The neutral diradical 5,5'-bis(1,3,2,4-dithiadiazolyl) [-], the first main group radical to exhibit a dramatic increase in paramagnetism on mechanical grinding

Abstract

Reduction of [([Formula: see text])2][As6]2 with triphenylantimony and tetrabutylammonium chloride produced the diradical 5,5'-bis(1,3,2,4-dithiadiazolyl) [[Formula: see text]-[Formula: see text]] in high yield as a black solid with widely ranging magnetic susceptibilities (e.g., 0.6 to 2.6 μB), which on oxidation with AsF5 regenerated [([Formula: see text])2][AsF6]2. The identity of [[Formula: see text]-[Formula: see text]] was established from EPR, vibrational, and mass spectra. Ab initio molecular orbital [MPW1PW91/6-311G(2df)] calculations show the lowest energy structure to consist of two coplanar rings separated by a C—C single bond (1.444 Å), reflected in the comparison of the vibrational spectra of the diradical with that of [([Formula: see text])2][AsF6]2 and the calculated spectra. Confidence in the calculated [MPW1PW91/6-311G(2df)] structure of the diradical is supported by the excellent agreement between the calculated and X-ray single crystal structure geometries of [[Formula: see text]]2 and [([Formula: see text])2]2+ in [([Formula: see text])2][AsF6]2. The molecular orbitals indicate the diradical is essentially disjoint, confirmed by a very small (0.07 kJ mol–1) GVB-PP(TC-SCF)/6-311G* calculated singlet–triplet energy gap. Accordingly, the EPR spectrum of the diradical (in tetrahydrofuran, 293 K) shows a simple 3-line pattern (g = 2.0043, a(14N) = 1.11 mT) with no observable exchange coupling between the two radical centers. Mechanical grinding of the diradical results in a large increase in paramagnetism (e.g., from 1.03 to 2.55 μB) that is unprecedented in main group chemistry. The X-ray diffraction data of the ground and unground powder are consistent with a second order phase change on grinding. Attempts to obtain crystals of the diradical by sublimation led instead to numerous decomposition and rearrangement products, including 4-cyano-1,2,3,5-dithiadiazolyl [NC-[Formula: see text]], monoclinic space group C2/c, a = 9.46(2) Å, b = 7.625(5) Å, c = 13.17(2) Å, β = 107.94(4)°, Z = 8. The structure consists of axially symmetric, co-facial, cis dimers linked to form strands through two sets of C[Formula: see text]Nδ–···Sδ+ contacts. More efficient and larger scale preparations of [SNS][AsF6] and [([Formula: see text])2][AsF6]2 are reported. Key words: 5,5'-bis(1,3,2,4-dithiadiazolyl), diradical, paramagnetism, mechanical grinding, second order phase change, 4-cyano-1,2,3,5-dithiadiazolyl.

Mechanically compliant single crystals of a stable organic radical

“…The total spin count of the sample was also measured before and aer manual grinding of the crystals, revealing a 5 and 12% increase in the spin count on grinding, respectively. Previous reports 50,51 have described an increase in paramagnetism on grinding for related radicals. In one case, 51 the authors suggested that the increase in paramagnetism was related to a change in associations between radical units on grinding, inducing a second-order phase transition observed by powder X-ray diffraction (PXRD).…”

RCNSSS+: A novel class of stable sulfur rich radical cations

“…The S-T gaps of the related diradicals 25-27 (Chart 6) were not experimentally determined, although some evidence of weak interaction between unpaired electrons was obtained for 25 [165] and 27 [166] by solution EPR studies. However the singlet-triplet gaps for 25-27 have been calculated [131] and the results are listed in Table 5.…”

“…Species 33 [168], 34 [169], 35 [170], and 36 [171] (Chart 8), can be isolated in the solid state but should rather be regarded as diradicaloids due to a moderate diradical character. Diradicals 25 [165], 26 [172], and 27 [166] contain *-* dimers in the solid state and are diamagnetic although 27 transforms into a weakly paramagnetic phase on grinding. The bis-verdazyl diradical 28 is also diamagnetic but as a result of intra-molecular antiferromagnetic coupling (2J = −760 cm −1 ) [173].…”

The Synthesis and Characterization of Stable Radicals Containing the Thiazyl (SN) Fragment and Their Use as Building Blocks for Advanced Functional Materials

“…108,115,150,151 One-electron reduction of bis(1,3,2,4-dithiadiazolium) dication 56 gives radical cation 58, in which the radical ring has rearranged to the 1,2,3,5-isomer; this product presumably proceeds via the bis(1,3,2,4) radical cation 57 (Scheme 9.16). The structure of 58 consists of trans-cofacial π dimers of the 1,2,3,5-radicals.…”

“…Attempted sublimation causes 59 to decompose to (among other things) the cyano-substituted-1,2,3,5-dithiadiazolyl 60. 108 High level computational studies indicate that 59 108 and 61 115 are, like 39, 104 biradical in nature with very weakly interacting spins.…”