Crystallographic studies of some tetracyaquidimethane complexes.

The solid state structures of charge-transfer complexes of 7,7,8,8-tetracyanoquinodimethane, TCNQ, have been reviewed and classified into three main structural types viz homosoric (pseudosoric), heterosoric, and nonsoric. In an attempt to more fully understand the factors influencing the type of structure adopted in the solid state and to relate the structural features to the electrical conductivities, the crystal structures of five TCNQ complexes have been determined by X-ray analysis:(i) Dimethyldibenzophospholium Bis-7,7,8,8-tetracyanoquinodimethanide, is a semi-conductor and belongs to the pseudosoric class of TCNQ complexes, with the cations and the TCNQ moieties forming segregated stacks. The TCNQ molecules are arranged in a diadic fashion with the diads being held together by an array of short intermolecular contacts to give two-dimensional sheets.(ii) Dimethyldiphenylphosphonium Bis-7,7,8,8-tetracyanoquinodimethanide is a good semi-conductor which adopts a monadic/diadic stacking of the TCNQ moieties into columns. The resulting one-dimensionality of the structure is attributed to the presence of statistically disordered cations. The influence of such cation disorder upon the electrical conductivities has been examined for related phosphonium complexes.(iii) N,N'-Dicyanophenyl-4,4'-bipyridylium Tetrakis-7,7,8,8-tetracyanoquinodimethanide, is found to be a pseudosoric complex with the TCNQ moieties contained within sheets in a tetradic manner. The poor semi-conducting properties are attributed to the presence of the cyanophenyl groupings which facilitate significant interactions between anion and cation stacks.(iv) 1,2-Diethyl-4,4' -bipyridiniumethane Tetrakis-7,7,8,8-tetra-cyanoquinodimethanide is a poor semi-conductor containing tetradic stacks of TCNQ, molecules within which there is poor overlap. Thetetrads are arranged in sheets containing extensive intermolecular electrostatic interactions.(v) 4,4'-Bipyridylethylene 7,7,8,8-tetracyanoquinodimethane, (DPE)-(TCNQ,), is a heterosoric complex and contains stacks of alternating donor (DPE) and acceptor (TCNQ) molecules. The large distances between donor and acceptor molecules within each stack and the geometry of the TCNQ molecule indicate little or no charge-transfer to be present.