Interconvertible vanadium-seamed hexameric pyrogallol[4]arene nanocapsules.

Research into stimuli-responsive controlled self-assembly and reversible transformation of molecular architectures has received much attention recently, because it is important to understand and reproduce this natural self-assembly behavior. Here, we report two coordination nanocapsules with variable cavities: a contracted octahedral V24 capsule and an expanded ball-shaped V24 capsule, both of which are constructed from the same number of subcomponents. The assemblies of these two V24 capsules are solvent-controlled, and capable of reversible conversion between contracted and expanded forms via control of the geometries of the metal centers by association and dissociation with axial water molecules. Following such structural interconversions, the magnetic properties are significantly changed. This work not only provides a strategy for the design and preparation of coordination nanocapsules with adaptable cavities, but also a unique example with which to understand the transformation process and their structure-property relationships.

Introduction

The design and synthesis of discrete metal-organic nanocapsules (MONCs) with specific geometries and cavities have been investigated extensively due to not only their interesting structures[1-3], but also their promising applications in supramolecular chemistry[4-6] and material science[7-9]. To date, a large number of MONCs have been synthesized from metal ions or metal clusters with different coordination environments and organic linkers with various shapes[10-12]. Of particular recent interest is control of the self-assembly of the MONCs by external stimuli[13,14] including light[15,16], electricity[17], pH[18], guests[19], and solvents[20]. Studies of this self-assembly may help us to understand and further mimic stimuli-responsive structural reorganization processes in biological systems. However, the structural transformations of the reported stimuli-responsive MONCs are usually accompanied by major changes in the species and number of subcomponents including metal centers and ligands. In contrast, exploration of stimuli-responsive MONCs with equal subcomponents[21,22], or MONC quasi-isomers[23,24] with the same metal centers, but some different coordinated components[15,16], which act similarly to natural macromolecules is still in its infancy. Recognition of the reversible structural interconversion between such isomers or quasi-isomers will not only provide new approaches to broaden the preparation of MONCs with different shapes, but also an understanding of their structure–property relationships, such as host–guest recognition, drug delivery and release, and supramolecular catalysis[16,25,26].

C-alkylpyrogallol[4]arenes (abbreviated as PgC, where n is the length of the associated alkyl tail), which are vase-shaped macrocyclic host molecules composed of 1,2,3-trihydroxybenzene units, have been determined over the past decade to be versatile building blocks for the construction of supramolecular complexes[27]. For example, PgC can assemble itself to form isolated MONCs[28,29], hydrogen-bonded capsules[30,4]arene nanocapsules: Structural and fluorescence studies. Angew. Chem. Int. Ed.. 2006 ">31], hydrogen-bonded/metal-organic nanotubes[32,33], and supramolecular organic frameworks[34].

Since the initial discovery by Atwood et al. in 2005[35] of PgC-based MONCs constructed from six PgC units and 24 Cu2+ ions, a number of studies have demonstrated that PgC can self-assemble into octahedral hexameric M24 (M=Mg, Co, Ni, Cu, and Zn)[36-40], spherical dimeric M8 (M=Co, Ni, Cu, and Zn)[41-43], “rugby ball” shaped hexameric Ga12[44] or mixed nanocapsules[45-47]. Interestingly, spherical Cu8 and Zn8 dimers can be linked by 4,4′-bipyridine ligands into a one-dimensional coordination polymer[48] and an MOF-like structure[49], respectively. However, PgC-based MONCs are limited to the aforementioned metal ions, and still have the possibility of synthesizing new PgC-based MONCs and exerting control over their self-assembly behavior.

Vanadium is of particular interest in this context owing to its various coordination behaviors, valence states, and promising applications in areas such as magnetism[50-52], optics[53], and catalysis[54]. Currently, the number of known vanadium capsules is limited[55-58] and here we report an interesting example of solvent-responsive assembly of coordination nanocapsule quasi-isomers with distinct geometries. This includes a contracted octahedral capsule (V24-oct) with the inner cavity of 1000 Å3, and an expanded ball-shaped capsule (V24-ball) with inner cavity of 1400 Å3, from the same number of subcomponents including 24 vanadium centers and 6 pyrogallol[4]arene units (Fig. 1). These two V24 capsules represent an example of a metal displaying versatility and forming different PgC-based hexamer capsules.

Fig. 1

Controlled self-assembly and interconversions of V24 capsules. Chemical structure of hexameric pyrogallol[4]arene V24 octahedron and V24 ball from 6 PgC3 ligands and 24 vanadium ions. Vanadium is green, oxygen red and carbon blue

Results

Synthesis and characterization of V24 octahedron and ball

C-Propylpyrogallol[4]arene (PgC3, Fig. 1) was synthesized as reported by Gerkensmeier et al.[59] by a condensation reaction of pyrogallol and butanal catalyzed by concentrated hydrochloric acid. The reaction of PgC3 with VOSO4·5H2O in CH3CN/H2O solution (10:1, v/v) at 80 °C for three days yields green rhombic crystals of V24-oct-α with the formula [V24O24(H2O)24(C40H40O12)6]∙(solvent). Single-crystal X-ray diffraction analysis shows that the V24-oct-α crystallizes in the trigonal system with space group R-3 and consists of 6 PgC3 units and 24 V ions arranged in 8 trinuclear V3 clusters capping the face of the octahedron (Fig. 2a). The overall geometry of this capsule is similar to the previously reported octahedral hexameric M24 (M=Mg, Cu, Co, Ni, and Zn) capsules[36-40]. Inspection of the crystal structure of V24-oct-α reveals that each V3 cluster is held together by three pyrogallol (Pg) units from different bowl-shape PgC3 ligands (Fig. 2b). The angle between the two opposite upper-rim oxygen atoms and the lower-rim centroid at the base of the PgC3 ligand is about 108.9° and the separation between two opposite faces of the octahedron, measured from the opposite centroids of the V3 clusters is approximately 14.3 Å (Supplementary Figures 4 and 5). These three V centers adopt octahedral geometries, and each one is coordinated with four phenoxyl oxygen atoms from two different PgC3 ligands, one interior water molecule, and one exterior oxygen atom (Fig. 2c). Further analysis shows that three V centers, situated at the vertices of an approximately equilateral triangle, in which V···V distances are in the range of 3.752–3.763 Å, are linked by three phenoxyl oxygen atoms to form a planar V3O3 array. In this array, the V···O distances range from 1.987–2.001 Å, the O−V−O angles range from 98.18–99.09°, and the V−O−V angles range from 140.08–141.17°. The capsule contains an internal cavity with a volume of ~ 1000 Å3, calculated using VOIDOO with a probe radius of 1.2 Å. Bond valence sum (BVS) calculations and EPR analysis reveal that the vanadium centers in V24-oct-α are at +4 oxidation states (Supplementary Table 2 and Supplementary Figure 1). In addition, the IR spectrum of V24-oct-α shows the characteristic V═O band in the frequency range 950–990 cm−1 (Supplementary Figure 2). It is interesting that the introduction of N,N-dimethylformamide (DMF) to the reaction of V24-oct-α produces its polymorph V24-oct-β which crystallizes in the triclinic space group P-1.

Fig. 2

Structural representations of V24-oct-α from X-ray diffraction data. a Molecular structure of V24-oct-α. b Metal-ligand arrangement and c coordination geometries of V ions within the capsule

Interestingly, changing the CH3CN/H2O solvent in the preparation of V24-oct-α to NMF/CH3OH (1:1, v/v; NMF=N-methylformamide) affords green tetragonal prism crystals of V24-ball-β: [V24O24(C40H40O12)6]∙(solvent), which crystallizes in the tetragonal space group P4/mnc and contains the same number of components as V24-oct-α (Fig. 3a). This capsule can be regarded as an expanded structure of the V24-oct-α for two main reasons. The angle between the two opposite upper-rim oxygen atoms and the lower-rim centroid at the base of the PgC3 ligand has expanded to 123.1° and the separation between two opposite centroids of V3 clusters in this ball increases to 16.7 Å, compared to V24-oct-α (Supplementary Figures 4 and 5). As a result of these expansions, the inner cavity volume of the V24-ball-β increases to ~ 1400 Å3, which is ~ 400 Å3 larger than the cavity in V24-oct-α. Upon close examination, their structural transformations can be seen to be due to the coordination geometry differences in V centers. In this case, all the V ions are five-coordinated in square-pyramidal coordination geometries and coordinated by four phenoxyl oxygen atoms from two different PgC3 ligands and one exterior oxygen atom (Fig. 3c). The changes of coordination geometry in the V ions have a large influence on the bond angles and the shape of the V3O3 array from the aforementioned V24-oct-α. Specifically, the V3O3 array in V24-ball-β is concavoconvex with V···O distances ranging from 1.953–2.037 Å, O−V−O angles ranging from 88.52–89.43°and V−O−V angles ranging from 134.38–137.12° (Fig. 3b). Except for the V···O distances, it is clear that the O−V−O and V−O−V angles in this capsule are much smaller than those in V24-oct-α quasi-isomer. BVS calculations and EPR analysis together with IR spectra reveal that the vanadium centers in V24-ball-β are at +4 oxidation states with a VO2+ form (Supplementary Table 3 and Supplementary Figures 1 and 3), which are the same to those in V24-oct-α. Whereas the structural differences in previously reported MONC isomers and quasi-isomers arise from the plasticity of the ligands[15,16,21,22], these two different types of V24 capsules represent an example of MONC quasi-isomers whose structural differences stem from the coordination diversity of metal centers. By replacing the NMF with DMF in the same reaction, its polymorph V24-ball-α was obtained and was found to crystallize in a cubic system with the space group Ia-3.

Fig. 3

Structural representations of V24-ball-β from X-ray diffraction data. a Molecular structure of V24-ball-β. b Metal-ligand arrangement and c coordination geometries of V ions within the capsule

Interconversions between V24 capsules

It has been observed that the five-coordinate square pyramidal and six-coordinated octahedral oxidovanadium complexes can interconvert by associating and disassociating an axial molecule[60-62]. With this in mind, we searched for conditions which promote the interconversion between the contracted V24 octahedron and the expanded V24 ball. Interestingly, we found that the axial water molecules of vanadium centers in V24 octahedron are removed in DMF/CH3OH (1:1, v/v) solution at 80 °C, the DMF working as a dehydrating agent;[63] while those vanadium centers in V24 ball-shaped capsule can capture the water molecules in DMF/CH3CN/H2O/NEt3 (20:80:10:1, v/v/v/v) solution at 80 °C (Fig. 4). The dissociation and association of axial water molecules in vanadium centers lead to their coordination geometries changing from square pyramidal and octahedral forms (Fig. 2c and Fig. 3c), respectively. When the vanadium centers adopt octahedral geometry, they and the equatorial coordinated oxygen atoms from the Pg units are almost coplanar (Supplementary Figure 6a). In contrast, when adopting square-pyramidal geometries, the vanadium ions and those oxygen atoms form a curved surface (Supplementary Figure 6b). Such transformations between the plane and curved surfaces result in the changes of inner cavities from contracted octahedra to an expanded ball in V24 capsules. As shown in Fig. 4, V24-oct-α and V24-ball-β can be easily converted into V24-ball-α and V24-oct-β, respectively, but the reverse is not observed. However, V24-oct-β and V24-ball-α can interconvert by regulating the solvents, which leads to form the V24 capsule partners showing different shapes. To sum up, the interconversions between the contracted and expanded V24 capsules have been successfully achieved by a process involving dissolution-reaction-recrystallization, which has been found to be an excellent method to explore the structural transformation of isolated coordination compounds as well as MONCs[64-66]. However, attempts to achieve the transformations through single-crystal-to-single-crystal phase transition under the stimulation of temperature and pressure were hindered by poor crystal quality, because packing of these V24 capsules is via weak supramolecular interactions, and the crystals of V24 capsules easily lose crystallinity after partial loss of the solvent.

Fig. 4

The transformation mechanism of V24 capsules. The transformation mechanism of V24 octahedron and ball can be achieved by controlling the geometries of vanadium ions by association and dissociation with axial water molecules in different solvent conditions

Magnetic properties of V24 capsules

Given the structural differences between these two V24 capsules, we compared their magnetic properties in order to yield important prototypes for exploring structure–property relationships. Here for clarity, we have provided only two phases (V24-oct-α and V24-ball-β) as examples, because the χmT vs. T data for V24-oct and V24-ball with two different phases show similar trends (Fig. 5 and Supplementary Figure 7). The magnetic property analyses of these two V24 capsules were performed on fresh samples from 2–300 K under a magnetic field of 1 kOe. For V24-oct-α, the room temperature χmT value of 9.23 cm3·K·mol−1 is close to the expected value of 9 cm3·K·mol−1 for 24 spin-only V4+ centers[50-52]. The value increases continuously with decreasing temperature, reaching a maximum of 10.01 cm3·K·mol−1 at 35 K and subsequently decreases sharply to 1.88 cm3·K·mol−1 at 2.0 K. The increase of the value of χmT upon reduction of the temperature at higher temperatures indicates intramolecular ferromagnetic exchange between neighboring metal ions. The 35–300 K magnetic data of this capsule was fitted to the analytical experimental equation (Eq. 1) deduced for compounds with three spin centers in an equilateral triangle[67], assuming the eight V3 clusters are noninteracting:

Fig. 5

Magnetic data for V24 capsules. Plots of χmT vs. T for V24-oct-α and V24-ball-β in a 1 kOe field

In Eq. 1, N is Avogadro’s number, β is Bohr’s magneton and k is Boltzmann’s constant. The best exchange interaction parameters obtained from fitting the χm data are J/k = 12.38 K and g = 1.93 (Supplementary Figure 8). The positive J value further suggests an intramolecular ferromagnetic interaction in V24-oct-α at higher temperatures.

For the V24-ball-β, the room temperature value of χmT of 3.97 cm3·K·mol−1 is much lower than the expected value (9 cm3·K·mol−1) for 24 spin-only V4+ centers. This decreases gradually to 3.72 cm3·K·mol−1 at ~ 20 K and then decreases rapidly reaching a value of 3.19 cm3·K·mol−1 at 2 K. Analysis of the V24-oct-α using the same equation (Eq. 1) yields J/k = −653.8 K and g = 2.04 (Supplementary Figure 9). Both the curve and negative J indicate dominant antiferromagnetic exchange interactions within this capsule since χmT at 300 K is much smaller than the expected values from 24 isolated V4+ spin carriers[68]. Notwithstanding all V centers being at +4 oxidation states in both V24 capsules, the variations between the five-coordinated square-pyramidal geometries in the V24-ball-β and the six-coordinated octahedral geometries in the V24-oct-α are indicative of a sensitive magnetic behavior of V4+ centers in different ligand field environments. Neither an obvious hysteresis loop or peaks for the out-of-phase component are observed for both capsules (Supplementary Figures 10-13), and this reveals no single molecule magnetic behavior above 2 K for both capsules.

Discussion

We have developed a strategy for the efficient construction of MONC quasi-isomers by controlling the coordination environments of the metal centers. In the present case, the adoption of octahedral and square-pyramidal geometries of vanadium centers results in a contracted V24 octahedron and an expanded V24 ball, respectively. V24-oct-β is the key motif in the interconversion between the contracted and expanded V24 capsules, which can be obtained by introducing DMF to the reaction of V24-oct-α and can also be prepared from V24-ball. The interconversions between V24-oct-β and V24-ball-α achieved by regulating solvents, leads to formation of the V24 capsule partners with different shapes. This work thus represents an example of MONCs whose structural differences arise from the coordination diversity of metal centers.

Methods

Materials and equipment

All reagents and solvents used in synthetic studies were obtained from commercial sources and employed without further purification. IR spectra were recorded in the range 4000−400 cm−1 with a Magna 750 IR spectrometer using KBr pellets. Electron paramagnetic resonance (EPR) spectra were recorded on a Bruker ER-420 spectrometer with a 100 kHz magnetic field in the X band at room temperature. Magnetic susceptibilities were determined on a Quantum Design PPMS-9T and MPMS-XL systems in the range of 2–300 K. All experimental magnetic data were corrected for the diamagnetism of the sample holders and of the constituent atoms according to Pascal’s constants. IR, EPR spectra and magnetic data were measured on the V24-oct-α and V24-ball-β samples.

Synthesis of PgC3 ligand

A solution of pyrogallol (20 g, 160 mmol) in ethanol (100 mL) and concentrated hydrochloric acid (10 ml) was mixed dropwise with butyraldehyde (11.4 g, 160 mmol) under N2 gas. This mixture was heated to reflux for 24 h, cooled, filtered, washed with water, a little cold ethanol and dried under vacuum. PgC3 was collected as a colorless powder. (13.6 g, 47%). 1H NMR (400 MHz, acetone-d6,): δ=0.95 (12H, t, CH3), 1.31 (8H, m, CH2), 2.26 (8H, q, CH2), 4.35 (4H, t, CH), 7.14 (4H, s, ArH), 7.18 (4 H, s, OH), 8.09 (8H, brs, OH) ppm.

Synthesis of V24 octahedron

Method 1: VOSO4·5H2O (0.4 mmol) and PgC3 (0.1 mmol) were added to CH3CN (5 mL), H2O (0.5 ml) and NEt3 (50 μL). The mixture was sealed in an 8 mL glass vial, which was heated at 80 °C for three days, affording the green rhombic crystals of V24-oct-α with a low yield (8% based on the PgC3 ligand). Enhancement of the synthetic yield can be achieved by the slow concentration of the filtrate at room temperature for one week, and in this way, the total yield of V24-oct-α was subsequently raised to 68%. Method 2: VOSO4·5H2O (0.4 mmol) and PgC3 (0.1 mmol) were added to DMF (1 mL), CH3CN (4 mL), H2O (0.5 ml) and NEt3 (50 μL). The mixture was sealed in an 8 mL glass vial, which was heated at 80 °C for 24 h. After slow concentration of the filtrate at room temperature for one week, green block crystals of V24-oct-β were collected in ~ 72% yield according to the PgC3 ligand.

Synthesis of V24 ball

Method 1: VOSO4·5H2O (0.4 mmol) and PgC3 (0.1 mmol) were added to DMF (2 mL) and CH3OH (2 mL). The mixture was sealed in an 8 mL glass vial, which was heated at 80 °C for 24 h. After slow concentration of the filtrate at room temperature for five days, cubic crystals of V24-ball-α were collected in ~ 76% yield based on the PgC3 ligand. Method 2: VOSO4·5H2O (0.4 mmol) and PgC3 (0.1 mmol) were added to NMF (2 mL) and CH3OH (2 mL). The mixture was sealed in an 8 mL glass vial, which was heated at 80 °C for 24 h. After slow concentration of the filtrate at room temperature for five days, green tetragonal prism crystals of V24-ball-β were collected in ~ 88% yield based on the PgC3 ligand.

Conversion from V24 octahedron to V24 ball

In an 8 mL glass vial, synthesized crystals of V24-oct-α (10 mg) or V24-oct-β (10 mg) were dissolved in DMF (1 mL) and CH3OH (1 mL), and the mixture was heated at 80 °C for 48 h. The solution was allowed to stand at room temperature for ten days to obtain ~ 8.5 mg green cubic crystals of V24-ball-α, 81% yield.

Conversion from V24 ball to V24 octahedron

In an 8 mL glass vial, synthesized crystals of V24-ball-α (15 mg) or V24-ball-β (15 mg) were dissolved in DMF (0.5 mL), CH3CN (2 mL), H2O (0.25 ml) and NEt3 (25 μL), and the mixture was heated at 80 °C for 48 h. The solution was allowed to stand at room temperature for one week to obtain ~ 12 mg green block crystals of V24-oct-β, 83% yield.

Single crystal X-ray diffractions

All X-ray single crystal data for V24 capsules were measured on diffractometers equipped with copper micro-focus X-ray sources (λ = 1.5406 Å) at 100.0(2) K. Diffraction data from V24-oct-α, V24-ball-β and V24-ball-α were measured on a SuperNova diffractometer, and that from V24-oct-β was collected on Bruker APEX-II CCD. The crystal structures were resolved by direct methods and all calculations were performed on the SHELXTL-2016 program package[69]. All non-hydrogen atoms were refined anisotropically with the exception of several highly disordered propyl carbon atoms and water molecules. Hydrogen atoms of the organic ligands were added in the riding model and refined with isotropic thermal parameters. Because of the diffuse electron density and the highly disordered/amorphous solvents, molecules of crystallization could not be fully located and were therefore not included for all structures (details are also provided in Supplementary Note 1). The crystal structures were treated by the “SQUEEZE” routine[70], a part of the PLATON package of crystallographic software, dramatically improving the agreement indices. We attempted to finish the refinement, but the R and wR factors were still high and some A-alerts were found by the (IUCr) checkCIF routine, all of which could be ascribed to the weak crystal diffraction, which is typical in giant supramolecular assemblies. Details on crystal data collection and refinement for these capsules are summarized in Supplementary Table 1.