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Stacking patterns of organic radicals in tetracyanoquinodimethane salts

Crystals of planar organic radicals are interesting because of pronounced π-stacking interactions. π-stacking usually occurs between aromatic rings mostly as weak interaction. However, it can also occur between planar radicals. 1 Recent studies in crystal engineering show that energy of some π-π contacts may be comparable to stronger intermolecular interactions such as hydrogen and halogen bonding. Close contact of two planar radicals usually involves spin coupling so the energy of such contacts may be as high as −20 kcal mol-1. 2 Furthermore, a long infinite stack of equidistant radicals contributes to special properties of such crystals like semiconductivity and antiferromagnetism.2 Therefore, these compounds have an important role in crystal engineering of functional materials such as optoelectronics, magnetic, and conductive molecular materials.1, 3 Here we report a study on π-stacking and multicentric bonding in salts of 7, 7, 8, 8- tetracyanoquinodimethane (TCNQ) radical anions with different organic cations: tetrazolium (1), 1, 4-dimethyl-DABCO cation (2), N, N- dipyridylmethanium (3), N, N-dimethyl-4, 4- bipyridinium (4), benzyl-N-methyl-4-pyridyl quinone (5) and N, N, N, N′, N′, N′-hexamethyl ethylenediamine (6) cations. Prepared crystals were studied by means of x-ray crystallography. In studied compounds stacking interactions are predominant with interplanar separations between rings of 3, 0 – 3, 2 Å. TCNQ moieties form dimers, stacks of dimers, trimers or tetramers with alternating shorter and longer separation between TCNQ rings or stacks in columns with equidistant separations (Fig 1). Besides that, in some salts N–H···N and C–H···N hydrogen bonds are present.

organic radicals, π-stacking, TCNQ

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