Tuning Intermolecular Magnetic Exchange Interactions in the Solids C_xF_2x(CNSSS)₂(AsF₆)₂:  Structural, EPR, and Magnetic Characterization of Dimeric (x = 2, 4) Diradicals

Abstract

A series of diradical containing salts CxF2x(CNSSS)2(**2+0(AsF6-)2 {x = 2, 1[AsF6]2; x = 3, 3[AsF6]2; x = 4, 2[AsF6]2} have been prepared. 1[AsF6]2 and 2[AsF6]2 were fully characterized by X-ray, variable-temperature magnetic susceptibility, and solid-state EPR measurements, further allowing us to extend the number of examples of the family of rare 7pi RCNSSS(*+) radical cations. 1[AsF6]2: a = 6.5314(7) A, b = 7.5658(9) A, c = 9.6048(11) A, alpha = 100.962(2) degrees , beta = 96.885(2) degrees , gamma = 107.436(2) degrees , triclinic, space group P, Z = 1, T = 173 K. 2[AsF6]2: a = 10.6398(16) A, b = 7.9680(11) A, c = 12.7468(19) A, beta = 99.758(2) degrees , monoclinic, space group P21/c, Z = 2, T = 173 K. In the solid-state, CxF2x(CNSSS)2(**2+) (x = 2, 4) formed one-dimensional polymeric chains of dications containing discrete centrosymmetric radical pairs in which radicals were linked by four centered two-electron pi*-pi* bonds [12+, d(S...S) = 3.455(1) A; 22+, d(S...S) = 3.306(2) A]. The exchange interactions in these bonds were determined to be -500 +/- 30 and -900 +/- 90 cm-1, by variable temperature magnetic susceptibility measurements, respectively, providing rare experimental data on the singlet-triplet gaps in the field of thiazyl radicals. For 2[AsF6]2, the thermally excited triplet state was unambiguously characterized by EPR techniques [/D/ = 0.0254(8) cm(-1), /E/ = 0.0013(8) cm(-1)]. These experimental data implied a weakly associated nature of the radical moieties contained in the solids 1[AsF6]2 and 2[AsF6]2. Computational analysis of the dimerization process is presented, and we show that the 2c 4 electron pi*-pi* bonds in 1[AsF6]2 and 2[AsF6]2 have ca. 50% and 40% diradical character, respectively. In contrast, 3[AsF6]2.SO2, containing diradical C3F6(CNSSS)2(**2+) with an odd number of CF2 spacers, showed magnetic behavior that was consistent with the presence of monomeric radical centers in the solid state.

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

“…At this value the observation of triplet species by EPR was not possible due to pronounced dipole-dipole interactions between triplets [132].…”

“…According to magnetic measurements, 24d[AsF 6 ] 2 is paramagnetic at r.t., but there is a drop in susceptibility around 200 K indicative of a possible phase transition. As indicated by variable temperature X-ray measurements, the origin of this phase transition was likely due to contraction of one pair of S· · ·S contacts from 4.1Å to 3.8Å on cooling from 223 to 123 K [132].…”

“…The radical rings associate in the same fashion as [XCNSSS] 2 2+ (X = Cl, Br, I) and [S 3 N 2 ] 2 2+ via long S· · ·S contacts of 3.455(1) and 3.306(2)Å. The slight difference in S· · ·S contacts in 24b[AsF 6 ] 2 and 24c[AsF 6 ] 2 is likely due to the difference in packing motifs [132].…”

“…EPR spectra of diradicals 24b 2+ -24f 2+ consist of one single line implying no significant interaction between the unpaired electrons [132,133]. In comparison, an EPR spectrum of 24a 2+ at −80 • C features two broad shoulders indicative of a dipolar exchange interaction between the unpaired spins (Fig.…”

“…The lack of resolution of triplet species at r.t. was likely caused by broadening of the resonance lines. Analysis of the temperature dependence of the (double integral) intensity of the S = 1 signal in the range 150-280 K gave a singlet-triplet gap of −910 ± 70 cm −1 [132].…”