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

A series of binuclear bicarbonato, trinuclear carbonato and binuclear hydro xo 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)(mu (3)-CO3)}(ClO4)(4).H2 O and {[Zn(tren)](3)(mu (3)-CO3)}(ClO4)(4).H2O were determined by X-ray ana lysis. UV-Vis spectra of [Cu(tren)(H2O)](ClO4)(2) were recorded as a functi on of pH in the absence and presence of NaHCO3, and reveal evidence for a c arbonation process in the pH range 6.0 to 9.5 and a hydrolysis process in t he range pH 9.5 to 12. C-13 NMR measurements on [Zn(tren)(H2O)](ClO4)(2) as a function of pH in the presence of (NaHCO3)-C-13, indicated that the Zn(I I) complex behaves similarly to the Cu(II) complex in solution, and that th e polynuclear carbonato complexes are only formed at low concentrations in solution. The kinetics of the reaction of [Cu(tren)(H2O)](2+) with HCO3- wa s 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 bipyrami dal structure, is proposed.