Abstract
K[(18-crown-6)-bis(tetrahydrofuran)anthracenide] was independently prepared by three groups, and its structure described by two of them. The third structure description, though listed in the Cambridge Crystallographic Data Centre (CSD) collection, contains no space group or atomic coordinates, but the cell constants leave no doubt that it is the same species as the two others, which were reported in 2006 and 2016. The compound crystallizes in space group P21, with Z′ = 2.0 at T = 123 K and R = 4.91% (I), and at 100 K and R = 4.44% (II); both impressive results in their individual quality and agreement, despite differences in experimental methods and the temperature of data collection. A more detailed examination of the published data for (I) and (II) reveals that the correct description for this very unusual, and thus far unique substance, is that it contains a radical anion crystallizing as a kryptoracemate rather than as a simple example of a Sohncke space group with Z′ = 2.0. The anthracenide anions present in (I) and (II) are virtually identical; in contrast, the internal pair of cationic species differ from one another in the dissymmetry of the flexible tetrahydrofuran ligands, having significantly different internal and external torsional angles. The two THF molecules attached to the K(crown-ether) cations are not centrosymmetrically related, and this is what makes this portion of the asymmetric unit responsible for the crystal being a kryptoracemate. Our presentation will be based on the more fully documented sample (II), unless specifically stated.
Highlights
The CSD [1] contains three entries labeled YETPAP [2], YETPAP01 [3], and YETPAP02 [4]describing the crystal structure of K[(18-crown-6)-bis(tetrahydrofuran)anthracenide]
In YETPAP, Rosokha and Kochi [2] described their unusual findings : “Arene cationradicals and anion-radicals result directly from the one-electron oxidation and reduction of many aromatic hydrocarbons, yet virtually nothing is known of their intrinsic stability and ‘aromatic character’”
Application of the structure-based “harmonic oscillator model of aromaticity” or the HOMA method leads to the surprising conclusion that the aromaticity of these rather reactive, kinetically unstable arene cation and anion radicals is higher than that of their parent contrary to conventional expectations”
Summary
Sometime later, Castillo et al [4] succeeded in isolating all the alkali metal derivatives and determined the structure of a number of them, of which the one that we selected is the one we label (II), which appears in the CSD [1] as YETPAP02 [4].
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