Pressure studies on the metallation of a supra-molecular porphyrin complex

Abstract

The complex formation of meso-tetrakis-(1-methylpyridinium-4-yl)porphine p-toluenesulfonate (TMPyP) with zinc ions was examined at various pressures. By the application of high pressure, the second-order reaction rate constant for the metallation was increased to give the negative volume of activation (ΔV{). This result suggested that the activated complex is more strongly hydrated than the reactants. By the presence of anionic polyelectrolytes such as polyacrylic acid sodium salt (NaPAA, polymers with five molecular weight ranges from 1.1×103 to 6.6×105 were used) and poly(l-glutamic acid sodium salt) (Mw = 3.7×104), the metallation reaction was largely accelerated, whereas neither poly(l-lysine HBr salt) (Mw = 3.6×103) nor poly(ethylene glycol) (Mw = 1.1×104) affected the reaction at all. The acceleration effect of polyanions could be intuitively attributed to the accumulation of the cationic TMPyP in the vicinity of the polyanion to afford a supra-molecular porphyrin complex (polyanion-TMPyP), resulting in the increase in the collision probability of TMPyP with a zinc ion which was also accumulated in the polyanion domain. The degree of acceleration by the presence of NaPAA was the largest when the molecular weight was 2.5×103. The volume of activation (ΔV{) in the presence of polyanions was less negative in comparison with that for the spontaneous reaction, due to the more pronounced dehydration of the activated complex than the reactants in the vicinity of the polyanion. A good correlation between the ΔV{ value and the entropy of activation (ΔS{) for the metallation in the presence of various polyanions supported that the acceleration effect could be related to the solvation effects.