Direct Spectrophotometric Measurement of Demetalation Kinetics of 5,10,15,20-Tetraphenylporphyrinatozinc(II) at the Liquid−Liquid Interface by a Centrifugal Liquid Membrane Method

Abstract

The equilibrium and kinetics of the protonation of 5,10, 15,20-tetraphenylporphyrin (H 2 TPP) and the demetalation of 5,10,15,20-tetraphenylporphyrinatozinc(II) (ZnTPP) at a dodecane−aqueous acid interface were investigated by means of a new in situ spectrophotometric method, the centrifugal liquid membrane method, which can provide the ultrathin two-phase liquid membrane system in a rotating glass cell. The consumption of H 2 TPP in the bulk dodecane phase and the production of the diprotonated aggregate, (H 4 TPP 2+ ) n, adsorbed at the liquid−liquid interface were directly measured from the spectral change. The equilibrium constants of the interfacial aggregation of H 4 TPP 2+ and the demetalation of ZnTPP were determined as log( K e1 /dm 6 mol - 2 ) = 2.14 ± 0.07 and log( K e2 /dm 9 mol - 3 ) = −6.05 ± 0.04 at 298 K, respectively. The observed rate constant of the demetalation of ZnTPP depended upon the first order of the acidity function, and it was suggested that the rate-determining step is the formation of the monoprotonated intermediate, [ZnTPPH] +, at the liquid−liquid interface. The demetalation rate constant of ZnTPP was determined as k 1 = (8.6 ± 1.3) × 10 - 5 dm 3 mol - 1 s - 1 at 298 K. In the aggregation of H 4 TPP 2+, the rate-determining step was controlled by molecular diffusion of H 2 TPP in the bulk dodecane phase.