Syntheses, Crystal Structures, and Nonlinear Optical Activity of Cs2Ba

May 31, 2017 - Cubic crystals of Cs2Ba[AnO2(C2H5COO)3]4, where An = U, Np, Pu, are constructed of typical mononuclear anionic complex units [AnO2(C2H5...
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Syntheses, Crystal Structures, and Nonlinear Optical Activity of Cs2Ba[AnO2(C2H5COO)3]4 (An = U, Np, Pu) and Unprecedented Octanuclear Complex Units in KR2(H2O)8[UO2(C2H5COO)3]5 (R = Sr, Ba) Viktor N. Serezhkin,† Mikhail S. Grigoriev,‡ Aleksey R. Abdulmyanov,† Aleksandr M. Fedoseev,‡ Anton V. Savchenkov,*,† Sergey Yu. Stefanovich,§ and Larisa B. Serezhkina† †

Samara National Research University, Samara 443086, Russia Russian Academy of Sciences, A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Moscow 199071, Russia § Department of Chemical Technology, and New Materials, Lomonosov Moscow State University, Moscow 119991, Russia ‡

S Supporting Information *

ABSTRACT: X-ray diffraction was applied to the elucidation of crystal structures of single crystals of Cs2Ba[AnO2(C2H5COO)3]4, where An = U(I), Np(II), Pu(III), and KR 2 (H 2 O) 8 [UO2(C2H5COO)3]5, where R = Sr(IV), Ba (polymorphs V-a and V-b). FTIR spectra were analyzed for the uranium-containing crystals I, IV, and V-b. Isostructural cubic crystals I−III are constructed of typical mononuclear anionic complex units [AnO2(C2H5COO)3]− and charge-balancing Cs and Ba cations. Features of actinide contraction in the six U−Np−Pu isostructural series known to date are analyzed. In crystal structures of IV and V two typical complexes [UO2(C2H5COO)3]− bind with a hydrated Sr or Ba cation to form the rare trinuclear neutral complex unit {R(H2O)4[UO2(C2H5COO)3]2}, where R = Sr, Ba. Two such trinuclear units and one typical mononuclear unit further bind with a K cation to form the unprecedented octanuclear neutral complex unit K[UO2(C2H5COO)3]{R(H2O)4[UO2(C2H5COO)3]2}2. As the derived polynuclear complexes of uranyl ion with carboxylate ligands in the crystal structures of IV and V are not the first but are rare examples, the equilibrium between mono and polynuclear complex units in aqueous solutions is discussed. The two polymorphic modifications V-a and V-b were studied at 100 K and at room temperature, respectively. Peculiarities of noncovalent interactions in crystal structures of the two polymorphs are revealed using Voronoi−Dirichlet tessellation. The nonlinear optical activity of noncentrosymmetric crystals I was estimated by its ability for second harmonic generation.

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INTRODUCTION To date the crystal structures of eight complexes of a uranyl ion with propionate ligands and cations of mono-, di-, and trivalent metal atoms are known. These include R[UO2(C2H5COO)3], where R = NH4+, K+, Rb+, Cs+, Tl+,1,2 {Ca(H2O)5[UO2(C2H5COO) 3 ]}[UO 2 (C 2 H 5 COO) 3 ]·H 2 O, 3 [La(C 2 H 5 COO) 2 (H2O)3][UO2(C2H5COO)3],4 and Mg(H2O)6[UO2(C2H5COO)3]2.5 Most of them are constructed of typical building unitstricarboxylate complexes of the uranyl ion [UO2L3]−, where L is an anion of a monocarboxylic acid.6−10 The only exception is the Ca-containing compound, in which one tricarboxylate complex of a uranyl ion binds with a hydrated Ca ion to form the binuclear cationic complex {Ca(H2O)5[UO2(C2H5COO)3]}+. The counterion in this compound is the typical anionic complex [UO2(C2H5COO)3]−. Rare examples of formation of polynuclear complexes of [UO2L3]− anions and cations of metal atoms are also observed for acetate- and butyrate-containing compounds. For example, Cs 2 {Sr[UO2 (CH 3 COO) 3 ] 4 }, [Sr(H 2 O) 6 ][UO 2(CH3 COO) 3 ]2{Sr(H 2O) 4[UO2 (CH 3COO)3]2 }2 , and © 2017 American Chemical Society

{Sr(H2O)4[UO2(C3H7COO)3]2}·2H2O contain trinuclear neutral {Sr(H2O)4[UO2(CH3COO)3]2} and {Sr(H2O)4[UO2(C3H7COO)3]2} complex units and pentanuclear anionic {Sr[UO2(CH3COO)3]4}2− complex units.11 The first of the compounds mentioned includes heterovalent Cs and Sr cations, while the remaining two compounds do not. Still, there are compounds of uranyl ion with acetate ligands and heterovalent metal atoms constructed of typical anionic complexes [UO2(CH3COO)3]−: (Cs0.5Ba0.25)[UO2(CH3COO)3]12 and (Rb0.50Ba0.25)[UO2(CH3COO)3].13 Two more examples of polynuclear complexes are the {Pb(H2 O) 3 (CH3 COO)[UO2(CH3COO)3]}14 and [Zn(H2O)4(EtOH)2)]{Ag(UO2(CH3COO)3)3}15 compounds with Pb-containing infinite polynuclear neutral chains and tetranuclear anionic complexes with monovalent Ag cations, respectively. The aim of the current work was to synthesize first complexes of a uranyl ion with propionate ligands and heterovalent Received: March 28, 2017 Published: May 31, 2017 7151

DOI: 10.1021/acs.inorgchem.7b00809 Inorg. Chem. 2017, 56, 7151−7160

Article

Inorganic Chemistry metal atoms. According to previous papers3,11,14,15 one could expect such compounds to be constructed of polynuclear building units, which is unusual and thus interesting. Our attempts resulted in single-crystal structures of five compounds: namely, Cs2Ba[AnO2(C2H5COO)3]4, where An = U (I), Np (II), Pu (III), and KR2(H2O)8[UO2(C2H5COO)3]5, where R = Sr (IV), Ba (V). Apart from enriching crystal structure databases with new entries on compounds of strategic actinide elements, the title compounds are promising due to their potentially high optical activity and are extremely valuable for analysis of actinide contraction in the U−Np−Pu series. These two points are discussed in more detail below. Curiously and unexpectedly, a great number of uranyl monocarboxylate complexes are noncentrosymmetric and crystallize in the cubic crystal system,5−7,16 although the CSD17 and ICSD18 contain respectively about 0.2 and 3.7% of crystal structures featuring a cubic crystal system, excluding the inversion center. Noncentrosymmetric crystals may show high nonlinear optical (NLO) activity.19−21 This fact enhances the value of carboxylate complexes of uranyl ions and motivated us in the search for new crystals with high NLO activity. The first mention of gyrotropy of Na[UO2(CH3COO)3] and R[UO2L3] crystals (R = NH4, Tl, Rb, Cs, L = propionate or n-butyrate ion) dates back half a century.2,22,23 Recently it was shown that uranyl complexes with carboxylate ligands may serve as a scaffold for NLO materials.16,24 As the title crystals Cs2Ba[UO2(C2H5COO)3]4 (I) turned out to be noncentrosymmetric, we found it reasonable to estimate their ability for second harmonic generation (SHG) and to compare their NLO activity with that of the other studied compounds. In addition, we were lucky to get the series of isostructural crystals Cs2Ba[AnO2(C2H5COO)3]4 with U, Np, and Pu atoms. This is the perfect series for analysis of actinide contraction of An(VI) atoms, as the only variable parameter in the composition of these crystals is the actinide atom. Research has shown that actinide contraction is more ambiguous than lanthanide contraction.25−28 In the case of lanthanide metals, 4f electrons do not noticeably participate in chemical bonding, while actinide metals can be divided into two groups. In lighter (early) actinide metals (from Th to Pu), 5f orbitals are delocalized and participate in metallic bonding. In heavier (late) actinide metals (from Am to Cf), only (spd)3 electrons act as binding electrons, whereas 5f electrons are not noticeably involved in bonding because of sharp contraction and localization of the 5f shell.25,26,29,30 The existence of actinide contraction in different compounds containing actinide elements in different oxidation states has been confirmed by both quantum-chemical calculations and crystal structure data.31−42 Excluding the title series of isostructural compounds, to date only five U−Np−Pu series of hybrid organic−inorganic compounds with hexavalent actinides have been reported: [Mg(H2O)6][AnO2(C2H5COO)3]2,5 AnO2(ClO4)2·5H2O,43 AnO2[B8O11(OH)4],44 Li2AnO2(PDC)2·2H2O45 (where PDC is the dipicolinic acid anion), and [AnO2(C2H5COO)2(H2O)2].46 Actinide contraction in these series of compounds is shown through shortening of actinyl AnO distances, while An−O distances in equatorial planes remain constant within the range of a standard deviation of bond length determination.5,32,38,43−47 However, the analysis of actinide contraction of hexavalent U, Np, and Pu atoms in hybrid compounds is hampered due to two aspects. First, the actinide contraction from U to Pu is very subtle and can be revealed in crystal structures only through precise X-ray experiments. Second, the

amount of Np- or Pu-containing compounds in the CSD17 is scarce due to their radioactivity. Thus, every new series of U−Np−Pu-containing isostructural compounds brings valuable insight into the chemistry of 5f elements.

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EXPERIMENTAL SECTION

Synthesis. Caution! Compounds of U, Np, and Pu represent a potential health risk owing to radioactivity. Although the uranium precursors used contain depleted uranium, standard safety measures for handling radioactive substances must be followed. Handling of both Np-237 and Pu-239 isotopes should be undertaken in a properly regulated and controlled radiological facility. Single crystals I−V were derived via isothermal evaporation in 20 mL glass vials at room temperature. Uranyl propionate (UO2(C2H5COO)2(H2O)2), neptunyl propionate (NpO2(C2H5COO)2(H2O)2), and plutonyl propionate (PuO2(C2H5COO)2(H2O)2) were synthesized according to the reported procedure.46 All other reagents were obtained commercially (Sigma-Aldrich, American Elements). Cs2Ba[UO2(C2H5COO)3]4 (I). Uranyl propionate (UO2(C2H5COO)2(H2O)2; 0.30 mmol, 136 mg) and propionic anhydride ((C2H5CO)2O; 1.00 mmol, 130 mg) were separately dissolved each in 2 mL of a water/ethanol (1/1) mixture with moderate heating. Barium hydroxide (Ba(OH)2·8H2O; 0.08 mmol, 24 mg) was dissolved with moderate heating in the second solution containing propionic anhydride. Cesium propionate (C2H5COOCs; 0.15 mmol, 31 mg) was dissolved in 1 mL of water. The three solutions were mixed together to form a transparent yellow solution. The final molar ratio of the reagents was 2/1/4 for Cs+/Ba2+/UO22+, respectively. After 5 days of evaporation yellow crystals in the form of tetrahedra with composition Cs2Ba[UO2(C2H5COO)3]4 (I) were formed. Yield: ∼70%. Gravimetric analysis of the uranium content in I gave a value of 40.2% (calculated 40.4%). Cs2Ba[NpO2(C2H5COO)3]4 (II) and Cs2Ba[PuO2(C2H5COO)3]4 (III). Neptunyl propionate (NpO2(C2H5COO)2(H2O)2; 0.1 mmol) or plutonyl propionate (PuO2(C2H5COO)2(H2O)2; 0.1 mmol) was dissolved in 4 mL of water. Cesium carbonate (Cs2CO3) and barium hydroxide (Ba(OH)2·8H2O) were separately dissolved each in 2 mL of a water/propionic acid (1/1) mixture. After full liberation of carbon dioxide (CO2), the three solutions were mixed together to form a transparent pale violet solution. The tested range of molar ratios of the reagents was (2−4)/(1−2)/1 for Cs+/Ba2+/AnO22+, respectively, where AnO22+ is an actinyl ion. After 1−2 days of evaporation pale violet crystals with composition Cs2Ba[NpO2(C2H5COO)3]4 (II) or Cs2Ba[PuO2(C2H5COO)3]4 (III) were formed. Yield: ∼55% for both compounds. Gravimetric analysis of the actinide content in II and III gave values of 40.2% for Np and 40.8% for Pu (calculated 40.3% and 41.0%, respectively). KSr 2 (H 2 O) 8 [UO 2 (C 2 H 5 COO) 3 ] 5 (IV). Uranyl propionate (UO2(C2H5COO)2(H2O)2; 0.30 mmol, 136 mg) and potassium propionate (C2H5COOK; 0.15 mmol, 17 mg) were separately dissolved each in 2 mL of water. Strontium oxide (SrO; 0.15 mmol, 16 mg) was dissolved in propionic acid (C2H5COOH; 0.6 mmol, 44 mg). The three solutions were mixed to form a transparent yellow solution. The final molar ratio of the reagents was 1/1/2 for K+/Sr2+/ UO22+, respectively. After 5 days of evaporation yellow crystals with composition K[UO2(C2H5COO)3] were formed. The structure of these crystals was solved earlier.1 After 7 days of evaporation yellow platelike crystals with composition KSr2(H2O)8[UO2(C2H5COO)3]5 (IV) were formed. Yield: ∼75%. Gravimetric analysis of the uranium content in IV gave a value of 42.3% (calculated 42.4%). KBa2(H2O)8[UO2(C2H5COO)3]5 (V-a, V-b). Uranyl propionate (UO2(C2H5COO)2(H2O)2; 0.40 mmol, 181 mg) and potassium propionate (C2H5COOK; 0.1 mmol, 11 mg) were separately dissolved each in 2 mL of water. Barium carbonate (BaCO3; 0.2 mmol, 39 mg) was mixed with propionic anhydride ((C2H5CO)2O; 0.4 mmol, 52 mg) and 2 mL of water with moderate heating until full liberation of carbon dioxide (CO2). The three solutions were mixed to form a transparent yellow solution. The final molar ratio of the reagents was 1/2/4 for K+/Ba2+/UO22+, respectively. After 3−4 days of evaporation 7152

DOI: 10.1021/acs.inorgchem.7b00809 Inorg. Chem. 2017, 56, 7151−7160

100(2) 0.32 × 0.24 × 0.18

13.120

100(2)

0.32 × 0.24 × 0.18

μ, mm−1

T, K

cryst size, mm

7153

−0.004(5)

Δρmax/Δρmin, e/Å3

3.015/−0.790

−0.033(18)

1.083

S

abs structure param x

1.049

0.0444

R1 on N2

57

wR2 on N1

1.625/−1.355

0.0346

0.0847

57

0.1183

no. of params refined

52

29575/1107, 0.0595/998

no. of rflns: collected/ unique (N1), Rint/with I > 1.96σ (I) (N2) 43513/1396, 0.0759/1248

4.25−29.95 −25 ≤ h ≤ 25, −25 ≤ k ≤ 25, −25 ≤ l ≤ 25

4.24−27.47

−22 ≤ h ≤ 23, −23 ≤ k ≤ 23, −23 ≤ l ≤ 23

θ range, deg

h, k, l range

6.513

Mo Kα, 0.71073

Mo Kα, 0.71073

radiation type, λ, Å

2.718

5758.5(4)

2.702

5801.72(10)

17.9241(4)

17.9241(4)

Dx, g/cm3

V, Å

17.96880(10)

c, Å

3

17.96880(10)

b, Å

17.9241(4)

cubic, I4̅3d, 4

cubic, I4̅3d, 4

17.96880(10)

cryst syst, space group, Z

a, Å

II Cs2Ba[NpO2(C2H5COO)3]4

I

Cs2Ba[UO2(C2H5COO)3]4

chem formula

1.653/−0.785

−0.033(14)

1.057

0.0339

0.0903

57

19028/851, 0.0429/794

−21 ≤ h ≤ 19, −20 ≤ k ≤ 21, −21 ≤ l ≤ 21

4.25−24.94

0.32 × 0.24 × 0.18

100(2)

6.686

Mo Kα, 0.71073

2.727

5787.9(10)

17.9545(10)

17.9545(10)

17.9545(10)

cubic, I4̅3d, 4

Cs2Ba[PuO2(C2H5COO)3]4

III

Table 1. Details of Data Collection and Structure Refinement Parameters for Crystals I−V IV

V-a

4.443/−3.585

1.029

0.0449

0.0995

481

5.639/−3.396

0.419(4) (refined as an inversion twin)

1.019

0.0368

0.0786

1820

164172/45950, 0.0491/40091

−21 ≤ h ≤ 21, −42 ≤ k ≤ 40, −47 ≤ l ≤ 48

71926/9007, 0.0756/6543

4.11−30.00

−44 ≤ h ≤ 44, −19 ≤ k ≤ 19, −19 ≤ l ≤ 19

0.36 × 0.20 × 0.10

100(2)

11.042

Mo Kα, 0.71073

2.377

16227.6(9)

34.7701(12)

30.0897(10)

15.5107(5)

orthorhombic, P212121, 8

KBa2(H2O)8[UO2(C2H5COO)3]5

4.10−27.50

0.34 × 0.20 × 0.20

100(2)

11.753

Mo Kα, 0.71073

2.369

7864.8(3)

15.1243(4)

15.0670(4)

34.5132(8)

orthorhombic, Pbcn, 4

KSr2(H2O)8[UO2(C2H5COO)3]5

V-b

3.294/−2.112

1.044

0.0469

0.0896

486

42470/7527, 0.0575/4966

−42 ≤ h ≤ 42, −18 ≤ k ≤ 16, −17 ≤ l ≤ 18

4.08−25.00

0.40 × 0.20 × 0.10

293(2)

10.389

Mo Kα, 0.71073

2.237

8624.2(5)

15.5817(4)

15.6332(5)

35.4042(12)

orthorhombic, Pbcn, 4

KBa2(H2O)8[UO2(C2H5COO)3]5

Inorganic Chemistry Article

DOI: 10.1021/acs.inorgchem.7b00809 Inorg. Chem. 2017, 56, 7151−7160

Article

Inorganic Chemistry

confirmed by comparison of simulated and experimental XRD patterns (see the Supporting Information).

yellow platelike crystals with composition KBa2(H2O)8[UO2(C2H5COO)3]5 (V) were formed. Yield: ∼75%. Gravimetric analysis of the uranium content in V gave a value of 40.9% (calculated 41.0%). The X-ray experiment showed the presence of two polymorphic modifications of V. Both polymorphs crystallize in the orthorhombic crystal system. The structure of the V-a polymorph at low temperature (100 K) is fully ordered and has the P212121 space group with Z = 8. The structure of V-b polymorph under ambient conditions (293 K) has the Pbcn space group with Z = 4 and one statistically disordered ethyl group of the propionate ion. The V-b polymorph is isostructural with the crystals IV. X-ray Diffraction Analysis. The structures of crystals I−V were studied using single-crystal X-ray diffraction analysis on a Bruker KAPPA APEX II automatic four-circle diffractometer with area CCD detector. The unit cell parameters were refined over the whole data set.48 Absorption correction was made using the SADABS program.49 The structures were solved by direct methods (SHELXS97)50 and were refined by the full-matrix least-squares method on F2 over the whole data set in the anisotropic approximation for all nonhydrogen atoms (SHELXL-2014).51 A significant decrease in experimental intensities on cooling to 100 K was observed for the crystal V-b. For that reason, the X-ray diffraction experiment for V-b was conducted at room temperature, while for all the remaining compounds the temperature 100 K was used. Hydrogen atoms of propionate ions in I−V were placed in geometrically calculated positions with isotropic temperature factors equal to 1.2 (CH2 groups) or 1.5 (CH3 groups) times the equivalent isotropic temperature factors of parent carbon atoms. The hydrogen atoms of water molecules in IV and V-b were localized in the difference Fourier syntheses and refined with isotropic temperature factors equal to 1.5 times the equivalent isotropic temperature factors of parent oxygen atoms. The O−H and H···H distances for water molecules in IV and V-b were restrained to 0.85(2) and 1.35(2) Å, respectively. The hydrogen atoms of water molecules in V-a were not localized. Details of data collection and structure refinement parameters for crystals I−V are given in Table 1. The main bond lengths and angles in the crystal structures of I−V are provided in the Supporting Information. The atomic coordinates were deposited at the Cambridge Crystallographic Data Centre: CCDC Nos. 1490890, 1490891, 1481247, 1490892, 1490893, 1490894 for I−IV, V-a, and V-b, respectively. In order to confirm phase purity of the bulk sample of crystals Cs2Ba[UO2(C2H5COO)3]4 (I), a powder X-ray diffraction pattern was collected prior to SHG measurements. Powder XRD analysis was performed using a Guinier Camera G670 equipped with curved germanium (111) monochromated Cu Kα1 radiation and imaging plate detector. A comparison of simulated and experimental XRD patterns of crystals I is provided in the Supporting Information. FTIR Spectroscopy. The uranium-containing crystals I, IV, and V-b were studied as pressed KBr pellets in the range of 500−3500 cm−1 using a PerkinElmer Spectrum 100 FTIR spectrometer. Assignment of absorption bands (provided in the Supporting Information) was carried out according to the published materials.53,54 Antisymmetric stretching vibrations of uranyl ions arise at 928 (I), 932 (IV), and 931 (V-b) cm−1. The ranges of bands of antisymmetric and symmetric stretching vibrations of carboxylate groups are indicative of bidentate cyclic type of coordination. Second Harmonic Generation. SHG was measured using the previously described procedure.55,56 Impulse radiation with wavelength λω 1.064 μm was achieved with a Nd:YAG laser. A repetition rate of 12.5 impulses/s and a duration of impulses in the range of 10−12 ns were used. Signals from powder samples were registered in the backward direction. Intensities of signals from samples at the second harmonic frequency (I2ω) were related to the SHG intensity of α-SiO2 powder taken as a standard (I2ω(SiO2)). To eliminate the influence of the grain size and the length of coherent interaction on the intensity of the SHG, all samples, as well as α-SiO2, were powdered to the grade of 3 μm.57 The nonlinear optical activity Q of a sample was calculated as Q = I2ω/I2ω(SiO2). The phase purity of the bulk sample of crystals was

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RESULTS AND DISCUSSION Crystal Structures. Crystals I−III are isostructural and crystallize in a cubic crystal system (space group I4̅3d) with the single crystallographic type of actinide atoms. Crystals IV and V-b are also isostructural and crystallize in an orthorhombic crystal system (space group Pbcn). Crystals IV and V-b contain 3 crystallographic types of U atoms, while V-a has 10. Coordination polyhedra of all actinide atoms in I−V are hexagonal bipyramids of AnO8 composition. Two actinyl oxygen atoms occupy axial positions of coordination polyhedra, while six oxygen atoms of three propionate ions occupy equatorial positions. All propionate ions realize a bidentate cyclic type of coordination with respect to actinide atoms (Figure 1). The

Figure 1. Anionic complex units [AnO2(C2H5COO)3]− (An = U, Np, Pu) in crystal structures of I−V. The An atom is depicted in a coordination polyhedron representation.

actinyl group in crystals I−V is almost linear and symmetric: the OAnO angles are in the range of 178−180°, the AnO distances are in the range of 1.71−1.78 Å, and the An−O distances are in the range of 2.43−2.53 Å (see the Supporting Information). The mean volume of Voronoi−Dirichlet polyhedra of uranium atoms in I, IV, and V is equal to 9.33(7) Å3, which is in good agreement with the known value of 9.3(2) Å3 for U(VI) atoms in UOn polyhedra with n = 5−9.58 Crystals I−III have 3D framework structures (Figure S2 in the Supporting Information). As in other known uranyl propionates,1,2 the actinide atoms in I−III occupy positions with C3 site symmetry. The structures of I−III are constructed of anionic complex units [AnO2(C2H5COO)3]− of the AB013 crystal chemical formula,59 where A = AnO22+ and B01 = C2H5COO−. The mentioned complex units [AnO2(C2H5COO)3]− are bound with mono- and divalent cations due to electrostatic interactions. The atoms of Cs and Ba are statistically disordered over one position with occupancies equal to 2/3 and 1/3, respectively. The coordination numbers of Cs and Ba atoms are equal to 8. The RO8 coordination polyhedra (R = Cs, Ba) have the form of a trigonal dodecahedron. Eight oxygen atoms of RO8 polyhedra belong to eight different propionate ions, which in turn belong to four neighboring [AnO2(C2H5COO)3]− complexes (Figure 2). The coordination numbers of all atoms in I−V, including K, Cs, Sr, and Ba atoms, were unambiguously determined using the method of intersecting spheres.60,61 The coordination numbers of Sr and Ba atoms in crystal structures of IV and V are equal to 8. The alkaline-earth metals in IV and V are bound with oxygen atoms of four bridging propionate ions, belonging to two neighboring [UO2(C2H5COO)3]− complex units, and with oxygen atoms of four water molecules (Figure 3). The coordination polyhedra of Sr and Ba atoms in IV and V have the form of trigonal dodecahedra. Thus, the crystal structures of IV and V are constructed of neutral complex groups of 7154

DOI: 10.1021/acs.inorgchem.7b00809 Inorg. Chem. 2017, 56, 7151−7160

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Inorganic Chemistry

Figure 2. Coordination environment of R atoms (R = Cs, Ba) in crystal structures of I−III. The An atoms are depicted in a coordination polyhedron representation. The H atoms of propionate ions are omitted for clarity.

Figure 4. Octanuclear neutral complex units KR2(H2O)8[UO2(C2H5COO)3]5 (R = Sr, Ba) in crystal structures of IV, V-a, and V-b. The U atoms are depicted in a coordination polyhedron representation. The H atoms of propionate ions are omitted for clarity. The vertical blue line with arrowheads represents the 2-fold rotation axis, which is present in isostructural crystals IV and V-b due to disorder of the upper propionate ion but is absent in the fully ordered crystal structure of V-a.

in the case of modification V-a. Note that the 2-fold rotation axis, which goes through potassium and one of the uranium atoms, is inherent for the metal−oxygen framework of the octanuclear complex unit and for most of the C and H atoms (Figure 4). However, the 2-fold rotation axis is not characteristic of the whole octanuclear complex due to the upper propionate ion in Figure 4, which restricts the 2-fold rotational symmetry. Apparently, in the crystal structures of IV and V-b the ethyl groups of the mentioned propionate ions (include C24 atom) have a sufficient amount of energy and space for rotation about a σ bond with the carboxylate group. This rotation results in statistical disorder of the ethyl group over two positions, which enables the 2-fold rotation symmetry of the octanuclear complex unit in crystal structures of IV and V-b (Figure 4). In the crystal structure of the V-a modification the ethyl group “freezes” in one position, which restricts the 2-fold rotation symmetry of the octanuclear complex unit. Lowering of symmetry of the octanuclear complex unit in V-a affects intermolecular interactions as opposed to intramolecular interactions. The method of molecular Voronoi− Dirichlet polyhedra62,63 was used for evaluation of partial contributions (Δ, %) of different types of noncovalent interactions in crystal structures of IV, V-a, and V-b. Only one out of two equal positions of disordered ethyl groups of propionate ions was considered for evaluation of noncovalent interactions for the crystal structures of IV and V-b. As hydrogen atoms of water molecules in V-a were not localized, the Hsite software (part of TOPOS) was used for the determination of positions of hydrogen atoms.64 Calculations showed that 3 and 4 out of 15 theoretically possible types of intermolecular interactions are realized in the case of isostructural crystals IV and V-b, respectively (Figure 5; see the Supporting Information). Hydrogen bonds H/O (∼48%) and dispersion interactions H/H (∼48%) are predominant in the binding of neutral octanuclear complex units KR2(H2O)8[UO2(C2H5COO)3]5 (R = Sr, Ba). Intermolecular interactions H/C and O/O are less significant (∼4 and 0.2%, respectively). Almost the same partial contributions are typical for the crystal structure of V-a. However, in the crystal structure of V-a two additional types of noncovalent contacts appear: C/O and H/Ba (Figure 5). This fact proves that octanuclear complex units in crystal structures of V-a and V-b realize different kinds of packings,

Figure 3. Coordination environment of R atoms (R = Sr, Ba) in crystal structures of IV and V. The U atoms are depicted in a coordination polyhedron representation. The H atoms of propionate ions are omitted for clarity.

{R(H2O)4[UO2(C2H5COO)3]2} composition (R = Sr, Ba). Similar neutral complexes were earlier observed in the case of acetate and n-butyrate complexes of the uranyl ion.11 Such trinuclear complexes have the A′A2B114B012M14 crystal chemical formula, where A′ = Sr2+, Ba2+, A = UO22+, B11, B01 = C2H5COO−, and M1 = H2O. The potassium atoms in crystals IV and V have coordination numbers equal to 6 and coordination polyhedra in the form of distorted octahedra. Four out of six oxygen atoms in the KO6 polyhedra belong to two trinuclear complexes {R(H2O)4[UO2(C2H5COO)3]2}. The remaining two oxygen atoms belong to a typical anionic complex unit [UO2(C2H5COO)3]− with AB013 crystal chemical formula, where A = UO22+ and B01 = C2H5COO−. Thus, the coordination formula of crystals IV and V may be written as K[UO2(C2H5COO)3]{R(H2O)4[UO2(C2H5COO)3]2}2. Overall, the structures of IV and V are constructed of unprecedented neutral octanuclear complex units KR2(H2O)8[UO2(C2H5COO)3]5, where R = Sr, Ba (Figure 4). Such units are bound into 3D frameworks through hydrogen bonds and other intermolecular interactions (Figure S3 in the Supporting Information). The hydrogen bonds form between H atoms of water molecules of a given octanuclear unit and O atoms of propionate ions of four neighboring octanuclear units. Parameters of the strongest hydrogen bonds in crystal structures of IV and V-b are given in the Supporting Information. The V-a modification of KBa2(H2O)8[UO2(C2H5COO)3]5 belongs to the space group P212121 and does not possess statistically disordered atoms. The potassium atoms, which represent the centers of neutral octanuclear complex units, occupy positions with C1 site symmetry in V-a and with C2 site symmetry in V-b and IV. This fact implies lower symmetry of the octanuclear complex unit KBa2(H2O)8[UO2(C2H5COO)3]5 7155

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However, in the pursuit of finding such a correlation we suggested that the NLO activity of compounds is strongly dependent on their cationic sublattices: i.e., atoms forming their frameworks. In the structures of all carboxylate complexes of uranyl ions under discussion one can identify cationic sublattices of atoms R and U, where R denotes atoms of alkali or alkaline-earth metals. Such sublattices represent the specific nature of the mutual spatial arrangement of cations R+ and complex anions [UO2L3]−. In terms of a stereoatomic model of crystal structures58,60−63 such sublattices can be characterized by the vector DUR‑U, which shows the displacement of uranium atoms from the centers of gravity of their Voronoi−Dirichlet polyhedra in the mentioned sublattices. Our calculations showed that the values of the DUR‑U vector for K[UO2(C3H 7 COO) 3 ], K[UO 2 (C 4 H 9 COO) 3 ], and Rb[UO 2 (C 4 H 9 COO)3] are equal to 0.98, 0.65, and 0.65 Å, while the corresponding value for Cs2Ba[UO2(C2H5COO)3]4 (I) is equal to 0.14 Å. Hence, a higher Q value corresponds to a lower DUR‑U value, and vice versa. From our point of view, this relationship is quite logical, as the value of DUR‑U shows the local electric field gradient at the nuclei of uranium atoms, created by the charges of its own electron shells as well as external electron shells, localized on neighboring R and U nuclei. Further analysis of correlation among composition, structure, and NLO activity of compounds is required and is planned for the near future (the results will be published elsewhere). Actinide Contraction. In addition to the title series of isostructural compounds, to date five more analogous U−Np− Pu series have been reported: [Mg(H 2O) 6 ][AnO 2 (C2 H5COO)3]2,5 AnO2(ClO4)2·5H2O,43 AnO2[B8O11(OH)4],44 Li2AnO2(PDC)2·2H2O,45 and [AnO2(C2H5COO)2(H2O)2].46 As is known, actinide contraction in compounds of actinyls appears through shortening of actinyl AnO distances, while An−O distances in equatorial planes remain constant within the range of a standard deviation of bond length determination.5,32,38,43−47 In crystal structures I−III the mean AnO and An−O distances change nonmonotonically in the U−Np−Pu series and their standard deviations are relatively high for making judgements on actinide contraction in the series (Table 2). Construction of Voronoi−Dirichlet polyhedra introduces two more valuable parameters for investigation of actinide contraction: VVDP and G3. The volume of the Voronoi−Dirichlet polyhedron VVDP of an actinide atom is an integral parameter,

Figure 5. Partial contributions (Δ, %) of intermolecular contacts in crystal structures of IV, V-a, and V-b.

which is, in our opinion, the main reason for the existence of two polymorphic modifications of compound KBa2(H2O)8[UO2(C2H5COO)3]5. Nonlinear Optical Activity. Two parallel SHG measurements for Cs2Ba[UO2(C2H5COO)3]4 (I) resulted in Q = 12.6 and 13.0, which is higher than Q for most of the studied coordination compounds of uranyl ion and is close to the highest observed value of Q = 16.0 for the acrylate complex K[UO2(CH2CHCOO)3].24 However, all of the studied carboxylate complexes of uranyl ions show significantly lower NLO activity in comparison with actual optically active materials.19−21 Probably, the most promising NLO materials to date are borates.20,65−67 Among coordination compounds of uranyl ions, borate complexes as well as some others also show fairly higher ability for SHG (although this ability is not always quantified due to the radioactivity of uranium-containing compounds) in comparison to crystals I with Q values reaching up to 300.68−71 SHG was estimated earlier for K[UO2(C3H7COO)3], K[UO2(C4H9COO)3], and Rb[UO2(C4H9COO)3]. Their characteristic values of Q are equal to 2.0, 2.2, and 0.9, respectively.16 As one can mention, in spite of the similar compositions and structures of these three compounds and the title compound Cs2Ba[UO2(C2H5COO)3]4 (I), the propionate complex performs better than butyrate and valerate complexes. A great deal of effort by many scientists was put into studying the correlation among composition, structure, and NLO activity of compounds; however, it is not that simple, in many cases due to the absence of a representative selection of compounds with studied structures and NLO activities.2,20,22,23,67,70

Table 2. Characteristics of An(VI) Atoms Estimating Actinide Contraction in Six U−Np−Pu Series of Isostructural Crystals mean d(AnO), Å/ mean d(An−O/N), Å compound [AnO2(C2H5COO)2(H2O)2]46 [Mg(H2O)6] [AnO2(C2H5COO)3]25 Cs2Ba[AnO2(C2H5COO)3]4 Li2AnO2(PDC)2·2H2O45

U

Np

1.7652/ 2.4783 1.7628/ 2.4827 1.767/2.451

1.7524/ 2.4735 1.749/2.481

AnO2(ClO4)2·5H2O

1.7769/ 2.5197 1.754/2.415

AnO2[B8O11(OH)4]44

1.750/2.499

43

1.739/2.447 1.7585/ 2.5163 1.7441/ 2.4157 1.742/2.503

σ[d(AnO)], Å/σ[d(An−O/N], Å Pu

1.751/ 2.476 1.740/ 2.474 1.750/ 2.449 1.747/ 2.525 1.732/ 2.409 1.727/ 2.497

U

Np

0.0013/ 0.0012 0.0024/ 0.0016 0.024/0.015

0.0011/ 0.0010 0.004/0.003

0.0017/ 0.0020 0.004/0.004 0.005/0.007

0.016/0.010 0.0013/ 0.0014 0.0009/ 0.0010 0.005/0.009

Pu 0.005/ 0.005 0.003/ 0.003 0.020/ 0.014 0.003/ 0.003 0.002/ 0.002 0.004/ 0.007

VVDP(An), Å3/G3(An)a U

Np

9.42/ 0.0834 9.48/ 0.0837 9.24/ 0.0834 9.74/ 0.0837 9.28/ 0.0840 9.48/ 0.0839

9.31/ 0.0835 9.39/ 0.0838 9.07/ 0.0836 9.62/ 0.0839 9.25/ 0.0841 9.47/ 0.0840

Pu 9.32/ 0.0836 9.29/ 0.0839 9.14/ 0.0836 9.63/ 0.0841 9.14/ 0.0842 9.33/ 0.0842

a

VVDP = the volume of the Voronoi−Dirichlet polyhderon of an An(VI) atom; G3 = dimensionless second moment of inertia of the Voronoi− Dirichlet polyhedron of an An(VI) atom. 7156

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Thus, one can conclude that actinide contraction from U to Pu is followed by a decrease in sphericity degree of Voronoi− Dirichlet polyhedra of actinide atoms due to an increase in the number of 5f electrons. As the actinide contraction from U to Pu is very subtle, the use of other parameters for its investigation, such as d(AnO), d(An−O), and VVDP is complicated by their high standard deviations and requires precise experimental data. Equilibrium between Mono- and Polynuclear Complex Units. The derived data for IV and V prove the point11 that aqueous solutions, containing ions of divalent metals R2+ and tricarboxylate complexes of uranyl ions [UO2L3]−, imply the following dynamic equilibrium between mono- and polynuclear complex units:

which depends on the lengths of all contacts of a central atom with its nearest neighbors in the crystal structure.60,61 The dimensionless second moment of inertia G3 of the Voronoi− Dirichlet polyhedron of an An(VI) atom represents the sphericity degree of the Voronoi−Dirichlet polyhedron and, thus, the environmental uniformity of a certain atom. The lowest theoretically possible value of G3 = 0.07697 characterizes a sphere.64 The values of VVDP and G3 are easily and precisely determined for the exact atom in the exact crystal structure. Thus, the errors of determination of VVDP and G3 depend solely on the errors of determination of atomic coordinates in crystal structures. A single-crystal X-ray diffraction experiment provides an averaged crystal structure with thermal ellipsoids showing potential displacement of atoms. Thus, the calculated values of VVDP and G3 are also averaged for the given crystal structure and are exact. However, it is useful to realize to what extent the displacement of atoms changes the values of VVDP and G3. Our estimations show that displacement of atoms in crystal structures of about 0.001 Å results in alteration of VVDP values by about 0.1 Å3 and G3 values by about 0.0001. These last two values may be interpreted as standard deviations of VVDP and G3 calculations for the compounds under discussion. The values of four parameters (namely mean AnO and An−O distances, VVDP and G3) for the six mentioned U−Np−Pu series of compounds are given in Table 2. Due to the actinide contraction, one would expect the VVDP values to decrease along the U−Np−Pu series. Although this trend is really observed for the series of compounds [Mg(H2O)6][AnO2(C2H5COO)3]2,5 AnO2(ClO4)2·5H2O,43 and AnO2[B8O11(OH)4],44 in the title series of compounds I−III a nonmonotonic change of VVDP takes place (Table 2). The same anomaly was mentioned earlier for the crystal structures of Li2AnO2(PDC)2·2H2O45 and [AnO2(C2H5COO)2(H2O)2].46 However, as seen from Table 2, the standard deviations for d(AnO) and d(An−O) and the standard deviation for VVDP of about 0.1 Å3 are relatively high for making judgements on actinide contraction in these series. The most precise and distinguishing parameter among those four given in Table 2 is the second moment of inertia G3 with a standard deviation of about 0.0001. The trend of increasing G3 values is realized for An(VI) atoms in all six studied series of compounds (Figure 6). The same values of G3

[R(H 2O)n ]2 + + k[UO2 L3]− ↔ {R(H 2O)n − m [UO2 L3]k }2 − k + mH 2O

(1)

where L is a monocarboxylate ion. The composition and the structure of crystals, which form in the process of isothermal evaporation of such solutions, depend on the nature of cations R2+ and carboxylate ligands L−. For example, in acetatecontaining solutions the equilibrium (1) is usually shifted to the left, as crystallized compounds are constructed of only mononuclear complex units [UO2L3]− and [R(H2O)n]2+ in a 2/1 ratio (R = Ba,12 Be,72 Mg,73 Co,73 Ni,74 Zn73). However, in rare cases, for example when R = Sr, the crystallized compounds in addition to mononuclear complex units consist of trinuclear {Sr[UO2L3]2(H2O)4} and pentanuclear {Sr[UO2L3]4}2− complex units corresponding to the equilibrium (1).11 In propionate-containing compounds the mononuclear complex units [UO2L3]− and [R(H2O)n]2+ in a 2/1 ratio form if R = Mg.5 However, if R = Ca2+, Sr2+, Ba2+ the equilibrium (1) in such solutions is shifted to the right. Thus, crystals of Ca[UO2(C2H5COO)3]2·6H2O3 are constructed of binuclear {Ca(H2O)5[UO2(C2H5COO)3]}+ and mononuclear [UO2(C2H5COO)3]− complex units and Sr- and Ba-containing crystals IV and V are constructed of trinuclear complex units {R(H2O)4[UO2(C2H5COO)3]2}2 (R = Sr, Ba). In the case of butyrate-containing compounds the equilibrium (1) may be shifted to the left (if R = Mg, crystals of Mg(H2O)6[UO2(C3H7COO)3]2 form)75 and to the right (if R = Sr, crystals of {Sr(H2O)4[UO2(C3H7COO)3]2}·2H2O form).11 From the series of Ba-containing crystals Ba[UO2(CH3COO)3]2, (Cs0.5Ba0.25)[UO2(CH3COO)3],12 and K[UO2(C2H5COO)3]{Ba(H2O)4[UO2(C2H5COO)3]2}2 (V) with acetate and propionate ions one can judge that, if R = Ba, the shift of the equilibrium (1) to the right is promoted by the increased hydrophobic nature of carboxylate ions L with the increased number of carbon atoms in their chains. It is worth noting that other types of rearrangements of groups of atoms in addition to transformations corresponding to the equilibrium (1) are possible during crystallization of the compounds under discussion. For example, association of polynuclear and mononuclear complex units through potassium atoms resulted in the formation of unprecedented octanuclear complex units in crystal structures of IV and V. It could be that the formation of neutral molecular complex units similar to that in IV, V or {Sr(H2O)4[UO2L3]2}·2H2O11 with anions of fulvic acids promote migration and diffusion of uranium in the biosphere.

Figure 6. Variation of G3 values for actinide atoms in the six U−Np− Pu series of isostructural crystals. The width of the line is in the order of standard deviation of G3 calculations.

for Np and Pu in the crystals Cs2Ba[AnO2(C2H5COO)3]4 are, probably, a result of a poorer crystal structure determination: the values of standard deviations of bond length determination are significantly higher for Cs2Ba[AnO2(C2H5COO)3]4 with respect to the remaining five series of compounds in Table 2. 7157

DOI: 10.1021/acs.inorgchem.7b00809 Inorg. Chem. 2017, 56, 7151−7160

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CONCLUSION

Isostructural cubic crystals Cs2Ba[AnO2(C2H5COO)3]4, where An = U (I), Np (II), Pu (III), are constructed of typical mononuclear anionic complex units [AnO2(C2H5COO)3]− and charge-balancing Cs and Ba cations. The actinide contraction in the six U−Np−Pu series of isostructural crystals studied to date is very subtle, which hampers its estimation due to high standard deviations of bond lengths and atomic volumes and requires precise experimental data. However, the second moment of inertia G3 seems to be the most precise and distinguishing parameter of actinides. The general trend an increase in the values of G3 in the U−Np−Pu series represents a decrease in environmental uniformity of actinide atoms due to an increase in the number of 5f electrons. Crystal structures of IV and V are constructed of unprecedented octanuclear neutral complex units K[UO2(C2H5COO)3]{R(H2O)4[UO2(C2H5COO)3]2}2. Analysis of noncovalent interactions by means of Voronoi−Dirichlet tessellation in crystal structures of the two polymorphs of V reveals different kinds of packings of octanuclear complex units. An equilibrium between typical mononuclear and rare polynuclear units in aqueous solutions of carboxylate complexes of the uranyl ion is proposed. Noncentrosymmetric crystals Cs2Ba[UO2(C2H5COO)3]4 show high nonlinear optical activity with respect to other analogous compounds but low activity with respect to the advanced optically active materials.

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ACKNOWLEDGMENTS

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REFERENCES

The reported study was funded by the RFBR according to the research project No. 16-03-00200 a. X-ray diffraction experiments were performed at the Center for Shared Use of Physical Methods of Investigation at the Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00809. Main bond lengths and angles in crystal structures of I−V, assignment of absorption bands in FTIR spectra of crystals I, IV, and V-b, parameters of hydrogen bonds in crystal structures of IV and V-b, characteristics of intermolecular interactions in crystal structures of IV, V-b, and V-a, simulated and experimental XRD patterns of I, representation of mutual arrangement of atoms in crystal structures of I−III, and packing of octanuclear neutral complex units in crystal structures of IV, V-a, and V-b (PDF) Accession Codes

CCDC 1481247 and 1490890−1490894 contain 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.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.V.S.: [email protected]. ORCID

Viktor N. Serezhkin: 0000-0001-7080-4563 Anton V. Savchenkov: 0000-0002-6048-3011 Notes

The authors declare no competing financial interest. 7158

DOI: 10.1021/acs.inorgchem.7b00809 Inorg. Chem. 2017, 56, 7151−7160

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