The crystal and molecular structure of an organic conductor from 4,4',5,5'‐tetramethyl‐Δ2,2'‐bis‐1,3‐diselenole and 7,7,8,8‐tetracyano‐p‐quinodimethane [TMTSF–TCNQ]

This report relates the synthesis of two salts, one conducting and one insulating, and the crystal structure of the conducting salt formed from TMTSF and TCNQ. The conducting salt crystallized in the triclinic system, space group P]-, with cell data: a=3.883 (1), b= 7.645 (5), c= 18.846(9) /~,, a = 77.34(5), fl = 89.67 (3), F = 94-63 (3) °, V = 543.97/~3, Z = 1, Dm 1.98 (1), De = 1.99 g cm -3. Intensities for 2216 non-zero reflections were collected by counter methods on an automated diffractometer. The structure was solved by standard heavy-atom methods and has been refined by full-matrix least squares to a final R value of 0.107. The TMTSF cations and TCNQ anions form separate, homologous stacks in the crystal with interplanar spacings of 3.60 and 3-26 ,~, respectively. The planes of the molecular ions are tilted in opposite directions relative to the short a axis to form a dihedral angle of 54.9 °. The intrachain stacking and molecular overlaps are close to those observed in the chemically similar salt HMTSF-TCNQ, but the interchain coupling in TMTSF-TCNQ appears to be substantially weaker. Since HMTSF-TCNQ remains conducting at low temperature whereas TMTSF-TCNQ does not, we infer that interchain interactions play a crucial role in the evolution of the metal-to-insulator transition in TMTSF-TCNQ. 417