Calix[4]Pyrrole for p-block elements: anti-van’t hoff-le bel configuration and ligand-element cooperativity revive the standard oxidation states.
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. dinitrogen) and catalytic transformations (e.g. water splitting and 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. Importantly, general feasibility was recently provided by the successful isolation of the first square-planar coordinated aluminum(III) anion in our laboratories, including the observation of its exciting reactivity. We believe that these objectives will initiate a renaissance of p-block element chemistry in their standard oxidation states, equipped with exceptional reactivity by strong structural constraint and ligand-element cooperativity.