Selective Coordination of Gallium(III), Zinc(II), and Copper(II) by an Asymmetric Dinucleating Ligand: A Model for Metallophosphatases.

Abstract

Complexation studies of the dinucleating ligand H3 L (H3 L=2-{[bis(pyridin-2-ylmethyl)amino]methyl}-6-{[bis(6-pivaloylamidopyridin-2-ylmethyl)amino]methyl}-4-methylphenol), with metal-binding sites A and B, which both provide four donors to a metal ion; a tertiary amine; two pyridines (substituted with amide hydrogen-bond donors in site B), and a bridging phenolate, with Zn(II) , Cu(II) , and Ga(III) are reported. The titration of H3 L with the three metal ions in solution was monitored by NMR spectroscopy or EPR and UV/Vis/near-IR spectroscopy, as well as by ESI-MS to analyze the selectivity of the two metal-ion sites A and B of this model ligand for metallophosphatases; the spectroscopic assignments are supported by X-ray crystallography results. The first Zn(II) ion coordinates to site A with unsubstituted pyridine donors and, upon addition of a second equivalent of Zn(II) , this coordinates to the sterically less accessible site B. From a similar titration with Ga(III) , it emerges that only a mononuclear complex is obtained, with the Ga(III) center coordinated to site A. When one equivalent of Ga(III) is reacted with the mononuclear Zn(II) complex, Zn(II) is forced by Ga(III) to exchange the site; this results in a dinuclear complex with Ga(III) in site A and Zn(II) in site B. With Cu(II) , two isomers are observed: one with and the other without a bridging phenolate; these differ significantly in their spectroscopic and magnetic properties.