Sulfur–nitrogen compounds

   Sulfur–nitrogen chemistry is an area that has seen major developments over the last few decades, in part because of the conductivity of the polymer (SN)_x. The following discussion is necessarily selective, and more detailed accounts are listed at the end of the chapter. Probably the best known of the sulfur–nitrogen compounds is tetrasulfur tetranitride, S₄N₄. It has traditionally been obtained using reaction 1.1, but a more convenient method is reaction 1.2. Tetrasulfur tetranitride is a diamagnetic orange solid (mp 451 K) which explodes when heated or struck; pure samples are very sensitive. It is hydrolysed slowly by water (in which it is insoluble) and rapidly by warm alkali (equation 1.3).

                                             (1.1)

                          (1.2)

                                 (1.3)

     The structure of S₄N₄, 1.1, is a cradle-like ring in which pairs of S atoms are brought within weak bonding distance of one another (compare with ½S₈_2‏, Figure 15.7). The S^_N bond distances in S₄N₄ indicate delocalized bonding with π-contributions (compare the S^_N distances of 163pm with the sum of the S and N covalent radii of 178 pm). Transfer of charge from S to N occurs giving S^δ+‏__N^δ- polar bonds. A resonance structure for S₄N₄ that illustrates the cross-cage S^_S bonding interactions is shown in 1.2.

(1.1)                                                     (1.2)

    Figure 15.18 gives selected reactions of S4N4; some lead to products containing S_N rings in which the cross-cage interactions of S₄N₄ are lost. Reduction (at N) gives tetrasulfur tetraimide, S₄N₄H₄, which has a crown-shaped ring with equal S^_N bond lengths. Tetrasulfur tetraimide is one of a number of compounds in which S atoms in S8 are formally replaced by NH groups with retention of the crown conformation; S₇NH, S₆N₂H₂, S₅N₃H₃ (along with S₄N₄ and S₈) are all obtained by treating S₂Cl₂ with NH₃.   No members of this family with adjacent NH groups in the ring are known.

(1.3)                                     (1.4)

   Halogenation of S4N4 (at S) may degrade the ring depending on X₂ or the conditions (Figure 15.18).

Fig. 15.18 Selected reactions of S₄N₄; the rings in S₄N₄H₄ and S₄N₄F₄ are non-planar.

     The ring in S₄N₄F₄ has a puckered conformation quite different from that in S₄N₄H₄. Fluorination of S₄N₄ under appropriate conditions (Figure 15.18) yields thiazyl fluoride, NSF, 1.3, or thiazyl trifluoride NSF₃, 1.4, which contain SN triple bonds (see problem 15.25a at the end of the chapter). Both are pungent gases at room temperature, and NSF slowly trimerizes to S₃N₃F₃; note that S₄N₄F₄ is not made from the monomer. The structures of S₃N₃Cl₃ (1.5) and S₃N₃F₃ are similar. The rings exhibit only slight puckering and the S^_N bond distances are equal in S₃N₃Cl₃ and approximately equal in the fluoro analogue. Oxidation of S₄N₄ with AsF₅ or SbF₅ gives [S₄N₄][EF₆]₂ (E = As or Sb) containing [S₄N₄]²⁺‏. This has the planar structure 1.6 in many of its salts, but [S₄N₄]²⁺‏ can also adopt a planar structure with alternating bond distances, or a puckered conformation. The [S₄N₃]⁺‏ cation (prepared as shown in Figure 15.18) has the planar structure 1.7 with delocalized bonding.

(1.5)                                          (1.6)                                (1.7)

    The S₄N₄ cage can be degraded to S₂ N₂ (Figure 15.18) which is isoelectronic with [S₄]^2‏+,  S₂N₂ is planar with delocalized bonding (S^_N = 165 pm), and resonance structures are shown in 1.8. At roomtemperature, this converts to the lustrous golden-yellow, fibrous polymer (SN)_x, which can also be prepared from S₄N₄. The polymer decomposes explosively at 520 K, but can be sublimed in vacuo at ≈ 410 K. It is a remarkable material, being covalently bonded but showing metallic properties: a one-dimensional pseudo-metal. It has an electrical conductance about onequarter of that of mercury in the direction of the polymer chains, and at 0.3K it becomes a superconductor. However, the explosive nature of S₄N₄ and S₂N₂ limits commercial production of (SN)_x, and new routes to (SN)_x or related polymers are goals of current research. In the solid state, X-ray diffraction data indicate that the S^_N bond lengths in (SN)_x alternate (159 and 163 pm) but highly precise data are still not available; the closest interchain distances are nonbonding S S contacts of 350pm. Structure 1.9 gives a representation of the polymer chain and the conductivity can be considered to arise from the unpaired electrons on sulfur occupying a half-filled conduction band .

(1.8)

(1.9)

   The reactions of S₇NH with SbCl₅ in liquid SO₂, or S₃N₃Cl₃ with SbCl₅ and sulfur in SOCl₂, lead to the formation of the salt [NS₂][SbCl₆] containing the [NS₂]⁺‏ ion, (1.10) which is isoelectronic (in terms of valence electrons) with [NO₂]‏.

(1.10)