Hydroflux Syntheses — Reactions in Ultrabasic Media at Moderate Temperatures

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Hydroflux synthesis, using very highly concentrated alkaline solutions, is a promising new time-, energy-, and atom-efficient synthesis approach, especially but not exclusively for oxides and hydroxides. The conditions differ from those in hydrothermal reactions as well as those in alkaline salt melts. Previous work has shown that the range of main-group and transition elements that can be reacted, as well as of the oxidation states that can be addressed, is much wider than initially expected. We will explore the potential of hydroflux synthesis and gain further knowledge about the crucial reaction parameters and the ongoing chemical processes. Key chemical, structural, and physical properties of the obtained solid products shall be determined. We will specifically follow two directions. First, the low activity of water in the hydroflux provides options for a virtually anhydrous acid-base chemistry at moderate temperature. Common mineral acids and their anhydrides will be reacted with the aim of crystallizing metal salts of complex anions with various degree of protonation and condensation. There is a high probability that the compounds formed are new, will have non-centrosymmetric structures and exhibit non-linear optical properties. Second, the redox chemistry under hydroflux conditions will be investigated. The tabulated electrochemical standard potentials no longer apply, particularly in systems that depend on pH. Together with the hygroscopic properties of hydroflux, this makes soluble oxometalates in intermediate oxidation states unexpectedly strong reducing agents whose potential will be explored. On the other hand, there is a lively redox chemistry between oxide, peroxide, superoxide and molecular oxygen depending on the pH and the alkali metal cation of the flux. By using solid oxygen sources or hydrogen peroxide solutions, the oxidation potential can be increased in a controlled manner, which will be studied in detail and exploited to stabilize high oxidation states of transition metals.