Exploiting the Equilibrium Dynamics in the Self-Assembly of Inorganic Macrocycles based upon Polyoxothiometalate Building Blocks

We report the preparation and characterization of two new polyoxothiometalate species, {Mo 16 } = {(Mo 2 O 2 S 2 ) 8 (OH) 16 (C 4 O 4 ) 2 } 4-(compound 1) and {Mo 12 } = {(Mo 2 O 2 S 2 ) 6 (OH) 12 (CH 3 COO) 2 } 2-(compound 2). Control of 10 synthesis is achieved by the addition of the organic-template and further re-arrangement is controlled simply by the addition of potassium to a precursor compound yielding compound 2. The structures of the compounds are discussed and we also show that ion-mobility mass spectrometry can be 15 used to resolve the species and their building blocks in solution.

synthesis is achieved by the addition of the organic-template and further re-arrangement is controlled simply by the addition of potassium to a precursor compound yielding compound 2. The structures of the compounds are discussed and we also show that ion-mobility mass spectrometry can be 15 used to resolve the species and their building blocks in solution.
Polyoxothiometalates 1 are self-assembled chemical systems with interesting diversity arising from the ability to generate libraries of building blocks in-situ and form complex structural 20 motifs via a condensation processes. 2It has been demonstrated recently that the use of organic templates triggers the generation of kinetically controlled diverse building blocks available to assemble into thermodynamically stable complex structures under a given set of experimental variables. 3Moreover, exploration of the [Mo 2 O 2 S 2 ] 2+ / C 4 O 4 2-system reveals a series of available stable species giving rise to interesting structures ranging from Möbius strip {Mo 10 } topologies to high nuclearity nanosized clusters {Mo 96 } of 3.8 nm. 4 Even though the available building blocks are virtually the same in each case, it was possible to 30 direct the self-assembly towards a new equilibrium-structure and isolate new clusters with distinct structural features.
However, the extraction of crucial information regarding the underlying self-assembly processes and the inherent ability of the [Mo 2 O 2 S 2 ] 2+ / C 4 O 4 2-chemical system to self-establish new equilibria by a combination of thermodynamic and topological control (e.g.stability of the system vs the structure of the templated system), is a great challenge.The main source of reliable information is offered mainly from the solid phase (primarily XRD) and the last years from solution studies utilizing high resolution electrospray ionization spectrometry (ESI-MS). 5For instance we have observed that the deconvolution of the spectra of such clusters is extremely challenging due to multiple species of very similar mass-to-charge ratios (m/z).This is despite the very different sizes/structures, yet still results in overlapping distribution envelopes. 6,7Thus, it is difficult to extract crucial information regarding the building block library formed in solution which could lead to better understanding of the assembly process.Ion mobility mass spectrometry (IMS-MS) has been proven an extremely useful analytical technique for the 50 investigation of biomolecules, polymers, fullerenes, 8 and less frequently in supramolecular chemistry.The resolution of multiple overlapping distribution envelopes originating from a plethora of species in the solution, in this case become possible by separating the above based on their size/shape and m/z value.
55 Herein, we report the preparation of two new thiometalate clusters and observation of structural transformations between the most stable building block libraries towards new species.Moreover, we present the high-resolution (HR-MS) of the isolated clusters, and demonstrate the use of IMS-MS for the 60 resolution and characterization of polyoxothiometalate clusters for the first time.Our motivation is based on the observation that this should facilitate their continued exploration and improve the understanding of the underlying assembly processes in a similar fashion to the polyoxometalate (POM) chemistry.The self-condensation process of the [Mo 2 O 2 S 2 ] 2+ system, depends on the pre-organization, dictated by the templating unit, and stability of the building block libraries generated from the reaction mixture. 1,2Moreover, the stability of the building blocks 75 depends greatly upon the available coordination sites, geometry, rigidity as well as orientation of the template.For example, utilization of various carboxylate based ligands or even XO 3 2-(X: S or Se) templates 1c showed greater flexibility whilst the squarate based templates appeared to be rigid with two additional 80 coordination sites and two different coordination modes, Fig. 1.This is potentially the reason that the generated building blocks, {(Mo 2 O 2 S 2 ) 4 (OH) 10 (C 4 O 4 )}, {Mo 8 }; {(Mo 2 O 2 S 2 ) 3 (OH) 8 (C 4 O 4 )}, {Mo 6 } (Figure 1), could be used constructively under wider range of experimental conditions giving rise to a plethora of structural 5 motifs.
We In this case, the system accessed a new favourable state due to the indirect increase of the concentration of the K + cations in the reaction mixture.In order to verify our initial claim, we followed the same experimental conditions under which the {Mo 14 } is normally formed in the presence of increased concentration of K + cations, which led to the direct formation of the {Mo 16 } 1 species (see SI). Interestingly, if we dissolve crystalline pre-formed and stable in solution {Mo 14 } cluster in 0.5 M KCl aqueous solution at pH 5, then it is converted to the {Mo 16 } ring.This structural 30 transformation is quite interesting and demonstrates not only the exceptional adaptability of the system but also reflects the kinetic stability of the {Mo 6 } and {Mo 8 } building blocks (Figure 1 Compound 1 crystallises in the C2/m space group exhibiting an 40 idealized D 2h symmetry.The cluster is constructed by eight [Mo 2 O 2 S 2 ] 2+ units bridged by two hydro-oxo groups and templated by two squarate anions forming an ellipsoid architecture.Interestingly in this case the {Mo 16 } consists of two {Mo 6 } building blocks (Figure 1) in contrast to the {Mo 14 } ring.45 Additionally, there is incorporation of two [Mo 2 O 2 S 2 ] 2+ cations between the {Mo 6 } building units which are responsible for the observed ring expansion.Moreover, a K + cation is captured within the cavity (K-O = 2.82(3), 2.87(4) Å) between the two squarate templates inducing further structural stability to the 50 oxothiometalate framework.
In an effort to generate a more flexible building block library, we used acetate anions as template for the self-condensation of [Mo 2 O 2 S 2 ] 2+ .In this case a dodecamolybdate ring {Mo 12 } = {(Mo 2 O 2 S 2 ) 6 (OH) 12 (CH 3 COO) 2 }•24H 2 O 2 crystallized within 55 two weeks (Fig. 2).The compound was synthesized from a [Mo 2 O 2 S 2 ] 2+ aqueous medium in the presence of CH 3 COONa at pH 4.5.X-ray diffraction analysis revealed an inorganic cyclic neutral skeleton {Mo 2n S 2n O 2n (OH) 2n } (n=6), which incorporates two acetate anionic templates.The wheel is formed by six 60 [Mo 2 O 2 S 2 ] 2+ building blocks connected to each other through double hydroxo bridges.Two types of Mo-Mo bonds are observed: a short Mo-Mo distance found to be ca.2.8 Å within the dinuclear unit, characteristic of a metal-metal bond, and the long Mo-Mo distance which found to be ca.3.4 Å which are in 65 agreement with the literature. 9The molecular arrangement of the wheel exhibits a 12-membered ring encapsulating two acetates residing parallel to the plane of the ring.The incorporation of the acetate template introduces distortion to the {Mo 12 } ring which lowers its idealized symmetry to C 2h . 9Two coordination water 70 molecules attach onto two Mo V atoms (Mo-O w = 2.42(6) Å) and possibly due to the steric hindrance introduced by the by -CH 3 groups of the acetate templates, four Mo V atoms found to be fivecoordinated.In an effort to verify the kinetic stability of the generated building block library, we disturbed the existing 75 equilibrium by increasing the concentration of K + /or adding Cs + cations and let the system access an alternative stable thermodynamic state.In this case no transformation has been observed.This is potentially due to the fact that the flexibility of the acetate templates induce structural instability of the respective 80 building block in marked contrast to the squarate based ones, Figure 1.
High resolution negative mode ESI-MS was used in conjunction with the structural analysis to confirm the identity of clusters 1 and 2. Most of the major peaks observed Fig. S7 are 85 broad and plausibly assigned as a continua resulting from a single anion with a range of different cations and/or neutral addenda (particularly solvent ions); this is characteristic of such compounds in general and a very frequent problem in polyoxothiometalate chemistry.Thus, Travelling Wave Ion 90 Mobility MS (TWIMS) was used here to resolve overlapping peak sets.IMS-MS (Figs. 3 and S8) and CID studies (vide infra) (Fig. S9) support the assignment of the broad peaks ca.1000 Da (z = -3) and 1500 Da (z = -2) as corresponding to the intact {Mo 16 } cluster with a variety of cations/solvent molecules (listed in blue fonts) (Table S1), and most other peaks are assigned as fragments (some of the largest listed in red fonts).Drift time through an N 2 -filled drift tube is plotted against m/z, with intensity represented in colour shading at each point (here in a linear scale); peaks corresponding to the same (or similarly-sized) ions fall on a shallow parabolic curve or line. 10The recorded spectrum revealed peaks falling into three sets -one consistent with intact {Mo 16 } clusters (marked in blue), and two consistent  Interestingly we were able to observe the existence of the 25 intact cluster and the derivatives of the {Mo 6 } building block after deconvolution of the MS spectrum unambiguously.By contrast, Compound 2 proved to be extremely fragile under ESI-MS conditions (Fig. S10): that we were not able to observe neither the intact {Mo 12 } 2 cluster, nor its acetate templated 30 building block derivatives, is an indication of its instability. In 2 O 2 S 2 ) 6 (OH) 12 (CH 3 COO) 2 } 2-(compound 2).Control of 10 65

Fig. 1
Fig. 1 Ball-and-stick representation of the available squaratetemplated building blocks {Mo 8 } and {Mo 6 }.The orientation of the squarate anion induces rigidity and steric hindrance which directs accordingly its assembly.(Mo: blue; S: yellow; O: red; C: black).

10 with
smaller fragments (marked in red), Fig.S8.Interestingly, IMS-MS reveals two conformations in the -2-charged ion at ca. 1500 Da; this may correspond to different arrangements of the bound water and counter-cations.Furthermore, a very faint peak can be observed on the same vertical line as there two large peaks 15 (z = 4), corresponding to an ion incorporating two such clusters; such dimers are commonly observed in the gas phase polyoxometalate clusters.10
have observed that the [Mo 2 O 2 S 2 ] 2+ / C 4 O 4 2-system is sensitive to quite small changes of reaction conditions that affect the thermodynamically stable state, by establishing a new equilibrium.Previously, we reported the template-driven The isolated crystalline material is possible to remain in equilibrium with the building block library formed at the early stages of the reaction for long period of times without any noticeable change.Disturbing this stable state by removing the crystalline {Mo 14 } product from the mother liquor, the system reestablished a new equilibrium utilizing the same building block library and gave rise to a new ring-shaped {Mo 16 } = 20 {(Mo 2 O 2 S 2 ) 8 (OH) 16 (C 4 O 4 ) 2 } 4-1 cluster of increased nuclearity. 4c summary, we have discussed the preparation of the two new polyoxothiometalates species by generating two building block libraries of different stability which led to the formation of hexadecanuclear, {(Mo 2 O 2 S 2 ) 8 (OH) 16 (C 4 O 4 ) 2 } 4-1, and a dodecanuclear, {(Mo 2 O 2 S 2 ) 6 (OH) 12 (CH 3 COO) 2 } 2-2, ring constructed by squarate and acetate templated building blocks respectively.Moreover, we observed experimentally an intriguing system showing a type of template-based adaptability via the interplay of the [Mo 2 O 2 S 2 ] 2+ / C 4 O 4 2-units where a structural 40 transformation of {Mo 14 } to {Mo 16 } took place utilizing different available constituents of the building block library