Electron paramagnetic resonance, kinetics of formation and decomposition studies of (Bis(hydroxyethyl)amino-tris(hydroxymethyl)-methane)oxochromate(V): A model chromium(V) complex for DNA damage studies

Abstract

A new chromium complex, (bis(hydroxyethyl)amino-tris(hydroxymethyl)methane)oxochromate(V), has been characterized by epr spectroscopy. The chromium(V) complex was formed by the ligand displacement reaction of bis(2-ethyl-2-hydroxybutanato) oxochromate(V) with bis(hydroxyethyl)amino-tris(hydroxy-methyl)methane (BT). Both epr and kinetic data indicate that the reaction proceeds through a chromium(V) intermediate. Kinetics of formation of the intermediate exhibit a rate saturation at higher [BT] (>30 mM) indicating a rate law constituting an equilibrium between the parent Cr(V) complex and the bis–tris ligand followed by a pure first order process. The g-value of the intermediate is consistent with a Cr(V) complex in which the BT is coordinated in a bidentate fashion replacing a coordinated hydroxy butanoic acid ligand, affording a mixed ligand complex. The equilibrium step ( K=36 M −1) consists of monodentate coordination by the BT ligand and the limiting first order rate constant (1.9 × 10 −2 s −1) manifests the rate of chelation by the polydentate ligand. The intermediate is converted to the product upon further chelation through the complete displacement of the remaining 2-ethyl-2-hydroxy butanoic acid by a first order process ( k=0.023 s −1). The epr data support a pair of products that are in rapid equilibrium. In these products, BT functions either as a tetra or a penta-dentate ligand coordinating through four or five alkoxy sites. The enthalpy and entropy of activations related to the two chelation steps were found to be 32 ± 2 kJ/mol and −(1.7 ± 0.2) × 10 2 J/mol K for the intermediate, and 36 ± 1 kJ/mol and −(1.5 ± 0.2) × 10 2 J/mol K for the product. Our data support an associative mechanism for the chelation steps. The Cr(V)–BT product is more stable than the parent complex. The second order disproportionation rate constant for the Cr(V)–BT complex was evaluated to be 0.1 M −1 s −1 compared to 8.0 M −1 s −1 for the parent complex. This is the first example of a chromium(V) complex with a non-macrocyclic ligand coordinating through oxygen donor atoms which is stable in aqueous solution at neutral pH over a long period of time.