Thiophosphoryl fluoride

Preparation

Thiophosphoryl fluoride was discovered and named by J. W. Rodger and T. E. Thorpe in 1888.

They prepared it by heating arsenic trifluoride and thiophosphoryl chloride together in a sealed glass tube to 150 °C. Also produced in this reaction was silicon tetrafluoride and phosphorus fluorides. By increasing the PSCl₃ the proportion of PSF₃ was increased. They observed the spontaneous inflammability. They also used this method:

3 PbF₂ + P₂S₅ → 3 PbS + PSF₃

at 170 °C, and also substituting a mixture of red phosphorus and sulfur, and substituting bismuth trifluoride.

Another way to prepare PSF₃ is to add fluoride to PSCl₃ using sodium fluoride in acetonitrile.

A high yield reaction can be used to produce the gas:

P₄S₁₀ + 1 2HF → 6 H₂S + 4 PSF₃

Under high pressure phosphorus trifluoride can react with hydrogen sulfide to yield:

PF₃ + H₂S → PSF₃ + H₂ (100 megabars at 200 °C)

Another high pressure production uses phosphorus trifluoride with sulfur.

Reactions

PSF₃ is decomposed by moisture and oxygen or heat. With heat, phosphorus, sulfur and phosphorus fluorides are formed:

PSF₃ → PF₃ + S

The hot gas reacts with glass producing SF₄, sulfur and elemental phosphorus. The pure gas is completely absorbed by alkali solutions. However, it does not react with ether, benzene, carbon disulfide, or pure sulfuric acid. It is stable against CaO, which can be used to remove impurities such as SiF₄ and PF₃. In air it burns spontaneously with a greyish green flame, producing solid white fumes. With dry oxygen combustion may not be spontaneous and the flame is yellow. On burning SO₂ and P₂O₅ are produced. The gas burns with one of the coldest flames known.

Reaction with water is slow:

PSF₃ + 4H₂O → H₂S + H₃PO₄ +3 HF

If PSF₃ is allowed to react with water in a lead glass container, the hydrofluoric acid and hydrogen sulfide combination produces a black deposit of lead sulfide on the inner surface of the glass.

It reacts with four times its volume of ammonia gas producing ammonium fluoride and a mystery product, possibly P(NH₂)₂SF.

PSF₃ is an initiator for the polymerization of tetrahydrofuran.

Sulfur removal

2 PSF₃ + SO₂ → 2 POF₃ + 3 S

This reaction indicates why PSF₃ is not formed from PF₃ and SO₂.

PSF₃ + SO₃ → POF₃ + S₂ and sulfur and sulfur sesquioxide (S₄²⁺ polysulfate) as additional products.

Fluorine substitution

PSF₃ + 2ICl → PCl₂F₃.PSF₃ combines with dimethylamine in solution to produce dimethylaminothiophosphoryldifluoride (CH₃)₂NPSF₂ and difluorophosphate and hexafluorophosphate ions.

Thiophosphoryl difluoride isocyanate can be formed by reacting PSF₃ with silicontetraisocyanate at 200 °C in an autoclave.

Cations

PSF₃ reacts with alkali solutions producing the fluoride and a thiophosphate (PSO₃³⁻).CsF + 2PSF₃ → CsPF₆ + CsF₂PS₂ (a thiophosphate). Showing the gas is related to the fluorothiophosphate cation :PF₂S₂⁻.PSF₄⁻ formed by reaction with SF₆⁻

Related compounds

One fluorine can be substituted by iodine to give iodothiophosphoryldifluoride PSIF₂. PSIF₂ can be converted to hydrothiophosphoryldifluoride PSHF₂ by reducing it with hydrogen iodide. In F₂PSSPF₂ one sulfur forms a bridge between two phosphorus atoms.

Physical properties

The thiophosphoryl trifluoride molecule shape has been determined using electron diffraction. The interatomic distances are PS 0.187±0.003 nm, PF 0.153±0.002 nm and bond angles ∠FPF=100.3±2 °C, Microwave rotational spectrum has line values 2657.63±0.04 for PS³²F₃, 2614.73±0.04 for PS³³F₃, 2579.77±0.04 for PS³⁴F₃ MC/sec.

The critical point is at 346K at 3.82 MPa. The liquid refractive index is 1.353

The enthalpy of vapourisation 19.6 kJ/mol at boiling point. Enthalpy of vapourisation at other temperatures H=28.85011(346-T)^0.38 kJ/mol

The molecule is polar. It has a non-uniform distribution of positive and negative charge which gives it a dipole moment. When an electric field is applied more energy is stored than if the molecules did not respond by rotating. This increases the dielectric constant. The dipole moment of one molecule of thiophosphoryl trifluoride is 0.640 Debye

Surface tension of the liquid is 26.3 dyne/cm.

The infrared spectrum includes vibrations at 275, 404, 442, 698, 951 and 983 cm⁻¹. These can be used to identify the molecule.