Do quaternary sulfur dications exist?

Question

We know that sulfur can form sulfides $\ce{R2S}$, with two substituents bonded to it. The simplest example of this would be hydrogen sulfide.

However, sulfur can also form sulfonium ions $\ce{R3S+}$, where 3 substituents are attached to the sulfur atom and a negatively-charged counteranion is present.

What I am asking is whether there is such a thing as sulfur bonded to 4 substituents, with each bond being a single bond, with 2 counteranions (either $\ce{(R4S^2+)(X^2-)}$ or $\ce{(R4S^2+)(X^-)2}$). Is there such a thing as that or something similar?

Answer 1

Ogawa et al. [1] were first to report a crystal structure (CSD-YAFNOI) of a compound with quaternary sulfur, bis(2,2′-biphenylylene)sulfurane:

Figure 1. Molecular structure of bis(2,2'-biphenylene)sulfurane (CSD-YAFNOI). Color code: $\color{#EEEEEE}{\Large\bullet}~\ce{H}$; $\color{#909090}{\Large\bullet}~\ce{C}$; $\color{#FFFF30}{\Large\bullet}~\ce{S}$.

Compound 1 was synthesized as follows (Scheme 1). Dibenzothiophene 5-oxide (200 mg, 1.0 mmol) in anhydrous tetrahydrofuran (THF, 10 ml) was treated with trimethylsilyl trifluoromethanesulfonate (0.25 ml, 1.3 mmol) under an $\ce{N2}$ atmosphere at −78 °C. After stirring at 0 °C for 30 min, the mixture was cooled to −78 °C and was treated with $\pu{1.0 mol dm-3}$ 2,2'-dilithiobiphenyl (1.0 ml, 1.0 mmol) in diethyl ether solution. The whole mixture was stirred at −78 °C for 1 h and at 0 °C for 30 min under an $\ce{N2}$ atmosphere. After evaporation of the solvent, the residue was washed with anhydrous diethyl ether (10 ml) and was extracted with anhydrous benzene (10 ml) under an $\ce{N2}$ atmosphere. The solvent was removed under reduced pressure, and the crude product was recrystallized from anhydrous THF at −20 °C to give 1 as orange rods in 96% yield.

Scheme 1 Reagents: i, trimethylsilyl trifluoromethansulfonate in THF; ii, 2,2'-dilithiobiphenyl in diethyl ether-THF

Further work by Sato et al. [2] resulted in a synthesis and crystal structure (CSD-NEDCEE) of bis(2,2′-biphenylylene)sulfuranyl bis(tetrafluoroborate).

Structurally, it's a similar compound with a greater, nearly 90° (in contrast to 60° twist angle in neutral bis(2,2′-biphenylylene)sulfurane), twist angle between 2,2′-biphenylylene ligands, however water molecules and $\ce{[BF4]}$-counterions appear heavily disordered:

Figure 2. Fragment of the molecular structure of bis(2,2′-biphenylylene)sulfuranyl bis(tetrafluoroborate) (CSD-NEDCEE) showing the bis(2,2′-biphenylylene)sulfuranyl cation. Oxygen atoms from water molecules as well as tetrafluoroborate anions are omitted for clarity. Color code: $\color{#EEEEEE}{\Large\bullet}~\ce{H}$; $\color{#909090}{\Large\bullet}~\ce{C}$; $\color{#FFFF30}{\Large\bullet}~\ce{S}$.

Recently, we have succeeded in the first isolation and structural determination of bis(2,2′-biphenylylene)sulfurane [10-S-4(C4)] (1) as a stable sulfurane(IV) having only carbon ligands.[…] We considered that this sulfurane would be a suitable precursor to provide the desired dication. Therefore, we tried the reaction of bis(2,2′-biphenylylene)sulfurane (1) with xenon difluoride ($\ce{XeF2}$) in the presence of $\ce{BF3 * OEt2}$ and indeed obtained the bis(2,2′-biphenylylene)sulfurane dication, [8-S4(C4)]²⁺ (2) as an amazingly stable bis(tetrafluoroborate) salt.[…] Here, we communicate the first isolation and structural determination of bis(2,2′-biphenylylene)sulfurane dication (2) having only carbon ligands. […]

The sulfurane 1 was reacted with 1 mol equiv of xenon difluoride in the presence of $\ce{BF3 * OEt2}$ in dry $\ce{CH3CN}$ at −40 °C (Scheme 1). After the solvent was removed at room temperature, the residue was washed with $\ce{CHCl3}$ at room temperature, and bis(2,2′-biphenylylene)sulfurane bis(tetrafluoroborate) (2) was isolated as a stable moisture-insensitive yellow powder in 62% yield.

Scheme 1

Subsequently, hexacoordinated derivatives – bis(2,2′-biphenylylene)dimethyl- and diphenylpersulfuranes – were synthesized and their molecular structures were elucidated [3].

References

1. Ogawa, S.; Matsunaga, Y.; Sato, S.; Iida, I.; Furukawa, N. First Preparation of a Sulfurane with Four Carbon–Sulfur Bonds: Synthesis and Molecular Structure of Bis(2,2′-Biphenylylene)Sulfurane. J. Chem. Soc., Chem. Commun. 1992, 0 (16), 1141–1142. https://doi.org/10.1039/C39920001141.
2. Sato, S.; Ameta, H.; Horn, E.; Takahashi, O.; Furukawa, N. First Isolation and Molecular Structure of Bis(2,2′-Biphenylylene)Sulfuranyl Bis(Tetrafluoroborate) [8−S−4(C4)]²⁺. J. Am. Chem. Soc. 1997, 119 (50), 12374–12375. https://doi.org/10.1021/ja971336k.
3. Sato, S.; Matsunaga, K.; Horn, E.; Furukawa, N.; Nabeshima, T. Isolation and Molecular Structure of the Organo-Persulfuranes [12−S−6(C6)]. J. Am. Chem. Soc. 2006, 128 (21), 6778–6779. https://doi.org/10.1021/ja060497y.