X‑Shaped Oligomeric Pyromellitimide Polyradicals

The
synthesis of stable organic polyradicals is important for the
development of magnetic materials. Herein we report the synthesis,
isolation, and characterization of a series of X-shaped
pyromellitimide (PI) oligomers (Xn-R, n = 2–4, R = Hex or Ph) linked together by
single C–C bonds between their benzenoid cores. We hypothesize
that these oligomers might form high-spin states in their reduced
forms because of the nearly orthogonal conformations adopted by their
PI units. 1H and 13C nuclear magnetic resonance
(NMR) spectroscopies confirmed the isolation of the dimeric, trimeric,
and tetrameric homologues. X-ray crystallography shows that X2-Ph crystallizes into
a densely packed superstructure, despite the criss-crossed conformations
adopted by the molecules. Electrochemical experiments, carried out
on the oligomers Xn-Hex, reveal that the reductions of the PI units
occur at multiple distinct potentials, highlighting the weak electronic
coupling between the adjacent redox centers. Finally, the chemically
generated radical anion and polyanion states, Xn-Hex•– and Xn-Hexn(•−), respectively, were probed extensively by UV–vis–NIR
absorption, EPR, and electron nuclear double resonance (ENDOR) spectroscopies.
The ENDOR spectra of the radical monoanions Xn-Hex•– reveal that the unpaired electron is largely localized on
a single PI unit. Further reductions of Xn-Hex•– yield EPR signals (in frozen solutions) that can be assigned
to spin–spin interactions in X2-Hex2(•−), X3-Hex3(•−), and X4-Hex4(•−). Taken together, these findings demonstrate that directly linking
the benzene rings of PIs with a single C–C bond is a viable
method for generating stabilized high-spin organic anionic polyradicals.