Solid-state chemistry underpins many critical technologies, including heterogeneous catalysis, batteries, and hydrogen storage materials. However, our understanding of chemical reactivity in the solid state remains limited, largely due to the inherent difficulty of monitoring reactions in these environments. If such processes could be emulated in solution, they would become accessible to the powerful suite of spectroscopic techniques available to solution-state chemists.
Recent work in our laboratory has developed a supramolecular strategy that enables precisely this. We have constructed a dicationic host assembly that mimics the local environment of anions in a solid-state lattice.1 The cavity is lined with four Lewis acidic metal centres (two Ca2+ and two K+), providing a highly stabilising, electrostatically defined environment. This platform enables the isolation and study of highly reactive main-group anions that are otherwise only accessible in the solid state.
One example is the stabilisation of the hexahydridosilicate dianion, [SiH6]2–, which is typically unstable in solution.2 Encapsulation within the host renders this species stable for months at room temperature. Upon heating, the trapped [SiH6]2– eliminates two equivalents of H₂ to generate the unprecedented Si(0) dianion, [SiH2]2–. Notably, this transformation is fully reversible: exposure of [SiH2]2– to H₂ regenerates [SiH6]2–. This represents the first example of a reversible Si(IV)/Si(0) redox cycle and highlights the potential for silicon anions to be used in hydrogen storage. The reactivity of this and related “trapped” main-group anions will be discussed.