Redox reactions of hexahydropyrene: Crystal structures of its radical-anion salts as well as of trihydropyrenylium tetrachloroaluminate and density-functional-theory calculations

Hexahydropyrene 1. a doubly propane-1,3-diyl-bridged peri-naphthalene deriv ative wtih 10 pi -electrons allows both oxidation to its cation as well as reduction to its radical-anion salts, which could be crystallized and struc turally characterized - a rather rare case for small unsaturated hydrocarbo ns. The unexpected formally threefold dehydrogenation by the oxidizing syst em AlCl3/H2CCl2, (Bock's reagent) generated the hitherto unknown 1,2,3-trih ydropyrene cation in two polymorphic crystals, which contain 12 pi -electro ns delocalized over three anellated six-membered rings comprising 13 pi -ce nters. Structural comparison of the altogether four crystallized redox prod ucts [K-solv(-)][M--] 2a, [K-solv(-)][M.-](infinity) 2b, [Na-solv(-)][M.-] 2c, and [(M - 3H)(-)][AlCl4-] 3 with the neutral hydrocarbon I reveals only small differences in bond lengths and angles, but establishes solvation co ntacts, pi(eta). . .K- coordination in the polymer 2b, the flattening of on e molecular half in the trihydropyren cation of 3 and ten H-bonds CH . . . Cl to the AlCl4- counter anion of 3. DFT/NBO Charge distributions, calculat ed based on the experimental structural parameters, show charge accumulatio n in the propanediyl bridges as welt as in the peripheral naphthalene C-C b onds of the radical anions, The largest changes result expectedly for the f ormally triply dehydrogenated 1. i.e. the trihydropyren cation of 3, with t wo slightly positive and partly considerably less negative pi -centers.