Article pubs.acs.org/IC
Spontaneous Partitioning of Californium from Curium: Curious Cases from the Crystallization of Curium Coordination Complexes Samantha K. Cary,† Mark A. Silver,† Guokui Liu,‡ Jamie C. Wang,† Justin A. Bogart,§ Jared T. Stritzinger,† Alexandra A. Arico,† Kenneth Hanson,† Eric J. Schelter,§ and Thomas E. Albrecht-Schmitt*,† †
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States § P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, United States ‡
S Supporting Information *
ABSTRACT: The reaction of 248CmCl3 with excess 2,6-pyridinedicarboxylic acid (DPA) under mild solvothermal conditions results in crystallization of the tris-chelate complex Cm(HDPA)3·H2O. Approximately half of the curium remains in solution at the end of this process, and evaporation of the mother liquor results in crystallization of the bis-chelate complex [Cm(HDPA)(H2DPA)(H2O)2Cl]Cl·2H2O. 248Cm is the daughter of the α decay of 252Cf and is extracted in high purity from this parent. However, trace amounts of 249,250,251 Cf are still present in all samples of 248Cm. During the crystallization of Cm(HDPA) 3 ·H 2 O and [Cm(HDPA)(H 2 DPA)(H 2 O) 2 Cl]Cl·2H 2 O, californium(III) spontaneously separates itself from the curium complexes and is found doped within crystals of DPA in the form of Cf(HDPA)3. These results add to the growing body of evidence that the chemistry of californium is fundamentally different from that of earlier actinides.
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INTRODUCTION
only three elements whose magnetic interactions are capable of controlling its crystal structure.6 Curium(III) is perhaps the most luminescent of all 5felement ions and emits characteristic orange light centered near 600 nm.7 This property is extraordinarily useful in a wide variety of applications that range from solution complexation studies to the environmental behavior of trivalent actinides to biological probes to understanding energy-transfer processes in f-block materials.7 Changes in the coordination of curium(III) can cause substantial shifts in the photoluminescence peak position and spectral shape.7 In contrast, gadolinium(III) compounds, while capable of emitting at visible wavelengths, have low quantum yields even when antennas are utilized for energy transfer and are often used as nonemitting hosts in europium(III)- and terbium(III)-doped materials.8 These aforementioned features point to a clear need to expand our understanding of curium(III)’s electronic structure and bonding in complex materials because these features may be indicators of ligand-field effects on the 5f electrons that are largely absent with the 4f electrons of gadolinium(III).9 While the luminescence of curium(III) species in solution has been well developed for many decades and continues to find new applications, there is a dearth of well-characterized curium(III) compounds. This can be ascribed to both the scarcity of the
Curium plays a central role in actinide chemistry in that it is isoelectronic with gadolinium, and both trivalent ions possess half-filled f7 shells. This allows curium(III) compounds and complexes to be used as benchmarks for comparisons with gadolinium and other lanthanide analogues as well as with both earlier and later actinides.1 Given the spherical symmetry of the f7 configuration and the general perception that both 4f and 5f orbitals are nonbonding, one might expect that gadolinium(III) would be an excellent analogue of curium(III) if the difference in their ionic radii is excluded. In fact, the electronic characteristics of curium(III) diverge from those of gadolinium(III) in a number of respects. For example, curium(III) doped into LaCl3 exhibits both splitting of the 8S7/2 ground state and enhanced crystal-field (CF) interactions compared to those found for gadolinium(III) doped into the same host lattice.2 Curium(III) materials are also known to exhibit small reductions in magnetic moments with respect to those calculated for the free ion, perhaps because of the splitting of the ground state. Examples of this include Cm2CuO4 and Cs2NaCmCl6, whose measured magnetic moments (μeff) are 7.89 and 7.64 μB, whereas the calculated moment for the free ion is 7.94 μB.3,4 In contrast, the effective moments exhibited by gadolinium(III) samples are virtually superimposable with the calculated value in most cases.5 The strongest testament of the unique electronic characteristics of curium is that it is one of © 2015 American Chemical Society
Received: September 4, 2015 Published: November 12, 2015 11399
DOI: 10.1021/acs.inorgchem.5b02052 Inorg. Chem. 2015, 54, 11399−11404
Article
Inorganic Chemistry only available long-lived isotope, 248Cm (t1/2 = 3.4 × 105 years), and the rarity of radiological facilities capable of handling greater than tracer levels of a moderately intense neutron emitter (8.3% of 248Cm decay is via spontaneous fission). Hence, there are only a handful of curium compounds where the structure, bonding, spectroscopy, and physical properties have been deeply probed, the best examples of which are Cm2O3,10 CmO2,11 CmCl3,12 Cm[M(CN)2]·3H2O (M = Ag, Au),13 [Cm(H2O)9][SO3CF3]3,1 Cm(IO3)3,6 Cm2CuO4,3 and Cm2[B14O20(OH)7(H2O)2Cl].6 The separation of middle-to-late actinides from one another has been a key challenge for decades in the development of the chemistry of these elements, their use as targets for the production of super heavy elements (i.e., transactinides), and advanced nuclear fuel cycles for separating lanthanides from actinides. Studies of the stability constants of trivalent lanthanides and actinides with α-hydroxyisobutyrate show that a monotonic trend exists with lanthanides.14 However, with diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid, this trend is not preserved; curium binds more weakly, and californium more strongly, than expected.14,15 These early studies were already suggestive of a change in chemistry occurring at californium that continues through mendelevium.14,15 We recently communicated a few features of the curium(III) tris-chelate, 2,6-pyridinedicarboxylate (dipicolinic acid, DPA) complex, Cm(HDPA)3, as a part of a comprehensive study that compared middle actinides with californium.16−18 Herein, we substantially expand on our analysis of this complex as well as elucidate the structure and properties of the bis-chelate complex [Cm(HDPA)(H2DPA)(H2O)2Cl]+. 248 Cm is milked from 252Cf sources in high purity. However, no separation is perfect, and various californium isotopes are carried along with the curium daughter at levels of
