Coordination-Induced Condensation of [Ta6O19]8 by Abramov

Table of Contents

Introduction
Methods
Results and Discussion
Conclusion
References

Introduction

One of the areas of polyoxometalate (POM) chemistry that has had very little research is the solution chemistry of polyoxotantalates. One of the main chemical components in this area comprises the hexatantalates. The chemical species [Ta₆O₁₉]⁸, which is a positively charged ion, has immense stability in elevated pH. The hexatantalates of several cationic compositions including Na₈[Ta₆O₁₉]·24.5H₂O and (NBu₄)₆[H₂Ta₆O₁₉]·10H₂O are known. Recent studies have seen the synthesis of a number of compounds by blending various complexes to form POM complexes. For example, {(p-cym)Ru}2 and the negatively charged ion [Nb₆O₁₉]⁸ have been blended to form Ru/POM ratios ranging from 1:1 to 4:1.6. Combinations of POM with {(arene)Ru}²⁺ have numerous applications including the synthesis of organometals containing POM. Such compounds find use as catalysts in chemical reactions involving various organic compounds.

The authors in this experiment sought to establish the coordination of the reaction between [(C₆H₆) RuCl₂] ₂and [Ta₆O₁₉]⁸.

Methods

Sodium hexatantalate was prepared by reacting Ta₂O₅ with molten sodium hydroxide. The compound was then extracted from the reaction mixture using water. [(C₆H₆)RuCl₂] ₂ was made using pre-established protocols. TG experiments were performed in an aluminum oxide crucible at temperatures ranging from 22 to 300^oC. Thereafter, infra-red spectra data was obtained at wavelengths ranging from 4000 to 400 cm^-1. Capillary electrophoresis was performed in fused silica capillary tubes with a borate buffer at a pH of 9.18, and ultraviolet visualization was done at 254nm. Electrospray ionization mass spectra data was obtained using a quadrupole-T-wave-time-of-flight. A solution of sodium iodide dissolved in propanol was mixed with water in the ratio of 1:1 and used to calibrate the instrument.

The nuclear magnetic resonance data of carbon 13 and hydrogen were obtained at room temperature after which apparent diffusion coefficients were found using modified algorithms founded on the highest entropy and Laplace change. Direct techniques were used to elucidate the structures of Na₁₀-1 and Na₄-2, which were then distinguished using the SHEXTL programs. Density maps were used to trace the locations of the disordered atoms, which were then distinguished using isotropic methods. However, Ru atoms were not refined. Anisotropic means were utilized in the processing of all hydrogen atoms present in Na_4-2. Geometric arrangements of the benzene hydrogen atoms in Na₁₀-1 and Na₄-2 were made thereby enabling the enhancing of the hydrogen atoms as they lay on central carbon atoms. The TOPOS 4.0 professional set was employed in the evaluation of the crystal arrangement patterns.

Na₁₀ [{(C₆H₆)RuTa₆O₁₈}2(¼-O)]·39.4H₂O (Na₄-1) was then synthesized by a precipitation reaction between Na₈[Ta₆O₁₉]·24.5H₂O and [(C₆H₆)RuCl₂] ₂. The mixture was kept at 80^oC for eight hours with frequent stirring after which the precipitate was filtered off. Na₄-2 was also synthesized in the same manner as Na₄-1, but using different concentrations of Na₈[Ta₆O₁₉]·24.5H₂O and [(C₆H₆)RuCl₂] ₂ (0.025mmol and 0.014mmol instead of 0.074mmol and 0.08mmol respectively). The resulting solution was left to crystallize at room temperature for 48 hours. The ensuing compounds were analyzed using infrared spectroscopy and NMR.

Results and Discussion

It was observed that the pH of the newly constituted solution of Na₈[Ta₆O₁₉]·24.5H₂O in water had a pH value of 11.5. The pH was attributed to the relocation of protons from water. Synchronization of {(C₆H₆)Ru}²⁺ to [Ta₆O₁₉]⁸ diminished the elevated negative charge of hexatantalate thereby lowering the extent of protonation. Consequently, the pH value lessened as the reaction progressed. Heating a blend of Na₈[Ta₆O₁₉]·24.5H₂O and [(C₆H₆)RuCl₂] ₂ in equal proportions in the presence of water molecules yielded a yellow-colored solution that matched a [(C₆H₆)RuTa₆O₁₉]⁶ (1a⁶) anion with one cap. Electrospray ionization mass spectroscopy was a useful tool in keeping an eye on the advancement of the reaction. In addition, there was a vibrant equilibrium of [{(C₆H₆)Ru}₂Ta₆O₁₉]⁴ and [Ta₆O₁₉]⁸ anions in solution because both moieties were evident in unadulterated samples of Na₁₀-1. Therefore, the creation of [{(C₆H₆) RuTa₆O₁₈}₂(¼-O)]¹⁰ could be categorized as coordination-triggered joining of two hexatantalate ions because the blending was absent in noncoordinated hexatantalate. The interaction of the two negatively charged species was attributed to a reduction in the overall charge due to the ratios of the constituents (1:1). In addition, the coordination of the electron-tolerant portion had an influence on the electronic intensity over the oxygen atoms of the POM section. The ESI-MS evaluation of the reconstituted Na₁₀-1 consistently showed the monomeric 1a⁶ anion only. That observation confirmed that the previous dimerization was a reversible process. In contrast, lowering the POM/Ru ratio to 1:2 caused the formation of [{(C₆H₆)Ru}₂Ta₆O₁₉]⁴, which was separated as Na4(trans-[{(C₆H₆)Ru}₂Ta₆O₁₉]·20H₂O (Na₄-2).

Conclusion

It was concluded that the stability between hexanuclear and oxo-bridged dodecanuclear chemical groups were common attributes of the Lindqvist anions provided that they constituted a transition metal from group 4 or 5. It was also observed that only the heterometals took part in the creation of the M2OM2 link in diverse combinations. Additionally, the outcomes of capillary electrophoresis confirmed the unprompted decomposition of Na₁₀-1 in aqueous solutions. Overall, the experiment confirmed that that the dimer [{(C₆H₆)RuTa₆O₁₈}2(¼-O)]¹⁰ could not be found in aqueous solutions because it was broken down to form [(C₆H₆)RuTa ₆O₁₉]⁶(1a⁶) as the prevailing moiety. In addition, the singly substituted 1a⁶ anion existed in equal proportions in cis and trans varieties with the doubly substituted anion [{(C₆H₆)Ru}₂Ta₆O₁₉]⁴ anion. Therefore, the steadiness of the hexatantalate component in pH ranges that exceeded the alkaline extent was attributed to the coordination of the {(C₆H₆)Ru}²⁺ part. Further elevations of pH with the addition of sodium hydroxide led to an intense ion containment.

References

Abramov, P. A., Sokolov, M. N., Floquet, S., Haouas, M., Taulelle, F., Cadot, E.,

Peresypkina, E. V., Virovets, A. V., Vicent, C., Kompankov, N. B., Zhdanov, A. A., Shuvaeva, O. V. & Fedin, V. P. (2014). Coordination-induced condensation of [Ta₆O₁₉]⁸: synthesis and structure of [{(C₆H₆)Ru}2Ta₆O₁₉]⁴ and [{(C₆H₆)RuTa₆O₁₈}2(¼-O)]¹⁰. Inorganic Chemistry. Web.