Chromium(III) complexes with lutidinic acid: kinetic studies in HClO4 and NaOH solutions

Abstract

Chromium(III)-lutidinato complexes of general formula [Cr(lutH)n(H2O)6−2n]3−n (where lutH− is N,O-bonded lutidinic acid anion) were obtained and characterized in solution. Acid-catalysed aquation of [Cr(lutH)3]0 leads to only one ligand dissociation, whereas base hydrolysis produces chromates(III) as a result of subsequent ligand liberation steps. The kinetics of the first ligand dissociation were studied spectrophotometrically, within the 0.1–1.0 M HClO4 and 0.4–1.0 M NaOH range. In acidic media, two reaction stages, the chelate-ring opening and the ligand dissociation, were characterized. The dependencies of pseudo-first-order rate constants on [H+] are as follows: kobs1 = k1 + k−1/K1[H+] and kobs2 = k2K2[H+]/(1 + K2[H+]), where k1 and k2 are the rate constants for the chelate-ring opening and the ligand dissociation, respectively, k−1 is the rate constant for the chelate-ring closure, and K1 and K2 are the protonation constants of the pyridine nitrogen atom and coordinated 2-carboxylate group in the one-end bonded intermediate, respectively. In alkaline media, the rate constant for the first ligand dissociation depends on [OH−]: kobs1 = kOH(1) + kO[OH−], where kOH(1) and kO are rate constants of the first ligand liberation from the hydroxo- and oxo-forms of the intermediate, respectively, and K2 is an equilibrium constant between these two protolytic forms. Kinetic parameters were determined and a mechanism for the first ligand dissociation is proposed. The kinetics of the ligand liberation from [Cr(lut)(OH)4]3− were also studied and the values of the pseudo-first-order rate constants are [OH−] independent.