Two Sb(V) compounds, meglumine antimoniate (Glucantime^TM) and sodium stibogluconate (Pentostam^TM), are currently frontline treatments for the neglected tropical disease leishmaniasis. Despite nearly a century of use, the structures of these pentavalent antimonial drugs are still unknown, as are their mechanisms of action. Although elucidation of their structures would help us to understand how the pentavalent antimonials work in the body, common spectroscopic techniques do not provide the whole picture. We sought to explore whether Sb XAS would be useful because of its fast spectroscopic time scale and its sensitivity to molecular geometry. We selected pentaphenylantimony (SbPh₅) as a model system. SbPh₅ can crystallize in either a square pyramidal (SQP) geometry as a nonsolvate or a trigonal bipyramidal (TBP) geometry as a cyclohexane hemisolvate, but –130 °C solution-phase ¹H NMR spectra reveal only a single averaged set of phenyl resonances. Thus, the geometry that this molecule assumes in solution remains unknown. We hypothesized that the solid-state Sb XAS spectra of the two forms of SbPh₅ would exhibit geometry-specific spectral differences. We prepared crystalline samples of both the nonsolvate and cyclohexane hemisolvate forms of SbPh₅, as well as a novel THF hemisolvate; single-crystal X-ray diffraction revealed that it, like the cyclohexane hemisolvate, possesses a TBP geometry. Sb L_III-edge XAS revealed that the three SbPh₅ solids show nearly identical spectra but that there is a distinct difference in the intensity of the lowest-energy pre-edge feature. The intensity of this first pre-edge feature, relative to the second pre-edge feature, is greater for the solids containing TBP SbPh₅ than for the solid containing SQP SbPh₅. The spectrum of a solution of SbPh₅ in THF matches those of the TBP SbPh₅ compounds, suggesting that the molecule assumes a TBP structure in solution. This conclusion was further supported by solid- and solution-phase Raman spectroscopic experiments. In summary, our Sb L_III-edge XAS and Raman spectroscopic data suggest that SbPh₅possesses the VSEPR-compliant TBP geometry in solution. Furthermore, these data display the validity of using XAS as a technique for investigating the structure of Sb-containing molecules in solution.

Monica Weng

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