Structural Studies of Perfluoroaryldiselenadiazolyl Radicals: Insights into Dithiadiazolyl Chemistry

Abstract

The synthesis and structural characterization of a series of perfluoroaryldiselenadiazolyls [DSeDA; p-XCFCNSeSeN (X = F, Cl, Br, CF, NO, and CN for 2a-2f, respectively)] are described. Concentration-dependent solution UV/vis measurements on 2a 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 (10-10 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-XCFCNSSN. The solid-state structures of 2a and 2d-2f all adopt spin-paired cis-cofacial dimers in which the dimers are held together via multicenter π*-π* "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 2e-2g 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···Se interactions. Conversely, p-NCCFCNSSN likely forfeits dimerization in the analogous packing motif in order to release strain but retains the favorable intermolecular CN···S interactions.

Molecular Electrostatic Potential as a Predictor of Supramolecular Synthons in Non‐Hydrogen‐Bonded Systems: Application to Heavier p‐Block Systems

“…The solid‐state structures of DTDA radicals are typically dictated by a combination of π* ··· π* dimers (a multi‐centre covalent bonding interaction) in which computational studies have shown dispersion plays a significant contribution , . In solution the dimerisation enthalpy has been estimated at 35 kJ mol –1 .…”

“…Most DTDA radicals dimerise in the solid state through what is considered to be a SOMO‐SOMO bonding interaction, sometimes referred to as “pancake bonding”, resulting in diamagnetic dimers (Scheme ). Computational studies indicate that dispersion plays a significant role in stabilising these π*–π* dimers , . DTDA radicals have been employed as building blocks for molecular conductors, magnets,, photoconductors, as paramagnetic ligands in coordination chemistry and, in the presence of a fluorophore, as multifunctional materials .…”

On the Design of Radical–Radical Cocrystals

“…Thes tructure of 6 is structurally similar to 5 with intradimer Se···Se contacts at 3.1330(8) and 3.1895 (7) , shorter than the symmetric 3.197(2) observed in pristine [C 6 F 5 CNSeSeN] 2 . [21] There is then as et of four close Se···O contacts (Figure 8(bottom)) in the range 2.897(5)-3.348(4) , mean 3.136 .D issolution of 6 in CH 2 Cl 2 afforded an EPR spectrum consistent with am ixture of C 6 F 5 CNSeSeN and TEMPO radicals ( Figure S5.4.3).…”

“…Thes imilarity in structure of many DTDAa nd DSDA radicals [21] prompted us to co-crystallize TEMPO with the diselenadiazolyl radical, [C 6 F 5 CNSeSeN] 2 .U sing as imilar methodology crystals of 6 were generated by vacuum sublimation. Thes tructure of 6 is structurally similar to 5 with intradimer Se···Se contacts at 3.1330(8) and 3.1895 (7) , shorter than the symmetric 3.197(2) observed in pristine [C 6 F 5 CNSeSeN] 2 .…”

Reversible Solution π‐Dimerization and Long Multicenter Bonding in a Stable Phenoxyl Radical

“…It has been reportedt hat the M05-2X functional is suitable for the description of long multicenter bonds of phenalenylr adical dimers and other hydrocarbons. [32,33] It is known that dispersion correction is significant for radical dimers [34,35] andt he M05-2Xf unctional does not include the dispersion correction terms. Also, it has been reported that M05-2X with D3 dispersion correction is over-parameterized, not affording results as good as in ap henalenylr adical dimer.…”