Q6(c). Tetrasulfur tetranitride: preparation, structure, action of chlorine

Preparation

1. Ammonia is passed into a solution of sulfur dichloride (or disulfur dichloride) in an inert solvent (CCl₄): 3pt] 6SCl₂ + 16NH₃ → S₄N₄ + 1/4S₈ + 12NH₄Cl (or 6S₂Cl₂ + 16NH₃ → S₄N₄ + S₈ + 12NH₄Cl) 3pt] NH₄Cl is filtered off and S₄N₄ crystallised as orange-yellow crystals.
2. S₄N₄ is endergonic (Δ_fG°=+536 kJ mol⁻¹) and may detonate on shock or heating; it is handled in small amounts.

Structure

1. An eight-membered S–N ring, cradle-shaped: the four N atoms lie in one plane (a square); the four S atoms form a flattened tetrahedron, two above and two below the plane.
2. The two S atoms on each side are only 258 pm apart, far less than the van der Waals contact: a weak transannular S···S bond. All S–N bonds are equal, so the π electrons are delocalised round the ring.
3. Lewis acids (BF₃, SbF₅, SO₃) form 1:1 adducts through one N atom, and the ring rearranges.

Action of chlorine

1. Chlorine passed into S₄N₄ in CCl₄ oxidises and cleaves the cage; the fragments rearrange to trithiazyl trichloride: 3pt] 3S₄N₄ + 6Cl₂ → 4(NSCl)₃ 3pt] Check: S 12 = 12, N 12 = 12, Cl 12 = 12.
2. (NSCl)₃ (S₃N₃Cl₃) is a six-membered ring of alternating S and N in a chair form, one Cl on each S (S(IV)). On warming it dissociates to monomeric thiazyl chloride, NSCl.

Application. S₄N₄ is the starting point of sulfur–nitrogen chemistry: passed over hot silver wool it gives S₂N₂, which polymerises to (SN)ₓ, a metallic polymer that becomes superconducting at very low temperature.