Dedicated to the memory of Linus Pauling on the occasion of the 100th anniversary of his birth
We present here an exceptionally long CÀC bonding interaction that has spectroscopic (IR and UV/Vis), structural, and magnetic properties expected for a bond and that complies with Paulings definition of a chemical bond. [1] Strong carbon ± carbon bonding is the essence of organic chemistry. The length of a CÀC single bond, that is, 1.54 found in the diamond allotrope of carbon, is among the essential information learned by all organic chemistry students. This is the length of a single bond between sp 3hybridized carbon atoms and is the longest of all common CÀC bonds, although elongated CÀC bonds as long as 1.73 have been reported. [2] These bonds form whenever two carbon atoms possessing an unpaired electron are close to each other (e.g., as occurs for two . CH 3 radicals, which form ethane). When the carbon atoms are radical anions (or radical cations), due to their strong coulombic repulsion, bonds do not form. Thus, many electron transfer salts involving radicals are stable, and as a consequence exhibit interesting electrical and optical properties. Herein, however, we show that CÀC interactions having all the properties of a chemical bond are present between pairs of [TCNE] À ions in some TCNE-based electron transfer salts, and that this type of bonding is present for many other reduced strong electron acceptors, for example, cyanil, [3] 7,7,8,8-tetracyano-p-quinodimethane (TCNQ), [4] perfluoro-7,7,8,8-tetracyano-p-quinodimethane (TCNQF 4 ), [5] and 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). [6] Strong organic electron acceptors (A) such as TCNE and TCNQ form stable electron transfer salts that contain [A] .À . These salts were crucial for the discovery and development of molecule-based metals, [7] which subsequently led to the discovery of molecule-based superconductors, [8] as well as molecule-based magnets. [9] [16] The chiral environment was obtained by the addition an appropriate amount of enantiopure (R)-(À)-1-(9-anthryl)-2,2,2,-trifluoroethanol to a CDCl 3 solution (see: W. H. Pirkle, J. Am. Chem. Soc. 1966, 88, 1837).
[17] Under conditions of fast exchange, two averaged lines should actually be observable, one for each pair of ortho carbon atoms of the two rapidly rotating phenyl groups. However, the separation of these two averaged lines (Dd 0.6) is too small to be resolved, as also shown by the computer line-shape simulation (see Figure 3).