Nanographene‐Based van der Waals Organic Framework With Permanent Porosity

ABSTRACT

Porous crystalline materials traditionally rely on robust coordination or covalent bonds. Herein, we report a new C ₃‐symmetric molecular nanographene 1 containing three hexabenzocoronene (HBC) units that self‐organizes into a permanently porous framework exclusively using weak van der Waals interactions. Single‐crystal x‐ray diffraction results reveal a propeller geometry with C ₃ symmetry, forming two‐dimensional honeycomb sheets parallel to the ab plane stabilized by π···π and C–H···π interactions. Nitrogen adsorption measurements at 77 K confirm a Type I(b) isotherm with a record‐breaking Brunauer–Emmett–Teller (BET) surface area of 1108 m² g⁻¹, the highest reported to date for purely organic van der Waals frameworks. Furthermore, compound 1 exhibits aggregation‐induced emission (AIE) and remarkable thermal stability up to 290° C. These results demonstrate that carefully designed nanographenes can achieve high structural predictability and robust porosity without the need for metal nodes or covalent linkages, opening new avenues for gas storage applications.

Unlocking record‐breaking porosity in a supramolecular organic framework (SOF). Through the strategic design of a C ₃‐symmetric nanographene, a robust supramolecular framework is achieved via exclusively weak van der Waals forces. This metal‐free scaffold reaches a benchmark BET surface area of 1108 m² g⁻¹ –the highest to date for this class–combining permanent 1D channels with AIE properties and stability up to 300 °C.

1. Introduction

Porous crystalline materials have attracted significant attention owing to their tuneable architectures and wide‐ranging applications in gas storage, separation, and lithium batteries [1, 2]. Among these, metal–organic frameworks (MOFs) are composed of metal nodes coordinated to organic linkers, forming extended networks with exceptionally high surface areas and adjustable porosity [3]. The concept of MOFs emerged in the mid‐1990s with the pioneering works of R. Robson [4], O. M. Yaghi [5], and S. Kitagawa [6]. They demonstrated how the metal–organic polymers could serve as crystalline networks with permanent porosity that reversibly adsorb and desorb gas molecules. One decade later, in 2005, covalent organic frameworks (COFs) emerged as a complementary class of porous materials, relying exclusively on covalent bonds between organic building blocks [7]. By avoiding metal centres, COFs offer high chemical stability and structural predictability while retaining tuneable pore sizes and topologies [8]. However, obtaining micrometre‐sized COF monocrystals suitable for single crystal x‐ray diffraction is highly challenging [9], and only a few examples have been structurally characterized so far [10, 11].

Parallel to the development of coordination networks, the design of porous materials has expanded to include frameworks assembled through weak non‐covalent interactions, such as hydrogen bonding (10–40 kJ/mol) [12] and van der Waals forces (2–10 kJ/mol) [13]. Hydrogen‐bonded organic frameworks (HOFs) are based on directional hydrogen bonding between carefully designed organic molecules, enabling the formation of crystalline yet dynamic three‐dimensional networks. In contrast, supramolecular organic frameworks (SOFs) take advantage of a broader variety of weak interactions—including π···π stacking, C–H···π interactions and van der Waals forces—to organize molecules into extended porous architectures [14, 15]. Usually, typical SOFs rely on the combination of different supramolecular interactions (e.g., hydrogen‐bonding and π···π stacking) to cooperatively stabilize the framework [16, 17]. There are only a few reported frameworks assembled exclusively through weak π···π [18, 19, 20, 21, 22, 23, 24, 25, 26, 27] and C–H···π [28, 29] interactions, where a metal centre provides structural directionality, and the overall architecture is stabilized by noncovalent contacts between the ligands. However, the number of reported examples is even more limited for SOFs constructed exclusively from purely organic building blocks, in which the absence of metal nodes makes structural directionality and long‐range order particularly challenging to achieve. Purely organic molecules exhibit a strong natural tendency to densely pack in the solid state to maximize dispersive forces. Overcoming this tendency to achieve permanent porosity without the intrinsic rigidity and well‐defined geometry that metal atoms provide—even when such metals do not actively participate as coordination nodes—represents a formidable challenge. For this reason, purely organic van der Waals frameworks constitute a distinct and highly demanding sub‐category of materials in which strictly metal‐free building blocks are employed (Figure 1) [30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42].

FIGURE 1.

Reported purely organic building blocks capable of self‐assembling into SOFs sustained exclusively by weak van der Waals interactions. Values in parentheses denote the BET specific surface area (m² g⁻¹). This work presents nanographene 1, which achieves a record‐breaking value of 1108 m² g⁻¹ for this class of materials.

It is worth mentioning that π···π interactions are particularly relevant in the context of molecular nanographenes (NGs), well‐defined nanometre‐scale fragments of graphene [43]. Their extended π‐conjugation and planar geometry make NGs ideally suited for π···π stacking. Although this has typically constituted a drawback due to the formation of insoluble materials, functionalization with bulky groups such as tert‐butyl or mesityl substituents represents a common strategy to avoid aggregation. π···π stacking, together with C–H···π interactions involving these solubilizing groups, constitute the supramolecular forces that allow chemists to resolve all‐carbon nanographene structures via single crystal x‐ray diffraction (SCXRD).

Advances in organic synthesis now enable precise control over NGs’ size, edge structure, and topology, yielding curved [44, 45, 46], helical [47, 48, 49], or multilayer architectures [50, 51, 52]. In this regard, a family of tetrahedral nanographenes composed of hexabenzocoronene (HBC) units linked to a central carbon atom or heteroatom—previously shown to exhibit properties such as AIE [53, 54], high performance as anodes in Li‐ion batteries [55], and the ability to act as ligands in homogeneous catalysis [56] — have been reported in the last years.

Although molecular nanographenes have previously been employed as linkers in the construction of MOFs [57, 58] and COFs [59, 60], strategies for integrating these species into SOFs remain largely unexplored. This is attributed to the reduced predictability in porosity and topology in SOFs, where non‐directional interactions nullify the characteristic synergy between tectons and synthons in MOFs, COFs, or HOFs [14].

Herein, we report the synthesis of a new C ₃‐symmetric molecular nanographene 1, containing three HBC units, capable of self‐organizing through van der Waals interactions, namely π–π and C–H···π forces, leading to a porous supramolecular organic framework.

Interestingly, the porous framework resulted to be quite robust, with a pore diameter of 1.7 nm, leading to a remarkable specific surface area of 1108 m² g^{− 1}. To the best of our knowledge, this is the highest value reported in the literature for van der Waals organic frameworks (Figure 1). As a proof of concept, nitrogen adsorption–desorption isotherms of nanographene 1 at 77 K were carried out confirming the size and stability of the porous framework. This result has also been confirmed by TGA and powder ‐ray diffraction studies.

During the preparation of this manuscript, Gong and coworkers reported the synthesis and radical reactivity of a related C ₃‐symmetric nanographene system endowed with a hydroxy group [61]. While their study provided valuable insights into the chemical reactivity of such architectures, our work focuses on the unprecedented self‐assembly of all‐carbon nanographene 1 into a permanently porous supramolecular framework.

2. Results and Discussion

2.1. Synthetic Procedure

The synthesis of nanographene 1 started from commercially available p‐dibromobenzene (Scheme 1). After monolithiation with n‐butyllithium, the threefold addition to diethyl carbonate led to the triaryl carbinol 2 in 89% yield. This intermediate was then subjected to dehydroxylation with formic acid, affording the corresponding triaryl methane 3 in 97% yield. Subsequently, a threefold Pd‐catalyzed Sonogashira cross‐coupling introduced the alkyne groups, producing the trialkynylphenyl methane 4 in 82% yield. The resulting compound underwent a [4 + 2] Diels–Alder cycloaddition with tetrakis(4‐tert‐butylphenyl) cyclopentadienone 5 as the diene, and the reaction, followed by in situ chelotropic CO extrusion, furnished the polyarene 6 in 71% yield after purification. Finally, Scholl cyclodehydrogenation of compound 6 using FeCl₃ as oxidant afforded the target nanographene 1 in 85% yield. During this graphitization step, the formation of 18 new C–C bonds were confirmed by HRMS (found: 2417.3440; calculated for C₁₈₇H₁₇₂: 2417.3459). Compound 1 was further characterized by ¹H and ¹³C NMR, FT‐IR and Single Crystal x‐ray Diffraction (SCXRD).

SCHEME 1.

Synthetic scheme towards the synthesis of nanographene 1.

The NMR spectra revealed the high symmetry of nanographene 1. In the aromatic region of the ¹H NMR spectrum, only five singlets were observed at 9.71, 9.31 (2H), 9.26, 9.23, and 9.21 ppm, corresponding to half of the HBC unit. On the other hand, the aliphatic region displayed only three singlets (1.82, 1.79, and 1.36 ppm) with relative intensities of 2:2:1, consistent with the 15 tert‐butyl groups present in the molecule. Notably, the aliphatic proton attached to the sp³ central carbon exhibited an unusually high deshielding, appearing at 7.81 ppm — a chemical shift more typical of aromatic protons.

Single‐crystal x‐ray diffraction analysis provided detailed insight into the supramolecular organization of compound 1 in the solid state (Figure 2a and Section 6 of the Supporting Information). It is interesting to note how the threefold symmetry of the nanographenes is clearly mirrored in the external morphology of the crystals (Figure S23). The asymmetric unit (Figure S24) contains only a third of the molecule, as it is located in a threefold axis coincident with the C–H bond from the central carbon atom, displaying a C ₃ point symmetry. This results in a propeller geometry, similar to the one found in the related OH‐containing molecule reported by Gong et al. [61] The HBC moieties display a remarkable planarity (Figure S25), as a result of the extensive π–π stacking interactions, with a close distance of 3.615 Å between the neighbor mean planes (calculated for the 42 atoms in the seven rings). Taking into account these π···π contacts, each nanographene acts as a 3‐connected node (Figure 2b) yielding two‐dimensional sheets parallel to the ab plane with a hcb honeycomb topology (point symbol 6³, Figures 2c and S26). The layers are reinforced by C–H···π contacts, which also join them in the c direction, stabilizing the framework and leading to the formation of large channels where highly disordered solvent molecules are lodged (Figures 2d, S27, and Table S2). The resulting supramolecular arrangement was found to be isostructural to the one displayed by the hydroxyl derivative [61].

FIGURE 2.

SCXRD analysis of nanographene 1. (a) Single‐molecule structure (view along the threefold axis). (b) π···π contacts between a molecule of 1 and three neighbour ones (H atoms and tert‐butyl groups are omitted for clarity) (c) Honeycomb layer parallel to the ab plane (view along c axis). (d) Stacking of the layers in the c direction (view along b axis).

The extraordinary stability of the crystals from compound 1 out of the mother liquor (unlike other nanographene crystal structures previously solved by our research group) highlights the key role of the sum of weak intermolecular forces in the formation of strong assemblies and renders this nanographene a promising material for gas adsorption applications.

2.2. Electrochemical Properties

The electrochemical properties of compound 1 were explored by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in toluene/acetonitrile (4:1) using a 0.1 M solution of tetrabutylammonium hexafluorophosphate as the supporting electrolyte at room temperature. An Ag wire was employed as a pseudo‐reference electrode, and all potentials were referenced to Fc/Fc⁺ (Figure 3a and Table 1).

FIGURE 3.

(a) Cyclic voltammetry and differential pulse voltammetry of 1 in toluene:acetonitrile 4:1 with a tetrabutylammonium hexafluorophosphate 0.1 M solution as the supporting electrolyte at room temperature. Glassy carbon was used as the working electrode, Ag as the pseudo‐reference electrode and platinum as the counter electrode. All redox potentials are referenced to Fc/Fc⁺. (b) Normalized absorption and emission spectra of 1 in THF. Emission spectrum was recorded with an excitation wavelength of 363 nm. (c) Emission spectra of 1 in THF with different water fractions (f _w) from 0.4 to 0.9. (d) Change in the color of the emission of 1 in THF with different water fractions.

TABLE 1.

Absorption, emission, and electrochemical data of 1 and ^t BuHBC.

Compound	Absorption λ a , (ε) b	Emission λ a	E 1/2 red c	E 1/2 ox c	∆HOMO‐LUMO d
1	346 (197745), 363 (338964), 392 (189599), 440 (5817), 447 (4396)	465, 477, 486, 495, 519, 530, 558, 569	−2.44, –2.79	0.76	2.96
t Bu‐HBC [53]	344 (63750), 360 (141700), 390 (46350), 439 (1600), 441 (1650), 445 (1800)	464, 475, 484, 493, 518, 528, 537, 556, 567	−2.45	0.75	2.82

^a nm.

^b M⁻¹ cm⁻¹.

^c V.

^d eV.

At negative potentials, a cathodic wave at –2.55 V was observed together with a quasi‐reversible anodic counterpart at –2.34 V, yielding a half‐wave reduction potential of E ¹ _{1/2 red} = –2.44 V. This value is nearly identical to the value of the ^t Bu‐hexasubstituted HBC derivative, ^t Bu‐HBC (–2.45 V) [53] indicating that both compounds exhibit similarly poor electron‐acceptor character. At more negative potentials, a second cathodic wave appears at –2.99 V with a quasi‐reversible anodic wave of –2.58 V leading to a half‐wave reduction potential of E ² _{1/2 red} = –2.79 V. Notably, only when the CV scan is extended to this second reduction event does a new anodic feature arise at –1.44 V, which we attribute to the decomposition of compound 1 under these conditions (Figure S20).

At positive potentials, compound 1 displays an anodic wave around 0.85 V, with a quasi‐reversible cathodic response at 0.67 V, corresponding to a half‐wave oxidation potential of E ¹ _{1/2 ox} = 0.76 V. This oxidation potential closely matches the value reported for ^t Bu‐HBC (0.75 V), further highlighting the influence of the Csp³ central atom in maintaining the electronic identity of the HBC units.

The HOMO–LUMO gap of compound 1 was determined experimentally from the onset potentials of the first oxidation and reduction waves (Figure S22), yielding a value of 2.96 eV. This bandgap is essentially identical to the value of ^t Bu‐HBC (2.82 eV) [53] which is consistent with the absence of effective conjugation between the three HBC units in 1.

2.3. Photophysical Properties

The optical profile of nanographene 1 was examined to evaluate the impact of the C ₃‐symmetric architecture on its electronic properties (Figure 3b and Table 1). In the ultraviolet region, the absorption spectrum is characterized by a dominant transition at 363 nm, accompanied by a shoulder at 346 nm. These features are assigned to symmetry‐allowed β bands. Additionally, a sharp signal appears at 392 nm, corresponding to the p band. This transition, while formally forbidden by symmetry, gains experimental intensity through vibronic coupling with the higher energy β states.

A comparative analysis with the pristine ^t Bu‐HBC [53] reveals that the absorption maxima remain nearly static, as the reference transitions (344, 360, and 390 nm) show negligible shifts upon incorporation into the 3D scaffold. This spectral congruence serves as direct evidence that the three hexabenzocoronene units behave as electronically independent chromophores. The sp³‐hybridized central carbon atom acts as an effective insulating node, preventing π‐homoconjugation between the branches and preserving the individual identity of each nanographene unit. This experimental observation could be accounted for by the unfavorable spatial arrangement of HBCs, which prevents the interaction of the π orbitals through space.

A standout feature of compound 1 is its significantly enhanced light‐harvesting capacity. The molar extinction coefficient (ε) for the primary transition at 363 nm is approximately three times higher than that of the isolated ^t Bu‐HBC. This purely additive behavior allows for the concentration of multiple photoactive subunits within a single molecular framework without compromising the high chromatic purity of the base nanographene.

In the visible range, the spectrum displays the characteristic α bands at 440 and 447 nm. These transitions are extremely weak and symmetry‐forbidden, showing low extinction coefficients. The absence of any significant broadening or bathochromic shifts in these bands further confirms the lack of homoconjugation through the tetrahedral core, ensuring that the intrinsic electronic properties of the HBC fragments are fully maintained.

The fluorescence profile of nanographene 1 was examined to evaluate the electronic influence of the C ₃‐symmetric assembly (Figure 3b and Table 1). In THF solution, 1 displays a vibronic emission pattern with maxima between 465 and 569 nm, a signature that closely mirrors the behavior of isolated ^t BuHBC unit. This spectral overlap confirms that, the central sp³‐hybridized carbon atom effectively decouples the three HBC chromophores.

A transition in the emissive properties is observed when moving from dilute solutions to the condensed phase, where compound 1 radiates an intense yellow light. To clarify the nature of this shift, the fluorescence was monitored in THF:H₂O mixtures with increasing water content (f _w). While the blue‐green monomeric emission prevails at low water fractions, the gradual formation of molecular aggregates triggers the appearance of a broad, red‐shifted band centred around 575–675 nm (Figure 3c,d). This phenomenon is characteristic of an AIE mechanism, likely driven by the restriction of intramolecular rotations and the establishment of specific supramolecular contacts within the assembly [53].

Quantitatively, the fluorescence quantum yield of 1 reaches 8.7% in solution and remains at a notable 4.9% in the aggregated state (f _w = 0.9). Although a partial reduction in efficiency occurs upon aggregation, the final value significantly surpasses that of the pristine ^t BuHBC reference.

2.4. Gas Adsorption Analysis

The porosity of nanographene 1 was assessed by nitrogen adsorption–desorption measurements at 77 K using a Micromeritics 3Flex 3500 analyser. Prior to analysis, the sample was activated overnight at 100 °C under vacuum in a VacPrep station and subsequently treated at 100 °C for 1 h immediately before measurement.

The resulting nitrogen adsorption isotherm reveals a sharp uptake at low relative pressures (P/P°<0.1), followed by a progressive filling that reaches a stable plateau (Figures 4a, S29). This profile corresponds to a Type I(b) isotherm which, according to IUPAC classification, is characteristic of materials possessing micropores at the upper limit of the microporous scale (typically between 1 and 2.5 nm) [62]. These are broader than the narrow micropores (width < ∼1 nm) found in Type I(a) systems, yet they still facilitate enhanced adsorbent–adsorbate interactions that lead to micropore filling at low relative pressures. Notably, the adsorption and desorption branches are fully reversible, confirming the absence of significant mesoporosity and demonstrating that the supramolecular framework maintains its structural integrity without undergoing deformation during the gas sorption cycle.

FIGURE 4.

(a) Nitrogen adsorption–desorption isotherms of nanographene 1 at 77 K (grey triangles: adsorption cycle; red triangles: desorption cycle). Inset: pore size distribution assuming a cylindrical pore model as estimated via NL‐DFT. P/P° is the ratio of gas pressure (P) to saturation pressure (P°). P° is measured in situ for each point. (b) Temperature‐dependent powder x‐ray diffraction patterns from 50 to 310° C, including the simulated diffractogram from SCXRD data and the diffractogram of a sample after heated at 120 °C under vacuum for 48 h.

The Brunauer–Emmett–Teller (BET) specific surface area was determined to be 1108 m² g–¹ using the BETSI method (Figures S30 and S31) [63]. To the best of our knowledge, this value represents the highest BET surface area reported to date for purely organic systems assembled exclusively through van der Waals interactions (Figure 1). The adsorption capacity reaches a plateau at 303 cm³ g⁻¹ (STP), corresponding to a total pore volume of 0.46 cm³ g–¹ (determined at P/P° = 0.95).

This experimental performance is further corroborated by theoretical porosity analysis using the Mercury pore analyser (Table S2). The calculated network‐accessible surface area of 1138 m² g⁻¹ is in striking agreement with the value calculated from experimental data, confirming the near‐quantitative accessibility of the framework. Furthermore, the simulation identifies a one‐dimensional (1D) pore network with a maximum pore diameter of 15.10 Å, which aligns with the wider microporous character assigned from the Type I(b) isotherm and closely fits with the experimental value of 1.7 nm.

Considering the calculated crystallographic density of 1.031 g cm⁻³ (Table S1) the experimental pore volume corresponds to a solvent‐accessible void volume of approximately 47%. This is consistent with the 43% void volume calculated from the crystal structure (Mercury, probe radius 1.20 Å, Figure S27). The agreement between the atomistic model and experimental gas sorption data confirms that, the C ₃‐symmetric design of nanographene 1 successfully prevents dense packing, sustaining a robust and permanent porosity without the need for directional coordination or covalent bonds.

The thermal and structural robustness of the supramolecular framework was evaluated by thermogravimetric analysis (TGA) and powder x‐ray diffraction (PXRD). TGA measurements under an inert atmosphere demonstrate the exceptional thermal resilience of 1, remaining stable until 500 °C. At this temperature, a weight loss of 23% is observed, corresponding to the initial thermal decomposition of the organic scaffold (Figure S33). This remarkably high decomposition temperature suggests the strong π–π interactions and the efficient packing of the nanographene units.

Crucially, powder x‐ray diffraction (PXRD) studies confirm that the crystalline phase remains entirely intact upon heating up to 300° C (Figure 4b), a temperature significantly higher than that required for the activation of the material (120 °C). The diffraction patterns prove the gradual loss of the guest molecules (responsible for the additional peaks) until 290 °C and the persistence of the crystalline arrangement. The PXRD pattern after total solvent evacuation under vacuum at 120° C during 48 h further demonstrates a reversible variation in the structure and the permanent nature of the porosity. Unlike many van der Waals‐based systems that undergo structural collapse or irreversible phase transitions upon guest removal, the C ₃‐symmetric scaffold of nanographene 1 provides enough structural rigidity to maintain open 1D channels. This permanent porosity is unequivocally validated by the high gas sorption experiment and the consistency between the experimental gas uptake and the crystallographically predicted void volume.

3. Conclusions

In summary, we have designed and synthesized the C ₃‐symmetric molecular nanographene 1, featuring three hexabenzocoronene (HBC) units linked to a central sp³ carbon node. Single‐crystal x‐ray diffraction data confirms that the C ₃ symmetry of the building block is faithfully translated into the solid state, forming two‐dimensional honeycomb sheets with a honeycomb (hcb) topology. This framework is stabilized exclusively by weak van der Waals forces, including π···π stacking and C–H···π interactions. These results demonstrate that a meticulous molecular design can achieve high structural predictability and long‐range order without relying on directional metallic coordination or covalent linkages.

The electrochemical and photophysical characterization reveals that the central sp³ carbon effectively decouples the HBC units, preserving their individual electronic identities while significantly enhancing the light‐harvesting c apacity of the assembly. Compound 1 exhibits a molar extinction coefficient three times higher than the monomeric reference and displays distinct AIE. The transition from a blue‐green monomeric emission in solution to an intense yellow radiation in the aggregated state highlights the potential of these scaffolds for advanced optoelectronic applications.

The permanent porosity of the framework was unequivocally validated by nitrogen adsorption–desorption measurements, which revealed a Type I(b) isotherm. With a record‐breaking BET surface area of 1108 m² g^{– 1} compound 1 sets a new benchmark for purely organic van der Waals frameworks. The remarkable consistency between the experimental pore volume (47%) and the theoretical void volume calculated from SCXRD data (43%) confirms that the internal 1D channels remain structurally intact and fully accessible upon solvent removal.

Finally, the supramolecular assembly demonstrates exceptional thermal and structural robustness, remaining crystalline up to 290 °C and resisting chemical decomposition until 500° C. This level of stability, coupled with high permanent porosity, positions C ₃‐symmetric nanographene 1 as a formidable new class of functional all‐carbon materials. By outperforming previously reported systems and rivalling the porosity of more complex hybrid networks, nanographene 1 illustrates the untapped potential of weak non‐covalent interactions in the engineering of robust, high‐performance gas‐storage materials.

Author Contributions

José Sánchez Costa: software, formal analysis, funding acquisition, project administration, writing – original draft. Josefina Perles: formal analysis, writing – original draft, validation, visualization, data curation, software, and writing – review and editing. Jesús M. Fernández‐García: conceptualization, investigation, writing – original draft, writing – review and editing, formal analysis, supervision, validation, and visualization. Rubén Turo‐Cortés: formal analysis, data curation, software, writing – original draft. Arturo Oró: investigation, writing – original draft, conceptualization, methodology, writing – review and editing, formal analysis. Nazario Martín: conceptualization, investigation, writing – original draft, writing – review and editing, formal analysis, project administration, supervision, resources, validation, and visualization.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

The authors have cited additional references within the Supporting Information [64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76].

Supporting File 1: anie72458‐sup‐0002‐SuppMat.docx.

Oró A., Turo‐Cortés R., Perles J., Costa J. S., Fernández‐García J. M., and and Martín N., Angewandte Chemie International Edition. 65, no. 27 (2026): e4089584. 10.1002/anie.4089584

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article. Deposition Number CCDC 2530875 contains the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachin formations zentrum Karlsruhe Access Structures service.