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.