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 KxR′ (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 KxR′ 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–Cphenyl bonds. For the crystalline decomposition derivative
products K­(RR*) and KxR′, 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.