User:Dangerousnoodles/Homoleptic multiple azide compounds

Lead
Binary polazides are compounds of the general formula M(N3)x where x>2. Without exception polyazides are highly shock sensitive compounds and need to be handled with the upmost caution. These compounds are candidates for high energy density materials. Binary azide compounds can take on several different structures including discrete compounds, or one- two, and three-dimensional nets.1

Synthesis
Incomplete halide/azide exchange is often seen when using the chloride derivatives instead of the fluorides.2

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.2 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.2, 3 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. It should be noted that the resulting compounds can still be highly shock sensitive and need to be handled with caution.

Group 2
A true polyazide of Ca(II) is known, and can be prepared from the reaction of Ca(N3)2 with [N(CH3)4]N3 in aqueous solution.4 The resulting compound [N(CH3)4][Ca(N3)3] forms a 3-dimensional network with [N(CH3)4]+ ions in the interstitial spaces. This structure is analogous to the related manganese one discussed below.

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.5 This couldn’t be proved in the case of Ti(N3)4, owing to difficulty in crystallization.6 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.6, 7 This deviation in theory is also seen in the N-donor adducts.3

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.6 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.2, 8 Similar to the neutral Ti(IV) azide, V(N3)4 is difficult to study due to high shock and temperature instability.2 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.

The neutral binary Nb(N3)5 and Ta(N3)5 also exist, and the acetonitrile adducts of these compounds contain a nearly linear azido trans to the coordinating acetonitrile.9 They represent the first evidence of linear M-N-N bonding. The corresponding anions [Nb(N3)6]-, [Nb(N3)7]2-, [Ta(N3)6]-, and [Ta(N3)7]2- are known and accordingly are much less shock sensitive.9, 10 The structure of the hexaazido monoanions are similar to other heptaazido monoanions with bent azido ligands despite being predicted to have perfect S6 symmetry in the gas phase for [Nb(N3)6].9 The heptaazido dianions possess monocapped triangular-prismatic 1/4/2 structures unlike the actinide trianion [U(N3)7]3- which crystallizes as a monocapped octahedron or pentagonal bipyramid.10 Several N-donor adducts are known to exist as well.11 Reactions of the neutral binary NbF5 and TaF5 in the presence of Me3SiN3 with N-donors containing small bite angles such as 2,2’-bipyridine or 1,10-phenanthroline result in self ionization products of the type [M(N3)4L2]+[M(N3)6]-(L= N-donor) whereas N-donors containing large bite angles such as 3,3’-bipryidine or 4,4’-bipyridine produces the neutral pentaazide adducts M(N3)5•L (L=N-donor).

Group 6
Both Mo(N3)6 and W(N3)6 have been synthesized, and W(N3)6 is stable enough to grow single crystals.12 Contrary to group 4 and group 5 binary azido compounds, the anionic [Mo(N3)7]- and [W(N3)7]- are less stable and more sensitive to handle than their neutral parent compounds. Upon warming solutions of the heptaazido anions in either MeCN or SO2 to room temperature, the tetraazido nitrido ions [NMo(N3)4]- and [NW(N3)4]- are formed with elimination of N2.

Group 7
Group 7 azide compounds are dominated by manganese chemistry. The first Mn polyazide compound was reported by Wöhler et al. in 1917 by reaction of MnCO3 with HN3 to form Mn(N3)2.13 Many divalent Mn salts have been synthesized and represent true polymeric systems. The azido moiety can bind as end-on (EO) (µ-1,1) or end-to-end (EE) (µ-1,3) to usually give ferromagnetic or antiferromagnetic coupling respectively.1, 14 1D chains are formed when 2,2’-bipyridine, a bidentate ligand, is used as the counter ion in the reaction between Mn(ClO4)2 • 6H2O and excess NaN3.15 This results in a chain with alternating EE and EO bridges which predictably gives alternating antiferromagnetic-ferromagnetic coupling. However, unfortunately except at absolute 0K one-dimensional systems show no magnetic ordering.16 Therefore, polymers of increasing dimensionality are of interest. A 2D system is formed upon reaction of MnCl2• 4H2O and NaN3 in the presence of 4,4’-dipyridino-N,N’diacetic acid which undergoes in situ  decarboxylation to afford alternating layers of [Mn(N3)4]2- with EE bridging azides and the 4,4’-dipyridine dication.17 The metal centers in this compound do show antiferromagnetic coupling but this is strictly not due to spin canting because of the uncommon centrosymmetry of the bridging azido ligands. Another 2D structure is accessed via the reaction of (PPh4)2MnCl2 with AgN3 to form the nonexplosive [PPh4]2[Mn(N3)4] which has alternating ion layers.18

The first example of a 3D azido compound was synthesized in the reaction of Mn(NO3)2 • 4H2O in hot aqueous [N(CH3)4][N3] saturated with HN3 to form [N(CH3)4][Mn(N3)3].19 This compound has a pseudo-perovskite structure with [N(CH3)4]+ ions in the cavities between the Mn centers. The azido moieties are arranged in an EE fashion, and indeed, this compound exhibits the expected antiferromagnetic behavior.16 The cesium analogue Cs[Mn(N3)3] is synthesized in the same manner but uses CsN3 instead of [N(CH3)4][N3] and is structurally unique from the tetramethylammonium version.14 For each 6 coordinate Mn, 4 of the azido linkages are EE and two are EO instead of all six being EE. This arrangement results in a honeycomb-like shape and a rare example of alternating ferro-antiferromagnetic interactions in 3D solid.

Examples of manganese azido compounds in higher oxidation states are relatively rare. The triazide acetonitrile adduct can be prepared using the fluoride exchange route to give Mn(N3)3CN as a dark red shock sensitive compound.20 Upon addition of PPh4N3 the compound disproportionates into an insensitive mixture of [PPh4]2[Mn(N3)2] and [PPh4]2[Mn(N3)6]. The Mn(IV) salt can be prepared on its own by using Cs2MnF6 as the starting material to give the highly explosive Cs2[Mn(N3)6].

Group 8
The first penta-coordinate azide and the ninth trigonal bipyramidal compound reported was the pentaazidoiron (III) ion [Fe(N3)5]2- and this compound can be made through either halid or nitrate elimination from an iron (III) starting material.21, 22 For applications however, iron azides tend to not be isolated but are instead generated in situ.23 NaN3 and iron (III) sulfate Fe2(SO4)3 are combined in methanol and added to an organoborane followed by slow addition of 30% hydrogen peroxide, presumably forming Fe(N3)3. When combined with alkenes, the azide will insert in an anti-markovnikov fashion.24 The role of the peroxide is not well understood but it is crucial for this reaction to occur.

A ruthenium tetrabutylammonium salt can be prepared by reacting K2[RuIVCl6] with NaN3 in ethanol and water.25 N2 gas is liberated and the reduced ruthenium (III) species [n-Bu4N]3[Ru(N3)6] is afforded. This compound has largely been studied in terms of its optical properties.

Group 9
Tetraazido cobalt(II) compounds have been isolated as both the tetraphenylphosphonium and tetraphenylarsonium salts from solutions of cobalt sulfate with a 15 time sexcess of NaN3 to yield [Ph4P]2[Co(N3)4] and [Ph4As]2[Co(N3)4] respectively.26 The autooxidation of solutions of  [Co(N3)4]2- can be used as a colormetric spot test for the presence of suflite ions 27

Tetrabutylammonium salts of rhodium(III) and iridium(III) azides are known and are prepared by reacting a large excess of NaN3 in an aqueous solution with the corresponding Na3[MCl6] • 12H2O metal chloride salt to form [n-Bu4N]3[Rh(N3)6] and [n-Bu4N]3[Ir(N3)6].25

Group 10
Nickel azide can be prepared by distilling HN3 onto nickel carbonate and precipitated with acetone to afford Ni(N3)2.28Sample of Ni(N3)2 will begin to rapidly decompose upon heating to 490K by roughly 35% followed by slow decomposition of the remaining material. It is thought that the first phase of decomposition results in microcrystals with metallic nickel on the outside and Ni(N3)2 at the core. The reaction slows because now decomposition must occur at the Ni/Ni(N3)2 interface.

[Pd(N3)4]2- anions are square planar and the degree of interaction between the anion and its corresponding cation can be determined by the amount of deviation in the torsion angles from the ideal geometry.29 Various platinates [Pt(N3)4]2- and [Pt(N3)6]4- are known and are prepared from Pt chloride salts with NaN3.25 Pt(II) salts tend to be far less stable than the Pt(IV) versions and either decompose fairly rapidly upon standing or explode.30 Their sensitivity in part has been explained by poor crystal packing.29

Group 11
The copper(I) binary azide CuN3 is a 1D ionic compound with chains that run diagonally to the unit cell.31 Many Copper (II) azides ions are known spanning the series of  [Cu(N3)3]-, [Cu(N3)4]2-, and [Cu(N3)6]2-.21 Three coordinate copper azide complexes form linear 1D chains with two EE and one EO azido ligands in contrast with the Mn analogue that forms a 3D structure.16 The dinuclear species and [Cu(N3)4]2- are both monomeric in nature.32

Silver (I) azide is a well known explosive compound and has been demonstrated to form a 2D coordination polymer with square planar Ag+ ions surrounded by azido ligands in an EE fashion.33

Gold(III) azide is known as the tetraethylammonium salt [Et4N][Au(N3)4] and also adopts a square planar structure.25However unlike the silver azide, the gold azide is not stable at room temperature and will decompose after a few days and its metal azide bonds have significant covalent character.

Group 12
While Zn(N3)2 has been known since the late 1890s, solvent free Zn(N3)2 was isolated for the first time in 2016 from a dry ethereal solution of HN3 and Et2Zn in n-hexane.34 Zn(N3)2 crystallizes in three different polymorphs α-Zn(N3)2 and the labile β-Zn(N3)2 and γ-Zn(N3)2 forms.

The first mercury (I) azide was realized by Curtius in 1890 by combining aqueous mercury(I) salts with alkali metal azides or by combining HN3 with elemental mercury to produce Hg2(N3)2 and the compound is stabilized by ionic bonds between the azido ligands N3- and Hg+ and covalent bonding between +Hg-Hg+.35, 36 Both mercury (I) and mercury(II) azides can be easily prepared by mixing the respective mercury nitrates with sodium azide in aqueous solution at roomtemperature.36 The mercury (II) azide Hg(N3)2 exists in two polymorphs α-Hg(N3)2 and β-Hg(N3)2. The β form is very labile and quickly turns into the α polymorphs at room temperature. However, the β polymorph can prepared in analogy to β-Pb(N3)2 by slow diffusion of aqueous NaN3 into a solution of Hg(NO3)2 separated by a layer of aqueous NaNO3, but crystals nearly always explode during formation leading to a mixture of α and β polymorphs.

Binary cadmium azide Cd(N3)2 can be prepared from CdCO3 and aqueous HN3.37 However, it is structural unrelated to the mercury or zinc anaolgues and is based on repeat units of Cd2(N3)10 double octahedrals.