Kinetic and equilibrium studies of the copper(II) promoted hydrolysis of salicyl phosphate. A rate acceleration of 108 for the hydrolysis of a phosphate monoester dianion

Abstract

The dissociation constants of salicyl phosphate (H₃L) at 10 °C and I= 0.1 mol dm^–3 have been determined (pK_a values 1.84, 3.57, and 6.18). At a 1 : 1 ligand to metal ratio the interaction of copper(II) with salicyl phosphate can be described by the equilibria given below. The pK_a values Cu²⁺+ H⁺+ L^3–⇌ CuHL logβ₁₁₁= 8.9 ± 0.1 Cu²⁺+ L^3–⇌[CuL]^– logβ₁₁₀= 3.8 ± 0.1 Cu²⁺+ H₂O + L^3–⇌[CuL(OH)]^2–+ H⁺ logβ_{11 – 1}=–2.1 ± 0.1 for the ionisation processes CuHL ⇌[CuL]^–+ H⁺ and [CuL(OH₂)]^–⇌[CuL(OH)]^2–+ H⁺ are 5.1 and 5.9 respectively. Kinetic studies establish that in the copper(II) promoted hydrolysis, the complex [CuL]^– is the active species with K_CuL= 6 × 10⁴ s¹ at 25 °C and I= 0.1 mol dm³. Copper(II) ions promote the hydrolysis of the dianion of the phosphate monoester by a factor of ca. 10⁸. Previous work in this area had indicated only a small rate acceleration (ca. 10 fold) as comparisons were made between the metal-ion promoted reaction and the intramolecular general acid catalysed hydrolysis of the phosphate monoester dianion.