Sulfur fluorides and oxofluorides

The fluorides SF₄ and S₂F₂ can be prepared from the reaction of SCl₂ and HgF₂ at elevated temperatures; both are highly unstable. Disulfur difluoride exists as two isomers; S₂F₂ (1.1) and F₂S=S (1.2), with S₂F₂ (made from AgF and S at 398 K) readily isomerizing to F₂S=S. The structure of S₂F₂ is like that of O₂F₂, with an internal dihedral angle of 888. The S^_S bond distances in both isomers are very short (compare ≈ 206pm for a single S^_S bond) and imply multiple bond character. For S₂F₂, contributions from resonance structures analogous to those shown for O₂F₂ are therefore important. Both isomers are unstable with respect to disproportionation into SF₄ and S, and are extremely reactive, attacking glass and being rapidly hydrolysed by water and alkali (e.g. equation 1.1).

(1.1)                         (1.2)

        (1.1)

    Sulfur tetrafluoride, SF4, is best prepared by reaction 1.2.  It is commercially available and is used as a selective fluorinating agent, e.g. it converts carbonyl groups into CF₂ groups without destroying any unsaturation in the molecule.   Representative reactions are shown in Figure 1.1; SF4 hydrolyses rapidly and must be handled in moisture-free conditions.

Fig. 1.1 Selected reactions of sulfur tetrafluoride.

           (1.2)

   The structure of SF₄, 1.3, is derived from a trigonal bipyramid and can be rationalized in terms of VSEPR theory. The S^_{_}F_ax and S^_F_eq bond distances are quite different (Table 1.). Oxidation by O₂ in the absence of a catalyst to form SOF₄ is slow. The structure of SOF₄, 1.4, is related to that of SF₄, but with S^_F_ax and S^_F_eq bond distances that are close in value.

(1.3)                   (1.4)                        (1.5)

     Among the sulfur fluorides, SF₆, 1.5, stands out for its high stability and chemical inertness. It can be made by burning S in F₂, and is commercially available, being widely used as an electrical insulator. Its lack of reactivity (e.g. it is unaffected by steam at 770K or molten alkalis) is kinetic rather than thermodynamic in origin. The value of Δ_rG^o for reaction 1.3 certainly indicates thermodynamic spontaneity. The bonding in SF₆ was discussed in Section 4.7.

       )1.3(

    The preparation of SF6 from S and F₂ produces small amounts of S₂F₁₀ and the yield can be optimized by controlling the reaction conditions. An alternative route is reaction 1.4. Selected properties of S₂F₁₀ are given in Table 1..

                                       )          1.4)

(1.6)

     Molecules of S₂F₁₀ have the staggered structure 1.6; the S^_S bond length of 221pm is significantly longer than the single bonds in elemental S (206 pm). It disproportionates when heated (equation 1.5) and is a powerful oxidizing agent. An interesting reaction is that with NH₃ to yield NSF₃ .

   )                                                                           1.4)

Many compounds containing SF₅ groups are now known, including SClF₅ and SF₅NF₂ (Figure 1.1). In accord with the relative strengths of the S^_Cl and S^_F bonds, reactions of SClF₅ usually involve cleavage of the S^_Cl bond (e.g. reaction 1.6).

                        (1.6)

(1.7)

   Sulfur forms several oxofluorides, and we have already mentioned SOF₄. Thionyl fluoride (or sulfinyl fluoride), SOF₂ (1.7), is a colourless gas (bp 229 K), prepared by fluorinating SOCl₂ using SbF₃. It reacts with F₂ to give SOF₄, and is slowly hydrolysed by water .The reaction of SOF₂ and [Me₄N]F at 77K followed by warming to 298K produces [Me₄N][SOF₃], the first example of a salt containing [SOF₃]^-. The anion rapidly hydrolyses (reaction 1.7 followed by reaction 1.8 depending on conditions) and reacts with SO₂ to give SOF₂ and [SO₂F] ^-.

                 (1.7)

                   (1.8)

  Sulfuryl fluoride (or sulfonyl fluoride), SO₂F₂ (1.8), is a colourless gas (bp 218 K) which is made by reaction 1.9 or 1.10.

                             (1.9)

                                              (1.10)

(1.8)

   Although unaffected by water, SO₂F₂ is hydrolysed by concentrated aqueous alkali. A series of sulfuryl fluorides is known, including FSO₂OSO₂F and FSO₂OOSO₂F. The latter compound is prepared by reaction 1.11; fluorosulfonic acid  is related to the intermediate in this reaction.

                             (1.11)

  The dissociation of FSO₂OOSO₂F at 393K produces the brown paramagnetic radical FSO₂O^. , selected reactions of which are shown in scheme 1.12.

                (1.12)

  The reaction of F₂ with sulfate ion yields [FSO₄] ^- which can be isolated as the caesium salt and is an extremely powerful oxidizing agent (equation 1.13).

             (1.13)