Lanthanides Singing the Blues: Their Fascinating Role in the Assembly of Gigantic Molybdenum Blue Wheels.

Molybdenum blues (MBs) are a distinct class of polyoxometalates, exhibiting versatile/impressive architectures and high structural flexibility. In acidified and reduced aqueous environments, isopolymolybdates generate precisely organizable building blocks, which enable unique nanoscopic molecular systems (MBs) to be constructed and further fine-tuned by hetero elements such as lanthanide (Ln) ions. This Review discusses wheel-shaped MB-based structure types with strong emphasis on the ∼30 Ln-containing MBs as of August 2021, which include both organically hybridized and nonhybridized structures synthesized to date. The spotlight is thereby put on the lanthanide ions and ligand types, which are crucial for the resulting Ln-patterns and alterations in the gigantic structures. Several critical steps and reaction conditions in their synthesis are highlighted, as well as appropriate methods to investigate them both in solid state and in solution. The final section addresses the homogeneous/heterogeneous catalytic, molecular recognition and separation properties of wheel-shaped Ln-MBs, emphasizing their inimitable behavior and encouraging their application in these areas.

Introduction

Orthomolybdate ([MoO4]2–) solutions exhibit a unique chemistry as they allow for the assembly of nanosized scaffolds with the general formula [XYHMoVIMoVO(H2O)] ({MoXY} (a = number of organic ligands; b = number of metallic hetero elements; c = degree of protonation; x and y = number of unreduced and reduced molybdenum, respectively; z = number of oxygens; v = number of coordinated water; n = resulting charge of the nanosized scaffold; w = sum of unreduced and reduced molybdenum). They are up to ∼4 nm large (for wheel-type clusters)[1] and exhibit unprecedented topology.[2] In a one-pot reaction an aqueous orthomolybdate solution is acidified (pH ≤ 4.5)[3] to initiate the polymerization process resulting in a number of isopolyoxomolybdates (IPOMos) (Figure ). Generally, polymerization processes of orthomolybdate [MoO4]2– occur in solutions with concentrations greater than 10–4 M (Figure ).[4] The speciation of [MoO4]2– in acidified solutions with different concentrations and ionic strengths has been comprehensively investigated.[5−8] In strongly alkaline environments, the monomeric [MoO4]2– anion predominates, while in strongly acidic solutions the dominant species are stable mono-[MoO2(OH)(H2O)3]+ or bi-[Mo2O5(OH)(H2O)5]+ oxocations able to form inorganic–organic complexes with organic ligands (e.g., dicarboxylic acids, hydroxyaldehydes, amines, etc.).[7,9−14] The major species formed first upon acidification of aqueous [MoO4]2– (Figure ) is heptamolybdate, H[Mo7O24](6– (Figure ), which reaches typically its maximum concentration at pH 5 and can be protonated upon further acidification.[4] When the environment becomes more acidic, the generation of β-[Mo8O26]4– prevails over H[Mo7O24](6– (x = 0–2; max. conc. of H[Mo7O24](6– is reached at pH 4.5) and reaches a maximum at pH ∼ 2.5. [Mo36O112(OH2)16]8– {Mo36} (Figure ), which is a dimer of [Mo18O56(H2O)8]4– with an inversion center, is the largest known IPOMo present in solution under nonreducing conditions below pH 2.8.[15]

Figure 1

IPOMos speciation in an aqueous solution with a MoVI concentration of 0.1–0.4 M.[4] The maximum strength of the blue color in each bar reflects the maximum concentration of the corresponding species in the respective pH region. The arrangement of the blue bars is merely based on the increasing nuclearity of the molybdate clusters and does not display any dominance over other species at a given pH range. x in H[Mo7O24](6– varies between 0 and 2. The crystallized IPOMos [Mo7O24]6–, β-[Mo8O26]4–, and [Mo36O112(OH2)16]8– ({Mo36}) are depicted in polyhedral representation. Coloring code: {MoO6}, rose and yellow; {Mo8O35}, blue with central {MoO7}-unit in cyan.

Upon acidification and subsequent addition of a reducing agent (e.g., N2H4, Na2S2O4) the orthomolybdate ([MoO4]2–) solution turns blue and the virtual linker units {MoO6} ({Mo1}), {Mo2O11} ({Mo2}) (Figure A)[16] and the fundamental building block {Mo8O35} ({Mo8}) with the pentagonal unit {MoO7Mo5O20} ({Mo(Mo)5}) (Figure A) are formed.[2] {Mo1}, {Mo2}, and {Mo8} can be linked together to form complex and large wheel-type structures (Figure ).[2]

Figure 2

Polyhedral representation of the building blocks and the lanthanide units. (A) Virtual linker units {MoO6} ({Mo1}) and {Mo2O11} ({Mo2}) and the fundamental building block {Mo8O35} ({Mo8}) form MB rings of different nuclearity.[2] (B) Lanthanide ions with a tricapped trigonal prismatic coordination sphere. Coloring code: {MoO6}, yellow; {Mo2O11}, red; {Mo8O35}, blue with central {MoO7}-unit in cyan; {LnO9}, green.

Figure 3

Simplified representation of all available types of MB/Ln-MB-based nanoclusters: (A) Complete MB (described in section ), (B) templated complete MB (section ), (C) lacunary MB (section ), (D) hybridized complete MB (section ), (E) templated and hybridized complete MB (section ), (F) lacunary hybridized MB (section ), (G) complete Ln-MB (sections . and 3.2.2.1), (H) templated complete Ln-MB (sections . and 3.2.2.2), (I) hybridized complete Ln-MB (sections . and 3.2.4.1.), and (J) templated and hybridized complete Ln-MB ring system (sections . and 3.2.4.2.). All cluster types are mainly synthesized in a one-pot synthesis, whereby the cluster type (G) may also be produced via a two-step synthesis by first synthesizing the cluster type C and then adding the lanthanide ions.

Solutions containing gigantic wheel-type clusters based on molybdenum oxide are referred to as “Molybdenum Blues” (MBs).[17] The reductively triggered blue color is a result of delocalized 4d electrons over the equator of these wheels.[17] MBs were first reported by Scheele,[18] but the structure of “amorphous molybdenum blues” remained unknown until the late 20th century when the giant wheel {Mo154} (Figure A) constructed from the building blocks {Mo1}, {Mo2}, and {Mo8} (Figure A) was synthesized and structurally elucidated by Müller et al.[19] The general formula for discrete pure molybdenum oxide-based wheels is [{MoVI2O5(H2O)2}2+{(MoVI/V8O26(μ3-O)2H(H2O)3MoVI/V)(4–}](2 (b = number of building units per set; m = number of protons; x = number of defect sites).[17] The pentagonal unit {(Mo)Mo5} is responsible for the distinctive circular structure of {Mo154}, which comprises a central pentagonal bipyramidal {MoO7}-unit (Figure A). The crystallization of {Mo154} (Figure A) was only made possible by destroying its hydration shell with high electrolyte concentrations.[17]

Figure 5

Polyhedral representation (and inner and outer diameter of the respective ring/rim) of the purely inorganic complete MB and all purely inorganic complete Ln-MB ring systems as of August 2021 described in this review starting with the (A) circle-shaped {Mo154}[19] (simplified archetype is shown in Figure A), (B) circle-shaped {Mo176}[1] (Figure A), (C) circle-shaped {Mo138}[20] (Figure C), (D) “Japanese rice-ball” {Mo120Ln6}[40] (Figure G), (E) egg-shaped {Mo96La8}[41] (Figure G), (F) egg-shaped {Mo100Ce6}[42] (Figure G), (G) ellipsoidal {Mo150La2}[40] (Figure G), (H) ellipsoidal {Mo134La10}[43] (Figure G), (I) circular-shaped {Mo92Ln9}[31] with LnIII = NdIII or SmIII (Figure G), (J) circular-shaped {Mo90Ln10}[31] with LnIII = LaIII, CeIII, or PrIII) (Figure G), and (K) ellipsoidal half closed {Mo130Ce6}[32] (Figure H). The open side is shown on the left and the closed side on the right. Coloring code: {MoO6}, yellow; {Mo2O11}, red; {Mo8O35}, blue with central {MoO7}-unit in cyan; {LaO9}, green.

Since 1995, a plethora of nanosized, molecular wheel-shaped clusters has been synthesized (Figure ), exhibiting different degrees of nuclearity (starting from {Mo138}[20] (Figure C) to {Mo176}[1] (Figure B)), whereby clusters of lower nuclearity ({Mo138} to {Mo152})[3] contain defect sites[3] (= vacancies within the MBs due to the lack of {Mo2}-units) (Figure C). {Mo176}[1] is the largest discrete wheel to date, which is constructed of two {Mo1}, {Mo2}, and {Mo8} building blocks more than {Mo154}.[19] The incorporation of these additional building blocks also results in the largest known nanocavity ({Mo154} exhibits a 20 Å and {Mo176} a 25 Å nanocavity; see Figure A and B).

Smaller fragments of suitable size, such as the IPOMo [Mo36O112(OH2)16]8– ({Mo36}),[15] can be templated into these wheel-shaped structures by stabilization in the central nanocavity of the wheel via supramolecular interactions, resulting in host–guest systems (e.g., {Mo36}⊂{Mo150} with the symbol “⊂” representing the inclusion of a template cluster in a ring) (Figure B).[21] This template effect was first realized by the incorporation of two nonisolable {Mo36O96(H2O)24} “hubcaps” into the nanocavity of the {Mo176}[1] nanowheel, thereby forming the “nanocookie” {Mo248}.[22]

Aside from generating defects in the cluster or introducing templates, another structural modification is the coordinative fixation of organic ligands onto {Mo2} ({MoVI2O5(H2O)2}2+)[17] units or replacement of {Mo2} units by metallic hetero elements of adequate size, such as lanthanide ions (Figure B) in the inner rings (the terms inner and outer ring/rim refer to the inner and outer regions of the wheels; Figure ). The coordinative attachment of organic ligands on MBs was achieved initially by hybridizing {Mo154} with cysteines.[23] This type of MBs (= organic–inorganic hybrids) (Figure D) represents a fusion of inorganic, nanosized giant clusters with organic components and possess characteristics that vary from their parent cluster, such as decreased solubility in comparison to the parent wheel[20,24] and a functionalized nanocavity (Figure B).

Figure 6

Polyhedral representation of a lacunary hybridized MB, hybridized complete MB, hybridized complete Ln-MB, and templated and hybridized complete Ln-MB ring systems described in this Review starting with (A) six acetates functionalizing {Mo138} forming the inorganic–organic hybrid {Mo138(acetate)6}[20] (simplified archetype is shown in Figure F), (B) the functionalized {Mo154} scaffold with eight l-histidines[24] (Figure D), (C) the host–guest complex {Mo36}⊂{Mo150} functionalized with six l-ornithine[38] (Figure E) [Reprinted with permission from ref (38). Copyright 2019 Wiley-VCH.], (D) the Λ-{Mo124Ce4}[24] (Figure I) scaffold functionalized with four d-arginines, (E) the Λ-{Mo124Ce4}[24] (Figure I) scaffold functionalized with six l-tryptophans, (F) the “Japanese rice-ball”-shaped α-{Mo8}@{Mo124Ce4(Orn)6}[38] (Figure J), (G) the elliptically shaped {Mo17}@{Mo150Ce2(Orn)6}[38] (Figure J), and (H) the elliptically shaped β-{PMo12}@{Mo150Ce2(Orn)6}[38] (Figure J) and (I) the Λ-{Mo124Ce4}[24] scaffold functionalized with six l-histidines with the chiral {Mo8} cluster entrapped in the nanocavity (Figure J). Coloring code: {MoO6}, yellow; {Mo2O11}, red; {Mo8O35}, blue with central {MoO7} unit in cyan; {CeO9}, green; Mo, blue spheres; C, gray spheres; O, red spheres; N, violet spheres.

The integration of lanthanide ions into MBs was accomplished first by the introduction of PrIII into an MB scaffold, forming the wheel {Mo120Pr6}[25] (Figure D), which exhibited a “Japanese rice-ball”-shape with an outer diameter of ∼31 Å, rather than a circular-shape as the parent wheel {Mo154} (Figure A).

The incorporation of lanthanide ions in MBs is particularly intriguing since new unique MB characteristics may arise. For example, when complexed, lanthanides can behave like typical Lewis acids and exhibit interesting catalytic properties.[26] In several organic transformation reactions, lanthanide-containing POMs show significant chemoselectivity.[27,28] Furthermore, lanthanide ions in POM-based nanostructures are also able to form inorganic–organic adducts, which can be exploited in chiral recognition.[29]

This Review addresses (i) the synthesis and reaction conditions of all available structural types of MB-based wheels (Figure ) with strong emphasis on their derivatization with lanthanide ions as the fascinating role of lanthanide ions in wheel-type MBs has not been covered yet in a review, (ii) thoroughly the geometrical changes and charge shifts to the parent {Mo154} cluster caused by the introduction of lanthanide ions, (iii) the inclusion of templates, and (iv) the introduction and role of (structure directing and chiral-inducing) organic ligands in MB-based architectures. As the main focus is on Ln-MB structures, techniques for confirming their intactness in solution as well as investigating their solution speciation (e.g., “blackberries”[30,31]) are presented using selected examples. The final section is devoted to potential applications of wheel-shaped Ln-MB architectures as homo- and heterogeneous catalysts as well as attractive components for molecular recognition. Additionally, specific Ln-MB syntheses are described as an elegant approach in lanthanide separation.[31]

General Synthesis Approach of MB/Ln-MB-Based Nanoclusters

The general approach to synthesize all types of MBs/Ln-MBs, which are shown in Figure , is quite straightforward. The partial reduction of molybdate(VI) solutions in an acidic environment is shared by all cluster type syntheses. Adjusted molybdate concentrations allow for the production of desirable clusters while preventing the formation of undesired byproducts.[22,32] A synthetic approach to force these cluster to crystallize is to destroy their hydration shell by a high electrolyte concentration.[17]

The amount of reducing agent has less of an effect on MB formation as the pH of the reaction solution decreases.[33] Above a pH value of ∼1.5, excessively high amounts of reducing agents (e.g., Na2S2O4) lead to brown reaction mixtures (a compound class containing the structural building block unit O–MoV–O–MoV–O)[17,19,33−36] rather than blue solutions. Typically, the pH is indirectly proportional to the nuclearity of the cluster (i.e., a decrease of the pH results in an increase of the clusters’ nuclearity)[3] and the molar ratio of molybdate/additional framework-building reagents (metallic heteroions, ligands) must be optimized for successful crystallization.[32,37]

Organic ligands for the synthesis of hybridized inorganic–organic MBs/Ln-MBs should have a carboxyl group in order to be able to attach to {Mo2} units (always depicted as red polyhedra in all the figures). Furthermore, amino groups in ligands (e.g., amino acids and peptides) are of essential importance particularly in Ln-MB syntheses when it comes to (a) stabilization of templates inside the rings,[38] (b) introduction of chirality,[24] and (c) modulation of the shape and size of Ln-MBs.[39]

Synthesis of MB/Ln-MB Based Nanoclusters

MB Based Nanoclusters

Complete MB ring systems (Figure A) such as {Mo154} are often synthesized at room temperature, with the pH value (typically below 1.4) being the most critical factor due to its strong effect on the nuclearity of the wheel.[1,19] The complete MB ring system can be transformed into templated complete MB ring systems (Figure B) or into lacunary MB ring systems by creating defects (= vacancies caused by a lack of {Mo2} building blocks) (Figure C). The inclusion of a template was first achieved by synthesizing the unprecedented structure {Mo248}, which is often described as a “nanocookie”[22] and the adjustment of the pH between 1.4 and 4.5 made it possible to synthesize MBs with a different number of defects.[3] The obtained defect-containing MB wheels are derived from the {Mo154}-cluster missing a certain number of {Mo2}-units (one unit at pH 1.4 and eight units at pH 4.5). They are specified with {Mo154–} (x = number of defect sites), with {Mo138} being the wheel bearing the largest number of defects (x = 16) to date (eight {Mo2}-units missing in total) (Figure C).[3] Complete MB ring systems (Figure A) can also be organically hybridized to build hybridized complete MB ring systems (Figure D). The ligation of {Mo154} to eight l-histidine (Figure B) molecules represents a good example for organically functionalized MBs.[24]

Figure 4

Overview of major synthesis aspects in the production of a (A) complete MB, (B) templated complete MB, (C) lacunary MB, (D) hybridized complete MB, (E) templated and hybridized complete MB ring system, and (F) lacunary hybridized MB ring system (synthesis temperature:T). Coloring code: {MoO6}, yellow; {Mo2O11}, red; {Mo8O35}, blue with central {MoO7}-unit in cyan.

Organic hybridization reactions can also be used to stabilize anionic templates in the center of MBs via hydrogen bonds resulting in templated and hybridized complete MB ring systems (Figure E). An example of such a host–guest complex is {Mo36}⊂{Mo150(ornithine)6} (Figure C).[38] The last representatives of lanthanide-free MB archetypes are lacunary hybridized MB ring systems, which contain defects within their scaffold as well as grafted carboxylic acids onto their remaining {Mo2}-units (Figure F). Lacunary hybridized MB ring systems were initially realized with the organic-chemical hybridization of a {Mo138} cluster leading to the inorganic–organic MB {Mo138(acetate)6}[20] (Figure A). Figure gives an overview of the main synthesis factors in the assembly of all structural types of MB-based clusters.

Ln-MB Based Nanoclusters

Preventing the integration of {Mo2} units ({MoVI2O5(H2O)2}2+), which contribute to the construction of the inner ring, into the wheel framework by the use of large electrophilic “open-shell” metal centers such as lanthanides (consistently incorporated as trivalent cations) alters the overall charge, molecular shape, and size of these nanostructures compared to the parent {Mo154} ring (Figure A). The introduction of LnIII ions into MB frameworks always causes a symmetry reduction compared to the highly symmetrical D7 {Mo154} nanocluster (Table ). As a result, lanthanide ions are often referred to as “symmetry breakers” in the context of MBs. Figure includes all lanthanide-containing MB structural archetypes (Figure G–J) that can currently be assembled and Table lists all crystal structures of lanthanide-containing molybdenum wheels as of August 2021, including information about the geometry, lanthanide pattern within the wheel, the most critical reaction parameters (such as total reagents and their stoichiometric ratios, pH (if specified), reducing agent/method, temperature, and reaction time), and applied methods to characterize the structure.

Table 1

Prominent Representatives of Purely Inorganic Complete Ln-MB Ring Systems Arranged in Descending Symmetry to the Parent MB {Mo154}

cluster	lanthanide ions	symmetry	ref
{Mo154}		D7d	(19)
{Mo120Ln6}	LaIII, CeIII, PrIII	D3	(25, 37, 40)
{Mo128Ln4}	EuIII	D2	(44)
{Mo90Ln10}	CeIII	D5d	(31)
[Mo130Ln6}	CeIII	C2v	(32)
{Mo124Ln4}	CeIII	C2	(24)
{Mo96Ln8}	LaIII	C2	(41)
{Mo100Ln6}	CeIII	C2	(42)
{Mo134Ln10}	LaIII	Ci	(43)
{Mo122Ln5}	CeIII	Cs	(39)

Table 2

List of All Purely Inorganic and Hybridized Inorganic–Organic Lanthanide-Containing Molybdenum Blue Wheel Crystal Structures According to the CCDC and ICSD Database (August 2021); Ln-MBs Are Listed Based on Their Shape, Starting with the “Japanese Rice-Ball”

The lanthanide substitution pattern of the respective wheel-type molybdenum blue is shown graphically. The lanthanide ions on the common inner rim of the wheel are shown in one color (green and pink). Important notes regarding the synthesis and characterization of the respective wheel are also indicated.

As the amount of organic ligand used has a significant influence on the final structure, the molar ratio of the main cluster-building reagents is indicated when an amino acid, peptide, or (di)carboxylic acid was used in the chemical synthesis.

X-ray diffraction.

Thermogravimetric analyses.

Infrared spectroscopy.

Bond valence sum.

Ultraviolet–visible-near-infrared spectroscopy.

Electrospray ionization-ion mobility spectrometry–mass spectrometry.

Extended X-ray absorption fine structure spectroscopy.

Lanthan-nuclear magnetic resonance spectroscopy. Building blocks marked with an asterisk (*) indicate an insertion between two {Mo8} building blocks.

Synthesis of Purely Inorganic Ln-MBs

″Japanese Rice-Ball”-, Egg-, Ellipsoid-, and Circular-Shaped Ln-MBs without Template Clusters

The “Japanese rice-ball”-shaped wheels {Mo120Ln6}[25,37,40] are complete Ln-MB ring systems (Figure G and 5D), which exist with PrIII, CeIII, and LaIII ions. The PrIII- and CeIII-containing “Japanese rice-ball”-shaped Ln-MBs were synthesized in a similar manner, but a significantly different synthetic protocol was used for the LaIII-containing cluster. For {Mo120Pr6}, Pr2(MoO4)3 was first produced by combining a Na2MoO4 solution with a Pr(NO3)3 solution. The obtained praseodymium molybdate Pr2(MoO4)3 was then suspended in an acidic solution, reduced, and heated at 70° for 45 min.[25] For the synthesis of the isostructural cluster {Mo120Ce6}, no Ce2(MoO4)3 was preformed. However, the synthesis was also performed at a rather high temperature (90 °C for 30 min) in an acidic and reduced medium immediately after all starting materials were produced, combined and had formed a cloudy suspension.[37] A [NH4]6[Mo7O24] solution was photochemically reduced for 2 days at room temperature to get {Mo120La6}.[40] All three “Japanese rice-ball”-shaped Ln-MBs crystallize within 1–2 weeks at room temperature (see Table for more details), whereby the lanthanide substitution pattern was built up in a one-pot synthesis and is therefore less controllable.

A complete Ln-MB ring system (Figure G) with the shape of an “egg” was first realized through the synthesis of {Mo96La8} (Figure E), in which a lacunary MB with an ambiguous number of defects was first dissolved and then treated with LaIII ions (see Table for more details).[41] This “egg formation” was enabled by a more predictable synthesis procedure using a two-step process. An MB containing defects was preformed in the first step, thus allowing 4f metals to react at specific positions within the wheel in the second step. The lacunary MB used in this synthesis was generated as a precipitate by reducing an acidified (pH 3.9–4.1) [Mo7O24]6–-containing solution photochemically for 10 days at a temperature between 10 and 15 °C leading to crystals of {Mo96La8} in 1 week at 4 °C. Notably, in a one-pot synthesis, the nanosized complete Ln-MB ring system (Figure G) {Mo100Ce6}[42] (Figure F) was generated which also exhibits the shape of an “egg”. {Mo100Ce6} was assembled by suspending a precipitate of Ce2(MoO4)3 in an acidic solution followed by reducing and heating at 80–85 °C for 1 h. The synthesis protocol of {Mo100Ce6} differs from that of {Mo120Pr6} in that {Mo100Ce6} was synthesized in a 10-fold higher concentration and an ∼4.5-fold lower acid concentration.

Elliptically shaped Ln-MB scaffolds belonging to complete Ln-MB ring systems (Figure G) were assembled for the first time in a one-pot approach through the use of EuIII ions, which have smaller ionic radii than those of CeIII and PrIII ions. For the assembly of {Mo128Eu4} a precipitate of Eu2(MoO4)3 was suspended in an acidic solution and then reduced with hydrazine and heated at 60–65 °C for 45 min (see Table for more details). The elliptical wheel {Mo128Eu4} (Figure B) led to the crystallization of the dimer {Mo256Eu8} after 2 weeks.[44] Remarkably, the self-assembly of the complete Ln-MB ring system (Figure G) {Mo150La2} (Figure G) with the shape of an ellipsoid was obtained in a photochemical reduction (irradiation for 2 days) of an acidified (pH 1.0) LaIII-containing solution with the IPOMo {Mo36} (Figure ).[40] A two-step reaction in which LnIII ions react in the vacancies of preformed wheels can also be applied to synthesize ellipsoidal structures.[43] This has been realized with the synthesis of the highly Ln-doped complete Ln-MB ring system (Figure G) {Mo134La10} (Figure H).[43] By treating the preformed wheel {Mo142}[45] with LaIII ions in a NaCl-solution without acidification at room temperature, crystals of {Mo134La10} form at 4 °C after 2 weeks.

Figure 7

Polyhedral and ball-and-stick representation of the (A) dimer {Mo100Ce6}2[42] (Figure G) and (B) ellipsoidal cluster {Mo128Eu4}[44] (Figure G) (with a side view shown in (C)). Coloring code: {MoO6}, yellow; {Mo2O11}, red; {Mo8O35}, blue with central {MoO7}-unit in cyan; {CeO9} and {EuO9}, green; Mo, blue spheres; Ce, green spheres; O, red spheres.

The “Japanese rice-ball”-, egg-, and ellipsoid-shaped Ln-MBs are all typically synthesized at temperatures 60-90 °C for 30−45min (in a one-pot approach). A fourth type of complete Ln-MB ring systems (Figure G), the circular-shaped {Mo92Ln9} (Figure I) and {Mo90Ln10} (Figure J) archetypes, can be generated when the synthesis temperature is increased to 100 °C for up to 4 days accompanied by controlled evaporation of the solvent (see Table for more details). This changes the reaction control from kinetic to thermodynamic control (Figure ), which is the key step in assembling these circular-shaped Ln-MBs.[31]

Figure 8

Four basic shapes of Ln-MBs. The “Japanese rice-ball“- (existing in the structural types shown in Figure G, I, and J), egg- (existing in the structural type shown in Figure G), and ellipsoidal-shaped Ln-MBs (existing in the structural types shown in Figure G–J) are all assembled in kinetically controlled syntheses; however, the circular-shaped Ln-MBs (existing in the structural type shown in Figure G) are synthesized in thermodynamically controlled reactions. The lanthanides that can be incorporated in the corresponding cluster types are indicated in the center of the shapes.

Ellipsoid-Shaped Ln-MBs with Template Clusters

The incorporation of a template cluster in an Ln-MB can be achieved without hybridizing the scaffold. This was accomplished with the assistance of oxalic acid, a dicarboxylic acid, which acts as a structure-directing ligand without attaching to {Mo2}-units to form templated complete Ln-MB ring systems (Figure H). This was discovered in the synthesis of {Mo130Ce6} (Figure K), which was assembled by suspending Ce2(MoO4)3 in an acidified aqueous solution, adding oxalic acid and heating to 90 °C for 2 h (see Table for more details). The crucial variable for the assembly of {Mo130Ce6} appears to be a structural ligand (in this case oxalic acid) in the proper ratio to the molybdate, which does not appear in the final structure but more likely, decomposes during synthesis. This hypothesis is based on the fact that oxalic acid decomposes in an acidic environment at 80 °C.[46] Single crystals suitable for X-ray diffraction were obtained at room temperature after 2 weeks.

Structural Description of Purely Inorganic Ln-MBs

The “Japanese rice-ball”-shaped Ln-MBs are composed of 12 {Mo1}, 6 {Mo2}, 12 {Mo8}, and 6 {Ln} building units (Table ). Six {Mo2}2+ groups are replaced by smaller LnIII ions (LnIII= LaIII, CeIII, or PrIII; according to reported crystal structures[25,31,42−44]). The average size of the integrated {LnO9} in the inner ring (O–LnIII–O) is 4.6 Å, while the corner-sharing {Mo2} units (O–Mo–O–Mo-O) are 7.3 Å, forcing the cluster into a “more contracted” structure exhibiting an irregular ring-shape and a lower symmetry (D3)[25] compared to that of the ideal circular parent structure {Mo154}[19] (D7d point group) (Figure A). All lanthanide centers on both the upper and lower surfaces of {Mo120Ln6}[25,37,40] exhibit a tricapped trigonal prismatic coordination sphere (Figure B). “Japanese rice-ball”-shaped Ln-MB clusters reported to date exhibit an inner and outer diameter of ∼17 and ∼31 Å, respectively, and their total charges varies between −4 and −6 (see Table ).

The egg-shaped Ln-MB {Mo96La8} (assembled in a two-step approach) consists of 10 {Mo1}, 2 {Mo2}, 10 {Mo8}, 2 {Mo1*} (building blocks marked with an asterisk indicate an insertion between two {Mo8} building blocks), and 8 {La} building units (Table ) and exhibits an inner and outer diameter of ∼14 and ∼30 Å, respectively, with four lanthanide ions and one {Mo2} unit coordinated in each inner rim (Figure E).[41] In comparison to the “Japanese rice-ball″-shaped scaffolds {Mo120Ln6} (LnIII= LaIII, CeIII, or PrIII) (Figure D), which were synthesized in a one-pot reaction, the “egg” {Mo96La8} is smaller and has a less uniform lanthanide pattern. {Mo96La8} was characterized as a heteropolyacid with a relatively high negative charge of minus 22 due to the lack of any ammonium counter-cations, as verified by elemental analysis (see Table ). In the egg-shaped structure {Mo100Ce6}, which is composed of 10 {Mo1}, 4 {Mo2}, 10 {Mo8}, 2 {Mo1*}, and 6 {Ln} building units (Table ), six CeIII ions are coordinated to the Ln-MB wheel, causing a contraction and consequently forcing the cluster to take on the shape of an egg with an inner and outer ring diameter of ∼16 and ∼31 Å, respectively, very close to that of the “Japanese rice-ball” (Figure F).[42] The lanthanide substitution pattern, however, differs from that of the “Japanese rice-ball”, especially in the distribution of the 4f-metals within the scaffold. The six CeIII ions form two triangles (see Table ), one located on the upper rim and the other one on the lower rim of the wheel. In each triangle, two CeIII ions are located adjacent to each other at one end of the wheel, whereas the third CeIII ion is situated at the opposite end. Without the addition of KCl, this third CeIII ion coordinates to a neighbor wheel via a Ce–O–Mo bond, leading to the formation of the dimer {Mo100Ce6}2[42] (Figure A).[42] The monomer {Mo100Ce6} exhibits a negative charge of four minus and the dimer {Mo100Ce6}2 of minus eight (see Table ).

The Ln-MB {Mo128Eu4}, exhibiting a negative charge of minus eight, is assembled under very similar conditions as {Mo120Pr6} and {Mo100Ce6}, however, the substitution of PrIII or CeIII by EuIII results in a significantly larger ellipsoidal-shaped cluster with 12 {Mo1}, 8 {Mo2}, 12 {Mo8}, 4 {Mo1*}, and 4 {Eu}-building units and an inner and outer ring diameter of ∼21 and ∼39 Å (Table ), respectively, rather than a “Japanese rice-ball” or egg-shaped cluster.[44] Notably, EuIII ions not only reduce the D7 symmetry of the parent {Mo154} wheel to D2 (Table ) but also prevent the parent-ring closure (Figure C). The ellipsoidal wheel {Mo150La2} is composed of 14 {Mo1}, 12 {Mo2}, 14 {Mo8}, and 2 {La} building units with an inner and outer diameter of ∼22 and ∼38 Å, respectively, and the second largest Ln-MB cluster to date with the greatest negative charge (−24) (Table ).[40] Two distorted tricapped-trigonal-prismatic LaIII centers (green polyhedra) are incorporated into the ring structure instead of two {Mo2}-units (red polyhedra) (Figure G) (see Table for more details). {Mo134La10}, also an ellipsoidal-shaped Ln-MB, is currently the largest Ln-MB wheel with an inner and outer diameter of ∼21 and ∼39 Å, respectively. It is composed of 14 {Mo1}, 4 {Mo2}, 14 {Mo8}, and 10 {Ln} building units with five lanthanide ions and two {Mo2}units coordinated in each inner rim (Figure H). Interestingly, in the synthesis of {Mo134La10}, when treating the six “defect pockets” (=6 missing {Mo2} units) of the MB {Mo142}[45] (={Mo154–12}) with LaIII ions, not only six lanthanide ions are incorporated in these vacant sites but four {Mo2} units are also substituted by LaIII ions, causing the wheel shape to change from circular to ellipsoid. This shape modification was confirmed by isothermal titration calorimetry.[43]

Intriguingly, Ln-MBs are able to assemble with all {Mo2}-units of the inner rim being replaced by lanthanide ions.[31] However, due to the lanthanide-induced contraction of the wheel structure, the replacement of all inner-rim {Mo2}-units was only feasible with the large ions LaIII, CeIII, and PrIII, yielding the first (and currently only) charge-neutral circular-shaped wheel {Mo90Ln10}[31] (LnIII = LaIII, CeIII, or PrIII) (Figure J). {Mo90Ln10} consists of 10 {Mo1}-, 10 {Mo8}-, and 10 {Ln}-building units (Table ) and exhibits an inner and outer diameter of ∼10 Å and ∼26 Å, respectively (Figure J). With NdIII and SmIII, which belong to the later lanthanides and therefore have a smaller radius, the scaffold {Mo92Ln9}[31] (LnIII = NdIII or SmIII) with a charge of minus one was obtained (Figure I) in which one {Mo2} building block unit is preserved in the wheel. {Mo92Ln9} is composed of 10 {Mo1}-, 1 {Mo2}-, 10 {Mo8}-, and 9 {Ln}-building units (Table ). The replacement of all inner-rim {Mo2}-units could not be accomplished in {Mo92Ln9}[31] as the wheel contraction would become too large (radii: SmIII < NdIII < PrIII < CeIII < LaIII < {Mo2}2+). The lanthanide substitution pattern of {Mo90Ln10} and {Mo92Ln9} does not induce a structural deformation compared to the origin cluster {Mo154}, in contrast to {Mo120Pr6},[25] {Mo100Ce6},[42] {Mo96La8},[41] and {Mo128Eu4}.[44]

Ellipsoid-Shaped Ln-MBs with Template Clusters

The ellipsoidal templated wheel {Mo130Ce6} is made up of 12 {Mo1}, 6 {Mo2}, 12 {Mo8}, and 6 {Ce} building units with an additionally enclosed {Mo10} fragment and exhibits an inner and outer diameter of ∼18 and ∼33 Å, respectively (Figure K). {Mo130Ce6} is composed of a {Mo120Ce6} ring that is half-closed by the polyoxomolybdate “cap” {Mo10}.

Fascinatingly, only the use of oxalic acid enabled the trapping of a {Mo10} fragment on just one side of the ellipsoidal {Mo120Ce6} ring without hybridizing the {Mo2} units organically (Figure K).[32] This entrapment, which is the only example of a semiclosed Ln-MB to date, probably occurs through either supramolecular interactions, such as hydrogen bonding, or oxalic acid serving as a transient anion template. In {Mo130Ce6}, four CeIII ions are coordinated on the open side of the ellipsoidal scaffold, and the remaining two CeIII ions are incorporated on the closed side containing the {Mo10} cap (Figure K).

The Four Basic Shapes of Ln-MBs: Which LnIII Ion Is Responsible for Which Shape?

LaIII, the lanthanide with the largest radius, was built into all four basic forms (Figure ) either photochemically and under moderate reaction conditions,[40] by occupying defect sites in preformed lacunary MBs,[43] or through a thermodynamically controlled self-assembly[31] (see Table ). The capability of LaIII ions to be introduced into MBs under mild reaction conditions (syntheses at room temperature) indicates that LaIII ions may be more reactive in MB systems than the rest of the lanthanide family. All four basic forms (Figure ) and all Ln-MB ring systems (Figure G–J) are synthesizable with CeIII ions, which leads to the assumption that CeIII ions are of ideal size and reactivity. CeIII containing Ln-MB reaction systems are likely to be the most versatile ones, which explain the relatively high number of CeIII containing Ln-MBs that have been reported (20 as of August 2021; see Table ).

PrIII ions, the third-largest lanthanides, are currently only known in “Japanese rice-ball”- and circular-shaped Ln-MBs.[25,31] The latter shape could hitherto only be assembled thermodynamically. The smaller lanthanide ions NdIII and SmIII have so far only been incorporated into circular-shaped Ln-MBs in thermodynamically controlled Ln-MB synthesis approaches.[31] Due to their ion size, NdIII and SmIII seem reluctant to be incorporated into “Japanese rice-balls” or “eggs”, which are typically formed in kinetically controlled synthesis approaches. Surprisingly, it is feasible to incorporate EuIII, the smallest lanthanide ion so far (which one would only expect to be incorporated thermodynamically into a circular-shaped Ln-MB ring due its size) into an ellipsoidal Ln-MB through a kinetically controlled reaction.[44] However, the incorporation of EuIII into an ellipsoidal Ln-MB prevented ring closure of the inner rim (Figure C), which is most likely owing to its relatively small size. An open inner rim in an Ln-MB is only known for EuIII ions.

The size of a lanthanide determines whether it can replace a {Mo2} unit in MBs. The smallest lanthanide ion integrated in the inner rim of MBs and identified by X-ray structure analysis is EuIII.[44] This suggests that the slightly larger lanthanide ions NdIII and SmIII can likewise serve as building units in kinetically controlled self-assemblies of Ln-MBs. The lanthanide ions from GdIII to LuIII appear to be too small to function as building units in Ln-MBs.

When incorporated in MBs lanthanide ions generate significant deformations relative to the circular parent cluster {Mo154}, resulting in the main basic shapes (“Japanese rice-ball”, “egg”, and ellipsoidal forms) (Figure ). However, if the degree of replacement of the {Mo2} units by LnIII ions is very high (9 or 10 LnIII ions per molecule), the resulting shape of the Ln-MB is nearly identical to that of {Mo154}. In order to visualize the effect of the integration site of built-in lanthanides generating deformations and to compare them directly, the cavity skeletons of the circular-shaped {Mo90Ln10}[31] (LnIII = LaIII, CeIII, or PrIII), the “Japanese rice-ball” {Mo120Pr6},[25] the egg-shaped {Mo100Ce6}[42] and {Mo96La8},[41] as well as the ellipsoidal {Mo128Eu4}[44] are illustrated in Figure . Due to the even distribution of both the PrIII ions within the inner surface of {Mo120Pr6}[25] and the LnIII ions in {Mo90Ln10}[31] as well as the antipodal location of the EuIII ions in {Mo128Eu4},[44] these wheels show a more regular structure than the remaining wheels in Figure . In contrast, the uneven distribution of three CeIII ions in the inner rim of {Mo100Ce6}[42] forms a triangle in which two CeIII ions are located adjacent to each other (Ce–O–Mo–O–Ce) while the third ion is positioned at the opposite side of the cavity. Also, the uneven lanthanide ion distribution in {Mo96La8},[41] where all four LaIII ions are asymmetrically positioned and connected to each other via O–Mo–O bridges results in an irregular rather than regular structure. Depending on the nature of the lanthanide, a specific replacement of {Mo2}-units can be accomplished, allowing for curvature tuning.

Figure 9

Ball-and-stick representation of the rim skeleton of the (A) circular-shaped {Mo154}[19] (Figure A), (B) “Japanese rice-ball″-shaped {Mo120Pr6}[25] (Figure G), (C) egg-shaped {Mo100Ce6}[42] (Figure G), (D) egg-shaped {Mo96La8}[41] (Figure G), (E) ellipsoidal-shaped {Mo128Eu4}[44] (Figure G), and (F) circular-shaped {Mo90Ln10}[31] (LnIII = LaIII, CeIII, or PrIII) (Figure G). The aspect ratio (longest/shortest distance of the inner cavity) is indicated in the center of the wheels. The symmetry-related LnIII ions are omitted for clarity. The metal–oxygen bonds of asymmetrical cavities are highlighted two-colored (green and pink). Coloring code: Mo, blue; Ln, green; O, red.

Synthesis of Hybridized Ln-MBs

As of August 2021, there are 11 crystal structures of hybridized complete Ln-MB ring systems (Figure I) and 7 templated and hybridized complete Ln-MB ring systems (Figure J) reported (Table ).

Wheel-shaped clusters have a chaotropic character, due to the large molecular surface with a relatively low charge density. Recent findings have demonstrated that formation in aqueous solutions of adducts with organic molecules (e.g., cyclodextrins), which originates from the general chaotropic nature of POMs, dramatically increases the kinetics of MB wheels and may open up new pathways in Ln-MB synthesis.[47]

Ln-MBs without Template Clusters

The setting of synthesis parameters is essential for the design of hybridized complete Ln-MB ring systems (Figure I). The pH value should be below 1.4 and the synthesis temperature about 90 °C for 1–2 h. To generate single-crystals for X-ray structure analysis, the lanthanide/ligand/molybdate ratio (see Table ) in the synthesis must be quite precise in order to prevent precipitation and poor-quality crystals.

When lanthanide ions are introduced into MBs, their symmetry is invariably reduced in comparison to the parent cluster {Mo154} (Table ), resulting in chiral racemic Ln-MB arrangements.[24] Enantiopure hybridized complete Ln-MB ring systems can be produced if a chiral amino acid is present in the self-assembly process of Ln-MBs. Since LnIII ions and chiral amino acids exert a synergistic effect in acidified solutions of reduced molybdates, the selective assembly of a series of enantiopure Ln-MBs with the formula Δ/Λ-{Mo124Ce4(amino acid)4-or-6} (Scheme ) was enabled.[24] A left-handed propeller-like configuration is represented by the symbol Δ, whereas a right-handed propeller-like arrangement is represented by Λ.

Scheme 1

Schematic Representation of the Stereoselective Synthesis of Chiral Ln-MBs through the Use of Chiral Amino Acids

(A) d-Histidine results in the Δ-{Mo124Ce4(d-histidine)6} enantiomer (Figure I), whereas d-tryptophan and d-arginine yield the Λ-{Mo124Ce4(d-tryptophan)4} and Λ-{Mo124Ce4(d-arginine)4} (Figure I), enantiomers.[24] (B) l-Histidine results in the Λ-{Mo124Ce4(l-histidine)6} enantiomer (Figure I), whereas l-tryptophan and l-arginine yield the Δ-{Mo124Ce4(l-tryptophan)4} and Δ-{Mo124Ce4(l-arginine)4} (Figure I), enantiomers.[24] (C) Without the addition of a chiral amino acid, {Mo126Ce4}[24] (Figure G) [Reprinted from ref (24). Copyright 2018 American Chemical Society.] is obtained as a racemate. Coloring code: {MoO6}, yellow; {Mo2O11}, red; {Mo8O35}, blue with central {MoO7}-unit in cyan; {CeO9}, green. Λ and Δ represent right-handed and left-handed propeller, respectively and organic ligands in the final structure are omitted for clarity.

Δ/Λ-{Mo124Ce4(amino acid)4-or-6} are chiral hybridized complete Ln-MB ring systems (Figure I) synthesized in a one-pot approach as follows: an acidified aqueous solution containing all main starting materials (CeCl3, Na2MoO4 and the respective proteinogenic amino acid) (see Table for more details) was reduced and heated at 90 °C for 1 h. Crystals suitable for X-ray diffraction were so far only obtained with the chiral arginine (after 1 week at room temperature), and tryptophan (after 5 weeks at room temperature) (Figure D and E).

The discovery that single enantiopure proteinogenic amino acids significantly affect the self-assembly and stereochemistry of Ln-MBs prompted researchers to investigate enantiopure oligopeptides, which extended the number of hybridized complete Ln-MB ring systems (Figure I) by six structures.[39] In a one-pot synthesis, an acidified aqueous solution containing all main starting materials (CeCl3, Na2MoO4 and the respective histidine-terminated oligopeptide) (see Table for quantity ratios) was reduced and heated at 90 °C for 2 h. Crystals of {Mo122Ce4(oligopeptide)3}, {Mo124Ce4(oligopeptide)4}, and {Mo126Ce4(oligopeptide)2} were obtained at room temperature after two to 8 weeks.[39] The reaction solution was occasionally heated for a longer period of time (2 h) than in the case for the hybridization of Ln-MBs with proteinogenic amino acids (1 h).

Ln-MBs with Template Clusters

To date, Ln-MBs with template clusters have only been assembled with the assistance of organic ligands.[24,32,38] It is quite difficult to ascertain which ligand will be able to enclose a polyoxometalate as a template. The only established requirement for the ligand (aside from the carboxyl group for grafting to the {Mo2} units) is the presence of a nitrogen-containing pendant group (e.g., histidine, ornithine). In an acidic environment, this nitrogen-containing group gets protonated and could therefore stabilize template clusters within the Ln-MB electrostatically and through hydrogen bonds. The use of chiral proteinogenic amino acids not only enables the organic hybridization of the inner rim of Ln-MBs, resulting in nanosized chiral Ln-MBs, but also the synthesis of chiral templated and hybridized complete Ln-MB ring systems (Figure J). For the self-assembly of Δ-{Mo8}@{Mo124Ce4(histidine)6} and Λ-{Mo8}@{Mo124Ce4(histidine)6}, an acidified aqueous solution containing all of the major starting materials (CeCl3, Na2MoO4, and l-histidine (for the Λ-enantiomer) or d-histidine (for the Δ-enantiomer) was reduced and heated at 90 °C for 1 h leading to suitable crystals for X-ray diffraction after 1 week at room temperature.[24] Besides the proteinogenic amino acids histidine,[24] arginine,[24] and tryptophan,[24] the nonproteinogenic amino acid l-ornithine (Orn)[38] also has a template-directing effect in the assembly of Ln-MBs. l-Ornithine enabled the synthesis of the “Japanese rice-ball”-shaped (almost ellipsoidal shaped) α-{Mo8}@{Mo124Ce4(Orn)6}[38] (Figure F), the elliptically shaped {Mo17}@{Mo150Ce2(Orn)6} (Figure G), and β-{PMo12}@{Mo150Ce2(Orn)6}[38] (Figure H). The critical synthetic variable in the self-assembly of these three templated and hybridized complete Ln-MB ring systems (Figure J) is the concentration of the lanthanide ion, ligand, and molybdate. For {Mo17}@{Mo150Ce2(Orn)6} and β-{PMo12}@{Mo150Ce2(Orn)6}, the concentrations of the ornithine and molybdate were ∼3-fold and that of the CeIII ions ∼1.5-fold higher than for the formation of α-{Mo8}@{Mo124Ce4(Orn)6} (see Table for details). In α-{Mo8}@{Mo124Ce4(Orn)6} and {Mo17}@{Mo150Ce2(Orn)6}, the templated polyoxomolybdate was formed in situ, whereby the enclosed template in β-{PMo12}@{Mo150Ce2(Orn)6} was a separately added preformed Keggin.[38] The ornithine and molybdate concentrations were significantly increased most likely to enhance the ligand’s template-directing effect and to enable both the encapsulation and stabilization of a larger negatively charged polyoxomolybdate cluster.

Structural Description of Hybridized Ln-MBs

Remarkably, organic hybridization of {Mo2} building blocks in Ln-MB assemblies predominantly yields ellipsoidal-shaped clusters. The hybridization of {Mo2} units in the cavity of ellipsoidal Ln-MBs seems to be less restricted by steric hindrances as in “Japanese rice-balls”, “eggs”, and circular-shaped clusters.

Structural Description of Ln-MBs without Template Clusters

All Ln-MBs without template clusters obtained through the hybridization with proteinogenic amino acids are hybridized complete Ln-MB ring systems (Figure I) sharing the same elliptical skeleton {Mo124Ce4}[24] with either four arginines[24] (Figure D) or six tryptophans[24] (Figure E) grafted to the cluster. The skeleton {Mo124Ce4} consists of 12 {Mo1}, 8 {Mo2}, 12 {Mo8}, and 4 {Ce} building units (Table ) and exhibits an inner and outer ring diameter of approximately 12 and 31 Å, respectively (Figure ). The four CeIII ions built into the scaffold are symmetrically located on the upper and lower rims of the wheel, and the MB-functionalizing amino acids arginine and tryptophan are coordinated to the organically chemically modifiable[23] {Mo2}-type building blocks via their carboxylate groups (Figure ).

Figure 10

Polyhedral representation of the {Mo124Ce4}[24] scaffold of the hybridized, elliptical Ln-MB frameworks (Figure I). Coloring code: {MoO6}, yellow; {Mo2O11}, red; {Mo8O35}, blue with central {MoO7}-unit in cyan; {CeO9}, green.

Different enantiomers are generated depending on the concentration, type, and configuration of the used amino acid. d- and l-amino acids are capable of producing the Λ and Δ enantiomer in the pure form, the configuration of the amino acid not favoring any specific enantiomer (Scheme ). In the absence of d- or l-amino acids, only the {Mo126Ce4} (Scheme ) wheel is obtained as a racemate, which is structurally similar to {Mo124Ce4}[24] (slightly compressed in contrast to {Mo124Ce4}). Incorporating histidine-terminated oligopeptides into Ln-MB systems revealed that the length and sequence of the peptides markedly influence the shape, Ln-substitution pattern, and stereochemistry of the resulting Ln-MBs.[39] For instance, if glycine (Scheme A) is replaced with serine/alanine (Scheme B) in a histidine-based dipeptide, an ellipsoidal Λ-nanowheel is obtained, rather than an ellipsoidal Δ-nanowheel. A very distinct “Japanese rice-ball″-structure is formed, when a histidine-based tripeptide contains two glycines (Scheme C). With four glycines in a histidine-based tetrapeptide, a one-sided deformed ellipsoid is constructed that resembles an “egg” (Scheme D). X-ray structure analysis showed that all investigated peptides on {Mo2} units coordinate exclusively on one side of the Ln-MBs in different stoichiometric ratios (Scheme ).

Scheme 2

Schematic representation of the histidine-terminated oligopeptide-controlled synthesis of Ln-MBs[39]

The oligopeptides used were (A) H-Gly-l-His-OH (GH) (Figure I), (B) H-l-Ala-l-His-OH (AH) (Figure I) and H-l-Ser-l-His-OH (SH) (Figure I), (C) H-Gly-Gly-l-His-OH (G2H) (Figure I), and (D) H-Gly-Gly-Gly-Gly-l-His-OH (G4H) (Figure I) and H-Gly-Gly-Gly-Gly-Gly-l-His-OH (G5H) (Figure I). The toroidal shapes of the wheels are displayed in blue. The lanthanide ions (CeIII) are shown in green; carbon, black; and nitrogen, purple.

The dipeptides H-Gly-l-His-OH (GH), H-l-Ala-l-His-OH (AH), and H-l-Ser-l-His-OH (SH) are attached to the ellipsoidal {Mo124Ce4}[39] scaffold, where GH mediates the self-assembly of the Δ-{Mo124Ce4} enantiomer, and AH and SH that of the Λ-{Mo124Ce4} enantiomer. Thus, the helicity of the chiral {Mo124Ce4}-Ln-MBs depends on the N-terminal rather than the C-terminal residue of the oligopeptide. This allows an elegant switch from the Δ to the Λ enantiomer by just altering the amino acid at the N-terminus without changing the stereochemistry of the entire peptide. The tripeptide H-Gly-Gly-l-His-OH mediates the self-assembly of the wheel-skeleton {Mo122Ce5}[39] and binds to it three times (Scheme C). {Mo122Ce5} is an unprecedented Ln-MB scaffold containing two differently shaped cavities in the same Ln-MB (“Japanese rice-ball”- and elliptical-shaped cavity, Figure ) and an odd number of lanthanide ions. The pentapeptide H-Gly-Gly-Gly-Gly-l-His-OH and hexapeptide H-Gly-Gly-Gly-Gly-Gly-l-His-OH mediate racemic Ln-MB mixtures, comprising the asymmetric ellipsoidal scaffold {Mo126Ce4}[39] (Scheme D). Since the oligopeptides differ in length and sequence, they form different numbers of hydrogen bonds when introduced into Ln-MBs, resulting in a variety of distinct structural characteristics.

Figure 11

Ball-and-stick representation of the “Japanese rice-ball”- and oligopeptide-hybridized elliptical-shaped cavities of {Mo122Ce5}.[39] The metal–oxygen bonds of the two different cavities are highlighted in different colors (green and pink). Coloring code: Mo, blue spheres; Ce, green spheres; C, gray spheres; O, red spheres; N, violet spheres.

Structural Description of Ln-MBs with Template Clusters

Notably, only the functionalization of {Mo124Ce4} scaffold with the proteinogenic amino acids l-/d-histidine transforms the nanocavity into a reaction tube, which can be described as a confined molecular reaction vessel, wherein the in situ assembled, chiral metal cluster {Mo8} (structurally related to γ-[Mo8O26]4−[48]) is embedded.[24] The inclusion of the chiral cluster {Mo8} into {Mo124Ce4} scaffold results in the formation of the supramolecular guest@host assembly {Mo8}@{Mo124Ce4(histidine)6}[24] (Figure I). The orientation of the introduced {Mo8} fragment within {Mo124Ce4(histidine)6} depends strongly on the spatial arrangement of the coordinating histidine ligands and their hydrogen bonds formed with {Mo8}.[24] The crystal structure analysis strongly indicates that all six histidine ligands must be protonated in order to minimize the repulsive electrostatic forces between the negatively charged assemblies (nanoring and {Mo8} cluster) and therefore increase system stability. Moreover, the orientation of the introduced {Mo8} fragment within {Mo124Ce4(histidine)6} depends strongly on the spatial arrangement of the coordinating histidine ligands and their hydrogen bonds formed with {Mo8}.[24]

The nonproteinogenic amino acid l-ornithine coordinates to {Mo2} units during the self-assembly processes and acts as a structure-directing ligand, triggering the formation of various guest@host architectures.[38] Three cerium-containing nanoclusters were obtained using l-ornithine: the “Japanese rice-ball”-shaped α-{Mo8}@{Mo124Ce4(Orn)6}[38] (Figure F), the elliptically shaped {Mo17}@{Mo150Ce2(Orn)6} (Figure G), and the elliptically β-{PMo12}@{Mo150Ce2(Orn)6}[38] (Figure H). In all three nanowheels, six l-ornithine ligands are attached to the inner rim. In addition, the negatively charged polyoxomolybdate template cluster in the cavity of each nanowheel is stabilized by electrostatic forces and hydrogen bonds. {Mo124Ce4(Orn)6} encapsulates the in situ formed octamolybdate-type anion α-[Mo8O26]4– (α-{Mo8}) (Figure F), whereas the nanoring {Mo150Ce2(Orn)6} can trap either the in situ formed [Mo17O52(H2O)10]2– ({Mo17}) (Figure G) or the separately added Keggin-type anion β-[PMo12O40]3– (β-{PMo12}) (Figure H).[38] So far, {Mo17} has only been isolated as a template in {Mo150Ce2(Orn)6} (Figure G). In the scaffold {Mo124Ce4(Orn)6}, two CeIII ions are located on each rim, separated by either one or two {Mo2} groups (Figure F). The scaffold {Mo150Ce2(Orn)6} contains a CeIII ion on the upper and lower rim, located opposite to each other (Figure G).

Characterization of Ln-MBs in the Solid State

Single Crystal X-ray Diffraction

First and foremost, for new Ln-MB-based nanoclusters, it is essential to obtain suitable crystals for single crystal X-ray structure analysis to determine their three-dimensional shape and size in the solid state. The size, stability, scattering power, and quality of the crystal are critical for a single crystal X-ray structure analysis. Since Ln-MBs are composed of heavy atoms and thus produce a greater diffraction pattern than molecules containing just light atoms, the crystal to be examined can be significantly smaller than that of organic compounds. Crystals of Ln-MB-based nanoclusters are typically measured around 200 K and below rather than at room temperature. To generate Ln-MB crystals suitable for X-ray diffraction, the crystallization process may need to be slowed down (for example, by allowing the solvent to evaporate more slowly or by lowering the crystallization temperature), or the synthesis parameters have to be adjusted. The majority of Ln-MB synthesis optimizations are based on the “trial and error” concept, with the aim of finding the optimal starting component ratio.[32,37]

FTIR and Raman Spectroscopy, Powder Diffractometry, and TGA

Infrared spectroscopy displays characteristic bands, which appear <1100 cm–1 (Mo=O vibrations are located at higher wave numbers than Mo–O–Mo) and confirms only the presence of a polyoxometalate scaffold, whereas Raman spectroscopy shows characteristic bands for wheel-type scaffolds.[49] Sharp bands are typically observed in the Raman spectra of MB/Ln-MBs at both low and high wavenumbers, which can be attributed to vibrations of {(MoV/VI)mO} (m = 2,3,4) fragments (Figure ) with the band at around 800 cm–1 being a symmetric stretching vibration of the equatorial {(MoV/VI)3μ3-O} fragment.[19,50] Raman spectroscopy also allows investigations of MBs/Ln-MBs in aqueous solution, since water molecules are only weakly Raman-active.

Figure 12

Raman spectrum of {Mo100Ce6}. Reprinted from ref (42). Copyright 2014 American Chemical Society.

Structural characterization of MBs/LnMBs in the solid state is completed by powder diffractometry to evaluate their phase purity, and thermogravimetric analysis is conducted to assess their crystal water content (by heating the MB/Ln-MB containing sample typically up to ∼150 °C).

Stability and Speciation Studies of Ln-MBs in Solution

The stability and speciation of Ln-MBs must first be studied when dissolved before they can be used as possible molecules for applications in solution. How can the integrity of Ln-MBs be confirmed? How are aggregates, such as the spontaneous self-assembly of homogeneous, hollow spherical “blackberry” structures,[30] identified? Which spectroscopic methods are relevant for the investigation of chirality in Ln-MBs? Is it feasible to show that the organic–inorganic hybridization in solution is preserved? Is it possible to determine the charge of these Ln-MB wheels at different pH values?

Among all the various investigation techniques for stability and speciation studies in solution, such as Raman-spectroscopy, ESI-MS and ES-IMS-MS, or UV–vis-Nir, small-angle X-ray scattering (SAXS) stands out when it comes to exploring nanoscopic structures (usually 1–100 nm) in solution.[51] The utilization of SAXS, a highly powerful, nondestructive technique enables the determination of size, shape, and interactions of nanoscopic POMs and additionally allows for a variety of in situ analyses. Investigations of solution-phase speciation under various conditions (pH, conc., type of counterions, aged solutions, etc.) can be conducted in order to obtain information about their morphology and assembly when dissolved (e.g., is the POM oligomerized or present as a monomer? Is an ecliptically stacked monomeric arrangement or rather a shifted face-to-face dimeric configuration preferred?).[52,53] Stability measurements can be performed to determine whether POMs are still intact under certain conditions (e.g., in catalytic processes).[51] Fundamental understanding of reaction mechanisms[51] can be provided and possible preferences for particular organic compounds and whether they accelerate the cluster formation of Ln-MBs can be examined.[47] Furthermore, the feasibility of registering the start and maximum of nanosized cluster formation by using SAXS, enables the determination of critical reaction parameters for their synthesis.[54] SAXS studies on Ln-MB systems have not yet been conducted.

ES-IMS-MS, Redox Titration, and UV–Vis-Nir

Electrospray ionization-ion mobility spectrometry-mass spectrometry (ESI-IMS-MS) is an excellent method for investigating the speciation of gigantic MBs in solution/gas phase. ESI-IMS-MS is superior to ESI-MS for the following reason: MB/Ln-MB building blocks generate multiple species with fairly similar mass-to-charge ratios (m/z) in very different sizes/structures, resulting in overlapping envelopes in the mass spectra.[55]Figure A shows an ESI-IMS-MS spectrum of the dimer {Mo100Ce6}2 and the dominating monomer {Mo100Ce6}.[42] The spectrum of the monomer {Mo100Ce6} was very similar to that of {Mo100Ce6}2 with the significant difference that no peak for the dimer was observed.

Figure 13

(A) ESI-IMS-MS of a solution containing the dimer {Mo100Ce6}2 with the dominating monomer {Mo100Ce6}.[42] [Reprinted with from ref (42). Copyright 2014 American Chemical Society.] (B) CD spectra of Δ-{Mo8@Mo124Ce4(histidine)6}, Λ-{Mo8@Mo124Ce4(histidine)6}, l-histidine, and d-histidine.[24] [Reprinted from ref (24). Copyright 2019, American Chemical Society.] C) 1H NMR spectra of {Mo8@Mo124Ce4(histidine)6} (black) and l-histidine (blue).[24] [Reprinted from ref (24). Copyright 2019, American Chemical Society.]

Redox titration is used to determine the number of MoV centers in the cluster (e.g., with a CeIV titration).[25] However, sufficient solubility of the compound is required for a redox titration; otherwise, the MoV centers must be identified using bond valence sum calculations based on the crystal structure.

Solution stability of MB/Ln-MB-based nanoclusters can be monitored with UV–vis-Nir, as they show characteristic absorption bands from 600 to 1100 nm, which are attributed to the intervalence charge transfer between MoVI and MoV.[17]

Circular Dichroism and 1H NMR Spectroscopy

Circular dichroism (CD) is suitable to determine if organic hybridization has occurred on chiral Ln-MBs and whether these are stable in solution. CD spectroscopy of Λ-{Mo8}@{Mo124Ce4(histidine)6} and Δ-{Mo8}@{Mo124Ce4(histidine)6} resulted in mutually mirror-inverted curves (Figure B) with a distinctive exciton splitting at 234 and 274 nm originating from histidine.[24] A red shift of about 20 nm relative to unattached histidine strongly suggests coordination on the {Mo124Ce4} scaffold and, as a result, a restriction of the histidine’s free rotation.

1H NMR spectroscopy is well suited for examining hybridized inorganic–organic Ln-MBs for organic attachment, provided the molecule to be examined is sufficiently soluble in deuterated solvents. The 1H NMR analysis of {Mo8@Mo124Ce4(histidine)6} (Figure C) displays the characteristic signals for histidine, with the difference that the signals obtained are both broader and upfield- and downfield-shifted when compared to unattached histidine.[24] The broadening of the signals as well as their upfield and downfield shift are probably caused by the shielding effect from the 24-electron reduced {Mo124Ce4}.[56]

“Blackberry”, Encapsulation Experiments, SLS, and DLS

Charged MB/Ln-MBs exhibit a unique behavior in solution, assembling into hollow, spherical blackberry-like aggregates.[30,31] The interactions involved for their formation are counterion-mediated electrostatic attractions, van der Waals forces, and hydrogen bonding.[30] A critical variable for the assembly of “blackberries” is the pH, which determines their size. Typically, the lower the pH, the smaller the overall negative charge (= weaker repulsive electrostatic interactions between adjacent anion clusters) and the larger the size of the “blackberry”.[30] Encapsulation tests using the positively charged cetrimonium bromide (CTAB) surfactant allow for the investigation of the total Ln-MB charge. For {Mo90Ce10}, the charge could be determined at various pH levels (pH 0.5–2), indicating that {Mo90Ce10} exists in solution as a decaanionic cluster above pH 2.[31] The concentration of the cations has the opposite effect, as demonstrated clearly by the increasing addition of divalent BaII cations, which exhibits enhanced attraction of macroion and counterion. The higher the concentration of BaII ions is, the larger the “blackberry” becomes, confirming that its assembly is a charge-regulated process.[31] The first “blackberry” assembly of an Ln-MB was confirmed for {Mo90Ce10} (at pH 1 in the presence of BaII) and verified by static light scattering (SLS).[31] The hydrodynamic radius of {Mo90Ce10} was identified by the use of dynamic light scattering (DLS).[31] POM superstructures, as well as structural changes induced by pH, counterion concentration, etc., can be observed and studied using SLS and DLS. Both light scattering techniques are able to determine dynamic properties of small POMs,[57] such as the Keggin archetype, and gigantic assemblies.

The Influence of Lanthanides on the Redox Characteristics of Ln-MB Nanoclusters

The partial reduction of MoVI-solutions results in Mo-scaffolds with a different degree of reduction (given in % in Table ), which is calculated by dividing the number of MoV centers by the total number of Mo centers in the wheel (Table ). When lanthanides are incorporated into MB scaffolds, the resulting Ln-MBs demonstrate almost the same degree of reduction as Ln-free MBs (Table ) with Ln-MBs containing a slightly smaller number of MoV centers within their scaffold compared with Ln-free MBs, as they are typically made up of a smaller number of building blocks.[1,15,40]

Table 3

All MB/Ln-MB-Based Nanoclusters with Approximate Degree of Reduction (Number (n) of Accepted e– Reflects the Number of MoV Centers in the Entire Wheel) Determined through Redox Titrationa

MB/Ln-MB scaffold	n of accepted e–	% of reduction	examples of reported applications	ref
{Mo36}⊂{Mo150}	20	0 in {Mo36} and 13 in {Mo150}		(21)
{Mo138}	24	17		(20)
{Mo154}	28	18	cathode material	(19, 61)
{Mo176}	32	18		(1)
{Mo180(ornithine)7}	32	18		(35)
{Mo150}	28	19		(21)
{Mo138(acetate)6}	28	20	sensor technology	(20, 61)
{Mo36}2⊂{Mo176}	80	67 in {Mo36}2 and 18 in {Mo176}		(22)
{Mo128Eu4}	24	18		(44)
[Mo130Ce6}	24	18		(35)
{Mo150(La)2}	28	18		(31)
{Mo96La8}	20	19		(32)
{Mo100Ce6}	20	19		(33)
{Mo120Pr6}	24	19		(43)
{Mo120Ce6}	24	19		(41)
{Mo120La6}	24	19		(31)
{Mo134La10}	28	19		(34)
{Mo90Ce10}	20	20		(30)

The structures are arranged in increasing degree of reduction of the MBs/LnMBs. The first half of the table contains MBs, and the second half contains Ln-MBs. The symbol “⊂” represents the inclusion of a template cluster within the wheel.

MoV centers, reflecting the scaffold’s degree of reduction, are integrated in Ln-MBs as MoV–O–MoVI bridges, exactly as in MBs, and present in a considerable number within the Ln-MB scaffold. These MoV–O–MoVI structural motifs are considered to be the catalytic centers, as proposed mechanistically in the {Mo154}-catalyzed oxidation of cyclohexane to cyclohexanol by Conte et al.[58]

A correlation has been established between the degree of reduction of gigantic Ln-free POMs and the yield of products in photoredox mediated organic coupling reactions. In the synthesis of 1,4-diphenyl substituted butane-1,4-dione, a yield of 87% was achieved with the keplerate {Mo132} (degree of reduction 45%) and a yield of 78% with the MB {Mo154} (degree of reduction 18%) as a catalyst.[59] As Ln-MBs and MBs exhibit a comparable degree of reduction (Table ), similar redox characteristics are to be expected.

Possible Applications and Relevance of Ln-MBs

Homogeneous and Heterogeneous Catalysis

The development of applications in materials using giant POMs, e.g., keplerates and [Mo154–} is described in the recently published book chapter by Long and Cronin.[61] While several intriguing examples of MBs as homogeneous and heterogeneous catalysts have been reported,[58,62−65] merely one report about a Ln-MB showing catalytical behavior exists.[24] However, based on their structure and stability in solution, potential applications can be foreseen. In homogeneous catalysis organosoluble POMs bearing lanthanides (e.g., the Wells–Dawson archetype) can exhibit chemoselectivity in organic reactions, such as in aldol, Diels–Alder-, Mannich-, and Mukaiyama-type reactions.[27,28] Chiral hybridized complete Ln-MB ring systems can enclose nanosized guest molecules within their cavity,[24] and as they contain lanthanides, which are Lewis acidic cations, these LnIII ions might therefore enable the performance of (selective) catalytic reactions on entrapped substrates in their “Japanese rice-ball”-, circle-, egg-, and ellipse-shaped (see Figure ) nanocavity and function as nanovessels for homogeneous catalysis in organic media. For their use in organic homogeneous catalysis, the Ln-MBs must be readily soluble in organic media. To increase the hydrophobicity of Ln-MBs, appropriate surface reagents can be used, which has already been implemented for the circular-shaped, lanthanide-free MB {Mo176}.[66] Using the amphiphilic complexing agents didodecyldimethylammonium (DDMA) bromide and dioctadodecyldimethylammonium (DOMA) bromide, the hydrophobic aggregates Ln-MB-DDMA/DOMA could be obtained in which the rigid, hydrophilic, inorganic nanocluster would have been encapsulated by a flexible hydrophobic outer shell.

In heterogeneous catalysis, MB nanorings have been shown to be effective oxidation catalysts[58,65] and that they can be used as a solid solution.[67] The chirality of Ln-MBs, caused with the help of LnIII ions,[24] could be exploited in heterogeneous catalysis, for instance, in asymmetric oxidation reactions. A Ln-MB-based solid solution for various oxidation reactions could be obtained as follows: (1) Ln-MBs could be embedded in silica using a sol–gel process as it has already been realized for the circular-shaped lanthanide-free MB {Mo176} (Figure ).[67] (2) Amorphous, optically clear objects (monoliths) of different sizes and shapes could be generated, creating a porous system that contains nanostructures with access to their inner surface (Figure ). The synthesized Ln-MB-silica could behave like a solid solution because the incorporated POMs would be homogeneously distributed in the silica matrix. Additional micropores of different sizes could be created by adding polyethylene glycol (PEG) without causing a macroscopic phase separation or the release/decomposition of the POMs. As the pores introduced by PEG would be much smaller than the cavities of the POMs, molecules could diffuse into the POM, without the POM being released. These POM hybrids could be described as precisely defined, monodisperse reaction systems in which the amount of molybdenum remains constant regardless of the chemical process.

Figure 14

Schematic representation of Ln-MB-silica. (A) Embedding of {Mo176}[1] in silica yields the hybrid material {Mo176}-silica.[67] (B) {Mo176}-silica hybrid material with additional micropores created by PEG.[67]

Molecular Recognition

Molecular recognitions are among the most significant processes of biological and supramolecular systems.[68] Chiral Ln-MBs are supramolecular wheels with unique functional sites inside a well-defined cavity and, therefore, should be capable of chiral recognition and sensing in host–guest systems. The following findings indicate that Ln-MBs can be used for chiral recognition: (1) Chiral lanthanide-containing POMs are capable of conducting chiral recognition. This has been observed for the chiral POM [Ce(α1-P2W17O61)(H2O)]7– by chiral amino acids; the resulting stereoselective interactions were confirmed by 13P NMR spectroscopy.[29] (2) Even achiral MBs can incorporate appropriate nanosized guests chemo-, regio-, and size-selectively[69] (Figure A), and selective recognition will most likely be enhanced by introducing chirality into the system. (3) The interaction of biphenyl dicarboxylic acid (bridging ligand), cyclodextrin (chiral species), and the lanthanide TbIII yielded polyrotaxane-type supramolecular assemblies, which enabled the recognition of small chiral molecules.[70] Wheels can also form polyrotaxane-type supramolecular assemblies.[71] The coassembly of the polymer p-phenylenebutadiynylene (PBn) (Figure B) with a MB yielded a one-dimensional, tubular arrangement of cofacially connected wheels. Both the rigidity and the pendant ammonium groups of the PBn have a high affinity for the wheel surface, which lead to threading interactions with the wheel and the formation of so-called inorganic–organic polypseudorotaxanes. The threading interaction is so strong that even electrostatically fixed metalloporphyrins can be pushed out of the wheel.

Figure 15

Schematic representation of {Mo176} complexating organic guest molecules selectively. (A) Inclusion of three metalloporphyrins with meso-aminophenyl substituents into the cavity of {Mo176}.[69] Only the amino groups at the para- and meta-position of the phenyl ring form hydrogen bonds inside the wheel. (B) {Mo176} and the polymer p-phenylenebutadiynylene (PBn) forming an inorganic/organic polypseudorotaxane.[71]

Chiral Ln-MBs could also potentially serve as inorganic counterparts of cucurbit[n]urils, which are organic macrocyclic molecules composed of several glycoluril units and known for their outstanding recognition properties.[72,73] In particular, the possibility to produce chiral Ln-MBs specially functionalized with different amino acids and peptide sequences makes them promising candidates for recognizing specific peptides and proteins. Thus, stable, highly scalable, and potentially more reliable synthetic receptors of inorganic–organic nature could be produced with the ability to specifically inhibit or enhance the biological activity of certain peptides or proteins. For example, smaller POM-peptide hybrids have been shown to inhibit the aggregation of β-amyloid peptides.[74]

Lanthanide Separation

Lanthanides are important metals in the design of special materials such as illuminant, colorants, permanent magnets, and lasers.[75−77] They coexist quite often isostructural in sources of occurrence due to their chemical similarities[78−80] (e.g., ion radii close together, lack of redox activity, etc.). As a result, the separation of neighboring 4f-elements in particular poses a significant challenge. A selective lanthanide separation based on purely inorganic chemical recognition has been realized through the crystallization of borates.[81] The separation was based on the binding differences of the LnIII ions. Remarkably, the thermodynamically controlled synthesis of the circular-shaped Ln-MBs {Mo90Ln10}[31] (LnIII = LaIII, CeIII, or PrIII) and {Mo92Ln9}[31] (LnIII = NdIII or SmIII) (both depicted in Figure J and I) can be used to selectively enrich lanthanides from binary lanthanide mixtures as well.[31] This was confirmed by ICP-OES analyses of lanthanide containing solid products formed in a reaction solution which contained two lanthanide species (Figure shows the binary system La/Sm, Eu, or Er). The separation factor of various binary lanthanide mixtures is associated with the difference in ionic radius between the lanthanides; that is, the larger the difference in ionic radius, the better the separation. Therefore, lanthanides with large ionic radii are preferred for thermodynamically regulated wheel construction. Lanthanide separation is typically performed in industry using hydrometallurgical techniques; however, on a laboratory scale, the Ln-MB-based lanthanide separation method could prove to be an elegant approach.

Figure 16

Comparison of the percentages of lanthanides in samples in various states obtained from binary lanthanide mixture in a thermodynamically controlled Ln-MB synthesis.[31]

Conclusion

The spectrum of Ln-MB wheel clusters is highly diverse, as they come in a variety of structural types, shapes (“Japanese rice-ball”, “egg”, “ellipse”, and “circle”) and sizes with an inner and outer diameter of 10–22 and 26–39 Å, respectively. These unique inorganic ring systems possess a nanometer-sized cavity that allows for unprecedented and intriguing host–guest chemistry. Specific scaffolds with the tuning of the nanometer-sized cavity can be conveniently achieved as all architecturally labile {Mo2}-type building blocks in Ln-MBs are both completely replaceable by lanthanide ions and organically modifiable. Not only the nanocavity but also the properties of the outer surface can be altered, specifically, by introducing organic surfactants to increase the hydrophobicity, which allows the adjustment of the cluster’s polarity. MBs with integrated lanthanide ions might be critical in applications such as catalysis, molecular recognition, and lanthanide ion separation, and as several spectroscopic techniques, including the powerful ESI-IMS-MS, have demonstrated, Ln-MBs preserve their integrity in solution, implying that they could endure robust reactions without decomposing. The archive of crystal structures of wheel-shaped Ln-MBs is still considerably expandable as many organic ligands, especially amine-containing ligands, remain to be explored. The conduction of SAXS studies of Ln-MBs in the solution phase is strongly recommended to explore their stability and speciation behavior in reactions and various media. We are confident that Ln-MBs will be used as an inorganic platform for the development of innovative molecular architectures and nanostructured materials as well as for the design of adaptive inorganic–organic chemical systems.