Role of Ammonium Ions in the Formation of ... - ACS Publications

Article pubs.acs.org/crystal

Role of Ammonium Ions in the Formation of Ammonium Uranyl Peroxides and Uranyl Peroxo-oxalates Florent Blanchard,†,‡ Marine Ellart,†,‡ Murielle Rivenet,† Nicolas Vigier,‡ Isabelle Hablot,‡ Bertrand Morel,‡ Stéphane Grandjean,§ and Francis Abraham*,† †

Université Lille Nord de France, UCCS - UMR CNRS 8181, ENSCL-USTL, BP 90108, 59652 Villeneuve d’Ascq Cedex, France AREVA-NC, TOUR AREVA, 1 Place Jean Millier, 92084 Paris La Défense, France § CEA, Marcoule Research Center, DEN/DRCP/DIR, F-30207 Bagnols sur Cèze, France ‡

S Supporting Information *

ABSTRACT: The flexibility of the ammonium ion environment is suitable for inducing the formation of several uranyl peroxides and peroxo-oxalates. For a given concentration of uranium and various oxalate/uranium ratios, by varying the pH with ammonium hydroxide, crystals of eight compounds have been isolated and characterized by X-ray diffraction, in addition to the studtite (UO2)(O2)·4H2O. All the compounds contain anionic uranyl polyhedra clusters with charge compensated by ammonium ions. Three are uranyl peroxides built from uranyl hexagonal bipyramids [(UO2)(O2)3]4− or [(UO2)(O2)2(OH)2]4− sharing peroxide or dihydroxyl equatorial edges to form cage clusters [(UO 2 ) 2 8 (O 2 ) 4 2 ] 28 − (U 28 ) and [(UO2)44(O2)66]44− (U44) in 7 and 3 respectively, or crown-shaped cluster [(UO2)32(O2(OH)2)52]40− (U32R) in 2. In these uranyl peroxides the pentagonal and hexagonal uranyl polyhedra rings are stabilized by ammonium ions. The other five compounds are uranyl peroxo-oxalates with various condensations of uranyl hexagonal bipyramids: (i) condensation of two [(UO2)(O2)(C2O4)2] bipyramids by peroxide ion sharing to form the dimer [(UO2)2(O2)(C2O4)4]6− (U2Ox4) in 8, (ii) assembly of five [(UO2)(O2)2(C2O4)] bipyramids linked by sharing peroxide to form the [(UO2)5(O2)5(C2O4)5]10− (U5Ox5) pentameric rings in 4 and 6, (iii) further condensation of 12 U5Ox5 rings through bis-bidentate oxalates to create the [(UO2)60(O2)60(C2O4)30]60− (U60Ox30) nanosphere in 5, (iv) replacing an oxalate by two hydroxide ions in U5Ox5 rings and sharing of the OH−OH bridge between two pentamers to form the dimer of pentamers [(UO2)10(O2)10(OH)2(C2O4)8]18− (U10Ox8) in 9. In the last four compounds, the ammonium ions stabilize the pentameric cycles. The ammonium ion has different effects, particularly in the case of pentamers, and thus provides access to a large panel of cluster sizes.

■

INTRODUCTION

precipitated by adding hydrogen peroxide to solution in the French industrial process. The so-obtained U(VI) peroxide, with general formula (UO2)(O2)·nH2O, is further calcined in order to recycle uranium into U3O8, used as feedstock for new UO2 fuel, or stored for future use. Additionally to its use in the back-end of the nuclear fuel cycle, (UO2)(O2)·nH2O also exists as a reaction intermediate in the synthesis of uranium oxide in the front-end of the nuclear fuel cycle. The only uranium peroxide compounds involved in the nuclear fuel cycle and identified to date are studtite, (UO2 )(O 2)·4H 2O, and metastudtite, (UO2)(O2)·2H2O, which results from the dehydration of studtite between 50 and 70 °C. Studtite and metastudtite are the only peroxides known as minerals. Their growth in nature results from the reaction of UO2 and H2O2, formed by alpha-radiolysis of water. The same

Both actinides oxalates and actinides peroxides play an important role in the nuclear fuel cycle and waste management. The interest of oxalic acid in recovering actinides from solution has been recognized for a long time and has motivated a recent review on actinides oxalates chemistry and applications.1 As example, the excellent complexing and precipitating properties of oxalic acid contribute to plutonium recovery at an industrial scale in the PUREX process where plutonium coming from dissolution of the spent fuel in nitric acid is recovered into a Pu(IV) oxalate solid phase. The oxalate is later transformed by calcination into PuO2, which is then recycled as feedstock for the fabrication of mixed-oxide fuel (MOX). Although the oxalic precipitation is well suited for plutonium recovery, the relative high solubility of U(VI) oxalate in acidic medium and the cumbersome reducing step necessary to produce U(IV) in order to precipitate the sparingly soluble U(IV) oxalate make it not the most appropriate way to recover uranium from nitric solution. It is one of the reasons why uranium is preferentially © XXXX American Chemical Society

Received: July 30, 2015 Revised: October 4, 2015

A

DOI: 10.1021/acs.cgd.5b01090 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Table 1. Crystal Data, Intensity Collection, and Structure Refinement Parameters for 2−5 CCDC number formula formula weight temperature/K crystal color crystal size/mm crystal symmetry space group a/Å b/Å c/Å α/° β/° γ/° volume/Å3 Z, ρcalculated/g·cm−3 μ/mm−1 Θ range/° limiting indices

collected reflections unique reflections R(int) parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff. peak and hole/e·Å−3

2: U32R

3: U44

4: U5Ox5_C2/m

5: U60Ox30

1408214 N20O172U32.71 10816.97 100 yellow 0.043 × 0.014 × 0.015 orthorhombic Immm 19.1276(8) 20.9557(9) 35.344(3) 90 90 90 14167(1) 2, 2.536 18.691 1.13−25.242 −22 ≤ h ≤ 14 −24 ≤ k ≤ 22 −40 ≤ l ≤ 39 28692 6245 0.0920 154 1.037 R1 = 0.0782 wR2 = 0.2145 R1 = 0.1275 wR2 = 0.2468 6.227 and −2.548

1408215 N44O222U44.53 14740.84 100 yellow 0.064 × 0.038 × 0.024 monoclinic P21/n 26.2761(8) 20.4710(6) 29.2307(9) 90 101.039(2) 90 15432.2(8) 2, 3.172 23.364 1.38−15.84 −20 ≤ h ≤ 20 −15 ≤ k ≤ 15 −22 ≤ l ≤ 22 91663 7316 0.1067 744 1.146 R1 = 0.0646 wR2 = 0.1805 R1 = 0.1027 wR2 = 0.2346 2.925 and −1.652

1408210 C10N10O48U5 2218.35 100 yellow 0.183 × 0.113 × 0.087 monoclinic C2/m 27.9494(11) 19.9702(8) 10.7251(4) 90 98.271(1) 90 5924.0(4) 4, 2.487 13.732 1.47−30.543 −39 ≤ h ≤ 39 −28 ≤ k ≤ 28 −15 ≤ l ≤ 15 90039 9320 0.0396 341 1.074 R1 = 0.0242 wR2 = 0.0687 R1 = 0.0286 wR2 = 0.0709 2.397 and −1.317

1408216 C60N152O360U60 22891.92 100 yellow 0.195 × 0.138 × 0.108 orthorhombic Fmmm 32.8783(6) 46.6442(9) 54.0834(10) 90 90 90 82941(3) 2, 1.833 11.735 0.85−26.573 −41 ≤ h ≤ 41 −58 ≤ k ≤ 58 −67 ≤ l ≤ 67 314506 22395 0.1218 392 1.078 R1 = 0.0727 wR2 = 0.1967 R1 = 0.1526 wR2 = 0.2481 2.746 and −2.080

(NH4)2[(UO2)2(O2)2(C2O4)]·4H2O24,25,29 were previously reported. They were synthesized by reacting UO22+ with oxalic acid and then hydrogen peroxide in the presence of ammonium ions, and by varying the pH and the concentrations of the starting solutions. The compounds were identified by absorption measurements, chemical analysis, IR and Raman spectroscopy, and thermal analysis, but in the absence of structural determination, the formulas remain uncertain. Like actinide oxalates, actinide peroxides, and peroxooxalates can be considered as good potential precursors of actinides oxides for advanced fuel fabrication as long as they contain no cation or thermally labile cations such as ammonium or hydrazinium. Several double ammonium or hydrazinium actinide oxalates have been reported.30−40 Among them the mixed actinide(IV)-actinide(III) oxalates with NH4+ or N2H5+ as charge compensators were thermally converted into mixed oxides.41−43 Owing to the importance of uranyl peroxides and peroxo-oxalates and their potential use as uranium oxide precursors, the ammonium-uranyl-peroxide-oxalate system was reinvestigated and reported in this paper, with a particular effort to grow single crystals for structural characterization.

reaction that might occur during the alteration of spent nuclear fuel (SNF)2 deposited in future deep geological repositories and lead to the formation of these peroxides has motivated several studies in recent years.3−17 Since 2005, the family of actinyl peroxide was extended to nanoclusters by the research group of P. C. Burns.18 The discovery of this beautiful and exciting family of materials has raised considerable interest in actinides peroxide clusters which were recently reviewed.19,20 The anionic charge of the clusters can be balanced by a large diversity of monovalent ions, but only one uranyl peroxide with NH4+ as charge compensator has been reported to date.21 The structure of this compound is based on a crown cluster containing 32 uranyl ions with composition [(UO2)32(O2)40+x(OH)24−2x]40−, x ∈ [0 ; 4], designated U32R. More recently the family of uranyl peroxide cluster was extended to mixed ligands uranyl peroxide. Among them four uranyl peroxo-oxalates have been structurally characterized. Three contain cage clusters built from 36, 50, and 60 uranyl ions and denoted U36Ox6,22 U50Ox20,23 and U60Ox30.22 The fourth compound, U120Ox90,23 is a peroxo-oxalate cage cluster, in a core−shell arrangement. It exhibits an U60Ox30 core with a shell formed of five-membered rings of uranyl bipyramids capping the five-membered rings of the core and linked to them by means of K+ cations. In all of these peroxo-oxalates compounds, the anionic charge of the clusters is balanced by alkali metal cations which preclude their use as a uranium oxide precursor. Indeed, three ammonium uranyl peroxo-oxalates (NH 4 ) 4 [(UO 2 ) 2 (O 2 )(C 2 O 4 ) 2 ]· 2H2O,24,25 (NH4)2[(UO2)(O2)(C2O4)]·(1−3)H2O,24−28 and

■

EXPERIMENTAL SECTION

Synthesis. Crystals of eight different compounds (2−9) were grown from solutions in 10 mL glass vials under ambient conditions. Solutions were prepared by dissolving uranyl nitrate hexahydrate (UO2(NO3)2·6H2O, 0.25 mmol, 125 mg) and ammonium oxalate hydrate ((NH4)2C2O4·H2O in varying oxalate/uranium ratio: 0, 0.7, 1.4 and 3) in 3.75 mL of deionized water. Then, with continuous B



Article

Table 2. Crystal Data, Intensity Collection and Structure Refinement Parameters for 6−9 CCDC number formula formula weight temperature/K crystal color crystal size/mm crystal system space group a/Å b/Å c/Å α/° β/° γ/° volume/Å3 Z, ρcalculated/g·cm‑3 μ/mm−1 Θ range/° limiting indices

collected reflections unique reflections R(int) parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff. peak and hole/e·Å−3

6: U5Ox5_C2/c

7: U28

8: U2Ox4

9: U10Ox8

1408211 C10N11.27O41.9U5 2138.51 293 yellow 0.237 × 0.148 × 0.124 monoclinic C2/c 30.761(4) 22.674(3) 20.944(3) 90 122.231(3) 90 12357(3) 8, 2.299 13.155 1.33−27.550 −39 ≤ h ≤ 39 −29 ≤ k ≤ 29 −27 ≤ l ≤ 27 154750 14203 0.0583 609 1.148 R1 = 0.0364 wR2 = 0.1122 R1 = 0.0558 wR2 = 0.1255 1.867 and −1.696

1408213 N28O148U28.25 9456.13 100 yellow 0.190 × 0.130 × 0.128 tetragonal I4̅ 20.5052(13) 20.5052(13) 24.7010(16) 90 90 90 10385.9(15) 2, 3.024 22.026 1.4−26.404 −24 ≤ h ≤ 21 −25 ≤ k ≤ 25 −30 ≤ l ≤ 30 45720 10635 0.0582 244 1.065 R1 = 0.0457 wR2 = 0.1181 R1 = 0.0570 wR2 = 0.1254 4.563 and −1.157

1408209 C8N6O24U2 1040.20 293 yellow 0.05 × 0.025 × 0.025 triclinic P1̅ 8.0403(11) 13.565(2) 14.163(2) 109.825(7) 105.333(7) 102.263(7) 1321.5(3) 2, 2.614 12.348 1.64−27.592 −10 ≤ h ≤ 10 −17 ≤ k ≤ 17 −18 ≤ l ≤ 18 36562 6024 0.0491 361 1.039 R1 = 0.0279 wR2 = 0.0658 R1 = 0.0439 wR2 = 0.0707 1.832 and −0.998

1408212 C16N18O74U10 4008.64 293 yellow 0.111 × 0.088 × 0.055 orthorhombic Pnnm 13.9505(8) 17.0655(9) 19.0417(11) 90 90 90 4533.3(4) 2, 2.937 17.909 1.60−27.188 −17 ≤ h ≤ 15 −21 ≤ k ≤ 18 −21 ≤ l ≤ 24 81168 5172 0.0754 157 1.149 R1 = 0.0653 wR2 = 0.1458 R1 = 0.1051 wR2 = 0.1603 4.722 and −2.202

not identified and can be N or O atoms which belong to NH4+ ions, H3O+ ions or H-bonded H2O, in this case they are arbitrarily named N. Crystallographic data are reported in Tables 1 and 2. The CIF files and structure determination details are provided as Supporting Information.

stirring, hydrogen peroxide (H2O2, 1 M, 1.25 mL) was added, in order to reach a peroxide/uranium ratio equal to 5. The pH of the solutions was varied between 4 and 11 by adding ammonia (NH4OH, 1 M). Two crystal growth methods were used: (i) methanol diffusion was realized by putting the vial containing the solution in a glass bottle partially filled with 5 mL of methanol, and the bottle was capped and single crystals grew from 24 h up to 1 week, depending on the experiment; (ii) the solution was covered with parafilm and a powder with sometimes some single crystals formed within a few weeks. This series of experiments allowed the growth of all the isolated crystals except for crystal 3 which was obtained by doubling the uranium concentration and using a 30% hydrogen peroxide solution. The conditions for growth of crystals selected for X-ray diffraction intensity measurements are detailed in the Supporting Information. Single-Crystal Data Collection. The single crystal diffraction intensities were measured by means of a Bruker X8 CCD 4K diffractometer using a MoKα radiation (0.71073 Å) with an optical fiber as collimator. The data of compounds 2−5 and 7 were collected at 100 K with the help of a N2 gas flow, while data of compounds 6, 8, and 9 were collected at room temperature. The intensities were extracted from the collected frames and corrected for absorption effects using the program SAINT V7.53a.44 The structure resolutions and refinements were performed with the SHELX software45 with the WINGX interface.46 An initial model consisting of uranium atoms is obtained with direct methods, while the remaining atoms were found from successive Fourier map analyses. As typical for uranium compounds, the H atoms were not located in the structures. In all the structures, the located counter-cations into the uranyl peroxo or peroxo-oxalate anionic cluster have been considered as ammonium ions, although in some cases, the presence of hydronium ions cannot be totally prohibited. The atoms external to the nanoclusters are often

■

RESULTS AND DISCUSSION Formation Range. For the four series of experiments, various uranyl peroxides and uranyl peroxo-oxalates were obtained depending on the C2O42−/UO22+ ratio and the pH (Figure 1). Without oxalate (C2O42−/UO22+ = 0), addition of hydrogen peroxide to the uranyl solution causes the precipitation of a solid (1) identified by powder X-ray diffraction as the peroxide (UO2)(O2)·4H2O. This compound remains insoluble until pH 9, whereas an orange solution is obtained at pH 10. By covering

Figure 1. Schematic representation of the solid compounds obtained in our experimental conditions for different oxalate/uranyl ratio and pH of the initial solution. C



Article

the vial with parafilm an amorphous powder A, accompanied by some platelet-shaped crystals of 2, is obtained after 2 days. The same results are obtained for C2O42−/UO22+ = 0.7, except that no crystals of 2 are observed in the amorphous powder. By doubling the initial uranium concentration and using a 30% hydrogen peroxide solution, crystals of 3 are obtained instead of 2. For a C2O42−/UO22+ ratio of 1.4, by diffusion of methanol, yellow needle-like crystals of 4 were obtained for pH between 4 and 8. Platelet-shaped crystals of 5 and yellow crystals of 6 in the shape of triangular prisms appeared simultaneously with crystals 4 in a narrow domain around pH ≈ 5 and pH ≈ 6.7, respectively. Between pH 8 and 9, yellow blocky crystals of 7 appeared after few days. Above pH 9, an amorphous phase is obtained. For C2O42−/UO22+ ratio of 3, by diffusion of methanol, yellow needle-like crystals of 8 were obtained between pH 4 and 4.5. Above pH = 4.5 and up to 8, crystals of 4 formed and above pH 8 yellow blocky crystals of 9 appeared after few days. Above pH 9, an amorphous phase is obtained. Structures Description. In all the obtained structures, the uranium polyhedron is a hexagonal bipyramid with uranyl ion coordinated in the equatorial plane by six oxygen atoms provided by peroxide or oxalate ions that act as bidentate ligands, by water molecules or by pair of hydroxide ions. The uranyl polyhedra are connected through peroxide or oxalate ions or OH − OH bridges to form infinite neutral chain in 1 or finite anionic ions or clusters in all the other compounds. U r a n y l P e r ox i d e s . ( 1 ) ( U O 2 ) ( O 2 ) · 4 H 2 O , ( 7 ) (NH4)28[(UO2)28(O2)42]·nH2O (U28-NH4), (2) (NH4)40[(UO 2 ) 32 (O 2 ,(OH) 2 ) 52 ]·nH 2 O (U32R-NH 4 ), and (3) (NH4)44[(UO2)44(O2)66]·nH2O (U44-NH4). The structure of the uranyl peroxide (UO2)(O2)·4H2O or studtite was previously determined from a mineral specimen.47 The primary structural block unit (PBU) is a hexagonal bipyramid [(UO2)(O2)2(H2O)2] (Figure 2a) where the equatorial coordination of the uranyl ion is provided by two bidentate peroxides and two water oxygens in trans positions. The PBU are connected through the peroxide ions to build infinite chains further linked via hydrogen bonds which involve both the water molecules belonging to the chains and the additional water molecules located in between them, so as to give the final formula [(UO2)(O2)(H2O)2]·2H2O (Figure 2d). The compounds 7, 2, and 3 contain clusters built of 28, 32, and 44 uranium polyhedra and designated as U28, U32R, and U44 respectively, according to the notation introduced by the group of Burns. U28 and U44 are closed clusters, and U32R is a crownshaped cluster. U28 cluster with composition [(UO2)28(O2)42]28− contains only triperoxo uranyl polyhedra edge-shared (Figure 2b) so as to design pentagons and hexagons. The sphere U28 is formed by 12 pentagons and 4 hexagons. The U28 cluster is similar to that reported for the first time by Burns and co-workers in 200518 with K and Li as counterions (U28-KLi) and later by Nyman et al.48 with K and larger ions Rb (U28-KRb) or Cs (U28-KCs) as counterions. In their paper on U28, Nyman et al. discuss the effect of the internal templating cations. In U28-KLi, K+ cations template the four hexagonal and the 12 pentagonal rings and are bonded to the yl oxygens that point inward the nanocage toward the center of the rings. In U28-KRb and U28-KCs, K+ ions template the 12 pentagonal rings and larger alkaline cations, Rb+ and Cs+, template the four hexagonal rings. In fact it is not known if the cations actually play a templating role or just a

Figure 2. In (UO2)(O2)·4H2O47 (1), the PBU [(UO2)(O2)2(H2O)2] (a) are connected through the peroxide ions to build infinite chains (d). Sharing of peroxide ions (in bold) between [(UO2)(O2)3] PBU (b) creates nanoclusters U28 (e) and U44 (f) in U28-NH4 (7) and U44-NH4 (3). In the crown-shaped Cluster U32R (g) in U32R-NH4 (2) some [(UO2)(O2)3] PBU (b) are replaced by [(UO2)(O2)2(OH)2] PBU (c).

stabilizing role, and hereafter it is considered that ammonium ions rather stabilize the rings. In the present compound U28NH4, NH4+ stabilizes both the pentagonal and the hexagonal rings (Figure 3a) and are displaced toward the center of the sphere of about 0.90−1.10 Å from the mean planes of the coordinated Oyl atoms for pentagonally bonded NH4+ ions and toward the outside of the sphere of about 0.65 Å for the hexagonally bonded NH4+ ions (Table 3). The overall geometry of the pentagonal cycle is similar whatever the countercation, K+ or NH4+, and to take into account the increase of the ionic radius from K+ to NH4+, the NH4+ cation is more displaced away from the Oyls mean plane leading to longer cation−Oyl distances (NH4+−Oyl distances in U28-NH4 in the range 2.85(4)−3.04(4) Å with an average value of 2.93 Å; K+−Oyl distances in U28-KLi18 in the range 2.59(4)−2.78(4) Å with an average value of 2.69 Å) and to shorter cation−cation distances (in the range 2.92(4)−3.11(5) Å, instead of about 4.0 Å for K+ in U28-KLi) and to the formation of a quasi-regular (NH4)12 cluster (Figure 4). This indicates that stabilizing of pentagonal macrocycles in U28 nanocages by the largest alkali cations (Rb+ and Cs+) does not appear possible due to cation−cation repulsion in agreement with the conclusions of Nyman et al.48 For the cation-stabilized hexagonal rings, the cation−Oyl distances are large and show little variation with the nature of the cation. The average values are 3.06(4), 3.13(4), 3.15(4), and 3.19(4) Å, for K+ (in U28-KLi18), Rb+ (in U28-KRb48), NH4+ (in U28NH4present work), and Cs+ (in U28-KCs48), respectively, the differences resulting from subtle variations of the Oyl−Oyl distances or the distance of the cation to the Oyls plane. For example, for NH4+ and K+, the average Oyl−Oyl distances are the same (3.08(4) Å), but the K+ cations are almost in the Oyls D



Article

Figure 3. Stabilizing effect of NH4+ cations in (a) U28-NH4 (7) and (b) U32R-NH4 (2). For clarity, In the UO8 polyhedra, only the Oyl coordinated to N atom are represented.

Table 3. Bond Data for Ammonium Ions Stabilizing the Rings in U28-NH4, U32R-NH4, and U44-NH4 NH4−Oyl bonds hexagonal ring

Pentagonal ring

range

average

d

U28

N1−Oyl N1−Ow1

3.07−3.27

3.15 2.78

0.65(3)

U32R

N3−Oyl N3−OH N5−Oyl N5−OH N6−Oyl N13−Oyl N19−Oyl

3.32−3.35

3.34 2.85 3.29 2.92 3.14 3.13 3.24

0.96(6)

U44

3.27−3.31 2.72−3.46 2.97−3.20 3.12−3.43

range

average

d

N2−Oyl N3−Oyl N4−Oyl N1−Oyl

2.88−3.04 2.85−2.97 2.87−3.00 2.82−3.00

2.95 2.93 2.90 2.91

1.10(3) 1.03(3) 0.91(3) 0.99(3)

N3−Oyl N4−Oyl N12−Oyl N18−Oyl N8−Oyl

2.73−3.11 2.84−3.00 2.89−2.99 2.79−3.15 2.72−3.11

2.94 2.90 2.95 2.96 2.91

1.13(6) 0.97(7) 1.13(8) 1.30(7) 1.22(6)

0.91(7) 0.51(8) 0.27(8) 0.68(6)

distances are greater (3.16(4) Å). The sum of the bond valences of ammonium ions calculated by using the bondvalence model of Brown49 and the parameters determined by Garcia-Rodriguez et al.50 are in the range 0.7−0.8 vu. Some ammonium coordination polyhedra being completed by oxygen atoms of the central uranium or water oxygens from the outside of the sphere not localized (see below) and not included in this calculation. On the other hand, the ammonium ion can participate in H-bonding; however in the present case, the coordination numbers (C.N.) and the geometry of the environments are close for the NH4+ ion and the alkali ions K+, Rb+, and Cs+, indicating that the pseudo alkali character of the ammonium ion dominates.51

Figure 4. Cluster (NH4)12 in U28-NH4 (7).

plane when NH4+ is at 0.65(3) Å from the Oyls plane. As for NH4+, Cs+ is out of the plane (of about 0.5 Å), but the Oyl−Oyl E



Article

Figure 5. Top view (a) and side view (b) of the hexagonal macrocycles with boat conformation in U44-NH4 stabilized by ammonium ions. The two N sites are half occupied.

U28-KRb or Cs U28-KCs.48 The centers of the U28-NH4 and U32R-NH4 clusters under study are occupied by about 25 and 50% by a uranium atom, respectively. In U44-NH4, about 0.65 uranium by U44 cluster occupies two positions displaced of ∼3.5 Å from the center of the cluster toward the bowls. These central uranium atoms are disordered on partially occupied sites, and the corresponding uranium polyhedron is probably orientationally disordered. In these conditions the localization of the ligand oxygen atoms is not possible. So the definition of the central uranyl complex, among several possibilities such as [(UO2)(O2)3]4− or [(UO2)(O2)2(H2O)2]2− suggested in U28 clusters,18,48 remains unsolved. Given the difficulty to distinguish with certainty between ammonium ions and water molecules outside the cluster, on one hand, and to determine the number of uranium and the formula of the uranyl complex disordered at the center of the cluster, approximate chemical formulas are given without taking into account the uranyl complex at the center of the capsules for the three compounds U28-NH4 (7), U32R-NH4 (2), and U28-NH4 (3). The Uranyl Peroxo-oxalates. (4) (NH 4 ) 10 [(UO 2 ) 5 (O2)5(C2O4)5]·8H2O (U5Ox5-NH4), (6) (NH4)10.5[(UO2)5(O2)5(C2O4)5](NO3)0,64·nH2O (U5Ox5′-NH4), (5) (NH4)60[(UO 2 ) 60 (O 2 ) 60 (C 2 O 4 ) 30 ]·nH 2 O (U60Ox30-NH 4 ), (8) (NH 4 ) 6 [(UO 2 ) 2 (O 2 )(C 2 O 4 ) 4 ]·2H 2 O (U2Ox4-NH 4 ), (9) (NH4)18 [(UO2)10(O2)10(OH)2(C2O4)8]·nH2O (U10Ox8NH4) In these uranyl peroxy-oxalates, two types of PBU involving both peroxo and oxalate ligands in the equatorial plane of the uranium hexagonal bipyramid are observed, [(UO2)(O2)(C2O4)2] in compound 8 and [(UO2)(O2)2(C2O4)] in compounds 4, 7, and 9 (Figure 6). In 8 two PBU are linked by sharing the peroxide ion to build a dimeric unit [(UO2)2(O2)(C2O4)4]6− denoted U2Ox4 similar to that found in the potassium compound K6[(UO2)2(O2)(C2O4)4]·4H2O studied by Sigmon et al.54 In this paper the authors conclude that the dihedral angle in the U−O2−U linkage is inherently bent as a result of a covalent interaction between the peroxide group and the uranyl ion. In the NH4 compound the U−O2−U angle is more strongly bent (142.6°) than in the K compound (152.9°) when the DFT calculations show that the U−O2−U angle increases with the ionic radius of the counterion.55 In fact the environments of K+ and NH4+ ions are different in the two compounds. In U2Ox4-K, the C.N. of K+ are 7 (for K1, K2, K3, K5, and K6) and 8 (for K4). In U2Ox4-NH4, the C.N. of NH4+ are lower, and N1 is bonded to four oxygen atoms forming a distorted tetrahedron and N2, N3, N4, N5 to five oxygen atoms, two belonging to the same

The cohesion between the spheres U28 are ensured by the monovalent cations and water molecules leading to different arrangements with monoclinic, orthorhombic, and tetragonal symmetry for U28-KLi, 18 U28-KRb/Cs, 48 and U28NH4present work, respectively. The U32R crown cluster in 2 (Figure 2g) contains eight triperoxide bipyramids, 16 uranyl bipyramids with two peroxides, and two OH−OH groups forming the three equatorial edges and eight uranyl bipyramids in which the O−O distance for the third equatorial edge determined from the crystallographic study is intermediate between peroxide and the OH−OH bridge. This has already been observed by Sigmon and Burns for U32R clusters obtained in the presence of NH4+ and Li+ cations.21 Thanks to the present crystallographic study of U32R-NH4 (2), all the NH4+ ions within the clusters were located, and they stabilize both the eight pentagonal and the four hexagonal rings that build the U32R cluster. As in U28NH4, in U32R-NH4, the N atoms of the NH4+ ions are displaced from the mean planes of the coordinated Oyl atoms toward the center of the sphere (pentagonal rings) and toward the outside of the sphere (hexagonal rings) (Figure 3b, Table 3). The coordination of the ammonium ions stabilizing the hexagonal rings is completed by two hydroxide oxygens. In U44-NH4 (3), the cluster U44 is shaped as a skein of wool with two bowls, built from a belt of three pentamers and three hexamers capped by three pentamers (Figure 2f). Six hexamers connect the two bowls. Unlike other pentamers and hexamers in which the uranium atoms are almost in a plane, in these last hexamers the six uranium atoms form a ring with a boat conformation. A theoretical study by DFT calculations performed on the [(UO2)6(O2)6(H2O)6 entity showed that hexagonal macrocycle is flexible and can adopt planar, boat, or chair conformations.52 The boat conformation has been found experimentally only in the U44 cluster. Ammonium cations stabilize both pentagonal and planar hexagonal rings and are located above the center of the rings. Ammonium ions stabilize also the hexagonal ring with boat conformation and are distributed on two sites half occupied (Figure 5) in position close to that theoretically determined for K+ ion.52 The U44 cluster has been previously described in the compound with approximate formula (Li,Na)44[(UO2)(O2)1.5]44·nH2O that crystallizes in the trigonal symmetry with space group R3̅.53 The actinyl nanoclusters contain or not an actinyl ion in the center. For example, the Np24 nanosphere is occupied in approximately 50% by a Np atom, although the center of the analogue U24 nanosphere is unoccupied.18 Similarly the U28 nanosphere is half occupied in U28-KLi18 and fully occupied in F



Article

dihedral angles U1−O2−U3 and U2−O2−U3 are greater for NH4+ than for K+ (Table 4) in agreement with the increase of the ionic radius, opposite to the U2−O2−U2 angle involving the two m related U2 atoms, this entity being flattened in the two compounds. In 6 the symmetry of the U5Ox5 ring is reduced to C1 with four U−O2−U dihedral angles in the range 147.2(3)−152.7(5)° and a fifth substantially smaller (140.7(4)°). The average NH4−O distance (2.91 and 2.94 Å in 4 and 6 respectively) is close to that obtained for ammonium stabilizing pentagonal rings in U28-NH4 (7) (2.90, 2.93, and 2.95 Å) and greater than in U5Ox5-K (2.78 Å).54 The main differences between compounds 4 and 6 concern the geometry of the pentameric ring and the role of the ammonium cation in the pentamer assembly. In 4 the atom N(1) is nearly in the plane of the Oyls, with an average N(1)−Oyl distance of 2.910(5) Å. Its coordination is completed by two oxalate oxygens belonging to another pentamer that leads to the formation of dimers within which the two pentamers are parallel and connected by four N−O bridges (Figure 7a). In compound 6 the ammonium cation is moved 0.46(1) Å away from the Oyls plane in opposite to the uranium atoms giving larger NH4−Oyl distances (average 2.95(1) Å). Its coordination is completed by two Oyl of another pentamer leading to the formation of dimers which differ from those described above (Figure 7b). For the two compounds, these two supplementary coordinated oxygen atoms allow the valence bond sum being equal to the theoretical value (0.99 and 0.97 vu for 4 and 6, respectively). In the potassium compound,54 the role of potassium is similar to that of ammonium in compound 4, but the pentamers are no longer parallel resulting in zigzag chains (Figure 7d). Another difference between compounds 4 and 6 is the presence of noncoordinated nitrate anions in 6. Compound 9 contains an unprecedented arrangement of 10 uranyl hexagonal bipyramids [(UO2)10(O2)10(OH)2(C2O4)8]18−. This double ring can be described from the precedent pentameric unit [UO2(O2)(C2O4)]510− by the replacement of one bidentate oxalate by two hydroxide anions and the sharing of this OH−OH bridge between two as-formed pentagonal rings. The dihedral U3A−(OH)2−U3B angle is bent (157.8(6)°), while in the cluster [(UO2)(O2)2(OH)2]6− found in K6[(UO2)(O2)2(OH)2]·7H2O60 it is 180°. However, optimized geometry by quantum chemical calculations for a hypothetical cluster of two uranyl ions polyhedra shared by an

Figure 6. [(UO2)(O2)(C2O4)2] hexagonal bipyramids share the peroxide ion (a) to build the dimer U2Ox4 in 8. Five [(UO2)(O2)2(C2O4)] hexagonal bipyramids are linked by sharing peroxide (b) to build U5Ox5 rings in 4 and 6 further connected through bis-bidentate oxalates (c) to create the U60Ox30 nanosphere in 5. In 9 one bidentate oxalate of the pentamers is replaced by a OH−OH bridge shared between two pentamers (d) to form the U10Ox8 unit.

oxalate ion, indicating the prevalence of H-bonding of NH4+ with four H-bonds for N1 and three normal H-bonds and a bifurcated H-bond for the other N atoms. The formula of 8 (NH4)6[(UO2)2(O2)(C2O4)4]·2H2O was announced in 190756 with higher numbers of water molecules (7 and 3, depending the synthesis conditions) and never confirmed to date. Similar dimers have been recently stabilized with various terminal ligands.57,58 The compound 4, (NH4)10[(UO2)5(O2)5(C2O4)5]·8H2O, was characterized by different authors24−27 and the monoclinic cell determined by Baskin et al.,26 but no structural study has been reported up to now. In this compound, as in 6, the PBU [(UO2)(O2)2(C2O4)] are linked by sharing the two peroxide ions to build a pentameric unit [UO2(O2)(C2O4)]510− denoted U5Ox5 similar to that found in the potassium compound K10[UO2(O2)(C2O4)]5·13H2O54 denoted hereafter U5Ox5-K. The pentagonal cluster has been recently stabilized using malonate instead of oxalate terminal ligand.59 In 4 and U5Ox5K,54 the U5Ox5 pentameric rings adopt Cm symmetry. The

Table 4. Dihedral Angles U−O2−U and Bond Data for Compounds Containing the U5Ox5 Cluster

a

U5Ox5-NH4 (4) and K for U5Ox5-K54 U5Ox5-NH4 U1−O2−U3 (2x) 149.3(2)° U2−O2−U3 (2x) 148.0(2)° U2−O2−U2 155.8(2)°

U5Ox5-K 143.6(2)° 142.5(2)° 158.4(2)°

Ma−O1 M−O6 (2x) M−O16 (2x)

2.936(9) Å 2.910(7) Å 2.897(5) Å

2.680(6) Å 2.752(5) Å 2.804(4) Å

average M−O21 (2x)

2.910(7) Å 3.043(9) Å

2.758(5) Å 3.155(6) Å

U5Ox5′-NH4 (6) U1−O2−U3 U2−O2−U3 U3−O2−U4 U4−O2−U5 U5−O2−U1 NH4−O2 NH4−O4 NH4−O6 NH4−O8 NH4−O10 NH4−O2 NH4−O4

140.7(4)° 149.6(3)° 150.7(2)° 147.2(3)° 152.7(3)° 2.995(11) 2.991(14) 2.901(14) 2.948(10) 2.891(18) 2.945(14) 2.983(18) 2.958(11)

Å Å Å Å Å

M = NH4 for U5Ox5-NH4 (4) and K for U5Ox5-K.54 G



Article

Figure 7. Stabilizing role of ammonium cation in (a) U5Ox5-NH4 (4), (b) U5Ox5′-NH4 (6), (c) U10Ox8-NH4 (9), and (d) of potassium cation in U5Ox5-K.54 For clarity, In the UO8 polyhedra, only the Oyl coordinated to N atom are represented.

(2.54(2)−2.65(3) Å). Some of these atoms are probably water or oxonium oxygens.

hydroxide bridge in the presence of different cations gives a U− (OH)2−U dihedral angle between 177.4 and 180° for A = Li, Na, Rb, and Cs, but only 156.8° for A = K.55 The atom N(2) is nearly in the plane of Oyls and complete coordination with two oxalate oxygens of another pentamer leading to the formation of dimers similar to those in 6 (Figure 7c). In compound 5, 12 U5Ox5 units condense by oxalate sharing to form the U60Ox30 nanosphere which adopt a fullerene topology already evidenced in the potassium oxalate based compound.22 The connection of the U5Ox5 pentameric rings through bis-bidentate oxalate anions creates U6Ox3 hexagonal rings built from three dimers of pentagonal bipyramids [(UO2)(O2)2(C2O4)] linked by sharing a peroxide ion further connected by oxalate ions. Both U5Ox5 and U6Ox3 rings are stabilized by ammonium ions (Figure 8). The role of

■

CONCLUSION Crystallization in the NH4/C2O4/O2/U(VI) system was carried out in fixed synthesis conditions by paying a special attention to the influence of the (C2O4)2−/(UO2)2+ ratio and pH. For pH > 9, an amorphous phase forms. It can be accompanied by crystals of U32R-NH4 peroxide built of a U32R crown-shaped cluster or with larger clusters of U44-NH4 built of U44 cages by increasing the hydrogen peroxide and uranium concentrations. At this stage, the characterization of the amorphous phases remains an interesting challenge. For lower pH values, the formed phases strongly depend on the (C2O4)2−/(UO2)2+ ratio and of the pH value. In the absence of oxalate ion or in the presence of a small amount of oxalate ion (C2O4)2−/(UO2)2+ = 0.7), ammonium is not involved in the formation of solid and studtite, and an extended one-dimensional compound, is obtained. Another peroxide U28-NH4 is formed for (C2O4)2−/(UO2)2+ = 1.4 and 8 < pH < 9. For lower pH and/or higher (C2O4)2−/(UO2)2+ ratio, the formation of peroxo-oxalates is favored. Five new uranyl peroxo-oxalates containing clusters built of 2 (U2Ox4), 5 (U5Ox5 and U5Ox5′), 10 (U10Ox8) or 60 (U60Ox30) uranyl polyhedra were found. The structure of all these peroxooxalates, except 8 in which clusters of two uranyls U2Ox4 are formed, contains clusters of pentamer cycles which can be associated with hexamer rings. Experiments in other conditions and with other cations will help to understand the factors that allow their formation and their different assemblies in U5Ox5, U5Ox5′, U10Ox8, and U60Ox30 clusters. At this stage of the work, it appears that the ammonium cations can stabilize the pentamer and hexamer rings and adapt their coordination to various curvatures and/or stabilizing effects. So, subtle changes in growth conditions may result in changes in the dimension and type of nanoclusters obtained. It is thought that other ammonium uranyl peroxides and ammonium uranyl peroxo-oxalates could be obtained by using different experimental conditions, which opens a new field of research on these potential uranium oxide precursors. The presence of the pentagonal dodecahedral cluster in the U60Ox30 sphere should also be confirmed and its nature clarified.

Figure 8. [(UO2)10(O2)10(OH)2(C2O4)8]18− ion in U10Ox8-NH4 (9) formed of two pentamers U5Ox4 shared by an OH−OH bridge.

ammonium in the U5Ox5 stabilizing is very similar to that observed in U28-NH4 (7), with ammonium displaced of 1.33(2), 1.23(3), and 1.25(3) Å from the Oyls mean plane and average N−Oyl distances of 2.94, 2.96, and 2.96 Å for N1, N2, and N3, respectively. The ammonium coordination is completed on the other side of the Oyls plane by an ammonium N atom at about 2.8 Å (Figure 9b). The N-ammonium atoms stabilizing the U6Ox3 rings are in the plane formed by the six oxalate oxygens (Figure 7c). A regular pentagonal dodecahedral cluster of atoms labeled N7 occupies the center of the U60Ox30 nanocage (Figure 10), this cluster being similar to the (H2O)20 cluster encapsulated in metal−organic frameworks61 with shorter interatomic distances H



Article

Figure 9. Stabilizing role of the ammonium cations in U60Ox30-NH4. (a) Top view and (b) side view of the environment of the ammonium N atoms stabilizing the U5Ox5 rings (for clarity, In the UO8 polyhedra, only the Oyl coordinated to N atom are represented), and (c) top view of the environment of the ammonium N atoms stabilizing the U6Ox3 rings.

■

Crystallographic information files (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

Figure 10. Regular pentagonal dodecahedral cluster of atoms labeled N7 located at the center of the U60Ox30 nanocage.

■

REFERENCES

(1) Abraham, F.; Arab-Chapelet, B.; Rivenet, M.; Tamain, C.; Grandjean, S. Coord. Chem. Rev. 2014, 266−267, 28−68. (2) Kubatko, K-A. H.; Helean, K. B.; Navrotsky, A.; Burns, P. C. Science 2003, 302, 1191−1193. (3) Sattonnay, G.; Ardois, C.; Corbel, C.; Lucchini, J. F.; Barthe, M.F.; Garrido, F.; Gosset, D. J. Nucl. Mater. 2001, 288, 11−19. (4) Amme, M.; Renker, B.; Schmid, B.; Feth, M. P.; Bertagnolli, H.; Döbelin, W. J. Nucl. Mater. 2002, 306, 202−212. (5) McNamara, B.; Buck, E.; Hanson, B. Mater. Res. Soc. Symp. Proc. 2003, 757, 401−406. (6) Clarens, F.; de Pablo, J.; Diez-Perez, I.; Casas, I.; Gimenez, J.; Rovira, M. Environ. Sci. Technol. 2004, 38, 6656−6661. (7) Hanson, B.; McNamara, B.; Buck, E.; Friese, J.; Jenson, E.; Krupka, K.; Arey, B. Radiochim. Acta 2005, 93, 159−168. (8) McNamara, B.; Hanson, B.; Buck, E.; Soderquist, C. Radiochim. Acta 2005, 93, 169−175. (9) Clarens, F.; de Pablo, J.; Casas, I.; Gimenez, J.; Rovira, M.; Merino, J.; Cera, E.; Bruno, J.; Quinones, J.; Martinez-Esparza, A. J. Nucl. Mater. 2005, 345, 225−231. (10) Clarens, F.; Giménez, J.; de Pablo, J.; Casas, I.; Rovira, M.; Dies, J.; Quiñones, J.; Martínez-Esparza, A. Radiochim. Acta 2005, 93, 533− 538.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01090. The crystallographic data for 2−9 have been deposited at the Cambridge Crystallographic Data Centre with CCDC 1408214 (2), 1408215 (3), 1408210 (4), 1408216 (5), 1408211 (6), 1408213 (7), 1408209 (8), and 1408212 (9). Synthesis conditions for growth of crystals, structure determination details, and tables of final atomic coordinates and displacement parameters for crystals 2 −9 (PDF) I



Article

(11) Jegou, C.; Muzeau, B.; Broudic, V.; Peuget, S.; Poulesquen, A.; Roudil, D.; Corbel, C. J. Nucl. Mater. 2005, 341, 62−82. (12) Corbel, C.; Sattonnay, G.; Guilbert, S.; Garrido, F.; Barthe, M.F.; Jegou, C. J. Nucl. Mater. 2006, 348, 1−17. (13) Ostanin, S.; Zeller, P. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 073101. (14) Meca, S.; Martinez-Torrents, A.; Marti, V.; Gimenez, J.; Casas, I.; de Pablo, J. Dalton Trans. 2011, 40, 7976−7982. (15) Kim, K.-W.; Hyun, J.-T.; Lee, K.-Y.; Lee, E.-H.; Lee, K.-W.; Song, K.-C.; Moon, J.-K. J. Hazard. Mater. 2011, 193, 52−58. (16) Mallon, C.; Walshe, A.; Forster, R. J.; Keyes, T. E.; Baker, R. J. Inorg. Chem. 2012, 51, 8509−8515. (17) Baker, R. J. Coord. Chem. Rev. 2014, 266−267, 123−136. (18) Burns, P. C.; Kubatko, K.-A.; Sigmon, G.; Fryer, B. J.; Gagnon, J. E.; Antonio, M. R.; Soderholm, L. Angew. Chem., Int. Ed. 2005, 44, 2135−2139. (19) Qiu, J.; Burns, P. C. Chem. Rev. 2013, 113, 1097−1120. (20) Nyman, M.; Burns, P. C. Chem. Soc. Rev. 2012, 41, 7354−7367. (21) Sigmon, G. E.; Burns, P. C. J. Am. Chem. Soc. 2011, 133, 9137− 9139. (22) Ling, J.; Wallace, C. M.; Szymanowski, J. E. S.; Burns, P. C. Angew. Chem., Int. Ed. 2010, 49, 7271−7273. (23) Ling, J.; Qiu, J.; Burns, P. C. Inorg. Chem. 2012, 51, 2403−2408. (24) Krishna Prasad, N. S. J. Inorg. Nucl. Chem. 1961, 21, 379−380. (25) Krishna Prasad, N. S. J. Inorg. Nucl. Chem. 1965, 27, 2311−2316. (26) Baskin, Y.; Krishna Prasad, N. S. J. Inorg. Nucl. Chem. 1964, 26, 1385−1390. (27) Bhattacharjee, M.; Chaudhuri, M. K.; Purkayastha, R. N. D. Inorg. Chem. 1986, 25, 2354−2357. (28) Sailaja, B. B. V.; Kebede, T.; Raju, G. S.; Rao, M. S. P. Thermochim. Acta 2002, 386, 51−57. (29) Hamaker, J. W.; Koch, C. W. MDDC-1211, 1947. (30) Alcock, W. J. Chem. Soc., Dalton Trans. 1973, 1610−1613. (31) Alcock, W. J. Chem. Soc., Dalton Trans. 1973, 1614−1615. (32) Alcock, W. J. Chem. Soc., Dalton Trans. 1973, 1616−1619. (33) Govindarajan, S.; Patil, K. C.; Poojary, M. D.; Manohar, H. Inorg. Chim. Acta 1986, 120, 103−107. (34) Poojary, M. D.; Patil, K. C. Proc. Indian Acad. Sci. (Chem. Sci) 1987, 99, 311−315. (35) Grigor’ev, M. S.; Bessonov, A. A.; Yanovskii, A. I.; Struchkov, Yu. T.; Krot, N. N. Sov. Radiochem. 1991, 33, 499−503. (36) Artem’eva, M. Yu.; Mikhailov, Yu. N.; Gorbunova, Yu. E.; Serezhkina, L. B.; Serezhkin, V. N. Russ. J. Inorg. Chem. 2003, 48, 1337−1339. (37) Chapelet-Arab, B.; Nowogrocki, G.; Abraham, F.; Grandjean, S. Radiochim. Acta 2005, 93, 279−285. (38) Chapelet-Arab, B.; Nowogrocki, G.; Abraham, F.; Grandjean, S. J. Solid State Chem. 2005, 178, 3046−3054. (39) Tamain, C.; Arab-Chapelet, B.; Rivenet, M.; Abraham, F.; Grandjean, S. Cryst. Growth Des. 2012, 12, 5447−5455. (40) Tamain, C.; Arab-Chapelet, B.; Rivenet, M.; Abraham, F.; Caraballo, R.; Grandjean, S. Inorg. Chem. 2013, 52, 4941−4949. (41) Arab-Chapelet, B.; Grandjean, S.; Nowogrocki, G.; Abraham, F. J. Alloys Compd. 2007, 444−445, 387−390. (42) Arab-Chapelet, B.; Grandjean, S.; Nowogrocki, G.; Abraham, F. J. Nucl. Mater. 2008, 373, 259−268. (43) Grandjean, S.; Arab-Chapelet, B.; Robisson, A. C.; Abraham, F.; Martin, Ph.; Dancausse, J.-Ph.; Herlet, N.; Léorier, C. J. Nucl. Mater. 2009, 385, 204−207. (44) SAINT Plus, Version 7.53a; Bruker Analytical X-ray Systems: Madison, WI, 2008. (45) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (46) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837−838. (47) Burns, P. C.; Hughes, K.-A. Am. Mineral. 2003, 88, 1165−1168. (48) Nyman, M.; Rodriguez, M. A.; Alam, T. M. Eur. J. Inorg. Chem. 2011, 2011, 2197−2205. (49) Brown, I. D. Acta Crystallogr., Sect. B: Struct. Sci. 1992, 48, 553− 572.

(50) Garcia-Rodriguez, L.; Rute-Pérez, A.; Pinero, J. R.; GonzalezSilgo, C. Acta Crystallogr., Sect. B: Struct. Sci. 2000, 56, 565−569. (51) Khan, A. A.; Baur, W. H. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1972, 28, 683−693. (52) Miro, P.; Pierrefixe, S.; Gicquel, M.; Gil, A.; Bo, C. J. Am. Chem. Soc. 2010, 132, 17787−17794. (53) Sigmon, G. E.; Unruh, D. K.; Ling, J.; Weaver, B.; Ward, M.; Pressprich, L.; Simonetti, A.; Burns, P. C. Angew. Chem., Int. Ed. 2009, 48, 2737−2740. (54) Sigmon, G. E.; Ling, J.; Unruh, D. K.; Moore-Shay, L.; Ward, M.; Weaver, B.; Burns, P. C. J. Am. Chem. Soc. 2009, 131, 16648− 16649. (55) Vlaisavljevich, B.; Gagliardi, L.; Burns, P. C. J. Am. Chem. Soc. 2010, 132, 14503−14508. (56) Mazzuchelli, A.; Bimbi, F. Atti. Accad, naz. lincei, Rend. classe Sci., Fis. mat. nat. 1907, 16, 576−584. (57) Thangavelu, S. G.; Cahill, C. L. Inorg. Chem. 2015, 54, 4208− 4221. (58) Qiu, J.; Vlaisavljevich, B.; Jouffret, L.; Nguyen, K.; Szymanowski, J. E. S.; Gagliardi, L.; Burns, P. C. Inorg. Chem. 2015, 54, 4445−4455. (59) Zhang, Y.; Bhadbhade, M.; Price, J. R.; Karatchevtseva, I.; Kong, L.; Scales, N.; Lumpkin, G. R.; Li, F. Polyhedron 2015, 92, 99−104. (60) Kubatko, K. A.; Forbes, T. Z.; Klingensmith, A. L.; Burns, P. C. Inorg. Chem. 2007, 46, 3657−3662. (61) Sang, R.-L.; Xu, L. CrystEngComm 2010, 12, 1377−1381.

J


Role of Ammonium Ions in the Formation of ... - ACS Publications

Recommend Documents