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The structural parameters and semiconductivity of crystals with stacked 5,6-dichloro-2,3-dicyanosemiquinone (DDQ) radicals were studied for a series of the first nine salts of DDQ with substituted N-ethyl- and N-methylpyridinium cations. Structures with stacks of equidistant radicals (interplanar separation ＜3.3 Å) are fairly good semiconductors (10−9 S cm−1), while those with Peierls-distorted stacks comprising diamagnetic dimers of radicals (interplanar separations are ＜3.1 Å within the dimers and ＞3.4 Å between the dimers) are insulators. DFT calculations indicate the covalent character (pancake bonding) of the interactions between the DDQ radical anions. The inductive effects of electronegative substituents (halogeno and cyano) on the quinoid ring stabilise the radicals, but also increase the HOMO–LUMO gap, resulting in lower electrical conductivity. [ABSTRACT FROM AUTHOR]

A survey of the Cambridge Structural Database (CSD) revealed the existence of a new supramolecular homosynthon in oxovanadium coordination compounds (OCCs) that is based on non-covalent V=O…C interactions. V=O…C close contacts were found in 38.7% of structures containing vanadyl or pervanadyl groups. The V=O…C interaction is clearly distinguishable from a putative V=O…H–C hydrogen bond upon analysis of OCC crystal structures deposited in the CSD. In the majority of structures, the OCCs assemble to form dimers or polymers. The V=O…C interactions were also found to lead to self-assembly of two new oxovanadium β-diketonate complexes into dimers in the solid state. [ABSTRACT FROM AUTHOR]