Molecular Saddles. 4.1 Redox-Active Cyclophanes by Bridging the 9,10-Bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene System: Synthesis, Electrochemistry, and X-ray Crystal Structures of Neutral Species and a Dication Salt

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Abstract
We report the synthesis of a new series of cyclophanes 11a−d by ester-forming macrocyclization reactions of diol 9 with the dicarbonyl chloride derivatives of benzene, thiophene, ferrocene, and diphenyl ether, 10a−d, respectively. Compounds 11a−d display a two-electron, quasireversible oxidation wave in the cyclic voltammogram to yield the dication species at Eoxpa = 0.70−0.72 V (for 11a−c) and 0.47 V (for 11d) (vs Ag/AgCl in acetonitrile). The raised oxidation potentials for 11a−c reflect the reduced stability of the twisted dication structure within the steric constraints of the smaller cyclophanes. Consistent with this, the value of ΔE (defined as Eoxpa − Eoxpc) decreases (i.e., reversibility of the oxidation process increases) in the sequence 11d > 11c > 11a > 11b as the bridging chain becomes shorter. X-ray crystal structures are reported for compounds 11a−d and the dication salt 11d2+(I3-)2·(CH2Cl2)2.25. For 11a−d the typical saddle-shaped conformation of the 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene moiety is observed, with the strain imposed by the cyclophane ring being accommodated in the structure of the bridging unit. In the dication 11d2+ both dithiolium rings are planar and the anthracene unit is essentially aromatic, with the conformation of the bridge basically the same as in neutral 11d.

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