ABSTRACT The kinetics and mechanism of the Co(II) metalloporphyrin Co(TMPyP) 4+ formation (where H 2 TMPyP 4+ = 5,10,15,20‐tetrakis(N‐methyl‐4‐pyridinium)porphyrin) in aqueous solution were investigated as a function of pH, ionic strength, and Co(II) concentration. Under acidic conditions (pH 3.00 to < 7.00), the Co(H 2 O) 6 2 + species predominates in solution, while at higher pH values, Co(H 2 O) 5 (OH) + and Co(H 2 O) 4 (OH) 2 0 become the dominant species. The reaction between H 2 TMPyP 4+ and Co(II) ions was monitored spectrophotometrically at λ max = 422 nm, which corresponds to the absorption maximum of H 2 TMPyP 4+ . The reaction followed first‐order kinetics with respect to H 2 TMPyP 4+ . Mechanistic analysis revealed that Co(H 2 O) 5 (OH) + is the most reactive species because its OH − ligand forms hydrogen bonds with the pyrrolic protons of the porphyrin core. The observed rate constant ( k obs ) reached a maximum of 1.69 × 10 − 2 s − 1 at pH 7.45, aligning with the peak abundance of the reactive Co(H 2 O) 5 (OH) + species. The formation rate constant ( k f ) was determined to be 1.69 × 10 −2 M −1 s −1 at I = 0.10 M (NaNO 3 ) and 25 ± 1°C. This aqueous kinetic method provides a greener and more physiologically relevant alternative to traditional organic‐phase synthesis.
Upoma et al. (Sat,) studied this question.