Structural Studies of Perfluoroaryldiselenadiazolyl Radicals: Insights into Dithiadiazolyl Chemistry

The synthesis and structural characterization of a series of perfluoroaryldiselenadiazolyls [DSeDA; p-XC6F4CNSeSeN (X = F, Cl, Br, CF3, NO2, and CN for 2a2f, respectively)] are described. Concentration- dependent solution UV/vis measurements on 2a follow the BeerLambert 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 g = 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, p-XC6F4CNSSN. The solid-state structures of 2a and 2d2f all adopt spin-paired cis-cofacial dimers in which the dimers are held together via multicenter pi*-pi* pancake bonding interactions. Conversely, 2b and 2c 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 2f 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 2a and 2d are isomorphous with their corresponding DTDA radicals, 2b, 2c, and 2e2g are all dimeric, in contrast to their DTDA analogues, which are monomeric in the solid-state. A study on 2f reveals that significant geometric strain accumulates in order to support the propensity for both cis dimerization and intermolecular CN center dot center dot center dot Se interactions. Conversely, p-NCC6F4CNSSN likely forfeits dimerization in the analogous packing motif in order to release strain but retains the favorable intermolecular CN center dot center dot center dot S interactions.