Kinetics and mechanism of substitution of bis(2,4,6-tripyridyl 1,3,5-triazine)iron(II) by 2,2′,6,2″-terpyridine

Abstract

The substitution of bis(2,4,6-tripyridyl 1,3,5-triazine)iron(II), \( {\text{Fe(TPTZ)}}_{ 2}^{{ 2 { + }}} \) by 2,2′,6,2″-terpyridine (terpy) occurs on a time scale of about 6 m. The kinetics of this reaction was followed by stopped-flow spectrophotometry in the pH range of 3.6–5.6 in acetate buffer. The data indicate that the reaction occurs in two consecutive steps: kinetic data for both steps were acquired simultaneously and analyzed independently. The first step is assigned to the reaction between \( {\text{Fe(TPTZ)}}_{ 2}^{{ 2 { + }}} \) and terpy to give Fe(TPTZ)(terpy)2+, followed by its reaction with another terpy molecule to give the final product, \( {\text{Fe(terpy)}}_{ 2}^{{ 2 { + }}} \). The rate of the reaction increases with increases in [terpy] and pH. The kinetic and activation parameters determined for both steps suggest that they involve both associative and dissociative paths. The ternary complex Fe(TPTZ)(terpy)2+ has been prepared, and the kinetics of its reaction with terpy suggest that this reaction is identical with the second step of the \( {\text{Fe(TPTZ)}}_{ 2}^{{ 2 { + }}} \)-terpy system, supporting the proposed mechanism.