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Equilibrium constants (log10 β1) for the formation of nine-coordinate adducts of tetrakis(thenoyltrifluoroacetonato)uranium(IV), [U(TTA)4] have been determined for some imidazole and antipyrine derivatives in benzene solution in which [U(TTA)4] itself is eight-coordinate. Comparing imidazoles with a 2-hydrogen and a 2-methyl substituent, the effect of steric hindrance reverses the order of stability expected from the pKa values of the ligands. For 4-aminoantipyrine complexes coordination occurs through only the carbonyl function as found from 1H NMR spectroscopy. Marked depression of log10β1 is observed for the hindered bases. There is evidence to suggest that the unhindered five-membered imidazoles have greater stability in their nine-coordinate adducts than six-membered pyridines with comparable pKa values. This could be a further demonstration of the separation of five- and six-membered heterocyclic bases through group specific factors. |
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