Crystal structure of Macrocyclic azidotetraamine Zinc(II) complex

The Zn(II)-containing carbonic anhydrase (CA) has stimulated much effort into the syntheses of simple model complexes that are designed to mimic the coordination environment of the active site. The Zn(II) in CA is coordinated to three histidine imidazols and a water molecule in a distorted tetrahedral geometry, which is considered to expand to five in a transient manner or by anion inhibitor binding. Recently, X-ray crystal analysis of the SCN- binding to CA showed the Zn(II) ion in an ill-defined five-coordinate complex with SCN- and a water bound. Kimura et al. discovered that a tetrahedral Zn(II)-OH2 triamine complex with 1,5,9-triazacyclododecane(L2) is a good model for the active center of CA. In this complex the pKa of the coordinated water molecule is close to that of CA, and the generated L2-Zn(II)-OH species acts as nucleophile in CA-catalyzing reaction, such as in hydrolysis of activated ester. Additional study on the crystal structure of the SCN- binding to Zn(II) complex of L2 showed a trigonal bipyramidal geometry with an equatorial and an apical Zn(II)-NCS bonds. We also reported the crystal structure of [Zn(L1)(NCS)][NCS], in which the Zn(II) ion reveals a square pyramidal geometry with an apical thiocyanate nitrogen atom. We report herein the preparation and crystal structure of the Zn(II) complex of L1 with azide ligands