Structural and mechanistic information on the reaction of bicarbonate with Cu(II) and Zn(II) complexes of tris(2-aminoethyl)amine. Identification of intermediate and product species

Abstract

A series of binuclear bicarbonato, trinuclear carbonato and binuclear hydroxo complexes were isolated in the reaction of [M(tren)(H₂O)](ClO₄)₂ (M = Cu(II), Zn(II) and tren = tris(2-aminoethyl)amine) with NaHCO₃ at pH ca. 6.5, 8.5 and 10.0, respectively, and physically characterized. The structures of two trinuclear carbonato complexes, {[Cu(tren)]₃(μ₃-CO₃)}(ClO₄)₄·H₂O and {[Zn(tren)]₃(μ₃-CO₃)}(ClO₄)₄·H₂O were determined by X-ray analysis. UV-Vis spectra of [Cu(tren)(H₂O)](ClO₄)₂ were recorded as a function of pH in the absence and presence of NaHCO₃, and reveal evidence for a carbonation process in the pH range 6.0 to 9.5 and a hydrolysis process in the range pH 9.5 to 12. ¹³C NMR measurements on [Zn(tren)(H₂O)](ClO₄)₂ as a function of pH in the presence of NaH¹³CO₃, indicated that the Zn(II) complex behaves similarly to the Cu(II) complex in solution, and that the polynuclear carbonato complexes are only formed at low concentrations in solution. The kinetics of the reaction of [Cu(tren)(H₂O)]²⁺ with HCO₃⁻ was studied by stopped-flow using a pH-jump technique. The results indicated that complex-formation of [Cu(tren)(H₂O)]²⁺ with HCO₃⁻ is too fast to be resolved kinetically. The observed kinetics and second-order rate constant of 494 ± 10 M⁻¹ s⁻¹ were assigned to the rate-determining formation of a binuclear carbonato complex. A plausible mechanism for the carbonation of CuN₄ complexes that include an axial water molecule in a trigonal bipyramidal structure, is proposed.