The Interplay of Inverted Redox Potentials and Aromaticity in the Oxidized States of New π-Electron Donors: 9-(1,3-Dithiol-2-ylidene)fluorene and 9-(1,3-Dithiol-2-ylidene)thioxanthene Derivatives

Derivatives of 9‐(1,3‐dithiol‐2‐ylidene)fluorene (9) and 9‐(1,3‐dithiol‐2‐ylidene)thioxanthene (10) have been synthesised using Horner–Wadsworth–Emmons reactions of (1,3‐dithiol‐2‐yl)phosphonate reagents with fluorenone and thioxanthen‐9‐one. X‐ray crystallography, solution electrochemistry, optical spectroscopy, spectroelectrochemistry and simultaneous electrochemistry and electron paramagnetic resonance (SEEPR), combined with theoretical calculations performed at the B3P86/6‐31G** level, elucidate the interplay of the electronic and structural properties in these molecules. These compounds are strong two‐electron donors, and the oxidation potentials depend on the electronic structure of the oxidised state. Two, single‐electron oxidations (${E{{{\rm ox}\hfill \atop 1\hfill}}}$${E{{{\rm ox}\hfill \atop 2\hfill}}}$) resulting in a single, two‐electron oxidation process. The latter is due to the aromatic structure of the thioxanthenium cation (formed on the loss of a second electron), which stabilises the dication state (102+) compared with the radical cation. This contrasts with the nonaromatic structure of the fluorenium cation of system 9. The two‐electron oxidation wave in the thioxanthene derivatives is split into two separate one‐electron waves in the corresponding sulfoxide and sulfone derivatives 27–29 owing to destabilisation of the dication state. A large gain in heteroaromaticity at the dication stage drives the oxidation of 9‐(1,3‐dithiol‐2‐ylidene)thioxanthene derivatives: a single two‐electron process is observed, providing a new example of inverted redox potentials (see scheme). In contrast, 9‐(1,3‐dithiol‐2‐ylidene)fluorene derivatives display two, stepwise, single‐electron oxidations. Theoretical calculations rationalise these experimental observations.