Page 1 of 9 1 2 3 4 5 6 7 Actinide separation inspired by self-assembled metal-polyphenolic 8 9 nanocages 10 †, §, †, ‡, § † # † † ⊥ 11 Lei Mei, * Peng Ren, Qun-yan Wu, Yu-bin Ke, Jun-shan Geng, Kang Liu, Xue-qing Xing, 12 Zhi-wei Huang, † Kong-qiu Hu, † Ya-lan Liu, † Li-yong Yuan, † Guang Mo, ⊥ Zhong-hua Wu, ⊥ John K 13 Gibson, ＆ Zhi-fang Chai, †, ¶ Wei-qun Shi †, * 14 15 † Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China 16 ‡ State key Laboratory of Nuclear Resources and Environment, School of Chemistry, School of Nuclear Science and Engineering, 17 East China University of Technology, Nanchang 330013, China. 18 ⊥Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China 19 # Spallation Neutron Source Science Center, Dongguan 523803, China 20 ¶ Engineering Laboratory of Advanced Energy Materials, Ningbo Institute of Industrial Technology, Chinese Academy of Sciences, 21 Ningbo 315201, China 22 ＆ Chemical Sciences Division, Lawrence Berkeley National Laboratory (LBNL), Berkeley, California 94720, USA 23 KEYWORDS: actinides; nano-extraction; uranyl-organic nanocage; self-assembly; pyrogallol[4]arene 24 25 26 ABSTRACT: The separation of actinides has a vital place in nuclear fuel reprocessing, recovery of radionuclides and 27 remediation of environmental contamination. Here we propose a new paradigm of nanocluster-based actinide separation, 28 namely nano-extraction, that can achieve efficient sequestration of uranium in an unprecedented form of giant coordination 29 nanocages using a cone-shaped macrocyclic pyrogallol[4]arene as the extractant. The U24-based hexameric 30 pyrogallol[4]arene nanocages with distinctive [U2PG2] binuclear units (PG = pyrogallol), that rapidly assembled in situ in 31 monophasic solvent, were identified by single-crystal XRD, MALDI-TOF-MS, NMR, and SAXS/SANS. Comprehensive biphasic 32 extraction studies show that this novel separation strategy has enticing advantages such as fast kinetics, high efficiency, and 33 good selectivity over lanthanides, and thus demonstrate its potential for efficient separation of actinide ions. 34 35 36 As a central component of nuclear fuel processing, actinide including inorganic-organic hybrid polyhedra13-15 and 37 separation is crucial for recovery and recycling of key inorganic nanoclusters bridged by oxide, peroxide, or 38 radionuclides.1-2 Other applications that require efficient hydroxide groups.16-19 In contrast to simple complexes, 39 radionuclide capture include remediation of such assemblies, especially metal-organic coordination 40 environmental contamination,3 emergency response to assemblies with nanometer-scale dimensions, impose 41 nuclear accidents,4-5 and uranium extraction from strict requirements on both molecular geometries of the 42 seawater.6-9 Two established approaches to actinide constituent ligands and their coordination preference for 43 separation are solvent extraction and solid adsorption, targeted metal ions (Figure 1a), an effect termed 44 with the former dominant due to its adaptability to “multivalent cooperativity”.20-23 Only metal-ligand couples 45 complex real-world systems and applicability to large- that meet both geometry and coordination criteria are 10 46 scale operations. candidates for effective multi-component nano-assembly. Such cooperativity could lead to highly selective formation 47 Traditional actinide extraction is achieved through selective complexation of actinide cations from mixtures of exquisite multinuclear assemblies by efficiently 48 excluding undesired ions. Another attribute of this 49 containing multiple metal ions by exquisitely designed organic extractants.2, 11-12 Nevertheless, the use of large approach is that multivalent interactions can result in 50 cluster compounds for actinide separations has been remarkable enhancement in stability of the produced 51 rarely pursued, despite extensive studies of well-defined supramolecular architectures, which facilitates both 52 actinide assemblies that remain intact in solution, extraction and sequestration. 53 54 55 56 1 57 58 59 60 Page 2 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Figure 1. Actinide sequestration based on self-assembled nanoclusters via multivalent cooperativity: (a) coordination and 41 capture of actinide ions via simple complexation, or as a well-defined nanocluster; (b) molecular structure of PgCn (n = 3, 5 or 9); (c) model of cone-shaped backbone of PgCn; (d) giant uranyl-polyphenolic coordination cage. 42 43 Motivated by characteristics of such cooperativity, a PgCn (n = 3, 5 or 9) with actinides ions, we have achieved 44 novel strategy for efficient separation of actinides by high-performance uranyl capture via uranyl-PgCn 45 multivalent assembly of well-defined metal-organic nanocages (Figure 1d) assembled in situ in monophasic or 46 nanoclusters is proposed herein. As a proof of this new biphasic solvent systems. This type of extraction as 47 concept, we demonstrate sequestration of uranyl ion nanoscale species is unprecedented for actinide ions and it 2+ 48 (UO2 ) by macrocyclic polyphenolic ligands, C- leads to a novel paradigm for hybrid nanocluster-based 49 alkylpyrogallol[4]arenes (abbreviated as PgCn, where n is actinide sequestration, which may be considered as “nano- 50 the number of carbon atoms in the alkyl chain, Figure 1b). extraction”. 51 The cone-shaped PgCn, which is the tetramer of the natural Coordination of small-molecule polyphenolic ligand PG 52 phenolic mimic pyrogallol (PG) with variable length side- with uranyl was considered first. We reported the crystal 53 chain groups (Figure 1c), has been demonstrated as a structure of the resulting complex, U_PG (Table S1), which versatile precursor for construction of metal-organic represent the first crystal structure assessment of uranyl 54 24-34 nanocapsules. Based on coordination assemblies of 35-36 55 coordination by polyphenol ligands. In U_PG, doubly- 56 2 57 58 59 60 Page 3 of 9 deprotonated PG molecules chelate a pair of uranyl ions to [U2PG2] motif is quite disparate from trinuclear [M3PG3] 1 form a [U2(PG)2] moiety (Figure S1). Density functional moiety that is ubiquitous in other metal-seamed hexameric 2 theory (DFT) calculations indicate that the residual pyrogallol[4]arene nanocapsules.24-25, 28, 30, 32, 34, 39-40 This 3 phenolic proton in U_PG likely occupies a protonated μ1- difference might reflect the larger effective radius and 4 OH site, which orients far away from the corresponding μ1- distinct coordination behavior of the uranyl moiety versus 5 OH site on the opposing PG ligand (type A in Figure S2 and bare transition metals, main group and alkaline earth 6 Table S2). This configuration leads to relatively lower metals. Different coordination linkages furthermore lead to 7 affinity of the protonated μ1-OH site towards uranium ( intriguing characteristics of the topological structures of 8 bonding character, Figure S3 and Table S3-S4). uranyl-polyphenolic nanocages. Considering vanadium- Based on the distinctive coordination of PG towards based hexameric pyrogallol[4]arene compounds for 9 28 uranyl, complexation and assembly of pyrogallol[4]arene comparison, there is a direct correspondence between 10 the 24 metal atoms that form truncated cubes in 11 and uranyl was pursued. Starting with the PgC3 ligand [U24(PgC3)6] and [V24(PgC3)6] (Figure S13), though the 12 (Figure S4), the UV-vis absorbance spectra of the solution after reaction between a pink solution of PgC3 and uranyl cone-shaped pyrogallol[4]arene ligands that cover the six 13 in DMF (Figure S5). It is attributed to strong ligand-to- faces of the cube adopt different orientations. Specifically, 14 metal charge transfer (LMCT) from pyrogallol anion to four arms of the pyrogallol[4]arene in [U24(PgC3)6] define 15 uranyl,37-38 and thus indicates effective complexation of four regular sides of a truncated cube (Figure S13b-d), 16 uranyl in monophasic solvent. The UV-vis spectra of crystal while those in [V24(PgC3)6] define four inclined sides of an 17 samples resulting from reactions of PgC3 with uranyl octagonal face (Figure S13f-h). The topological difference 18 similarly exhibit strong LMCT absorbance from 600 nm to between [V24(PgC3)6] and [U24(PgC3)6] can be considered 19 350 nm (Figure S6). Molecular structures were as a reorganization of tetratopic cone-shaped 20 subsequently pursued for the products of reactions pyrogallol[4]arenes through an in-plane rotation 21 between uranyl and PgCn. The crystal structures of U_PgC3 accompanied by slight reorganization (Figure S13i). 22 and U_PgC5 show that both have as primary building Crystals of U_PgCn (n = 3, 5 or 9) were dispersed in 23 blocks isolated uranyl-pyrogallol[4]arene coordination organic solvents for solution characterization. All three 24 cages, [(UO2)24(H2O)24(PgCn)6] (n = 3 or 5; Figure 2a-c and complexes in DMF show wide absorbance in the UV-visible region, with a maximum absorption at ~394 nm (Figure 25 Figure S7), with differences in the side chain length n resulting in different lattice packing modes (Figure S8). S14). Cyclic voltammetry shows that the uranium centers 26 Although several pyrogallol[4]arene-based metal-organic in all three complexes exhibit the +6 oxidation state 27 nanocapsules (MONCs) have been reported24-28, 30, 32, 34, 39-40, characteristic of uranyl (Figure S15). Tyndall scattering 28 the [U24(PgCn)6] motif is the first metal-pyrogallol[4]arene phenomenon (Figure S16) is consistent with nanoscale 29 coordination cages based on actinide nodes. Crystals of features of [U24(PgCn)6]. Small angle X-ray scattering (SAXS) 30 U_PgC9 suitable for single crystal X-ray diffraction were and neutron scattering (SANS) were employed to probe 31 hard to be obtained (Figure S9), presumably due to the nanocage aggregation, in lieu of dynamic light scattering 32 long nonyl side chains hindering close packing of the that is inapplicable to light-absorbing nano-systems.