Pressure-Induced Polymerization of Monosodium Acetylide: A Radical Reaction Initiated Topochemically

Pressure-induced polymerization (PIP) of metal acetylides is a novel method to synthesize a metal-carbon framework and polycarbide materials with unique structures and properties. However, the pressure required for the PIP of C-2(2-) is too high for large-scale synthesis. In this work, we investigated the PIP of monosodium acetylide (NaC2H) by performing in situ Raman spectroscopy, infrared spectroscopy, X-ray diffraction, and impedance spectroscopy up to 30 GPa and ex situ gas chromatography-mass spectrometry on the recovered sample. NaC2H experiences a phase transition at 7 GPa and polymerizes at 14 GPa, which is the lowest PIP pressure of acetylide to date and already in the working range of a large volume press. At the reaction threshold, the nearest intermolecular C center dot center dot center dot C distance is about 2.9 angstrom, which is almost the same as that of CaC2 and indicates a topochemical initiation. The PIP is mainly a free radical addition process. The termination of the free radicals limits the composition of the produced polycarbide anions CxHyn- within x - 2 <= y + n <= x + 2. Our work discloses the threshold of the intermolecular distance for the PIP of acetylide and proposes the reaction mechanism, which furthers the investigation of its high-pressure chemical reaction.