Spectrophotometric investigations of highly fluorescent metal chelating molecules are of relevance due to their potential application in novel, selective fluorescence-based sensors. Benzene and naphthalene chromophores are highly fluorescent while hydroxamic acids are widely used as ligands for complexation of transition metals. In order to develop fluorescence probes, several phenyl derivatives of N-phenylbenzohydroxamic acid and an aminodihydroxamic acid linked with a naphthalene chromophore were synthesized and their selective ionophoric properties towards iron(III) and manganese(II) ions were investigated using fluorescence and absorption spectroscopy. Both methods confirm the formation of 1:1 and 1:2 complexes for iron(III) and a 1:1 complex for manganese(II). The complex that is formed depends on the concentration of the ligand and pH of the medium. The amino dihydroxamic acid exhibits a prominent selectivity towards iron(III) with a two-step 1:1 and 1:2 quenching mechanism at pH 3 and towards manganese(II) with a 1:1 quenching mechanism at a probe concentration of 1 x 10(-5) mol dm(-3) at pH 9.5 The logarithm of overall formation constants of 1:1 and 1:2 complexes of iron(III) were estimated as 3.30 and 9.05, respectively.

Abstract:The determination of ionization constants and the chelating properties exhibited by a series of derivatives of N-phenylbenzohydroxamic acid, N-phenyllaurohydroxamic acid and 1-naphthalenemethylimminodiacetohydroxamic acid type ligands toward Fe(III) , Cu(II) and Ni(II) ions were studied by pH-metric method. The data obtained by pH-metric method were analyzed by three standard methods namely, Bjerrum's method, Irving and Rossotti method, and Sarkar and Kruck method. The ionization constants of hydroxamic acids and the stability constants of metal-ligand complexes were calculated using the above three methods and it was found that the values obtained closely agreed with each other. The evaluation of the calculated stability constants shows that the substituent effect on the N-phenyl ring leads to more basic character at the carbonyl oxygen and it influences significantly the stability of the complex species formed by the hydroxamate moieties. The stability constant and the species distribution of Fe(III) -1-naphthalenemethylimminodiacetohydroxamic acid system at physiological pH range (6.8 -7.2) suggest that 1-naphthalenemethylimminodiacetohydroxamic acid is an effective source for the iron overload.Keywords: Bjerrum method, hydroxamic acid, Irving and Rossotti method, Sarkar and Kruck method, stability constant IntroductIonHydroxamic acids are a group of naturally occurring or synthetic weak organic acids having the general formula RC(=O)N(R')OH. They are weaker proton donors than the structurally related carboxylic acids RC(=O)OH. These acids are widespread in the tissues of plants, in metabolites of bacteria and fungi, including complex compounds. With regard to the significant complex-forming ability of hydroxamic acid, the biological activity of the ligands must be closely correlated with the formation of their RESEARCH ARTICLE the pK a values of ligands and stability constants of the complexes of Fe(III), cu(II) and ni(II) with some hydroxamic acids: a comparative study of three different potentiometric methods

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N-Phenyllaurohydroxamic acid reacts with titanium(1V) in 9-10 M hydrochloric acid to give a complex that is completely extractable into solvents such as hexane and chloroform. The chloroform extract of the titanium complex, on second extraction from a dilute hydrochloric acid medium (0.1-0.5 M), in the presence of phenylfluorone and isoamyl alcohol, forms an intensely coloured complex possessing an absorption maximum at 540 nm. Even though the molar absorptivity of the complex under optimum conditions a t 540 nm is 2.33 x 105 1 mol-l cm-l, the measurements are more precise at 560 nm with a molar absorptivity of 1.23 x lo5 1 mol-l cm-l. The system obeys Beer's law for up to 0.4 p.p.m. of titanium(1V). Considerable amounts of many cations and anions including a 350-fold molar excess of iron(II1) can be tolerated. Interference from zirconium(1V) can be mitigated or even eliminated by the addition of fluoride ions. The method can be applied to the determination of titanium present a t 10 parts per log.

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