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.