Kinetics of b-haematin formation from suspensions of haematin in aqueous benzoic acid

Kinetics of b-haematin (synthetic malaria pigment) formation from haematin have been studied in the presence of aqueous benzoic acid and derivatives of benzoic acid. Formation of the b-haematin product is demonstrated by X-ray diffraction and IR spectroscopy. Reactions were followed by determining the fraction of unreacted haematin at various time points during the process via reaction of extracted aliquots with pyridine. The kinetics can be fitted to the Avrami equation, indicating that the process involves nucleation and growth. Reaction kinetics in stirred benzoic acid are similar to those previously observed in acetic acid, except that benzoic acid is far more active in promoting the reaction than acetic acid. The reaction reaches completion within 2 h in the presence of 0.050 M benzoic acid (pH 4.5, 60 • C). This compares with 1 h in the presence of 4.5 M acetic acid and 4 h in the presence of 2 M acetic acid. The reaction rate in benzoic acid is not affected if the stirring rate is decreased to zero, but very vigorous stirring appears to disrupt nucleation. The rate constant for b-haematin formation in benzoic acid has a linear dependence on benzoic acid concentration and follows Arrhenius behaviour with temperature. There is a bell-shaped dependence on pH. This suggests that the haematin species in which one propionate group is protonated and the other is deprotonated is optimal for b-haematin formation. When the reaction is conducted in para-substituted benzoic acid derivatives, the log of the rate constant increases linearly with the Hammett constant. These findings suggest that the role of the carboxylic acid may be to disrupt hydrogen bonding and p-stacking in haematin, facilitating conversion to b-haematin. The large activation energy for conversion of precipitated haematin to b-haematin suggests that the reaction in vivo most likely involves direct nucleation from solution and probably does not occur in aqueous medium

Color-Coded Ligands: Tracking the Catalyst using Highly Pigmented Porphyrazine Ligands in Biphasic Reactions

We demonstrate the concept that highly colored imidazolium porphyrazine ligands and complexes may be useful for visualizing the location of a catalyst and/or ligand. This was analytically demonstrated by the application of UV–vis spectrophotometry to detect the ligand and ICP-OES spectroscopy for quantification of the Pd in the biphasic systems. In the first instance, a toluene/water system was used, in which the complex and ligand preferred the organic phase. Water-soluble substrates were made to react under these conditions by employing the complexes as catalysts in Heck and Suzuki C–C bond forming reactions. In the second instance, an ionic liquid containing biphasic mixture was used, in which the ligand/catalyst was retained in the ionic liquid. Catalyst recycling experiments were met with limited success