Doctor of Philosophy (PhD)
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chemistry, rhodizonate, croconate, oxalate
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The rhodizonate dianion is of importance for forensic testing, specifically with regard to the sodium rhodizonate test for lead in gunpowder residues. In an effort to understand this test from a chemical perspective, the coordination of rhodizonate and its decomposition products, croconate and oxalate, with several metals has been investigated with a variety of structural and spectroscopic methods.
FT-IR data of the three sequential steps of the sodium rhodizonate test show coordination between Pb(II) atoms, tartrate anions, and rhodizonate dianions, with the tartrate being substituted for Cl- once a 5% HCl solution is added to produce the confirmatory blue-purple color for Pb. XPS data confirms the coordination of Cl- to Pb(II) with a shift from the major XPS peak of the Pb 4f7/2 peak from 138.6 eV to 139.0 eV. UV-Vis titrations show that Pb(II) and rhodizonate bind in a 1:1 ratio in water/ethanol (50/50 by volume). This solvent system was essential for generating these results, as in pure water, the formed Pb-rhodizonate complex immediately precipitates.
Rhodizonate was also shown to complex with lanthanides. UV-Vis titrations show that rhodizonate coordinates to lanthanides in 1:1 ratio in water/ethanol (50/50 by volume). However, rhodizonate has higher affinity for Pb(II) over several Ln(III).
Metal-rhodizonates with N-donating co-ligands such as 1,10-phenanthroline, 2,2’-bipyridine, and imidazole, were also shown to coordinate to toxic metals, such as Hg(II) in a 1:1 metal-rhodizonate/co-ligand ratio in water/ethanol (50/50 by volume), regardless of whether the co-ligand was added to a solution of metal-rhodizonate, or the metal was added to a solution of rhodizonate and co-ligand.
Dilute solutions (~10-5 M) of sodium rhodizonate, which are suitable for UV-Vis spectroscopic work will typically decompose and lose their characteristic orange color in about 2 h, though full decomposition into croconate takes upwards of one week. We have now shown that by dissolving rhodizonate in water/ethanol (50/50 by volume), the stability of rhodizonate in solution is extended by several hours, allowing for accurate UV-Vis spectroscopic measurements, titration studies, and determination of binding constants.
Crystals of the five-member croconate, a decomposition product of rhodizonate, with Pb(II) and Cd(II) show marked differences. Pb(II)-croconate crystals feature 3D-layers of alternating Pb(II) and croconate, while Cd(II)-croconate crystals feature 1D-layers of alternating Cd(II) and croconate held together through a hydrogen bonding network. Metal-rhodizonate solutions left over longer times will further decompose into oxalates, which can then form, 3D metal-oxalate frameworks. The 3D structures are unique among metal oxalates, forming large pores, in contrast to 1D and 2D metal-oxalates previously reported in the literature.
Previously Published In
Silverman, J. A., Mathivathanan, L., Govor, E. V., Raptis, R. G., Kavallieratos, K., Acta Cryst C, 2019, 75, 935-940. https://doi.org/10.1107/S2053229619007277.
Silverman, Joshua A., "The Coordination Chemistry of the Rhodizonate Dianion and Its Decomposition Products" (2021). FIU Electronic Theses and Dissertations. 4714.
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