Kinetic and equilibrium study of the reaction of [ meso-tetrakis( p-sulfonatophenyl)porphinato]diaquachromate(III) with pyridine in aqueous solution

Abstract

The reaction of pyridine (py) in aqueous solution with [ meso-tetrakis( p-sulfonatophenyl)porphinato]diaquachromate(III) (CrTPPS(H 2O) 2 3−) has been studied at 15, 25 and 35 °C in μ = 1.00 M (NaClO 4) from pH = 5.00 to 1.00 M NaOH. The free py concentration, [py], was varied from 1 × 10 −4 to 1.00 M. The species CrTPPS(H 2O) 2 3−, CrTPPS(OH)(H 2O) 4−, CrTPPS(OH) 2 5−, CrTPPS(py)(H 2O) 3−, CrTPPS(OH)py 4− and CrTPPS(py) 2 3− were all observed. However, the ligation reactions of CrTPPS(H 2O) 2 3−, CrTPPS(OH)(H 2O) 4− and CrTPPS(py)(H 2O) 3− were the only kinetically important ones observed. The values of the various stability constants, rate constants and activation parameters are reported. The porphyrin ligands labilize the Cr(III) to axial substitution and the OH − ligand labilizes it even more. But, there are no apparent trends reflected in the values of the activation parameters of the various paths. A comparison to reactions with NCS − and imidazole imply that the reactions are dissociatively activated.