Preparation and Characterization of (CNSSS)₂(A)₂ (A = AsF₆⁻, SbF₆⁻, Sb₂F₁₁⁻) Containing the O₂-like 5,5′-Bis(1,2,3,4-trithiazolium) Dication: The Second Example of a Simple Nonsterically Hi

The reaction of NC−CN with a 1:1 mixture of S4(MF6)2 and S8(MF6)2 (M = As, Sb) (stoichiometrically equivalent to four “S3MF6” units) results in the quantitative formation of S3NCCNS3(MF6)2 [7(MF6)2], which is the thermodynamic sink in this reaction. The Sb2F11− salt 7(Sb2F11)2 is prepared by the addition of an excess of SbF5 to 7(AsF6)2. Crystal structure determinations for all three salts show that 72+ can be viewed as two R-CNS3+ radical cations joined together by a C−C single bond. The two rings are coplanar and in a trans orientation due to electrostatic Nδ−···Sδ+ interactions. The classically bonded alternative (quinoidal structure), in which the octet rule is obeyed, is not observed and is much higher in energy based on calculated estimates and a simple comparison of π bond energies. Calculated molecular orbitals (MOs) support this, showing that the MO corresponding to the quinoidal structure lies higher in energy than the nearly degenerate singly occupied MOs of 72+. The vibrational spectra of 72+ in all salts were assigned based on a normal-coordinate analysis and theoretical vibrational frequencies calculated at the PBE0/6-31G* level. In the solid state, 72+ is a planar disjoint diradical with essentially degenerate open-shell singlet and triplet states. The disjoint nature of the diradical cation 72+ is established by magnetic susceptibility studies of the Sb2F11− salt doped in an isomorphous diamagnetic host material (CNSNS)2(Sb2F11)2 [10(Sb2F11)2]. Intramolecular spin coupling is extremely weak corresponding to a singlet−triplet gap (ΔEST = 2J) of <±2 cm−1. CASPT2[12,12]/6-311G* calculations support a triplet ground state with a small singlet−triplet gap. The single-crystal electron paramagnetic resonance (EPR) of 7(Sb2F11)2 doped in 10(Sb2F11)2 is in agreement with the triplet state arising from the weak coupling between the unpaired electrons residing in pπ orbitals in each of the rings. Variable-temperature susceptibility data for bulk samples of 7(A)2 (A = SbF6−, AsF6−, Sb2F11−) are analyzed by employing both 1D chain and 2D sheet magnetic models. These studies reveal significant intermolecular exchange approximating that of a 1D chain for the SbF6− salt with |J| = 32 cm−1. The exchange coupling is on the same order of magnitude as that for the AsF6− salt, although in this case it is likely that there are complex exchange pathways where no particular one is dominant. Intermolecular exchange in the Sb2F11− salt is an order of magnitude weaker. In solution, the EPR spectrum of 72+ shows a broad triplet resonance as well as a sharp resonance that is tentatively attributed to a rotomer of the 72+/anion pair, which is likely the origin of the green species given on dissolution of the red 72+ salts in SO2/AsF3/MF5. We account for the many similarities between O2 and 72+, which are the only simple nonsterically hindered nonmetal diradicals to retain their paramagnetism in the solid state. 72+ is also the first isolable, essentially sulfur-based diradical as evidenced by calculated spin densities.