Diethylenetriaminepentaacetic acid (DTPA) is a frequently used chelator in the nuclear and medical industries, especially for the complexation of trivalent actinides. However, structural data on these complexes in the solid-state have long remained elusive. Herein, a detailed structural analysis of the presented crystal structures of [C(NH2)3]4[Nd(DTPA)]2·nH2O and [C(NH2)3]4[Am(DTPA)]2·nH2O, where [C(NH2)3]+ is guanidinium, details the subtle differences in the Lewis acidity between a lanthanide/actinide pair of similar ionic sizes. Contractions in nitrogen-metal bond lengths between neodymium(III) and americium(III) were observed, while the metal-oxygen bonds remained relatively consistent, highlighting the marginal favorability for actinide complexation over the lanthanides with moderately soft N-donors. Spectroscopic analysis shows significant splitting of many transitions and relatively strong electronic interactions with traditionally low-intensity transitions in the americium complex, as is demonstrated in the 7F0→7F5 transitions. Pressure-induced spectroscopic analysis showed surprisingly little effect on the americium complex, with 5f→5f transitions either not shifting or marginally shifting from 2 to 3 nm at 11.93 ± 0.06 GPa─atypical of a soft, N-donor americium complex under pressure. Large voids occupied by water molecules in between the complexes within the crystal structure may be responsible for the lack of pressure response in the 5f→5f transitions.

Three Sm(II) dibenzo-24-crown-8 (db24c8) complexes were synthesized in anhydrous, air-free conditions via the reaction of SmI2 with db24c8 and tetrabutylammonium tetraphenylborate ([TBA][BPh4]; where needed) in acetonitrile (CH3CN), dimethoxyethane (DME), and tetrahydrofuran (THF) to yield [Sm(db24c8)(CH3CN)2][BPh4][I]·CH3CN, [Sm(db24c8)(DME)]I2, and [Sm(db24c8)(THF)2]I2, respectively. In each case, a 10-coordinate, staggered dodecahedral (2:6:2) environment is formed around the Sm2+ center that is completed by either two solvent molecules (CH3CN or THF) or one bidentate solvent molecule (DME). Inner-sphere solvent molecules can be excluded by reacting SmI2 with db24c8 in 1:3 THF:toluene to yield Sm(db24c8)I2. This molecule features a distorted, eight-coordinate, hexagonal pyramidal Sm2+ metal center, where the coordinated db24c8 molecule shows a torsion angle unexpectedly close to the 180° antiperiplanar arrangement and two uncoordinated db24c8 oxygen atoms. Solution UV-vis-NIR measurements demonstrate that Sm2+ is a good size match for the cavity of various db24c8 conformations and that Eu2+ and Yb2+ exhibit competition between acetonitrile solvation and the Eu2+ and Yb2+/db24c8 complexes in solution. During excitation by 546 nm light, both [Sm(db24c8)(DME)]I2 and [Sm(db24c8)(THF)2]I2 exhibit mixed 5d → 4f and 4f → 4f emission at 20 °C and exclusively 4f → 4f at -180 °C, whereas Sm(db24c8)I2 only shows 5d → 4f emission regardless of temperature. Photoluminescence from [Sm(db24c8)(CH3CN)2][BPh4][I]·CH3CN is quenched.

An octa-hydrated curium compound [Cm(H2O)8](Hdtp)(dtp)·H2O (Cm1, H2dtp = 2,3-di(tetrazol-5-yl)pyrazine) along with its lanthanide analogues [Ln(H2O)8](Hdtp)(dtp)·H2O (Ln1, Ln3+ = La3+-Nd3+, Sm3+-Lu3+) were synthesized and characterized using single crystal X-ray diffraction and spectroscopic methods. Bond length analysis of VIIICm(III)-OH2 (where VIII refers to the coordination number) was compared to VIIILn(III)-OH2 (Ln3+ = Nd3+ and Sm3+), indicating similar VIIIM(III)-OH2 bond lengths because of their similar eight-coordinate ionic radii of these VIIIM(III) cations. Owing to the reduced coordination number, the VIIICm(III)-OH2 bond lengths were shorter than previously reported IXCm(III)-OH2 bonds in [Cm(H2O)9](CF3SO3)3. The octa-aquo complexes were also characterized by solid-state UV-vis-NIR spectroscopy in addition to variable-temperature and variable-pressure photoluminescence. Variable-pressure absorption spectra of Cm1 were compared with Ln1 and show that the Cm(III) f → f transitions have a stronger dependence on pressure than that observed in Ln1 (Ln3+ = Nd3+ and Sm3+). The experimental and computational analyses reveal that the monotonic decrease in the computed energy difference between the ground state and the first excited state corresponds to the observed red shift of the photoluminescence peak. This is accompanied by a gradual reduction in the average Cm(III)-OH2 bond length and a delocalization of spin densities, alongside an intensified interaction involving the 5f orbitals under increasing pressure. These changes accommodate the new geometry and collectively modify the energy landscape, resulting in peak broadening and quenching.

Mixed-donor ligands, such as those containing a combination of O/N or O/S, have been studied extensively for the selective extraction of trivalent actinides, especially Am3+ and Cm3+, from lanthanides during the recycling of used nuclear fuel. Oxygen/sulfur donor ligand combinations also result from the hydrolytic and/or radiolytic degradation of dithiophosphates, such as the Cyanex class of extractants, which are initially converted to monothiophosphates. To understand potential differences between the binding of such degraded ligands to Nd3+ and Am3+, the monothiophosphate complexes [M(OPS(OEt)(2))(5)(H2O)(2)](2-) (M3+ = Nd3+, Am3+) were prepared and characterized by single-crystal X-ray diffraction and optical spectroscopy and studied as a function of pressure up to ca. 14 GPa using diamond-anvil techniques. Although Nd3+ and Am3+ have nearly identical eight-coordinated ionic radii, these structures reveal that while the M-O bond distances in these complexes are almost equal, the M-S distances are statistically different. Moreover, for [Nd(OPS(OEt)(2))(5)(H2O)(2)](2-), the hypersensitive I-4(9/2) -> (4)G(5/2) transition shifts as a function of pressure by -11 cm(-1)/GPa. Whereas for [Am(OPS(OEt)(2))(5)(H2O)(2)](2-), the F-7(0) -> F-7(6) transition shows a slightly stronger pressure dependence with a shift of -13 cm(-1)/GPa and also exhibits broadening of the 5f -> 5f transitions at high pressures. These data likely indicate an increased involvement of the 5f orbitals in bonding with Am3+ relative to that of Nd3+ in these complexes.

To develop the structural chemistry of radium, the halide compounds RaX2·H2O and RaX2·2H2O (X- = Cl- and Br-) have been synthesized and characterized and serve as benchmarks for comparisons with more complex compounds in the future. In contrast with historic reports on the structural chemistry of radium, the Ra2+ chlorides differ from their Ba2+ analogues. For MCl2·H2O (M2+ = Ba2+, Ra2+), the variance between the metal coordination environments manifests as a small, local distortion that becomes more apparent in the extended structure. However, differences between RaCl2·2H2O and BaCl2·2H2O are more pronounced with a 10-coordinate Ra2+ cation being observed instead of a nine-coordinate Ba2+ in BaCl2·2H2O. RaBr2·nH2O (n = 1 or 2) are isomorphous with the Ba2+ analogues. Raman spectroscopy was used as an additional probe of these compounds and reveals substantial shifts and different vibrational modes between RaX2·H2O and RaX2·2H2O compared to BaX2·2H2O.

To develop the structural chemistry of radium, the halide compounds RaX2H2O and RaX22H2O (X- = Cl- and Br-) have been synthesized and characterized and serve as benchmarks for comparisons with more complex compounds in the future. In contrast with historic reports on the structural chemistry of radium, the Ra2+ chlorides differ from their Ba2+ analogues. For MCl2H2O (M2+ = Ba2+, Ra2+), the variance between the metal coordination environments manifests as a small, local distortion that becomes more apparent in the extended structure. However, differences between RaCl22H2O and BaCl22H2O are more pronounced with a 10-coordinate Ra2+ cation being observed instead of a nine-coordinate Ba2+ in BaCl22H2O. RaBr2nH2O (n = 1 or 2) are isomorphous with the Ba2+ analogues. Raman spectroscopy was used as an additional probe of these compounds and reveals substantial shifts and different vibrational modes between RaX2H2O and RaX22H2O compared to BaX22H2O.

The coordinative properties of 12-crown-4 (12c4) with Sm2+, Eu2+, and Yb2+ have been examined using nonaqueous and inert atmosphere conditions and led to the isolation of five complexes: Ln(12c4)(THF)(2)I-2 (Ln = Sm 1, Eu 2), [Ln(12c4)(2)(CH3CN)][Ph4B](2) (Ln = Sm 3, Eu 4), and [Yb(12c4)(2)][Ph4B](2) (5). Most complexes were prepared via the salt metathesis of LnI(2) with tetrabutylammonium tetraphenylborate ([TBA][Ph4B]) and 12-crown-4 in acetonitrile, while some were crystallized from THF. The half-sandwich compounds 1 and 2 crystallize with trans iodide orientation and exhibit mixed d-f and f-f and d-f photoluminescence when excited with 546 and 365 nm light, respectively. The full sandwich compounds 3 and 4 feature [Ln(12c4)(2)(CH3CN)](2+) complex cations, where two 12c4 molecules are not sufficiently large to coordinatively saturate these larger cations without further ligation, while the smaller Yb2+ cation is fully encapsulated in two 12c4 molecules (5). Solution UV-vis-NIR studies show that when 12c4 is added to acetonitrile solutions of LnI(2), the f-d transitions shift to higher energies, suggesting destabilization of the lower lying d orbitals and increased stability of the divalent state by complexation to 12c4 in solution.

Understanding the effects of pressure on actinide compounds is an integral part of safe nuclear waste storage in deep geologic repositories and provides a means of systematically altering the structure and properties. However, detailing how the effects of pressure evolve across the actinide series in the later elements is not typically undertaken because of the challenges of conducting research on these unstable isotopes. Here, a family of bimetallic actinide complexes, [(An(pmtz)(2)(H2O)(3))(2)(mu-pmtz)](2)(pmtz)(2)nH(2)O (An(3+) = Cm3+, Bk3+, and Cf3+, pmtz(-) = 5-(pyrimidyl)tetrazolate; Cm1, Bk1, and Cf1), are reported and represent the first structurally characterized bimetallic berkelium and californium compounds. The pressure response as determined from UV-vis-NIR transitions varies for Cm1, Bk1, and Cf1. The 5f -> 5f transitions in Cm1 are notably more sensitive to pressure compared to those in Bk1 and Cf1 and show substantial bathochromic shifting of several 5f -> 5f transitions. In the case of Bk1, an ingrowth of a metal-to-ligand charge-transfer transition occurs at elevated pressures because of the accessible Bk3+/Bk4+ couple. For Cf1, no substantial transition shifting or emergence of MLCT transitions is observed at elevated pressures because of the prohibitive energetics of the Cf3+/Cf4+ couple and reduced sensitivity of the 5f -> 5f transitions to the local coordination environment because of the more contracted 5f shell versus Cm3+ and Bk3+.

The salt metathesis reaction of Na(pmtz)H2O [pmtz(-) = 5-(pyrimidyl)tetrazolate] and PuBr3nH(2)O in an aqueous media leads to the formation of the mononuclear compound [Pu(pmtz)(3)(H2O)(3)](3 + n) H2O (Pu1, n = similar to 8) that is isotypic with the lanthanide compounds [Ln(pmtz)(3)(H2O)(3)](3 + n) H2O (Ln = Ce-Nd). Dissolution and recrystallization of Pu1 in water yields the dinuclear compound {[Pu(pmtz)(2)(H2O)(3)](2)(mu-pmtz)}(2)(pmtz)(2)14H(2)O (Pu2), which is isotypic with the lanthanide compounds {[Ln(pmtz)(2)(H2O)(3)](2)(mu-pmtz)}(2)(pmtz)(2)14H(2)O (Ln = Nd and Sm). Like their nine-coordinate ionic radii, the M-O and M-N bond lengths in Pu1/Pu2 and Nd1/Nd2, respectively, are within error of one another. The Laporte-forbidden 4f -> 4f and 5f -> 5f transitions are also assigned in the UV-vis-NIR spectra for these f-element tetrazolate coordination compounds.

The crystal structure of the title compound, hexaaquanickel(II) dichloride–1,4,7,10,13,16-hexaoxacyclooctadecane–water (1/2/2), [Ni(H2O)6]Cl2·2C12H24O6·2H2O, is reported. The asymmetric unit contains half of the Ni(OH2)6 moiety with a formula of C12H32ClNi0.50O10 at 105 K and triclinic (P1) symmetry. The [Ni(OH2)6]2+ cation has close to ideal octahedral geometry with O—Ni—O bond angles that are within 3° of idealized values. The supramolecular structure includes hydrogen bonding between the water ligands, 18-crown-6 molecules, Cl− anions, and co-crystallized water solvent. Two crown ether molecules flank the [Ni(OH2)6]2+ molecule at the axial positions in a sandwich-like structure. The relatively symmetric hydrogen-bonding network is enabled by small Cl− counter-ions and likely influences the more idealized octahedral geometry of [Ni(OH2)6]2+.

The crystal structure of the title compound, C15H20N2 or DippIm, is reported. At 106 (2) K, the molecule has monoclinic P21/c symmetry with four molecules in the unit cell. The imidazole ring is rotated 80.7 (1)° relative to the phenyl ring. Intermolecular stabilization primarily results from close contacts between the N atom at the 3-position on the imidazole ring and the C—H bond at the 4-position on the neighboring DippIm, with aryl–aryl distances outside of the accepted distance of 5 Å for π-stacking.

Californium (Z = 98) is the first member of the actinide series displaying metastability of the 2+ oxidation state. Understanding the origin of this chemical behavior requires characterizing Cf(II) materials, but isolating a complex with this state has remained elusive. The source of its inaccessibility arises from the intrinsic challenges of manipulating this unstable element as well as a lack of suitable reductants that do not reduce Cf(III) to Cf(0). Herein we show that a Cf(II) crown-ether complex, Cf(18-crown-6)I2, can be prepared using an Al/Hg amalgam as a reductant. While spectroscopic evidence shows that Cf(III) can be quantitatively reduced to Cf(II), rapid radiolytic re-oxidation back to the Cf(III) parent occurs and co-crystallized mixtures of Cf(II) and Cf(III) complexes are isolated if the crystallization is not conducted over the Al/Hg amalgam. Quantum chemical calculations show that the Cf‒ligand interactions are highly ionic and that 5f/6d mixing is absent, resulting in remarkably weak 5f→5f transitions and an absorption spectrum dominated by 5f→6d transitions.

The cyclization of salophan with triethyl orthoformatein the presenceof HBr results in a bisphenol benzimidazolium carbene precursor 1,3-bis(2-hydroxybenzyl)-2H-benzimidazolium bromide (H(3)OBBIm). By reactingH(3)OBBIm in a one-pot synthesis with Ln(N(SiMe3)(2))(3) (Ln = La-Sm excl. Pm) and an excessof KN(SiMe3)(2), the N-heterocycliccarbene (NHC) complexes, [Ln(OBBIm)(2)(N '')][K2THF x ], can be prepared and isolated.These complexes were characterized by single-crystal X-ray diffraction,solution- and solid-state UV-vis-near-infrared spectroscopy,and H-1 NMR. These complexes possess distorted pentagonalbipyramid geometries.

The crystallization, single crystal structure, and Raman spectroscopy of Ra(NO3)2 have been investigated by experimentation and theory, which represent the first pure radium compound characterized by single crystal X-ray diffraction. The Ra2+ centers are bound by six chelating nitrate anions to form an anticuboctahedral geometry. The Raman spectrum acquired from a single crystal of Ra(NO3)2 generally occurs at a lower frequency than found in Ba(NO3)2, as expected. Computational studies on Ra(NO3)2 provide an estimation of the bond orders via Wiberg bond indices and indicate that Ra-O interactions are weak with values of 0.025 and 0.026 for Ra-O bonds. Inspection of natural bond orbitals and natural localized molecular orbitals suggest negligible orbital mixing. However, second-order perturbation interactions show that donation from the lone pairs of the nitrate oxygen atoms to the 7s orbitals of Ra2+ stabilizes each Ra-O interaction by ca. 5 kcal mol-1.

Two cis-dioxomolybdenum complexes based on salan ligands with different backbones are reported. The first complex, dioxido{2,2′-[l,2-phenylenebis(iminomethylene)]bis(phenolato)}molybdenum(VI) dimethylformamide disolvate, [Mo(C20H18N2O2)O2]·2C3H7NO (PhLMoO2, 1b), features a phenyl backbone, while the second complex, (6,6′-{[(cyclohexane-1,2-diyl)bis(azanediyl)]bis(methylene)}bis(2,4-di-tert-butylphenolato))dioxidomolybdenum(VI) methanol disolvate, [Mo(C36H56N2O2)O2]·2CH3OH (CyLMoO2, 2b), is based on a cyclohexyl backbone. These complexes crystallized as solvated species, 1b·2DMF and 2b·2MeOH. The salan ligands PhLH2 (1a) and CyLH2 (2a) coordinate to the molybdenum center in these complexes 1b and 2b in a κ2N,κ2O fashion, forming a distorted octahedral geometry. The Mo—N and Mo—O distances are 2.3475 (16) and 1.9567 (16) Å, respectively, in 1b while the corresponding measurements are Mo—N = 2.3412 (12) Å, and Mo—O = 1.9428 (10) Å for 2b. A key geometrical feature is that the N—Mo—N angle of 72.40 (4)° in CyLMoO2 is slightly less than that of the PhLMoO2 angle of 75.18 (6)°, which is attributed to the flexibility of the cyclohexane ring between the nitrogen as compared to the rigid phenyl ring in the PhLMoO2.

The structure of a trinuclear zinc complex, hexakis(μ2-2-anilinobenzoato)diaquatrizinc(II), [Zn2(C13H10NO2)6(H2O)2] or (NPA)6Zn3(H2O)2 (NPA is 2-anilinobenzoate or N-phenylanthranilate), is reported. The complex crystallizes in the triclinic space group P-1 and the central ZnII atom is located on an inversion center. The NPA ligand is found to coordinate via the carboxylate O atoms with unique C-O bond lengths that support an unequal distribution of resonance over the carboxylate fragment. The axial H2O ligands form hydrogen bonds with neighboring molecules that stabilize the supramolecular system in rigid straight chains, with an angle of 180° along the c axis. π stacking is the primary stabilization along the a and b axes, resulting in a highly ordered supramolecular structure. Docking studies show that this unique supramolecular structure of a trinuclear zinc complex has potential for binding to the main protease (Mpro) in SARS-CoV-2 in a different location from Remdesivir, but with a similar binding strength.

A berkelium(III) mellitate, Bk2[C6(CO2)6](H2O)8·2H2O, was synthesized and rapidly crystallized by reacting mellitic acid, C6(CO2H)6, and BkBr3·nH2O in an aqueous medium. Single crystal X-ray diffraction shows that the compound crystallizes as a three-dimensional framework isostructural with Pu(III), Am(III), and Cm(III) mellitates. UV-vis-NIR spectroscopic studies as a function of pressure were performed using a diamond anvil cell and show that the 5f → 5f transitions of Bk3+ display enhanced hypsochromic shifting when compared to other An(III) mellitates.

Two neptunium(III) mellitates, 237Np2(mell)(H2O)9·1.5H2O (Np-1α) and 237Np2(mell)(H2O)8·2H2O (Np-1β), have been synthesized from 237NpCl4(dme)2 by reduction with KC8 and subsequent reaction with an aqueous solution of mellitic acid (H6mell). Characterization by single-crystal X-ray crystallography and UV-vis-NIR spectroscopy confirms that the neptunium is in its +3 oxidation state and both polymorphs are isostructural to the previously reported plutonium mellitates. Of the two morphologies, Np-1α is indefinitely stable in air, while Np-1β slowly oxidizes over several months. This is due to the change in the energy of the metal-ligand charge-transfer absorption exhibited by these compounds attributed to differing numbers of carboxylate bonds to Np(III), where in Np-1β the energy is low enough to result in spontaneous oxidation.

The crystal structures of ligand precursor bis(imidazolium) salts 1,1′-methylenebis(3-tert-butylimidazolium) dibromide monohydrate, C15H26N4+·2Br−·H2O or [tBuNHC2Me][Br]2·H2O, 1,1′-(ethane-1,2-diyl)bis(3-tert-butylimidazolium) dibromide dihydrate, C16H28N4+·2Br−·2H2O or [tBuNHC2Et][Br]2·2H2O, 1,1′-methylenebis[3-(2,4,6-trimethylphenyl)imidazolium] dibromide dihydrate, C25H30N42+·2Br−·2H2O or [MesNHC2Me][Br]2·2H2O, and 1,1′-(ethane-1,2-diyl)bis[3-(2,4,6-trimethylphenyl)imidazolium] dibromide tetrahydrate, C26H32N42+·2Br−·4H2O or [MesNHC2Et][Br]2·4H2O, are reported. At 293 K, [tBuNHC2Me][Br]2·H2O crystallizes in the P21/c space group, while [tBuNHC2Et][Br]2·2H2O crystallizes in the P21/n space group at 100 K. At 112 K, [MesNHC2Me][Br]2·2H2O crystallizes in the orthorhombic space group Pccn while [MesNHC2Et][Br]2·4H2O crystallizes in the P21/c space group at 100 K. Bond distances and angles within the imidazolium rings are generally comparable among the four structures. All four bis(imidazolium) salts co-crystallize with one to four molecules of water.

The title compound, bis­(1,2-diphenyl-2-sulfanyl­idene­ethane­thiol­ato-κ2S,S′)(1,3,5-tri­aza-7-phosphaadamantane-κP)cobalt(II) dichloromethane hemisolvate, [Co(pdt)2(PTA)]·0.5C2H4Cl2 or [Co(C14H10S2)2(C6H12N3P)]·0.5C2H4Cl2, contains two phenyl­dithiol­ene (pdt) ligands and a 1,3,5-tri­aza-7-phosphaadamantane (PTA) ligand bound to cobalt with the solvent 1,2-di­chloro­ethane mol­ecule located on an inversion center. The cobalt core exhibits an approximately square-pyramidal geometry with partially reduced thienyl radical monoanionic ligands. The supra­molecular network is consolidated by hydrogen-bonding inter­actions primarily with nitro­gen, sulfur and chlorine atoms, as well as parallel displaced π-stacking of the aryl rings. The UV–vis, IR, and CV data are also consistent with monoanionic di­thiol­ene ligands and an overall CoII oxidation state.