Isolation of kinetically-stabilised diarylchalcogenide radical cations

Abstract

Chalcogenide radical cations [R₂E]^•+ (E = S, Se, Te) are commonly short-lived intermediates of fundamental interest. Sulfide radical cations in particular are associated in vivo with oxidative stress and neuropathological processes. Having succeeded in the preparation of meta-terphenyl-based dichalcogenide radical cations [R₂E₂]^•+ (E = S, Se, Te), and a telluride analogue [R₂Te]^•+ in the past, we aimed to complete the series regarding sulfur and selenium. Here we report on the single-electron oxidation of diarylchalcogenides M^SFluindPhE (E = S, Se, Te; M^SFluind = dispiro[fluorene-9,3'-(1',1',7',7'-tetramethyl-s-hydrindacen-4'-yl)-5',9"-fluorene]) using XeF₂ in the presence of K[B(C₆F₅)₄], which afforded deeply coloured and isolable radical cation salts [M^SFluindPhE][B(C₆F₅)₄] (E = S, Se, Te). Structural and electronic properties were characterised by electron paramagnetic spectroscopy, cyclic voltammetry, optical absorption spectroscopy and single crystal X-ray diffraction (E = Se, Te), combined with extensive quantum mechanical computations.

Fig. 1. Selected radical cations of chalcogenides known in the literature.

While the Methionine and 1,5-dithiacyclooctane radical cations I^,, II have been observed spectroscopically, stabilised chalcogen radical cations have been structurally characterised for dichalcogenides (e.g. IIIa/b^,), as well as for monotellurides (IV, R = mesityl, V).

Fig. 2. Synthesis and structural characterisation of chalcogen radical cations.

A Synthesis of 1E and [1E][B(C₆F₅)₄] (E = S, Se, Te). B Mulliken spin density (left) of [1S]^•+ (blue—ositive, green—negative, contour value 0.005). AIM molecular graph (right) of [1S]^•+ with bond critical points as red spheres and bond paths in orange, as well as IGM based on a Hirshfeld partition of the molecular density. IGMH iso-surfaces at s(r) = 0.005 colour coded with sign(λ2)ρ in a.u. Blue surfaces refer to attractive forces and red to repulsive forces. Green indicates weak interactions. C Molecular structures of [1Se][B(C₆F₅)₄] and [1Te][B(C₆F₅)₄] showing 50% probability ellipsoids and the essential atomic numbering. Hydrogen atoms, solvent molecules and counter-ions are omitted.

Fig. 3. Electronic characterisation of chalcogen radical cations.

A Cyclic voltammograms of 1E (E = S, Se, Te) measured in CH₂Cl₂ with c(1E) = 0.1 mM, c([ⁿBu₄N][PF₆]) = 0.1 M, scan rate = 100 mV s⁻¹ and ferrocene (FcH) as an internal standard. Half-wave potentials E_1/2 of the 1^st oxidations are given, as well as the oxidation peak potential of the 2^nd oxidation of 1Te. B UV-Vis-NIR-absorption spectra of solutions of [1E][B(C₆F₅)₄] (E = S, Se, Te) in CH₂Cl₂ (c = 0.05 mM) (* marks a band attributed to an unidentified impurity). The NIR-absorption bands attributed to HOMO-LUMO-transitions (800–1500 nm) are marked. C X-band (left) and Q-band (right) EPR spectra of frozen solutions of [1E][B(C₆F₅)₄] (E = S, Se, Te) in CH₂Cl₂/THF (1:1) mixture measured at 60, 22, 7.4 K (X-band) and 70, 45, 25 K (Q-band), respectively, displayed on the same magnetic field axis. The figure inset is a magnetic field expansion of [1S][B(C₆F₅)₄] to display the axial character of the spectrum. Experimental data is shown in black, while simulations are shown in red. EPR simulation parameters are detailed in Supplementary Table S73 (* marks impurity in the Q-band spectrum ~0.8%).