Solution Studies of Meso-Tetra(p-sulphophenyl)porphine: Aggregation, Metallation and Potential Analytical Applications

Abstract

<p>Meso-tetra(p-sulphophenyl)porphine (TPPS₄), a synthetic water-soluble porphyrin, has been prepared in high purity and characterized in greater detail than previously reported. Several metal complexes of TPPS₄ have been prepared by a novel heterogeneous reaction. This simple reaction was also used in the synthesis of complexes of other porphyrins.</p> <p>The solution behaviour of TPPS₄ was investigated by spectrophotometry and nuclear magnetic resonance methods, with a view toward ultimate analytical applications. In neutral or alkaline ionic media, TPPS₄ was found to dimerize, with KD = 1.58 x 10⁴ Mˉ¹. At higher concentrations, TPPS₄ was shown to aggregate further to tetrameric species, with KTT = 4-15 Mˉ¹. In acidic media, the behaviour of TPPS₄ was found to be complex and completely different from that observed in neutral media. A model involving micellization of TPPS₄ has been advanced to account for observations made in both the present work and that of other workers.</p> <p>The kinetics of metal-ion incorporation, as exemplified by the reaction of Cu(II) with TPPS₄, was studied at pH 3.55. In general, the findings agreed with those of other workers, but an initial slow reaction, not previously reported, was found to occur under conditions favouring aggregation of TPPS₄.</p> <p>The reactivity of TPPS₄ towards metal ions was examined in both neutral and acidic (pH 3.55) solutions. TPPS₄ showed kinetic and thermodynamic selectivity toward a relatively small group of metal ions, with the greatest selectivity towards Cu(II). The anionic metal complexes were found to be extractable as ion-associated species (with tetraphenyl-arsonium nitrate, for example) into organic solvents such as 1-pentanol. Based on these findings, an extraction-spectrophotometric method was developed for the determination of Cu(II), with a detection limit of 1.1 ppb. This limit compares favourably with those of atomic absorption and inductively-coupled plasma emission. With further development, the use of TPPS₄ as a complexing and preconcentration reagent in trace metal analysis appears to be most promising.</p>