Synthesis and properties of a dizinc complex of a novel macrocyclic ligand with two alcohol-pendants: a model for alkaline phosphatase

Abstract

A novel 24-membered macrocyclic compound having two hydroxyethyl pendants L, 3,6,9,17,20,23-hexazatricyclo[23.3.1.1.^11,15]triconta-1(29),11(30),12,14,25,27-hexaene-6,20-bis(2-hydroxyethyl), has been synthesized as a dinucleating ligand. A new complex [Zn₂H₋₂L]Br₂(H₂O)₂ has been synthesized as a model to mimic the active center of alkaline phosphatase. The bridging coordination of the two alkoxides to two Zn^II ions was confirmed by the crystal structure of [Zn₂H₋₂L]Br₂(H₂O)₂. The protonation constants of L as well as complexation constants of L with Zn^II have been determined at 298.1 K by means of potentiometric titration. [Zn₂H₋₂L]²⁺ and [Zn₂H₋₂L(OH)]⁺ are the dominant species in aqueous solution at 7 < pH < 9. The kinetics of promoted hydrolysis of p-nitrophenyl acetate has also been studied; the second-order rate constant is 0.018 M⁻¹ s⁻¹ at pH = 9.0.