Chalcogen controlled redox behaviour in peri-substituted S, Se and Te naphthalene derivatives

Abstract

Cyclic and square wave voltammetry of (PhE)₂peri-disubstituted naphthalene[1,8-cd]dichalcoganyls and acenaphthene[5,6-diyl]dichalcoganyls (E = S, Se, Te, 12 compounds) is reported. Mixed (E₁ = Se, Te; E₂ = Br, I) naphthalene[1,8-cd]halochalcoganyls were also investigated, as well as an exemplar bearing two PhS(O) groups and another bearing one PhSe and one Ph₂P(S) substituent. The voltammetry, in CH₂Cl₂/0.4 M [ⁿBu₄N][PF₆] at both platinum and glassy carbon macro-disk working electrodes, shows two sequential chemically reversible and electrochemically quasi-reversible oxidation processes, and the lack of accessible reductions. Additional oxidations above +1.5 V vs. Fc^+/0 have not been investigated in detail. In situ and ex situ EPR spectroscopy conclusively demonstrate that both anodic processes are 1e transfers; persistent radical cations could be generated for all the dichalcoganyls except when E₁ = E₂ = Te; for the latter case thermally stable dications are generated instead. The complex possible solution conformations of these compounds in 0, +1 and +2 charge states were modelled with DFT at the B3LYP-D3(BJ)/6-31+G(d) level of theory in a CH₂Cl₂ PCM continuum solution model and adiabatic ionisation energies calculated, which correlate linearly (R = 0.88) with the E^a1_p values. Crystal structures of four solvolysis and hydrolysis products of the ditellurium dications are reported and were modelled computationally. Interpretative comparisons to unsubstituted naphthalene[1,8-cd]dichalcogenoles are reported and the crystal structure of naphtho(1,8-cd)(1,2-dithiolium) tetrafluoroborate has been obtained. This is the first structure reported for any salt of this cation radical. Electron transfer mechanisms of both the (PhE)₂ and E₂peri-disubstituted naphthalene series are correlated using a redox molecular orbital interpretative framework.