Experimental and Computational Study on the Structure and Properties of Herz Cations and Radicals: 1,2,3-Benzodithiazolium, 1,2,3-Benzodithiazolyl, and Their Se Congeners

Salts of 1,2,3-benzodithiazolium (1), 2,1,3-benzothiaselenazolium (3), and 1,2,3-benzodiselenazolium (4) (Herz cations), namely, [1][BF4], [1][SbCl6], [3][BF4], [3][GaCl4], [3][SbCl6], and [4][GaCl4], were prepared from the corresponding chlorides and NaBF4, GaCl3, or SbCl5. It was found that [1][SbCl6] and [3][SbCl6] spontaneously transform in MeCN solution to [1]3[SbCl6]2[Cl] and [3]3[SbCl6]2[Cl], respectively. [1][BF4], [1]3[SbCl6]2[Cl], [3][BF4], [3]3[SbCl6]2[Cl], and [4][GaCl4] were structurally characterized by X-ray diffraction (XRD). In solution, these [BF4](-) and [GaCl4](-) salts as well as [1][GaCl4], [2][GaCl4], [3][GaCl4], [3][Cl], and [4][Cl] were characterized by multinuclear nuclear magnetic resonance (NMR). The corresponding Herz radicals 1(•)-4(•) were obtained in toluene and DCM solutions by the reduction of the appropriate salts with Ph3Sb and characterized by EPR. Cations 1-4 and radicals 1(•)-4(•) were investigated computationally at the density functional theory (DFT) and second-order Møller-Plesset (MP2) levels of theory. The B1B95/cc-pVTZ method was found to satisfactorily reproduce the experimental geometries of 1-4; an increase in the basis set size to cc-pVQZ results in only minor changes. For both 1-4 and 1(•)-4(•), the Hirshfeld charges and bond orders, as well as the Hirshfeld spin densities for the radicals, were calculated using the B1B95/cc-pVQZ method. It was found for both the cations and the radicals that replacing S atoms with Se atoms leads to considerable changes in the atomic charges, bond lengths, and bond orders only at the involved and the neighboring sites. According to the calculations, 60% of the positive charge in the cations and 80% of the spin density in the radicals is localized on the heterocycles, with the spin density distributions being very similar for all radicals 1(•)-4(•). For the cations 1-4, the NICS values (B3LYP/cc-pVTZ for B1B95/cc-pVTZ geometries) lie in the narrow range from -5.5 ppm to -6.6 ppm for the carbocycles, and from -14.4 ppm to -15.5 ppm for heterocycles, clearly indicating the aromaticity of the cations. Calculations on radical dimers [1(•)]2-[4(•)]2 revealed, with only one exception, positive dimerization energies, i.e., the dimers are inherently unstable in the gas phase.