Oxidative hydrogenation of polyanions with gallium, silicon, germanium and tin - functional materials by subtle redox chemistry

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Zintl phases are compounds between an alkaline or alkaline earth metal M with an element E of group 13-16 and usually contain polyanions [Ex-]n with covalent E-E bonds. Hydrogen may react either by oxidizing these polyanions thus increasing their connectivity or by insertion or addition of hydrogen to the polyanions. In a previous project, preliminary rules for the formation of main group element-element and element-hydrogen bonds were established. The hydrogenation of Zintl phases with silicon and germanium zigzag chains (ME with M = Ca, Sr, Ba; E = Si, Ge) by in situ X-ray and neutron powder diffraction, electron microscopy, thermal analysis, solid-state nuclear magnetic resonance spectroscopy, total neutron scattering, quantum-mechanical modelling and Raman spectroscopy revealed new Zintl phase hydrides (MEH4/3, MEH5/3 and MEH6/3) with condensed hydrogen-containing polyanions and intermediates with lower hydrogen content. In the series M(I)-M(II)-M(III), i. e. alkali-alkaline earth-rare earth, the chemical bonding of H atoms shifts from covalent to ionic and metallic. In this project, we will refine this rule-of-thumb for hydrogenation reactions of Zintl phases, enabling us to rationalize existence, thermodynamic stability, chemical bonding, crystal and electronic structure, and hydrogen capacity of hydrogenated Zintl phases. Initially about 45 compounds MxEy with E = Ga, Si, Ge, Sn and M = alkali-alkaline earth-rare earth or a mixture of two of them, featuring various polyanions (zigzag chains, helices, five- and six-membered rings, dumb-bells, 1D ribbons and 3D nets) will be studied by the above mentioned methods. The detailed knowledge of the rules governing the formation of E-E and E-H bonds will enable us to predict further hydrogenation reactions of Zintl phases, i. e. allow for a connectivity tuning in polyanions of Zintl phase hydrides by oxidation with hydrogen. The potential in the preparation of materials for hydrogen storage and low-dimensional materials shall be explored. On one hand, practical aspects of Zintl phases as hydrogen stores will be tested. By volumetric and gravimetric method, thermodynamics, kinetics, reversibility and cycleability for hydrogen storage will be studied for CaSi and further candidates identified in the first part of the project. On the other hand, Zintl phase hydrides will be used as precursors for 1D and 2D materials. Hydrolysis, ammonolysis and methylation reactions may lead to 1D and 2D polymers, which could be interesting intermediates to silicane, germanane, and stannanane or related materials. The controlled oxidation of polyanions by hydrogen, to be developed in this project, has got a great potential for the fine tuning of structural, electronic and magnetic properties of solids. The results of this project are expected to stimulate materials related research with compounds containing polyanions of triels and tetrels, especially gallium, silicon, germanium, tin.