A Toroidal Zr70 Oxysulfate Cluster and Its Diverse Packing Structures

Abstract Herein, we report the discovery of a toroidal inorganic cluster of zirconium(IV) oxysulfate of unprecedented size with the formula Zr70(SO4)58(O/OH)146⋅x(H2O) (Zr70), which displays different packing of ring units and thus several polymorphic crystal structures. The ring measures over 3 nm across, has an inner cavity of 1 nm and displays a pseudo‐10‐fold rotational symmetry of Zr6 octahedra bridged by an additional Zr in the outer rim of the ring. Depending on the co‐crystallizing species, the rings form various crystalline phases in which the torus units are connected in extended chain and network structures. One phase, in which the ring units are arranged in layers and form one‐dimensional channels, displays high permanent porosity (BET surface area: 241 m2 g−1), and thus demonstrates a functional property for potential use in, for example, adsorption or heterogeneous catalysis.

Toroidal (ring-shaped) units of molecular dimensions are visually appealing due to their peculiar symmetry,h ave an intrinsic high surface area due to their geometry and the possibility of forming host-guest ensembles by filling of their inner cavity.T hese traits are favorable for the formation of regular, yet multifunctional surfaces,which can be employed in for example,a dsorption and catalysis.R ing-shaped inorganic entities on the nanometer scale,a lbeit rare,a re normally found as (ionic) polyoxometalates (POMs) and coordination complexes (containing organic ligands), prominent examples of which include the Mo 154 "big-wheel", [1] Pd macrocycles discovered by directed solution techniques, [2] and more recently agigantic molecular wheel of Gd 140 [3] and aU 70 unit. [4] Zr-based clusters have gained considerable attention after the discovery of carboxylate oxoclusters and, especially,Z rbased Metal-Organic Frameworks (MOFs). Theh exameric dodecacarboxylate cluster found in most Zr-based MOFs was first isolated from an organic solution in 1997, [5] then as aglycine cluster from aqueous phase (coincidently with noncoordinated sulfate) in 2008, [6] as ac ombined carboxylate/ sulfate cluster as part of aMOF structure in 2015, [7] and lastly as an isolated carboxylate/sulfate cluster in 2018. [8] The hexameric cluster motif is also found in newly reported doubly [9] and five-fold fused clusters. [10] Zr-based MOFs can also be obtained with Zr-coordinated sulfate introduced as ap recursor or postsynthetically,a nd the materials show similar characteristics as sulfated zirconia catalysts. [11] Theaqueous chemistry of Zr/Hf sulfates has been studied in detail, and several crystalline species of oligomeric Zr oxysulfates have been described, with and without additional ligands.N otably,t he first large oxysulfate,Z r 18 (OH) 26 O 20 -(H 2 O) 23.2 (SO 4 ) 12.7 (Zr 18 ), was isolated and characterized in 1987. [12] It has recently been found that pre-nucleation, or the assembly of building units (BUs) in solution, plays alarge role in determining which species are found in the solid phase,and that the compositions of these BUs depend on the Zr:SO 4 ratio. [13] However,itisnot fully understood whether the BUs present in solution during the formation of solid Zr oxysulfate clusters have well-defined compositions and structures,o ri f their state is more fluid/amorphous. TheZ r 70 unit reported herein was discovered by serendipity while screening synthesis conditions for promoted Zrbased catalysts.Ahydrothermal reaction between Zirconium-(IV) sulfate and Magnesium(II) nitrate yielded large single crystals of an unknown phase with alarge unit cell, as evident by sharp low-angle reflections in its PXRD pattern (Figure S6). Structural analysis by single crystal X-ray diffraction (SC-XRD) revealed ac omplex structure consisting of large toroidal clusters (see Figure 1) 6 ] y hereafter called Zr 70 -mP-Mg (where mP signifies ap rimitive monoclinic crystal lattice). Each toroidal cluster in the Zr 70 -mP-Mg crystal structure was linked to two others by two bridging sulfates each, forming as taircase-like infinite polymeric chain (shown in Figure 2). Thechains stack in parallel (symmetric to each other by 2-fold rotation), in aherringbone-like packing mode ( Figure 3).
Curiously,m agnesium is not ap art of the ring structure, but occupies the interstitial space as ah exaaqua-complex stabilized by hydrogen bonds to the neighboring Zr 70 units. This realization prompted an investigation as to whether the Zr 70 toroids would form under similar conditions,b ut with other reagents in addition to Zr 4+ sulfate.F rom previous reports it is known that the Zr:SO 4 ratio should be lower than the 1:2s toichiometry found in the precursor to promote the formation of oligomeric species in solution. Since the Zr 70 had formed just by adding magnesium nitrate,itwas assumed that other metal nitrates would promote asimilar reaction. In such as olution of two salts,t he complexation between the secondary metal and sulfate in the solution would adjust the Zr:SO 4 ratio in situ. In parallel, additions of Zirconium(IV) oxynitrate (ZrO(NO 3 ) 2 )w as also investigated to obtain solutions with adjusted Zr:SO 4 ratios without the presence of secondary cations.A ll syntheses were conducted at hydrothermal conditions (185 8 8C) using Te flon-lined steel autoclaves,a nd the obtained products are summarized in Table 1.   From SC-XRD structure determination, four phases were identified, consisting of the same toroidal Zr 70 cluster arranged in different packing modes.T he packing is clearly influenced by the identity of the co-reagent. All M II nitrates direct to am onoclinic (mP) or closely related triclinic (aP) phase in which the Zr 70 rings are fused to each neighboring unit with two bridging sulfate anions,and the resulting chains packed in ah erringbone pattern. Nitrates of sodium and aluminum direct to an orthorhombic (oP) structure in which each torus is close to perpendicular to its nearest neighbor, but without directly bridging units.Inall of these cases,metal ions (as aqua-complexes) and water molecules occupy the interstitial space between the Zr 70 units.W hen Zr(SO 4 ) 2 is reacted with ZrO(NO 3 ) 2 or with benzyltriphenylphosphonium chloride (BTPPC,k nown to co-crystallize with organic cations), atetragonal phase (tI) is obtained, in which the rings are arranged in parallel square-grid layers (Figures 3a nd 4).
As triking difference between the three phases is the density of the packing of rings,t he relative magnitude of which can be derived from the unit cell volumes (see Table 2). Thehighest density phase,the stacked chains of fused rings,is obtained when M 2+ cations are present, and is the only phase in which the rings are connected by strong bonds via bridging sulfate ions.T he lowest density is found in the Zr 70 -tI phase, where the only cation present is Zr 4+ .T he low density of this phase indicate that the presence of other ions facilitates the denser packing modes,p erhaps by balancing the negative surface charge of the Zr 70 ring.
Thestructure of the Zr 70 torus is seemingly identical in all the reported structures,consisting of 10 repeating sub-units of Zr 7 (SO 4 ) 5-6 O 14 (Figure 1) related by a1 0-fold rotation axis. The1 0Z ra toms of the inner rim are connected by two parallel bridging sulfate ions each, whereas the 20 Zr of the outer rim of the ring are connected by either singly or doubly bridging sulfate ions,f or at otal of five or six sulfate per repeating sub-unit. Determined by the crystal structure refinements,t here are 4t o5Zr-Zr pairs in the outer rim connected by asingly bridging (bidentate) sulfate per toroid, whereas the two parallel sulfate that occupy the rest of the outer rim are tridentate (shown in Figure 5). Ther eason for this disorder is unclear, but could for example originate from the structure of the BUs present in the solution, provide the appropriate charge balance or minimize the strain in the toroid. All structures also contain monodentate sulfate ions on the side of the torus.
Zirconium is predominantly 8-coordinated in the structure,b ut some 7-coordinated species occur in the case of singly-bridging sulfate groups.T he arrangement of Zr atoms within each sub-unit can be regarded as an octahedron of six Zr-atoms,w ith m 3 -O or m 4 -O capping each facet. To account for the curvature of the toroid, the outer rim of the ring contains an additional Zr bridging the Zr 6 octahedra (Figures 1a nd S1).
In the aP and mP phases,each individual torus is bound to two neighbouring units with two bridging sulfate anions.T he single crystals of these phases break into fiber-like fragments when stressed (e.g.w hen prodded with an eedle), indicating  Overview of the synthesis screening and the obtained products. s: structure determined by SC-XRD, m: unit cell determined from SC-XRD, n: nanocrystallinephase also obtained (in separate syntheses).

,In(NO 3 ) 3 Unknown nanocrystallinesolid
[a] The abbreviatedn ames contain the bravais lattice of the obtained product:aP: triclinic, mP:p rimitive monoclinic, oP:primitive orthorhombic, tI:body-centeredtetragonal. significant anisotropy in its mechanical properties caused by the unidirectional strong bonds.T hese related phases are the only ones that have not been observed to crystallize at ambient temperature,w hich suggests that the bridged chains might need hydrothermal conditions to form. By contrast, both Zr 70 -tI and Zr 70 -oP-Na were observed to crystallize by slow evaporation of the mother liquor after hydrothermal treatment. Although this demonstrates the rings solubility in amixture of sulfuric and nitric acid, all Zr 70 materials reported herein shows very poor solubility in water and ethanol. From the diffraction data of the Zr 70 -oP-Na phase,itwas possible to resolve positions of many interstitial atoms (aquasodium complexes and water), showing anetwork of sodium ions and water between two rings ( Figure 5). Only in small parts of the structure is the disorder too large to be resolved. Judging from the positions of the Na + ions,w edged between the anion-rich surfaces of the rings,they seem to facilitate this packing mode.
Zr 70 -tI consists of layers of parallel tori arranged in asquare-grid pattern, co-planar with the a-b plane of the unit cell (see Figures 3a nd 4). Thes quare-grid arrangement indicates as tructure-directing interaction between the rings, since other packing modes (e.g.hexagonally arranged layers) would have provided higher density.T his is corroborated by the presence of monodentate sulfate ions only present on the axial surfaces that are not in close proximity to neighboring rings.T he layers of rings are stacked along the c-axis, alternating in an A-B-A-B configuration where the centroid of each torus of the Alayer is aligned with the space between four rings of the B-layer. This arrangement provides the basis of the tetragonal body-centered crystal structure.
TheZ r 70 -tI structure shows complex disorder.T he diffraction pattern indicates the space group I4/mmm,but this is not compatible with the point symmetry of Zr 70 :t he 4-fold rotation axis of the space group is co-aligned with the 10-fold rotation axis of the torus,a nd these rotation axes are incompatible.T he resulting crystal structure features disordered Zr 70 units,with (at least) two arrangements of the torus occupying the same volume of the unit cell. To verify that this disorder was intrinsic to the material (and not an artefact related to twinning);m ore than ten different crystals from three different batches were tested. PXRD refinements also show perfect agreement with I4/mmm ( Figure S5). The symmetry of the Zr 70 and its crystal structures are further elaborated in the supporting information (see for example, Figure S1).
As previously mentioned, the formation of the rings is likely enabled by ad isplacement of the Zr:SO 4 ratio.W hen as econd cation is present in the solution, this could form aqueous complexes with sulfate thus lowering the concentration of available sulfate.W hen the Zr oxynitrate and sulfate are mixed, the ratio is shifted by the increased concentration of Zr.I nt he case of Zr sulfate and BTPPCl, as econd solid phase of (BTPP) 2 (SO 4 )i sf ormed alongside Zr 70 -tI, highlighting how sulfate is abstracted by the secondary cation.
One systematic study of species in aqueous solutions across varying Zr:SO 4 ratios reports multimeric species in solution whose structure depend on this ratio,a nd finds similar characteristics between the large multimeric species in solution (probed by SAXS and HEXS) and a18-meric cluster precipitating from the examined solution. [13] This Zr 18 cluster has the formula [Zr 18 (OH) 26 O 20 (H 2 O) 23.2 (SO 4 ) 12.7 ]Cl 0.6 ·n H 2 O, which gives aZr:SO 4 ratio of 1:0.7, close to the corresponding ratio of 1:0.8 for Zr 70 .T he 18-mer shows strong structural resemblance of Zr 70 ,such as the arrangement of the central Zr atoms and the singly-and doubly-bridging sulfate ions in the outer rim ( Figure 6). Although the solution chemistry of Zirconium oxysulfates is well understood at ambient conditions,t he hydrothermal conditions seem to enable further oxolation of the oligomeric solution species into rings.T hese similarities imply that Zr 18 could be aprecursor of Zr 70 ,orthat the two have common solution precursors.
TheZ r 70 -tI phase displays infinite cylindrical channels along the c-axis,o fa pproximately 1nmi nd iameter. N 2 adsorption measurements show ac onsiderable permanent porosity (SA BET of 241 m 2 g À1 )w hich corresponds to av olumetric porosity of around 20 %( based on the solvent accessible pores of an N 2 -sized probe molecule of the crystal structure). Depending on the size of the adsorbate,the pores are accessible through the holes of the tori, or through smaller pore windows on the sides of the structure (Figure 7). The Figure 5. Partial unit cell of Zr 70 -oP-Na, from two sides, showing structurally resolved sodium ions (green). The sodium ions whose positions can be determined by diffraction, occupy the regions of close proximity between Zr 70 rings, presumably facilitating denser packing than in Zr 70 -tI. kinetic diameter of the side windows vary greatly,depending on the conformation of the closest sulfate groups.
These findings highlight an important difference between Zr 70 and the recently reported isostructural U 70 ,s ince different BUs have been identified in the two systems.Inthe case of Uranium, the anionic species [U 6 O 4 (OH) 4 (SO 4 ) 12 ] 12À can be isolated. Although numerous hexameric Zr units are known, the dodecasulfate has not yet been reported.
An important requirement for widespread application of am aterial is the ability to control its phase and morphology, by identifying the factors that govern it. One such example is provided with "POMzites", which consists of ring-shaped POMs (P 8 W 48 O 184 ) 40À linked by transition metal ions which direct different network structures. [14] It is likely that further ring packing topologies of Zr 70 will be discovered in the future,a nalogous to how 14 different ring packing modes have been discovered for the POMzites.
In summary,w eh ave reported an ew,i norganic toroidal Zr 70 oxysulfate cluster that can form av ariety of phases,o ne of which exhibits significant permanent porosity.A lbeit structurally complex, Zr 70 consists of simple building units and is very simple to synthesize.T his Zr 70 oxysulfate cluster likely forms by oxolation of already known oligomeric species,f ollowing in situ adjustment of the Zr:SO 4 ratio in the solution. Thediversity of packing structures exhibited by the Zr 70 units,i ncluding one with considerable permanent microporosity,i nc ombination with the described structuredirecting parameters (i.e.the nature of the co-reacting metal ion and hydrothermal conditions) may provide ar obust platform for the development of new,Z r 70 -based materials with novel functional properties.