Syntheses, Structures, and Comparisons of Heterometallic Uranyl

Article pubs.acs.org/IC

Syntheses, Structures, and Comparisons of Heterometallic Uranyl Iodobenzoates with Monovalent Cations Mark Kalaj,§ Korey P. Carter,§ Anton V. Savchenkov,‡ Mikaela M. Pyrch,§ and Christopher L. Cahill*,§ §

Department of Chemistry, The George Washington University, 800 22nd Street, NW, Washington, D.C. 20052, United States Samara National Research University, Samara, 443086, Russian Federation

‡

S Supporting Information *

ABSTRACT: The syntheses and crystal structures of six new heterometallic compounds containing the UO22+ cation, o-, m-, and p-iodobenzoic acid ligands, and Tl+, Rb+, and Cs+ cations which adopt the role of both charge balancing cation and secondary metal center are described, as are the luminescent properties for Tl+ containing compounds 1, 4, and 6. The structures of compounds 1−3 are isomorphous and contain uranyl monomers bound by o-iodobenzoic acid ligands with Tl+, Rb+, and Cs+ cations acting as secondary metal centers. Compounds 4 and 5 are also isomorphous and feature m-iodobenzoic acid ligands bound to the uranyl cation along with Tl+ and Rb+ cations. Compound 6 is unique in this series as it is assembled from a dimeric uranyl unit and features p-iodobenzoic acid ligands and Tl+ cations which function as charge balancing secondary metal centers. Single crystal X-ray diffraction analysis of these materials suggests that the secondary metal cations are incorporated based on the size of their ionic radius (Tl+ < Rb+ < Cs+), which is directly related to the size of the “pocket” observed in 1−6. Further, Voronoi−Dirichlet tessellation and Hirshfeld surface analysis were used to probe the coordination environment of the secondary metal centers as part of ongoing efforts to develop metrics for determining the coordination number of secondary metal cations in similar systems.

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considerations.32−36 Thallium(I), referred to as a “pseudo” alkali metal due to its preferred monovalent oxidation state and similar ionic radii to K+ and Rb+,37 also features a stereoactive lone pair in the +1 oxidation state, thus potentially leading to unique influences on the coordination environment and the properties of a structure.32,33 Rb+, Cs+, and Tl+ were therefore selected as secondary metal centers due to their oxidation state in order to charge balance the anionic uranyl-iodobenzoate units, as well as to probe the effects of the cation size on resulting structural topologies and physical properties. Herein we set out to explore a system of heterometallic uranyl compounds featuring both o-, m-, and p-iodobenzoic acid ligands and monovalent cations of varying size (Tl+, Rb+, and Cs+). Changes in the iodine position on the benzoic acid linkers and in the size of the secondary metal center yield three distinct heterometallic units, and we thus report the syntheses, crystal structures, and modes of supramolecular assembly for a family of six new uranyl hybrids featuring o-, m-, and p-iodobenzoic acid ligands and alkali/pseudoalkali secondary metal centers. Heterometallic uranyl hybrid materials of this kind introduce the question of whether the secondary metal is responding to a space filling opportunity or truly driving the formation of a structural motif. Whereas this latter process is challenging to confirm definitively and is not the topic of this manuscript, probing coordination environments of such species is relevant, as there is a lack of consensus as to what constitutes a “bond” to

INTRODUCTION Heterometallic chemistry within hybrid materials incorporating hexavalent uranium is an area of continued interest as it presents new opportunities for unique structure types and unexpected properties.1−5 The literature is rich with examples of heterometallic uranyl hybrid materials utilizing heterofunctional, multitopic ligands as such organic moieties feature multiple sites for potential metal−ligand coordination.6−16 However, efforts to promote heterometallic structures with homofunctional ligands that contain a single type of metal coordination site (i.e., a single carboxylic acid) remain underexplored, and some studies have shown that a coligand may be necessary to generate a heterometallic material.17−19 Halogenated benzoic acid ligands have a metal binding site (carboxylic acid) and a terminal site (halogen) and are thus not typically used to promote extended structures, yet recent studies from our group have shown that the “terminal” end of the ligand can be used as supramolecular synthon site for assembly via noncovalent interactions in both lanthanide and actinide systems.20−27 Herein, we expand our work with halogenated benzoic acids and show that the “terminal” halogens are not only supramolecular synthon sites but also potential covalent bonding participants in heterometallic systems. Whereas the inclusion of d-block metals in uranyl heterometallic compounds is often part of an effort to yield new topologies or alter spectroscopic properties,7,28−31 the inclusion of alkali metals (Rb+ and Cs+) or a “pseudo” alkali metal (such as Tl+) is generally a result of charge balancing © 2017 American Chemical Society

Received: May 12, 2017 Published: July 25, 2017 9156

DOI: 10.1021/acs.inorgchem.7b01208 Inorg. Chem. 2017, 56, 9156−9168

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Inorganic Chemistry Table 1. Crystallographic Data for Compounds 1−6 1

2

3

chem formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z T (K) λ (Mo Kα) Dcalc (g cm−3) μ (mm−1) Rint R1 [I > 2σ(I)] wR2 [I > 2σ(I)]

C42H24I6O16Tl2U2 2430.81 triclinic P1̅ 13.4179(6) 13.7605(6) 14.4961(7) 96.926(4) 93.485(3) 97.732(6) 2624.7(2) 2 293(2) 0.71073 3.076 15.864 0.0413 0.0485 0.1176 4

C42H24I6O16Rb2U2 2193.01 triclinic P1̅ 13.475(7) 13.755(7) 14.500(9) 96.861(3) 93.119(2) 97.772(3) 2637.0(3) 2 293(2) 0.71073 2.762 11.543 0.0276 0.0437 0.1001 5

C42H24I6O16Cs2U2 2287.89 triclinic P1̅ 13.5084(5) 13.7022(5) 14.8145(6) 96.630(3) 92.921(2) 97.930(4) 2691.46(18) 2 293(2) 0.71073 2.823 10.846 0.0412 0.0370 0.0775 6

chem formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z T (K) λ (Mo Kα) Dcalc (g cm−3) μ (mm−1) Rint R1 [I > 2σ(I)] wR2 [I > 2σ(I)]

C21H12I3O8TlU 1215.41 triclinic P1̅ 7.869(5) 10.920(5) 15.786(6) 73.202(4) 84.567(5) 78.809(5) 1272.8(11) 2 293(2) 0.71073 3.171 16.357 0.0382 0.0342 0.0606

C21H12I3O8RbU 1096.51 triclinic P1̅ 7.9088(4) 11.0930(5) 15.6478(8) 72.989(11) 84.105(12) 79.058(11) 1287.26(14) 2 293(2) 0.71073 2.829 11.823 0.0372 0.0301 0.0555

C42H24I6O16Tl2U2 2430.81 triclinic P1̅ 8.5626(7) 16.5503(14) 19.5515(16) 74.357(10) 83.527(11) 89.266(9) 2650.6(4) 2 293(2) 0.71073 3.046 15.709 0.0369 0.0460 0.0976

(Sigma-Aldrich, 99.7%), and cesium nitrate (Sigma-Aldrich, 99.99%), were purchased and used as received. Synthesis. M2[(UO2)(o-IBA)3]2 where M = Tl+ (1), Rb+ (2), Cs+ (3). Compound 1, Tl2[(UO2)(o-IBA)3]2, was synthesized by combining UO2(CH3COO)2·2H2O (0.053 g, 0.12 mmol), o-iodobenzoic acid (0.063 g, 0.25 mmol), TlNO3 (0.066 g, 0.25 mmol), sodium hydroxide (12.5 μL, 5 M), and distilled water (1.5 g, 83.3 mmol) in a Parr autoclave (pH 4.1) and then heating statically at 150 °C for 48 h. Upon removal from the oven, the sample was allowed to cool over 4 h, and the Parr autoclave was opened after approximately 16 hours. Orange rectangular plate crystals were obtained from the bulk product after decanting the supernatant liquor, washing three times with distilled water and ethanol, and air-drying at room temperature overnight. Compounds 2 and 3, M2[(UO2)(o-IBA)3]2 where M = Rb+ (2), Cs+ (3), were prepared following the same procedure as 1 with RbNO3 (0.036 g, 0.24 mmol) and CsNO3 (0.048 g, 0.25 mmol) replacing the TlNO3 with reaction pH values of 4.0 and 3.8, respectively. X-ray quality yellow plate crystals were obtained from the bulk product for both materials. M[(UO2)(m-IBA)3] where M = Tl+ (4), Rb+ (5). Compound 4, Tl[(UO2)(m-IBA)3], was synthesized by combining UO2(CH3COO)2·2H2O (0.053 g, 0.12 mmol), m-iodobenzoic acid

secondary metal cations within uranyl hybrid materials.32,33,35,38−40 As such, we explored bonding and interaction criteria in 1−6 via two independent approaches: Voronoi− Dirichlet (VD) tesselation41,42 and Hirshfeld surface (HS) analysis.43,44 Finally, single crystal luminescence spectra of compounds 1, 4, and 6 were collected as these results allow for a deeper discussion of uranyl structure−property relationships, more specifically, evaluating the effect of heavy atoms on uranyl luminescence, which is explored herein through the three uranyl-thallium heterometallic materials.

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EXPERIMENTAL SECTION

Materials and Methods. Caution: Whereas the uranium oxyacetate dihydrate [UO2(CH3COO)2]·2H2O and uranyl nitrate hexahydrate [UO2(NO3)2]·6H2O used in this study contained depleted U, standard precautions for handling radioactive and toxic substances should be followed. All organic materials, o-iodobenzoic acid (o-IBA) (Sigma-Aldrich, 98%), m-iodobenzoic acid (m-IBA) (Alfa Aesar, 98+%), and piodobenzoic acid (p-IBA) (Sigma-Aldrich, 98%), and all inorganic materials, thallium nitrate (Acros Organics, 99.5%), rubidium nitrate 9157


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Inorganic Chemistry (0.063 g, 0.25 mmol), Tl(NO3) (0.066 g, 0.25 mmol), sodium hydroxide (12.5 μL, 5M), and distilled water (1.5 g, 83.3 mmol) in a Parr autoclave (pH 3.9) and then heating statically at 150 °C for 48 h. Upon removal from the oven, the sample was allowed to cool over 4 h, and the Parr autoclave was opened after approximately 16 hours. Orange rod crystals were obtained from the bulk product after decanting the supernatant liquor, washing three times with distilled water and ethanol, and air-drying at room temperature overnight. Compound 5, Rb[(UO2)(m-IBA)3], was prepared following the same procedure as 4 with RbNO3 (0.036 g, 0.24 mmol) replacing the TlNO3 and the reaction pH was 4.2. X-ray quality yellow rod crystals were obtained from the bulk sample. [Tl(UO2)(p-IBA)3]2 (6). Compound 6, [Tl(UO2)(p-IBA)3]2, was synthesized by combining UO2(CH3COO)2·2H2O (0.053 g, 0.12 mmol), p-iodobenzoic acid (0.063 g, 0.25 mmol), TlNO3 (0.066 g, 0.25 mmol), sodium hydroxide (12.5 μL, 5 M), and distilled water (1.5 g, 83.3 mmol) in a Parr autoclave (pH 4.3) and then heating statically at 150 °C for 48 h. Upon removal from the oven, the sample was allowed to cool over 4 h, and the Parr autoclave was opened after approximately 16 hours. Orange rhomboidal plate crystals were obtained from the bulk product after decanting the supernatant liquor, washing three times with distilled water and ethanol, and air-drying at room temperature overnight. Characterization. X-ray Structure Determination. Single crystals from each bulk sample were isolated and mounted on MiTeGen micromounts. Structure determination for each of the single crystals was achieved by collecting reflections using 0.5° ω scans on a Bruker SMART diffractometer equipped with an APEX II CCD detector using MoKα (λ = 0.71073 Å) radiation at 293(2) K. Data for each compound were integrated using the SAINT software package45 contained within the APEX II software suite46 and absorption corrections were applied using SADABS.47 The crystals selected from the bulk products of compound 4 and 6 were two component nonmerohedral twins that were addressed using TWINABS.48 Compounds 1−5 were solved via direct methods using SIR 92,49 whereas compound 6 was solved via the Patterson Method (SHELXS2014).50 All six compounds were refined using SHELXL-201450 in the WinGX51 software suite and in each structure, all non-hydrogen atoms were located via difference Fourier maps and refined anisotropically. Aromatic hydrogen atoms were placed at their idealized positions by employing the HFIX43 instruction and allowed to ride on the coordinates of their parent carbon atom ((Uiso) fixed at 1.2Ueq). Whole molecule disorder of one of the o-iodobenzoic acid ligands in 1−3 was modeled via AFIX, EADP, and ISOR commands, whereas splitting of iodine atoms on the disordered o-iodobenzoic acid moieties was accounted for using PART and EADP commands. Whereas Na+ (from NaOH in syntheses) was not anticipated to incorporate into 1− 6 considering its much smaller ionic radius (1.02 Å for CN = 6),37 we did freely refine site occupancy factors (SOFs) for the Tl+, Rb+, and Cs+ positions as confirmation, and these values converged to near unity for each compound. Data collection and refinement details for compounds 1−6 are included in Table 1. Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) data on the bulk reaction product of compounds 1−6 were used to examine the bulk purity of each sample. All data were collected on a Rigaku Miniflex (Cu Kα, 2θ = 3−60°) and were analyzed using the JADE software program.52 The bulk products of compounds 1−6 were found to contain multiple solid-state phases including excess o-, m-, and p-IBA. Attempts to remove these additional phases from 1−6 were made by washing and grinding in a range of organic solvents, and by manipulating reaction conditions (temperature, reaction time, molar ratios), yet these were unsuccessful. The syntheses reported above are therefore representative and have consistently yielded single crystals of the title compounds. Further characterization of compounds 1, 4, and 6 (luminescence) was undertaken, yet required selection of individual single crystals. Spectroscopic Characterization. Low temperature (77 K) solidstate luminescence measurements were obtained for 1, 4, and 6 on a Horiba JobinYvon Fluorolog-3 spectrophotometer. Data were manipulated using the FluoroEssence software package, and the final

plots of the resulting solid-state spectra were made in Microsoft Excel. Samples were prepared by placing 8−10 single crystals (by visual inspection) into a quartz NMR tube and immersing the NMR tubes in liquid N2. Experimental parameters (slit width, integration time, etc.) were adjusted on a sample-by-sample basis to obtain suitable data with a sufficiently high signal-to-noise ratio.

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RESULTS Description of Structures. Single crystal X-ray crystallography analyses reveal three unique coordination environments in this family of heterometallic hybrid materials. Compounds 1−6 each contain uranyl cations bound by o-, m-, or piodobenzoic acid ligands, as well as a thallium, rubidium, or cesium charge balancing cation. Local structures are described in detail for compounds 1, 4, and 6 as they represent each of the unique coordination environments. M2[(UO2)(o-IBA)3]2 where M = Tl+, Rb+, Cs+ (1−3). Single crystal X-ray diffraction analyses reveals that compounds 1−3 are isomorphous and crystallize in the space group P1̅; thus only the local structure and supramolecular modes of assembly for 1 are described here. Compound 1 features two crystallographically unique, anionic uranyl monomers where each uranium atom has adopted a hexagonal bipyramidal coordination geometry (Figure 1). Each uranium atom is

Figure 1. Polyhedral representation of compound 1 highlighting local coordination environments of UO22+ and Tl+ cations. Yellow polyhedra are U(VI) centers, whereas red spheres are oxygen atoms, purple spheres are iodine atoms, and gray spheres are thallium(I) atoms. Black lines represent carbon−carbon bonds. All H atoms have been omitted for clarity.

coordinated to eight oxygen atoms, two axial uranyl atoms (O1−O2, O9−O10), and six equatorial oxygen atoms (O3− O8, O11−O16) from three bidentate o-IBA ligands at average distances of 1.753 Å (axial) and 2.473 Å (equatorial). The two anionic uranyl monomers are charge balanced by two crystallographically unique Tl+ cations (Tl1 and Tl2), and after a first look at the coordination environments one may describe Tl1 as having a coordination number of six with Tl−O (O3, O7, O12, O13, O14, O16) distances ranging from 2.9724(1) Å to 3.2660(2) Å, with an average of 3.051 Å. Similarly, Tl2 is observed to adopt a coordination number of 9158


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Figure 2. (Top) Polyhedral representation of compound 2 highlighting Rb+ coordination environments. Magenta spheres represent rubidium(I) atoms. (Bottom) Polyhedral representation of compound 3 illustrating Cs+ coordination environments. Blue spheres represent cesium(I) atoms.

five with Tl−O (O4, O5, O10, O11, O15) distances ranging from 2.9271(1) Å to 3.1441(2) Å, with an average distance of 3.014 Å. These values are all well within the sum of the van der Waals radii for thallium(I) and oxygen (3.48 Å), and a search of the Cambridge Structural Database (CSD, v 5.38, November 2016),53 for all contacts between thallium(I) and oxygen atoms (with an upper bound of 3.48 Å) yielded 409 results ranging from 2.479 to 3.475 Å, with an average of 3.013 Å, thereby (initially) confirming our CN assignments for Tl1 and Tl2. A similar search of the CSD on Tl(I)−iodine contacts yielded zero results, and we thus refrain from including Tl(I)−I contacts (Tl2−I1 (3.6236(2) Å), Tl2−I2 (3.6926(2) Å), Tl1− I5 (3.6787(2) Å), and Tl2−I6 (3.8043(2) Å)) in our preliminary coordination sphere assignments absent more rigorous characterization (below). Looking at the global structure of compound 1 (Figure S1, Supporting Information), uranyl monomers are linked to form a one-dimensional (1D) chain propagating in the [001] direction via Tl−O and Tl−I

bonds. Halogen bonding interactions between a carboxylic acid oxygen atom (O16) and the adjacent iodine atom (I3) of an oiodobenzoic acid ligand supplement the formation of the 1D chains of 1, and the corresponding I−O interaction distance and angle are 3.426(8) Å (98.4% sum of the van der Waals radii) and ∠C−I---O 147.3(3)° (Figure S1, Supporting Information). The uranyl coordination geometry in compound 2, Rb2[(UO2)(o-IBA)3]2, is nearly identical to that in 1, yet we do observe differences in the local environments of the two Rb+ (Rb1 and Rb2) cations (Figure 2). Looking at the range of possible contacts at the Rb+ metal centers, one may initially describe Rb1 as having CN = 5 with five Rb−O (O3, O7, O9, O11, O15) distances ranging from 2.9231(18) Å to 3.150(2) Å, with an average of 2.986 Å. Similarly, Rb2 can be described as having a CN = 6 with six Rb−O (O5, O8, O12, O13, O14, O16) distances ranging from 2.9536(18) Å to 3.189(2) Å, with an average of 3.041 Å. These CN assignments were initially 9159


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Figure 3. (Top left) Polyhedral representation of local geometry of 4 highlighting uranyl coordination environment. (Top right) Local geometry of 4 detailing thallium(I) coordination sphere. (Bottom) Compound 4 viewed in the (010) plane highlighting the halogen bonding interactions that assemble monomers into a 2D sheet.

This search query yielded 6723 results ranging from 2.638 to 3.519 Å, with an average contact distance of 3.202 Å. A similar search on Cs−I contacts yielded 41 hits with contacts ranging from 3.672 to 3.980 Å with an average contact distance of 3.894 Å. M[(UO2)(m-IBA)3] where M = Tl+, Rb+ (4 and 5). Moving from o-IBA to m-IBA yields compounds 4 and 5, which are isomorphous and crystallize in the space group P1̅, and thus only the local structure of 4 will be described. Compound 4 features a single crystallographically unique, anionic uranyl monomer where the uranium atom has once again adopted hexagonal bipyramidal coordination geometry. The uranium atom in 4 is coordinated to two axial uranyl oxygen atoms (O1 and O2) at distances of 1.742(5) Å and 1.779(5) Å, respectively, and six equatorial oxygen atoms (O3−O8) from chelating m-IBA ligands at an average distance of 2.467 Å. (Figure 3). This uranyl monomer is charge balanced by one Tl+ cation (Tl1) and using the same search parameters as detailed for compound 1, one may conclude that the Tl+ cation likely adopts a coordination number of five as it displays contacts to five oxygen atoms (O1, O4, O5, O6, O8) ranging from 2.662(5) Å to 3.174(5) Å, with an average distance of 2.919 Å. Compound 4 also displays a Tl1−I2 contact at 3.577(2) Å, and similar to 1, this was not included in the coordination sphere assignment of 4 as a CSD information on Tl(I)−I contacts

verified after a CSD search (similar to that for 1) of Rb−O contacts with the sum of the van der Waals radii of rubidium and oxygen (4.55 Å) set as an upper limit. This search yielded 3825 hits ranging from 2.556 to 3.520 Å, with an average contact distance of 3.036 Å. A similar search of the CSD on Rb−I contacts yielded 23 results ranging from 3.568 to 3.959 Å, with an average of 3.562 Å. Rb−I contacts Rb1−I3 (3.735(2) Å) and Rb1−I4 (3.931(2) Å) are therefore also likely in the coordination sphere of Rb1, suggestive of an updated CN of 7. Compound 3, Cs2[(UO2)(o-IBA)3]2, also features uranyl coordination environments similar to those in 1 and 2, yet we once again note differences in the likely coordination environments of the monovalent Cs+ cations (Cs1 and Cs2) as compared to the Tl+ and Rb+ cations of 1 and 2 (Figure 2). One may describe Cs1 as having CN = 8 with five Cs−O contacts ranging from 3.0730(1) Å to 3.2804(1) Å (O2, O5, O7, O11, O15) with an average distance of 3.132 Å and three Cs−I (Cs1−I2, Cs1−I4, Cs1−I6) contacts at distances of 3.8683(2) Å, 3.8698(2) Å, and 3.8426(2) Å, respectively, whereas Cs2 displays contacts to seven oxygen atoms (O1, O3, O4, O6, O8, O13, O16) ranging from 3.0350(1) Å to 3.4339(1) Å with an average contact distance of 3.203 Å. These numbers are well within the range of Cs−O contacts found via CSD search (again with the sum of the van der Waals radii of cesium and oxygen (4.95 Å) was set as an upper limit). 9160


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Figure 4. (Left) Polyhedral representation of local coordination geometry of 6 detailing uranyl coordination environments. (Right) Local geometry of 6 highlighting contacts with thallium(I) cations. Selected noninteracting iodine atoms have been omitted for clarity.

Figure 5. Compound 6 viewed in the (100) plane highlighting the three unique halogen−halogen interactions that assemble unique 2D sheets of 6 into a 3D network.

distance of 2.385 Å. Charge balancing these anionic dimers are two unique Tl+ cations (Tl1 and Tl2), and a CSD search similar to that in compounds 1 and 4 suggests each may adopt a coordination number of six. Tl1 features contacts with six oxygen atoms (O1, O6, O7, O10, O12, O15) ranging from 2.877(5) Å to 3.399(5) Å with an average distance of 2.996 Å, and Tl2 displays contacts with six oxygen atoms (O1, O3, O5, O10, O14, O16) ranging from 2.856(5) Å to 3.373(5) Å with an average distance of 3.005 Å. Both Tl+ cations appear to act as (structural) secondary metal centers by (likely) linking the uranyl dimers into a 2D sheet (Figure 4). The 2D sheets of compound 6 are assembled to form a supramolecular three-dimensional (3D) network via three unique halogen−halogen interactions originating from p-IBA ligands on neighboring units (Figure 5). The corresponding I5−I6, I2−I3, and I1−I1 interaction distances are 3.9592(10) Å (99.8% sum of the van der Waals radii), 3.8661(10) Å (97.6% sum of the van der Waals radii), and 3.7848(10) Å (95.6% sum of the van der Waals radii), respectively. Halogen−halogen interactions generally adopt one of two geometries, Type I or Type II, based on the criteria delineated by Desiraju and colleagues.54 In 6 we observe one Type I interaction (I1−I1) with corresponding angles (θ1 and θ2) of 136.5(3)° (∠C5− I1---I1) and two Type II interactions (I5−I6 and I2−I3) with corresponding angles of 164.9(2)° (∠C33−I5I6) and 127.1(2)° (∠C40−I6I5) for the I5−I6 interaction, and

(somewhat surprisingly) yielded no hits. Compound 5 includes a Rb+ cation (Rb1) in place of the Tl+ cation in 4 and a similar search of the CSD as detailed for compound 2 suggests the Rb+ cation may adopt a coordination number of six. Similar to 4, the Rb+ cation in 5 displays contacts to five oxygen atoms (O1, O4, O5, O6, O8) ranging from 2.784(3) Å to 3.068(4) Å with an average distance of 2.908 Å, yet in 5 we also note a Rb1−I2 contact at a distance of 3.691(3) Å. The monomers of 4 and 5 are linked via the secondary Tl+ and Rb+ (respectively) cations into 1D chains in the [100] direction and then further are assembled to form a supramolecular two-dimensional (2D) sheet in the (010) plane via weak halogen bonding interactions between a carboxylic acid oxygen atom (O7) and an iodine atom (I3) on adjacent chains (Figure 3). The corresponding I− O interaction distance and angle are 3.499(5)Å (99.9% sum of the van der Waals radii) and ∠C−I---O 158.9(2)°. [Tl(UO2)(p-IBA)3]2 (6). Switching from m-IBA to p-IBA yields compound 6, which crystallizes in the space group P1̅. Compound 6 features two crystallographically unique uranyl dimers (U1, U1′, U2, U2′) where each uranium atom has adopted pentagonal bipyramidal coordination geometry (Figure 4). Both unique uranium metal cations are coordinated to seven oxygen atoms, two axial uranyl atoms (O1, O2, O9, and O10) at an average distance of 1.769 Å as well as five equatorial oxygen atoms (O3−O8, O11−O15) from three pIBA ligands each adopting a different coordination mode (bridging bidentate, bidentate, and monodentate) at an average 9161


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Inorganic Chemistry Chart 1. Summary of Observed Local Coordination Environments for Compounds 1−6a

a

Figures in the box are secondary metal free and those of UO2-p-iodobenzoic acid dimer, [UO2(p-IBA)2(H2O)]2, are re-drawn from ref 20.

Tl+ and Rb+ cations with m-iodobenzoic acid, yet is not large enough to incorporate the larger Cs+ cation, and with piodobenzoic acid only accommodate the smaller Tl+ cation. A closer look at the ionic radii of the three secondary metal cations suggests that a size related trend is plausible as a Cs+ cation with a coordination number of six displays an ionic radius of 1.67 Å, whereas the ionic radii of the Rb+ and Tl+ cations at the same coordination number are 1.52 and 1.50 Å, respectively.37 Voronoi−Dirichlet (VD) Tesselation and Hirshfeld Surface (HS) Analysis. As part of efforts to improve our understanding of the role/function of secondary metal centers in compounds 1−6, we investigated their interactions with neighboring atoms. The secondary metal centers Tl+, Rb+, and Cs+ have low ionization energies and are known to form largely ionic bonds. Additionally, these metal centers feature spherical coordination environments and experience minimal crystal field effects; thus they are expected to form series of contacts with neighboring atoms, thereby making it challenging to delineate whether interactions should or should not be identified as chemical bonds. Herein we explored this question using two independent approaches to determine coordination numbers (CN) of metal atoms: VD tessellation41,42 with the method of intersecting spheres (MIS)56 and HS analysis.43,44 Whereas application of VD analysis of uranyl materials is known, we have included a brief, didactic introduction to this methodology for context.42,57,58 The VD polyhedron of an atom A surrounded by atoms Y is a convex polyhedron of minimum volume, containing this atom, and bound by perpendicular planes that pass through middle points of segments A−Y that connect atom A with all other atoms Y. Each face of a VD polyhedron belongs to two atoms (central

165.5(2)° (∠C12−I2I3) and 121.3(2)° (∠C19−I3I2) for the I2−I3 interaction. Structural Discussion. As compounds 1−6 were synthesized from similar reaction conditions, the resulting structure types and coordination environments provide an opportunity to assess the influence of the secondary metal centers (Tl+, Rb+, Cs+) on the resulting structural topologies (Chart 1). Compounds 1−6 feature uranyl cations coordinated by iodofunctionalized benzoic acid ligands (o-, m-, and p-IBA) to form anions charge balanced by monovalent secondary metal cations (Tl+, Rb+, Cs+). In compounds 1−3 (with o-IBA) we observe two unique uranyl monomers that adopt hexagonal bipyramidal coordination geometries and are structurally isomorphous with the exception of the secondary metal center. Moving to the mIBA ligand yields compounds 4 and 5 where the uranyl cation has once again adopted hexagonal bipyramidal coordination geometries. These compounds are charge balanced by Tl+ and Rb+ cations, respectively, and under analogous reaction conditions with the Cs+ cation, the product is [UO2(mIBA)]n, a coordination polymer that fails to incorporate the larger Cs+ secondary metal center (Chart 1).55 With the p-IBA ligand we only observe compound 6, which is made up of two unique bidentate-bridged uranyl dimers that are subsequently charge balanced, and further connected, by four Tl+ cations (two crystallographically unique) that adopt coordination numbers of six. Similar synthetic efforts with Rb+ and Cs+ cations yielded a previously characterized p-iodobenzoic acid bridged dimer that failed to incorporate either of the two secondary metals centers.20 These results with the m-iodo- and p-iodobenzoic acid ligands suggest that within this family of hybrid materials there may be a “pocket” for incorporating the secondary metal center that is able to accommodate the smaller 9162


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Figure 6. HS and VD representations of Tl1 (top), Rb1 (middle), and Cs1 (bottom) atoms in compounds 1−3 with o-iodobenzoic acid ligands. Flat regions/faces corresponding to contacts with hydrogen atoms are marked with asterisks (*). Regions/faces corresponding to metal−metal interactions are marked with pound signs (#).

Figure 7. HS and VD representations of Tl1 (top) and Rb1 (bottom) atoms in compounds 4 and 5 with m-iodobenzoic acid ligands. Flat regions/faces corresponding to contacts with hydrogen atoms are marked with asterisks. Regions/faces corresponding to metal−metal interactions are marked with pound signs.

Figure 8. HS and VD representations of Tl1 atoms in compound 6 with p-iodobenzoic acid ligands. Flat region/face corresponding to contacts with hydrogen atoms are marked with asterisks.

one iodine atom at 3.68 Å. The results of VD analysis are preferred over the results of HS analysis as the former accounts 9163


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both the VD polyhedron and HS feature a small face/region, corresponding to Tl---Tl contacts at 4.45 Å (marked with pound signs on Figure 7), indicative of a weak metal−metal interaction. The Rb1 atom in compound 5 is the most interesting example in this family of compounds as its environment is highly distorted, and the sphericity of the HS is the lowest among all secondary metal atoms in this study (Figure 7). Both methods of analysis agree, however, that neighboring atoms at 3.86, 4.39, 3.80, and 3.70 Å are not chemically bonded with the Rb1 atom. Hence, the coordination polyhedron consists of five oxygen atoms at distances 2.78−3.07 Å and one iodine atom at 3.69 Å. Possibly, the reason for such significant distortion of the Rb1 environment is the absence of chemical bonds with oxygen atoms from one side of the atom due to the close Rb---Rb contact at 4.48 Å oriented exactly in the direction of the nonbonded oxygen atoms. The corresponding face on the VD polyhedron of Rb1 atom is marked with a pound sign, whereas on the HS the same region has very large curvature, thereby implying no interaction at all (Figure 7, green color). Interestingly, both VD and HS analyses indicate similar coordination environments for the Tl1 and Rb1 atoms in compounds 4 and 5 with m-iodobenzoic acid (the pairs of corresponding VD polyhedra and HS in Figure 7 are depicted at the same angle and are very similar). Taking into account the high distortion of the VD polyhedron and HS of the Rb1 atom (which is unusual for alkali metals forming ionic bonds), one can infer that for compound 5, the Rb1 atom acts more like the “pseudo” alkali metal Tl1 in compound 4 with a stereoactive lone electron pair, then a typical alkali metal cation. The VD analysis shows almost equal volumes of Tl1 and Rb1 in compounds 4 and 5: 25.0 and 24.9 Å3 respectively, while the HS analysis indicates the smaller size of Rb1 in contrast with the trend of its slightly increased ionic radius (see Chart 1): 39.2 and 30.0 Å3, respectively. The Tl1 atom in compound 6 is surrounded by the largest number of neighboring oxygen atoms, eleven, within the range of 2.57−3.70 Å. However, according to the MIS only six of them are chemically bonded with the Tl1 center, which are the nearest oxygen atoms up to 3.34 Å. HS of Tl1 atom in 6 features flat regions for ten out of 11 oxygen atoms (Figure 8). Surprisingly, the remaining atom O5 at 3.56 Å is not the furthest in distance away from Tl1 as two oxygen atoms are further at 3.67 Å (O9) and 3.70 Å (O14), respectively, both of which correspond to quite large red, flat regions (Figure 8). However, the MIS shows that all the three mentioned atoms O5, O9, and O14 at extended distances are not chemically bonded with the Tl1 center, which is due to the stereoactive lone electron pair of Tl1. Thus, according to the MIS, the coordination polyhedron of Tl1 atom in 6 consists of six oxygen atoms at distances ranging from 2.57−3.34 Å. The size of Tl1 atom in compound 6 is equal to 23.8 and 37.6 Å3 according to VD and HS analysis, respectively. Thus, both VD and HS analyses indicate sequential volume reduction of Tl1 atoms when switching from o- to m- and p-iodobenzoic acid ligands in compounds 1, 4, and 6 (26.9 → 25.0 → 23.8 Å3 according to VD analysis and 41.4 → 39.2 → 37.6 Å3 according to HS analysis) and of Rb1 atoms when switching from o- to miodobenzoic acid ligands in compounds 2 and 5 (32.7 → 24.9 Å3 according to VD analysis and 33.4 → 30.0 Å3 according to HS analysis). These trends are consistent with the above proposed idea of a decreasing “pocket” size for incorporation of secondary metal centers when switching from o- to m- and p-

for the stereoactive lone electron pair on the thallium(I) center, which is localized between the Tl1 and I6A_a atoms (Figure 6). On the contrary, HS analysis shows red flat surfaces for all three iodine atoms, thus underestimating the increased length of the two contacts with I3 and I6A_a and the effects of stereoactive lone electron pair (Figure 6). The Rb1 atom in compound 2 has eight oxygen atoms (at distances of 2.92−5.42 Å) and five iodine atoms (at distances of 3.74−4.57 Å) in the neighboring environment. The HS has red flat regions for five contacts with oxygen atoms (at distances 2.92−3.15 Å) and for all five contacts with iodine atoms, thus giving a CN of 10. The MIS yields a CN equal to 8 (5O and 3I), excluding the furthest contacts with iodine atoms I3 and I6 respectively at 4.05 and 4.57 Å (Figure 6). Moreover, the coordination environment and results of VD and HS analyses of the Cs1 atom in compound 3 are very similar to that of Rb1 in 2. The Cs1 atom has eight oxygen (at distances 3.07−5.51 Å) and five iodine (at distances 3.84−4.39 Å) atoms in the neighboring environment. The HS has red flat regions for 10 contacts: five with oxygen atoms (at distances 3.07−3.28 Å) and five with all the mentioned iodine atoms. The resulting CN according to HS analysis is equal to 10. The MIS shows CN equal to 8 (5O and 3I), excluding the furthest contacts with iodine atoms I5 and I13 at 4.09 and 4.39 Å, respectively (Figure 6). Overall, Figure 6 allows for comparison of the environment of the secondary metal centers in compounds 1−3 with the same o-iodobenzoic acid ligands. Both VD and HS analyses show a very similar coordination environment of Rb1 and Cs1 atoms as the pairs of corresponding VD polyhedra and HS are depicted at the same angle and are very similar. The only noticeable difference between the Rb+ and Cs+ environments is the presence of weak Cs---Cs interactions at 5.44 Å in compound 3, which are discoverable only by the MIS as noncovalent interactions (marked with pound signs in Figure 6). On the other hand, both VD and HS analyses indicate a very different coordination environment for Tl1 in comparison with that of alkali metal atoms (see Figure 6). Thus, Tl1 can be considered as a “pseudo” alkali metal only due to the size and charge; however the nature of the stereoactive lone electron pair promotes its different crystal chemical role in the series of three isomorphous compounds. The volumes of VD polyhedra of secondary metal centers are consistent with their increasing ionic radii (see Chart 1): 26.9, 32.7, and 36.5 Å3 respectively for Tl1, Rb1, and Cs1, whereas the volumes of HS do not follow this trend: 41.4, 33.4, and 41.9 Å3 respectively for Tl1, Rb1, and Cs1. Figure 7 shows VD and HS representations of secondary metal atoms in compounds 4 and 5 with m-iodobenzoic acid ligands. The Tl1 atom in compound 4 is surrounded by eight oxygen atoms at distances 2.66−3.93 Å and one iodine atom at 3.58 Å. The MIS shows five chemical bonds with oxygen atoms (with the furthest atom at 3.17 Å) and one with an iodine atom. However, the HS features flat red regions for all the nine contacts, including the three longest contacts with oxygen atoms at 3.81, 3.93, and 3.73 Å, respectively, although the surface area for the first of them is very small (Figure 7). This case is similar to that of Tl1 atom in compound 1: HS underestimates the stereoactive lone electron pair, which is pointed approximately in the direction of Tl---Tl interaction in Figure 7. On the other hand, the MIS accounts for this effect and for the extended lengths of corresponding contacts, thereby indicating no chemical bonding with these atoms. Interestingly, 9164


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coloration of these crystals (orange) and the paucity of uranylTl+ hybrid materials. Whereas homometallic uranyl materials typically exhibit characteristic green emission as a result of ligand-to-metal charge transfer transitions between uranyl bonding and nonbonding orbitals,62,63 the only reported example of luminescence in a uranyl-Tl(I) heterometallic compound suggested that Tl+ quenched uranyl emission in materials featuring both cations.64 A look at the emission spectra for 1, 4, and 6 reveals characteristic emission (four to five major vibronic peaks) upon excitation at 420 nm (Figure 9). The spectrum of 6 also features additional shoulder peaks,

iodobenzoic acid ligands in crystal structures of the title compounds (Chart 1). It should be noted that in addition to the faces/regions corresponding to interactions with oxygen and iodine atoms, the VD polyhedra and HS feature smaller faces/flat regions corresponding to contacts with hydrogen (marked with asterisks on Figures 6−8) as well as carbon atoms. This makes it difficult to count the CN using HS, as one must take into account all regions corresponding to interactions with hydrogen atoms and manually exclude them. The corresponding faces of VD polyhedra are also large and they usually have even more faces than HS have flat regions due to the sharpness of the VD polyhedra. However, the MIS does not automatically count Tl---H and Tl---C interactions as chemical bonds, as the corresponding spheres of Slater and spherical domain radii of Tl, H, and C atoms do not intersect to an appropriate extent. Analysis of coordination environments using the two approaches allowed for evaluation of the role/function of secondary metal centers in compounds 1−6. The presented VD and HS diagrams (Figures 6−8) illustrate the value of VD tessellation and the MIS in analyzing coordination environments of atoms, even in such challenging cases as alkali metals and “pseudo” alkali metals featuring stereoactive lone electron pairs, as HS consistently overestimated secondary metal center coordination numbers and a focus on only crystallographic parameters (via CSD search) tended to underestimate secondary metal cation CN, particularly for Tl+ (Table 2).

Figure 9. Low temperature, solid-state emission of 1 (blue), 4 (red), and 6 (green) upon excitation at 420 nm.

Table 2. Charge Balancing Secondary Metal Cation CN (in 1−6) as Determined via CSD, VD, and HS Analysis secondary metal cation

CN (CSD)

CN (VD)

CN (HS)

Tl1 (1) Tl2 (1) Rb1 (2) Rb2 (2) Cs1 (3) Cs2 (3) Tl1 (4) Rb1 (5) Tl1 (6) Tl2 (6)

6 5 7 6 8 7 5 6 6 6

7 8 8 6 8 7 6 6 6 6

10 10 10 10 10 10 9 6 10 10

which are notable as these peaks are generally attributed to additional coupling of the uranyl bending mode and/or the vibrational modes of the equatorial ligands with the uranyl excited state.63 It is difficult to define their origin exactly in the absence of rigorous theoretical treatment, however. These results are interesting as they indicate that, at least with regard to luminescence in these materials, Tl+ is more like the larger alkali metal cations that typically do not impact uranyl luminescence, as opposed to transition metal cations (i.e., Cu2+), which are known to quench (or significantly diminish) uranyl luminescence.6,30,31,65

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CONCLUSIONS The syntheses and crystal structures of six uranyl compounds containing o-iodo-, m-iodo-, and p-iodobenzoic acid and the secondary metal centers Tl+, Rb+, and Cs+ are reported. Modes of supramolecular assembly for 1−6 have been described, VD tessellation with the method of intersecting spheres and HS analysis calculations for all compounds have been analyzed, and luminescence spectra for compounds 1, 4, and 6 are discussed. A look at the local structures of this family of uranyl hybrid materials suggests that one important factor for the incorporation of a secondary metal center (Tl+ < Rb+ < Cs+) is the ionic radii of the metal cation of interest. Additionally, VD tessellation with the method of intersecting spheres analysis on these compounds were used in an effort to develop a metric for delineating whether an interaction in a heterometallic system is chemical bond or a noncovalent interaction and was found to be the most appropriate method for CN determination, as compared to only crystallographic observations (as analyzed via the CSD) or HS analysis. Low temperature luminescence spectroscopy on the Tl+ containing compounds (1, 4, and 6) displays characteristic uranyl emission upon excitation at 420 nm. Follow up studies utilizing VD

This is not a surprising result as the developers of the atomic HS note HS can reveal “possible covalent interactions”;43,44 however the final decision on the CN is up to the researcher, which inevitably leads to subjectivity of analysis and necessitates individual visual examination of each compound. On the contrary, the VD tessellation method features rigorous geometrical determination of CN and automatic examination, applicable to large databases and this allowed for identification of Tl+, Rb+, and Cs+ bonds in 1−6 that were beyond CSD search criteria (i.e., Tl+−I bonds). Although HS analysis accurately shows interacting atoms, a further method, such as VD analysis, should be employed for classifying the interacting atoms into chemically bonded/nonbonded atoms. Additional figures illustrating HS and VD representations of Tl2, Rb2, and Cs2 atoms in compounds 1−3 and 6 have been prepared, and they are included in the Supporting Information (Figures S2− S3, Supporting Information). Luminescence. Low temperature (77 K) solid-state luminescence studies were carried out on several single crystals from the bulk phases of 1, 4, and 6 due to the unusual 9165


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(7) Knope, K. E.; Cahill, C. L. Synthesis and Characterization of 1-, 2-, and 3-Dimensional Bimetallic UO22+/Zn2+ Phosphonoacetates. Eur. J. Inorg. Chem. 2010, 2010 (8), 1177−1185. (8) Knope, K. E.; de Lill, D. T.; Rowland, C. E.; Cantos, P. M.; de Bettencourt-Dias, A.; Cahill, C. L. Uranyl Sensitization of Samarium(III) Luminescence in a Two-Dimensional Coordination Polymer. Inorg. Chem. 2012, 51 (1), 201−206. (9) Kerr, A. T.; Kumalah, S. A.; Holman, K. T.; Butcher, R. J.; Cahill, C. L. Uranyl Coordination Polymers Incorporating η5-Cyclopentadienyliron-Functionalized η6-Phthalate Metalloligands: Syntheses, Structures and Photophysical Properties. J. Inorg. Organomet. Polym. Mater. 2014, 24 (1), 128−136. (10) (a) Savchenkov, A. V.; Klepov, V. V.; Vologzhanina, A. V.; Serezhkina, L. B.; Pushkin, D. V.; Serezhkin, V. N. Trinuclear {Sr[UO2L3]2(H2O)4} and pentanuclear {Sr[UO2L3]4}2‑ uranyl monocarboxylate complexes (L-acetate or n-butyrate ion). CrystEngComm 2015, 17 (4), 740−746. (b) Vologzhanina, A. V.; Savchenkov, A. V.; Dmitrienko, A. O.; Korlyukov, A. A.; Bushmarinov, I. S.; Pushkin, D. V.; Serezhkina, L. B. Electronic Structure of Cesium Butyratouranylate(VI) as Derived from DFT-assisted Powder X-ray Diffraction Data. J. Phys. Chem. A 2014, 118, 9745−9752. (11) Thuéry, P.; Rivière, E.; Harrowfield, J. Uranyl and Uranyl−3d Block Cation Complexes with 1,3-Adamantanedicarboxylate: Crystal Structures, Luminescence, and Magnetic Properties. Inorg. Chem. 2015, 54 (6), 2838−2850. (12) Thuéry, P.; Harrowfield, J. Modulation of the Structure and Properties of Uranyl Ion Coordination Polymers Derived from 1,3,5Benzenetriacetate by Incorporation of Ag(I) or Pb(II). Inorg. Chem. 2016, 55 (13), 6799−6816. (13) Falaise, C.; Delille, J.; Volkringer, C.; Vezin, H.; Rabu, P.; Loiseau, T. Series of Hydrated Heterometallic Uranyl-Cobalt(II) Coordination Polymers with Aromatic Polycarboxylate Ligands: Formation of UOCo Bonding upon Dehydration Process. Inorg. Chem. 2016, 55 (20), 10453−10466. (14) Cole, E.; Flores, E.; Basile, M.; Jayasinghe, A.; de Groot, J.; Unruh, D. K.; Forbes, T. Z. Directing dimensionality in uranyl malate and copper uranyl malate compounds. Polyhedron 2016, 114, 378− 384. (15) Thuéry, P.; Harrowfield, J. AgI and PbII as Additional Assembling Cations in Uranyl Coordination Polymers and Frameworks. Cryst. Growth Des. 2017, 17 (4), 2116−2130. (16) Zhao, R.; Mei, L.; Hu, K.-Q.; Wang, L.; Chai, Z.-f.; Shi, W.-Q. Two Three-Dimensional Actinide−Silver Heterometallic Coordination Polymers Based on 2,2′-Bipyridine-3,3′-dicarboxylic Acid with Helical Chains Containing Dimeric or Trimeric Motifs. Eur. J. Inorg. Chem. 2017, 2017 (11), 1472−1477. (17) Thuéry, P. Sulfonate Complexes of Actinide Ions: Structural Diversity in Uranyl Complexes with 2-Sulfobenzoate. Inorg. Chem. 2013, 52 (1), 435−447. (18) Zhao, F.-H.; Li, H.; Che, Y.-X.; Zheng, J.-M.; Vieru, V.; Chibotaru, L. F.; Grandjean, F.; Long, G. J. Synthesis, Structure, and Magnetic Properties of Dy2Co2L10(bipy)2 and Ln2Ni2L10(bipy)2, Ln = La, Gd, Tb, Dy, and Ho: Slow Magnetic Relaxation in Dy2Co2L10(bipy)2 and Dy2Ni2L10(bipy)2. Inorg. Chem. 2014, 53 (18), 9785−9799. (19) Zhang, Y.; Zheng, J.-M. Three 3d−4f Tetranuclear Complexes Based on 2,3,5-Trichlorobenzoic Acid: Syntheses, Structures, and Magnetic Properties. Aust. J. Chem. 2016, 69 (4), 446−450. (20) Deifel, N. P.; Cahill, C. L. Combining coordination and supramolecular chemistry for the formation of uranyl-organic hybrid materials. Chem. Commun. 2011, 47, 6114−6116. (21) Carter, K. P.; Pope, S. J. A.; Cahill, C. L. A series of Ln-pchlorobenzoic acid-terpyridine complexes: lanthanide contraction effects, supramolecular interactions and luminescent behavior. CrystEngComm 2014, 16 (10), 1873−1884. (22) Carter, K. P.; Zulato, C. H. F.; Cahill, C. L. Exploring supramolecular assembly and luminescent behavior in a series of RE-pchlorobenzoic acid-1,10-phenanthroline complexes. CrystEngComm 2014, 16 (44), 10189−10202.

tessellation with the method of intersecting spheres to investigate chemical bonding in heterometallic uranyl hybrid materials at the secondary metal site are in progress.

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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.7b01208. ORTEP figures of all compounds, tables of selected bond lengths, VD analysis details, and figures illustrating HS and VD representations of Tl2, Rb2, and Cs2 atoms in compounds 1−3, 6 (PDF) Accession Codes

CCDC 1547989−1547994 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: [email protected]. Phone: (202) 994-6959. ORCID

Korey P. Carter: 0000-0003-4191-0740 Anton V. Savchenkov: 0000-0002-6048-3011 Christopher L. Cahill: 0000-0002-2015-3595 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported as part of the Material Science of Actinides, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0001089. K.P.C. would also like to acknowledge George Washington University for a Presidential Merit Fellowship award. A.V.S. acknowledges financial support by the base part of the government mandate of the Ministry of Education and Science of the Russian Federation (number for publications 4.6558.2017/7.8).

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REFERENCES

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