Pressure-Induced Amidine Formation via Side-Chain Polymerization in a Charge-Transfer Cocrystal

Compression of small molecules can induce solid-state reactions with products that are difficult or impossible to obtain through solution-phase synthesis. Of particular interest is the topochemical-like reaction of arenes to produce polymeric nanomaterials rich in sp3 carbon. However, high reaction onset pressures and poor control over high-pressure reaction selectivity remain significant challenges to be addressed. Herein, the incorporation of electron withdrawing/donating groups into π-stacked arenes is proposed as a strategy to reduce reaction barriers and onset pressures. Charge transfer cocrystals represent systems with optimal π-stacking and reduced energy barriers for intermolecular cycloaddition reactions, however, competing side-chain reactions between functional groups must also be considered. For the case of a diaminobenzene:tetracyanobenzene cocrystal, amidine formation between side groups is the first reaction to occur with an onset pressure near 9 GPa, as characterized using vibrational spectroscopy, X-ray diffraction, and computational studies. High-pressure reactivity is system-dependent and while functionalized arenes are predicted to exhibit reduced-barrier energy cycloaddition pathways, directed reactions between side groups can be used as a novel strategy for the formation unique polymeric materials.