Tetracyanoethylene Tetracyanoquinodimethane

Although organic anion radicals are oxygen sensitive, they have been isolated as crystalline salts from a variety of electron acceptors (e.g., chloranil, tetracyanoethylene, tetracyanoquinodimethane, perylene, naphthalene, anthracene, tetraphenylethylene, etc.) and their structures have been established by X-ray crystallography.180... 

An updated book on organic radical ions3 comprises an extensive (presumably almost complete) set of hyperfine data and summarizes methods for their generation. Another book concentrates on chemical properties of radical ions4. Valuable information on timescales and the energetics leading to the dimerization of radical anions and cations (and neutral radicals) based on tetracyanoethylene, tetracyanoquinodimethane, substituted benzoquinones (DDQ and chloranil) as acceptors and octamethylbiphenylene is reported by Kochi and coworkers5. 

The anion radical of the 2,4-dinitrochlorobenzene and the cation radical of the triethylamine pass into the solvent volume. In this case, both acceptors of an electron (p-ben-zoquinone, tetracyanoethylene, tetracyanoquinodimethane) and donors of an electron (potassium iodide, ferrous sulfate, /V,/V-tetramcthyl-p-phcnylenediaminc) inhibit the substitution (Shein 1983). 

Acceptors. Most common acceptor molecules such as tetracyanoethylene or tetracyanoquinodimethane are commercially available. However, TCNQ can be synthesized in high yield by a two-step synthesis involving a condensation of malonitrile with 1,4-cydohexanedione followed by treatment with an oxidizing agent such as bromine or AJ-bromosuccinamide in pyridine solvent (23) (Fig. 6). 

True CT complexes are formed between unsaturated electron acceptors, among which the derivatives with cyano and nitro groups predominate. The most common strong electron acceptors are 1,2,4,5-tetracyanobenzene, 7,7,8,8-tetracyanoquinodimethane (1), tetracyano-p-benzoquinone (2), tetracyanoethylene (TCNE) (3) and many electron-donor... 

Tetracyanoquinodimethane (1) and tetracyanoethylene (3) are able to form CT complexes with crown ethers which are electron-donor molecules25. A recent study has recorded the spectral properties and stability constants of 89 tetracyanoethylene CT complexes with donors26. The main interaction in these complexes is an electron transfer (ji -> it ) between the HOMO of the donor and the LUMO of the acceptor. 

Tetracyanoethylene (TCNE), tetracyanoquinodimethane (TCNQ), and various mono-, di-, and trinitroaromatic compounds are the preferred electron-acceptor molecules for the detection of donor sites on oxide surfaces. Mostly TCNE has been used as a poison in catalytic research. Electronic and ESR spectra of the adsorbed acceptor molecules are characteristic of the surface anion radicals which are assumed to be formed according to... 

The synthesis of the strong cyanocarbon acceptors tetracyanoethylene (TCNE, 8) in 1957 [7], and 7,7,8-8-tetracyanoquinodimethan (TCNQ, 9a) in 1960 [8] heralded intensive studies of organic ionic crystals, with unusual magnetic properties [9-11] and also unusually high conductivity in some of... 

Tetracyanoethylene (tone) and 7,7,8,8-tetracyanoquinodimethane (tcnq) both form adducts with stannocene and plumbocene that are of uncertain structure. The stannocene derivatives SnCp2 (tcne) (n = 1 or 2) and SnCp2 (tcnq) are proposed to be cyclopentadienyl 7r-charge transfer complexes (42). The plumbocene derivatives PbCp2(tcne) (n = 0.5 or 1) and PbCp2(tcnq) apparently involve charge transfer and are proposed to be polymeric (41). 

Metal-vapour-deposition methods have begun to provide new low-valent complexes of uncommon stoichiometry. The preparation of gram quantities of paramagnetic (ji = 2.02 BM) [Co(bipy)2] has been reported and its reactions with Br2, tetracyanoethylene and tetracyanoquinodimethane studied.283 Electrochemically it behaves identically to [Co(bipy)3]2+ (equation 47).283... 

TTT is a powerful electron donor which forms radical-cation salts with many strong electron acceptors such as 2,5-cyclohexadiene-l,4-diylidene dimalononitrile (commonly named tetracyanoquinodimethane or TCNQ), o-chloranil, o-bromanil, o-iodanil, and tetracyanoethylene. These salts are prepared by mixing the components in an appropriate solvent and recovering the product as precipitate. With TCNQ the complex is a 1 1 association however it has the stoicheiometry (TTT) 3 A where n = 1 when Ais o-chloranil, o-bromanil, or o-iodanil and where n = 2 when A is tetracyanoethylene. ... 

Bisbenzene chromium reacts with good 77-acceptor Lewis acids to form complexes (CgHg)2Cr L" (L = tetracyanoethylene, trinitrobenzene, -quinone, chloranil) in which electron transfer from the (CgHg)2Cr to the Lewis acid has taken place. The complexes are best described as bisbenzene chromium cation salts of radical anions (765). The crystal structure of one such compound [(MeCgHg)2Cr] (TCNQ) (TCNQ = 7,7,8,8-tetracyanoquinodimethane) has been determined and consists of Stacks of TCNQ anions and bisbenzene chromium cations with interplanar spacings of 3.42 A (577). 

Cleavage of the SnSn bond can also be brought about by electron transfer reactions with reagents such as tetracyanoethylene (TCNE), tetracyanoquinodimethane, and tetra-chloroquinone for example, TCNE and MegSn2 gives the radical adduct... 

Figure 3.3 The Ea of benzonitrile, 1,2 dicyanoethylene, 1,2 dicyanobenzene, maleic anhydride, benzoquinone, s-tetracyanobenzene, fluoranil, chloranil, hexacyanobenzene, tetracyanoquinodimethane, tetracyanoethylene, and hexacyanobutadiene reported in 1975 [64] from reduction potentials and charge transfer absorption maximum versus the current best values. These are precision and accuracy plots. The slopes are determined with a fixed intercept of zero, so that the displacements are random uncertainties.

Figure 4.15 Electron affinities of charge transfer complex acceptors calculated from C2 = 2.9 versus the current best adiabatic electron affinities. This is a precision and accuracy plot. The zero intercept slope indicates that the same quantities are measured. The compounds are maleic anhydride, tetrachlorophthalic anhydride, benzoquinone, trinitro-flourenone, s-trinitrobenzene, chloranil, tetracyanoquinodimethane, and tetracyanoethylene in order of their electron affinities.

Pal et al. [89] have reported on substituted ferrocenyl compounds, where one of the cyclopentadienyl rings is linked to an aromatic Schiff base, that were synthesized and analyzed for their second-order nonlinearity ((3). Their results indicate that the metal to ligand charge transfer (MLCT) transition dominates their second-order response. These compounds form charge transfer (CT) complexes with acceptors such as Ij, p-chloranil (CA), 2,3-dichloro-5,6-dicyano-l,4-bcnzoquinonc (DDQ), tetracyanoethylene (TCNE), and 7,7,8,8-tetracyanoquinodimethane (TCNQ). The CT complexes exhibit much higher second-order response. Bisferrocenyl complexes where two ferrocene moieties are linked through the same aromatic Schiff base... 

Quite recently this notion was dramatically illustrated by the report of the electrical properties of some complexes made from relatives of tetracyanoethylene, namely, tetracyanoquinodimethane (TCNQ) (36-38). This material can be made into crystals of two different kinds. 

Nucleophilic attack at nitrogen has also been identified in a study of the reactions of triarylphosphines with tetracyanoethylene in aqueous acetonitrile. In contrast, the related reactions with tetracyanoquinodimethane (TCNQ) involve one electron transfer from phosphorus to the TCNQ molecule. Full details of the reactions of tertiary and ditertiary phosphines with bromophenyldiazirines have now appeared. Interest in the Staudinger reaction of tertiary phosphines with azides has also been maintained. A spectroscopic study has shown that the sequence of addition of reactants alters the course of the Staudinger reaction of azides in the presence of acyl derivatives. The Staudinger reaction of a-azidophenylacetonitrile with triphenylphosphine unexpectedly results in the formation of the salt (135). Applications of the Staudinger reaction in synthesis... 

Large hyperpolarizabilities have been reported for CT complexes formed between ferrocene imines and acceptors such as tetracyanoethylene (TCNE), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and -chloranil (CA) for example, compound 21 shows /3 = 33 x 10 esu (HRS, 1,064nm), but complexes with TCNE, TCNQ, and CA show values of 126, 204, and 279 x 10 esu (/ o is estimated to be enhanced from 5.5 to 33-53 x 10 esu). Presumably, the NLO active states are associated with the intermolecular CT in these complexes. 

---