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A Uranyl Metal Organic Framework Arising from the Coordination of a Partially Hydrolyzed Tetrauranyl Node with the Tautomerically Diverse 1,4-(diamidoximyl)benzene Ligand Manish Kumar Mishra, Yogesh Patil, Steven P. Kelley, and Robin D Rogers Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00544 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 27, 2019

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Crystal Growth & Design

A Uranyl Metal Organic Framework Arising from the Coordination of a Partially Hydrolyzed Tetrauranyl Node with the Tautomerically Diverse 1,4-(diamidoximyl)benzene Ligand Manish Kumar Mishra,a† Yogesh P. Patil,a Steven P. Kelley,b and Robin D. Rogersa,c* aCollege

of Arts & Sciences, The University of Alabama, Tuscaloosa, AL 35487, USA

bDepartment c525

of Chemistry, University of Missouri, Columbia, MO 65211, USA.

Solutions, Inc., P.O. Box 2206, Tuscaloosa, AL 35403, USA

Abstract: We report a unique uranium-based metal organic framework afforded from the spontaneous reaction of 1,4-(diamidoximyl)benzene (1,4-(DAO)Bz) with uranyl chloride. The structure is the first reported example to show either hydrolysis alongside amidoxime coordination or the presence of multiple amidoxime protonation states in a single complex. The ability of the ligand to adopt three chemically distinct forms affords it enough flexibility to link the typically planar uranyl nodes in three orthogonal directions, giving rise to a cationic framework with large channels and suggesting applications for this system in crystal engineering. The extraction of uranium from seawater is a long-standing field of interest as an economical alternative to terrestrial mining. In spite of the very low concentration of uranium in sea water (3 ppb as [UO2(CO3)4]4-), it is a rich source compared to terrestrial ores.1,2 After extensive research on nearly 200 sorbents, amidoxime functionalized fibers were found to be promising extractants for uranium from seawater.3,4 Amidoxime is a robust ligand which can coordinate in variety of different ways with metal ions arising from this ligand’s reactivity, tautomerism, and the presence of multiple possible coordinating groups, amine and oxime.5 The recent research to improve the extraction of uranium from seawater has hypothesized that the way the structure of the ligands allows cooperativity in chelating uranium is a major feature of amidoxime based sorbents.6,7,8 We have thus sought to understand how multiple adjacent amidoxime groups may influence each other to improve the selectivity of these sorbents for the uranyl ion. Other than two reports demonstrating unidentate coordination to [UO2]2+ to a zwitterionic amidoxime through the oxygen atom, (Scheme 1a)9 all reported [UO2]2+ 1 ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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complexes with amidoximes coordinate to the anionic amidoximate in an η2 fashion (Scheme 1b).10,11 A chelating coordination mode with cyclic imidedioxime has also been crystallographically characterized (Scheme 1e)12 and is believed to be the form of the ligand responsible for selectivity in these sorbents. There are also a few binding modes which have not yet been observed for [UO2]2+, including unidentate (Scheme 1c) and chelating coordination (Scheme 1f).11 Our group has recently studied the effect of coordination cooperativity of two adjacent diamidoximes on a single ligand which allowed coordination in the chelating η2-fashion (Scheme 1d).13 H2N

NH2 R HN

O

O

HN

O

R

(NO3)2 O

O

O

NH

U

N

R

O

O

OH

NH2

R

H2N

HN

N

N

O

O

O

NH2 N

O

O

U

H2 N

H2O

O

N

U

N

N N H

OH2

d) Chelate coordination

R

O

O

N

N O

O

U

O O

N

N R

H 2N

O O

H

H

N

R NH2

c) Unidentate coordination (Predicted but not yet observed)

R= CH3 or C6H5 b) 2-Coordination

R= CH3 or C6H5 a) Unidentate coordination

N U

NH2

R

N

O

H O

N

R

U HO

NH

O

OH

O

R NH2

NH2

NH2

NH2

R

O

O

N

O

N H2

R

H

e) Tridentate/Chelate coordination with cyclic amidoxime

f) Chelate coordination (Predicted but not yet observed)

Scheme 1. Possible coordination modes in uranyl amidoxime complexes. In

furthering

our

studies,

we

investigated

the

coordination

of

1,4-

(diamidoximyl)benzene (hereafter 1,4-(DAO)Bz; Scheme 2) to understand cooperativity and electronic effects on the coordination behavior of the amidoxime group. The amidoxime groups in 1,4-(DAO)Bz are separated by a rigid aromatic core which can tautomerize to its zwitterionic form as shown in Scheme 2. We had anticipated that the two amidoxime groups would not strongly influence each other’s coordination due to the para positioning. The 1,4-(DAO)Bz ligand was prepared from commercially available 1,4-dicyanobenzene by reaction with hydroxylamine (see Supporting Information for experimental details).14 The 2 ACS Paragon Plus Environment

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synthesis was verified by 1H and

13C

NMR and single crystal X-ray diffraction (SCXRD) on

crystals obtained by slow evaporation of an acetone solution which confirmed the structure previously reported.14 H2N HO

N

N

OH

NH2

1,4-(diamidoximyl)benzene [1,4-(DAO)Bz]

H2N O

N H

HN

O

NH2

Zwitterionic Tautomer

Scheme 2. Molecular structure and zwitterionic tautomer of 1,4-(diamidoximyl)benzene (1,4-(DAO)Bz). 1,4-(DAO)Bz was then added to a concentrated (25.31 mM) aqueous solution of UO2Cl2·3H2O via stirring until saturation, at which point there was an immediate color change of the reaction mixture from greenish-yellow to dark yellow with red precipitate formation. The filtered supernatant produced yellow colored plate-shape crystals upon slow evaporation after standing for two weeks along with crystals of 1,4-(DAO)Bz as identified by powder X-ray diffraction of the bulk materials (see Supporting Information). Single crystal X-ray diffraction (SCXRD) analysis of the crystals revealed the formula to be [(UO2)4(O)2(C8H8N4O2)(C8H10N4O2)3(H2O)2]·Cl·nH2O (hereafter UMOFUA), where C8H8N4O2 and C8H10N4O2 represent dianionic and zwitterionic 1,4(DAO)Bz, respectively. UMOFUA crystallizes in the triclinic space group P-1 with the asymmetric unit containing two independent and chemically distinct UO22+ cations, 1.5 zwitterionic and 0.5 dianionic 1,4(DAO)Bz ligands, one μ3-O2- anion bridging three UO22+ cations, one noncoordinated Cl- anion, and two coordinated water molecules. Ten noncoordinated, disordered guest molecules modelled as neutral water molecules reside inside the framework (see Supporting Information). The structure is composed of planar, oxide-bridged tetrauranyl cores surrounded by eight coordinating 1,4(DAO)Bz ligands in varying protonation states and two water molecules (Figure 1).

3 ACS Paragon Plus Environment


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Figure 1. Coordination geometry of the planar tetrauranyl core present in UMOFUA (top and side view): The colors represent the coordination modes of the 1,4-(DAO)Bz ligands: Unidentate dianions (red, residing on i), μ2-zwitterion (yellow, residing on i), and unidentate and μ2zwitterionic (blue). The water and chloride molecules have been omitted for clarity. It is at once apparent that none of the amidoxime ligands coordinate in the expected η2-fashion, and we explored the structure further to ascertain the reason. The oxide-bridged tetrauranyl cluster (Fig. 2) is isostructural with those seen in compounds bridged by chloride,15 isocyanate,16 and 1,8octanedicarboxylate.17 It has also been observed in several inorganic structures, all containing chloride.18 Compounds containing this cluster appear to form exclusively as precipitates; it has not been proposed as a solution form of uranyl and in at least one report it is undetectable in the mother solution from which it crystallized.15 The crystallization of these compounds is similar to what we observed: An initially acidic uranyl chloride solution is treated with a base, separated from a precipitate, and allowed to stand until crystals form.19,20 Given that crystals of 1,4-(DAO)Bz form alongside this compound it is likely that the solution (after precipitation) contains discrete, soluble uranyl amidoximate complexes, but with time some of the anionic 1,4-(DAO)Bz ligands are protonated and exchanged with hydroxide or oxo ligands in hydrolysis reactions to form the far less soluble UMOFUA which precipitates immediately.


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Figure 2: Structure of the tetrauranyl core moiety. On the basis of charge balance and the presence of hydrogen bond acceptors around the ligands, it can be shown that the 1,4-(DAO)Bz ligands are in two different protonation states, either zwitterionic (yellow and blue, Figure 1) or doubly deprotonated, dianionic (red, Figure 1). There are no prior reports of this occurring in a [UO2]2+ complex; all examples in the Cambridge Structural Database (CSD)21 either contain exclusively anionic or exclusively zwitterionic amidoxime ligands. Furthermore, this crystal structure contains the first example of an anionic amidoxime coordinating to [UO2]2+ in a unidentate, as opposed to η2, fashion. Although the partial hydrolysis and tautomerization of 1,4-(DAO)Bz in UMOFUA are typical in uranyl crystal chemistry as discussed above, this was nevertheless unexpected since none of the studies on uranyl complexes with small molecule amidoximate ligands have reported this. This suggests something unusual about the 1,4-(DAO)Bz ligand itself, and we attribute this to reduced acidity of the amidoxime groups caused by their para-positioning on the ring. Although they cannot engage in intramolecular noncovalent interactions, it would be expected that due to the resonance with the C=N double bond, these groups should act as strong, para-directing electron donors in an aromatic molecule. We suspect that the 1,4-(DAO)Bz molecules in solution are not deprotonated but do adopt their zwitterionic form. Since only the amidoxime portion of the molecule interacts with [UO2]2+, they can behave as monovalent anions and allow the formation of a similar tetranuclear cluster to that stabilized by other monovalent, anionic ligands. The 1,4-(DAO)Bz ligands on the clusters are still able to freely exchange protons with the solution and thus eventually adopt the charge state that allows crystallization of UMOFUA. This also suggests that amidoxime



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ligands could be designed to take advantage of the strong uranyl amidoxime interaction yet permit the crystallization of coordination polymers of interest in crystal engineering. Although we anticipated that 1,4-(DAO)Bz could act as a bridging ligand, the structure quite unexpectedly crystallized as a non-interpenetrated MOF. The channels in this structure, which are occupied by chloride ions and water molecules, extend infinitely along the a-axis with an estimated inner diameter of 6.76 Å (Figure 3a) and occupy a total volume of 782 Å3 per unit (40.9% of the unit cell volume, see Supporting Information). Three-dimensional uranium-based-MOFs with genuine porous structures are quite rare, and to date there are only about fourteen known uranium MOFs all based on carboxylate ligands.22-28 However, it is interesting to note that the tetrauranyl core has been observed to form coordination polymers bridged by dicarboxylate anions in a similar fashion to 1,4-(DAO)Bz, and these structures contain large channels that were templated by cucurbituril molecules.17 This suggests channel formation in these structures may be a conserved feature of the tetrauranyl core as a node. (a)

(b)

(c)

(d)

Figure 3 The three-dimensional porous non-interpenetrated framework of UMOFUA viewed down all three crystallographic axes: a-axis (a), b-axis (b), and c-axis (c). Extended UMOFUA


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viewed down the a-axis (d). The pores are occupied by chlorides and water molecules which have been omitted for clarity. A full material properties characterization of UMOFUA is outside the scope of this study, but we do note some structural features that have intriguing implications for applications of amidoxime-linked uranyl MOFs. The tetrauranyl core, which as mentioned above is not unique among uranyl structures, acts as a six-connected radiating node for which we are unaware of any transition metal analogs. Furthermore, the structure could conceivably be made cationic or anionic via (de)protonation of the noncoordinating nitrogen atoms, which could lead to some very interesting capabilities as an ion exchange material. In conclusion, we have synthesized a rare uranium-based metal-organic framework from the reaction of uranyl chloride with the diamidoxime ligand, 1,4-(DAO)Bz which is only possible because of the coordinative and tautomeric flexibility of the ligand. This unusual structure provides three crystallographic firsts; the first direct evidence of an amidoxime ligand coordinating to a metal in two different forms (zwitterionic and anionic), the first unidentate coordination of the anionic amidoximes with uranyl ions which has been predicted, but not yet experimentally observed, and the first six-connected radiating metal node for which we are unaware of any transition metal analogs. The speciation is not of direct relevance to applications in extracting uranium from seawater, since hydrolysis is suppressed either by the extremely low concentration of uranyl ions or the presence of strong acids or chelating agents in stripping solutions. However, this work introduces the possibility of tailoring even the exceptionally strong amidoxime-uranyl interaction for use as a tool in designing new amidoxime-functionalized materials. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.xxxxxxx. Experimental details on the synthesis and characterization of UMOFUA with 1HNMR, FT-IR, Powder X-ray Diffraction (PXRD) data, and crystal structure refinement details with additional structural figures reported in the paper.



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Accession Code CCDC 1862662 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] ORCID Manish Kumar Mishra: 0000-0002-8193-3499 Steven P. Kelley: 0000-0001-6755-4495 Robin D. Rogers: 0000-0001-9843-7494 Present Address †M.K.M.:

Department of Pharmaceutics, College of Pharmacy, University of Minnesota,

Minneapolis, MN 55455, U.S.A Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We thank the U.S. Department of Energy Nuclear Energy University Program (DOE-NEUP) for financial support (DE-NE0008427). REFERENCES 1. Choppin, G.; Rydberg, J.; Liljenzin, J. O. Radiochemistry and Nuclear Chemistry, 2nd Ed., Butterworth-Heinemann, Great Britain, 1995, p. 107. 2. Saito, K.; Miyauchi, T. J. Chemical Forms of Uranium in Artificial Seawater. Nucl. Sci. Technol., 1982, 19, 145-150. 3. Schenk, H. J.; Astheimer, L.; Witte, E. G.; Schwochau, K. Development of Sorbers for the Recovery of Uranium from Seawater. 1. Assessment of Key Parameters and Screening Studies of Sorber Materials. Sep. Sci. Technol., 1982, 17, 1293-1308. 8 ACS Paragon Plus Environment

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4. Astheimer, L.; Schenk, H. J.; Witte, E. G.; Schwochau, K. Development of Sorbers for the Recovery of Uranium from Seawater. Part 2. The Accumulation of Uranium from Seawater by Resins Containing Amidoxime and Imidoxime Functional Groups. Sep. Sci.Technol., 1983, 18, 307-339. 5. Bolotin, D. S.; Bokach, N. A.; Kukushkin, V. Yu. Coordination Chemistry and MetalInvolving Reactions of Amidoximes: Relevance to The Chemistry of Oximes and Oxime Ligands. Coord. Chem. Rev., 2016, 313, 62-93. 6. Abney, C. W.; Mayes, R. T.; Saito, T.; Dai, S. Materials for the Recovery of Uranium from Seawater. Chem. Rev., 2017, 117, 13935–14013. 7. Kang, S. O.; Vukoic, S.; Custelcean, R.; Hay, B. P. Cyclic Imide Dioximes: Formation and Hydrolytic Stability. Ind. Eng. Chem. Res., 2012, 51, 6619–6624. 8. Parker, B. F.; Zhang, Z.; Rao, L.; Arnold, J. An Overview and Recent Progress in The Chemistry of Uranium Extraction from Seawater. Dalton Trans., 2018, 47, 639–644. 9. Witte, E. G.; Schwochau, K. S.; Henkel, G.; Krebs, B. Uranyl Complexes of Acetamidoxime And Benzamidoxime. Preparation, Characterization, and Crystal Structure. Inorg. Chim. Acta 1984, 94, 323-331. 10. Barber, P. S.; Kelley, S. P.; Rogers, R. D. Highly Selective Extraction of the Uranyl Ion with Hydrophobic Amidoxime-Functionalized Ionic Liquids via η2 Coordination. RSC Adv., 2012, 2, 8526–8530. 11. Vukovic, S.; Watson, L. A.; Kang, S. O.; Custelcean, R.; Hay, B. P. How Amidoximate Binds the Uranyl Cation. Inorg. Chem., 2012, 51, 3855–3859. 12. Tian, G.; Teat, S.; Zhang, Z.; Rao, L. Sequestering Uranium from Seawater: Binding Strength and Modes of Uranyl Complexes with Glutarimidedioxime. Dalton Trans., 2012, 41, 11579–11586. 13. Kelley, S. P.; Barber, P. S.; Mullins, P. H. K.; Rogers, R. D. Structural Clues to UO22+/VO2+ Competition in Seawater Extraction using Amidoxime-Based Extractants. Chem. Commun., 2014, 50, 12054-12057. 14. Bruton, E. A.; Brammer, L.; Pigge, F. C.; Aakeroy, C. B.; Leinen, D. S. Hydrogen Bond Patterns in Aromatic and Aliphatic Dioximes. New J. Chem. 2003, 27, 1084-1094.



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For Table of Contents Use Only A Uranyl Metal Organic Framework Arising from the Tautomeric and Coordinative Diversity of 1,4-(diamidoximyl)benzene Manish Kumar Mishra,a,† Yogesh P. Patil,a Steven P. Kelley,b and Robin D. Rogersa,c* aCollege

of Arts & Sciences, The University of Alabama, Tuscaloosa, AL 35487, USA

bDepartment c525

of Chemistry, University of Missouri, Columbia, MO 65211, USA.

Solutions, Inc., P.O. Box 2206, Tuscaloosa, AL 35403, USA

Corresponding author: E-mail: [email protected] 11 ACS Paragon Plus Environment


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Synopsis: A rare uranium-based metal-organic framework results from the coordinative and tautomeric

flexibility

of

the

1,4-(diamidoximyl)benzene,

which

provides

three

crystallographic firsts; the first direct evidence of an amidoxime ligand coordinating to a metal in two different forms (zwitterionic and anionic), the first unidentate coordination of the anionic amidoximes with uranyl ions, and the first six-connected radiating metal node.


A Uranyl Metal Organic Framework Arising from ... - ACS Publications

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