Anion-Coordination-Driven
Assembly of Anionic Hexagonal and Square Architectures and the Structural Interconversion
Cong Zhao1†, Jie Zhao1†, Dong Yang1*, Tanya K. Ronson2, Le Yu1, Huizheng Zhang1, Wenyao Zhang1, Fen Zhao1, Wei Sun1, Xiao-Juan Yang1& Biao Wu1*
1Key Laboratory of Synthetic and Natural Functional Molecule of the Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710069,2Department of Chemistry, University of Cambridge, Cambridge CB2 1EW
*Corresponding authors:[email protected];[email protected];†C. Zhao and J. Zhao contributed equally to this work.
Cite this: CCS Chem. 2021, 3, 1990–1999
A biphenyl-bridged bis-tris(urea) ligand L was ratio- nally designed with a favorable angle to construct a hexagon-shaped A6L6 (A = anion) complex upon assembly with phosphate anions (PO43−) via anion- coordination-driven assembly (ACDA). Due to the moderate flexibility of L, another well-defined dis- crete architecture, a square-like A4L4 complex, has also been obtained from ligand L and PO43−. Inter- conversion between these two self-assemblies can be readily realized by solvent regulation. In addition, the two anionic architectures display different bind- ing abilities for selected cationic guest molecules, enabling the uptake of a desired guest from a mixture of guests.
Keywords: tris(urea), anion coordination, self- assembly, supramolecular transformation, host– guest chemistry
Introduction
Self-assembly from relatively simple components allows efficient construction of complex supramolecular archi- tectures.1–5Metal-coordination-driven macrocycles have attracted great interest over the past few decades, not only for their aesthetic attributes but also because of their applications in recognition,6 biomedicine,4,7
holographic storage,8 and so on.9–11 Many successful strategies have been developed for the achieve- ment of such polygons as triangles,12,13 squares,14,15 pentagons,16–19 hexagons20–25 and even octagons26 and larger systems.27–29 Among the different macrocycles, hexagonal patterns are prevalent in nature, existing from the benzene ring to the bees’ honeycomb to the Giant’s Causeway in Northern Ireland. Metal-coordination-driven
hexagon-shaped architectures can be achieved by dif- ferent methods,20–24 including bis-monodentate ligands with unsaturated transition-metal complex fragments,23 bidentate binding sites with tetrahedral/octahedral metal ions (Ag+, Cu2+, Fe2+ for example),20,22,24 and bis-tridentate ligands with octahedral metal ions (such as Ru2+, Fe2+, or Cd2+).21,30–32 The latter approach, where the two tridentate binding sites (such as terpyridine) must necessarily be connected by a linker with an angle of 120° to construct the structurally rigid hexameric macrocycles, is less frequent, possibly due to the syn- thetically challenging nature of the ligands and the larger metallocycle. Nevertheless, using higher-hapticity triden- tate coordinate sites for the self-assembly of hexagons requires fewer components and exhibits stronger bind- ing ability than the monodentate and bidentate counter- parts and may afford more predictable outcomes, thus deserving deeper exploration and understanding.
Considerable effort has been devoted to elucidating the principles underpinning the self-assembly of dis- crete structures. Even though the construction of well-defined supramlecular architectures utilizing anion-coordination-driven assembly (ACDA) has so far not been as extensively studied as conventional metal-coordination,33–39 studies on the assembly of oligo-urea ligands and anions have demonstrated that the coordination behavior of the phosphate anion is analogous to that of octahedral transition metals.40–44 This tetrahedron-shaped phosphate anion (PO43−) can readily form both AL3complexes with the “bidentate”
bis(urea) moiety and AL2fragment with the“tridentate”
tris(urea) coordination sites.44,45 However, while suc- cessful assembly of a series of anionic architectures based on bis(urea) moieties and phosphate (through the AL3coordination motif) has been achieved, only one discrete A4L4 (A = anion) square-shaped macrocycle constructed from tris(urea) and phosphate anions (through the AL2motif) has been reported. Expanding anionic assemblies, especially from tris(urea) building blocks, still needs more effort in order to approach the complexity of biological molecules.
Encouraged by our previous work,45we aim to further explore the predictable geometry of anion-coordinated complexes for the programmable construction of new topologies from the tris(urea) building block, which has displayed strong coordination affinity to phosphate. By examining the single crystal structures of our previously reported tris(urea) complexes,44,45 we observed angles between the linkers and PO43− ranging from 73.7° to 115.8° (Scheme 1a). However, changing the position of the connection (represented by R in Scheme1) between the two tris(urea) groups could alter the angle to 108.2– 125.7° (Scheme1b), revealing the possibility of controlling the assembled structure by adjusting the position of the linker. The biphenyl-bridged bis-tris(urea) ligand L (Supporting Information Scheme S1 and Figures S1–S6)
was thus designed to construct a hexagon-shaped phos- phate complex by coordination with PO43− ions as the internal angles of a regular hexagon are 120°. To our surprise, due to the moderate flexibility of L, we could also achieve a square-like phosphate complex by chang- ing the solvent. The host–guest properties of the assem- blies were investigated with a selection of cationic guests revealing marked differences in the binding preferences of the two assemblies.
Experimental Methods
All chemical reagents were purchased from commercial sources and used as received. All solvents and other reagents were of reagent grade quality. NMR spectra were obtained at 298 K by using Bruker AVANCE III 400/600 MHz (Bruker, Switzerland) spectrometers un- less noted otherwise.1H and13C NMR chemical shifts were reported relative to residual solvent peaks [1H NMR:
2.50 ppm for dimethyl sulfoxide (DMSO)-d6, 1.94 ppm for CD3CN and 7.26 ppm for CDCl3respectively;13C NMR:
39.5 for DMSO-d6]. Electrospray ionization mass spec- trometry (ESI-MS) measurements were carried out using a Bruker Daltonics micrOTOF-Q II (Bruker Daltonics Corp., Bremen, Germany) mass spectrometer. UV–vis spectra were done on Agilent Cary-100 (United States) spectrometer. Fluorescence spectra were recorded by a Horiba Fluorolog-3 (HORIBA Scientific, Canada) spec- trometer. Single-crystal diffraction analyses were done on a Bruker D8 Venture photon II (Bruker, Germany) diffractometer. Experimental details for all new com- pounds and anionic assemblies with synthesis, charac- terization, and crystal diffraction data (CIF) are available in theSupporting Information.
Results and Discussion
Synthesis and characterization of A
6L
6hexagonal 1 and A
4L
4square 2
Hexagon 1 was synthesized by treating L with 1.0 equiv of [K([18]crown-6)]3PO4in acetonitrile. After stirring over- night at room temperature, a clear solution was obtained.
Different cations (such as TMA+, TBA+, TPA+, and PPh4+) were added for crystallization separately. Slow vapor diffusion of diethyl ether into one of these solutions provided yellow crystals suitable for single-crystal X-ray diffraction (Supporting Information Table S7). The result- ing structure showed an A6L6hexagon-shaped architec- ture [K([18]crown-6)]17[PPh4][(PO4)6(L)6] (Figure 1a and Supporting Information Figures S69 and S70), with six PO43− ions positioned at the corners with alternative ΔΛΔΛΔΛ configurations and six ligands located at the edges, which were held together by 72 hydrogen bonds (Supporting Information Table S8). The PO43−···PO43−sep- aration distances ranged from 14.5 to 14.9 Å, exhibiting RESEARCH ARTICLE
crystallographic centrosymmetry with P–P–P angles of 94.9°, 107.4°, and 108.7° (Figure 1b). Four of the PO43−
ions were approximately coplanar with the remaining two located above and below the plane with an angle of 62.7°, composing a chair conformation (Figure1c). The angles between the linkers and PO43−ranged from 115.0° to 120.5°
(Figure1d).
Notably, all 12 monourea arms (i.e. the shorter portion of each tris(urea) unit) were laid inside the hexagon with six of them pointing upward from the hexagon plane and the other six pointing downward. The 12 monourea arms together with the biphenyl moieties of the ligand formed a discus-shaped cavity with a radius of 1.2 and 1.3 nm thick at its center. Due to the limited resolution of the crystal data, only seven [K([18]crown-6)]+countercations and one tetraphenylphosphonium ion (added as PPh4Br for crystallization) were found to be distributed around the hexagon (Supporting Information Figure S71).
Nevertheless, the anionic hexagonal backbone could be unambiguously established. This is the first example of hexagon-shaped A6L6macrocycle by ACDA. In addition, in this case a connective linker with an angle of 120° was not mandatory for the tris(urea) anion binding ligands.
An A4L4 complex (2) was also achieved by mixing L and 1.0 equiv of [K([18]crown-6)]3PO4 in chloro- form. Single-crystal X-ray diffraction analysis proved the formation of the square complex [K([18]crown- 6)]12[(PO4)4(L)4] (Figure 2a and Supporting Information Table S7 and Figures S72 and S73). Four PO43−ions were located at the vertices and four ligands lay on the edges of the square, respectively. Pairs of adjacent PO43− centers showed the opposite chirality, and the square was achiral with ΔΛΔΛ configurations.
Each PO43−ion was coordinated with two tris(urea) sub- units from two adjacent ligands through 12 hydrogen bonds (Supporting Information Table S9). The Scheme 1 | (a) The angles between the linkers and PO43−in the PO43−complexes assembled from tris(urea) ligand and PO43−. (b) Design of biphenyl-bridge bis-tris(urea) ligandL and illustration of the hexagonal A6L6complex1 and the square A4L4complex2, as well as the interconversion between these two self-assemblies.
PO43−···PO43− separation distances varied from 13.5 to 13.6 Å and the four PO43− ions were noncoplanar with P–P–P angles of 85.3° and 86.6° (Figures2band2c).
Compared to our previously reported A4L4grid com- plex formed from a bis–tris(urea) ligand in which the tris (urea) binding site was connected from the end by a more flexible p-xylylene linker,44 this A4L4 square
exhibited crystallographic C2 symmetry. One of the PO43− ions (P2 or P2′) deviated greatly from the plane formed by the other three PO43− ions with an angle of 42.3° (Figure 2c), showing butterfly conformation. Four alternating monourea groups pointed inside the square, while the other four were laid outside the square. Mean- while, four of the [K([18]crown-6)]+counter cations were located inside the square and interacted with the oxygen atoms of the terminal nitro groups of the ligands, the other eight were outside the square coordinating with both the terminal nitro groups and the oxygen atoms of urea groups of the ligands (Supporting Information Figures S72 and S74a). The angles between the linkers and PO43−were 115.2° and 116.8° (Figure2d), which were slightly smaller than that in hexagonal 1. This result indi- cates that the curvature of the ligand could compensate for the insufficiency in the angle of phosphate coordina- tion for the formation of A4L4square 2.
The 1H NMR spectrum of 1 was recorded in CD3CN (Supporting Information Figures S8–S10). Compared to the free ligand, large downfield shifts of all urea NH protons (1H NMR of ligand L were measured in DMSO- d6 for solubility reasons, Figures 3b and 3c) were ob- served, indicating strong hydrogen bonds between PO43−
and the urea groups. 2D NMR spectra provided further evidence for the formation of 1 in solution (Supporting Information Figures S14–S16). Cross-peaks were ob- served between H3 of the o-phenylene rings and H8/10 of the nitrophenyl ring in the nuclear overhauser effect spectroscopy (NOESY) spectrum, revealing strong through-space interactions in solution (Supporting Figure 2 | (a) Crystal structure of the A4L4square2. Red
lines connect PO43−centers to highlight the square frame- work. (b) Top view. (c) Side view of the square frame- work. The relative positions of each PO43−are the same in (a) and (b). (d) The angles between the linkers and PO43−
in2. Solvent molecules and counterions of crystallization are omitted for clarity.
Figure 1 | (a) Crystal structure of the A6L6 hexagon1. Red lines connect PO43− centers to highlight the hexagonal framework. (b) Top view. (c) Side view of the chair conformation. The relative positions of each PO43−are the same in (a) and (b). (d) The angles between the linkers and PO43−in1. Solvent molecules and counterions of crystallization are omitted for clarity.
RESEARCH ARTICLE
Information Figure S15). The result was consistent with the solid-state structure. Diffusion ordered spectroscopy (DOSY) confirmed the formation of a single species with a diffusion coefficient of 3.0 × 10−10 m2s−1 (Supporting Information Figure S16), corresponding to an approxi- mate solvodynamic radius of 2.1 nm46,47 and consistent with the size in solid state (2.4 nm measured from crystal structure). The [K([18]crown-6)]+counterions exhibited a different diffusion coefficient (7.0 × 10−10m2s−1) from the ligand, indicating that the counterions were not as- sociated with 1 in solution. High-resolution ESI-MS further demonstrated the existence of the A6L6species with a peak for [(PO4)6(L)6([K([18]crown-6)]11)]7−, observed at m/z 1534.64 versus calculated 1534.42 (Supporting Information Figure S20 and Table S1).
The formation of 2 in solution was also confirmed by one-dimensional (1D,Supporting Information Figures S11– S13) and two-dimensional (2D, Supporting Information Figures S17–S19) NMR spectra. All the urea NH groups in 2 showed significant downfield shifts compared to the free ligand (Figures3aand3c) upon coordination to PO43−ions.
It is worth noting that all protons of 2 split into two sets of signals in the1H NMR spectrum due to the asymmetrical structure of the square in solution (Supporting Information Figure S13). The asymmetry could result from the distribution of the monourea groups in ligand L, one point- ing inside the square and the other lying outside.
Cross-peaks were observed between H4 of the o-phenylene rings and H10/11 of the nitrophenyl ring in the NOESY spectrum (Supporting Information Figure S18).
DOSY showed that all the split signals belonged to a single species with a diffusion coefficient of 2.5 × 10−10 m2 s−1 (Supporting Information Figure S19), which corresponded
to an approximate solvodynamic radius of 1.6 nm, consis- tent with the size in solid state (1.6 nm measured from crystal structure). The [K([18]crown-6)]+counterions were associated with 2 in this case. The possible reason could be the existence of stronger interactions between the [K([18]
crown-6)]+counterions and 2 which were favored by the less polar solvent. The high-resolution ESI-MS of 2 exhib- ited intense signals for various [A4L4] species, including those at m/z= 1411.82 (x = 7) versus calculated 1411.78, m/
z = 1840.78 (x = 8) versus calculated 1840.76, and m/
z= 2555.33 (x = 9) versus calculated 2555.38 correspond- ing to the species [(PO4)4L4([K([18]crown-6)])x](12–x)−
(Supporting Information Figure S21 and Table S2). Density functional theory (DFT)-minimized structures of 1 and 2 also predicted the stable existence of the A6L6hexagon and A4L4 square-shaped structures, respectively (Supporting Information Figures S75 and S76).
Structural interconversion
Due to the formation of A6L6hexagon 1 and A4L4square 2 in different solvents, the interconversion between 1 and 2 by solvent regulation was subsequently studied. 1H NMR was utilized to study the conversion of 2 to 1 by changing the volume ratio of the deuterated solvents, that is, CDCl3 and CD3CN (Figure 4 and Supporting Information Figure S22). Addition of CD3CN to the solu- tion of 2 in CDCl3resulted in gradual structural transfor- mation. Upon increasing the ratio of CD3CN, a new set of peaks which belong to 1 gradually appeared (marked with red color in Figure4), and the signals belonging to 2 decreased at the same time. The A4L4square 2 largely transformed into 1 after the volume ratio of CD3CN Figure 3 |1H NMR spectra (400 MHz, 298 K) of (a)2 in CDCl3. (b)1 in CD3CN; (c)L in DMSO-d6.
exceeded 50% and completely converted into A6L6 hexagon 1 in CD3CN (100%) solution.
UV–vis absorption spectra of 1 (in CH3CN) and 2 (in CHCl3) were also studied (Supporting Information Figure S23). Two strong absorption peaks (240 and 354 nm) were observed for 1, while three absorption peaks (248, 292, and 368 nm) were observed for 2. The possible reason for the difference of UV–vis absorption spectra of 1 and 2 may be the difference in the coordination environment of phosphate.48The change in the absorp- tion peaks of the two structures in different solvents enabled the structural transformation to be studied by UV–vis spectra. The UV–vis spectra of 2 were then collected in solvent mixtures of CH3CN and CHCl3
with different volume ratios (Supporting Information Figure S24). Upon increasing the volume ratio of CH3CN, the two absorption peaks at 292 and 368 nm gradually merged to one peak, which was consistent with the UV–vis absorption spectrum of 1 in CH3CN. The changes in the UV–vis spectra give more evidence of the structural transformation between 1 and 2.
Host –guest chemistry
We hypothesized that the anionic frameworks of 1 and 2 may be suitable hosts for large cationic guests. Methy- lene blue (MB) is one of the most used chemical indi- cators and dyes in various industries, resulting in effluents which form a major source of environmental pollution.49Hence, it is necessary to bind and separate such pollutant organic dyes from industrial wastewater.
Wefirst investigated the binding abilities of host 1 with MB. NMR, UV–vis, and fluorescence were utilized to study the host–guest properties. Obvious upfield shifts (ΔδHβ= −0.74, ΔδHγ= −1.13 ppm;Supporting Information
Figures S25 and S26) corresponding to the trapped MB were observed after addition of 1.0 equiv of MB to 1 (0.5 mM) in CD3CN. Cross-peaks were observed be- tween Hα (CH3) of MB and H10 of L in the NOESY spectrum, indicating interactions between 1 and the trapped MB (Supporting Information Figures S39 and S40). The binding constant Ka between 1 and MB was determined to be (1.38 ± 0.07) × 103 M−1 in CD3CN (Supporting Information Figure S62). DOSY indicated that the 1H NMR peaks assigned to 1 and MB exhibited the same diffusion rate, proving the formation of only a single species (Supporting Information Figure S42).
UV–vis and fluorescence studies were further carried out in CH3CN (Supporting Information Figures S57 and S58). The absorption peak of MB at 657 nm showed a redshift to 668 nm, revealing the existence of a host–guest interaction. Fluorescence quenching of MB could be observed when 1 was added to a CH3CN solu- tion of MB, that is, formation of the 1 MB complex.
A possible explanation is the existence of photoinduced electron transfer (PET) between host and guest.50–52
In addition to MB, another pollutant [Rhodamine 6G (Rh6G)] and two guest molecules with similar size to MB [1,4-benzyl-1,4-diazabicyclo[2.2.2]octane bromide (BnDBn2+ and Supporting Information Figure S7) and tetraphenylphosphonium bromide (PPh4+)] were also studied for comparison (Figure5).1H NMR spectra were recorded upon addition of 1.0 equiv of guest molecules (Rh6G, BnDBn2+, and PPh4+) to 1 (0.5 mM) in CD3CN, and different degrees of upfield shifts corresponding to the trapped guests were observed (Supporting Information Figures S25–S34 and Tables S3 and S4). Cross-peaks were also observed between protons of these guests and H10/11 of the nitrophenyl ring of L in NOESY spectra, indicating host–guest interactions (Supporting Information Figures S39–S52). DOSY spectra confirmed Figure 4 |1H NMR spectra (400 MHz, 298 K) demonstrat-
ing the structural conversion from square2 (0.5 mM) to hexagon1 by changing the ratio of CD3CN/CDCl3.
Figure 5 | Prospective cationic guests employed in host–guest studies. Guests which were observed to bind to1 (blue box) and 2 (red box).
RESEARCH ARTICLE
the formation of 1⊃ guest complexes and the diffusion coefficients are summarized in Supporting Information Table S5. The binding constants for these guest mole- cules were determined by 1H NMR titrations in CH3CN (Supporting Information Figures S61–S68 and Table S6).
The host–guest interactions between square 2 and the aforementioned guest molecules were also studied. 1H NMR spectra were recorded upon addition of 1.0 equiv of the guest molecules to 2 (0.5 mM) in CDCl3(Supporting Information Figures S37 and S38). All the guest mole- cules showed extremely small chemical shift changes except for MB (Supporting Information Figures S35 and S36). Hα (CH3), Hβ, and Hγ of MB displayed evident upfield shifts (ΔδHα= −0.50, ΔδHβ= −0.92, ΔδHγ= −0.88;
Supporting Information Figure S35), attributed to the shielding effect of the aromatic rings of 2. NOESY spectra (Supporting Information Figures S53–S55) indicated the formation of 2⊃ MB due to the observed cross-peaks between Hα (CH3) of MB and H3/4/5 of L. The existence of a single species was further confirmed by DOSY, from
which the same diffusion rate for host and guest was observed (Supporting Information Figure S56). Similar to 1⊃ MB, the UV–vis absorption peak of MB at 657 nm red shifted to 668 nm in UV–vis spectra, and fluorescence quenching of MB occurred after interaction with 2 (Supporting Information Figures S59 and S60).
The difference in binding ability of 1 and 2 for the same guest molecules enabled the selective encapsulation of specific guest molecules by structural transformation.
Guest molecules MB and BnDBn2+ were chosen for this study. 1.0 equiv of MB and 1.0 equiv of BnDBn2+ were added to 1 in CD3CN and 2 in CDCl3, respectively (Figures6band6e). Both MB and BnDBn2+proton signals experienced upfield shifts in the presence of 1 in CD3CN, while only those of MB upfield shifted in the presence of 2 in CDCl3(Figures6cand6d). MB and BnDBn2+could be encapsulated by 1 with a distribution ratio of 47 : 53 (Figure 6c), showing 1 has no selectivity for these two guest molecules. However, 2 could bind MB selectively.
The results suggest that conversion from 1 to 2 could Figure 6 | (a) Illustration of selective guest binding by1 and 2. (b–e)1H NMR spectra (400 MHz, 298 K) of (b)1 in CD3CN; (c)1 with 1 equiv MB and 1 equiv BnDBn2+in CD3CN; (d)2 with 1 equiv MB and 1 equiv BnDBn2+in CDCl3(e)2 in CDCl3.
enable a desired guest to be trapped from a mixture of guests (Figure6a).
Conclusion
We have described the solvent-controlled assembly of anion-coordination-driven A6L6 hexagon 1 and A4L4
square 2 from a biphenyl-bridged bis-tris(urea) ligand L and phosphate anions. The favorable angle of the ligand ensures the formation of the hexagon, while the moderate flexibility allows structural transformation to the square by modulation of the volume ratio of chloro- form and acetonitrile. The hexagon 1 shows no selectivity in the encapsulation for MB, Rh6G, BnDBn2+, and PPh4+, while the square 2 only binds MB, revealing the possibility of encapsulation and release of a desired guest from a mixture of guests by conversion from one host to anoth- er. The results shed light on the important role of solvent in directing the outcomes of ACDA processes and ex- pand the supramolecular toolkit for stimulus-induced transformations between self-assembled architectures.
Future work will focus on exploring the functions of these and related assemblies and their incorporation into sti- muli responsive purification systems.
Supporting Information
Supporting Information is available and includes com- plete experimental details, Cartesian coordinates, and X-ray data for 1 and 2.
Con flict of Interest
The authors declare no conflict of interest.
Funding Information
This work was supported by the National Natural Science Foundation of China (nos. 21971210, 91856102, and 21772154) and the Natural Science Foundation of Shaanxi Province (no. 2019KJXX-062).
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