Bonds or not bonds? Pancake bonding in 1,2,3,5-dithiadiazolyl and 1,2,3,5-diselenadiazolyl radical dimers and their derivatives

Abstract

Unusually long bonds or short intermolecular contacts occur in the title compounds reminiscent of pancake bonding. Pancake bonding interactions seem analogous to π-stacking interactions, but they display much shorter contact distances than normally seen in van der Waals (vdW) dimers. The interpretation of these SN and SeN containing structures has been an outstanding challenge for some time. The antibonding (π*) singly occupied molecular orbital (SOMO) of the radical is the source of two-electron multicenter bonding (2e/mc). Preferred conformations thus can be traced back to SOMO-SOMO overlap. We used several computational methods to understand the nature of pancake bonding in the title compounds including four wave function methods (WFT) and a dozen density functional theories (DFT) including empirical dispersion corrections. We used experimental data and high level CCSD(T)/6-311++G(d,p) and MRPT2/6-311++G(d,p) calculations for comparison. The analysis provided the interpretation a wealth of experimental data including conformational preferences of these SN and SeN containing radical dimers leading to a better overall understanding of pancake bonding. Analysis of the various components of the inter-radical interactions showed that SOMO-SOMO bonding interaction and dispersion interaction contribute to the binding energy and neither of these interactions alone is sufficient to bind the dimer. The dimer is predicted to show weak diradical character.

The key role of vibrational entropy in the phase transitions of dithiazolyl-based bistable magnetic materials

“…All-electron DFT calculations on periodic systems and highlevel correlated wavefunction-based (CASPT2) calculations carried out on an isolated stack of four TTTA radicals validate the previous results (Supplementary Notes 4 and 5, Supplementary Table 2 and Supplementary Figs 9-18). The CASPT2 calculations prove that the driving force for the dimerization process taking place during the optimization of HT-0 and HT-0-iso is the formation of long multicenter bonds [42][43][44][45] between TTTA radicals (Supplementary Note 6 and Supplementary Fig. 19).…”

Effects of Halo-Substitution on 2′-Chloro-5′-halo-phenyl-1,2,3,5-dithiadiazolyl Radicals: A Crystallographic, Magnetic, and Electron Paramagnetic Resonance Case Study

“…2b and 2c) become energetically favored and the dimer is destabilized. Computational studies reflect some admixing of the excited S 1 term into the S 0 ground state affording some diradical character, 23,24 and experimental studies by us [26][27][28] and others 23,29 on DTDA radicals and related members of this family of heterocyclic radicals have identified the presence of low-lying thermally accessible triplet excited state configurations (T 1 ).…”

Radical–Radical Recognition: Switchable Magnetic Properties and Re-entrant Behavior

“…At 100 K, the mean distance between the planes of the DTDA heterocycles is ~3.2 Å, well within the sum of van der Waals radii for the constituent N and S atoms and consistent with the formation of diamagnetic π-dimers, via so-called "pancake bonding". [13][14][15]35 The dimerization of the pyDTDA moieties serves to assemble the complexes into supramolecular chains propagating along the ("pancake bonded") [pyDTDA]2 dimer with a neighboring complex (mean distance between planes ~3.2 Å at 220 K) and one pyDTDA ligand that is no longer engaged in pancake bonding and is part of an "open" [pyDTDA]2 pair (Figure 3b). While the "closed" dimers maintain the same trans-cofacial geometry as those described above, the "open" radical pairs are characterized by an increase in the interplanar distance between neighboring DTDA heterocycles accompanied by a lateral translation, or in-plane slippage of the heterocycles with respect to one another (Figure 3b).…”

“…At a molecular level, these phase transitions involve the rupturing, or "opening", of so-called "pancake bonds" [13][14][15] between π-dimers of neighboring pyDTDA radical ligands, which is itself very unusual. As a result, the phase transitions also manifest as stepwise increases in the magnetic moment as the structure transitions from "closed", diamagnetic [pyDTDA]2 dimers to "open", paramagnetic [pyDTDA]2 pairs.…”

“…The cis-cofacial, twisted-cofacial, and trans-cofacial geometries are simply "pancake bonds" involving the σ overlap of all four lobes of the SOMOs, rotated by roughly 0°, 90°, and 180° respectively, and consistent with the roughly square shape of the SOMO. 14 The trans-antarafacial motif is the σ overlap of only two lobes of the SOMO, those centered on the S atoms.…”