Tetracyanoquinodimethane

Tetracyanoquinodimethane (TCNQ) is the organic compound with the formula (NC)2CC6H4C(CN)2. This cyanocarbon, a relative of para-quinone, is an electron acceptor that is used to prepare charge transfer salts, which are of interest in molecular electronics.

Tetracyanoquinodimethane (TCNQ)
Ball-and-stick model of the tetracyanoquinodimethane molecule

Sample of TCNQ
Names
Other names
(2,5-Cyclohexadiene-1,4-diylidene)
-dimalononitrile, 7,7,8,8-Tetracyanoquinodimethane
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.014.704
UNII
Properties
C12H4N4
Molar mass 204.19 g/mol
Appearance green colored powder or orange crystals
Melting point 293.5 to 296 °C (560.3 to 564.8 °F; 566.6 to 569.1 K)
Boiling point sublimes
Hazards
not listed
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Preparation and structure

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

C6H8O2 + 2 CH2(CN)2 → C6H8(C(CN)2)2 + 2 H2O
C6H8(C(CN)2)2 + 2 Br2 → C6H4(C(CN)2)2 + 4 HBr

The molecule is planar, with D2h symmetry.[2]

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.[3]

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.[4]

References
  1. Acker, Donald S.; Hertler, Walter R. (1962). "Substituted Quinodimethans. I. Preparation and Chemistry of 7,7,8,8-Tetracyanoquinodimethan". Journal of the American Chemical Society. 84 (17): 3370. doi:10.1021/ja00876a028.
  2. Long, Robert E.; Sparks, Robert A.; Trueblood, Kenneth N. (1965). "The crystal and molecular structure of 7,7,8,8-tetracyanoquinodimethane". Acta Crystallographica. 18 (5): 932. doi:10.1107/S0365110X65002256.
  3. Kaim, Wolfgang; Moscherosch, Michael (1994). "The coordination chemistry of TCNE, TCNQ and related polynitrile π acceptors". Coordination Chemistry Reviews. 129 (1–2): 157–193. doi:10.1016/0010-8545(94)85020-8.
  4. Torrance, Jerry B. (1979). "The difference between metallic and insulating salts of tetracyanoquinodimethone (TCNQ): how to design an organic metal". Accounts of Chemical Research. 12 (3): 79–86. doi:10.1021/ar50135a001.
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