Comparison of the structural, electrochemical, and spectroscopic properties of two cryptates of trivalent uranium

We describe a study of the influence of cryptand denticity on the structural, electronic, and electrochemical properties of UIII-containing cryptates. Two cryptands (2.2.2 and 2.2.1) are reported. The cryptand with the smaller denticity leads to negative electrochemical potentials and shorter bond lengths that are consistent with a better fit for UIII than the larger cryptand. These studies provide insight into the rational design of cryptand-based ligands for trivalent uranium.


Introduction
8][19] Further, the coordination chemistry of U III , Np III , and Pu III with 2.2.2-cryptand has been reported recently, 20 expanding cryptand chemistry into the actinides.2][23][24] For example, 2.2.1cryptand fits better with Eu III , and 2.2.2-cryptand fits better with Eu II ; moreover, the Gibbs free energy of Eu III (2.2.1-cryptand) is 1.8 times greater than that of Eu III (2.2.2-cryptand), and the dissociation constant of Eu III (2.2.1-cryptand) is 2.7 × 10 3 times smaller than that of Eu III (2.2.2-cryptand). 24Because the size of U III (1.025 Å) is closer to the size of Eu III (0.947 Å) than Eu II (1.17 Å) 25 and the charge density of U III is closer to Eu III than Eu II , we suspected that 2.2.1-cryptand would be a good ligand for U III .Additional support for this suspicion is in our recent report that the flexible counterpart of 2.2.2-cryptand, tris[2-(2-methoxyethoxy)ethyl]amine (TDA-1), forms U IIIcontaining complexes with smaller coordination numbers (nine) compared to all reported 2.2.2-cryptates (with coordination numbers of ten). 26This report of an acyclic ligand implies that trivalent uranium can be encapsulated by cryptands with smaller denticities than that of 2.2.2-cryptand.Therefore, based on the studies of Eu III cryptand chemistry and U III chemistry with acyclic TDA-1, we hypothesized that 2.2.1-cryptand is a better match for U III than 2.2.2-cryptand.Here, we report U III -containing cryptates of 2.2.2-and 2.2.1-cryptand (Fig. 1) to investigate how ligand denticity affects the structural, spectroscopic, and electrochemical properties of U III .

Results and discussion
To evaluate the structural properties of U III -containing cryptates, crystals were grown from N,N-dimethylformamide (DMF) or acetonitrile (CH 3 CN) (Schemes 1 and 2), and the structures of [U III 1(DMF) 2 ]I 3 , [U III 2(CH 3 CN) 2 ]I 3 , and [U III 2(DMF) 2 ]I 3 were solved from the crystals (Fig. 2).All three complexes contained coordinated solvent molecules.The structure with 1 is like reported structures of 2.2.2-cryptates of trivalent uranium that share ten-coordinate structures with bound solvent molecules, iodide, 18,20 triflate, 22 or water molecules. 18,20The geometry of [U III 1(DMF) 2 ]I 3 , analyzed by SHAPE (v.2.1), 27 is sphenocorona.We also performed SHAPE analysis for the reported structure of [U III 1I(CH 3 CN)]I 2 with one inner-sphere iodide and one inner-sphere molecule of CH 3 CN (CCDC number 2020050), 19 and we found that it also has the sphenocorona geometry.The crystal structure that we report here contains two coordinated molecules of DMF instead of iodide, water, or triflate, but the change in monodentate donors does not change the geometry about U III .In contrast to  In addition to solid-state characterization, electronic spectroscopic characterization was performed for U1I 3 and U2I 3 to analyze how ligand denticity influences solution properties of the complexes.We performed UV-visible and near-IR experiments using elementally pure U III -containing complexes of 1 and 2 that does not include coordinated solvent molecules.Those complexes are referred to as U1I 3 and U2I 3 .UV-visible and near-IR electronic absorption data were collected from 350 to 1400 nm in CH 3 CN (Fig. 3).In the visible region, color-producing bands in U1I 3 appeared for the green-yellow solutions in acetonitrile with maximum absorbances at 395 nm (ε = 978 M −1 cm −1 ).U2I 3 forms reddish-pink solutions in CH 3 CN with color-producing bands having maxima at 407 nm (ε = 1008 M −1 cm −1 ) with shoulders at 521 nm (ε = 603 M −1 cm −1 ).Weak bands appear in the near-IR region for U1I 3 and U2I 3 like the other complexes of trivalent uranium with 5f 3 electronic configurations with Laporte-forbidden f-f transitions. 18,28,29These bands in near IR-region are broader than the UI 3 bands in acetonitrile (Fig. S3 †) and the reported acyclic U III TDA-1. 26The differences in the optical properties of U III 1I 3 and U III 2I 3 in solution, specifically the large differences in shifts of color producing bands, indicate an influence of the change of ligand denticity from octadentate to heptadentate on 5f-6d orbital energy gaps.
To study the electrochemical behavior of U III -containing cryptates, cyclic voltammetry was performed for U1I 3 and U2I 3 in CH 3 CN (Fig. 4).Oxidation peaks corresponding to the U III/IV couple appear at −0.26 V versus ferrocene/ferrocenium (Fc/Fc + ) for U1I 3 .The oxidation potential of the U III/IV couple of U1I 3 in CH 3 CN is like the reported oxidation potential (−0.31 V versus Fc/Fc + ) of U III 2.2.2-cryptate. 19The cyclic voltammogram of U2I 3 contains an oxidation peak corresponding to the U III/IV couple at −0.46 V versus Fc/Fc + .Both U1I 3 and U2I 3 contain an oxidation peak corresponding to a U III -to-U IV oxidation that is not observed in the voltammogram of UI 3 (Fig. S2 †).Cyclic voltammograms of U1I 3 and U2I 3 were performed using elementally pure powdered compounds that do not contain coordinated solvent molecules.Both U1I 3 and U2I 3 can coordinate at least one acetonitrile molecules in that solid state as evidenced by reported crystal structures 19 and this study.Similarly, in solution, acetonitrile can coordinate to U1I 3 , U2I 3 , and UI 3 .However, cyclic voltammetry of U1I 3 , U2I 3 , and UI 3 in this study were performed in the same solvent, acetonitrile; consequently, the variability in shifts of oxidation potentials that arises from the coordination of solvent is minimized.Therefore, the observed 0.2 V difference in oxidation potentials between U1I 3 and U2I 3 in acetonitrile most likely arises from changes in the ionization energies resulting from changes in the ligand structure.These results indicate that oxidation potentials of U2I 3 shift to more negative potentials with the decrease of coordination number compared to the octadentate cryptand in U1I 3 .The negative shift in oxidation potential is likely due to the smaller denticity of 2 compared to 1 and consequent size match between U III and 2. A similar relationship is observed in the reported study between Eu III -containing complexes of 1 and 2. 30 The formal potential of Eu III 2 (−425 mV versus saturated calomel electrode) is more negative than that of Eu III 1 (−225 mV versus saturated calomel elec-

Dalton Transactions Paper
trode).Therefore, the negative shift in cyclic voltammetry of U2I 3 is consistent with the electrochemistry of 4f systems.Interestingly, U1I 3 and U2I 3 have electrochemical potentials among the most positive of reported U III/IV couples of monometallic complexes of uranium. 31The reported U III complexes that coordinated to negatively charged donor atoms increase the electron density of uranium and consequently result in more negative electrochemical potentials. 31Therefore, the observed positive shift in electrochemical potential is not surprising for U III -containing cryptates of 1 and 2 when compared to complexes of cyclopentadienyls, bis(trimethylsilyl)amides, tris(aryloxides), and β-diketiminates. 31However, to the best of our knowledge, the relationship of cryptand denticity on electrochemical studies of actinide series has not been reported; therefore, the insight gained from the electrochemical studies described here provides information regarding the influence of cryptand denticity on the tuning of the electrochemical potential of U III .

General methods
All air-and moisture-sensitive reactions were performed using standard Schlenk technique with Ar or using an inert atmosphere dry glove box under an atmosphere of N 2 .
Elemental analyses were performed by the Microanalytical Facility at the University of California, Berkeley.Electronic absorption spectra were collected using a Jasco UV-Vis-NIR spectrophotometer, and metal concentrations were determined using energy-dispersive X-ray fluorescence spectroscopy with a Shimadzu EDX-7000 spectrometer at the Lumigen Instrument Center in the Department of Chemistry at Wayne State University.
Cyclic voltammetry was performed using a three-electrode setup with a Pine Wavenow USB potentiostat under an atmosphere of Ar with a glassy carbon working electrode, a freshly prepared Ag/AgCl wire pseudo reference electrode (Ag wire with AgCl coating was prepared by dipping a polished Ag wire in bleach for 5-10 min), and a Pt-wire auxiliary electrode.Acquisition parameters of [U III 1]I 3 and [U III 2]I 3 were eight segments, an initial potential of −0.5 or 0.0 V (rising), an upper potential of 0.7 V, a lower potential of −0.5 V or 0.0 V against Ag/AgCl pseudo reference electrode, and a scan rate of 100 mV s −1 .Tetrabutylammonium trifluoromethanesulfonate (0.1 M) was used as the electrolyte, and analyte concentrations were 3.0-3.5 mM.All cyclic voltammograms were recorded in CH 3 CN referenced to an internal standard of Fc/Fc + .

Conclusions
This study reports differences in the structural, spectroscopic, and electrochemical properties of U III encapsulated into

Paper
Dalton Transactions neutral, redox-inactive cryptands.The smaller denticity and cavity size of heptadentate 2 enabled greater bonding interactions with U III compared to octadentate 1.This evidence of a favorable size match between 2.2.1-cryptand and U III is supported by shorter bond lengths and negative shifts of the electrochemical potentials of U III 2 compared to U III 1.These findings provide valuable insight into the encapsulation of trivalent uranium that has potential use areas such as actinide separations relevant to the management of radioactive waste.
Scheme 1 Synthesis of U III -containing complex of 1.

Table 1
Average bond lengths of U III 1, U III 2, and U III (TDA-1)