Seeking easily accessible construction strategy to create 3D stable crystalline CSFs, our attention was initially captivated by robust HOFs, such as HOF-1 that relies on the H-bonded assembly of organic tectons with tetrahedral geometry to form 3D crystalline networks26,27. Taking inspiration from characteristic feature of HOF-1 (Supplementary Fig. 1), we envisioned that harnessing tetrahedrally symmetric clusters as modular building blocks may facilitate building robust CSFs with distinctive topologies and architectures. To this end, we synthesized a copper cluster [Cu5L4(P(C6H4F)3)4]PF6 (1) (2-mercapto-1-phenylimidazole = HL, and P(C6H4F)3 = tris(4-fluorophenyl)phosphine) through reducing the copper salts with NaBH4 in the presence of HL and P(C6H4F)3 under ambient conditions. The structure of 1 was first elucidated by NMR spectroscopy, of which 1H NMR exhibits a set of shifted ligand signals (Supplementary Fig. 2), verifying the protection of the cluster surface by the mixed ligands. The precise mass and composition of 1 was further confirmed by electrospray ionization mass spectrometry (ESI-MS) in positive mode (Supplementary Fig. 3). The parent cluster ion was found at the most dominant peak, m/z 2283.0142, which could be identified with isotopic envelopes corresponding to [Cu5L4(P(C6H4F)3)4]+ (calc. m/z = 2283.0307). DFT-optimized cluster structure of [Cu5L4(P(C6H4F)3)4]+, characterized as an energy minimum, was found to be of S4 symmetry (Supplementary Fig. 4), which agrees quite well with the X-ray structure (vide infra). TD-DFT calculated results match well with experimental UV/Vis absorption spectrum of 1 measured in CH2Cl2, which showed two major UV absorption bands at 358 and 368 nm and a tail up to about 400 nm (Supplementary Fig. 5).
Colorless crystals of 1 suitable for X-ray crystallography were obtained by slow vapor diffusion of n-hexane into a concentrated CH2Cl2 solution of the corresponding cluster (PF6− salt) at ambient temperature. Microscopic image of the single crystals, exhibited rhombic dodecahedra shape with twelve well-defined diamond facets (Fig. 2A). Single-crystal X-ray diffraction (SCXRD) analysis revealed that 1 crystallizes in the higher-symmetry cubic crystal system with a non-centrosymmetric space group P—43n (No. 218, Supplementary Table S1). Its overall composition contains a cationic cluster [Cu5L4(P(C6H4F)3)4]+, as well as one PF6− counterion and interstitial CH2Cl2 molecules. As portrayed in Fig. 2B, the structural anatomy of [Cu5L4(P(C6H4F)3)4]+ could be viewed as a Cu5 kernel wrapped by peripheral four P(C6H4F)3 and four deprotonated L− ligands. The metal core adopts a unique centered tetrahedral geometry. The Cu-Cu bond length in the metal skeleton from the central Cu atom to the vertexes of Cu4 tetrahedron is equal (2.655(1) Å), suggestive of the presence of significant intramolecular cuprophilic interactions. It is worthy of note that such centered tetrahedral Cu5 skeleton is very unusual, because a Cambridge Structural Database (CSD) search28 yields previously reported discrete Cu5 clusters featuring a planar conformation or a bipyramidal geometry (Fig. 2C)29,30. The center Cu atom is tetrahedrally bonded to all four L− molecules via four Cu-S bonds, while each terminal Cu atom in the Cu4 tetrahedron is tetrahedrally completed by two µ3-S atoms and one N atom from three different L− molecules plus one P atom from one P(C6H4F)3. Thus the entire cationic cluster lies on a special position with S4 symmetry.
Inspection of the rhombic dodecahedral crystal habit in the case of 1 is unexpected among the crystalline forms of coinage metallic clusters, which is evocative of an example of icosahedral viruses crystallized into cubic crystals in a similar dodecahedral shape31. These inspire us to conjecture that extraordinary arrangement and packing of cluster tectons might exist. To our delight, the most attractive structural feature of 1 is that twelve copper clusters per unit cell self-assemble into an enclosed pseudoicosahedral capsule denoted as Cu60 (Fig. 2D) with approximate diameter of 3.0 nm. The surface of the pseudoicosahedron contains two types of triangular faces (Fig. 2E-2F): 8 equilateral triangles and 12 isosceles triangles, each equilateral triangle with side lengths of 16 Å (the centroid-centroid distances of two adjacent clusters) and each isosceles triangle with side and base lengths of 16 and 13 Å, respectively. Notably, based on ChatGPT prediction32, this is the only possible combinatorial outcome for the irregular icosahedron buckled through these triangles and isosceles triangles. The hollow icosahedral capsule Cu60 could accommodate a sphere with a diameter of about 25 Å, corresponding to a sphere volume of ~ 8177 Å3. Our observation of the supramolecular icosahedral assembly that involves the solid-state aggregation of individual clusters is unprecedented and intriguing. More importantly, the atom-precise icosahedral structure will provide much more detailed information for further identifying the origin of this Platonic polyhedral assembly.
Figure 2G shows the electrostatic potential (ESP) mapped onto the electron isodensity surface of [Cu5L4(P(C6H4F)3)4]+. It is found that the surface ESP is highly positive and may be strongly inclined to interact with negatively charged species (i.e., anions) to make supramolecular assemblages. As a result, PF6− counterions lie at the centre of equilateral triangles of the icosahedron, wherein half fluoride atoms of each PF6− anion interact with three [Cu5L4(P(C6H4F)3)4]+ to form a supramolecular trimer. Each copper cluster located in the vertice of icosahedron could be visualized as two parts, a CuP(C6H4F)3 and a Cu3L4(P(C6H4F)3)3 moiety, which points inside and outside the surface of icosahedral capsule, respectively. A closer inspection of the intermolecular contact patterns reveals each F atom of the PF6− participates in two hydrogen bonds with two fluorobenzenes from two neighboring CuP(C6H4F)3 moieties by means of C-H···F charge-assisted hydrogen bonds (dH···F = 2.56–2.61 Å and θC−H···F = 127.3°-130.5°). These small H-F separations match well with previous studies of other fluorobenzenes in crystals33. In total, the icosahedral capsule is held together by way of 48 CH···F hydrogen bonds. It is especially interesting to note the uniqueness of such cluster-based icosahedral H-bonded capsule Cu60 is reminiscent of copies of identical protein clusters assembled into icosahedral viral capsids1. As mentioned at the outset, supramolecular coordinative icosahedral entities have been identified, such as molybdenum oxide cluster Mo1325 and metal-organic capsule Fe126,7. However, bonding energy for hydrogen bonds (25–40 kJ mol− 1)34,35 is much smaller than those of coordinate covalent bonds (90–350 kJ mol− 1)35, making well-defined H-bonded icosahedrons more challenging to assembly than their coordination counterparts. From the view of capsule size, the diameter of the supramolecular H-bonding capsule Cu60 is 3.0 nm, making it comparable to that of the nanosized icosahedral Mo132 (2.9 nm).
To explore the disassembly behavior of the icosahedral nanoconstruct in solution, we carried out CSI-MS characterization, as a variant of ESI-MS operating at low temperature. The CSI-MS was measured by dissolving crystals of 1 in CH2Cl2, in which the ion source temperature is 0 ºC. Positive-mode CSI-MS affords three sequentially charged ion species (2 + to 4+), centered at m/z 3497.526, 3903.006 and 4711.996, that we attributed to cluster-based H-bonded oligomers, including trimeric {[Cu5L4(P(C6H4F)3)4]3·PF6}2+, pentameric {[Cu5L4(P(C6H4F)3)4]5·(PF6)2}3+, and octameric {[Cu5L4(P(C6H4F)3)4]8·(PF6)4}4+, respectively. The m/z values along with isotopic distribution patterns of each charge state closely match the simulated ones (Fig. 3), verifying the assignment. These H-bonded oligomers showcase the increasing cluster aggregations concomitant with the associated increase in anions PF6− present which bring discrete clusters together into the oligomers. The optimized H-bonded oligomeric structures (Supplementary Fig. 6) are quite similar to those observed in the crystallographic data. These H-bonded species are considered as geometric fragments of the icosahedral units, which is primarily important to understand the mechanism of formation of the icosahedral capsule from this reaction system. Especially, detection of the most dominant peak of the trimeric {[Cu5L4(P(C6H4F)3)4]3·PF6}2+, is significant, representing the basic trigonal unit of the icosahedron. CSI-MS results indicate that charge-assisted CH···F contacts play a pivotal role in the assembly of clusters with anions into identifiable H-bonded oligomers in solution, prior to crystallization.
Fascinatingly, each pseudoicosahedral capsule―considered to be a supramolecular secondary building unit―connects with six other icosahedral capsules through sharing bases of isosceles triangles of the pseudoicosahedron, thereby yielding an extended three-periodic zeolitic-like HOFs (Fig. 4A), which is the missing link between coinage-metal clusters and HOFs. Of note, topologically, regarding the discrete cluster as a 10-connected node, the whole architecture of the CSF is a unimodal 10-connected framework with the Schläfli symbol (316.424.55). When viewed in projection down the a axis, the intricate 3D CSF are seen to contain two types of 1D infinite channels (α and β), as illustrated in Fig. 4B. The α-type channel has cross section of 3.7 Å×7.2 Å to which the phosphine moieties of clusters are exposed, and guest CH2Cl2 solvents were inside the channel. The β-type channel is formed with phenylimidazole arms of clusters and its channel cross section is 7.0 Å×7.0 Å. Surprisingly, in the tunnel β, the clusters are arranged to form 1D spirals, with four clusters adopting different rotations per spiral of unit cell length. The spirals in turn are intertwined to give the two strands of an infinite double helix with the same right handedness (Fig. 4C). The helical features of the double helix are defined by a pitch length of 5.2 nm and width of 3.6 nm. The entwined strands are stabilized by charge assisted CH···F H-bonding interactions between the cationic clusters and anions. The tunnels alongside these double helices are filled with disordered CH2Cl2. The existence of guest molecules CH2Cl2 was also supported by 1H NMR (Supplementary Fig. 2). It is significant to observe that identical supramolecular double helices are produced along a, b and c-axes, respectively, therefore producing unusual mutually perpendicular double helices in three dimensions (Fig. 4D). It is worth of noting that it is only recently that 1D double-helical self-assembly has been observed in the realm of coinage metallic clusters aggregates13–15, but no example extending into 3D crystallographic axes have appeared until now. Hence, the superstructure of 1 contains another remarkable structural feature of double-stranded helicates which extend in three perpendicular directions, forming 3D orthogonal double-helical patterns, which is advocative of the assembly of double-stranded DNA into various nanostructures in different dimensions36–38.
Despite the lack of strong metal-coordination bonds, it is the CSF, achieved by charge-assisted hydrogen bonds, that might exhibit improved stability because of the additional electrostatic attraction between the components39,40. To confirm the idea, we accessed the chemical durability of crystals of 1 in aqueous solutions with a broad pH range (pH 1–14). After solid 1 was soaked in solutions above-mentioned for three days, there is no obvious difference in morphology and color (Supplementary Fig. 7). Crystals of 1 still retained their single crystallinity, enabling the SCXRD and their unit-cell parameters are almost identical to that of as-synthesized 1. Moreover, the powder XRD (PXRD) patterns of the sample after prolonged treatment in acidic and basic conditions agree well with the simulated, suggesting that the integrity of the framework is preserved toward both acids and bases (Fig. 5A).
Thermogravimetric analysis (TGA) profile (Fig. 5B) of freshly prepared crystals 1 showed the first continuous weight loss stepping from room temperature to ~ 170 ºC corresponds to the loss of CH2Cl2 molecules. The superstructure of 1 began to decompose at temperature higher than ~ 215 ºC. To thoroughly understand the thermal stability of the supramolecular framework of 1, we measured variable-temperature PXRD, which revealed that no phase transition or architecture collapse at elevated temperature range 30–200 ºC (Fig. 5C). The PXRD results reflect an uncharacteristic robustness of the cluster-based H-bonded assembly even after removal of the solvates from channels, where many other noncovalent coinage-metal cluster-based frameworks would fail.
By slowly heating crystals of 1 to 170 ºC under inert atmospheres or in vacuo, 1 could be fully desolvated to obtain CH2Cl2-free crystals 1-d. The complete removal of guest CH2Cl2 was also confirmed by 1H NMR spectroscopy (Supplementary Fig. 2), where the chemical shift at 5.25 ppm corresponding to CH2Cl2 disappears completely after activation. The desolvated crystals 1-d exhibited high thermal stability with negligible weight loss occurred until 215 ºC (Fig. 4B). No significant loss of crystallinity was observed when the CH2Cl2 molecules were removed gently. The resultant crystals 1-d was successfully examined by X-ray crystallographic analysis, explicitly confirming the single-crystal-to-single-crystal transformation. 1-d retained the same P—43n space group as 1, with a shortening of lattice parameters from 26.1645 Å for (1) to 25.8013 Å (1-d). The mechanical contraction of the network, with ~ 2% reduction of the initial unit cell volume, occurs without damaging the crystal upon the removal of CH2Cl2. The atomic resolution of SCXRD allows us to discern subtle variations in the CSF before and after heating. As illustrated in Fig. 3B, the portal of open channel α′ in 1-d kept almost unchanged as observed in the original channel α in 1. Closer examination of channel β′ along the a-direction (Fig. 3C), however, revealed structural subtleties, wherein the chelating arrangement of the imidazole plane restricts its rotation, but a clear rotation of the phenyl group attached to the imidazole moiety partially blocks the portal of the open channel β′, resulting in the shrinkage of the resulted window size (4Å×4Å). CO2 adsorption-desorption measurement for 1-d at 273 K showed a fully reversible type-I Langmuir profile (Supplementary Fig. 8), verifying the supramolecular architectural stability and permanent microporous characteristic of 1-d. It is worth noting that the framework of 1-d can revert back to its original architecture of 1 when recrystallized from CH2Cl2, featuring structural dynamism of the CSF.
In general, the cluster-cluster interactions can be tailored via modification of the ligand shell. For 1, due to the inductive effect associated with fluorine atom at the para-position of benzene, the positively polarized surfaces of the fluorinated aromatics are prone to interact with electron-rich anions. We reasoned that substitution of the para-F atom with H may be expected to disrupt CH···F hydrogen bonds, further markedly affecting the structure of the resulting CSFs. To verify this, we tried to synthesize another analog [Cu5L4(PPh3)4]PF6 (2), by replacing P(C6H4F)3 with triphenylphosphine (PPh3). X-ray diffraction analysis at 173 K for single crystal of 2 established that it occurs in the centrosymmetric tetrahedral space group P4/ncc (No. 130, Supplementary Table S2) with the composition of [Cu5L4(PPh3)4]PF6·CH2Cl2, proved by ESI-MS (Supplementary Fig. 9). As anticipated, the cationic cluster [Cu5L4(PPh3)4]+ is isostructural with [Cu5L4(P(C6H4F)3)4]+. In comparison with 1, however, SCXRD revealed that 2 exhibits distinctly different assembly behavior, whereby [Cu5L4(PPh3)4]+ clusters spontaneously organize into another 3D CSF. The topology of 2 can be described as a compressed pcu (primitive cubic) net with nodes as [Cu5L4(PPh3)4]+ clusters. The basic repeating unit of the 3D CSF contains a double cubane-like cage fused by two same compressed cubic supramolecular cages (Fig. 6A), of which each is formed by eight clusters through weak tecton-tecton contacts (i.e., CH···π and H···H interactions) (Supplementary Fig. 10). Interestingly, self-assembled hexafluorophosphate-dichloromethane anionic clusters of [PF6•(CH2Cl2)4]− are docked in the confined cavities of the double cubane-like cage, respectively. Structural characterization reveals that PF6− anion in the [PF6•(CH2Cl2)4]− only uses one fluoride atom as quadruple H-bond acceptors to bond with four CH2Cl2 molecules as single H-bond donor, thereby resulting in four weak nonconventional C-H···F hydrogen bonds (dH···F = 2.58 Å and θC−H···F = 144°). Strikingly, [PF6•(CH2Cl2)4]− has a C4 symmetry with a 4-fold axis passing through F-P-F of PF6−, thus producing two equivalent enantiomers, R-[PF6•(CH2Cl2)4]− and S-[PF6•(CH2Cl2)4]− (Fig. 6B). Notably, this is the first structural evidence on chiral PF6−-CH2Cl2 solvated anionic clusters imprisoned in the cage.
The calculated independent gradient model based on Hirshfeld partition (IGMH) analysis on the supramolecular species [Cu5L4(P(C6H4F)3)4]+•PF6− indicates the apparent strong CH···F interactions in the green section occur between C-H groups of the fluorobenzene rings and F atoms from PF6− anion (Fig. 6C), which is crucial for the enhanced stability of the rigid supramolecular framework in 1. In contrast, 2 does not form the target [Cu5L4(PPh3)4]+•PF6− due to negligible weak CH···F interactions between PF6− and phenyl ring (Fig. 6C). On the contrary, the PF6− in 2 has a high tendency to be solvated by CH2Cl2 molecules, leading to the formation of solvated anions as discussed above. With the above considerations in mind, we surmised that the absence of strong charge-assisted CH···F H-bonding interactions in 2 will make the CSF more labile. As anticipated, the crystals 2 are extremely fragile, quickly cracked and lost their crystallinity after being taken out of the mother liquor.
Solid-state UV/Vis diffuse-reflectance spectroscopy of the crystalline sample of 1 displayed that it is transparent in the visible region of 400–800 nm (Fig. 6D). We investigated the steady-state photoluminescence (PL) spectrum of 1 at room temperature, which presented a weak green emission band at around 547 nm upon excitation at 365 nm, with a large Stokes shift (~ 182 nm). Additionally, the temperature-dependent PL emission was also measured from 300 to 80 K (Fig. 6D). The PL spectrum blue-shifted with decreasing temperature from 547 nm at 300 K to 507 nm at 80 K. The corresponding PL intensity enhanced nearly 11-fold. The increased PL intensity upon cooling is a typical phosphorescence characteristic owing to the effective reduction of nonradiative decay at low temperature, whereas the 40-nm hypsochromic-shifted emission may be caused by the restriction of the rotation of the phenyl group attached to the imidazole moiety at low temperature. The emission decay curve monitored at 547 nm gave relatively long lifetimes of 13–16 ms at 80–170 K (Supplementary Table S3), which is even visible to the naked eye (inset of Fig. 6D). Both long emission lifetime (millisecond) and large Stokes shift in 1 suggest the phosphorescence nature of the green luminescence, primarily originating from a process involved ligand to metal charge transfer (LMCT) or ligand-to-metal-metal charge transfer (LMMCT)41–43.