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

Abstract

Abstract 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 (E p = 1.14 V vs. SCE) forming 7 . Removal of a second electron (E p = 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].