Sodium metal mirror reduction of biphenylene in aprotic THF solution containing [2.2.1]cryptand yields as a structurally characterized product the 9,9dimethylsilafluorene radical anion salt containing a (H 3 C) 2 Si expanded cyclobutadiene ring. In addition, the Si content can be detected by total reflection X-ray (TXRF) analysis. When the use of any silicon grease was avoided for all glass fittings, the solvent-separated biphenylene radical anion contact pair crystallized from an aprotic diglyme solution.

Edgar Heilbronner zum 80. Geburtstag gewidmet Redox Reactions of Hexahydropyrene: Crystal Structures of Its Radical-Anion Salts as well as of Trihydropyrenylium Tetrachloroaluminate and Density-Functional-Theory CalculationsHexahydropyrene 1, a doubly propane-1,3-diyl-bridged peri-naphthalene derivative wtih 10 p-electrons allows both oxidation to its cation as well as reduction to its radical-anion salts, which could be crystallized and structurally characterized ± a rather rare case for small unsaturated hydrocarbons. The unexpected formally threefold dehydrogenation by the oxidizing system AlCl 3 /H 2 CCl 2 (Bocks reagent) generated the hitherto unknown 1,2,3-trihydropyrene cation in two polymorphic crystals, which contain 12 p-electrons delocalized over three anellated six-membered rings comprising 13 p-centers. Structural comparison of the altogether four crystallized redox products [K solv 3 with the neutral hydrocarbon 1 reveals only small differences in bond lengths and angles, but establishes solvation contacts, p(h 6 )´´´K coordination in the polymer 2b, the flattening of one molecular half in the trihydropyren cation of 3 and ten H-bonds CH´´´Cl to the AlCl 4 À counter anion of 3. DFT/NBO Charge distributions, calculated based on the experimental structural parameters, show charge accumulation in the propanediyl bridges as well as in the peripheral naphthalene CÀC bonds of the radical anions. The largest changes result expectedly for the formally triply dehydrogenated 1, i.e. the trihydropyren cation of 3, with two slightly positive and partly considerably less negative p-centers.Ausgangspunkte. ± Ungesättigte p-Kohlenwasserstoffe lassen sich in aprotischen Lösungen durch Alkalimetalle zu ihren Molekülanionen reduzieren und die entstehenden Kontaktionen-Multipel kristallisieren sowie strukturell charakterisieren [2] (für ausgewählte neuere Veröffentlichungen über Kontaktionenpaare, s. [3]). Demgegenüber werden Molekülkation-Salze weit seltener isoliert und Paare reduzierter sowie oxidierter Molekülion-Salze desselben p-Kohlenwasserstoffes sind nach Recherche in der Cambridge Structural Database-Raritäten [4].Das 1,2,3,6,7,8-Hexahydropyren (1), ein in seinen peri-Positionen zweifach Propan-1,3-diyl-überbrücktes Naphthalin-Derivat, liegt im Kristall in Sessel-Konformation vor [5] (Schema 1 und Fig. 1) und in Lösung vermutlich im Gleichgewicht mit der WannenKonformation. Für das Molekül-Radikalanion lässt sich das Konformationsgleichgewicht durch Temperatur-abhängige ESR/ENDOR-Messungen nachweisen und die