Thermodynamics and Kinetics of the Nickel(II)−Salicylhydroxamic Acid System. Phenol Rotation Induced by Metal Ion Binding

The kinetics and the equilibria of Ni(II) binding to p-hydroxybenzohydroxamic acid (PHBHA) and salicylhydroxamic acid (SHA) have been investigated in an aqueous solution at 25 °C and I = 0.2 M by the stopped-flow method. Two reaction paths involving metal binding to the neutral acid and to its anion have been observed. Concerning PHBHA, the rate constants of the forward and reverse steps are k₁ = (1.9 ± 0.1) × 10³ M^-1 s^-1 and k_-1 = (1.1 ± 0.1) × 10² s^-1 for the step involving the undissociated PHBHA and k₂ = (3.2 ± 0.2) × 10⁴ M^-1 s^-1 and k_-2 = 1.2 ± 0.2 s^-1 for the step involving the anion. Concerning SHA, the analogous rate constants are k₁ = (2.6 ± 0.1) × 10³ M^-1 s^-1, k_-1 = (1.3 ± 0.1) × 10³ s^-1, k₂ = (5.4 ± 0.2) × 10³ M^-1 s^-1, and k_-2 = 6.3 ± 0.5 s^-1. These values indicate that metal binding to the anions of the two acids concurs with the Eigen−Wilkins mechanism and that the phenol oxygen is not involved in the chelation. Moreover, a slow effect was observed in the SHA−Ni(II) system, which has been put down to rotation of the benzene ring around the C−C bond. Quantum mechanical calculations at the B3LYP/lanL2DZ level reveal that the phenol group in the most stable form of the Ni(II) chelate is in trans position relative to the carbonyl oxygen, contrary to the free SHA structure, where the phenol and carbonyl oxygen atoms both have cis configuration. These results bear out the idea that the complex formation is coupled with phenol rotation around the C−C bond.