Complex Uranyl Dichromates Templated by Aza-Crowns

Three new uranyl dichromate compounds templated by aza-crown templates were obtained at room temperature by evaporation from aqueous solutions: (H2diaza-18-crown-6)2 [(UO2)2(Cr2O7)4(H2O)2](H2O)3 (1), (H4[15]aneN4)[(UO2)2(CrO4)2(Cr2O7)2(H2O)] (H2O)3.5 (2) and (H4Cyclam)(H4[15]aneN4)2[(UO2)6(CrO4)8(Cr2O7)4](H2O)4 (3). The use of aza-crown templates made it possible to isolate unprecedented and complex one-dimensional units in 2 and 3, whereas the structure of 1 is based on simple uranyl-dichromate chains. It is very likely that the presence of relatively large organic molecules of aza-crown ethers does not allow uranyl chromate chain complexes to condense into the units of higher dimensionality (layers or frameworks). In general, the formation of 1, 2, and 3 is in agreement with the general principles elaborated for organically templated uranyl compounds. The negative charge of the [(UO2)(Cr2O7)2(H2O)], [(UO2)2(CrO4)2 (Cr2O7)2(H2O)] and [(UO2)3(CrO4)4(Cr2O7)2] one-dimensional inorganic motifs is compensated by the protonation of all nitrogen atoms in the molecules of aza-crowns.


Introduction
The crystal chemistry of hexavalent uranium oxysalts is remarkable for its extremely rich diversity of structural types [1].Among these, uranyl compounds containing tetrahedrally coordinated hexavalent cations (S, Cr, Se, Mo) form one of the most numerous groups [2].Structural chemistry of uranyl oxysalts is mostly represented by layered structural architectures which results from strong directional anisotropy of the bond distribution in UO 2 2+ coordination geometries.However, each of the chemical classes of compounds bearing T 6+ O 4 tetrahedral anions (i.e., uranyl sulfates, chromates, selenates and molybdates) exhibits its own characteristic structural trends and features.Uranyl sulfates demonstrate remarkable structural diversity [1].The sulfate anions may coordinate to the uranyl group in both monodentate and bidentate manner via corner and edge sharing, respectively, between the UO 7 pentagonal bipyramid and the SO 4 tetrahedron.However, just a few of uranyl polysulfates (disulfates in fact) are known to date [3].Polymerization is unknown for selenates in uranyl compounds.The chemistry of organically templated uranyl selenates is also diverse and many contributions were reported in the last decade [4].Bidentate bridging of SeO 4 2− anion with the UO 7 bipyramid cannot be realized due to the relatively large size of Se 6+ cation and high repulsive forces between the U 6+ and Se 6+ .Uranyl molybdates tend to form framework structures [5] due to the flexibility and broad variation of U-O-Mo bond angles.Uranyl chromates constitute a particularly versatile class among the compounds considered as they exhibit all the features listed above: both mono- [6][7][8][9][10][11][12][13][14][15][16] and bidentate [17][18][19] linkage modes of CrO 4 tetrahedra to UO 7 uranyl bipyramids, formation of corner-sharing tetrahedral chromate polyanions [19][20][21], flexibility and variation of U-O-Cr angles similar to those in uranyl molybdates and formation of framework structures [22], and variability of coordination environments [23][24][25].The tendency of the CrO 4 2− to form isopolyanions in acidic media leads to formation of structures involving simultaneously several polychromate moieties such as Cr 2 O 7 2− dimers, Cr 3 O 10 2− trimers and Cr 4 O 13 2− tetramers [26].
Organically templated uranyl-based materials [27] are characterized by the structures consisting of weakly bonded inorganic and organic substructures.The majority of organic molecules used as templates in uranyl compounds are structurally non-rigid nitrogen-based hydrogen bond donors.It is of interest to explore a template effect when employing organic species with restricted adaptability and fixed mutual positioning of the donor atoms.Macrocycles including crown ethers and particularly their aza derivatives seem to be the most proper candidates due to their applications in actinide partitioning [28,29], given their high selectivity for uranium [30] and neptunium [31,32].Recently, crown ether complexes with alkali metals [33] and aza-crown ether complexes with Ni [34] were used as assemblers and linkers in uranyl-organic coordination polymers.In addition to metal-organic compounds, a number of works were devoted to the preparation of uranyl oxysalts templated by crown ethers [35][36][37][38].In some cases, the use of crown ethers as templates has resulted in highly porous framework structures [39].We note that, in most of the reported structures of crown ether-templated uranyl compounds, both inorganic units and the crown ether molecules remain electroneutral leading to formation of the so-called organic-inorganic composite structures [4,19,38].This clear demonstration of the template effect in organically templated uranyl-based materials sheds light at further perspectives of the macrocycles in the structural design of uranyl compounds.Therefore, one can expect that incorporation of nitrogen donor atoms, capable of accepting protons, into the structures of macrocyclic templates would further expand the structural chemistry of the templated uranium compounds.Uranyl compounds templated by aza-crowns have not been reported to date.Chromates were chosen due to their higher structural diversity in terms of composition, including formation of polyanions as discussed above.
Herein, we report syntheses and structural data for the first three uranyl dichromate compounds based on aza-crown templates: (H   4 (3) obtained at room temperature in the aza-crown-CrO 3 -(UO 2 )(NO 3 ) 2 systems by evaporation from aqueous solutions.A small set of relatively simple aza-crown molecules has resulted in two new unprecedented and complex one-dimensional units in the structures of 2 and 3, respectively.The structure of 1 is based on more simple uranyl-dichromate chains previously reported in an imidazole-templated compound [20].

Synthesis
Caution: Depleted uranium is radioactive and chemically toxic so its compounds should be handled with care.Chromium (VI) compounds are carcinogenic.Suitable safety measures for precautions and protection should be taken.

X-ray Experiments
The crystals of 1-3 selected for data collection were examined under an optical microscope and mounted on glass fibers.The single crystal X-ray data were collected at 120 K on a Bruker SMART diffractometer equipped with an APEX II CCD detector operating with MoK α radiation at 50 kV and 40 mA.A single dark red translucent prismatic crystal of 1 with dimensions of 0.10 × 0.12 × 0.04 mm 3 , a dark red plate of 2 with dimensions of 0.12 × 0.12 × 0.02 mm 3 and one of 3 with similar habit measuring 0.20 × 0.20 × 0.10 mm 3 were chosen.For each crystal, more than a hemisphere of data was collected with a frame width of 0.5 • in ω, and 10 s counting time spent for each frame.The data were integrated and corrected for absorption using a multi-scan type model using the Bruker programs APEX and SADABS [40].The crystals of 1-3 decay after approximately 3 h of X-ray exposure.Their structures were solved by direct methods.1).All structures were successfully refined with the use of SHELX software package [41].For 1, all atoms except split sites Cr8 and O27 were refined anisotropically.The final model for 2 included anisotropic displacement parameters for all atoms.For 3, only all of the U and Cr and most of O atoms in uranyl chromate units could be refined anisotropically.The hydrogen atoms in protonated aza-crowns molecules were added to their ideal positions using HFIX command.Hydrogen atoms belonging to H 2 O molecules could not be localized in all structures.Further details of the data collection and refinement are given in Table 1 and selected bond lengths in Tables 2-4.Crystallographic data in cif format have been deposited with the Cambridge Crystallographic Data Center (1883671 (for 1), 1883670 (for 2), 1883669 (for 3)).Copies of the data can be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK, fax: +44-1223-366033, email: deposit@ccdc.cam.ac.uk or on the web at http://www.ccdc.cam.ac.uk.

Cation Coordination
The structure of 1 (Figure 1) contains two symmetrically independent U 6+ cations and eight Cr 6+ cations.Each of two U atoms is strongly bonded to two O atoms thus forming linear UO Crystallographic data in cif format have been deposited with the Cambridge Crystallographic Data Center (1883671 (for 1), 1883670 (for 2), 1883669 (for 3)).Copies of the data can be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK, fax: +44 1223 366033, email: deposit@ccdc.cam.ac.uk or on the web at http://www.ccdc.cam.ac.uk.
In all three structures, the C-C, C-N and C-O bond lengths as well as the bond angles in aza-crown molecules are within the limits typically observed for these molecules.Some strong distortions in several bond-lengths values are due to the instability of grown crystal under X-ray beam.In all three structures, the C-C, C-N and C-O bond lengths as well as the bond angles in aza-crown molecules are within the limits typically observed for these molecules.Some strong distortions in several bond-lengths values are due to the instability of grown crystal under X-ray beam.(11) Cr8A-O32 1.804(15) Cr8B-O31 1.512(13) Cr1-O8 1.603(12) Cr5-O11 1.587( 14) Cr8B-O27B * 1.613(7) Cr1-O12 1.604(13) Cr5-O29 1.585 (12) Cr8B-O17 1.682(15) Cr1-O1 1.621(11) Cr5-O20 1.642 (12) Cr8B-O32 1.729( 14   The monoclinic structure of 2 contains two symmetrically independent U 6+ cations also forming linear UO2 2+ ion each coordinated by five oxygen atoms at the equatorial vertices of pentagonal The monoclinic structure of 2 contains two symmetrically independent U 6+ cations also forming linear UO 2 2+ ion each coordinated by five oxygen atoms at the equatorial vertices of pentagonal bipyramids similar to those observed in 1. Water molecules are absent in the coordination sphere of U1 atom in 2, whereas U2 coordination environments are similar to those in 1 with the formation of UO 6 (H 2 O) pentagonal bipyramids (Table 3, Figure 3a,b).bipyramids similar to those observed in 1. Water molecules are absent in the coordination sphere of U1 atom in 2, whereas U2 coordination environments are similar to those in 1 with the formation of UO6(H2O) pentagonal bipyramids (Table 3, Figure 3a,b).
There are four distinct Cr 6+ sites tetrahedrally coordinated by four O atoms each.There are six Cr sites in the structure of 2. The Cr1 and Cr2 form isolated CrO4 tetrahedra, whereas the Cr3, Cr4, Cr5 and Cr6 sites belong to dichromate groups, Cr2O7 2− .Cr-Obr (Obr = bridging O atom in Cr2O7 2− group) bonds are significantly elongated in comparison to the Cr-Ot (Ot = terminal O atom) (Table 3).There are three symmetrically independent U atoms in the triclinic structure of 3 (Table 4), all forming linear uranyl cations equatorially coordinated by five O atoms to form pentagonal bipyramids.Water molecules are absent in the coordination sphere of uranium atoms in 3.
[H 4 [15]aneN 4 ] 4+ aza-crown moieties and additional water molecules form layers parallel to ab plane (Figure 4c).Aza-crown molecules are interconnected with uranyl chromate units by the relatively weak hydrogen bonds only.
There are four distinct Cr 6+ sites tetrahedrally coordinated by four O atoms each.There are six Cr sites in the structure of 2. The Cr1 and Cr2 form isolated CrO 4 tetrahedra, whereas the Cr3, Cr4, Cr5 and Cr6 sites belong to dichromate groups, Cr 2 O 7 2− .Cr-O br (O br = bridging O atom in Cr 2 O 7 group) bonds are significantly elongated in comparison to the Cr-O t (O t = terminal O atom) (Table3).