Biphenylene Dimer. Molecular Fragment of a Two-Dimensional Carbon Net and Double-Stranded Polymer

Biphenylene dimers, which may be considered as “macrocyclic” 2
× 2 fragments of net 1 and dimeric
fragments of a double-stranded polymer, 5, are prepared.
X-ray crystallography on two polymorphs of the dimer
9
reveals that its bond alternation is analogous to that in biphenylene;
the tetraphenylene part of 9 has approximately
C2h symmetry with a small
dihedral angle (<12°) between the benzene rings in the adjacent
biphenylenes. Cyclic
voltammetry of 9 gives two reversible waves corresponding to
a radical anion and a dianion; the lithium and potassium
salts of these anions,
9•-,M+
(M = Li, K) and
92-,2M+
(M = Li, K), were generated and characterized by
ESR,
NMR, and UV−vis spectroscopies. The spectral data for the
dianions are compatible with classical structures; no
evidence for “in-plane aromaticity” is found. For polymer
5, a localized band and large density of states near
the
Fermi level are found, using extended Hückel theory
calculations.