Electrochemical evidence for hypervalent (10‐C‐5) [1] pentacoordinate carbon

The electrochemical oxidation of 1,8‐bis(4‐methylphenylthio)‐9‐(2, 6‐dimethoxyphenyl)‐10‐phenylanthracene (2) is discussed. It undergoes facile, reversible two‐electron electrooxidation in two closely spaced one‐electron steps at E ′ = 0.98 V and E ′ = 1.02 V (vs. SCE). This behavior is discussed in terms of relief of three‐center antibonding interaction involving the 1‐, 8‐, and 9‐substituents of 2 with two sulfurs and a carbon of the 2,6‐dimethoxyphenyl group, which becomes a stabilized three‐center bonding interaction in dication 1. The central atom in the three‐center bond is a hypervalent pentacoordinate carbon atom. A related two‐center antibonding interaction present in 1‐(4‐methylphenylthio)‐9‐(2,6‐dimethoxyphenyl)‐10‐phenylanthracene (5) is relieved by facile one‐electron oxidation (Ep = 1.14 V vs. SCE) forming 7. Removal of a second electron (Ep = 1.34 V vs. SCE) from anthracene 5 results in an unstable dication, 8. Other analogous anthracene derivatives lacking both thioaryl groups (4) or the 9‐aryl group (4) prove much more difficult to oxidize, both showing oneelectron reversible oxidation potentials of 1.27 V (vs. SCE) similar to 9‐phenylanthracene (1.34 V vs. SCE) and 9,10‐diphenylanthracene (1.22 V vs. SCE). These results provide strong evidence for the postulated structure of dication 2 which incorporates hypervalent pentacoordinate carbon, a 10‐C‐5 species [2,3].