Discrete polyoxometalates: from geochemistry via crystal engineering to functional materials

U. Kortz

Jacobs University, Department of Life Sciences and Chemistry, Campus Ring 1, 28759 Bremen, Germany u.kortz@jacobs-university.de

Polyoxometalates (POMs) are a class of discrete, anionic metal-oxides with an enormous structural diversity and a multitude of interesting properties leading to potential applications in different areas including catalysis, nanotechnology and medicine [1]. Noble metal-containing POMs are in particular attractive for homogeneous and heterogeneous catalytic applications. However, the number of well characterized noble-metal POMs is rather small [2]. POMs based exclusively on Pd²⁺ addenda (polyoxopalladates, POPs) were discovered in 2008 [3]. The area of POP chemistry has developed rapidly ever since, due to the fundamentally novel structural and compositional features of POPs, resulting in unprecedented electronic, spectroscopic, magnetic, and catalytic properties [4]. In terms of POP structural types, the symmetrical 12-palladate nanocube {MPd₁₂L₈} and the 15-palladate nanostar {MPd₁₅L₁₀} are the most abundant. Especially for the {MPd₁₂L₈} nanocube, many derivatives with various central guests including d and f block metal ions and various capping groups are known [4]. We demonstrated the use of {MPd₁₂L₈} as discrete molecular precursors for the formation of supported palladium metal nanoparticles as hydrogenation catalysts, and we discovered an important dependence of the catalytic properties on the type of internal metal guest and external capping group [5]. We also managed to construct 3D coordination networks using externally functionalized POPs, resulting in metal-organic framework (MOF)-type assemblies (POP-MOFs) with interesting sorption and catalytic (C-C coupling) properties [6].

[1] Pope, M. T. (1983). Heteropoly and isopoly oxometalates. Springer Verlag.

[2] Izarova, N. V.; Pope, M. T.; Kortz, U. (2012). Angew. Chem. Int. Ed. 51, 9492.

[3] Chubarova, E. V.; Dickman, M. H.; Keita, B.; Nadjo, L.; Mifsud, M.; Arends, I. W. C. E.; Kortz, U. (2008). Angew. Chem. Int. Ed. 47, 9542.

[4] Yang, P.; Kortz, U. (2018). Acc. Chem. Res. 51, 1599.

[5] Ayass, W. W.; Miñambres, J. F.; Yang, P.; Ma, T.; Lin, Z.; Meyer, R.; Jaensch, H.; Bons, A.-J.; Kortz, U. (2019). Inorg. Chem. 58, 5576.

[6] Bhattacharya, S.; Ayass, W. W.; Taffa, D. H.; Schneemann, A.; Semrau, A. L.; Wannapaiboon, S.; Altmann, P. J.; Pöthig, A.; Nisar, T.; Balster, T.; Burtch, N. C.; Wagner, V.; Fischer, R. A.; Wark, M.; Kortz, U. (2019). J. Am. Chem. Soc. 141, 3385.

Titan, the largest moon of Saturn, has been revealed by the Cassini-Huygens mission to be a fascinating and quite Earth-like world. Among the parallels to Earth, which includes the lakes, seas, fluvial and pluvial features on its surface, is an inventory of organic minerals [1]. However, where on Earth these organic minerals are only found in niche environments, on Titan they are likely to be the dominant surface-shaping materials. Titan’s organic minerals are formed primarily from photochemistry induced by UV radiation and charged particles from Saturn’s magnetosphere, which cause molecular nitrogen and methane (the primary components of the upper atmosphere) to generate into various CHN-containing species that deposit onto the surface [2].

Despite the ubiquity of these organic minerals upon the surface, it is difficult to understand their influence on the landscape and as, in some cases, even their crystal structure is unknown let alone wider physical properties[3]. Hence we have undertaken an experimental program to address this, and are currently focusing on the missing crystal structure and physical property understanding of a number of molecular solids and co-crystals that are likely to be organic minerals upon Titan. Using a combination of neutron diffraction, X-ray diffraction and Raman scattering we have studied molecular solids including ethane, acrylonitrile, acetonitrile, butadiene and propyne, and explored what co-crystal form from the inventory of Titan’s molecules. This contribution will report highlights from these investigations.

[1] Lopes, R.M., Malaska, M.J., Schoenfeld, A.M., Solomonidou, A., Birch, S.P.D., Florence, M., Hayes, A.G., Williams, D.A., Radebaugh, J., Verlander, T. and Turtle, E.P. (2020) Nature Astronomy 4(3) pp.228-233. [2] Hörst, S. M. (2017). Journal of Geophysical Research: Planets 122 (3), 432-482 [3] Maynard-Casely, H.E., Cable, M.L., Malaska, M.J., Vu, T.H., Choukroun, M. and Hodyss, R. (2018) 103(3), pp.343-349 [4] Balzar, D. & Popa, N. C. (2004). Diffraction Analysis of the Microstructure of Materials, edited by E. J. Mittemeijer & P. Scardi, pp. 125-145. Berlin: Springer.

Over the last decade, crystal flask that converts one chemical species to another one in single-crystal-to-single-crystal (SCSC) manner has attracted much attention in crystal engineering. An important key for the construction of crystal flask is a porous space which can induce a chemical from outside of a crystal.¹ To design such a porous space, our group has intensively studied the metalloligand approach in which a pre‐prepared homometallic complex with coordination donor sites is reacted stepwise with secondary metal ions.² We established the construction of a variety of metalloarchitectures by the metalloligand approach with using thiolato groups derived from amino acids and phosphine ligands. Recently, our group has successfully prepared a microporous material of a nanometer-sized Au^ICd^II 116-nuclear cage-of-cage structure (1_CdNa) from the reaction of the tripodal-type trigold(I) metalloligand, [Au^I₃(tdme)(D-Hpen)₃] (tdme = 1,1,1-tris(diphenylphosphinomethyl)ethane, D-H₂pen = d-penicillamine), with Cd^II(NO₃)₂.³ The cage-of-cage structure was constructed from 12 building units of Au^I₆Cd^II₃ cage complex through hierarchical aggregation. Interestingly, 1_CdNa has large interstices connected by 3D channels which allow the easy incorporation and accommodation of guest molecules. Therefore, it was found that 1_CdNa underwent the stepwise SCSC transmetallation reactions to form Au^ICu^II metallocage (1_Cu). Furthermore, we found that the crystals of 1_Cu have the ability to accommodate MoO₄^2– ions (2_Mo1) and condense them to form Mo₇O₂₄^6– (2_Mo7) and β-Mo₈O₂₆^4– (2_Mo8) by the addition of protons in the solid state. These results show the availability of the large crystal interstices in 1_Cu as crystal flask, which serves as a reaction field for accommodated chemical species in crystal. Such a crystal flask reaction of polyoxomolybdate will give an important insight for not only material science but also biosynthesis in Mo-storage protein (MoSto) which contains Mo₈, Mo_5-7 and Mo₃ clusters. The detail will be discussed in the presentation.

[1] Inokuma, Y., Kawano, M. & Fujita, M. (2011). Nat. Chem. 3, 349. [2] Yoshinari, N. & Konno, T. (2016). Chem. Rec. 16, 1647. [3] Imanishi, K., Wahyudianto, B., Kojima, T., Yoshinari, N. & Konno, T. (2020). Chem. Eur. J. 26, 1827. [4] Kowalewski, B., Poppe, J., Demmer, U., Warkentin, E., Dierks, T., Ermler, U. & Schneider, K. (2012). J. Am. Chem. Soc. 134, 9768.

Stress-accumulated electrical charge to close wounds in living tissue, yet to-date this piezoelectric effect has not been realised in self-repairing synthetic materials which are typically soft amorphous materials requiring external stimuli, prolonged physical contact and long healing times (often >24h). Here we overcome many of these challenges using piezoelectric organic crystals, which upon mechanical fracture, instantly recombine without any external direction, autonomously self-healing in milliseconds with remarkable crystallographic precision (Figure 1). Atomic-resolution structural studies reveal that a 3D hydrogen bonding network, with ability to store stress, facilitates generation of stress-induced electrical charges on the fractured crystals, creating an electrostatically-driven precise recombination of the pieces via a diffusionless instant self-healing, as supported by spatially-resolved birefringence experiments. Perfect, instant self-healing creates new opportunities for deployment of molecular crystals using crystal engineering principles in robust miniaturised devices, and may also spur development of new molecular level repair mechanisms in complex biomaterials [1].

References:

[1] Bhunia S, Chandel, S., Karan, SK., Dey, S., Tiwari, A., Das, S., Kumar, N., Chowdhury, R., Mondal, S., Ghosh, I., Mondal, A., Khatua, BB., Ghosh, N., Reddy, C. M. Autonomous self-repair in piezoelectric molecular crystals. (2021) Science. 373 , 321-327

The crystalline sponge method^[1],[2] allows the absolute structural determination of non-crystalline compounds such as powder, amorphous solid, liquid, volatile matter or oily state. In this method, metal-organic frameworks (MOFs) are used as ‘crystalline sponges’ which can absorb target sample (guest) molecules from their solution into the pores and allow them to arrange themselves in a regular pattern with the help of specific interactions between MOF pores and the guests, such as π-π, CH-π, and charge-transfer interactions. This technique was first introduced in 2013^[1] and since then has grown rapidly and proved helpful in the structure elucidation of liquids and other volatile compounds.

In this work, the crystalline sponge [{(ZnI₂)₃(tris(4-pyridyl)-1,3,5- triazine)₂·x(solvent)_n] (1) was used to produce three novel encapsulation complexes of terpenes, such as geraniol-monoterpenoid, farnesol-sesquiterpenoid and β-damascone-tetraterpenoids.^[3] Along with the structure determination of the terpenoids, non-bonding CH-π, π-π interactions were identified in the host-guest complexes shown in Figure 1, which were responsible for holding the guests in the specific position with respect to the framework. Since pores of sponge 1 were hydrophobic, no hydrogen bonding between host and guest was observed.

In addition, new crystalline sponges were explored {[Co₂(bis-(3,5-dicarboxy-phenyl) terephthalamide)(H₂O)₃]·solvent_x}^[4] (2) and [Cd₇(4,4’,4’’-[1,3,5-benzenetriyltris(carbonylimino)]trisbenzoic acid)(H₂O)]·solvent _x]^[5] (3). With sponge 2 two novel inclusion complexes with 3-Phenyl-1-propanol and 2-Phenylethanol were obtained. Sponge 2 has three coordinated water molecules indicating the hydrophilic nature, which was further observed in the inclusion complexes where the hydroxyl group of guest molecules were found to form hydrogen bonds with the framework of 2. Further, 3 shows great potential to act as a crystalline sponge because of larger pore size than 1 and the hydrophilic nature which will allow a wide range of guest molecule for encapsulation and their structure elucidation.

The first developments on metal-organic frameworks, or MOFs, were made approximately four decades ago, marking the discovery of a novel class of porous materials. Initially thought to be ordered and truly crystalline, there is now an increasing realisation that defects and disorder are prevalent in MOFs, and that nontrivial arrangements can be important in physical properties [1]. Though, disorder in MOFs does not necessarily imply randomness. In fact, depending on the interactions between components, MOF structures can exhibit short-range order and long-range disorder simultaneously. This is what we refer to as correlated disorder [2]. Thorough understanding is achieved through investigation of the interactions involved in causing these states, with the aim of controlling physical properties via their manipulation.

Generally, there are three types of disorder observed in MOFs: vacancy defects, compositional disorder, and orientational/conformational disorder [3]. The latter forms the focus of this project. A key factor is the distinction between molecular point symmetry (i.e. the local structure) and that of the corresponding position in the lattice (i.e. the average structure). Namely, molecular components are arranged on a lattice of which the geometry is determined by that of the MOF, giving control over the molecular orientational degrees of freedom [4]. Since MOF geometry is often highly symmetric, lowering the symmetry of nodes and/or linkers can enable targeted design of correlated conformational disorder (Fig. 1).

In this work, correlated disorder is introduced to the highly symmetric (hypothetical) parent framework ZnO₄(BTC) (BTC = benzene tricarboxylic acid) by reducing the linker symmetry from D_3h to C_2v, giving ZnO₄(1,3-BDC) (BDC = benzene dicarboxylic acid) (Fig. 1). The resulting linker disorder is characterised via diffuse scattering observed in single crystal X-ray diffraction (SCXRD) patterns, 3DΔ-pair distribution functions (3DΔ-PDFs), and Monte Carlo code. Ultimately, we want to understand how to control defective structures in MOFs to optimise their properties, enabling utilisation in real-life applications.

[1] Bennett, T. D., Cheetham, A. K., Fuchs, A. H., Coudert, F.-X. (2017). Nat. Chem. 9, 11.[2] Keen, D. A. and Goodwin, A. L. (2015). Nat. 521, 303.[3] Meekel, E.G. and Goodwin, A.L. (2021). CrystEngComm.[4] Simonov, A. and Goodwin, A. L. (2020). Nat. Rev. Chem.