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

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-hy droxybutanato) oxochromate(V) with bis(hydroxyethyl)amino-tris(hydroxy-meth yl)methane (BT). Both epr and kinetic data indicate that the reaction proce eds through a chromium(V) intermediate. Kinetics of formation of the interm ediate exhibit a rate saturation at higher [BT] (>30 mM) indicating a rate law constituting an equilibrium between the parent Cr(V) complex and the bi s-tris ligand followed by a pure first order process. The g-value of the in termediate is consistent with a Cr(V) complex in which the BT is coordinate d in a bidentate fashion replacing a coordinated hydroxy butanoic acid liga nd, affording a mixed ligand complex. The equilibrium step (K=36 M-1) consi sts of monodentate coordination by the BT ligand and the limiting first ord er rate constant (1.9 x 10(-2) s(-1)) manifests the rate of chelation by th e polydentate ligand. The intermediate is converted to the product upon fur ther 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 pro ducts, BT functions either as a tetra or a penta-dentate ligand coordinatin g through four or five alkoxy sites. The enthalpy and entropy of activation s related to the two chelation steps were found to be 32 +/- 2 kJ/mol and - (1.7 +/- 0.2) x 10(2) J/mol K for the intermediate, and 36 +/-: 1 kJ/mol an d -(1.5 +/- 0.2) x 10(2) J/mol K for the product. Our data support an assoc iative mechanism for the chelation steps. The Cr(V)-BT product is more stab le than the parent complex. The second order disproportionation rate consta nt 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 chromiu m(V) complex with a non-macrocyclic ligand coordinating through oxygen dono r atoms which is stable in aqueous solution at neutral pH over a long perio d of time. (C) 1998 Elsevier Science Inc. All rights reserved.