Spin-Delocalization in a Helical Open-Shell Hydrocarbon

Neutral
open-shell molecules, in which spin density is delocalized
through a helical conjugated backbone, hold promise as models for
investigating phenomena arising from the interplay of magnetism and
chirality. Apart from a handful of examples, however, the chemistry
of these compounds remains largely unexplored. Here, we examine the
prospect of extending spin-delocalization over a helical backbone
in a model compound naphtho­[3,2,1-no]­tetraphene,
the first helically chiral open-shell hydrocarbon, in which one benzene
ring is fused to [5]­helicene, forming a phenalenyl subunit. The unpaired
electron in this molecule is delocalized over the entire helical core
composed of six rings, albeit in a nonuniform fashion, unlike in phenalenyl.
In the case of a monosubstituted derivative, the uneven spin-distribution
results in a selective σ-dimer formation in solution, as confirmed
by 2D NMR spectroscopy. In contrast, the dimerization process is suppressed
entirely when four substituents are installed to sterically hinder
all reactive positions. The persistent nature of the tetrasubstituted
derivative allowed its characterization by EPR, UV–vis, and
CD spectroscopies, validating spin-delocalization through a chiral
backbone, in accord with DFT calculations. The nonuniform spin-distribution,
which dictates the selectivity of the σ-dimer formation, is
rationalized by evaluating the aromaticity of the resonance structures
that contribute to spin-delocalization.