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

Reduction of [(CNSNS)(2)][As-6](2) with triphenylantimony and tetrabutylammonium chloride produced the diradical 5,5'-bis(1,3,2,4-dithiadiazolyl) [SNSNC-CNSNS] in high yield as a black solid with widely ranging magnetic susceptibilities (e.g., 0.6 to 2.6 mu(B)), which on oxidation with AsF5 regenerated [(CNSNS)(2)][AsF6](2). The identity of [SNSNC-CNSNS] 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 Angstrom), reflected in the comparison of the vibrational spectra of the diradical with that of [(CNSNS)(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 [CNSSN](2) and [(CNSNS)(2)](2+) in [(CNSNS)(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(N-14) = 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 mu(B)) 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-CNSSN], monoclinic space group C2/c, a = 9.46(2) Angstrom, b = 7.625(5) Angstrom, c = 13.17(2) Angstrom, beta = 107.94(4)degrees, Z = 8. The structure consists of axially symmetric, co-facial, cis dimers linked to form strands through two sets of C = Ndelta-...Sdelta+ contacts. More efficient and larger scale preparations of [SNS][AsF6] and [(CNSNS)(2)][AsF6](2) are reported.