Tetracyanoquinodimethane

Preparation and structure

TCNQ is prepared by the condensation of 1,4-cyclohexanedione with malononitrile, followed by dehydrogenation of the resulting diene with bromine: : :

The molecule is planar, with D2h symmetry.

Reactions

Like tetracyanoethylene (TCNE), TCNQ is easily reduced to give a blue-coloured radical anion. The reduction potential is about −0.3 V relative to the ferrocene/ferrocenium couple. This property is exploited in the development of charge-transfer salts. TCNQ also forms complexes with electron-rich metal complexes.

Charge transfer salts

TCNQ achieved great attention because it forms charge-transfer salts with high electrical conductivity. These discoveries were influential in the development of organic electronics. Illustrative is the product from treatment of TCNQ with the electron donor tetrathiafulvene (TTF), TCNQ forms an ion pair, the TTF-TCNQ complex, in which TCNQ is the acceptor. This salt crystallizes in a one-dimensionally stacked polymer, consisting of segregated stacks of cations and anions of the donors and the acceptors, respectively. The complex crystal is an organic semiconductor that exhibits metallic electric conductivity.

Related compounds

• Tetracyanoethylene, another cyanocarbon that functions as an electron acceptor.
• Tetrathiafulvalene, another organic compound that functions as an electron donor.

References

References

1. "7,7,8,8-Tetracyanoquinodimethane".
2. (1962). "Substituted Quinodimethans. I. Preparation and Chemistry of 7,7,8,8-Tetracyanoquinodimethan". Journal of the American Chemical Society.
3. (1965). "The crystal and molecular structure of 7,7,8,8-tetracyanoquinodimethane". Acta Crystallographica.
4. (1994). "The coordination chemistry of TCNE, TCNQ and related polynitrile π acceptors". Coordination Chemistry Reviews.
5. Torrance, Jerry B.. (1979). "The difference between metallic and insulating salts of tetracyanoquinodimethone (TCNQ): how to design an organic metal". Accounts of Chemical Research.