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scientific commentaries\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

Journal logoSTRUCTURAL
CHEMISTRY
ISSN: 2053-2296
Volume 78| Part 6| June 2022| Pages 322-323
https://doi.org/10.1107/S2053229622004491

Comment on molybdenum polyoxo clusters: from the `Blues' to the `Reds'

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Bernt Krebsa*

aWestfälische Wilhelms-Universität Münster, Institut für Anorganische und Analytische Chemie, Corrensstrasse 28/30, D-48149 Münster, Germany
*Correspondence e-mail: krebs@uni-muenster.de

Edited by P. Raithby, University of Bath, United Kingdom
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Molybdenum `Blues', `Browns' and the recently discovered `Reds' are unquestionably some of the most impressive nanoscale architectures in polyoxometalate (POM) chemistry (Müller & Gouzerh, 2012[Müller, A. & Gouzerh, P. (2012). Chem. Soc. Rev. 41, 7431-7463.]; Lin et al., 2020[Lin, J., Li, N., Yang, S., Jia, M., Liu, J., Li, X.-M., An, L., Tian, Q., Dong, L.-Z. & Lan, Y.-Q. (2020). J. Am. Chem. Soc. 142, 13982-13988.]). These three polyoxomolybdate classes, with the general formula [XaYbHcMoVIxMoVyOz(H2O)v]n− (a and b = number of organic ligands and heteroelements, respectively; c = degree of protonation; x and y = number of MoVI and reduced MoV centres, respectively) are classified as gigantic mixed-valence (MoV/VI) polyoxomolybdate clusters with various topologies ranging from `wheels' to `balls', `cages' and the `blue lemon', known to be the largest inorganic mol­ecule to date (Lin et al., 2020[Lin, J., Li, N., Yang, S., Jia, M., Liu, J., Li, X.-M., An, L., Tian, Q., Dong, L.-Z. & Lan, Y.-Q. (2020). J. Am. Chem. Soc. 142, 13982-13988.]; Müller & Gouzerh, 2012[Müller, A. & Gouzerh, P. (2012). Chem. Soc. Rev. 41, 7431-7463.]). While the subclasses of molybdenum `Blues', `Browns' and `Reds' have the reduction of an acidified solution containing orthomolybdate ([MoO4]2−) or hepta­molybdate (Hx[Mo7O24](6–x)–) in common, they can be distinguished by three characteristic features. Firstly, `Blues', `Browns' and `Reds' exhibit distinct degrees of reduction, with the `Blues' being the least (∼18%) and the `Reds' the most (up to ∼81%) reduced representatives (Ribó et al., 2022[Ribó, E. G., Bell, N. L., Long, D.-L. & Cronin, L. (2022). Angew. Chem. Int. Ed. https://doi.org/10.1002/ange.202201672.]).

Secondly, in terms of molybdenum building blocks, wheel-shaped molybdenum `Blues' are composed of {MoO6} ({Mo1}), corner-sharing {Mo2O11} ({Mo2}) and the essential building block {Mo8O35} ({Mo8}), while the ball-shaped `Browns' consist of {Mo1}, edge-sharing {Mo2O10} ({Mo2′}) and {Mo6O27} ({Mo6}) units (Fig. 1[link]) (Müller & Gouzerh, 2012[Müller, A. & Gouzerh, P. (2012). Chem. Soc. Rev. 41, 7431-7463.]).

[Figure 1]
Figure 1
Polyhedral representation of the building blocks in mixed-valence polyoxomolybdates. Colour code: {MoO6} ({Mo1}), yellow; corner-sharing {Mo2O11} ({Mo2}), red; edge-sharing {Mo2O10} ({Mo2′}, ochre; {Mo6O27} ({Mo6}) and {Mo8O35} ({Mo8}), blue, with the central {MoO7} unit in cyan.

Remarkably, molybdenum `Reds' exclusively comprise {Mo1} and {Mo2′} units (Fig. 1[link]) (Lin et al., 2020[Lin, J., Li, N., Yang, S., Jia, M., Liu, J., Li, X.-M., An, L., Tian, Q., Dong, L.-Z. & Lan, Y.-Q. (2020). J. Am. Chem. Soc. 142, 13982-13988.]), hence requiring a comparably trickier synthetic approach than their blue counterparts, which necessitates the use of additional cluster-stabilizing coordinated ions (e.g. SO32−, 3d- and 4f-metal ions) (Lin et al., 2020[Lin, J., Li, N., Yang, S., Jia, M., Liu, J., Li, X.-M., An, L., Tian, Q., Dong, L.-Z. & Lan, Y.-Q. (2020). J. Am. Chem. Soc. 142, 13982-13988.]; Ribó et al., 2022[Ribó, E. G., Bell, N. L., Long, D.-L. & Cronin, L. (2022). Angew. Chem. Int. Ed. https://doi.org/10.1002/ange.202201672.]) to aggregate the {Mo1} and dumbbell-shaped {Mo2′} into larger clusters, consequently promoting mol­ecular growth (Ribó et al., 2022[Ribó, E. G., Bell, N. L., Long, D.-L. & Cronin, L. (2022). Angew. Chem. Int. Ed. https://doi.org/10.1002/ange.202201672.]).

Contrary to molybdenum `Reds', additional ions (e.g. 3d- and 4f-metal ions) react differently in `Blue' systems as they are utilized here for fine-tuning the mol­ecular shape (Garrido Ribó et al., 2020[Garrido Ribó, E., Bell, N. L., Xuan, W., Luo, J., Long, D.-L., Liu, T. & Cronin, L. (2020). J. Am. Chem. Soc. 142, 17508-17514.]) and alternating the mol­ecule's physical characteristics rather than spanning the cluster framework.

For instance, large electrophilic `open-shell' metal centres, such as 4f-metal ions, alter the overall charge, mol­ecular shape and size, and always cause a symmetry reduction of molybdenum `Blues' when introduced into their frameworks (Al-Sayed & Rompel, 2022[Al-Sayed, E. & Rompel, A. (2022). ACS Nanosci. Au. https://doi.org/10.1021/acsnanoscienceau.1c00036.]). In `Blue' systems, however, 3d-metal ions seem to behave in a significantly opposite manner from their 4f counterparts considering their relatively small size which leaves them without any impact on the assembly of the {Mo1}, {Mo2} and {Mo8} building blocks (Fig. 1[link]).

Thirdly, in terms of functionalizability, the `Blues' and `Browns' can be organically functionalized, resulting in reaction vessels for studying confined mol­ecules. Organic hybridization is typically accomplished by attaching carboxyl groups to the cluster and nitro­gen-containing pendant groups in carb­oxy­lic acids. Such constructs are capable of promoting mol­ecular growth and triggering the formation of various guest@host architectures (Xuan et al., 2017[Xuan, W., Pow, R., Long, D. L. & Cronin, L. (2017). Angew. Chem. Int. Ed. 56, 9727-9731.], 2019[Xuan, W., Pow, R., Zheng, Q., Watfa, N., Long, D. L. & Cronin, L. (2019). Angew. Chem. Int. Ed. 58, 10867-10872.]; Imai et al., 2009[Imai, H., Akutagawa, T., Kudo, F., Ito, M., Toyoda, K., Noro, S., Cronin, L. & Nakamura, T. (2009). J. Am. Chem. Soc. 131, 13578-13579.]). In stark contrast to their `Blue' counterparts, nothing is yet known about the organic hybridizability of molybdenum `Reds'. In the event of a molybdenum `Red' hybridization, the dumbbell-shaped and edge-sharing {Mo2′} (Fig. 1[link]) building blocks would have to be functionalized organically.

In this context, Al-Sayed et al. (2022[Al-Sayed, E., Tanuhadi, E., Giester, G. & Rompel, A. (2022). Acta Cryst. C78, 299-304.]) recently made a significant contribution, reporting a new binding mode of an organic hybridization reagent in mixed valence polyoxomolybdates (Al-Sayed et al., 2022[Al-Sayed, E., Tanuhadi, E., Giester, G. & Rompel, A. (2022). Acta Cryst. C78, 299-304.]). The organic functionalization of a dumb­bell-shaped and edge-sharing {Mo2′} building block occurred through chelation of a pyridine derivative, as shown by the isolation of the [Mo2O2(OH)4(C6H4NO2)2]2+ unit (Fig. 2[link]) acting as a charge-balancing dication for the molybdenum `Blue' cluster Na4[Mo2O2(OH)4(C6H4NO2)2]2[Mo120Ce6O366H12(OH)2(H2O)76]∼200H2O. As `Reds' comprise a multitude of {Mo2′} units, the outstanding work of Al-Sayed et al. (2022[Al-Sayed, E., Tanuhadi, E., Giester, G. & Rompel, A. (2022). Acta Cryst. C78, 299-304.]) represents a major synthetic step forward, since it showcases the first example of the N,O-chelation of {Mo2′} units (Fig. 2[link]).

[Figure 2]
Figure 2
Ball-and-stick representation of the organofunctionalized [Mo2O2-(OH)4(C6H4NO2)2]2+ unit acting as a charge-balancing dication for the Japanese rice-ball-shaped Molybdenum Blue Na4[Mo2O2(OH)4(C6H4NO2)2]2[Mo120Ce6O366H12(OH)2(H2O)76]∼200H2O. Colour code: Mo black, O red, C grey, N blue and H white.

To understand the fundamental principles of a multicomponent system driving the assembly of these aesthetically pleasing constructions, as well as the prospects for forming novel ones, is a major undertaking. Identifying novel steps in the formation process of mixed-valence polyoxomolybdates, such as an effect of a structure-directing ligand or implementing a unique functionalization of building blocks, will enable the construction of POM clusters with unprecedented sizes and topologies, thereby shedding light upon the mostly elusive self-assembly mechanisms, ultimately perhaps paving the way towards novel POM architectures with nuclearities surpassing the famous Na48[HxMo368O1032(H2O)240(SO4)48]∼1000H2O (Mo368) `blue lemon' (Müller et al., 2002[Müller, A., Beckmann, E., Bögge, H., Schmidtmann, M. & Dress, A. (2002). Angew. Chem. Int. Ed. 41, 1162-1167.]).

More than two decades separate the discovery of the `Blues' and the `Reds'. Is that the end of the molybdenum classes, or will the synthetic chemists have yet another new colour to enjoy?

References

First citationAl-Sayed, E. & Rompel, A. (2022). ACS Nanosci. Au. https://doi.org/10.1021/acsnanoscienceau.1c00036.  Google Scholar
 First citationAl-Sayed, E., Tanuhadi, E., Giester, G. & Rompel, A. (2022). Acta Cryst. C78, 299–304.  CSD CrossRef IUCr Journals Google Scholar
 First citationGarrido Ribó, E., Bell, N. L., Xuan, W., Luo, J., Long, D.-L., Liu, T. & Cronin, L. (2020). J. Am. Chem. Soc. 142, 17508–17514.  Web of Science PubMed Google Scholar
 First citationImai, H., Akutagawa, T., Kudo, F., Ito, M., Toyoda, K., Noro, S., Cronin, L. & Nakamura, T. (2009). J. Am. Chem. Soc. 131, 13578–13579.  Web of Science CrossRef PubMed CAS Google Scholar
 First citationLin, J., Li, N., Yang, S., Jia, M., Liu, J., Li, X.-M., An, L., Tian, Q., Dong, L.-Z. & Lan, Y.-Q. (2020). J. Am. Chem. Soc. 142, 13982–13988.  Web of Science CrossRef ICSD CAS PubMed Google Scholar
 First citationMüller, A., Beckmann, E., Bögge, H., Schmidtmann, M. & Dress, A. (2002). Angew. Chem. Int. Ed. 41, 1162–1167.  Google Scholar
 First citationMüller, A. & Gouzerh, P. (2012). Chem. Soc. Rev. 41, 7431–7463.  Web of Science PubMed Google Scholar
 First citationRibó, E. G., Bell, N. L., Long, D.-L. & Cronin, L. (2022). Angew. Chem. Int. Ed. https://doi.org/10.1002/ange.202201672.  Google Scholar
 First citationXuan, W., Pow, R., Long, D. L. & Cronin, L. (2017). Angew. Chem. Int. Ed. 56, 9727–9731.  Web of Science CSD CrossRef ICSD CAS Google Scholar
 First citationXuan, W., Pow, R., Zheng, Q., Watfa, N., Long, D. L. & Cronin, L. (2019). Angew. Chem. Int. Ed. 58, 10867–10872.  Web of Science CSD CrossRef CAS Google Scholar

This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.

Journal logoSTRUCTURAL
CHEMISTRY
ISSN: 2053-2296
Volume 78| Part 6| June 2022| Pages 322-323
https://doi.org/10.1107/S2053229622004491
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