Characterization of the diradical •NSNSC-CNSSN• and [NSNSC-CNSSN][MF6]n (n=1, 2).: The first observation of an excited triplet state in dimers of 7π -CNSSN• radicals

Preparation and full characterization of the main-group diradical (NSNSC)-N-center dot-CNSSN center dot, 8, the MF6- salt (As, Sb) of radical cation +NSNSC-CNSSN center dot, 8(center dot+), and the AsF6- salt of the dication +NSNSC-CNSSN+, 8(2+), are presented. 8, a = 6.717 (4), b = 11.701(2), c = 8.269(3) A, alpha = gamma = 90, beta = 106.69(3)degrees, monoclinic, space group P2(1)/n, Z = 4, T = 203 K; 8SbF(6), a = 6.523(2), b = 7.780(2), c = 12.012(4) A, alpha = 91.994(4), beta = 96.716(4), gamma =109.177(4)degrees, triclinic, space group P (1) over bar, Z = 2, T = 198 K; 8[AsF6](2), a = 12.7919(14), b = 9.5760(11), c = 18.532(2) A, alpha = gamma = 90, beta = 104.034(2)degrees, monoclinic, space group Pn, Z = 6, T = 198 K. Preparation of 8MF(6) was carried out via a reduction of [CNSNS](2)[MF6](2) (M = As, Sb) with either ferrocene or a SbPh3-NBu4Cl mixture. In the solid state, diamagnetic 8SbF(6) contains centrosymmetric dimers [8(center dot+)](2) linked via two-electron four-centered pi*-pi* interactions with a thermally excited triplet state as detected by electron paramagnetic resonance (EPR). This is the first observation of a triplet excited state for a 7 pi 1,2,3,5-dithiadiazolyl radical dimer. The singlet-triplet gap of the [-CNSSN center dot](2) radical pair was -1800 +/- 100 cm(-1) (-22 +/- 1 kJ/mol) with the ZFS components parallel to D parallel to = 0.0267(6) cm(-1) and parallel to E parallel to = 0.0012(1) cm(-1), corresponding to an in situ dimerization energy of ca. -11 kJ/mol. Cyclic voltammetry measurements of 8[AsF6](2) showed two reversible waves associated with a stepwise reduction of the two isomeric rings [E-1/2 (+2/+1) = 1.03 V; E-1/2 (+1/0) = 0.47 V, respectively]. 8MF(6) (M = As, Sb) was further reduced to afford the mixed main-group diradical 8, containing two isomeric radical rings. In solution, 8 is thermodynamically unstable with respect to (NSSNC)-N-center dot-CNSSN center dot, but is isolable in the solid state because of its low solubility in SO2. Likewise, 8SbF(6), 8 is dimeric, with pi*-pi* interactions between different isomeric rings, and consequently diamagnetic; however, a slight increase in paramagnetism was observed upon grinding [from C = 6.5(3) x 10(-4) emu center dot K/mol and temperature-independent paramagnetism (TIP) = 1.3(1) x 10(-4) emu/mol to C = 3.2(1) x 10(-3) emu center dot K/mol and TIP = 9.0(1) x 10(-4) emu/mol], accompanied by an increase in the lattice-defect S = (1)/(2) sites [from 0.087(1) to 0.43(1)%]. Computational analysis using the multiconfigurational approach [CASSCF(6,6)/6-31G*] indicated that the two-electron multicentered pi*-pi* bonds in [8(center dot+)](2) and [8](2) have substantial diradical characters, implying that their ground states are diradicaloid in nature. Our results suggest that the electronic structure of organic-radical ion pairs, for example, [TTF center dot+](2), [TCNE center dot-](2), [TCNQ(center dot-)](2), [DDQ(center dot-)](2), and related pi dimers, can be described in a similar way.