We hypothesised that there is space for a greater diversity of structures in this immediate reduction zone. Here we show it is possible to make highly reduced mix‐valence POMs by presenting new classes of polyoxomolybdates: [MoV52MoVI12H26O200]42− {Mo64} and [MoV40MoVI30H30O215]20− {Mo70}, 81 % and 57 % reduced, respectively. The {Mo64} cluster archetype has a super‐cube structure and is composed of five different types of building blocks, each arranged in overlayed Archimedean or Platonic polyhedra. The {Mo70} cluster comprises five tripodal {MoV6} and five tetrahedral {MoV2MoVI2} building blocks alternatively linked to form a loop with a pentagonal star topology. We also show how the reaction yielding the {Mo64} super‐cube can be used in the enrichment of lanthanides which exploit the differences in selectivity in the self‐assembly of the polyoxometalates.The assembly of nanoscale polyoxometalate (POM) clusters has been dominated by the highly reduced icosahedral {Mo 64} cube and {Mo70} star, are obtained through near‐hydrothermal conditions with incorporations of nickel and lanthanide ions and possess a set of building blocks different to those present in Mo blues and browns. With a few of other rare structures, this group of polyoxomolybdates is redefined as Mo reds based on the major differences in building blocks and reduction degrees.Novel types of highly reduced Mo polyoxometalates, the {Mo Exploring the assembly‐space of metal oxides is important to understand self‐assembly and discover new structural archetypes. In the case of polyoxometalates (POMs), the self‐assembly is normally critically controlled by the degree of reduction, type of anionic templates, and the nature of the cations. POMs are a diverse family of molecular metal oxides which have already been used for or have potential in catalysis,5} units, as well as the {Mo2} and {Mo1} units seen in Molybdenum blues {Mo154}368}.V2} unit is found to be a new type of building block. This basic unit having a MoV−MoV bond of ≈2.5 Å  in solution. The building blocks commonly found are the pentagonal {(Mo)MoV−MoV bond and creates a set of structures commonly known as the Molybdenum browns, due to their brown colour. Many of the other building blocks are shared between both sets of reduced molybdates, as seen in the well‐studied {Mo132},5} pentagonal unit being key for molecular growth.37}42}240} has been a new addition to this now growing group of mix‐valence molybdates.d and f metal ions in the synthesis of two new classes of highly reduced, mix‐valence polyoxomolybdates: [MoV52MoVI12H26O200]42− {Mo64} and [MoV40MoVI30H30O215]20− {Mo70} and redefine this group of molybdates as Molybdenum reds.This building block localizes the two reducing electrons to form the MoV52MoVI12H26O200]42− and [MoV40MoVI30H30O215]20− cluster anions were obtained with incorporation of Ni2+ and a range of Ln3+ ions and charge balanced by Na+ cations as follows:The 2+ seems to rapidly lead to the initial formation of the anionic ϵ‐Keggin‐type cluster [Mov12H12O40]8− 30]8− forms. We then investigated whether the same Mo/Ni‐only cube framework would form in the absence of La3+ ions but instead the Ni2+ complex of ϵ‐Keggin [{Ni(H2O)3}4Mov12H12O40] {Mo12Ni4} , they could be selectively filtered by using a larger pore sized filter mesh which allowed the precipitate and smaller crystals to wash off while retaining compound 1. Compounds 2 and 3 were synthesized and purified in the same manner with Ce3+ and Pr3+ replacing La3+ and the clusters adopt the same cubic structure as 1. 1 and 2 are isomorphous and crystallise in similar unit cells, however 2 also forms another phase compound 2′, which crystallizes in a unit cell isomorphous to 3. Hence compounds 2′ and 3 share an almost identical cluster framework to 1 and 2 but they deviate slightly in the overall size of the cluster due to the smaller lanthanide ion. This is consistent with the crystal morphology observed; compounds 1 and 2 grow as large red square plates while 2′ and 3 are found as long red rods.This excess, in addition to an even higher pH, results in a substantial amount of precipitate in the form of La4 was obtained under similar reaction conditions to those for compounds 1–3, with two key differences: the pH and the amount of reducing agent used. A lower pH value is needed to obtain 4 as well as a reducing stoichiometry similar to that of the final structure . That said however, where these two syntheses differ the most is on the yield and purity of the final product. Where for compounds 1 to 3 the crystals that appeared were relatively big and easy to separate from their by‐products in the reaction mixture, here the crystals of 4 were found as tiny rods in a midst of other crystals from minor molybdenum‐based salts, mostly MoO3,4 extremely difficult. A possible reason for this could be that the overall structure may not be very stable, or rather, it may be a high energy intermediate between other, more thermodynamically stable, compounds.Compound 70H30O215]20− {Mo70} in compound 4 exhibits a pentagonal star‐like structure with building blocks similar to that of {Mo240} cage reported earlier, see Figure 240}.5 and 6 have structures derived from 4 where the Ce3+ are replaced by Nd3+ and Sm3+ respectively and they retain the {Mo70} star‐like framework but have significant differences in lanthanide ion coordination. For compound 4, the large cerium ions are difficult to fully incorporate into the star rim resulting in the 5th Ce3+ ion in the rim moving out of plane and bridging between two adjacent star structures. As such, the Ce3+ ion on this position of a crystallographic inversion centre is shared by two star molecules. In contrast, the rim lanthanides in both 5 and 6 are all within the plane and are fully occupied. Due to these differences the three compounds, 4, 5, and 6, crystallise in very different crystal systems, see Table S4.The cluster 8−, {Mo64Ni8Ln6} , containing a [MoV52MoVI12H26O200]42− {Mo64} polyoxomolybdate . The structure is composed by 5 types of building blocks. Firstly, 48 MoV atoms form the main framework of the structure in the shape of a truncated cuboctahedron 3} is fitted to coordinate to three μ2‐O atoms on the {MoV6} and form a {NiO3(H2O)3} moiety of C3v symmetry, which resembles one of the four faces of a Ni doped ϵ‐Keggin, {Mo12Ni4}.ca 10.4 Å indicating a cluster size of over 1000 Å3 in volume. Attached to the main {MoV48} framework, 12 {MoVI} sites, each below a cube edge centre, are distributed in a cuboctahedron } sites, each on a face‐centre of the cube, form an octahedron } units link together 4 {MoV6} groups as well as 4 {MoVI} units by coordinating to 4 μ2‐O atoms each from a {MoV6} and 4 terminal O atoms each from a {MoVI} unit, leaving the final H2O ligand towards outside of the cluster. The coordination sphere of {LnO8(H2O)} displays regular C4v symmetry. Finally, over each edge centre of the cube an additional {MoVO2} can be found. However, the occupancy of these 12 {MoVO2} units is only 1/3, meaning only 4 {MoVO2} coexist in one cube which, with maximum symmetry, adopt a tetrahedral supramolecular arrangement 3}, an octahedron of 6 {LnIII(H2O)}, a cuboctahedron made of 12 {MoVI} and a disordered tetrahedron of 4 {MoVO2} spread over 12 positions. A perchlorate anion was identified at the centre of the cubic cluster, although it was difficult to model during the structure refinement due to disorder.Single‐crystal X‐ray structural analysis4 crystallises in the monoclinic system with C2/c space group and displays a [MoV40MoVI30H30O215]20− {Mo70} cluster structure consisting of 5 {Mo10} units and 5 {Mo4} linking units. Each {Mo10} can be considered as a tripodal {MoV6} unit  and a {MoO4} template . The position of this MoVI site is very similar to how the Ni sits over the centre of a coplanar {MoV6} building block in the cube structure, but with a coordination sphere of {MoVIO5(H2O)} . As the tripodal {MoV6} is not stable on its own, the {MoVI3} add‐on group can be regarded as a finishing unit for the exposed two “legs” of the tripod. The tripodal {MoV6} building block has been observed in structures like the {Mo240}, {Mo37} or {Mo42}, see Figure Compound V6} building block, a tetrahedral {MoO4} template also tightly contacts with the edge sharing dimer {MoVI2}, taking a position normally occupied by a heteroatom such as P, S or Si. Five {Mo4}={MoV2MoVI2}  units connect the aforementioned {Mo10} building units making the main framework of the structure.ca 23.5 Å at its most elongated points and 8.9 Å at its shortest. The inner cavity is defined by two concentric rings delineated by the {Mo4} units and the central {MoO4} tetrahedral templates, with a radius of 4.4 Å and 5.5 Å, respectively.In the centre of tripodal {Mo5 and 6 also display the same {Mo70} star cluster structure however, they crystallise in different crystal systems and space groups (P21/c for 5 and P21/n for 6). The main differences between these compounds come from the final components of the structure: lanthanide ions appended to the {Mo70} framework. In the case of compound 4, 4.5 {CeIII(H2O)n} (n=4 to 6) per cluster can fit in the space in between two adjacent {Mo10} units and on top of a {Mo4} BB. The half equivalent comes about due to the fact that a disordered {CeIII(H2O)n} is located around a crystallographic inversion centre shared by and bridging two {Mo70} clusters. Between these two {Mo70} clusters there are other four Ce3+ bridges in proximate area of this inversion centre. Each Ce3+ typically connects to one {Mo70} main framework through three points: three terminal oxo each from the two neighbouring {Mo10} and one {Mo4} below. The remaining coordination sites of Ce3+ are occupied by water ligands or oxo ligands from nearby {Mo70} cluster thus forming multiple bridges between clusters. Only one of the five Ce3+ on the cluster does not form bridges; thus, each cluster of 4 has seven Ce3+ bridges between clusters that form a tightly linked 1D chain .Compounds 5 and 6, five {LnIII(H2O)n} (Ln=Nd and Sm) are found attached to the {Mo70} cluster. Here only two of the five {LnIII(H2O)n} are bound to oxo ligands from nearby {Mo70} clusters. Thus, each cluster has only four bridges forming a 1D chain . Common to all three compounds is an additional {LnIII(H2O)9} with full or partial occupancy at the centre of the cluster. The slight differences in the packing between compounds 4, 5 and 6 can be attributed to the different sizes of the lanthanide ions around the cores of the {Mo16}In the case of compounds 102},368} blue lemon.46}54},132}, is the main representative of this group and it has been a subject of great study for ligand exchange and host–guest chemistry.V2} building block, formed by two MoV, where the reducing electrons are localised and a Mo−Mo bond forms. In both the Mo blues and Mo browns, one of the most characteristic building blocks to form is the pentagonal {(Mo)Mo5}. This building block usually interconnects other building units and promotes molecular growth. Its versatility allows it to be present in structures as different as the {Mo154},132}57}.When looking at mix‐valence Mo clusters, traditionally, these have been categorised into 2 main groups. “Mo blues” are the most extensive group and also the less reduced one, with a reduction degree around 35 %. In this group we find the ring structures, the blue ball, {MoV content >50 %) than the ones described here. These were, probably for a lack of a better classification, categorized as Mo browns. The first one is the ϵ‐Keggin [Mov12H12O40]8−. This structure has 100 % of its Mo centres reduced but it is unstable so it does not appear on its own but rather stabilized by other metal ions like Ni2+, Co2+, MoVI and also by lanthanides.[37} structure.42}, an extended structure of a ϵ‐Keggin core, represents the highest reduced Mo compound known to date (85 %) but it should be noted that it relies on organic ligands for its formation.Until recently, there were only three examples of mixed valence Mo compounds with a higher reducing ratio Mo1 was first encountered as {Mo64Ni8La6} it was hypothesized that several iso‐structures could be achieved by changing both the Ni2+ for a different 1st row TM and the La3+ for other lanthanide ions. This proved to be only partially true since no compound was successfully isolated when using other TM salts. On the other hand, the cube structure was also found for Ce, compound 2, and for Pr, compound 3, but interestingly, for no other lanthanides. This last finding raised the question that perhaps the cube framework and its synthesis can be a means of separating different lanthanides depending on their ability to form the compound or not. With that in mind, we sought to achieve separation between close lanthanide ions and also between light and heavy lanthanides to see if there was a potential new approach.When compound 1). The third one was a 3 : 1 mixture using 0.75 equiv and 0.25 equiv of La and Er, respectively. The mixture using 0.5 equiv of each lanthanide failed to produce any quality crystals but the other two did, and in comparable yield to a regular synthesis. For the 1 : 1 mixture using 1 equiv of each lanthanide, the separation factor was quite promising and comparable to previous studies. That said, the more surprising results came from the 3 : 1 mixture where a separation factor close to 26 was obtained. It was expected that with a higher content of La, the separation would be better, but such a separation factor shows the selectivity that this reaction has for light lanthanides. With these promising results achieved for one binary mixture, a set of different La based mixtures were tested to compare and test the selectivity between early and later lanthanides. Table A mixture of La and Er was chosen as a starting point since this mixture has given good results in previous work.64Ni8Ln6} (1–3), through near hydrothermal conditions and using lanthanides  as molecular growth agents. Strongly acidifying the reaction mixture in the presence of reducing agent promotes the reduction of the molybdenum centres from MoVI to MoV, which form dimeric {Mo2V} building blocks. These can come together into Keggin‐like structural units, which will control the self‐assembly process. In addition, through similar synthetic conditions a star‐shape POM framework, {Mo70Ln5‐6}  has been isolated. This structure is believed to be a high‐energy intermediate of the recently reported {Mo240} cage. Its discovery has been a means to re‐evaluate the {Mo240} and its formation, establishing tetrahedral Mo units as templates as opposed to SO32−/SO42−. The cube and star are two new additions to the now growing set of structures we here define as Mo reds. The differences in the morphology and characteristic building blocks between Mo blues, browns and these new set of highly reduced polyoxomolybdates has led us to naturally classify them as a subspecies of mixed‐valence Mo‐based POMs. In addition, the cube structure has proven to be highly selective towards lanthanide size only incorporating the larger ones in its framework. We have exploited this fact through lanthanide enrichment tests where we can draw a direct correlation between the difference in ionic radii of the lanthanides in the mixture and their separation factors. The positive results obtained in these tests confirm the great potential of using self‐assembly in lanthanide separation. In conclusion, this work demonstrated the first example of incorporating different types of transition metal ions to produce high nuclearity Mo reds. We predict that there are still plenty of interesting reduced POMs that will be accessed by use of other combination of metal ions as templates and charger balancers. These materials will be of high interest as they will combine the properties of highly reduced POMs and the metal ions themselves.To conclude, we have synthesised highly reduced cube‐shaped polyoxometalates, {MoThe authors declare no conflict of interest.1As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. 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