Kinetic studies on pH controlled partial dechelation of the trans-imidazole-[Cr(His)2]+ cation

Partial dechelation of one tridentate N,N??,O-histidine in trans-imidazole-Cr(His)₂]⁺ (where N and N?? are amine and imidazole nitrogen atoms, respectively, and O is carboxylate oxygen) has been studied. The aquation process is both acid and base catalyzed and the product with didentate His is converted back into the substrate at pH 4?C7. Kinetics of the chelate ring opening were studied spectrophotometrically within the 0.01?C1.0?M HClO₄ and 0.1?C1.0?M NaOH ranges under first-order conditions. The following dependences of k _obs on H⁺] and OH^?] were determined: $$ \left( {k_{\text{obs}} } \right)_{\text{H}} = a + b\left {{\text{H}}^{ + } } \right] + c/\left {{\text{H}}^{ + } } \right] $$ $$ \left( {k_{\text{obs}} } \right)_{\text{OH}} = a^{\prime } + b^{\prime } \left {{\text{OH}}^{ - } } \right]. $$ Mechanisms of the chelate ring opening in acidic and alkaline media are proposed, and activation parameters for the process in acidic solution have been determined. Additionally, kinetic studies were performed on the second dechelation stage in acidic media, which is ca. 15 times slower than the first one.