Phosphorus–nitrogen compounds: part 71. Novel unsymmetrically-substituted dispiro-cyclotriphosphazenes: synthesis, characterization, antituberculosis activity, and phototunable charge storage studies

Abstract

The aim of this research is to synthesize and characterize new organic–inorganic hybrid multi-heterocyclic dichlorodispiro(N/N)cyclotriphosphazenes and to elucidate their antituberculosis activities. To achieve this aim, primarily, trans (6a–11a) and cis (6b–11b) unsymmetrical dichlorodispiro(N/N)cyclotriphosphazenes were obtained from ferrocenyldiamines (4 and 5) and benzyl-pendant armed tetrachloromonospirophosphazenes as starting materials. Several spectroscopic techniques (mass spectrometry, FTIR, and ¹H, ¹³C, and ³¹P NMR), as well as elemental analysis, were employed to characterize the new phosphazenes. All unsymmetrically substituted phosphazenes are expected to be racemic mixtures (RR′/SS′ and RS′/SR′) due to two different stereogenic P-centers. In addition, the molecular and crystal structures of cis-6b, cis-10b, and cis-11b isomers were elucidated via single-crystal X-ray crystallography. They belong to centrosymmetric C2/c, P2₁/c, and P space groups, respectively; thus, their crystal lattices must contain both the enantiomers, RR′/SS′ or SR′/RS′. Moreover, only one oxidation–reduction wave was observed for the ferrocenyl-dispirophosphazenes in the reversible cyclic voltammograms. The HOMO and LUMO energy values of the phosphazenes were calculated using the oxidation potential (E_ox) and the optical band gap values using Tauc plots, respectively. To determine the phototunable charge storage properties of the four dispirophosphazenes, their memory functions were investigated under UV illumination. The antituberculosis activities of the dichlorodispiro(N/N)cyclotriphosphazenes trans-6a, cis-6b, trans-10a, and cis-10b against Mycobacterium tuberculosis H37Rv (ATCC 27294) strain were evaluated. The results showed that among these compounds, trans-6a (MIC value 18 μg mL⁻¹) was much more effective than the other phosphazenes.