Structural Studies of Perfluoroaryldiselenadiazolyl Radicals: Insights into Dithiadiazolyl Chemistry

The synthesis and structural characterization of a series of perfluoroaryldiselenadiazolyls [DSeDA; <i>p</i>-XC₆F₄CNSeSeN (X = F, Cl, Br, CF₃, NO₂, and CN for <b>2a</b>–<b>2f</b>, respectively)] are described. Concentration-dependent solution UV/vis measurements on <b>2a</b> follow the Beer–Lambert law and the transitions assigned through time-dependent density functional theory (TD-DFT) studies, indicating little propensity for dimerization in solution (10^–3–10^–4 M). Solution electron paramagnetic resonance (EPR) spectra reveal that these radicals exhibit a broad featureless singlet around <i>g</i> = 2.04 but form well-resolved anisotropic EPR spectra in frozen solution, from which spin densities were determined and found to reflect an increase in the spin density at the chalcogen in relation to the corresponding dithiadiazolyl (DTDA) radicals, <i>p</i>-XC₆F₄CNSSN. The solid-state structures of <b>2a</b> and <b>2d</b>–<b>2f</b> all adopt spin-paired cis-cofacial dimers in which the dimers are held together via multicenter π*−π* “pancake bonding” interactions. Conversely, <b>2b</b> and <b>2c</b> exhibit an orthogonal mode of association, which is unique to DSeDA chemistry but which also affords a singlet ground state evidenced by SQUID magnetometry. The more sterically demanding diselenadiazolyl radical <b>2f</b> was also prepared and exhibits a trans-antarafacial dimerization mode. DFT studies [UPBE0-D3 ccPVTZ-PP­(-F)++] on the model radical HCNSeSeN confirm that each dimer is a stable energy minimum on the potential energy surface, reproducing well the experimental geometric parameters with relative stability in the order cis-cofacial > orthogonal > trans-antarafacial. Computational studies reflect stronger dimerization for DSeDA radicals in relation to their sulfur analogues, consistent with the experimental observation: While <b>2a</b> and <b>2d</b> are isomorphous with their corresponding DTDA radicals, <b>2b</b>, <b>2c</b>, and <b>2e</b>–<b>2g</b> are all dimeric, in contrast to their DTDA analogues, which are monomeric in the solid-state. A study on <b>2f</b> reveals that significant geometric strain accumulates in order to support the propensity for both cis dimerization and intermolecular CN···Se interactions. Conversely, <i>p</i>-NCC₆F₄CNSSN likely forfeits dimerization in the analogous packing motif in order to release strain but retains the favorable intermolecular CN···S interactions