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Alkali salts of tetrachloro- and tetrabromosemiquinone radical anions

p-Tetrabromosemiquinone (bromanil) and p-tetrachlorosemiquinone (chloranil) radical anions are well-known for their stability. Potassium salt of tetrachlorosemiquinone was first prepared in 1912 and described as a green salt with composition C6Cl4O2K. Since then, it was further studied by EPR, IR and Raman spectroscopies and theoretical methods ; an X-ray crystallographic study was also attempted. The crystal structures, however, remained elusive. Therefore we dedicated to strictly controlled synthesis and careful intensity measurements. Freshly prepared semiquinone crystals are red, but turn to green within a few minutes in air at room temperature. The colour change is due to a disproportionation of the radical into a quinhydrone-like substance. Only red crystals diffract and have to be flash-cooled for the X-ray measurements. Due to small size of the crystals, a high-intensity X-ray beam is needed. The accurate 3D data have been obtained for five compounds: two solvent-free tetrabromosemiquinone salts (C6Br4O2Rb and C6Br4O2Na) and three acetone solvates (C6Cl4O2K• CH3COCH3, C6Cl4O2Rb• CH3COCH3 and C6Br4O2K• CH3COCH3). The isostructural relation between C6Cl4O2K• CH3COCH3 and its ammonium homologue was deduced according to the unit cell parameters. All 3D structures of semiquinone radicals determined so far involved a radical stabilised by a charge-transfer interaction such as coordination bonds or low-barrier hydrogen bonds. The first 3D structures of semiquinone radicals which do not participate in charge-transfer interactions are reported. Geometry of the quinoid rings and C– O bond lengths (1.25 – 1.26 Å ) differ slightly from the hydrogen bonded semiquinone (C– O bond lengths of 1.29 – 1.30 Å ), and these results agree well with quantum chemical models. Apparently, hydrogen bonding significantly affects not only C– O bonds, but also the bonds of a carbon skeleton.

semiquinone; radical anion; crystallography; alkali salt

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