Kinetics and Mechanism of Cobalt(II) Porphyrin [Co(TMPyP)] ⁴⁺ Formation in the Aqueous Solution

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.