Atomically-precise colloidal nanoparticles of cerium dioxide

Abstract

Synthesis of truly monodisperse nanoparticles and their structural characterization to atomic precision are important challenges in nanoscience. Success has recently been achieved for metal nanoparticles, particularly Au, with diameters up to 3 nm, the size regime referred to as nanoclusters. In contrast, families of atomically precise metal oxide nanoparticles are currently lacking, but would have a major impact since metal oxides are of widespread importance for their magnetic, catalytic and other properties. One such material is colloidal CeO₂ (ceria), whose applications include catalysis, new energy technologies, photochemistry, and medicine, among others. Here we report a family of atomically precise ceria nanoclusters with ultra-small dimensions up to ~1.6 nm (~100 core atoms). X-ray crystallography confirms they have the fluorite structure of bulk CeO₂, and identifies surface features, H⁺ binding sites, Ce³⁺ locations, and O vacancies on (100) facets. Monodisperse ceria nanoclusters now permit investigation of their properties as a function of exact size, surface morphology, and Ce³⁺:Ce⁴⁺ composition.

Introduction

Since its introduction in 1976 as an oxygen-storage component to ensure the efficient activity of the noble metals used in three-way catalysis in automobile exhausts^1,2,3, cerium(IV) dioxide (CeO₂, ceria) has become of considerable utility as a catalyst or co-catalyst in industrial, petrochemical and environmental processes^2,3,4,5,6,7. In addition, CeO₂-containing materials are often used in oxide fuel cells⁸, precision polishing materials^9,10, UV filters¹⁰, corrosion prevention¹¹, and other applications^1,12,13.This widespread use of Ce is partially due to its significant abundance (0.0046% by weight of the Earth’s crust) and its Ce³⁺/Ce⁴⁺ redox couple, which is crucial to many applications by facilitating the formation of CeO_2−x , containing highly reactive defect sites comprising O vacancies and attendant Ce³⁺ ions^1,12,14. Ceria can thus act as an efficient oxygen buffer, assisted by oxygen mobility within its layered fluorite structure. In fact, bulk ceria naturally contains relatively few Ce³⁺/O-vacancy defect sites at ambient temperatures, but their number increases at higher temperatures where Ce⁴⁺ reduction and oxygen release are favoured. Catalysis by bulk ceria is therefore normally carried out at temperatures > 450 °C.

In the last decade, interest in ceria nanoparticles (CNPs) has seen explosive growth due to their much greater reactivity and increased catalytic efficiencies at lower temperatures^9,12. Significant CNP activity at or near room temperature has also been established¹⁵, as has facet-dependent reactivity^12,16. For example, appreciable oxygen storage capacity is observed at 150 °C on the cubic (100) face of nanoceria crystals, which is ~250 °C lower than for irregularly shaped nanoceria or the bulk material¹⁷. CNPs are also under investigation as photovoltaic materials in solar cells whereas bulk CeO₂ has no photovoltaic response¹⁸. Using CNPs instead of a cerium oxide support increases by two orders of magnitude the activity of a Au catalyst for the selective oxidation of CO¹⁹. In addition, the higher reactivity of CNPs at ambient temperatures is permitting many important biomedical applications to be developed, such as scavenging of reactive oxygen species (ROS)^12,20,21,22. The CNP activity and toxicity to living tissue clearly depend on particle size and surface composition (e.g., Ce³⁺/Ce⁴⁺ ratios), but as is normally the case in all areas of nanoparticle science, the problems of polydispersity, agglomeration, and surface variations have plagued detailed study of these parameters^10,23,24. For CNPs, it is particularly challenging to determine the concentration and locations of Ce³⁺, attendant O vacancies, and protonated O (i.e., OH⁻, H₂O) species on the surface, and the relationship between them¹⁰. A more controlled approach to monodisperse CeO₂ nanoclusters and nanoparticles is clearly needed, especially at the ultra-small, sub-20 nm sizes that are of growing importance, particularly for biomedical applications.

We now describe development of procedures using simple Ce⁴⁺ salts and organic reagents that yield a family of monodisperse ceria nanoclusters of different sizes depending on the carboxylic acid employed. Such an approach was recently accomplished for monodisperse metal nanoclusters, primarily of Au, stabilized by thiolate ligands^25,26. In our work, the ligands of choice for metal oxide nanoclusters are carboxylates, especially since oleic and similar acids are common surfactants in metal oxide nanoparticle syntheses^20,27. The solubility and monodisperse nature of the products we obtain allows molecular crystals to be grown, leading to structural characterization of the nanoclusters and their surface features to atomic precision by single-crystal X-ray diffractometry. The nanoclusters are [Ce₂₄O₂₈(OH)₈(PhCO₂)₃₀(py)₄] (1; Ce₂₄), [Ce₃₈O₅₄(OH)₈(EtCO₂)₃₆(py)₈] (2; Ce₃₈) and [Ce₄₀O₅₆(OH)₂(MeCO₂)₄₄(MeCO₂H)₂(py)₄]/[Ce₄₀O₅₆(OH)₂(MeCO₂)₄₄(MeCN)₂(py)₄] (3a/b; Ce₄₀), where py is pyridine. 3a/3b denote the two independent nanoclusters in the asymmetric unit of 3, which differ slightly in the organic ligation. 2 also contains two independent nanoclusters (2a/2b), but they have the same formulation.

Results

Nanocluster structures

Several pertinent points about the complete structures of 1–3 (Fig. 1a–c) will be summarized to allow for convenient comparisons. They all comprise Ce _x O _y cores (excluding carboxylate O atoms) with metal (x) and total (x + y) atom counts of 24/60, 38/100 and 40/98 for 1–3, respectively, and they exhibit the same fluorite structure as bulk CeO₂, i.e., alternating layers of 8-coordinate cubic Ce⁴⁺ and 4-coordinate tetrahedral O²⁻ions. Some surface Ce⁴⁺ ions are 7- or 9-coordinate (vide infra). From the viewpoint of Fig. 1d–f, the cores consist of five Ce layers in an A:B:C:B:A pattern (1, A = 2, B = 6, C = 8; 2, A = 4, B = 9, C = 12; 3, A = 4, B = 10, C = 12), leading to the Ce₃₈ core of 2 being essentially spherical (1.12 × 1.12 × 1.12 nm) whereas those of Ce₂₄ (1, 0.75 × 1.10 × 1.10 nm) and Ce₄₀ (3, 1.13 × 1.13 × 1.61 nm) are ellipsoidal. The Ce₃₈ can also be described as a ‘truncated octahedron’, a structure that is one of those recently predicted by DFT studies to be favoured for Ce _x O _y fragments of CeO₂ in this size range²⁸. 2 contains only Ce⁴⁺, but 1 and 3 each also contain two 10-coordinate Ce³⁺ ions at opposite ends of the cores, as suggested by DFT calculations on Ce³⁺ in Ce _x O _y fragments of CeO₂ ^28,29. The Ce oxidation states were confirmed by bond valence sum (BVS) calculations (Supplementary Table 2) and the detection of Ce³⁺ (S = ½) EPR spectra for 1 and 3. The latter were measured on microcrystalline powders at 5 K (Supplementary Figs. 1 and 2) and are comparable with the few Ce³⁺ EPR spectra reported for CeO₂ nanoparticles or Ce³⁺ doped into polymeric species^30,31,32. Nanoclusters 1–3 are large enough to display multiple facets (Fig. 1g–i), as do CNPs; the different faces for CeO₂ and Ce₄₀ are defined in Supplementary Fig. 3. 1 and 2 display only (100) and (111) facets, whereas 3 exhibits these and also (110) facets. Finally, the cores are enveloped within monolayer organic shells of carboxylate and py groups, which exhibit only minor positional disorder in some C and O atoms (Fig. 1a–c).

Fig. 1

Structures of ceria nanoclusters. a–c show the complete structures of 1 (Ce₂₄), 2(Ce₃₈), and 3a (Ce₄₀), respectively. H atoms have been omitted for clarity. Atom sizes of C, N, and O are made small to emphasize Ce locations. Colour code: Ce^IV gold, Ce^III sky blue, O red, N blue, C grey. d–f show their Ce/O cores from the same viewpoint (including carboxylate O atoms that are bridging) using the same colour code except that protonated O atoms (i.e., OH⁻ ions) are indicated in purple. g–i show the cores again, from approximately the same viewpoint but with surface facets colour-coded: (100) facets are blue; (110) facets are violet; (111) facets are green. Only carboxylate O atoms that are bridging are included

The structural results thus strongly support the description of 1–3 as atomically precise ceria nanoclusters in the ultra-small size range corresponding to the smallest CNPs synthesized to date, and stabilized to agglomeration by the organic monolayers. CNPs at this sub-20 nm size are being heavily targeted for use in various applications, especially in the biomedical field because they show enhanced catalytic activity and regenerative properties^21,33,34. 1–3 are larger than the few previously known Ce/O molecular species, most of which are Ce₆ species^35,36 and some with tridentate amino-alcohol N,O,O-chelates³⁷. It should be noted that the large family of monodisperse, crystalline polyoxometalates, some with very high metal nuclearities and sizes approaching 4 nm, have been known for many decades, but they do not possess the structure of bulk metal oxides and therefore cannot be described as their nanoparticles.

Surface features

X-ray crystallography has allowed definition to atomic resolution of the surfaces, which are crucial to CNP reactivity. The overall question is how the geometry and environments of surface Ce and O atoms differ from those of body atoms. Indeed, surface Ce⁴⁺ geometries in 1–3 differ markedly from the 8-coordinate cubic of body Ce⁴⁺ ions. Even those still 8-coordinate are significantly distorted, while many are 9-coordinate and there is even rare 7-coordination for Ce12/Ce22 in 3a/3b, respectively; coordination numbers are listed for all Ce atoms in Supplementary Table 2. This variety reflects both the greater degrees of freedom at the surface and at the carboxylate ligation. Nevertheless, all body and surface Ce atoms are essentially at the positions they would occupy in bulk CeO₂, as shown by the overlays in Supplementary Fig. 4. The larger nanoclusters Ce₃₈ (2) and Ce₄₀ (3) show very little deviation of Ce and O atoms from their positions in bulk ceria; the smallest, Ce₂₄ (1), appears more pliable by showing greater deviation, but it is still small. Thus, the Ce _x O _y cores of 1–3 really can be described as fragments of bulk ceria, stabilized/passivated by the monolayer of carboxylate and pyridine ligands.

There are four types of carboxylate binding in 1–3 (Fig. 2a–d): chelating (η ²) and three doubly or triply bridging modes, allowing for flexibility and versatility in binding to one, two, or V-shaped sets of three surface Ce ions. The carboxylates can thus accommodate the multi-faceted surface structure, including points of high curvature (Supplementary Fig. 5), with terminal py groups completing ligation where necessary. Both types of μ₂-carboxylates occur in all three nanoclusters and bridge Ce₂ edges joining two facets, one of which is always a (111) facet (Table 1 and Fig. 2e–g). Interestingly, the η ²:μ₂ mode is found only at (100)(111) and (110)(111) edges, whereas the μ₂ mode is found only at (111)(111) and (110)(111) edges. In contrast, μ₃-carboxylates occur only in 3, bridging a V-shaped edge of the (110) facets. The η ²-chelating mode is also only found in 3, always bound to one Ce of a (100) Ce₄ square (vide infra). Terminal py ligands occur in all three nanoclusters, always ‘capping’ a (111) hexagon, i.e., attached to its central Ce (Fig. 2e). The two independent Ce₃₈ nanoclusters in 2 are identical in formula and structure, but the two Ce₄₀ units in 3 provide the benefit of slightly differing organic monolayer shells, revealing one way the latter can vary for a given nanocluster core. Thus, the chelating carboxylates (Fig. 2a) on two Ce⁴⁺ ions (Ce9) in 3a are each replaced by a terminal MeCN (on Ce32) in 3b, converting 9-coordinate Ce9 into 8-coordinate Ce32.

Fig. 2

Ligand binding modes on the surface of ceria nanoclusters. The different binding modes of surface carboxylate and pyridine groups in 1–3: a chelating (η ²); b µ₂-bridging; c η ²-chelating and µ₂-bridging; d µ₃-bridging; e Ce₃₈ (2) showing terminal pyridines ‘capping’ (binding to the center of) the (111) hexagons, and µ₂-carboxylates bridging edges joining two (111) facets; f Ce₃₈ (2) showing η ²:μ₂-carboxylates at edges joining (100) and (111) facets; and g Ce₄₀ (3) showing η ²:μ₂- and μ₃-carboxylates on edges of (110) facets, and µ₂-carboxylates bridging edges joining (110) and (111) facets. Colour code: Ce^IV gold, Ce^III sky-blue, O red, N blue, C grey, (100) facets dark blue; (110) facets violet; (111) facets green

Table 1 Type of surface ligands in nanoclusters 1–3

There are two distinct Ce surface subunits in 1–3 resulting from the fluorite structure, Ce₃ triangles and Ce₄ squares, and these will be described in turn. Ce₃ triangles are very common surface units in (111) and (110) facets and are bridged primarily by pyramidal μ₃-O^2- ions (Table 1), from tetrahedral body O²⁻ ions now binding one less Ce. Some in 1 and 2 are instead bridged by μ₃-OH⁻ ions (Fig. 3a): The four in 1 are obvious from their O-H···N hydrogen bonding to lattice py molecules (O···N = 2.7–2.9 Å), which thus anchors the H⁺ on O15 and O16 and gives the expected O BVS of 1.21 (Supplementary Table 3). In contrast, the two μ₃-OH⁻ in 2 are disordered since there is no reason for H⁺ to favour particular μ₃-O²⁻ ions when so many are essentially equivalent. Slightly lowered BVS values (1.52–1.72) for the four μ₃-O²⁻ ions at O18/O39 and the four at O49/O60 in 2a and 2b, respectively (Supplementary Table 4), suggest that the 2H⁺ are randomly distributed primarily among these positions to give partial μ₃-OH⁻ occupancies.

Fig. 3

Structural features on the surface of ceria nanoclusters. a μ₃-OH⁻ on (111) Ce^IV ₃ triangle; b μ₄-OH⁻ on a (100) Ce^IV ₄ square; c μ₄-OH⁻ on a (100) Ce^IIICe^IV ₃ square; d η ²-carboxylates in 3a acting as lids on adjacent (100) Ce^IV ₄ and Ce^IIICe^IV ₃ squares; e the analogous situation in 3b to that in d, with an MeCN ligand replacing one η ²-carboxylate as lid; and f μ₄-OH⁻ lids on a V-shaped (100) Ce^IIICe^IV ₃double-square in 1 linked at the Ce^IIIcorner. Color code: Ce^IV gold, Ce^III sky-blue, O red, OH⁻ purple, N blue, C grey. H atoms have been omitted for clarity

In body Ce₄ squares, each edge is oxide-bridged, but at the surface the edges are carboxylate-bridged. These are the (100) facets (Fig. 1g–i) and occur in three slightly different forms. The six separated Ce⁴⁺ ₄ squares in 2 (Fig. 3b), the two Ce⁴⁺ ₃Ce³⁺Ce⁴⁺ ₃ V-shaped double-squares fused at a Ce³⁺ corner in 1 (Fig. 3f), and two Ce⁴⁺ ₃ Ce³⁺ squares in 3 (Fig. 3c) are all bridged by a μ₄-OH⁻ ion with rare tetragonal pyramidal geometry (the O is 0.7–0.8 Å above the Ce₄ plane). All μ₄-OH⁻ ions have similar O BVS values of 0.52–0.71 (Supplementary Tables 3–5), intermediate between those of OH⁻ and H₂O. In 1 (but not 2 or 3), the μ₄-OH⁻ protons (H12 and H14) were observed in difference Fourier maps, confirming them (and by extension those in 2 and 3) to be OH⁻, not H₂O. The Ce⁴⁺··OH⁻and Ce³⁺··OH⁻distances are extremely long (2.7–3.0 Å; Supplementary Table 7) and suggest minimal Ce-O bonding; for comparison, Ce⁴⁺-μ₃-O²⁻ = 2.2–2.3 Å, Ce⁴⁺-μ₄-O²⁻ = 2.3–2.35 Å, and Ce⁴⁺-μ₃-OH⁻ = 2.3–2.45 Å. The very-long Ce··μ₄-OH⁻ distances suggest an essentially free OH⁻ ion acting as a weakly docked ‘lid’ on the Ce₄ surface (and thus rationalizing its small BVS). Space-filling representations (Supplementary Fig. 6) show the OH⁻ to be encapsulated by the surrounding carboxylates and cannot move from its μ₄ central position to become more strongly bound μ₂ or μ₃.

3 also contains planar double-square units (Fig. 3d, e), and these do not contain μ₄-OH⁻ ions. Instead, those in 3a (Fig. 3d) have η ²-carboxylates attached to one Ce that act as lids, tilting inwards so that one O atom approaches the mid-point of each square; the three resulting Ce··O separations (~3.0 Å) indicate extremely weak contacts (Supplementary Table 8). In 3b, one η ²-carboxylate of each double-square is replaced by an MeCN, as described above, and this again tilts over the center of the square to act as a lid, giving a very unusual bent binding mode. The three resulting Ce··N separations (>3.0 Å) again indicate only very weak contacts. Interestingly, these planar double squares in 3 are each fused at their Ce³⁺ corners to the μ₄-OH⁻-bridged Ce⁴⁺ ₃Ce³⁺ squares (Fig. 3c) described above, so that 3 contains two asymmetric L-shaped (86.1°) triple squares with the Ce³⁺ lying at the inner point of the L. For charge balance, 3a must also contain two additional H⁺. Since the O BVS values indicate they are not on surface μ₃-O²⁻ ions, we suspected them to be on ligand groups. Indeed, three carboxylate O atoms attached to Ce³⁺, namely O40, O40′ and O83, form a triangle and all show lowered BVS values of 1.78, 1.78 and 1.65, respectively, suggesting an H⁺ is capping each of the two triangles in 3a by interacting with the O atoms in a trifurcated fashion (Supplementary Table 5). The formulation of 3b can now be rationalized as resulting from loss of some of the chelating MeCO₂ ⁻ groups in 3a, assisted by protonation to MeCO₂H by these H⁺, and replacement by MeCN solvent molecules in 3b.

The two Ce³⁺ each in 1 and 3 are thus all at surface sites, as suggested to also be the case in CNPs^10,38. The lower Ce³⁺charge favors fewer O²⁻ ligands than Ce⁴⁺ and thus disfavors body sites. In contrast, 2 contains no Ce³⁺. Interestingly, all Ce³⁺ occur within (100) Ce⁴⁺ ₃Ce³⁺ square facets. In 3 (Fig. 3c), the Ce³⁺··OH⁻ and Ce⁴⁺··OH⁻ distances are identical (~2.7 Å), again supporting weak contacts by the μ₄-OH⁻. In 1, the V-shaped double-square joined at the Ce³⁺corner (Fig. 3f) has a Ce³⁺··OH⁻ distance of ~ 2.7 Å on one side, but this causes a longer Ce³⁺··OH⁻ on the other (~3.0 Å; Supplementary Table 7). It is also extremely interesting that when Ce³⁺ ions are present, the surface H⁺ (i.e., OH⁻ ions, and H⁺ hydrogen bonding to carboxylate groups) are located very close to them. The presence and positions of H⁺ on nanoparticles are extremely challenging to determine³⁹, but in nanoclusters 1 and 3 most of them are directly observed and clearly accumulate on O atoms near Ce³⁺ (Fig. 1d, f). The effect is likely synergistic, i.e., the lower Ce³⁺ charge favors accumulation of H⁺ nearby, which in turn mollify the O²⁻ and carboxylate charges and stabilize the lower Ce³⁺ charge. In contrast, with no Ce³⁺ in 2, the H⁺spread out over the surface (Fig. 1e), although they again favor Ce₄ squares. H⁺ are expected to be mobile on the nanocluster surfaces, as recent work has concluded from studies of hydrogen mobility (‘hopping’) on surface O atoms of CeO₂ thin films⁴⁰. Double protonation of an O²⁻ and desorption of surface H₂O was suggested as the means of forming O vacancies.

Discussion

We have shown that a bottom-up synthetic approach in solution at ambient temperatures using readily available reagents can be successfully applied to obtain a family of monodisperse metal oxide nanoparticles of ultra-small dimensions. This thus achieves for metal oxides what was previously accomplished for the distinctly different area of metal nanoparticles, particularly of Au. In the present work, monodisperse CeO₂ nanoclusters with the fluorite structure and monolayer organic ligand shells can be synthesized and structurally characterized to atomic resolution. They exhibit multifaceted structures consisting mainly of (100) and (111) facets, but 3 also has (110) facets giving noticeable surface kinks/edges/trenches. The surface location of any Ce³⁺ ions and the H⁺ positions on μ₃- and μ₄-OH⁻ groups, as well as ligand groups, are particularly welcome to know. The μ₄-OH⁻are weakly attached with long Ce···O distances to the (100) facets, acting as lids on Ce₄ squares, as also do O (carboxylate) and N (MeCN) lids on other (100) facets. Such surface features are likely of great relevance to CNP reactivity: Under heterogeneous catalysis conditions, or in solution or colloidal suspension, one can envisage the ready loss or ‘opening’ of such weakly interacting lids (e.g., by protonation of OH⁻, detachment of MeCN, or tilting away of the chelating carboxylate, perhaps by becoming monodentate) exposing Ce₄ square faces for reaction. We thus propose these weakly lidded Ce₄ sites as resting states of some of the catalytically highly reactive, surface O-vacancy sites in CNPs. In addition, when Ce³⁺ ions are present, their locations in 1 and 3 corner-linking two (100) Ce³⁺Ce⁴⁺ ₃ squares, and the concomitant accumulation nearby of mobile H⁺, on μ₃-OH⁻, μ₄-OH⁻ and/or ligand groups, together offer a possible picture for the high catalytic activity of surface Ce³⁺ in CNPs. Similarly, the kinks/edges/trenches associated with the (110) facets in 3 suggest additional sites of increased reactivity, as seen for (110) facets of CNPs, and they have also been identified in CNPs as nucleation sites for heterometals^41,42, a process we are trying to mimic with 3. We note that there is a general consensus that the (111) facet of CNPs is the most thermodynamically stable while the (100) facet is highly reactive due its lower stability and is therefore a proposed site for O vacancies and Ce³⁺ ions^17,29,43, observations that are consistent with the surface features we have identified in 1–3.

Even on the basis of only the three nanoclusters described herein, it is already apparent how CNPs with similar sizes can have very different properties and reactivities. Although 2 and 3 are essentially the same size and metal nuclearity, they differ significantly in their overall shape, the variety of facets they exhibit, the resulting surface morphology, and their Ce³⁺ content. On the other hand, the availability now of samples of identical, monodisperse nanoclusters makes possible the study of activity vs. exact size, surface morphology and Ce³⁺ content. In addition, while dispersions of CNPs in water are often unstable, leading to agglomeration that can affect their transport, distribution and reactivity, particularly for ultra-small CNPs in biomedical studies, 3 is completely water soluble and affords the opportunity to study reactivity in biologically relevant media^27,44.

Finally, 1–3 contain either Ce⁴⁺ ₄-μ₄-OH⁻ or Ce³⁺Ce⁴⁺ ₃-μ₄-OH⁻(100) squares, or both, and this variation may also be responsible for the recognized redox-state dependent ROS-scavenging ability and toxicity of CNPs with different amounts of surface Ce^{3+ 21,22,45,46}. In contrast, recent suggestions that 1.1–3.5 nm CNPs should have a defect-fluorite structure and a large surface Ce³⁺:Ce⁴⁺ ratio are not supported by 1–3 ^47,48. Ce³⁺ is certainly on the surface, but no correlation between size and the number of Ce³⁺ is seen, with Ce₂₄ and Ce₄₀ having two each, but Ce₃₈ none. In fact, given that Ce³⁺ ions in 1–3 always occur at the centre of V-shaped double-square subunits (Fig. 3f and similar), we hypothesize that the higher symmetry, essentially spherical 2 contains no Ce³⁺ because its surface structure contains no such double squares. Furthermore, the conventional wisdom that smaller CNPs have more surface Ce³⁺ may reflect the greater number of such V-shaped units present in smaller nanoparticles of lower symmetry as a result of their increased number of points of high curvature. We are currently seeking to extend the family to larger nanoclusters and higher Ce³⁺:Ce⁴⁺ ratios, and exploring the reactivity of 1–3 with ROS.

Methods

Syntheses

[Ce₂₄O₂₈(OH)₈(PhCO₂)₃₀(py)₄] (1) was prepared by the reaction of (NH₄)₂[Ce(NO₃)₆] and PhCO₂H in a 1:2 molar ratio in pyridine at room temperature. The golden-yellow solution was stirred for 30 mins, diluted with 2 volumes of MeCN, and maintained undisturbed for 1 week. The resulting yellow square plates of 1∙9py were collected by filtration, washed with MeCN, and dried in vacuum. The yield was 14% based on Ce. Anal. Calcd (Found) for dried 1∙2py (C₂₄₀H₁₈₈Ce₂₄N₆O₉₆): C, 35.79 (35.63); H, 2.35 (2.00); N, 1.04 (0.98).

[Ce₃₈O₅₄(OH)₈(EtCO₂)₃₆(py)₈] (2) and[Ce₄₀O₅₆(OH)₂(MeCO₂)₄₄(MeCO₂H)₂(py)₄]/ [Ce₄₀O₅₆(OH)₂(MeCO₂)₄₄(MeCN)₂(py)₄](3) were prepared by the reactionof (NH₄)₂[Ce(NO₃)₆], the corresponding RCO₂H, and NH₄I in a 1:4:1 molar ratio in pyridine/H₂O (10:1 v/v) at room temperature. The golden-yellow solutions were stirred for 30 min, diluted with two volumes of MeCN, and maintained undisturbed for 4 weeks. The resulting yellow square plates (2∙16MeCN) or rods (3∙48MeCN) were collected by filtration, washed with MeCN, and dried in vacuum. The yields were 49% and 35% for 2 and 3, respectively. Anal. Calcd (Found) for 2∙7H₂O (C₁₄₈H₂₄₂Ce₃₈N₈O₁₄₁): C, 18.30 (17.90); H, 2.51 (2.36); N, 1.15 (1.05). Anal. Calcd (Found) for 3∙8H₂O (C₁₁₂H₁₇₈Ce₄₀N₅O₁₅₆): C, 13.88(13.79); H, 1.85 (1.78); N, 0.72 (0.76). The indicated atomic composition of 3 is calculated using the average of the 3a and 3b formulas.

Fuller details of the three syntheses and infra-red spectral data for 1–3 are available in Supplementary Methods.

X-ray crystallography

Single-crystal X-ray diffraction studies at −173 °C were performed on a Bruker DUO diffractometer using MoK_α (λ = 0.71073 Å) or CuK_α (λ = 1.54178 Å) radiation (from an ImuS power source), and an APEXII CCD area detector (Supplementary Methods and Supplementary Table 1). The metric parameters of the refined structures were used to determine the Ce oxidation states and the O protonation levels by bond valence sum (BVS) calculations (Supplementary Tables 2–5).

Data availability

The crystallographic information files (CIFs) for [Ce₂₄O₂₈(OH)₈(PhCO₂)₃₀(py)₄]∙9py (1∙9py), [Ce₃₈O₅₄(OH)₈(EtCO₂)₃₆(py)₈]∙16MeCN (2∙16MeCN), and [Ce₄₀O₅₆(OH)₂(MeCO₂)₄₄(MeCO₂H)_2/0(MeCN)_0/2(py)₄]∙48MeCN (3∙48MeCN) have been deposited at the Cambridge Crystallographic Data Centre with deposition codes CCDC 1529955-1529957 for 1–3, respectively.