Homoleptic azido compounds are chemical compounds in which the only anion or ligand is the azide group, -N3. The breadth of homoleptic azide compounds spans nearly the entire periodic table. With rare exceptions azido compounds are highly shock sensitive and need to be handled with the upmost caution. Binary azide compounds can take on several different structures including discrete compounds, or one- two, and three-dimensional nets, leading some to dub them as "polyazides". Reactivity studies of azide compounds are relatively limited due to how sensitive they can be. The sensitivity of these compounds tends to be correlated with the amount of ionic or covalent character the azide-element bond has, with ionic character being far more stable than covalent character. Therefore, compounds such as silver or sodium azide – which have strong ionic character – tend to possess more synthetic utility than their covalent counterparts. A few other notable exceptions include polymeric networks which possess unique magnetic properties, group 13 azides which unlike most other azides decompose to nitride compounds (important materials for semiconductors), other limited uses as synthetic reagents for the transfer for azide groups, or interest in high energy density materials.
Synthesis
There are several general routes and strategies employed when synthesizing homoleptic azido compounds. Salt metathesis between an azide salt like sodium azide or silver azide and the metal chloride is how a lot of the earlier azides were prepared. Another popular route include acid-base reactions hydrazoic acid HN3 and either hydrido or lewis base complexes. However, modern methods often rely on halide-azide exchange with trimethylsilyl azide SiMe3N3 with the metal fluorides as incomplete halide/azide exchange is often seen when using the chloride derivatives.
Neutral binary azides are rather difficult to study due to the fluxional nature of the azido ligands and their lack of thermal and shock stability. Their lack of stability is in part due to the covalent binding of the azido ligand to the metal center which favors a single and a triple bond in the azide moiety. Increasing the ionic character of the azido group – either by the introduction of anion formation or N-donor adducts – favors two double bonds instead. This ionic bonding motif therefore increases the activation barrier for breaking of the N-N bond to release N2 and helps to stabilize the compounds. The resulting compounds can still be highly shock sensitive and need to be handled with caution.
Transition metals
Group 4
Group 4 polyazides of the formula M(N3)4 are predicted to have linear or near linear M-N-N angles unlike their main group counterparts which are predicted to have bent M-N-N angles. This couldn’t be proved in the case of Ti(N3)4, owing to difficulty in crystallization. However, incorporation of large spacer counterions or N-donor adducts makes the compounds far easier to work with. In the cases of [PPh4]2[M(N3)6] (M=Ti, Zr, Hf), only the axial ligands exhibit near linear M-N-N angles whereas the equatorial ligands are closer to bent angles. This deviation in theory is also seen in the N-donor adducts.
The main hypothesis given for why these compounds do not have linear M-N-N angles despite theoretical calculations is that these adducts are not tetrahedral. In the homoleptic tetrahedral compounds, the nitrogen closest to the (+IV) metal center is positioned in such a way that the three valence electron pairs can donate to the vacant d orbitals on the metal and therefore the azido can act as a tridentate donor ligand in which case the expected coordination would be linear. Since the adduct compounds are not tetrahedral, the azido group can only act as a monodentate donor with two sterically active electron pairs which result in a bent M-N-N bond angles.
Group 5
The neutral binary V(IV) azide as well as V(III), V(IV), and V(V) azido ions are known. Similar to the neutral Ti(IV) azide, V(N3)4 is difficult to study due to high shock and temperature instability. However, [V(N3)6]2- paired with a large, inert counterion is relatively stable and crystalizeses as a near perfect octahedral. In contrast to V(IV), the neutral binary V(V) could not be synthesized and attempts result in the reduction of V(V) to V(IV) with the elimination of N2 gas. Fortunately, the oxidation potentials of anions are lower than that of their parent compounds so [V(N3)6]- can be formed. Unlike [V(N3)6]2-, [V(N3)6]- is highly shock sensitive and distorted from octahedral symmetry with three long and three short M-N bonds in mer positions.