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] 2 and [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 300oC. 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 Na4-2. 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(μ-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 80oC 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(μ-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 M′−O−M′ 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(μ-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(μ-O)]10–. Inorganic Chemistry. Web.