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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 (LDOPT). The compounds are obtained by reaction of LDOPT 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 pi-system of LDOPT. Depending on the aprotic etheral solvent and the hardness of the alkaline metal agent a significant structural perturbation of the conjugated carbon framework of LDOPT is observed resulting in a bowl shaped curvature of the anionic pi-perimeter, in contrast to its fully planar neutral state. Reduction of LDOPT with lithium in DME results in the formation of the solvent-separated molecular structure of {[(DME-κ2O)3Li+]2(LDOPT2−)}2 1 containing naked isolated units of dianionic LDOPT. A similar structural arrangement is observed for the corresponding sodium compound {[(DME-κ2O)3Na+]2(LDOPT2−)}2 2 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(LDOPT2−)}0.5 3. The deformation of the peripheric phenylene rings of [LDOPT2−] 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 LDOPT. No preferential endo- or exo-site coordination of the alkaline metal cations Li+ and Na+ on the curved dianionic pi-perimeter is observed.
