Biphenylene dimer. Molecular fragment of a two-dimensional carbon net and double-stranded polymer

Biphenylene dimers, which may be considered as 'macrocyclic' 2 x 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 C(2h) 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 9^2-,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 Huckel theory calculations.