Bowl shaped deformation in a planar aromatic polycycle upon reduction. Li and Na separated dianions of the aromatic polycycle 5,6:11,12-di-o-phenylene-tetracene

Abstract

Herein we report the synthesis and crystal structures of three light alkali metal salts of the dianion of the polycyclic aromatic hydrocarbon 5,6:11,12-di-o-phenylenetetracene (L_DOPT). The compounds are obtained by reaction of L_DOPT with an excess of lithium or sodium metal in different O-donor solvents (DME, diglyme) and crystallize as naked, solvated-cation separated dianions exhibiting no interaction between the alkali metal ion and the aromatic π-system of L_DOPT. Depending on the aprotic etheral solvent and the hardness of the alkaline metal agent a significant structural perturbation of the conjugated carbon framework of L_DOPT is observed resulting in a bowl shaped curvature of the anionic π-perimeter, in contrast to its fully planar neutral state. Reduction of L_DOPT with lithium in DME results in the formation of the solvent-separated molecular structure of {[(DME-κ²O)₃Li⁺]₂(L_DOPT²⁻)}₂1 containing naked isolated units of dianionic L_DOPT. A similar structural arrangement is observed for the corresponding sodium compound {[(DME-κ²O)₃Na⁺]₂(L_DOPT²⁻)}₂2 in which, however, a lesser curvature of the isolated dianionic ligand skeleton compared to 1 is observed. In contrast to 1 and 2 reduction with sodium in diglyme results in the formation of {[(diglyme-κ³O)₂Na⁺]₂(L_DOPT²⁻)}_0.53. The deformation of the peripheric phenylene rings of [L_DOPT²⁻] in 3 is not as pronounced as compared to 1 and 2. Nevertheless, molecular structures of 1–3 deviate from full-planarity as observed in the parent neutral L_DOPT. No preferential endo- or exo-site coordination of the alkaline metal cations Li⁺ and Na⁺ on the curved dianionic π-perimeter is observed.