Structural Studies of Perfluoroaryldiselenadiazolyl Radicals: Insights into Dithiadiazolyl Chemistry.

Rebecca L Melen,Christopher M Pask,Robert J Less,Jeremy M Rawson

doi:10.1021/acs.inorgchem.6b01771

Abstract

The synthesis and structural characterization of a series of perfluoroaryldiselenadiazolyls [DSeDA; p-XC6F4CNSeSeN (X = F, Cl, Br, CF3, NO2, 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 (10-3-10-4 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 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-NCC6F4CNSSN likely forfeits dimerization in the analogous packing motif in order to release strain but retains the favorable intermolecular CN···S interactions.

Highlights

The magnetic[1,2] and transport[3] properties of dithiadiazolyl (DTDA) radicals have attracted considerable attention since the first structural report of a DTDA radical, [PhCNSSN]2, in 1980.4 More recently, their coordination chemistry as paramagnetic ligands in molecule-based magnetic materials has been exploited by Preuss,[5] and approaches to control their solid-state structure through crystal engineering design strategies have been reviewed by Haynes.[6]
We found that the cc-PVTZ-PP(-F)++ basis set provided very good agreement with the “all-electron” 6311++G(d,p) used previously,[12] in terms of both the computed geometry and dimerization energy for cisoid (HCNSeSeN)[2]
The total computed dissociation energy to break the CN··· Se interaction was 26 kJ mol−1, of which the dispersion (D3) contribution was 5 kJ mol−1. These computational studies indicate that the dimerization energy of DSeDA radicals is ca. 3 times higher than that observed for DTDA radicals, with the cisoid, trans-antarafacial, and orthogonal dimers all appearing as stable entities on the potential energy surface

Summary

Introduction

The magnetic[1,2] and transport[3] properties of dithiadiazolyl (DTDA) radicals have attracted considerable attention since the first structural report of a DTDA radical, [PhCNSSN]2, in 1980.4 More recently, their coordination chemistry as paramagnetic ligands in molecule-based magnetic materials has been exploited by Preuss,[5] and approaches to control their solid-state structure through crystal engineering design strategies have been reviewed by Haynes.[6]. 3.0 Å), which are significantly less than the sum of the van der Waals radii (3.6 Å) but more than a covalent S−S bond (2.0 Å). Structural studies on these DTDA dimers reflect a range of dimerization modes (cis, twisted, trans etc.) in which there is a strong orientational preference, supporting the hypothesis that dimerization is associated with a bonding interaction between the two singly occupied molecular orbitals of a2 symmetry. More recent magnetic and electron paramagnetic resonance (EPR) studies by Passmore, Rawson, and their co-workers have revealed that many of these dimers exhibit the onset of paramagnetism near room temperature due to the population of a thermally accessible triplet state.[10,11] This casts some speculation over the nature of the ground-state electronic configuration as to whether it is a closed-shell “spin-paired” singlet, an open-shell “antiferromagnetically coupled” singlet, or an admixture of these two Received: July 27, 2016 Published: November 1, 2016

Results

Discussion

Conclusion