Selective coordination of gallium(III), zinc(II), and copper(II) by an asymmetric dinucleating ligand: a model for metallophosphatases

Complexation studies of the dinucleating ligand H3L (H3L= 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 H3L 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