Oxalate-templated synthesis of di-zinc macrocycles

Abstract

Herein we report that oxalate serves as a template in the formation of di-zinc macrocycles featuring bis(imino)pyridine chelating units linked by p-xylylene or m-xylylenes linkers. The resulting complexes were characterized by NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray crystallography. NMR spectroscopy indicates highly symmetrical (C2v or D2h) structures of the macrocyclic complexes in solution. Solid-state structures, determined by X-ray crystallography (obtained only for the m-xylylene bridged macrocycles), revealed hexa-coordinate zinc centers with μ2-κ2,κ2-bound oxalate, meridional NNN chelates, and an additional ligand (DMF or H2O) bound to zinc centers. Cyclic voltammetry demonstrates series of reductions associated with the redox-active bis(imino)pyridine chelates; no oxidation was observed up to 1V. We have attempted to extract oxalate from the macrocycle using calcium bromide, ethylenediaminetetraacetic acid (EDTA), trimethyl silyl chloride (TMSCl), acetic acid, or hydrochloric acid (HCl). Of the above, EDTA, TMSCl, acetic acid, and HCl in ether failed to remove oxalate or to form a new isolable product, while calcium bromide transformed μ2-κ2,κ2-bound oxalate into the μ2-κ1,κ2-bound form. The addition of aqueous HCl enables observation of the oxalate-free complex by mass spectrometry as one of the major products.