New insight into the coordination and reactivity of hetero-substituted maltol and method development for complex mixture analysis of asphaltene.

Abstract

The metal complexes of maltol and its hetero-substitutions have a wide variety of bioinorganic applications. However, the latest addition to this family of molecules, 3-hydroxy-2-methyl-4H-pyran-4-selenone (selenomaltol), had not previously had its metal complexes characterized. To remedy this, transition metal complexes were made using Fe(III), Ni(II), Cu(II) and Zn(II) to characterize the homoleptic-complexes of selenomaltol and compare them to previously published maltol-like ligands. The selenomaltol complexes were found to be similar to thiomaltol complexes. In addition, new insights were gained into the redox chemistry of the thiomaltol and selenomaltol ligands, and the complexes of selenomaltol and thiomaltol were found to be more aromatic than the free ligand. The increased aromaticity of the complexes over the ligands was thought to be due to the complexes stabilizing the pyrylium resonance structure of hetero-substituted maltols. To explore if the free ligand exhibited this resonance, NMR spectroscopy and reactivity studies were performed. Polar solvents seemed to favor the pyrylium resonance, and significantly impacted the nucleophilicity of the hetero-substituted maltol ligands. This reactivity study led to the generation of the two new ligands 3-hydroxy-2-methyl-4-(methylsulfanyl)pyrylium and 3-hydroxy-2-methyl-4-(methylselanyl)pyrylium. This new ligand class is of interest as it contains a stable pyrylium species, which can be used to generate a new family of ligands for use in bioinorganic chemistry. The second project explored was on the development of a new method for investigating asphaltenes, the heaviest and most problematic fraction of crude oil, by NMR spectroscopy and mass spectrometry. To study asphaltenes by mass spectrometry, we reacted a sample of asphaltene with elemental bromine (Br2). This was done to selectively substitute the aromatic protons on asphaltenes to probe their structure. The subsequent high-resolution mass spectra were visualized by Kendrick Mass Defect (KMD) plots. These reactivity studies showed fewer aromatic protons available to react than predicted by current models of asphaltene. To more accurately characterize the number of aromatic protons present, a new method for performing 1H NMR spectroscopy was developed, which removed paramagnetic materials from the sample.

Description
Keywords
Selenomaltol. Maltol. NICS. Crude oil. KMD. NMR. Asphaltene.
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