Periodic Reporting for period 2 - pCx4All (Calix[4]pyrrole for p-block elements: anti-van’t Hoff-Le Bel configuration and ligand-element cooperativity revive the standard oxidation states.)
Reporting period: 2022-05-01 to 2023-10-31
State-of-the-art strategies for p-block element-based bond activations predominantly rely on the activity of low-valent species (e.g. silylenes). However, during bond activation, those species usually collapse into the deep thermodynamic sinks of their standard oxidation states, precluding any catalytic cycles. The present proposal PCX4ALL develops new concepts that add unique reactivity to the p-block elements Al, Ga, Si, Ge, Sn and P in their stable oxidation states. This is achieved by the generation of planar anti-van’t Hoff/Le Bel configurations, ligand-element cooperativity, and p-block valence isomerism. All those original features are enabled by calix[4]pyrrole – a well-established ligand for transition metals – which has never been used for p-block elements. Challenging bond activations (e.g. N2) and catalytic transformations (e.g. H2O splitting, acceptorless dehydrogenative oxidation of alcohols) are tackled with the most abundant elements of the earth crust (e.g. a square-planar, tetracoordinated silicon(IV) or aluminum(III)). The concepts are supplemented by photo-induced reactivity and extended into the field of dynamic covalent chemistry. An integrated synthetic, spectroscopic and theoretical research approach guides the way from fundamental understanding to the application in catalysis and materials.
During the first reporting period of this project, substantial progress has been made along most research directions outlined in the Description of Action, and beyond. Overall, the calix[4]pyrrolato ligand turned out as versatile and effective for the stabilization of unusual structures and for the initiation of new reactivity modes of p-block elements in their normal (group) oxidation states. Beyond the proposed applications towards aluminum, gallium, silicon, germanium, tin and phosphorous as central elements, we could further extend the range towards the heavier congeners indium, arsenic, antimony and bismuth. Additionally, besides the originally proposed substrates for activation, we could develop the conceptualities of element-ligand cooperation toward dioxygen activation or the catalytic application in ammonia borane dehydrocoupling, and we could include elements also in their lower oxidation states (tin(II)/germanium(II)). Moreover, the project beneficially influenced a broader perspective that deals with the deformation of p-block elements in general.
Beyond the state of the art are recent discoveries on all-metal aromaticity, wherein the calix[4]pyrrolato units act as supramolecular entities to stabilize delicate cationic metal rings. Furthermore, we discovered a unique generation and stabilization protocol for acyclic amino carbenes and their transfer onto transition metals. We expect to exploit these new directions more toward a profound understanding of electronic structures and the herby-created novel reactivities. In addition, the supramolecular trapping of cationic clusters is a valuable goal to extend until the end of this project.