except solid (NH 4 ) 2 Eu(O 2 CMe) 5 and Eu(H 2 O) 9 (OTf) 3 , where a 3 nm bandpass was used. The raw data from the PTI Quanta were exported into Origin 2019. Then, the data were t with a double exponential decay function (IRF and decay), from which kinetic lifetimes were extracted. Radiological samples were contained using screw-top quartz cuvettes. Cuvettes were loaded in a HEPA ltered open-front hood and surveyed using a Ludlum 3030 smear counter before transport to the instrument.

Radiological containment for XAS samples
The custom-made XAS holders and handling procedures provided adequate containment (three layers) and administrative/engineering controls that guarded against release of radiological material during shipment and data acquisition. The holder consisted of a Teon body with a 5 mm well for Cm 3+ and a 2 mm well for Ac 3+ equipped with a set of Teon windows (1 mil) and a Kapton window (1 mil). Solutions were introduced into the holder through an injection hole sealed with a Teon gasket that was held in place by an aluminum plate. This primary holder was held within a secondary container, which in turn was nested within the tertiary container. The secondary and tertiary containers were best described as a set of aluminum holders equipped with Kapton windows (2 mil) and rubber gaskets.

XAS data acquisition
The X-ray absorption spectra (XAS) were collected at the Stanford Synchrotron Radiation Lightsource (SSRL) under dedicated operating conditions (3.0 GeV, 5%, 500 mA) on end station 11-2. This beamline was equipped with a 26-pole and a 2.0 Tesla wiggler. Using a liquid nitrogen-cooled double-crystal Si(220) (F ¼ 90 and 0 for Ac and Cm, respectively) monochromator and employing collimating mirrors, a single energy was selected from the incident white beam. Vertical acceptance was controlled by slits positioned before the monochromator. All measurements were conducted with the monochromator crystals fully-tuned. For these experiments, higher harmonics from the monochromatic light were removed using a 370 mm Rh coated harmonic rejection mirror. The Rh coating was 50 nm with 20 nm seed coating and the substrate was Zerodur. The harmonic rejection cut-off was set at 21 700 eV by the mirror angle, thereby controlling which photons experience total external reection.
The samples were attached to the beamline 11-2 XAS rail. The rail was equipped with three ionization chambers through which nitrogen gas continually owed. One chamber (10 cm long) was positioned before the sample holder, to monitor the incident radiation (I 0 ). The second chamber (30 cm long) was positioned aer the sample holder, such that sample transmission (I 1 ) could be evaluated against I 0 , while a third chamber (I 2 , 30 cm long) positioned downstream from I 1 so that the XANES of a calibration foil could be measured in situ during the XAS experiments against I 1 . All actinide L 3 -edge XAS samples were measured by monitoring sample uorescence against the incident radiation (I 0 ). An additional 100 element Ge uorescence detector was positioned at 90 to the incident radiation (I 0 ) and windowed on either the Cm La 1 -emission line (14 961 eV) or the Ac La 1 -emission line (12 652 eV). With this designation, actinide L 3 -edge XAS spectra (15 871 eV and 18 970 eV for Ac and Cm, respectively) were recorded in uorescence mode as the ratio of uorescence intensity over the intensity of the incident radiation (I 0 ). High-energy contributions to the uorescence signal were removed by equipping the Ge detector with Soller slits and either a Sr lter (3 absorption lengths for Cm) or a Br lter (3 absorption lengths for Ac).

XAS data analysis
Data manipulations and analyses were conducted as previously described. 36,40 The Ac 3+ L 3 -edges XAS data were calibrated to the Rb K-edge (15 200 eV) from a RbCl pellet diluted in BN to 1 absorption length and Cm 3+ L 3 -edges XAS spectra were calibrated to the Zr Kedge from a Zr metal-foil (18 013.3 eV). All calibration samples were measured in situ. To correct for detector dead time, nonlinear response curves were dened from 0 to $70% dead (windowed counts of the emission line versus the total incoming counts into the solid-state detector) using an Y 3 mm lter ($400 above the Y Kedge) for Cm and a Se lter ($400 eV above the Se K-edge) for Ac. Each channel was manually surveyed for outliers, which were omitted. The deadtime correction was applied before averaging the individual channels. Then, the 8 individual scans were aligned with the in situ calibration foil and averaged using IFEFFIT 73 within the Athena soware package. The XAS data were analyzed by tting a line to the pre-edge region, which removed the background from experimental data in the spectra. Then, a third-order polynomial t was chosen for the post-edge region. The difference between pre-and post-edge lines was set to unity at the rst inection point, normalizing the absorption jump to 1.0. To remove contributions from low frequency noise, a spline function was t over the absorption background of an isolated atom and subtracted from the data. The EXAFS data were then analyzed by shell-by-shell tting methods using IFEFFIT 73 soware and FEFF8 calculations. 74,75 The spectra were k 3 -weighted and Fourier transformed prior to nonlinear least squares curve tting. The energy phase shi parameter (DE o ) was rened as a global parameter and then xed for the remainder of the curve-tting analyses. The amplitude reduction factor (S 0 2 ) was set to 0.9 in accordance with our previous An-Cl, Anaquo, An-NO 3 studies and numerous other actinide EXAFS reports. 34,35 The actinide coordination number (N), scattering path length (R), and mean-squared displacements (s 2 ), were used as variables. Values from N, R and s 2 were rened as free variables. In the Cm 3+ case, atomic coordinates for the FEFF8 calculations were obtained from the single-crystal X-ray data (CIFs) for Cm(O 2 CMe) 5 2À (described above). For Ac 3+ , the central Cm 3+ cation was substituted in silico to generate a hypothetical Ac(O 2 CMe) 5