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Molecular assemblies of a series of mixed tetravalent uranium and trivalent lanthanide complexes associated with the dipicolinate ligand, in aqueous medium Nicolas P Martin, christophe volkringer, Natacha Henry, Sylvain Duval, Dimitrije Mara, Rik Van Deun, and Thierry Loiseau Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01617 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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Crystal Growth & Design

Molecular assemblies of a series of mixed tetravalent uranium and trivalent lanthanide complexes associated with the dipicolinate ligand, in aqueous medium

Nicolas P. Martin,a Christophe Volkringer,ab Natacha Henry,a Sylvain Duval,a Dimitrije Mara,c Rik Van Deun,c and Thierry Loiseaua

a

Université de Lille, Centrale Lille, ENSCL, Univ. Artois, UMR CNRS 8181 - UCCS - Unité de Catalyse et Chimie du Solide, F-59000 Lille, France. b

Institut Universitaire de France (IUF), 1, rue Descartes, 75231 Paris cedex 05, France. Luminescent Lanthanide Lab (L3), Department of Chemistry, Ghent University, Krijgslaan 281, S3, 9000 Ghent, Belgium. c

*

To whom correspondence should be addressed. E-mail: [email protected]. Phone: (33) 3 20 434 122, Fax: (33) 3 20 43 48 95.

To be submitted to Crystal Growth & Design. Version November 21, 2017 Revised version February 14, 2018 Final version March 2, 2018

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Crystal Growth & Design 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 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: The investigations of the chemical system involving tetravalent uranium associated to trivalent lanthanides (La, Ce, Nd, Sm-Er, Lu) with dipicolinic acid (H2dpa) led to the identification of six distinct types of heterometallic coordination complexes (I-VI). These compounds, obtained at room temperature from aqueous solution, have been characterized by single-crystal X-ray diffraction. They consist of tris-chelated dipicolinate uranium sub-units {U(dpa)3} interacting with poly oxo/aquo or mixed polyoxo/aquo-chelated dipicolinate lanthanide sub-units {LnO1-3(H2O)5-8 or Ln(dpa)O0-3(H2O)4-6} in different arrangements. Complex I (La,Ce) exhibits the assembly of molecular dinuclear [ULn] and [ULnU] trinuclear units, with linkage between U and Ln via bidentate carboxylate groups of the dipicolinate molecules. The same carboxylate bridging connection mode creates a linear chain-like coordination polymer in complex II (La,Ce), with a strict alternation U-Ln sequence. Complex III (Ce,Nd) contains an octanuclear motif based on six-ring [Ln4U2] decorated by two peripheral {U(dpa)3} sub-units, resulting in a [Ln4U4] entity, through carboxylate bridges. Complex IV crystallizes by using a wide series of lanthanide cations (Nd, Sm-Tb), and consists of a simple assembly of discrete {U(dpa)3} sub-units with {Ln(Hdpa)(H2O)6}. The complex V (Tb-Er) is composed of a hexanuclear motif based on four-ring [U2Ln2] with two peripheral {U(dpa)3} sub-units (final complex [U4Ln2]). The last complex (VI) occurs with lutetium and is built up from a dinuclear unit [ULu] connected through one carboxylate group. Thermal decomposition of samples of complexes II, III and IV has been analyzed by thermogravimetry and in-situ XRD. The luminescence spectrum of the europium-based complex IV has been measured. Keywords: tetravalent uranium, trivalent lanthanide, dipicolinic acid, coordination polymers, thermal analysis, europium luminescence

Introduction For the last decades, the chemistry of coordination polymers bearing actinides has been intensively investigated with a special attention on the elaboration of uranium-based complexes. For this specific element, the studies have mainly concerned the synthesis of a wide variety of uranyl-organic frameworks, i.e reporting the use of the hexavalent state for this 5f cation, in association with polydentate O- and/or N-donor organic ligands.1–6 Moreover, some efforts have been successfully devoted to the construction of coordination networks using tetravalent uranium with polytopic carboxylate linkers. In this series, polynuclear inorganic cores have been described with dicarboxylate molecules in order to generate original open-framework solids.7–11 The other challenge is to identify the coordination polymers involving a mixture of 5f elements, with different oxidation states. This situation typically occurs in the spent nuclear fuel reprocessing cycle, in which there is a requirement for the separation and/or co-precipitation of the different actinides (ex: U/Pu in 1


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the PUREX process). In this context, many studies have reported the synthetic conditions for the formation of mixed An(IV)/An(III) coordination complexes (An = actinide) in the presence of oxalic acid, which is used as a precipitating agent in aqueous nitric acid solutions.12,13 Similar strategies also detailed the synthesis and structural characterizations of mixed U(IV)/Ln(III) oxalates,14–19 since lanthanides (Ln) could be employed as surrogate element for trivalent heavy actinides (Pu, Am…), and are also present in the spent nuclear fuel reprocessing step as fission products. Apart from this study dedicated to the actinides oxalates, reports describing mixed actinide-lanthanide coordination compounds are quite scarce in literature. Some contributions are devoted to uranyl-lanthanide heterometallic complexes, involving polydentate carboxylate,20–27 N-heterocyclic ring carboxylate ligands28,29 or sulfonates.30,31 Other chemical systems have been explored by using complexing agents such as germanates32 or vanadates33,34 dealing with the remediation of actinides in environment, or forming molecular cage clusters based on uranyl peroxide building units.35,36 Regarding the tetravalent oxidation state for actinides, there are very few studies, except the investigations in the previously mentioned oxalate system. Thus, AlbrechtSchmitt and coworkers described a series of mixed Ce(IV)-Pu(IV) systems combined to diphosphonates exhibiting a disordered heterometallic compound.37 Beside the oxalate ligand, the dipicolinic acid (or 2,6-pyridinedicarboxylic acid) is a candidate of choice as a multi-chelating linker through several coordination modes via O- or N-bridges, for associating distinct metals in a given coordination complex. Many works have demonstrated the capacity of the dipicolinate ligand to generate heterometallic coordination polymers containing both transition and lanthanide metals.38–44 In this context, we explored the possibility of combinations of such ligand with uranium(IV) in the presence of trivalent lanthanide metals. In actinide chemistry, the dipicolinic acid has been employed for the crystallization of many coordination complexes involving the uranyl moiety45–56 together with transition metals (3d),57–59 neptunium(V),60,61 americium(III),62 curium(III)62,63 and californium(III).62 However, the dipicolinate ligand also form stable molecular coordination complexes with tetravalent actinides such as thorium(IV)64 or uranium(IV)65 with the [An(dpa)2(H2O)n] stoichiometry (dpa = dipicolinate; n = 3 (for U) or 4 (for Th)), the anionic moieties containing uranium(IV)66 [U(dpa)3]2or mixed uranium(IV/VI),67 [U(dpa)3UO2(dpa)(H2O)]2-. The complexation of the tetravalent actinide Th, U, Np and Pu with dipicolinate has been studied in aqueous solution, with the occurrence of the different species [An(dpa)]2+, [An(dpa)2]0 and [An(dpa)3]2-.68 The recent contribution of AlbrechtSchmitt and co-workers described an unprecedented mixed valence Pu(IV)/Pu(III) dipicolinate coordination polymer69 and displays the continuous relevance for the use of such organic O,N-donor molecule with actinides elements. Moreover, the interaction with this ligand towards actinides might be of significant interest since this N-heterocyclic dicarboxylate can be considered as a fragment of dicarbonic amide derivatives used as

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extracting agent for reprocessing liquid wastes containing mixtures of actinides (and lanthanides) from spent nuclear fuel.70–72 The present study deals with the synthesis of a series of heterometallic uranium(IV)lanthanide(III) dipicolinate coordination complexes, covering all the members of the lanthanide row. In our investigations, we isolated six types of molecular assemblies based on the association of the {U(dpa)3} moieties associated to Ln centers, which can be chelated by dipicolinate or not. We successfully identified the complex I: [U(dpa)3Ln(H2O)8]2[(U(dpa)3)2Ln(H2O)7][(U(dpa)3)2Ln(H2O)6]·21-20H2O with Ln = La (1), Ce (2), the complex II: [U(dpa)3Ln(Hdpa)(H2O)4]·6H2O, with Ln = La (3), Ce (4), the complex III: [(U(dpa)3)2Ln(dpa)(H2O)4Ln(H2O)7]2·18-24H2O, with Ln = Ce (5), Nd (6), the complex IV: [U(dpa)3][Ln(Hdpa)(H2O)6]·6-7H2O, with Ln = Nd (7), Sm (8), Eu (9), Gd (10), Tb (11), the complex V: [(U(dpa)3)2(Ln(H2O)5)]2-·2H3O+·8H2O, with Ln = Tb (12), Dy (13), Ho (14), Er (15) and the complex VI: [U(dpa)3Lu(Hdpa)(H2O)4]·11H2O (16). Four of these complexes are closely related to a series of compounds previously described by Prasad and Rajasekharan, in mixed cerium(IV)-lanthanide(III) dipicolinates,73 in which the tetravalent cerium is replaced by uranium in our work, with the same connection fashion. The structural description is reported for each type of complex. For some of them (type II, III and IV), the thermal decomposition is analyzed by thermogravimetry and X-ray diffraction. The luminescence property of the europium-based complex type IV is also indicated.

Experimental section Synthesis. Caution! Natural U precursors are radioactive and chemically toxic reactants, so precautions with suitable equipments and facility for radiation protection are required for handling these substances. The compounds have been hydrothermally synthesized under autogenous pressure using 20 mL glass vials with Teflon caps by using the following chemical reactants: uranium tetrachloride (UCl4, obtained from the protocol7 using the reaction of hexachloropropene with uranium oxide UO3), dipicolinic acid (C7H5NO4, noted H2dpa, Aldrich, 98%), lanthanum chloride heptahydrate (LaCl3·7H2O Alfa Aesar, 99%), cerium chloride anhydrous (CeCl3, Aldrich, 99.9%), neodymium chloride anhydrous (NdCl3, Aldrich, 99.9%), samarium chloride hexahydrate (SmCl3·6H2O, Aldrich, ≥99%), europium chloride hydrate (EuCl3·xH2O, Aldrich 99.9%), gadolinium chloride hexahydrate (GdCl3·6H2O, Aldrich 99%), terbium chloride hexahydrate (TbCl3·6H2O, Aldrich 99.9%), dysprosium chloride hexahydrate (DyCl3·6H2O, Aldrich 99.9%), holmium chloride hexahydrate (HoCl3·6H2O, Aldrich 99.9%), erbium chloride hexahydrate (ErCl3·6H2O, Alfa Aesar 99.9%) lutetium chloride heptahydrate (LuCl3·7H2O, Alfa Aesar 99.9%) and deoxygenated de-ionized water. The starting chemical reactants (except UCl4) are commercially available and have been used without any further purification. The reactant mixtures concerning the syntheses using UCl4 have been manipulated and weighted in a glove box under argon atmosphere. They have been then 3


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placed in closed glass vials, which are removed from the glove box and then heated in an oven (under ambient atmosphere). Complex I: [U(dpa)3Ln(H2O)8]2[(U(dpa)3)2Ln(H2O)7][(U(dpa)3)2Ln(H2O)6]·21-20H2O, Ln = La (1) or Ce (2): a mixture of 50 mg (0.13 mmol) UCl4, 150 mg (0.90 mmol) dipicolinic acid, 50 mg (0.13 mmol) LaCl3·7H2O or 40 mg (0.16 mmol) CeCl3 and 5 mL (278 mmol) H2O was placed in a closed glass vial and then heated statically at 130°C for 1 hour in order to dissolve all the reactants. The green solution was then placed at 22°C in a water bath during several days. The resulting products of I were then filtered off, washed with water and dried at room temperature under ambient air. Optical microscopy shows the formation of flat needle-shape crystallites (Figure S1a) in mixture with powdered products of complexes II (La (3) and Ce (4)). In case of cerium-based complex, the crystallization of compound 5 (type III) also appears. Complex II: [U(dpa)3Ln(Hdpa)(H2O)4]·6H2O, Ln = La (3) or Ce (4): a mixture of 50 mg (0.13 mmol) UCl4, 90 mg (0.54 mmol) dipicolinic acid, 100 mg (0.27 mmol) LaCl3·7H2O or 60 mg (0.24 mmol) CeCl3 and 5 mL (278 mmol) H2O was placed in a closed glass vial and then heated statically at 130°C for 1 hour in order to dissolve all the reactants. The green solution was then placed at 22°C in a water bath during several days. The resulting products of II were then filtered off, washed with water and dried at room temperature under ambient air. Optical microscopy shows the formation of large green blocks crystallites of 100-1000 µm (Figure 1). The corresponding X-ray diffraction powder pattern of the pure phase 3 is indicated in supplementary information (Figure S2a). Complex III: [(U(dpa)3)2Ln(dpa)(H2O)4Ln(H2O)7]2·18-24H2O, Ln = Ce (5) or Nd (6): a mixture of 50 mg (0.13 mmol) UCl4, 90 mg (0.54 mmol) dipicolinic acid, 100 mg (0.4 mmol) CeCl3 or 100 mg (0.40 mmol) UCl4 , 90 mg (0.54 mmol) dipicolinic acid, 55 mg (0.20 mmol) NdCl3 and 5 mL (278 mmol) H2O was placed in a closed glass vial and then heated statically at 130°C for 1 hour in order to dissolve all the reactants. The green solution was then placed at 22°C in a water bath during several days. The resulting products of III were then filtered off, washed with water and dried at room temperature under ambient air. Optical microscopy shows the formation of large green blocks crystallites of 100-1000 µm (Figure 1). The corresponding X-ray diffraction powder pattern of the pure phase 6 is indicated in supplementary information (Figure S2b). Complex IV: [U(dpa)3][Ln(Hdpa)(H2O)6]·6-7H2O, Ln = Nd (7), Sm (8), Eu (9), Gd (10), Tb (11): a mixture of 70 mg (0.18 mmol) UCl4, 90 mg (0.54 mmol) dipicolinic acid, 50 mg (0.20 mmol) NdCl3 or 55 mg (0.19 mmol) SmCl3·2H2O or 55 mg (0.18 mmol) EuCl3·2H2O or 60 mg (0.16mmol) GdCl3·6H2O or 60 mg (0.16 mmol) TbCl3·6H2O and 5 mL (278 mmol) H2O was placed in a closed glass vial and then heated statically at 130°C for 1 hour in order to dissolve all the reactants. The green solution was then placed at 22°C in a water bath during several days. The resulting products of IV were then filtered off, washed with water and dried 4



at room temperature under ambient air. Optical microscopy shows the formation of large green blocks crystallites of 100-1000 µm (Figure 1). The corresponding X-ray diffraction powder pattern of the pure phase 9 is indicated in supplementary information (Figure S2c). Complex V: [(U(dpa)3)2(Ln(H2O)5)]2-·2H3O+·8H2O, Ln = Tb (12), Dy (13), Ho (14), Er (15): a mixture of 50 mg (0.13 mmol) UCl4, 90 mg (0.54 mmol) dipicolinic acid, 50 mg (0.13 mmol) TbCl3·6H2O or 50 mg (0.13 mmol)DyCl3·6H2O or 50 mg (0.13 mmol) HoCl3·6H2O or 50 mg (0.13 mmol) ErCl3·6H2O and 5 mL (278 mmol) H2O was placed in a closed glass vial and then heated statically at 130°C for 1 hour in order to dissolve all the reactants. The green solution was then placed at 22°C in a water bath during several days. Only the resulting complex 12 was filtered off, washed with water and dried at room temperature under ambient air. The other compounds 13-15 are air sensitive and only single-crystals have been picked up from the mother solution for their X-ray diffraction analyses under nitrogen gas flow at 100K. Optical microscopy shows the formation of large green blocks crystallites (Figure 1). Complex VI: [U(dpa)3Ln(Hdpa)(H2O)4]·11H2O Ln = Lu (16): a mixture of 50 mg (0.13 mmol) UCl4, 90 mg (0.54 mmol) dipicolinic acid, 50 mg (0.13 mmol) LuCl3·6H2O and 5 mL (278 mmol) H2O was placed in a closed glass vial and then heated statically at 130°C for 1 hour in order to dissolve all the reactants. The green solution was then placed at 22°C in a water bath during several days. The resulting complex 16 (green flat needle-like crystallites – Figure 1) is air sensitive and only its single-crystal X-ray diffraction analysis was performed under nitrogen gas flow at 100K. Crystals of compound [U(dpa)2(H2O)3]·3,5H2O isolated by Haddad et al.65 were also obtained with compound 16 (Figure 1). When obtained as pure phase, the reaction yields for the formation of such mixed U(IV)Ln(III) dipicolinate complexes from slow evaporation technique were in the range 40-60%, based on uranium, after 1 day synthesis. The reaction pH of the reactants mixtures giving rise to the crystallization of different complexes I-VI was measured to be close to 0 after the heating step (1 hour). It did not vary for one given complex to another one, indicating that the pH value is constant during the study of our whole chemical system.

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Figure 1: Optical photographs of crystals of compounds: a) 1, b) 3, c) 6, d) 10, e) 13 and f) mixture of 16 (brown) and [U(dpa)2(H2O)3]·3.5H2O (green).

Single-crystal X-ray diffraction Crystals of the compounds bearing uranium (1-5,7-16) were analyzed on a Bruker DUOAPEX2 CCD area-detector diffractometer at 300K or 100K (in case of unstable crystals), using microfocus Mo-Kα radiation (λ = 0.71073 Å) with an optical fiber as collimator (at UCCS, University of Lille). For reasons of diffractometer availability, a crystal of compound 6 was analyzed on a Bruker X8-APEX2 CCD area-detector diffractometer at 100K using microfocus Ag-Kα radiation (λ = 0.56086 Å) with an optical fiber as collimator. Crystals of 116 were selected under a polarizing optical microscope and glued on a Kapton micromounts for single-crystal X-ray diffraction experiments. Several sets of narrow data frames (10 s per frame) were collected at different values of θ for two initial values of φ and ω, respectively, using 0.3° increments of φ or ω. Data reduction was accomplished using SAINT V7.53a.74 The substantial redundancy in data allowed a semi-empirical absorption correction (SADABS V2.1075) to be applied, on the basis of multiple measurements of equivalent reflections. The structure was solved by the intrinsic phasing method from the SHELXT program, developed by successive difference Fourier syntheses, and refined by full-matrix least-squares on all F2 data using SHELX76 (OLEX277 interface) or the JANA200678 program suite. Hydrogen atoms of the pyridine ring and bonded water molecules were included in calculated positions and allowed to ride on their parent atoms (except for complex I). The final refinements include anisotropic thermal parameters of all non-hydrogen atoms, except the oxygen atoms of the free water molecules. Due to its very low symmetry (S.G.: P1), crystals of complex I were systematically refined by using JANA software for applying the rigid body method. SQUEEZE processes were used on any structures of complex V as detailed in supplementary information. The crystal data of one member of each complex I-VI are given in Table 1, whereas the crystal data of all compounds are gathered in supplementary information (Tables 6



S1a-c). Supporting information is available in CIF format. CCDC file numbers: 15822791582294. X-ray thermodiffraction. X-ray thermodiffractometry was performed for compound 3 (complex II), 6 (complex III) and 9 (complex IV) under 5 L.h-1 airflow in an Anton Paar HTK1200N of a D8 Advance Bruker diffractometer (θ-θ mode, CuKα1/α2 radiation) equipped with a Vantec1 linear position sensitive detector (PSD). Each powder pattern was recorded in the range 5-50° (2θ) (at intervals of 20°C between RT and 800°C) with a 1s/step scan, corresponding to an approximate duration of 37 min. The temperature ramps between two patterns were 5°C.min-1. Thermogravimetric analysis. The thermogravimetric experiment has been carried out on a thermo-analyzer TGA 92 SETARAM under air atmosphere with a heating rate of 5°C.min-1 from room temperature up to 800°C. Infrared spectroscopy. Infrared spectra of compounds 3 (complex II), 6 (complex III) and 9 (complex IV, see Supplementary Information) were measured on a Perkin Elmer Spectrum TwoTM spectrometer between 4000 and 400 cm-1, equipped with a diamond Attenuated Total Reflectance (ATR) accessory. Luminescence measurements. The luminescence of a solid powdered sample of compound 9 (complex IV) was studied. This sample was put between quartz plates (Starna cuvettes for powdered samples, type 20/C/Q/0.2). Luminescence measurements were performed on an Edinburgh Instruments FLSP920 UV-vis-NIR spectrometer setup. A 450W xenon lamp was used as the steady state excitation source. Luminescence decay times were recorded using a 60W pulsed Xe lamp, operating at a frequency of 100 Hz. A Hamamatsu R928P photomultiplier tube was used to detect the emission signals in the visible range. The emission spectrum reported in this paper has been corrected for detector sensitivity.

Results Structure descriptions The investigations of the chemical system mixing tetravalent uranium with trivalent lanthanides from La up to Lu in association with the dipicolinate ligand, revealed the formation of six varieties of coordination complexes (I-VI). For each type, we describe in detail one member for a given lanthanide atom, related to the crystallographic data indicated in Table 1.

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The structure of the complex I [U(dpa)3Ln(H2O)8]2[(U(dpa)3)2Ln(H2O)7][(U(dpa)3)2Ln(H2O)6]·20-21H2O is described in the non-centric triclinic space group P1 for the lanthanide cations La3+ (1) and Ce3+ (2). It contains two sets of dinuclear moieties and two sets of trinuclear moieties, which are closely related to each other, respectively (Figure 2). For these different di/trinuclear units, the uranium centers are tris-chelated by three dipicolinate ligands, with U-O and U-N bond distances in the range 2.303(10)-2.464(11) Å and 2.492(5)-2.569(5) Å (for the La-compound for instance). For the two dinuclear units, the remaining non-bonded C-O lengths of the monodentate carboxylate groups are in the range 1.23(3)-1.24(4) Å, indicating the nonprotonated state of the dipicolinate ligand. However, one of the carboxylate arms acts as a bidentate bridge between the uranium and the lanthanide center. The coordination sphere around the lanthanide atoms (Ln1 and Ln2) is completed by eight water molecules in terminal positions, and is defined by a typical tricapped trigonal prismatic polyhedron. For the Lacompound, the La-Ow bond distances vary from 2.498(12) up to 2.663(13) Å, whereas the unique La-Ocarboxyl bonds are 2.45(3) and 2.527(9) Å, for La1 and La2, respectively. The existence of the bidentate bridging connection mode results in the formation of a dinuclear cationic species [U(dpa)3Ln(H2O)8]+, which could be closely related through an inversion center. The second type of polynuclear unit is composed of two uranium centers linked to each other through a lanthanide atom. As for the set of dinuclear moieties, one of the carboxylate arms adopts a bidentate connection fashion between the uranium and lanthanide cations, which gives rise to the formation of a trinuclear anionic entity [(U(dpa)3)2Ln(H2O)6/7]- (Figure 2). The two distinct crystallographically independent trimers differ only by the coordination spheres around the lanthanide atoms. Indeed, the lanthanide centers are bonded to two carboxyl oxygen atoms from the carboxylate groups of the dipicolinate molecules, with La-Ocarboxyl bond distances in the range 2.449(10)-2.514(13) Å and 2.527(9)-2.568(11) Å, for La3 and La4, respectively. The difference comes from the number of water molecules attached to the lanthanide, which are six aquo and seven aquo species for La3 and La4, respectively. The corresponding coordination environments are bicapped trigonal prismatic and tricapped trigonal prismatic, respectively. La3-Ow bond distances are in the range 2.498(12)-2.571(12) Å and La4-Ow bond distances are in the range 2.548(8)-2.648(13) Å. The different coordination of these two lanthanide induces a slight distortion of the bonding between the uranium and lanthanide centers, since the Ocarboxyl-LaOcarboxyl bonding angle is 150.4(4)° for La3 and 138.5(3)° for La4. This geometric feature breaks the symmetry operations between the two distinct trinuclear units, with the rejection of an inversion center, and leading to the non-centric space group P1. The resulting crystal structure is neutral and consists of the molecular assemblies of discrete positively charged dinuclear units and negatively charged trinuclear units (Figure 3). Water species are found intercalated between these polynuclear units.

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Figure 2: Views of the different molecular moieties occurring in the complex I with the lanthanum (1) and cerium (2) compounds. (top) Representation of the two distinct cationic dinuclear species, [U(dpa)3Ln(H2O)8]+. (bottom) Representation of the two distinct trinuclear anionic species [(U(dpa)3)2Ln(H2O)6]- and [(U(dpa)3)2Ln(H2O)7]-, which differ by the number of aquo groups surrounding the lanthanide centers Ln3 (LnO2(H2O)6, bicapped trigonal prism) and Ln4 (LnO2(H2O)7, tricapped trigonal prism). Color codes: U: green; Ln: purple; O: red; N: blue; C: grey.

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Figure 3: Polyhedral representation of the structure of complex I, along the a axis. It shows the arrangements of the dinuclear units, [U(dpa)3Ln(H2O)8]+ (brown polyhedra for U; pink polyhedra for Ln) and the trinuclear anionic species (green polyhedra for U, purple polyhedra for Ln).

The complex II ([U(dpa)3Ln(Hdpa)(H2O)4]·6H2O) is formed with the lanthanide cations La3+ (3) and Ce3+ (4), and crystallized in the centric triclinic space group P-1 as for the other complexes III-VI. It is related to the assembly reported in the mixed Ce4+/Ln3+ dipicolinate compounds.73 Its crystal structure consists of two distinct crystallographically independent uranium and lanthanide atoms with a nine-fold coordination, defining a tricapped trigonal prismatic geometry (Figure 4). The uranium center {UN3O6} is bound to three tridentate dipicolinate molecules with U-O and U-N distances in the range 2.3457(18)-2.4205(18) Å and 2.516(2)-2.557(2) Å, respectively, for the Ce-based compound. The carboxylate groups of one dipicolinate ligand as well as one carboxylate group of the two other dipicolinate ligands act as monodentate bridges with the uranium atom. The corresponding terminal C=O bond lengths are in the range 1.231(3)-1.236(3) Å. The two remaining carboxylate arms of two different dipicolinates adopt a bidentate bridging mode between the uranium and the lanthanide center. The lanthanide atom is nine-fold coordinated to one tridentate dipicolinate molecule, two carboxyl oxygen atoms from adjacent dipicolinate ligands (bound to uranium) and four water species in terminal position. For the Ce-compound, the interatomic distances are 2.597(2) Å for Ce-N, 2.4583(19)-2.5033(18) Å for Ce-Ocarboxyl and 2.510(2)-2.721(2) Å for Ce-Ow. The carboxylate groups of the dipicolinate molecule chelating the lanthanide center are both monodentate and the terminal C-O bonding distances are slightly longer with the values of 1.266(3) Å (for the Ce-compound). This indicates the existence of the protonated state for the carboxylate function, which would be delocalized on the two carboxylate arms of this dipicolinate. Indeed, the chemical formula of the complex is [U(dpa)3Ln(Hdpa)(H2O)4] and neutral when considering the monoprotonated dipicolinate molecule coordinated to the Ln center. The occurrence of the anti-anti bidentate connection mode (U-O-C-O-Ln) of two of the dipicolinate molecules around the uranium atom gives rise to the generation of an infinite polymeric chain, with a strict alternation of uranium and lanthanide centers developing along the [-101] direction (Figure 4).

10



Figure 4: (top) view of the two distinct environments for uranium and lanthanide (La (3), Ce (4)) in complex II. Disorder of the terminal aquo species is not shown for clarity. (bottom) Polyhedral representation of the polymeric chain running along the [-101] direction, showing the strict alternation of {UN3O6} and {LnNO4(H2O)4} units linked to each other via the carboxylate group of the dipicolinate molecule. Color codes: U: green; Ln: purple; O: red; N: blue; C: grey; H: light grey.

Figure 5: View of the structure of complex II, showing the stacking of the infinite polymeric chain [ULn]∞ perpendicular to the [-101] direction. Color codes: U: green; Ln: purple; O: red; N: blue; C: grey.

The crystal structure is based on the assembly of the such infinite ribbons [U(dpa)3Ln(Hdpa)(H2O)4] (noted [ULn]∞), which are stacked along the [101] direction (Figure 5), via weak π-π interactions between two neighboring aromatic rings of the dipicolinate ligands (C···C distance close to 4.00 Å; Figure S4). Six distinct water molecules 11


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have been revealed from the single-crystal XRD analysis and are found intercalated between the chains. They interact through hydrogen bonds with the terminal aquo species bonded to the lanthanide center, as well as with the non-bonded carboxylate C-O of the dipicolinate. The complex III ([(U(dpa)3)2Ln(dpa)(H2O)4Ln(H2O)7]2·18-24H2O) is obtained with trivalent cerium (5) and neodymium (6) and its crystal structure is close to that reported by Prasad and Rajasekharan in their description of mixed Ce4+/Ln3+ dipicolinate compounds.73 It is composed of a molecular assembly of octanuclear moities [U4Ln4] intercalated by water molecules. It consists of two sets of crystallographically independent uranium and lanthanide atoms. Both uranium centers U1 and U2 are nine-fold coordinated (tricapped trigonal primatic geometry: {UN3O6}) by three tridendate dipicolinate ligands (Figure S5). For the Ndcompound, typical U-O and U-N bond distances are in the range 2.330(2)-2.419(2) Å and 2.519(2)-2.571(2) Å, respectively. The lanthanide center Ln1 is bonded to one tridendate dipicolinate molecule, two carboxyl oxygen atoms from two distinct dipicolinate ligands and four terminal aquo species, defining a tricapped trigonal prismatic polyhedron {LnNO4(H2O)4}). In the Nd-compound, the Nd-O, Nd-N and Nd-Ow bond lengths are 2.437(2)-2.540(2), 2.549(2) and 2.466(2)-2.587(2) Å, respectively. The coordination sphere for Ln2 is different, since it is linked to two carboxyl oxygen atoms from two distinct dipicolinate ligands and seven terminal aquo species. It is therefore nine-fold coordinated in a tricapped trigonal prismatic surrounding ({LnO2(H2O)7}), with Nd-O bond lengths in the range 2.433(2)-2.524(2) Å and Nd-Ow bond lengths in the range 2.432(2) 2.603(2) Å. The originality is the connection mode of the carboxylate groups of some dipicolinate ligands. They act either as a monodentate connection fashion (for two of the three dipicolinates around uranium) or a bidentate connection mode bridging uranium (U1) and lanthanide (Ln1 or Ln2) centers, or lanthanide (Ln1) and lanthanide (Ln2) centers. The type of connection generates a six-membered elliptic ring core [U2Ln4] containing the following sequence U1···Ln1···Ln2···U1···Ln1···Ln2 (Figure 6). One of the two carboxyl oxygen atoms attached to Ln1 belongs to a distinct type of dipicolinate ligand, which tris-chelates an additional uranium center U2, resulting in the formation of an octanuclear brick with an Sshape (Figure 7). The monodentate carboxylate groups have terminal C=O bonding, with distances in the range 1.221(4)-1.254(3) Å, corresponding to a non-protonated state for the dicarboxylate linker. The association of the deprotonated dipicolinate molecules together with the U4+ and Ln3+ leads to a neutral octanuclear moiety, which is stacked along the a axis, in a supramolecular assembly (Figure 7).

12



Figure 6: (top) Views of the sub-unit [U2Ln4] forming a six-membered ring through bidentate bridging mode between Ln1 and Ln2 centers, and U and Ln centers, in complex III. (bottom) Views of the octanuclear moiety [U4Ln4] built up from the hexameric sub-unit [U2Ln4], associated to two peripheral {U(dpa)3} units linked via a bidentate carboxylate group between Ln1 and U2. Color codes: U: green; Ln: purple; O: red; N: blue; C: grey; H: light grey.

Figure 7: Representation of the stacking of the S-shape octanuclear units along the a axis on complex III.

The complex IV ([U(dpa)3][Ln(Hdpa)(H2O)6]·6-7H2O) crystallizes for the series of lanthanides such as Ln = neodymium (7), samarium (8), europium (9), gadolinium (10) and terbium (11). The crystal structure contains discrete molecular units of uranium bonded to 13


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three dipicolinate anions and lanthanides bound to one dipicolinate anion and water species. For uranium, the coordination sphere is completed by three tridentate dipicolinate ligands, in a tricapped trigonal prismatic geometry (Figure 8). The U-O and U-N bond distances are in the same range for all the complexes of type IV: in the case of compound 9 (Eu), for U-O, it corresponds to the range 2.330(2)-2.448(2) Å and for U-N, they are in the range 2.534(3)2.550(2) Å. The remaining non-bonded C-O bonding distances are in the range 1.219(4)1.244(4) Å, corresponding to the deprotonated state for the dipicolinate anion. The lanthanide center is also nine-fold coordinated by one tridentate dipicolinate ligand and six terminal aquo groups, describing a tricapped trigonal prismatic environment. Following the well-known lanthanide contraction effect, the Ln-N bond lengths vary from 2.600(3) Å for Nd (7) down to 2.525(3) Å for Tb (11), the Ln-O bond distances change from 2.425(3)-2.536(2) Å for Nd (7) down to 2.386(2)-2.469(2) Å for Tb (11) and Ln-Ow bond distances vary from 2.476(3)2.516(3) Å for Nd (7) down to 2.402(3)-2.452(2) Å for Tb (11). The two remaining nonbonded oxygen atoms from the dipicolinate ligand exhibit two different situations: one of the C-O groups exists in the protonated state, with the C-O distance of 1.288(4) Å (for compound 9, for instance) whereas the second one occurs in the deprotonated state with a shorter C-O bond distance of 1.235(4) Å. It results that a monoprotonated dipicolinate molecule acts as tris-chelate with the lanthanide center. The structure of complex IV consists of the threedimensional assembly of negatively charged units {U(dpa)3}2- compensated by the positively charged units {Ln(Hdpa)(H2O)6}2+ (Figure 8). Six (or seven in case of compound 11) distinct water molecules have been located between the two moieties {U(dpa)3}2- and {Ln(Hdpa)(H2O)6}2+, and contribute to the cohesion of the structure through a network of hydrogen bond interaction. This series of anionic-cationic sequences of discrete units has been reported in the mixed Ce4+/Ln3+ dipicolinate compounds,73 with lanthanides up to dysprosium. In our investigations, complex IV is formed up to terbium, since our efforts to obtain the dysprosium form have failed. Furthermore, the crystals of compound 11 (Tb) are quite instable in air atmosphere and required a collection of the X-ray data at 100K under nitrogen gas flow.

14



Figure 8: (top) Views of the discrete molecular units in complex IV: left: representation of the coordination environment for U1 with three tris-chelate dipicolinate molecules {U(dpa)3}; right: representation of the coordination environment for Ln1 with one tris-chelate dipicolinate molecule and six water species {Ln(Hdpa)(H2O)6}. (bottom): Representation of the molecular assembly of the {U(dpa)3} and {Ln(Hdpa)(H2O)6} moieties in complex IV; along the a axis. Color codes: U: green; Ln: purple; O: red; N: blue; C: grey; H: light grey.

The complex V [(U(dpa)3)2(Ln(H2O)5)]2-·2H3O+·8H2O) appeared for the series of heavier lanthanides with terbium (12), dysprosium (13), holmium (14) and erbium (15). The crystal structure is based on two crystallographically independent uranium U1 and U2 atoms and one lanthanide Ln1 atom (Figure S6). Both uranium centers are nine-fold coordinated with three tridentate dipicolinate anions described in a tricapped trigonal prismatic polyhedron. Typical U-O and U-N bond distances are observed in the range 2.313(2)-2.458(2) Å and 2.510(3)2.558(3) Å, respectively for the dysprosium compound, for instance. The lanthanide center is eight-fold coordinated (bicapped trigonal prism) by three carboxyl oxygen atoms and five aquo species in terminal positions. For the Dy compound, the Ln-O bond lengths are 15


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2.355(2)-2.396(2) Å and the Ln-Ow bond distances are in the range 2.358(2)-2.402(2) Å. In complex V, one of the two carboxylate arms of two dipicolinate ligands around the U1 center acts as an anti-anti bidentate bridge between U1 and Ln1, in order to create a square ring with the U1···Ln1···U1···Ln1 sequence (Figure 9). Since the two carboxyl oxygen atoms bound to Ln1 come from the bidentate carboxylate groups of dipicolinate engaged in the fourmembered ring, the third carboxyl oxygen atom belongs to another carboxylate arm from a distinct dipicolinate molecule, tris-chelating a second uranium center (U2). In this configuration, the lanthanide atom is linked through bidentate carboxylate bridges to three uranium atoms and this generates a hexanuclear cluster [U4Ln2], consisting of a 4-ring core with two peripheral {U(dpa)3} units (Figure 9). The resulting hexamer is composed of four anions {U(dpa)3}2- and two cations {LnO3(H2O)5}3+, corresponding to an excess of two negative charges. The examination of the terminal C-O bond lengths of the different dipicolinate ligands shows typical distance values in the range 1.220(4)-1.247(4) Å, reflecting their fully deprotonated states. Considering this fact, the required positive charges may come from the occurrence of protonated water species, but this cannot be observed due to the difficulty to locate from the single-crystal XRD analysis, the hydrogen atoms bonded to free water species and intercalated between the hexanuclear units (Figure 10). A similar configuration has been observed in the isostructural series of mixed Ce4+/Ln3+ dipicolinate compounds.73

Figure 9: View of the molecular hexanuclear unit [U4Ln2] in complex V, built up from a square ring containing Ln1 alternating with U1 centers, and two peripheral U2 centers. Color codes: U: green; Ln: purple; O: red; N: blue; C: grey; H: light grey.

16



Figure 10: View of the structure of complex V, showing the arrangement of the hexanuclear moieties [U4Ln2] along the a axis.

The structure of complex VI ([U(dpa)3Lu(Hdpa)(H2O)4]·11H2O) is observed only with lutetium (16). It is composed of one uranium atom and one lutetium atom, which are located on two distinct crystallographically independent positions. As previously observed in the other complexes of this series, the coordination around the uranium atom is completed by three tridentate dipicolinate ligands with U-O and U-N bond lengths in the range 2.322(4)2.454(3) Å, and 2.543(4)-2.565(4) Å, respectively. The lanthanide atom is eight-fold coordinated in a bicapped trigonal prismatic geometry with one tridentate dipicolinate anion (Lu-N = 2.417(4) Å; Lu-O = 2.286(3)-2.406(3) Å), four terminal water molecules (Lu-Ow = 2.272(4)-2.326(3) Å) and one carboxyl oxygen atom (Lu-O = 2.323(3) Å). Unlike the situation in complex IV, the two metallic centers are bridged one to each other through a carboxylate group of a dipicolinate ligand chelating the uranium atom in a syn-anti connection mode, making a dinuclear molecular moiety (Figure 11). The analysis of the bond distances for C-O from the different dipicolinate anions shows that the ligands around uranium are fully deprotonated (non-bonded C-O = 1.221(6)-1.244(6) Å) whereas the dipicolinate chelating the lanthanide atom exhibits a monoprotonated state (one C-O bond is 1.232(6) Å; the second C-O is 1.271(6) Å). From this consideration, the dinuclear core [ULu] is neutral with the anionic part {U(dpa)3}2- compensated by the cationic part {Lu(Hdpa)(H2O)4}2+. The crystal structure is thus built up from the assembly of such molecular dinuclear units isolated by water species (Figure 11), forming a hydrogen bond interactions scheme. This complex is also closely related to that of type II, since only one dipicolinate ligand acts as bidentate bridge between U and Ln centers. In complex II, there are two such bidentate ligands, which allows for the generation of a one-dimensional polymeric chain.

17


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Figure 11: (top) View of coordination environments of uranium and lutetium centers in the dinuclear unit of the complex VI. (bottom) Polyhedral representation of the complex VI, along the a axis. Color codes: U: green; Lu: purple; O: red; N: blue; C: grey; H: light grey.

Discussion The investigation of the U(IV)-Ln(III)-H2dpa system in aqueous media gives rise to the identification of six distinct types of heterometallic compounds U-Ln synthesized at room temperature. In all cases, tetravalent uranium atoms exhibit a similar coordination environment, with a typical nine-fold coordinated fashion (tricapped trigonal prism) through the chelation of three dipicolinate ligands (via one nitrogen atom and two carboxyl oxygen atoms). The coordination mode of lanthanide atoms differs since they may be coordinated through chelating or/and bidentate dipicolinate ligands, and their coordination number is related to their ionic radii. This evolution is represented in Figure 12, showing the lanthanide 18



centers chelated by one dipicolinate ligand, carboxyl oxygen atoms, and water molecules in complexes II, III, IV and VI. In compounds I and V, the lanthanide atoms are only coordinated by carboxyl oxygen atoms (2 or 3) and water molecules. However, this geometric trend differs from the tetrad effect79–81 for which the lanthanides can be separated in four distinct groups (first group: La-Ce-Pr-Nd; second group: (Pm)-Sm-Eu-Gd; third group: GdTb-Dy-Ho; fourth group: Er-Tm-Yb-Lu), based on the filling rate of the 4f electron shell (one quarter, half, three-quarter and complete, respectively). In our investigations, the first quarter (La-Ce-Nd) corresponds to the complexes I, II and III. But, we observe a continuous evolution from La to Nd, with couples of (La, Ce) for complexes I and II, couples (Ce, Nd) for complexe III. The complex IV exists for the series Nd-Tb, covering the end of the first quarter and the middle of the third quarter. The complex V is present for the series Tb-Er from the middle of the third quarter to the beginning of the last quarter. Al last, the complex VI has been isolated only for lutetium, corresponding to the last member of the fourth quarter. A similar deviation to the tetrad effect has been reported in the recent series of lanthanide borates,82 indicating the important influence of the ionic radii in the chelating mode of ligand (in our case dipicolinate) around the lanthanide centers.

Figure 12: Evolution of the coordination mode of the lanthanide cations chelated by one dipicolinate ligand within different complexes II, III, IV and VI.

Within complex II (La, Ce), the lanthanide atom is nine-fold coordinated with one dipicolinate ligand, four aquo groups and two carboxyl oxygen atoms: [Ln(dpa)(H2O)4O2]. One of the lanthanide atoms in complex III (Ce, Nd) has the same environment, whereas the second one is surrounded by only oxygen atoms from carboxyl (2) and water species (6). In complex IV (Nd, Sm, Eu, Gd, Tb), the coordination number remains unchanged for the lanthanide atom, but the nature of the surrounding atoms is quite different. There is no more carboxyl oxygen atom connected to this cation, but only aquo groups complete its coordination sphere, in order to lead to the moiety [Ln(dpa)(H2O)6] in complex IV, instead of [Ln(dpa)O2(H2O)4] in complex III. In contrast, with the smaller lanthanide lutetium, the complex VI exhibits a coordination number 8 for this cation, with only one carboxyl oxygen atom together with the surrounding four water species, [Ln(dpa)O(H2O)4]. A change of coordination state is also observed with other lanthanide atoms, which are not complexed by a 19


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dipicolinate ligand, as it is observed in I, III and V. In these cases, the lanthanide centers are coordinated by two or three carboxyl oxygen atoms and five, six or seven water species. In complex I (La, Ce), the coordination number varies from 8 [Ln(H2O)6O2] to 9 [Ln(H2O)7O2]. The nine-fold environment is also found in complex III (Ce and Nd) for one of the two crystallographically independent lanthanide atoms. For smaller lanthanide cations (Tb, Dy, Ho, Er) in complex V, their coordination sphere is built up from five aquo groups and three carboxyl oxygen atoms ([Ln(H2O)5O3]) with a coordination number of 8. The series of these heterometallic complexes differs by the assembly of the uranium- and lanthanide-centered subunits (Figure 13). Indeed, a U/Ln ratio of 1/1 is observed in the complexes of type II, III, IV and VI, but with distinct atomic arrangements. A onedimensional coordination polymer is found in II, with strict alternation of U and Ln centers, bridged through bidentate carboxylate groups of dipicolinate molecules. At the opposite, the complex type IV is built up from the molecular assembly of discrete anionic {U(dpa)3}2- and cationic Ln(Hdpa)(H2O)6}2+ units. An intermediate condensation mode occurs in complex type VI, for which a molecular dinuclear entity occurs, with only one bridging carboxylate group between the U and Lu atoms. The latter can be considered as a fragment of the infinite chains existing in complex type II. A more condensed system is observed in complex type III, in which an octanuclear moiety consists of a six-membered ring with two uranium and four lanthanide centers, linked to each other via the bidentate carboxylates, and connected to two peripheral U centers. A second variety of ring-type entity is formed in a hexanuclear moiety (complex type V), with a central four-membered ring containing a uranium-center in strict alternation with the lanthanide one, and two additional peripheral uranium centers, resulting in a U/Ln ratio of 2/1 for the complex V. Finally, the complex I contains two distinct molecular moieties. One is similar to that encountered in the complex VI, and consists of a dinuclear sub-unit of one uranium center and one lanthanide center, bridged through a bidentate carboxylate group. The second one is a trinuclear entity with one lanthanide center, linked to two uranium centers. The total U/Ln ratio is 3/2.

20



Figure 13: Representation of the arrangements of the mixed uranium-lanthanide dipicolinates in the different types of complexes: dinuclear/trinuclear motif (complex I); 1D chain (complex II); octanuclear motif (complex III); isolated motif (complex IV); hexanuclear motif (complex V) and dinuclear motif (complex VI).

Thermal behavior characterizations of complexes II, III and IV. The thermal behavior has been characterized for a few compounds, which have been obtained as pure phases, and are stable under air atmosphere (which is not the case for the complexes V and VI series). It concerns the La-sample of complex II, the Nd-sample of complex III and Eu-sample of complex IV. These three compounds have been analyzed by thermogravimetric method under air atmosphere and X-ray thermodiffraction. The thermal curves of the three compounds (II, III and IV type) are shown in Figure 14 and indicate a similar two-steps weight loss event up to 800°C, even if the molecular crystal structure of these highly hydrated solids differs. The first weight loss is assigned to the departure of water molecules and occurs between 50 and 150°C. For complex II, [U(dpa)3La(Hdpa)(H2O)4]·6H2O, the observed weight loss value is 15%wt, and corresponds to the amount of 10 H2O per ‘U-La’ unit (calcd: 14.8%). It is therefore attributed to the removal of the free (6 H2O) and attached (4 H2O) water molecules. The first experimental weight loss value of the complex III, [(U(dpa)3)2Nd(dpa)(H2O)4Nd(H2O)7]2·24H2O, is 17.1 %wt and is in good agreement with the expected value of 17.7 %, for 46 H2O per ‘U4-Nd4’ unit, coming from 24 free H2O and 22 bonded H2O species. For the complex IV, [U(dpa)3][Eu(Hdpa)(H2O)6]·6H2O, the first weight loss is observed at 16.2 %wt. This is also assigned to the departure of the water molecules, with 6 free H2O and 6 bonded H2O species (calcd: 17.2 %) per ‘U-Eu’ unit. For the three complexes, the heating step to 150°C gives rise to a total dehydration process, which induces the degradation of the crystal network of the molecular assembly of the U-Ln dipicolinate complexes.

Figure 14: Thermogravimetric curves of the complexes II (La – blue line), III (Nd – red line) and IV (Eu – green line) under air atmosphere (heating rate 5°C.min-1). 21


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Upon heating, the X-ray thermodiffraction experiment shows the disappearance of the Bragg peaks followed by the crystallization of an unknown transient phase (Figure 15 – green zone). It is interesting to notice that the Bragg peaks of the intermediate persist up to 260°C for the complex II (La), before it reveals the decomposition of the crystalline compounds. Indeed, the second weight loss event occurs from 250 up to 420-430°C (Figure 14) and is attributed to the removal of the organic ligand. The observed remaining weight values are 36.3 %wt, 39.0% wt and 35.4 %wt, for complexes II, III and IV, respectively. Based on the stoichiometric amounts of the simple oxides from the chemical formula of the crystal structures, the calculated values are 35.6 % for 1 UO2 + ½ La2O3 from complex II, 37.6 % for 2 UO2 + 1 Nd2O3 from complex III and 34.2 % for 1 UO2 + ½ Eu2O3 from complex II. However, only the formation of the UO2 type form (Bragg peaks at d ≈ 3.14, 2.72 and 1.94 Å) is observed by X-ray thermodiffraction (Figure 15), which indicates its crystallization at a temperature around 400°C for complexes III (Nd) and IV (Eu), whereas it is only observed from 600°C for complex II with La. Indeed, the structure of UO2 (fluorite type) is well known for incorporating lanthanide elements in a solid solution system over a wide composition range.83,84 This UO2-like product is stabilized by the incorporation of lanthanide at the expense of U3O8, even in presence of air atmosphere.85 Such a structural transformation has been previously reported for the thermal degradation of a mixed uranyl-lanthanide mellitate coordination polymer, in which the crystallization of U3O8 is observed in a first step. It is then transformed into the mixed (U,Ln)O2 oxide at higher temperature under air atmosphere.24 However, an earlier work of the decomposition of mixed U(IV)/Ln(III) oxalates has shown that their thermal treatment resulted directly in the formation of the fluorite product (U,Ln)O2.19 The final product at 800°C is assumed to be a mixed (UxLn1-x)O2-δ fluorite compound. EDX analyses of the final residues at 800°C indicate the following chemical compositions of (U0.504La0.496)O2-δ from complex II, (U0.498Nd0.502)O2-δ from complex III and (U0.513Eu0.487)O2-δ from complex IV. These U/Ln ratios are close to 50/50, as expected from the initial stoichiometry found in the coordination complexes. SEM photographs of calcined products at 800°C show similar morphologies to those of the initial products (Figure 16). The parallelepiped-like shape is preserved but some cracks are observed on the surface, in particular for compound 6.

22



Figure 15: Evolution of the X-ray diffraction powder patterns (copper radiation) as a function of temperature of the complexes II (La – top), III (Nd – middle) and IV (Eu – bottom). The UxLn1-xOy compositions are taken from pdf files 04-008-2454, -2453 and -7002 for II, III and IV samples, respectively. XRD patterns are indicated every 20°C.

Figure 16: SEM images of the calcined crystallites of the complexes II (La – left), III (Nd – middle) and IV (Eu – right) after a thermal treatment at 800°C under air.

Infrared analysis Infrared spectra of compounds 3, 6 and 9 are presented in supplementary (Figure S7). Each solid shows a very similar infrared signature. The presence of water molecules is confirmed by a broad band around 3200 cm-1 and corresponding to the O-H symmetric stretching. At 3091 cm-1, one can distinguish a very small vibration assigned to the stretching of C-H groups of the inorganic linker. The connection between the carboxylate functions and the inorganic cations is characterized by two important groups of vibrations centred at 1613 and 1365 cm-1, associated to asymmetric and symmetric stretching of coordinated carboxylate functions

23


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ν(COO), respectively. In this range, the intense peak at 1425 cm-1 is attributed to vibrations of the benzene ring. Luminescence study Only the luminescence

properties

of

the

Eu-containing

complex

of

type

IV

([U(dpa)3][Ln(Hdpa)(H2O)6]·6H2O - 9) have been investigated. This complex was selected, as it is known that the trivalent europium ion can act as a structural probe, because of its nondegenerate emissive 5D0 electronic state and its non-degenerate 7F0 ground state, yielding relatively simple PL emission spectra. The combined normalized excitation and emission spectra for this sample can be found in Figure 17. As it can be seen, the excitation spectrum (left part of the figure) is characterized by a relatively narrow band located at 278 nm, corresponding to the π* ← π absorption of the dipicolinate ligand. The emission spectrum (right part of the figure) contains three easily distinguishable peaks, at 592.6 nm, 614.4 nm and 693.6 nm, respectively, corresponding to electronic transitions from the europium ion’s 5D0 excited state to the lower lying 7F1, 7F2 and 7

F4 states, respectively. The intensity of the peak at 614.4 nm is clearly much higher than the other peaks in the spectrum. This peak corresponds to the hypersensitive 5D0 → 7F2 transition, which is particularly sensitive to the europium ion’s environment: in a centrosymmetric system, where the europium ion occupies an inversion center, this peak should be absent, or very weak, whereas in systems with low symmetry, it typically has a high intensity. The parameter R = I(5D0 → 7F2)/I(5D0 → 7F1) describing the intensity ratio between the peaks at 614.4 nm and 592.6 nm, is low for highly symmetrical systems and high for low symmetry environments. This ratio related to the intensity of the 5D0 → 7F1 transition, which is a magnetic dipole transition, is roughly unaffected by the europium ion’s surroundings. In this case, the R parameter equals to 4.19, which is a relatively high value, corresponding to a low symmetry, in accordance with literature values for similar materials.86 In addition, no f ← f absorption transitions are distinguishable in the excitation spectrum, indicating that energy transfer from the ligand to the europium ion is efficient in sensitizing europium luminescence. The luminescence performance of the europium ion in its specific environment can be estimated by calculating the intrinsic quantum yield QLn, which expresses how well the radiative processes (emitting light) compete with non-radiative processes (loss of excitation energy through vibrational relaxation, i.e. dissipation of heat), as formulated by equation 1. (1) Here, kr and knr are the rate constants of the radiative and non-radiative processes, respectively. The radiative rate constant can be calculated using equation 2:

24



Page 26 of 38

(2) The radiative lifetime τrad kan be estimated by equation 3: (3) In this formula, AMD,0 is the probability of spontaneous emission of the 5D0 → 7F1 magnetic dipole transition, equal to 14.65 s-1, n is the refractive index, assumed to be in the order of 1.5, Itot is the total integrated emission of the 5D0 → 7FJ transitions (J = 0-6) and IMD is the integrated emission of the 5D0 → 7F1 transition. The intrinsic quantum yield QLn can also be determined through equation 4: (4) In this equation, τobs is the observed, experimentally determined luminescence lifetime. Figures S8a and S8b (Supporting Information) show the decay trace of the 5D0 → 7F2 transition, excited with a pulsed xenon lamp (pulse frequency 100 Hz) at 278 nm and monitored at 614.4 nm. As it can be seen from these figures, a double exponential function is needed to satisfactorily fit the trace, yielding two lifetime components τ1 = 798 ± 24 µs (45%) and τ2 = 156 ± 9 µs (55%), averaging to an overall τobs = 448 ± 16 µs. Using this value in combination with the τrad value calculated from equations 2 and 3, an intrinsic QLn of 15.1% is found. The luminescence data for this compound are summarized in Table 2. These rather low values are comparable with other europium coordination compounds, in which the europium ion is coordinated by a number of water molecules, which are efficient quenchers of luminescence.86

25


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 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity (a.u.)

Page 27 of 38

200

250

300

350

400

550

600

650

700

750

Wavelength (nm) Figure 17: Combined excitation (left, monitored at 614.4 nm) – emission (right, excited at 278 nm) spectrum of the Eu-containing complex IV ([U(dpa)3][Eu(Hdpa)(H2O)6]·6H2O - 9).

Conclusion In this contribution, we analyzed the crystal structures of sixteen heterometallic U(IV)-Ln(III) (Ln = La, Ce, Nd, Sm-Er, Lu) coordination complexes bearing the dipicolinate ligand. These compounds, obtained at room temperature in aqueous solution, exhibit a wide variety of molecular assemblies, which can be listed in six different types I-VI. The complexes are constructed from tetravalent uranium centers surrounded by three tris-chelating dipicolinate ligands {U(dpa)3} and polyoxo/aquo or mixed polyoxo/aquo-chelated dipicolinate trivalent lanthanide centers {LnO1-3(H2O)5-8 or Ln(dpa)O0-3(H2O)4-6}. These two sub-units are found in isolated species in the crystal structures of the series of complex type IV. But, the metallic cations are also linked to each other through the bidendate modes of one or several carboxylate groups from the dipicolinate ligands. It results in a diversity of arrangements, with dinuclear entities [U-Ln] (I, VI), trinuclear entities [U-Ln-U] (I) or infinite chains with the sequence [-U-Ln-U-Ln-]∞ in complex type II. Cyclic complexes are also obtained either from a six-membered ring (III) or a four-membered ring (V) of uranium- and lanthanidecentered units linked via bidentate carboxylate groups and two additional {U(dpa)3} units are also attached via bidentate carboxylates at the periphery, giving rise to the development of a molecular octanuclear unit [U2(U2Ln4)] in complexes type III or a hexanuclear unit 26



[U2(U2Ln2)] in complexes type V. Under air atmosphere, the thermal degradation of some of these complexes (II, III, IV) induces the structural transformation into fluorite type with the chemical formula U≈0.5Ln≈0.5O2-δ, in accordance with the original stoichiometry. This indicates the crystallization of mixed U-Ln oxide, which could be of great interest in the view of uranium recovery from spent nuclear fuel, in mixtures with trivalent lanthanides (as fission products) or other actinides with different oxidation states (i.e. plutonium, if lanthanides are used as surrogate elements). The use of the dipicolinate linker thus shows its ability to ensure the association of hetero-metals in one given crystal structure, with direct bonding via carboxylate groups. The structural versatility of this O,N-donor ligand is very wide towards actinides, as indicated in the reports of mixed uranyl-transition metal57–59 or mixed valence Pu(III)-Pu(IV)69 coordination complexes. It is also interesting to notice the structural similarities of the U(IV)-Ln(III) dipicolinate complexes with those described with the cerium(IV)73 replacing the tetravalent actinide. But other arrangements have been discovered in our study (complexes type I and VI), which highlights the richness and the complexity of such a chemical system combining actinides with other hetero-metals or with mixed valency. In this sense, the structural organization of the Pu(III)-Pu(IV) dipicolinate shows a new type of infinite triple ribbon of {PuIV(dpa)3} and {PuIIIO4(H2O)4} sub-units, which was not encountered in our system with U(IV)/Ln(III).

Conflicts of interest There is no conflict of interest to declare. Acknowledgments The authors would like to thank Mrs. Nora Djelal & Laurence Burylo, Mr. Philippe Devaux for their technical assistances with the SEM images, TG measurements and powder XRD and Dr Pascal Roussel for his fruitful discussions about the crystal structure determination (UCCS). The Chevreul Institute (FR 2638), "Fonds Européen de Développement Régional (FEDER)", "CNRS", "Région Nord Pas-de-Calais" and "Ministère de l'Education Nationale de l'Enseignement Supérieur et de la Recherche" are acknowledged for funding of X-ray diffractometers. DM and RVD thank Ghent University’s Special Research Fund for a doctoral scholarship (project BOF15/24J/049).

Supporting Information. The supporting Information is available free of charge in the ACS Publications website at DOI: 10.1021/acs.cgd.??? Optical photographs, powder X-ray diffraction patterns of compound 3, 6 and 9, infrared spectra of 3, 6 and 9, luminescence decay curve of 9, 27


Page 28 of 38

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Accession Codes CCDC 1582279-1582294 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

28



Page 30 of 38

Table 1. Crystal data and structure refinements for the complexes I-VI in the uranium-lanthanide dipicolinate system. For complexes I-V, only the crystal data of one sample of the series is indicated. For more details with the whole series, see supplementary information. 1 (complex I)

3 (complex II)

6 (complex III)

9 (complex IV)

13 (complex V)

16 (complex VI)

Formula

C126N18O121La4U6

C28H20N4O26LaU

C98H86N14O102Nd4U4

C28H24N4O28EuU

C84H56N12O78Dy2U4

C56H42Lu2N8O62U2

Formula weight

5685.2

1205.42

4620.88

1254.50

3758.52

2644.97

Temperature/K

293

300.33

100

296.15

100.01

99.99

Crystal type

green needle

green block

green plate

green plate

green block

green plate

Crystal size/mm

0.071 x 0.079 x 0.089

0.07 x 0.114 x 0.166

0.248 x 0.204 x 0.151

0.121 x 0.139x 0.186

0.145 x 0.167 x 0.179

0.106 x 0.206 x 0.225

Crystal system

triclinic

triclinic

triclinic

triclinic

triclinic

triclinic

Space group

P1

P-1

P-1

P-1

P-1

P-1

a/Å

16.5276(4)

12.8597(5)

14.8944(16)

12.0871(4)

14.3980(12)

13.0826(5)

b/Å

16.9453(4)

13.0375(5)

15.1661(16)

12.9180(5)

16.1097(13)

13.1980(5)

c/Å

19.4782(5)

13.2067(5)

17.5494(17)

14.6723(6)

17.0781(15)

14.3851(6)

α/°

96.7551(14)

98.309(2)

106.770(4)

109.831(2)

115.169(4)

113.484(2)

β/°

99.9349(13)

119.231(2)

100.779(4)

105.550(2)

113.423(4)

97.098(2)

γ°

118.2241(12)

90.328(2)

100.871(5)

99.973(2)

92.529(4)

98.492(2)

Volume/Å3

4608.1(2)

1904.49(13)

3602.5(7)

1986.07(13)

3179.5(5)

2207.06(15)

Z, ρcalculated/g.cm-3

1, 2.0487

2, 2.102

1, 2.130

2, 2.098

1, 1.963

1, 1.990

µ/mm-1

6.275

5.456

4.712

5.743

6.346

5.992

Θ range/°

2.306 – 24.490

2.317 – 30.113

3.143– 21.964

2.793– 30.463

2.400– 28.260

2.431 – 30.368

Limiting indices

-20 ≤ h ≤ 20 -21 ≤ k ≤ 21 -24 ≤ l ≤ 24

-18 ≤ h ≤ 18 -18 ≤ k ≤ 18 -18 ≤ l ≤ 18

-19 ≤ h ≤ 19 -20 ≤ k ≤ 20 -23 ≤ l ≤ 23

-17 ≤ h ≤ 17 -18 ≤ k ≤ 18 -20 ≤ l ≤ 20

-19 ≤ h ≤ 19 -21 ≤ k ≤ 21 -22 ≤ l ≤ 22

-16 ≤ h ≤ 16 -16 ≤ k ≤ 16 -17 ≤ l ≤ 17

Collected reflections

103453

67639

238074

68114

82270

53645

Unique reflections

36810 [R(int) = 0.0543]

10285 [R(int) = 0.0402]

17854 [R(int) = 0.0544]

11998 [R(int) = 0.0293]

15709 [R(int) = 0.0315]

8944 [R(int) = 0.0336]

Parameters

643 / 3

545

1011

535

766

551

Goodness-offit on F2

1.38

1.037

1.049

1.044

1.033

1.065

Final R indices [I>2σ(I)]

R1 = 0.0491 wR2 = 0.0527

R1 = 0.0266 wR2 = 0.0647

R1 = 0.0196 wR2 = 0.0451

R1 = 0.0237 wR2 = 0.0625

R1 = 0.0219 wR2 = 0.0520

R1 = 0.0293 wR2 = 0.0676

R indices (all data)

R1 = 0.0874 wR2 = 0.0558

R1 = 0.0334 wR2 = 0.0680

R1 = 0.0229 wR2 = 0.0473

R1 = 0.0283 wR2 =0.0647

R1 = 0.0263 wR2 = 0.0534

R1 = 0.0360 wR2 = 0.0709

Largest diff. peak and hole/e.Å-3

1.61 and -1.96

2.437and -1.505

2.183 and -1.059

1.613 and -0.975

1.810 and -0.932

3.717 and -0.818

Table 2. Photo-physical parameters of compound [U(dpa)3][Eu(Hdpa)(H2O)6]·6H2O) (9). R

τobs (ms)

τrad (ms)

kr (s-1)

knr (s-1)

QLn (%)

4.19

0.45 ± 0.02

2.97

336.4

1891.5

15.1

29


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For Table of Contents Use Only Molecular assemblies of a series of mixed tetravalent uranium and trivalent lanthanide complexes associated with the dipicolinate ligand, in aqueous medium Nicolas P. Martin, Christophe Volkringer, Natacha Henry, Sylvain Duval, Dimitrije Mara, Rik Van Deun, and Thierry Loiseau.

Table of Content graphic

Synopsis. Mixed uranium(IV)-lanthanide(III) dipicolinate complexes have been isolated throughout the Ln series from La to Lu, with the identification of six distinct molecular entities from mono-, di-, tri-, hexa and octanuclear units to chain-like motifs. The diversity of this structural system is correlated to the ionic radius of the lanthanide cation.

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Molecular assemblies of a series of mixed ... - ACS Publications

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