Engineering Highly Reduced Molybdenum Polyoxometalates via the Incorporation of d and f Block Metal Ions

Abstract The assembly of nanoscale polyoxometalate (POM) clusters has been dominated by the highly reduced icosahedral {Mo132} “browns” and the toroidal {Mo154} “blues” which are 45 % and 18 % reduced, respectively. 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: [MoV 52MoVI 12H26O200]42− {Mo64} and [MoV 40MoVI 30H30O215]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 {MoV 6} and five tetrahedral {MoV 2MoVI 2} 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.


Synthetic procedures
In a 20mL vial equipped with a magnetic stirrer bar, Na2MoO4·2H2O (50 mg, 0.2 mmol) and NiCl2·6H2O (95 mg, 0.4 mmol) were dissolved in 1.5 mL of H2O. The mixture was then acidified by adding 1 mL of 6 M HClO4. This was followed by the addition of LaCl3·7H2O (30 mg, 0.08 mmol) and N2H4·2HCl (5 mg, 0.05 mmol) dissolved in 1.1 mL of H2O. The mixture was sealed with a metallic lid with a rubber septum and heated at ̴ 90°C for ̴ 2h. The mixture changes from light green to a red-brown colour. Then, the pH is increased slowly by first, adding 0.95 mL of 6 M NaOH at 1 mL/min, which produces yet another colour change to deep blue, and then 1 M NaOH (approx. 0.35 mL) at 0.3 mL/min to bring the final pH to 2.8. The In a 20mL vial equipped with a magnetic stirrer bar, Na2MoO4·2H2O (50 mg, 0.2 mmol) and NiCl2·6H2O (95 mg, 0.4 mmol) were dissolved in 1.5 mL of H2O. The mixture was then acidified by adding 1 mL of 6 M HClO4. This was followed by the addition of CeCl3·7H2O (30 mg, 0.08 mmol) and N2H4·2HCl (3 mg, 0.03 mmol) dissolved in 1.1 mL of H2O. The mixture was sealed with a metallic lid with a rubber septum and heated at ~ 90 °C for ~ 2 h. The mixture changes from light green to a red-brown colour. Then, the pH is increased slowly by first, adding 0.95 mL of 6 M NaOH at 1 mL/min, which produces yet another colour change to deep blue, and then 0.2 mL 1 M NaOH at 0.3 mL/min. The sample is then degassed in a sonicator for 10 min, flushed with Ar gas and sealed again. The sample is left in an oven at 100°C undisturbed for 4 days for crystal growth. Small red rod-like crystals of compound 4 appeared after 4 days.

Formula determination
The determination of the formulas of the mix-valence Mo clusters has been well established in the past and requires a series of analytical techniques including IR and UV−vis spectroscopy, bond valence sum analysis (BVS), elemental analysis and thermogravimetric analysis (TGA), in addition to single-crystal X-ray diffraction analysis. [5] Here 1 was selected to exemplify the general approach used to determine the formula of all the compounds 1 to 3.
Firstly, BVS calculations were carried out on all the Mo and O centres, revealing that 1 is composed of a 52-electron reduced anionic cubic structure containing 26 singly protonated oxygen atoms. [6] BVS indicated that the reduced Mo centres are localised at the {Mo V 2} pairs that have a Mo-Mo bond around 2.5 Å, consistent with previous work. [7] UV−vis spectroscopy showed an absorption band centred at around 310 nm, which can be attributed to the Mo-Mo charge transfer and the IR showed characteristic bands that can be assigned to the symmetric and asymmetric bending of O-Mo-O bridges at 959 cm -1 and 878 cm -1 , respectively. A strong broad peak around 1100 cm -1 can be assigned to both Mo=O bonds scattered throughout the structure and to a ClO 4anion present in the cavity at the centre of the cluster. Finally, a peak at 704 cm -1 can be attributed to La-OH2 bond stretches. Elemental analysis confirms the framework of 1 consists of a Mo:Ni:La ratio of 64:8:6. This is consistent with the structural refinement done using the single-crystal Xray diffraction data. Taking into consideration of the information obtained from the calculations S5 above, along with elemental analysis, it is possible to determine the main framework and overall charge for 1 as [Mo64Ni8La6H26O200(H2O)30] 8plus a charge separated ClO4 -. [8] The charge is countered with nine Na atoms, as confirmed by elemental analysis and is consistent with the protonation level determined by BVS. Finally, the TGA curve of 1 exhibits a total weight loss of 11.5% from r.t. to 200 °C, which corresponds to ∼85 water molecules that include 30 ligand water molecules on the cluster. On the basis of the discussion above, the formula of 1 could therefore be determined as Na9[Mo64Ni8La6H26O200(H2O)30][ClO4]·55H2O. Formulae for compounds 2 and 3 were determined in the same way as for 1.
Formulae of compounds 4, 5 and 6 were primarily determined by crystallography due to difficulty in isolating enough materials for chemical analyses. The contents of Ni, lanthanide, Cl and Mo were found in structure refinements with occupancies refined freely first and then fixed. The number of protons on clusters were tentatively determined from BVS calculated for oxo ligand protonation for compound 4 and 5. For compound 6, due to poor crystal diffraction data, no BVS calculation was performed but the number of protons was adopted from compound 4 and 5 based on structure similarity.

Ln Mixtures synthesis
For all binary mixtures the same procedure was followed In a 20 mL vial equipped with a magnetic stirrer bar, Na2MoO4·2H2O (50 mg, 0.2 mmol) and NiCl2·6H2O (95 mg, 0.4 mmol) were dissolved in 1.5 mL of H2O. The mixture was then acidified by adding 1 mL of 6 M HClO4. This was followed by the addition of LnCl3·nH2O (0.08 mmol), Ln2Cl3·nH2O (0.08 mmol) and N2H4·2HCl (5 mg, 0.05 mmol) dissolved in 1.1 mL of H2O. The mixture was sealed with a metallic lid with a rubber septum and heated at ~ 90 °C for ~ 2 h. The mixture changes from light green to a red-brown colour. Then, the pH is increased slowly by first, adding 0.95 mL of 6 M NaOH at 1 mL/min, which produces yet another colour change to deep blue, and then 1 M NaOH (approx. 0.35 mL) at 0.3 mL/min to bring the final pH to 2.8. The sample is then degassed in a sonicator for 10 min, flushed with Ar gas and sealed again. The sample is left in an oven at 100°C undisturbed for 4 days for crystal growth. Big red rodlike crystals of were collected were collected by first washing the solid with iced cold water and removing the suspended impurities and then by filtration through a filter with 100 µm pore size. The crystals were S6 then left to dry in a fume hood cupboard for 2 days. In case of the 3:1 mixture, the same procedure was followed with the exception of using 0.06 mmol of LnCl3·nH2O and 0.02 mmol of Ln2Cl3·nH2O.   For compound 6, due to poor crystal diffraction data, no BVS calculation was performed but the number of protons was adopted from compound 4 and 5 based on structure similarity.   Table S3. Crystal data and structure refinement details for compounds 2' and 3.

{Mo70} star VS {Mo240} cage
As mentioned in paper, there is a strong resemblance between the star framework (compounds 4, 5 and 6) and the pentagonal window of the previously reported {Mo240} cage (Figure 4) In the case of the {Mo240} cage, [16] the authors posited not only that that this position is occupied by a SO3 2-/SO4 2template but also that the reduction of SO4 2to SO3 2is key for the formation of the overall structure.
While this is, in principle, a plausible deduction, the fact the star presents such a resemblance to the cage window portion but with a MoO4 2in the same position presents a disjunctive. found in literature. [19] In the reported {Mo240} structure, S atoms were reported with unusually high thermal parameters.
If the structure data is re-refined with free S occupancies, results show that the total S content is likely not full in these positions but occupied at around 50%, on average. If these S sites are assigned as Mo in the rerefinements, the total occupancy averages 20%. Table S7 lists the polyhedral perimeters of the star structures and {Mo240} as shown in Figure S7, showing how similar the star base frameworks in both the star {Mo70} and ball {Mo240}.    9. Thermogravimetric analysis (TGA) Figure S8. TGA graph of 1. The weight loss from r.t. to 200 °C is 11.49%, which corresponds to ~86 guest water molecules. Figure S9. TGA graph of 2. The weight loss from r.t. to 200 °C is 11.27%, which corresponds to ~84 guest water molecules. Figure S10. TGA graph of 3. The weight loss from r.t. to 200 °C is 11.35%, which corresponds to ~85 guest water molecules. Figure S11. IR spectra for compound 1-3. Figure S12. UV spectra for compound 1-3 and {Mo2O4(H2O)4} 2+ solution.