Kinetics and mechanism of aquation of chromium(III) complexes with 3-hydroxypicolinic and 2-hydroxynicotinic acids in HClO_{4{ solutions

Abstract

Two complexes, [Cr(3-hpic)_{3}]^{0} and [Cr(2-hnic)_{3}]^{0} (where 3-hpic = hydroxypicolinic acid and 2-hnic = 2-hydroxynicotinic acid anions), were prepared and characterized in solution. The 3-hpic ligand forms a 5-membered chelate ring via pyridine nitrogen and carboxlate oxygen atoms, whereas the 2-hnic ligand forms a 6-membered chelate ring via carboxylate and phenolate oxygen atoms. The kinetics of the acid-catalyzed aquation were studied spectrophotometrically in the 0.1–1.0 HClO_{4} range, at I = 1.0 . The rate equations for the first aquation step – the chelate-ring opening – was determined and a mechanism was proposed. In the case of [Cr(3-hpic)_{3}]^{0}, the reversible chelate-ring opening at the Cr—N bond precedes much slower than the second aquation step – a one-end bonded ligand liberation. The equation rate is of the form: k _{obs} = k _{1} + k _{-1} /Q _{1}[H^{+}], where k _{1} and k _{-1} are the rate constants for the forward and the reverse processes in the unprotonated substrate and Q _{1} is the protonation constant of the non-bonded pyridine nitrogen atom. In the case of [Cr(2-hnic)₃]^{0}, the chelate-ring opening at the Cr—O (phenolate) bond is the rate-determining step. The observed pseudo-first order rate constant increases as [H^{+}] increases: k _{obs} = k _{0} + k _{H} Q _{H}[H^{+}], where k _{0} and k _{H} are the rate constants of the spontaneous and acid catalyzed processes and Q _{H} is the protonation constant of the coordinated phenolate oxygen atom. The results lead to the conclusion that an aquation mechanism depends on the coordination mode of the ligand.