Multiple chromisms associated with dithizone
Ntoi, Lumanyano L. A.
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Ever since discovery of the intensely colored dithizone ligand, H2Dz, in the late 1800s it had been used extensively in aqueous solution trace metal analyses. However, hardly any effort had gone into developing the water-soluble derivative. The main aim of this study was to synthesize such derivatives by symmetrically placing para substituents that enhances water-solubility. The three substituents that were involved were the carboxylic acid, sulfanilic acid, and sulfonylacetamide groups. First step syntheses, up to nitroformazan formation, was successful for all three derivatives. Quality single crystals were grown for the former and latter species, but crystal data reported only for the carboxylic acid derivative, the only set being of publishable standard. The p-COOH derivative was the only one to be successfully synthesized and characterized up to the final dithizone and metal complex products. Spectral properties of both the free metals and ligands, as well as its complexes, limited the number of metals to be involved in consequent mole-ratio and continuous variation studies. These studies were embarked on, in order to find well-working and inexpensive undergraduate spectrometry practical’s – in water media – that may be employed in training classes, apart from its wider analytical applications. The metals that were involved were Co, Ni, Pb, Hg and Ag, and for comparison purposes, the unsubstituted dithizone ligand was also employed, next to its p-COOH derivative. Resulting metal to ligand coordination ratio’s were in agreement with mass spectrometry data, however, in some cases surprising, as discussed in detail in the Results chapter. During these investigations seven chromisms associated with dithizone were observed, some expectantly and some unexpectantly. Presently there exists no other known compound with such varied and sensitive chromic behaviour. The new p-COOH-H2Dz derivative gave rise to many of these discoveries. For comparison purposes the p-SCH3-H2Dz, p-CH3-H2Dz and unsubstituted derivatives were also employed where necessary. Also the derivative that are methylated at the backbone sulphur of the ligand, SCH3-HDz, were brought into comparison The former three were found to be halochromic, solvatochromic, electrochromic and concentratochromic as free ligands, while p-SCH3-H2Dz also revealed thermochromic behaviour, in ethanol alone. The mercury complexes of all three species were photochromic, as was the free SCH3-HDz ligand itself – for the first time seen in a purely organic dithizone species. The latter was also chronochromic, i.e. changing color over time. The back reactions of the photochromic species were studied kinetically, during which solvent, concentration, temperature and varied substituents were compared.