Redox-controlled potassium intercalation into two polyaromatic hydrocarbon solids


Alkali metal intercalation into polyaromatic hydrocarbons (PAHs) has been studied intensely after reports of superconductivity in a number of potassium- and rubidium-intercalated materials. There are, however, no reported crystal structures to inform our understanding of the chemistry and physics because of the complex reactivity of PAHs with strong reducing agents at high temperature. Here we present the synthesis of crystalline K2Pentacene and K2Picene by a solid–solid insertion protocol that uses potassium hydride as a redox-controlled reducing agent to access the PAH dianions, and so enables the determination of their crystal structures. In both cases, the inserted cations expand the parent herringbone packings by reorienting the molecular anions to create multiple potassium sites within initially dense molecular layers, and thus interact with the PAH anion π systems. The synthetic and crystal chemistry of alkali metal intercalation into PAHs differs from that into fullerenes and graphite, in which the cation sites are pre-defined by the host structure.

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Figure 1: Available void space in carbon-based molecular crystal structures.
Figure 2: Characterization data for K2Pentacene.
Figure 3: Crystal structure of K2Pentacene.
Figure 4: Characterization data for K2Picene.
Figure 5: Crystal structure of K2Picene.
Figure 6: Local structure in K2Picene.


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We acknowledge financial support from the UK Engineering and Physical Sciences Research Council (EP/K027255 and EP/K027212), the European Union/JST SICORP-LEMSUPER FP7-NMP-2011-EU-Japan project (contract no. NMP3-SL-2011-283214), the Mitsubishi Foundation, the Japan Society for the Promotion of Science under the Scientific Research on Innovative Areas ‘J-Physics’ Project (no. 15H05882), the ‘World Premier International (WPI) Research Center Initiative for Atoms, Molecules and Materials,’ the Ministry of Education, Culture, Sports, Science, and Technology of Japan and the Japan Science and Technology Agency under the ERATO Isobe Degenerate π-Integration Project. We thank the Diamond Light Source for access to synchrotron X-ray facilities, and C. C. Tang, C. A. Murray and P. Adamson for beamline assistance. We thank Katalin Kamarás for help with the infrared measurements. We thank M. Persson for the use of his XVASP code and related external routines for projecting density of states onto molecular orbitals. We thank the Royal Society for a Newton International Fellowship (G.K.). M.J.R. is a Royal Society Research Professor.

Author information

K.P. and M.J.R. conceived and designed the project. M.J.R. directed and coordinated the research. F.D.R. carried out the initial syntheses using KH that identified both phases, performed the initial structure determination of both phases and performed Raman measurements. M.J.P. completed the structural analyses with C.I.H. and C.C. C.I.H. performed the final syntheses and, with M.J.P., finalized the link between synthesis and structure. G.F.S.W. defined the structural evolution of the intermolecular interactions. S.K. and A.Y.G. developed early reaction protocols, and R.H.C. undertook early structural work. D.A. computed the evolution of the void space within the material that connects the molecular packing to the cation distribution. M.S.D. performed and interpreted electronic structure calculations. G.K. performed and interpreted Raman and infrared measurements. F.D.R. and M.J.R. wrote the first draft of the paper, which was then completed with input from all the authors.

Correspondence to K. Prassides or M. J. Rosseinsky.

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Romero, F., Pitcher, M., Hiley, C. et al. Redox-controlled potassium intercalation into two polyaromatic hydrocarbon solids. Nature Chem 9, 644–652 (2017).

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