Hydrogen azide and azide salts

   Sodium azide, NaN₃, is obtained from molten sodium amide by reacting NaNH₂ with NaNO₃ at 450 Kand treatment of NaN₃ with H₂SO₄ yields hydrogen azide, HN₃.

     Hydrogen azide (hydrazoic acid) is a colourless liquid (mp 193 K, bp 309 K); it is dangerously explosive (Δ_fH^o(l, 298 K) = ‏264 kJ mol^-1) and highly poisonous. Aqueous solutions of HN₃ are weakly acidic.

   The structure of HN₃ is shown in Figure 1.1a, and a consideration of the resonance structures in Figure 1.1b provides an explanation for the asymmetry of the NNN-unit. The azide ion is isoelectronic with CO₂, and the symmetrical structure of [N₃]^- (Figure 1.1c) is consistent with the bonding description in Figure 1.1d. A range of azide salts is known; Ag(I), Cu(II) and Pb(II) azides, which are insoluble in water, are explosive, and Pb(N₃)₂ is used as an initiator for less sensitive explosives. On the other hand, group 1 metal azides decompose quietly when heated. The reaction between NaN₃ and Me3SiCl yields the covalent compound Me3SiN3 which is a useful reagent in organic synthesis. Reaction below occurs when Me₃SiN₃ is treated with [PPh₄]‏[N₃] ^- in the presence of ethanol.

   The [N₃HN₃]^- anion in the product is stabilized by hydrogen bonding (compare with [FHF]^- . Although the position of the H atom in the anion is not known with great accuracy, structural parameters for the solid state structure of [PPh₄][N₃HN₃] (Figure 1.2) are sufficiently accurate to confirm an asymmetrical N^_H^……N interaction (N^……N = 272 pm).

  The azide group, like CN^. (though to a lesser extent), shows similarities to a halogen and is another example of a pseudohalogen. However, no N₆ molecule (i.e. a dimer of N₃^- and so an analogue of an X₂ halogen) has yet been prepared. Like halide ions, the azide ion acts as a ligand in a wide variety of metal complexes, e.g. [Au(N₃)₄]^-, trans-[TiCl₄(N₃)₂]^2-, cis-[Co(en)₂(N₃)₂]‏, trans-[Ru(en)₂(N₂)(N₃)]‏ (which is also an example of a dinitrogen complex, Figure 1.3a) and [Sn(N₃)₆]^2- (Figure 1.3b).

   The reaction of HN₃ with [N₂F][AsF₆]  in HF at 195K results in the formation of [N₅][AsF₆]. Designing the synthesis of [N₅]⁺‏ was not trivial. Precursors in which the N≡N and N=N bonds are preformed are critical, but should not involve gaseous N₂ since this is too inert. The HF solvent provides a heat sink  for the exothermic reaction, the product being potentially explosive. Although [N₅][AsF₆] was the first example of a salt of [N₅]‏⁺ and is therefore of significant interest, it is not very stable and tends to explode. In contrast, [N₅][SbF₆]  is stable at 298K and is relatively resistant to impact. Solid [N₅][SbF₆] oxidizes NO, NO₂ and Br₂ (scheme below), but not  Cl₂ or O₂.

   The reaction of [N₅][SbF₆] with SbF5 in liquid HF yields [N₅][Sb₂F₁₁], the solid state structure of which has been determined, confirming a V-shaped [N₅]⁺‏ ion (central N^_N^_N angle =1118). The N^_N bond lengths are 111pm (almost the same as in N₂) and 130pm (slightly more than in MeN=NMe), respectively, for the terminal and central bonds. Resonance stabilization is a key  factor in the stability of [N₅]⁺‏ and provides a degree of multiple-bond character to all the N^_N bonds.   

    The three resonance structures shown in blue contain one or two terminal sextet N atoms. Their inclusion helps to account for the observed N_terminal^_N^_N_central bond angles of 1688.

   The reaction of sodium azide with aryldiazonium salts yields arylpentazoles (equation below), from which it has been possible to generate the cyclic anion [N₅]^- through molecular fragmentation in an electrospray ionization mass spectrometer.

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