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. Author manuscript; available in PMC: 2019 Aug 21.
Published in final edited form as: Chem Commun (Camb). 2018 Aug 21;54(68):9422–9425. doi: 10.1039/c8cc05160g

Uranyl dication mediated photoswitching of a calix[4]pyrrole-based metal coordination cage

Juhoon Lee a, James T Brewster II a, Bo Song b, Vincent M Lynch a, Inhong Hwang a, Xiaopeng Li b, Jonathan L Sessler a,✉
PMCID: PMC6128147  NIHMSID: NIHMS984534  PMID: 30079408

Abstract

A set of self-assembled tri- and tetrapodal metal coordination cage structures (cage-1 and cage-2, respectively) constructed from the uranyl dication (UO22+) and a dibenzoic acid functionalised cis-calix[4]pyrrole (1) are described. The inherent photochemical reactivity of the uranyl dication mediates the transformation of cage-1 to cage-2 via the activation of molecular oxygen.

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The actinide elements demonstrate remarkable chemistry that in many instances is not replicated in the case of ostensibly related transition metal or lanthanide species. The ability to support high coordination numbers and stabilise a range of accessible oxidation states often gives rise to unique chemical reactivity.1–5 Uranium in particular has emerged as a viable tool in the design of chemical transformations showing promise in the activation of CO, CO2, N2, alkenes, and alkanes.1,6,7–9 A number of elegant studies have also explored the inherent photochemical activity of the uranyl dication (UO22+), which to date has been exploited for the activation of simple alkanes,10 water,11 and molecular oxygen.12

Efforts have also been made to incorporate actinide elements into coordination polymers, metal organic frameworks (MOFs), and metal coordination cages (MCCs).13–16 However, examples of such species remain limited and generally involve polyoxometalate-type cage clusters.17 Furthermore, actinide-based metallocage systems often fail to recapitulate the broad utility demonstrated in transition metal species, such as robust and quantitative self-assembly, stimuli-responsiveness, and applicability as, e.g., sensors and molecular machines.18,19 Here we report a new structurally characterized tetrapodal uranyl-derived, calix[4]pyrrole diacid-based cage system (cage-2, Fig. 1) that is formed via a photoactivated structural transformation mediated by uranyl and molecular oxygen from an initial tripodal cage system (cage-1) whose structure was inferred from analytical data.

Figure 1.

Figure 1.

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Simplified cartoon representations of the tri- and tetrapodal cages, cage-1 and cage-2, respectively. Yellow polyhedron: uranyl ion, red dot: oxygen, arch shape: compound 1. The structure of cage-2 was confirmed by a single crystal X-ray diffraction analysis. That of cage-1 was inferred from analytical data.

In previous studies we found that calix[4]pyrrole (C[4]P) could be exploited as a secondary building unit (SBU) in conjunction with praseodymium(III) to create a metal-macrocycle framework (MMF).20 The chemically responsive behaviour between Pr(III) and fluoride anion was described, but no photochemical properties were observed. We thus envisaged that marriage of the uranyl cation with a judiciously designed calix[4]pyrrole might furnish new cage frameworks that would respond to uranyl-mediated photochemistry.

To test this possibility, the dibenzoic acid functionalised cis-calix[4]pyrrole (1) was prepared and, based upon, X-ray crystallographic analysis it was postulated to maintain the appropriate directionality of the carboxylate moieties that would allow for the preparation of MCC systems. Gratifyingly, self-assembly of cage-2 was observed in the solid state by single crystal X-ray analysis upon the reaction of calix[4]pyrrole 1 with the uranyl dication in the presence of pyridine. Single crystal of the tetrapodal structure cage-2 was isolated from aforesaid reaction solution that had been concentrated via slow evaporation while it was exposed to ambient light and air. NMR spectroscopy and electrospray ionisation mass spectrometry (ESI-MS) studies revealed that a tripodal cage-1 was initially formed in solution. The initial construct was observed to undergo a structural reorganization to a dimeric uranyl-peroxo cage-2, a process we believe is facilitated by the coordinated uranyl dication.

The synthesis of cis-calix[4]pyrrole dibenzoic acid (1) is shown in Scheme 1. Briefly, methyl 4-acetylbenzoate (2) was reacted with pyrrole as the solvent and trifluoroacetic anhydride (TFAA) to give compound 3 in 89% yield. TFAA-catalysed condensation of 3 with acetone (acetone : methanol, 1:1, v/v) gave the cis form of the disubstituted bis-(methyl benzoate) calix[4]pyrrole (4) in 9% yield after silica gel column chromatography. Saponification of the ester produced the desired compound 1 in 95% yield. Slow evaporation of a concentrated acetonitrile solution of 1 yielded single crystals suitable for X-ray diffraction analysis.

Scheme 1.

Scheme 1.

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Synthesis and crystal structure of compound 1.

Upon exposing compound 1 (7.5 μmol), uranyl nitrate (5.0 μmol) and pyridine (0.6 mmol) in acetonitrile containing 1.5% DMF (2 mL total volume) to ambient light and air, over the course of three weeks, yielded block-shape crystals suitable for single crystal X-ray diffraction analysis.

The resulting structure revealed that four calix[4]pyrrole bis-carboxylate ligands (1) are bound to two bridged uranyl-peroxo species within a tetrapodal metallo-construct, cage-2 ([(UO2)2(O2)2(1)4]4–; Fig 2). The pyrrolic NH protons of each calix[4]pyrrole subunit are hydrogen bonded to a DMF molecule and are found in the respective cone conformations (Fig. S6). The U-O (yl), the U-O (oxo) and the O-O distances were 1.74, 2,30 and 1.47 Å, respectively which are similar to previously reported crystal structures.12,24–26

Figure 2.

Figure 2.

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Top view (left) and side views (right) of the single-crystal structure of cage-2. Solvent molecules have been omitted for clarity.

As assembled, cage-2 is tetra-anionic and requires four counter cations to balance the overall charge. The X-ray crystal structure reveals that a pyridinium cation is incorporated within the “cup” of each calix[4]pyrrole (Fig 2). These pyridinium cations originate, presumably, from protonation of the pyridine base.

Evidence that 1 supports the formation of a uranyl coordination cage in solution came from 1H NMR spectroscopic analyses carried out in CD3CN containing 1.5% N,N’-dimethylformamide-d7 (DMF-d7) added to improve the solubility. As shown in Figure 3, when exposed to both uranyl nitrate and triethylamine (or pyridine; Fig. S1), a proton signal corresponding to the ortho-position (HB1) on the benzoate unit undergoes a downfield shift (ΔδHB1 = 0.44 ppm). This happens rapidly on the laboratory time scale (i.e., by the time mixing is complete and an NMR spectrum can be recorded). In contrast, no substantial chemical shift changes were observed when 1 was exposed separately to either uranyl nitrate or triethylamine (Fig. 3). The change in the chemical shift of the HB1 signal was ascribed to the proton adjacent to the carboxylate moiety becoming deshielded as a result of uranyl complexation. Downfield shifts for the NH proton and β-pyrrolic proton resonances were also observed. These changes are ascribed to hydrogen bonding between the pyrrolic NHs and the carbonyl group of the DMF-d7 co-solvent, as well as cation-π interactions between the pyrrole rings and a protonated triethylamine moiety. These changes are taken as evidence that the calix[4]pyrrole unit adopts a cone-like conformation with the benzoate anion substituents oriented in a face-to-face parallel fashion. This stands in contrast to the 1,3-conformation seen for the free calix[4]pyrrole, where the two benzoic acid subunits are oriented in opposite directions from one another (cf. Scheme 1). Indeed, the conformational lability of C[4]P from the 1,3-alternate to the cone conformation in the presence of Lewis basic anions and ammonium and related salts is well documented.21 The conformational locking seen upon uranyl complexation is expected to impart rigidity to the normally flexible ligand 1 and facilitate the formation of the coordination cage.

Figure 3.

Figure 3.

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Partial 1H NMR spectra of (a) 1 only (3.75 mM), (b) 1 + 0.66 equiv. of UO2(NO3)2·6H2O, (c) 1 + 38.22 equiv. of TEA (triethylamine), and (d) 1 + 0.66 equiv. of UO2(NO3)2·6H2O + 38.22 equiv. of TEA, recorded at room temperature in CD3CN (1.5% DMF-d7)

DOSY spectral analysis of the uranyl complex solution revealed one set of signals for the complex, which indicates only a single complex species dominates at equilibrium (Fig. 4).

Figure 4.

Figure 4.

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DOSY spectrum of 1 (3.75 mM) + 0.66 equiv. of UO2(NO3)2·6H2O + 38.22 equiv. of TEA, recorded at room temperature in CD3CN (1.5% DMF-d7).

Initial ESI-MS analysis reveal a species consistent with the proposed tripodal structure (i.e., cage-1) with a peak at 2456.36 amu (cf. Fig S5, where three calix[4]pyrroles are bound to two separate uranyl dication via the benzoate ligands, is a major component in solution at initial time points. In light of the aforementioned NMR spectroscopic studies, we suggest that in this structure, the calix[4]pyrrole bis-carboxylate ligands occupy equatorial positions around the uranyl cation and are present in a 3:2 (ligand: uranyl) ratio within a tripodal construct, namely cage-1 ([(UO2)2(1)3]2–, Fig. 1). As formed, cage-1 is expected to bear an overall 2- charge. This charge is presumably balanced by triethylammonium or pyridinium cations formed through protonation of the added bases.

Taking these findings into consideration, we conclude that the reaction of compound 1 and uranyl in the presence of base forms cage-1. The dimeric uranyl-peroxo species, cage-2, is then thought to arise from cage-1 through uranyl-mediated photoactivation of molecular oxygen. Similar processes are well documented.24–29 Moreover, Cahill and co-workers have postulated similar mechanistic explanations in the case of their studies of related transformations; these latter suggestions were based in part on 1H NMR spectral studies.12,23

Uranyl-peroxo species have been previously prepared by reacting H2O2 with the uranyl cation.22 An ESI-MS analysis of cage-1 mixed with H2O2 revealed peaks consistent with the formation of cage-2 (Fig 5). These findings provide support for the notion that the observed transformation of cage-1 into cage-2 occurs upon exposure of the starting cage to ambient light in the presence of air; it can also be induced via direct exposure to hydrogen peroxide, although this latter method yields a mixture of products.

Figure 5.

Figure 5.

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ESI-MS of cage-2. MeOH adducts were attributed to the solvent utilized in the ESI measurements.

As noted above, cage-1 and cage-2 are formally anionic. They are thus soluble in methanol allowing high resolution ESI-MS analyses to be carried out. In the case of cage-1, a doubly charged negative ion peak at m/z = 1227.5 was seen. The isotope pattern agreed well with theoretical modelling and was found to correspond to a chemical composition of [C120H114N12O16U2]2– as expected for cage-1. After mixing cage-1 with H2O2, a quadrupally charged negative ion peak at m/z = 924.3 was observed. In this case, the isotope pattern proved consistent with theoretical modelling and was found to correspond to the chemical composition expected for cage-2, namely [C160H152N16O28U4]4–. ESI-MS analyses thus provide support for the suggestion that both cage-1 and cage-2 may be obtained under solution phase conditions. Note that in the mass spectra of both cage-1 and cage-2, we observed signals at + 32 and +64 amu relative to the dominant peaks. These latter features are attributed to formation of adducts containing one and two methanol molecules, respectively, under conditions of the mass spectrometric ionisation.

In summary, we report a new responsive coordination cage system (cage-1) that is readily prepared from a meso-substituted cis-dibenzoic acid calix[4]pyrrole (1) and a uranyl dication source. By harnessing the inherent chemical reactivity of the uranyl dication, this dianionic complex may be converted readily to a larger uranyl-bridged peroxo tetrapodal cage structure (cage-2). This conversion can be induced by exposing cage-1 air and ambient light or via deliberate treatment with hydrogen peroxide. The present study serves to illustrate further the relatively untapped chemical reactivity of the actinide elements and their potential utility in creating stimulus-responsive and structurally modifiable metal coordination cages.

The work in Austin was supported by the Office of Basic Energy Sciences, U.S. Department of Energy (DOE) (grant DE–FG02–01ER15186 to J.L.S.). Support from the Robert A. Welch Foundation (F–0018 to J.L.S.) is also acknowledged. X.L. gratefully acknowledges the support from NSF (CHE-1506722) and the National Institutes of Health (1R01GM128037). J.T.B. would like to thank The University of Texas at Austin Environmental Science Institute for a Scientist in Residence (SciRes) Fellowship.

Supplementary Material

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Footnotes

Conflicts of interest

There are no conflicts to declare.

† Electronic Supplementary Information (ESI) available: Synthetic details, experimental procedures, and characterization data. Structural details for 1, and cage-2. CCDC 1850738–1850739. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/x0xx00000x

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Supplementary Materials

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NIHMS984534-supplement-cif.cif (957.7KB, cif)
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NIHMS984534-supplement-esi.pdf (2.1MB, pdf)

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