Sulfur tetrafluoride
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Through Sulfur, Reagent and Oxidation state
Structure
Sulfur in SF4 is in the +4 oxidation state, with one lone pair of electrons. The atoms in SF4 are arranged in a see-saw shape, with the sulfur atom at the center. One of the three equatorial positions is occupied by a nonbonding lone pair of electrons. Consequently, the molecule has two distinct types of F ligands, two axial and two equatorial. The relevant bond distances are S–Fax = 164.3 pm and S–Feq = 154.2 pm. It is typical for the axial ligands in hypervalent molecules to be bonded less strongly.
The 19F NMR spectrum of SF4 reveals only one signal, which indicates that the axial and equatorial F atom positions rapidly interconvert via pseudorotation.
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Synthesis and manufacture
At the laboratory scale, sulfur tetrafluoride is prepared from elemental sulfur and cobaltic fluoride
- S + 4CoF3 → SF4 + 4CoF2
SF4 is industrially produced by the reaction of SCl2 and NaF with acetonitrile as a catalyst
- 3 SCl2 + 4 NaF → SF4 + S2Cl2 + 4 NaCl
At higher temperatures (e.g. 225–450 °C), the solvent is superfluous. Moreover, sulfur dichloride may be replaced by elemental sulfur (S) and chlorine (Cl2).
A low-temperature (e.g. 20–86 °C) alternative to the chlorinative process above uses liquid bromine (Br2) as oxidant and solvent:
- S(s) + 2 Br2(l; excess) + 4KF(s) → SF4↑ + 4 KBr(brom)
Use in synthesis of organofluorine compounds
In organic synthesis, SF4 is used to convert COH and C=O groups into CF and CF2 groups, respectively. The efficiency of these conversions are highly variable.
In the laboratory, the use of SF4 has been superseded by the safer and more easily handled diethylaminosulfur trifluoride, (C2H5)2NSF3, "DAST": This reagent is prepared from SF4:
- SF4 + (CH3)3SiN(C2H5)2 → (C2H5)2NSF3 + (CH3)3SiF
Other reactions
Sulfur chloride pentafluoride (SF
5Cl), a useful source of the SF5 group, is prepared from SF4.
- SF4 + Cl2 + CsF → SF5Cl + CsCl
Hydrolysis of SF4 gives sulfur dioxide:
- SF4 + 2 H2O → SO2 + 4 HF
This reaction proceeds via the intermediacy of thionyl fluoride, which usually does not interfere with the use of SF4 as a reagent.
When amines are treated with SF4 and a base, aminosulfur difluorides result.
Toxicity
SF
4 reacts inside the lungs with moisture, forming sulfur dioxide and hydrogen fluoride which forms highly toxic and corrosive hydrofluoric acid