Nitrogen-Rich Heterocycles – A Study of the Use of Tetrazole and Pentazole as Ligands

Conventional energetic materials are often hazardous to health, which can be caused by the inclusion of heavy metals or the release of harmful by-products such as perchlorates. Nitrogen-rich compounds have seen intensive research in the hope that they offer a “green” alternative to the energetic materials commonly used today.
The initial area of concentration of this investigation focussed on the refinement of the current preparation of homoleptic hexakistetrazolato main group complexes of the type (PPN)2[E(CHN4)6] (E = Si, Ge) in order to obtain accurate analytical data. This preparation did not produce the unreported tin analogue (PPN)2[Sn(CHN4)6]. The reaction between 1-trimethylsilyl-tetrazole and tin(IV) fluoride provided the required driving force to allow the isolation and characterisation of (PPN)2[Sn(CHN4)6]. This preparative technique was subsequently extended to the low-valent tin(II) fluoride in order to produce the first reported homoleptic tetrazolato tin co-ordination polymer. An additional route to the homoleptic poly(tetrazolato) main group compounds was also explored. It has been shown that by heating an acetonitrile solution containing main group co-ordinated azides, the azide groups can undergo a [3+2] cycloaddition, producing a methyltetrazolate ligand. This transformation has been previously observed for a series of bis(tetrazolato) diazido silicon complexes Si(N3)2(5-R-CN4)2(L), where R = CH3, C6H5, and L = 2,2’-bipyridine, 1,10-phenanthroline. However the transformation has not been catergorically proven for the homoleptic compounds. For this reason various azido co-ordinated species were synthesised and reacted with acetonitrile. The resulting precipitates were analysed using infra-red (IR) and nuclear magnetic resonance (NMR) spectroscopy.
A final study attempted the isolation of a free pentazole ring through the reduction of arylpentazoles. The reducing agents sodium naphthalenide and the (mesnacnac)Mg−Mg(nanacmes) dimer were used, whilst the potential for selective cleavage of the C-N bond and stabilisation of the produced pentazolate anion was evaluated using IR and 1H NMR spectroscopy. In parallel to this various re-crystallisations of arylpentazoles were attempted in order to widen the availability of pure starting materials.