Reactivity of Solid Rubrene with Potassium: Competition between Intercalation and Molecular Decomposition

We present the synthesis and characterization of the K⁺-intercalated rubrene (C₄₂H₂₈) phase, K₂Rubrene (K₂R), and identify the coexistence of amorphous and crystalline materials in samples where the crystalline component is phase-pure. We suggest this is characteristic of many intercalated alkali metal-polyaromatic hydrocarbon (PAH) systems, including those for which superconductivity has been claimed. The systematic investigation of K-rubrene solid-state reactions using both K and KH sources reveals a complex competition between K intercalation and the decomposition of rubrene, producing three K-intercalated compounds, namely, K₂R, K(RR*), and K _xR' (where R* and R' are rubrene decomposition derivatives C₄₂H₂₆ and C₃₀H₂₀, respectively). K₂R is obtained as the major phase over a wide composition range and is accompanied by the formation of amorphous byproducts from the decomposition of rubrene. K(RR*) is synthesized as a single phase, and K _xR' is obtained only as a secondary phase to the majority K₂R phase. The crystal structure of K₂R was determined using high-resolution powder X-ray diffraction, revealing that the structural rearrangement from pristine rubrene creates two large voids per rubrene within the molecular layers in which K⁺ is incorporated. K⁺ cations accommodated within the large voids interact strongly with the neighboring rubrene via η⁶, η³, and η² binding modes to the tetracene cores and the phenyl groups. This contrasts with other intercalated PAHs, where only a single void per PAH is created and the intercalated K⁺ weakly interacts with the host. The decomposition products of rubrene are also examined using solution NMR, highlighting the role of the breaking of C-C_phenyl bonds. For the crystalline decomposition derivative products K(RR*) and K _xR', a lack of definitive structural information with regard to R* and R' prevents the crystal structures from being determined. The study illustrates the complexity in accessing solvent-free alkali metal salts of reduced PAH of the type claimed to afford superconductivity.